Ph.D. Thesis Defense Announcement 

Influence of Carbon and Oxygen for Photocatalytic Nitrogen Fixation by Titania 

Yu-Hsuan Liu 

Marta C. Hatzell (ME); Xing Xie (CEE) 

Sotira Yiacoumi (CEE), Xing Xie (CEE), Yongsheng Chen (CEE),Andrew J. Medford (CHBE) 

 Date & Time: Monday, November 29th, 2021 at 9am 

Location: https://bluejeans.com/901358576/2767?src=calendarLink 

Photocatalytic nitrogen fixation is an approach that may enable the synthesis of ammonia (NH3) from abundant feedstock (nitrogen and water) using only renewable energy (sun). Despite the potential sustainability of this approach, there is seldom photocatalyst which exhibits desirable nitrogen fixation activity. Therefore, the focus of this thesis is on understanding what and how catalytic processes enable the light-driven ammonia production.To do this, we propose to focus efforts on understanding fixed nitrogen production on a model catalyst (titania). We first investigate the influence of carbonaceous hole-scavengers (e.g. methanol,ethanol, propanol, formic acid) have in mediating indirect or direct photo-catalytic nitrogen fixation. We demonstrate that the rate of ammonia varies with the addition of different hole scavengers. The presence of various organic scavengers could obviously increase the NH3 yield by providing sacrificial electron donors to the holes in photocatalysts and reducing the recombination of electron-hole pair. However, in the absence of organic hole scavengers, electron-hole recombination would occur and further limit NH3 formation. We further investigate the role of adsorbed carbonaceous intermediates (contaminants) through recycling photocatalyst reacted with hole scavengers in solutions without hole scavengers. The increased performance exhibited with the hole scavenger (increased by 50% for Fe-TiO2) was maintained after removal of hole scavengers. This indicates that the contaminant or trace adsorbed carbon species continued to influence nitrogen fixation. Current photocatalytic experiments only provide indirect evidence as to the role carbon play in photocatalytic nitrogen fixation. Thus, we aim to probe the interaction between nitrogen, carbon, and titania using in-situ electron paramagnetic spectroscopy (EPR) and analytical spectroscopy (IR, NMR). Finally, decarbonized ammonia production through photocatalytic nitrogen fixation is appealing, as it may allow for farm-scale fertilizer production using earth-abundant feedstocks, energy, and catalysts. Yet, the viability of decentralized ammonia production systems is largely dependent on the cost of a complete photocatalytic system reaching a Haber-Bosch parity point. Here, we demonstrate that an air separation unit for a farm scale low-cost photocatalytic ammonia synthesis system can account 70% of the total system cost. This promotes the need for catalyst, which can tolerate trace oxygen or can even operate under aerobic conditions to attain Haber-Bosch cost parity. We further demonstrate the change in catalytic activity of a prototypical undoped and metal-doped titania photocatalyst under aerobic and anaerobic conditions. Among various metal-doped titania, vanadium and ruthenium doped titania demonstrated no performance decline under aerobic conditions.