Vadim V. Guliants

The effect of the phase composition of model VPO catalysts for partial oxidation of n-butane

X-ray diffraction, Raman spectroscopy, 3’P MAS-NMR and spin-echo NMR indicated that model vanadium phosphorus oxide (VPO) precursors and catalysts contained various minor phases depending oxboth the synthetic approach and P/V ratios used. Raman spectroscopy revealed the presence of a number of micro-crystalline and amorphous V(W) and V(V) phases not evident by XRD. The presence of VOPO, phases was detrimental to the performance of the VP0 catalysts for KN-butane oxidation. The best model organic VP0 catalyst contained only vanadyl pyrophosphate with the highest degree of stacking order and virtually no VOPO, phase impurity. Raman spectroscopy detected vanadyl metaphosphate. VO(PO,),, in the catalysts derived from aqueous precursors possessing P/V ratios greater than I. Pure vanadyl metaphosphate catalyst was inactive in n-butane oxidation. s’P NMR demonstrated the absence of vanadyl metaphosphate and other impurity phases in the best catalyst derived from organic precursors at P/V = 1.18. The experimental data strongly indicate that the best VP0 catalysts for n-butane oxidation contain only vanadyl pyrophosphate with well-ordered stacking of the (200) planes.

In situ Raman spectroscopy studies of bulk and surface metal oxide phases during oxidation reactions

Bulk V-P-O and model supported vanadia catalysts were investigated with in situ Raman spectroscopy during n-butane oxidation to maleic anhydride in order to determine the fundamental molecular structure-reactivity/selectivity insights that can be obtained from such experiments. The in situ Raman studies of the bulk V-P-O catalysts provided information about the bulk crystalline phases, the hemihydrate precursor and its transformation to vanadyl pyrophosphate. However, the Raman experiments could not provide any molecular structural information about the amorphous and surface phases also present in this bulk metal oxide catalyst because of the strong Raman scattering from the crystalline phases. In contrast, in situ Raman studies of the model supported vanadia catalysts, where the active phase is present as a two-dimensional surface metal oxide overlayer, provided new insights into this important hydrocarbon oxidation reaction. In addition, the surface properties of the supported vanadia catalysts could be molecularly engineered to probe the role of various functionalities upon the structure-reactivity/selectivity relationship of n-butane oxidation to maleic anhydride. These fundamental studies revealed that the oxidation of n-butane required only one surface vanadia site and that the critical rate determining step involved the bridging V-O-P or V-O-support bonds. The selective oxidation of n-butane to maleic anhydride could occur over one surface vanadia site as well as multiple adjacent surface vanadia sites, but the reaction is more efficient with multiple sites. The n-butane oxidation TOF increased with the introduction of both surface Briinsted and Lewis acid sites, but only the surface Lewis acid sites increased the maleic anhydride selectivity.

Evolution of the active surface of the vanadyl pyrophosphate catalysts

Bulk crystallinity of vanadyl(IV) pyrophosphate catalysts forn-butane partial oxidation increased up to 23 days on stream as determined by XRD and Raman spectroscopy, while selectivity reached steady state after 8–10 days. Electron microscopy detected a 15 Å amorphous layer terminating the (200) planes of (VO)2P2O7 in fresh catalysts that was not observed in the equilibrated catalysts. It is suggested that ordering of (200) planes at the surface of (VO)2P2O7 is responsible for selective oxidation.

Bulk structure and catalytic properties of mixed Mo-V-Sb-Nb oxides for selective propane oxidation to acrylic acid

