Catalysts and mechanisms for the selective heterogeneous hydrogenation of carbon-carbon triple bonds

SUMMARY The selective hydrogenation of carbon-carbon triple bonds to the corresponding double bonds, the semi-hydrogenation process, plays a very important role in polymer and ﬁne chemical industry. Various heterogeneous catalysts have been exploited for the selective hydrogenation of alkynes and alkynols in the gas and liquid phase. Herein, the recent progress in developing semi-hydrogena-tion catalysts, from traditional Pd-based monometallic catalysts to intermetallic compounds and single-atom catalysts, is summarized. The activation of dihydrogen during hydrogenation and the full hydrogenation mechanism, along with relevant research methodolo-gies, are discussed. This review provides a comprehensive overview on the catalysts and mechanisms of industrially relevant semi-hydro-genation processes, addresses some existing debates, and sheds light on future catalyst design for hydrogenation. 40,41 and ‘‘green oil’’ 21,42 problems in the gas phase, as well as hydrogen solubility 43 and metal leaching 44 in the liquid phase are analyzed. In a similar fashion, several kinds of typical hydrogenation mechanisms like the Horiuti-Polanyi mechanism (so-called dissociative mechanism), 40,45 the associative mechanism, 45 and the Eley-Rideal mechanism 46 are discussed, and their applicability in hydrogenation reactions are compared. Multiple techniques, including in situ Fourier transform infrared spectroscopy (FTIR), 29,32,33,47,48 temperature-programmed desorption (TPD), 32–35,40,48,49 H 2 -D 2 exchange, 50–52 solid-state nuclear magnetic resonance (NMR) 48,53,54 spectroscopy, as well as density functional theory (DFT) calculations, 29,31–33,36 are proposed for insight into the mechanism and structure-perfor-mance relationship in selective hydrogenations. heterogeneous hydrogenation systems. 99,126,131 On the premise of CLPs, Guo and co-workers demonstrated that the Ni-doped ceria, namely Ni@CeO 2 (111), exhibited high activity in acetylene hydrogenation. 99 DFT calculations revealed that the oxygen vacancies facilitated the heterolytic dissociation of dihydrogen, producing the Ce-H and O-H, respectively. The CLPs pattern of dihydrogen activation avoided the overstabilization of C 2 H 3 * intermediate and corresponded to the low barrier for the formation of ethylene. Similar structure-performance relationship was disclosed over Pt@Y zeolite, conﬁrming the wide applicability of CLPs and the corresponding selective speciﬁcity in hydrogenation reactions. 131


ORIGIN AND EVOLUTION OF CATALYSTS
Exploring an efficient catalyst to obtain both high activity and good selectivity toward alkenes remains a key challenge in the semi-hydrogenation process. 55 Two types of catalysts, namely homogeneous and heterogeneous catalysts, are the major contributors. Despite the high selectivity and explicit mechanism of homogeneous catalysts, the difficulties in separation and recycling restrict their industrial applications to some extent. 56 Compared with homogeneous catalysts, heterogeneous catalysts not only share the advantages of recyclability and regeneration but also the explicit and uniform location of active metal sites in some heterogeneous catalysts provide visualized models favoring the investigation of the structure-performance relationships. 57,58 On this basis, various heterogeneous catalysts, including traditional Pd-based catalysts, intermetallic compounds or alloys, and SACs, gradually spring up as promising candidates for selective hydrogenations (Figure 1 and Table 1).

Traditional Pd-based catalysts
Pd is one of the most efficient components in hydrogenation processes, inspired by its superior ability in dihydrogen dissociation. 106,107 However, the selectivity toward C=C bonds exhibits a huge fluctuation on account of the desorption energy of alkenes on the Pd sites. 108 Specifically, ethylene has the following three adsorption patterns: ethylidyne adsorbed on Pd-trimers, di-s-bonded C 2 H 4 adsorbed on Pd-dimers, and p-bonded C 2 H 4 adsorbed on Pd single atoms, respectively 108 (Figure 2A). Noteworthily, the desorption energy of ethylene is lower than that for the over-hydrogenation only when C 2 H 4 is p-bonded on Pd single atoms, thus leading to high selectivity toward ethylene. In contrast, when the ethylene is bonded on Pd species in ethylidyne or di-s-bonded C 2 H 4 modes, the excessive hydrogenation is more favorable than the desorption of ethylene, which is detrimental to the selectivity and causes the production of the over-hydrogenation product, ethane. In this regard, despite the excellent activity of Pd-based catalysts, the selectivity toward alkenes is severely deteriorated under the ensemble effect of Pd species. Correspondingly, several components, like Pb, 6 S, 2,7-10 C (subsurface carbon), 11 CO,12,13 and other p-block elements [14][15][16] have been adapted to promote the selectivity toward alkenes by covering the corner of edge sites of Pd counterparts. The Lindlar catalyst, typically Pd/CaCO 3 modified by both lead and quinoline, has been regarded as the benchmark in the selective hydrogenation reaction. 6 The selectivity toward alkenes is promoted since the Pd species are partially poisoned by lead and quinoline. Similarly, sulfur-containing substances like thiols can be exploited as a ''toxicant'' in Pd-complex systems. 1,[7][8][9][10][11]50 Anderson and co-workers discovered a particular palladium sulfide phase (Pd 4 S) that was eligible in the semi-hydrogenation reactions. [7][8][9] The Pd 4 S active sites not only exhibited high conversion and selectivity in the semi-hydrogenation of dienes and alkynes but also showed high endurance even at high pressure (up to 18 bar), which is a challenging topic but is overlooked in the hydrogenation industry. 8 Inspired by the promising performance of the Pd 4 S phase in the high-pressure hydrogenation reactions, Javier and co-workers designed a nanostructured Pd 3 S phase with controlled crystallographic orientation, denoted as Pd 3 S@C 3 N 4 , 2 which showed unparalleled performance in the liquid-phase hydrogenation reactions. In fact, the above-mentioned sulfur-modified Pd catalysts 7-9 mimic enzyme catalysts by imitating the tactics of ensemble and electronic density control. The stable phase of Pd-S compounds provides plenty of space for tailoring the active site with the most selective ensembles at the molecule level. Recently, Zheng and co-workers demonstrated a distinct Pd-sulfide/thiolate interface, denoted as Pd@SPhF 2 , showing good performance in the

