Non-equilibrium Plasmas in Gases at Atmospheric Pressure

This chapter discusses the non-equilibrium (NEQ) in plasma discharges operating at atmospheric pressure, providing an overview of their fundamental chemical and physical aspects. In the first part of the chapter, we recall the basic notions of chemical kinetics and dynamics. In particular, we discuss why chemical reactions occur and the basis of transition state (TS) theory. We focus on aspects of chemistry peculiar to out-of-the-equilibrium environments, like chemical processes involving vibrationally excited states and ions. After this brief recall of basic notions of chemistry, we discuss the crucial role of the electron energy distribution function (EEDF) in determining the way the discharge power is transferred to the different degrees of freedom of the plasma constituents. It turns out that the input energy can be partitioned among the various quantum states of the gas constituents so that non-Boltzmann distributions occur. This possibility allows boosting the reactivity by exploiting mode-selective chemistry. An exciting opportunity, so much investigated at present, is given by the potential role of the vibrational excitation of CO2 in favoring the reduction reaction to CO+O. The second part of the chapter discusses key experimental aspects of plasma-mediated processes. Using gas discharges to stimulate the desired chemical reactions requires the accurate assessment of several process parameters, such as conversion, yield and selectivity, plasma power, specific energy input and process efficiency. These quantities, which are regularly employed in plasma chemistry, are often poorly specified. Therefore, we review their definition and the procedure to retrieve them from the experimental observables, discussing how to avoid common missteps. We also present a selection of optical diagnostics techniques, finalized at investigating the space and time evolution of physical and chemical observables. Indeed, revealing the NEQ nature of non-thermal plasmas requires dedicated diagnostics to detect the local value of critical quantities. Finally, since the plasma activation of carbon dioxide is a hot field nowadays, we discuss a selection of the most recent research works on plasma CO2 conversion, with particular attention to the case of the nanosecond repetitive discharges that represent one of the most challenging environments for the microscopic investigation of plasma discharges