Quantitative determination of the number of active surface sites and the turnover frequencies for methanol oxidation over metal oxide catalysts: I. Fundamentals of the methanol chemisorption technique and application to monolayer supported molybdenum oxide catalysts

Abstract

A detailed study of the methanol chemisorption and oxidation processes on oxide surfaces allowed the development of a method to quantify the number of surface active sites (Ns) of metal oxide catalysts. In situ infrared analysis during methanol adsorption showed that molecular methanol and surface methoxy species are co-adsorbed on an oxide surface at room temperature, but only surface methoxy species are formed at 100°C. Thermal stability and products of decomposition of the adsorbed species were determined with temperature programmed reaction spectroscopy (TPRS) experiments. Controlled adsorption with methanol doses resulted in a stable monolayer of surface methoxy species on the oxide surfaces. The stoichiometry of methanol chemisorption resulted in one surface methoxy adsorbed per three Mo atoms for polymerized surface molybdenum oxide structures, regardless of surface molybdenum oxide coordination. The activity of the catalysts per surface active sites (turnover frequencies TOF) was calculated in order to quantitatively compare the reactivity of a series of monolayer supported molybdenum oxide catalysts. The TOF value trends reflect the influence of the bridging Mo-O-Support bond and the electronegativity of the metal cation of the oxide support.