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The above strategies provide us some enlightenment about how the surface coordination environment regulates the reaction performance. In addition to organic sulfides mentioned above, various metal sulfides show a promotion effect on alkene selectivity. 53,94 Li and co-workers disclosed that CuS nanoplates contributed to the dispersion of Pd elements by forming stable anchored active sites, accordingly attaining high selectivity, activity, and stability simultaneously. 94 Similarly, Zheng et al. demonstrated that the modification of rhodium sulfide over Pd nanosheets facilitated the formation of surface PdS x ensembles, which not only promoted the hydrogenation activity but also accounted for the isomerization of cis alkenes to trans alkenes. 53 Likewise, the subsurface carbonous compounds can promote the  11 The carbonaceous substance from feeding organic molecules could occupy the interstitial lattice sites and separated the adjacent Pd ensembles, accounting for the enhancement in selectivity ( Figure 2B). The importance of subsurface chemistry is evident, and is beneficial for the in-depth understanding of surface and subsurface dynamics as well as the rational design of catalysts.
Other p-block elements, like boron (B), can also boost alkene selectivity by covering the redundant Pd active sites. 14,15 Edman Tsang and co-workers employed BH 3 THF as a reagent and designed a novel catalyst, Pd int -B, Pd nanoparticles modified with interstitial boron atoms. 14 The catalyst exhibited good chemical and thermal stability owing to the strong electronic interaction within the host-guest sites between Pd and BH 3 THF. Meanwhile, Philip and co-workers compared the selective hydrogenation activity over boron-modified Pd catalysts using DFT calculations. 15 It is clear that the Pd(111)-B, Pd(111) surface modified with boron atoms, shows higher activity than clean Pd (111) in the hydrogenation of acetylene and 1,3-butadiene, validating the feasibility of boron modification for the purpose of increasing the hydrogenation performance. Apart from the above inorganic elements, CO has been selected to be a good promoter in the hydrogenation process, especially in industrial applications. Javier et al. investigated the activity of Pd-based catalyst in the absence and presence of CO with the assistance of DFT calculations. 12 It was demonstrated that the CO tends to reduce the Pd ensembles by forming densely packed overlayers. Thus, the over-hydrogenation and the polymerization can be prevented by suppressing the adsorption of unsaturated reactants, dihydrogen, and the hydrocarbon intermediates. However, the ratio of CO/H 2 should be controlled within a certain limit (usually below 0.1) to prevent the formation of oligomers. 13

Intermetallic compounds
The above poison strategy aims at promoting the selectivity by covering partial Pd active sites. However, due to sacrificing a mass of active phases and the utilization of toxic compounds, the strategy is deemed to be environmental unfriendly, with low atomic efficiency. Therefore, an alternative method employing a second or third metal, called alloys or intermetallics ( Figure 3A), was proposed. [16][17][18][19][20][21]60,61,71,93,105 On this basis, various Pd-based alloys/intermetallics like PdAg, 16 PdCu, 17 PdAu, 18 Pd-Zn, 19 Pd-Ga, 20 and Pd-In 21 have been investigated. Some non-precious metal alloys have also been developed on the premise of the site separation strategy. Under the construction of alloys/intermetallics, the continuous bulk metal ensembles can be separated or isolated by the second metal, promoting the selectivity toward alkenes.
Zhang and co-workers reported a series of IB-metal-alloyed Pd-based catalysts, namely PdAu/SiO 2 , 18 PdAg/SiO 2 , 16 and PdCu/SiO 2 , 17 and compared their activities in the selective hydrogenation of acetylene. The similar activation energies among the three catalysts indicated an analogous mechanism, but shared different selectivity toward ethylene, which might imply different electronic effects between Pd and the second metals ( Figures 3B and 3C). Specifically, the PdCu/SiO 2 catalyst possessed the highest electron density of Pd species, and thus suppressed the adsorption of C=C bonds and resulted in the higher selectivity toward ethylene (Figure 3D). In addition to the elements of group IB, Zhang and co-workers constructed PdZn intermetallic nanostructure through alloying Pd with Zn. 19 The Pd species in the PdZn catalyst weakened the p-bonding adsorption of ethylene and further prevented its over-hydrogenation, resulting in high selectivity toward ethylene. The ingenious arrangement of Pd species provided two adjacent but isolated sites, which was feasible for the adsorption and activation of acetylene through moderate s-bonding patterns, and responsible for the superior activity thereof (Figures 2A and  3E). Recently, Su and co-workers found that adding a Ga phase could destroy successive Pd atom ensembles, and designed supported Pd 2 Ga intermetallic catalysts 20 showing high thermal stability under employed reaction conditions by taking the advantage of covalent interactions between nanocrystals. However, the selectivity toward ethylene over the PdGa catalyst remained to be improved, probably due to the nonuniformity of nanoparticles prepared by a wet impregnation method.
Recently, Fan and co-workers developed a calcite-supported PdBi intermetallic compound, PdBi/calcite. Owing to the isolated and electron-rich Pd sites, the PdBi/calcite catalyst showed weak adsorption of ethylene and superior stability (over 99% ethylene selectivity at full conversion) over a wide range of reaction temperatures (423-573 K). 105 In addition to Pd-based alloy catalysts, some non-precious intermetallics have been investigated to replace the use of noble metals. Nørskov and co-workers screened about 70 bimetallic compounds, including Fe, Ni, Co, Cu, Pd, Pt, Ag, Au, Zn, Cd, Hg, Ga, Tl, Ge, Sn, and Pb, for the hydrogenation process ( Figures 3F and 3G). 60 Among these catalyst models, the low-cost Ni-Zn catalyst exhibited comparable activity (turnover rate) and selectivity. Theoretical predictions provided a novel and efficient strategy for the rational design of alloy hydrogenation catalysts free of noble metals. In addition, Javier et al. developed a ternary Cu-Ni-Fe catalyst that exhibited good performance in the semi-hydrogenation of propyne (selectivity of 80% at full conversion). 71 The three metal phases performed their own function, where Cu was used for a basement, Fe severed as the structural promoter to enhance propylene selectivity, and Ni facilitated the spillover of hydrogen and further prevented the oligomerization. The ternary catalyst was a great achievement, promoting the selectivity without the use of noble metal species and the potential poisoning step. Under the guidance of the site-isolation concept, a low-cost replacement for Pd-based catalyst, Al 13 Fe 4 , was constructed by Armbrü ster et al. 61 The electron structure of both Fe and Al was changed by the tight chemical bonding, and thus exhibited remarkable performance in the semi-hydrogenation of acetylene. Inspired by the above results, Wang et al. synthesized intermetallic Ni x Ga y and Ni x Sn y nanocrystals via a solution-based co-reduction strategy, 93 favoring the formation of alloys with uniform sizes. The isolated sites between Ni and Sn/Ga as well as the electronic effect within the active sites accounted for the good catalytic performance, making Ni x M y good candidates for the Pd-based catalysts.

