Ultrathin PdAu Shells with Controllable Alloying Degree on Pd

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Communication

Ultrathin PdAu Shells with Controllable Alloying Degree on Pd Nanocubes towards Carbon Dioxide Reduction Xintong Yuan, Lei Zhang, Lulu Li, Hao Dong, Sai Chen, Wenjin Zhu, Congling Hu, Wanyu Deng, Zhi-Jian Zhao, and Jinlong Gong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11771 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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Journal of the American Chemical Society

Ultrathin PdAu Shells with Controllable Alloying Degree on Pd Nanocubes towards Carbon Dioxide Reduction Xintong Yuan1, Lei Zhang1, Lulu Li, Hao Dong, Sai Chen, Wenjin Zhu, Congling Hu, Wanyu Deng, Zhi-Jian Zhao and Jinlong Gong* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China

Supporting Information Placeholder ABSTRACT: Electrocatalytic reduction of carbon dioxide (CO2ER)

to reusable carbon resource is a significant step to balance the carbon cycle. This Communication describes a seeded growth method to synthesize ultrathin Pd-Au alloy nanoshells with controllable alloying degree on Pd nanocubes. Specifically, Pd@Pd3Au7 nanocrystals (NCs) show superior CO2ER performance with a 94% CO faraday efficiency (FE) at -0.5 V vs. reversible hydrogen electrode (RHE) and approach 100% CO FE from −0.6 to −0.9 V. The enhancement primarily originates from ensemble and ligand effects, i.e., appropriately proportional Pd-Au sites and electronic back donation from Au to Pd. In situ attenuated total reflection-infrared (ATR-IR) spectra and density functional theory (DFT) calculations clarify the reaction mechanism. This work may offer a general strategy for the synthesis of bimetallic NCs to explore the structure-activity relationship in catalytic reactions.

Various metal candidates (e.g., Au,1-4 Zn,5-6 Cu,7-8) have been investigated for CO2ER.9 Particularly, Pd has relatively high binding energy with *COOH. However, the scaling relationship between the binding affinities of *COOH and *CO is an inevitable thermodynamic limitation.10 Alloying degree can tune the intermediates’ binding energies.11-15 Unlike Pd, Au binds *CO weakly, facilitating *CO desorption.16 It has been reported that Au@Pd catalysts with different thickness of Pd shells lead to different products.17-19 A mixture of C1 to C5 hydrocarbons or HCOOH is produced on PdAu alloy films.20-21 However, development of an effective Au-Pd catalyst to boost the selectivity toward single product and improve the current density at lower potentials remains challenging. Surface-controlled NCs can also adjust the intermediates’ binding strength. Previous studies have shown that {100} facets of Au,4 Ag,22 and Pd10, 23 have a higher catalytic activity in CO2ER than {111} facets. Au can epitaxially be grown on Pd nanocubes to form core-shell structures.24-25 Based on the principles of seed-mediated growth and galvanic displacement, an ultra-thin PdAu interlayer should be formed when the shell is as thin as a few atomic layers. The epitaxial growth of Pd-Au alloy shells on Pd nanocubes with {100} facets can provide unique electronic modification.26 In addition, the designed Pd@PdAu NCs can achieve controllable alloying degree with a certain number of atomic layers. This Communication describes the relationship between intermediates’ Gibbs free energy (△G) and d band center shifting of Pd. By controlling the ensemble and ligand effects of the Pd-Au alloying degree, we weaken the Pd-C bond energy and adjust the target intermediates’ adsorption. The as-prepared Pd@Pd3Au7 NCs show a 94% CO FE at -0.5 V vs. RHE

