(317e) Thermally Induced Restructuring of Core@Shell Nanoparticles As a Strategy for Increasing Catalyst Stability and Activity

The restructuring of catalytic nanomaterials that occurs during prolonged operation at high temperature remains a significant challenge in catalysis. Typically, thermally induced restructuring results in the sintering of active metal and the loss of support porosity, which compromises both the structure and number of active sites that can be used for reaction. Such outcomes significantly inhibit both catalyst activity and selectivity. The challenges imposed by sintering processes are magnified by the growing cost and scarcity of many of the metals that comprise industrial catalysts.

In this work, we investigate how initial catalyst nanostructure affects the outcomes of high temperature restructuring.¹ By comparing core@shell nanoparticles that have active metal cores encapsulated by porous metal oxide shells, with catalysts prepared by wet impregnation, we show divergent restructuring outcomes that depend on the placement of the active metal.

Elevated temperature aging (800ËšC) of a Pd@CeO₂ core@shell catalyst redisperses core Pd species throughout the encapsulating CeO₂, increasing active site dispersion from 33% to 88%. The highly dispersed restructured Pd clusters exhibit an average diameter of 1.3 nm, and coordinate strongly with the reducible CeO₂ support, as characterized by CO chemisorption, transmission electron microscopy, and x-ray energy dispersive and photoelectron spectroscopies. The Pd redispersion also appears to stabilize the CeO₂ crystallites that comprise the shell, making them less prone to high temperature sintering. These restructuring processes appreciably increase catalytic activity, as indicated by a 45ËšC decrease in the temperature required for 90% conversion in a probe CO oxidation reaction. The turnover frequency for CO oxidation increases from 0.2 s^-1 to 0.39 s^-1 and the apparent activation energy decreases from 42 to 33 kJ/mol after aging. Such changes in intrinsic catalytic activity suggest that the enhanced activity seen in the restructured catalysts is not solely due to an increase in active site dispersion. Such outcomes are in stark contrast to those observed for Pd/CeO₂ catalysts prepared by impregnation, which exhibit appreciable pore closure, sintering and impeded activity after identical elevated temperature aging. By extending this investigation to a number of aging conditions and active metal/ support combinations, we hope to better understand how directed restructuring of carefully designed nanomaterials can be used as a novel strategy to enhance catalyst functionality.

[1] Hill, Schwank, et. al. ACS Catal. (2020). 10.1021/acscatal.9b05224.