Light‐Enhanced Conversion of CO2 to Light Olefins: Basis in Thermal Catalysis, Current Progress, and Future Prospects

Carbon dioxide (CO2) valorization to light olefins via sustainable energy input poses great industrial significance for the synthesis of key chemical feedstocks and reduces emission of this potent greenhouse gas. Solar energy, harnessed using light‐capturing catalytic materials, can negate external heat requirements for the energy‐intensive reaction. Presently, photothermal CO2‐Fischer–Tropsch synthesis (FTS)‐dedicated studies remain limited and are focused on the nonselective synthesis of C2+ hydrocarbons. A possible extension in catalyst design may be leveraged upon re‐examination of the better‐established thermal CO2‐FTS in conjunction with studies on photothermal FTS. To this end, herein, a narrative on the prominent chemical mechanisms and existing strategies for Fe‐based catalyst design within thermal CO2‐FTS as a foundation is established. Then, with the intent of regulating product selectivity, a gap in the adaptation of encapsulated structures involving zeolitic frameworks for CO2‐FTS is discussed. Next, current photothermal studies on C2+ hydrocarbon synthesis via FTS, CO2‐FTS, and relevant thermal‐assisted photocatalytic systems involving CO2 conversion are examined. Finally, the possible applications of structures encapsulated by porous media for boosting light utilization for photothermal CO2‐FTS are considered. Overall, the potential for the uptake of strategies aimed at producing multifunctional, light‐responsive future catalysts suitable for CO2‐FTS is explored.


Introduction
The release of anthropogenic carbon dioxide (CO 2 ) far exceeds Earth's uptake in the natural carbon cycle. CO 2 has been widely recognized as a potent greenhouse gas and is a major precursor toward global warming, with fossil fuels usage being the biggest contributor toward its release. [1] The atmospheric concentration of CO 2 has increased steadily over the past century, reflecting the rise in global CO 2 emissions. The annual CO 2 emission stood at over 34.8 billion tons in 2020. [2,3] This is considered a primary culprit toward an enhanced absorption of longer wavelengths of sunlight, leading to a heat-trapping effect, as well as the rise in ocean temperatures and acidity, which can yield serious impact toward the marine ecosystem. [4,5] As such, a significant reduction in the atmospheric emission of CO 2 is necessary.
Effective reduction strategies can include a transition to greener or less carbon-intensive sources of energy, as well as the capturing and storage of CO 2 from point sources or directly from the atmosphere. [6] In view of the continuous dependence on fossil fuel-derived energy in the near future, the storage of this greenhouse gas would invoke a prohibitive cost for maintenance and transportation. [7] As such, the recycling of CO 2 into products of higher value has been considered as an important method to facilitate effective carbon management, in line with the stringent emission scenario for keeping the global temperature increase below 2°C and the impending target for carbon neutrality by 2050. [4,8] There exists a substantial volume of work dedicated toward the conversion of CO 2 to C 1 products such as CH 4 and methanol due to their high energy density, applicability toward synthetic chemistry, and storage benefits. [9] Furthermore, it has been widely recognized that the generation of C 2þ products can offer higher value as both chemical feedstock and energy materials that permeate day-to-day life. Of these, light olefins (C 2 -C 4 alkenes) are widely used as chemical building blocks in areas such as polymer, alcohol, and solvent synthesis. [10,11] Currently, ethylene and propylene are produced at a rate of 150 and 80 billion tons per year, respectively, with the demand for the former having been projected to grow by 1.5-4.1% per year. [12] At present, they are mainly manufactured through steam cracking of hydrocarbons ranging from natural gas liquid (ethane, propane, butane) to petroleum liquids (naphtha and gas oil). Alternatively, Fischer-Tropsch synthesis (FTS) using syngas (CO þ H 2 ) is a versatile strategy to produce hydrocarbons ranging from olefins to aromatics. However, the overall processing associated with these technologies (including feedstock DOI: 10.1002/sstr.202200285 Carbon dioxide (CO 2 ) valorization to light olefins via sustainable energy input poses great industrial significance for the synthesis of key chemical feedstocks and reduces emission of this potent greenhouse gas. Solar energy, harnessed using light-capturing catalytic materials, can negate external heat requirements for the energy-intensive reaction. Presently, photothermal CO 2 -Fischer-Tropsch synthesis (FTS)-dedicated studies remain limited and are focused on the nonselective synthesis of C 2þ hydrocarbons. A possible extension in catalyst design may be leveraged upon re-examination of the better-established thermal CO 2 -FTS in conjunction with studies on photothermal FTS. To this end, herein, a narrative on the prominent chemical mechanisms and existing strategies for Fe-based catalyst design within thermal CO 2 -FTS as a foundation is established. Then, with the intent of regulating product selectivity, a gap in the adaptation of encapsulated structures involving zeolitic frameworks for CO 2 -FTS is discussed. Next, current photothermal studies on C 2þ hydrocarbon synthesis via FTS, CO 2 -FTS, and relevant thermal-assisted photocatalytic systems involving CO 2 conversion are examined. Finally, the possible applications of structures encapsulated by porous media for boosting light utilization for photothermal CO 2 -FTS are considered. Overall, the potential for the uptake of strategies aimed at producing multifunctional, light-responsive future catalysts suitable for CO 2 -FTS is explored.
production) generates a significant amount of CO 2 (more than 300 Mt per year) and is highly reliant on fossil fuel feedstocks. [13] The upcycling of CO 2 into these materials via a nonfossil fuel-based route would therefore offer the opportunity to relieve the growing supply-demand issue of fossil fuel-derived products. This presents a possible solution for establishing sustainable and realistic pathways for the abatement of CO 2 .
The synthesis of these chemicals requires successful activation of the fully oxidized and stable CO 2 molecules. The conversion of CO 2 to hydrocarbons can proceed via several different routes. Among the most explored are the CO 2 -modified FTS route and a methanol-mediated route (CO 2 conversion to methanol, followed by methanol to hydrocarbon: CO 2 -MTH). With both traditionally driven by an external heat supply, incorporating sustainable energy sources is imperative for relieving the dependence on fossil fuel-derived energy during the conversion process. This invokes greater significance toward harnessing solar energy as a direct input for driving these energy-intensive reactions, in line with drawing closer to a net neutral carbon emissions scheme. To achieve this, the development of low-cost, effective, and stable catalysts to facilitate production at a high efficiency that can also harness solar energy is paramount. This is being realized through the recent emergence of studies on photothermal CO 2 -FTS. [14] At the outset of photothermal CO 2 -FTS, the characteristics and existing understanding of the thermal system (without light) serve as a basis for studying the (beneficial) roles of light illumination/excitation and for inspiring the design of bifunctional photothermal catalysts. This necessitates an examination of the prominent chemical pathways involved in the CO 2 activation and carbon coupling toward the synthesis of C 2þ hydrocarbons in thermal catalysis (TC) (covered in Section 2). Following consideration of the chemical pathways, Section 3 discusses the widely studied CO 2 -FTS catalyst active sites and the common strategies employed for the design/modification of Fe-based thermal CO 2 -FTS catalysts. Conventionally, product distribution and olefinicity may be influenced by the selection of primary active sites and auxiliary components such as promoters and support structures or matrices. In addition, zeolite incorporation downstream to the primary CO 2 -FTS active phases could influence product distribution through complex secondary reactions (e.g., cracking or consumption of the undesirable CO). [15][16][17][18] Further adoption of zeolite-encapsulated structures within CO-fed FTS has shown to significantly narrow product distribution. [19][20][21] However, similar structures are yet to be discussed in detail in relation to CO 2 -FTS. As a plausible multifunctional catalyst design, the potential application of zeoliteincorporated structures for CO 2 -FTS is discussed in depth within Section 3.2, leveraged from current applications in CO-fed FTS.
Past reviews targeting CO 2 valorization to higher-value products under illumination have focused predominantly on the production of C1 products (CO, CH 4 , CH 3 OH), with comparatively less attention paid to C 2þ products (i.e., photothermal CO 2 -FTS). [22][23][24][25][26][27] For instance, of the latter, Albero et al. previously facilitated a discussion on photocatalytic CO 2 reduction to C 2þ products, focusing on the utilization of light-induced charges. [10] In exploring further avenues, which extend this work, Section 4 reflects on the current progress in light-responsive catalyst design for the synthesis of C 2þ hydrocarbons using solar energy. Section 4.2 provides an overview on the current progress within the better-established CO-fed FTS to elucidate possible influences of light toward hydrocarbon chain growth, olefinicity, and material candidates for effective light harvesting; these are aspects highly relevant to CO 2 -FTS postactivation to inspire further developments in catalyst design. Section 4.3 addresses recent progress in photothermal CO 2 -FTS and highlights key challenges faced within the space. Following this, progress in thermal-assisted photocatalytic CO 2 conversion to C 2þ products is briefly outlined as inspiration toward the possible roles of light other than a source of heat, as dictated by material selection. Finally, Section 4.4 postulates the possible application of encapsulated structures by porous media as an aspect of nanostructure engineering within photothermal settings; applications toward other organic reactions were reviewed to exemplify unique approaches to enhance light-to-heat efficiency and light absorption via multiple scattering.

Reaction Mechanism
Several different reaction mechanisms have been proposed in the hydrogenation of CO 2 to C 2þ hydrocarbons, with CO 2 -FTS and methanol-mediated routes being the two most extensively explored. Each relies on different key intermediaries and mechanisms toward hydrocarbon synthesis, thereby producing substantially distinct product distributions. Recent emergence of a ketene intermediary pathway, and a possible concurrence in CO 2 -FTS through CO carbonylation, have drawn attention due to the resulting selective synthesis of ethylene. Other novel routes for CO 2 hydrogenation to light olefins may include the conversion of CO 2 to ethanol, followed by dehydration resulting in high selectivity (>90%) of ethylene. [28] The following section aims to highlight the propagation mechanisms involved under the predominantly explored CO 2 -FTS and methanol-mediated route, followed by discussion on recent efforts exploring the ketene intermediary pathway.

Carbon Dioxide-Fischer-Tropsch Synthesis
The CO 2 -FTS pathway proposes that CO 2 is first reduced to CO via the reverse water-gas shift (RWGS) reaction (Equation (1)) for the subsequent hydrogenation of CO into hydrocarbons via FTS (Equation (2)). Adsorbed CO can be activated into surface *CH x species, which services carbon chain growth for C 2þ synthesis. [29] CO 2 -FTS is often regarded as the more favorable route from economic and environmental standpoints in comparison to the indirect case, which requires the individual operation of RWGS and FTS in separate processes. [29][30][31][32] The inertness of CO 2 often translates to high-energy requirements for its activation as RWGS is endothermic in nature. Conversely, the establishment of C─C bonds required for chain growth, as governed by FTS, is exothermic. [29,32] This necessitates the selection of suitable reaction conditions and feedstock ratio for the single-reactor process. Nevertheless, CO is present in most cases as a byproduct. CO 2 -FTS is therefore considered kinetically challenging given the rate imbalance between CO generation and slow hydrogenation that is paramount for C-C coupling. It must also be noted that the overall hydrocarbon distribution produced through the FTS stage will be governed by the classical Anderson-Schultz-Flory (ASF) distribution ( Figure 1a). The weight fraction (W n ) of a carbon chain with n carbon can be correlated to chain growth probability (α) (Equation (3)). α is dependent on the rate of chain propagation (r p ) and termination (r t ) (Equation (4)). [33] Carbon chain growth is often described as stepwise addition of C 1 species, where the fractional yield follows an exponential decrease with chain length, and α may be assumed to be independent of carbon number. [34] This assumption often aligns better for higher-molecular-weight (MW) products where C 1 addition constitutes the main synthesis mechanism. Deviation in product yield from the assumed constant chain growth probability with lower n (n < 4) is common due to the multiple pathways for synthesis and further consumption (e.g., site-dependent CH 4 synthesis, readsorption of light olefins for chain growth). [34] The theoretical maximum selectivity for C 2 -C 4 is limited to 58%, inclusive of olefin and paraffins. [32] Figure 1a illustrates the prominent propagation mechanisms proposed for FTS. Davis previously published a review on the development of FTS mechanisms. [35] Briefly, the carbide mechanism stands as the most frequently referenced for hydrocarbon synthesis via FTS-related routes. Propagation is carried out with *CH 2 being the monomeric species provided that excessive hydrogenation to CH 4 does not occur. *CH x may be formed via the dissociative adsorption of CO forming *C and *O, with the removal of surface oxygen as water. [36,37] Alternatively, the CO insertion mechanism features the insertion of CO into surface-adsorbed H. Further hydrogenation services the removal of terminal oxygen through dehydration for *CH x formation. Propagation occurs through further insertion of CO into the metal-CH 3 group. The two aforementioned mechanisms are the most discussed. Zhou et al. provided experimental evidence supporting the plausibility of both carbide and CO insertion mechanisms over Fe catalysts. [37] Other routes can include the enolic mechanism whereby adsorbed CO is hydrogenated to *HCOH species. Propagation occurs through dehydration via OH removal with H from a neighboring unit (self-condensation), leading to carbon coupling among the two substrates. [37] A more detailed mechanism relating to the main actives site used is further discussed in Section 3.1. CO 2 -FTS requires a balance between the tandem steps to ensure an adequate activated C:H on the catalyst surface to promote chain propagation as opposed to overhydrogenation to light paraffins or surplus CO desorbing as a byproduct. [38,39] Reaction parameters influence FTS kinetics to alter product distributions. CO 2 -FTS is commonly conducted under industrially mild conditions (e.g., 250-400°C, 2-5 MPa), at a H 2 :CO 2 ratio of %3. [29,40] Chain growth probability decreases with the abundance of Figure 1. Schematic illustrations of a) carbon chain propagation pathways under FTS route, drawn based on other studies. [35][36][37] Theoretical carbon number distribution predicted by Anderson-Schultz-Flory model under FTS. Reproduced with permission. [33] Copyright 2018, Royal Society of Chemistry; b) Dual-cycle mechanism under the MTH. Adapted with permission. [51] Copyright 2012, John Wiley and Sons; c) the ketene pathway, drawn based on Ref. [55]. hydrogen, increasing the H 2 :CO 2 feed ratio that invariably increases the selectivity of lighter fractions, specifically CH 4 . [41,42] Increasing the reaction temperature can improve CO 2 conversion and CO dissociation; this comes at a trade-off as the rate of hydrogenation also increases, shifting product yield toward lighter fractions and often resulting in a dominance of CH 4 . [43] Increasing the reaction pressure can enhance CO 2 and CO conversion with a corresponding increase in reactant concentration per reactor volume. Nevertheless, excessively high pressure (e.g., >3 MPa) may promote secondary hydrogenation of light olefins, where mass transfer restrictions under the formation of heavier hydrocarbon products are more difficult to egress, or irreversibly adsorbed carbon species can occlude active surfaces. [44,45] Gas space velocity impacts residence time. An increase in residence time promotes reactant interaction with catalyst surfaces and the formation of surface carbonyl species for C-C coupling rather than desorption as products. However, the secondary conversion of light olefins can also occur from prolonged exposure to the hydrogenating environment. [42,43] Olefin activity for secondary reactions, such as insertion into longer chains or hydrogenations into paraffins, also contributes toward the final selectivity. [41,46] Overall, catalytic materials with an affinity toward the acidic CO 2 and mild hydrogenation ability, coupled with appropriate reaction conditions, are vital in achieving well-balanced kinetics and avoiding unfavorable secondary reactions.

