Hongyan Fang ,Jingjing Jiang ,Dingsheng Wang ,Xiangwen Liu ,Dunru Zhu ,Yadong Li ,＊

1 State Key Laboratory of Materials-oriented Chemical Engineering,College of Chemical Engineering,Nanjing Tech University,Nanjing 211816,China.

2 Institute of Analysis and Testing,Beijing Academy of Science and Technology(Beijing Center for Physical and Chemical Analysis),Beijing 100094,China.

3 Department of Chemistry,Tsinghua University,Beijing 100084,China.

Abstract: Traces of acetylene impurities in the feed gas during the subsequent industrial production process of polyethylene will inactivate ethylene polymerization.The semi-hydrogenation of acetylene to ethylene has been proved to be one of the most effective technologies for the purification of ethylene.Pd catalysts have been playing a leading role in industrial applications due to their excellent performance.However,as Pd is a precious metal,Pd catalysts are expensive.Thus,it is very important to design low-cost,high-selectivity,and high-conversion acetylene semihydrogenation catalysts.Here,we summarize the influence of single-metal catalysts based on the acetylene semi-hydrogenation mechanism.The hydrogenation ability of the catalysts should be neither too high nor too low.When other metals are added to palladium catalysts,bimetallic catalysts are formed,which can be classified into typical substitutional solid-solution alloy catalysts,intermetallic compound catalysts,and single-atom alloy catalysts.Regarding the influence of bimetallic catalysts on the performance of acetylene hydrogenation,metals other than Pd have different effects on the acetylene hydrogenation process due to the different structure and environment.While,the structure of the catalyst and the chemical environment ultimately affect the electronic structure of the active center of the catalyst.Based on this,we conclude that the key to the semi-hydrogenation of acetylene is the charge density of the active center of the catalyst,such as dual-atom sites and nano-single atoms;the electrons control the active center of the catalyst.Finely turning the electronic structure of single metal active sites will improve their catalytic activity,selectivity,and stability of the catalyst for acetylene semi-hydrogenation.Additionally,we propose a possible future direction for the development of high-performance acetylene semi-hydrogenation catalysts.Future catalysts for acetylene semi-hydrogenation able to precisely control the active sites to improve their catalytic activity,selectivity,and stability are the focus of researchers,such as the precise control of single-atom-site,dual-atom-site,and nano-singleatom-site catalysts.

Key Words: Acetylene;Ethylene;Selective hydrogenation;Catalyst

1 Introduction

Ethylene produced by petroleum cracking contains 0.5%-3% of acetylene,which inactivates ethylene polymerization in Ziegler-Natta catalysts.In order to obtain polymer-grade ethylene,acetylene in the stream must be reduced to below 5 ppm(volume fraction)1-5.Generally,acetylene can be removed from ethylene-rich feed streams through heating,physical adsorption,chemical adsorption,or catalysis6-14.In industry,organic solvent extraction methods,such asN,Ndimethylformamide(DMF)orN-methyl pyrrolidone(NMP),can also be used to recover acetylene from crude ethylene for use as the raw material in organic synthesis15.However,the extraction method used has considerable toxic and side effects,which are harmful to humans and the environment.Acetylene recovery is also quite complex,and there are several disadvantages in this technology.The semi-hydrogenation of acetylene to ethylene has been proved to be one of the most efficient techniques to purify ethylene and is widely used in industrial production16-19.On the one hand,this process can remove impurity acetylene;on the other hand,it can increase the content of ethylene in feed gas20.

Acetylene semi-hydrogenation actually consists of the conversion of acetylene to ethylene with as few side reactions as possible.The following reactions may occur in acetylene in the selective hydrogenation reaction of acetylene17,21,22:

Main reaction:

Side reactions:

Among these reactions,reaction(1)is the main reaction,and ethylene is formed through the hydrogenation reaction of acetylene.However,side reactions have both competitive reactions and parallel reactions,which are more complex.Reactions(2)-(5)are side reactions,where path(3)produces ethane due to excessive hydrogenation.In path(5),an oligomeric reaction occurs,forming green oil substances,which ultimately leads to a reduced catalyst activity23-25.

C2H2+ * ? C2H2*

H2(g)+ 2* ? 2H*

C2H2* + H* ? C2H3* + *

C2H3* + H* ? C2H4* + *

C2H4* → C2H4(g)+ *

Generally,the hydrogenation of acetylene is considered to follow the Horiuti-Polanyi mechanism,that is,acetylene and hydrogen are co-adsorbed at the active sites on the surface of the catalyst,and the semi-hydrogenation of acetylene reaction is divided into the five following steps26-31:

(1)Acetylene and hydrogen diffuse to the surface of the catalyst and adsorb C2H2and H2on the active sites of the catalyst.

(2)The hydrogen adsorbed at the active sites of the catalyst is decomposed into hydrogen atoms.

(3)Each C2H2molecule adsorbed at the active sites of the catalyst combines with a neighboring hydrogen atom to form an intermediate C2H3* and a vacancy.

(4)The intermediate combines with a neighboring hydrogen atom to form ethylene,which is adsorbed at the active sites of the catalyst,and also forms a vacancy.

(5)Due to the low adsorption ability of the ethylene molecules at the active center,ethylene is desorbed from the active sites of the catalyst,and the active sites continue to perform the subsequent acetylene hydrogenation process.

In general,the adsorption capacity of acetylene and ethylene on the active center determines the activity and selectivity of the catalyst4,32-34.The timely desorption of molecules from the catalyst is the key to improving the selectivity of ethylene,and the geometry and electronic structure of the catalyst are key factors affecting the selective hydrogenation adsorption of acetylene35-39.

Through kinetic analysis and research on the acetylene hydrogenation with Pd,which was pioneered by Cremer’s group,many types of Pd-based catalysts have been designed using different particle sizes,supports,functional groups influencing the catalyst morphology,and hydrogenation activities40.In subsequent studies,researchers found that in the selective hydrogenation of acetylene,the catalytic hydrogenation activity of various metals can be roughly arranged in the following order:Pd ＞Pt ＞Ni～Rh ＞Co ＞Fe ＞Cu～Au41,42.Due to the excellent hydrogenation activity of Pd,Pd is often used as an active component for the selective hydrogenation of acetylene.Generally,most single Pd catalysts show a low ethylene selectivity,and the ethylene selectivity decreases sharply with increasing time of the reaction cycle43-45.Researchers have devoted extensive efforts to developing hydrogenation catalysts with a better catalytic ability and selectivity46-50.

Through in-depth studies of acetylene semi-hydrogenation catalysts,researchers have found that the size and dispersion of monometallic catalysts change their reaction at the active sites29,48,51,52.Therefore,according to their size distribution,catalysts for the semi-hydrogenation of acetylene can be divided into three categories,namely nanoparticles,nanoclusters,and single-atom-site catalysts(SASCs).However,most monometallic catalysts show a low ethylene selectivity23.Several researchers introduced a second or even a third metal to improve the catalytic selectivity and stability of these catalysts.

Researchers also introduced a second or third metal into monometallic catalysts,changed the valence state of the main metal to tune the activity of the Pd—Pd bonds,and prepared bimetallic catalysts.These have a unique effect on the selective hydrogenation of acetylene due to the synergistic effect between the two elements,which is in turn due to electron interaction53-58.In addition to this,according to the semi-hydrogenation mechanism,which requires the hydrogenation to be neither too strong nor too weak,an argument was proposed regarding single-component and two-component catalysts.With improved material understanding,for single metals,nano-single-atom-site catalysts(NSASCs)have emerged,in which single atoms and nanoparticles simultaneously exist.The metal valence charge density of such single atoms and nanoparticles of the same metal is completely different;thus,the single atoms can be treated as another metal with a different metal valence charge density.In this way,bimetallic-like modulation can be achieved59-62.The difference between dual-atom-site catalysts(DASCs)and bimetal is that the metal active sites of DASCs are separated,while bimetals are bonded together.The method of tuning the hydrogenation properties achieved by DASCs is different from that obtained with bimetal nanoparticles or clusters and is isolated dispersion on the carrier surface63-65.The carrier and the active component can form a new structure under reaction conditions through the metal-carrier interaction,which affects the catalytic performance of the carrier.

In this review,we summarize the assemblies of metal sites in single-metal catalysts,bimetallic catalyst,and multiple-metal catalysts,as well as explain the correlation between the structure of the catalysts and the selective acetylene semi-hydrogenation.Finally,by reviewing the development of this field over the past decade,we present the future prospects and challenges of acetylene hydrogenation both from fundamental and applicative perspectives.

