Reactions of Radicals at Surfaces

Tomer Zidki, Biological Chemistry, Ariel University, Raanana, Israel
Ronen Bar-ziv, Chemistry, Nuclear Research Centre Negev, Beer-Sheva, Israel

Radicals are formed near surfaces in a variety of processes, e.g. in catalytic processes, in electrochemistry, in photo-catalytic processes, in environmental processes etc. It was therefore decided to measure the mechanisms and kinetics of reaction of M°-nanoparticles (NPs), M = Ag; Au; Cu; Pt; Pd, Pt/Au-alloy-NPs and TiO₂-NPs with methyl radicals. (All the M°-NPs were prepared by reduction of the corresponding salts with NaBH₄.) These reactions are very fast, approaching the diffusion-controlled limit, forming long-lived transients with (M°-NP)-(CH₃)_n s bonds. These transients decompose to yield C₂H₆ for Ag°-, Au°- and TiO₂-NPs, CH₄ for Cu°-NPs, for Pt°- and Pd°-NPs most methyls remain bound to the NPs, and are released as methane when H₂ is added to the suspension, though some C₂H₆, C₂H₄ and oligomerization products are formed.

Hydrous oxides of transition metals and in particularly of Co are found to be good catalysts for the water oxidation reaction. However, the catalytic reaction takes place at pH > 7 where the degree of Co hydrolysis is rather high which results in precipitation and de-activation of the catalyst. The deposition of Co hydroxide on silica NPs prevents these precipitation and de-activation. The resultant catalyst is optically transparent over the entire UV−Vis range and is suitable for mechanistic investigations by time-resolved spectroscopic techniques such as stopped flow, flash photolysis, and pulse radiolysis. The catalyst also shows high stability; no deactivation or cobalt precipitation was observed upon multiple cycling.