Benchmarking Heterogeneous Photocatalysts for the Nitrogen Reduction Reaction

Ammonia is the key ingredient for living organisms, and artificial catalytic synthesis of ammonia (known as the Haber-Bosch process) is undoubtedly a milestone in scientific research in the 20th century and the cornerstone of today's society. [1] However, the Haber-Bosch process is energy-intensive (2% of the global energy consumption) and is accompanied by large CO 2 emissions (1% of global CO 2 emission). [2] To catch up with the goal of “Net Zero CO 2 Emissions” by 2050, it is imperative to develop a carbon-free and sustainable ammonia production process. The photocatalytic ammonia synthesis method relies only on renewable feedstocks (water and air) and sustainable energy input (light) and can be operated under ambient conditions. The current progress in photocatalytic ammonia synthesis suggests, however, there exists a large gap in performance before commercial use is viable. To replace the Haber-Bosch process and produce commercial fertilizer, the solar-to-ammonia (STA) efficiency needs to be at least 0.1% and the highest reported efficiency carried out in a lab setting is approaching 0.04% [2][3] With advances in measurement techniques, recent studies in the field of electrocatalytic ammonia synthesis have shown that many reported highly active catalysts have later been shown to be inactive toward nitrogen reduction. [4] This false positive result is mainly due to the adventitious ammonia in the environment. A series of discussions were triggered, including identifying and eliminating contaminations, developing robust measurements for low-concentration ammonia detection, and establishing rigorous testing protocols. [4-7] At present, the ammonia yield of electrocatalysis is at least an order of magnitude higher than that of photocatalysis, so the problem of nitrogen contamination will only be more serious in the field of photocatalysis. [7] Unfortunately, such discussions have not received equal attention in this field, and few if any rigorous testing protocols have been employed in existing reports. It is imperative to benchmark and develop a robust testing protocol for the growing field of photocatalytic ammonia synthesis. Here, we developed a rigorous setup to suppress all known ammonia contaminations (e.g., ammonia and NO x ), systematically discussed the effect of various gas pretreatment methods, and established the background contamination level in the system. We synthesized three highly active photocatalytic materials (Fe-BiOBr, OVs-TiO 2 , and Fe 2 O 3 /g-C 3 N 4 ) from the literature and measured their ammonia yield using the setup with strict controls. [8-10] To avoid potential interference effects during the measurements, we cross-compared different ammonia-detecting methods (Indophenol blue method and ion chromatography). [6] In addition, we applied isotope labeling with quantitative NMR to confirm the genuine production of photo-fixed ammonia. Going a step further, we methodically compare the effects of two experimental parameters (temperature and gas flow rate) on the performance of photocatalytic ammonia synthesis. These parameters vary greatly across reports, making it difficult to objectively analyze their impacts on the production of photo-fixed ammonia. Based on our findings, we aim to provide a solid benchmark for this burgeoning field and draw more attention to the importance of rigorous testing. Reference: Smil, Vaclav. Enriching the earth: Fritz Haber, Carl Bosch, and the transformation of world food production . MIT press, 2004. Medford, Andrew J., and Marta C. Hatzell. "Photon-driven nitrogen fixation: current progress, thermodynamic considerations, and future outlook." Acs Catalysis 4 (2017): 2624-2643. Yuan, Jili, et al. 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