The Impact of the Bulk Structure on Surface Dynamics of Complex Mo–V-based Oxide Catalysts

Abstract

Mixed oxides composed of the four metals Mo, V, Te, and Nb are known to be efficient catalysts in selective oxidation of lower alkanes. The outstanding catalytic performance of such mixed oxides is attributed to the presence of the so-called M1 crystal phase that contains all four elements in the metal positions of the structure. In the present work, an M1 phase composed only of Mo and V has been prepared by hydrothermal synthesis. High crystallinity was achieved by applying a synthesis temperature of 200 °C. The phase-pure mixed MoV oxide was studied as catalyst in the oxidation of propane. In contrast to previous reports, the desirable oxidation product acrylic acid is formed over the Te-free M1 structure in significant amounts, implying that Te is not necessarily required as a component of the active ensemble responsible for selective oxygen insertion. The MoV M1 oxide is, however, less selective compared to that of the M1 structure composed of the four metals Mo, V, Te, and Nb. The reason has been determined by applying a combination of synchrotron-based single crystal structure analysis and near-ambient-pressure X-ray photoelectron spectroscopy. Determination of the crystal structure of MoV M1 oxide reveals partial occupation of sites in the hexagonal channels of the M1 structure by V, which are occupied by Te in MoVTeNb M1 oxide. Hydrolysis of the M-O bonds (M = V, Te) under reaction conditions leads to migration of the metal in the hexagonal channels to the surface. Accumulation of more than 50 at % V on the surface of the MoV M1 oxide most likely causes postcombustion of formed acrylic acid, whereas enrichment of Te at the surface of MoVTeNb M1 oxide results in dilution of surface V5+ species and, consequently, high selectivity

Site specific and localized structural displacements in open structured multimetallic oxides

“…A covalent bond character of the hexagonal channel cation with lattice oxygen, which surrounds the S2 site, has already been discussed for (Mo,V)O x . 12 An unequal Te occupancy in the hexagonal channels of orthorhombic (Mo,V,Te)O x has been previously reported. 9,20 Our finding of Te disorder in the hexagonal channels of various synthesized (Mo,V,Te,Nb)O x with different composition shows that these structural irregularities are a general feature in Te-containing molybdenum/vanadium mixed oxides.…”

“…These pentagonal building blocks are connected via corner-sharing M 2 O 6 (M 2 = Mo, V) octahedra resulting in the formation of hexagonal and heptagonal structural channels. [10][11][12] These channels can be partially occupied by cationic species of tellurium, antimony, vanadium or ammonium. 7,[10][11][12][13] Furthermore, complex molybdenum and vanadium mixed oxides represent non-stoichiometric compounds.…”

“…[10][11][12] These channels can be partially occupied by cationic species of tellurium, antimony, vanadium or ammonium. 7,[10][11][12][13] Furthermore, complex molybdenum and vanadium mixed oxides represent non-stoichiometric compounds. Thus, in order to accomplish charge neutrality, the intrinsic oxidation states of molybdenum and vanadium within these materials depend on the metal and oxygen composition.…”

“…17,18 They have been the subject of numerous studies that focused on the understanding of their catalytic and structural properties also including operando/ in situ investigations. 7,[9][10][11][12][13]16,[19][20][21][22][23][24][25][26][27][28][29] A crystallographic model is shown in Fig. 1A.…”

“…The displacement vector of shifted metal cations on this site points toward the centers of the hexagonal channels. 12,13 Their spatial distribution and a detailed understanding of how these displacements affect the local chemistry and properties of the material has yet not been reported. However, this local information would be an important prerequisite to improve our current understanding of heterogeneous catalysis.…”