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

Emeritus Professor in Chemistry at the University of Manchester, J. Christopher Whitehead introduces his new Topical Review in Journal of Physics D: Applied Physics:

A plasma-catalysis reactor

A plasma-catalysis reactor Image © J. Christopher Whitehead

Plasma created by an electrical discharge in a gas has been shown to be an effective environmental tool for reducing pollutants in waste gas streams and in for converting greenhouse gases such as carbon dioxide into clean fuels such as methanol and hydrogen (when combined with methane or water) and a range of high-value chemical products. Electrical discharge in gases can produce a range of species including very energetic electrons, ions, excited states and free radicals. In one commonly used form of plasma, there is a disequilibrium that allows these high energy electrons and other species to coexist without heating up the gas. This could not be achieved thermally by conventional heating, and non-thermal plasma has been an important tool for over 150 years in creating ozone for the sterilisation of drinking water, the clean-up of contaminated waste water and the elimination of odours. Catalysis is used widely in industry to improve the efficiency of many chemical processes by lowering reaction energy barriers, thereby improving yields with lower energy costs. However, many common catalytic processes still work at very high temperatures and pressures making their engineering very difficult and creating conditions in which the lifetime of the catalysts are reduced by poisoning, the build-up of ‘coke’, or sintering into small fragments.

It was noticed over fifty years ago that when a catalyst was placed in an electrical discharge, there was a synergy between the plasma and the catalyst that improved the performance of the overall process by increasing yields, lowering the operating temperatures and pressures, improving energy efficiency and increasing the durability of the catalyst. Since then, many researchers have demonstrated the benefits of the hybridisation of catalysis with plasma for a wide range of systems. What is less clear is the mechanism by which this occurs. How does the plasma affect the properties of the catalyst and how does placing a catalyst in a discharge affect its electrical performance? Which of these effects is dominant?

This article critically reviews the current state of knowledge in the field (the ‘known knowns’ and the ‘known unknowns’) and identifies where the gaps in our knowledge lie. The review suggests some experimental techniques used in other fields that could be applied to plasma catalysis to address some of the ‘unknowns’ and guide researchers into new areas with new questions (the ‘unknown unknowns’). One senior researcher in the field recently remarked that “plasma catalysis may be regarded as the last unvisited continent”.

JChristopherWhiteheadAbout the author

Professor Christopher Whitehead is an Emeritus Professor in Chemistry at The University of Manchester where until recently he was Head of the School of Chemistry. He has worked in the area of plasma research for the remediation and conversion of waste gases for the last twenty years with a recent focus on plasma catalysis. Prior to that, he studied the formation and reaction of excited states in the gas-phase related to combustion, chemical lasers and atmospheric chemistry.

CC-BY logoThis work is licensed under a Creative Commons Attribution 3.0 Unported License. Reactor and author images © J. Christopher Whitehead.

Categories: Journal of Physics D: Applied Physics

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