The bulk mixed Mo–V–Sb–Nb–O catalysts, which are a candidate catalytic system for the selective oxidation of propane to acrylic acid, were investigated to elucidate the bulk structure and catalytic behavior of these complex materials. These mixed oxides were prepared via a redox reaction between V 5+ and Sb 3+ in the presence of Mo 6+ and Nb 5+ and characterized by potentiometric titrations, XRD, Raman spectroscopy, electron microscopy (TEM), and bulk elemental analysis. A potentiometric titration method was used to determine concentrations of metal cations in various oxidation states. XRD and Raman spectroscopy identified Mo 6V9O40 ,M oO 3 ,S bVO 4 ,a nd a Nb-stabilized defect phase of a V-rich molybdate as the major phases present. Electron microscopy illustrated the heterogeneity of the bulk oxide phases present in the model Mo–V–Sb–Nb–O system on the submicron scale. MoO3 comprised the bulk of this mixed metal oxide system, while the surface region of these model catalysts contained mixed Mo–V–Sb–Nb oxides. The rutile SbVO4 phase was inefficient in propane oxidation to acrylic acid, while mixed Mo–V–Nb oxides were capable of producing acrylic acid at ∼ 20 mol% yield.  2003 Elsevier Science (USA). All rights reserved.

Solid state chemistry of bulk mixed metal oxide catalysts for the selective oxidation of propane to acrylic acid

Abstract Syntheses of Mo–V–Sb–Nb–O bulk materials, which are candidate catalyst systems for the selective oxidation of propane to acrolein and acrylic acid, were made using soluble precursor materials. The products were characterized by X-ray powder diffraction and Raman spectroscopic studies. The objectives of this work were to explore the utility of liquid phase automated synthesis for the preparation of bulk mixed metal oxides, and the identification of the oxide phases present in the system. This is the first published study of the phase composition for these materials. After calcination of these bulk oxides under flowing nitrogen at 600°C, and using stoichiometric ratios of Mo–V–Sb–Nb (1:1:0.4:0.4) and Mo–V–Sb–Nb (3.3:1:0.4:0.4) it was demonstrated that a mixture of phases were obtained for the syntheses. X-ray powder diffraction studies distinguished SbVO 4 , Mo 6 V 9 O 40 , MoO 3 , and a niobium-stabilized defect phase of a vanadium-rich molybdate, Mo 0.61–0.77 V 0.31–0.19 Nb 0.08–0.04 O x , as the major phases present. Complementary data were provided by the Raman spectroscopic studies, which illustrated the heterogeneity of the phases present in the mixture. Raman also indicated bands attributable to the presence of phases containing terminal MO bonds as well as M–O–M polycrystalline phases. Previous studies on this system have identified SbVO 4 and niobium-stabilized vanadium molybdate species as the active phases necessary for the selective oxidation of alkanes.

Structure–reactivity relationships in oxidation of C4 hydrocarbons on supported vanadia catalysts

Abstract Molecular vanadia species supported on the surface of metal oxides are used as catalysts for many hydrocarbon oxidation reactions. This paper discusses (i) the structure of the surface vanadia species in fresh and dehydrated catalysts, (ii) the structural and oxidation state changes during C4 hydrocarbon oxidation, and (iii) the roles of the oxide support, the surface oxygen species, and the acidity of the supported vanadia catalysts in selective oxidation catalysis. The molecular structure and reactivity information provides new fundamental insights into the catalytic properties of surface vanadia species during C4 hydrocarbon oxidation. On the other hand, the studies of the model supported vanadia catalysts shed new light on the origins of the catalytic activity of the bulk vanadium–phosphorus oxide (VPO) catalysts for partial oxidation of n -butane to maleic anhydride. The new fundamental structure–activity and selectivity relationships of surface vanadia species on oxide supports provide a foundation for the molecular engineering of supported catalysts for selective oxidation of hydrocarbons.

The role of surface basal planes of layered mixed metal oxides in selective transformation of lower alkanes: propane ammoxidation over surface ab planes of Mo-V-Te-Nb-O M1 phase.

The surface ab planes of the M1 phase exposed selectively after atomic layer deposition (ALD) of alumina followed by crushing showed significantly improved selectivity to acrylonitrile during propane ammoxidation. The results demonstrated the importance of surface ab planes for the activity and selectivity of the M1 phase in propane ammoxidation and general utility of surface modification by ALD in studies of catalytic behavior of surface planes in layered mixed metal oxides.

