Selective Hydrogenation of Acetylene to Ethylene over BimetallicCatalystsQingyuan Li,† Yuxin Wang,† George Skoptsov,‡ and Jianli Hu*,†

†Department of Chemical and Biomedical Engineering, West Virginia University, Morgantown, West Virginia 26506, United States‡H Quest Vanguard, Inc., Pittsburgh, Pennsylvania 15238-1333, United States

ABSTRACT: Ni/α-Al2O3 catalyst and a series of bimetallic catalystsincluding Pd−Ag, Ni−Pd, Ni−Zn, Ni−Ag, and Ni−Ga were synthesized,characterized, and tested in selective hydrogenation of acetylene to ethylene.The bimetallic catalyst Ni−Ga exhibited almost the same ethylene selectivitycompared to Pd−Ag based catalyst. The effect of the Ni/Ga ratio on thecatalytic activity and selectivity to ethylene in acetylene hydrogenation wasevaluated. Characterization by transmission electron microscopy, X-raydiffraction, hydrogen temperature-programmed reduction, and X-ray photo-electron spectroscopy was carried out to identify active phases on Ni−Gabased catalysts, which was correlated with catalytic performance and thereaction mechanism occurring over the catalyst. The presence of Ga in Ni−Ga lattice structure confined the movement of dissociated H* and reduced the adsorption binding energy of ethylene, whichcould prevent overhydrogenation of acetylene.

1. INTRODUCTION

In catalytic hydrocarbon cracking processes, there is always thepresence of acetylene that needs to be removed fortechnological and economic considerations. The effectiveway of separating acetylene is to selectively hydrogenate it toethylene. The concentration of acetylene needs to be reducedto less than 2 vol % before polymerization of ethylene.1,2 Inrecent years, new processes such as microwave or/and plasma-assisted methane conversion produce acetylene, and theamount of acetylene appears to be higher than that producedfrom the traditional industrial process.3,4 Hence, selectivehydrogenation of acetylene becomes critical in upgrading thevalue of the product. For example, according to a report, in2013, the size of the global ethylene market was estimated tobe US$156.0 billion, and it is projected that by 2020 themarket value will reach US$234.2 billion. Meanwhile, theglobal acetylene gas market was just valued at US$5610 millionin 2018 and is likely to reach US$6090 million by the end of2025.5,6 There are numerous monometallic catalysts such asNi,7 Au,8 and Pd9 which were reported active for hydro-genation of acetylene. However, these monometallic catalystsusually suffer from high activation energy or formingoligomeric species which lead to low selectivity to ethyleneand catalyst deactivation.5 To overcome these drawbacks ofmonometallic catalysts, bimetallic and even trimetallic catalystssuch as Pd−Ga,10 Pd−Ag,11,12 Pd−Au,13 Pd−Cu,14 Pd−Fe,15Co−Pd,16 Pd−Zn,17 Au−Ag,18 Au−Ni,19 and Pd−Au−Ag20have been synthesized and studied in selective hydrogenationreactions. For example, adding Ag to Pd can suppress theoverhydrogenation activity of Pd only and increase theselectivity to ethylene. This is mainly because adding Ag toPd can decrease the oxidation state of unoccupied d orbital in

Pd atoms.9 This phenomenon decreased the adsorptionbonding energy of unsaturated hydrocarbons on the catalystsurface which led to the higher ethylene selectivity.Intermetallic Pd−Ga catalyst also exhibited the same level ofcatalytic activity and ethylene selectivity compared to Pd−Agcatalysts. According to the literature, in a well-definedintermetallic structure, the isolation of Pd active sites canreduce the overhydrogenation activity.21 However, thebimetallic or trimetallic catalysts mentioned above are allprecious metal based catalysts, which is not cost-effective.For Ni-based metallic catalysts, not only density functional

theory (DFT) was used to discuss the Ni-based metalliccatalysts, but also experiments were carried out to prove thetheoretical calculations.21−25 For example, intermetallic Ni−Zncatalyst was recognized as a promising catalyst when comparedwith precious metal based catalysts. Studt et al. used a DFTmodeling approach to analyze the Ni−Zn catalyst system inselective hydrogenation of acetylene, and then synthesized Ni−Zn supported on MgAl2O4 support to measure the activity inacetylene hydrogenation in a gas mixture of 1.33% ethylene,0.0667% acetylene, and 0.67% hydrogen.26 Spanjers et al. usedisotopic labeling and DFT calculation approaches to evaluateacetylene hydrogenation activity over intermetallic Ni−Zncatalysts. They found that the formation of ethane was not aquantitative measure of ethylene selectivity, because theformation of higher molecular weight products from acetyleneis possible.27 According to the literature,42 Ni−Ga intermetallic

