Al2O3 catalyst via segregation for efficient purification of ethene feedstock

Chemical Engineering Science 210 (2019) 115216

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Nanoscale surface engineering of PdCo/Al2O3 catalyst via segregation for efficient purification of ethene feedstock Rui Ma a,b, Tianxing Yang a,b, Jianhua Sun c, Yufei He a,b,⇑, Junting Feng a,b, Jeffrey T. Miller d, Dianqing Li a,b,⇑ a

State Key Laboratory of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China Beijing Engineering Center for Hierarchical Catalysts, Beijing University of Chemical Technology, Beijing 100029, China c Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China d Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, United States b

h i g h l i g h t s
H2-induced segregation of Pd was observed in Pd-Co/Al2O3 catalysts.
Isolated-Pd site with enriched electron is constructed on Pd15Co85-700 catalyst.
The TOF of Pd15Co85-700 catalyst is 75% higher than Pd15Co85-400 catalyst.
The Pd15Co85-700 catalyst exhibits 88% ethene selectivity, 87% higher than Pd15Co85-400 catalyst.

a r t i c l e

i n f o

Article history: Received 24 December 2018 Received in revised form 1 August 2019 Accepted 7 September 2019 Available online 9 September 2019 Keywords: PdCo/Al2O3 catalyst Surface segregation Composition dependency Redispersion Acetylene selective hydrogenation

a b s t r a c t Surface engineering of bimetallic nanoparticles are crucial for harnessing the true catalytic potential. Herein, H2-induced segregation of palladium was employed to tune the surface structure of PdCo/ Al2O3 catalysts with different Pd/Co atomic ratios. The composition dependency of segregation was demonstrated by XRD, XPS, and HAADF-STEM characterization. The results from in situ CO-FTIR analysis show that Pd ensemble sites were formed on Pd75Co25/Al2O3; while, isolated Pd sites were formed on Pd15Co85/Al2O3 after 700 °C hydrogen post-treatment. The post-treated catalysts exhibited significantly increased activity for acetylene selective hydrogenation due to Pd segregation on the surface. In addition, the Pd15Co85-700 catalyst displayed 88% ethene selectivity at 100% conversion of acetylene, which was related to the redispersion of Pd and Co during the H2 treatment. Kinetic analysis provided valuable insight into the nature of the active sites resulting from the surface segregation. The Pd15Co85-700 catalyst also showed favorable stability during 100 h reaction. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Alloying precious metal with low cost 3d-transition metals, for example, Fe, Co and Ni, is a key methodology for the improvement of the intrinsic catalytic performance and the atom economy (Stamenkovic´ et al., 2006; Zhang and Fang, 2009; Peng et al., 2014; Nie et al., 2015; Hwang et al., 2012). Since catalytic reactions occur on the surface of nano-particles (NPs), the surface geometric and/or electronic structure is the most important parameter, which determines the catalytic performance of a bimetallic catalyst (Chen and Rodionov, 2016; Tao et al., 2012). Impregnation methods of preparation are commonly used to prepare catalysts on a large ⇑ Corresponding authors at: State Key Laboratory of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China. E-mail addresses: [email protected] (Y. He), [email protected] (D. Li). https://doi.org/10.1016/j.ces.2019.115216 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved.

scale. However, because of the differences in redox potential, kinetic reducibility, diffusivity and thermodynamic stability between precious and non-noble metals, precise control of the synthesis to obtain bimetallic catalysts with the optimum surface composition, structure and catalytic performance is generally not possible (Wang and Li, 2011). This then leads to inefficient catalytic performance and use of precious metal. In addition, as-prepared bimetallic catalysts also exhibit limited stability owing to the high mobility of 3d-transition metals in air, which often leads to changes in surface composition upon reduction (Kronawitter et al., 2011; Kim et al., 2012). Thus, precise control of the surface structure and composition at the nanoscale is necessary for harnessing the true catalytic potential of bimetallic catalysts. Post-treatment upon as-prepared bimetallic NPs, such as selective leaching (Cui et al., 2013) and electrochemical dealloying (Mani et al., 2008), has been studied to control surface composition

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of bimetallic catalysts. However, the removal of metal from bimetallic NPs could lead to the inefficient utilization of the metals. In 2008, Somorjai and co-workers (Tao et al., 2008) reported that the surface composition and chemical state of Rh0.5Pd0.5 bimetallic NP could undergo changes in response to reactive environment, illustrating the flexibility and tunability of the structure in bimetallic NPs. Based on the phenomenon, Sun (Zhang et al., 2017) recently tuned the surface composition of Pt-Ni catalyst by heat treatment under different reaction atmospheres and found that Pt could migrate to the surface when annealing NPs in H2; while, Ni segregated on the surface in O2. These changes in surface composition showed different CO poisoning tolerance in the order of Ni-increased > homogeneous Pt-Ni alloy > Pt-increased. These results indicate that surface segregation by post treatment procedures can be a viable strategy to control the surface composition and catalytic performance. In principal, surface segregation is dependent on a decrease of the surface free energy (Brongersma and Sparnaay, 1978; Christoffersen et al., 2002), which is affected by many factors, including size, morphology, and composition of bimetallic NPs. At present, composition dependency of segregation over bimetallic NPs is mainly investigated using Monte Carlo (MC) simulation and density functional theory (DFT) (Ramírez-Caballero et al., 2010; Han et al., 2005; Zou et al., 2017; Ramírez-Caballero et al., 2010; Peng et al., 2015; An et al., 2017), however, there are few experimental studies on the composition dependency of segregation over bimetallic NPs. Hence, more detailed experiments are necessary to obtain deeper understanding of segregation among bimetallic NPs and their influence on catalytic performance. Semi-hydrogenation of alkyne to alkene is one of the most widely studied reactions. Typically, acetylene selective hydrogenation is used to purify crude ethene containing a small amount of acetylene, which deactivates ethene polymerization catalysts (Pei et al., 2015; McCue et al., 2014; Prinz et al., 2014). While monometallic Pd is unselective and rapidly deactivates, the modification of Pd by transition metals, including Ag, Co, Ga, Ni and Cu, is commonly used to reduce active Pd ensemble size or modify Pd surface electronic structure to enhance catalytic selectivity and stability (Feng et al., 2017; Huang et al., 1998; Shin et al., 1998; Rodríguez et al., 1997). Unfortunately, the hydrogenation rate of acetylene generally decreases in the presence of transition metal due to the increasing barrier for H2 dissociation (Harris and Andersson, 1985). In addition, the ethene selectivity is also highly sensitive to the surface properties of NPs. Thus, fine control of surface composition of Pd bimetallic catalysts via surface segregation may improve the selectivity and stability without sacrificing activity. In this work, PdCo/Al2O3 catalysts with different Pd:Co atomic ratios were prepared and applied to selective hydrogenation of acetylene. In addition, the as-prepared PdCo/Al2O3 catalysts were further post-treated in H2 atmosphere at 700 °C to induce Pd surface segregation. The evolution of the surface structure, such as composition, distribution and chemical state, was characteristic by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and in situ Fourier-transformed infrared absorption spectroscopy of CO experiments. Acetylene selective hydrogenation kinetic and time on stream performance were obtained to demonstrate which compositions led to significant improvements in selectivity, activity and stability.

