Mechanism and kinetics of methanol synthesis on zinc oxide

Abstract

The kinetics of the adsorption and surface reactions of hydrogen, water, carbon dioxide, carbon monoxide, formaldehyde and methanol on what is mainly the prism face of zinc oxide have been studied using temperature programmed desorption and reaction. Both hydrogen and carbon dioxide show a multiplicity of desorption energies, adsorption into those states showing the highest binding energies being activated. Temperature programming after the room-temperature adsorption of formaldehyde or methanol shows evidence of surface reaction, with the formation of a formate intermediate. The surface reaction mechanisms of this formate intermediate and their kinetics are identical regardless of whether it was formed from methanol adsorption or formaldehyde adsorption. The formate appears to be the pivotal intermediate in zinc oxide catalysed synthesis gas chemistry, decomposing (a) to carbon monoxide and hydrogen or (b)(depending on the hydrogen coverage) to methanol. The same formate intermediate is formed by co-adsorption of carbon dioxide and hydrogen while hydrogen/carbon monoxide dosing does not result in its formation; indeed carbon monoxide itself did not adsorb on the defected zinc oxide.