Performance of Cu-Alloyed Pd Single-Atom Catalyst for

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Performance of Cu Alloyed Pd Single-Atom Catalyst for SemiHydrogenation of Acetylene under Simulated Front-End Conditions Guang Xian Pei, Xiaoyan Liu, Xiaofeng Yang, Leilei Zhang, Aiqin Wang, Lin Li, Hua Wang, Xiaodong Wang, and Tao Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03293 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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Performance of Cu Alloyed Pd Single-Atom Catalyst for Semi-Hydrogenation of Acetylene under Simulated Front-End Conditions Guang Xian Pei,†,‡ Xiao Yan Liu,*,† Xiaofeng Yang,† Leilei Zhang,† Aiqin Wang,† Lin Li,†Hua Wang,† Xiaodong Wang,† and Tao Zhang*,† †

State Key Laboratory of Catalysis, iChEM (Collaborative Innovation Center of Chemistry for

Energy Materials), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China. ‡

University of Chinese Academy of Sciences, Beijing 100049, China.

ABSTRACT. Selective hydrogenation of acetylene to ethylene is an industrially important reaction. The Pd-based catalysts have been proved to be efficient for the acetylene conversion, while enhancing the selectivity to ethylene is challenging. Here, we chose Cu as the partner of Pd, fabricated alloyed Pd single-atom catalyst (SAC) and investigated its catalytic performance for the selective hydrogenation of acetylene to ethylene under simulated front-end hydrogenation process in industry, that is, with high concentration of hydrogen and ethylene. The Cu alloyed Pd SAC showed ~85% selectivity to ethylene and 100% acetylene elimination. Compared with the Au or Ag alloyed Pd SAC, the conversion of the Cu alloyed analog exceeded both of them while the selectivity rivaled that of the Ag alloyed Pd SAC and surpassed that of the Au alloyed Pd

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SAC. As Cu is a low-cost metal, Cu alloyed Pd SAC will minimize the noble metal usage and possess high utilization potential for industry. The Cu alloyed Pd SAC was verified by EXAFS, with the Pd/Cu atomic ratio lowered to 0.006, corresponding to the loading of Pd at 494 ppm. The microcalorimetric measurement results demonstrated that the adsorption of C2H4 over the Cu alloyed Pd SAC was weaker than that over the catalyst with large Pd ensembles, thus the selectivity to ethylene was greatly enhanced. At the same time, the adsorption of H2 was stronger than that over the corresponding monometallic Cu catalyst, thus the activation of H2 was obviously promoted. Based on the above results, a possible reaction path over the Cu alloyed Pd SAC was proposed. Furthermore, by systematical comparison of the IB metal alloyed Pd SACs, we found that the apparent activation energies of the IB metal alloyed Pd SACs were close to each other, indicating similar active sites and/or catalytic mechanisms over the three catalysts. The isolation of the Pd atoms by the IB metal contributed to both of the conversion and selectivity distinctly. Further DFT calculation results suggested that the electron transfer between the IB metal and Pd might be responsible for their different selectivity to ethylene. KEYWORDS. Cu · Pd single atom · XAS · acetylene hydrogenation · excess ethylene · microcalorimetry · H2 adsorption · IB metal 1. INTRODUCTION Supported noble metal nanocatalysts are widely used in a variety of important reactions1-2. In most cases, only the atoms on the surface of the nanoparticles act as the active sites, while those inside the nanoparticles are spectators which leads to a waste of the noble metal. Synthesizing highly dispersed noble metals will improve their atomic efficiency. The maximum limit is downsizing the size of metal particles to single atoms, which will maximize the utilization of noble metals and lower the cost of the catalysts3-4. Therefore, preparation of SACs is highly


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desirable. However, when the particle size decreases, the surface energy will increase correspondingly, which leads to aggregation of the highly dispersed metal atoms4. Recently, several methods have been developed to stabilize the single atom, for example, anchoring it on the surface of the support5-7, inserting it into the framework of the support8-10 or alloying it with other metal to form the alloyed SAC11-13. The last one is one of the extreme cases for bimetallic catalysts, in which one metal is completely isolated by the second one. Due to the synergistic effect between the two metals, the alloyed SAC exhibits different geometric and electronic structure from the single atom which was anchored on the support, and may induce excellent catalytic performance for various reactions11-15. Sykes and co-workers did a series of important work by alloying the single atoms of one metal with the other one in both model and real catalytic systems13-17. They found that when alloyed the Pd single atom into the Cu (111) surface, it was facile in H2 dissociation and spillover of hydrogen atoms onto the near Cu surface, rendering its performance for selective hydrogenation of acetylene and styrene better than those of the pure Cu and Pd systems13. By using the galvanic replacement method, they synthesized Cu alloyed Pd single atom supported on Al2O3 and found it showed both high conversion and styrene selectivity for selective hydrogenation of phenylacetylene14. Besides, the alloyed Pt single atoms with Cu obtained by the same method were also reported to be highly active and selective for the hydrogenation of butadiene to butenes15. These work indicated that the alloyed SAC might be one of the most promising candidates for selective hydrogenation reactions. Partial hydrogenation of acetylene is an industrially important reaction. As small amounts of acetylene (~1 %) contained in the ethylene stream are poisonous to the catalysts for ethylene polymerization, it must be diminished to an acceptable level18-20. Front-end and/or tail-end configurations are involved in the elimination of acetylene21-23. The front-end process contains


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excess hydrogen in the feed gas, while stoichiometric amount of hydrogen is dosed in the tailend process with purified gas composition. Thus, to achieve simultaneously high conversion and ethylene selectivity is much more challenging under the front-end conditions than that of the tailend process21-23. Palladium-based catalysts with promoters are chosen for both processes24-27. However, in spite of high acetylene conversion, they suffer from low selectivity to ethylene. In addition, most of the promoters block the surface of Pd nanoparticles, which induces a waste of the noble metal27-28. Therefore, it is highly desirable to design novel catalysts to get rid of these deficiencies and achieve high conversion and selectivity simultaneously. In our previous work29-30, silica supported Au or Ag alloyed Pd SAC was prepared and utilized for semi-hydrogenation of acetylene simulating the industrial front-end conditions with both high ethylene and hydrogen concentration21-23. We found that compared with the monometallic Pd catalyst, the selectivity to ethylene was significantly improved over the Au or Ag alloyed Pd SAC while maintaining high acetylene conversion (~90%). Especially for the Ag alloyed Pd SAC, ~80% ethylene selectivity could be achieved and remain unchanged within a wide temperature window.30 Although the alloyed Pd SACs were formed in these two systems, the prices of Au and Ag were relatively high. As another IB element, Cu exhibits similar nature to Au and Ag while it is cost-effective; this makes it a good candidate for synthesizing alloyed Pd SAC and exploring the potential application for the selective hydrogenation of acetylene. Furthermore, systematic investigation of the geometric and electronic effect of the IB metal alloyed Pd SACs will give new insights into the origin of their good catalytic performance. Therefore, it is necessary to investigate the Cu alloyed Pd SAC for the selective hydrogenation of acetylene.