2 Single-metal catalysts

The electronic structure and geometric shape of single-metal catalysts change with their size,from a single atom(0.1-0.5 nm)to a cluster(generally,particles with a size less than 1 nm can transform into clusters)and to a nanoparticle(＞1 nm)66-70.Reducing the particle size from a few nanometers to the subnanometer range not only improves the metal use efficiency under high dispersion but also changes the morphology and electronic properties of the metal.For the semi-hydrogenation of acetylene,nanoparticle catalysts have strong M—M bonds(M=Pd,Ni,or Cu),and the further hydrogenation of ethylene to ethane dominates the whole process,resulting in a reduced selectivity of ethylene.However,the M—N/C/S bonds in SASCs,and the hydrogenation of ethylene to ethane on single atoms require overcoming higher barriers to limit acetylene excessive hydrogenation to methane,thus improving the selectivity and conversion of ethylene.

Therefore,the catalytic performance can be well adjusted.Here,we discuss the catalytic applications of nanoparticles,nanoclusters,and SASCs,showing how the differences in the electronic structure during the selective acetylene semihydrogenation affect the catalytic process and,consequently,their activity and selectivity for acetylene hydrogenation.

2.1 Nanocluster and nanoparticle catalysts

For nanoclusters and nanoparticles,the size effects on the electronic structure and geometry are quite complicated due to the orbital overlapping between the metal atoms,which influences the catalyst morphology and hydrogenation activity71,72.Taking Pd as an example,Pd-based catalysts are used owing to their high intrinsic activity,but the selectivity of ethylene is very low,which is due to the fact that the Pd nanoparticles tend to aggregate into bulk metals,resulting in over-hydrogenation.Hence,a series of heterogeneous catalysts based on Pd nanoclusters or nanoparticles encapsulated into the networks and obtained from different carriers have been intensively studied over the past several years73-76.Here,only a few representative examples are listed for discussion.

In 2015,Li and co-workers reported a hollow zeolitic imidazolate framework-8(ZIF-8)structure under mild and humid conditions and encapsulated Pd nano-particles(NPs)in the ZIF-8 cavity(Fig.1a-f)77.The as-prepared Pd@H-Zn/Co-ZIF showed improved the acetylene conversion(＞80%)and ethylene selectivity(＞80%)(Fig.1g,h).For ZIF-embedded Pd NPs,smaller acetylene diffuses more easily into the internal Pd NPs than ethylene.Ethylene is formed through the hydrogenation of acetylene,but it is difficult to form ethaneviathe further hydrogenation of ethylene at the active sites,promoting selective hydrogenation.The hollow interior of HZn/Co-ZIF may increase the inward diffusion rate of acetylene,and ethylene can be preferentially desorbed from the Pd surface,which is conducive to the efficient and selective semihydrogenation of acetylene.

Fig.1 (a-d)TEM and HAADF-STEM images,(e-f)EDX mapping of Pd@H-Zn/Co-ZIF and Pd@S-Zn/Co-ZIF.(g-h)Acetylene conversion and ethylene selectivity of Pd based catalysts 77.

In 2018,Zheng and Fu developed a Pd/C catalyst based on the precise regulation of the nanometer boundary,which exhibited better alkyne semi-hydrogenic alkene performance than Lindlar catalysts(a heterogeneous catalyst that is used mainly for the selective hydrogenation of alkynes to alkenes)78.They studied ultrathin,two-dimensional Pd nanopels modified with thiol to change the surface of the Pd catalyst at the molecular level.The resulting catalyst showed excellent selectivity forcis-alkene(97%)in the intermediate alkyne hydrogenation.It is worth mentioning that the activity of the catalyst remained almost unchanged after thiol modification:The test was repeated 10 times,and little attenuation of the activity and selectivity was observed.The thiol treatment leads to a phase transition of the Pd NPs,and the generated palladium sulfide(PdSx)forms a special interface with the residual thiol root on the surface,which not only changes the electronic properties of Pd but also forms the steric hindrance.This interface can effectively inhibit the further hydrogenation of the C ＝C bonds,significantly improving the catalyst selectivity.

Gong and co-workers reported a Pd@zeolite-encapsulated catalyst(Pd@SOD)obtained by directly annealing the pristine catalyst in air at 350 °C for 3 h and reduced in flowing H2/N2at 150 °C for 1 h at a heating rate of 2 °C·min-179.In the encapsulated structure,the Pd nanoclusters had a size distribution of 0.8-0.3 nm,and the Pd load was 0.1 wt%(Fig.2a,b).The Pd nanoclusters were confined within the sodalite(SOD)zeolite;acetylene can react with the OH species on the SOD surface,which are generated from the hydrogen activated by the encapsulated Pd sites,producing ethylene with a high selectivity of up to 94.5%.In addition,Pt@SOD was also synthesized using the same method.Reactivity tests showed that the ethylene selectivity of Pt@SOD could reach 84.0%(Fig.2c).The reason for this encapsulated structure having a high ethylene selectively is that the hydrogen molecules can enter the pores of the SOD zeolite and be activated by the encapsulated Pd nanoclusters;OH species are then formed on the SOD surface through a hydrogen spillover process.Acetylene consequently reacts with the OH species on the SOD surface.On the other hand,the spatial restriction of the SOD zeolite impedes the direct interaction of acetylene and ethylene with the Pd nanoclusters through the semi-hydrogenation of acetylene(Fig.2d).

Fig.2 (a-b)HAADF-STEM images of Pd@SOD(fresh)and Pd/SOD(fresh).(c)Acetylene conversion and selectivity to ethylene and ethane over Pd@SOD and Pd/SOD catalysts.(d)Synergism between metal catalysis and the spatial restriction effect of a small-pore zeolite in acetylene hydrogenation 79.

Kolen’ko and co-workers successfully synthesized a model 5% Pd/Al2O3nanocatalyst using the high-throughput flame spray pyrolysis(FSP)method80.The material has a high conversion rate(97%),a moderate selectivity(62%),and a turnover frequency for ethylene formation of 5 s-1;thus,it is a good acetylene semi-hydrogenation catalyst.The experimental data were supported by the computational modelling of the catalytic properties.Density functional theory(DFT)calculations indicated that the catalytic activity of acetylene hydrogenation was comparable at the Pd30cluster and Pd surface,as expected based on the similard-band centers of these systems.However,microkinetic simulations predicted a higher activity and selectivity for the Pd30clusters,and the predicted trends were consistent with the experimental results.The selectivity of ethylene determinedviathe microkinetic simulations is in good agreement with previous simulations and surface experiments in the same temperature range.The quantitative difference between the experimental(reduced Pd/Al2O3)and theoretical(Pd30/Al2O3(100))results can be explained by the particle size:The clusters in Pd30/Al2O3(100)are smaller than the Pd nanoparticles on reduced Pd/Al2O3,which explains the lower experimental selectivity.The larger Pd nanoparticles favor the formation of theβ-hydride phase,which provides active hydrides to the surface,leading to the complete hydrogenation of acetylene to ethane,thus reducing the ethylene selectivity.

Zhang and co-workers reported a catalyst for the semihydrogenation of acetylene with a highly dispersed and stable PdC phase supported byα-Al2O3,which was prepared using an N-containing silane coupling agent as the modifier81.The catalyst modified byN-(2-aminoethyl)-3-aminopropyltrime thoxysilane(AAS)is more uniform,and it no longer contains large metal particles like the catalysts prepared by the traditional impregnation method.The addition of AAS can effectively improve the dispersion of Pd on the carrier.The ethylene selectivity of the catalyst is 15% higher than that of the conventional PdAg alloy.The excellent ethylene selectivity is mainly due to the special PdC-phase nanoparticles with lattice relaxation and distortion.The unique structure of the catalyst leads to a weaker adsorption of ethylene to avoid further hydrogenation,oligomerization,and hydrogen spillover on the surface of the support and leads to a strong interaction between the Pd nanoparticles and the surface,which inhibits the aggregation of the metal particles,thereby ensuring long-term operational stability.

Nanoparticles and nanoclusters are packaged into the carrier to form nanoparticles or nanocluster catalysts,because this type of catalyst unique structure can make hydrogen enter the interior of the catalysts and can activate the metal active sites.Hydrogen and acetylene are adsorbed on the surface of the active sites,and C2H4on the catalyst adsorption energy is low,so it is easy to desorb from catalysts active site.The carrier can not only react with the intermediate encapsulation in the interior but also avoid further hydrogenation to generate ethane.This determines that nanoparticles and nanocluster catalysts have a higher semihydrogenation selectivity compared to the normal catalysts.

