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 (D 2 O*) 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 2 , CO, CH 2 CH 2 and CH 4 continued to increase with co-adsorbed D 2 O*, up to two D 2 O overlayers, despite separate increasing rates; above two D 2 O overlayers, those of H 2 , CO and CH 2 CH 2 were approximately saturated while that of CH 4 decreased. The increased (or saturated) production probabilities are rationalized with an increased (saturated) concentration of surface hydroxyl (OD*, formed by O* abstracting D from D 2 O*), 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 4 could also involve the competition for H* with the formation of H 2 and HDO.