UC Riverside

The conversion of C1 molecules such as carbon dioxide and methane into syngas, hydrocarbons and other valuable chemicals requires catalysts that are resistant to deactivation by coking and sintering. Furthermore, the cost-effectiveness and sustainability necessitate minimizing or eliminating the use of precious metal catalysts. The in-situ synthesis of metallic nanoparticles via in-situ exsolution in reducing environments is an emerging strategy for developing sustainable catalysts. Exsolved catalysts are anchored strongly on parent perovskite support, can be tailored for optimized activity and stability and are regenerable through redox cycling.

In this dissertation, the exsolution of nickel containing catalytic nanoparticles in ABO3-type simple perovskites is studied. Various strategies to manipulate the dynamics of exsolution are employed and their effects on the catalyst performance are evaluated. The introduction of vacancies on the A-site creates oxygen vacancies which favors the exsolution of reducible metals on the B-site. Substituting multiple-metals on the B-site resulted in the formation of alloyed nanoparticles that enable synergistic effects and enhance catalyst activity and stability. Conditions of perovskite synthesis and reduction also affect material characteristics and the nucleation and growth dynamics of the nanoparticles. Each of these strategies has an impact on the size, dispersion and composition of the resulting exsolved nanoparticle and thus on catalyst performance. The experiments conducted in this study elaborate the implications of each strategy by evaluating and comparing catalyst activity and stability for reactions such as dry methane reforming. A combination of in-situ and ex-situ characterization techniques capture the kinetics of the exsolution and shed light on the dynamic nature of exsolving metals subject to different redox conditions.

This dissertation lays the foundation of rationally designed catalysts developed from the perovskite platform. The characteristics of exsolved nanoparticles are controllable and thus desirable properties such as selectivity and stability can be exacted from these materials. The stability and regenerability of these materials can also enable a sustainable use of earth-abundant metals as catalysts.