STUDY OF ATOMS, IONS AND CLUSTERS OF SUPPORTED BY THE METHOD OF DIFFUSE REFLECTANCE ELECTRON SPECTROSCOPY

Author Institution: Department of Chemistry, Tomsk Institute of Civil EngineeringVariations in atomic, ion and cluster states of silver catalysts for methanol oxidation under the effect of the reaction medium have been studied, using diffuse reflectance electron spectroscopy. Metal atoms and particles were formed on the surface of oxide supports. Spectra of silver catalysts exhibit bands at 240-250, 270, 315-320, 370-390 and 400-460 nm. IR and UV measurements permitted to identify the following silver states: bands at 240-250 nm belong to isolate $Ag^{+}$ ions, the 315 nm band can be ascribed to a metallic silver (film and large particles) and the 370-390 nm and 460 nm bands belong to charged silver clusters $Ag^{n}$ and crystallites of various sizes. EPR studies have not revealed higher valence states in the catalysts. Prolonged treatment of the catalysts by the reaction methanol-air mixture greatly decreases the relative intensity of the band at 315-320 nm, whereas that of the bands at 240-250 and 380 nm rise and there appears a band at 270 nm, characteristic of the charged complexes $Ag_{2}{^{+}}_{3}$. Increase in the relative intensity of bands at 370-390 and 400-420 nm ascribed to clusters and silver crystallites is accounted for by the development of catalyst surface during the reaction accompanied by the formation of a great number of the above particles already registered using the electron microscopic method. Thus under the effect of reaction medium an active surface of silver catalysts is formed, which manifests itself in the fornation of charged form of $Ag^{+}$ and $Ag^{+}_{n}$. Whereas the portion of uncharged metallic silver decreases. Results of the experiments permit to make some conclusions about the nature of active centres of silver catalysts