The internal energy of CO2 produced by the catalytic oxidation of CH3OH by O2 on polycrystalline platinum

Abstract

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 CO(2) 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 CH(3)OH 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(3)OH 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(3)OH 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).