CO 2 -MTH
For CO 2 -MTH, the initial step involves CO 2 hydrogenation into methanol, followed by methanol transformation to hydrocarbon (MTH). [47,48] This offers an alternate pathway synthesizing hydrocarbons from CO 2 in a single reactor that is not limited by the ASF product distribution. [49] The requirement of a tandem reaction necessitates the use of a bifunctional catalyst, typically involving selected mixed metal oxides (MMO) for methanol production, followed by the diffusion of gaseous products into a zeolite component for conversion into hydrocarbons. [48] Methanol synthesis from CO 2 is an exothermic reaction and competes with the endothermic RWGS that occurs in parallel. [50] CO is, therefore, a major byproduct. The consumption of methanol within a downstream zeolite may suppress the extent of the RWGS. [48,50] Within a zeolite material, the MTH reaction is complex and there exist many potential mechanisms. [7] However, the hydrocarbon pool (HCP) mechanism is the most widely accepted. [47] Yarulina et al. provided a comprehensive review on the mechanisms involved within MTH. [51] Methanol may first be dehydrated to form dimethyl ether (DME) or methoxy species at Brønsted acid sites within zeolites to assist with the direct formation of C─C bonds during early stages of the reaction. [52] This facilitates the formation of unsaturated, coke-like HCP species (often olefinic and/or aromatic in nature) that are confined within zeolite pores. [29,47,51] At olefinic sites (trapped light olefins), chain growth may proceed via a series of methylation reactions followed by the cracking of long-chain lengths to generate short-chain olefin products. Alternately, H transfer and cyclization may occur synthesizing paraffins and aromatics, respectively (Figure 1b). At aromatic sites (often methylbenzene species), methylation via the dehydration of added methanol species forms the basis for carbon chain growth. [51] Hydrocarbon products are formed upon dealkylation. The coexistence of these two competing processes has led to the development of a dual-cycle mechanism ( Figure 1b). Overall, zeolite topology and pore distribution influence the dominant HCP species, product egression, and, concomitantly, product distribution. [51,53] Recent reviews have highlighted the significance of zeolite deactivation. Low-temperature conditions (<350°C) may lead to deactivation via the formation of large, saturated cyclic hydrocarbons that are inactive to methanol addition. [54] High temperature deactivation can also occur via coke deposition. [48]

Ketene Intermediate Pathway
The formation of ketene was proposed to occur through CO insertion to surface-stabilized CH x species (Figure 1c). [55] The ketene intermediary route for FTS was first discussed by Jiao et al. via the use of a dual-bed system comprising ZnCrO x and mesoporous SAPO (MSAPO) zeolite, a setup akin to that of the MTH route. This led to hydrocarbon selectivity of 94% at 17% CO conversion. Ethylene accounted for 80% of all synthesized hydrocarbons. [56] Direct detection of the transient ketene was difficult and relied on the identification of surface acetates, while others have suggested acetic acid as indirect evidence for the presence of ketene. [57,58] This route was later studied using a ZnCrO x /mordenite (MOR) system, where a 73% ethylene selectivity was demonstrated. Shape-selective channel blocking using pyridine and Na ions was employed to probe ketene interaction at different pore sizes within MOR. Ketene preferentially interacted with acid sites in the 8-MR side pockets of the MOR structure, highlighting the structural-performance relationship of zeolitic frameworks. [59] Recent works by Zhu et al. identified that coordinately unsaturated Ga 3þ , in conjunction with Zn and oxygen vacancies over reducible ZnGaO x spinels, played a role in steering the reaction pathway. Here, the methanol-initiating formate intermediate changed to ketene acetates offering high selectivity toward ethylene upon further diffusion into SAPO-34 zeolite. [60] As a novel pathway toward olefin synthesis relative to MTH and FTS, despite the level of interest surrounding the ketene intermediate, it has been much less targeted in the past in terms of catalyst design.

Catalyst Design
Catalyst design is defined by the pathway being targeted. In the context of CO 2 -FTS, this typically necessitates catalysts, which facilitate both CO 2 activation (via RWGS) and subsequent FTS activity. CO 2 -MTH, in contrast, involves MMO phases favoring methanol synthesis, followed by a secondary zeolite phase containing a suitable framework, which enables chain growth via the HCP mechanism. In the case of the former, activity and product selectivity may be tuned toward a specific fraction based on various auxiliary components. This section provides an overview on the key components considered in the catalyst design toward CO 2 -FTS and CO 2 -MTH reactions targeting light olefins.

The Basis of Active Sites
A high degree of CO 2 activation and ensuing stabilization of activated species are essential to encourage carbon chain growth and avoid the desorption of CO as a product. A moderate interaction between *CH x and active sites is desirable toward coupling. In addition, a controlled hydrogen availability can offer the opportunity for C-C coupling to occur rather than rapid hydrogenation to CH 4 . [61] Previous incorporation of materials with high hydrogenation abilities (such as Ni) into catalyst designs greatly favored the selectivity toward CH 4 as opposed to higher hydrocarbons. [62,63] Ru is considered as an attractive base metal due to high activity at comparatively low operation temperatures although, in conjunction with higher costs, Ru seemed to favor the synthesis of C 5þ fractions. [64,65] As such, low-cost, FTS active catalysts with Co or Fe species as the main active phase are the two most explored in the field.

Cobalt
Co is a widely investigated FTS catalyst, which exhibits favorable selectivity toward longer-chain paraffins, good stability, resistance to coking, low activity toward WGS for CO 2 generation, and a comparatively higher FTS activity than Fe. [66][67][68][69][70] The coexistence of metallic Co and Co 3 O 4 can enhance the olefin-toparaffin ratio (o/p) by providing weaker C 2þ adsorption with a higher-energy barrier for hydrogenation. [70] Recent work has identified the differing roles of supported metallic Co and CoO sites on CO 2 activation. Where dissociation is favored over metallic Co sites, CoO invokes hydrogen-assisted pathways via surface carbonate, formate, and formyl intermediates conducive to carbon coupling. [71] Nevertheless, Co is often considered less suitable in the presence of CO 2 feed compared to Fe. The use of mixed CO/CO 2 /H 2 feed over Co has shown suppression toward CO 2 conversion due to competitive adsorption between CO and CO 2 where, in some cases, the catalyst is effectively "inert" to CO 2 . [72,73] Riedel et al. demonstrated that on increasing the CO 2 concentration in a syngas feed over Co/MnO, the reaction would shift from an FTS regime toward methanation. [39] The authors discussed that CO selectively inhibited both methanation and product desorption over Co, which were essential for C-C coupling. Within this study, the chain growth probability for CH 4 dropped drastically with the decrease in CO content in the feed. In contrast, the chain growth probability of C 2þ products was well retained, which suggested that methanation and FTS occurred independently on separate active sites. Upon decreasing CO partial pressure, the density of active sites favoring FTS decreased. [39] Zhang et al. obtained similar results to Riedel et al. in terms of the methanation tendencies of a CO 2based feed over Co; a CH 4 selectivity of over 70% was obtained with the use of a H 2 /CO 2 feed over Co/SiO 2 . The authors speculated the synthesis of methanol as an intermediate under a H 2 /CO 2 feed due to difficulties in cleaving the second C─O bond. This was followed by the hydrogenation of methanol to CH 4 , and the coverage of hydrogen was postulated to be significantly higher than that of CO 2 . [73] Visconti et al. discussed the low stability of adsorbed CO 2 over Co, leading to lower surface C:H, which favored lighter products, particularly CH 4 . The low C 2þ selectivity was accredited to a lack of CO due to poor CO 2 activation, which limited the degree of polymerization to heavier products. [72] Experimental work by He et al. supported the occurrence of a formate-mediated route over Co 6 /MnO x via in situ fourier transformed infrared spectroscopy (FTIR). The authors documented that reaction with a 13 CO-labeled feed (0.2 MPa of 13 CO, 3.8 MPa of CO 2 , 4 MPa of H 2 ) showed that 13 C did not enter the products derived from CO 2 and vice versa. They subsequently speculated that both 13 CO and CO 2 consumption occurred via separate pathways over segregated and different active sites. [74] Though it may be correlated to the low RWGS activity over Co, the finding shared a similar concept with previous studies by Riedel et al. and Zhang et al. [39,73] Within the original study by Riedel et al., the shift from FTS to methanation with the change in feedstock was not observed over Fe-based catalysts. The chain growth probability was well retained across different carbon numbers, meaning hydrocarbon distribution was not affected by the feed. Thus, it was concluded that CO partial pressure did not significantly influence the reaction regime over Fe and that the conversion of both CO 2 and CO shared the same route. [39] Given the observed activity toward methanation with the use of a CO 2 feed over Co, the current review will be centered around Fe-based materials.