Received: August 20, 2019Revised: October 27, 2019Accepted: October 28, 2019Published: October 28, 2019

Article

pubs.acs.org/IECRCite This: Ind. Eng. Chem. Res. 2019, 58, 20620−20629

© 2019 American Chemical Society 20620 DOI: 10.1021/acs.iecr.9b04604Ind. Eng. Chem. Res. 2019, 58, 20620−20629

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compound can be formed where the geometric and electronicproperties of Ni are modified due to the presence of Ga.Specifically, small Ni ensembles are formed due to thepenetration of contiguous bulk Ni clusters by Ga whichincreases the electron density of Ni through charge transferfrom Ga atoms. The presence of Ga also isolates the active siteof Ni isolation. These hypotheses were supported by DFTcalculations.42 Hu et al. synthesized Ni3Ga hollow micro-spheric particles with multishelled intermetallic structure usinga hydrothermal method for the selective hydrogenation ofalkyne and found the ethylene selectivity was much higher thanthat of commercial Pd/C catalysts.28 The Ni-based catalystsnot only decreased the usage of precious metal such as Pd andAg, but also enhanced the hydrogen coverage which coulddecrease the characteristic acetylene oligomerization duringthe selective hydrogenation process.29 In other words, Ni-based catalysts at least showed potential catalytic activity forselective hydrogenation of acetylene. Hence, Ni-based catalystswere chosen for this work.In this study, a gas mixture (CH4, C2H2, C2H4, H2, and

argon) with a concentration of C2H2 up to 15% (molepercent) was used as the feedstock. A series of Ni-basedbimetallic catalysts such as Ni−Pd/α-Al2O3, Ni−Zn/α-Al2O3,Ni−Ag/α-Al2O3, and Ni−Ga/α-Al2O3 with different Ni/Garatios were synthesized via different preparation methods.Under the same acetylene conversion, the catalytic activity andselectivity toward ethylene over Ni−Ga/α-Al2O3 catalyst werecomparable with those of Pd−Ag/α-Al2O3 catalyst. Catalystcharacterization was carried out to study the correlation ofcatalytic performance with surface property to elucidate thereaction mechanism.

2. EXPERIMENTAL SECTIONCatalyst Preparation. The Ni/α-Al2O3 and Ni-based

bimetallic catalysts such as Ni−Zn/α-Al2O3, Ni−Pd/α-Al2O3,Ni−Ag/α-Al2O3, and Ni−Ga/α-Al2O3 were synthesized byincipient wetness coimpregnation technique. The chemicalsused in the synthesis included α-Al2O3 (Sigma-Aldrich),Ni(NO3)2·6H2O (Acros Organics), Zn(NO3)2·6H2O (FisherChemical), Ga(NO3)3·xH2O (Alfa Aesar), Pd(NO3)2·2H2O(Sigma-Aldrich), and AgNO3 (Sigma-Aldrich). In a typicalsynthesis procedure, for Pd−Ag catalyst, m(Pd−Ag)/m(α-Al2O3) was set at 1.0 wt % where the molar ratio of Pd:Ag was7:2. For the Ni-based catalysts, m(Ni)/m(α-Al2O3) was set at1.5 wt %. The molar ratio of Ni:Zn was 1:2, Ni:Pd was 22:1,Ni:Ag was 1:1, and Ni:Ga was 1:1, 5:3, and 3:1, respectively. α-Al2O3 was impregnated in the metal nitrate solution, then agedat room temperature for 12 h, and subsequently dried in air at110 °C overnight. The catalyst samples such as Ni/α-Al2O3,Ni−Zn/α-Al2O3, Ni−Pd/α-Al2O3, and Ni−Ag/α-Al2O3 werecalcined at 550 °C for 4 h followed by hydrogen reduction at400 °C for 2 h before acetylene hydrogenation reaction. ForNi−Ga/α-Al2O3 catalysts, two different procedures werecarried out. One procedure was to calcine the sample at 500°C for 4 h, followed by reduction in hydrogen in a fixed-bedreactor operated at 400, 550, and 700 °C for 2 h beforeacetylene hydrogenation reaction. In additional to the abovetreatment, another pretreatment procedure for Ni−Ga/α-Al2O3 catalysts was conducted by directly reducing the catalystsamples under hydrogen in a fixed-bed reactor operated at 400,550, and 700 °C for 2 h.Catalyst Characterization. Transmission electron micros-