SACs
Compared with the ''selective poison'' strategy, the construction of alloys/intermetallics promotes the atomic utilization efficiency to a great extent. However, multi-element alloys are usually prepared through wet impregnation, leading to inhomogeneous active sites and showing negative impacts on catalytic performance. Thereupon, the singleatom strategy with the maximal economic efficiency emerged. 22 By means of advanced techniques like Cs-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray adsorption spectroscopy (XAS), and FTIR spectroscopy with CO adsorption, the explicit structure of SACs can be revealed. On this basis, the single-atom catalyst systems can be modeled by DFT calculations and the structure-performance relationship can be further interpreted.

SACs on inert carriers
Graphene, nitrogen-rich carbon (C 3 N 4 ), aluminum oxide (Al 2 O 3 ), and silicon oxide (SiO 2 ) are widely utilized as relatively inert supports for metal species. [23][24][25][26][27][28][29]109,110 These inert supports can modify the configuration of metal species' morphologic and electronic aspects, forming metal/support interfaces, which are important to selective hydrogenation. Lu and co-workers prepared atomically dispersed Pd on graphitic carbon nitride (g-C 3 N 4 ), Pd 1 /C 3 N 4 , through an atomic layer deposition (ALD) technique, which showed 95% ethylene selectivity in the semi-hydrogenation of acetylene. 109 The Pd 1 /C 3 N 4 also exhibited a higher stability (more than 100 h) in either reducing or oxidizing conditions than g-C 3 N 4 -supported Pd NP catalysts, and thus appearing to be a promising candidate for promoting selectivity as well as coking resistance in hydrogenation systems. A similar structure-performance relationship was also disclosed over Pd 1 /graphene where the alkyne adsorbed through mono-p-adsorption rather than a di-p-adsorption manner and thus showed excellent selectivity. 23,24 In addition, Javier and co-workers established a stable singlesite Pd catalyst, namely [Pd]mpg-C 3 N 4 , which was confirmed to be efficient for the three-phase hydrogenation of alkynes and thus provide more potential in industry applications. 28 Recently, Ma et al. constructed the single Pd atoms supported on nanodiamond-graphene, Pd 1 /ND@G. 26 The atomically dispersed Pd atoms over Pd 1 /ND@G were strongly anchored on the support through Pd-C bonds, which not only avoided the formation of b-H species but also favored the desorption of ethylene, and thus showed remarkable selectivity. Similarly, Li and co-workers demonstrated a mesoporous N-doped carbon nanosphere, Pd/MPNC, which had high activity, excellent ethylene selectivity, and good long-term stability in the semi-hydrogenation of acetylene. 111 Considering the high cost of noble metals, Lu and co-workers designed a novel metal trimer catalyst (Ni 1 Cu 2 /g-C 3 N 4 ) exhibiting high efficiency in the semi-hydrogenation of acetylene. 29 The Cu atomic grippers could boost the loading of Ni species through dynamic and synergetic metal-support interactions, providing an atom-by-atom fabrication approach for the rational design of catalysts. Similar performance was also disclosed on Cu 0.5 /Al 2 O 3 25 and Cu 1 /ND@G 27 when applied in the semi-hydrogenation of acetylene.

SACs on porous supports
Porous materials such as zeolites, metal-organic frameworks (MOFs), 30 and zeolite imidazolate frameworks (ZIFs) 91 with ordered channel structure as well as unique confinement effects have become indispensable supports in heterogeneous catalysis. 57 These porous supports can provide accessible space to stabilize metal species and construct single-site catalysts. For example, Corma et al. designed Fe III -(OH) single sites embedded within MOFs, which exhibited good acetylene semi-hydrogenation performance under simulated front-end industrial conditions. 30 Inspired by the channel confinement effect of the zeolite, Gong and co-workers demonstrated that Pd clusters encapsulated within sodalite (SOD) zeolite, namely Pd@SOD, could catalyze the semi-hydrogenation of acetylene. 31 The narrow channels of SOD cages (0.28 3 0.28 nm) restricted the free diffusion of acetylene and ethylene, but allowed dihydrogen to enter the pore channels smoothly. Under such circumstances, the dihydrogen molecules underwent cleavage on encapsulated Pd clusters, accompanied by OH species transferring to the surface of SOD cages through spillover and reacting with acetylene. Recently, Li and co-workers constructed Ni(II) species confined within different zeolites, Ni@FAU 32 and Ni@CHA, 33 respectively. These two zeolite catalysts exhibited comparable hydrogenation performance but followed different mechanism patterns, which might originate from the faint difference of the confining environments between faujasite and chabazite zeolites ( Figure 4A). Owing to the stronger local electric field of chabazite than that of faujasite, the Ni(II) sites were more tightly confined in Ni@CHA and triggered the direct dissociation of dihydrogen. By contrast, Ni@FAU with a lower coordination number possessed higher affinity for acetylene and further induced the acetylene-promoted hydrogenation mechanism.
Single-atom alloys Intermetallics or alloys provide an alternative strategy toward selective hydrogenation and thus inspired the construction of the single-atom-alloy catalysts. [16][17][18][19][20][21]60,61,71,93,105 In essence, the single-atom-alloy catalysts can be rationally designed by regulating the formation and aggregation energies within host/guest metals in equilibrium, [34][35][36][37][38][39] thus reducing the dosage of noble metals to a great extent. Equipped with simple and specific configurations, the single-atom alloys accommodate the ideal catalyst models for surface science investigations and DFT calculations, beneficial to in-depth investigations of the structure-performance relationship. 112 As for the selective hydrogenation of alkynes, the facile dissociation of dihydrogen and the easy desorption of hydrocarbon intermediates are crucial factors controlling the selectivity, which are still difficult to realize simultaneously. To solve the problem, Sykes and co-workers constructed the Pd/Cu(111) single-atom-alloy catalyst with isolated Pd atoms over a Cu (111) surface. 34 Through low-temperature scanning tunneling microscopy (STM), it was disclosed that molecular hydrogen first dissociated on the single-dispersed Pd sites among 0.01 ML Pd/Cu (111) catalyst. Then the individual H adatoms underwent transfer from Pd sites to bare Cu (111) terraces (that is, hydrogen spillover) ( Figure 4B), providing the first direct observation of hydrogen spillover. The lower energy barrier for the dissociation of dihydrogen and the easy desorption of alkenes over Pd/Cu(111) than over bare Cu (111) surfaces were also confirmed via TPD and DFT calculations. Similarly, the Pt/Cu(111) singleatom-alloy catalyst with low concentration of Pt doped on Cu (111) surface was also found to be active for the facile dissociation of dihydrogen and the subsequent ll OPEN ACCESS hydrogenation of 1,3-butadiene. 35 Inspired by the unique characteristics of trace amount metal supported on Cu (111), Zheng et al. designed Pd atoms supported on different crystal faces, Pd 1 /Cu(100) and Pd 1 /Cu (111), and their hydrogenation behaviors were compared. 36 It was disclosed that extremely diluted Pd 1 /Cu (111) was inert for the hydrogenation of phenylacetylene unless Cu(100) was introduced ( Figure 4C). That is, the spillover of dihydrogen could only occur on the Cu(100) face despite the clear evidence of H adatoms spillover from Pd single sites to the bare Cu(111) face. 36 The essence of dihydrogen spillover over Pd/Cu single-atom alloy remains a controversy. Nevertheless, under the guidance of the single-atom-alloy strategy, diverse metal species in the form of single-dispersed atoms stabilized on peculiar crystal facets, for example Rh/Cu(111), 37 Pd/Au(111), 38 and Pt/Ag(111), 39 were constructed for various reactions not limited to selective hydrogenations.
Reducible supports with strong metal-support interaction Apart from inert supports as mentioned above, many reducible metal oxides, like titanium oxides (TiO 2 ), 113   (SMSI). For example, Francisco and co-workers found that moderate thermal treating of Pt/TiO 2 favored the deep diffusion of Pt phase into the bulk TiO 2 structure. 113 Correspondingly, the strong intimate relationship between Pt and TiO 2 could be induced, facilitating the following hydrogenation process. Similarly, Wang et al. reported that Pd/In 2 O 3 synthesized through facile wet impregnation was a promising alternative for the hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to the corresponding alkenol 2-methyl-3-buten-2-ol (MBE), a key component for the fabrication of vitamin E. 102 The In 2 O 3 support or substrate could form a loose layer to cover the exposed Pd active site under employed conditions on the premise of SMSI, ensuring the high selectivity toward alkenols. On the basis of SMSI, Choi et al. disclosed a novel mechanism, dynamic metal-polymer interactions (DMPI), which can be regarded as the organic version of SMSI. 50 The organic support polyphenylene sulfide was flexible and could cover Pd active sites under employed conditions via the strong metal-polymer interactions. This controllable interface could regulate the passages of reactants and prevent the over-hydrogenation. Apart from the concept of SMSI and DMPI, Zhu and co-workers designed a porous yolk-shell structure via reverse strong metal-support interaction. 104 The fully encapsulated core-shell configuration Pd-FeO x nanoparticles transformed into a porous yolk-shell structure Pd-Fe 3 O 4 -H under H 2 treatment. Being exposed to the reactants, the Pd active sites exhibited excellent catalytic performance in the selective hydrogenation of acetylene with a turnover frequency of (TOF) of 6.46 s À1 under the rule of SMSI in a reverse route. In addition to the common reducible support mentioned above, Ga 2 O 3 exhibited strong electron transfer as well as the decaying adsorption of CO and H 2 , which should be a promising candidate for the SMSI. 115 On this basis, Xu and coworkers constructed Pd/Ga 2 O 3 and disclosed that the formation of SMSI was triggered by the coexistence of PdGa alloy as well as the Ga 2 O 3 , thus providing both high propylene selectivity and propyne conversion under mild reaction conditions (303 K, atmospheric pressure). Noteworthily, the unique SMSI feature was also disclosed over layered double hydroxide (LDH). 116 Feng et al. reported that the Pd/ MgAl-LDH-Al 2 O 3 exhibited high activity and selectivity in semi-hydrogenation of acetylene owing to the reduced acidity as well as the SMSI.

