Authors

Abstract

This work investigated the effect of the addition of a second metal (Cu, Ag, Zn, Sn) on the performance of Pd/ZrO2 catalyst for HDO of phenol at 573 K in the gas phase. The incorporation of dopants resulted in the formation of Pd–X (Cu, Ag, Zn) alloys, which reduced the reaction rate for HDO and increased the selectivity to hydrogenation products (cyclohexanone and cyclohexanol). The lower activity of the bimetallic catalysts was due to the segregation of the second metal on the surface of the Pd particle. For PdSn/ZrO2, alloying was also observed but tin oxide was still present on the surface after reduction at 773 K. For Pd and PdSn/ZrO2, the oxophilic sites represented by Zr and Sn cations promotes the hydrogenation of the carbonyl group of the keto-tautomer intermediate formed, producing benzene as the main product. All catalysts significantly deactivated during the reaction but the deactivation degree depended on the type of the metal. Pd/ZrO2 and PdZn/ZrO2 and PdAg/ZrO2 exhibited approximately the same deactivation degree. However, the loss of activity was less pronounced for PdSn/ZrO2 catalyst. Pd dispersion significantly decreased during the reaction, indicating that the sintering of Pd particles is one of the causes for catalyst deactivation.