Role of hydroxyl on metal surface in hydrogenation reactions

The important role of surface hydroxyl in metal catalysis is attracting more attentions but rarely studied for liquid-phase hydrogenation reactions. Here we report such deliberate research with two model substrates, p-chloronitrobenzene and cinnamaldehyde. Graphene-supported iridium nanoparticles with hydroxyl on the metal surface were prepared by hydrothermal synthesis, and the surface hydroxyl can be removed by a mild pre-reduction. The catalyst with surface hydroxyl exhibited around three times higher rate for the hydrogenation of p-chloronitrobenzene than the catalyst without hydroxyl, and an opposite trend was found for the hydrogenation of cinnamaldehyde. Kinetic study revealed that although the surface hydroxyl halved the coverage of H on iridium, it increased the rate constant by a factor of eight through reducing the reaction barrier for the hydrogenation of p-chloronitrobenzene. Theoretical calculation confirmed that the surface hydroxyl can polarize the nitro group via a weak hydrogen-bond interaction and thus facilitate its hydrogenation.