Structure characterization of orthorhombic phase in MoVTeNbO catalyst by powder X-ray diffraction and XANES

The structure of the pure orthorhombic phase of the MoVTeNbO catalyst used for propane selective oxidation to acrylic acid was characterized by a combination of XANES and XRD. The vanadium K absorption pre-edge peak position due to 1s-3d transition obtained by XANES shows that average oxidation state of V is 4.1+. The Rietveld XRD analysis refined the unit cell parameters, chemical formula, and geometric arrangement of the catalyst, particularly the positions and fractional occupancies of metal atoms. The unit cell is Pba2 with a = 2.1116 nm, b = 2.6594 nm, c = 0.4007 nm. The unit formula is Mo_0.55⁵⁺Mo_6.76⁶⁺V_1.52⁴⁺V_0.17⁵⁺Te_0.69⁴⁺Nb_1.00⁵⁺O_x^2- (28.34 < x < 28.69). The structure refinement of five-, six-, and seven-membered rings of MO₆ (M = Mo or V) determined that Nb⁵⁺ is localized in the pentagonal channel surrounded by MO₆ octahedra, that Te⁴⁺ is localized in the hexagonal channel surrounded by MO₆, and that the heptagonal channel is not occupied. Results revealed that Te⁴⁺ is only localized in the hexagonal channel at 70% occupancy. Te in the catalyst proves to be highly stable.