GAS-PHASE AND LIQUID-PHASE SEMI-HYDROGENATIONS
Gas-phase hydrogenation Generally, the hydrogenation of alkynes is a two-phase system, where the gaseous acetylene/propyne/vinylacetylene and dihydrogen react over the solid catalyst. 1,42,60 The target products ethylene/propylene/1,3-butadiene are key components for the manufacture of polyolefins. 117 Alkenes are obtained in large scale through the cracking of naphtha, which contains a considerable number of impurities; i.e., 0.5%-8% alkynes. 1 These highly unsaturated compounds may cause serious damage in olefin polymerization; for example, the break of polymerase chains and the deactivation of the Ziegler Natta catalyst. 118 Thus, the content of triple-bond chemicals in an ethylene/propene stream must be reduced to an acceptable level (<5 ppm). Typically, the front-end and tail-end semi-hydrogenation is supposed to be the most efficient route to the elimination of trace alkynes. 17,119 The front-end hydrogenation, which requires high concentration of dihydrogen in the feed gas, easily causes the over-hydrogenation of acetylene and the ''runaway'' of reaction temperature, and thus makes it more challenging to obtain high selectivity in the front-end semi-hydrogenation. 40 In contrast, the ethylene feed gas in the tail-end process has been purified before the hydrogenation of trace amounts of alkynes. Therefore, the tail-end hydrogenation generally requires ll OPEN ACCESS near-stoichiometric dihydrogen (dihydrogen/alkynes = 1-2), which is easier to realize and more popular in practice. 120 For the selective alkyne hydrogenation in the gas phase, the selectivity toward the target alkene products is affected by many factors; for example, the drastic reaction temperature, 65 the excess amount of hydrogen, and the inner construction of catalysts. 91 On this basis, multiple hydrogenation catalysts have been investigated to avoid the over-hydrogenation process, which is not explained in detail here. Besides, some side reactions, such as couple crossing or polymerization of alkynes, are nonnegligible. Under certain circumstances, especially on the catalysts that have strong affinity toward ChC bonds, alkynes tend to form coke and C 4 -C 6 hydrocarbons (green oils) in the presence of alkenes. 121 The above adverse reactions may cause a huge waste of alkene cuts and will lead to catalyst deterioration as well as breaking off the polymer chains. Thus, it is urgent to design suitable catalysts that can suppress the side reactions and ensure high selectivity toward alkenes.

Liquid-phase hydrogenation
The selective hydrogenation of ChC bonds can also occur in the liquid phase to produce fine chemicals. Nowadays, the hydrogenation of alkynols is considered a fundamental process for the synthesis of fine chemicals and intermediate chemicals. 48 One important example is the synthesis of intermediate substance like linalool and MBE, which are prevalently utilized in the production of vitamin E, vitamin K, and provitamin b-carotene. 4,5,14 The demand of efficient hydrogenation catalysts in the liquid phase is now at a high pitch. Generally, the durability of hydrogenation catalysts in the liquid phase does not draw much attention owing to the relatively mild conditions compared with the hydrogenation in the gas phase. 55 However, there are some severe problems to solve in liquid-phase hydrogenations. For example, the dihydrogen solubility in liquid phase is limited, thus hampering the conversion of alkynes. 43 On the other hand, the selectivity toward alkenes/alkanols remains to be promoted since the active sites exhibit stronger adsorption affinity of liquid medium than that of dihydrogen in the gas phase. Therefore, the alkenes/alkanols tend to be over-hydrogenated to saturated alkanes/alcohols instead of being desorbed as products, corresponding to low selectivity. Besides, it is important to identify whether the active sites are heterogeneous or not, which is crucial for industry applications, and this may require hot filtration test in batch reaction or long-term fixed-bed reaction. However, many liquid-phase hydrogenation catalysts equipped with organic modifiers have a huge risk of leaching, which further leads to declines in catalytic activity and/or selectivity when applied in practical operation. That is to say, the exploitation of hydrogenation catalysts with anchored active sites appropriate for hydrogenation in the liquid phase is still a challenging task.

