Atomically Precise Single-Site Catalysts via Exsolution in a Polyoxometalate-Metal-Organic-Framework Architecture.

Single-site catalysts (SSCs) achieve a high catalytic performance through atomically dispersed active sites. A challenge facing the development of SSCs is aggregation of active catalytic species. Reducing the loading of these sites to very low levels is a common strategy to mitigate aggregation and sintering; however, this limits the tools that can be used to characterize the SSCs. Here we report a sintering-resistant SSC with high loading that is achieved by incorporating Anderson-Evans polyoxometalate clusters (POMs, MMo6O24, M = Rh/Pt) within NU-1000, a Zr-based metal-organic framework (MOF). The dual confinement provided by isolating the active site within the POM, then isolating the POMs within the MOF, facilitates the formation of isolated noble metal sites with low coordination numbers via exsolution from the POM during activation. The high loading (up to 3.2 wt %) that can be achieved without sintering allowed the local structure transformation in the POM cluster and the surrounding MOF to be evaluated using in situ X-ray scattering with pair distribution function (PDF) analysis. Notably, the Rh/Pt···Mo distance in the active catalyst is shorter than the M···M bond lengths in the respective bulk metals. Models of the active cluster structure were identified based on the PDF data with complementary computation and X-ray absorption spectroscopy analysis.

Computational Insights into Iron Heterometal Installation in Polyoxovanadate-Alkoxide Clusters.

Polyoxovanadate-alkoxide clusters are a new class of electroactive species with applications in a wide variety of fields from redox catalysis to energy storage. Heterometallic installation in these species can be used to modulate the redox properties of polyoxovanadate-alkoxide clusters and thus their applications. However, the formation mechanism of heterometallic polyoxovanadate alkoxides during the solvothermal process is unknown, limiting our understanding regarding what thermodynamic driving forces and/or kinetic barriers are present in the heterometal insertion. Here, we present a computational study on the nucleation pathways of the iron-functionalized mixed-valent hexameric [VV2VIV3O5(µ6-O)(µ2-OCH3)12(FeIIICl)] polyoxovanadate-alkoxide cluster.