2. Experiment section 2.1. Materials Palladium chloride (PdCl2, 99.9%), sodium chloride (NaCl, 99%), hexahydrate cobalt chloride (CoCl26H2O, 99%) were purchased

from Aldrich and used as received. Al2O3 was purchased from Sasol Co. and calcined at 800 °C in air before use. The water used in all the experiments was deionized. 2.2. Catalyst preparation The alumina supported Pd75Co25 and Pd15Co85 catalysts (4 wt.% total metal loading, 75:25 and 15:85 wt.% of Pd:Co) were prepared via impregnation method. In a typical synthesis of Pd75Co25/Al2O3, 1 g alumina was dispersed in 100 mL deionized water. Na2PdCl4 (5.638 mL, 50 mmol/L) and CoCl26H2O (0.040 mg) were added to the dispersed alumina solution. The mixture was stirred for 2 h to allow completely loading of metal ions, followed by filtration and wash with de-ionized water. The powder was dried overnight at 90 °C, followed by heating in a tube furnace at 400 °C under flowing 10% H2/N2 for 3 h. The catalyst was denoted as Pd75Co25400. The Pd15Co85-400 catalyst was prepared by the same method except for the amount of metal precursors. For comparison, the monometallic Pd-400 and Co-400 catalyst with 4% metal loading were also prepared. To induce the surface segregation of Pd, asprepared Pd75Co25-400 and Pd15Co85-400 samples were further treated at 700 °C under pure H2 for another 3 h. The samples were donated as Pd75Co25-700 and Pd15Co85-700 catalyst. 2.3. Catalyst characterization Elemental analysis of Pd and Co was performed using a Shimadzu ICPS-7500 inductively coupled plasma atomic emission spectrometer (ICP-AES). The pore volume and surface area were determined using the Brunauer-Emmett-Teller (BET) method based on the adsorption isotherm. Temperature programmed H2reduction (H2-TPR) profiles of the PdCo/Al2O3 precursors with different metal ratios were recorded using Micrometric ChemiSorb 2750 chemisorption instrument equipped with a thermal conductivity detector. TPR was carried out with a heating ramp rate of 10 o Cmin1 in a stream of 10% H2 in Ar from 50 to 700 °C. The XRD study was carried out with a Rigaku Miniflex powder X-ray diffractometer equipped with a Cu Ka radiation source (k = 0.15418 nm) by applying a scanning rate of 0.1 deg/min in the 2h range of 30– 60°. Raman spectra were collected at 4 cm1 resolution using a HORIBA Scientific LabRAM ARAMIS Raman Microscope equipped with a 532 nm laser excitation operated at 8 mW. X-ray photoelectron spectroscopy (XPS) was conducted on an Axis Supra spectrometer (Kratos, Japan) using Al Ka radiation (1486.6 eV). The binding energy of catalysts was calibrated using the energy of the C 1 s peak (284.6 eV) as a reference. High-resolution transmission electron microscopy (HRTEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDX) were performed on a FEI Titan Themis microscope fitted with aberration-correctors for the probe forming lens and imaging lens, and a Super-X EDX system, operated at 300 kV. In situ transmitted reflectance infrared Fourier transform spectroscopy of CO chemisorption over catalysts were carried out on a Bruker Tensor 27 instrument under room temperature. CO-temperature-programmed desorption (TPD) of the catalysts was conducted in a Micrometric ChemiSorb 2920, about 0.1 g of fresh catalyst was pretreated at corresponding temperature (400 or 700 °C) for 30 min in 10% H2/N2 and then cooled to room temperature. This procedure was used to simulate the reduction treatment typically used in the preparation of catalysts to minimize structural changes during TPD measurements. Carbon monoxide and hydrogen chemisorption was performed on a Micrometric Chemisorb 2920 equipped with 50 lL loop. Thermal analysis of coke deposition on used catalysts was performed on a TG/ DTA X70 Thermogravimetric analyzer.

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2.4. Acetylene selective hydrogenation Vapor phase selective hydrogenation of acetylene was performed in a fixed-bed quartz microreactor with relative pressure 0.4 MPa. 50 mg for all catalysts and 1.35 g (1.0 mL) quartz sand (40–70 sieves) were mixed and loaded at the center of the 7 mm quartz tube. 167.5 mL/min total volume gas flow consisting of 33% C2H4/0.33% C2H2/0.66% H2/balance N2 to give a space velocity (GHSV) of 10,056 h1 and H2: C2H2 ratio of 2:1. The gas composition from the microreactor outlet was analyzed by online gas chromatography equipped with a flame ionization detector online using PLOT capillary column (0.53 m  50 mm). Before starting the reaction, the catalyst was pretreated at 150 °C for 1 h. In the test process, at least five tests for every point were executed in order to obtain reproducible values and carbon balance determined from the effluent gas was 100 ± 5%. In order to justify the accuracy/reproducibility of catalytic data, five tests for point are given to show the steady state of our measurements using GCFID (detailed discussion in Section 3.2). For calculation of H2 dependencies, partial pressures were ranged between 0.0013 (0.33%) and 0.072 (1.79%) MPa while holding PC2H2 constant at 0.0033 MPa (1%). During the measurement of turnover frequency (TOF) and activation energies (Ea), the acetylene conversion were determined using less catalyst (10–50 mg) over a relatively wide temperature range. TOF and Ea at differential conversion conditions (<15%) were then determined between 28 and 55 °C. Acetylene conversion, ethene selectivity and TOF for C2H2 conversion were calculated as follows:

C2H2 Conversion ¼

C2H4 Selectivity ¼

TOF ¼

C2H2 ðinletÞ- C2H2 ðoutletÞ C2H2 ðinletÞ C2H4 ðoutletÞ- C2H4 ðinletÞ C2H2 ðinletÞ- C2H2 ðoutletÞ

number of acetylene molecules reacted number of Pd surface sites  timeðsÞ

3. Results and discussion 3.1. The evolution of structure and composition over PdCo/Al2O3 bimetallic catalysts The metal loading of catalysts was determined by ICP-AES measurement and the results were listed in Table 1. The experimental loading for each sample is slightly lower than the nominal value (4%) but remains within the margin of experimental error. Temperature-programmed reduction experiment was performed to evaluate the reducibility of Pd and Co species. As shown in Fig. 1, the peaks in the temperature programed reduction (TPR) at 500 and 600 °C for Co/Al2O3 precursor can be assigned to the reduction of cobalt species while the negative peak below 100 °C over Pd/Al2O3 sample is corresponding to the decomposition of PdHx (Ferreira et al., 2005). The reduction of Pd and Co in the bimetallic NPs show large differences compared to the single metals. For the PdCo/Al2O3 catalysts, the reduction of Co shifts toward significantly lower temperature, which decreases with increasing

Fig. 1. H2-TPR profiles of (a) Pd/Al2O3, (b) Co/Al2O3, (c) Pd15Co85/Al2O3, and (d) Pd75Co25/Al2O3 precursors.

