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

Abstract

It was recently reported that the reaction of methyl radicals with Pt(0) nanoparticles (NPs), prepared by the reduction of Pt(SO4)2 with NaBH4, is fast and yields as the major product stable (Pt(0)-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 Pt(0)-NPs. The results show that the coating can cover up to 75% of the surface Pt(0) atoms. The rate constant of the reaction, k((.)CH3+Pt(0)-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 Pt(0) surfaces might poison the NPs catalytic activity. When the Pt(0)-NPs are prepared by the reduction of a different precursor salt, PtCl6(2-), nearly no C2 H4 and oligomers are formed and the methyl coating covers a larger percentage of the surface Pt(0) atoms. The difference is attributed to the morphology of the Pt(0)-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 Pt(0) nanocrystal edges created between (111) facets. In addition, the results show that Pt(0)-NPs react very differently compared with other noble metals, for example, Au(0) and Ag(0); this difference is attributed in part to the difference in the bond strength, (M(0)-NP)-CH3, and should be considered in heterogeneous catalytic processes involving alkyl radicals as intermediates.

Keywords: heterogeneous catalysis; methyl radicals; nanoparticles; platinum; reaction mechanisms.