Single-site Catalysts by Metal-ligand Complexation at Surfaces: From Model Systems in Vacuum to High-pressure Catalysis on Oxide Supports

A grand challenge in heterogeneous catalysis is to achieve high levels of selectivity by _x000D_controlling the chemical uniformity of metal catalyst sites at surfaces . Our group is working to _x000D_apply principles of on -surface metal -organic redox assembly to develop a new approach to this _x000D_problem. Metal -organic coor dination networks at surfaces hold promise for selective chemical _x000D_function , but there is a limited understanding of the chemical reactivity of these systems. We _x000D_tested chemical activity of vanadium single -site complexes that are stabilized by tetrazine -bas ed _x000D_ligands. We demonstrate activity toward dioxygen activation and a high degree of selectivity _x000D_compared to vanadium nanoparticles. Reaction with O 2 causes an increase in V oxidation state _x000D_from V II to V IV, resulting in a single strongly bonded V -oxo produc t and spillover of O to the Au _x000D_surface [1] . The metal centers are stabilized in extended, ordered metal -organic complexes that _x000D_self -assemble through an on -surface redox process on the Au( 100) surface and are characterized _x000D_by X -ray photoelectron spectroscopy, scanning tunneling microscopy, high -resolution electron _x000D_energy loss spectroscopy, and density functional theory. New results extend these chemical _x000D_studies to more complex system s that include bimetallic sites and redox isomer systems [2] , _x000D_which will also be highlighted in this presentation. _x000D_We have also developed synthesis schemes to assemble quasi -square planar metal -organic _x000D_complexes on high surface area powdered oxides through a mod ified wet -impregnation -style _x000D_method. X-ray photoelectron spectroscopy measurements demonstrate loading of metal and _x000D_ligand on the surface and synchrotron -based X -ray absorption spectroscopy measurements of the _x000D_coordination shell of the metal centers demons trates single site formation rather than _x000D_nanoparticle assembly [3] . These systems are shown to be active for the catalysis of _x000D_hydrosilylation reactions at a level that is competitive with current homogeneous catalysts.