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

Highlights: • The activity otherness at some specific temperatures illustrated the effects of TiO₂. • The double peaks and symmetry phenomena were observed with the increase of TiO₂. • Surface area, adsorbed oxygen content and surface acid had influence on the activity. • The symmetry phenomena were ascribed to the different controlling factors. • Double peaks were related to low-temperature acid and otherness between Pd⁰ and Pd²⁺. - 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(X¹) (X 3-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.