Abstract
The increasing demand for CO₂ utilisation technologies, combined with the thermodynamic stability of CO₂, has driven the development of highly active, selective, and stable catalysts for the production of value-added chemicals. In particular, the development of catalysts that use a tandem approach for CO₂ hydrogenation to short-chain hydrocarbons via CO₂ Fischer–Tropsch synthesis(FTS) remains a major challenge. Exsolution has emerged as a powerful strategy for generating robust, socketed nanoparticles with tunable size, population, and composition. However, the application of exsolved catalysts for hydrocarbon synthesis has been practically non-existent due to a lack of systematic understanding of exsolution driving principles in complex hosts, the absence of rational design frameworks, and limited mechanistic insights into the reaction pathways on exsolved surfaces. This thesis presents a detailed investigation into the rational design of iron-based multi-metallic exsolved catalysts for CO2 hydrogenation to light hydrocarbons. A strategic framework is established to link host composition, microstructure, and dopant chemistry with exsolution behaviour and catalytic performance. Operando characterisation was employed to link structural evolution with catalytic performance.
Systematic deconvolution of synthesis parameters reveals that microstructure refinement and dopant levels govern the interplay between nanoparticle nucleation and growth, establishing fundamental design rules for tailoring the active surface. Engineering the host lattice to tune oxygen vacancy concentrations and introducing nickel as a secondary metal enables 100% CO selectivity by suppressing methanation. In titanium-stabilised ferrite host, the selective exsolution of Fe, Cu and Ni-based bi and trimetallic nanoparticles is achieved, resulting in high activity toward C₂–C₅ hydrocarbons even at atmospheric pressure. Exsolution-driven compositional tuning was shown to modulate olefin/paraffin selectivity through hydrogen spillover and CO2 activation. Contrary to the use of perovskite hosts in exsolution, changing the host to spinels fundamentally changes the chemistry, showing that the oxide host is an active design element that governs not only exsolution behaviour, but also the dynamic phase evolution of the active sites. A major breakthrough was the discovery of reaction-induced exsolution in Cr stabilized spinel ferrites, eliminating the need for energy-intensive hydrogen pre-reduction. Under CO2/H2 atmosphere at 4bar, exsolved metallic species dynamically transform into iron carbides, which serve as the active sites for C–C bond formation and hydrocarbon chain growth. This proves that the support can be engineered to trigger site-specific chemistries tailored for complex catalytic reactions. Operando studies reveal that CO₂ hydrogenation over exsolved catalysts involves a bifunctional tandem mechanism in which multifunctional exsolved centres undergo continuous redox cycling to facilitate the two-step transformation of CO2 into hydrocarbons.</p><p>Overall, this work establishes a rational design blueprint for robust, adaptive catalysts capable of converting CO₂ into high-value hydrocarbon feedstocks, advancing sustainable catalytic technologies.