Plasma Chemistry

the branch of chemistry that studies chemical processes in low-temperature plasma, including the laws that govern reactions in plasma and the fundamentals of plasmochemical technology. Plasmas are artificially produced in plasmatrons at temperatures that range from 10³ to 2 × 10⁴ K and pressures that range from 10^–6 to 10⁴ atmospheres. Interaction between the reagents in plasma results in the formation of final, or terminal, products; these products can be removed from the plasma by rapid cooling, or quenching. The basic feature of all plasmochemical processes is that reactive particles are generated in significantly higher concentrations than under ordinary conditions of chemical reactions. The reactive particles that are produced in a plasma are capable of effecting new types of chemical reactions; the particles include excited molecules, electrons, atoms, atomic and molecular ions, and free radicals. Indeed, some of these particles can only exist in the plasma state.

Reactions. As a rule, plasmochemical reactions occur under nonequilibrium conditions, for example, when (1) the subsystems of a single reacting multicomponent system have different translational energies, (2) the rotational, vibrational, and electronic energies of the system differ greatly from one another, and (3) the energy levels of the system do not conform to the Boltz-mann distribution (seeKINETIC THEORY OF GASES). Nonequilibrium conditions may be induced by various physical forces, such as electromagnetic fields, rapid changes in pressure, and supersonic effusion. The nonequilibrium state can also be a result of the chemical reaction itself, which is a threshold process and as such proceeds with a decrease in the number of molecules that have sufficient energy to exceed the threshold energy; thus, the form of the energy-distribution function of the molecules is altered as the reaction proceeds. For example, at low pressures in glow discharges, as well as in high-frequency and ultrahigh-frequency discharges, the average energy of the electrons ranges from 3 to 10 electron volts (eV), while the energy distribution functions of these electrons depart significantly from the Maxwell distribution; furthermore, the average vibrational energy of molecules and radicals is either less than or equal to 1 eV, while the average translational and rotational energies approximately equal 0.1 eV.

Mechanisms of reactions. Plasmochemical reactions exhibit a number of peculiarities, depending on a variety of factors. Dissociation reactions, in particular, those that result in the formation of free radicals, may be the rate-controlling steps in a plasmochemical process. These dissociation reactions are initiated by the excited charged particles that are present in low-temperature plasma, for example, vibrationally and electronically excited molecules and electrons. In both the ground and excited states, electron collisions accelerate vibrational relaxation and dissociation of molecules. Electron collisions are a rate-determining factor in isothermal plasma, in which the degree of ionization exceeds 10~³, while in those plasmas whose electrons and heavy particles are at widely different temperatures, collisions are a determining factor at any degree of ionization. Nonadiabatic transformations become increasingly important in cases of dissociation and recombination in electronically excited states.

Dissociation in electronically excited states is a two-step process: the electronic excitation occurs first and is followed by the dissociation of the excited particles. Owing to predissociation, these excited particles can be either stable or unstable. Interactions between molecules and electronically excited ions also become essential in the dissociation reactions of plasma.

Plasmochemical reactions can occur, as a rule, over several pathways, and this characteristic is a determining factor in all experimental reactions in low-temperature plasma. The reactions may be directed along a particular pathway by changing the conditions under which the plasma is generated and by controlling the plasma’s composition.

Kinetics. The kinetics of chemical processes in nonequilibrium plasma differs from ordinary chemical kinetics. Nonequilibrium kinetics must take into account the quantum structure of molecules and atoms, that is, the distribution of each component in every energy state, as well as the interactions between energy states and the pathways of chemical reactions. The equations that apply to ordinary kinetics are replaced in the case of plasma kinetics by the Pauli equations. Each Pauli equation relates the rate of change in the concentration of a given type of reacting molecule, atom, ion, or radical in the z’th energy state to the concentration of particles in all possible energy states. The rate of concentration change is also related by the Pauli equation to the probability of transitions between states, to the collision frequency of particles, and to the rate of excitation of a given energy level. Furthermore, the Pauli equation does not make use of the ordinary rate constant of a reaction; rather, it uses a coefficient that is characteristic of the given ith energy level. In the simplest case, integration of the Pauli equations by computer provides a complete description of the plasmochemical reaction in a given system.

Technology. Plasmochemical technology is a new branch of industrial chemical technology. Its special nature is determined by the peculiarities of the mechanisms and kinetics of plasmochemical reactions, as well as by the specific characteristics of chemical processes in low-temperature plasma and in plasma jets. The high rates of a plasmochemical process, whose duration is from 10^–5 to 10^–2 sec, make it possible to reduce the size of industrial equipment and apparatus. For example, the plasma reactor that pyrolyzes methane at an annual production rate of 25,000 tons is 65 cm long and has a diameter of 15 cm. Since the duration of reagent mixing in plasma jets is so close to the duration of the plasmochemical reaction, an appreciable number of plasma processes are limited by optimum turbulent mixing to the molecular level.

Plasmochemical reactions are quenched when the rate of formation of the desired products is in a maximum range. As a rule, plasmochemical processes are easy to control, model, and optimalize. In many cases, plasmochemical technology makes it possible to produce very valuable materials, for example, finely dispersed powders, films, and coatings. Sometimes new properties can be imparted to a substance; for example, tungsten can be made to resist recrystallization and creep and to acquire anisotropy of electron emission properties.

Many plasmochemical processes have been effected on an industrial or semi-industrial scale, including the production of acetylene and technical-grade hydrogen from natural gas and the production of acetylene, ethylene, and hydrogen from petroleum hydrocarbons that are found in distillates and raw petroleum. Plasmochemical technology has been applied to the production of many important substances, including titanium dioxide pigment and synthesis gas for the manufacture of vinyl chloride. It is also used to fix atmospheric nitrogen in the production of nitric acid.

Plasma chemistry became an independent branch of science during the 1960’s, when fundamental work was carried out in the USSR, USA, and the Federal Republic of Germany.