Minimizing the impacts of the ammonia economy on the nitrogen cycle and climate

Purification and Value-Added Conversion of NOx under Ambient Conditions with Photo-/Electrocatalysis Technology

As primary air pollutants from fossil fuel combustion, the excess emission of nitric oxides (NOx) results in a series of atmospheric environmental issues. Although the selective catalytic reduction technology has been confirmed to be effective for NOx removal, green purification and value-added conversion of NOx under ambient conditions are still facing great challenges, especially for nitrogen resource recovery. To address that, photo-/electrocatalysis technology offers sustainable routes for efficient NOx purification and upcycling under ambient temperature and pressure, which has received considerable attention from scientific communities. In this review, recent advances in photo-/electrocatalysis technology for the purification and value-added conversion of NOx are critically summarized. The target products and reaction mechanisms for NOx conversion systems, together with the responsible active sites, are discussed, respectively. Then, the realistic environmental practicability is proposed, including strict performance evaluation criteria and application in realistic conditions for NOx purification and upcycling by the application of photo-/electrocatalysis. Finally, the current challenges and future opportunities are proposed in terms of catalyst design, NOx conversion enhancement, reaction mechanism understanding, practical application conditions, and product separation techniques.

Multivariate Approaches Boosting Lithium-Mediated Ammonia Electrosynthesis in Different Electrolytes

Ammonia electrosynthesis through the lithium-mediated approach has recently reached promising results towards high activity and selectivity in aprotic media, reaching high Faradaic efficiency (FE) values and NH3 production rates. To fasten the comprehension and optimization of the complex lithium-mediated nitrogen reduction system, for the first time a multivariate approach is proposed as a powerful tool to reduce the number of experiments in comparison with the classical one-factor-at-a-time approach. Doehlert design and surface response methodology are employed to optimize the electrolyte composition for a batch autoclaved cell. The method is validated with the common LiBF4 salt, and the correlations between the FE and the amount of lithium salt and ethanol as proton donor are elucidated, also discussing their impact on the solid electrolyte interphase (SEI) layer. Moreover, a new fluorinated salt is proposed (i.e., lithium difluoro(oxalate) borate (LiFOB)), taking inspiration from lithium batteries. This salt is chosen to tailor the SEI layer, with the aim of obtaining a bifunctional interfacial layer, both stable and permeable to N2, the latter being an essential characteristic for batch systems. The SEI layer composition is confirmed strategic and its tailoring with LiFOB boosts FE values.

Synthesis and electrochemical properties of molybdenum nitrido complexes supported by redox-active NHC and MIC ligands

We report the synthesis of a series of molybdenum nitrido complexes supported by bis-phenolate N-heterocyclic and mesoionic carbenes (NHC & MIC). The reaction between MoN(OtBu)3 and the corresponding azolium salts [H3L1]Cl and [H3L2]Cl (with L1 = bis-phenolate triazolylidene and L2 = bis-phenolate benzimidazolylidene) gives clean access to the corresponding NHC/MIC complexes 1-Cl and 2-Cl. Electrochemical investigations of these complexes showed that they can be reversibly reduced at potentials of -1.13 and -1.01 V vs. Fc/[Fc]+ and the reduced complexes [1-Cl]- and [2-Cl]- can be cleanly isolated after chemical reduction with one equivalent of decamethylcobaltocene. Exchange of the halide atoms is furthermore reported to give a series of nitrido complexes supported by tert-butanolate (1-OtBu and 2-OtBu), perfluoro-tert-butanolate (1-OtBuF9 and 2-OtBuF9), tritylate (1-OCPh3 and 2-OCPh3), mesitolate (1-OMes and 2-OMes), thio-tert-butanolate (1-StBu), thiotritylate (1-SCPh3 and 2-SCPh3) and thiomesitolate complexes (1-SMes). The electrochemical properties of all complexes were evaluated and compared. All isolated complexes were characterized by multinuclear and multidimensional NMR spectroscopy and (if applicable) by EPR spectroscopy. Furthermore, the reactivity of 1-Cl and 2-Cl in the presence of protons and decamethylcobaltocene was investigated, which shows facile extrusion of ammonia, yielding diamagnetic bis-molybdenum(III) complexes 3 and 4.

Cost-competitive decentralized ammonia fertilizer production can increase food security

The current centralized configuration of the ammonia industry makes the production of nitrogen fertilizers susceptible to the volatility of fossil fuel prices and involves complex supply chains with long-distance transport costs. An alternative consists of on-site decentralized ammonia production using small modular technologies, such as electric Haber-Bosch or electrocatalytic reduction. Here we evaluate the cost-competitiveness of producing low-carbon ammonia at the farm scale, from a solar agrivoltaic system, or using electricity from the grid, within a novel global fertilizer industry. Projected costs for decentralized ammonia production are compared with historical market prices from centralized production. We find that the cost-competitiveness of decentralized production relies on transport costs and supply chain disruptions. Taking both factors into account, decentralized production could achieve cost-competitiveness for up to 96% of the global ammonia demand by 2030. These results show the potential of decentralized ammonia technologies in revolutionizing the fertilizer industry, particularly in regions facing food insecurity.

Climate Impacts of Hydrogen and Methane Emissions Can Considerably Reduce the Climate Benefits across Key Hydrogen Use Cases and Time Scales

Recent investments in "clean" hydrogen as an alternative to fossil fuels are driven by anticipated climate benefits. However, most climate benefit calculations do not adequately account for all climate warming emissions and impacts over time. This study reanalyzes a previously published life cycle assessment as an illustrative example to show how the climate impacts of hydrogen deployment can be far greater than expected when including the warming effects of hydrogen emissions, observed methane emission intensities, and near-term time scales; this reduces the perceived climate benefits upon replacement of fossil fuel technologies. For example, for blue (natural gas with carbon capture) hydrogen pathways, the inclusion of upper-end hydrogen and methane emissions can yield an increase in warming in the near term by up to 50%, whereas lower-end emissions decrease warming impacts by at least 70%. For green (renewable-based electrolysis) hydrogen pathways, upper-end hydrogen emissions can reduce climate benefits in the near term by up to 25%. We also consider renewable electricity availability for green hydrogen and show that if it is not additional to what is needed to decarbonize the electric grid, there may be more warming than that seen with fossil fuel alternatives over all time scales. Assessments of hydrogen's climate impacts should include the aforementioned factors if hydrogen is to be an effective decarbonization tool.