Amorphous features of working catalysts:: XAFS and XPS characterization of Mn/Na2WO4/SiO2 as used for the oxidative coupling of methane

Mn/Na2WO4/SiO2 catalyst for the oxidative coupling of methane has been investigated by using the L-edge and the K-edge XAFS and XPS, The surface of the fresh catalyst is found to be dominated by oxygen-enriched amorphous phases consisting of discretely distributed tetrahedral WO4 and octahedral MnO6 groups, The average bond lengths, which is 1.77 Angstrom for W-O and 2.00 Angstrom for Mn-O, are fully identical to those in pure Na2WO4 and Mn2O3. However, on the used catalyst after a 450 h run, almost no tungsten species is detectable. Instead, tetrahedral manganese sites are observed, The surface of the used catalyst is dominated by highly dispersed MnO4 groups consisting of tetrahedral and coordinatively unsaturated octahedral sites. The average Mn-O bond length increases to 2.09 Angstrom while the Mn 2P(3/2) binding energy increases from 641.4 eV (Mn3+) to 641.7 eV (Mn2+), These results suggest that the combination of tetrahedral and octahedral metallic cores with different oxidation states from each other is responsible for the catalysis in the oxidative coupling of methane, and that the high activity of the manganese-tungsten catalyst is due to the surface oxygen species, which are pre-activated from lattice oxygens to "the nearest oxygens" by the surface in an amorphous state. (C) 1998 Academic Press.