Vibrationally Promoted Dissociation of Water on Ni(111)

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Science  02 May 2014:
Vol. 344, Issue 6183, pp. 504-507
DOI: 10.1126/science.1251277

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Vibrating Water Apart

The main route for producing hydrogen for industrial chemical synthesis is steam reforming, in which water and methane react at high temperatures on nickel catalysts to produce hydrogen and carbon dioxide. For both water and methane, the initial dissociation step can be promoted by the translational energy of a molecule as well as its internal vibrational energy, and fundamental studies of these reactions try to determine the relative contributions of these pathways. Although the methane reaction has been well studied, only recently have lasers been available to excite the higher stretching vibrations of water. Hundt et al. (p. 504) now report a joint experimental and theoretical study of D2O dissociation on the Ni(111) surface. For a given input of energy, vibrational energy was more effective for surmounting the reaction barrier than translational energy.


Water dissociation on transition-metal catalysts is an important step in steam reforming and the water-gas shift reaction. To probe the effect of translational and vibrational activation on this important heterogeneous reaction, we performed state-resolved gas/surface reactivity measurements for the dissociative chemisorption of D2O on Ni(111), using molecular beam techniques. The reaction occurs via a direct pathway, because both the translational and vibrational energies promote the dissociation. The experimentally measured initial sticking probabilities were used to calibrate a first-principles potential energy surface based on density functional theory. Quantum dynamical calculations on the scaled potential energy surface reproduced the experimental results semiquantitatively. The larger increase of the dissociation probability by vibrational excitation than by translation per unit of energy is consistent with a late barrier along the O-D stretch reaction coordinate.

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