Computational Screening of Bifunctional Catalysts for CO and CH4 oxidation

Modern advances in density functional theory (DFT) and computing power have allowed us to investigate catalytic reaction at surfaces in great details and with reasonable chemical accuracy. Based on fundamental knowledge of reaction mechanism and key surface properties, new catalysts can be designed and further tuned for optimal performance. In the recent literature, several examples of catalysts that perform multiple site-specific functionalities under steady-state reaction conditions have been reported. The most common systems are bifunctional catalysts where each of the two distinct sites preferentially catalyzes different reaction steps independently. In this dissertation, DFT calculations were used in combination with microkinetic modeling to explore bifunctional catalyst design strategies for CO and CH4 oxidation.
Preliminary study suggested that there are theoretical limits for the achievable activity improvement and bifunctional catalysts do not necessarily outperform single-site catalysts. For CO oxidation on bimetallic surfaces, it was found that the optimal activity is not significantly altered when bifunctional mechanism are considered, but equally active bifunctional catalysts may be tailored from less active and cheaper components. In particular, when CO oxidation was probed on the novel RuPt core-edge nanocluster catalyst, a bifunctional mechanism that involves the delivery of two reactants from two different spatial domains to a reacting interface was used to explain the significant activity improvement. In the special case of CO oxidation on Au/TiO2 system, a new mechanism was proposed and water was identified as a co-catalyst in the reaction. To investigate the importance of the material gap in computational catalyst screening, complete CH4 oxidation was evaluated on different representative models of Pd catalysts. It was observed that, although quantitative results may vary significantly, the trend in reactivity is the same across all surface models. Extensive promoter screening was also performed on PdO(101) surfaces, and calculated data for CH4 activation suggested that there exists an additive effect upon promoter substitution at two distinct Pd sites on PdO(101). Our efficient screening strategy has led to predictions of several promising promoters for Pd catalyst for complete CH4 oxidation on the basis of intrinsic activity and resistance toward water inhibition and sulfur poisoning.