and a near 100% CO FE from -0.6 to -0.9 V, which appears to be the most efficient among all of Au-Pd and other comparable bimetallic catalysts toward CO2ER for CO production (Table S1). The morphological and structural evolution of Pd@PdAu NCs was derived from a kinetic controlled nucleation and growth. Each {100} facet on Pd cubes served as a nucleation site for newly formed Pd and Au atoms. When Pd nanocubes were mixed with HAuCl4, Pd0 on Pd cubes could be substituted by Au because the standard reduction potential of the Pd2+/Pd pair is lower than that of AuCl4-/Au pair.27 The inductively coupled plasma optical emission spectrometry (ICP-OES) results show that when HAuCl4 was added, the concentration of Pd2+ increased slightly (Table S2). Then, the dissolved Pd2+ and AuCl4- were reduced to form the PdAu alloy shells. Figure 1a shows the growth process. By adjusting the volumes of precursors, the shell thickness could be controlled at an atomic level. When a thicker shell (~3 nm) was formed, the Pd@Au core-shell structures could be confirmed from transmission electron microscopy (TEM) and X-ray diffraction (XRD) due to the limitation of Pd0 dissolution (Figure S1). Figures 1b-d show TEM images of Pd@PdAu NCs with Pd-Au shell of 3, 7 and 13 atomic layers. The ratios of Pd to Au in the shells are 7:3, 3:7 and 1:9 based on the results of TEM and X-ray photoelectron spectroscopy (XPS) (Figure S2). As the average sizes of the prepared Pd@PdAu NCs are similar, the size effect can be precluded (Figure S3). Pd@PdAu NCs with other shell thicknesses can also be precisely synthesized (Figure S4). HR-TEM images (Figure 1e-g) reveal the changes in the shell thickness of the different NCs, and the lattice spacing of 1.99 ± 0.04 Å can be assigned to the {200} planes of face-centered cubic (fcc) Pd-Au alloy with different alloying degrees. High-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) and elemental mapping images (Figure 1h) illustrate compositional distributions. The results demonstrate Pd and Au are well overlapped in the shells. It should be pointed out that the closer the position is to the outermost layer, the higher the ratio of Au in the alloy would be. The alloying ratio at different positions on one Pd@Pd3Au7 particle was detected by HAADF-STEM and energy dispersive X-ray spectroscopy (EDS) (Figure S5). The line-scanning profile (Figure 1i) shows that the onset positions of the Pd and Au signals occur simultaneously and two peaks of Au represent the thickness of the shells, indicating the formation of Pd-Au alloy shells.

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Total mass current density and partial CO geometric current density of Pd@Pd3Au7 NCs achieve 155.3 mA g-1 and 18.79 mA cm-2 at -0.9 V, displaying overwhelming advantages over other catalysts (Figure S7). The post-testing CO2ER activity (Figure 2d) and structure of the catalysts morphology (Figure S8) were well maintained after an 8-hours test at 0.7 V. We also prepared Pd cubes, Au cubes, and PdxAuy alloy NCs (Figure S9) to explore the origin of enhanced activity of Pd@Pd3Au7 NCs. As shown in Figure S10, the size effect was excluded. The CO2ER performance of Pd@Pd3Au7 NCs exceeds Pd cubes and Au cubes, due to the ensemble and ligand effects. The ensemble effect is a dilution of the more catalytically active component (Pd) by Au. As surface proportion of Au increases, contiguous Pd ensembles disappear and more isolated Pd ensembles are surrounded by Au. The ligand effect is electronic perturbation of Pd by Au, which involves direct electron transfer and bond length changes.29 We also tested the CO2ER performance of a series of PdAu alloy NCs. The composition of PdxAuy was obtained by ICP-OES (Table S2) and EDS (Table S3). Pd3Au7 alloy has the highest CO FE among PdxAuy alloy NCs (Figure S11). However, its performance is not comparable to Pd@Pd3Au7 NCs. This result also illustrates the significance of the core-shell structure with the controllable alloying degree and {100} facets.

