Controlling Nanoscale Restructuring to Improve Catalyst Activity, Stability, and Material Utilization

Abstract

Society relies on catalytic processes for many important chemical transformations, which has contributed to the increasing cost, scarcity, and demand for precious metals. Synthesis techniques can improve the utilization of these metals by controlling the catalyst structure. However, precisely designed structures often degrade during operation, as high-temperature (≥800°C) working conditions agglomerate active metal into large particles, in a process known as sintering. Sintering decreases the dispersion, or fraction of available active sites and, in turn, the activity and material efficiency. Consequently, there is significant interest in delaying or reversing the effects of high-temperature sintering to improve the dispersion and stability of catalytic sites. This dissertation investigates how nanoscale architecture and aging conditions can direct the restructuring of catalytically active sites away from sintering and towards outcomes that improve metal dispersion, stability, and utilization. This is done through the development of a core@shell architecture, where a sintering–prone active metal core is encapsulated by a porous, metal oxide shell. Palladium (Pd) is used as the model active metal, because of its high cost and widespread use in industrial catalysis. Core@shell catalysts are investigated alongside catalysts prepared through conventional synthesis strategies that create architectures where active metal nanoparticles decorate the external surface of metal oxide supports. This comparison demonstrates that the trajectory of restructuring depends greatly on the initial nanoscale architecture. Elevated temperature (800°C) aging redisperses active metal within core@shell catalysts. The redispersion is more pronounced, and nearly complete, when palladium is encapsulated by reducible ceria (Pd@CeO2), as opposed to nonreducible silica (Pd@SiO2). This difference in redispersion is due to the participation of oxygen from the CeO2 lattice, which promotes redispersion. Aging at 800°C increases dispersion from 33% to 88% in the Pd@CeO2 system, improving catalytic performance by over two-fold. This result contrasts with the conventionally prepared catalysts, whose dispersion decreases from 31.8% to 10% under identical conditions. The stability of these redispersed sites is examined through repeated aging at 800°C. Palladium continues to migrate in Pd@SiO2 because of poor Pd–SiO2 bonding, which leads to agglomeration. In contrast, redispersed sites formed in aged Pd@CeO2 remain stable due to the interactions between palladium and the oxygen present in reducible CeO2. The importance of palladium–oxygen bonding is studied by aging at temperatures (1000°C) that surpass the decomposition temperature of palladium oxide. Although 1000°C aging sinters dispersed palladium sites, these sites can be regenerated by returning to 800°C aging conditions. Consequently, redispersion and sintering can be described by the thermodynamics of palladium–oxygen bond formation and decomposition, respectively. This suggests a generalizable redispersion mechanism that relies on the thermally induced oxidation and disintegration of core metal into highly mobile species. The generalizability of this approach is investigated using a Au@CeO2 catalyst, as gold exhibits much poorer oxidation thermodynamics when compared to palladium. While metals with unstable oxides, such as gold, do not redisperse at high temperatures, alloying such elements with readily dispersible metals facilitates the redispersion of both species. Altogether, this dissertation develops the relationship between nanoscale structure, aging conditions, and restructuring outcomes. The findings presented provide guidelines on how controlled thermal energy inputs can create high dispersions of stable, catalytically active sites. These protocols are simple and require little intervention, which makes them attractive as highly scalable synthesis techniques or protocols that can regenerate catalyst performance on-stream.