Dependence on co-adsorbed water in the reforming reaction of ethanol on a Rh(111) surface.

We have studied the reforming reaction of ethanol co-adsorbed with atomic oxygen (O*, * denotes adspecies) and deuterated water (DO*) on a Rh(111) surface, with varied surface probe techniques under UHV conditions and with density-functional-theory calculations. Adsorbed ethanol molecules were found to penetrate readily through pre-adsorbed water, even up to eight overlayers, to react at the Rh surface; they decomposed at a probability promoted by the water overlayers. The production probabilities of H, CO, CHCH and CH continued to increase with co-adsorbed DO*, up to two DO overlayers, despite separate increasing rates; above two DO overlayers, those of H, CO and CHCH were approximately saturated while that of CH decreased. The increased (or saturated) production probabilities are rationalized with an increased (saturated) concentration of surface hydroxyl (OD*, formed by O* abstracting D from DO*), whose intermolecular hydrogen bonding with adsorbed ethanol facilitates proton transfer from ethanol to OD* and thus enhances the reaction probability. The decreasing behavior of CH could also involve the competition for H* with the formation of H and HDO.