Lightning in a bottle: Plasma enables selective, electrolytic production of ammonia from nitrogen and water.
Replacing the solid metal cathode in an electrolytic cell with a plasma (gas discharge) allows the reduction of metal salt solutions to metal nanoparticles. Here, we apply gravimetric analysis to quantify the charge transfer (faradaic) efficiency of this reduction process and understand reaction pathways at the plasma-liquid interface. Silver powder synthesized from aqueous solutions of silver nitrate is collected and the weight is compared to the theoretical amount of silver obtained by applying Faraday's law of electrolysis. We find that the faradaic efficiencies are near 100% when the initial silver nitrate concentration is large (>100 mM), but are much less than 100% at low salt concentrations (<15 mM) or at high applied currents (>6 mA). These results are explained in terms of reaction kinetics involving solvated electrons and two important reactions: the reduction of silver ions (Ag + ) and the second order recombination of solvated electrons which releases hydrogen gas. Plasmas formed at the surface of liquids including water have the potential to carry out electrochemical reactions through the interaction between gas-phase and solution species.1-3 In a direct-current (DC) configuration where the plasma is one electrode and a solid metal or plasma is another electrode with an aqueous electrolyte in between, charge is transferred from gas-phase electrons or ions in the plasma to aqueous ions in solution through charge transfer reactions at the plasma-liquid interface. 4,5 In contrast to typical electrochemical reactions at the interface of a solid metal electrode and ionic solution, the reduction or oxidation reactions in plasma-assisted electrochemistry occur substrate-free. For example, solutions containing metal cations can be reduced by a cathodic plasma to produce colloidallydispersed metal nanoparticles rather than thin films of metal deposited on a substrate. [6][7][8] This approach to synthesizing metal nanoparticles is high-purity and environmentally-friendly, avoiding chemical reducing agents such as sodium borohydride, 9 can eliminate the need for organic capping agents by electrostatically stabilizing the particles, 10,11 and has the potential to create novel materials such as metal oxides via non-equilibrium, solution-phase radical chemistry. 12,13 While the ability of plasmas to function as an electrochemical electrode and reduce metal cations to metal nanoparticles has been clearly demonstrated, the fundamental reaction mechanisms remain elusive, limiting the ability to optimize or tailor the process. A critical reason is that plasmas contain a variety of reactive species that originate from the inert carrier gas and, in many applications, ambient air, including electrons, ions, and metastable neutrals, and these reactive species have a distribution of energies.14 Many of these species can react directly with and dissociate other gas or vapor phase molecules such as water, 15 or dissolve into the solution and then react with solution species to create a number of products.16 For example, in ...
Fuel microchannels for regenerative cooling are receiving increasing attention in advanced aviation technologies. Those microchannels allow heat integration between the endothermic cracking of the jet fuels and their subsequent combustion. In this work, a detailed elementary-step kinetic model is developed to gain insights into the cracking chemistry of a Jet A surrogate (n-dodecane, isooctane, n-propyl benzene, and 1,3,5-trimethylbenzene), which allows for further optimization of those aviation technologies. A dedicated procedure is described for the automated generation of kinetic models for multi-component mixtures with the open-source Reaction Mechanism Generator (RMG) software. The full kinetic model is validated against experimental measurements in multiple reactor geometries, under various experimental conditions, including both a surrogate mixture and a commercial Jet A. The experimental data include new experimental measurements for the pyrolysis of a Jet A surrogate in a tubular reactor with a detailed product analysis using comprehensive 2D GC. The good performance of the kinetic model for data from a broad range of experimental conditions demonstrates the advantage of a kinetic model with detailed chemistry against empirical kinetic models that are limited in their applicability range. Further analysis of the important chemistry in the kinetic model shows that it is essential to account for cross-reactions between the different surrogate components.
The electrolysis of water is an important reaction in many aqueous electrochemical systems, including those where an atmosphericpressure microplasma jet is formed at a liquid surface. Here, we quantitatively study the hydrogen gas evolved from this plasma electrolytic system. Unlike conventional water electrolysis with a metal cathode in contact with the solution, more hydrogen gas is produced than expected based on the quantity of electricity passed, and the apparent faradaic efficiency exceeds 100%. By varying the solution temperature and carrying out kinetic analysis, we show two parallel reaction mechanisms exist , specifically faradaic liquid-phase reactions involving solvated electron-mediated reduction of hydronium ions and non-faradaic gas-phase reactions involving electron impact dissociation of water vapor, that lead to the distinct behavior.
Ammonia which is a precursor to fertilizers and food and is one of the most important chemicals in the world is currently synthesized on a large scale by the Haber-Bosch (H-B) process. Recent research has been aimed at developing alternative approaches to synthesizing ammonia to address several issues associated with the H-B process: 1) the very high pressures and high temperatures which lead to enormous capitals costs, 2) the need for hydrogen gas which comes from steam-methane reforming and contributes to high energy costs and environmental impact through greenhouse gas emissions, and 3) the incompatibility with renewable sources of energy that could lower energy costs and environmental impact while also offering more localized, point-of-use sources of ammonia. Here, we report a hybrid electrochemical system to produce ammonia where one of the metal electrodes is replaced by a plasma (gas discharge). Similar to more typical electrochemical systems, ammonia synthesis is carried out at ambient conditions and only requires electricity. However, the interaction of a plasma and water leads to the formation of solvated electrons, one of the strongest reducing agents known with a reduction potential of ~-2.8 V. Production rates and selectivity (i.e. charge-transfer efficiency) were determined as a function of various process parameters including current, time, and electrolyte composition. We will also discuss a potential reaction mechanism based on the observed results.
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