Te Nb Research Articles

Quantum Chemical Modeling of the Full Catalytic Cycle for Selective Oxidation of Propane to Propene on the M1 Phase of Mo–Te–Nb–O Mixed-Metal Oxide Catalysts

Quantum Chemical Modeling of the Full Catalytic Cycle for Selective Oxidation of Propane to Propene on the M1 Phase of Mo–Te–Nb–O Mixed-Metal Oxide Catalysts

Preparation and improved photochemical properties of V-doped Nb2Te4O13

Mixed oxides combining d0-and p-cations with stereoactive lone pair electrons exhibit excellent polarization abilities for potential photoelectric applications. This work reports the photochemical properties of V-doped Te–Nb mixed oxide Nb2Te4O13, which has the typical framework constructed by NbO6 (d0-) octahedra, TeO4, and TeO3 (p-) polyhedra. The samples were prepared using the sol-gel method, which resulted in the final nanoparticles. The luminescence spectra, lifetimes of photoinduced electrons and holes, and conductivity properties of the samples were characterized. Nb2Te4O13 has an indirect transition with a band energy of 2.98 eV V5+-doping shows a narrowing effect on the band gap, enhancing the harvest of the optical absorption in the visible light region. Photocatalysis tests were conducted via the photodegradation of RhB solutions under visible-light irradiation. V5+-doping depresses the intrinsic luminescence efficiency, while it significantly enhances photodegradation. It is suggested that the induced redox couples V5+/4+ and defects are responsible for the improvements of photocatalysis of Nb2Te4O13. This work offers an effective method to modify photochemical properties via redox couples and defects induced by impurity ion doping.

Nonequilibrium Thermodynamics and Stoichiography in Studying Solid-Phase Processes of Fabrication of Functional Materials

The application of general principles of nonequilibrium thermodynamics and stoichiography allows obtaining novel information on the solid-phase transformations of multielement and heterophase substances and materials. Without solid-phase standards, the use of stoichiography makes it possible to detect, identify, and quantify known and unknown crystalline and amorphous phases of constant or variable composition. For the first time, reliable results are presented concerning the evolution of solid products in the course of preparation of the Mo–V–Te–Nb–O catalyst of propane ammoxidation.

Deactivation of a mixed oxide catalyst of Mo–V–Te–Nb–O composition in the reaction of oxidative ethane dehydrogenation

The operational stability of a mixed oxide catalyst of Mo–V–Te–Nb–O composition in the oxidative dehydrogenation of ethane (ratio of C2H6: O2 = 3: 1) is studied in a flow reactor at temperatures of 340–400°C, a pressure of 1 atm, and a WHSV of the feed mixture of 800 h−1. It is found that the selectivity toward ethylene is 98% at 340°C, but the conversion of ethane at this temperature is only 6%; when the temperature is raised to 400°C, the conversion of ethane is increased to 37%, while the selectivity toward ethylene is reduced to 85%. Using physical and chemical means (XPS, SEM), it is found that the lack of oxidant in the reaction mixture leads to irreversible changes in the catalyst, i.e., reduced selectivity and activity. Raising the reaction temperature to 400°C allows the reduction of tellurium by ethane, from the +6 oxidation state to the zerovalent state, with its subsequent sublimation and the destruction of the catalytically active and selective phase; in its characteristics, the catalyst becomes similar to the Mo–V–Nb–O system containing no tellurium.

Promotional effect of cerium on Mo–V–Te–Nb mixed oxide catalyst for ammoxidation of propane to acrylonitrile

The MoV0.31Te0.23Nb0.24 mixed oxide (M) catalysts with different cerium amounts for propane ammoxidation to acrylonitrile were synthesized by the co-precipitation method. The effect of cerium content on the physicochemical property of the M catalyst was investigated by low-temperature N2 adsorption, XRD, XPS and H2-TPR. The results show that the presence of cerium in the M catalyst can significantly enhance its catalytic performance for propane ammoxidation. 46% acrylonitrile yield can be obtained over the MCe0.05 catalyst with Ce/Mo=0.05 (mol) at SV of 1200mL/(h.g), in which there are most optimal surface compositions, such as more M1 and M2 active phases, and higher Te4+ concentration and appropriate V content. The presence of Ce3+/Ce4+ pairs promotes the redox capability of the M catalyst and more reasonable atom distribution on the catalyst surface.