Pd fraction, suggesting the formation of PdCo alloys. The formation of the later inhibits the formation of PdHx, which is thought to lead to over-hydrogenation and low selectivity (Fagherazzi et al., 1995). In PdCo bimetallic catalysts, the reduced Pd favors adsorption of molecular hydrogen which is subsequently involved in the reduction of Co species (Lee et al., 2003; Wang et al., 2000). Since the same amount of sample (0.1 g) is loaded in TPR analysis, the intensity of reduction peak of Co in Pd75Co25/Al2O3 sample is much lower than that in Co/Al2O3 sample. In addition, the absence of a peak in the range of 500–600 °C suggests that no isolated Co NPs are formed in the PdCo bimetallic catalysts. The N2 adsorption-desorption curve, pore diameter, pore volume and specific surface area were determined using the BET method, and the results are showed in Fig. S1 and Table S1. From Fig. S1, the N2 adsorption-desorption isotherms of the obtained Al2O3, Pd75Co25-400, Pd75Co25-700, Pd15Co85-400 and Pd15Co85700 are all of Type IV with an obvious hysteresis loop (Bakala et al., 2008). The shape of the hysteresis loop in each case was a superposition of Types H1 and H3 (Khalfaoui et al., 2003; Shiratori et al., 2009), indicating that loading metal and posttreatment of as-prepared samples barely influence pore structure of Al2O3 support. As shown in Table S1, compared with alumina, the supported catalysts show decreased specific surface areas, pore volume and increased average pore diameter, resulting from the partial block of pore structure by the loading metal or the slight sintering during the 700 °C post-treatment. Moreover, all supported catalysts showed similar surface area and pore structure. The morphology and size distribution of the nanoparticles were determined by STEM measurements. More than 300 particles in different regions are randomly selected to measure the distribution and mean size of nanoparticles. As shown in Fig. 2b, Pd-400 exhibits inhomogeneous dispersed Pd nanoparticles with wide size distribution and an average size of 6.9 nm. When the Pd-400 sample is further treated at 700 °C under H2 atmosphere, Fig. 2d, the size of the Pd NPs increases (13.6 nm). In case of Pd75Co25-400 and

Table 1 Pd and Co content (wt.%) in catalyst. Sample

Pd (wt.%)

Co (wt.%)

Pd:Co (wt.%)

Pd:Co (at.%)

Pd-400 Pd75Co25-400 Pd15Co85-400

3.87 2.87 0.59

– 0.97 3.36

– 2.96 0.17

– 1.64 0.10

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Pd15Co85-400 bimetallic catalysts, the size of nanoparticles is mainly around 8–12 nm and the mean diameter of the Pd75Co25400 and Pd15Co85-400 NPs is 10.4 and 11.5 nm respectively (Fig. 2f and j). It can be seen that the introduction of Co leads to larger particle size that increase with increasing Co fraction consistent with previous studies (Vasquez et al., 2015; Cui et al., 2014). It should be noted that a uniform distribution of the PdCo nanoparticles on the Al2O3 without particle growth after post-treatment are observed over Pd75Co25-700 and Pd15Co85-700 (12.2 nm and 11.8 nm) in Fig. 2h and l, indicating that the PdCo bimetallic NPs possess higher thermal stability unlike mono-metallic NPs. The structures of alumina-supported PdCo NPs were determined using XRD. To trace the structural changes of the catalyst, XRD patterns were recorded using a continuous mode from 30° to 60°. As shown in Fig. 3, the Al2O3 support exhibits the characteristic reflections of c-alumina at 37°, 39°, 46°, d-alumina at 33°, 46°, 47°, and h-alumina phase at 31°, 36°, 37°, 45°. Compared to the XRD pattern of Al2O3, peaks are recorded at 40.4 and 46.9° on Pd75Co25 samples. It is noted that these two peaks are shifted to higher angles with respect to the corresponding peaks of the pure Pd (40.1 and 46.6°), indicating that Co is incorporated into the Pd fcc structure with a concomitant lattice contraction. Furthermore, the diffraction peaks of the Pd75Co25-700 sample shift to higher angle (0.1°) with comparison to Pd75Co25-400, indicating a further lattice

Fig. 3. X-ray diffraction patterns of the (a) Al2O3, (b) Pd75Co25 400, (c) Pd75Co25 700, (d) Pd15Co85 400, and (e) Pd15Co85 700 samples, with the black and red vertical line corresponding to the peak positions of pure Pd and Co, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. STEM images of Al2O3 supported (a, b) Pd-400, (c, d) Pd-700, (e, f) Pd75Co25-400, (g, h) Pd75Co25-700, (i, j) Pd15Co85-400, and (k, l) Pd15Co85-700 nanoparticles and corresponding distribution of particle size. Scale bar, 100 nm.

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contraction. Based on the Vegard’s law (Leppert et al., 2012; Li et al., 2016), the molar ratio of Pd/Co in Pd75Co25-400 sample is calculated to be 2.17, which is slightly higher than the atomic ratio of Pd/Co (1.64) determined by ICP-AES, suggesting that not all cobalt in the NPs is alloyed with palladium. The remaining is present as cobalt oxides, which are observed by Raman spectroscopy, shown in Fig. S2. After post-treatment at 700 °C in H2, the atomic ratio of Pd/Co decreases (1.69), indicating nearly all Co is incorporated into the Pd fcc structure. No peaks associated with Pd NPs are detected on Pd15Co85 sample. The state of Pd and Co was determined by XPS. As shown in Fig. 4A, for Pd75Co25-400, peaks at 777.5, 781.1 and 783.8 eV correspond to the coexistence of Co0, Co3+ and Co2+ species. For Pd15Co85-400 the dominant Co species are cationic (Biesinger et al., 2011), which may result from the relatively low reduction temperature (400 °C) and higher Co/Pd atomic ratio. The XPS result of cationic Co on Pd15Co85-400 agrees with the Raman results. After post-treating at 700 °C under H2 atmosphere, cationic Co species are reduced to metallic state (more detailed information is shown in Table S2), and the metallic Co is the dominant Cospecies.

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The Pd XPS also shows that the amount of metallic Pd increase after post-treatment (Table S2). Moreover, the binding energy of Pd0 of Pd15Co85-700 shifts to lower binding energy from 335.2 to 334.9 eV in Pd 3d5/2 compared to that of Pd15Co85-400 as well as monometallic Pd catalysts (Fig. S3). This shift in binding energy can be attributed to the electron transfer from Co to Pd. The binding energy increases as the increased amount of metallic Co increases in the bimetallic NPs (Qin et al., 2015). The change of electronic structure on Pd is not detected in Pd75Co25 possibly because of the low Co content. The weight percent of Pd and Co on the surface of PdCo NPs were calculated from the XPS compositions. Based on the energy of Al Ka X-rays (1486.6 eV) the generated photoelectrons is estimated with a mean free path of 3 nm. In Fig. 4C and D, there is an increase of Pd on the surface of PdCo NPs after the posttreatment. For example, the surface Pd/Co atomic ratio of Pd75Co25700 and Pd15Co85-700 is 2.3 and 0.12 respectively. The surface Pd/ Co ratio in Pd75Co25-700 is 32% higher than that in Pd75Co25-400, while the value is 14% in Pd15Co85. The surface Pd/Co ratio is also higher than the average composition determined by ICP-AES analysis (1.64 and 0.10). The XPS results clearly demonstrate that there

