High levels of reaction selectivity for heterogeneous catalysis are generally difficult to achieve with traditional metal nanoparticle catalysts, due to the variety of metal binding sites available. Motivated by the desire towards uniform reaction sites, there is growing interest in developing single-site catalysts (SSCs). Our group approaches this challenge using redox self-assembly of metal-organic systems at surfaces. Our previous work has demonstrated the formation of metal single-sites on single crystal surfaces in UHV by co-depositing metals (Pt, V, Cr, or Fe) and oxidizing ligands with suitable binding pockets (JACS 2014, 136, 9862; JCP 2015, 142, 101913; JACS 2015, 137, 7898; JPCC 2017, 121, 13183; Chem. Sci. 2018, 9, 1674). A uniform chemical character of the metal cations is controlled by the coordinating ligands. Here, we present the application of this self-assembly strategy on high-surface-area powdered oxide supports. Simultaneous wet impregnation of metal precursors and well-designed ligands creates oxide-supported metal-ligand SSCs (J. Catal. 2018, 365, 303). Structures of resulting SSCs are elucidated by a wide range of characterization techniques, including XAS, XPS, TEM, XRD, and CO adsorption DRIFTS. This synthesis strategy has been successful with multiple metals (Pt, Ir, and Rh), ligands, and supports (MgO, CeO2, Al2O3, and TiO2); however, we have also discovered limitations in its scope, which we interpret in the context of competing metal-ligand-support interactions. We have shown successful catalysis with our metal-ligand SSCs in various reactions, ranging from alkene hydrosilylation in a solution-phase batch reactor to hydrogenation reactions in a gas-phase flow reactor. Catalytic results from these reactions highlight several advantages of SSCs over traditional metal nanoparticle catalysts, including metal dispersion, higher selectivity, and improved durability.

Metal-ligand self-assembly on powdered supports: A novel strategy towards heterogeneous single-site catalysts

2019 Spring National Meeting

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