2.2 Single-atom-site catalysts

SASCs with homogeneous isolated active sites and unique electronic structures are highly desired in acetylene semihydrogenation71,82-87.Zhang and co-workers reported the single-atom catalyst Pd1/ZnO,which exhibited excellent catalytic performance in the selective hydrogenation of acetylene4.Li’s group synthesized a series of Pd and Ni single atoms dispersed on different substrates and systematically studied the structure-activity relationship of single-atom catalysts on different substrates for the semi-hydrogenation of acetylene1,88-91.Different from traditional metal nanoparticle catalysts,single-atom catalysts exhibit excellent activity and selectivity for the hydrogenation of acetylene to ethylene.

In 2018,Chen and colleagues reported a highly efficient Pd/Ni(OH)2catalyst containing ultralow levels of Pd(mass:50 ppm)to selectively hydrogenate acetylene to ethylene(Fig.3ag)89.The turnover frequency for the acetylene conversion of the 0.005% Pd/Ni(OH)2catalyst was twice that of the equivalent 0.8% Pd/Ni(OH)2catalyst.An acetylene-to-ethylene selectivity of 80% was achieved over a wide range of temperatures.Thermodynamic studies show that acetylene has more influence than ethylene on the adsorption and activation of Pd.In the selective hydrogenation of acetylene,ethylene is competitively adsorbed on the active Pd sites,resulting in a decrease in ethylene selectivity.The 0.005% Pd/Ni(OH)2catalyst has a better activity and selectivity than the 0.8% Pd/Ni(OH)2catalyst,which may be related to the atomic dispersion of the Pd sites,and the isolated Pd atomic catalyst shows a good ability to activate acetylene,which is conducive to the adsorption of acetylene and the subsequent desorption of ethylene from the catalyst surface,greatly improving the selective hydrogenation of acetylene to ethylene(Fig.3f-i).The Fourier-transform infrared(FT-IR)spectra of Pd/SiO2and the corresponding acetylene conversion results reveal that the atomically dispersed Pd catalysts have a strong OH--dependent activity(Fig.3j,k).Recent DFT calculations and experimental results have shown that the activation of hydrogen by atomically dispersed Pd catalysts usually involves a heterogeneous dissociation process that produces negatively charged Pd-Hδ-.Studies have shown that decoupling Pd combinations and increasing the number of isolated Pd atoms effectively limit the number of ethylene intermediates,carbides,active hydrogen species,and other intermediate products that are not conducive to the selective hydrogenation of acetylene.

Fig.3 (a)Acetylene selective catalytic hydrogenation schematic illustration of Pd catalyst.(b-g)TEM images of the 0.8% Pd/Ni(OH)2 catalyst.(f-i)On 0.005% Pd/Ni(OH)2 catalyst,acetylene conversion and ethylene selectivity with temperature change and time stability test.(j-k)FT-IR spectra and acetylene conversions as a function of temperature 89.

Li and co-workers reported a strategy to convert noble metal nanoparticles(Pd,Pt,and Au)into thermally stable single atoms(SAs)above 900 °C for 3 h in an inert atmosphere(Fig.4a-d)92.In situobservations show that there is a competition between aggregation and atomization in the dynamic transformation process of nanoparticles into single atoms.DFT calculations show that during this high-temperature dynamic conversion process,after the moving Pd atoms are captured by the N-doped carbon material,a more thermally stable Pd—N4structure is formed.The semi-hydrogenation of acetylene shows that this Pd catalysts not only has an excellent thermal stability but also a better catalytic activity and selectivity than Pd nanoparticles.At 120 °C,the acetylene conversion and ethylene selectivity reached 96.0% and 93.4%,respectively(Fig.4e,f).Compared with Pd-NPs,Pd-SAs have a better catalytic performance,possibly because Pd-NPs have strong Pd—Pd bonds,and the hydrogenation of ethylene to ethane dominates the whole process,resulting in a reduced selectivity of ethylene.However,there are Pd—N bonds in Pd-SAs,and the hydrogenation of ethylene to ethane in Pd-SAs requires overcoming the 0.93 and 1.22 eV barriers to limit its excessive hydrogenation to methane;regarding Pd—N4,H2prefers the N position,while C2H2prefers the Pd position.Thus,although the co-adsorption energy of C2H2and H2on Pd1—N4is smaller than on the Pd(111)surface,the potential for molecular collisions between C2H2and H2is much greater on Pd1—N4,resulting in a higher activity.

Fig.4 (a-d)HAADF-STEM images and high-resolution HAADF-STEM images(insets)of Pd-nanoparticles@ZIF-8,intermediate I,intermediate II and Pd single atoms.(e-f)Acetylene conversion and ethylene selectivity of Pd-SAs-900,Pd/CN-800,Pd/CN-700,Pd/CN-600 and Pd-NPs/CN as a function of reaction temperature 92.

Wang and co-workers reported a method based on hardtemplate Lewis acid doping(HTLAD)for the preparation of a Pd single atom site catalyst(Fig.5a)90.The Pd atoms are anchored to the inner wall of a mesoporous nitrogen-doped carbon nanosphere(ISA-Pd/MPNC),the catalyst with a specific surface area of 633.8 m2·g-1and a pore wall thickness of 1-2 nm(Fig.5b-e).The activity of the ISA-Pd/MPNC catalyst prepared in this work in the acetylene semi-hydrogenation compared to the Pd single-atomic catalyst with non-interporous carbon loading.Obviously,the ISA-Pd/MPNC catalyst had better activity than the ISA-Pd/non-MPNC catalyst(83%vs.32%)and better ethylene selectivity than the NP-Pd/MPNC catalyst(82%vs.17%)(Fig.5h).C2H4to hydrogen C2H5in ISA-Pd requires 1.01 eV,which is much higher than the C2H4attachment energy(0.01 eV),and the energy of TS3is higher than the gaseous C2H4,which also indicates that C2H4prefers to unattach(Fig.5f-g).Thus,ISA-Pd has good selectivity in acetylene semihydrogenation reactions.For NP-Pd,the calculation is contrary to the above,indicating that C2H4is easier to continue hydrogenation to form ethane on NP-Pd.

Fig.5 (a)Schematic illustration of the overall synthetic procedure for ISA-Pd/MPNC sample.(b-e)Morphological and structural characterizations of ISA-Pd/MPNC sample.(f-g)Step-by-step hydrogenation mechanism of C2H2 on ISA-Pd and NP-Pd.(h)Catalytic performance of the three samples for semi-hydrogenation of acetylene 90.

Fu and co-workers successfully built a single-atom Pd catalyst in a polyoxometalate-based metal-organic framework(POMOF)(Fig.6a)1.Its unique internal environment can not only provide anchor sites for single-atom Pd but also cause the selective adsorption and enrichment of acetylene molecules,which enables POMOF to separate acetylene from the acetylene/ethylene gas mixture and limit it near single-atom Pd(Fig.6b-d).After the completion of the semi-hydrogenation reaction,the generated ethylene is preferentially discharged from the pores;thus,over-hydrogenation is avoided,and a high selectivity of 92.6% is reached(Fig.6e-g).First-principles simulations show that the adsorbed acetylene/ethylene molecules can form hydrogen bonding networks with O atoms in SiW12O4and generate dynamic confined regions that preferentially release the generated ethylene.Furthermore,at the Pd sites,the hyper-hydrogenation of ethylene has a higher reaction barrier than the semi-hydrogenation of acetylene.The idea of combining POMOF and single-atom Pd provides an effective avenue for the regulation of the semi-hydrogenation selectivity and stability.

Fig.6 (a)Schematic illustration of the synthetic procedure of Pd1@Cu-SiW.(b-c)PXRD patterns and FTIR spectra of Cu-SiW,Pd(acac)2@Cu-SiW,and Pd1@Cu-SiW,respectively.(d)HAADF-STEM images and the element mapping.(e)Adsorption isotherms of acetylene and ethylene for Cu-SiW at 298 K.(f-d)Conversion and selectivity as a function of temperature for acetylene hydrogenation over Pd1@Cu-SiW,Pd1@NENU-1,and Pd1@Y,respectively.(g)Durability test on Pd1@Cu-SiW at 110 °C 1.

