Understanding confinement effects and nano structuring in heterogeneous catalysis

Abstract

During the COVID-19 lockdown we were all confined to our houses. While this puts a considerable strain on our lives, the chores around the house do tend to get done more efficiently. Catalytic reactions can benefit from confinement in a similar way: forcing the reactants closer to the catalyst, thereby controlling reactivity and selectivity. This dissertation investigates the nature of such confinement effects in heterogeneous catalysis, and provides a framework for analysing these effects. To establish confinement within our catalysts, we took two experimental routes: one where we actively created confinement by adding a barrier at different distances from the active site, and one where we used surface modification of novel two-dimensional materials (MXenes and MAX phases) to create confined spaces between the layers or on the surface. To identify the subtle changes in reactivity between catalysts, we have developed a novel device that precisely measures the kinetics of our gas-producing model reactions. We gained important insights about the role of the Arrhenius pre-exponential factor in surface catalysis and the range over which confinement effects are important. Finally, this dissertation establishes MXenes and MAX phases as a new types of catalytic materials and supports in heterogeneous catalysis. Using oxidative treatment, we could influence the type and number of acid sites on MXene, thereby controlling its reactivity and selectivity. The oxidation sensitivity of MXene also provides a new tool for tuning the electronic structure at metal particles, resulting in better catalysts for energy applications and on-demand hydrogen generation