MECHANISM OF SELECTIVE HYDROGENATION Activation of dihydrogen
The adsorption and activation of dihydrogen is considered the crucial step in the hydrogenation process, which can be classified as homolytic dissociation and heterolytic cleavage, respectively. 11 The homolytic dissociation of dihydrogen generally happens on the surface of VIII group metals such as Pt, 122 Pd, 123 and Rh. 37 These metals have segmental occupied d orbitals, where the s electrons from dihydrogen can be accommodated. On the other hand, metals can donate d electrons to the antibonding orbits of dihydrogen ( Figure 5A). Accordingly, two hydrides are formed by homolytic cleavage of the weak H-H bond. It is acknowledged that metal ensembles, where more than two metal atoms are present in the vicinity, are more beneficial for the homolytic cleavage of dihydrogen than single-atom metal species. 96 However, this might lead to over-hydrogenation, since the hydride formed after homolytic dissociation will migrate to the near-surface region of metal counterparts, forming the subsurface hydride, which is detrimental to the selectivity. 11 Analogously, hydrogen species can be heterolytically cleaved into H + /H À through the concerted effect of metal species and basic materials such as supports. The

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formed H + and H À then bond with metal atoms and a proton acceptor, for example N, 125 O, 33 and C 23 atoms, respectively. It is well-known that heterolytic dissociation of dihydrogen usually happen in homogeneous catalytic systems like enzymes, 125 the frustrated Lewis pairs (FLPs), and the classical Lewis pairs (CLPs). 99,126,127 However, some heterogeneous catalysts, like supported metal oxides, 98,128,129 SACs, 28,96,98,130 and metal encapsulated zeolites, 33,131 are also prone to dissociate dihydrogen heterolytically. , which provided strong evidence for the heterolytic cleavage of hydrogen. 24 Unlike the homolytic dissociation of dihydrogen, which needs adjacent metal atoms, heterolytic dissociation of hydrogen requires the joint efforts of both metal and support, which serve as the hydride acceptor and proton acceptor, respectively. Lacking metal ensembles, the analogous FLPs/CLPs patterns of hydrogen dissociation enable the higher selectivity toward alkenes in alkyne hydrogenations, and therefore become a hot spot in heterogeneous hydrogenation systems. 99,126,131 On the premise of CLPs, Guo and co-workers demonstrated that the Ni-doped ceria, namely Ni@CeO 2 (111), exhibited high activity in acetylene hydrogenation. 99 DFT calculations revealed that the oxygen vacancies facilitated the heterolytic dissociation of dihydrogen, producing the Ce-H and O-H, respectively. The CLPs pattern of dihydrogen activation avoided the overstabilization of C 2 H 3 * intermediate and corresponded to the low barrier for the formation of ethylene. Similar structure-performance relationship was disclosed over Pt@Y zeolite, confirming the wide applicability of CLPs and the corresponding selective specificity in hydrogenation reactions. 131 Plausible reaction mechanisms For insight into the mechanism of alkyne hydrogenation, some unique catalyst systems and their corresponding reaction pathways are discussed in the following sections. The Langmuir-Hinshelwood mechanism 124,132 with the co-adsorption of reactants on surface plays a dominating role in heterogeneous hydrogenation catalysis. In contrast, the Eley-Rideal mechanism, 133 featuring the direct reaction between a molecular reactant with an adsorbed one, is rarely reported for alkyne hydrogenation.

(Pseudo-)Horiuti-Polanyi mechanism
The Langmuir-Hinshelwood mechanism, which remains the most common one among heterogeneous catalytic systems, can be divided into two routes ( Figure 5B). Proposed in 1934, the Horiuti-Polanyi mechanism, or the so-called dissociative mechanism, plays a primary role in hydrogenation processes. 134 It entails dihydrogen homolytic dissociation on the metal surface, followed by the successive addition of H atoms to the adsorbed alkynes. This routine is firstly disclosed on the surface of Ni 86 and Pd 135 with intrinsic ability to split dihydrogen, leaving the hydrogen atoms bonded with metal and establishing the stable metal hydrides (M-H). Subsequently, the so-called dissociative mechanism was discovered over Au nanoparticles by Javier and co-workers. 42 Despite the filled d orbitals of Au-based catalyst and the restricted activity toward dihydrogen dissociation, the catalyst exhibited decent performance in the selective hydrogenation of propyne with $90% conversion and selectivity to propylene. Additionally, the Horiuti-Polanyi mechanism was found to ll OPEN ACCESS be applicable over alloys and metal phosphides such as Pd 3 Ga 7 69 and Ni 2 P, 92 involving molecular hydrogen dissociation followed by the successive addition of hydrogen atoms to the tri-bonded compounds.
However, for SACs without adjacent metal-metal pairs available for the homolytic dissociation of dihydrogen, the dissociation of dihydrogen must take place via an alternative pathway, namely heterolytic dissociation. 96 Typically, the dihydrogen molecules undergo heterolytic cleavage, leaving one of the hydrogen atoms bound to the metal atom and the second one to the heteroatom of the support like N, C, or O. Inspired by this kind of dissociation routine, various SACs, e.g., Pd 1 -O/graphene, 23 Pd 4 S, 2 Ni@CHA, 33 and Pd/mpg-C 3 N 4 , 28 were rationally designed. For example, in a typical system of Ni(II)-encapsulated zeolite, namely Ni@CHA, the dihydrogen firstly undergoes heterolytic cleavage under the effect of coordinately unsaturated Ni(II) site and the surrounding oxygen atoms in the six-membered ring of chabazite zeolite. 33 This type of hydrogen heterolytic dissociation was verified not only via DFT calculations but also through isotope-labeled FTIR spectra, providing strong experimental evidence for the CLPs. As shown in Figure 5C, the stretching pattern of bridging hydroxy ions (n O-H = 3,610 cm À1 ) shifted to the red region (n O-D = 2,600 cm À1 ) in the deuterium labeling experiment, according with the theoretical ratio of the frequencies for harmonic oscillation of H 2 and D 2 molecules against a rigid wall (1.41). 136 Similarly, Lu and co-workers demonstrated that the hydrogenation of 1,3-butadiene over Pd 1 -O/Graphene followed the typical Horiuti-Polanyi mechanism where the heterolytic dissociation of dihydrogen was found to be the rate-determining step. 23 Considering the different dissociation patterns of dihydrogen with the Horiuti-Polanyi mechanism, the hydrogenation with heterolytic cleavage of dihydrogen was denoted as pseudo-Horiuti-Polanyi mechanism by Lu et al.
Noteworthily, the Horiuti-Polanyi mechanism occurs over metal surfaces in most cases, which is feasible for the cleavage of dihydrogen. However, this kind of hydrogenation mechanism generally leads to mostly cis alkene products in the hydrogenation of internal alkynes, even though the trans alkenes are thermodynamically more stable than the cis ones. 137,138 On this basis, Zheng et al. reported that the defective Rh 2 S 3-x exhibited high selectivity toward trans alkenes in the hydrogenation of internal alkynes. 53 The dihydrogen underwent dissociation at the defects of the solid surface and formed the frustrated hydrogen pair, which could modulate the cis-to-trans isomerization without over-hydrogenation. That is to say, the isomerization of cis/trans alkenes can be modulated by altering the hydrogenation mechanism, providing a new thought for the rational design of novel catalysts.

