A fundamental challenge in surface chemistry and catalysis remains the determination of three-dimensional structures with atomic-level precision. Here we determine the three-dimensional structure of organometallic complexes on amorphous silica surfaces using solid-state NMR measurements, enabled through a dynamic nuclear polarization surface enhanced NMR spectroscopy approach yielding a 200-fold increase in NMR sensitivity for the surface species. This results, in combination with EXAFS, in detailed structures for the surface complexes determined with sub angstrom resolution. We observe well-defined conformations folded towards the surface goverened by specific structure directing interactions with surface oxygen atoms. We also address the challenge of elucidating the binding mode of carboxylate-containing ligands on catalytic gold nanoparticles (AuNPs) which is crucial to understand their stabilizing role. A detailed atomic-level picture of the three-dimensional structure and coordination modes of citrate, acetate, succinate and glutarate to AuNPs is obtained by carbon-13 and sodium-23 solid-state MAS NMR in combination with computational modelling, XPS and TEM measurements. The binding between the carboxylate and the AuNP surface is found to occur in three different modes. These three modes are simultaneously present at low citrate to gold ratios, while a monocarboxylate monodentate (1κO1) mode is favored at high citrate to gold ratios. The surface AuNP atoms are found to be predominantly in the zero oxidation state after citrate coordination, although trace amounts of Auδ+ are observed. Sodium-23 NMR experiments show that Na+ ions are present near the gold surface, indicating that carboxylate binding occurs as a 2e- L-type interaction for each oxygen atom involved.

Direct observation of three-dimensional atomic-level structures on the surface of catalytic nanoparticles

255th National Meeting (2018)

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