Tungsten Oxide Phase Research Articles

Structural Studies of Tungsten–Titanium Oxide Thin Films

Structural studies of tungsten–titanium oxide thin films grown on alumina substrates have been performed on samples prepared by reactive magnetron sputtering of a W/Ti alloy (90% W and 10% Ti). Structural changes undergone by samples heated at 773 and 1073 K have been studied by using X-ray diffraction and X-ray photoemission techniques. The results, which have been also discussed with the aid of a structural model, indicate that the original amorphous phase transforms by annealing into a crystalline phase of tungsten oxide with a degree of order depending on the annealing temperature. The relationship between titanium and the structure of the film is discussed in terms of disorder effects induced by the Ti ions in the WO3lattice. The ordering is ascribed to the segregation of Ti ions toward the surface upon annealing at 1073 K.

Temperature-programmed reduction and oxidation of nickel supported on WO3–Al2O3composite oxides

A series of Ni/WO3–Al2O3 catalysts have been prepared by systematically varying the weight loading of WO3–Al2O3 and the pH during adsorption/impregnation. The catalysts developed an increased interaction between the impregnated nickel and the supported metal oxide; the charge on the mixed oxide resulted in the impregnated nickel cation to be preferentially adsorbed onto tungsten sites of the composite supports. The amount of nickel associated with tungsten oxide was found to be a function of both the tungsten oxide weight loading and the impregnation pH. An increase in the negative charge due to hydrolysis of the tungsten oxide phase in the aqueous impregnant is proposed to result in a tungsten oxide associated nickel that is twice as large for the catalysts prepared at pH 6 than that at pH 4. The Ni/support interaction formed during adsorption/impregnation and drying were studied by temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO). The tungsten oxide and associated nickel were found to form a nickel tungstate compound which reacted with H2 at ca. 1050 K but could be regenerated by TPO. An Ni/Al2O4 type surface compound was also formed that was reduced at ca. 1150 K.

The effect of second-phase oxides on the catalytic properties of dispersed metals: Cobalt supported on 12% WO 3/Al 2O 3

The differences in the amphoteric properties Of WO 3 and Al 2O 3 were exploited and cobalt precursors were selectively mounted on the tungsten oxide phase of a 12% WO 3/Al 2O 3 composite. Temperature-programmed reduction (TPRd), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and catalyst performance testing using H 2/CO methanation as a test reaction revealed that this composite-supported catalyst's properties were significantly different from cobalt supported on alumina. Cobalt crystallites as well as a CoAl 2O 4 type of surface species are formed on the Al 2O 3 support. The presence of WO 3 as a second-phase oxide offers the possibility for a greater number of surface compounds to be formed on the composite. Specifically, cobalt-tungsten interaction species are formed. We provide evidence that strongly suggests the active catalytic center in the conversion of H 2 and CO to methane is a (cobalt-tungsten) interaction species. These active centers. are present on CO/WO 3, Co/12% WO 3/Al 2O 3, and a commercial cobalt/tungsten oxide compound. A portion of the cobalt: in these active centers is not reduced to the metallic state.

Laser Raman characterization of tungsten oxide supported on alumina: Influence of calcination temperatures

The influence of calcination temperature upon the solid state chemistry of WO 3 on Al 2O 3 was examined with laser Raman spectroscopy. Laser Raman spectroscopy revealed the amorphous and crystalline structural transformations occurring in the WO 3 on Al 2O 3 oxide system. Below monolayer coverage of tungsten oxide on alumina, the tungsten oxide phase is present as a highly dispersed and amorphous surface complex on the support. A close-packed monolayer of the tungsten oxide surface complex on alumina is formed as the surface area of the alumina support decreases at high calcination temperatures. The lower the tungsten oxide loading, the more severe the calcination temperature must be to reach the close-packed monolayer. The close-packed tungsten oxide monolayer accommodates the further desurfacing at still higher temperatures by forming the bulk tungsten oxide phases WO 3 and Al 2(WO 4) 3. The Al 2(WO 4) 3 phase is formed from the reaction of WO 3 crystallites with the Al 2O 3 support. The parameter controlling the phases present in the WO 3 on Al 2O 3 system is the surface density of the tungsten oxide species on the alumina surface.