Current industrial production of ammonia via the Haber−Bosch process has a massive carbon footprint because the hydrogen gas feedstock comes from the reformation of fossil fuel which releases large amounts of carbon dioxide. One possible solution is to provide hydrogen gas by water electrolysis that is powered by renewable sources. A more radical solution is to use water as the hydrogen source in a process-intensified scheme whereby ammonia is produced by directly reacting nitrogen gas and water. Here, we investigated an atmospheric-pressure plasma process to continuously produce ammonia from nitrogen gas and atomized water droplets. The successful production of ammonia was carefully confirmed by several control experiments. We find that other forms of fixed nitrogen are synthesized as well, predominantly nitrate and some nitrite. To understand the role of water droplets, we characterized their lifetimes in the reactor and compared the ammonia production with a water vapor feed. The results show that the ammonia forms through gas-phase chemistry and the liquid phase does not appear to substantially contribute. Additional insight into the reaction mechanism was provided by replacing the water with hydrogen gas or oxygen gas as the feedstock. As expected, no nitrates or nitrites were synthesized with hydrogen gas, and the ammonia production was comparable, indicating similar mechanisms may be occurring that involve feedstock dissociation to atomic hydrogen. No ammonia or nitrates/nitrites were detected with oxygen gas, which suggests that water provides a unique reaction pathway for nitric and nitrous acid to be formed in the gas phase. Our study demonstrates that nitrogen gas and water droplets can serve as feeds in a plasma process for the sustainable and distributed production of ammonia or other fixed nitrogen compounds.

While plasmas are now routinely employed to synthesize or remove nano- to micron-sized particles, the charge state (polarity and magnitude) of the particles remains relatively unknown. In this study, charging of nanoparticles was systematically characterized in low-temperature, atmospheric-pressure, flow-through plasmas previously applied for synthesis. Premade, charge-neutral nanoparticles of MgSO4, NaCl, and sea salt were introduced into the plasma to decouple other effects such as the reactive vapor precursor, and MgSO4 was selected as the focus because of its stability (i.e., no evaporation) in the plasma environment. The charge fraction and distribution of the particles was examined at the reactor outlet for different particle diameters (10–250 nm) as a function of plasma power and two types of power source, alternating current (AC) and radio frequency (RF). We found that the overall charge fraction increased with increasing plasma power and diameter for the RF plasma. A similar increasing trend was observed for the AC plasma with increasing particle diameter in the range of 50–250 nm, but the charge fraction increased with decreasing particle diameter in the range of 10–50 nm. The charge distribution was revealed to be bipolar, with particles supporting multiple charges for both the RF and AC plasmas, but the RF plasma produced a higher fraction of multiple charges. Differences in the characteristic timescales for particle charging in the AC and RF plasmas are a possible explanation of the trends observed in the experiments.

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The nucleation and growth of nanoparticles in the gas phase using atmospheric-pressure plasma systems is an important approach to synthesizing novel dimensionally-controlled materials.Here, we investigated the effect of the nanoparticles on a typical type of continuous-flow, substrate-free plasma at atmospheric pressure to understand their potential contribution to electron density changes. A tandem plasma system was set up consisting of one plasma reactor that produced carbonaceous nanoparticles from mixtures of argon and hexane, and another identical plasma reactor where the as-grown particles were injected and non-intrusive electrical and optical measurements were performed. The electron densities obtained from conductivity measurements and a plasma fluid model were found to decrease in the presence of nanoparticles. However, control experiments revealed that the main source of the electron depletion was residual vapor or small molecule products (nanoclusters) and not the particles themselves. These results were validated by constant number Monte Carlo simulations which showed that at the experimentallymeasured conditions, the nanoparticles were not of sufficiently high enough concentration to reduce the electron density; however, if residual vapor molecules and clusters are ionizable, they remain in sufficient concentration to deplete electron densities when compared to pristine plasma densities. Our study shows that at atmospheric pressure, because of their typically larger electron density values, particle-producing plasmas are distinct from those at low pressure, and nanoparticle formation does not have the same impact while molecular-scale species may be a more important consideration.