OXIDE SUPPORT INTERACTION AND SURFACE RECONSTRUCTION IN THE NA2WO4/SIO2 SYSTEM

The surface dispersion state and oxide/support interaction in the MnOx-Na2WO4/SiO2 catalyst system for oxidative coupling of methane have been studied by XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), and PASCA (positron annihilation spectroscopy for chemical analysis). It was observed that Mn existed as Mn2O3-free crystallites and Na2WO4 Was dispersed on the silica support preferentially. A close-packed monolayer of Na2WO4 was formed at a loading of 4 wt %. Strong interaction between Na2WO4 and silica is indicated in the region of 0-1.5 wt % Na2WO4, while a weaker interaction is exhibited in the region 1.5-4.0 wt % Na2WO4. A phase transformation of SiO2 from silica gel to cristobalite upon calcination at 1023-1123 K took place as the consequence of this strong interaction. Reconstruction of surface WO4 tetrahedral units into a structure containing a W=O and three W-O-Si surface bonds was demonstrated. These species are believed to be relevant to the oxidative coupling of methane. A possible model for the structure of the new surface cluster compound is proposed.