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 ALD overcoat is able to improve the alkene product selectivity of a supported Pd catalyst in acetylene (CH) hydrogenation. We further demonstrate that the AlO ALD overcoat blocks the access of CH (kinetic diameter of 0.33 nm), O (0.35 nm), and CO (0.38 nm) but allows H (0.29 nm) to access Pd surfaces. A H-D exchange experiment suggests that H might dissociate heterolytically at the Pd-AlO interface. These findings are in favor of a hydrogen spillover mechanism.