Kinetics of free radical generation in the catalytic oxidation of methanol

The formation of free radicals over the surface of platinum-containing catalysts in the methanol oxidation reaction depending on the temperature, the composition of the reaction mixture, and the procedure used for introducing platinum was studied by the matrix isolation method technique. The nature and transformations of surface intermediates depending oil the temperature and the presence of oxygen in the gas phase were studied by Fourier transform IR spectroscopy. The main surface intermediate was the methoxy group. The following three types of these groups were stabilized in alumina-based catalysts: (1) CH3O-Al-oct (v(s)(C-H) = 2806 cm(-1)), (II) CH3O-Al-tetr (v(s)(C-H) = 2825 cm(-1)), and (III) CH3O < (Al)(2) (v(s)(C-H) = 2845 cm(-1), delta(as)(C-H) = 1460 cm(-1), delta(s)(C-H) = 1440 cm(-1), r(\\) (CH3) = 1185 cm(-1), and v(C-O) = 1095 cm(-1)). At the same time, isolated methoxy groups (v(as)(C-H) = 2997 cm(-1), v(as)(C-H) = 2959 cm(-1), v(s)(C-H) = 2857 cm(-1), and delta(CH3) = 1450 cm(-1)) and hydrogen-bonded groups (v(O-H) = 3400-3550 cm(-1)), which resulted from chemisorption at siloxane bridges, were stabilized in silica gel-based catalysts. It was found that CH3O. and CH3OO. radicals were formed only over the surfaces of pure supports (SiO2 and Al2O3) and their mechanical mixtures with platinum. The total concentration of radicals was described by an extremal function of the composition of reactants, whereas the relative concentration depends on the nature of the support. This is conceivably due to the effect of coordinatively unsaturated cations of the support, which are formed by dehydroxylation in the course of catalyst pretreatment. An increase in the rate of formation of gas-phase radicals on mixed catalysts was explained by special properties of the platinum/support interface region, at which surface intermediates were formed in superequilibrium concentrations under reaction conditions.