Yan and co-workers reported an ionic liquid-stabilized single-atom catalyst(ILSSAC),which stabilizedviaelectrostatic interactions93.The catalyst was tested online for 90 h,and it was found to exhibit an acetylene conversion down to only 92% and a high ethylene selectivity of over 75%.DFT calculations reveal that the activation energy for the transformation of two isolated Pt atoms into a Pt dimer increases remarkably from 0.11 to 0.72 eV under the protection of[Bmim][BF4].The unprotected single-atom catalyst partially agglomerates into metal nanoparticles after hydrogenation,while the ionic liquid-modified single-atom catalyst retains the original atomic-level dispersion state.Ionic-liquid modification can significantly improve the reactivity of single-atom catalysts.Kinetic studies have shown that ionic liquids do not affect the hydrogenation reaction mechanism but can improve the reactivity by reducing the reaction activation energy.Moreover,ionic-liquid modified catalysts do not have M—M bonds,which proves that they are more stable.

Liu and Ma reported a method where dispersed single-atom Pd catalysts were loaded on defective graphene.Characterization measurements revealed that the single-atom Pd catalysts were embedded in graphene through the formation of Pd—C bonds,with Pd interacting with the C surrounding the graphene defect(Fig.7a-f).This catalyst achieved 90% of acetylene hydrogenation to ethylene with a conversion rate of 100% and remained stable for at least 30 h at 180 °C(Fig.7g-i)94.Such a special structure promotes the detachment of the C2H4*species from the catalyst surface and suppresses further deep hydrogenation,which is crucial for the selectivity of the hydrogenation reaction.According to the control reaction kinetics,the lack ofβ-H in the Pd1/ND@G catalyst(compared with conventional Pd catalysts and Pdn/ND@G)prevents the over-hydrogenation of acetylene to ethane.Furthermore,acetylene gas molecules are preferentially adsorbed on the Pd atoms of Pd1@Gr.The adsorption energy of acetylene on the Pd atoms of Pd1@Gr(-0.61 eV)is less than that on the Pd(111)surface(-1.79 eV).The hydrogen molecules dislike heterogeneity,leaving a combination of an H atom with a C atom and that of and H atom with a Pd atom.From vinyl molecules to ethylene,the process undergoes further hydrogenation with a 0.85 eV barrier and an exothermic reaction with a generated energy of 1.30 eV.Although the further hydrogenation of ethylene to ethane at the dispersed Pd active sites is still thermodynamically exothermic,the energy barrier of the adsorbed C2H4intermediate for further hydrogenation to ethane(1.17 eV)is much higher than that of surface C2H4desorption;thus,ethylene is further desorbed rather than hydrogenated to form ethane,thereby improving the selectivity of acetylene semi-hydrogenation(Fig.7j).

Fig.7 (a-d)HAADF-STEM images of Pd1/ND@G.(e-f)STEM and HAADF-STEM images of Pdn/ND@G.(g-i)Conversion and selectivity for acetylene hydrogenation over Pdn/ND@G and Pd1/ND@G catalysts;and durability test on Pd1/ND@G at 180 °C for 30 h.(j)Energy profile of acetylene hydrogenation on the Pd1/ND@G catalyst 94.

Lin and co-workers reported that Ni-doped CeO2acts as a catalyst for acetylene semi-hydrogenation in an attempt to promote the formation of surface oxygen vacancies through Ni doping95.The smaller Ni ions replace the Ce ions in the Ce lattice,and the transmission electron microscopy(TEM)images show only the edges of the Ce lattice,with no separate Nicontaining particles detected(Fig.8a,b).They showed that Nidoped CeO2had a higher hydrogenation of acetylene selectivity than CeO2alone.In an H2environment,there may be a certain number of oxygen vacancies on the CeO2crystal planes;firstly,H2forms O—H and Ce—H species at the oxygen vacancies through heterolytic cracking,and then acetylene is gradually hydrogenated to form ethylene.Since the number of oxygen vacancies on the CeO2(111)planes is very limited,more oxygen vacancies need to be created to increase the number of active sites and improve the reactivity.Additionally,as the coordination of nickel is saturated,the intermediates of the oligomer are prevented from binding at the Ni sites.DFT calculations show that,unlike conventional single-atom catalysts,the single-atom-site Ni in this system is not directly involved in the catalytic process but acts as SASCs to help generate and modify the active sites.Experimental studies confirm these design principles and demonstrate a much higher activity for Ni-doped CeO2in the selective hydrogenation of acetylene(Fig.8c,d).

Fig.8 (a)XRD patterns of CeO2 and Ni@CeO2.(b)HRTEM image of the Ni-doped CeO2 sample.(c-d)Acetylene hydrogenation reactivity of 0.5,1.0,and 1.5 wt% Ni@CeO2 mixed nitrate samples and CeO2 95.

Li and co-workers reported a composite of single Ni atoms supported by N-doped carbon(Ni SAs/N-C),which was successfully synthesizedviaa pyrolysis process,during which the aggregation of Ni atoms was strictly limited by the free N sites distributed on the ZIF-8 frameworks(Fig.9a,b)88.The synthesis was based on thein situreduction of the Ni atoms at a high temperature.Importantly,the Ni SAs/N-C catalysts for the semi-hydrogenation of acetylene delivered an excellent acetylene conversion and a high selectivity for ethylene(both over 90%)even at a high temperature of 200 °C(Fig.9c,d).This result may be due to the fact that the catalyst contains Ni-Nxsites that promote the specific adsorption of acetylene and the desorption of ethylene,and the uniform coordination of the N atoms around the Ni center regulates thed-orbital of the Ni atoms,thus resulting in an excellent ethylene selectivity for the pair.It can be seen that the N-coordination of a single Ni atom plays an important role in determining the catalytic performance of the acetylene semi-hydrogenation.

Fig.9 (a)TEM image of Ni SAs/N-C.(b)Preparation scheme of Ni SAs/N-C.(c-d)The curves of acetylene conversion and ethylene selectivity of the catalyst with the flow reaction temperature 88.

Furthermore,other research groups have conducted extensive work on single-atom catalysts for the semi-hydrogenation of acetylene.Liu and co-workers reported a new atomically dispersed Cu catalyst,namely Cu1/nanodiamond-graphene(ND@G),supported by a defective ND@G,which was reduced in flowing H2(10 vol% in He,flow rate=50 mL·min-1)at 200 °C for 1 h(Fig.10a)16.The single-atom catalyst exhibits excellent catalytic performance for the selective conversion of acetylene to ethylene,with a high conversion(95%),a high selectivity(98%),and a good stability(for more than 60 h)(Fig.10d).The isolated Cu atom is fixed on the defect site of graphene by coordination with three C atoms.The catalyst does not have Cu—Cu bonds,which ensures the effective activation of acetylene and the easy desorption of ethylene and is the key to the outstanding activity and selectivity of the catalyst(Fig.10b,c).Moreover,the transition energy of ethylene hydrogenation(TS2,1.27 eV)is greater than that of gas-phase ethylene(1.08 eV)on Cu1/ND@G,indicating that ethylene favors desorption in the subsequent hydrogenation process,which is conducive to improving the selectivity of ethylene.

Fig.10 (a)TEM characterization of ND@G support and Cu1/ND@G and Cun/ND@G catalysts.(b)Cu K-edge XANES profiles for Cu1/ND@G,Cun/ND@G,Cu foil,and CuO.(c)Cu K-edge EXAFS spectra in R space for Cu1/ND@G,Cun/ND@G,Cu foil,and CuO.(d)Catalytic performance of Cu1/ND@G and Cun/ND@G 16.

In acetylene-selective hydrogenation reactions,according to the mechanism of the acetylene semi-hydrogenation reaction,the catalytic reaction takes place on the surface of the catalyst,so the size of the catalyst metal particles affects the overall performance of the acetylene hydrogenation.For a given metal catalyst(Table 1),such as Pd,the performance of Pd clusters is better than that of nanoparticles because the Pd—Pd chemical bond is the same,and for a given content of Pd clusters,there are more Pd—Pd bonds.Furthermore,the higher the conversion rate of hydrogenation to ethylene,the greater the gas adsorption.It is also possible to continue the hydrogenation process to produce ethane.When the particles of the catalyst become increasingly small to the point that each atom becomes an active center,there is no Pd—Pd bond.This is what is known as SASCs,in which the metal atoms on the catalyst surface are isolated from each other.Ethylene adsorption on the isolated metal atoms is weak;thus,single-atom catalysts permit the easy desorption of ethylene and limit the further hydrogenation of ethylene to ethane,thus greatly improving the selectivity of the reaction.

