A Career in Catalysis: Kazunari Domen

Professor Kazunari Domen has devoted his research career in catalysis to development of water splitting photocatalysts, mechanocatalytic water splitting, transition metal oxide mesoporous materials, transmission infrared spectroscopy, infrared reflection absorption spectroscopy (IRAS), time-resolved infrared laser spectroscopy, surface nonlinear spectroscopy such as interfacial sum-frequency spectroscopy (SFG), carbon-based acid materials, and layered solid acid materials. In photocatalysis, layered oxide photocatalysts and oxynitride and oxysulfide photocatalysts with visible-light response have been developed. Solar water splitting with 100 m² of a scale was demonstrated using a powdered RhCrO_x/SrTiO₃:Al/CoOOH photocatalyst with almost 100% of a quantum yield. Mesoporous Ta₂O₅ with different wall thicknesses was also investigated as a photocatalyst for water splitting. The amorphous Ta₂O₅ phase of the thick-walled sample was crystallized with maintaining the original porous structure for the improvement of its photocatalytic activity. In contrast to the photocatalytic water splitting, mechanocatalytic water splitting under dark conditions was found using several metal oxides such as Cu₂O, NiO, Co₃O₄, and Fe₃O₄ suspended in distilled water with magnetic stirring in a glass reaction vessel at room temperature without any inputs of other electrical or photonic energies. The observed mechanocatalytic water splitting into H₂ and O₂ was triggered by the conversion of the mechanical energy provided by the friction of these oxide powders against the bottom wall of the reaction vessel with a stirring bar. In spectroscopies to see a mechanism of catalysis, transmission infrared (IR) spectroscopy was applied for the clarification of the adsorption structure and reaction mechanisms on powdered catalysts represented by transition metal oxides and zeolites, in addition to the adsorption of diatomic molecules at the low-temperature range to probe active surface sites. Catalytic reaction mechanisms of adsorbed species on single crystal surfaces were also investigated using infrared reflection absorption spectroscopy (IRAS) and interfacial sum-frequency spectroscopy (SFG). In solid acid catalysts, a carbon-based solid acid, an amorphous carbon material consisting of small polycyclic aromatic carbon sheets with attached SO₃H groups, worked as a solid “sulfuric acid”, which exhibited high catalytic performance for various acid-catalyzed reactions, including hydrolysis of cellulose. Transition metal oxide nanosheets such as HTiNbO₅, HNb₃O₈, and HNbWO₆ were also applicable for solid acid catalysts. The strong Bro̷nsted acid sites emerged on the surface in the form of bridging hydroxyl groups, M-OH-M′. Layered mixed molybdates, HNbMoO₆ and HTaMoO₆ were found to exhibit remarkable activity due to their unique intercalation behavior.