Nature and distribution of tungsten oxides in porous Vycor glass

Abstract Tungsten oxides, including WO3 and mixed valent oxides, are formed in unpolished porous Vycor glass (PVG) by ultraviolet photolytic decomposition of adsorbed tungsten hexacarbonyl. The relative amounts of the oxides formed are determined by a competition between aggregation and oxidation, and both are influenced by CO pressure gradients created by the photoinduced release of CO within the nanoporous silica matrix. These gradients sweep the hexacarbonyl precursor into the outer regions of the PVG, increasing amounts of tungsten nearer the surface, thereby enhancing the rate of aggregation relative to the rate of oxidation and promoting the formation of the mixed valent oxides. Thermolysis of the physisorbed hexacarbonyl, on the other hand, produces predominantly WO3. Thermolysis promotes desorption of the physisorbed W(CO)6, reducing the amount of tungsten available, permitting oxidation to be the predominant process over aggregation, leading to WO3 formation.