In the CCIC group we use computational methods to understand and predict chemical processes that occur on solid-gas and solid-liquid interfaces. In particular, our work focuses on heterogeneously catalyzed reactions relevant for energy production, energy storage, photocatalysis, pollution mitigation and the production of useful chemicals. Density Functional Theory (DFT) and kinetic modeling form the basic tool set in our group.
Methane Activation and Conversion
Natural gas is an abundant resource in the U.S. but its use is currently limited to electricity generation and hydrogen generation through steam-reforming. Using natural gas in internal combustion engines is easily feasible, however, the exhaust emissions are rich in methane, which is a potent green house gas. For natural gas powered vehicles to take over a larger market share, new emissions control catalysts are needed. Alternatively, methane can be used as C1 feedstock for the catalytic production of hydrocarbons, olefins, alcohol, aromatics and many other chemicals.
The main challenge in any methane conversion process is the initial activation of methane, or breaking the strong C-H bond (435 kJ/mol). As of today, the potential for methane as a feedstock for the production of useful chemicals has not yet been fully realized and an economically viable methane to higher value chemicals upgrade process could revolutionize the chemical industry. The CCIC group uses computational catalyst screening techniques to find new catalyst formulations that lower the required temperature for C-H bond activation, which will improve the conversion during methane combustion in an exhaust catalytic converter, and increase the selectivity to the desired products, when methane is used as feedstock. The types of catalytic systems include metal alloys, metal/metal-oxide interfaces, and zeolites.
Unifying Principles in Hydrotreating Catalysis
Fast pyrolysis of biomass, a renewable and sustainable resource, is a promising low-cost technology that produces bio-oil suitable for use as transportation fuel after an appropriate upgrade step. The upgrade can be achieved by reducing the high oxygen content of up to 35 – 40 wt.% through hydrotreatment over heterogeneous catalysts, but the complexity of bio-oils with ca. 400 different oxygenated compounds and the fact that this technology has only recently gained interest are both responsible for the lack of fundamental knowledge in this field. In contrast, the petroleum industry has been using hydrotreating reactors with cobalt and nickel promoted molybdenum sulfide based catalysts for the removal of sulfur impurities for decades, and the catalyst structure, nature of the active site, and elementary reaction steps are largely understood.
Our group's efforts build on the hypothesis that the hydrotreating processes for the removal of oxygen and sulfur are fundamentally similar at the atomic-scale and existing knowledge from the treatment of petroleum derived feedstock can be leveraged for the design of novel catalysts for the upgrade of bio-oil. Electronic structure simulations and kinetic modeling will be used to improve our mechanistic understanding of bio-oil hydrotreatment and to derive characteristic catalyst properties that are responsible for high activity and selectivity. From the resulting structure-function relationships we can extract common features of hydrotreating catalysts and develop unifying principles that lead to the accelerated design of novel materials for bio-oil upgrade.
Vehicle Emissions Control
Advanced combustion engines provide more fuel flexibility, higher net efficiencies, and lower NOx and particulate matter emissions than current diesel engine technologies. However, the exhaust gas must still be treated to meet emissions regulation standards. Catalytic converters for traditional gasoline and diesel vehicle exhaust aftertreatment exist, but the emerging advanced engines technologies pose new challenges to the catalytic converter. For example, higher efficiency implies that less waste heat is produced and the exhaust must be treated at a lower temperature.
Presently available catalysts are not active at lower temperatures and therefore, engines must be operated with excess fuel, to increase the exhaust temperature and to heat the catalyst to the required temperature. Combustion at lower temperature also results in higher concentrations of carbon monoxide and unburned hydrocarbons that must be abated. The exhaust gas composition also depends strongly on the fuel type and quality. Given the abundance of low cost natural gas, a transition to natural gas powered vehicles has become very attractive and the appropriate natural gas engines exist. However, these engines produce much higher levels of methane, which acts as a green house gas and is about 20 times as potent than CO2. The CCIC group belongs to a team of researchers at UH and the Texas Center for Clean Engines, Emissions & Fuels, and together we work on tailoring vehicle exhaust catalytic converters for new engine technologies and fuels.