Understanding the kinetic behavior of a Mo–V–Te–Nb mixed oxide in the oxydehydrogenation of ethane

Two kinetic models based on Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms were developed to describe the oxydehydrogenation of ethane to yield ethylene over a Mo–V–Te–Nb catalyst. Obtained in a lab-scale fixed-bed reactor, experimental data at the steady-state were used to estimate the kinetic models parameters via a nonisothermal regression. Experiments were performed using an ethane, oxygen and nitrogen mixture as feedstock, spanning temperatures from 673 to 753K, inlet partial pressures of oxygen and ethane between 5.0 and 22.0kPa, and space–time from 10 to 70gcath(molethane)−1. Ethylene, CO and CO2 were the only detected products, the selectivity for ethylene ranged from 76% to 96% for an ethane conversion interval 4–85%. A series of tests feeding ethylene instead of ethane were also performed at 713K, varying inlet partial pressures and space–time in the same ranges as was done for ethane. Ethylene conversion was relatively low, 3–14%, the dominant product being CO with CO/CO2 ratios from 0.73 to 0.79. The LH mechanism was found to represent better the experimental data. The oxydehydrogenation of ethane was the reaction with the lowest activation energy, 108–115kJmol−1. Except for the conversion of ethane into CO2, deep oxidations were detected as very energetically demanding steps, 156–193kJmol−1. Competitive adsorption between reagents and products occurred in the two mechanisms particularly at relatively high reaction severity, water re-adsorption being weaker in comparison with COx re-adsorption.

Propane Ammoxidation over Mo–V–Te–Nb–O M1 Phase Investigated by DFT: Elementary Steps of Ammonia Adsorption, Activation and NH Insertion into π-Allyl Intermediate

The selective ammoxidation of propane into acrylonitrile catalyzed by the bulk Mo–V–Te–Nb–O system has received significant attention because it is more environmentally benign than the current process of propene ammoxidation and relies on more abundant propane feedstock. The reaction mechanism is proposed to consist of a series of elementary steps including propane oxidative dehydrogenation, ammonia and O2 activation, and NHx insertion into C3 intermediates. In this study density functional theory calculations have been performed to investigate the energetics of ammonia adsorption and activation in the proposed active center in the ab plane of the M1 phase. The formation of NH x (x = 0, 1, 2, 3) species is found to be highly favored on reduced, oxo-depleted metal sites. The reduced Mo site is determined to be the most favorable site for ammonia activation by comparing the reaction energy profiles for the sequential dehydrogenation of ammonia on the various metal sites. The activation barrier for the initial H abstraction from ammonia was found to depend strongly on the surface sites that stabilize H and NH2, and is as low as 0.28 eV when NH2 is stabilized by the reduced Mo site and H is abstracted by the telluryl oxo group. The subsequent step of surface NH insertion into a π-allyl gas intermediate was also found to have a low activation energy barrier of 0.03 eV on the reduced Mo site.

Propane ammoxidation over Mo–V–Te–Nb–O M1 phase: Density functional theory study of propane oxidative dehydrogenation steps

Propane ammoxidation to acrylonitrile catalyzed by the bulk Mo–V–Te–Nb oxides has received considerable attention because it is more environmentally benign than the current process of propylene ammoxidation and relies on a more abundant feedstock. This process is proposed to consist of a series of elementary steps including propane oxidative dehydrogenation (ODH), ammonia and O2 activation, NHx insertion into C3 surface intermediates, etc. Density functional theory calculations were performed here to investigate the three sequential H abstraction steps that successively convert propane into isopropyl, propene, and π-allyl on cation sites in the proposed selective and active center present in the ab plane of the Mo–V–Te–Nb–O M1 phase. The initial H abstraction from propane was found to be the rate-limiting step of this process, consistent with both the proposed reaction mechanism for propane ammoxidation on the Mo–V–Te–Nb oxides and current understanding of V5+ as the active site for alkane activation on V-based oxides. Te=O was found to be significantly more active than V5+=O for the H abstraction from propane, which suggests that the surface and bulk Te species may be different. The role of Mo=O is most likely limited to being an H acceptor from isopropyl to form propene under ammoxidation conditions.

