Vitor Silveira: Shedding Light on Nitrate-to-Ammonia Conversion: Plasmonic catalysis for fertilizer synthesis and beyond

Abstract

There is a pressing need to find new and decentralized ways to produce ammonia. The electrochemical nitrate reduction reaction (NitrRR) is receiving increasing attention as an alternative due to the higher reactivity and solubility of nitrates in comparison to nitrogen gas. However, the hydrogen evolution reaction (HER) remains a concern at the potentials required for NitrRR in water electrolytes. The use of the photocatalytic pathway is an interesting option for improving catalytic processes due to the possibility of promoting alternative reaction mechanisms that lead to higher selectivity and increased reaction rates.

Noble metal nanoparticles are promising materials for photocatalytic applications using sunlight due to the effect of surface plasmon resonance. Plasmonic nanoparticles are capable of efficiently harvesting visible light and using their energy to generate high-energy carriers (or hot carriers). The feasibility of effectively utilizing plasmon-generated hot carriers for catalytic processes remains a matter under debate in the literature. The discussion is based on the short-lived nature of these carriers, whose decay leads to elevated temperatures in the surroundings of plasmonic nanoparticles. 

The results presented in this thesis are organized into four chapters, each addressing fundamental questions related to plasmonic catalysis and ammonia synthesis. It starts by studying the direct use of plasmon-generated hot carriers in reactions such as hydrogen evolution reaction and carbon dioxide reduction. Beyond the practical applications, the focus of this chapter is to demonstrate how a combination of carefully designed photoelectrodes and advanced characterization techniques can provide insights into the mechanisms underlying the contribution of plasmon-generated hot carriers. The following chapters are related to the utilization of photo-excited carriers for the nitrate reduction reaction. It goes through selectivity aspects, the study of nitrate hydrogenation, and the study of the photocatalytic active sites on the titania surface.