What Determines Electrochemical Surface Processes on Carbon-Supported PdAu Nanoparticles?

Abstract

Supported bimetallic nanoparticles have good activities in heterogeneous catalysis and electrocatalysts. Among those systems, PdAu shows improved stability and enhanced catalytic activity towards electrochemical reactions such as oxygen reduction, formic acid oxidation and hydrogen evolution. The aim of this work was to study comprehensively the influence of ligand and ensemble effects on surface processes, such as oxide formation/reduction, oxidation of adsorbed CO and adsorption of hydrogen, taking place on carbon-supported PdAu nanoparticles of different compositions. Towards that end, we thoroughly characterized the properties of PdAu/C catalysts with nominal Au contents of 20 to 50 % (in atoms) prepared by the same procedure and having similar average particle diameters. The combination of results obtained by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicates that nanoparticles had Pd-enriched surfaces and Au-rich interior. X-ray absorption spectroscopy (XAS) measurements around the Pd L3 edge evidenced that Au promotes an increase in the electronic occupation of the Pd 4d band, consistent with XPS results showing varying amounts of surface-like and bulk-like Pd oxide and 2 metallic Pd. We show herein that ligand effects determine the formation of a Pd oxide layer on PdAu/C catalysts and ensemble effects govern hydrogen and CO adsorption. CO oxidation is delayed as the Au content increases as result of the decreasing availability of oxygen-containing species.

Effect of Pd Coordination and Isolation on the Catalytic Reduction of O2 to H2O2 over PdAu Bimetallic Nanoparticles

“…Pd and Au are miscible and form a substitutional, exothermic alloy (−2 > Δ H mix > −9 kJ mol –1 ) for all compositions. − As such, Monte Carlo simulations of PdAu surfaces predict the formation of significant numbers of isolated Pd monomers when the mole fraction of Au greatly exceeds that of Pd. − Most results indicate that Pd atoms gain d-electrons as the Au contents in PdAu alloys increase. This conclusion agrees with clear evidence from ex situ XANES spectra that reveal a less intense white line at the Pd L 3 edge for Pd 1 Au 1 nanoparticles compared to monometallic Pd . Hence, numerous spectroscopic and computational investigations indicate that alloying Pd with Au modifies the electronic structure of Pd atoms in manners that seem relevant for catalysis.…”

“…This conclusion agrees with clear evidence from ex situ XANES spectra that reveal a less intense white line at the Pd L 3 edge for Pd 1 Au 1 nanoparticles compared to monometallic Pd. 22 Hence, numerous spectroscopic and computational investigations indicate that alloying Pd with Au modifies the electronic structure of Pd atoms in manners that seem relevant for catalysis.…”

“…PdAu alloys can provide catalytic rates and selectivities that exceed those of monometallic Pd or Au catalysts; however, ascertaining the reasons for these differences remains a challenge. The formation of PdAu alloys modifies both the electronic and geometric structure of Pd, which has been demonstrated by investigations that involve X-ray photoelectron spectroscopy (XPS), − X-ray absorption spectroscopy (XAS), ,− and Fourier transform infrared spectroscopy (FTIR) of adsorbed CO. ,,,,,,,− These two effects are entangled and complicate the interpretation of data obtained by these methods, because the atomic composition and arrangement of PdAu alloy surfaces affect the electronic structure. Pd and Au are miscible and form a substitutional, exothermic alloy (−2 > Δ H mix > −9 kJ mol –1 ) for all compositions. − As such, Monte Carlo simulations of PdAu surfaces predict the formation of significant numbers of isolated Pd monomers when the mole fraction of Au greatly exceeds that of Pd. − Most results indicate that Pd atoms gain d-electrons as the Au contents in PdAu alloys increase.…”

PdAu Alloy Nanoparticles for Ethanol Oxidation in Alkaline Conditions: Enhanced Activity and C1 Pathway Selectivity

“…The electronic interaction between Pd and Au was also studied by XPS. Compared with pure Au’s binding energy (BE = 84.2 eV), the Au 4f peaks of the PdAu samples all shifted negatively, indicating more negatively charged Au in the alloy. When it comes to Pd 3d peaks, the trend is not as straightforward.…”

A novel electrochemical cell for operando X-ray absorption measurements at low energies: Probing electrochemically induced electronic changes in palladium

“…Despite the fact that measurements around the Pd L3 edge are rather scarce for carbon-supported Pd and Pd-based nanoparticles, analysis of https://doi.org/10.1016/j.elecom.2018.07.023 Received 10 July 2018; Received in revised form 25 July 2018; Accepted 26 July 2018 the XANES region of spectra acquired under high-vacuum conditions has shed some light on the role of the electronic occupation of the Pd 4d band in electrocatalytic reactions [11,12] and electrochemical surface processes [13].…”