High-Performance Catalyst Design for Efficient H2 and CO2 Utilization via Active-Site Engineering

Abstract

The present dissertation investigated the rational design of high-performance catalysts for efficient hydrogen utilization through systematic active-site engineering, addressing key challenges associated with low-temperature CO2 methanation and NH3 decomposition. Rather than treating catalytic performance as a simple consequence of metal identity or loading, this work examined how local catalytic environments govern hydrogen activation, reactant adsorption, intermediate stabilization, and reaction turnover. To this end, five distinct catalyst systems were developed, each representing a different active-site engineering strategy, including support–active metal interaction, support-defined redox modulation, interfacial-site optimization, synthesis-defined bonding-state control, and composition-triggered surface reorganization. The results collectively demonstrate that hydrogen utilization efficiency can be markedly improved when the electronic and structural characteristics of active sites are deliberately tailored to the kinetic requirements of the target reaction. Mechanistic analyses throughout this dissertation further provide a unified framework for understanding how catalytic surfaces should be designed to maximize the effective use of hydrogen under practical reaction conditions. In the first study, the characteristics of active sites were deliberately tuned through support–active metal interaction in Ru-based NH3 decomposition catalysts. Because the catalytic behavior of Ru in NH3 decomposition is highly sensitive to electron density and local coordination, CeO2 was introduced as a reducible support to construct a catalytically distinct Ru–O–Ce interfacial environment. Compared with Ru/Al2O3 and Ru/SiO2, the Ru/CeO2 catalyst exhibited much stronger interfacial redox behavior, enhanced charge transfer, and a more electron-rich Ru surface, as evidenced by H2-TPR, XPS, and CO-DRIFTS. These electronically modified Ru sites exhibited stronger back-donation capability and facilitated the activation of adsorbed NH3, resulting in lower-temperature NHx formation and faster H2/N2 evolution in NH3-TPD-MS and NH3-DRIFTS analyses. Consequently, Ru/CeO2 showed significantly lower apparent activation energy and markedly improved low-temperature NH3 decomposition activity. This study established that support–active metal interaction is a decisive design parameter for controlling the intrinsic properties of Ru active sites and thereby enhancing hydrogen release behavior. In the second study, support-defined active-site modulation was applied to Ni-based CO2 methanation through the use of PrOx as a redox-active support. The main objective was to overcome the low-temperature limitation of conventional Ni catalysts by simultaneously promoting CO2 adsorption/activation and the formation of metallic Ni sites for H2 dissociation. PrOx was found to undergo facile Pr4+/Pr3+ transition and strong oxygen-vacancy generation under reductive conditions, creating defect-rich environments with high oxygen mobility. These support-derived defects provided methanation-relevant medium basic sites for CO2 adsorption while also weakening the Ni–O interaction and improving Ni reducibility. As a result, Ni/PrOx exhibited earlier CH4 formation, higher low-temperature CH4 selectivity, and a substantially lower T50 than Ni/SiO2 and Ni/Al2O3. This study demonstrated that support-defined restructuring of the active surface can enhance hydrogen utilization by coupling CO2 activation and H2 activation within the same catalytic environment. In the third study, direct interfacial-site engineering was realized in Ni–Pr/SiO2 catalysts by systematically tuning the Ni:Pr ratio. In contrast to the support-driven strategy above, this system was designed to isolate the role of Ni–Pr interaction itself by using inert SiO2 as the support. The results revealed that the catalytic advantage of the Ni–Pr system originates not from the simple presence of Pr, but from the optimized formation of Ni–O–Pr interfacial sites that electronically and structurally modify adjacent Ni centers. At the optimized Ni:Pr ratio of 10:1, the catalyst exhibited the most favorable balance among CO2 adsorption, electron-enriched Ni0 formation, surface hydrogen activation, and structural coupling between Ni and Pr. Excessive Pr addition, by contrast, promoted segregation and reduced the effectiveness of catalytically relevant interfacial sites. The optimized catalyst therefore delivered the highest low-temperature CH4 formation and superior stability, demonstrating that interfacial-site density and effectiveness are critical determinants of hydrogen utilization efficiency. In the fourth study, the active-site concept was extended beyond post-synthetic interaction toward synthesis-defined bonding-state control through nickel phyllosilicate formation. By applying one-pot sol–gel/hydrothermal synthesis, Ni species were incorporated into the silica framework to generate structurally embedded Ni–O–Si active sites, which are fundamentally different from conventional metallic Ni particles dispersed on silica. This system further demonstrated that active-site effectiveness is governed not only by bonding structure but also by accessibility under reaction conditions. Through TMB-assisted control of micellar organization and pore development, the catalyst structure was optimized to simultaneously stabilize Ni-phyllosilicate, suppress NiO crystallization, and generate radially open mesoporous channels that improve CO2 and H2 transport. The optimized catalysts exhibited enhanced low-temperature methanation activity together with strong cyclic and long-term durability under severe conditions. This study showed that efficient hydrogen utilization requires the coupled optimization of active-site identity, structural stability, and transport accessibility. In the fifth study, composition-dependent surface reorganization was demonstrated in co-precipitated NixMg1−xO catalysts, providing an integrated design framework in which composition, bonding configuration, and surface chemistry evolve cooperatively. A distinct compositional threshold was identified, above which the catalyst surface reorganized from a largely oxide-like state into a cooperative structure composed of accessible metallic Ni domains and defect-rich residual Ni–O–Mg environments. This transition was accompanied by a sharp increase in readily reactive hydrogen species, the emergence of intermediate-strength CO2 adsorption sites relevant to methanation, and a pronounced decrease in apparent activation energy. In situ CO2-DRIFTS further revealed that the post-threshold surface enabled rapid turnover of oxygenated and carbonyl intermediates rather than their accumulation, indicating efficient coupling between CO2 activation and hydrogenation. Consequently, the Ni-rich catalysts exhibited outstanding low-temperature CO2 methanation activity and stability. This study established that hydrogen utilization can be qualitatively transformed by reorganizing the entire catalytic surface into a cooperative metallic–oxide environment. In conclusion, this doctoral research demonstrates that high-performance hydrogen-utilization catalysts can be rationally designed by deliberately engineering the local catalytic environment rather than by relying on simple loading effects or empirical compositional changes. Across all five studies, superior catalytic performance was achieved by tailoring one or more essential active-site properties, including electron density, metal reducibility, defect chemistry, interfacial coordination, bonding-state stability, pore accessibility, and dynamic surface reorganization. These modifications consistently improved the ability of the catalyst to activate hydrogen, stabilize methanation- or decomposition-relevant intermediates, and sustain rapid surface turnover under kinetically demanding conditions. Accordingly, the central contribution of this dissertation is the establishment of active-site engineering as a unifying design principle for efficient hydrogen utilization. The mechanistic insights obtained here provide practical guidelines for future catalyst development in CO2 hydrogenation, hydrogen release, and broader carbon-neutral catalytic processes.