Sustainable Catalytic Systems for Ammonia Synthesis

Abstract

The Haber-Bosch ammonia synthesis process is one of the most impactful catalytic reactions that sustains the global population. However, this reaction is extremely energy-intensive and emits excess CO2 that is detrimental to the environment. Additionally, anthropogenic nitrogen fixation creates nitrate pollutants that contaminate water sources and becomes a health hazard for humans upon consumption. To address these problems, we explore catalysts and alternative reaction systems to reduce the environmental impact of fertilizer use for humans. To improve upon the Haber-Bosch ammonia synthesis, we explored a series of metal (e.g., Fe, Co, and Ru) supported molybdenum carbides (Mo2C) and nitrides (Mo2N) that have previously been shown to be active for ammonia synthesis at ambient pressures (Chapter 3). We hypothesized that metal sites will break the nitrogen triple bond and support sites will perform the hydrogenation steps. This synergistic interaction will improve ammonia synthesis activity at ambient operating conditions. Kinetic experiments at 400 °C and 1 atm for supported Mo2C and Mo2N display little to no catalytic improvement beyond activity from the supports. Future in situ work is recommended to understand the role of nitrogen atoms in the nitride participating in the ammonia synthesis reaction via a Mars-van Krevelen mechanism. Alternative catalytic systems that recycle nitrate to ammonia can be more environmentally friendly, and both thermocatalytic nitrate reduction reaction (TNO3RR) and electrocatalytic nitrate reduction reaction (ENO3RR) are explored in this work. Previous research from the Goldsmith Group predicts that a Pt3Ru alloy could be active for ENO3RR. To experimentally confirm these results, different compositions of PtxRuy/C (x = 48–100%) were synthesized, characterized, and tested for their activity and selectivity for nitrate reduction at different operating potentials (Chapter 4). The PtxRuy/C alloys are more active than Pt/C, with Pt78Ru22/C being six times more active than Pt/C at 0.1 V vs. RHE with 93–98% faradaic efficiencies towards ammonia. Experimental and computational results show similar qualitative trends, with the maximum catalytic activity occurring at Ru content of ~25 at%. This maximum is rationalized by an optimum in nitrate and hydrogen binding energies where there is a transition of the rate-determining step from nitrate dissociation to a new rate-determining step. This work confirms previous computational models and demonstrates how electrocatalyst activity can be optimized by changing the adsorption strength of reacting species through alloying, providing further insights for future catalyst design for optimal nitrate reduction and ammonia production. By comparing ENO3RR with TNO3RR, we obtain additional mechanistic insights for the similarities and differences between the two reactions (Chapter 5). The results show that increasing the driving force of hydrogen (H2 partial pressure for TNO3RR and applied potential for ENO3RR) and nitrate concentration increases the reaction activity. Additionally, the activity order of catalyst composition also remains the same. Despite these similarities, the effects of pH and the apparent activation energy have a different effect on PtRu/C for TNO3RR and ENO3RR activity, suggesting reaction effects and changes to the mechanism unique to ENO3RR. Additional work to isolate pH effects on ENO3RR, such as ionic strength, hydrogen equilibrium potential, the point of zero free charge, is important to understand nitrate reduction mechanisms. By comparing these reactions at similar conditions, this work allows us to evaluate TNO3RR and ENO3RR systems for industrial implementation.