Date of Award

Spring 2023

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

First Advisor

Gonghu Professor Li

Abstract

Heterogeneous catalysts have been extensively used in various chemical transformations that have important ecological and economical relevance. The rational design of such catalysts, however, is still not effectively implemented due to the ambiguity that encircles their structure-activity correlations. The origin of synergetic effects which make the binary system (metal/metal oxide) distinct from the individual components continues to elude researchers. In this work, binding and structural properties that emerge from metal/metal oxide interaction were investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS). Different catalytic structures, including highly dispersed Cu sites on TiO2, were prepared and characterized with spectroscopic techniques. These catalytic materials were studied in the activation of small molecules, including CO2, CO, H2O, and H2. XAS demonstrated that in the supported metal catalyst systems, the underlying support enables the deposited surface metal clusters to assume local structures that are different from the corresponding pure bulk metal or metal oxide. Such structures were found to be responsible for novel binding, redox, and optical properties in the binary catalyst. In DRIFTS studies, carbon monoxide was utilized as a probe molecule to track the binding sites as other molecules are competitively adsorbed, and as the surface is brought gradually to operando catalytic conditions. The investigation revealed the existence of bifunctional active sites in which the metal sites catalyze the dissociation of CO2 and H2 molecules whereas interfacial sites (Cu+ and Ti3+, for instance, in Cu/TiO2) bind and activate Lewis basic molecules such as H2O and CH3OH. Surface defects, such as oxygen vacancies often formed upon annealing in an oxygen-deficient atmosphere, could also play an important role in the activation of various reactant molecules.

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