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 (D₂O*) 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 H₂, CO, CH₂CH₂ and CH₄ continued to increase with co-adsorbed D₂O*, up to two D₂O overlayers, despite separate increasing rates; above two D₂O overlayers, those of H₂, CO and CH₂CH₂ were approximately saturated while that of CH₄ decreased. The increased (or saturated) production probabilities are rationalized with an increased (saturated) concentration of surface hydroxyl (OD*, formed by O* abstracting D from D₂O*), whose intermolecular hydrogen bonding with adsorbed ethanol facilitates proton transfer from ethanol to OD* and thus enhances the reaction probability. The decreasing behavior of CH₄ could also involve the competition for H* with the formation of H₂ and HDO.