copy (TEM) was employed to measure the metal distribution

on the surface of catalyst support. X-ray diffraction (XRD) wasemployed to measure the crystalline structures of the catalystsand alloy formation. Transmission electron microscopymeasurement was performed on a JEOL (JEM 2100) TEMinstrument operated at 200 kV accelerating voltage andcharacterized the catalysts size and dispersion. XRD patternsand crystal types of as-prepared samples were analyzed usingX-ray diffraction instrumentation (PANalytical X’Pert Pro).The XRD measurement was operated under 45 kV and 40 mAusing Cu Kα radiation. The diffraction angles (2θ) werescanned from 5 to 80°. The scanning speed was set at 5°/minusing an X’celerator solid-state detector. The measurement ofhydrogen temperature-programmed reduction (H2-TPR) wasconducted to analyze the reducibility of the catalysts using acommercial instrument, Micromeritics Autochem 2950. BeforeTPR measurement, 0.2 g of catalyst was pretreated underargon flow (50 mL/min) at 300 °C for 1 h, and then cooled to50 °C. The TPR measurement was carried out in thetemperature range from 100 to 900 °C. The temperatureramping rate was set at 10 °C/min in the presence of the gasmixture 10% H2 in argon at a flow rate of 50 mL/min. The X-ray photoelectron spectroscopy (XPS) measurement wascarried out using a PHI 5000 Versa Probe system (PhysicalElectronics) to study the valences of Ni and Ga. The referencepeak selected was the C 1s peak at 284.8 eV for the calibrationof binding energies of all XPS spectra.

Catalytic Activity Evaluation. The selective hydro-genation of acetylene was carried out in a fixed-bed reactormade of a quartz tube. The feedstock was a gas mixtureconsisting of acetylene (15%), ethylene (0.8%), methane(17.95%), argon (8.031%), and hydrogen as balance. In atypical experiment, 0.5 g of catalyst was loaded in a reactor andreduced under hydrogen (5 mL/min) for 2 h. The catalyst wasthen cooled to room temperature when the feedstock gasmixture was introduced at a space velocity of 4800 mL/g·h.The products were analyzed at different temperatures such as80, 120, 160, 200, 240, and 280 °C. The products wereanalyzed by an online gas chromatograph (GC) manufacturedby PerkinElmer (Model Clarus 500). The GC is equipped witha number of separation columns including HayeSep N 60/80,HayeSep T 60/80, Molecular Sieve 5A 45/60, and MolecularSieve 13X 45/60 packed columns. A thermal conductivitydetector (TCD) is installed for the product detection.The conversion of acetylene (Xacetylene), the selectivity to

ethylene (Sethylene), the selectivity to ethane (Sethane), and theselectivity to other products (Sothers) were calculated as follows:

=−

×XC H (in) C H (out)

C H (in)100%acetylene

2 2 2 2

2 2

=−

−×S

C H (out) C H (in)C H (in) C H (out)

100%ethylene2 4 2 4

2 2 2 2

=−

−×S

C H (out) C H (in)C H (in) C H (out)

100%ethane2 6 2 6

2 2 2 2

= − −S S S1others ethylene ethane

where C2H2(in) and C2H2(out) are designated as moles ofacetylene in the products and in the feed, respectively. Thesame designation applies to C2H4(in) and C2H4(out),C2H6(in), and C2H6(out).

