Supported catalysts are an important class of materials and understanding their properties is key for the preparation of more efficient systems. In an effort to elucidate deactivation mechanisms occurring in supported catalysts, we employed colloidal nanocrystals to begin with uniform active phases and interrogate performance and stability of Pd-based catalysts for hydrocarbon activation reactions, methane combustion in particular. Using this approach, we were able to independently control Pd particle size and particle loading, which would not be possible through conventional impregnation methods. Counterintuitively, we found that increasing the density of Pd nanocrystals on a pure alumina support led to more stable materials after aging treatments at elevated temperatures. Catalysts with a low Pd weight loading, which are usually considered more stable, instead deactivated after just few minutes at high temperatures (775 °C). In order to explain this trend, we performed detailed characterization and EXAFS and HR-TEM analysis confirmed the presence of single-atom species in the low-density materials, whereas higher weight loading samples still contained uniform nanocrystals. The atomic species were stabilized by defect sites present on the alumina surface. A quantitative statistical model helped elucidate the deactivation mechanism, which is due to the formation of atomic species that are not active for methane combustion and that are in dynamic equilibrium, thus leading to stable materials at high weight loadings. This work demonstrates that deactivation of supported catalysts may occur through increase in dispersion, rather than sintering, and that single-atom species are important intermediates in supported catalysts that may determine catalytic performance and stability.

Single-atom species determine the deactivation of supported catalysts

2019 Fall National Meeting

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