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.