Towards a fundamental understanding of plasma - TiO2 catalyst interaction for greenhouse gas conversion
29 May 2017
UAntwerpen, Campus Middelheim, G0.10 - Middelheimlaan 1 - 2020 Antwerpen (route: UAntwerpen, Campus Middelheim
Erik Neyts & Annemie Bogaerts
PhD defence Stijn Huygh - Faculty of Science, Department of Chemistry
The continuously increasing anthropogenic emission of greenhouse gases has induced substantial changes in the magnitude of the greenhouse effect. This has already resulted in changes in the global climate. To fight the global climate change, proper actions need to be taken. The decrease of the greenhouse gas emissions and concentrations is required to prevent further changes to our climate.
Conventional conversion methods of greenhouse gases have their disadvantages, such as a high energy requirement, sintering, and coke formation of the employed catalysts. It has been suggested that plasma-catalysis is a promising alternative to the conventional reforming processes. In plasma-catalysis one combines the catalyst with plasma technology. The plasma will activate the feed gas by electron impact reactions, while the catalyst will lower the energy barrier for surface reactions. The hybridization of a plasma with a catalyst can possibly result in synergistic effects due to the interactions of the plasma with the catalyst and vice versa. However, due to the high complexity of the plasma-catalytic system, there is still a lack of fundamental knowledge on the plasma-surface interactions.
This thesis focusses on the interaction of plasma-generated species with a titanium dioxide catalyst surface for the dry reforming of methane, i.e., CO2 + CH4 à 2 CO + 2 H2. By applying Density Functional Theory calculations on the adsorption and desorption processes, together with the surface reactions of adsorbed plasma species, a more profound molecular understanding of the processes is obtained. By combining the plasma with the catalyst surface, the temperature threshold required for the dry reforming is significantly lowered compared to thermal catalysis. This is the result of circumventing the rate limiting dissociative adsorption of methane. The use of titanium dioxide in plasma catalysis, compared to the conventionally used nickel catalyst in thermal reforming, has the advantage that coke formation will be prevented by the low stability of carbon on the titanium dioxide catalyst. Carbon will become hydrogenated or desorb as carbon monoxide.
To obtain the reduction of carbon dioxide to carbon monoxide on the titanium dioxide surface, I found that oxygen vacancies are required to obtain significant reaction rates. However, a regeneration mechanism of these oxygen vacancies is required. In the plasma-catalytic dry reforming of methane this regeneration mechanism would be provided by using methane derived radicals as reductants.
Based on the reaction mechanisms found in this thesis, it is expected that the selectivities to specific end products of the plasma-catalytic dry reforming of methane can be tuned based on the properties of the plasma-catalytic system. The surface reactions exhibit a different dependency on the temperature. The range in which the temperature of the feed gas can be adjusted is enlarged in plasma-catalysis as both the rate-limiting step and coke formation are removed compared to thermal reforming. The surface reactions further depend on the concentration of adsorbed plasma species, and thus on their rate of adsorption and desorption. The rate of adsorption in turn depends on the density of the different species in the plasma phase. This will be determined by the formation reactions, which depend on the plasma properties. The results of this thesis are the first step to find the missing links required to control and steer the plasma-catalytic dry reforming of methane.