Fig. 4. The XPS spectra of (A) Co 2p region, (B) Pd 3d region, and (C), (D) comparison of XPS-determined Pd wt.% (red) and Co wt.% (green) over (a) Pd75Co25-400, (b) Pd75Co25700, (c) Pd15Co85-400, and (d) Pd15Co85-700 bimetallic catalysts. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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is a rearrangement of the elemental distribution after the posttreatment under H2 atmosphere, leading to a Pd-enriched surface in PdCo bimetallic NPs. Furthermore, the surface composition of the Pd-enriched PdCo bimetallic NPs is strongly dependent on the Pd/Co ratio in the as-prepared bimetallic NPs. This Pd surface enrichment is greater for compositions with higher Pd content, e.g., Pd75Co25. This Pd surface segregation and enrichment is driven by the higher adsorption enthalpy of H on Pd than on Co (Popova and Babenkova, 2008). The relatively large particle size of PdCo nanoparticle (>10 nm) also contributes to the atomic diffusion the segregation rate (Xiao et al., 2006). The structural evolution of Pd-Co bimetallic NPs was further investigated by HAADF-STEM combined with EDS analysis. Fig. 5a shows that the Pd75Co25-400 NPs have the lattice fringe with a spacing of 0.221 nm, which can be assigned to the (1 1 1) plane of the Pd-Co alloy (Xue et al., 2016). This alloy structure is also confirmed by the elemental line profiles of Pd and Co, shown in Fig. 5b, where Pd and Co evenly distributes in the nanoparticle. With increasing the percentage of Co, i.e., Pd15Co85-400, small patches of cobalt oxides covering the NPs can be observed, which is consistent with the results from the Raman and XPS measurements. The lattice fringes at 0.206 nm are simlar to Co (1 1 1) (0.205 nm), due to the high percentage of Co in the Pd15Co85-400 NPs. After posttreatment, Fig. 5e, the line-scan in Pd75Co25-700 shows a 1.5 nm Pd enriched surface. The corresponding EDS elemental mapping images of Pd and Co also reveals that Pd dominates the outer layers of Pd75Co25-700 NPs. For Pd15Co85-700 (Fig. 5g), the lattice constant was 0.209 nm, larger than Co (1 1 1), suggesting the incorporation of Pd to Co into these NPs. Compared to Co (0.206 nm), the

increased lattice constant, Fig. 5c, also indicates the redistribution of Pd and Co atoms after the post-treatment. The line scan, Fig. 5h, also shows enrichment of Pd on the surface of the NPs. The STEM analysis are consistent with the XPS indicates post-treatment at 700 °C in H2 leads to significant Pd surface segregation in the PdCo NPs. FTIR spectroscopy of CO chemisorption is a well established method to determine changes the geometric structure of Pd in bimetallic NPs (Collins et al., 2012; Feng et al., 2013). Fig. 6 compares FTIR spectra of CO absorbed on Pd75Co25-400, Pd75Co25700, Pd15Co85-400 and Pd15Co85-700 catalysts. CO adsorbs on Pd NPs in two modes, linear (ca. 2030 cm1) and bridge (ca. 1950 cm1) sites (McKenna et al., 2012). In Fig. S4, only bridge bonded CO is observed on Pd/Al2O3; while CO does not adsorb on Co/Al2O3. Compared to mono-metallic Pd/Al2O3, the emergence of CO linear adsorption on Pd75Co25-400 can be attributed to the introduction of Co. However, owing to the high concentration of Pd, bridge-bonded CO is still dominant in Pd75Co25-400 NPs. With the decrease of Pd/Co ratio, no obvious enhancement of linear adsorption of CO in Pd15Co85-400 catalyst is observed. Interestingly, the CO linear adsorption on Pd75Co25-700 disappears after the post-treatment. Based on the STEM result, it is suggested that Pd dominates on the surface, and only Pd ensembles exist. Post treatment of Pd15Co85-700, however, gives mostly linear bonded CO, indicating few Pd ensembles due to rearrangement of Pd and Co. This latter configuration, where the Pd atom are surrounded by Co atoms, may result from the lower activation energy for Pd surface diffusion than the bulk (Chen et al., 2012; Yang et al., 2013).

Fig. 5. HAADF-STEM images and corresponding EDS line-scan profiles of (a, b) Pd75Co25-400, (c) Pd15Co85-400, (d, e) Pd75Co25-700, and (f, g, h) Pd15Co85-700 samples.

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Fig. 6. In situ Fourier-transformed infrared absorption spectra of CO adsorption on (a) Pd75Co25-400, (b) Pd15Co85-400, (c) Pd75Co25-700, and (d) Pd15Co85-700 catalysts.

Temperature programmed desorption of CO, Fig. S5, shows that not only the composition of the surface altered, but the strength of the binding sites is also changed. Pd15Co85-400, Pd75Co25-400 and Pd15Co85-400, Figs. S5a, b and c, respectively, desorb CO at about 175 °C, while CO is desorbs below 100 °C in Pd15Co85-400 indicating weaker adsorption sites in the latter. The combined results demonstrate that post-treatment not only leads to surface segregation and redistribution of Pd, but also to new surface sites, CO adsorption located at 2055 cm1, with lower heats of adsorption, Fig. S5d. 3.2. Catalytic performance of PdCo/Al2O3 bimetallic catalysts Acetylene selective hydrogenation was performed to understand how these structural changes affect the catalytic performance. The accuracy/reproducibility of catalytic measurements are shown in Fig. S6. The concentration of ethene and acetylene are quite stable indicating steady-state performance. Under typical industrial conditions, the catalytic performance of PdCo-400 bimetallic catalysts with different Pd:Co ratio, corresponding post-treated PdCo-700 catalysts and monometallic catalysts are displayed in Fig. 7. The monometallic Co-400 and Co-700 catalysts show negligible catalytic activity at these temperatures. In Fig. 7B, the conversion of C2H2 on Pd75Co25-400 catalyst monotonically increases and reaches 100% at 95 °C. As for Pd15Co85-400 catalyst, the C2H2 conversion profile shifts toward higher temperature (100% acetylene conversion at 102 °C). It can be found that the conversion of acetylene over PdCo-400 catalysts are obviously lower than that over monometallic Pd-400 catalyst at same temperature. This phenomenon is caused by the increased barrier for H2 dissociation arising from the introduction of Co. After the post-treatment, PdCo-700 catalysts present significantly increased conversion of acetylene while the mono-metallic Pd catalyst shows slightly lower conversion due to larger particles and lower dispersion. The Pd75Co25-700 catalyst displays higher conversion than monometallic Pd catalyst even with lower Pd loading. The Pd loading in Pd15Co85-700 catalyst is only 1/6 than that in Pd-700. With similar acetylene conversions, the rate/g Pd of the former is about 6 times that of the latter. Chemisorption of CO was employed to quantify the Pd sites on the surface of catalyst. As showed in Table 2, PdCo-400 samples show small amount of CO uptake, but the values almost triple after