Iron
Associated Reaction Mechanisms and Phase Transitions: Fe in the form of Fe 3 O 4 is active toward the RWGS reaction and, thus, can be utilized for CO 2 activation. [62,75] FTS-active iron carbide species can be generated by the carburization of Fe 3 O 4 with CO following its sequential reduction to metallic Fe during an early induction period of the reaction. [31] However, reoxidation is also possible under high concentrations of CO 2 and H 2 O. [31] The coexistence of Fe 3 O 4 and Fe carbides offers considerable benefits in the direct hydrocarbon synthesis from CO 2 and is the primary active phase used in majority of the CO 2 -FTSrelated studies. Metallic Fe has also been suggested to contribute to an appreciable level of olefin production, although this may also be accompanied by relatively high levels of CO and CH 4 . [76,77] The mechanism of CO 2 -FTS over Fe is complex and involves various active phases of Fe and an inherently wide FTS product distribution. Lee et al. constructed a mechanism for CO 2 -FTS, underpinned by a series of hydrogenation-dehydration steps over an Fe-based catalyst, portraying polymerization as the major route favoring hydrocarbons (Figure 2a). [78] Further, it outlined alternative routes of hydrogenation leading to byproduct synthesis. The mechanism first assumed an associative pathway where CO 2 was reduced by Fe, which could react with H 2 for O removal (via dehydration) forming surface CO. Depending on the location of hydrogenation at different stages, carboxylic acid, formaldehyde, or methanol could be formed as byproducts. Entering the FTS stage, surface CO could then undergo hydrogenationdehydration forming surface methylene (Fe-CH 2 ) as the propagating species facilitating chain growth following further CO 2 reduction to CH x . Termination through hydrogen abstraction (dissociative β-H abstraction) can produce olefin species as the product, while further hydrogenation leads to paraffin desorption. [79] Overall, chain propagation is reliant on the stabilization of surface intermediates. Deep hydrogenation of CH 2 can synthesize CH 4 instead. This stresses the importance of regulating hydrogen availability during olefin synthesis considering that termination via further hydrogenation (associative α-H addition) favors paraffin production. [78][79][80] Nie et al. conducted density function theory (DFT) studies comparing CO 2 adsorption and activation over Fe(100), Fe 3 O 4 (111), and Fe 5 C 2 (510), with each common to conventional Fe catalysts for CO 2 -FTS. CO 2 activation pathways were shown to be surface dependent, eliciting different adsorption geometries. Fe 3 O 4 (111) was deemed the preferred CO 2 adsorption site, leading to the formation of CO 3 À species with high symmetry that relied on H-assisted dissociation pathways. Decoration by K can modulate surface charge density and introduce attractive forces between K and CO 2 in proximity, which promotes C─O bond stretching for O removal. [81] In terms of chain growth mechanism during FTS, Zhang et al. offered a DFT study on olefin synthesis from syngas over Fe 5 C 2 (510) facets where the effect of hydrogen coverage was considered. Their work is often viewed as being complementary to Nie et al.'s work on CO 2 adsorption and activation over Fe. [33,38] [83] Overall, polymerization via FTS can be described as a series of competing CO dissociation, hydrogenation, and C-C coupling reactions and highlights the influence of availability and adsorption strength of hydrogen on hydrocarbon synthesis. [82] An FTS-like pathway not involving a CO intermediate has also been discussed, assuming the hydrogenation of CO 2 first to formate (HCOO*) followed by HCOOH. Further, hydrogenation-dehydration yields HCO* followed by the same steps undertaken to synthesize CH x . DFT modeling has demonstrated that the addition of Cu at a 4/9 monolayer configuration could heighten the dissociation barrier for CO 2 with decreased adsorption strength ( Figure 2b). [84] The benefit of this pathway stems from the suppression of CO in the final products, and its occurrence heightened by preventing the direct dissociation of CO 2 through decreased adsorption strength. Nevertheless, this may go hand in hand with CO 2 adsorption geometry, whereby H-assisted dissociation would be favored with the formation of surface carbonate species and can be influenced by the facets exposed. [81] It should also be noted that the monomers involved, propagation mechanism, and the ASF product distribution were anticipated to follow that of the typical FTS. [74] Overall, a satisfactory rate of CO 2 conversion into *CH x is pivotal. [29] The stabilization of CO, CHO*, and other intermediates is required to promote oxygen removal and facilitate C-C coupling for chain growth. Fe 5 C 2 as the key hydrogenation species may benefit H-assisted activation. Furthermore, a controlled hydrogenation is needed to avoid initial paraffin synthesis as well as the secondary hydrogenation of olefins. These together require a higher ratio of activated C:H, which may be adjusted by catalyst selection or by modulating the active site density and size. Nevertheless, detailed operando assessments on the exact mechanism for chain propagation accompanying catalyst design would be beneficial. As described by the ASF model, standalone FTS in the hydrocarbon synthesis stage does not offer excellent selectivity toward specific fractions. Enhancements may be attained by implementing auxiliary components to improve activated C:H ratio, product adsorption strength, or zeolitic components facilitating secondary reactions such as CO carbonylation or acid-catalyzed cracking. This will be discussed further in Section 3.2. Figure 2. a) Schematic reaction mechanism of Fe-catalyzed CO 2 -FTS system. Reproduced with permission. [78] Copyright 2004, Elsevier. b) Methylene formation pathway in CO 2 -FTS via CO intermediate over Fe (100) surface, and the formate pathway exemplified over Cu-Fe(100) surface. Reproduced with permission. [186] Copyright 2017, American Chemical Society. It is generally accepted that CO 2 is first reduced to CO* over Fe 3 O 4 sites, while Fe 5 C 2 plays an important role in subsequent CO hydrogenation/dissociation and C-C coupling. [80,85] A dynamic change to the Fe components during CO 2 -FTS reaction has often been observed. Riedel et al. proposed the evolution of surface reactions as five distinct episodes correlated to the construction of active Fe species (Figure 3a). Episode I is characterized almost exclusively by reactant adsorption and carbidation. Episodes II and III describe the initial emergence and then dominance of the RWGS reaction, producing more CO rather than its consumption as surface carbon species. FTS activity emerges during episode IV, followed by its stabilization in episode V. Phase change during episodes I-III was minimal. The initiation of FTS coincides with the increase in Fe 5 C 2 at the expense of α-Fe with α-Fe further diminished during the stabilization of FTS in episode V. It was concluded that metallic Fe was not active toward FTS. The authors also speculated about the formation of an amorphous FeO x , presumably an Fe II oxide following the reduction sequence of Fe III ! Fe II ! Fe 0 . Compared to using a CO 2 feed, using a syngas feed resulted in shorter episode durations, although it had also introduced an episode VI involving deactivation due to carbon deposition at high CO partial pressure. [86] The difference in episode duration signified a longer induction period to reacting stable FTS when using a reduced Fe catalyst within a H 2 /CO 2 reaction atmosphere. [87] Recent works by Zhu et al. expanded on the dynamic evolution of surface Fe species, correlating it to surface chemical potentials. Initial reactant introduction over metallic Fe constructed a high carbon potential (μ C ) environment favoring carbon permeation and the formation of carbides whereby hydrogenation began to take place. With this, oxidizing species such as water vapor are released into the environment, increasing the oxidation potential (μ O ) together with CO 2 and promoting Fe oxide formation. Oxide dominance inhibits hydrogenation, where μ C is temporarily restored to again favor carbide formation. Phase evolution continues at equilibrium influenced by gas-phase compositions and reaction conditions (Figure 3b). An oxide-carbide core-shell structure would ensue as the balance of surface oxidation (from the H 2 O byproduct) and carburization reaches equilibrium under a specific reaction environment. [88] Liu et al. studied the effect of Fe 3 O 4 crystal size on CO 2 hydrogenation reaction. The study found that reduction and carburization of Fe species hampered with an increase in crystal size of the pristine Fe 3 O 4 . Larger particle size led to higher α-Fe content observed from the spent catalysts, which lowered CO 2 conversion and o/p. [89] Smaller particle size was proposed to aid carbon diffusion, thereby increasing the carbon ratio of the bulk phase. [90] As such, control over the oxide crystallite size represents an additional strategy to mediate the degree of reduction and carburization, tuning the ratio of oxide-carbide phase present. [89] Notwithstanding this, Zhu et al. found that the synthesis of hydrocarbons from CO 2 was favored over CO with increasing Fe diameter from 2.5 to 12.9 nm. The benefit arose from enhanced synthesis of formate species leading to direct methanation, as well as the increase in terrace site density with particle size, which was conducive to carbon coupling. [91] A limit of 6 nm has been frequently cited as the critical size in standalone FTS, below which the selectivity toward CH 4 increases due to a higher density of corner and edge sites that are more favorable for methanation. [90,91] In general, the coexistence of different Fe species, including metallic Fe, Fe oxides (α-Fe 2 O 3 , γ-Fe 2 O 3 , Fe 3 O 4 , and FeO), and carbide phases (ε-Fe 2 C, ε 0 -Fe 2.2 C, Fe 7 C 3 , θ-Fe 3 C, and χ-Fe 5 C 2 ), can be expected during the CO 2 hydrogenation reaction over Fe-based catalysts. [63,92] Zhang et al. identified the difference in the reduction pathways of Fe oxide when starting with α-Fe 2 O 3 and γ-Fe 2 O 3 precursors. Operando Raman spectroscopy and X-Ray diffraction (XRD) revealed that, while α-Fe 2 O 3 precursors favored the formation of χ-Fe 5 C 2 following prereduction and carburization during reaction with CO 2 /H 2 , γ-Fe 2 O 3 favored the formation of θ-Fe 3 C. Comparatively, θ-Fe 3 C promoted the synthesis of C 5þ products due to stronger CO 2 adsorption, while χ-Fe 5 C 2 favored the formation of light olefins due to a higher effective energy barrier during the first carbon coupling, accompanied by a weaker hydrogenation ability. [93] Figure 3. a) Episodes of reaction and Fe phase evolution during CO 2 -FTS. Reproduced with permission. [86] Copyright 2003, Springer Nature. b) Proposed phase diagram of Fe during CO 2 -FTS, where red/blue dots represent conditions imposed by product gas conducive to carburization/oxidation, respectively. Reproduced under the terms of CC BY-NC license. [88] Copyright 2022, The Authors, Published by American Association for the Advancement of Science.
The relationship between iron carbide phases and activity performance is frequently studied for both CO 2 -FTS and FTS. Fe carbide formation often relies on pretreatment under syngas or a CO-containing high-carbon atmosphere. de Smit et al. discussed the effect of μ C during pretreatment imposed by the reaction atmosphere on catalyst composition and the resultant FTS performance. A low μ C induced the formation of amorphous Fe x C phases and θ-Fe 3 C where the latter was speculated to favor carbonaceous deposits. In contrast, high-μ C pretreatments, facilitated by lower temperatures and higher CO content, could encourage the better hydrogenation of carbonaceous products given a higher ratio of χ-Fe 5 C 2. Concurrently, this resulted in higher porosity where a buildup of H 2 O pressure served as an oxidant. [85] Chang et al. elaborated on the FTS activities for various carbides (ε-Fe 2 C, χ-Fe 5 C 2 , Fe 7 C 3 ) produced from different pretreatment methods over an Fe/SiO 2 precursor. It was shown that Fe 7 C 3 -enriched catalysts delivered the highest FTS intrinsic activity, and ε-Fe 2 C-enriched catalysts produced lower CH 4 selectivity while favoring longer-chain products. Nevertheless, χ-Fe 5 C 2 was more stable under conventional CO-fed FTS conditions, justifying its dominance across related investigations. [90] Liu et al. demonstrated that carbon-encapsulated Fe 5 C 2 was active toward both RWGS and FTS, where olefin production could be further promoted by K decoration. The authors also discussed a minor route involving direct CO 2 hydrogenation without the involvement of CO, which primarily yielded CH 4 , although it was suppressible with promotion by K. Physically mixing Fe 3 O 4 and Fe 5 C 2 at different compositions showed that an increase in Fe 5 C 2 content accelerated CO 2 conversion. CO selectivity was lowered, while hydrocarbon selectivity was raised due to enhanced hydrogenation ability. However, CH 4 selectivity also increased with Fe 5 C 2 content. The findings accentuate the need to control Fe oxide and carbide ratios in favor of synthesizing higher hydrocarbons and olefins, both of which peaked at an oxide: carbide ratio of 2:1 within the study. [94] Active Fe surfaces are characterized by dynamic, intricate compositions that are highly dependent on the chemical potential of the surrounding environment, which is easily influenced by reaction conditions. Fe-catalyzed CO 2 -FTS is viewed as a concerted effort between Fe oxides responsible for activation via the RWGS reaction and Fe carbides responsible for hydrogenation toward hydrocarbon synthesis or H-assisted CO 2 activation. The transitional state of Fe enables a variety of oxide/carbide species to be present with differing performance results, highlighting the distinct need for control.
Deactivation: Fe 5 C 2 , as an active reaction site for FTS, is prone to oxidation within the CO 2 /H 2 reaction environment. Zhang et al. provided deeper insights into the deactivation mechanism for Fe-based catalysts in CO 2 -FTS, which can proceed via three separate routes: 1) the phase transition of carbide into inactive sites; 2) the deposition of carbonaceous materials; and 3) sintering. The oxidation of Fe 5 C 2 to Fe 3 O 4 under a reaction environment was considered to be a major source of deactivation relative to other possible means when using an oxygen-rich CO 2 /H 2 feed where both CO 2 and H 2 O can serve as oxidants. [95] Local gas mixtures may become more oxidizing with reaction progression as they exit the catalyst bed. [96] Moreover, with increasing particle size, oxidation may be further projected at the core with reactant diffusion into the particle. [96] Concurrently, the formation of secondary carbide phases such as Fe 3 C with weaker FTS activity relative to Fe 5 C 2 may be observed, followed by further oxidation to Fe 3 O 4 (Fe 5 C 2 ! Fe 3 C ! Fe 3 O 4 ). [95] Zhang et al. showed that a separate regeneration process under 10% CO (350°C, 5 h) was sufficient to reproduce Fe 5 C 2 from Fe 3 O 4 -enriched spent catalysts, while the removal of Fe 3 C required a two-step CO 2 -CO (oxidation-carburization) regeneration approach (450°C, 3 h; 350°C, 5 h). Deactivation via the blockage of active sites from carbon deposition was observed to be limited within the 120 h operation under a low μ C during the study. The low-μ C reaction environment led to a low quantity of amorphous polymeric carbon and graphitic carbon layers that are typically considered to be detrimental to performance. Nevertheless, the effect of carbon buildup during a long-term reaction would be significant. [95] 3.2. Auxiliary Components

