Integrating Photocharging of TiO₂ Aerogels and N₂-Reduction in Taylor Flow for On-Demand Ammonia Synthesis

Akbar Valaei, Dirk Ziegenbalg,

Chemical engineering department, Ulm university, Ulm, Germany

Abstract

Ammonia is not only a necessary feedstock for several industry, like fertilizer, textile, and pharmaceutical, but it is also considered a carbon-neutral energy carrier. Mass production of ammonia contributes to more than 1% of fossil energy consumption and plays a significant role in global greenhouse gas emissions. Therefore, exploring alternative processes to the Haber-Bosch method is of considerable scientific interest [1]. This research presents a two-reactor system designed for the systematic investigation of process parameters in photocatalytic ammonia synthesis, using TiO₂ aerogel. TiO₂ aerogels are able to store large amounts of photogenerated electrons in surface trap states upon illumination in water−methanol dispersions [2]. The developed system realizes the processes of photocharging and nitrogen reduction in two different capillary reactors. The first reactor is dedicated to photocharging TiO₂ aerogels, preparing it for N₂ reduction. In this reactor, TiO₂ aerogels are exposed to controlled UV-LED lighting conditions, focusing on maximizing the storage of photogenerated charge carriers within the TiO₂ structure. The second reactor is dedicated to reducing nitrogen with the stored electrons in the aerogel by utilizing Taylor-flow conditions for enhanced mass transfer and nitrogen reduction efficiency. This reactor is engineered to maximize the contact between nitrogen and the photocharged aerogels, ensuring a more efficient and controlled reduction of nitrogen to ammonia. This study includes characterization of the photocharging reactor and an analysis of the operational parameters of both reactors. The number of photons incident on the suspension is the most important factor for determining the photocharging efficiency and comparing batch and capillary reactors. A multidimensional radiometric measurement setup developed by Sender et al.[3], [4] was used to measure the radiation field of the various reactor configurations.  The effects of photocharging conditions and flow rates on the overall efficiency of ammonia production were investigated. Concluding this contribution, the potential of this two-reactor system in ammonia production will be discussed. This system offers a more flexible approach to ammonia production, and fundamental understanding of both photocharging and N₂ reduction process.