Surface Reactions at Metal Oxides: Relaxation Spectroscopy and Charge Transfer
Ga203 films are exposed to square pulses of methane. The reaction products formed on the surface are detected by a mass spectrometer. The desorption energies and the frequency factors are calculated for H20 (Ea=0.2 eV, fa=2×103 s-1) and C02 (Ea=1.0 eV, fa=100×103 s-1). Semiempirical Hartree-Fock calculations with the MNDO-PM3 method give information about the adsorption process. The energy gain of physisorption amounts to 0.27 eV. The heat of formation of the chemisorbed CH3 group and the H atom is 4.05 eV lower in comparison to the value of the physisorbed methane. After the COlatt. group formation the energy gain is 7.33 eV. The binding energy of the formed COlatt. is calculated to 2.07 eV. The charge transfer is estimated using the mulliken charges. A reaction scheme for the catalytic oxidation of methane by lattice oxygen is proposed. The conductance increase of Ga203 on exposure to methane can be explained by the creation of oxygen vacancies and by the formation of absorbed hydrogen, both acting as surface donors.
KeywordsTitanium Methane Graphite Platinum Gallium
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