Reactions of Methyl Radicals with Iridium, Osmium, and Ruthenium Nanoparticles Suspended in Aqueous Solutions: Surface Effects

Abstract

The reactions of methyl radicals with noble metal nanoparticles, M⁰-NPs, M = Ru, Os, and Ir, were studied. The M⁰-NPs were prepared by reduction of the corresponding salts with NaBH₄. HR-TEM, XPS, and Raman analysis of the M⁰-NPs thus formed point out that the Ru⁰-NPs and Os⁰-NPs formed are covered with an oxide/hydroxide layer. The Ir⁰-NPs are only partially covered by such a layer. The methyl radicals oxidize all of the M⁰-NPs studied herein. Thus, the mechanism differs from the mechanisms of reaction with M⁰-NPs, M = Ag, Au, and Pt. This phenomenon is attributed to the differences in the reduction potentials of these metals. The rate constant of the reaction of methyl radicals with Ir⁰-NP is very high, similar to that reported for other M⁰-NPs, M = Ag, Au, and Pt. This is attributed to the interaction of the radicals with the electrons in the conduction bands of the metals. On the other hand, the rate constants of the reactions of methyl radicals with M⁰-NPs, M = Ru and Os, are considerably lower and similar to those reported for titania. The total yield of methyl radicals, the yield of methane + twice the yield of ethane, observed in the presence of the M⁰-NPs, is somewhat smaller than expected. This is attributed to either the formation of a mixture of side products, at concentrations below the limit of observation under our experimental conditions, or some energy absorption by the M⁰-NPs.