Understanding Surface Chemistry of the BaTiO2.5H0.5 Perovskite Oxyhydride from First Principles
- Romero, Kristen Wang
- Advisor(s): Jiang, De-en
With global energy consumption on the rise, it is crucial to design new heterogeneous catalysts that can efficiently produce desired products at a lower energy input. A way to control a catalyst’s activity and selectivity is by tuning the local environment around the catalytic active site. The BaTiO3-xHx perovskite oxyhydride is an example of an anion-tuned ABO3 perovskite where some lattice O2- are replaced by H-. The presence of these lattice hydrides in BaTiO3-xHx not only make the material a more efficient catalyst support for CO2 methanation and ammonia synthesis than BaTiO3, but BaTiO3-xHx (with x = 0.5) is also active for ammonia synthesis. Hence, in this thesis, we aim to study the surface chemistry of a prototypical perovskite oxyhydride, BaTiO¬2.5H0.5 (BTOH), to understand the role BTOH lattice hydrides play in hydrogenation reactions. We begin by identifying stable terminations of BTOH surfaces under catalytically relevant conditions from first principles density functional theory (DFT). Our results show that the (010)-Ba2O2, (210)-Ti2O2, and (211)-Ba2O4H surface terminations are the most stable under relevant catalytic conditions. Next, we employ DFT methods for the mechanistic investigation of ammonia synthesis, acetylene semi-hydrogenation, and selective hydrogenation of crotonaldehyde over stable BTOH terminations. From these studies, we find that the lattice hydrides can: 1) be directly incorporated in the catalytic reaction, and 2) have an influence on the BTOH surface structure to favor the formation of surface vacancies under specific conditions which in turn are beneficial in reaction activity and selectivity. In ammonia synthesis, these vacancies are key in assisting N-N bond cleavage while in the acetylene semi-hydrogenation reaction, they help stabilize the vinyl intermediate. Additionally, surface vacancies offer a site for facile heterolytic H2 dissociation and in crotonaldehyde selective hydrogenation, the resulting surface hydride and proton that form are found to drive the hydrogenation of crotonaldehyde at the C=O bond rather than at the C=C bond. Overall, the results presented in this work shed light on the role of BTOH lattice hydrides and vacancies in various hydrogenation reactions and suggest the potential use of BTOH and other perovskite oxyhydride materials as catalysts for general hydrogenation reactions.