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Mechanism for the catalytic exchange of methane with deuterium on Pt(111) surfaces

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Abstract

The reaction of methane with deuterium on Pt(111) single crystals was studied using thermal programmed desorption (TPD) under ultra-high vacuum conditions. Methyl groups, believed to result from the dissociative adsorption of methane under catalytic conditions, were deposited directly by thermal decomposition of methyl iodide. TPD experiments after coadsorption with deuterium show that methane formation occurs in very high yields. Furthermore, the desorbing molecules display a bimodal distribution very similar to that obtained under atmospheric pressures, with maxima for CH3D and CD4. This distribution can be explained by a competition between two surface reactions, namely, direct incorporation of hydrogen in methyl groups and methylene formation followed by rapid interconversion to methylidyne before final hydrogenation to methane. We found that the rates for both reactions depend linearly on the coverage of methyl groups, but while the hydrogenation rate is also proportional to the coverage of deuterium, in the case of multiple H-D exchange an almost zero order dependence at low temperatures slowly becomes first order as the reaction temperature is increased. The activation energies for CH3D and CD4 formation were estimated to be 17.0 and 6.5 kcal/mol respectively even though the reaction rates displayed comparable values because of a compensation effect in the preexponential factors. Finally, the relative yields for CH2D2, CHD3 and CD4 are close to the equilibrium values, a result that is consistent with the proposed methylene-methylidyne interconversion step in our mechanism.

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Zaera, F. Mechanism for the catalytic exchange of methane with deuterium on Pt(111) surfaces. Catal Lett 11, 95–104 (1991). https://doi.org/10.1007/BF00866906

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Keywords

  • Methyl
  • methyl iodide
  • methane
  • H-D exchange
  • platinum