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the post-treatment process. Based on the corrected Pd/CO adsorption stoichiometry, the Pd15Co85-700 catalyst possesses the highest dispersion (39.7%) among the obtained catalysts. The increased CO uptake and the dispersion suggest that H2-induced surface segregation of Pd on NPs could obviously gain the number of active sites. This superior Pd dispersion is considered as one of the key reasons for the enhancement in catalytic activity. Generally, the formation of C2H4 is primary product at low conversion of C2H2; however, the product rapidly shifts towards C2H6 with the increasing conversion. In Fig. 8, monometallic Co catalysts present high ethene selectivity with quite low acetylene conversion (<5%). Pd-400 and Pd-700 show similar selectivity and only 20% ethene is obtained under the complete consumption of acetylene. The ethane selectivity of mono-metallic Pd and bimetallic PdCo catalysts are shown in Fig. S7. It is apparent that after the surface engineering Pd15Co85-700 catalyst exhibits 88% C2H4 selectivity at 100% conversion of C2H2 while Pd75Co25-700 shows slight decrease of C2H4 selectivity. The improvement of ethene selectivity on Pd15Co85-700 catalyst can be partly ascribed to the isolation of the palladium active sites. As reported, these isolated sites could lead to a weak p-bonded acetylene adsorption, and the p-bonded species are more easily desorption from Pd sites with comparison to r-bonding mode and then contribute to the increase of the ethene selectivity (Huang et al., 2007; Bond and Wells, 1966; Gislason et al., 2002; Lamberov and Egorova, 2007; Mei et al., 2009). Moreover, electronic effect is also introduced and electronic-rich Pd atoms on the surface of Pd15Co85-700 catalyst could favor desorption of ethene (Pei et al., 2015). The comparison of catalytic performance of Pd bimetallic catalysts is given in Table 3. It can be seen that our Pd15Co85-700 is among the most selective acetylene hydrogenation catalysts reported. Compared with Pd0.01Ag/SiO2, PdS4/CNF and PdZn/ZnO catalyst with high selectivity of ethene, the Pd15Co85-700 catalyst can achieve excellent C2H4 selectivity under mild reaction condition (Pei et al., 2015; McCue et al., 2016; Zhou et al., 2016). Moreover, compared to PdAg/TiO2, PdS4, and PdGa/Al2O3 catalysts (Riyapan et al., 2016; Ota et al., 2011; Osswald et al., 2008), less usage of Pd is required in our system for the complete removal of acetylene. Meanwhile, it can be seen that series of Pd-based single-atom alloy catalysts, including Pd0.01Ag/SiO2, PdCu0.006/SiO2 and Pd0.01Au/SiO2, are more efficient under high temperature and high H2/C2H2 ratio. The post-treated Pd15Co85-700 shows much 87% higher ethene selectivity compared to as-prepared catalysts Pd15Co85-400 and mono-metallic Pd demonstrating the potential for surface engineering via segregation, which may lead to improved practical catalysts. 3.3. Kinetic analysis of PdCo/Al2O3 bimetallic catalysts Kinetic analysis was undertaken to provide further understanding of the active sites. Because H2 activation plays an important role in the reaction, the reaction order of H2 over the Pd15Co85700 catalyst and Pd-700 catalyst was determined. The plots shown in Fig. 9a give good fit for reaction order of 1.23 for Pd-700 catalyst and 1.18 for Pd15Co85-700 catalyst, and the value is in agreement with that of Pd/Al2O3 catalyst (1.0–1.4) measured by Bond (Bond and Wells, 1966). This result indicates that the activation of H2 is less affected by the introduction of Co in Pd15Co85-700 catalyst and the improvement of catalytic activity of PdCo 700 catalyst can be attributed to the enrichment of Pd sites on the surface of particle, which have been shown in the XPS and HAADF-STEM analysis. Considering the different Pd content used in the catalyst tests, turnover frequency (TOF), as the intrinsic activity of the catalyst, was calculated to evaluate the catalytic performance of Pd-Co catalysts (He et al., 2012; Niquille-Röthlisberger and Prins, 2006). It is

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Fig. 7. Plots of acetylene conversion versus temperature over (A) monometallic Pd and Co catalysts and (B) PdCo bimetallic catalysts. The reaction temperature was varied from 30 to 100 °C with H2: C2H2 ratio of 2:1, space velocity (GHSV) of ca. 10,050 h1 and relative pressure of 0.4 MPa.

Table 2 The physicochemical properties of supported PdCo.

a b

Catalysts

CO uptake (lmol/g)

Dispersion (%)a

TOF (s1)b

Ea (kJ/mol)

Pd75Co25-400 Pd75Co25-700 Pd15Co85-400 Pd15Co85-700

2.8 6.9 1.1 3.1

17.6 29.1 14.0 39.7

0.035 0.029 0.077 0.135

58.5 49.9 47.9 41.1

Calculated by assuming a CO to surface Pd atom ratio of 1:1 over Pd15Co85-700 sample while 1:2 over other three catalysts. Estimated at 40 °C.

Fig. 8. Plots of ethene selectivity versus acetylene conversion on (A) monometallic Pd and Co catalysts and (B) bimetallic PdCo catalysts.

important that TOF should be based on rate measurement without influence of mass or heat transfer. Therefore, TOF from steady-state rate measurements at low conversion (<15%) was employed and the plot of acetylene conversion versus temperature to measuring TOF was presented in Fig. S8. The Weisz-Prater and Mears analysis were also performed for eliminating the effect of heat and mass transfer over Pd15Co85-700 catalyst (shown in the SI) (Weisz and Prater, 1954; Oyama et al., 2008).

The Weisz-Prater criterion,

C WP ¼

r 0 AðobsÞ qc R2 De C As

gives 3.6  102 which is much less than 1, suggesting no internal diffusion limitation.

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R. Ma et al. / Chemical Engineering Science 210 (2019) 115216 Table 3 Comparison of catalytic performance over Pd-based catalysts.

a b

H2/C2H2

GHSV (h1)

Pd usage (mg)

85 102 88

2 2 2

10,050 10,050 10,050

3.0 3.0

95 160 200 200 100 250 150 160

5 20 10 10 3.0 1.8 20 20

Catalysts

Selectivity (%)a

T (oC)

Pd15Co85-700 Pd15Co85-400 Commercial PdAg/Al2O3 (BCH-20B) PdAg/TiO2 (Riyapan et al., 2016) Pd0.01Ag/SiO2 (Pei et al., 2015) PdGa-nano (Ota et al., 2011) PdGa/Al2O3 (Osswald et al., 2008) PdCu0.006/SiO2 (Pei et al., 2017) PdS4/CNF (McCue et al., 2016) PdZn/ZnO (Zhou et al., 2016) Pd0.01Au/SiO2 (Lucci et al., 2016)

88 47 51 60 88 65–80 70 68 94 91 56.4

b

60,000 – 24,000 60,000 – 60,000

12.6 0.3 8.6 13.5 0.2 4.7 0.9 0.2

Selectivity at >99% acetylene conversion. Temperature at >99.5% acetylene conversion.

Fig. 9. (A) The H2 pressure dependencies for Pd15-700 and Pd15Co85-700 at 50 °C. (B) Arrhenius plots for acetylene conversion over four Pd-Co catalysts.

The Mears criterion,

r 0 A R2 1 þ 0:33cv < C Ab De jn  cb bb jð1 þ 0:33nxÞ gives 2.08  104 < 1.67, suggesting interphase and intraparticle heat transfer or mass transport limitations is not significant. The Pd15Co85-700 catalyst gives a TOF of 0.135 s1, which is almost twice higher than that of Pd15Co85-400 catalyst or other widely reported Pd-based bimetallic catalysts under the identical conditions (McKenna and Anderson, 2011; Kim et al., 2004; Mears, 1971). Furthermore, apparent activation energy (Ea) value for PdCo bimetallic catalysts is obtained by plotting ln(TOF) versus 1/T, as presented in Fig. 9B. It can be seen that the Ea value of Pd15Co85-700 catalyst is 34.1 kJ/mol, which is much lower than that of Pd15Co85-400 catalyst. Similar trend of Ea can be seen on Pd75Co25 samples. The decrease of Ea could be ascribed to the enrichment of Pd on the surface of the NPs and/or the redispersion of Pd and Co post-treated under H2 atmosphere. Moreover, the decreased activation barrier originating from post-treatment on as-prepared catalysts would then give an increase in hydrogenation activity. 3.4. Stability of Pd15Co85-700 catalyst A catalyst with high activity and selectivity becomes economically advantageous in the technological process only if it is stable

