Title:

Investigating the structure, mobility, and activity of major and minor species in gallium-based catalysts using operando spectroscopy

Abstract:

The identification of the chemical structures of active sites in heterogeneous catalysts is a fundamental and persistent challenge in catalysis science. Heterogeneous catalysts are structurally complex materials that often contain multiple coexisting metal species whose distributions depend on the reaction conditions and the support chemistry. Not all metal sites are catalytically active. Therefore, identifying the active sites amid a range of possibilities is essential for the rational design of catalysts. Only by knowing which species control reactivity and the mechanisms by which they operate can catalyst composition and structure be deliberately engineered to improve activity, selectivity, stability, and overall chemical process efficiency.

Accurate active site identification requires quantitative, element-specific characterization methods that can probe catalysts under their working conditions. In-situ and operando X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), is uniquely suited for this purpose due to its sensitivity to the electronic and local chemical structure of a specific element in complex materials, as well as its compatibility with high temperatures and reactive gas environments typical of heterogeneous catalysis. However, rigorous spectroscopic interpretations are essential to avoid making incorrect structural assignments that can lead to misleading structure-function relationships.

These challenges are particularly evident in gallium (Ga)-based propane dehydrogenation (PDH) catalysts, which are used for the industrial production of on-purpose propylene. Propylene is an important chemical feedstock for the large-scale manufacturing of polypropylene and other commodity chemicals. Growing global demand for polypropylene has driven recent expansion of PDH technologies. While oxide-supported Ga-based catalysts exhibit promising activity and selectivity for PDH, and are used industrially, decades of research have produced competing chemical assignments and a proliferation of proposed reaction mechanisms. Much of this disagreement arises from inconsistent interpretation of Ga K-edge XANES and EXAFS recorded for catalysts under reaction conditions.

There are two main goals of this thesis. First, rigorous spectroscopic criteria are established using a combined experimental and theory driven approach to unambiguously characterize the electronic and chemical structure of Ga species in heterogeneous catalysts. Second, these criteria are applied, in parallel with other spectroscopic techniques, to study Ga speciation in catalysts under reaction conditions. Both the active sites and spectator species which do not participate in the catalytic cycle are identified and their abundances are quantified. During this investigation, dynamic and mobile behavior of Ga species is observed, revealing important implications for catalyst speciation and function under reaction conditions.