Oral Dissertation Defense

Abstract

Anthropogenic activities contribute more fixed nitrogen to the global nitrogen cycle than can be reduced naturally. Developing a deep understanding of nitrogen-based chemistry is essential to balancing the global nitrogen cycle, and understanding the interconversion of NO₃^–, NH₃, and N₂ and developing new catalytic materials are critical to enabling this process. In this defense, I explore metal alloy, metal sulfide, and metal oxynitride catalyst materials in the context of two nitrogen cycle reactions: nitrate reduction (NO₃RR) and nitrogen reduction (NRR) to form ammonia. NO₃RR reduces excess nitrate (NO₃^–) in water, helping decrease infant methemoglobinemia, ovarian cancer, eutrophication, and mass death in aquatic ecosystems. Alloying Pt and Ru in certain compositions increases the rate of NO₃RR beyond the activity of pure Pt or Ru alone. This improved activity can be rationalized by first-principles calculations and microkinetic modelling. Metal sulfides, specifically Rh sulfides, also catalyze NO₃RR but can additionally resist being poisoned or deactivated in the presence of Cl^–, a common halide ion present in contaminated water sources. Sulfur vacancies in the Rh sulfide lattice contribute largely to the observed NO₃RR activity. Finally, I explore the thermodynamic stability of single perovskite oxynitrides, which have been proposed as NRR electrocatalysts. The presence of certain arrangements of the O and N anions, as well as of certain pairs of metal cations, correlate with a high degree of thermodynamic stability. Successfully discovering highly active, selective, and stable electrocatalysts will enable us to balance the global nitrogen cycle. Effective NO₃RR and NRR electrocatalysts can help mitigate the ecological and health risks from the nitrogen cycle imbalance in an energy-efficient and economically viable way.