Thermocatalytic Degradation of Gaseous Formaldehyde Using Transition Metal‐Based Catalysts

Abstract Formaldehyde (HCHO: FA) is one of the most abundant but hazardous gaseous pollutants. Transition metal oxide (TMO)‐based thermocatalysts have gained much attention in its removal due to their excellent thermal stability and cost‐effectiveness. Herein, a comprehensive review is offered to highlight the current progress in TMO‐based thermocatalysts (e.g., manganese, cerium, cobalt, and their composites) in association with the strategies established for catalytic removal of FA. Efforts are hence made to describe the interactive role of key factors (e.g., exposed crystal facets, alkali metal/nitrogen modification, type of precursors, and alkali/acid treatment) governing the catalytic activity of TMO‐based thermocatalysts against FA. Their performance has been evaluated further between two distinctive operation conditions (i.e., low versus high temperature) based on computational metrics such as reaction rate. Accordingly, the superiority of TMO‐based composite catalysts over mono‐ and bi‐metallic TMO catalysts is evident to reflect the abundant surface oxygen vacancies and enhanced FA adsorptivity of the former group. Finally, the present challenges and future prospects for TMO‐based catalysts are discussed with respect to the catalytic oxidation of FA. This review is expected to offer valuable information to design and build high performance catalysts for the efficient degradation of volatile organic compounds.


Introduction
Formaldehyde (FA) is represented as one of the simplest forms of volatile organic compounds (VOCs). FA is often regarded as the priority target for treatment because of its ubiquity, abundance, and negative effects on human health. [1] Sources of indoor FA are diverse to include cooking, smoking, building decoration, and furnishing materials (like glue, varnishes, plastic castings, rubber, and wooden furniture). [2] In light of the adverse health effects of FA, the World Health Organization has set its short-term (30 min) exposure limit in indoor environment at DOI: 10.1002/advs.202300079 0.1 mg m −3 . [3] There is even more strict technical standard of 0.03 mg m −3 for the emission control in interior decoration systems such as residential buildings in China. [4] As people are spending more of their time indoors, it is critical to control FA levels in indoor air to avoid their adverse effects on human health.
Recently, the thermocatalytic degradation approach is recognized as an effective option to achieve the complete decomposition of gaseous FA into harmless products (e.g., H 2 O and CO 2 ) under atmospheric pressure with heat energy. [5] As thermocatalysis can proceed in the absence of a light source, it can be regarded as a superior option over photocatalytic degradation method. [6] Generally, the thermocatalytic degradation of gaseous FA has been achieved using noble (e.g., platinum, gold, palladium, rhodium, and silver) and/or transition metal (TM) (e.g., manganese) based materials. [7] However, their real-world applications are often restricted by the combined effects of both low abundance of noble metals and their high costs. [8] As a result, TM-based catalysts have gained interest as alternative options due to their high stability, good catalytic activity, abundant resource, and costeffectiveness. [3] TM catalysts can offer unpaired d electrons or empty d orbitals to efficiently attract target molecules during the catalysis process. [9] As such, the catalytic destruction of pollutants over these TMs can be promoted by forming chemical bonds and/or by lowering the activation energy of the reaction. [9] Moreover, the catalytic activities of TM-based composites were also reported to be higher than those of their pure forms such as Co 3 O 4 , CeO 2 , and MnO 2 . [10] Such enhancement was mainly due to the improved oxidation capacity of the composites with higher FA adsorptivity and/or oxygen vacancies. [11] The objectives of this review are to highlight the recent progress in the development of TM-based thermocatalysts (e.g., manganese, cerium, cobalt, and their composites) for the degradation of gaseous FA. The discussion has been extended to cover their structural activity relationship and their FA degradation mechanism. Effective strategies (e.g., exposed specific crystal faces, metallic/non-metallic modification, and alkali/acid treatment) developed for improving their catalytic activities are also discussed. Moreover, the FA degradation efficiencies of TMbased thermocatalysts are also evaluated in terms of the key performance metrics (e.g., T 90 and kinetic reaction rate) to properly evaluate their potential for real-world applications. Finally, the

