Catalytic consequences of the thermodynamic activities at metal cluster surfaces and their periodic reactivity trend for methanol oxidation

Abstract

The periodic reactivity trend and the connection of kinetics to the thermodynamic activity of oxygen are established for the oxidation of methanol on metal clusters. First-order rate coefficients are a single-valued function of the O2 -to-CH3OH ratio, because this ratio, together with the rate constants for O2 and CH3OH activation, determine the oxygen chemical potential, thus the relative abundance of active sites and bulk chemical state of the clusters. CH3OH activation rate constants on oxygen-covered Ag, Pt, and Pd and on RuO2 clusters vary with the metal-oxygen binding strength in a classical volcano-type relation, because the oxygen-binding strength directly influences the reactivities of oxygen as H abstractors during the kinetically relevant CH3OH activation step. The differences in oxygen thermodynamic activity lead to five orders of magnitude variation in rates (Pt>Pd>RuO2>Ag, 373 K), because of its strong effects on the activation enthalpy and more prominently activation entropy in CH3OH activation.

Keywords: heterogeneous catalysis; methanol; oxidation; oxygen chemical potential; periodic reactivity.