Raman-Spectrokinetics to Determine Structure-Reactivity Relationships in Catalytic Reactions with Supported Metal Oxide Catalysts
Type of DegreePhD Dissertation
Restriction TypeAuburn University Users
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The relationship between catalyst surface structure and reactivity/selectivity still represents one of the most important questions to be addressed in modern heterogeneous catalysis. Such fundamental information would allow for the rational design of new and improved catalytic materials. It is now well known that catalyst surfaces are dynamic and can be affected by the experimental conditions (e.g., temperature, pressure, and gas or liquid composition). Therefore, it is critical to establish structure-performance relationships for catalysts operating under reaction conditions given their dynamics by simultaneously collecting catalyst characterization and online product analysis data, an approach that has been referred to as operando spectroscopy. By simultaneously performing time-resolved in-situ spectroscopy and online product analysis, it is possible to directly relate the catalyst surface/bulk structure with catalyst performance. Typically, the kinetic data is obtained by using an auxiliary piece of equipment/detector (e.g., FTIR, UV-Vis, MS). Being able to collect the kinetic data directly from the spectroscopic technique would represent an essential advance following a natural evolution in the field of operando techniques. Raman Spectroscopy stands as one of the more powerful techniques for the surface structural characterization of heterogeneous catalysts due to the critical change in polarizability most of the transition metal oxides show. A custom made operando Raman experimental setup was designed and built to time resolve the fast redox processes occurring in metal oxide catalysts, which are highly relevant in catalytic partial oxidation reactions. Using a supported ternary metal oxide catalysts (V-Nb/SiO2), the new time-resolved operando Raman methodology, coined in this study as Raman-spectrokinetics, was proved to evaluate synergistic effects as a function niobia promoter loading in a vanadium supported catalyst. For the first time, oxidation kinetic data from the vanadium active site was obtained directly from the Raman spectra. The results obtained using Raman-spectrokinetics allowed us to follow the way vanadium interacts with niobia during the time-resolved redox process studied. In order to understand the structural catalysts features occurring at the surface of the V-Nb/SiO2 ternary metal oxide catalyst series and how they are correlated with the synergistic effects found by Raman-spectrokinetics, a combination of in-situ Raman spectroscopy, with in-situ FTIR and NAP-XPS was performed. Among the V-Nb/SiO2 catalyst series, the 4V/4.2Nb/SiO2 showed the highest reducibility among the whole series, demonstrating an enhanced synergistic effect as a result of the close interaction between the (VO4)n and (NbO4)m surface groups during the formation of a joint monolayer at the surface of the silica support. Finally, support and loading effects were studied in a series of supported vanadium oxide catalysts using SiO2, Al2O3, ZnO2, Nb2O5, and TiO2, aiming to evaluate the maximum potential of the Raman spectrokinetics approach. Furthermore, activation energy values and reaction order with respect to oxygen were determined with kinetic data calculated directly from the Raman spectroscopic data. The highest and lowest activation energy values were found for V/Al2O3 (50.2 kJ mol-1), and V/ Nb2O5 (15.9 kJ mol-1) respectively. Furthermore, all the other metal oxide supported catalysts (SiO2, ZrO2, Nb2O5, and TiO2) presented similar activation energies of about 20 kJ mol-1, which are lower to the reported for bulk samples. Thus, Raman-spectrokinetics served as a versatile approach to obtain surface kinetic information from supported metal oxide catalysts, providing the needed chemical insights about the surface properties of molecularly dispersed metal oxide catalysts that previously were not attainable by other characterization methods.