ESR STUDIES OF WELL-DISPERSED AG CRYSTALLITES ON SIO2

摘要

ESR was utilized to study Ag on SiO2 over a dispersion range of 0.04 to 0.32, which gave average Ag crystallites sizes between 4 and 31 nm. For all samples only a single sharp signal was observed at g = 2.0028 +/- 0.0002 with a peak-to-peak linewidth of 1.5-6.0 G, whether after reduction at 773 K or after adsorption of O-2, N2O or C2H5Cl. This signal was shown to be associated with conduction electrons existing only in small nanocrystallites (ca. 7 nn or smaller) in which quantum size effects occur to increase the spin relaxation time and provide a CESR signal. The use of O-17 verified the peak was not due to any paramagnetic oxygen species. No correlation between line width or g shift acid average crystallite size (below 7 mn) was observed, although a model invoking quantum size effects has predicted one. Adsorption of O-2 or N2O at either 300 or 443 K produced identical results-the peak intensity was decreased and no new peaks were detected. Exposure to H-2 at 443 or 300 K provided almost complete or partial restoration of the signal, respectively. Adsorption of C2HSCl at 300 K had no effect, but at 523 K adsorption on clean Ag markedly decreased intensity, and adsorption on an oxygen-covered Ag surface further decreased signal strength, thus indicating the strong interaction Cl atoms can achieve when such promoters are used in the partial oxidation of ethylene. (C) 1995 Academic Press, Inc. [References: 42]