(98b) Understanding the Criteria for Controlling Catalyst Restructuring and Redispersion

Sintering impedes effective material utilization in catalytic systems. While sintering causes macroscale effects such as reduced activity, the solution lies at the nanoscale, as the organization of active and support components can drastically influence thermal restructuring outcomes. To this end, this work investigates how structure and aging conditions can direct catalyst restructuring towards favorable outcomes that improve active metal dispersion, stability, and utilization.

We investigate several active metals to determine the characteristics that influence redispersion. Thermodynamics of metal oxidation play a key role as metals with stable oxides (Pd) are more prone to disintegration and redispersion. Notably, while metals with unstable oxides (Au) do not redisperse under the same conditions, alloying such elements with readily dispersible metals can cause co-redispersion of both species during elevated temperature aging. This provides an alternative to conventional redispersion strategies that use thermal treatment with chemicals that exhibit adverse environmental properties. We further demonstrate that cycling between conditions that form and decompose the dispersible metal oxide phase can produce a distribution of site types. Such a distribution is favorable for managing simultaneous reactions, such as those relevant to automotive emissions control.

Taking CeO₂ as a model system, we use thermal pretreatments to develop a series of support structures with well-defined densities of defect and under-coordinated sites, which trap mobile species at elevated temperatures. Using this catalog of structures, we develop a descriptor that informs the optimal loading of active metal to achieve complete redispersion on a support through thermally-induced restructuring.

Using well-defined catalytic systems in concert with structure-probing reactions and in-situ microscopic and spectroscopic techniques, this work develops the relationship between nanoscale structure, aging protocols, and restructuring outcomes. Our findings provide a criterion that informs how controlled thermal energy inputs can be deployed as highly-scalable industrial synthesis techniques or protocols that recover catalyst performance in-operando.