野崎 安衣

Pt is an excellent and widely used hydrogen evolution reaction (HER) catalyst. However, it is a rare and expensive metal, and alternative catalysts are being sought to facilitate the hydrogen economy. As tungsten carbide (WC) has a Pt-like occupied density of states, it is expected to exhibit catalytic activity. However, unlike Pt, excellent catalytic activity has not yet been observed for mono WC. One of the intrinsic differences between WC and Pt is in their magnetic properties; WC is non-magnetic, whereas Pt exhibits high magnetic susceptibility. In this study, the WC lattice was doped with ferromagnetic Co nanocrystals to introduce an ordered-spin atomic configuration. The catalytic activity of the Co-doped WC was ~30% higher than that of Pt nanoparticles for the HER during the hydrolysis of ammonia borane (NH3BH3), which is currently attracting attention as a hydrogen fuel source. Measurements of the magnetisation, enthalpy of adsorption, and activation energy indicated that the synergistic effect of the WC matrix promoting hydrolytic cleavage of NH3BH3and the ferromagnetic Co crystals interacting with the nucleus spin of the protons was responsible for the enhanced catalytic activity. This study presents a new catalyst design strategy based on the concept of an internal magnetic field. The WC-Co material presented here is expected to have a wide range of applications as an HER catalyst.

In recent years, chemical hydrogen storage systems utilizing compounds with a high hydrogen capacity have attracted significant attention. In the present study, Ru-Fe supported on porous CeO2 was prepared for application in hydrogen generation systems using ammonia borane. The catalytic activity of the material for partial oxidation was elucidated. Specifically, the oxidation pathway was evaluated by thermodynamic calculations using the Ru-Fe-O ternary phase diagram. It was determined that the Ru-Fe alloy exhibited enhanced catalytic activity compared to pure Ru particles. In the alloy, the Ru component was self-organized, forming nailed particles in the oxidation films during preferential oxidation of Fe. The exposed nailed Ru particles prevented self-aggregation and coarsening, which was proposed as one of the reasons for the excellent catalytic activity. Moreover, it was speculated that in the exposed Ru-Fe alloy matrices, the density of states in the vicinity of the Fermi level was high due to the charge transfer from Fe to Ru. This meant that the number of molecular levels contributing to the catalytic reaction increased. It was concluded that a synergistic effect of the intrinsic charge state of the Ru-Fe alloy and the self-organized Ru assembly in the oxidation film resulted in remarkable catalytic activity.