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Herein, with only ppm levels of Pd addition, we synthesized Cu alloyed Pd SAC supported on silica gel and tested it for the semi-hydrogenation of acetylene under conditions approximate to the front-end hydrogenation process. The results showed that the Cu alloyed Pd SAC could attain ~85% ethylene selectivity with 100% acetylene conversion. Even though the content of Pd was very low, we succeeded in providing strong evidence for the formation of Cu alloyed Pd SAC by comprehensive characterization with EXAFS, in-situ FT-IR and microcalorimetry. Based on the work we have done on the IB metal alloyed Pd SACs, the origin of the good catalytic performance for semi-hydrogenation of acetylene was systematically discussed. 2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A series of CuPd/SiO2 catalysts with different Pd/Cu atomic ratios were prepared by incipient wetness co-impregnation method. For the first step, the silica gel (Provided by Qingdao Ocean Chemical Plant with a Brunauer-Emmett-Teller (BET) surface area of 463 m2·g-1) was impregnated with a mixture of Pd(NO3)2 and Cu(NO3)2·3H2O solution to insure a nominal Cu loading of 5 wt% and varied Pd/Cu atomic ratios. All the samples were subsequently dried at 80 °C for 10 h before calcination in air at 400 °C for 2 h. The as-prepared catalysts are denoted as CuPdx/SiO2 with x referring to the Pd/Cu atomic ratio. For comparison, monometallic Pd0.006/SiO2 catalyst with the same nominal Pd loading as CuPd0.006/SiO2 was also prepared by using the same method (Table 1). 2.2. Catalyst Characterization. Actual metal loadings of all the as-prepared catalysts were analyzed by inductively coupled plasma spectrometry (ICP-AES) on a Thermo IRIS Intrepid II XSP instrument. The results were shown in Table 1.


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Temperature-programmed reduction (TPR) experiments were undertaken on a Micromeritics AutoChem II 2920 automated characterization system. First of all, approximately 170 mg of the calcined sample was loaded into a U-shaped quartz tube, after pretreating with Ar for 10 min, the gas flow was switched to 10 vol% H2/Ar, and the sample was heated to 500 °C at 10 °C·min-1. X-ray diffraction (XRD) patterns were recorded on a PW3040/60 X’ Pert Pro Super (PANalytical) diffractometer operating at 40 kV and 40 mA equipped with a Cu Kα radiation source (λ= 0.15432 nm). The scanning angle (2θ) was 10°-80°. Fourier-Transform Infrared Spectra (FT-IR) were recorded on a Bruker EQUINOX 55 infrared spectrometer with a DTGS detector. Prior to CO chemisorption, the as-prepared samples were pretreated at 250 °C under flowing 80 vol% H2/He (20 mL·min-1) for 1 h, followed by evacuation at the same temperature for 0.5 h. After being cooled to room temperature, the background spectrum was collected and subtracted automatically from the subsequent spectra. CO adsorption experiments were undertaken sequentially on a single sample. The spectra were collected after introduction of CO, followed by evacuation for 5 min. The probe gas CO was puriﬁed with a liquid nitrogen trap. X-ray adsorption spectroscopy (XAS) including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra at Pd K-edge were collected at the BL14W1 at Shanghai Synchrotron Radiation Facility (SSRF), Shanghai institute of applied physics (SINAP), China. A double Si (311)-crystal monochromator was employed for energy selection. Energy calibration was undertaken by using Pd foil. The spectra were recorded at room temperature under the fluorescence mode with a solid state detector. Prior to the experiments, the samples were reduced at 250 oC for 1 h then purged with He for 10 min. After cooling to room temperature, the reactor was evacuated and transferred to the glove box. Then,


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all the samples were sealed in Kapton films in the glove box. Athena software package were used for the data analysis. Microcalorimetric measurements of C2H4 and H2 adsorption were undertaken at 40 °C with a BT 2.15 heat-flux calorimeter connected with a gas handling system. A volumetric system equipped with MKS 698A baratron capacitance manometers was used to guarantee a precise pressure measurement (±1.33×10-2 Pa).31 Prior to the measurements, the samples were pretreated at 250 oC under flowing H2 for 1 h and evacuated for another 0.5 h at the same temperature to remove adsorbed H2, then cooled to room temperature in vacuo, and isolated subsequently by refilling with He and sealing in thin quartz tube. The sealed quartz tubes were placed into the calorimetric cell which is immersed within the isothermal calorimetric block. 2.3. Catalytic Performance Evaluation. Selective hydrogenation of acetylene in excess ethylene was carried out in a quartz reactor. Prior to the experiments, 30 mg of catalysts were pretreated with flowing 80 vol% H2/He (20 mL·min-1) at 250 oC for 1 h followed by purging with He (20 mL·min-1) at the same temperature. After cooling to room temperature, a gas mixture with a space velocity of 60,000 mL·h-1·g-1 was introduced into the reactor, simulating the frontend hydrogenation conditions with 1.0 vol% C2H2, 20.0 vol% H2 and 20.0 vol% C2H4 balanced with He. The gas purities were as follows: H2 (UHP, 99.999%), He (UHP, 99.999%), C2H2 and C2H4 (mixture of 4.76 vol.% C2H2 in C2H4 with 50 ppm C2H6 impurity from Dalian Special Gases Co., LTD.). Mass ﬂow controllers were used to control all gas flows. The reaction temperature was held constant for 25 min before ramping to the next temperature point. The gas components from the microreactor outlet were analyzed by on-line gas chromatography (Agilent Technologies 6890N) equipped with a FID detector.


