STRUCTURE SENSITIVITY AND SELECTIVITY OF PROTON ABSTRACTION FROM ALKYNES ON THE POLAR FACES OF ZINC-OXIDE
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The terminal alkynes, C2H2, CH3CCH, and C6H5CCH, decomposed on the (0001)-Zn polar face of ZnO, but not on the (0001)-O polar face. These alkynes were ultimately dehydrogenated on the (0001)-Zn polar face to produce water and to deposit carbon atoms on the surface. Adsorbed carbon was oxidized by lattice oxygen during TPD experiments, giving rise to CO and CO2 desorption peaks at 780 K. _Carbon oxidation was also found to be accompanied by zinc metal evolution. In contrast to the (0001)-Zn face, the (0001)-O face was found to be inactive for dehydrogenation of alkynes, and XPS results indicated that the alkynes did not adsorb dissociatively on this surface. Acetylene and phenylacetylene formed the corresponding acetylide species via dissociative adsorption on the active Zn-polar face. Methylacetylene adsorbed dissociatively on the (0001)-Zn polar face via abstraction of a methyl proton to form stable propargyl species exclusively. This result contrasts with the selectivity of proton abstraction both in the gas phase and on Ag(110), for which abstraction of the acetylenic proton is preferred. These results suggest that covalent contributions to the surface-adsorbate bond on ZnO may play a significant role in determining the selectivity of acid-base reactions on the (0001)-Zn surface. 1987 American Chemical Society.
