Paul Rumbach

The solvation of electrons by an atmospheric-pressure plasma

Solvated electrons are typically generated by radiolysis or photoionization of solutes. While plasmas containing free electrons have been brought into contact with liquids in studies dating back centuries, there has been little evidence that electrons are solvated by this approach. Here we report direct measurements of solvated electrons generated by an atmospheric-pressure plasma in contact with the surface of an aqueous solution. The electrons are measured by their optical absorbance using a total internal reflection geometry. The measured absorption spectrum is unexpectedly blue shifted, which is potentially due to the intense electric field in the interfacial Debye layer. We estimate an average penetration depth of 2.5±1.0 nm, indicating that the electrons fully solvate before reacting through second-order recombination. Reactions with various electron scavengers including H+, NO2−, NO3− and H2O2 show that the kinetics are similar, but not identical, to those for solvated electrons formed in bulk water by radiolysis.

Fundamental properties of field emission-driven direct current microdischarges

For half a century, it has been known that the onset of field emission in direct current microdischarges with gap sizes less than 10 μm can lead to breakdown at applied voltages far less than predicted by Paschens law. It is still unclear how field emission affects other fundamental plasma properties at this scale. In this work, a one-dimensional fluid model is used to predict basic scaling laws for fundamental properties including ion density, electric field due to space charge, and current-voltage relations in the pre-breakdown regime. Computational results are compared with approximate analytic solutions. It is shown that field emission provides an abundance of cathode electrons, which in turn create large ion concentrations through ionizing collisions well before Paschens criterion for breakdown is met. Breakdown due to ion-enhanced field emission occurs when the electric field due to space charge becomes comparable to the applied electric field. Simple scaling analysis of the 1D Poisson equation de...

Decoupling Interfacial Reactions between Plasmas and Liquids: Charge Transfer vs Plasma Neutral Reactions

Plasmas (gas discharges) formed at the surface of liquids can promote a complex mixture of reactions in solution. Here, we decouple two classes of reactions, those initiated by electrons (electrolysis) and those initiated by gaseous neutral species, by examining an atmospheric-pressure microplasma formed in different ambients at the surface of aqueous saline (NaCl) solutions. Electrolytic reactions between plasma electrons and aqueous ions yield an excess of hydroxide ions (OH(-)), making the solution more basic, while reactions between reactive neutral species formed in the plasma phase and the solution lead to nitrous acid (HNO2), nitric acid (HNO3), and hydrogen peroxide (H2O2), making the solution more acidic. The relative importance of either reaction path is quantified by pH measurements, and we find that it depends directly on the composition of the ambient background gas. With a background gas of oxygen or argon, electron transfer reactions yielding excess OH(-) dominate, while HNO2 and HNO3 formed in the plasma and by the dissolution of nitrogen oxide (NOx) species dominate in the case of air and nitrogen. For pure nitrogen (N2) gas, we observe a unique coupling between both reactions, where oxygen (O2) gas formed via water electrolysis reacts in the bulk of the plasma to form NOx, HNO2, and HNO3.

Evidence for the electrolysis of water by atmospheric-pressure plasmas formed at the surface of aqueous solutions

The formation of atmospheric-pressure plasmas with liquid electrodes holds great importance for a number of emerging technologies and applications, yet fundamental questions remain about the nature of the interactions at the plasma/liquid interface. In particular, when the liquid serves as the anode, the plasma supplies gas-phase electrons to the liquid surface, and how these electrons interact with the liquid has not been fully explained. In this work, we present experimental evidence that in the case of water, plasma electrons are involved in electrolytic reactions leading to the conversion of protons (H + ) to hydrogen gas. Reactions associated with water electrolysis are indirectly characterized by pH measurements that show qualitatively and quantitatively that the liquid at the plasma interface increases in basicity, consistent with the reduction of protons by plasma electrons. Mass spectrometry measurements confirm the evolution of hydrogen gas, directly providing evidence of water electrolysis. This work highlights the critical role that plasma electrons can play in plasma/liquid interactions with broad implications for any plasma system involving an aqueous electrode, including emerging applications in plasma medicine and plasma‐liquid materials synthesis.