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According to our former study2930, 32-34, C2H4 and C2H6 were the only C2 products detected by GC. The oligomers formed during the hydrogenation process could be ignored due to the short contact time. The conversion and selectivity were calculated as follows: Conversion =

C2H2(feed) − C2H2 × 100% C2H2(feed)

 C2H6 − C2H6(feed)   × 100% Selectivity = 1 − C2H2(feed) − C2H2  

(1)

(2)

The apparent activation energies were determined on the same reaction system between 130 and 180 °C with the same feed gas composition as the above activity evaluations. To maintain acetylene conversion < 15 %, the gas flow rates were varied from 60 mL·min-1 to 150 mL·min-1. 2.4. Density Functional Theory (DFT) Calculation. The relativistic density functional theory calculations were performed with the Vienna Ab-initio Simulation Package (VASP) .35-38 The projector augmented wave (PAW) method with a kinetic energy cut-off of 500 eV was employed for representing the core and valence electrons. In the calculations, the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was used.39 The surface Brillouin zone was sampled by a Monkhorst-Pack grid of size of 3×3×1. By minimizing the forces on the atoms below 0.02 eV/Å, ground-state atomic geometries were obtained. According to the experimental characterization results, the Pd single atom alloyed into the (111) surfaces of the IB metal was used to model the catalyst structure. The bulk structures with the optimised interatomic distances of Cu-Cu (2.576 Å), Ag-Ag (2.933 Å), Au-Au (2.939 Å), and Pd-Pd (2.788 Å) were used for surface construction. A four slabs of a 3×3 unit cell and the top two layers allowed relaxing with a 15 Å vacuum layer was used to model the (111) surfaces for ethylene adsorption and bader analysis 40-41.


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3. RESULTS AND DISCUSSION 3.1. Formation of Cu Alloyed Pd SAC. 3.1.1. TPR and XRD. H2-TPR profiles for all the as-prepared catalysts are presented in Figure 1. For monometallic Cu/SiO2 catalyst, only one reduction peak around 310 oC appeared, indicating most of the copper oxide existed in similar mode42. With ppm levels of Pd addition (Table 1), the peak shifted to lower temperatures (between 200 oC and 300 oC), indicating the reduction of copper oxide was promoted, which may be resulted from the spillover of hydrogen from Pd to the neighboring CuO43-44. Accompanied by this, a new peak between 100-150 oC appeared for the CuPd0.015/SiO2 and CuPd0.025/SiO2 catalysts, which could be attributed to the reduction of PdO45-47. These peaks shifted towards lower temperatures with increasing Pd content, indicating interaction existed between Cu and Pd. The XRD patterns for all the reduced samples showed diffraction peaks at 43.3o, 50.4o and 74.1o (Figure S1), corresponding to the crystal planes of Cu (111), (200) and (220), respectively. Due to the high dispersion or low content of Pd, there was no signal for palladium species. The particle sizes estimated by the Scherrer equation demonstrated that the addition of only ppm levels of Pd could decrease the sizes of the bimetallic particles obviously (Table 1), which also suggested that there was interaction between Pd and Cu. Table 1. Elemental analysis of CuPd/SiO2 catalysts. Entry

1 2 3 4

Catalysts

Cu/SiO2 CuPd0.006/SiO2 CuPd0.015/SiO2 CuPd0.025/SiO2

Nominal loading Cu:Pd (atomic ratio) -160 65 40

Pd (ppm) -523 1288 2093

Actual loadinga Cu (wt%) 5.28 4.96 5.19 4.80


Pd (ppm) -494 1097 2287

Particle sizeb (nm) 42.7 40.7 38.9 28.5

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5 Pd0.006/SiO2 -523 -448 Determined by ICP-AES; b Estimated by Scherrer equation: D = k·λ/(β·cosθ), according to the XRD patterns.

--

a

0.1

a

TCD signal (a.u.)

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b c d 100

200

300

o

400

500

Temperature ( C)

Figure 1. TPR profiles for the catalysts with different Pd/Cu atomic ratios: (a) Cu/SiO2, (b) CuPd0.006/SiO2, (c) CuPd0.015/SiO2, (d) CuPd0.025/SiO2 catalysts. 3.1.2. In-situ FT-IR. To explore the surface configuration of the CuPd/SiO2 catalysts, in-situ FT-IR coupled with CO adsorption was carried out (Figure 2). For CO adsorption on the Pd0.006/SiO2 catalyst (with the same Pd content as CuPd0.006/SiO2, Table 1), only one adsorption band at 1870 cm-1 appeared, which could be ascribed to the tricoordinated CO adsorbed on threefold Pd, indicating large Pd ensembles existed in this catalyst48. However, when comes to the bimetallic system, there was no signal for the adsorption of CO on Pd over both of the CuPd0.006/SiO2 and CuPd0.025/SiO2 catalysts. Similar to the Cu/SiO2 catalyst, only one band at 2133 cm-1 existed. This result indicated that the Pd atoms were isolated by Cu and/or the concentration on the surface of the catalysts was too low to be detected by in-situ FT-IR via CO adsorption. Consistent with our previous AuPd/SiO2 and AgPd/SiO2 systems, with decreasing Pd


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concentration, the Pd atoms were gradually isolated by Au or Ag and only linearly or no CO adsorption band could be detected29-30.

Figure 2. FT-IR spectra of CO adsorption over the (a) Cu/SiO2, (b) CuPd0.006/SiO2, (c) CuPd0.025/SiO2 and (d) Pd0.006/SiO2 catalysts. 3.1.3. XAS (XANES and EXAFS). Due to the low Pd content of the CuPd/SiO2 bimetallic catalysts, it is difficult to probe the state of Pd by in-situ FT-IR via CO adsorption. XAS is a unique tool for studying the local structure around selected element at the atomic scale even with low element content49-51. Herein, XAS was employed to analyze the chemical environment of Pd in our catalysts. The k3-weighted EXAFS spectra at the Pd K-edge for the Pd0.006/SiO2, CuPd/SiO2 and Pd foil were shown in Figure 3. The oscillation manners of Pd K-edge in the k-space for the CuPd/SiO2 bimetallic catalysts were different from that of the Pd foil (Figure3a, c, e, g) due to disturb of the Cu neighbours. The Fourier transforms of the EXAFS spectra in the R-space (Figure3b, d, f, h)