Table 1 Some recently reported catalysts for selective acetylene semi-hydrogenation.

3 Bimetallic catalysts

Bimetallic catalysts have been shown to present quite distinct properties compared with single-metal catalysts regarding the semi-hydrogenation of acetylene.They are characterized by the synergistic effects between different elements,which result from their electronic interactions96,97.Depending on how the two different metals coexist,bimetallic catalysts can be classified into typical substitutional solid-solution alloy catalysts,intermetallic compound catalysts,and single-atom alloy catalysts(SAACs)58,98-102.A solid solution in which two or more metals not only dissolve each other when melted but also remain miscible when solidified is called a solid-solution alloy.When metal atoms form an ordered atomic structure and occupy the lattice with a specific atomic stoichiometric number,the obtained system is often referred to as an intermetallic compound103-109.For the former case,solid-solution alloys are formed by guest metals with a similar atomic size,electronic parameters,and crystal structure,randomly replacing atoms in the parent metal or entering the parent metal gap.By contrast,the characteristics of the two metals are obviously different,but a new ordered crystal structure is formed110.SAACs are a subclass of single-atom catalysts;they are composed of reactive dopant metals with atoms dispersed in the second metal body111,112.SAACs with unique geometries play a special role in heterogeneous catalysis due to the easy dissociation of the target product and the weak binding of the reaction intermediates61,113.In recent years,an increasing number of researchers have focused on bimetallic catalysts for the semi-hydrogenation of acetylene.This section introduces the research progress on solidsolution alloy catalysts,intermetallic compound catalysts,and SAACs for application to the semi-hydrogenation of acetylene.

3.1 Solid-solution alloy catalysts

In recent years,bimetallic Pd catalysts have been developed by adding a second active metal element,such as Au,Ag,Co,Cu,Cr,Ga,K,Ni,Pb,Rh,Si,and V.This method is considered to be one of the most effective approaches to improve the ethylene selectivity103,114,115.Group IB metals(Au,Ag,and Cu)are the most studied components for doping,especially Cu and Ag33,116.

Han and co-workers introduced Ag into a Pd/TiO2catalystviaphoto-deposition(pd)117.Forpd-Pd(0.95)@Ag(0.97)/TiO2andim-Pd(0.94)-Ag(0.96)/TiO2,the ethylene selectivity decreases with the increase in acetylene conversion.However,considering the same acetylene conversion,pd-Pd(0.95)@Ag(0.97)/TiO2shows a much higher ethylene selectivity thanim-Pd(0.94)-Ag(0.96)/TiO2.This apparent difference in ethylene selectivity can be well explained by the structure of the bimetallic species on the surface.Forim-Pd(0.94)-Ag(0.96)/TiO2,the surface metal consists of a well-mixed Pd-Ag combination,so that both the body and the surface of the Pd particles are diluted with Ag.By contrast,the surface metal ofpd-Pd(0.95)@Ag(0.97)/TiO2consists of a bimetallic species with a core-shell structure(Pd@Ag),which is due to the deposition of Ag at specific locations on the Pd surface.Since Ag only modifies the surface of Pd particles,a more effective blocking of the highly coordinated Pd can be achieved forpd-Pd(0.95)@Ag(0.97)/TiO2than forim-Pd(0.94)-Ag(0.96)/TiO2at a similar Ag loading,as shown by the quantitative CO adsorption and the corresponding infrared(IR)spectroscopy results.Thus,pd-Pd(0.95)@Ag(0.97)/TiO2exhibits a stronger ethylene selectivity thanim-Pd(0.94)-Ag(0.96)/TiO2.For the selective semihydrogenation of acetylene,the performance of the catalyst prepared using this method is better than that of the catalyst prepared using the traditional impregnation(im)method because the deposition of Ag on the Pd surface blocks the sites of the highly coordinated Pd.

Zhang and co-workers deposited Ag and Au on a Pd/SiO2catalyst preparedviaelectroless deposition118.The coverage of Ag and Au on the Pd surface was measured through selective adsorption.It was found that the loading of metals(Ag and Au)needed to be increased to obtain a high coverage of Ag and Au.The acetylene conversion rate decreased with the increase in Ag and Au coverage on the Pd surface,but the ethylene selectivity followed the opposite trend.The TOF value of the acetylene conversion calculated by taking the number of Pd surface atoms as the number of active sites first increases and then decreases with the increase in the Ag and Au coverage and reaches the maximum when the coverage is 0.9.For a high Ag coverage,most of the Pd surface is covered by Ag,leaving only small Pd clusters and even single-atom sites of Pd on the surface.In this case,only one Pd site may be required,and acetylene is adsorbed in the form ofπ-bonds on the active sites,which favors the formation of ethylene.Therefore,it can be seen that isolated Pd sites or small clusters are the reasons for the selective hydrogenation reaction of acetylene.

Tsang and co-workers reported an approach for altering the catalytic properties of Pd in the partial hydrogenation of acetylene119.They modified the lattice gap of Pd with light elements,such as Li and B,to obtain an alloy catalyst.The performance of the Pd catalyst for the hydrogenation of acetylene was enhanced through the subsurface modification of Pd(ethylene selectivity ＞80%).The Li- and B-modified Csupported Pd catalyst obtainedviasolid-state synthesis can exist stably in air and wet atmosphere.However,the unmodified Pd retained an ethylene selectivity of only 50% during the stabilization period.The presence of Li and B on the subsurface inhibits the entrance of H atoms into the Pd lattice to form palladium hydride and significantly improves the ethylene selectivity.In addition,the subsurface Li increases the electron density of the Pd surface and enhances the adsorption of acetylene on the surface.On the other hand,the subsurface B decreases the electron density of the Pd surface at the Fermi level and weakens the adsorption force on the surface.

Feng and co-workers also reported the preparation of a flowercluster PdAu alloy and a sea urchin image alloy,which were used to catalyze acetylene hydrogenation99.The obtained nanoflowers were in fact composed of octahedral building units with a size of 3-6 nm.Very importantly,Positron annihilation spectroscopy(PAS)and high-resolution transmission electron microscope(HRTEM)images revealed that abundant defective sites existed on the PdAu nanoflowers,and a synergetic effect caused by the cooperation between the building units was revealedviaCO-IR.The synthesized Fl-PdAu/MMO catalyst showed a good catalytic performance,including a high activity,selectivity,and stability for the partial hydrogenation of acetylene.The improved activity was ascribed to the high density of crystalline defects in the PdAu nanoflowers,which facilitated hydrogen activation.The geometry of the nanoflower catalysts was proposed to improve the ethylene selectivity.Moreover,the structural stability of the PdAu nanoflowers promoted the catalytic stability by inhibiting aggregation to form bulk PdAu crystals.

Feng and co-workers prepared an acetylene semihydrogenation catalyst with a uniform nanoalloy structure from a layered double hydroxide(imp-NiCu/MMO)120.Compared with single-metal Ni catalysts,the introduction of Cu enhanced the selectivity of ethylene and decreased the deactivation rate of the long-term reaction.Furthermore,to gain a deep understanding of the structure-dependent performance,the conventional impregnation method was also used to prepare a bimetallic NiCu catalyst for comparison.Interestingly,imp-NiCu/MMO presented a mixture of monometallic Ni,monometallic Cu,and bimetallic NiCu.The catalytic results indicated that a 100% conversion was achieved at 160 °C with an improved selectivity(＞70%)compared with that of the NiCu nanoalloy catalyst,while the bimetallic NiCu catalyst with a mixed structure showed both poor conversion(50%)and selectivity(65%).The enhanced activity was attributed to the small particle size(3.2 nm)and high degree of metal dispersion(31.4%).The improved selectivity and anti-coking ability were related to the alloying of Ni with Cu.

After extensive research and comparisons,it can be inferred that the reasons for the influence of the acetylene semihydrogenation reaction on the incorporation of different metals in Pd-based catalysts to form bimetallic alloy catalysts are also different.For the different metal elements incorporated,we can briefly summarize their influencing factors.Naturally,not all catalysts fit this pattern;after all,the synthesis method,structure,and other aspects are different for different catalysts.It can be seen that Ag doping mainly affects the electronic structure of the Pd catalysts.The addition of Cu tends to modify the geometry of the Pd catalyst.The presence of Au reduces the C coverage on the surface of the catalyst,thereby inhibiting ethane production.In fact,bimetallic alloy catalysts improve the selective hydrogenation reaction of acetylene due to two main reasons,namely the electron effect and the geometric effect between the two metals.On the one hand,the Ag-doped modified Pd catalyst transfers the charge from the Ag atom to the Pd atom,which increases thed-band electron density of Pd.On the other hand,the modification of Pd by Ag can weaken the adsorption strength of hydrogen,acetylene,ethylene,and the hydrocarbon intermediates on the PdAg catalysts and promote the desorption of hydrogen and ethylene,thereby inhibiting the overhydrogenation of acetylene to ethane.

