Adsorption studies of H2 isotopes on ZnO: Coverage-induced IR frequency shifts and adsorbate geometry

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Abstract
The coverage dependence of the IR stretching frequencies for dissociative type I adsorption of H2 and D2 on ZnO powders has been measured using transmission IR spectroscopy. By comparing the frequency shifts observed when the isotopic composition of the adsorbate is varied at constant total coverage with the shifts observed when the total coverage of either pure component is varied, we can separate the dynamic and static contributions to the coverage‐induced frequency shifts. The ZnH and ZnD shifts are due primarily to electrodynamic interactions. The observed shifts are in good agreement with the Hammaker model for dynamic dipole–dipole interactions, if adsorption is assumed to occur on (2×2) reconstructed ZnO(0001) surface planes. In contrast, the OH and OD shifts are due to electrostatic and inductive interactions. The electrostatic contribution can be estimated using a modification of Buckingham’s treatment of local environment effects. A qualitative model based on electron localization effects is presented to explain the observed inductive contribution.