Prof. Hiroshi Kitagawa has explored solid-solution alloying and multi-element nanomaterials using combinations of elements that do not mix in bulk, employing non-equilibrium synthesis methods such as the hydrogen process, spray simultaneous reduction, and subcritical/supercritical flow methods. He has successfully synthesized numerous new materials and developed new functions, primarily catalysts. His main achievements are introduced below.

1. Development and Functional Exploration of Binary Solid-Solution Nanoalloys Using Immiscible Elements
Unlike phase-separated alloys, where two or more metals (or alloys) form distinct domains that separate, solid-solution alloys exhibit homogeneous atomic-level mixing of two or more elements. Solid-solution alloys allow continuous control of physical properties through their composition and enable maximization of synergistic effects between elements. However, most binary alloys exhibit phase separation near room temperature and ambient pressure, making synergistic effects between elements difficult to achieve. To overcome this, Dr. Kitagawa developed two non-equilibrium synthesis methods; the hydrogen process and the spray simultaneous reduction method. These techniques enable solid-solution alloying (inter-elemental fusion) of combinations of elements that do not mix in bulk, leading to the successful development of numerous new materials. He succeeded in achieving atomic-level alloying between the inherently immiscible palladium (Pd) and platinum (Pt) by hydrogen absorption and desorption in phase-separated Pd/Pt core/shell nanoparticles (hydrogen process method). He discovered that the solid-solution nanoalloy PdPt possesses over twice the hydrogen storage capacity of Pd, which is well known as a hydrogen storage metal. Furthermore, he succeeded in achieving atomic-level alloying between silver (Ag) and rhodium (Rh), elements adjacent to Pd on the periodic table yet highly immiscible. Although neither Ag nor Rh exhibits hydrogen storage property, the AgRh solid-solution nanoalloy formed by mixing them at the atomic level does show hydrogen storage capacity. He found that the Ag_0.5Rh_0.5 alloy possesses the maximum hydrogen storage capacity. Hard X-ray photoelectron spectroscopy revealed that the Ag_0.5Rh_0.5 alloy possesses an electronic state (density of state near the Fermi level) highly similar to that of Pd, drawing attention as a synthetic pseudo-palladium. Furthermore, he have successfully achieved atomic-level solid-solution formation between ruthenium (Ru) and Pd, which occupy positions adjacent to Rh in the periodic table. Rh is known for its high catalytic activity in cleaning nitrogen oxides (NOx), whereas Ru and Pd exhibit low activity. The PdRu solid-solution nanoalloy, formed by mixing Ru and Pd at the atomic level, demonstrated NOx purification capabilities surpassing those of Rh. The density of state calculations revealed it possesses electronic states near the Fermi level highly similar to Rh, drawing attention as an artificial rhodium alloy. Based on these results, Dr. Kitagawa succeeded in continuously controlling hydrogen storage capacity by replacing part of the Pd with Ir, Pt, or Au―elements adjacent to Pd in the 5d transition metal series of the periodic table―thus achieving band-filling control via a rigid band model approach. Furthermore, by atomically alloying a mere 6% of iridium (Ir)―an element with relatively low water electrolysis activity and high cost but high durability, and non-soluble with Ru―into Ru (which has high water electrolysis activity and low cost but poor durability), he successfully developed Ru₀.₉₄Ir₀.₀₆, exhibiting both high water electrolysis activity and durability at low cost.

2. Development of Highly Stable and Active Ternary Catalyst Solid-Solution Alloys Utilizing Configurational Entropy Effects
Aiming to increase configuration entropy and achieve high durability, Prof. Kitagawa developed a ternary solid solution nanoalloy (PdRuIr) by solid-solving Ir into an artificial rhodium alloy (PdRu). He demonstrated that this alloy exhibits extremely high ternary catalytic activity. Furthermore, he derived a method utilizing both configuration entropy and mixing enthalpy, providing guidelines for designing highly durable, high-efficiency functional solid solution alloys. Based on these guidelines, he successfully developed automotive exhaust catalysts in collaboration with industry through materials informatics utilizing machine learning and digital screening, combined with high-throughput screening.

3. Creation of Multi-Element Nanomaterials via Supercritical/Subcritical Solvothermal Continuous Flow Synthesis
Prof. Kitagawa successfully developed a supercritical/subcritical solvothermal continuous flow synthesis apparatus capable of simultaneously flowing catalyst slurries. Leveraging extensive insights gained from non-equilibrium batch synthesis of solid-solution nanoalloys, he developed the world's first fully automated high-throughput synthesis apparatus dedicated to nanoalloy production. This apparatus enables synthesis under high temperatures (up to 500 °C) and pressures (up to 40 MPa). This system instantly converts a solvent containing various dissolved metal ions into a supercritical or subcritical fluid. This process simultaneously reduces each metal ion to its atomic state and alloys them (reaction time <1 s). By rapidly cooling the mixture to room temperature within approximately 10-20 s, it enables the synthesis of solid-solution nanoalloys with minimum sizes at the 1 nm level. Furthermore, this apparatus incorporates a robotic arm and an automated preparation system of catalyst with slurry as supports. Continuous operation enables the synthesis of approximately 30 samples per day of multi-element nanoalloy catalysts, significantly exceeding the previous limit of one sample per day. Through the development of this unique apparatus, we have achieved world-first syntheses of; a 15-element nanoalloy (BiCoCuFeGaInIrNiPdPtRh RuSbSnTi) composed of elements from Groups IV to XV, and a multi-element nanoalloy (FeCoNiRuRhPdOsIrPt) composed of iron-group metals and platinum-group metals. These alloy catalysts exhibit exceptionally high catalytic activity in challenging electrochemical reactions such as the complete oxidation of ethanol and hydrogen evolution. Furthermore, using a continuous-flow supercritical hydrothermal process without calcination, we have successfully synthesized multi-element perovskite oxides, multi-element metal selenides, borides, and amorphous metal phosphides.

As described above, Prof. Hiroshi Kitagawa has pioneered new research fields through original ideas at the intersection of nanomaterial chemistry, solid-state chemistry, coordination chemistry, molecular science, inorganic chemistry, materials science, structural organic chemistry, catalytic chemistry, and chemical engineering. His research has received high international acclaim for its pioneering spirit and originality. Therefore, his achievements are recognized as worthy of the Chemical Society of Japan Award.