3.2 Intermetallic compound catalysts

Intermetallic compounds used as catalysts have a fixed composition,a uniform dispersion,and special geometric and electronic structures.When used as catalysts for the selective semi-hydrogenation of acetylene,they can effectively weaken the adsorption strength of ethylene on the catalyst surface,so as to improve the selectivity of the hydrogenation reaction.In recent years,increasing attention has been paid to study intermetallic catalysts121-123.Compared with alloys,intermetallic compounds are single-phase materials with an ordered crystal structure,which can adjust uniform active sites and separate active sites.Furthermore,they are strongly affected by the covalent interaction between different atoms and have excellent stability124-126.Considerable work has been devoted to optimizing the geometry of Pd catalysts(Pd—Pd coordination)and the electronic(d-band center)structure of Pd sites.Typical strategies include blocking active Pd sites using additives and isolating Pd atoms in support structures or intermetallic structures(e.g.,PdIn,PdBi,PdZn,and PdAg)9,53,127.

Li and co-workers predicted through theoretical calculations that the PdIn(110)crystal plane has a higher acetylene hydrogenation selectivity than the Pd3In(111)crystal plane.They then successfully synthesized the corresponding PdIn and Pd3In intermetallic compounds and directionally exposed the PdIn(110)and Pd3In(111)crystal planes(Fig.11a-f)127.Experiments show that PdIn intermetallic compound catalysts can achieve 95% acetylene conversion and 92% ethylene selectivity at 90 °C.A universal experiment on the catalytic hydrogenation of various alkynes with PdIn intermetallic compounds was carried out.It was found that both the conversion of acetylene and the selectivity of ethylene could exceed 97%(Fig.11g,h).The Pd atoms in PdIn are effectively and completely isolated by the In atoms,and electrons are transferred from In to Pd,which weakens the adsorption of acetylene and ethylene,and inhibits the formation of hydrides and green oil.Furthermore,the PdIn(110)surface with the active sites of single-atom Pd shows a high ethylene selectivity in the acetylene semi-hydrogenation reaction.

Fig.11 (a-f)Structural analysis of Pd3In IMNCs.(g-h)Acetylene conversion and ethylene selectivity in long-term selective hydrogenation of acetylene at 90 °C on the three as-obtained catalysts 127.

Subsequently,Li and co-workers reported a metal-organic framework(MOF)-confined co-reduction strategy to prepare sub-2 nm intermetallic PdZn nanoparticles by employing the well-defined porous structures of calcinated ZIF-8(ZIF-8C)and anin situco-reduction(Fig.12a)128.Sub-2 nm intermetallic PdZn nanoparticles show excellent catalytic properties for the selective hydrogenation of acetylene.These properties result from the fact that the acetylene hydrogenation and ethylene desorption are more favorable paths for ultrasmall particles than larger-size intermetallic PdZn.The PdZn-sub-2@ZIF-8C catalyst was obtainedviathein situreduction of Pd and Zn within the ZIF-8C skeleton using the confinement effect of the ZIF-8C structure.The porous and homogeneous ZIF-8C structure effectively restricts particle aggregation,resulting in metal particles with a uniform size and catalysts with a high thermal stability.The catalyst performance tests showed that the PdZn-1.2(nanoparticle size is 1.2 nm)catalyst exhibited excellent acetylene conversion(around 70%)and ethylene selectivity(＞80%)at 115 °C(Fig.12b,c).The conversion and selectivity did not change after a prolonged reaction,indicating the excellent stability of the catalyst(Fig.12d,e).It can be seen that the confinement effect of the ZIF-8C structure facilitates the formation of homogeneous-size metal particles,which results in the excellent thermal stability of the catalysts.

Fig.12 (a)Schematic preparation process of the PdZn-sub-2@ZIF-8C using a MOF-confined co-reduction strategy.(b-c)Acetylene conversion and ethylene selectivity as a function of temperature over the ZIF-8C support and the prepared intermetallic PdZn.(d)The specific rate of the prepared intermetallic PdZn at 70 °C.(e)Plots of stability test of the PdZn-1.2@ZIF-8C at 115 °C 128.

Osswald and co-workers prepared unsupported PdGa and PtGa intermetallic compounds using a heat treatment129.The Pd/Ga content ratio in the PdGa intermetallic compounds was found to have a certain effect on the selective hydrogenation of acetylene.In the PdGa intermetallic compound structure,the Pd atoms are completely isolated by the Ga atoms.Compared with Pd/Al2O3and Pd20Ag80,PdGa and Pd3Ga7intermetallic compound catalysts showed similar specific surface activity,but the ethylene selectivity and the catalyst stability were greatly improved.As the Pd active sites are completely isolated,the acetylene molecules can only form weakπ-bonds when adsorbed on the Pd active sites,and the formation of the Pd hydride phase is inhibited,which significantly improves the selectivity of ethylene.By forming a Pd-C phase below the surface,which isolates a large amount of dissolved and very active hydrogen from the surface,Pd can be converted into a selective catalyst.In other words,only PdGa and Pd3Ga7the hydrogenation of alkynes to alkenes occurs at room temperature.The stable site separation,changes in the electronic structure,and the absence of hydride formation make PdGa and Pd3Ga7ideal,selective,and long-term stable hydrogenation catalysts.

Later,Luo and co-workers prepared an unsupported InPd2intermetallic compound catalyst using the same method130.InPd2showed a high activity and selectivity(up to 93%)toward ethylene in the temperature range from 478 to 508 K.In addition,the compound revealed a high stability over a period of 20 h in a stream at 473 K,with a selectivity of 80% and a conversion greater than 90%.Due to the isolation of the active sites,InPd2behaves similarly to GaPd2in the semi-hydrogenation reaction.However,the selectivity of InPd2may be slightly increased over that of GaPd2due to the narrowerd-band of the former and the 1.2% increase in the closest Pd-Pd distance.

Zou and co-workers reported a PdBi intermetallic compound catalyst(PdBi/calcite)synthesized on the calcite,which exhibits acetylene hydrogenation catalytic sites with mutually separated electron-rich Pd atoms(Fig.13a-g)131.PdBi/calcite catalysts were synthesized using a two-step process consisting of deposition and a reduction reaction,in which Bi and Pd were continuously deposited on calcite.The PdBi intermetallic compound was obtained when the Bi amount was 4.9 wt% and the Pd amount was 0.9 wt%;it was treated for 1 h in H2/Ar gas.In the PdBi catalyst,the adsorption of ethylene molecules is very weak,while the catalyst structure is very stable and avoids the formationofβ-PdHx.The PdBi/calcite catalyst could achieve a C2H4selectivity of above 99% in a wide operating temperature range(150-300 °C).The conversion of C2H2in the temperature range of 150-300 °C reached ～100%(Fig.13h-k).In addition to inhibiting the formation ofβ-PdHx,in PdBi/calcite,the Pd atoms are isolated by the Bi atoms,so they cannot be combined with ethylene through adsorption of theσ-bonds.By contrast,they are bound to ethylene by a weakπ-bond adsorption,promoting ethylene desorption from the catalyst surface,thereby improving the selectivity.

Fig.13 (a-g)Microstructures of PdBi/Calcite.(h)C2H2 conversion and C2H4 selectivity as a function of reaction temperature over PdBi/Calcite.(i)Plot of C2H4 conversion versus reaction temperature over PdBi/Calcite and Pd/Calcite in ethylene hydrogenation.(j)Plot of C2H2 conversion and C2H4 selectivity at 120,140,and 160 °C.(k)Catalytic performance of PdBi/Calcite without pretreatment in H2/Ar at 200 °C to form PdBi IMCs(denoted as PdBi/Calcite-wp)131.