Associative mechanism
Despite the most crucial step of dihydrogen dissociation in the hydrogenation process, there are some relatively inert metal sites that are invalid for the direct splitting of dihydrogen. In this sort of catalyst, the hydrogen cannot be dissociated by active center independently but with the assistance of alkynes, denoted as associative mechanism ( Figure 5B). The alkyne-assisted pathway was first disclosed on Ag nanoparticles by Javier et al. in 2013. 45 The adsorption sites of alkynes and the energy barriers over Ag(211) under a different mechanism were compared using DFT calculations. As shown in Figure 5D, the associative mechanism required relatively lower activation barriers than the classical Horiuti-Polanyi mechanism. Later, this acetylene-promoted associative mechanism was reported with over Pd@SOD 37 and Cu 1 /ND@G 27 systems. On the premise of the associative mechanism, it was found that the above SACs exhibited higher selectivity than the corresponding metal ensembles.

OPEN ACCESS
However, the associative mechanism is merely confirmed by DFT calculations, lacking direct experimental evidence. 45 This can be ascribed to the fact that the intermediate phases of Horiuti-Polanyi mechanism and associative mechanism are identical or similar; for example, (C=CH*, CH-CH 2 *), making it difficult to distinguish by means of spectroscopic protocols. 32,33 Recently, Li et al. conducted the H 2 -D 2 -C 2 H 2 pulse-response experiments, providing the first in-depth evidence on the alkyne-associative mechanism. 32 The signals of HD (m/z = 3) in the absence and presence of acetylene were compared over Ni@CHA and Ni@FAU catalysts, respectively. As shown in Figure 5E, no HD signal was detected over Ni@FAU under the mixture flow of H 2 -D 2 at rational temperature, indicating that the dihydrogen could not be efficiently activated under employed condition. However, it appeared immediately upon the introduction of acetylene, confirming that the dihydrogen was dissociated after the injection of acetylene molecules. That is, the hydrogen and deuterium underwent dissociation and formed HD only after the combination of Ni(II) sites and alkynes. The so-called alkyne-promoted/assisted mechanism is different from the conventional Horiuti-Polanyi mechanism, 134 which is rarely seen in heterogeneous hydrogenation but widely observed in homogeneous systems. 89,139,140 For the Ni@CHA catalyst, the HD signals were clearly detected in the H 2 -D 2 stream at the very beginning and gradually decreased upon the introduction of acetylene due to the hydrogenation of acetylene by HD, deriving a typical Horiuti-Polanyi mechanism. In short, the H 2 -D 2 -C 2 H 2 pulse-response experiments provide direct evidence to distinguish the associative mechanism and Horiuti-Polanyi mechanism. Under the guidance of the associative mechanism, considerable amounts of trans alkenes could be obtained from the hydrogenation of internal alkynes like di-phenylacetylene and 1-phenylpropyne, in accordance with the views of Zheng et al. 53 That is, the associative mechanism may lead to the formation of trans alkenes in the selective hydrogenation of internal alkynes.
The associative mechanism, where the dihydrogen dissociated heterolytically, is widely observed in organometallic chemistry and homogeneous catalysis. For example, the associative mechanism has been found to boost the trans-alkene selectivity in various metal complexes like (IMes)Ag*Rp, 89 Ni*Ln 139 and Cp*Ru. 140 However, this mechanism is rarely seen in a heterogeneous hydrogenation system. The alkyne-promoted mechanism provides a specific hydrogenation process that compensates for the conventional Horiuti-Polanyi mechanism. The associative pattern was also disclosed to be suitable in heterogeneous ammonia synthesis by Li et al. [141][142][143] The associative mechanism was also found to be efficient in ammonia synthesis over Ru/H-ZSM-5 142 or Fe 3 /q-Al 2 O 3 143 catalyst, further proving its wide application in hydrogenation reactions.
Hu and co-workers summarized the Horiuti-Polanyi mechanism and non-Horiuti-Polanyi mechanisms in hydrogenation catalysis from DFT calculations. 144,145 The universality of the Horiuti-Polanyi routine, proposed 100 years ago, has been confirmed over various types of catalysts. For the catalysts showing weak adsorption or dissociation ability of dihydrogen, for example Ag(211) and Ni (111), the prevalence of associative mechanism may occupy the dominating role.