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3. RESULTS AND DISCUSSION

Catalyst Screening Tests. Usually, Pd-based catalyst hasbeen widely used in selective hydrogenation of acetylene.9

Because of the high concentration of acetylene in the feed,initial catalyst formulation was focused on bimetallic Pd−Ag/α-Al2O3. As shown in Figure 1, the effect of temperature onacetylene conversion and ethylene selectivity in the hydro-genation of acetylene was measured over as-prepared catalysts.For most of the catalysts tested such as Ni−Zn/α-Al2O3, Ni−Pd/α-Al2O3, and Ni−Ag/α-Al2O3 catalysts, complete con-version was achieved at temperatures over 100 °C. The highestselectivity to ethylene of 85% was achieved over Pd7Ag2/α-Al2O3 catalyst. The results are consistent with those reportedin the literature.10,11 In addition, the selectivity to undesiredproducts such as ethane was less than 10%. It appears thatPd7Ag2/α-Al2O3 catalyst is suitable for the selective acetylenehydrogenation at high concentration. Although Pd−Agexhibits desired properties, an economical nonprecious metalsuch as Ni is favored for commercial application. In thefollowing section, the catalytic activity of Ni-based bimetalliccatalysts for the selective acetylene hydrogenation is discussed.The catalytic activity and ethylene selectivity over Ni/α-

Al2O3 and Ni-based bimetallic catalysts were compared, andthe results are shown in Figure 1. When the reaction

temperature was raised, for all the catalysts tested, theconversion of acetylene could reach 100%. However, theselectivities to ethylene and ethane were very differentdepending on the catalyst formulation. For the monometallicNi/α-Al2O3 catalyst, at temperature lower than 160 °C,selectivity to ethane was dominant along with a small amountof methane formation. When the temperature was increasedfrom 160 °C, the selectivity to ethylene increased sharply, andreached 64% at 280 °C. Correspondingly, the selectivity toethane dropped when the temperature continued to increase to280 °C. The selectivity to ethylene over Ni22Pd/α-Al2O3

catalyst was much lower than that over Ni/α-Al2O3 catalyst,which is probably due to overhydrogenation of acetylene andethylene polymerization which formed oligomers (green oil).30

As shown in Figure 1D, the selectivities to other products overNi22Pd/α-Al2O3 catalyst are higher than those of monometallicNi/α-Al2O3 catalyst. As can be seen in Figure 1B, theselectivity to ethylene over Pd−Ni catalyst was lower than thatover Ni-based catalyst. This means more hydrogen isconsumed during the reaction over Pd−Ni catalyst. Theselectivity to ethane was almost the same for both Pd−Ni andNi-based catalysts; therefore, selectivity to other products overPd−Ni catalyst was higher.

Figure 1. C2H2 selective hydrogenation over as-prepared catalysts (0.1 MPa, Pd7Ag2/α-Al2O3, gas hourly space velocity (GHSV) = 120 000 mL/g·h; other Ni-based catalysts, GHSV = 4800 mL/g·h).

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Figure 2. Effect of ratio of Ni:Ga on selective hydrogenation of acetylene over Ni−Ga based catalysts (0.1 MPa, GHSV = 4800 mL/g·h).

Figure 3. TEM images of Ni−Ga/α-Al2O3 catalysts and particle size distributions (right bottom corner of each image). (A) NiGa/α-Al2O3. (B)Ni5Ga3/α-Al2O3. (C) Ni3Ga/α-Al2O3. (D) NiGa/α-Al2O3 calcined at 550 °C for 4 h.

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Although Zn-doped Ni was considered as a promisingcatalyst for acetylene selective hydrogenation, the ethyleneselectivity was strongly dependent on the ratio of Zn to Ni andreaction systems.27 Hence in the case of NiZn2/α-Al2O3catalyst, the selectivity to ethylene was not identical to thereported literature.27 According to a literature report, theintroduction of Ag to Ni/SiO2 could reduce the formation ofethane and methane; hence the selectivity of ethylene could beenhanced.31 It is clearly shown in Figure 1 that NiAg/α-Al2O3catalyst exhibits improved selectivity to ethylene as comparedwith monometallic Ni/α-Al2O3 catalyst. Usually Ag is inactivefor acetylene hydrogenation. The addition of Ag is speculatedto limit overhydrogenation of acetylene, therefore improvingthe selectivity to ethylene.11

