The Origin of the Ligand Effect in Metal Oxide Catalysts: Novel Fixed-Bed in Situ Infrared and Kinetic Studies during Methanol Oxidation

Abstract

Supported and bulk metal oxide catalysts generally exhibit a “ligand effect” in the selective oxidation of methanol to formaldehyde. This phenomenon is identified by orders of magnitude variations in the turnover frequency (TOF; TOF = activity per active surface site) of the same active metal oxide metal atom, for example, V or Mo active sites, as changes are made in the choice of support cation or bulk mixed metal oxide co-cation. The mechanistic origin of this ligand effect has been investigated in the present study using a novel in situ IR cell designed to operate as a fixed-bed catalytic reactor with coupling to an online gas chromatograph for analysis of gas-phase product distributions. The active site determinations required for calculation of TOFs were performed using methanol chemisorption and IR spectroscopy. Moreover, under actual methanol oxidation reaction conditions, quantification of the adsorbed, steady-state concentrations of methoxylated surface intermediates by in situ IR spectroscopy allowed for decoupling of the reaction mechanism to yield individual estimates of the adsorption and surface reaction kinetic parameters. Comparisons of the relative values of the adsorption equilibrium constant, Kads, and the kinetic rate constant for the surface decomposition step, krds, indicate that the TOF clearly correlates with the rate constant for the surface decomposition step. For example, supported vanadia catalysts exhibit a 12-fold increase in the surface decomposition rate constant, krds, for vanadia supported on silica, alumina, titania, and ceria supports. Conversely, the adsorption constant, Kads, is relatively invariant for vanadia supported on silica, alumina, and titania, decreasing in value by about half only for the vanadia/ceria catalyst. It also appears that the origin of the ligand effect is fundamentally related to the electronegativity of the ligand cation, the more electropositive ligands (titania and ceria support cations) always having higher TOFs than the more electronegative ligands (alumina and silica support cations). Most likely, the ligand cation electronegativity affects the magnitude of the rate-determining surface reaction step, krds, via its influence on the ability of the active metal cation to decompose the adsorbed methoxylated surface intermediates by hydride abstraction of methyl hydrogen.