Reaction Mechanism on TM-Based Thermocatalysts
A better knowledge of the catalytic oxidation mechanism of FA by TM-based thermocatalysts can help in the design of highly active, stabile, and efficient thermocatalysts. As thermocatalysts are exposed to the external heat energy, electrons (e − ) from valence band are excited to the conduction band to leave positive holes (h + ) in the valence band even at low temperature (e.g., room temperature). [12] The concentration of e − /h + in thermocatalysts generally increases at elevated temperature (e.g., 50°C). [12] The thermally generated charge carriers further move to the surface of the catalyst to react with adsorbed oxygen molecules and water for the generation of surface adsorbed oxygen species such as superoxide anions (O 2 − , e − + O 2 → O 2 − ) and hydroxyl radicals ( • OH, h + + H 2 O → • OH). Further, water can also be converted into O 2 and • OH with the assistance of O 2 − . [12b] The target pollutants can thus be catalytically oxidized to CO 2 and H 2 O by these surface adsorbed oxygen species. [12b,13] The production of surface adsorbed oxygen species is also affected by the amount of oxygen vacancies (OVs) of the thermocatalysts (e.g., Co 3 O 4 and MnO 2 ). [14] The surface oxygen vacancies possess localized electrons that can easily charge adsorbed oxygen ( • O 2− , O 2 2− , or O 4 2− ) via the molecular oxygen activation channel. [15] Hence, the O 2 molecules can accept delocalized electrons from oxygen vacancies (i.e., Lewis bases) to be converted into active species. [4,14a] The creation of surface defects (e.g., through doping and pyrolysis) over the thermocatalysts is also recognized as an effective way to achieve abundant oxygen vacancies. [16] In addition, higher concentrations of oxygen vacancies in the thermocatalysts can narrow the band gap to enhance the catalytic activity against VOC degradation. [17] The lattice oxygen species were also found to enhance the formation of surface adsorbed oxygen species through its complex interaction with oxygen vacancy and molecularly oxygen. [18] As such, the FA catalytic activity of catalysts was enhanced. [18] For instance, the FA catalytic activity for four MnO 2 catalysts (i.e., , , , and -MnO 2 ) was in line with their amount of lattice oxygen with the following relative order: -MnO 2 > -MnO 2 > -MnO 2 > -MnO 2 . [19] The thermocatalytic oxidation of FA over transition-metal based catalysts (e.g., manganese and cobalt oxides) was found to follow a Mars-van Krevelen mechanism in which OVs can play a vital role during the degradation process. [4,7f,16b,20] In such a mechanism, pollutants are initially oxidized by the surface adsorbed oxygen species to lead to the reduction of metal sites (Figure 1). Then, the reduced meal centers are re-oxidized by O 2 . [21] A detailed multi-step catalytic process for oxidation of FA through Mars-van Krevelen mechanism (e.g., over TM cobalt-manganese oxides) has been proposed (Figure 2). [22] First, FA and oxygen molecules are adsorbed on the catalysts surface and active sites (e.g., oxygen vacancies), respectively. The oxygen molecules are dissociated and activated into surface adsorbed oxygen species (e.g., O 2 − and O − ) by oxygen vacancies at low/high temperature (O 2 + OVs → O 2 − , O − ). [23] Then, the adsorbed FA molecules rapidly react with surface adsorbed oxygen species to be converted into dioxymethylene intermediates and further into formate species so as to form hydrocarbonates. These hydrocarbonate species are to be ultimately oxidized into H 2 O and CO 2 . [22] Similar oxidation pathways were also found when pure/surface defected manganese and cobalt catalyst were used for the thermal oxidation of FA. [4,16b,20,24] In addition, thermocatalytic degradation of FA over noble metal catalysts (e.g., Pt/TiO 2 ) can also be explained by a similar Mars-van Krevelen mechanism. [25] More specifically, the FA molecules were adsorbed on the surface of catalysts and then directly oxidized into formate species by surface adsorbed oxygen species. The formate species were subsequently decomposed into H 2 O and CO which was further oxidized into