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showed that the distance in the first shell was shorter than that of the Pd foil, suggesting the formation of Pd-Cu alloy. From the data fitting results in Table 2, we can see that for the monometallic Pd0.006/SiO2 catalyst, both Pd-O and Pd-Pd coordination existed with the coordination number (CN) of 1.5 and 6.3, respectively. As can be seen from the TPR profiles, PdO would be reduced completely after reduction at 250 oC. The residual Pd-O bonds may be resulted from the strong interaction between the palladium and the silica support at the interfaces52. For the bimetallic catalysts with relatively high Pd content (CuPd0.015/SiO2), both Pd-Cu and Pd-Pd bonds existed with the CN of 9.8 and 1.5, respectively. Furthermore, only PdCu bond existed in the CuPd0.006/SiO2 catalyst, indicating complete isolation of Pd atoms by Cu. The fitted results were in good agreement with the experimental spectra (Figure 3 and Figure S2), confirming the formation of Cu alloyed Pd SAC for the CuPd0.006/SiO2 catalyst. For the CuPd0.006/SiO2 and CuPd0.015/SiO2 catalysts, neither Pd-O nor Pd-Si coordination was detected, suggesting that there was no Pd loaded onto the silica support. This could be ascribed to the facile formation of CuPd alloy.53 Table 2 EXAFS data fitting results at Pd K-edge for the Pd0.006/SiO2, CuPd0.015/SiO2 and CuPd0.006/SiO2 catalysts. CNa

R(Å)b

σ2×103 (Å2)c

R-factor ΔE0 d (%) (eV) Pd foil Pd-Pd 12 2.74 5.3 1.7 0.33 Pd0.006/SiO2 Pd-O 1.5 2.01 6.8 -4.8 0.28 Pd-Pd 6.3 2.73 7.0 -4.8 CuPd0.015/SiO2 Pd-Cu 9.8 2.58 7.2 2.0 0.46 Pd-Pd 1.5 2.69 7.2 2.0 CuPd0.006/SiO2 Pd-Cu 11.6 2.58 6.2 2.1 0.31 a b Coordination number for the absorber-backscatterer pair; Average absorber-backscatterer Samples

Shell

distance; cDebye-Waller factor;

d

Inner potential correction. The accuracies of the above

parameters were estimated as CN, ±20%; R, ±1%; σ2, ±20%; ∆E0, ±20%. The data range used for data fitting in k-space (∆k) and R-space (∆R) are 3.0-13.1 Å-1 and 1.3-3.1 Å, respectively.


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32

(a)

3

-4

16

Chi*k

(b)

Pd foil Fit

FT Magnitude (Å )

24

8

-16

8

0 12

10

(c)

(d) Pd0.006/SiO2

-4

FT Magnitude (Å )

Pd0.006/SiO2 Fit

3

5

Chi*k

Pd foil Fit

24 16

0 -8

0 -5

8

Fit

4

0

(e)

(f )

CuPd0.015/SiO2

-4

FT Magnitude (Å )

-10 20

Fit

Chi*k

3

10

0

CuPd0.015/SiO2

20

Fit 10

-10 24

0 30

CuPd0.006/SiO2

(h) CuPd0.006/SiO2

FT Magnitude (Å )

(g)

-4

Fit 12

Fit

20

3

Chi*k

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0

10

-12

0 4

6

8 -1 k (Å )

10

12

14

0

1

2

3 R (Å)

4

5

6

Figure. 3 The k3-weighted (a, c, e, g) EXAFS spectra in the k-space and (b, d, f, h) the corresponding Fourier transform spectra (without phase correction) in R-space of (a, b) Pd foil, (c, d) Pd0.006/SiO2, (e, f) CuPd0.015/SiO2 and (g, h) CuPd0.006/SiO2 catalysts. In Figure 4, we compared the normalized XANES spectra at Pd K-edge of the Pd0.006/SiO2, CuPd0.015/SiO2 and CuPd0.006/SiO2 catalysts with that of Pd foil. After reduction at 250 oC, the adsorption edge for the Pd0.006/SiO2 catalyst was at 24355.8 eV, which was higher than that of Pd foil (24350.0 eV), that is, part of Pd was in oxidized state. Consistent with the data fitting results


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in Table 2, there was Pd-O bond in the Pd0.006/SiO2 catalyst. The adsorption edges for the CuPd0.015/SiO2 and CuPd0.006/SiO2 catalysts were at 24348.0 eV and 24348.1 eV, respectively, which were lower than that of the Pd foil, indicating that palladium in these two catalysts was slightly negatively charged54. This was in accordance with the XPS results reported by Venezia et al., in which they also demonstrated that the Pd was negatively charged when it was alloyed with Cu on the pumice support55.

Normalized Absorption (a.u.)

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Pd foil Pd0.006/SiO2

24330

24340

CuPd0.015/SiO2 CuPd0.006/SiO2

24350

24360

24370

24380

Energy (eV) Figure. 4 Normalized XANES spectra at Pd K-edge of the Pd foil, the Pd0.006/SiO2, CuPd0.015/SiO2 and CuPd0.006/SiO2 catalysts. 3.2. Catalytic Performance. Selective hydrogenation of acetylene simulating the front-end conditions was employed for the activity tests. The catalytic performances of the CuPd/SiO2 and monometallic Cu/SiO2 catalysts are shown in Figure 5. We can see that as a function of reaction temperature, the selectivity to ethylene was extremely high over the Cu/SiO2 catalyst, whereas the acetylene conversion was negligible until 200 oC. This might be caused by the large