Selective Hydrogenation of Acetylene on Ni−Ga/α-Al2O3. Gallium was introduced onto Ni-base catalyst to studythe effect of Ga on the selective hydrogenation of acetylene. Asillustrated in Figure 2A, the conversion of acetylene wasincreased with the increase in reaction temperature. For Ni−Ga catalysts with different Ni:Ga ratios, 100% conversion wasachieved at 240 °C. Although the conversion reached 100% atabout 240 °C, the catalytic activity was decreased with theincrease in the molar ratio of Ni:Ga. As shown in Figure 2A,the conversion follows the order Ni3Ga/α-Al2O3 > Ni5Ga3/α-Al2O3 > NiGa/α-Al2O3. Even though the catalytic activity ofNi3Ga/α-Al2O3 catalyst was higher than other ratios, the

selectivity to ethylene over this catalyst was lower than those ofNi5Ga3/α-Al2O3 and NiGa/α-Al2O3 catalysts (Figure 2B). It isnoteworthy that the selectivity to ethylene nearly reached 80%over NiGa/α-Al2O3 catalyst, and the selectivity to ethane waseven lower than 10% (Figure 2C) within the range oftemperatures tested.In order to study the effect of catalyst pretreatment on the

selective hydrogenation of acetylene, NiGa/α-Al2O3 wasprecalcined at 550 °C for 4 h in air, and then reduced byhydrogen at 700 °C for 2 h before the acetylene hydrogenationreaction. As shown in Figure 2A, the catalytic activity of theprecalcined catalyst is slightly higher than that of NiGa/α-Al2O3 without the precalcination. However, the selectivity toethylene is much lower and obviously the selectivity to ethaneis higher for the precalcined catalyst.

Catalyst Characterization. Catalyst characterization byTEM was carried out to ascertain the correlation betweencatalytic activity and interaction among metal particles on thesurface of the catalysts. In the TEM measurement shown inFigure 3, the particle size distribution was measured based on30 particles for each Ni−Ga sample and is shown on the rightbottom corner of each image in Figure 3. The composition ofNiGa/α-Al2O3 catalyst was measured by energy-dispersive X-ray spectroscopy (EDS). The measured atomic composition ofNiGa/α-Al2O3 catalyst was as follows: C (31.92%), O (8.81%),Al (1.65%), Ni (29.57%), Cu (7.76%), and Ga (20.28%). The

Figure 4. XRD patterns of different Ni−Ga catalysts. (1) α-Al2O3; (2) Ni3Ga/α-Al2O3 reduced at 700 °C for 2 h; (3) Ni5Ga3/α-Al2O3 reduced at700 °C for 2 h; (4) NiGa/α-Al2O3 reduced at 700 °C for 2 h; (5) NiGa/α-Al2O3 reduced at 550 °C for 2 h; (6) NiGa/α-Al2O3 reduced at 400 °Cfor 2 h; (7) NiGa/α-Al2O3 calcined at 550 °C for 4 h and then reduced at 700 °C for 2 h; (8) NiGa/α-Al2O3 calcined at 550 °C for 4 h and thenreduced at 550 °C for 2 h; (9) NiGa/α-Al2O3 calcined at 550 °C for 4 h and then reduced at 400 °C for 2 h.

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Ni:Ga atomic ratio for the NiGa/α-Al2O3 catalyst was a littlebit higher than 1. This could be due to the partial oxidation ofsome of Ni when the sample was exposed to air and NiOencapsulated under crystalline Ni−Ga could not be furtherreduced by hydrogen at 700 °C.Figure 4 exhibits the XRD patterns of α-Al2O3 and Ni−Ga

based catalysts. As can be seen from Figure 4A, the diffractionpeaks located at 25.58, 35.15, 37.78, 43.36, 52.55, 57.50, 61.30,66.52, 68.21, and 76.87° are ascribed to α-Al2O3. It is difficultto observe diffraction peaks of either Ni or Ga. This is probablybecause of low concentration of metals on the support or highdispersion of metals on the α-Al2O3 support. It was reportedthat Ni−Ga catalyst with a Ni:Ga = 1:1 atomic ratio formedvarious structures at different temperatures.32 For example,NiO crystallites first formed when the temperature exceeded200 °C. When the temperature was higher than 350 °C, theformation of α′-Ni3Ga, δ-Ni5Ga3, and β-NiGa phases wereobtained. The β-NiGa phase became dominant at 700 °C,which corresponds to a Ni/Ga ratio of 1:1in the originalsample. For the Ni−Ga catalysts with Ni/Ga ratios of 5:3 and3:1, the main products were Ni5Ga3 and Ni3Ga phases,respectively.Figure 4B shows the enlarged XRD spectra for some samples

