(586g) Catalyst Changes at the Atomic Level Studied Using Nanocrystal Catalysts
Supported catalysts are an important class of materials and understanding their properties at the atomic scale 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. We interrogate performance and stability of catalysts for hydrocarbon activation reactions, methane combustion in particular, by studying the materials before, after and during catalysis. 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. 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. When Pd nanocrystals are instead supported onto silica, the catalysts are stable up to temperatures of 875 °C. The eventual sintering is caused by metal vaporization, which is a mechanism that we unequivocally demonstrate using tailored support particles in addition to uniform nanocrystals. Our work shows that deactivation of supported catalysts may occur through increase in dispersion, rather than sintering, and that the choice of metal and support is critical for performance and stability for catalysts prepared with control at atomic scale.