for long-term use. For this reason, 100 h test over Pd15Co85-700 catalyst and mono-metallic Pd-700 catalyst was performed at 80 °C with a GHSV space velocity of 10,056 h1 and H2:C2H2 ratio of 2. Under these conditions, the initial acetylene conversions were slightly higher than 95%. In Fig. 10, the acetylene conversion over Pd15Co85-700 maintains the conversion at 96% for the 100 h evaluation. However, the mono-metallic Pd-700 deactivates after about 45 h with the acetylene conversion decreasing to 70% after 100 h. While both catalysts had similar conversions, the ethane selectivity of Pd15Co85-700 was significantly higher (greater than 90%) than that for Pd-700 (ca. 25%). For Pd-700, there was a small increase in the ethene selectivity throughout the 100 h test. The increased selectivity could be ascribed to the formation of PdCx phase arising from the diffusion of decomposed carbon into Pd lattice during the reaction (Balmes et al., 2012). The formed PdCx phase can block the low coordination Pd atoms and/or modify the electronic structure of Pd NPs, and then reduce the adsorption strength of the reactant molecules (Han et al., 2004). Thermogravimetric analysis of the spent catalyst to determine the amount of carbon deposition on the spent catalysts is show in Fig. 10B. There was a mass-loss (peaks from 200 to 350 °C) of 9% on Pd15Co85-700 and 35% on Pd-700. This mass-loss arises from the oxidation of the surface carbon which is responsible for catalyst deactivation (McCue et al., 2014; Lamberov and Egorova, 2007). Much lower carbon deposition of Pd15Co85-700 is consistent with the lower deactivation rate. In addition, STEM analysis shows no obvious

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R. Ma et al. / Chemical Engineering Science 210 (2019) 115216

Fig. 10. (A) Acetylene conversion and ethene selectivity versus reaction time over alumina supported mono Pd and bimetallic Pd15Co85-700 catalysts. (B) Thermal analysis of coke deposition on catalysts after 100 h usage. STEM images of Pd15Co85-700 (C) and (D) Pd catalysts after 100 h usage.

aggregation and growth of the Pd15Co85-700 NPs, Fig. 10C and Fig. S6; while some sintering is observed for mono-metallic Pd. The the mean size of the latter increase from 13.6 nm to 16.8 nm after reaction.

Declaration of Competing Interest There are no conflicts to declare. Acknowledgements

4. Conclusions The surface structure and composition of as-prepared PdCo-400 bimetallic catalysts with different Pd/Co ratios is tuned by H2induced surface segregation at high temperatures, e.g., 700 °C. Characterization by XPS, HAADF-STEM and in situ CO-IR, shows that Pd surface segregation and enrichment occurs in PdCo NPs and this process is strongly dependent on the initial nanoparticle composition. The surface Pd/Co ratios on Pd75Co25 and Pd15Co85 nanoparticles increase by 32% and 14%, respectively, after the post-treatment. A 1.5 nm Pd enriched shell is formed on Pd75Co25700 sample; while surface isolated Pd sites with higher electron density are formed on Pd15Co85-700. The enrichment and redistribution of Pd atoms on the NP surface exhibits significantly increased activity in acetylene selective hydrogenation, which can be attributed to the increased Pd dispersion and higher TOR. The Pd15Co85-700 catalyst displays 88% ethene selectivity under industrially relavant conditions, which is among the highest reported. The Pd15Co85-700 catalyst also exhibits excellent carbon deposition resistance and activity stability during 100 h reaction. This surface engineering through surface segregation of noble metal and transition metal bimetallic catalysts could provide a feasible strategy for the design of other, higher performing industrial catalysts.

This work was supported by the National Key R&D Program of China (2016YFB0301600), the National Natural Science Foundation, the Fundamental Research Funds for the Central Universities (BUCTRC201725, JD1816) and the Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.ces.2019.115216. References An, H., Ha, H., Yoo, M., Kim, H.Y., 2017. Understanding the atomic-level process of CO-adsorption-driven surface segregation of Pd in (AuPd)147 bimetallic nanoparticles. Nanoscale 9, 12077–12086. Bakala, P.C., Briot, E., Millot, Y., Piquemal, J.-Y., Brégeault, J.-M., 2008. Comparison of olefin metathesis by rhenium-containing c-alumina or silica-aluminas and by some mesoporous analogues. J. Catal. 258, 61–70. Balmes, O., Resta, A., Lundgren, E., 2012. Reversible formation of a PdCx phase in Pd nanoparticles upon CO and O2 exposure. PCCP 14, 4796–4801. Biesinger, M.C., Payne, B.P., Grosvenor, A.P., Lau, L.W.M., Gerson, A.R., Smart, R.S.C., 2011. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe Co, and Ni. Appl. Surf. Sci. 257, 2717– 2730.