CO 2 . [25a] Interestingly, as the existence of strong metal-support interactions in Pt/TiO 2 catalysts can cause the partial reduction of the Ti 4+ species into Ti 3+ , it can favorably enhance the FA catalytic activity through the generation of OVs. [25a] If the surface of the thermocatalysts possesses abundant surface hydroxyl (-OH) groups, the adsorption of FA on the thermocatalyst's surface can be enhanced to induce its subsequent degradation with the assistance of surface -OH. [14b] In addition, the formate intermediate species might be directly oxidized to CO 2 and H 2 O by surface -OH groups (HCOO − + -OH → CO 2 + H 2 O) during the FA oxidation process. [26] Therefore, the surface -OH groups are considered the main factor for determining the rate of FA oxidation. [26a] Figure 2. Illustration of FA oxidation over cobalt-manganese oxides catalysts, Reproduced with permission. [22] Copyright 2022, Elsevier. proposed for developing TM-based catalysts such as sol-gel, hydrothermal, precipitation, and template methods. [24b,29] As the preparation of TM-based thermocatalysts has been discussed previously, interested readers may refer to the synthesis methods described elsewhere. [5a,7f,30] Monometallic TMO-based thermocatalysts can exist in several crystallographic structures. [19] In particular, manganese oxide can exist in many different forms of crystals (e.g., -MnO 2 , -MnO 2 , -MnO 2 , and -MnO 2 ), which consist of [MnO 6 ] octahedra to share corners and edges in their structure (Figure 3). [31] The presence of variable chemical valences and defects of manganese oxides will help increase the mobility of surface oxygen to boost their FA degradation ability with the enhanced oxygen storage capacity. [28a,32] For instance, the birnessite MnO 2 , with edge-sharing octahedral MnO 6 layers, was reported to exhibit 100% FA degradation capacity at room temperature. [23c] Such birnessite structures can provide higher water content (like interlayers and adsorbed water) to amplify the thermocatalytic activity. [23c] Water molecules can also help generate the consumed surface -OH groups (during the FA oxidation) through the reaction with surface active oxygen species The monometallic TMO-based thermocatalysts can also be designed into various morphologies (e.g., sheet, cube, and rod). [20,28b,29b,33] For instance, the CeO 2 spherical-like aggregate of nanoplates exhibited excellent FA oxidation activity (e.g., relative to CeO 2 nanorods and nanocubes) with the aid of the abundant surface hydroxyl and oxygen groups (Figure 4): It was reported to achieve a maximum conversion of 87% against 500 ppm FA at www.advancedsciencenews.com www.advancedscience.com Figure 4. The SEM, TEM, and HRTEM images of CeO 2 : a1-a3) CeO 2 spherical-like aggregate of nanoplates, b1-b3) CeO 2 nanorods, and c1-c3) CeO 2 nanocubes, Reproduced with permission. [34] Copyright 2022, Elsevier.
120°C with a GHSV of 10 L g −1 h −1 . [34] A stable fluorite structure of CeO 2 (consisting Ce 4+ and Ce 3+ ions) with a 2D sheet-like morphology and high concentration of OVs was also observed to completely degrade 50 ppm FA at 310°C. [16a] In another study, a 3D MnO 2 structure with an interconnected network structure was developed through a freeze-drying method. [33b] The 3D monolith network facilitated the diffusion of reactants onto the active sites of the catalyst. As such, the 3D MnO 2 was able to fully destruct FA (100 ppm) at 80°C under a gas hourly space velocity (GHSV) of 180 L g −1 h −1 . [33b] In addition, Co 3 O 4 nano-rods were built to have a large surface area and a high content of surface Co 3+ . Such characteristics offered large numbers of oxygen anionic sites to promote the adsorption of H 2 O molecules. The adsorbed water molecules can further be dissociated to form • OH active species for the oxidation of FA. [28b] Despite the extensive research on monometallic TMO-based thermocatalysts ( Table 1), they suffer from several disadvantages (e.g., poor ability of O 2 activation and limited oxygen vacancies) that affect their FA degradation efficiency. [3,4] For example, high temperature (>90°C) (e.g., urchin-like MnO 2 , 2D-Co 3 O 4 , and CeO 2 nanorod) is often required to achieve complete FA oxidation (Table 1). [29b,34,35] Hence, researchers have produced thermocatalysts that are more effective than monometallic TM-based thermocatalysts by introducing impurities and other enhancing agents as discussed below.

