Strain Effects on the Oxidation of CO and HCOOH on Au-Pd Core-Shell Nanoparticles

8 Figuras, 1 Tabla, 2 Esquemas
The mechanism of CO and HCOOH electrooxidation in an acidic solution on carbon-supported Au–Pd core–shell nanoparticles was investigated by differential electrochemical mass spectrometry and in situ Fourier transform infrared (FTIR) spectroscopy. Analysis performed in nanostructures with 1.3 ± 0.1 nm (CS1) and 9.9 ± 1.1 nm (CS10) Pd shells provides compelling evidence that the mechanism of adsorbed CO (COads) oxidation is affected by structural and electronic effects introduced by the Au cores. In the case of CS10, a band associated with adsorbed OH species (OHads) is observed in the potential range of CO oxidation. This feature is not detected in the case of CS1, suggesting that the reaction follows an alternative mechanism involving COOHads species. The faradaic charge associated with COads oxidation as well as the Stark slope measured from FTIR indicates that the overall affinity and orbital coupling of CO to Pd are weaker for CS1 shells. FTIR spectroscopy also revealed the presence of HCOOads intermediate species only in the case of CS1. This observation allowed us to conclude that the higher activity of CS10 toward this reaction is due to a fast HCOOads oxidation step, probably involving OHads, to generate CO2. Density functional theory calculations are used to estimate the contributions of the so-called ligand and strain effects on the local density of states of the Pd d-band. The calculations strongly suggest that the key parameter contributing to the change in mechanism is the effective lattice strain.
V.C. gratefully acknowledges the UK National Academy for their support through the International Newton Fellowship program (NF120002). D.P. and D.J.F. gratefully acknowledge funding from EPSRC (EP/K007025/1). J.J.L.H. is grateful for the Ph.D. scholarship partly funded by Sasol UK and NERC. E.P. acknowledges financial support from the Ministry of Economy and Competitiveness through Project ENE2014-52158-C2-2-R. D.J.F. is also grateful for the Research Fellowship from the Institute of Advanced Studies of the University of Bristol. TEM studies were performed at the University of Bristol Chemistry Imaging Facility with equipment funded by UoB and EPSRC (EP/K035746/1 and EP/M028216/1). We are grateful to Prof. Jeremy Sloan (University of Warwick, Coventry, U.K.) and Dr. Christoph Mitterbauer (FEI Co.) for the high-resolution TEM images of the CS1 nanoparticles. This work is part of the research network of the Deutsche Forschungsgemeinschaft FOR1376. E.S. acknowledges PIP-CONICET 112-201001-00411 and PICT 2012-2324; whereas P.M.Q. acknowledges PICT-2014-1084 (Agencia Nacional de Promoción Científica y Tecnológica, FONCYT, préstamo BID). A grant of computing time from the Baden-Württemberg grid is gratefully acknowledged. All the data presented in this paper can be freely accessed from the Bristol’s Research Data Repository (http://data/bris.ac.uk/10.5523/bris.1om4r04s216tn164tab9nfjyfg).