Coating Platinum Nanoparticles with Methyl Radicals: Effects on Properties and Catalytic Implications

It was recently reported that the reaction of methyl radicals with Pt⁰ nanoparticles (NPs), prepared by the reduction of Pt(SO₄)₂ with NaBH₄, is fast and yields as the major product stable (Pt⁰-NPs)- (CH₃)_n and as side products, in low yields, C₂H₆, C₂H₄, and some oligomers. We decided to study the effect of this coating on the properties of the Pt⁰-NPs. The results show that the coating can cover up to 75 % of the surface Pt⁰ atoms. The rate constant of the reaction, k(^.CH₃+Pt⁰-NPs), decreases with the increase in the surface coverage, leading to competing reaction paths in the solution, which gradually become dominant, affecting the composition of the products. The methyl coating also affects the zeta potential, the UV spectra, and the electrocatalytic reduction of water in the presence of the NPs. Thus, the results suggest that binding alkyl radicals to Pt⁰ surfaces might poison the NPs catalytic activity. When the Pt⁰-NPs are prepared by the reduction of a different precursor salt, PtCl₆^2-, nearly no C₂H₄ and oligomers are formed and the methyl coating covers a larger percentage of the surface Pt⁰ atoms. The difference is attributed to the morphology of the Pt⁰-NPs: those prepared from Pt(SO₄)₂ are twinned nanocrystals, whereas those prepared from PtCl₆^2- consist mostly of single crystals. Thus, the results indicate that the side products, or most of them at least, are formed on the twinned Pt⁰ nanocrystal edges created between (111) facets. In addition, the results show that Pt⁰-NPs react very differently compared with other noble metals, for example, Au⁰ and Ag⁰; this difference is attributed in part to the difference in the bond strength, (M⁰-NP)- CH₃, and should be considered in heterogeneous catalytic processes involving alkyl radicals as intermediates.