IR spectroscopic and calorimetric study of water and formic acid adsorption on a vanadium-titanium catalyst

Abstract

Formaldehyde is oxidized to formic acid at a high selectivity over vanadium -titanium catalysts. IR spectroscopic and calorimetric techniques were used for studying the structure and energy parameters of the surface formic acid and water complexes on the of 11 wt % V2O5/89 wt % TiO2 catalyst. A decrease in the adsorption-desorption heat of formic acid from 100 down to 50 kJ/mol was observed at an increase in the surface coverage. The adsorption heat of formic acid in the form of bidentate formates is 70 kJ/mol under the conditions of the steady-state oxidation of formaldehyde that indicates, according to the earlier DFT calculations, a weak bonding of formate to the active site (V5+) to provide desorption of the intact acid from the vanadium-titanium catalyst surface. The weakly bonded formates are involved in the formation of formic acid during the catalytic oxidation of formaldehyde and provide the high selectivity of the reaction. The close adsorption heats of water (74 kJ/mol) and of formic acid indicate their possible competitive adsorption and may account for the favorable effect of water on the selectivity of formaldehyde oxidation to formic acid.