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activation barrier of H2 over Cu, as previously reported that high temperature was needed to induce the hydrogenation reaction over the Cu/SiO2 catalyst56-58. With the addition of ppm levels of Pd (Table 1), the catalytic performances of the CuPd/SiO2 catalysts at low temperature were enhanced dramatically. Total acetylene conversion could be achieved at 120 oC over the CuPd0.015/SiO2 and CuPd0.025/SiO2 catalysts, but the selectivity to ethylene was lower than -80%. With increasing reaction temperature, their selectivity decreased further to ~-200%. In accordance with most of the previous results, the ethylene in the feed gas was also hydrogenated to ethane, leading to a waste of the feed gas59-61. To our delight, when comes to the Cu alloyed Pd SAC (CuPd0.006/SiO2), total acetylene conversion could be achieved at 160 oC with ~85% selectivity to ethylene. Compared with the CuPd0.025/SiO2 and CuPd0.015/SiO2 catalysts, though the reaction temperature to achieve total acetylene elimination over the CuPd0.006/SiO2 catalyst was high (160 vs. 120 oC), its ethylene selectivity was much more excellent than those of the CuPd0.025/SiO2 and CuPd0.015/SiO2 catalysts (Figure 5b). This may be attributed to the increased dispersion of the Pd active sites.62-64 When the reaction temperature was further increased, the ethylene selectivity over the CuPd0.006/SiO2 catalyst could maintain at ~80% with 100% acetylene conversion (Figure 5). After increasing the gas velocity to 240,000 mL·h-1·g-1, the stability test was undertaken over the CuPd0.006/SiO2 catalyst at 160 oC (Figure S3), its conversion and selectivity remained almost unchanged within 24 h. As we summarized previously30, in the recent reports, Pd catalysts with the modification of another element were used for the elimination of acetylene in ethylene-rich stream. For these systems, high acetylene conversion was always accompanied by low ethylene selectivity, especially with the increased temperature, large amount of ethylene would be hydrogenated to ethane. In contrast, in our present work, both high acetylene conversion and ethylene selectivity were attained


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simultaneously within wide temperature window, which effectively restrained the waste of the ethylene in the feed gas.

(a)

Conversion (%)

100 80

CuPd0.025/SiO2 CuPd0.015/SiO2 CuPd0.006/SiO2

60

Cu/SiO2

40 20 0 40

80

120

160

o

200

240

Temperature ( C)

(b) 100 95 90

Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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85 80 75

-100 CuPd0.025/SiO2

-150

CuPd0.015/SiO2 CuPd0.006/SiO2

-200

Cu/SiO2

-250 40

80

120

160o

200

240

Temperature ( C) Figure 5. (a) Acetylene conversion and (b) ethylene selectivity as a function of reaction temperature over the CuPd0.025/SiO2, CuPd0.015/SiO2, CuPd0.006/SiO2 and Cu/SiO2 catalysts. To better understand the relationship between the structures of the catalysts and the catalytic performances, the influence of the Pd/Cu atomic ratios on the conversion and selectivity of the


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catalysts at 160 oC were summarized in Figure 6. It showed that over the CuPd/SiO2 catalysts, the conversion of acetylene could attain 100%. As indicated by the XAS results, with decreasing Pd/Cu atomic ratios, the Pd atoms were better isolated by Cu. Correspondingly, the selectivity to ethylene increased greatly, from lower than -200% to ~85%.

100

100

90 80 80 60

70

40 20

-200

C2H4 Selectivity (%)

C2H2 Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 0.000

0.005

0.010

0.015

0.020

0.025

-250 0.030

Pd/Cu atomic ratio

Figure 6. Acetylene conversion (Orange: solid) and ethylene selectivity (Green: half down) at 160oC as a function of the Pd/Cu atomic ratio. The ethylene selectivity over the Cu alloyed Pd SAC was higher than that of the Au alloyed Pd SAC (AuPd0.025/SiO2), over which the selectivity decreased obviously with increasing reaction temperature (Figure S4)29. Moreover, though the wide temperature window of the high ethylene selectivity over Cu alloyed Pd SAC was similar to that of the Ag alloyed Pd SAC (AgPd0.01/SiO2)30, Cu alloyed Pd SAC displayed complete acetylene elimination. The catalytic performances at 160 oC over the Cu/Ag/Au alloyed Pd SACs were summarized in Figure 7. Over the Cu alloyed Pd SAC, 85% ethylene selectivity was attained with 100% acetylene conversion. The Ag alloyed Pd SAC showed 80% selectivity to ethylene when the acetylene conversion


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attained ~90%. While only ~25% selectivity to ethylene could be gained at similar acetylene conversion over the Au alloyed Pd SAC. 100

Conv. Sel.

80

60

(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

20

0 CuPd0.006/SiO2

AgPd0.01/SiO2

AuPd0.025/SiO2

Figure 7. Acetylene conversion and ethylene selectivity at 160oC over the CuPd0.006/SiO2, AgPd0.01/SiO2 and AuPd0.025/SiO2 catalysts. 3.3. Microcalorimetry of C2H4 and H2 Adsorption. To get insight into the origin of the catalytic performance, microcalorimetric measurements were undertaken over the CuPd0.025/SiO2, CuPd0.006/SiO2 and Cu/SiO2 catalysts. As ethylene is the target product for the selective hydrogenation of acetylene, the adsorption of ethylene will influence the selectivity to ethylene primarily. From Figure 8a, we can see that the initial adsorption heats of C2H4 over the CuPd0.025/SiO2, CuPd0.006/SiO2 and Cu/SiO2 catalysts were 94.9 kJ·mol-1, 71.9 kJ·mol-1 and 59.5 kJ·mol-1, respectively. That is to say, the C2H4 adsorption strength decreased with decreasing Pd content, which led to high selectivity over the CuPd/SiO2 catalysts with low loading of Pd (Figure 5b). This is in accordance to our previous study29-30. Generally, three modes for C2H4


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adsorption existed: ethylidyne, di-σ bonded and π-bonded species65-67. The ethylidyne species with very high initial adsorption heat (185 kJ·mol-1) was resulted from the dissociation adsorption of C2H4 on the three-fold adjacent metal atoms29,65, which will easily induce the over hydrogenation of acetylene to ethane. For the di-σ and π-bonded adsorption species, the C2H4 was attached primarily on two adjacent and single metal atom sites, respectively66-67. As was reported by Li et al., π-bonded adsorption of C2H4 exhibited the lowest initial adsorption heat (74 kJ·mol-1) among these three adsorption modes65. Our results demonstrated that no ethylidyne adsorbed C2H4 existed over all of these catalysts. Moreover, only π-bonded C2H4 species existed over the Cu alloyed Pd SAC with its initial adsorption heat of 71.9 kJ·mol-1, it is likely reflected the EXAFS results that the Pd atoms were completely isolated by Cu in the CuPd0.006/SiO2 catalyst. Correspondingly, the surface coverage of C2H4 over the three catalysts also decreased with decreasing Pd content. As shown in Figure 8b, the coverage of C2H4 were 7.5 and 5.0 µmol·g-1 over the CuPd0.025/SiO2 and CuPd0.006/SiO2 catalysts, respectively. Over the CuPd0.006/SiO2 and Cu/SiO2 catalysts, the surface coverage of C2H4 was similar to each other (5.0 vs. 5.4 µmol·g-1), consistent with their higher selectivity to ethylene.


