Catalytic decomposition of formic acid on sodium tungsten bronzes

Abstract

The kinetics of the catalytic decomposition of formic acid on sodium tungsten bronzes, Na_xWO₃ with x in the range 0.11–0.85, and on tungstic oxide have been investigated manometrically in a static system at 150–250°C with acid pressures of 25–30 Torr. The decomposition products were CO₂, CO, H₂O and H₂, the mole ratios CO₂ : CO and H₂ : CO₂ being determined with a mass spectrometer. Initial (zero order) rates were used in conjunction with the ratios CO₂: CO to estimate the relative dehydration and dehydrogenation activities of the catalysts as a function of sodium content. Electrical conductivity measurements (a.c. and d.c.) were carried out on the catalysts under reaction conditions.

By use of Fuchs' model of the bronze structure, in which clustering, in contrast to random distribution of sodium ions is postulated, the catalytic activity of the bronzes is interpreted in terms of geometric rather than electronic factors: dehydration of formic acid involves adsorption on adjacent sodium and oxygen vacancies, while dehydrogenation requires adsorption on a sodium cluster site. The marked change from dehydration to dehydrogenation activity, which occurs when the x-value increases beyond ∼ 0.7, is related to the reported ordering of the residual sodium vacancies which occurs near x= 0.75.