Zoom link: https://kaust.zoom.us/j/4022310687 

Abstract

Ammonia (NH₃) plays a crucial role in modern industry, serving as a crucial building block for fertilizers, chemicals, and explosives. More recently, its potential as a hydrogen carrier has positioned it as a key player in the transition to a sustainable energy future. The widespread production of NH₃ relies on thermocatalytic processes, which have been fundamental in enabling large-scale synthesis and decomposition. The Haber-Bosch process, in particular, has been a technological triumph, providing the world with synthetic nitrogen fertilizers that sustain global food production. Similarly, NH₃ cracking has emerged as a viable method for on-demand hydrogen release. However, despite their effectiveness, these processes remain highly energy-intensive, requiring extreme temperatures and pressures that limit efficiency and sustainability. This dissertation explores an innovative photo-thermal catalytic approach to NH₃ synthesis and decomposition, leveraging the synergistic effects of light and heat to enhance catalytic efficiency and reduce energy demands.

The study systematically investigates three different catalyst systems: Ru-based catalysts supported on CeO₂ for low-temperature NH₃ synthesis, metal-organic framework-derived Co-based catalysts for NH₃ decomposition, and Fe@C catalysts obtained from pyrolyzed metal-organic frameworks for optimized NH₃ cracking. Advanced characterization techniques, including in-situ spectroscopy, transient photocurrent measurements, and computational modeling, provide mechanistic insights into the role of light-induced processes in accelerating reaction kinetics and enhancing catalyst stability.

Results demonstrate that K-promoted Ru/CeO₂ catalysts achieve a remarkable NH₃ production rate of 20 mmol g⁻¹ h⁻¹ under photo-thermal conditions, surpassing conventional thermocatalytic benchmarks. In NH₃ decomposition, MOF-derived Co-based catalysts exhibited different conversions depending on their respective topology and graphitic nitrogen content; while Fe@C catalysts enabled efficient hydrogen production at significantly lower temperatures compared to conventional thermo-catalysis. Light intensity experiments confirm that non-thermal effects, including charge carrier excitation and local heating, play a crucial role in accelerating reaction dynamics.

By closing the NH₃ chemistry loop through both synthesis and decomposition, this work establishes a framework for integrating photo-thermal catalysis into future ammonia-based energy cycles. The findings provide valuable insights into catalyst design principles and highlight the potential of light-assisted catalysis in advancing sustainable chemical processes. This dissertation contributes to the growing field of photo-thermal catalysis, intending to pave the way towards a greener, more energy-efficient NH₃ production and utilization.