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-1

Differential heat (kJmol )

(a) 100 CuPd0.025/SiO2

80

CuPd0.006/SiO2 Cu/SiO2

60

40

20

0 0

2

4

6 8 10 -1 Coverage (µmolg )

12

14

(b) 16 14 -1

Coverage (µmolg )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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12 10 8 CuPd0.025/SiO2

6

CuPd0.006/SiO2

4

Cu/SiO2

2 0 0

1

2

3 4 Pressure (Torr)

5

6

Figure 8. (a) Differential heat of C2H4 adsorption and (b) surface coverage of C2H4 over the CuPd0.025/SiO2, CuPd0.006/SiO2 and Cu/SiO2 catalysts. The dissociation of H2 on the surface of the catalysts is also very important for this reaction. Herein, microcalorimetry of H2 adsorption was employed to provide additional information related to the activation of H2. As shown in Figure 9, the initial adsorption heat of H2 over the CuPd0.025/SiO2 catalyst with relatively high Pd content was relatively high (45.3 kJ·mol-1). It may be ascribed to the chemisorption of H2 on large Pd ensembles, on which the dissociation of H2 is


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barrierless but the produced hydrogen atoms bond strongly, thus it will induce the hydrogenation of both acetylene and ethylene simultaneously13 (Figure 5b). Over the monometallic Cu/SiO2 catalyst, only physically adsorbed H2 presented with very low initial adsorption heat, indicating H2 adsorbed very weakly on the Cu/SiO2 catalyst68. Thus, it is very difficult for the activation of H2 over the monometallic Cu/SiO2 catalyst. In terms of the CuPd0.006/SiO2 catalyst, the initial adsorption heat was 37.6 kJ·mol-1, which was much higher than that of the Cu/SiO2 catalyst (Figure 9). This result suggested that, compared with the monometallic Cu/SiO2 catalyst, the Cu alloyed Pd single atoms greatly promoted the activation of H2 over the CuPd0.006/SiO2 catalyst. As was also demonstrated by Kyriakou et al., the individual and isolated Pd atoms in the Cu (111) surface could promote the dissociation of H2 and spillover onto the near Cu surfaces13, 69-70. Therefore, the Cu alloyed Pd SAC integrated the advantages of the monometallic Pd/SiO2 and Cu/SiO2 catalysts for facilitating the dissociation of H2 and weakening the adsorption of C2H4, respectively. Thus, the catalytic performance for selective hydrogenation of acetylene to ethylene under ethylene-rich stream was enhanced greatly over the Cu alloyed Pd SAC.


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60 CuPd0.025/SiO2

50

-1

)

CuPd0.006/SiO2

Differential heat (kJmol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cu/SiO2

40 30 20 10 0 0

2

4 6 -1 Coverage (µmolg )

8

10

Figure 9. Differential heat of H2 adsorption over the CuPd0.025/SiO2, CuPd0.006/SiO2 and Cu/SiO2 catalysts. From the above results, we could predict the reaction mechanism for the semi-hydrogenation of acetylene over the Cu alloyed Pd SAC, as shown in Scheme 1: (a) isolated Pd atoms in the surface of Cu promoted the dissociation of H2 and the produced H subsequently spillover onto the near Cu surface; (b) high acetylene conversion was guaranteed with the spillover of the produced H; (c) the weakly π-bonded ethylene promoted the desorption of ethylene from the surface of the catalyst, inhibiting the unselective path involving the formation of ethylidyne, thus, over hydrogenation was prevented and the selectivity to ethylene could be greatly improved. It has been reported that when forming the alloyed Pd SAC with Ag, both the dissociation of H2 and the hydrogenation of acetylene to ethylene took place over the isolated Pd atoms instead of the nearby Ag sites.71-72 This was in agreement with our previous experimental results that the monometallic Ag/SiO2 catalyst showed negligible activity for selective hydrogenation of acetylene.30 Both the DFT results and experimental results for the PdZn intermetallic catalyst with isolated Pd sites demonstrated that the hydrogenation of acetylene took place on the Pd sites


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other than the Zn surfaces.73 Thus, in our present system, Pd single atoms may play a key role not only in the dissociation of H2, but also in the hydrogenation of acetylene to ethylene. However, we didn’t exclude the reaction took place over the Cu surfaces, as the monometallic Cu/SiO2 catalyst was active when the reaction temperature was high56-58.

Scheme 1. Proposed reaction pathway for semi-hydrogenation of acetylene over silica supported Cu alloyed Pd SAC: (a) dissociation of H2 and spillover of H atoms, (b) adsorption and hydrogenation of acetylene, (c) desorption of π-bonded ethylene. 3.4. Comparison of IB Metal Alloyed Pd SACs. To investigate the intrinsic origin of the good catalytic performance over the Cu alloyed Pd SAC, the apparent activation energy for C2H2 hydrogenation over the CuPd0.006/SiO2 catalyst was tested and compared with those of the AgPd0.01/SiO2 and AuPd0.025/SiO2 catalysts, which were the Ag and Au alloyed Pd SACs we previously reported (Table S1) 29-30. As shown in Figure 10, the apparent activation energy of the CuPd0.006/SiO2 catalyst was 37.7 kJ·mol-1, similar to those of the AgPd0.01/SiO2 and AuPd0.025/SiO2 catalysts (38.2 kJ·mol-1 and 37.3 kJ·mol-1, respectively). That is, the ability for acetylene activation was almost equal over the IB metal alloyed Pd SACs, indicating that all of the three catalysts have similar active sites and/or similar catalytic mechanisms. As has been


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evidenced that the Pd atoms in three catalysts were in isolated mode, thus the IB metal may played the same role in isolating the Pd atoms. 30 25

