Nitrogen is fundamental to all of life and many industrial processes. The interchange of nitrogen oxidation states in the industrial production of ammonia, nitric acid, and other commodity chemicals is largely powered by fossil fuels. A key goal of contemporary research in the field of nitrogen chemistry is to minimize the use of fossil fuels by developing more efficient heterogeneous, homogeneous, photo-, and electrocatalytic processes or by adapting the enzymatic processes underlying the natural nitrogen cycle. These approaches, as well as the challenges involved, are discussed in this Review.

Design principles for reducible metal nitride catalysts are developed and demonstrated for ambient-pressure solar-driven N2 reduction into NH3.

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6About 40% of our food would not exist without synthetic ammonia (NH 3 ) for fertilization. Yet, NH 3 7 production is energy intensive. About 2% of the world's commercial energy is consumed as fossil fuels 8 for NH 3 synthesis based on the century-old Haber-Bosch (H.-B.) process. The state of the art and the 9 opportunities for reducing the fossil energy footprint of industrial H.-B. NH 3 synthesis are discussed. It is 10shown that even a hypothetical utterly revolutionary H.-B. catalyst could not significantly reduce the 11 energy demand of H.-B. NH 3 as this is governed by hydrogen production. Renewable energy-enabled, 12fossil-free NH 3 synthesis is then evaluated based on the exceptional and continuing cost decline of 13 renewable electricity. H.-B. syngas (H 2 , N 2 ) is assumed to be produced by electrolysis and cryogenic air 14 separation, and then supplied to an existing H.-B. synthesis loop. Fossil-free NH 3 could be produced for 15 energy costs of about $232 per tonne NH 3 without claiming any economic benefits for the avoidance of 16 about 1.5 tonnes of CO 2 released per tonne NH 3 compared to the most efficient H.-B. implementations. 17Research into alternatives to the H.-B. process might be best targeted at emerging markets with currently 18 little NH 3 synthesis capacity but significant future population growth such as Africa. Reduced capital 19 intensity, good scale-down economics, tolerance for process upsets and contamination, and intermittent 20 operability are some desirable characteristics of NH 3 synthesis in less developed markets, and for 21 stranded resources. Processes that are fundamentally different from H.-B. may come to the fore under 22 these specific boundary conditions. 23

The coupled reverse osmosis-electrodialysis (RO/ED) method was used to isolate dissolved organic matter (DOM) from 16 seawater samples. The average yield of organic carbon was 75 ± 12%, which is consistently greater than the yields of organic carbon that have been commonly achieved using XAD resins, C 18 adsorbents, and cross-flow ultrafiltration. UV-visible absorbance spectra and molar C/N ratios of isolated samples were consistent with the corresponding properties of DOM in the original seawater samples, indicating that DOM samples can be isolated using the coupled RO/ED method without any bias for/against these two properties. Five of the samples were desalted sufficiently that reliable measurements of their 13 C and 1 H NMR spectra and their Fourier transform ion cyclotron resonance (FTICR) mass spectra could be obtained. The 13 C and 1 H NMR spectra of RO/ED samples differed distinctly from those of samples that have been isolated in much lower yields by other methods. In particular, RO/ED samples contained a relatively lower proportion of carbohydrate carbon and a relatively greater proportion of alkyl carbon than samples that have been isolated using cross-flow ultrafiltration. From the FTICR mass spectra of RO/ED samples, samples from the open ocean contained a much lower proportion of unsaturated compounds and a much higher proportion of fatty acids than coastal samples.