J. Christopher Whitehead

The 2012 Plasma Roadmap

Low-temperature plasma physics and technology are diverse and interdisciplinary fields. The plasma parameters can span many orders of magnitude and applications are found in quite different areas of daily life and industrial production. As a consequence, the trends in research, science and technology are difficult to follow and it is not easy to identify the major challenges of the field and their many sub-fields. Even for experts the road to the future is sometimes lost in the mist. Journal of Physics D: Applied Physics is addressing this need for clarity and thus providing guidance to the field by this special Review article, The 2012 Plasma Roadmap.

Dry reforming of methane over a Ni/Al2O3 catalyst in a coaxial dielectric barrier discharge reactor

A coaxial double dielectric barrier discharge (DBD) reactor is developed for plasma-catalytic conversion of CH4 and CO2 into syngas and other valuable products. A supported metal catalyst (Ni/Al2O3) reduced in a methane discharge is fully packed into the discharge region. The influence of the Ni/Al2O3 catalyst packed into the gas gap on the electrical characteristics of the discharge is investigated. The introduction of the catalyst pellets leads to a transition in discharge behaviour from a typical filamentary microdischarge to a combination of spatially limited microdischarges and a predominant surface discharge on the catalyst surface. It is also found that the breakdown voltage of the CH4/CO2 discharge significantly decreases when the reduced catalyst is fully packed in the discharge area. Conductive Ni active sites dispersed on the catalyst surface contribute to the expansion of the discharge and enhancement of charge transfer. In addition, plasma-catalytic dry reforming of CH4 is carried out with the reduced Ni/Al2O3 catalyst using a mixing ratio of CH4/CO2 = 1 and a total flow rate of 50 ml min−1. An increase in H2 selectivity is observed compared with dry CH4 reforming with no catalyst, while the H2/CO molar ratio significantly increases from 0.84 to 2.53 when the catalyst is present.

Plasma catalysis: A solution for environmental problems

The combination of a nonthermal, atmospheric plasma with a catalyst is investigated as a means of destroying pollutants in waste gas streams. Using the examples of dichloromethane (DCM) and toluene in air streams, it is shown that the destruction of the pollutant can be increased whilst lowering the operating temperature, giving increasing energy efficiency. Unwanted by-products can also be reduced selectively by appropriate choice of catalyst and of the plasma–catalyst configuration. By studying the temperature dependence of plasma catalysis, some ideas can be obtained about the nature of the interaction between plasma and catalyst in the processing.

Plasma–catalysis: the known knowns, the known unknowns and the unknown unknowns

This review describes the history and development of plasma-assisted catalysis focussing mainly on the use of atmospheric pressure, non-thermal plasma. It identifies the various interactions between the plasma and the catalyst that can modify and activate the catalytic surface and also describes how the catalyst affects the properties of the discharge. Techniques for in situ diagnostics of species adsorbed onto the surface and present in the gas-phase over a range of timescales are described. The effect of temperature on plasma–catalysis can assist in determining differences between thermal catalysis and plasma-activated catalysis and focuses on the meaning of temperature in a system involving non-equilibrium plasma. It can also help to develop an understanding of the gas-phase and surface mechanism of the plasma–catalysis at a molecular level. Our current state of knowledge and ignorance is highlighted and future directions suggested.

Electrical and spectroscopic diagnostics of a single-stage plasma-catalysis system: effect of packing with TiO2

The influence of adding TiO2 on the electrical and spectroscopic characteristics of a N2 dielectric barrier discharge (DBD) has been investigated in a single-stage plasma-catalysis system. The introduction of the catalyst into the electrode gap leads to a transition in the discharge behaviour. The presence of the TiO2 pellets in the discharge significantly increases the vibrational temperature of N2 in the DBD, which suggests that the interaction of plasma and catalyst has a strong effect on the electron energy distribution function in the discharge with an increase in electron density in the high-energy tail of the distribution function.