Ea=37.7 kJ/mol

20

Ln (k)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ea=38.2 kJ/mol

15 Ea=37.3 kJ/mol

10 CuPd0.006/SiO2

5

AgPd0.01/SiO2 AuPd0.025/SiO2

0 2.15

2.20

2.25

2.30

2.35

2.40

2.45

2.50

2.55

-1

1000/T (K ) Figure 10. Arrhenius plots for C2H2 conversion over the CuPd0.006/SiO2, AgPd0.01/SiO2 and AuPd0.025/SiO2 catalysts. As was demonstrated in our previous work29 that with the similar high acetylene conversion, monometallic Pd/SiO2 catalysts showed extremely low ethylene selectivity, which could be attributed to the strong ethylene adsorption strength. After the gradual isolation of Pd atoms by IB metal, the ethylene adsorption strength weakened obviously, which corresponding to the significantly enhanced ethylene selectivity. Compared with the corresponding monometallic Pd/SiO2 catalyst, the selectivity to ethylene over the IB metal alloyed Pd SACs was increased by about 3 orders of magnitude (-853% vs. 25~85%)29-30, thus the isolation of Pd atoms contributed to the enhanced catalytic performance distinctly. Correspondingly, these three catalysts owed similar ethylene adsorption strength (Figure S5), all of which were obviously lower than that of the monometallic Pd/SiO2 catalyst29, 68. This was also verified by the DFT calculation results


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shown in Table 3: the carbon-carbon double bonds of the adsorbed ethylene were all at 1.39 Å over the Pd1Au(111), Pd1Ag(111) and Pd1Cu(111) surfaces, all of which were shorter than that of the ethylene adsorbed on the Pd(111) surface (1.45 Å). Our calculated results also showed that the ∆E for the adsorption of carbon-carbon double bond over the Pd(111) was -0.91 eV, which was almost doubled those of the Pd1Au(111), Pd1Ag(111) and Pd1Cu(111). This suggested that the isolation of Pd by the IB metal could greatly weaken the adsorption of the C=C on the surfaces of the catalysts. Thus, the geometric effect between Pd and the IB metal played an important role in promoting the selectivity to ethylene. Table 3. DFT modeling of ethylene adsorption over the Pd1Au(111), Pd1Ag(111), Pd1Cu(111) and Pd(111) surfaces. The dark green, yellow, light blue and red balls denote the Pd, Au, Ag and Cu atoms, respectively. Surfaces

Pd1Au(111)

Pd1Ag(111)

Pd1Cu(111)

Pd(111)

1.39 -0.53

1.39 -0.46

1.39 -0.51

1.45 -0.91

Adsorption of C2H4

d C=C (Å) ∆E (C2H4, eV)

Despite of the similar active sites, their catalytic performances were somewhat different. As shown in Figure 7, when the acetylene conversion was >90%, the selectivity to ethylene over the Cu or Ag alloyed Pd SAC was higher than that of the Au alloyed Pd SAC under the same reaction condition. The SiO2 supported IB metal showed negligible activity for selective hydrogenation of acetylene when the reaction temperature was ≤200 oC (Figure S6), therefore, the difference mainly came from the alloyed Pd single atoms. To illuminate the difference of the IB metal alloyed Pd single atoms, density functional theory (DFT) calculation was carried out.


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As shown in Figure 11, when forming the alloyed Pd SACs with Cu or Ag, electron transfer from Cu or Ag to Pd occurred and resulted in negatively charged Pd atoms (-0.37e and -0.22e, respectively), which is in good accordance with our XANES result in Figure 4 and the XPS result from our former Ag alloyed Pd SAC30. However, in terms of the Au alloyed Pd SAC, electron would probably transferred from Pd to Au (Figure 11). This is also consistent with the order of their electronegativity: Au (2.54) > Pd (2.20) > Ag (1.93) ~ Cu (1.90). Therefore, Pd tends to be electron-rich when alloyed with Ag or Cu. It has been proved that negatively charged Pd could enhance the ethylene selectivity41, 74-75. It seems that the higher electron density of Pd atoms within the alloy surface would repel the carbon-carbon double bond of ethylene, thus the π-bonded ethylene will bond weakly over the electron rich Pd atoms. This may account for the relatively higher ethylene selectivity of the Ag or Cu alloyed Pd SAC than that of the Au alloyed one. Therefore, the electron transfer between the IB metal and Pd might lead to the difference in the selectivity to ethylene over the corresponding alloyed Pd SACs.

Figure 11. DFT calculation results of electron states of isolated Pd atoms when alloyed into Au(111), Ag(111) and Cu(111) surfaces.


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For another way, the acetylene conversion over the IB metal alloyed Pd SACs were different, complete acetylene conversion could be achieved over the Cu alloyed Pd SAC. Nevertheless, the acetylene conversion for Au or Ag alloyed Pd SAC was ~90%. The combined DFT and STEM results from the Sykes and co-workers demonstrated that isolated Pd atoms were efficient in dissociation of H213,69-70, however, the spillover of the produced H atoms to the nearby element surfaces could occur on the Cu(111) surfaces but not the Au(111) surfaces, which may contribute to the excellent catalytic performance over the Cu alloyed Pd SAC. Besides, in the PdZn intermetallic catalyst, it was proved that proper distance of the Pd atoms was responsible for high acetylene conversion73. Among the IB metal, maybe only the atomic diameter of Cu is proper to isolate the Pd single atoms with proper distance, while the atomic diameters of Au and Ag are too large to do so. Thus, the Cu alloyed Pd SAC could achieve the highest acetylene conversion among the IB metal alloyed Pd SACs. 4. CONCLUSIONS By utilizing simple incipient wetness co-impregnation method, we succeeded in synthesizing silica gel supported Cu alloyed Pd SAC, over which ~85% ethylene selectivity could be achieved with total acetylene elimination for the selective hydrogenation of acetylene simulating the frontend conditions. The isolation of Pd by Cu and the electron transfer from Cu to Pd not only promoted the dissociation of H2, but also resulted in the weak adsorption of C2H4, rendering high selectivity to ethylene at high acetylene conversion. Systematic study of the Pd SACs in this work demonstrated that the isolation of Pd atoms by the IB metal played a key role in enhancing the ethylene selectivity, while the electronic effect had relatively minor effect on their catalytic performances. The apparent activation energy of Cu alloyed Pd SAC for C2H2 hydrogenation was similar to that of Au or Ag alloyed Pd SAC, indicating similar active sites and/or catalytic


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mechanisms of the IB metal alloyed Pd SACs. This study will inspire the exploration of other alloyed SACs which are efficient for selective hydrogenation reactions. ASSOCIATED CONTENT Supporting Information. XRD patterns for all the reduced samples, time on stream activity for CuPd0.006/SiO2, ect. This material is available and free of charge at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are grateful to the financial supports provided by the National Natural Science Foundation of China (21303194, 21373206, 21476227, 21522608, 21503219 and 21573232), Youth Innovation Promotion Association of the Chinese Academy of Sciences (2014163), the Hundred Talents Program of Dalian Institute of Chemical Physics, the National Key Projects for Fundamental Research and Development of China (2016YFA0202801), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020100) and the department of science and technology of Liaoning province under contract of 2015020086-101. The authors also thank the BL14W at the SSRF for the XAS experiment.


