The double peaks and symmetric path phenomena in the catalytic activity of Pd/Al2O3-TiO2 catalysts with different TiO2 contents

Abstract

Physicochemical properties of Pd/Al2O3-TiO2 catalysts with different amounts of TiO2 contents were investigated by XRD, nitrogen adsorption-desorption, FTIR, NH3-TPD, H2-TPR and XPS techniques. Catalysts of different compositions were tested in the ethanol oxidation reaction to study the effects of TiO2 contents. Double peaks and symmetric path phenomena were observed at certain temperatures with the increase in TiO2 contents. The symmetric peak phenomena and the diverse activity fluctuations have been ascribed to the controlling factors such as temperature and compositions. With the increase in TiO2 content, the surface area, adsorbed oxygen contents and surface acid quantity decreased gradually. The large surface area and adsorbed oxygen contents were conducive to the performance, while increased acid amounts were not beneficial for ethanol oxidation. At 150 and 175 °C, Pd/AT(X11Pd/AT(X): Pd/Al2O3-TiO2(X = X wt% of TiO2).) (X < 50) catalysts performed better in ethanol oxidation because of the main factors of surface area and adsorbed oxygen. Owing to the stronger effects of acid quantity compared to other factors, Pd/AT(X) (X < 50) catalysts showed lower catalytic performance at 200 and 225 °C. The higher contents of low-temperature acid in the NH3-TPD results and the larger difference between Pd0 and Pd2+ in XPS analyses were the origins of the double peaks of higher activity for the Pd/AT20 and Pd/AT80 catalysts.