Synthesis, Structural Characterization, and Reactivity of Metal Oxide Supported Pt Atoms and Sub-Nanometer Pt Clusters
In this dissertation synthetic protocols were developed to produce homogeneous and stable catalyst designs for important chemical reactions in environmental emission control. Specific focus was placed on the development of catalysts that exhibit perfect metal utilization and do not sinter under conditions relevant for catalysis. The stability of these materials enabled rigorous characterization and meaningful reactivity measurements that demonstrate the superior efficiency of the catalyst design.
The work presented in this dissertation is primarily divided into three major components. The first component encompasses the development of the synthesis approach to engineer atomically dispersed metals on oxide supports motivated by their promise to alleviate the demand for scarce metals used in industrial chemical production and environmental emissions control. We put forth a synthesis approach that enables the maintained existence of atomically dispersed Pt catalysts, Ptiso that exist in a homogeneous spread by depositing ~1 precious metal atom per support particle with an initial case study using anatase TiO2 as the metal oxide support.
Due to the difficulty in producing catalysts exhibiting these properties, significant variations exist in the conclusions of literature regarding the reactivity of dispersed metal atoms on oxide supports. The second component of this thesis was dedicated to rigorously characterizing Ptiso species and drawing definitive conclusions on its catalytic activity. Site-specific infrared spectroscopy was used as a primary means to provide unique spectroscopic signatures and evidence of the site homogeneity coupled with aberration corrected transmission electron microscopy to provide direct evidence and corroborate the site assignments. Carefully run kinetic studies demonstrated Ptiso exhibited a 2-fold improvement in turnover frequency over ~1 nm metallic Pt clusters and yet share an identical reaction mechanism for the environmentally important CO Oxidation reaction.
The final component of this thesis was dedicated to a detailed analysis of how the pretreatment and reaction environmental conditions influence the local coordination. Accompanying the infrared spectroscopy and electron microscopy, x-ray absorption spectroscopy provided additional details allowing the unique identification of three Ptiso coordination geometries. It was shown that the oxidation state and local coordination of Ptiso species on TiO2 can be controlled from a highly oxidized to an almost metallic state by systematically decreasing the local Pt-O coordination number through exposure of the catalyst to varying levels of oxidative or reductive treatments. It was then demonstrated that the local environment of the Ptiso species controls the strength of interaction with CO and CO oxidation reactivity. These results demonstrate the importance of creating homogeneously dispersed isolated atoms on supports and considering the response of local coordination to environmental conditions when developing structure-function relationships.