Effect of Bias and Aerobic Conditions on Photocatalytic Nitrogen Fixation By Titania

Abstract

Photo-catalytic nitrogen fixation on titania (TiO2) was discovered for decades ago by an Indian Soil Scientist (N Dhar) [1]. Dhar’s early work suggested that titania in soil produces fixed nitrogen (ammonia) from air. Despite these findings, a clear understanding regarding how nitrogen fixation occurs on titania remain unclear. Previous studies has demonstrated nitrogen photofixation in both gas and liquid phase and in various environmental conditions (temperature, pressure, relative humidity) [2]. Investigation have also centered on the use of various proton source (hydrogen, water) and hole scavengers. Under all conditions, the prevailing consensus is that ammonia is formed photo-catalytically. However, very few investigations have detailed the impact of oxygen, as a competing reactant. Most investigations used nitrogen that is separated from air. Separation nitrogen from air is energy intensive and expensive. If photocatalytic technologies are to be developed for ammonia production, omitting the air separation process and operating with air is necessary to drive down the cost. Therefore, it is important to understand the impact of oxygen plays in suppressing photo(electro)catalytic nitrogen fixation. Here we will present data from a series of photocatalytic experiments conducted in aerobic conditions (Air) and anerobic conditions (N2). We demonstrate the production rates and efficiencies as applied bias and light intensity varies. Results show that ammonia production in air flow nearly in all cases are 30% lower than nitrogen flow. Literature also indicated that oxygen will suppress the ammonia formation because oxygen will be easier to reduce in conductive band on titania than nitrogen due to more long pair ions on oxygen [3]. Therefore, we also investigate the potential for both nitrogen and oxygen reduction through a series of rotating ring disk experiments. Through biasing the ring potential, we evaluate the product of nitrogen reduction (ammonia) and oxygen reduction (hydrogen peroxide). Reference [1] N. Dhar, E. Seshacharyulu and N. Biswas, Proc. Natl. Acad. Sci., India, 1941,7, 115-131. [2] G. Schrauzer and T. Guth, J, Am. Chem. Soc., 1977, 99, 7189-7193. [3] H. Hirakawa, M. Hashimoto, Y. Shiraishi, T. Hirai, J, Am. Chem. Soc., 2017, 139(31), 10929-10936.