Bifunctional metal–acid sites on nickel boride catalysts: Phenol hydrodeoxygenation and water-promoted C = C hydrogenation

Abstract

Modification of earth-abundant metal catalysts with optimized catalytic performance can be potential alternatives to precious metal catalysts for more sustainable processes. Using a chemical reduction method, nickel boride (Ni3B) alloy catalysts were prepared with a Ni/B ratio of 3. Ex-situ and in-situ characterization indicates that the structure of Ni3B changes from amorphous to crystalline with increasing pretreatment temperatures. The presence of B does not seem to alter the electronic properties of Ni. However, after pretreatment, oxidized B species (i.e., BOx and H3BO3) ex-solve from the bulk, leading to the formation of a B-enriched surface. In-situ titration and dehydration reaction experiments demonstrate the presence of acid sites, which play an important promoting role in the hydrogenation of C = C bonds. Moreover, Ni3B catalysts show a remarkable activity towards phenolic ring hydrogenation with the added ability to deoxygenate the secondary products via dehydration on the acid sites. Under water-free reaction conditions, the Ni3B catalysts exhibit similar hydrogenation activity to that of a pure Ni catalyst. Remarkably, the addition of small amounts of water leads to a > 100-fold increase on Ni3B catalysts, while it inhibits the reaction on bare Ni catalyst by blocking active sites. These observations highlight the cooperative role of bifunctional sites in Ni3B catalysts and solvent composition for C = C hydrogenation reactions.