Bonding and motional aspects of CO adsorbed on the surface of Pt nanoparticles decorated with Pd

Spontaneous deposition of Pd onto catalytic grade Pt nanoparticles has been shown to yield Pt/Pd catalysts having enhanced catalytic activity toward formic acid oxidation, when compared to pure Pt- and Pd-black. Here, we report the results of electrochemical nuclear magnetic resonance (EC NMR) and electrochemical measurements of CO chemisorbed onto these Pt/Pd catalysts, to probe the nature of the CO chemisorption bond, as well as the motional behaviour of adsorbed CO. The C-13 NMR spectra are broad and can be deconvoluted into two peaks, assigned to CO adsorbed on Pt and Pd sites. From the temperature dependence of the spin-lattice relaxation rates, we conclude that CO chemisorbed on Pd undergoes fast diffusion. The activation energy (E-a) obtained from these results for CO on Pd is smaller than that found for CO adsorbed onto Pd nanoparticles supported on alumina. A two-band model analysis of the NMR data shows that the 5sigma orbital of CO makes a significant contribution to the chemisorption bond of CO on Pd, which agrees well with theoretical predictions. The interaction of Pd with Pt leads to a reduction in the Fermi level local density of states (E-f-LDOS) at the Pd sites, which reduces the strength of CO and, most likely, OH adsorption. This electronic modification is proposed to be responsible for the improved catalytic performance of Pt/Pd in formic acid oxidation.