Huang and co-workers reported the synthesis of intermetallic PtSn nanoparticles confined in mesoporous silica wells(MSWs)for use in the semi-hydrogenation of acetylene132.Pt nanoparticles(2.9 nm)supported on amine-functionalized silica spheres with dimensions of ～180 nm were enmeshed in a mesoporous silica shell.After the addition of Snviaa polyol synthesis method,they obtained the final PtSnx@MSW catalysts(x=0.30,0.50,0.70,1.0,and 2.0)(Fig.14a).In the gas-phase semi-hydrogenation of acetylene,the PtSn@MSW with Sn/Pt molar ratios equal to or greater than 1 displayed a higher selectivity of ethylene.Additionally,increasing the Sn content decreased the acetylene conversion efficiency.As the MSWs provide a confined environment for the catalyst,the PtSn@MSW intermetallic compound catalysts exhibited longterm stability and regeneration(Fig.14b,c).

Fig.14 (a)Schematic synthesis of Pt-Sn bimetallic catalysts encapsulated in mesoporous silica wells.(b)Acetylene semi-hydrogenation stability for 100 mg of PtSn1.0 containing different amounts of Pt.(c)Conversion and selectivity of PtSn1.0 @MSW for semi-hydrogenation of acetylene 132.

Compared with Pd-based intermetallic compounds,Ni-based intermetallic compounds are cheaper,so they also are more promising for applications.Based on hydrotalcite(LDH)precursors,Wang and co-workers synthesized a series of supported NiGa intermetallic compounds(Ni3Ga,Ni5Ga3,and NiGa)via in situreduction(Fig.15a-h)91.They found that Ni-Ga intermetallic compounds with completely isolated Ni sites exhibit excellent performance(～82%)of the acetylene semihydrogenation reaction(Fig.15i-m).The fully isolated Ni sites not only show preferential acetylene adsorption but also enhance ethylene desorption.The electrons transfer from Ga to Ni in the Ni-Ga intermetallic compounds and the separation effect of the Ga atoms on the active sites are the reason for the significant improvement in alkene selectivity.

Fig.15 (a-h)AC-HAADF-STEM image and FFT pattern of Ni5Ga3 catalyst and NiGa catalyst.(i)Acetylene conversion and ethylene selectivity as a function of reaction temperature for acetylene hydrogenation in the absence of ethylene,(j)C2H2-TPD(upper)and C2H4-TPD(bottom)profiles,(k)Acetylene conversion and ethylene selectivity as a function of reaction temperature in the presence of ethylene,and(l)Acetylene conversion as a function of time on stream at 190 °C over the Ni,Ni5Ga3 and NiGa catalysts.(m)TGA profiles of the used Ni,Ni5Ga3 and NiGa catalysts 91.

Intermetallic compounds are ordered alloys in which the host metal M1 is isolated by a second metal M2.Thus,in palladium based intermetallic compound catalysts,the Pd active site is usually isolated by the other metal,and ethylene can be adsorbed in three different ways on the host metal Pd,namely the hypoethyl method,di-σmethod,andπ-bond bonding mode.Ethylene has the lowest adsorption strength when it combines with a single Pd atomviaaπ-bond.Thus,when ethylene cannot be combined with Pd atom through the adsorption ofσ-bonds,it will instead be combined with the host metalvia π-bonds,and its desorption energy barrier will be smaller than the energy barrier for further hydrogenation.Therefore,ethylene will desorb from the catalyst surface without further hydrogenation,resulting in a high ethylene selectivity.

3.3 Single-atom alloy catalysts

SAACs usually consist of a bimetallic system,in which a small amount of metals with a higher activity is added to the surface of the main metal133-137.Moreover,a few active metal atoms surrounded by the main metal atoms are separated,which is also thermodynamically stable.Obviously,this method is conducive not only to the full utilization of each precious metal atom in the industrial hydrogenation reaction but also to the selective hydrogenation of acetylene.This method can effectively eliminate the multi-bond adsorption of acetylene on isolated precious metal atoms59,111,138,139.

Pei and co-workers reported a single-atom alloy(SAA)structure,in which a Pd SAA structure was formed by alloying Pd with Au/SiO2139.When the atomic ratio of Pd/Au is below 0.025,the Pd SAA structure is easily formed.The ethylene selectivity of the Pd SAA catalyst is much higher than that of the single-metal Pd/SiO2catalyst,which may be due to the weak adsorption strength of the Pd SAACs on ethylene.As a result,Au plays a key role in isolating Pd atoms and preventing the excessive hydrogenation of acetylene.During acetylene hydrogenation in an ethylene-rich flow,the Pd SAACs of a single-metal Au or Pd system has a better catalytic performance compared to other Pd SAACs .In addition,Pei and co-workers prepared AuPd,AgPd,and CuPd SAACs and studied their catalytic performance for the selective hydrogenation of acetylene to ethylene under a simulated industrial hydrogenation process(i.e.,conditions with high concentrations of H2and C2H4).Under the same conditions,compared with Ag-alloyed Pd and Au-alloyed Pd,Cu-alloyed Pd has a higher acetylene conversion and selectivity(Cu-alloyed Pd ≈ Ag-alloyed Pd ＞Au-alloyed Pd);the adsorption of Cu-alloyed Pd on C2H4is weaker than that of the Pd-based catalyst,which is also the reason for the high ethylene selectivity6,139,140.It can be concluded that the three catalysts have similar active centers(isolated Pd atoms)and similar catalytic mechanisms(that is,Cu,Ag,and Au play the same role in isolating the Pd atoms).This can prevent the excessive hydrogenation of acetylene and greatly improve the ethylene selectivity.

In single-atom alloys,electrons are transferred from the secondary metal to the host metal Pd,which causes the Pd atom to become negatively charged.When Pd is alloyed with other metals,it tends to be rich in electrons.The higher electron density of the Pd atoms on the surface of the alloy repels the C＝C bond of ethylene.Thus,π-bonded ethylene weakens the binding on the electron-rich Pd atoms.Therefore,the electron transfer between group ⅠB metals and Pd may be responsible for the relatively high selectivity of ethylene.

4 Others

The introduction of a second metal into the catalyst can change the electronic structure of the active sites on the catalyst surface,thereby changing the acetylene adsorption and ethylene desorption performance,and thus improving the ethylene selectivity.Similarly,the introduction of a third metal in a bimetallic catalyst can also improve the catalyst performance.Bridier and co-workers tried to add a third metal element(Fe)to NiCu,so that it could be used as a reinforcement to increase the exposure ratio of Ni and Cu141.The addition of Fe effectively reduces the formation of ethane and improves the catalytic performance.Feng and co-workers,prepared sea urchin-like and octahedral PdAuAg tri-metallic catalystsviathe co-reduction method142.At 120 °C,the acetylene conversions of the sea urchin-like PdAuAg catalyst(m-PdAuAg2)and the octahedral PdAuAg2catalyst(c-PdAuAg2)were 91.4% and 81.2%,respectively.For a given acetylene conversion rate,m-PdAuAg2shows a higher ethylene selectivity.From the above results,it can be concluded that the morphology of tri-metallic catalysts has a greater impact on the catalyst performance than other influencing factors,which may be due to an excessive metal amount may affecting the exposure of the active metal sites.

In the design of selective hydrogenation catalysts for acetylene,on the one hand,introducing a large number of second and third metals into single-metal catalysts changes the distribution state and the valence electron structure of the active metal.Thus,the structure of the reactant adsorbed on the catalyst surface can be changed to improve the performance of the catalyst.On the other hand,the design of the catalysts can also directly regulate the catalytic active sites of the catalyst,and further tune the catalyst performance through the interaction between the active sites.For example,the introduction of a second metal atom can adjust the structure of the catalyst active site M1 or provide a dual-atom-site M1M2 to expand the catalytic range of the single-atom active sites.NSASCs play different roles in the acetylene semi-hydrogenation reaction,improve the ethylene selectivity and reaction activity,and are finally able to remove acetylene.

4.1 Dual-atom-site catalysts

Although both DASCs and bimetallic catalysts are materials synthesized through the incorporation of two metals,there are obvious differences between them:DASCs have two active sites,while bimetallic catalysts have often only one143-149.DASCs not only combine two active metal atoms with acetylene semihydrogenation capability but also provide new surface adsorption sites/configurations,metal electronic structures,and reaction pathways,which are usually not available for a singlemetal active site.Thus,DASCs are an effective way to overcome the shortcomings of isolated active sites and enhance the semihydrogenation reactivity.DASCs have recently attracted significant attention as an extension of SASCs.Compared with SASCs,DASCs have higher metal loads as well as more complex and flexible active sites,providing more opportunities for achieving a better catalytic performance150-154.

