Rational Catalyst Design for Direct Conversion of CH4 and CO2

Possibilities for mitigation of carbon dioxide and methane levels in the atmosphere are of major global interest. One of the alternatives that attracts much scientific attention is their chemical utilization, especially because both of these gases are components of the natural gas, and rapid and extensive shale gas development makes them abundant raw materials. Development of an effective catalytic process that could be scaled-up for industrial purposes remains a great challenge for catalysis. Understanding of the mechanisms of molecular activation as well as of the reaction pathways over active centers on heterogeneous catalysts needs to be advanced. To that end this work focused on initiating the development of a bifunctional catalyst for low-temperature: 200C - 450C) direct conversion of methane and carbon dioxide by tailoring the structure of each active component using the insights from molecular modeling. Pt nanoparticles supported on ceria support have been selected based on molecular modeling and density functional theory analysis that provided the guidance for the catalyst structure optimization. Tetrahedral Pt nanoclusters, with a high fraction of edge and corner sites that would supposedly promote methane activation, were prepared by carefully adjusting the concentration ratio between Pt precursor and the capping polymer. Ceria nanorods, exhibiting more reducible facets that would activate carbon dioxide, were prepared by hydrothermal method. Conventional incipient wetness and modified polyol method were also used for the preparation of supported round shape Pt samples, for the comparison. Catalyst activity was evaluated by studying the H2 evolution rates during the exposure of the catalyst to the methane flow, in a small packed bed reactor, at the atmospheric pressure and temperatures up to 450C. Insights into the structure of the adsorbed carbonaceous species, formed during methane chemisorption, were gained from temperature programmed reduction profiles. The effects of catalyst structure, reaction temperature, CH4 partial pressure and the Pt loading on the methane activation were outlined. Results revealed that a strong metal-support interaction, characteristic for the ceria supported samples, had a pronounced impact not only on the total amount of chemisorbed CH4, but also on the structure of the adsorbed carbonaceous film. This has been attributed to the high concentrations of oxygen vacancies at the interface between ceria and Pt. The promoting effect of ceria was further confirmed in the experiments involving supported tetrahedral Pt nanoclusters. However, the application of these nanoparticles was limited by the instability of their shapes under reaction conditions. New synthesis methods that would increase the catalyst stability and prevent reconstruction of cluster shapes need to be developed. Further studies of the reactivity of the adsorbed carbonaceous species with the carbon dioxide and the selectivity towards desired oxygenates are needed. An integrated method involving both experimental and modeling efforts could enable more rational design of new materials with improved activity and selectivity and could potentially cut the costs and duration of extensive trial-and-error approach commonly practiced in industry. The possibility of tailoring catalyst activity and selectivity through shape and size- control could lead to the more efficient catalyst utilization