The effect of air on solvated electron chemistry at a plasma/liquid interface

Plasmas in contact with liquids initiate complex chemistry that leads to the generation of a wide range of reactive species. For example, in an electrolytic configuration with a cathodic plasma electrode, electrons from the plasma are injected into the solution, leading to solvation and ensuing reactions. If the gas contains oxygen, electronegative oxygen molecules may react with the plasma electrons via attachment to reduce the electron flux to the solution reducing the production of solvated electrons or produce reactive oxygen species that quickly scavenge solvated electrons in solution. Here, we applied a total internal reflection absorption spectroscopy technique to compare the concentration of solvated electrons produced in solution by an argon plasma containing various amounts of oxygen, nitrogen, and air. Our measurements indicate that in oxygen or air ambients, electron attachment in the plasma phase greatly attenuates the electron flux incident on the liquid surface. The remaining electrons then solvate but are quickly scavenged by reactive oxygen species in the liquid phase.

The Coupling of Ion-Enhanced Field Emission and the Discharge During Microscale Breakdown at Moderately High Pressures

Recent studies have shown that, in microscale electrode gaps, the traditional Paschens curve fails as the left branch sharply decreases with electrode spacing, thus resulting in the modified Paschens curve. This deviation from Paschens curve is attributed to ion-enhanced field emission and notably breaks pressure times distance (pd) scaling. Here, 1-D particle-in-cell/Monte Carlo collision simulations at moderately high pressures are used to predict breakdown and reproduce the modified Paschens curve, which is in good agreement with existing theory. These simulations reveal that the net positive space charge that accumulates in the electrode gap enhances the electric field, subsequently enhancing field emission from the cathode. Because the emitted electrons generate additional ions in the discharge, a positive feedback mechanism occurs, where the field-emitted electrons produce the ions that enhance the electric field. It is revealed that this coupling between field emission and the discharge is necessary in order for breakdown to occur.

Visualization of Electrolytic Reactions at a Plasma-Liquid Interface

A dc microplasma jet in flowing argon (Ar) gas is used to electrolyte aqueous sodium chloride (NaCl) solutions. Electrolytic reactions at the plasma-liquid interface make the solution more basic and result in H2 gas evolution. The pH change is visualized using a pH sensitive dye, which becomes dark green under basic conditions. The basic solution created at the plasma-liquid interface is subsequently transported to the bottom of the reactor vessel by an apparent electrohydrodynamic flow.

Overcoming ammonia synthesis scaling relations with plasma-enabled catalysis

AbstractCorrelations between the energies of elementary steps limit the rates of thermally catalysed reactions at surfaces. Here, we show how these limitations can be circumvented in ammonia synthesis by coupling catalysts to a non-thermal plasma. We postulate that plasma-induced vibrational excitations in N2 decrease dissociation barriers without influencing subsequent reaction steps. We develop a density-functional-theory-based microkinetic model to incorporate this effect, and parameterize the model using N2 vibrational excitations observed in a dielectric-barrier-discharge plasma. We predict plasma enhancement to be particularly great on metals that bind nitrogen too weakly to be active thermally. Ammonia synthesis rates observed in a dielectric-barrier-discharge plasma reactor are consistent with predicted enhancements and predicted changes in the optimal metal catalyst. The results provide guidance for optimizing catalysts for application with plasmas.Plasma catalysis holds promise for overcoming the limitations of conventional catalysis. Now, a kinetic model for ammonia synthesis is reported to predict optimal catalysts for use with plasmas. Reactor measurements at near-ambient conditions confirm the predicted catalytic rates, which are similar to those obtained in the Haber–Bosch process.

Electrostatic Debye layer formed at a plasma-liquid interface

We construct an analytic model for the electrostatic Debye layer formed at a plasma-liquid interface by combining the Gouy-Chapman theory for the liquid with a simple parabolic band model for the plasma sheath. The model predicts a nonlinear scaling between the plasma current density and the solution ionic strength, and we confirmed this behavior with measurements using a liquid-anode plasma. Plots of the measured current density as a function of ionic strength collapse the data and curve fits yield a plasma electron density of ∼10^{19}m^{-3} and an electric field of ∼10^{4}V/m on the liquid side of the interface. Because our theory is based firmly on fundamental physics, we believe it can be widely applied to many emerging technologies involving the interaction of low-temperature, nonequilibrium plasma with aqueous media, including plasma medicine and various plasma chemical synthesis techniques.

Corrigendum: The solvation of electrons by an atmospheric-pressure plasma.

Nature Communications 6 Article number:7248 (2015); Published: 19 June 2015; Updated 6 June 2016. We have discovered an error in the original data analysis of our publication, which significantly impacts the model parameters derived from experiment. Specifically, the correction changes our estimations of the average penetration depth l and scavenging rate constants, but these new estimates do not change the overall physics conveyed in the Article.