such as α-Al2O3, Ni3Ga/α-Al2O3 which was reduced at 700 °C(2 h), Ni5Ga3/α-Al2O3 reduced at 700 °C (2 h), NiGa/α-Al2O3 reduced at 700 °C (2 h), and NiGa/α-Al2O3 calcined at550 °C (4 h) and then reduced at 700 °C (2 h), shown inFigure 4A. Although the highest diffraction peak of Ni−Gaalloy appears at about 2θ = 43°, it is just overlapped with thecharacteristic peak of α-Al2O3 at 43.36°. This is why the XRDprofile only showed α-Al2O3 diffraction peaks. However,according to the Ga−Ni phase diagram, when the Ni:Garatio is 1:1, 5:3, or 3:1, β-NiGa, δ-Ni5Ga3, and α′-Ni3Ga phaseswere formed at 700 °C, respectively.33 Figure 4C shows theNiGa/α-Al2O3 catalyst directly reduced at 400, 550, or 700 °Cbefore the selective hydrogenation reaction. It is noteworthythat the diffraction peak of NiGa/α-Al2O3 catalyst reduced at700 °C shifted slightly toward higher degrees as compared tothe catalysts reduced at 400 and 550 °C. This is mainlybecause, at the Ni/Ga ratio of 1:1, α′-Ni3Ga and δ-Ni5Ga3phases were formed when the reduction temperature exceeded350 °C, whereas on raising the temperature to 700 °C only theβ-NiGa phase was observed over Ni−Ga (1:1 ratio) sample.33

Figure 4D shows XRD patterns of Ni−Ga catalyst calcined at550 °C for 4 h and then reduced at 400, 550, or 700 °C beforethe selective hydrogenation. This is probably because the NiGaphase (formed at 700 °C) dominates the catalyst whereas theformation of Ni5Ga3 phase and formation of Ni3Ga phase aremuch less. The XRD patterns were identical to that of thesample without being precalcined. This is probably due to theformation of the face-centered-cubic structure of α′-Ni3Gaphase and the orthorhombic structure of δ-Ni5Ga3 phase.

33

Figure 5 shows the H2-TPR profiles of Ni−Ga bimetalliccatalysts with three different ratios from 170 to 850 °C. Thepeak observed at 262 °C is attributed to the reduction of NiOspecies.34 The peak located at 380 °C is associated with thereduction of NiO and the formation of α′-Ni3Ga alloy phase.33A broad peak located between 510 and 550 °C corresponds tothe initial formation of β-NiGa, δ-Ni5Ga3, and α′-Ni3Gaphases, respectively.33 It was reported that Ga/SiO2 precursorshowed two peaks at about 450 and 800 °C, which appearedvery broad and were attributed to the reduction of Ga2O3(GaO)+ species and the larger bulk Ga2O3 cluster.35−37 In

Figure 5, no peaks were observed between 450 and 800 °C,indicating the disappearance of monometallic Ga, andconfirming the formation of Ni−Ga alloy (at 380 °C) onNiGa/α-Al2O3, Ni5Ga3/α-Al2O3, and Ni3Ga/α-Al2O3 catalysts.For the NiGa/α-Al2O3 catalyst calcined at 550 °C for 4 h, thebroad peak located at 535 °C can be attributed to thereduction of NiO and the formation of Ni−Ga alloy.33 Theshoulder peak located at about 650 °C may be due to theformation of β-NiGa phase.33

