Journal
JOURNAL OF PHYSICAL CHEMISTRY B
Volume 108, Issue 52, Pages 20228-20232Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jp0403893
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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.
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