Amorphous Features of Working Catalysts: XAFS and XPS Characterization of Mn/Na2WO4/SiO2as Used for the Oxidative Coupling of Methane

• Published in: Journal of catalysis 1998-01, Vol.173 (2), p.399-408
• Main Authors: Kou, Yuan, Zhang, Bing, Niu, Jian-zhong, Li, Shu-ben, Wang, Hong-li, Tanaka, Tsunehiro, Yoshida, Satohiro
• Format: Article
• Language: English
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1006/jcat.1997.1900

Mn/Na2WO4/SiO2catalyst for the oxidative coupling of methane has been investigated by using theL-edge and theK-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 WO4and octahedral MnO6groups. The average bond lengths, which is 1.77 Å for W–O and 2.00 Å for Mn–O, are fully identical to those in pure Na2WO4and 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 MnO4groups consisting of tetrahedral and coordinatively unsaturated octahedral sites. The average Mn–O bond length increases to 2.09 Å while the Mn 2p3/2binding 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.