Bi-TMO-Based Thermocatalysts
Bi-TMO-based catalysts have been reported to exhibit superior catalytic activities for FA compared to their mono metallic counterparts. [13a,36] Bi-TMO-based catalysts are generally fabricated through a sol-gel method, a template method, precipitation, and a hydrothermal method. [13b,36b,37] The improved FA oxidation capabilities of bi-TMO-based thermocatalysts (e.g., MnO x -CeO 2 , Co 3 O 4 -CeO 2 , MnO 2 -Fe 2 O 3 , CuO-MnO 2 , and Co 3 O 4 -ZrO 2 ) is due to their high surface oxygen mobility and OVs, which help promote the transportation of charges during redox cycles. [3,36a] In this regard, mono-TMO-based thermocatalysts such as cerium oxides are frequently bound with other TMOs (e.g., cobalt oxides). [37a] In such a case, the combination of two metal oxides helps synergize the overall thermocatalytic degradation capabilities of FA. For instance, cerium oxides in CeO 2 -Co 3 O 4 thermocatalysts offer high oxygen storage capacity, good redox performance, and high lattice oxygen activity, while Co 3 O 4 provides strong oxidation activity with good electron transfer properties [4] ( Table 1). Similarly, the variable valence, easy defect formation, and high activity of manganese oxides give them high potential for bi-TM-based thermocatalysts. [4,38] For example, the complete oxidation of FA at lower temperature (100°C) can be achieved by the synergy between manganese and cobalt oxides in Co x Mn 3-x O 4 catalysts compared to each of their pristine forms, that is, MnO x (170°C) or CoO x (180°C). [38] The synergy was obtained through a series of redox cycles including Mn 4+ /Mn 3+ and Co 3+ /Co 2+ involving the activation of oxygen molecule by Co and its transfer to Mn. [38] Generally, the FA oxidation reaction over bi-TMO-based thermocatalyst (e.g., Co x Mn 3-x O 4 ) follows the formate decomposition route (i.e., HCHO→ HCOO − → CO→ CO 2 ). [38] The ratio of TMs in bi-TMO-based catalysts can significantly influence their catalytic activities against FA. [13a,37a,39] For instance, Ni 0.8 Co 2.2 oxides synthesized by co-precipitation method at 300°C (Ni 0.8 Co 2.2 -CP-300) had the best FA catalytic activity (complete removal of 100 ppm FA at 90°C) among all Ni x Co 3-x -CP-300 (x = 0-1) catalysts (Figure 5a). [40] The outstanding activity of Ni 0.8 Co 2.2 -CP-300 was ascribable to the surface oxidant Co 3+ and abundant hydroxyl as evidenced in X-ray photoelectron spectroscopy (XPS) spectrum (Figure 5b,c). [40] As another example, the effect of Ce amount on the thermocatalytic performance of CeO 2 -Co 3 O 4 was investigated.
[37a] Accordingly, the best FA oxidation performance (100% of FA removal at 80°C) was found for a Co/(Co + Ce) atomic ratio of 0.95. Moreover, according to O 2 -temperature-programmed desorption (TPD) analysis, the large desorption peak of O 2 in CeO 2 -Co 3 O 4 (ratio of 0.95) thermocatalyst also indicated an increased O 2 adsorption capacity, which helped produce surface active oxygen. [37a] In contrast, the pure Co 3 O 4 (containing no Ce) showed only 35% FA removal under the same temperature (80°C). Such a difference could be attributed to the increase of surface adsorbed oxygen species as evidenced by H 2 -temperature programmed reduction analysis. However, at higher Co/(Co + Ce) atomic rations (>0.95) the phase separation resulted in lower FA catalytic activities for the catalyst. [37a] Likewise, when the ratio of Co:Mn was 3:1, Mn x Co 3-x O 4 possessed the highest amount of surface oxygen and exhibited the best catalytic activity against FA relative to other Co:Mn ratios (e.g., 8:1, 2:1, and 1:1). As such, Mn x Co 3-x O 4 exhibited the best catalytic performance to achieve 100% oxidation of 80 ppm FA at a lower temperature (75°C) than that of its pure counterparts (i.e., MnO 2 (90°C) and Co 3

TMO-Based Composite Thermocatalysts
Despite the good performance of pure TMO-based thermocatalysts (e.g., mono-and bi-TMO), their application in powder or particle forms often gives rise to dust contamination, which complicates the process. [41] In addition, as other TMO forms (e.g., nanoparticles, nanorods, and nanoplates) tend to agglomerate, their catalytic performance can be degraded. [42] Therefore, various nanomaterials (e.g., carbon spheres and polyester fiber) with advanced properties (e.g., high specific surface area and porosity) have been coupled with TMOs to fabricate TMObased composites. [41b,43] In addition, the utilization of various nanomaterials as matrix/substrate has also been recognized as an effective route to increase the FA oxidation performance over TMO-based catalysts by imparting abundant surface oxygen vacancies. [10a,10b,44] The TMO-based composite catalysts are generally formulated through solution mixing, calcination, and in situ approaches. [41b,43,45] To prepare the MnO 2 modified activated carbon (MnO 2 /AC) spheres, the AC spheres were impregnated with Mn(NO 3 ) 2 ·4H 2 O solution and then dried at 105°C for 24 h. The final product was obtained through a calcination process at 300°C for 3.5 h under a nitrogen atmosphere. [43] For the in situ approach, the TMOs can be synthesized in the presence of the matrix for the generation of stable composites. [41] The synthesis approach has also influenced the thermocatalytic properties of TMO-based composites. For example, the crystallinity of Co 3 O 4 was lowered after the in situ growth of Co 3 O 4 nanowires on the Ni foam surface. This approach helped introduce more oxygen vacancies for oxidation of FA. [10b] Accordingly, the reduced (r)-Co 3 O 4 NW@Ni foam composites possessed more www.advancedsciencenews.com www.advancedscience.com active oxygen species with high mobility and reactivity with the aid of increased surface OVs. Such abundant surface OVs could lower O 2 adsorption energy to make r-Co 3 O 4 NW@Ni foam composites easily adsorb and store more active oxygen species. As such, r-Co 3 O 4 NW@Ni foam composites exhibited the best FA oxidation performance with a much reduced T 10 (i.e., temperature for 10% FA conversion) of 75°C compared to the pristine Co 3 O 4 (132°C). [10b] Matrices including carbonaceous materials (e.g., activated carbon fibers, granular activated carbon, and activated carbon spheres), polyester fibers, and cellulose fibers, generally show improved surface area and porosity in the synthesis of TMObased composite catalysts. [41,43,45,46] As an example, CeO 2 was anchored on the 3D hierarchical nitrogen-doped porous carbon (3D-CeO 2 @CN) and used for FA oxidation. [10c] In this case, CeO 2 provided a high number of oxygen vacancies, abundant active surface oxygen, and high reducibility. Electron transfer from the N atoms of the surface CN further resulted in more oxygen defects and surface oxygen on CeO 2 compared to pure CeO 2 . The 3D hierarchical structure of CN helped stabilize CeO 2 , facilitating the mass transfer of FA molecules. As such, the prepared 3D-CeO 2 @CN was able to completely oxidize 90 ppm FA at 170°C, which was about 130°C lower than that of pure CeO 2 catalysts at a GSHV of 100 L g −1 h −1 . [10c] The surface functionalization of the matrix is regarded as an effective approach to enhance the compatibility of TMOs with nanomaterial matrices. [41b] For example, the carboxyl and hydroxyl functionalization of a polyethylene terephthalate (PET) surface (i.e., matrix) helped form a firmly attached thin MnO x layer on the surface of PET fibers. The synthesized MnO x /PET composite showed no agglomeration of powder MnO x catalysts with the reduced air pressure drop. In addition, the formulated composite was capable of degrading ≈94% of FA (0.6 mg m −3 ) at room temperature. [41b] Graphene has also been employed to couple with TMOs for accelerating charge transport during redox processes against FA using its exceptional electron conductivity. [26a] In comparison to the pure MnO 2 , the 2D structure of graphene can facilitate the adsorption of FA and O 2 molecules to expose more active sites for catalysis. Hybridized areas of graphene-MnO 2 provide important interfaces where the conducting graphene greatly accelerated the charge transfer between Mn 4+ and Mn 3+ species. [26a] As such, the catalytic performance of MnO 2 /graphene hybrids achieved completed conversion of FA (100 ppm) at 65°C, which lasted up to 70 h. [26a] Likewise, the heterostructure of nanosheet MnO 2 encapsulating N-doped graphene spheres (GS) was deposited in a network-like sponge (acting as support) to prepare 3D structure MnO 2 -GS sponge composites for FA oxidation (Figure 6a). [10a] Such a 3D structure facilitated the catalytic degradation of FA by exposing more active sites to FA molecules. The enhanced potential of MnO 2 -GS sponge composites was also reported, as it could favorably adsorb FA molecules by amino groups on N-doped GS surface through the formation of imide products (Figure 6b). [10a] Accordingly, the MnO 2 -GS sponge composites showcased 96.7% conversion of FA at low temperature (<35°C), which was far better than that of the pristine MnO 2 nanosheets (95.3% conversion at 40°C) (Figure 6c). [10a] In addition, N-doped carbon nanotubes (NCNT) have also received interest because of their large surface reactive sites suitable for high catalytic efficiency. [47] For instance, NCNT increased the number of structural defects with increases in electron transfer at the interfaces between NCNT and MnO 2 of MnO 2 /NCNT composites. [48] In addition, oxygen molecules were readily activated on NCNT through the formation of active superoxide species to promote the regeneration of MnO 2 . As such, the prepared MnO 2 /NCNT composites showed good activity and selectivity for FA oxidation. [48b]

Factors Influencing the Catalytic Activity of TMO-Based Thermocatalysts
The catalytic efficiency of TMO-based thermocatalysts can be influenced by several factors such as crystal form, catalyst preparation process (e.g., synthesis methods and calcination temperature), physiochemical properties (e.g., morphology, surface area, and particle sizes), and other experimental operation parameters (e.g., formaldehyde feed rate and humidity levels). [3,5a] Several articles have reviewed the effect of such factors on catalytic performances of TM-based thermocatalysts. [3][4][5]49] For instance, the effects of preparation method, morphology, specific area, and experimental parameters (e.g., water vapor content, initial FA concentration, and space velocity) were described with respect to the thermocatalytic performance of TMO catalysts. [49b] In addition, the effect of crystal form, surface morphology, microstructure, and temperature on the FA thermocatalytic performance of manganese oxides catalysts was also discussed. [5a] The catalytic performance of TMOs against FA was also significantly affected by many other factors such as exposed crystal facets, metallic/nonmetallic modifications, type of precursors, alkali/acid treatment, and type of matrix/substrate. Hence, the following sections are organized to address the various factors that can influence thermocatalytic performance of TM-based materials.