Eley-Rideal mechanism
Proposed by Eley and Rideal in 1938, the Eley-Rideal mechanism illustrates the route that only one of the reactant molecules adsorbs on catalyst surface and the other one participates in the reaction without adsorption (usually from the gas phase). 46,146 The reaction scheme of ChC bond hydrogenation is briefly described in Figure 5B. Typically, the Eley-Rideal mechanism can be divided into two categories: (1) adsorbed alkynes molecules, *ChH reacting with dihydrogen in the gas phase, ll OPEN ACCESS H 2(g) ; (2) adsorbed hydrogen molecules, *H 2 reacting with alkynes in the gas phase, ChH (g) .
(1) *ChH reacting with H 2(g) It is acknowledged that the Eley-Rideal mechanism is extensively applied in the hydrochlorination of acetylene, in which the *C 2 H 2 reacting with HCl (g) . 147 However, as for the selective hydrogenation process, the activation and dissociation of H 2 molecules remains very significant in the hydrogenation process. 54 Thus, the reaction pattern of *C 2 H 2 reacting with H 2(g) is rarely seen in alkyne hydrogenation. In a very early study, 148 Butt and co-workers investigated the performance of benzene hydrogenation over supported Ni catalysts. The kinetic results of benzene hydrogenation matched well with a typical Eley-Rideal mechanism, proceeding via the molecular addition of dihydrogen to adsorbed benzene.
However, there remain some controversies about the feasibility of Eley-Rideal mechanism (*ChH reacting with H 2(g) ), especially in terms of alkyne hydrogenation. The pathway of adsorbed acetylene reacting with molecular dihydrogen from the gas phase was found to be inapplicable by Hafner et al. using DFT calculations. 149,150 The high activation energy of 200 kJ/mol as well as the strong Pauli repulsion between dihydrogen and acetylene molecules suppressed the impending hydrogenation process theoretically. Therefore, more straightforward approaches to distinguish the peculiar Eley-Rideal mechanism and to investigate the origin thereof are urgent required.
(2) ChH (g) reacting with *H 2 As mentioned above, the dissociation of dihydrogen represents an essential step in the hydrogenation of alkynes, whether in homolytic or heterolytic patterns. 11 Therefore, the hydrogenation pattern of adsorbed dihydrogen molecules reacting with alkynes in gas phase is easier to accept in the Eley-Rideal mechanism. For example, Zheng et al. demonstrated that the poisoned Pd-based catalyst, Pd 4 S@SPhF 2 , might follow the typical reaction scheme. 10 The direct participation of internal alkynes from the gaseous phase might be speculated since neither PhChCCH 3 nor PhCH = CHCH 3 could adsorb on the catalyst surface.
There remain some controversies about the feasibility of the Eley-Rideal mechanism. Hafner et al. assumed the pathway of adsorbed acetylene reacting with molecular dihydrogen from the gas phase using DFT calculations. 149,150 This route was claimed to be inapplicable in alkyne hydrogenation process limited by the high activation energy of 200 kJ/mol as well as the strong Pauli repulsion between dihydrogen and acetylene molecules. Therefore, more straightforward means are required to distinguish the peculiar Eley-Rideal mechanism and to investigate the origin thereof.

RESEARCH METHODOLOGY
For a better understanding of the selective hydrogenation reaction, comprehensive characterization of the catalytic configuration and reaction process is highly desired, which is a hot and challenging topic. For the structure and configuration of catalysts, several advanced characterization strategies, such as Cs-corrected HAADF-STEM; 26,27 FTIR spectroscopy with CO adsorption; 22 and XAS, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES), 17 33 The competitive adsorption of reactants, i.e., dihydrogen and acetylene, could be analyzed, providing useful information on the detail reaction pathway. 32,33 As shown in Figure 6A, the chemisorbed acetylene on Ni@CHA (3,010 and 2,925 cm À1 ) could be weakly captured only after the pretreatment with helium but completely disappeared after the pretreatment of dihydrogen. That is, the Ni (II) sites showed stronger affinity of dihydrogen that acetylene and subsequently hindered the adsorption of latter. On the contrary, the stretching bands of C-H (acetylene) were strongly bonded to Ni(II) sites in Ni@FAU whether pretreatment was in helium or dihydrogen. The distinct adsorption behaviors toward reactants over Ni@CHA and Ni@FAU may imply the diverse hydrogenation mechanisms.

TPD
As mentioned above, the Ni(II) species confined within faujasite and chabazite displayed different adsorption affinity of acetylene and dihydrogen shown by FTIR spectra ( Figure 6A). 32,33 The intrinsic competitive adsorption behaviors on Ni@FAU and Ni@CHA can be interpreted from TPD experiments. As shown in Figure 6B, the desorption temperature of dihydrogen was higher than that of acetylene over Ni@CHA catalyst, indicating the stronger affinity of dihydrogen than acetylene. It revealed that the dihydrogen could be easily dissociated over Ni@CHA with trace hydrogen spillover around the zeolite (small peak at $500 K). In contrast, the Ni@FAU catalyst only exhibited a moderate desorption peak of acetylene but no desorption signals of dihydrogen, suggesting the very weak adsorption of dihydrogen. On the premise of the different desorption behaviors between Ni@FAU and Ni@CHA, two different hydrogenation mechanisms were disclosed. 32,33,57 Similarly, on the premise of the TPD investigation, Zheng et al. disclosed that the Pd@SPhF 2 exhibited sore adsorption of dihydrogen but no desorption peak of alkynes (PhChCCH 3 ), suggesting a typical Eley-Rideal mechanism. 10

H-D exchange
The dissociation of dihydrogen is acknowledged to be prerequisite in most hydrogenation reactions. The kinetic isotopic effect can provide strong evidence that the dissociation of dihydrogen is the rate-determining step, which also complies with the strong Pauli repulsion between dihydrogen and acetylene. 149,150 Additionally, the dissociation sites can be investigated by comparing the HD formation rate. Lu and co-workers compared the HD formation rate over Pd 1 -O/graphene and Pd-NPs/graphene via H-D exchange. 23 As shown in Figure 6C, the HD formation rate over Pd-NPs/graphene was about 12 times higher than that over Pd 1 -O/graphene, indicating that the hydrogen dissociation over Pd 1 single atoms was extremely hindered and the rate-determining step could be identified. The HD formation rates decreased on both catalysts after the introduction of butadiene, which could be attributed to the stronger adsorption of butadiene on Pd 1 single atoms and Pd-NPs than dihydrogen.
Choi and co-workers performed the H-D isotope exchange over Pd/PPS and Pd/SiO 2 catalysts. 50 As shown in Figure 6D, the absence of hydrogen activation ability over Pd/PPS was verified since no HD formation could be detected when feeding an H 2 -D 2 mixture to the catalyst. The signal of HD appeared immediately upon the feeding of acetylene, revealing the activation of dihydrogen in the presence of coadsorbed acetylene. The H-D isotope exchange results provided strong evidence for the alkyne-promoted hydrogenation process over Pd/PPS catalyst (i.e., the associative mechanism). Similarly, Li et al. disclosed the strong affinity of acetylene over Ni@FAU catalyst and confirmed the alkyne-promoted mechanism though H 2 -D 2 -C 2 H 2 pulse-response experiments as discussed previously ( Figure 5E). 32