Promoter Species
Fe phases are often decorated by basic species (Li, Na, K, Rb, or Cs) on the catalyst surface. [97,98] Na and K remain the most explored. In most CO 2 -FTS systems, an alkali metal is added as an electron donor to effectively: 1) adsorb the acidic CO 2 molecules; 2) lower the H concentration on the catalyst surface, which favors the production of olefins over paraffins and suppresses CH 4 production; and 3) facilitate active Fe 5 C 2 phase formation. [99][100][101][102][103] Several studies have correlated the content of Na promoter with the extent of Fe carburization within a CO 2 /H 2 feed. Wei et al. correlated the content of residual Na (from Fe 3 O 4 synthesis using NaOH as a precipitating agent) with the degree of carburization. An increase in Na content (Na/Fe wt%) from 0.08 to 1.18 wt% promoted the carburization process during the CO 2 -FTS reaction, as revealed from Mössbauer spectra of the spent catalysts. Increasing the Na loading up to 1.18 wt% increased CO 2 conversion, C 2 -C 4 selectivity, as well as product o/p ratio, benefiting from the enhanced dissociative adsorption of CO (Figure 4a). [101] The observed promotional effects from Na were echoed by the results reported by Liang et al., where an increase in Na loading from 0 to 0.5 wt% boosted the carbide content from 7.5% to 27.8% (Figure 4b). Olefin selectivity concomitantly increased from 6.0% to 64.3%, while CH 4 and paraffin production were suppressed. Nevertheless, an optimal alkali metal loading will exist as enrichment could eventually lead to active site obstruction. [104,105] Lu et al. further demonstrated that Na decoration on Fe 3 O 4 could lead to an increased Fe 2þ /Fe 3þ ratio, alluding to an increased oxide vacancy presence, which enhanced CO 2 adsorption. [83] K has been widely applied to significantly boost the o/p ratio over Fe catalysts. [77,106,107] Satthawong et al. accredited the promotional effects of K to a decrease in weakly adsorbed hydrogen at higher concentrations of the promoter. A moderated surface hydrogen concentration combined with enhanced CO 2 adsorption created a favorable environment to initiate C-C coupling reactions. As such, unfavorable activities such as the secondary hydrogenation of olefins, as well as methanation, can be suppressed (Figure 4c,d). [106,108] The adsorption strength of oligomers may weaken in the presence of K, thereby favoring the production of light olefins through mediated desorption and propagation. A similar case was made for Na. [109] Barrios et al. demonstrated that K decoration resulted in better activated C 1 species utilization (from CO 2 ) toward the C-C coupling reactions. Using ZrO 2 -supported Fe 2 O 3 promoted by different metals (FeM/ZrO 2 ), light olefin selectivity increased with CO 2 conversion. Further promotion by K (FeMK/ZrO 2 ) altered the trend as light olefin selectivity decreased with CO 2 conversion due to the production of heavier products. The authors proposed that under the influence of alkaline promoters, the effects of oligomerization to heavier products may be more significant than the overhydrogenation of C 2 -C 4 species or the readsorption of light olefins products in shifting the product distribution away from light olefins. [108] K in excess may also obstruct the Fe sites available for activation. As the interaction between Fe and K inhibits H consumption, the initial reduction of Fe oxides may be hampered. As such, a compromise between chemisorption, reduction, and carburization may need to be considered upon K decoration to modify the surface C:H in favor of carburization and C-C coupling. [110] Recent work by Yang et al. suggested that the Fe binding energy in Fe 5 C 2 can be influenced by the alkaline species added (Li, Na, K, Rb, or Cs) and the loading, the extent of which could be correlated to the Allen electronegativity scale. A more electronegative species with lower-energy valence electrons could better enhance CO 2 adsorption and dissociation, while decreasing the adsorption strength of olefins. The surface coverage of activated C species would increase, though the rate of CO and H 2 adsorption and activation would be hampered. An appropriate balance of these items would concomitantly modulate key factors such as C:H ratio and o/p (Figure 4e,f ). [98] Collectively, surface decoration by alkaline promoters at an optimized loading has been shown to improve CO 2 adsorption, enhancing the generation and stability of carbide phases, as well as performance toward light olefin production.
In terms of structural promoters, Mn inclusion has been found to increase Fe dispersion and act as a steric promoter to enhance the C-C coupling rate. [75,97,111] Xu et al. reported that a combination of both Na and Mn offered a synergistic effect to increase the exposure of stabilized Fe 5 C 2 , enhancing the olefin composition within the products. [111] In contrast, Cu was found to enhance Fe reducibility, as the CuO first undergoes reduction to facilitate H adsorption in turn promoting Fe reduction at lower temperatures and increasing the carburization rate as a result. [108,112] Co has also been explored as a secondary metallic promoter to improve CO hydrogenation. [113] Zn incorporation can facilitate the formation of ZnFe 2 O 4 to suppress sintering and sustain the surface area of Fe sites with greater dispersion. [107,112] However, carbide formation may be hindered due to the diminished reducibility of ZnFe 2 O 4 . Zhang et al. briefly discussed the possibility of recovering reducibility at higher Zn concentrations by invoking ZnO formation, although this may increase CH 4 synthesis under enhanced H 2 dissociation and promoted RWGS reaction. [107] Overall, promoter addition can exert structural or electronic influences toward Fe species. The . a) Influence of Na loading on CO 2 -FTS product distribution. Reproduced with permission. [101] Copyright 2011, Royal Society of Chemistry. b) Influence of Na loading on Fe 5 C 2 content. Reproduced with permission. [104] Copyright 2018, American Chemical Society; c) Schematic modulation of surface H content by K. d) Variation in product distribution with K loading. Reproduced with permission. [106] Copyright 2015, Elsevier. e) CO 2 -FTS performance. f ) Fe-phase distribution with different alkaline promoters. Reproduced with permission. [98] Copyright 2022, John Wiley and Sons. subsequent influences moderate key factors, including active site dispersion, ease of reduction, reactant adsorption (controlling surface C:H ratio), as well as the carburization extent to favor a targeted product fraction.

Metal Oxide Supports
Metal oxide supports are often employed to introduce an abundance of metal-oxide heterojunction and defect sites to enhance CO 2 adsorption and activation, increase the dispersion of metal deposits, and concomitantly, the number of active sites. [70] The enhanced dispersion of species effective for CO 2 dissociation also increases the ratio of activated C:H to promote chain growth. Support porosity may influence product contribution, as light olefins may readsorb or become confined by smaller pores, where secondary hydrogenation into paraffins can also occur. Correspondingly, the synthesis of C 5þ products may proceed more readily within large pores. [114] Support materials such as SiO 2 , Al 2 O 3 , TiO 2 , and ZrO 2 have previously been examined for CO 2 -FTS. [115] Selecting a support material also depends on the level of Fe-support interaction. Stable species such as Fe(II) silicate and Fe(II) aluminate may form over SiO 2 and Al 2 O 3 to hinder the initial Fe reduction sequence and subsequent carbide formation. [115,116] In contrast, the moderate interaction offered by ZrO 2 in facilitating deposit anchorage without hampering Fe reducibility has enabled its application as a suitable support material. [108] 3.

Porous Carbon Framework
Porous carbon materials such as mesoporous carbon supports (MCS) offer low support interaction with Fe oxide deposits, promoting Fe oxide reduction and carburization. A sufficient carbide content combined with suitable mass transfer properties enables efficient product egression through the porous matrix. In contrast to MCS, activated carbon (AC), which primarily contains micropores, may inhibit mass transfer kinetics in cases where higher-order products are desired. It is also more prone to coking. [117] Furthermore, 3D carbonaceous frameworks may enable the confinement of Fe actives sites, tuning deposit size and dispersion, which can be preserved by framework stability under reducing environments to restrict Fe sintering. [118,119] Wu et al. explored honeycomb-structured graphene (HSG) as a support framework for K-promoted Fe catalysts. In their study, a 1.5 wt% K loading on Fe 3 O 4 supported by HSG (FeK1.5/HSG) produced a light olefin selectivity of 59% with a CO 2 conversion of 46% and a Fe time yield (FTY) of 123 μmol CO2 g Fe À1 s À1 . The total C 2 -C 4 selectivity inclusive of paraffins was 70%, far exceeding the ASF distribution. This was accredited to the 3D mesoporous-macroporous columnar architecture of HSG, which inhibited Fe sintering while retaining efficient mass transport. Interestingly, C 5þ synthesis was only evident at a K loading >1.5 wt%. In contrast, the use of AC, a 3D scaffold comprising micropores and mesopores, led to a lower conversion (36%), much lower FTY (25 μmol CO2 g Fe À1 s À1 ), and light olefin selectivity of 53%. The difference in pore size distribution between HSG and AC highlighted the benefits of active site confinement preserving structure and dispersion with low support interaction, while enabling reactant delivery facilitating carburization and subsequent reactions. Furthermore, egression of olefinic products could occur without being overhydrogenated. [118] Adopting a similar approach, Zheng et al. investigated K-decorated Fe 3 O 4 -FeC x confined within N-doped mesoporous carbon structures, where N behaved as surface electron donors over inert carbonaceous frameworks. The catalyst provided 87.3% selectivity toward the C 2 -C 4 fraction with an o/p of 4.7 and CO 2 conversion at 54.5%. Low selectivity toward C 5þ products was also obtained in the absence of K despite high CO 2 conversion and low CO and CH 4 selectivity. [43] Gu et al. suggested an enhanced and intimate interaction between the active phase and promoters under nanoconfinement as a reason for boosted activity and light olefin selectivity. [120] Bao et al. studied the confinement effect on Fe nanoparticles using carbon nanotubes (CNTs), which offered enhanced selectivity toward C 5þ materials, being double that of Fe nanoparticles loaded on the external surface of the CNTs. The enhancement was attributed to a confinement effect that prolonged the contact time of intermediates at active sites favoring the synthesis of long hydrocarbon chains. [121] The finding was furthered by Cheng's study on size-uniform Co confined by mesoporous SiO 2 supports. The SiO 2 framework invoked stronger C* adsorption, which ensured a high surface intermediate concentration that was localized at active sites to favor chain growth. [122] Wu et al. demonstrated that both single and multiwalled carbon nanotubes (SWNT, MWNT) can be used as supports for Fe and were conducive to hydrocarbon synthesis from CO 2 . The larger wall curvature for SWNT results in an increased electron density on the exterior surface. The enhanced electron density was postulated to favor oligomerization via stronger intermediate adsorption, which then enabled the formation of heavier products. [123] In conjunction with enhanced carburization activity, a suitable transfer rate of reactants and products to and from active sites may be modulated by the rational selection of hierarchical structures, which offer a suitable pore size distribution. The hierarchical scaffold may tune surface interactions with activated carbon intermediates to an extent, which favors the synthesis of light olefins. Nevertheless, the specific mechanisms involved need to be explored in greater detail. Further understanding on whether the high C 2 -C 4 selectivity derives from a balance between adsorption strength and chain propagation specific to carbon frameworks would be highly beneficial.

