Elucidating the Roles of Amorphous Alumina Overcoat in Palladium-Catalyzed Selective Hydrogenation

Amorphous alumina overcoats generated by atomic layer deposition (ALD) have been shown to improve the selectivity and durability of supported metal catalysts in many reactions. Several mechanisms have been proposed to explain the enhanced catalytic performance, but the accessibilities of reactants through the amorphous overcoats remain elusive, which is crucial for understanding reaction mechanisms. Here, we show that an AlO_x ALD overcoat is able to improve the alkene product selectivity of a supported Pd catalyst in acetylene (C₂H₂) hydrogenation. We further demonstrate that the AlO_x ALD overcoat blocks the access of C₂H₂ (kinetic diameter of 0.33 nm), O₂ (0.35 nm), and CO (0.38 nm) but allows H₂ (0.29 nm) to access Pd surfaces. A H-D exchange experiment suggests that H₂ might dissociate heterolytically at the Pd-AlO_x interface. These findings are in favor of a hydrogen spillover mechanism.