Collisional removal rates for electronically excited CH radicals (B) and (C)

The quenching of electronically excited CH in its B2Σ– and C 2Σ+ states by O2, H2, CO2, N2O, NO, CH4, C2H6, C3H8 and C2H4 has been studied using 248 nm laser photolysis of CHBr3 as the source of CH (B and C). For the B state, quenching is largely by chemical reaction, with the possible exception of N2O, with rate constants that exceed those for A-state quenching and are comparable to or greater than those for the ground state, CH (X), reactions. The hydrocarbon molecules and nitric oxide are particularly efficient in quenching the B state and it is thought that in addition to reaction, collisional removal of CH (B) can take place via and E → E energy transfer to the A state with a corresponding excitation of a C—H or N—O stretching vibration. This is a near-resonant process which can occur with high efficiency. The C-state quenching-rate constants are lower or similar to those for A-state quenching for most of the molecules studied, with the exception of O2 and H2. For the quenching of CH (C) by alkanes, the evidence would suggest that quenching is by collisional removal rather than by reaction.

Rotational and vibrational energy transfer in CH(A2Δ)

Rate constants have been obtained for rotational and vibrational energy transfer within the A 2Δ state of CH by collisions with Ar and N2. By using laser photolysis of CHBr3 as the source of CH(A), it was possible to produce a wide range of rotational and vibrational levels (N⩽ 25, v⩽ 2). The rate constants for rotational relaxation are comparable to gas kinetic rates (k < 2.5 × 10–10 cm3 molecule–1 s–1) and decrease with increasing rotational quantum number. The efficiency of vibrational relaxation is ca. 10 times less than that of rotational relaxation (k < 6.5 × 10–12 cm3 molecule–1 s–1).

Molecular beam studies of free-radical processes: photodissociation, inelastic and reactive collisions

In this review, the production and detection of a wide range of unstable free-radical species under molecular beam conditions are described. The use of such molecular beam methods to study the photodissociation and inelastic and reactive scattering of free radicals in recent years is reviewed. A selection of the many experiments on the photodissociation of radicals that have been performed recently using molecular beam conditions is described to illustrate the range and scope of such experiments. For the comparatively smaller fields of free-radical inelastic and reactive scattering studied using molecular beam techniques, a comprehensive review is presented. In all cases, the experimental results are interpreted and discussed with reference to recent related theoretical calculations on the electronic structure and dynamics for the systems. A particular aim is to illustrate how the observed features of the experiments are related to information on the topography of and the couplings between the appropriate potential energy surfaces and the associated dynamics.

Remediation of dichloromethane (CH2Cl2) using non-thermal, atmospheric pressure plasma generated in a packed-bed reactor.

This work describes the application of a non-thermal plasma generated in a dielectric barrier packed-bed plasma reactor for the remediation of dichloromethane (CH2Cl2, DCM). The overall aim of this investigation is to identify the role of key process parameters and chemical mechanisms on the removal efficiency of DCM in plasma. The influence of process parameters, such as oxygen concentration, concentration of initial volatile organic compounds (VOCs), energy density, plasma residence time, and background gas, on the removal efficiency of 500 ppm DCM was investigated. Results showed a maximum removal efficiency with the addition of 2-4% oxygen into a nitrogen plasma. It is thought that oxygen concentrations in excess of 4% decreased the decomposition of chlorinated VOCs as a result of ozone and nitrogen oxide formation. Increasing the residence time and the energy density resulted in increasing the removal efficiency of chlorinated VOCs in plasma. A chemical kinetic model has been developed on the basis of the proposed reaction scheme, and the calculation of end product concentrations are in general good agreement with the observed values. With the understanding of the effect of the key parameters, it has been possible to optimize the remediation process.

Modelling of non-thermal plasma aftertreatment of exhaust gas streams

A reaction mechanism has been developed that is appropriate for the plasma aftertreatment of diesel exhaust gas. It is based on a simulated gas mixture containing propene, nitric oxide, nitrogen dioxide, oxygen and nitrogen. The reaction mechanism has been used to determine the end-products from the plasma processing and their concentrations using a chemical kinetics modelling procedure. It has been validated by a range of experiments using the same gas mixture with a packed bed, a dielectric barrier plasma reactor and a wide range of end-product analysis techniques. Using a wide range of experimental conditions has enabled us to validate the model and its predictions and to critically evaluate several alternative reaction mechanisms for the oxidation of propene and the formation of end-products in a more systematic and reliable manner than before.