Zeolite-Incorporated Catalysts
Zeolite-incorporated bifunctional catalysts typically comprise two phases: 1) an FTS-active or methanol-generating MMO phase, followed by 2) a zeolite phase. Regardless of the route taken, the manner of integration of the oxide and zeolite phases is important (Figure 5a). Briefly, the dual-bed configuration comprises separate metal/metal oxide and zeolite layers. Granule mixing, in contrast, increases the proximity between the two phases, which can enhance intermediate consumption through better diffusion into zeolites for improved hydrocarbon selectivity. [49,124] Mortar or powder mixing offers even closer interaction between the two components, although it can potentially deactivate the zeolites via surface coverage or inhibition of acid sites in cases where basic promoters are employed. [30] This can lead to enhanced CH 4 formation (Figure 5a). Aside from the manner of packing, modulation of bed height or flow rate may also influence the degree of hydrogenation achievable for intermediary species, thus modifying o/p. [56,124] Applications via Carbon Dioxide-Fischer-Tropsch Synthesis: The role of zeolites in CO 2 -FTS is to alter presynthesized hydrocarbons with a complex distribution via secondary reactions such as cracking, oligomerization, aromatization, or hydrocarbon poollike mechanisms. [15] The aluminosilicate frameworks of zeolites are comprised of SiO 2 and AlO 2 units, where the latter is negatively charged, typically compensated by exchangeable nonframework cations. [125] When exchanged with a proton (H), Brønsted acid sites comprising highly polarized hydroxyl groups are formed and acidity typically decreases with increasing Si/Al ratio. [126,127] The majority of the Brønsted acid sites are located within pore channels that must cater for the diffusion of hydrocarbons and products to avoid coking. Different sized pore openings can be found in different framework types. Typically, they can be described as zeolites with small pores (eightmembered ring or 8-MR), medium pores (10-MR), large pores (12-MR), or extra-large pores (>12-MR). [127] Effect of Porosity and Acidity on Secondary Reactions: The selection of an appropriate zeolite framework is governed by the influence of pore structure on product distribution, as well as the density and strength of internal acid sites. These factors can modulate mass transfer and secondary reactions occurring within the zeolite phase. Furthermore, coke formation may be more apparent over topologies with 2D channel structures relative to 3D structures. [128] While studying CO 2 -FTS, Ramirez et al. demonstrated that relative to a standalone Fe 2 O 3 @KO 2 , incorporating hydrogen exchanged (H) Zeolite Socony Mobil-5 (ZSM-5) through mortar mixing provided an enhanced selectivity toward aromatics. In contrast, using MOR gave a slight enhancement in light olefin selectivity (Figure 5b,c). The difference was attributed to the level of steric effects imposed by each topology. The 10-MR topology of HZSM-5 offered better steric hindrance to promote oligomerization and aromatization from lighter fractions, with the latter yielding isobutane as a byproduct. In the case of MOR, lower hindrance within the 12-MR channels resulted in decreased interaction with acid sites, where secondary reactions occurred to a lesser extent. [18] Where zeolites are used as supports, the loss of external acid sites at the interface may be an issue due to surface coverage by deposits blocking the pore entrances. The effect of pore filling would also need to be considered for large pore topologies (e.g., MOR). [128] Xu et al. demonstrated that on using HZSM-5 as a support for CO 2 -FTS, interfacial Brønsted acid sites may act as electron acceptors to compete against charge donation from Fe to CO for dissociation post-RWGS. As a consequence, Figure 5. a) Packing configurations and their effect on the synthesis of hydrocarbon products. Reproduced with permission. [30] Copyright 2017, Springer Nature. Influence of b) H-ZSM5 and c) MOR on hydrocarbon product distribution by Fe 2 O 3 @KO 2 . Reproduced with permission. [18] Copyright 2019, American Chemical Society. d) Influence of different framework types on hydrocarbon product selectivity downstream to CO 2 -FTS. Reproduced with permission. [15] Copyright 2021, Springer Nature. e) Influence of Ca-modified ZSM-5 on hydrocarbon product distribution derived from CO 2 -FTS using Fe 2 O 3 /KO 2 . Reproduced with permission. [16] Copyright 2020, Elsevier. active site reducibility and carburization were diminished. [129] Within CO 2 -FTS, fast-emerging effort has been devoted to synthesizing aromatics and gasoline range products using Fe-based catalysts supported by hierarchical HZSM-5 with reduced acidity. [30,[130][131][132][133][134] Comparatively, similar integrations targeting light olefins from CO 2 -FTS have been rare.
In terms of CO-fed FTS for synthesizing light olefins, Kang et al. inspected the effect of acidity in FeCuK/ZSM-5 composites at various Si/Al ratios (Si/Al ¼ 25, 40, 140). The selectivity for C 2 -C 4 and the overall olefin percentage decreased with increasing Si/Al. An increase in Si/Al within HZSM-5 has been frequently associated with a heightened ratio of medium-strong acid sites relative to weak acid sites, making it favorable for oligomerization and aromatization. [30,135,136] High-Si/Al variants can also enhance Fe dispersion, though a stronger Fe-support interaction may lower Fe reducibility and tarnish conversion performance. [135,136] Moreover, excessively strong acid sites may result in overcracking to produce light paraffins as well as CH 4 . Hou et al. screened a range of different zeolites downstream to FeZnNa for CO hydrogenation in a dual-bed configuration (330°C, 2 MPa, 1:1 H 2 :CO). Light olefin selectivity increased in all zeolite-incorporated cases compared to the standalone FeZnNa and was attributed to extensive cracking of C 5þ products. Structures possessing strong acidity combined with small pore size were speculated to increase intrazeolite residence time. The longer residence time can promote overcracking of the limited number of molecules with access to internal acid sites to favor CH 4 and light paraffins. [137] Other studies on the synthesis of C 5þ using hierarchical ZSM-5 also commented on the concurrence of overcracking in microporous channels, though to lower extents. [30,133,134] The overall density and relative concentrations of strong to weak acid sites, together with channel structures, should influence the type and extent of secondary reactions taking place. An appropriate molecular fit may be established to enable adequate interaction with internal acid sites in a manner suitable for transporting the initial hydrocarbon distribution.
On examining the synthesis of light olefins from CO 2 within a zeolite-incorporated dual-bed system, Ding et al. discussed the concurrence of CO 2 -FTO (Fischer-Tropsch to olefin) and CO 2 -MTO (methanol to olefin) within a reduced Fe-Cu-K/SAPO-34 admixture. The admixture delivered a 49.7% CO 2 conversion and 62.9% selectivity to light olefins, with CO selectivity at less than 10%. The authors described a promotional effect from SAPO-34, which led to the formation of an Fe-rich Cu-Fe alloy following the thermal migration of Cu during reduction. This led to the formation of low-valence, electron-rich Fe species conducive to CO 2 adsorption. Complimentary DFT studies demonstrated the favorability of Cu-Fe (100) facets for CO 2 and CO adsorption and H 2 activation. Further, in situ infrared (IR) spectroscopy highlighted the prominence of the CO intermediate pathway over the admixed catalyst, and a heightened CH* formation relative to the zeolite-free case, promoting light olefin production. In addition, an in situ-formed Fe-Cu alloy (Cu-rich counterpart) enabled methanol synthesis. A secondary MTO reaction upon diffusion into SAPO-34 subsequently promoted high light olefin selectivity given the nature of reaction. [138] The occurrence of complex intrazeolite chemistry is clearly illustrated in this instance.
Intrazeolite chemistry involving FTS-generated products is complex and difficult to observe. Recent work by Ramirez et al. further correlated zeolite topology and framework as selectivity descriptors for CO 2 -FTS during secondary reactions within the zeolite microenvironment (Figure 5d). Eight different zeolites were screened, each in dual-bed configuration downstream to Fe 2 O 4 @KO 2 . All zeolite-incorporated cases showed a decrease in CO selectivity despite a similar CO 2 conversion relative to the standalone Fe 2 O 4 @KO 2 . A secondary consumption of CO within the zeolite phase was highlighted as a result. 8-MR and 12-MR frameworks (MOR, SAPO-34, ZSM-58, β-zeolite, zeolite Y) appeared to favor light olefins (Figure 5d). In contrast, in the case of 10-MR frameworks (ZSM-22, FER, and ZSM-5), light olefins consumption to synthesize longer olefins were postulated (e.g., over ZSM-22). Hydrogen transfer was suspected to dominate within the FER-type zeolite and ZSM-5 to favor the synthesis of paraffins and aromatics. Controlled catalytic reactions over standalone MOR, ZSM-22, FER, and ZSM-5 using a 13 CO/ethylene mixed feed demonstrated the direct consumption of 13 CO within the zeolite. CO facilitated a carbonylative environment forming zeolite acetate, diacetyl, and acetone species with ketene being a key intermediate. The idea of "molecular fit" was further substantiated with the authors claiming that only 10-MR frameworks could facilitate the transition states required for further oligomerization. Straight, 10-MR channels favored the synthesis of long aliphatic products. Alternatively, the presence of an additional set of sinusoidal 10-MR channels running perpendicular to the wide, straight channels in ZSM-5 provided intersecting cavities large enough to accommodate aromatization. Nevertheless, the density and strength of acid sites also influenced the nature of the hydrocarbon pool, thereby manipulating product distribution. [15] As such, a delicate and systematic tuning of zeolite acid properties specific to the framework under study may be required during catalyst development to further probe the intrazeolite chemistry in favor of specific fractions.
Overall, efficient mass transport and effective interaction with internal acid sites may come as a trade-off as easy diffusion could reduce the opportunity for cracking over acidic sites. Alternatively, excess confinement effects can result in undesirable interactions leading to chain growth, aromatization, overcracking, or secondary hydrogenation at the expense of light olefins. The zeolite framework structure also has direct influence on deactivation. [139] Considering this, hierarchical structures may be employed to enhance mass transfer and mediate the extent of interaction with internal acid sites. The careful introduction of mesopores may also enhance molecular entrance into zeolites with small pore openings (8-MR) catering for cracking over regions with more stringent shape selectivity. Though it must be noted that FTS is favored at much lower temperatures (250-400°C) in comparison to the catalytic cracking of hydrocarbons over zeolites. The latter often requires temperatures ranging from 500 to 650°C for extensive cracking into C 2 -C 4 fractions within industrial settings. [139] This suggests that, while beneficial, cracking may not be the primary mechanism to rely on when tuning hydrocarbon distribution toward light olefins in CO 2 -FTS.
Modulating Acidity: To improve the selectivity toward lighter fractions in CO 2 -FTS via cracking, the temperature mismatch against FTS must addressed. To resolve this, Ramirez et al.  [17] Enhanced cracking activity has also been achieved over ZrS supported on MCM-41 zeolite, which led to significant cumene cracking with high propylene selectivity at 300-350°C. [140] The same group subsequently explored a Ca-modified ZSM-5 in a dual-bed configuration with Fe-K for CO 2 -FTO to enhance light olefin selectivity at 375°C (Figure 5e). Ca deposition via wetness impregnation decreased the density of Brønsted acid sites and suppressed the degree of H-transfer reactions, inhibiting undesirable aromatization and oligomerization that can otherwise dominate within pristine ZSM-5. This gave rise to a light olefin selectivity of 38%, with a CO 2 conversion of around 50%. An increase in C 2 -C 4 selectivity was also attributed to a combination of CO consumption through ketene-like intermediates, and cracking of heavier fractions at the remaining Brønsted acid sites in the zeolite, although the occurrence of the latter process was likely to be limited. [16] Ion exchange is another viable method to alter the acidity of zeolites. An increase in cation basicity (correlated to molar mass) modulates the strength of acid sites. [137] Nam et al. explored the activity and product distribution for alkaline ion (Na, Li, K, Rb)exchanged Y zeolites with 17% Fe loading. The modified cases enhanced CO 2 adsorption and selectivity toward hydrocarbons relative to the typical HY zeolite. This was attributed to the interaction between Fe deposits and trace alkali ions at the metal support interface, leading to stabilization of the Fe 3 O 4 and carbides in a manner similar to deposited promoter species. [141] Nam et al. later demonstrated the efficacy of Fe-Ce loaded on KY zeolite (Y zeolite, ion exchanged with K), where Ce and K induced a synergistic effect to further enhance CO 2 conversion. [142] Wu et al. concluded that ion exchange may better influence the number and ratio of Brønsted, and Lewis acid sites present, with less influence on their strength. The accessibility and density of acid sites can also be influenced by the size of the exchanged cation. [126] Liu et al. investigated Fe-impregnated ZSM-5-type zeolites for both FTS and CO 2 -FTS. Zeolite acidity was modified by ion exchange to give NaZSM-5 and HZSM-5. For CO-fed FTS, the less-acidic Fe/NaZSM-5 facilitated higher light olefin selectivity relative to Fe/HZSM-5. This was attributed to a greater extent of carburization, followed by the cracking of long-chain primary hydrocarbons into secondary olefinic products. In terms of CO 2 hydrogenation, the enhanced basicity over NaZSM-5 (without prior carburization using CO) strengthened CO 2 interaction to instead favor the RWGS reaction, which produced CO as the major product. [143] Overall, acid-base modification of zeolites represents a feasible strategy to fine tune hydrocarbon selectivity at lower temperatures. However, it should be noted that an increase in zeolite acidity may neutralize the effectiveness of nearby alkali-promoted FTS active phases. When considering ion exchange, depending on the Si/Al ratio and exchange conditions, desilication under alkaline conditions could lead to mesopore introduction or structural collapse. [126] Encapsulating Metal Active Sites: Zeolite-encapsulated metal catalysts have also been considered for hydrocarbon synthesis. Core-shell catalysts comprising zeolite-coated Fe-based centers enforce product restructuring as the hydrocarbons synthesized at the core must exit through the zeolite shell. Thus, a more effective redistribution of product selectivity can be obtained. Furthermore, the segregation of primary active sites can prevent agglomeration and corrosion during reaction. [144] In a study of CO 2 -FTS targeting C 2 -C 4 hydrocarbons, Xie et al. employed a Pt/CeO 2 core encapsulated by mesoporous silica (mSiO 2 ) with surface Co decoration (CeO 2 À Pt@mSiO 2 À Co). The core-shell structure facilitated CO 2 activation at the core, followed by FTS at Co sites dispersed over catalyst surface. A high local concentration of CO at catalyst surface resolved the methanation tendencies of Co sites at the use of a CO 2 feed, which resulted in over 60% C 2 -C 4 product selectivity. [145] A well-structured catalyst design appears beneficial to the tandem steps in CO 2 -FTS. In recent years, Ni encapsulation by zeolite frameworks has been explored to mitigate active site sintering during CO 2 hydrogenation to CH 4 . [146] In addition, decoration by Mg can increase the basicity of porous framework to boost CO 2 adsorption. [147] Despite the benefits and current application in CO 2 conversion, the exploration of encapsulated structures for CO 2 -FTS remains limited. Further review on relevant configurations applied to CO-fed FTS may help in gauging possible structural influences over performance.
In an effort to target light olefins from CO-fed FTS, Qiu et al. synthesized a core-shell catalyst comprising Fe 3 C encapsulated by SAPO-34 (Fe/C@Si-SAPO). At 340°C, 1 MPa, and a H 2 :CO ratio of 1, the catalyst yielded a 52.6% selectivity toward C 2 -C 4 (o/p ¼ 2.3) and a 3.6% selectivity toward C 5þ products. Comparatively, a mechanical mixture of Fe/C and SAPO-34 yielded C 2 -C 4 and C 5þ selectivities of 44.6% and 21.7%, respectively. The SAPO-34 shell was speculated to impose a strict confinement effect capable of completely suppressing C 6þ products and allow only the diffusion of lighter fractions (Figure 6a,b). A high CH 4 selectivity was attributed to better H 2 diffusion relative to CO into the catalyst core, though this could be suppressed by K addition. [21] A similar result for product distribution was observed by Wang et al. when using a catalyst comprising a FeMnK/Al 2 O 3 core and a silicalite-2 (S-2) shell (FeMnK/ Al 2 O 3 @S2(N)) ( Figure 6c). [19] S-2 (3D framework, 10-MR openings) is an Al free, hydrophobic framework with low acidity. [148] At 340°C, 2.0 MPa, and a H 2 :CO ratio of 1, the presence of the S-2 shell significantly improved light olefin selectivity, which increased from 19.2% for the standalone FeMnK/Al 2 O 3 to 33.2%. CO conversion was lowered due to interactions between the iron oxides and silica, which hindered the reduction of iron oxide to carbides. Diffusion limitations imposed by the porous shell may have also contributed to difficulties in carburization. [19] A more thorough dispersion of a metal catalyst throughout the zeolite phase has also attracted attention due to shape-selective effects on FTS products. Amoo et al. employed a Fe catalyst encapsulated by either Na-or H-exchanged zeolite Y that enabled high-intrazeolite dispersion of immobilized Fe nanoparticles at 3 nm (Figure 6d). The encapsulation prevented the loss of internal area from pore blockage or filling by external deposits relevant to conventionally supported cases. Three encapsulated cases (FeChar@NaY, Fe@NaY, and Fe@HY) obtained at various stages of synthesis were employed for catalytic testing. All yielded high selectivity toward C 2 -C 4 at 300°C and 3 MPa. CO conversions of 84.4%, 91.2%, and 70.3% were obtained with selectivities toward C 2 -C 4 (CO 2 free) at 60.7%, 65.5%, and 64.4% (o/p ¼ 0.74, 1.34, 0.69) for FeChar@NaY, Fe@NaY, and Fe@HY, respectively. Each combination outperformed supported controls (Figure 6e). The reduced acidity of NaY relative to HY suppressed the secondary hydrogenation of olefins. [149] Though not explicitly discussed, possible promotional effects of the Na in proximity to Fe may have also contributed by enhancing CO activation. Alternate investigations involving the use of hierarchical zeolite Y with tunable microporosity in turn favored the synthesis of liquid gasoline products, reaching up to 65.7% in hydrocarbon selectivity. This was accompanied by a 12.3% conversion at 240°C and 3 MPa. [150] Wang et al. studied Ru@NaY for the synthesis of gasoline range products via FTS. The importance of pore sizes on product distribution was highlighted by NaZSM-5 (10-MR) encapsulation that led to an enhanced C 2 -C 4 yield, while NaY (12-MR) favored gasoline range products. Diffusion-dependent activity was also observed as the micropores of NaZSM-5 offered narrower channels, which concomitantly hindered CO conversion. Furthermore, the coupled effects of spatial confinement at synthesis and acid-catalyzed cracking to C 5 -C 11 were noted for more acidic H-exchanged variants. [151] Overall, the influence of intrazeolite confinement effects at the point of synthesis may be highlighted here. Within encapsulated structures, product distributions may first be tuned through spatial effects at internal active sites. Access to acid sites via outward diffusion through zeolite may be less restricting as compared to supported cases where molecular entrance can be restricted by wider product distribution.
www.advancedsciencenews.com www.small-structures.com with a selectivity of 41.2% among hydrocarbons, though C 5þ also increased in the presence of mesopores (Figure 6g). Comparatively, FeMnK/S-1 largely favored the synthesis of C 5þ products. Estimates on the adsorbed H:C ratio at Fe active sites were in the order of FeMnK@S-1 > FeMnK@Hol-S-1 > FeMnK/S-1 > FeMnK@HM-S-1. Intrinsic to core-shell structures, the authors noted a higher internal H 2 :CO than the feed due to better H 2 diffusion. [20] Confinement effects derived from porous structures can therefore mediate product distribution by modulating mass transfer of the reactants and products. This can regulate the local H:C ratio or product residence time relevant to the occurrence of secondary hydrogenation.
Hydrophobic zeolite shells could inhibit secondary diffusion of the water byproduct into downstream-encapsulated structures. This heightens the synthesis of C 2þ products during FTS by suppressing the CO 2 -producing water-gas shift. [152] CO 2 -FTS with an even higher water content derived from initial activation could benefit from this effect, as water diffusion away from core Fe sites and downstream structures would directly influence active-phase composition, which is highly relevant for catalyst performance.
Using porous materials as a host matrix to FTS-active metals seemingly imposes confinement effects influencing the local reactant ratio to directly control the degree of polymerization at synthesis. Nevertheless, with decreased reactant diffusion, conversion would also be affected. Extrapolating from these syngas-based studies, the internal dilution effects may be expected to occur more severely for a CO 2 -based feed with molecule size being the prime factor. Carbon delivery to Fe sites would be critical for early carburization to take place, which may lead to an extended induction period at the start of the reaction. This further accentuates the importance of pore tuning to control mass diffusion, lowering internal H:C to encourage C-C coupling rather than deep hydrogenation to CH 4 and light paraffins, which appear to be a common issue with zeoliteencapsulated structures.
In the absence of acidic properties, secondary reactions such as hydrogenation, oligomerization, and aromatization at the expense of olefins would be hindered, while being more beneficial to the diffusion of the acidic CO 2 to internal active sites. However, strong interactions between silica (High Si:Al, low acidity structures) and Fe-based active sites may hamper carburization. Furthermore, the opportunity for beneficial secondary carbonylation and acid-catalyzed cracking would be lowered. Efficient cracking seemingly requires a delicate balance between acidity and framework structure to cater for the entrance of larger molecules (C 5þ ) and sufficient interaction with internal acid sites, which may be difficult to control in the scope of light olefin synthesis. Furthermore, high-temperature requirements under typical industrial settings for acid-catalyzed cracking to the extent of light olefins are unfavorable for the CO 2 -FTS reactions. Due to the acidity of CO 2 , low-acidity or nonacidic frameworks may be considered more beneficial in terms of reactant delivery. Otherwise, the internal C:H balance vital for in situ carburization and chain growth may be further tarnished. Further relevance of zeolite hydrophobicity may stem from the role of H 2 O. Effective elimination of H 2 O theoretically shifts the equilibrium toward RWGS, beneficial for activation, while removing additional means of oxidation. [153] Both acidity and hydrophobicity can be modulated by the Si:Al ratio.
Applications via Methanol-Mediated Route: Briefly, the MTO route can result in better selectivity toward specific hydrocarbon fractions. Hydrocarbon products are intrinsically grown via HCP species within zeolite channels from methanol intermediates. This involves using a methanol-synthesizing MMO catalyst in conjunction with a zeolite component, coined OXZEO catalysts. Typically, HZSM-5 with medium pore size favors the production of gasoline range products, while those with a smaller pore size, such as SAPO-34, favors the production of C 2 -C 4 olefins. [47] Thus, the structural effects of zeolites may be exploited with better efficacy. For example, Liu et al. showed that ZnGa 2 O 4 / SAPO-34 delivered an 86% selectivity toward light olefins among hydrocarbons, with 46% CO selectivity at 13% CO 2 conversion via the MTO route. [124] Zeolite characteristics such as particle size can modulate the residence time of hydrocarbons inside the zeolite phase. Zeolite acidity can also influence the product distribution. The density of Brønsted acid sites decreases with increasing Si:Al ratio. [30] Wang et al. demonstrated that an increase in Si content for SAPO-5 from 0 to 0.3% in a In 2 O 3 -ZrO 2 /SAPO-5 mixture led to a 6.7% CO 2 conversion, with excellent C 2 -C 4 selectivity (83% among hydrocarbons). The 0% Si case led to an 80% hydrocarbon selectivity toward CH 4 . [49] The CO 2 -MTO route was the prominent pathway studied in recent years for the direct conversion of CO 2 to light olefins given high selectivity in light olefins among hydrocarbon products. [80] This was further complimented by successful commercialization of the MTO process with the development of pilot plants operating under the conditions of 1-5 atm at 400-550°C, all utilizing SAPO-34 or modified analogs. [154] Nevertheless, the overall tandem route is challenged by a low CO 2 conversion combined with high CO selectivity. This derives from RWGS dominance over the exothermic methanol synthesis at the higher reaction temperatures required for MTO to occur. [17,138,155] 4. FTS and Carbon Dioxide-Fischer-Tropsch Synthesis Driven by Solar Energy