Figure 1. (a) Schematic, (b-d) TEM and (e-g) HR-TEM images of Pd@Pd7Au3, Pd@Pd3Au7, Pd@Pd1Au9 NCs. (h) HAADF-STEM image and corresponding elemental mapping of Pd@Pd3Au7 NCs. (i) HAADF-STEM-EDS line-scanning profile as marked in (h). (j) XRD patterns. We then identify the structural differences among Pd@PdAu NCs by employing XRD (Figure 1j). When the shells are only 3-atomic-layer thick, the diffraction peaks of Au cannot be observed. With the increase of the shell thickness, the diffraction peaks appear between the peaks of fcc Au and Pd, confirming the formation of Pd-Au alloy. The peaks shift to lower angle positons as the shells become thicker, indicating more Au atoms are doped in the shells. Ultraviolet-visible (UV-Vis) extinction spectrums (Figure S6) show Pd cubes have no localized surface plasmon resonance (LSPR), and Au cubes have a characteristic LSPR at 550 nm. For Pd@PdAu NCs, the major LSPR peaks associated with the Au are observed at ~550 nm. As the thickness of the shells increases, the LSPR peaks exhibit blue-shift continuously, which is caused by more electron donation from Pd to Au.24 The electron transfer can also be characterized by XPS (Figure S2). The Pd 3d3/2 peak shifts from 340.0 (Pd cubes) to 339.4 eV (Pd@Pd1Au9 NCs) upon alloying with Au; while a shift from 82.8 to 82.6 eV can be seen for the Au 4f7/2. The lower binding energies (BEs) for both Pd 3d3/2 and Au 4f7/2 are in accordance with the net charge flowing into Pd (higher Pd d electron densities) and Au (gaining s and p electrons from Pd).28-29 This result indicates the electronic structure of the surface Pd atoms can be modified by alloying with Au. It has been demonstrated that charge is partially compensated by a depletion of Au 5d electrons when Au gains s, p electrons in Pd-Au alloy.30 The blue-shift of the LSPR peaks and the negative shift of the BEs of Pd 3d3/2 both indicate a strong ligand effect between Pd and Au. Pd@Pd3Au7 NCs show a significantly enhanced catalytic activity toward CO2ER in an H-cell with CO2-saturated 0.1 M KHCO3, i.e., 94% CO FE at -0.5 V vs. RHE and near 100% CO FE from -0.6 to -0.9V (Figure 2a). Linear sweeping voltammetry (LSV) shows Pd@Pd3Au7 NCs have the highest current density and lowest overpotential (Figure 2b).

Figure 2. (a) CO FE, (b) LSV curves, (c) CO FE at -0.5 V vs. RHE of different catalysts, (d) stability test of Pd@Pd3Au7 NCs.

Figure 3. (a) Tafel plots, (b) in situ ATR-IR of Pd@Pd3Au7 NCs, (c) proposed CO2ER pathway on Pd@Pd3Au7 NCs. To probe the mechanism of CO2ER, the partial CO current density was tested under low potentials for Tafel plots (Figure 3a). The Tafel slope of Pd@Pd3Au7 NCs is 68.6 mV dec-1, indicating a one-electron pre-equilibrium step prior to a rate-determining step. The conversion

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Journal of the American Chemical Society from *COOH to *CO is an energetically downhill process, and *CO desorption on Pd is an endergonic process. The *CO desorption was detected by electrochemical CO stripping voltammetry (Figure S12). The Pd cubes showed a sharp peak at 0.91V, while a negative potential shift of CO adsorption peaks indicated a weak adsorption.31 With increasing the Au ratio, the desorption of *CO from the catalyst surface became easier until no adsorption of *CO was observed on Pd@Pd1Au9 NCs. Hence, the reaction path on Pd@Pd3Au7 NCs includes (i) a fast and reversible e- transfer to CO2 to form CO2•-, (ii) CO2•- and a proton combine to form *COOH, and then (iii) additional H+ and e- transfer to complete the reduction to CO (Figure 3c). To explore the origin of activity of Pd@Pd3Au7 NCs, surface-adsorbed species at different potentials were detected employing in situ ATR-IR. The negative peak at 2341 cm-1 is assigned to the consumption of CO2. The positive peaks at ~1650 cm-1 are δ(H-O-H) of H2O. The positive bands at ~1926 cm-1 and 1864 cm-1 appear from -0.4 V and are assigned to linear-bonded *CO and bridge-bonded *CO (Figure 3b).32-34 Corresponding peaks are also observed on Pd@Pd7Au3 NCs, Pd3Au7 alloy NCs and Pd cubes. However, a relatively negative potential is needed to observe *CO peaks for these catalysts, which indicates Pd@Pd3Au7 NCs are the most active catalysts for the formation of *CO (Figure S13).