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REFERENCES (1) Flytzani-Stephanopoulos, M. Acc. Chem. Res. 2014, 47, 783-792. (2) Takei, T.; Akita, T.; Nakamura, I.; Fujitani, T.; Okumura, M.; Okazaki, K.; Huang, J.H.; Ishida, T.; Haruta, M. In Advances in Catalysis; Gates, B. C., Jentoft, F. C., Eds.; Elsevier Academic Press Inc: San Diego, 2012; Vol. 55, pp 1-126. (3) Qiao, B. T.; Wang, A. Q.; Yang, X. F.; Allard, L. F.; Jiang, Z.; Cui, Y. T.; Liu, J. Y.; Li, J.; Zhang, T. Nat. Chem., 2011, 3, 634-641. (4) Yang, X.-F.; Wang, A.-Q.; Qiao, B.-T.; Li, J.; Liu, J.-Y.; Zhang, T. Acc. Chem. Res. 2013, 46, 1740-1748. (5) Lin, J.; Wang, A. Q.; Qiao, B. T.; Liu, X. Y.; Yang, X. F.; Wang, X. D.; Liang, J.; Li, J.; Liu, J. Y.; Zhang, T. J. Am. Chem. Soc. 2013, 135, 15314-15317. (6) Wei, H. S.; X. Y. Liu; Wang, A. Q.; Zhang, L. L.; Qiao, B. T.; Yang, X. F.; Huang, Y. Q.; Miao, S.; Liu, J. Y.; Zhang, T. Nat. Commun.2014, 5, 5634. (7) Matsubu, J. C.; Yang,V. N.; Christopher, P. J. J. Am. Chem. Soc. 2015, 137, 3076-3084. (8) Yang, M.; Li, S.; Wang, Y.; Herron, J. A.; Xu, Y.; Allard, L. F.; Lee, S.; Huang, J.; Mavrikakis, M.; Flytzani-Stephanopoulos, M. Science 2014, 346, 1498-1501. (9) Sun, S.; Zhang, G.; Gauquelin, N.; Chen, N.; Zhou, J.; Yang, S.; Chen, W.; Meng, X.; Geng, D.; Banis, M. N.; Li, R.; Ye, S.; Knights, S.; Botton, G. A.; Sham, T. K.; Sun, X. Sci. Rep. 2013, 3,1775. (10) Yan, H.; Cheng, H.; Yi, H.; Lin, Y.; Yao, T.; Wang, C.; Li, J.; Wei, S.; Lu, J. J. Am. Chem. Soc. 2015, 137, 10484-10487. (11) Zhang, H. J.; Watanabe, T.; Okumura, M.; Haruta, M.; Toshima, N. Nat. Mater., 2012, 11, 49-52.


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(12) Zhang, L.; Wang, A.; Miller, J. T.; Liu, X.; Yang, X.; Wang, W.; Li, L.; Huang, Y.; Mou, C.-Y.; Zhang, T. ACS Catal. 2014, 4, 1546-1553. (13) Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Sykes, E. C. H.; Flytzani-Stephanopoulos, M. Science 2012, 335, 1209-1212. (14) Boucher, M. B.; Zugic, B.; Cladaras, G.; Kammert, J.; Marcinkowski, M. D.; Lawton,T. J.; Sykes, E. C. H.; Flytzani-Stephanopoulos, M. Phys.Chem.Chem. Phys., 2013, 15, 12187-12196. (15) Lucci, F. R.; Liu, J.; Marcinkowski, M. D.; Yang, M.; Allard, L. F.; FlytzaniStephanopoulos, M.; Sykes, E. C. H. Nat. Commun. 2015, 6, 8550. (16) Wang, Z.-T.; Darby, M. T.; Therrien, A. J.; El-Soda, M.; Michaelides, A.; Stamatakis, M.; Sykes, E. C. H. J. Phys. Chem. C 2016, 120, 13574-13580. (17) Liu, J.; Lucci, F. R.; Yang, M.; Lee, S.; Marcinkowski, M. D.; Therrien, A. J.; Williams, C. T.; Sykes, E. C. H.; Flytzani-Stephanopoulos, M. J. Am. Chem. Soc.2016,138, 6396-6399. (18) Johnson, M. M.; Walker, D. W.; Nowack, G. P. US Patent 4404124, 1983. (19) Rahimpour, M.R.; Dehghani, O.; Gholipour, M.R.; Shokrollahi Yancheshmeh, M.S.; Seifzadeh Haghighi, S.; Shariati, A. Chem. Eng. J. 2012, 198-199, 491-502. (20) Cao, X.; Mirjalili, A.; Wheeler, J.; Xie, W.; Jang, B. W.-L. Front. Chem. Sci. Eng. 2015, 9, 442-449. (21) Blankenship, S. A.; Boyer, J. A.; Gildert, G. R. EU Patent 1773487, 2006. (22) Godínez, C.; Cabanes, A.L.; Víllora, G. Chem. Eng. Comm., 1998, 164, 225-247. (23) Godínez, C.; Cabanes, A.L.; Víllora, G. Chem. Eng. J. 1995, 34, 459-468. (24) Moses, J. M.; Weiss, A. H.; Matusek, K.; Guczi, L. J. Catal., 1984, 86, 417-426. (25) Kim, W. J.; Moon, S. H. Catal. Today 2012, 185, 2-16. (26) Borodziński, A.; Bond, G. C. Catal. Rev. Sci. Eng., 2008, 50, 379-469.


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Cu Alloyed Pd Single-Atom Catalyst 90 80 80 60

70

C2H2+H2→C2H4

40 20

-200

C2H4 Selectivity (%)

100

100

C2H2 Conversion (%)

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0 -250

0

0.006

0.015

0.025

Pd/Cu atomic ratio


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Performance of Cu-Alloyed Pd Single-Atom Catalyst for

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