NMR measurements
The heterolytic dissociation of dihydrogen favors the high selectivity toward ethylene owing to the lack of b-H species on metal surface. However, it is difficult to capture the M-H À species limited by the characterization methods. Theoretically, the solid-state 1 H magic-angle spinning (MAS) NMR spectroscopy can provide clear evidence on the heterolytic cleavage of dihydrogen. Geoffrey and co-workers demonstrated that the dihydrogen underwent heterolytic cleavage on the In 2 O 3-x (OH) y catalyst, forming In-OH 2 + and In-H À , respectively. 54 The stretching bands of In-OH 2 + and In-H À attributed to 1,220 and 1,300 cm À1 could be clearly captured through FTIR spectroscopy.
In the 1 H MAS NMR measurements ( Figure 6E), the chemical shifts at 4.05 and 1.14 ppm, attributed to the In-OH 2 + and In-H À , respectively, could be captured at room temperature, in good consistency with FTIR results. On these grounds, the NMR measurement makes a quantitative compensation for the characterization of dihydrogen hydrolysis. Recently, Zheng and co-workers demonstrated that the dihydrogen could be heterolytically dissociated into the frustrated hydrogen pair over defective Rh 2 S 3-x catalyst. 53 The frustrated hydrogen pair could stereo-selectively mediate the cis-totrans isomerization of alkene via 1 H MAS NMR. As shown in Figure 6F, the chemical shift signals at 6.25 and 6.13 ppm corresponding to the a-C-H (H c ) and b-C-H (H d ) confirmed the formation of trans-1-phenyl-1-propene, with weak signals at 6.32 and 5.65 ppm attributed to the a-C-H (H a ) and b-C-H (H b ). The deuterium labeling experiments ( Figure 6F) demonstrated that the intensity of H d decreased drastically after the D 2 was charged while the intensities of others specie kept nearly unchanged. In such a way, the isomerization process was illustrated where the alkene inserted into the metal-D bond with the elimination of the original b-C-H (H d ) after the rotation of C-C single bond.

DFT calculations
In addition to the above-mentioned experimental protocols, DFT calculations play a significant role in studying the selective hydrogenation process, providing comprehensive information from precise structure of catalyst to the reaction mechanism as well as structure-performance relationship. First, the structure and configuration of heterogeneous catalysts such as zeolite, 33 oxides, 86,128 carbon-nitride, 28 and alloys 36 can be optimized and modeled. Li et al. optimized the structure of Ni@CHA by calculated energies, and Ni 2+ sites were found to sit stably in the sixmembered rings with Al atoms in the para or meta position ( Figure 7A). Second, various modes of reactant adsorption as well as the dissociation of dihydrogen among multiple active sites can be interpreted. For instance, Zheng et al. measured different energy barriers of dihydrogen spillover among different Cu sites in Pd/Cu 86 catalyst 36 ( Figure 7B). The small energy barriers (0.11-0.25 eV) verified the facile spillover of H atoms. Third, the spectroscopy signature of adsorption and reaction intermediates can be validated. As shown in Figure 7C, the stretching bands of CH 2 * and CH 3 * were well predicted via DFT calculations, in accordance with the ll OPEN ACCESS experimental observations and assignments. 33 Noteworthily, with the optimized structures, the hydrogenation routine can be predicted accurately. For example, Li et al. compared the energy barrier of Horiuti-Polanyi mechanism and associative mechanism in acetylene hydrogenation over Ni@FAU, revealing the priority of associative mechanism ( Figure 7D). This is in line with the H 2 -D 2 -C 2 H 2 pulse-response experiments where dihydrogen is activated with the assistance of adsorbed acetylene molecules.
Finally, DFT calculations can provide multiple insights for the rational design of hydrogenation catalysts. Nørskov et al. performed DFT calculations to identify relations in heats of adsorption of hydrocarbon molecules and fragments on metal surfaces. 60 As a result, cheap Ni-Zn alloys were predicted to be efficient catalysts for acetylene semi-hydrogenation ( Figures 3F and 3G), which were successfully verified by experimental studies. Hopefully this strategy might change the current trial-anderror mode of catalyst development.

CONCLUSIONS AND OUTLOOK
In this review article, we have summarized recent progress in the selective hydrogenation of ChC bonds to the corresponding C=C bonds. This is not only an industrially relevant process known as semi-hydrogenation but also a popular model reaction. Efficient heterogeneous catalysts are being pursued and the detailed mechanism is still hotly discussed. Various strategies, including covering/poisoning the corner/edge active sites by organic compounds, segregation of active sites by adding the second metal, and the site-isolation approach by forming SACs, were discussed with concrete examples. Then, some key issues in the semi-hydrogenation process, for example the thermal run-away in front-end hydrogenation, the formation of green-oil and coke in tail-end hydrogenation, the solubility of dihydrogen, and the potential leaching of active sites in liquid-phase hydrogenation, are listed, which remain key challenges in industrial semi-hydrogenations. Finally, the detailed reaction mechanism of selective hydrogenation was discussed. Typical mechanisms, including the Horiuti-Polanyi mechanism, associative mechanism, and Eley-Rideal mechanism, are compared and the origin thereof discussed. Understanding reaction mechanisms and the further structure-performance relationships undoubtedly gives huge benefits to the rational design of robust catalysts for semi-hydrogenations. To achieve this goal, some important research methodologies, including spectroscopic investigations and theoretical calculations, were briefly detailed.
The semi-hydrogenation of acetylene to ethylene in the gas phase and the semi-hydrogenation of MBY to MBE in the liquid phase have been industrialized on a large scale for decades, both using Pd-based catalysts. The innovation of catalytic materials offers great opportunities to construct a new generation of semi-hydrogenation catalysts with improved performance and economy. SACs appear to be ideal solutions to the new semi-hydrogenation processes. However, there is still a long way from the laboratory to industry. Objectively speaking, new semi-hydrogenation catalysts with comprehensive performance (substrate conversion, product selectivity, catalytic stability, and catalyst cost) surpassing the commercial ones are yet to be reported and confirmed. Apart from the thermo-catalytic semi-hydrogenation systems, electrocatalytic and photocatalytic or photothermal catalytic semi-hydrogenations are attracting growing attention in recent studies. For example, layered double hydroxide (LDH)-derived copper catalysts 152 and Cu dendrites deposited through an electrochemical method 117 show remarkable performance in the electrocatalytic semi-hydrogenation of acetylene to ethylene. Pd 1 /TiO 2 , 153 Au-Pd/C-TiO 2 , 154 and Pd 1 /N-graphene 155 catalysts show good performance in the semi-hydrogenation of acetylene under photothermal irradiation. Interestingly, Pt/TiO 2 photocatalyst shows high substrate conversion and styrene selectivity in the hydrogenation of phenylacetylene under 385-nm monochromatic light irradiation, in significant contrast to the thermo-catalytic process. 156 These achievements might pave the way to new semi-hydrogenation processes, especially in the liquid phase.