Catalytic oxidation of alcohol on metal or metal-oxide clusters occurs via kinetically coupled steps of reductant and oxidant activation to evolve partially oxidized intermediates (e.g. formaldehyde or methyl formate from methanol) as the desired products. Oxidation of these intermediates in sequential, undesired steps leads to carbon dioxide and water. In this contribution, we adapt kinetic, isotopic, and site titration methods to probe the periodic reactivity trend and the catalytic effects of chemical states, oxygen binding strength, and operating oxidant-to-reductant ratio during methanol oxidation on Pt, Pd, Ru, and Ag metal or oxide clusters and then describe their mechanistic synergies. First-order rate coefficients are a single-valued function of the oxidant-to-reductant ratio, because this ratio determines the oxygen and carbon chemical potentials at catalytic surfaces, which in turn dictates the relative abundances of chemisorbed oxygen atoms, vacant sites, and carbon atoms as well as and the bulk chemical state of the clusters. We describe the periodic reactivity trend and structure sensitivity effects, interpret them in terms of activation enthalpy and entropy, and then draw a connection between these trends and the chemical potential at cluster surfaces, a thermodynamic quantity that prescribes the coverages of reactive species and site occupancies. CH3OH conversion rates on oxygen saturated surfaces exhibit a classical volcano-type relation with the oxygen binding strength, because oxygen atom acts as the active site in the kinetically-relevant H abstraction step and influences the extent of methoxy stabilization at the methanol activation transition state. The kinetic phenomena relating the catalytic activities and the thermodynamic quantities of active sites are demonstrated here for several metal and metal oxide clusters and appear to be general for oxidation reaction systems (alcohol, alkene, and alkane oxidation).  [1] Tu, WF. and Chin, Y-H. J. Catal, 312 (2014) 1.

Connection of thermodynamic quantities with reactivity trends during catalytic oxidation of methanol on metal and metal-oxide clusters

248th National Meeting (2014)

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