Synthesis and Characterization of Support-Modified Nanoparticle-Based Catalysts and Mixed Oxide Catalysts for Low Temperature CO Oxidation

Heterogeneous catalysts are responsible for billions of dollars of industrial output and have a profound, if often understated, effect on our everyday lives. New catalyst technologies and methods to enhance existing catalysts are essential to meeting consumer demands and overcoming environmental concerns. This dissertation focuses on the development of catalysts for low temperature carbon monoxide oxidation. CO [carbon monoxide] oxidation is often used as a probe reaction to test overall oxidation activity of a given catalyst and is an important reaction in the elimination of toxic pollutants from automotive exhaust streams. The work included here presents three new heterogeneous catalysts developed over the last 4 years in our group.

The first type Au/SiO₂ [gold/silica] catalyst synthesized using a new method for the deposition of Au nanoparticles onto SiO₂ via a nitrogen-containing polymer, C₃N₄ [carbon nitride]. C₃N₄-modification of SiO₂ allows us to ignore unfavorable electrostatic effects that hinder standard Au deposition onto this support. While removal of the C₃N₄ is necessary for good CO oxidation, this new method is an improvement over the standard deposition-precipitation procedure for supports with low isoelectric point that enables the successful deposition of Au nanoparticles onto SiO₂.

The second type includes precious metal catalysts deposited on an “inert” silica support but promoted by the addition of an “active” metal oxide. Here we present a Au/FeO_x/SiO₂ [gold/iron oxide/silica] and a Pd/ZrO₂/SiO₂ [palladium/zirconia/silica] catalyst which show increased activity and stability effects due to the presence of the metal oxide promoter. They are synthesized by a C₃N₄-deposition and sol-gel methods, respectively. These catalysts were also tested in simulated automotive exhaust streams. The results show that inhibition effects play a major role in the activity of these catalysts.

The third type of catalyst is a mixed oxide catalyst, CuO-Co­₃O₄-CeO₂ [copper oxide-cobalt oxide-cerium oxide], developed with the goal of overcoming the inhibition effects seen in the previous precious metal catalysts. The catalyst was synthesized by co-precipitation method and shows exceptional activity for CO oxidation under simulated exhaust conditions. Also noteworthy is the observation that this catalyst also shows a lack of inhibition by a common exhaust component, propene.