R. Ma et al. / Chemical Engineering Science 210 (2019) 115216 Bond, G.C., Wells, P.B., 1966. The hydrogenation of acetylene: Ⅱ. The reaction of acetylene with hydrogen catalyzed by alumina-supported palladium. J. Catal. 5, 65–73. Brongersma, H.H., Sparnaay, M.J., 1978. Surface segregation in Cu-Ni and Cu-Pt alloys; A comparison of low-energy ion-scattering results with theory. Surf. Sci. 71, 657–678. Chen, Z., Cui, Z.M., Li, P., Cao, C.Y., Hong, Y.L., Wu, Z.Y., Song, W.G., 2012. Diffusion induced reactant shape selectivity inside mesoporous pores of Pd@meso-SiO2 nanoreactor in Suzuki coupling reactions. J. Phys. Chem. C 116, 14986–14991. Chen, T.Y., Rodionov, V.O., 2016. Controllable catalysis with nanoparticles: bimetallic alloy systems and surface adsorbates. ACS Catal. 6, 4025–4033. Christoffersen, E., Stoltze, P., Nørskov, J.K., 2002. Monte Carlo simulations of adsorption-induced segregation. Surf. Sci. 505, 200–214. Collins, S.E., Delgado, J.J., Mira, C., Calvino, J.J., Bernal, S., Chiavassa, D.L., Baltanás, M. A., Bonivardi, A.L., 2012. The role of Pd-Ga bimetallic particles in the bifunctional mechanism of selective methanol synthesis via CO2 hydrogenation on a Pd/Ga2O3 catalyst. J. Catal. 292, 90–98. Cui, C.H., Gan, L., Heggen, M., Rudi, S., Strasser, P., 2013. Compositional segregation in shaped Pt alloy nanoparticles and their structural behavior during electrocatalysis. Nat. Mater. 12, 765–771. Cui, C.H., Gan, L., Neumann, M., Heggen, M., Cuenya, B.R., Strasser, P., 2014. Carbon monoxide-assisted size confinement of bimetallic alloy nanoparticles. J. Am. Chem. Soc. 136, 4813–4816. Fagherazzi, G., Benedetti, A., Polizzi, S., Mario, A.D., Pinna, F., Signoretto, M., Pernicone, N., 1995. Structural investigation on the stoichiometry of b-PdHx in Pd/SiO2 catalysts as a function of metal dispersion. Catal. Lett. 32, 292–303. Feng, J.T., Ma, C., Miedziak, P.J., Edwards, J.K., Brett, G.L., Li, D.Q., Du, Y.Y., Morgan, D. J., Hutchings, G.J., 2013. Au-Pd nanoalloys supported on Mg-Al mixed metal oxides as a multifunctional catalyst for solvent-free oxidation of benzyl alcohol. Dalton Trans. 42, 14498–14508. Feng, Q.C., Zhao, Z., Wang, Y., Dong, J.C., Chen, W.X., He, D.S., Wang, D.S., Yang, J., Zhu, Y.M., Zhu, H.L., Gu, L., Li, Z., Liu, Y.X., Yu, R., Li, J., Li, Y.D., 2017. Isolated single-atom Pd sites in intermetallic nanostructures: high catalytic selectivity for semihydrogenation of alkynes. J. Am. Chem. Soc. 139, 7294–7301. Ferreira, A.P., Henriques, C., Ribeiro, M.F., Ribeiro, F.R., 2005. SCR of NO with methane over Co-HBEA and PdCo-HBEA catalysts: the promoting effect of steaming over bimetallic catalyst. Catal. Today 107, 181–191. Gislason, J., Xia, W.S., Sellers, H., 2002. Selective hydrogenation of acetylene in an ethylene rich flow: Results of kinetic simulations. J. Phys. Chem. A 106, 767– 774. Han, Y.F., Kumar, D., Sivadinarayana, C., Clearfield, A., Goodman, D.W., 2004. The formation of PdCx over Pd-based catalysts in vapor-phase vinyl acetate synthesis: Does a Pd-Au alloy catalyst resist carbide formation. Catal. Lett. 94, 131–134. Han, B.C., Van der Ven, A., Ceder, G., Hwang, B.-J., 2005. Surface segregation and ordering of alloy surfaces in the presence of adsorbates. Phys. Rev. B 72, 205409. Harris, J., Andersson, S., 1985. H2 dissociation at metal surfaces. Phys. Rev. Lett. 55, 1583. He, Y.F., Feng, J.T., Du, Y.Y., Li, D.Q., 2012. Controllable synthesis and acetylene hydrogenation performance of supported Pd nanowire and cuboctahedron catalysts. ACS Catal. 2, 1703–1710. Huang, D.C., Chang, K.H., Pong, W.F., Tseng, P.K., Hung, K.J., Huang, W.F., 1998. Effect of Ag-promotion on Pd catalysts by XANES. Catal. Lett. 53, 155–159. Huang, W., McCormick, J.R., Lobo, R.F., Chen, J.G.G., 2007. Selective hydrogenation of acetylene in the presence of ethylene on zeolite-supported bimetallic catalysts. J. Catal. 246, 40–51. Hwang, S.J., Kim, S.K., Lee, S.C., Jang, J.H., Kim, P., Lim, T.H., Sung, Y.E., Yoo, S.J., 2012. Role of electronic perturbation in stability and activity of Pt-based alloy nanocatalysts for oxygen reduction. J. Am. Chem. Soc. 134, 19508–19511. Khalfaoui, M., Knani, S., Hachicha, M.A., Lamine, A.B., 2003. New theoretical expressions for the five adsorption type isotherms classified by BET based on statistical physics treatment. J. Colloid Interf. Sci. 263, 350–356. Kim, W.J., Kang, J.H., Ahn, I.Y., Moon, S.H., 2004. Deactivation behavior of a TiO2added Pd catalyst in acetylene hydrogenation. J. Catal. 226, 226–229. Kim, D.S., Kim, J.H., Jeong, I.K., Choi, J.K., Kim, Y.T., 2012. Phase change of bimetallic PdCo electrocatalysts caused by different heat-treatment temperatures: effect on oxygen reduction reaction activity. J. Catal. 290, 65–78. Kronawitter, C.X., Bakke, J.B., Wheeler, D.A., Wang, W.C., Chang, C.L., Antoun, B.R., Zhang, J.Z., Guo, J.H., Bent, S.F., Mao, S.S., Vayssieres, L., 2011. Electron enrichment in 3d transition metal oxide hetero-nanostructures. Nano Lett. 11, 3855–3861. Lamberov, A.A., Egorova, S.R., Il’yasov, I.R., Gil’manov, K.K., Trifonov, S.V., Shatilov, V. M., Ziyatdinov, A.S., 2007. Changes in the course of reaction and regeneration of a Pd-Ag/Al2O3 catalyst for the selective hydrogenation of acetylene. Kinet. Catal. 48, 136–142. Lee, T.J., Nam, I.S., Ham, S.W., Baek, Y.S., Shin, K.H., 2003. Effect of Pd on the water tolerance of Co-ferrierite catalyst for NO reduction by CH4. Appl. Catal. B: Environ. 41, 115–127. Leppert, L., Albuquerque, R.O., Kümmel, S., 2012. Gold-platinum alloys and Vegard’s law on the nanoscale. Phys. Rev. B 86, 241403. Li, Z.P., Yu, X.Y., Paik, U., 2016. Facile preparation of porous Co3O4 nanosheets for high-performance lithium ion batteries and oxygen evolution reaction. J. Power Sources 310, 41–46. Lucci, F.R., Darby, M.T., Mattera, M.F.G., Ivimey, C.J., Therrien, A.J., Michaelides, A., Stamatakis, M., Sykes, E.C.H., 2016. Controlling hydrogen activation, spillover, and desorption with Pd-Au single-atom alloys. J. Phys. Chem. Lett. 7, 480–485.

