Understanding and improving catalytic hydrocarbon combustion using well-defined noble metal-based nanocrystals
Keywords: Chemical Engineering.

The increasingly stringent regulations on exhaust emissions create significant demands for high-performing automotive emission control catalysts. Emission control catalysts typically consist of large quantities of noble metals (e.g. platinum and palladium), which are expensive and environmentally damaging materials to extract. To develop efficient catalysts where the use of noble metals is optimized, fundamental understanding of catalytic hydrocarbon combustion would be beneficial. Yet, hydrocarbons with varying molecular structures pose a variety of challenges for this process. Therefore, this dissertation aims to study the mechanistic difference between catalytic combustion of alkane and alkene, and propose design for improved emission control catalysts. Propane and propene were chosen as the model compounds. A library of uniform Pd/Pt nanocrystal catalysts with control over composition and size were employed to study the structure-property relationships on the combustion of propene and propane. Since high levels of water always exist in automotive exhausts, the catalytic reactions in this dissertation were always performed in the presence of water, providing a complete understanding of the role of water on reaction kinetics. The first portion of this dissertation provides insights and comparison of structure-property relationships in propane and propene catalytic combustion. Synthetic conditions were optimized to generate uniform Pd/Pt nanocrystals with control over Pd/Pt ratios. Using the uniform nanocrystals, several important variables including Pd/Pt composition, support, phase and aging stability were studied. The important findings are outlined here: first, Pt-rich Pd/Pt/Al2O3 and Pt/Al2O3 were found to be the best performing samples for propene and propane combustion, respectively. From DFT calculations, propene was found to chemisorb, while propane only physisorb on the noble metal surface, which results in the opposite trends in the rate order results. Finally, equimolar Pd/Pt/Al2O3 and Pt/Al2O3 were found to exhibit the best catalytic performance after aging in propene and propane combustion, respectively. A relationship between structural sensitivity and the degree of aging resistance was found to correlate the aging stability results for both reactions. The second portion of the dissertation identifies the active sites for propene combustion. A library of Pd/Pt nanocrystals with equimolar ratio ranging from 2.3 to 10.2 nm was prepared. From the turnover frequencies and rate order results, it is observed that larger Pd/Pt nanocrystals show higher reactivity in propene combustion and sensitivity to the change in the partial pressure of reactants. We employed DFT calculations to demonstrate that water drives surface reconstruction and exposes undercoordinated sites, which are more efficient at breaking bonds in representative elementary steps in propene combustion, compared to high coordinated sites. We further developed a coordination-based model to reveal that the edge sites with (7-7) as the coordination numbers are the active-site ensemble for propene combustion. The third portion of this dissertation unravels the role of support acidity in propane combustion. A library of Pt/support with controlled Brønsted acidity was prepared with uniform Pt nanocrystals. The sample with higher Brønsted acid sites was found to have higher activity in propane combustion, as well as higher resistance to water poisoning. Using the Langmuir-Hinshelwood model, we demonstrated that supports with higher Brønsted acid site density are more hydrophobic and help reduce water coverage on Pt sites, resulting in more available sites and higher reaction rates in propane combustion. The last part of the dissertation proposes better emission control catalysts by Pt-based bimetallic nanocrystal catalysts. A seed-mediated colloidal synthesis method to produce uniform PtxM100-x (M = Cu, Co, Ni and Mn) nanocrystals with controlled size and composition was introduced. Together with DFT calculations, we created an experimental-guided volcano map to offer guidance to design catalysts with desired electronic structures, that are promising for emission control performances. Moreover, Pt/Cu was identified as the most active bimetallic sample in propene combustion. We further demonstrated that Pt/Cu have desired binding energies to C* and O*, creating more active surfaces for propene combustion. In summary, this dissertation focuses on the understanding of catalytic hydrocarbon combustion and the design of improved catalysts for emission control applications. Well-defined catalytic systems were created through the use of colloidal nanocrystals with control over size, shape and composition. With such systems, active sites and important metal-support interactions were identified for both propene combustion and propane combustion, respectively. Finally, Pt-based bimetallic nanocrystal systems were proposed to offer guidance for improved emission control catalysts