It has been reported that ethanol and ethanol-water mixtures can be converted directly into H2 with ~100% selectivity and >95% conversion by catalytic partial oxidation on rhodium-ceria catalysts. [1] The oxidation products and intermediates from ethanol, ketones, and aldehydes, have been studied for their importance as chemical intermediates in industry.[2] Despite the numerous studies which have been conducted so far, microscopic understanding of the chemistry between ethanol and catalyst as well as the role of the water remains. To shed new light on these, the ethanol adsorption and dissciation on Rh(111) surface with and without presence of the water were studied by density functional theory calculations. [3] Our studies indicate that the H-bond plays important role on the stability and reactivity of the adsorbed ethanol.
First, the ethanol adsorption on Rh(111) surface are studied. Various structures of ethanol, including monomers, dimers, and one-dimensional chains, were studied in details. It is found that ethanol molecules prefer to adsorb at atop sites and bind to the surfaces through the oxygen atom, independent on the coverages and adsorption modes. Formation of H-bond is energetically favorable, and becomes dominated for adsorbed 1D ethanol chain. The softness of the hydroxyl stretching frequency is found after adsorption, and corresponding red shifts are sensitive to the adsorption modes and the formation of the H-bond, which can be served the fingerprint of the formation of the H-bond of alochol molecules.
With addition of the water, mixed one-dimensional chains between adsorbed ethanol and water molecules linked by the H-bond are found to be energetically favorable. The proton transfer becomes facile within the H-bond chain, which facilitates the dehydrogenation of ethanol to ethoxyl. Compared to the adsorbed ethanol monomer, calculated barrier for the dehydrogenation of the adsorbed ethanol molecules with presence of the water is reduced by 0.17 eV.
Reference:
[1] Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E. Science 2004, 303, 993. [2] Papageorgopoulos, D. C.; Ge, Q.; King, D. A. J. Phys. Chem. 1995, 99, 17645. [3] Hammer, B.; Hansen, L. B.; Nørskov, J. K. Phys. Rev. B 1999, 59, 7413.
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