No abstract

The conversion of plastic waste into high-quality lubricants is more profitable than sorting the waste or converting it into liquid fuels. A life cycle assessment showed that the emissions were lower than those of petroleum lubricants of similar quality.

When a nonthermal plasma and a liquid form part of the same circuit, the liquid may function as a cathode, in which case electrons are emitted from the liquid into the gas to sustain the plasma. As opposed to solid electrodes, the mechanism of this emission has not been established for a liquid, even though various theories have attempted to explain it via chemical processes in the liquid phase. In this work, we tested the effects of the interfacial chemistry on electron emission from water, including the role of pH as well as the hydroxyl radical, the hydrogen atom, the solvated electron, and the presolvated electron; it was found that none of these species are critical to sustain the plasma. We propose an emission mechanism where electrons, generated from ionized water molecules in the uppermost monolayers of solution, are emitted into the plasma directly from the conduction band of the water.

Plasma electrolysis, where a solid electrode in an electrolytic cell is replaced by a plasma (or gas discharge), differs from conventional electrolysis by not being dictated by the surface characteristics of an electrode, but by the chemical species injected into the solution from the plasma. Reduction in a plasma cathode configuration occurs mostly by plasma-injected solvated electrons (e − aq ), which may engage in side reactions, such as the second order recombination of e − aq , that ultimately reduce the faradaic efficiency for the production of a desired product. In this work, we show that the depletion of reactants at the plasma-liquid interface due to insufficient transport can reduce the predicted faradaic efficiency for a plasma cathode at low concentrations. Measurements of the faradaic efficiency using the dissociative electron attachment to chloroacetate and the ferri/ferrocyanide redox couple confirm this behavior. The effect of other mechanisms on the faradaic efficiency, such as competing oxidation reactions with the hydroxyl radical, are also evaluated and found to be far less significant. Unlike conventional electrolysis, stirring the solution does not increase the faradaic efficiency, but increasing the species concentration does.

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated electrons produced by a low-temperature plasma in contact with an aqueous solution. Solvated electrons are powerful reducing agents, and it has been postulated that they play an important role in the interfacial chemistry between a gaseous plasma or discharge and a conductive liquid. However, due to the high local concentrations of reactive species at the interface, they have a short average lifetime (~1 µs), which makes them extremely difficult to detect. The TIRAS technique uses a unique total internal reflection geometry combined with amplitude-modulated lock-in amplification to distinguish solvated electrons' absorbance signal from other spurious noise sources. This enables the in situ detection of short-lived intermediates in the interfacial region, as opposed to the bulk measurement of stable products in the solution. This approach is especially attractive for the field of plasma electrochemistry, where much of the important chemistry is driven by short-lived free radicals. This experimental method has been used to analyze the reduction of nitrite (NO2(aq)), nitrate (NO3(aq)), hydrogen peroxide (H2O2(aq)), and dissolved carbon dioxide (CO2(aq)) by plasma-solvated electrons and deduce effective rate constants. Limitations of the method may arise in the presence of unintended parallel reactions, such as air contamination in the plasma, and absorbance measurements may also be hindered by the precipitation of reduced electrochemical products. Overall, the TIRAS method can be a powerful tool for studying the plasma-liquid interface, but its effectiveness depends on the particular system and reaction chemistry under study.