Elucidating trends and transients in CO₂ dissociation

The purpose of this dissertation is to — on occasion very literally — shine a light on processes that occur in non-thermal plasmas containing CO₂, mostly for CO₂ conversion. In particular, the focus lies on the transient behaviour of these discharges: how do these systems evolve over time before they settle in a (non-thermal) equilibrium. In addition to that, it analyses trends in the field of plasma-catalytic CO₂ conversion as a whole to evaluate the current state-of-the-art, but also presents a new platform for the community to contribute and collaborate on, to facilitate cross-comparison between disparate experiments. The first part consists of experiments performed on: (a) an atmospheric pressure nanosecond repetitively pulsed (NRP) discharge for CO₂ conversion, and (b) a test bed system for a remote CCP plasma source for plasma-enhanced atomic layer deposition (PE-ALD) of trimethylaluminium (TMA). The common theme in these experiments is a focus on the application of time-resolved, in situ diagnostics to study transient behaviour in the systems under investigation. The main diagnostics employed for such measurements are optical emission spectroscopy (OES) and laser induced fluorescence (LIF), which can provide complementary results when used in conjunction. In particular, this work presents the following results: A study of the evolution of emission from an NRP discharge (using OES), establishing both electron densities (by Stark broadening of atomic oxygen and carbon lines) and gas temperatures (by the N₂ second positive system) as the discharge evolves from a breakdown phase to a spark phase. It furthermore explores the changes to these properties when operating in burst mode, where a subsequent pulse experiences a memory effect from the preceding one, which has been shown to be conducive to efficient conversion in literature. A study into the effect on energy efficiency of CO₂ conversion by alternating the power modulation in an NRP discharge. Crucially, using CET-LIF (collisional energy transfer LIF) and OES it is shown that while power deposition to the discharge occurs in the order of 100 ns in the discharge, CO₂ dissociation occurs on a timescale beyond a microsecond. This indicates that instead of direct electron impact, molecular-excitation kinetics play an important role under these conditions for CO₂ dissociation. By shortening the time between pulses in a burst (down to 33 us in the work), these mechanisms can be further enhanced, by prolonging the quasi-‘metastable’ state of the system. The application of LIF in a PE-ALD process plasma along with OES, where diffusion profiles were measured close to the substrate surface with local time-resolved measurements of the OH ground state density. These indicate that the investigated surface reactions finish on a timescale of 100 ms, faster than would be indicated by OES which effectively measures emission from the bulk plasma after diffusion of reaction products away from the surface. The second part of this work is an open access database on plasma(-catalytic) CO₂ conversion that is instrumental in identifying and verifying trends in experimental data, but also stresses the importance of rigorous reporting of essential parameters in literature. The approach in literature is diverse: some studies focus more on a mechanistic understanding of the fundamental processes, whilst others already focus on process tailoring and optimization for industrial applications. Trends observed in earlier review papers are observed as well and can now be trivially reproduced. The database platform (https://db.co2pioneer.eu) is put forward as a new tool for the community to easily cross-compare and contextualize experimental outcomes and strongly encourages new contributions. Based on the 196 papers included at the time of publication, a number of observations and recommendations can already be made. Chief among those is a clear and present need in the field for a more fundamental understanding of plasma-catalysis interaction, to develop techniques and criteria that are properly suited to test the synergy of both, rather than relying on methods from e.g. traditional thermal-catalysis. Also in this instance, local, time-resolved diagnostics may play a key role, but their implementation will be challenging.