Effect of Alkali Metal/Nitrogen Modification on Catalytic Activity
The introduction of alkali metal/nitrogen has been reported for enhancing the catalytic activity of oxides materials by imparting abundant OVs and enhancing FA adsorption. [53] As such, the modification of TMO-based thermocatalysts with metal-like alkali metal (e.g., K + ) and nitrogen is an effective option to enhance the formation of abundant OVs, to facilitate adsorbed O 2 activation, and to promote the adsorption of FA molecules for the effective thermocatalysis of FA. [53c,54] For instance, the introduction of K + ions over Co 3 O 4 catalysts both increased the proportion of Co 3+ /Co 2+ (0.42) relative to pure Co 3 O 4 (0.30) and improved the number of OVs on the surface of modified Co 3 O 4 . This further boosted the oxidation ability of the modified thermocatalysts -MnO 2 : c) Illustration of synthesis procedure, d) XRD patterns, and e) FA removal efficiency between different exposed facets, Reproduced with permission. [52] Copyright 2023, Elsevier.
toward FA molecules. [54d] As a result, the K + modified Co 3 O 4 was able to completely convert 100 ppm FA to CO 2 at a lower temperature (i.e., 100°C) than the pure Co 3 O 4 (120°C). [54d] Interestingly, the isolated and localized K + modification of layered MnO 2 exhibited a distinct effect on FA removal for the generated thermocatalyst.
[54a] As the isolated K + was dissociated between layers via weak chemical bonds, the desorption of H 2 O molecules was restricted to hinder the FA catalytic process. In contrast, the coordination of localized K + with oxygen atoms at the vacancy sites was a useful option to activate O 2 with poor adsorption of H 2 O molecules. As such, the FA catalytic activity of the result-ing thermocatalyst was enhanced in a localized K + modified layer MnO 2 . [54a] Similar alkali-promoted effects were also observed for noble metal supported catalysts (e.g., K-Ag/Al 2 O 3 ). [55] Recently, the nitrogen modified -MnO 2 was reported to enhance the thermocatalytic decomposition of FA by achieving complete oxidation of 60 ppm FA under a GHSV of 90 L g −1 h −1 at 90°C. [54e] In this respect, the interstitial sites of thermocatalysts (Mn-N-O or Mn-O-N) were found to be the most suitable sites for introducing non-metal nitrogen on -MnO 2 . This was due to the potential of such interstitial sites for OV formation as well as FA/O 2 adsorption/activation, which are beneficial for FA oxidation.

