Suggestions for Future Work 146

Whilst I was doing the work for this thesis there were many occasions when I would find myself thinking, “What if…?”

In order to accelerate the progress of packed bed plasma research, some fundamental problems need to be addressed. As stated very early into this thesis, the behavior of packed bed reactors is currently very poorly understood. Significant progress is being made with understanding and identifying catalyst – plasma interactions, whilst progress on understanding the dynamics of packed bed plasmas is very slow. I believe this is a problem that needs to be addressed if catalytic plasma reactors are to ever make it out of the laboratory and into industrial use. There are very few direct comparisons that can be made between results obtained with different catalytic experiments, therefore there needs to be a mechanism for benchmarking results obtained in different reactors.

Some of my suggestions for future work reflect a requirement for fundamental research, whilst others are suggestions for novel approaches to drive progress forwards by potentially opening new avenues of research.

8.1 Improving plasma properties in packed bed reactors

It has been demonstrated that packed bed reactors using small particle sizes are likely to be most beneficial for CO2 conversion if discharges can be ignited in the void spaces of the

particles. When increasingly small particle sizes are used, the large plasma – wall interface area will increase the loss rate of electrons, ions and excited species. Therefore, in order to have high electron and ion densities in order to maintain the plasma discharge the effects of materials with low work functions should be investigated. When an ion or electron collides with a material with a low work function, if the energy of that ion or electron exceeds the work function of the material, one or more electrons will be emitted back into the plasma. In my opinion, this is an essential piece of research that may have a significant impact if it can lead to a significant improvement in electron densities in packed bed reactors

8.2 Discharge phenomena in packed beds

The formation and mechanism of discharges in packed bed DBDs is very much unknown. High speed photography, high speed optical emission spectroscopy, and electrical characterisation

stacked pellets and packed beds arranged in monolayers, or thin layers dependent upon particle size. Discharge behavior should be studied varying applied field strength, gas composition, packing material, and applied frequency.

8.3 Catalytic activity of γ-Al

O

for CO

reduction in non-

thermal plasmas

A question that keeps recurring amongst discussion between myself and other researchers is the possibility that γ-Al2O3 might be catalytic for CO2 reduction in plasma. The work of Roland et al

[82] demonstrating that a paramagnetic surface species is formed in γ-Al2O3 when it is subjected

to non-thermal plasma might indicate that it can become catalytic towards CO2 reduction, which

may explain some of the surprisingly good results that have been observed using Al2O3 for CO2

reduction. In terms of methodology to test this hypothesis, plasma DRIFTS-MS recently demonstrated at QUB [83], in-situ Raman spectroscopy, molecular beam mass spectrometry, and optical emissions spectroscopy could all be useful. I would also be interested in using DRIFTS- MS to measure changes in the relative height of vibrational lines of CO2 and CO on the surfaces

of potential catalysts. As far as I am aware, this has not been demonstrated previously.

8.4 Larger scale testing of packed bed reactors

The majority of publications on packed bed reactors are performed using very small reactors with packed bed sizes of no more than 3 – 20 ml. I would be very interested to see some of these reactors operated on a slightly larger scale, perhaps about a 100 – 300 ml sized reactor. As demonstrated by this thesis, gap sizes should be kept small (0.5 – 2mm perhaps) with high driving voltages (60 – 120 kV peak to peak), with powers of 500 W to 2 kW. Driving frequencies should also be significantly increased from those studied in this thesis, perhaps up to 150 kHz. Although the work of this thesis recommends the use of small particles for studying packed bed reactor materials, pressure drop will become an important factor when the reactor is scaled up significantly. Structured catalyst monoliths designed specifically for plasma reactors should be developed to optimise gas contacting, gas flow through the bed, and catalytic performance.

8.5 In-situ oxygen separation from CO

plasma reduction

This is a project that has attracted some interest already [118, 119], and that I have also done some work on myself. Solid oxide electrolysis cells could be coupled with non-thermal plasma technologies in order to actively remove oxygen from the reaction site as it is being created. If

this was operated at high temperature for co-electrolysis (CO2 and H2O) there could be many

potential benefits, including direct reforming to fuels. The high temperature of operation is likely to reduce the power required to generate a plasma to a fraction of the normally required energy input [120, 121]. In addition to this, the plasma may be used to improve reaction kinetics, negate the requirement for a catalyst, help overcome kinetic limitations due to rates of gas diffusion, and potentially even address problems of solid oxide cell durability.