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For decades, reports have suggested that photo-catalytic nitrogen fixation by titania in an aqueous environment is possible. Yet a consensus does not exist regarding how the reaction proceeds. Furthermore, the presence of an aqueous protonated solvent and the similarity between the redox potential for nitrogen and proton reduction suggest that ammonia production is unlikely. Here, we re-investigate photo-catalytic nitrogen fixation by titania in an aqueous environment through a series of photo-catalytic and electrocatalytic experiments. Photo-catalytic testing reveals that mineral phase and metal dopants play a marginal role in promoting nitrogen photofixation, with ammonia production increasing when the majority phase is rutile and with iron dopants. However, the presence of a trace amount of adsorbed carbonaceous species increased the rate of ammonia production by two times that observed without adsorbed carbon based species. This suggests that carbon species play a potential larger role in mediating the nitrogen fixation process over mineral phase and metal dopants. We also demonstrate an experimental approach aimed to detect low-level ammonia production from photo-catalysts using rotating ring disk electrode experiments conducted with and without illumination. Consistent with the photocatalysis, ammonia is only discernible at the ring with rutile phase titania, but not with mixed-phase titania. Rotating ring disk electrode experiments may also provide a new avenue to attain a higher degree of precision in detecting ammonia at low levels.
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A rotating ring disc electrode study of photo(electro)catalyst for nitrogen fixation
There are numerous reports of photo(electro)catalysts demonstrating activity for nitrogen reduction to ammonia and a few reports of photo(electro)catalysts demonstrating activity for nitrogen oxidation to nitric acid. However, progress in advancing solar-to-fertilizer applications is slow, due in part to the pace of catalyst screening. Most evaluations of photo(electro)catalysts activity occur using batch reactors. This is because common product analyses require accumulation of ammonia or nitric acid in the reactor to overcome instrument detection limits. The primary aim here is to examine the use of an electroanalytical method, rotating ring disk electrode voltammetry (RRDE), to detect ammonia produced by a nitrogen fixing photo(electro)catalyst. To examine the potential for RRDE, we investigated a photo(electro)catalyst known to reduce nitrogen to ammonia (titania), while varying the applied electrochemical potential and degree of illumination on the disk. We show that the observed ammonia oxidation at the ring electrode corresponds strongly with ammonia measurements obtained from the bulk electrolyte. Indicating that RRDE may be effective for catalyst screening. The chief limitation of this approach is the need for an alkaline electrolyte. In addition, this approach does not rule out the presence of adventitious ammonia.
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- PAR ID:
- 10423444
- Date Published:
- Journal Name:
- Faraday Discussions
- ISSN:
- 1359-6640
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2FD00168C