Overview
When conducted under a thermal catalytic route, the temperature for FTS reactions can vary between 250 and 400°C depending on the catalyst employed, accompanied by high-pressure conditions (2)(3)(4)(5). [40,156] The use of sustainable energy sources such as sunlight is, therefore, crucial to alleviate the dependence on fossil fuel-derived power by minimizing the need for an external energy input, in line with the goal of mitigating anthropogenic carbon emissions. This has led to an increase in research directed toward utilizing solar energy to offset external thermal requirements by incorporating light harvesting properties into FTS-active catalysts.
Mechanistically, UV illumination over light-responsive, emiconducting materials generates charge carriers through interband/intraband excitations. Moreover, plasmonic materials enabling broad absorption within the substantial vis-NIR contributions of sunlight can facilitate the generation of hot carriers www.advancedsciencenews.com www.small-structures.com through localized surface plasmon resonance (LSPR) effects. These excited charges can directly or indirectly influence surface reactions. [157,158] Photogenerated charge carriers can directly contribute toward surface reactions, and hot carriers may rapidly thermalize through relaxation to result in localized hotspots. [22] Furthermore, the utilization of dark-colored materials with good vis-NIR absorption can enable photothermal effects through intrinsic solar heating. These beneficial attributes of light, when harnessed through appropriate catalyst design, can offset external heat input and potentially alter dominant reaction pathways to influence product distribution. [159,160] They are highly relevant to the energy intensive conditions required by the nonselective CO 2 -FTS.
To the best of our knowledge, the light-driven/assisted MTH pathway has remained unexplored. This may be correlated to the need for higher-temperature conditions and the difficulty in heat transfer with conventional packed bed and dual-bed reactors. Consequently, this section aims to provide an overview on the current progress toward photothermal FTS and CO 2 -FTS, illuminated by wide-spectrum, simulated sunlight. The overview is then followed by a discussion on the possible extensions in catalyst design strategy for driving CO 2 -FTS using solar energy.