High-throughput experimentation in multicomponent bulk mixed metal oxides: Mo-V-Sb-Nb-O system for selective oxidation of propane to acrylic acid

Abstract The bulk mixed Mo-V-Sb-Nb oxides, which is a candidate catalytic system for the selective oxidation of propane to acrylic acid, were prepared using an automated synthesis workstation. Two catalyst compositions, Mo 0.3 V 0.3 Sb 0.125 Nb 0.125 and Mo 1.0 V 0.3 Sb 0.125 Nb 0.125 , were characterized by XRD and Raman spectroscopy and shown to contain Mo 6 V 9 O 40 , Mo 0.61–0.77 V 0.31–0.19 Nb 0.08–0.04 O x (a niobium-stabilized defect phase of a vanadium-rich molybdate), Mo 3 Nb 2 O 11 , and MoO 3 as the major phases. The results of the structural characterization and kinetic screening demonstrated good reproducibility of the phase compositions and catalytic properties of the model Mo-V-Sb-Nb-O system and provided new insights into the transformation processes that occur in these mixed oxides with time on stream. These results suggest the promise of emerging high-throughput in situ characterization techniques for establishing fundamental structure–activity/selectivity relationships and developing novel multicomponent mixed metal oxides for selective oxidation of lower alkanes.

Hydrogen Production through Water–Gas Shift Reaction over Supported Cu, Ni, and CuNi Nanoparticle Catalysts Prepared from Metal Colloids

Colloidal Cu, Ni, and CuNi nanoparticles obtained through chemical reduction were impregnated onto γ‐Al2O3 to prepare supported catalysts. Conventional catalysts were also prepared for comparison through support impregnation with metal salt solutions. A combination of TEM and STEM, thermogravimetric analysis, powder X‐ray diffraction, X‐ray photoelectron spectroscopy, and N2 adsorption analysis was used to characterize the alumina‐supported nanoparticle catalysts. Monodisperse Cu and Ni nanoparticles were uniformly distributed on γ‐Al2O3 as observed by STEM equipped with a high‐angle annular dark field (HAADF) detector. The powder XRD and HAADF–STEM provided evidence supporting the formation of CuNi particles containing the Cu core and the CuNi alloy shell. The resulting supported Cu and Ni nanoparticle catalysts were found to be 2.5 times more active in the water–gas shift (WGS) reaction per unit mass of the active metal as compared with catalysts prepared by means of the conventional impregnation method. Supported Cu catalysts showed promising activity at low temperatures (125–250 °C), whereas supported Ni catalysts showed high activity at high temperatures (275–400 °C). The presence of a surfactant during the preparation of supported Cu nanoparticles resulted in a higher degree of Cu dispersion and thus enhanced WGS activity. Stabilized Ni/Ni(OH)2 nanoparticles were also synthesized to prepare well‐dispersed supported Ni catalysts. In the case of bimetallic catalysts, Cu5Ni5 catalysts showed a higher specific reaction rate per unit surface area of the active metal than did monometallic catalysts. These findings strongly suggested that supported Cu, Ni, and CuNi nanoparticle catalysts prepared from metal colloids are promising, highly active WGS catalysts.

Better Catalysts through Microscopy: Mesoscale M1/M2 Intergrowth in Molybdenum–Vanadium Based Complex Oxide Catalysts for Propane Ammoxidation

In recent decades, catalysis research has transformed from the predominantly empirical field to one where it is possible to control the catalytic properties via characterization and modification of the atomic-scale active centers. Many phenomena in catalysis, such as synergistic effect, however, transcend the atomic scale and also require the knowledge and control of the mesoscale structure of the specimen to harness. In this paper, we use our discovery of atomic-scale epitaxial interfaces in molybdenum-vanadium based complex oxide catalysts systems (i.e., Mo-V-M-O, M = Ta, Te, Sb, Nb, etc.) to achieve control of the mesoscale structure of this complex mixture of very different active phases. We can now achieve true epitaxial intergrowth between the catalytically critical M1 and M2 phases in the system that are hypothesized to have synergistic interactions, and demonstrate that the resulting catalyst has improved selectivity in the initial studies. Finally, we highlight the crucial role atomic scale characterization and mesoscale structure control play in uncovering the complex underpinnings of the synergistic effect in catalysis.