SOLID-STATE AG-109 NMR CHARACTERIZATION OF SILVER DISPERSED ON OXIDE SUPPORTS

Magic angle spinning Ag-109 solid-state NMR spectroscopy, transmission electron microscopy, X-ray diffraction, and gas adsorption measurements were used to characterize Ag particles supported on SiO2, eta-Al2O3, and zeolite A. The samples were prepared either by a standard incipient wetness technique or by ion exchange followed by drying and reduction in H-2. Ag-109 NMR spectroscopy was used to follow the state of the silver throughout the preparation process. Ag-109 spectra were obtained for metallic Ag and for pure powder samples of AgNO3 and AgCl as well as for Ag+ in solution; however, no resonances were observed for powder samples of Ag2O and Ag2O2. Prior to drying, chemical shifts were found to be close to those of aqueous AgNO3 solutions (approximately 0 ppm), and after drying at 373 K, the chemical shift moved upfield to approach that of bulk AgNO3. After reduction, a single Knight-shifted metallic resonance at +5252 +/- 10 ppm was observed for all samples which contained Ag particles larger than 50 nm in diameter. Low-loading, well-dispersed samples with smaller Ag particles yielded no observable Ag-109 resonance. It is proposed that the absence of an observable Ag-109 signal in the latter samples is due to a surface boundary effect on conduction electrons, common to other metal conductors, which broadens the signal. The adsorption of O2, Cl2, and HCl on the surface of the large Ag crystallites had no effect on the Ag-109 spectrum.