Photoactive Catalysts for Syngas-Fed Fischer-Tropsch Synthesis
Currently, design strategies for photoactive FTS and CO 2 -FTS catalysts mainly involve using materials that are both FTS active and light responsive to capture solar energy, often in the form of heat to facilitate photothermal catalysis. The dominance of Fe-and Co-based catalysts has been translated across to the design of photoactive catalysts for C 2þ hydrocarbon synthesis. Photothermal CO 2 -FTS remains understudied in comparison to photothermal FTS using syngas. With FTS being the chain growth mechanism in CO 2 -FTS, a review over the current progress toward FTS driven by solar energy is beneficial to inspire catalytic material selection and understand possible influences of light on product distribution.
In the case of Fe-based catalysts facilitating FTS under illumination, several studies have highlighted the light responsivity of presynthesized Fe 5 C 2 . Gao et al. studied Fe 5 C 2 as a photothermal catalyst for syngas hydrogenation, where both Fe 5 C 2 and supported Fe oxides exhibited good light absorbance well into the IR range. Fe 5 C 2 facilitated a CO conversion of 49%, with an o/p of 10.9 under illumination (200-1100 nm, 0.5 h, 2.9 Wcm À2 ) (Figure 7a,b). A small amount of oxygen adsorbed during the reaction was found to favor the C-C coupling reaction and ethylene desorption. However, further passivation of Fe 5 C 2 significantly hampered CO conversion, where the o/p was lowered to 6.6. Conducting the thermal reaction at temperatures comparable to the illuminated case yielded distinctly different products. A CH 4 selectivity of 94% among hydrocarbons was obtained without light olefin production. Illumination appeared to favor light olefin synthesis, although photogenerated charges in this case were expected to be scarce. [161] Furthering the applications of Fe 5 C 2 toward photothermal FTS favoring light olefins, Li et al. explored the use of Fe 5 C 2 supported on N-doped carbon which offered a 55.3% hydrocarbon selectivity toward light olefins, with a CO conversion of 22.3% (Xe lamp, 0.5 h 2.96 W cm À2 ). The N-doped carbon-supported catalyst provided a 3.5-fold increase in activity relative to the neat Fe 5 C 2 , which was attributed to the role of N as an electron donor to enhance CO activation. Similar activities under both illuminated and dark (i.e., nonilluminated) conditions suggested the dominance of a thermal pathway where light acted as a source of heat. [162] In terms of Fe oxides, Li et al. demonstrated that Fe-Fe 3 O 4 / ZnO-Al 2 O 3 could enable photothermal FTS that was enhanced by photogenerated charges. Under illumination (Xe lamp, 2 h, 5.0 W cm À2 ), a 20.9% CO conversion accompanied by light olefin selectivity at 42.4% (o/p ¼ 4.7) could be achieved. In contrast, thermal reactions in the dark yielded a lower CO conversion (9.9%) with a dominance of CO 2 and CH 4 in the products. Illumination was shown to increase the energy barrier for CO 2 synthesis, while strengthening ethylene interaction with Fe 3 O 4 , which lifted the energy requirement for further hydrogenation, thereby increasing o/p. [61] The light responsivity of Fe 5 C 2 and Fe/Fe oxide interfaces is clearly demonstrated.
When considering Co-based catalysts, Li et al. demonstrated that the coexistence of Co and Co 3 O 4 on a ZnO/Al 2 O 3 support enabled photothermal FTS, providing 41.5% hydrocarbon selectivity toward C 2 -C 4 products with an o/p of 6.1. However, this was accompanied by a CO 2 selectivity of 47.6% and a conversion below 16%. [70] Ning et al. studied SrTiO 3 semiconductorsupported Cu and Co (Cu-Co/SrTiO 3 ) for FTS. Co, as the primary FTS active site, was proposed to capture photogenerated charges from SrTiO 3 as well as hot carriers and the heat generated through hot carrier thermalization from the plasmonic Cu (Figure 7c). Activity comparison across trials that were strictly thermal (i.e., nonilluminated) and illumination under different wavelengths conveyed the role of photogenerated charges that assisted hydrocarbon synthesis. Vis-IR irradiation enabled the generation of plasmonic hot carriers from Cu which then likely migrated toward the FTS-active Co and assisted the activation of adsorbed CO for hydrocarbon synthesis. In contrast, lowtemperature photocatalysis (PC) invoked by UV illumination favored CH 4 synthesis coupled with low CO conversion. Finally, full-range solar illumination (UV-vis-IR) synergistically enhanced C 2þ selectivity (Figure 7d). [159] The findings highlight the potential for tuning product distribution using photothermal and photocatalytic processes under solar illumination, which could be harnessed through an appropriate selection of lightresponsive auxiliary components.

Photoactive Catalysts for Carbon Dioxide-Fischer-Tropsch Synthesis
Current progress in photothermal CO 2 -FTS is dominated by Fe-based catalysts. Chen et al. demonstrated that a Co-Fe alloy/Al 2 O 3 catalyst could deliver 78.6% CO 2 conversion under UV-vis illumination (Xe lamp, 2 h, 5.2 W cm À2 ), with a 5% selectivity toward CO and 35% hydrocarbon selectivity toward C 2þ products. Combining performance results and DFT calculations, the authors postulated that the Co-Fe alloy was more conducive to the coupling of CH x species than monometallic Co or Fe surfaces. Illumination induced a bed temperature of over 300°C. The reaction proceeded via a photothermal route based on a www.advancedsciencenews.com www.small-structures.com similar performance obtained in the dark controlled at similar temperatures (Figure 7e,g). [156] Furthering the direct application of Fe oxide and carbide phases, Song et al. demonstrated that both Fe 3 O 4 and Fe 3 C can induce photothermal effects capable of driving CO 2 -FTS. However, C 2þ selectivity decreased with increasing light intensity. Relative to TC in the dark, illuminating the Fe 3 C increased CO 2 conversion. However, CH 4 production was also heightened. The authors postulated that the production of photogenerated charges under illumination participated in both reactant activation and rapid hydrogenation conducive to methanation (Figure 7h,j). [76] Liu et al. also commented on the direct participation of photogenerated charge carriers for CO 2 hydrogenation over Na-promoted Co@C catalysts. [163] Puga et al. explored the use of nickel-iron oxides for photothermal CO 2 hydrogenation. Under simulated sunlight (Xe lamp, 1 h, 5 W cm À2 ), the addition of Ni suppressed CO byproduct formation. However, CH 4 selectivity increased significantly given the methanation tendency of Ni. The highest C 2þ selectivity was obtained over an FeO x control accompanied by a noticeable o/p ratio (2.9) as opposed to negligible olefin content over Ni-Fe bimetallic catalysts. [62] Li et al. explored using Fe-FeO x / MnO-Al 2 O 3 for photothermal CO 2 hydrogenation, with carbon coupling being mediated at Fe metal-oxide interfaces. An appropriate balance between metallic Fe (favored methanation) and FeO x (favored RWGS) mediated the degree of hydrogenation, crucial for the synthesis of C 2þ products. [164] The study yielded a C 2þ selectivity of 52.9% among hydrocarbons, accompanied by a CO 2 conversion of 50.1%. Table 1 demonstrates that conversion and selectivity performance for CO 2 -FTS via light-assisted pathways vary greatly depending on the reaction parameters such as duration under batch mode and catalytic materials employed. Catalyst composition dictates the extent and the role of captured light, which can further influence catalyst bed temperature. Hydrogenating photothermal materials, such as metallic Ni, boosts conversion and however reduces C 2þ selectivity. It is evident that current work has a strong focus on Co and Fe, with demonstrated photothermal capabilities and FTS activity. To date, there has been an emphasis on Fe toward CO 2 -FTS to better service CO 2 activation. Furthermore, in cases where photogenerated charges are involved, conversion and hydrogenation can also be enhanced, though selectivity toward C 2þ products may become hampered. As photothermal CO 2 -FTS remains in its early stages, studies utilizing CO 2 as a feedstock have primarily focused on the nonselective synthesis of C 2þ hydrocarbons. CO and CH 4 have remained as dominant products. Where reported, C 2þ products mainly comprised C 2-4 paraffins. Olefin has not been explicitly explored as a target material ( Table 1). As such, further investigation toward promoting the C-C coupling reaction and more selective synthesis of light olefins from CO 2 -FTO via solar energy would be highly beneficial in developing a more realistic and sustainable application. Figure 7. a,b) UV-vis-IR absorption, cycling performance of FTS over Fe 5 C 2 under illumination (200-1100 nm, 2.9 W cm À2 ). Reproduced with permission. [161] Copyright 2018, Elsevier; c) Photogenerated charge transfer over Cu-Co/STO, and d) FTS performance under TC, solar catalysis (SC), and PC. Reproduced with permission. [159] Copyright 2022, Elsevier. e) Illustration of Co-Fe alloy catalysts, f ) selectivity performance toward CO 2 -FTS by Co-Fe-600, g) performance of illuminated and thermal reactions in the dark. Reproduced with permission. [156] Copyright 2017, John Wiley and Sons. h) UV-Vis-IR absorbance, i) temperature evolution of Fe 3 O 4 and Fe 3 C under UV-vis illumination (2.05 W cm À2 ), and j) CO 2 -FTS performance under TC and PC catalysis. Reproduced with permission. [76] Copyright 2020, American Chemical Society. Fe 3 O 4 -Fe 5 C 2 mixed-phase catalysts have not been explored in detail so far for CO 2 -FTS under illuminated scenarios, despite the individual photothermal activity shown by Fe oxides and carbides. A critical examination on the role of Fe 5 C 2 as a key active site within this system would be necessary. Furthermore, the feasibility for in situ evolution of carbide phases under illumination as compared to thermal conditions could potentially offer greater insight toward the synthesis technique required for the multiphase catalyst, particularly if ex situ carburization is needed. This would necessitate the adoption of high-temperature H 2 reduction of Fe oxides beforehand. Activity assessment under illumination would be critical in validating the applicability of the Fe oxides and carbides as core active sites for future designs of light-responsive CO 2 -FTS catalysts. The intrinsic product distribution attainable under illumination can also be established from this.
Current progress in light-driven FTS has demonstrated that a rational incorporation of light harvesting semiconductor and/or plasmonic materials could further harness photogenerated charges beneficial to C-C coupling reactions. [159] Similar strategies may be valuable for C 2þ production from CO 2 . This could facilitate exploration toward photoactive promoters, supports, or light transparent frameworks in conjunction with adjusted feedstock ratio, improving CO 2 activation, and enable product tunability. For example, NiO deposited on Nb 2 O 5 (NiO/Nb 2 O 5 ) has exhibited an appreciable amount of ethane production under thermal-assisted photocatalytic (heat externally supplied, UV-illuminated) CO 2 hydrogenation. [165] Cu dispersed within TiO 2 nanotubes (Cu/TiO 2 ) has demonstrated potential for thermal-assisted photocatalytic CO 2 reduction with water, synthesizing light olefins. [166] Other semiconducting materials such as carbon-doped In 2 S 3 (C-In 2 S 3 ) and WO 3Àx have also been applied to the thermal-assisted photocatalytic CO 2 reduction with water, producing appreciable amounts of C 2 hydrocarbons along with CH 4 under illumination (UV-vis and UV-vis-NIR, respectively). [24,167] Photogenerated charges in these cases participated in water splitting and CO 2 activation. The presence of oxygen vacancies can widen the absorption spectra for an enhanced photothermal effect and heighten CO 2 adsorption promoting carbon coupling. [167] These studies, though not explicitly targeting chain growth via the FTS mechanism, may provide clues to possible avenues for carbon coupling within photothermal systems.

Perspectives on the Extension of Encapsulated Structures for Photothermal Carbon Dioxide-Fischer-Tropsch Synthesis
Beyond material selection, appropriate engineering of the catalyst structure via encapsulation by porous media has effectively shifted the FTS product distribution toward the C 2 -C 4 range. Furthermore, the bifunctionality of zeolite-based catalysts offering secondary reactions to alter the product distribution over photothermal CO 2 -FTS represents an explorable direction in the rational design of feasible heterogeneous catalysts. Physical mixtures and dual-bed configurations may be less attractive  (Figure 8a,c). The presence of the SiO 2 shell prevented energy loss through conduction and infrared radiation from the hot Ni core, and the core temperature increased with shell thickness (Figure 8b). In addition, the confinement bolstered Ni active sites against sintering during exposure to hightemperature conditions, enabling better stability. [168] This study underscores insulation as an additional aspect in boosting light-to-heat efficiency. Further attention may also be given to the morphological control of internal deposits, increasing the surface area of active sites for CO 2 -FTS and enhancing heat transfer. Insulation by regular, porous frameworks that can accommodate for light-harvesting components to enable compartmentalization, prevent agglomeration, and sintering illustrates a possible avenue for catalyst design toward photothermal applications. However, despite the extensive application of zeolites in thermal catalytic FTS processes, their role within photothermal processes remains understudied. [14] 4.

