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

The mechanism of CO and HCOOH electrooxidation in acidic solution at carbon-supported Au-Pd core-shell nanoparticles was investigated by differential electrochemical mass spectrometry (DEMS) and in-situ Fourier transform infrared (FTIR) spectroscopy. Analysis performed in nanostructures with Pd shells of 1.3±0.1 (CS1) and 9.9±1.1 nm (CS10) provide compelling evidence that the mechanism of adsorbed CO (CO_ads) 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 (OH_ads) 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 COOH_ads species. The faradaic charge associated with CO_ads oxidation as well as the Stark slope measured from FTIR indicate that the overall affinity and orbital coupling of CO to Pd is weaker at CS1 shells. FTIR spectroscopy also revealed the presence HCOO_ads intermediate species only in the case of CS1. This observation allowed concluding that the higher activity of CS10 towards this reaction is due to a fast HCOO_ads oxidation step, probably involving OH_ads, to generate CO₂. DFT 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 parameters contributing to the change in mechanism is the effective lattice strain.