Insights into\nSustainable Nitrogen Fixation by Gas-phase\nSpectroscopic Measurements and Global Modeling of Reaction Intermediates\nin Humid Nitrogen Plasma

There is growing interest in reacting molecular nitrogen\nand water\nto sustainably synthesize fixed forms of nitrogen such as ammonia.\nIn particular, low-temperature plasmas can activate these relatively\ninert feedstocks at or near room temperature without a catalyst. However,\nbecause of the enhanced reactivity and nonequilibrium chemistry, a\ndiverse range of products is formed, and the underlying reaction mechanisms\nare exceedingly complex. In this work, we studied a simplified reactor\nconsisting of a gaseous plasma containing controlled mixtures of nitrogen\ngas and water vapor. Densities of key chemical species such as N,\nH, OH, NH, and NO were measured by emission and laser-based spectroscopy\nas a function of the relative humidity. A global model was constructed\nand the reaction network was validated by comparing calculated species\ndensities with experiments. We discover that N, a key initial intermediate\nfor ammonia, strongly decreases in the presence of water vapor, and\nas a result, ammonia formation becomes limited at high relative humidity.\nThis decrease is surprisingly not because N itself reacts, but because\none of its main sources, an excited molecular nitrogen state, is reacted\naway. In addition, oxidation pathways for nitrogen, which lead to\nNO and related products, are found to be favored over reduction pathways\nbecause the corresponding reverse reactions are less significant.\nTogether, this understanding helps explain previously reported observations\nof selectivity toward nitrogen oxides over ammonia, particularly at\nhigher relative humidities.