UC Santa Barbara

Heterogeneous catalysts are vital for the industrial production of fuels, chemicals, and commodities that continue to increase standards of living across the globe. By facilitating chemical reactions with accelerated rates and promoting high selectivity to desired products, catalysts ensure efficient utilization of raw materials with minimal energy inputs. Heterogeneous catalysts often consist of an active transition metal phase dispersed across a thermally stable high surface area support, such as metal oxides. Understanding how the structure of metal species correlates with catalytic properties is an essential element of catalyst research that builds both fundamental intuition and informs the practical design of next-generation catalyst materials. In order to develop these structure-function relationships, accurate characterization of the active catalyst species under reaction conditions is crucial.In this dissertation we explore how catalyst structures change under reaction conditions and correlate these structural changes to changes in catalytic performance. Observing how these materials evolve requires characterization at a variety of length scales that range from angstrom length bonds to long range ordering on the order of microns. Using a combination of tools including spectroscopic and microscopic characterization techniques, theoretical electronic structure calculations, and kinetic analysis through reactivity measurements we can gain detailed insights into the roles these structures play during chemical conversion processes. We employ this suite of tools to investigate a variety of different catalyst systems. Chapters 3 and 4 focus on characterization of dilute alloy catalysts that consist of noble coinage metal (e.g., Cu, Ag, Au) nanoparticles doped with low concentrations (e.g., < 1:100 molar ratio) of transition metals (e.g., Pt, Pd, Ni, etc.). In chapter 3 we reveal how adsorbate induced surface segregation can cause Pt atoms in AgPt alloys to migrate to the surface and form a variety of structures. Using CO oxidation as a probe reaction, we demonstrate how surface segregation can have a pronounced effect on catalytic activity. We further show that the local coordination environment of dopant Pt atoms can significantly impact their electronic structure, and potentially offer a handle to further tune catalyst properties. In chapter 4 we explore longer range restructuring in the form of sintering for a similar CuPt dilute alloy catalyst. Sintering of metal nanoparticles results in deactivation through the loss of surface area over time, and coinage metal nanoparticles are particularly susceptible to sintering due to their low cohesive energy and high mobility. Both reactivity measurements of high-pressure methanol synthesis and characterization show that dilute doping of Cu nanoparticles with Pt can significantly inhibit sintering, resulting in small nanoparticles that are stable even under high temperature conditions. Evidence from a combination of experimental and theoretical techniques suggest that Pt atoms locally inhibit the motion of undercoordinated Cu atoms on the surface of nanoparticles, minimizing the detatchment of these undercoordinated atoms that leads to sintering through an Ostwald ripening mechanism. The surface segregation effects identified in chapter 3 and the sintering resistant properties observed in chapter 4 are general characteristics for dilute alloy nanoparticle catalysts and electronic structure calculations provide predictions for systems that are likely to behave in a similar manner. In chapter 5 we investigate how bulk iron catalyst particles evolve during the conversion of CO into solid carbon and CO2 via the Boudouard reaction. We find that particles that initially start with micron size dimensions are fragmented under reaction conditions to produce particles on the order of ~50 nm. At low temperatures the reaction proceeds at steady state yielding extended carbon filaments several microns in length. Conversely at high temperatures, rapid reaction rates result in graphitic carbon overlayers that quickly deactivate the catalyst. The combination of characterization and reactivity reveal an underlying principle for these catalyst systems: there is balance between controlled carbon nucleation and catalyst longevity that must be understood for the design of hydrocarbon conversion processes. In chapter 6 we take a step back from specific catalysts and reactions and examine a common tool for catalyst characterization under reaction conditions: X-ray absorption spectroscopy. Using modelling techniques to analyze extended X-ray absorption fine structure (EXAFS) spectra, we quantitatively determined limits of detection for identifying catalyst structures using EXAFS when a sample contains a mixture of different active site structures. We found that often times minority species may go completely undetected using EXAFS analysis, but demonstrate scenarios where X-ray absorption near edges spectroscopy (XANES) can provide acute sensitivity to small changes in catalyst structures. Regardless, our analysis emphasizes the need for a variety of complimentary techniques to adequately characterize catalyst systems – an approach employed for all studies throughout this dissertation.