The chemical compositions and elemental valences ofNiGa/α-Al2O3, Ni5Ga3/α-Al2O3, Ni3Ga/α-Al2O3, and NiGa/α-Al2O3 catalyst samples were analyzed by XPS. Figure 6presents the Ni 2p3/2 and Ga 2p3/2 XPS spectra which werefitted with Gaussian fitting. For transition metals, the bindingenergy of surface atoms is different from that of the bulkatoms, because of the decrease in coordination number of thesurface atoms.38 Hence, the binding energy located at 853.3 eVin Figure 6A is attributed to Ni0 which was about 0.3 eVdeviation from bulk Ni atom (NiO).39 The peak located at855.8 eV for NiGa/α-Al2O3 catalyst was also shown for thecatalyst of NiGa/α-Al2O3 precalcined at 550 °C beforereduction, indicating the presence of the same oxidationstate of Ni, which could be ascribed to Ni2+. In order toremove the passivated layer on the surface of catalysts, thesample was sputter-cleaned by Ar+ ion beam. The Ni2+ andGa3+ species were still measurable on all Ni−Ga catalysts,indicating they were not completely removed from the catalystsurface by the sputtering of Ar+. For Ni5Ga3/α-Al2O3 andNi3Ga/α-Al2O3 catalysts, the binding energy was shifted to alower binding energy, indicating an electron density increaseon Ni atom in Ni−Ga alloy. Furthermore, the satellite peakswere found at about 861 eV, which was originated from thedifference between the ground state and the excited stateduring the photoelectron emission. The satellite peaks exhibithigher binding energy than the main peak.As shown in Figure 6B, the binding energies for NiGa/α-

Al2O3, Ni5Ga3/α-Al2O3, and Ni3Ga/α-Al2O3 catalysts were1117.6, 1117.3, and 1117.4 eV, respectively, which werebetween Ga3+ (1118.0 eV) and metallic Ga0 (1116.7eV).37,40,41 The results indicate that the Ga species in NiGa/α-Al2O3, Ni5Ga3/α-Al2O3, and Ni3Ga/α-Al2O3 catalystspossessed positive charges between 0 and +3. In addition, itwas reported that, in Ni−Ga intermetallic compound, electron

Figure 5. H2-TPR for Ni−Ga/α-Al2O3 catalysts.

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transfer from Ga atom to Ni atom took place, which wasproved by extended X-ray absorption fine structure (EXAFS)and DFT calculations.42

The effect of reduction temperatures (400 and 550 °C) oncatalytic performance was evaluated using NiGa/α-Al2O3catalyst calcined at 550 °C. It can be seen in Figure 7A thatthe conversion of acetylene could reach 100% when thereaction temperature reached 240 °C. However, theselectivities of ethylene and ethane were very different. ForNiGa/α-Al2O3 catalyst, α′-Ni3Ga phase was easily formedwhen the catalyst was reduced at 400 °C for 2 h, while both δ-Ni5Ga3 and β-NiGa phases were formed on raising thereduction temperature to 550 °C. This is the main reason whythe selectivity to ethylene was higher when the NiGa/α-Al2O3catalyst was reduced at 550 °C as compared to the catalyst thatwas reduced at 400 °C. The result is consistent with thediscussion of Figure 2. For NiGa/α-Al2O3 catalyst calcined at550 °C, though the conversion of acetylene reached 100%when the reaction temperature reached 240 °C, the selectivityto ethylene was still higher than that of NiGa/α-Al2O3 catalystsreduced at 400 and 550 °C. This may be due to the acidity ofthe NiGa/α-Al2O3 catalysts calcined at 400 and 550 °C ascompared to the catalysts directly reduced at 400 and 550 °C.In addition, the reduction temperatures needed for directlyreduced NiGa/α-Al2O3 catalyst and precalcined NiGa/α-Al2O3

catalyst were very different. The product distribution in termsof selectivity to ethylene, ethane, and other products obtainedfrom these two catalysts was different.

Function of Bimetallic Structure and Correlationswith Reaction Pathway. Krajcı et al. introduced a theory ofsite isolation to describe the special electronic environmentsurrounding the active sites and correlation with catalyticperformance.43 On NiGa catalyst, the site isolation could beachieved through the ordered arrangement of Ga atoms in theNi lattices, which was largely because bonding between Ni andGa tends to be more covalent (p−d hybridization) than that ofmetallic Ni−Ni (d−d hybridization).44 According to theliterature reports, the improvement of ethylene selectivity onPdGa catalyst was due to the absence of Pd−Pd neighboringstructure in the crystal formation which led a decrease inavailability to adsorbed hydrogen that was responsible foroverhydrogenation. In contrast, the formation of strongcovalent bonding between the Pd and Ga was observed.45,46

Consequently, Pd hydrides were absent and segregation wasreduced. Similarly, Ni−Ga confined the movement ofhydrogen which prevented it from overhydrogenatingacetylene. This explains why the selectivity to ethylene overNiGa/α-Al2O3 catalyst is higher than that over Ni/α-Al2O3catalyst (Figures 1B and 2B). The modification of Pd surfaceby Ag led to a weaker adsorption of ethylene, and henceincreased the selectivity in the hydrogenation of acetylene.26,47