Zeolite-Incorporated Structures
An early review by Corma and Garcia outlined the transparency of zeolites to light above 240 nm, enabling a certain extent of light exposure for the embedded components. [169] Light-responsive phases may form a part of the zeolite framework, be anchored, or deposited within cavities. Past investigations utilizing Reproduced with permission. [168] Copyright 2021, Springer Nature. d) Pomegranate-like oxide@zeolite structure. e) UV-vis absorbance of encapsulated pomegranate structures. Reproduced with permission. [175] Copyright 2017, Elsevier. f ) Local EM field enhancement for Au@TiO 2 yolk-shell catalyst from FDTD simulations. g) Product distribution of yolk-shell Au@TiO 2 catalyst. Reproduced with permission. [144] Copyright 2015, Royal Society of Chemistry. h) Schematic of multiple scattering within needle array of Co-PS@SiO 2 . i) Light absorption tunable through distance between needles. Reproduced with permission. [184] Copyright 2020, John Wiley and Sons.  [170] Li et al. explored the use of Zn-containing ZSM-5 for ethane synthesis via the nonoxidative coupling of methane (NOCM) at room temperature. UV-vis illumination enabled electron transition from the zeolite framework, reducing Zn 2þ to Zn þ as the main active sites. 23.8% CH 4 conversion was achieved under 8 h of UV illumination, with ethane being produced almost exclusively besides H 2 . The synthesis of longer hydrocarbon species from ethane coupling was suppressed by the shape selectivity of ZSM-5. [171] Similar structures involving UV-excited charge states affixed within zeolitic frameworks have been explored in the past, facilitating CO 2 reduction and NO x decomposition. [172][173][174] Ma et al. demonstrated the fixation of Pt/TiO 2 and TiO 2 inside S-1 zeolite (Pt/TiO 2 @S-1 and TiO 2 @S-1), forming pomegranate-like structures that were active under UV-vis irradiation. The absorption spectra of Pt/TiO 2 and TiO 2 were retained despite being encapsulated, while the zeolite acted as a shape-selective sheath to control the diffusion of organic pollutants (Figure 8d,e). [175] Light delivery to internal metal active centers with the possibility to promote CO 2 reduction and other organic reactions has been highlighted, while exploiting the rigid, shape-selective characteristics of zeolitic frameworks. Nevertheless, the application of similar structures toward photothermal catalysis has been limited.
Zeolite-confined composites for light-driven applications are primarily limited to UV illumination. Light responsivity in these cases is derived from the confined active phase, while exploiting the shape selectivity and guest-host interactions with the surrounding zeolite framework. Light transparency of zeolites, in conjunction with a rational selection of active materials, may enable further exploration of composites capable of harnessing the substantial vis-NIR contributions of sunlight for photothermal applications. Complimentary verification of structural influences would be indispensable for the design of encapsulated structures delivering light to primary active sites. Other factors such as the loading and dispersion of photothermally active components on temperature evolution and insulation may need to be further explored. Enhanced CO 2 activation may be speculated with the integration of photocatalytic porous shells for relay catalysis commencing at reactant entrance, followed by FTS at the core. To this end, the influence of photogenerated charges on the synthesized hydrocarbons requires greater exploration to avoid charge-mediated secondary reactions at the expense of target products.

Incorporation of Zeolite Imidazolate Framework-8
Other optically transparent and porous materials may also be examined in the synthesis of novel structures where photoresponsivity and confinement effects surrounding active sites can be simultaneously achieved. This expands the range of plausible materials beyond zeolites.
Zeolite imidazolate framework-8 (ZIF-8) has been frequently explored as a host matrix for plasmonic materials in various photothermal applications due to its porosity. ZIF-8 is a type of MOF comprising tetrahedrally coordinated Zn or Co nodes connected by 2-methylimidazolate (mIM) ligands (Figure 9a). [176] Zhang et al. encapsulated Au and Pt nanocages within a ZIF-8 matrix (Au&Pt@ZIF) for CO 2 hydrogenation to methanol. Au facilitated photothermal effects, while Pt behaved as the active centers, with ZIF-8 being used as a heat insulator. [177] Yang et al. conducted the hydrogenation of 1-hexene under full-spectrum light irradiation at 100 mW cm À2 over Pd@ZIF-8 under aqueous conditions. ZIF-8 encapsulation enhanced the dispersion of Pd nanocubes and mass transport to the Pd sites. The resultant structure yielded a broad light absorption spectrum akin to that of Pd nanocubes and product yield increased linearly with illumination intensity (Figure 9b,d). [178] ZIF-8-encapsulated structures could therefore also assist photothermal CO 2 -FTS, given the preservation of absorption characteristics of internal active materials through light transparency.
The direct application of MOF as a catalytic component for FTS is rare. Davoodian et al. investigated the use of ZIF-8 and ZIF-7 as supports for Co for thermal FTS. Thermogravimetric analysis (TGA) of ZIF-8 and ZIF-7 showed significant weight loss at 480 and 550°C, respectively, attributed to structural collapse due to organic linkage degradation. Reaction at 220°C and 2 MPa saw rapid decline at around 25 h on stream, which was correlated to the destruction of both MOF structures. [179] As such, the stability of ZIF-8 structures under mild FTS reaction temperatures may influence its direct application. Moreover, Hu et al. implemented ZIF-8 as a support to α-Fe 2 O 3 for thermal FTS, which offered sufficient stability at 3 MPa and 300°C. Nevertheless, an internal structure conducive to H 2 storage facilitated secondary hydrogenation of olefinic products. [180] As such, the extent of hydrogenation may require further modulation. Comparatively, there has been greater focus on the use of MOFs as a sacrificial template for synthesizing catalysts with high metal dispersion due to its coordinated metallic framework. [181]

Other Encapsulated Structures for Carbon Dioxide Photoreduction
Consideration of other less acidic materials may require better focus on shape selectivity through tunable porous structures. Oxide-encapsulated structures have been frequently explored for light-driven CO 2 reduction. A brief overview of these identifies the potential for enhanced light absorption internal to the capsule nanostructure via specific and unique mechanisms.
Yolk-shell structures featuring a unique configuration where a void space exists between the core and hollow shell (core@ void@shell) have been investigated. The introduction of void space can enable multiple reflections of the penetrating light where the increased path length can enhance light absorption for internal architectures and can be amplified through the incorporation of multiple shells. [182,183] A control over void volume may influence the confinement effect on reactions occurring at the core surface. In relation to CO 2 hydrogenation, Tu et al. explored an Au@TiO 2 yolk-shell catalyst for light-driven CO 2 reduction to CH 4 and C 2 H 6 . The structure enabled a stronger plasmonic near-field effect in the region closely surrounding the yolk-shell point of contact. The production of CH x was subsequently heightened to promote C-C coupling for C 2 H 6 synthesis (Figure 8f,g). CH 4 was preferentially produced further away with lower concentrations of CH x . [144] Nevertheless, charge and heat transfer across the core and shell and further interactions requiring intimate contact such as spillover effects may be inhibited with reduced interfacing. Feng et al. explored a unique plasmonic superstructure comprising Co nanocrystals encapsulated within silica needles (Co-PS@SiO 2 ) for photothermal CO 2 hydrogenation. Plasmonic hybridization effects within each capsule of interconnecting Co nanocrystals combined with multiple reflections within the array led to amplified light absorption (Figure 8h,i). Illumination at 20 sun enabled a temperature of 383°C within 3 min, as well as a hotter Co core. CO 2 hydrogenation at 1 bar produced primarily CO, accompanied by CH 4 , trace C 2 H 4 , and C 2 H 6 . However, its stability was challenged by coke buildup in the voids of the Co core and Co migration through the SiO 2 shell. [184] Encapsulating active sites capable of broad light absorption may offer additional pathways to improve sunlight use for photothermal applications with reduced heat loss. [168] For porous materials, an abundance of pore cavities, combined with light scattering due to a difference in the refractive index between the solid phase and void space, allows for light penetration to a certain extent, particularly when the thickness is controlled at nanoscale. [185] This is particularly evident in encapsulated structures that demonstrated overall light absorption characteristics similar to the core photoresponsive components. Implementing porous shells/matrices inevitably decreases incident light intensity reaching the internal photoresponsive active sites, resulting in lower initial light absorption. A tradeoff between light delivery, porosity, and shell thickness can be expected. In addition, porosity and shell thickness simultaneously influence mass transport and insulation properties. Structural engineering at the nanoscale goes hand in hand with material selection for enhanced optical transparency, forming a critical part of catalyst design that is fit for purpose. A plausible strategy for amplifying light absorption may include the deliberate introduction of void spaces to effectively harness multiple scattering. For the use of zeolites, a carefully mediated confinement effect in favor of C 2 -C 4 products, combined with internal acid sites for the formation of organic-inorganic supramolecular active centers toward carbonylation, would be beneficial. Although this may clash with transport of the acidic CO 2 to internal active sites. Catalyst architecture and material selection based on framework characteristics and activity toward secondary reactions may form some of the key items to be considered within this space. Upon application of the final catalyst material, reactor design for maximized light delivery into the catalyst bed must also be considered to enhance efficiency for solar utilization.

Conclusion and Outlook
An overview on the major mechanistic pathways and the key factors in catalyst design toward light olefin synthesis from CO 2 has been presented. The methanol-mediated pathway, though capable of producing light olefins at high selectivity among hydrocarbons, is limited by low CO 2 conversion accompanied by high CO production. Alternatively, CO 2 -FTS reactions over Fe-based catalysts typically produce a wide distribution of hydrocarbon products as governed by the ASF distribution. Olefin synthesis, via CO 2 -FTS over Fe-based materials, has been shown to rely on the presence of key carbide phases and appropriate C:H ratios for moderate hydrogenation ability to enable C-C coupling without overhydrogenating to paraffins. Auxiliary components such as basic promoters, support structures, and frameworks (e.g., oxide supports, carbonaceous, and zeolitic frameworks) are commonly employed. Appropriate support structures/frameworks can improve Fe dispersion to promote reduction. Basic promoters aid CO 2 adsorption to enhance conversion and carburization. Zeolites, depending on the manner of incorporation, offer secondary mechanisms such as cracking or control over the internal C:H ratio and hydrocarbon synthesis via spatial confinement effects. Current consideration toward catalyst design lies in promoting CO 2 adsorption, moderating the C:H ratio and the degree of hydrogenation to assist Fe-phase transformation and controlling secondary reactions. These aspects, coupled with industrially mild reaction conditions, are crucial to the synthesis of light olefins from CO 2 , desorption and product elution to ensure selectivity.
Despite existing studies on zeolite-incorporated Fe-based catalysts for thermal CO 2 -FTS, encapsulated structures continue to be underexplored. The inclusion of a high-surface-area porous support matrix can offer dispersion of key metallic active sites via compartmentalization, which inhibits sintering and improves stability. Past applications of zeolite-encapsulated Fe structures for syngas-fed FTS have demonstrated effective modulation of product distribution toward light olefins. Further development in zeolite-incorporated catalysts targeting light olefin synthesis via CO 2 -FTS may require careful consideration surrounding the selection of framework structure, acidity, and the manner of incorporation. Altogether, these types of secondary reactions offered by zeolites may be tuned to favor lighter fractions.
Photothermal CO 2 -FTS is in the initial stages of development, and investigations are currently scarce relative to the thermally driven case. Recent progress has primarily relied on Fe-based catalysts, targeting the general synthesis of C 2þ hydrocarbons rather than a shift toward specific fractions or olefinicity. In addition to the search for distinct and functional material combinations active under illumination, further investigation on the use of Fe oxide-carbide-based catalysts would be beneficial. Additional studies on the intrinsic behavior of Fe active sites under illumination would generate experimental evidence to further discern the potential of various Fe phases as base materials. Further understanding on the performance of Fe phases under illumination can facilitate exploration toward other auxiliary components such as light-harvesting support structures or promoter species to enhance activity. The current progress on thermal CO 2 -FTS, photothermal FTS, and thermal-assisted photocatalytic CO 2 reduction may serve as good references for material selection.
Zeolite-containing nanostructures targeting photothermal CO 2 -FTS remain unexplored. Combined with the selection of appropriate framework structures and acidity, the deliberate introduction of metal content into zeolite structures has the potential to offer additional photocatalytic properties. Composite structures consisting of a promoted Fe-based catalyst as a local heater and a zeolite/porous framework offering further insulation may hold value as plausible components for catalyst design for light-assisted CO 2 -FTS. Such encapsulated structures may enable unique insulation or confinement effects that strictly limit hydrocarbon distribution to enhance the synthesis of light olefins. Nevertheless, an increase in local hydrogen ratio near active sites remains a key aspect to be controlled. To this end, the careful selection of core active materials for boosting initial light olefin synthesis, coupled with appropriately controlled thickness and porosity of the lateral matrix, may enhance light absorption and reduce product exposure to a hydrogenating environment. Additional engineering of the catalytic nanostructure such as the deliberate introduction of void space may further enhance light absorption. Overall, an in-depth discussion on the combined influence of key items such as framework structure, Si/Al ratio, and porosity on reaction performance, as well as intrazeolite chemistry, would be imperative for such designs.
In regard to complex intrazeolite chemistry, further establishment of the carbonylative pathway involving ketene-like intermediates within the zeolite phase may allow secondary CO consumption to be better harnessed. The successful integration of related reactions with high specificity toward C 2þ products may compensate the low degree of hydrocarbon cracking at suitable temperatures for CO 2 -FTS. Nevertheless, inherent complexities in intrazeolite chemistry will require further efforts to be untangled via in-depth mechanistic studies.