Plasma catalysis is gaining increasing interest for various gas conversion applications, such as CO2 conversion into value-added chemicals and fuels, CH4 activation into hydrogen, higher hydrocarbons or oxygenates, and NH3 synthesis. Other applications are already more established, such as for air pollution control, e.g. volatile organic compound remediation, particulate matter and NOx removal. In addition, plasma is also very promising for catalyst synthesis and treatment. Plasma catalysis clearly has benefits over ‘conventional’ catalysis, as outlined in the Introduction. However, a better insight into the underlying physical and chemical processes is crucial. This can be obtained by experiments applying diagnostics, studying both the chemical processes at the catalyst surface and the physicochemical mechanisms of plasma-catalyst interactions, as well as by computer modeling. The key challenge is to design cost-effective, highly active and stable catalysts tailored to the plasma environment. Therefore, insight from thermal catalysis as well as electro- and photocatalysis is crucial. All these aspects are covered in this Roadmap paper, written by specialists in their field, presenting the state-of-the-art, the current and future challenges, as well as the advances in science and technology needed to meet these challenges.

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A self-consistent model is developed to study the temporal variation of the gas temperature in millisecond single dc pulsed discharges and their afterglows in air-like mixtures (N 2 -20%O 2 ) at low pressures. The model is based on the solutions to the time-dependent gas thermal balance equation, under the assumption of a parabolic gas temperature profile across the discharge tube, coupled to the electron, vibrational and chemical kinetics. Modelling results provide a satisfactory explanation for recently published time-resolved experimental data for the gas temperature in a 5 ms pulsed air plasma with a current of 150 mA and the corresponding afterglow at a pressure of 133 Pa (1 Torr). It is shown that the main heating mechanisms during the first millisecond of the pulse come predominantly from O 2 dissociation by electron impact through the pre-dissociative excited state O 2 (B 3 − u ) and the quenching of nitrogen electronically excited states N 2 (A 3 + u , B 3 g , a 1 − u , a 1 g , w 1 u ) by O 2 , agreeing with other studies on fast gas heating in air plasmas. As the pulse duration increases, other gas heating sources become important, namely V-T N 2 -O energy exchanges, recombination of oxygen atoms at the wall, N 2 (A) quenching by O( 3 P) and reaction N( 4 S) + NO(X) → N 2 (X, v ∼3) + O, contributing altogether to an additional smooth increase in the gas temperature until the end of the pulse. In the first instants of the early afterglow, the gas temperature decreases very rapidly as a consequence of the minor role played by electronic collisions and due to a fast decay of N 2 electronic states. For afterglow times up to 10 ms, the gas temperature continues to decrease, following the time-dependent kinetics of [N 2 (X,v)], [N( 4 S)], [O( 3 P)] and [NO(X)]. Sensitivity of the model to different input parameters such as thermal accommodation coefficient and probabilities for atomic recombination at the wall are reported.