Molecular-beam studies of methanol partial oxidation on Cu(110)

Abstract

A thermal molecular-beam system has been used to examine the oxidative dehydrogenation of methanol on the Cu(110) surface. The initial sticking probability for oxygen is 0.21 (±0.01) at room temperature and shows a near linear dependence of sticking coefficient on atomic coverage due to an island growth mechanism with dissociation on an oxygen dilute inter-island phase. Beam temperature variations show this adsorption to be activated. The reaction of methanol in the beam with a predeposited patch of oxygen depends strongly on surface temperature and oxygen coverage. There is a change in stoichiometry in the reaction from 2CH₃OH + O_(a)→ 2H₂CO + H₂+ H₂O at 330 K to CH₃OH + O_(a)→ H₂CO + H₂O above ca. 450 K. Oxygen promotes the methanol adsorption and reaction at low coverages, but shows poisoning effects at half a monolayer [saturation, p(2 × 1) structure] where the rate of reaction is very much reduced, and there is an induction time before products are seen in the gas phase (ca. 30 min at 333 K under these conditions). This is explained by stabilisation of the methoxy species when the adjacent (110) trough sites are blocked by either adsorbed oxygen or hydroxyl groups; a kinetic model is being developed to describe these complex kinetics, based on a slow production of vacant sites in these (110) troughs. This is shown to describe the kinetics quite well in a semi-quantitative manner. Use of CD₃OD in the beam shows a marked isotope effect, whereas CH₃OD does not, again indicating that it is methoxy decomposition which limits the product evolution rate.