Liu and Ma prepared a DASC with a bonded Pd1-Cu1atom pair anchored on an ND@G carrier,called Pd1Cu1/ND@G(Fig.16a,b)155.Compared with single-atom Pd or Cu catalysts,diatomic catalysts exhibit higher reactivity at lower temperatures,with 100% acetylene conversion at 110 °C and an ethylene selectivity up to 92%(Fig.16c,d).It was found that the unique diatomic configuration provides sterically favorable sites for the simultaneous adsorption of reactants,thereby changing the competitive adsorption process of reactants to a noncompetitive process.In other words,the Pd1-Cu1diatomic pair can adsorb H2and acetylene at the same time,which weakens the stereoscopic effect and promotes the occurrence of activation/dissociation.Additionally,the modulated electronic structure due to atomic pairing is conducive to a stronger adsorption of the reactants;it alters the response path,thereby reducing the barrier for the transition from C2H2to C2H4.All of the above factors can promote the adsorption of acetylene as well as accelerate the adsorption and dissociation of H2.

4.2 Nano-single-atom-site catalysts

NSASCs are a type of catalyst containing both single-atom sites and nanoparticle sites,which directly or indirectly participate in the catalytic process,regulating active site or accelerating the reaction rate from a steric structure156,157.

Qiao and Li reported a simple reductive wrapping strategy to successfully construct Pd1/TiO2SASCs by selectively wrapping nanoparticles while exposed single atoms using different conditions for the occurrence of a single-atom-nanoparticle metal-carrier strong interaction(SMSI),which can significantly enhance the selectivity of the acetylene semi-hydrogenation reaction(Fig.17a)158.In addition,the excellent photocatalytic properties of TiO2carriers were utilized to significantly increase the reactivity of the Pd1/TiO2SASCs to achieve an efficient conversion of the acetylene hydrogenation process down to 70 °C(Fig.17b,c).DFT calculations indicate that irradiation causes bandgap excitation of TiO2,resulting in electron-hole pairs.H2dissociates into the separated Pd site and TiO2,the C2H2molecules are adsorbed on the isolated Pd atoms,and then the photogenerated electrons are transferred to the isolated Pd atoms,promoting the activation of the adsorbed C2H2.The H species then react with C2H2activated on monoatomic Pd to form Pd-C2H4species and then achieve C2H4desorption.

Fig.17 (a)Structural characterization of Pd/TiO2 serial catalysts.(b)Catalytic performance of Pd/TiO2 serial catalysts.(c)Durability test on Pd/TiO2-600H at 120 °C for 40 h 158.

Xu and Li reported a simplein situencapsulation strategy for the assembly of multifunctional nanocatalysts,namely Au@Pt nanotubes(NTs)@ZIFs,in which multiple hollow Au@Pt NTs are fully incorporated into a ZIF matrix(ZIF-67 and ZIF-8)159.They fabricated a core-shell hollow Au@Pt NTs@ZIFs nanocomposite as an efficient catalyst for acetylene semihydrogenation.The hollow Au@Pt NTs were synthesized through the epitaxial growth of Pt shells on Au nanorods followed by oxidative etching of the Au@Pt nanorods.The obtained hollow Au@Pt NTs were then uniformly encapsulated in the ZIFsvia in situcrystallization(Fig.18b).By combining the high activity of the bimetallic NTs with the gas enrichment properties of the porous MOFs,the hollow Au@Pt NT@ZIF catalysts show a superior catalytic performance for acetylene,in terms of both selectivity and activity,to their monometallic Au and solid bimetallic nanorods@ZIF counterparts(Fig.18a).The presence of highly reactive Pt around Au facilitates the conversion of C2H2hydrogenation to C2H4due to the synergistic effect of the bimetallic nanostructures.This catalyst design idea is considered to be an important step in the development of efficient nanocomposite catalysts.

Fig.18 (a)Acetylene conversion and ethylene selectivity over the catalysts NPs@ZIF-67 and NPs@ZIF-8.(b)Schematic illustration of the formation of Au,Au@Pt NR,and hollow Au@Pt NTs and their ZIF nanocomposites 159.

DASCs and NSASCs can provide a higher metal loading and more complex and flexible active sites,which can promote the adsorption of H2and acetylene on the catalyst at the same time.Furthermore,the adsorption mechanism has also changed,and the electronic structure produced by the pairing of the two atoms(nanoparticle and single-atom sites)also has an impact on the adsorption mechanism.Thus,two-site catalysts not only have more adsorption sites but also a different reaction path,electronic structure,etc.,which greatly improves the selective hydrogenation capacity of the catalyst.

5 Conclusions and outlook

Single metal catalysts,nanoparticle catalysts and nanocluster catalysts have strong M—M bonds(M=Pd,Ni,or Cu),which make the hydrogenation of acetylene to ethane dominant throughout the entire catalytic process and correspondingly result in a reduced selectivity of ethylene.SASCs,and the hydrogenation of ethylene to ethane on single atoms require overcoming higher barriers to limit acetylene excessive hydrogenation to ethane,thus improving the selectivity and conversion of ethylene.However,the stability of SASCs needs to be improved.Compared with single metal Pd catalysts,the addition of a second or third metal element can optimize the geometric structure(Pd—Pd coordination)and electronic(dband center)structure of the Pd catalyst surface,and change the activity and selectivity of the single metal catalyst for the acetylene semi-hydrogenation reaction,and reduce the cost of the Pd catalyst.Furthermore,the influence of the acetylene semihydrogenation reaction on the incorporation of different metals in Pd-based catalysts to form bimetallic alloy catalysts are also adjustable.In addition,the stability and selectivity of singleatom alloy catalysts are superior to solid-solution alloys and intermetallic compounds catalysts.However,dual-atom-site catalysts have more advantages in fine tuning than traditional bimetallic ones.The tuning of the hydrogenation properties achieved by the dual-atom-site are not only adjust the types of different metal elements,but also the non-metallic coordinated with metals,such as using N,P,S to adjust the coordination environment of unsaturated metal sites.

In conclusion,with the development of methods for the synthesis and characterization of catalysts,we gained a deeper understanding of the catalytic process of acetylene semihydrogenation,the interaction mechanism with the reactants,and the dynamic evolution of the active sites(such as changes in the coordination environment and migration of the metal atoms during the reaction process).Acetylene semi-hydrogenation is currently one of the most effective methods for acetylene removal,and most catalysts currently used in the industry are still Pd-based catalysts.However,it is still a challenge to balance the selectivity and conversion of the catalyst for acetylene semihydrogenation.For example,for SASCs,the adsorption of ethylene with weakπ-bonds can significantly improve the reaction selectivity.However,due to the poor ability of SASCs centers to decompose H2,the reaction activity is low.Therefore,future catalysts for acetylene semi-hydrogenation able to precisely control the active sites to improve their catalytic activity,selectivity,and stability are the focus of researchers,mainly in the following areas:

(1)The environment of the active sites of the catalyst is very important in the semi-hydrogenation reaction.A precise regulation of the coordination environment,electronic structure,and the unique interaction between the supports and the metal sites of SASCs is essential to make the catalysts exhibit unique catalytic activities while improving the utilization of noble metal atoms and reducing the cost.For example,introducing a second metal atom to form DASCs on the basis of SASCs,adjusting the electronic structure of single metal active sites to improve the catalytic activity of SASCs for acetylene semi-hydrogenation.

(2)Introducing NPs on the basis of SASCs or DASCs to form NSASCs,so that SASCs and NPs can coexist or DASCs and NPs can coexist.In this way,the respective advantages of SASCs,DASCs,and NPs can be exploited in the same catalyst.The synergy catalysis of NSASCs will be the future direction of the selective semi-hydrogenation of acetylene.SASCs,DASCs,and NPs have different roles in the reaction.For example,in catalysts supported by Pd single atoms and transition metal nanoparticles,dispersed Pd single atoms were able to enhance the catalytic performance,and supported nanoparticles were able to suppress the undesirable palladium hydride formation,thereby improving the selectivity of ethylene.In a word,the ultimate goal of the research on acetylene semi-hydrogenation catalysts is to precisely control the active sites of the catalyst and achieve a catalyst with a high catalytic activity,selectivity,and stability.

Author Contributions:Conceptualization &Resources,Supervision,Li,Y.D.;Writing - Original Draft,Fang,H.Y.;Investigation &Software,Jiang,J.J.;Writing - Review &Editing,Liu,X.W.;Suggestion - Review &Editing,Wang,D.S.;Supervision &Suggestion,Zhu,D.R.