For Ni−Ga catalyst, it could be possible that the addition ofGa to Ni might facilitate the desorption of ethylene from thesurface of NiGa/α-Al2O3 catalyst, and reduced overhydroge-nation of acetylene.Based on the results of catalytic performance and character-

ization on the catalysts, the pathway of selective hydrogenationof acetylene to ethylene over NiGa/α-Al2O3 catalyst wasproposed and is illustrated in Figure 8. The acetylenehydrogenation was initiated by adding a surface hydrogen(H*), which was formed from H2 dissociation on the Nisurface, to the adsorbed acetylene on the catalyst to form vinylintermediate (*CHCH2) on the surface. The vinylintermediate then reacted with another H* to form ethylene.Due to the site isolation of Ga which lowered the adsorptionenergy of ethylene on the surface of Ni−Ga catalyst, thedesorption of ethylene readily took place.37 This led to a highselectivity toward ethylene. The results shown in Figures 1Band 2B are consistent with the above pathway analysis. Inaddition, the heat of adsorption of acetylene on Ni surface isabout 150−280 kJ/mol according to the experimentalmeasurements7,48,49 and the DFT calculations,26,50 while theheat of adsorption of H2 is only 80 kJ/mol.50 Thus, during thereaction, H2 first dissociated to form H*, and then the H*reacted with acetylene to form the vinyl intermediate on thesurface of Ni. With the further addition of dissociated H* tothe vinyl intermediate, ethylene was formed and desorbed fromthe Ni surface.

4. CONCLUSIONSThe catalytic performance in acetylene hydrogenation wascompared over a series of α-Al2O3 supported bimetalliccatalysts such as Pd−Ag, Ni−Pd, Ni−Zn, Ni−Ag, and Ni−Ga. The conversion of acetylene reached 100% by increasingthe reaction temperature up to 240 °C. However, theselectivities to ethylene and ethane over these catalysts didnot follow the same trend due to the extent of over-hydrogenation to ethane. The selectivity to ethylene over

Figure 6. XPS for Ni−Ga/α-Al2O3 catalysts.

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traditional Pd−Ag catalyst was almost higher than 80% underthe reaction conditions tested. Nonprecious bimetallic NiGabased catalysts exhibited higher ethylene selectivity ascompared with other nonprecious metal catalysts such asNi−Pd, Ni−Zn, and Ni−Ag based catalysts, and could reach80% at 280 °C. The selectivity to ethane over NiGa basedcatalyst was lower than 10% from 80 to 280 °C. Comparedwith Ni5Ga3 and Ni3Ga based catalysts, the NiGa basedcatalyst also exhibited higher ethylene selectivity and lower

ethane selectivity. It could be possible that the presence of Gain Ni−Ga lattice confined the movement of hydrogen,preventing overhydrogenation of acetylene. It could bepossible that NiGa reduced ethylene adsorption bindingenergy on the surface, facilitating desorption and avoidingoverhydrogenation.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: john.hu@mail.wvu.edu.ORCIDQingyuan Li: 0000-0002-4245-7370Jianli Hu: 0000-0003-3857-861XNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSH Quest Vanguard, Inc., is a privately held technologycompany, based in Pittsburgh, PA, focused on the develop-ment and commercialization of novel hydrocarbon conversiontechnologies utilizing microwave plasma. This material is basedon work supported by the U.S. Department of Energy, Office

Figure 7. Different preparation processes of Ni−Ga based catalysts for C2H2 selective hydrogenation. (5) NiGa/α-Al2O3 reduced at 550 °C for 2 h;(6) NiGa/α-Al2O3 reduced at 400 °C for 2 h; (8) NiGa/α-Al2O3 calcined at 550 °C for 4 h and then reduced at 550 °C for 2 h; (9) NiGa/α-Al2O3calcined at 550 °C for 4 h and then reduced at 400 °C for 2 h.

Figure 8. Pathway of acetylene hydrogenation to ethylene over Ni−Ga catalyst.

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of Science, through Subaward Agreement No. 58495899 withH Quest Vanguard, Inc., under the Prime Award DE-SC0017227 Phase II SBIR.

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