Methanol adsorption and decomposition on rhodium

Abstract

The decomposition of methanol on rhodium probed from ~200 atomic sites of the (001) or (111) planes of Rh field emitter crystals but randomly with regard to crystallographic zones was studied by pulsed field desorption mass spectrometry. High electric field pulses were used to quantitatively desorb the final products, carbon monoxide and hydrogen, thus achieving steady-state conditions. Substantial amounts of methoxy (mainly desorbed as CH3 + ions) were also present at the surface. Applying a steady electric field, Fr $ ̌= 4 V/nm, between the field pulses, led to a deceleration of the decomposition reaction and to an increase of the amount of adsorbed CH3O and CH2O species. There were indications that the rate-determining step of the reaction is CH bond cleavage in adsorbed methoxy to form the CH2O intermediate. This was supported by the observation of a kinetic isotope effect in the formation of CD2O and CHDO from methy-d2-alcohol, CHD2OH. Here, the CH bond breaking to form the CD2O was found to be twice as fast as the breaking of the CD bond which results in CHDO. Field ion microscopy was applied to investigate the influence of the reaction on the structure of the whole hemispherical single crystal surface. There were neither topographic changes nor corrosion of the Rh surface after field-free exposure to 2 Pa methanol at temperatures Up to 423 K. © 1989.