11

Mani, P., Srivastava, R., Strasser, P., 2008. Dealloyed Pt-Cu core-shell nanoparticle electrocatalysts for use in PEM fuel cell cathodes. J. Phys. Chem. C 112, 2770– 2778. McCue, A.J., McRitchie, C.J., Shepherd, A.M., Anderson, J.A., 2014. Cu/Al2O3 catalysts modified with Pd for selective acetylene hydrogenation. J. Catal. 319, 127–135. McCue, A.J., Guerrero-Ruiz, A., Rodríguez-Ramos, I., Anderson, J.A., 2016. Palladium silphide-A highly selective catalyst for the gas phase hydrogenation of alkynes to alkenes. J. Catal. 340, 10–16. McKenna, F.M., Anderson, J.A., 2011. Selectivity enhancement in acetylene hydrogenation over diphenyl sulphide-modified Pd/TiO2 catalysts. J. Catal. 281, 231–240. McKenna, F.M., Mantarosie, L., Wells, R.P.K., Hardacre, C., Anderson, J.A., 2012. Selective hydrogenation of acetylene in ethylene rich feed streams at high pressure over ligand modified Pd/TiO2. Catal. Sci. Technol. 2, 632–638. Mears, D.E., 1971. Tests for transport limitations in experimental catalytic reactors. Ind. Eng. Chem. Process Des. Dev. 10, 541. Mei, D.H., Neurock, M., Smith, M., 2009. Hydrogenation of acetylene-ethylene mixtures over Pd and Pd-Ag alloys: first-principles-based kinetic Monte Carlo simulations. J. Catal. 268, 181–195. Nie, Y., Li, L., Wei, Z.D., 2015. Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction. Chem. Soc. Rev. 44, 2168–2201. Niquille-Röthlisberger, A., Prins, R., 2006. Hydrosulfurization of 4,6dimethyldibenzothiophene and dibenzothiophene over alumina-sopported Pt, Pd, and Pt-Pd catalysts. J. Catal. 242, 207–216. Osswald, J., Kovnir, K., Armbrüster, M., Giedigkeit, R., Jentoft, R.E., Wild, U., Grin, Y., Schlögl, R., 2008. Palladium-gallium intermetallic compounds for the selective hydrogenation of acetylene: Part Ⅱ: Surface characterization and catalytic performance. J. Catal. 258, 219–227. Ota, A., Armbrüster, M., Behrens, M., Rosenthal, D., Friedrich, M., Kasatkin, I., Girgsdies, F., Zhang, W., Wagner, R., Schlögl, R., 2011. Intermetallic compound Pd2Ga as a selective catalyst for the semi-hydrogenation of acetylene: from model to high performance system. J. Phys. Chem. C 115, 1368–1374. Oyama, S.T., Zhang, X.M., Lu, J.Q., Gu, Y.F., Fujitani, T., 2008. Epoxidation of propylene with H2 and O2 in the explosive regime in a packed-bed catalytic membrane reactor. J. Catal. 257, 1–4. Pei, G.X., Liu, X.Y., Wang, A.Q., Lee, A.F., Isaacs, M.A., Li, L., Pan, X.L., Yang, X.F., Wang, X.D., Tai, Z.J., Wilson, K., Zhang, T., 2015. Ag alloyed Pd single-atom catalysts for efficient selective hydrogenation of acetylene to ethylene in excess ethylene. ACS Catal. 5, 3717–3725. Pei, G.X., Liu, X.Y., Yang, X.F., Zhang, L.L., Wang, A.Q., Li, L., Wang, H., Wang, X.D., Zhang, T., 2017. Performance of Cu-alloyed Pd single-atom catalyst for semihydrogenation of acetylene under simulated front-end conditions. ACS Catal. 7, 1491–1500. Peng, H.L., Liu, F.F., Liu, X.J., Liao, S.J., You, C.H., Tian, X.L., Nan, H.X., Luo, F., Song, H. Y., Fu, Z.Y., Huang, P.Y., 2014. Effect of transition metals on the structure and performance of the doped carbon catalysts derived from polyaniline and melamine for ORR application. J. Am. Chem. Soc. 4, 3797–3805. Peng, L.X., Ringe, E., Van Duyne, R.P., Marks, L.D., 2015. Segregation in bimetallic nanoparticles. PCCP 17, 27940–27951. Popova, N.M., Babenkova, L.V., 2008. TPD studies of hydrogen adsorption on group VIIImetals. React. Kinet. Catal. Lett. 319, 187–192. Prinz, J., Pignedoli, C.A., Stöckl, Q.S., Armbrüster, M., Brune, H., Crcöning, Q., Widmer, R., Passerone, D., 2014. Adsorption of small hydrocarbon on the three-fold PdGa sufaces: the road to selective hydrogenation. J. Am. Chem. Soc. 136, 11792– 11798. Qin, Y.L., Liu, Y.C., Liang, F., Wang, L.M., 2015. Preparation of Pd-Co based nanocatalysts and their superior applications in formic acid decomposition and methanol oxidation. ChemSusChem 8, 260–263. Ramírez-Caballero, G.E., Ma, Y.G., Callejas-Tovar, R., Balbuena, P.B., 2010. Surface segregation and stability of core-shell alloy catalysts for oxygen reduction in acid medium. PCCP 12, 2209–2218. Riyapan, S., Zhang, Y.Y., Wongkaew, A., Pongthawornsakun, B., Monnier, J.R., Panpranot, J., 2016. Preparation of improved Ag-Pd/TiO2 catalysts using the combined strong electrostatic adsorption and electroless deposition methods for the selective hydrogenation of acetylene. Catal. Sci. Technol. 6, 5608– 5617. Rodríguez, J.C., Marchi, A.J., Borgna, A., Monzón, A., 1997. Effect of Zn content on catalytic activity and physicochemical properties of Ni-based catalysts for selective hydrogenation of acetylene. J. Catal. 171, 268–278. Shin, E.W., Choi, C.H., Chang, K.S., Na, Y.H., Moon, S.H., 1998. Properties of Simodified Pd catalyst for selective hydrogenation of acetylene. Catal. Today 44, 137–143. Shiratori, N., Lee, K., Miyawaki, J., Hong, S.H., Mochida, I., An, B., Yokogwa, K., Jang, J., Yoon, S.-H., 2009. Pore structure analysis of activated carbon fiber by microdomain-based model. Langmuir 25, 7631–7637. Stamenkovic´, V.R., Mun, B.S., Mayrhofer, K.J.J., Ross, P.N., Markovic, N.M., 2006. Effect of surface composition on electronic structure, stability, and electrocatalytic properties of Pt-transition metal alloys: Pt-skin versus Ptskeleton surfaces. J. Am. Chem. Soc. 128, 8813–8819. Tao, F., Grass, M.E., Zhang, Y.W., Butcher, D.R., Renzas, J.R., Liu, Z., Chung, J.Y., Mun, B. S., Salmeron, M., Somorjai, G.A., 2008. Reaction-driven restructuring of Rh-Pd and Pt-Pd core-shell nanoparticles. Science 332, 932–934. Tao, F.F., Zhang, S.R., Nguyen, L., Zhang, X.Q., 2012. Action of bimetallic nanocatalysts under reaction conditions and during catalysis: evolution of chemistry from high vacuum conditions to reaction conditions. Chem. Soc. Rev. 41, 7980–7993.

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Vasquez, Y., Sra, A.K., Schaak, R.E., 2015. One-pot synthesis of hollow superparamagnetic CoPt nanospheres. J. Am. Chem. Soc. 127, 12504–12505. Wang, X., Chen, H.Y., Sachtler, W.M.H., 2000. Catalytic reduction of NOx by hydrocarbons over Co/ZSM-5 catalysts with different methods. Appl. Catal. B: Environ. 26, 227–2299. Wang, D.S., Li, Y.D., 2011. Bimetallic nanocrystals: liquid-phase synthesis and catalytic applications. Adv. Mater. 23, 1044–1060. Weisz, P.B., Prater, C.D., 1954. Interpretation of measurements in experimental catalysis. Adv. Catal. 6, 143–196. Xiao, S., Hu, W., Luo, W., Wu, Y., Li, X., Deng, H., 2006. Size effect on alloying ability and phase stability of immiscible bimetallic nanoparticles. Eur. Phys. J. B 54, 479–484. Xue, H.R., Tang, J., Gong, H., Guo, H., Fan, X.L., Wang, T., He, J.P., Yamauchi, Y., 2016. Fabrication of PdCo bimetallic nanoparticles anchored on three-dimensional ordered N-doped porous carbon as an efficient catalyst for oxygen reduction reaction. ACS Appl. Mater. Inter. 8, 20766–20771.

Yang, B., Burch, R., Hardacre, C., Headdock, G., Hu, P., 2013. Influence of surface structures, subsurface carbon and hydrogen, and surface alloying on the activity and selectivity of acetylene hydrogenation on Pd surfaces: a density functional theory study. J. Catal. 305, 264–276. Zhang, J., Fang, J.Y., 2009. A general strategy for preparation of Pt 3d-transition metal (Co, Fe, Ni) nanocubes. J. Am. Chem. Soc. 131, 18543–18547. Zhang, Z.C., Tian, X.C., Zhang, B.W., Huang, L., Zhu, F.C., Qu, X.M., Liu, L., Liu, S., Jiang, Y.X., Sun, S.G., 2017. Engineering phase and surface composition of Pt3Co nanocatalysts: a strategy for enhancing CO tolerance. Nano Energy 34, 224–232. Zhou, H.R., Yang, X.F., Li, L., Liu, X.Y., Huang, Y.Q., Pan, X.L., Wang, A.Q., Li, J., Zhang, T., 2016. PdZn intermetallic nanostructure with Pd-Zn-Pd ensembles for highly active and chemoselective semi-hydrogenation of acetylene. ACS Catal. 6, 1054–1061. Zou, L.F., Li, J., Zakharov, D., Saidi, W.A., Stach, E.A., Zhou, G.W., 2017. Atomically visualizing elemental segregation-induced surface alloying and restructuring. J. Phys. Chem. Lett. 8, 6035–6040.