The internal energy of CO_2 produced by the catalytic oxidation of CH_3OH by O_2 on polycrystalline platinum

摘要

The dynamics of steady state catalytic methanol oxidation on a polycrystalline Pt surface over a range of surface temperatures and reactant flow conditions were investigated by monitoring the kinetics with mass spectrometry and the internal state distribution of nascent CO2 with tunable diode laser absorption spectroscopy. The results indicate that CO_2 formation proceeds via three distinct reaction pathways. The first produced CO_2, which is vibrationally excited relative to CO_2 in thermal equilibrium with the surface and shows preferential excitation in the asymmetric stretch. This pathway proceeds via the decomposition of CH3OH and the subsequent oxidation of nascent CO adsorbed to Pt in a weakly held precursor state. CO_2 production via this pathway is favored at high surface temperatures and high oxygen coverage. The second forms CO_2, which is vibrationally deactivated relative to CO_2 in thermal equilibrium with the surface and exhibits no preferential excitation among its three nondegenerate vibrational modes or the rotational energy. This pathway involves the decomposition of CH_3OH and subsequent oxidation of nascent CO adsorbed to Pt in a more strongly held chemisorbed state. CO_2 production via this pathway is favored at low surface temperatures and low oxygen coverage. The third forms CO_2 with preferential excitation in the asymmetric stretch but with less overall vibrational excitation than CO_2 from the first pathway and more vibrational excitation than CO_2 from the second. This third pathway occurs via the complete dehydrogenation of CH_3OH and subsequent oxidation of nascent CO adsorbed to Pt in a bridged state bound through both ends of the molecule. CO_2 production via this pathway is favored at intermediate surface temperatures and oxygen coverage, conditions which favor overall oxidation to form CO_2.