Effect of Precursors on Catalytic Activity
The utilization of different precursors in the preparation of TMObased catalysts was also found to influence the redox ability, the amount of active oxygen species, crystal forms, and physiochemical properties. [56] The utilization of bases during the synthesis of TMO-based catalysts promoted their FA catalytic oxidation properties through increases in their OH − content and surface area. [57] As such, the effect of alkali precipitants (e.g., NH 3 ·H 2 O, KOH, NH 4 HCO 3 , K 2 CO 3 , and KHCO 3 ) was investigated with regard to the thermal catalytic activity of Co 3 O 4 catalysts toward gaseous FA. [56a] The Co 3 O 4 synthesized using KHCO 3 possessed abundant hydroxyl groups with a high ratio of Co 3+ /Co 2+ , exhibiting high catalytic activity against gaseous FA. Moreover, the BET specific areas and pore volumes of KHCO 3 derived Co 3 O 4 (97.9 m 2 g −1 /0.411 cm 3 g −1 ) were higher than that of Co 3 O 4 prepared by NH 3 ·H 2 O (67.7 m 2 g −1 /0.135 cm 3 g −1 ), KOH (54.8 m 2 g −1 /0.105 cm 3 g −1 ), NH 4 HCO 3 (95.4 m 2 g −1 /0.393 cm 3 g −1 ), and K 2 CO 3 (78.7 m 2 g −1 /0.337 cm 3 g −1 ). Such larger specific areas and pore volumes of KHCO 3 derived Co 3 O 4 can also facile the FA transportation onto active sites for oxidation, thereby enhancing the FA oxidation process. In contrast, low crystalline -MnO 2 was obtained when potassium permanganese (PP) was used as a manganese ionic source (Figure 8a). Among these, the MnO x /PG-PP possessed large specific surface area, crystalline -MnO 2 , highly distributed active components (e.g., surface active oxygen species), high content of Mn 4+ species, and increased lattice oxygen content. [56c] Consequently, the MnO x /PG-PP exhibited the best catalytic activity (95% removal of 1 ppm FA at 25°C) even after 600 min among all other MnO x /PG-(MA, MS, and MN) catalysts (below 30% after 600 min) (Figure 8b). [56c]

Effect of Alkali/Acid Treatment on Catalytic Activity
Alkali/acid treatments can be used to improve the FA catalytic performance of TMO-based catalysts through the introduction of -OH groups, defect sites, and OVs. [13b,23b,58] For instance, the post alkali (NaOH) treatment of NiCo 2 O 4 endowed it with more hydroxyl groups and Co 3+ /Ni 2+ content compared to the untreated NiCo 2 O 4 . [13b] In another study, birnessite-type MnO 2 treated by tetrabutylammonium hydroxide exhibited a lower apparent activation energy for FA oxidation than the pristine MnO 2 . [58] This is because tetrabutylammonium hydroxide helped generate irregular surface pits on MnO 2 , which both provided larger specific surface areas and led to the formation of more high valent manganese species (e.g., Mn 4+ and Mn 3+ ) and surface oxygen. As a result, the FA adsorption and oxidation properties of MnO 2 were enhanced considerably. [58] Likewise, ammonia was also utilized to control the OV formation onto -MnO x supported by AC ( -MnO x /AC) for the enhancement of FA catalytic removal. [23b] Accordingly, the -MnO x /AC-N 2 (ammonia used at 0.07 mol) had recorded higher ratio values for adsorbed oxygen (O ads )/lattice oxygen (O latt ) (0.71) and Mn 3+ /Mn 4+ (1.25) than those of pristine -MnO x /AC (0.44 and 0.73, respectively) (Figure 9a,b), which supports the suitable conditions for the favorable formation of OVs. As such, an increased relative abundance of OVs in -MnO x /AC-N 2 (i.e., compared with -MnO x /AC) was evident in electron paramagnetic resonance (EPR) spectra (Figure 9c). The abundant OVs were thus helpful to improve the catalytic activity of -MnO x /AC-N 2 against FA as they promoted the adsorption, activation, and migration of oxygen molecules to form more active oxygen species (e.g., O 2 • ).
[23b] The surface structure and redox properties of TMO-based catalysts can also be affected by acid treatment. [8,59] In particular, the H 2 SO 4 treatment of MnO x -CeO 2 catalysts showed higher surface area (310%) and stronger redox properties than their untreated counterparts. [8] It was observed that the textural and redox properties of MnO x -CeO 2 samples were altered considerably when the Mn content was above 50%. The higher surface area of acid treated thermocatalysts may reflect the dissolution of Mn 2+ ions by H 2 SO 4 acid. In addition, acid treatment oxidized the manganese species to a higher oxidation state via Mn dismutation reaction (Mn 3 O 4 + 4H + = MnO 2 + 2Mn 2+ + 2H 2 O) improving the redox properties of catalysts. [8] The extension of acid treatment time can also improve textual properties (e.g., BET surface area and pore volume) of the -MnO 2 catalyst while developing interconnected macro-mesoporous networks [59b] (Figure 9d). The BET surface area/pore volume of the resulting materials increased from 18 m 2 g −1 /0.1 cm 3 g −1 to 181 m 2 g −1 /0.73 cm 3 g −1 as the acid treatment duration increased from 0 h to 22 h. Such alteration of -MnO 2 catalyst led to the noticeable improvement in the FA catalytic activity. [59b]

Performance Comparison of TM-Based Catalyst for FA Degradation
Quantitative evaluation of thermocatalytic performance of TMObased catalysts is important to properly assess their practical feasibility toward FA oxidation. In this regard, the catalytic performances of different types of TMO-based catalysts were compared in terms of the key metrics (e.g., T 90 and kinetic reaction rate) as summarized in Tables 1 and 2. Note that T 90 value is the temperature corresponding to 90% removal efficiency of FA gas over TMO-based catalysts. [26b] The kinetic reaction rate (r, equation 1) is also a useful tool to offer a meaningful comparison between different catalytic systems as it can be used to integrate the interaction between important process variables (e.g., catalyst mass, pollutant conversion level, pollutant concentration, and pollutant feeding rate). [60] Reaction rate ( r, mmol mg −1 h −1 ) To make this comparison more meaningful, all TMO-based catalysts (mono-metallic, bi-metallic, and TMO-based composites) have been classified into two categories based on their working temperature as low (room temperature) and high temperature (>100°C) thermocatalysts (Tables 1 and 2). As such, the kinetic reaction rate (r) values at room temperature (RT) and at high temperature (e.g., 100°C) are utilized to compare their catalytic performances.

