Surface texturing of Mo–V–Te–Nb–O x selective oxidation catalysts

Abstract

The paper concentrates on the study of Mo-V-Te-Nb oxide mixtures by electron microscopy combined with catalytic investigation of these materials in the partial oxidation of propane. Surface texturing of catalyst particles composed of two phases referred to in the literature as M1 and M2 is revealed by high-resolution transmission electron microscopy of high performing catalysts. The chemical composition of the catalyst surface is modified by treatment in water to obtain a significant increment in yield of acrylic acid. A chemical realization of the site isolation concept recurring on a supramolecular arrangement of catalyst and reactant rather than on atomic site isolation is suggested. A complex Mo-V-Te-Nb-Ox precursor phase carries nanoparticles made from a network of oxoclusters active as catalyst for the conversion of propane to acrylic acid. The designed synthesis of the multielement oxide bulk and of the surface structure with a different composition than the precursor phase improved the performance by a factor of 4.

Active Sites for Propane Oxidation: Some Generic Considerations

“…This pentameric site is located within the motif of M1 as the bridge between two adjacent heptagonal channels. Figure 2 illustrates that in agreement with experimental derivations [42,49,53,71] the assumed active site does not only occur on the basal plane (001) but also on the stepped large prism faces of the oxide needles if we assume a low-energy process for creating a surface in the M1 structure. Figure 2a reminds us on the basal plane projection of the orthorhombic structure of M1.…”

“…It occurs that the bulk crystalline phase provides the rigid base. The variation of the site occupation [16,45,46] as an ingredient or reactivity leading to metastability controls the formation and abundance of isolated nanodomains with the correct chemical composition [13,26,49,53] yielding the active catalyst. The cation composition of the pentameric site is of a mixture of V and Mo according to the original literature [15] with highly specific requirements for assumed catalytic function; not all V x Mo Y combinations would be active based upon assumptions of rigid oxidation states and neglecting the concept of adaptive sites.…”

“…The high level of structural definition has inspired theoretical work [11,32,41,48] aiming at a detailed quantum chemical explanation [11,12,41] of the mode of operation; such work is only possible with a reasonable knowledge about the local chemistry and with the atom coordinates available now in high precision from structure determination. Transmission electron microscopy, a technique since long of great value [2,42,47,49,50] in the analysis of the complex oxide catalysts, helped [51] also here to elucidate the surface termination [52] of M1.…”

“…For these operations neither the influx of active species during the unit conversion time (as long as the educt is adsorbed) nor the participation of electrons from outside of the active site are required. This is the consequence of the site isolation concept [53,54] being so powerful in guiding the development of selective oxidation catalysts. It is expected that the active site behaves dynamical and changes its electronic and geometric structure during the catalytic reaction.…”

Preparation of Phase-Pure M1 MoVTeNb Oxide Catalysts by Hydrothermal Synthesis—Influence of Reaction Parameters on Structure and Morphology

“…M2 on its own does not activate propane, but selectively oxidizes the propylene intermediate. Correlations between phase composition and catalytic properties are still actively debated [13][14][15][16]. However, it has been generally accepted that the high selectivity towards acrylic acid is associated with the M1 phase [3,17].…”