Abstract:
In heterogenous catalysis, the high dispersion of metal species is generally desired, for the high metal utilization efficiency and unique electronic/structural properties of the active sites. Nevertheless, the structure of highly dispersed metal active sites, including single metal atoms (M1) and sub-nanometer metal clusters (Mn), is often difficult to characterize, and tends to be highly dynamic under reaction and pre-treatment conditions. Such behaviors can determine the catalyst performance and reaction mechanisms, but require state-of-the-art in-situ/operando spectroscopic and microscopic techniques to elucidate. XAS is regarded as one of the most powerful techniques for this purpose due to the comprehensive information it provides and availability of in-situ setups.
This seminar presents a series of studies on this topic using resources at multiple DOE’s user facilities, including the APS. Combining in-situ XAS with other characterization techniques, we demonstrated that Pt1 on anatase TiO2 aggregate into disordered, flat Ptn under H2, which are re-dispersed into Pt1 by oxidation. The aggregation-dispersion cycle pulls Pt1 from in-surface to on-surface sites, enhancing their activity in CO2 hydrogenation. The same cycle occurs to Pd1/TiO2 as well, but does not alter the coordination environment of Pd1, or thus their CO2 hydrogenation activity. Nonetheless, the reduction of TiO2 under H2 creates electron-rich Pd sites with partially cleaved Pd−O interface, which are highly active for the reaction and captured by both in-situ XAS and theory. On the other hand, we demonstrated that under H2, Run exist as disordered 2D-rafts on CeO2 but as rigid 3D-particles on anatase TiO2. The different morphology of Ru determines that the former is highly active and selective in polyolefin hydrogenolysis for plastic waste upcycling, while the latter is inactive for the reaction. These studies unveil the dynamic structure of active sites on highly dispersed heterogeneous metal catalysts under conditions relevant to their applications, and the catalytic consequence of such behaviors. It also showcases the unique power that in-situ XAS has in studying these catalysts.
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