of Pd-C is regulated to some extent, which ensures superior catalytic activity and selectivity for CO2ER. These results are in accordance with the ATR-IR results. In summary, we synthesized controllable Pd@PdAu structures with different shell thickness and researched their CO2ER performance. A significantly enhanced activity of CO2ER was observed on Pd@Pd3Au7 NCs, achieving CO FE of 94% at -0.5 V vs. RHE and approaching 100% from -0.6 to -0.9 V. The synergistic ensemble and ligand effects utilize neighboring Pd-Au sites and electron transfer between them to weaken the Pd-C bond, which balances the △G of *COOH and *CO. In situ ATR-IR and DFT calculations assisted in elucidating the mechanism of CO2ER. This work provides a general strategy to rationally design and synthesize bimetallic catalysts to boost CO2ER.

ASSOCIATED CONTENT Supporting Information Experimental details and supporting data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ORCID Jinlong Gong: 0000-0001-7263-318X Lei Zhang: 0000-0001-7146-0184 Zhi-jian Zhao: 0000-0002-8856-5078

Author Contributions 1

These authors contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Figure 4. Free energy profile of (a) CO2ER and (b) HER. (c) Projected density of states of different catalysts, gray dashed indicates EF, black bar indicates its d band center. DFT calculations were conducted to further elucidate the mechanism credited for the low potential and high CO FE on Pd@Pd3Au7 NCs. The calculation models are shown in Figure S14, S15. Figure 4a shows that *COOH on Pd@Pd3Au7 is easier to be stabilized compared with Pd@Pd1Au9, Au cube and Pd3Au7 alloy. Moreover, the desorption of *CO over Pd@Pd3Au7 occurs more readily than Pd@Pd7Au3 and Pd cube. Hence, the adsorption of *COOH and desorption of *CO over Pd@Pd3Au7 are at a moderate level. The shifting of the intermediate binding strength can be explained by the classical d band theory. With the increase of an Au content, the d band center of Pd moves down relative to the Fermi level (EF) (Figure 4c, Table S4), which weakens the Pd-C bond (Figure S16).35 Therefore, CO can form at low potentials. Compared with alloys, Pd@Pd3Au7 core-shell structure also makes it easier to transfer electrons, which is beneficial to the formation of *COOH (Figure 4, S17). Calculated △G for hydrogen evolution reaction (HER) indicates that Pd@Pd3Au7 requires a higher overpotential for the formation of *H than Pd@Pd7Au3, Pd7Au3 alloy and Pd cube (Figure 4b). According to the calculations, with an appropriate proportion of Pd to Au, Au atoms can transfer electrons to Pd sites, which leads to the change of d band center of Pd. Therefore, the chemical adsorption

We acknowledge the National Key Research and Development Program of China (2016YFB0600901), the National Science Foundation of China (U1463205, 51302185, 21525626 and 21606169), and the Program of Introducing Talents of Discipline to Universities (B06006) for financial support.

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This Communication describes a seeded growth method to synthesize ultrathin Pd-Au alloy nanoshells with controllable alloying degree on Pd nanocubes for electroreduction of carbon dioxide.

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