Conclusions and Outlook
The present review was organized to report the recent scientific developments of TMO-based thermocatalysts used for the degradation of gaseous FA. TMO-based catalysts have drawn great attention because of their availability, thermal stabilities, abundance, and cost-effectiveness. The mechanisms of FA removal over the TMO-based catalysts have also been discussed. To provide an in-depth discussion on applied TMO-based catalysts for thermal degradation of gaseous FA, monometallic, bimetallic, and composite-based TMO thermocatalysts were compared. Accordingly, the FA catalytic performance of the TMO-based catalysts was significantly affected by a number of factors (e.g., exposed crystal facets, alkali metal/nitrogen modification, alkali/acid treatment, and precursor type) that influence the generation of abundant TM cations, provide a high content of oxygen vacancies, activation of adsorbed O 2 , and enhancement of FA adsorption. Finally, the quantitative performance evaluation of diverse applied TMO-based catalysts was carried out based on their kinetic reaction rate values. The FA catalytic performance of the TMO-based catalysts was evaluated using kinetic reaction rate as the suitable metric under both low and high reaction temperature conditions. Based on this evaluation, TMO-based composites catalysts outperformed other types of TMO-based catalysts (i.e., mono-TMO and bi-TMO based catalysts) under all temper-ature conditions. As such, the TMO-based composites catalysts appeared to be the more feasible option than mono-and bi-TMO based catalysts for FA degradation. These observations may offer clear guidance for better design strategies for developing an effective thermocatalytic reactor for large scale applications.
Despite the great potential of TMO-based thermocatalysts for FA removal, more efforts are needed to expand their real-word applications as explained below.
1) It was observed that TMO composite thermocatalysts possessing functional matrices (e.g., cellulose and activated carbon) exhibited superior FA degradation performance compared to pristine TMO catalysts. Therefore, we recommend investigating the effect of other advanced materials like metal organic frameworks (e.g., large specific area and chemical stability) as matrices to further improve the catalytic performances of pure TMO-based thermocatalysts. 2) An ample understanding of the interactions between pollutants and TMO-based catalysts may help researchers develop efficient catalysts. For this purpose, the use of computational simulations such as density function theory and molecular dynamic simulations may have to be considered. 3) As most TMO-based thermocatalysts are commonly prepared with a batch synthesis mode, they may suffer from some drawbacks (e.g., low efficiency, lack of flexibility, and tenability/controllability toward better product properties). For the upscaled application of TMO-baed thermocatalysts, the novel synthesis routes are to be developed with the proper control of properties (e.g., size/shape and oxygen vacancies). 4) In most of the lab-scale studies, thermocatalytic removal of FA on TMO-based catalysts has been assessed under highly favorable reaction conditions (e.g., single pollutant system, high FA concentrations (>100 ppm), and large catalysts mass). However, FA is frequently present in the sub-ppm or ppb level in the real indoor environment. To obtain more practical information for the removal of FA under real-world conditions, the performances of TMO-based catalysts should be evaluated properly to reflect the real-world conditions (e.g., low concentration of FA and the presence of interfering pollutants). 5) Based on the performance evaluation, the TMO-based composite catalysts are found to have superior degradation efficiencies at elevated temperature. Hence, in the pursuit of the cost-effective TMO-based catalysts, it is desirable to develop some strategies (e.g., alkali/acid treatment) for their practical operation under low temperature (e.g., room temperature) conditions.