Over the last century, the industrialization of agriculture and the consumption of fossil fuels have resulted in a significant imbalance and redistribution in nitrogen-containing resources. This has sparked an interest in developing more sustainable and resilient approaches for producing nitrogencontaining commodities such as fertilizers and fuels. One largely neglected but emerging approach is photocatalytic nitrogen fixation. There is significant evidence that this process occurs spontaneously in terrestrial settings, and it has been demonstrated in numerous engineered systems. Yet many questions still remain unanswered regarding the rates, mechanisms, and impacts of photocatalytically producing fixed nitrogen "out of thin air". This work reviews the fascinating history of the reaction and examines current progress toward understanding and improving photofixation of nitrogen. This is supplemented by a quantitative review of the thermodynamic considerations and limitations for various reaction mechanisms. Finally, future prospects and preliminary performance targets for photocatalytic nitrogen fixation are discussed.

A nanostructured, carbon-based physical catalyst electrochemically reduces N2 to ammonia under ambient conditions.

The design of electrocatalysts capable of selectively reducing nitrate to ammonia is gaining interest as a means of transforming waste into fertilizers. However, most prior investigations of prototypical electrocatalysts, such as polycrystalline Pd and Pt, have focused on unraveling the mechanisms responsible for the selective reduction of nitrate to nitrogen gas. Such polycrystalline noble metals demonstrate notoriously low activity for nitrate reduction (nitrate to nitrite) and high activity for nitrite reduction (nitrite to nitrogen). Here, we aim to elucidate the effect Pd surface structure has on nitrate and nitrite reduction and to determine what role catalyst structural design can play in enabling selective reduction of nitrate to ammonia. Through synthesizing nanocatalysts with controlled facets (e.g., nanocubes, cuboctahedrons, octahedrons, and concave nanocubes), we demonstrate that Pd(111) > Pd(100) > Pd­(hk0) for nitrate reduction activity and Pd(100) > Pd­(hk0) > Pd(111) for nitrite reduction activity in an alkaline electrolyte. Octahedrons without Pd (100) facets exhibited nearly selective production of NO2 – with little to no measurable NH3 or N2. However, nanocubes that expose only the Pd(100) facet exhibited high activity for NO2 – reduction to NH3. Cuboctahedrons that expose both Pd(111) and Pd(100) facets demonstrated the highest production of ammonia (306.8 μg h–1 mgPd –1) with a faradaic efficiency of 35%. Density functional theory (DFT) simulations reveal that *NO3 dissociation to *NO2 + O* is more favorable on Pd(111) than Pd(100), explaining the faster nitrate reduction kinetics on the Pd(111) facet observed in the experiments. The simulations also show that *NO2 binds less strongly to Pd(111) compared to Pd(100). Thus, nitrite formed via nitrate dissociation readily desorbs from the Pd(111) surface, which explains why Pd(111) selectively reduces nitrate to nitrite. The results show that cuboctahedron is bifunctional in nature, with the (111) facet catalyzing the conversion of NO3 – to NO2 – and the (100) facet catalyzing the conversion of NO2 – to NH3.

Access to nitrogen-based fertilizers is critical to maximize agricultural yield, as nitrogen is the most common rate-limiting nutrient. Nearly all nitrogenbased fertilizers rely on ammonia and nitric acid as feedstocks, and thus the demand for these chemicals is heavily dependent on the global population and food demand. Over the next three decades, the global population will continue to dictate the market size and value for ammonia and nitric acid, which consequently will have a significant impact on our energy infrastructure. Here, we discuss the potential for carbon-free electrocatalytic nitrogen reduction, nitrogen oxidation, and nitrate reduction to meet fertilizer manufacturing demands. We also explore various growth scenarios to predict the 2050 market size and value for ammonia and nitric acid. We highlight that if the current approaches for manufacturing ammonia and nitric acid remain constant, carbon emissions from the production of fixed fertilizer feedstocks could exceed 1300 Mt CO 2eq /yr, prompting a strong need for green alternatives.

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