Structure–Activity–Selectivity Relationships in Propane Dehydrogenation over Rh/ZrO₂ Catalysts

A few years ago, we introduced alternative-type bulk ZrO₂-based catalysts for nonoxidative propane dehydrogenation (PDH). Currently, they belong to the state of the art catalysts owing to their environmental compatibility, high activity, propene selectivity, and durability. However, the structure–activity–selectivity relationships are still not appropriately understood. To close such gaps, we focused on elucidating the role of surface defects (coordinatively unsaturated Zr (Zr_cus) sites) and supported Rh nanoparticles (NPs) in Rh/ZrO₂ for activity and selectivity in the PDH reaction. Relevant physicochemical properties were analyzed by complementary experimental techniques, while details of catalyst functioning on an elementary-step level were derived from density functional theory calculations. Two types of Zr_cus sites responsible for propane dehydrogenation were suggested to exist on the surface of ZrO₂. Those located at Rh NPs reveal higher intrinsic activity owing to the positive effect of the metal on hydrogen desorption, which is the rate-limiting step in the PDH reaction over bare ZrO₂. However, when the reduction degree of ZrO₂ is increased, propene strongly adsorbs on Rh, resulting in blockage of sites for hydrogen recombination. Consequently, the accelerating effect of the metal is hindered. Moreover, the strong propene adsorption plays a negative role in propene selectivity due to favoring conversion of the adsorbed propene into coke. The most active Rh/ZrO₂ catalyst revealed higher activity in comparison with the state of the art Ru/YZrO_x and an analogue of commercial K-CrO_x/Al₂O₃. It was also durable over 60 PDH/regeneration cycles at 550, 600, and 625 °C lasting 11 days in total.