Combinatorial design and preparation of transition metal doped MoVTe catalysts for oxidation of propane to acrylic acid

In this study Mo–V–Te–Nb based multi-component catalysts were designed and tested using combinatorial and high-throughput methods. Based on the composition of the M1 matrix phase new compositions were designed containing additional promoters Mn, Ni, W and In. The following promoters were tested too in preliminary experiments and disregarded because of the detrimental effect on acrylic acid yields: Cu, Sb, Fe, Sm, Sn, Bi, Co, and Cr. In addition, citric acid in different ratios was used in the synthesis as a structure-directing agent. An optimization variable has been defined as the molar ratio of a given component to Mo in the synthesis mixture. Consequently, the experimental space had eight variables. The discrete levels of variables are established in such a way that the size of the multi-dimensional experimental space was in the range of 200000 theoretical experiments. Five generations were designed using an optimization platform consisting of artificial neural networks and holographic optimization algorithm. Altogether 215 catalysts were prepared and tested. The elite list in each generation was created according to the yield of acrylic acid (AA). The yield of AA over the best catalyst after five generations was 59%. On the basis of holographic maps correlations between the composition of the synthesis mixture and yield of AA were visualized. Two catalysts families amongst the good performing catalysts have been distinguished that differ from each other in their low and high V content as referenced to molybdenum.

A combined HAADF STEM and density functional theory study of tantalum and niobium locations in the Mo–V–Te–Ta(Nb)–O M1 phases

Abstract The M1 phases of the Mo–V–Te–Nb mixed-metal oxides are highly promising catalysts for direct propane (amm)oxidation. Nb is beneficial to the activity and selectivity for acrylonitrile, but direct evidence for Nb location is lacking because structural methods cannot distinguish Nb (Z = 41) from Mo (Z = 42). We have employed high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and density functional theory (DFT) to study the Ta (Z = 73) location in a Mo–V–Te–Ta–O M1 phase produced via hydrothermal synthesis, which possesses uniform Ta distribution compared to M1 phases previously synthesized via slurry evaporation. The HAADF STEM and DFT results indicate that Ta predominantly occupies the pentagonal bipyramidal site (S9). Moreover DFT results validate the previous hypothesis that Ta and Nb both prefer the S9 site.

Propane Ammoxidation Over the Mo–V–Te–Nb–O M1 Phase: Reactivity of Surface Cations in Hydrogen Abstraction Steps

Density functional theory calculations (GGA-PBE) have been performed to investigate the adsorption of C3 (propane, isopropyl, propene, and allyl) and H species on the proposed active center present in the surface ab planes of the bulk Mo–V–Te–Nb–O M1 phase in order to better understand the roles of the different surface cations in propane ammoxidation. Modified cluster models were employed to isolate the closely spaced V=O and Te=O from each other and to vary the oxidation state of the V cation. While propane and propene adsorb with nearly zero adsorption energy, the isopropyl and allyl radicals bind strongly to V=O and Te=O with adsorption energies, ΔE, being ≤−1.75 eV, but appreciably more weakly on other sites, such as Mo=O, bridging oxygen (Mo–O–V and Mo–O–Mo), and empty metal apical sites (ΔE > −1 eV). Atomic H binds more strongly to Te=O (ΔE ≤ −3 eV) than to all the other sites, including V=O (ΔE = −2.59 eV). The reduction of surface oxo groups by dissociated H and their removal as water are thermodynamically favorable except when both H atoms are bonded to the same Te=O. Consistent with the strong binding of H, Te=O is markedly more active at abstracting the methylene H from propane (Ea ≤ 1.01 eV) than V=O (Ea = 1.70 eV on V5+=O and 2.13 eV on V4+=O). The higher-than-observed activity and the loose binding of Te=O moieties to the mixed metal oxide lattice of M1 raise the question of whether active Te=O groups are in fact present in the surface ab planes of the M1 phase under propane ammoxidation conditions.

Reflux method as a novel route for the synthesis of MoVTeNbO x catalysts for selective oxidation of propane to acrylic acid

This work reports the applicability of reflux to the synthesis of Mo–V–Te–Nb–O metal oxide catalysts for the first time. The well-formed and defined orthorhombic M1 phase, i.e. Te 2 M 20O 57 ( M = Mo, V, Nb), was found to be better developed when a longer time was used to reflux the mixed metal clusters. The catalysts efficient for propane selective oxidation to acrylic acid, reaching ca. 60% of selectivity to acrylic acid at propane conversion of ca. 50% and at a reaction temperature of 693 K. In addition, a parallel between catalytic activity and catalyst reducibility is also concluded.

Performance of WO x-added Mo–V–Te–Nb–O catalysts in the partial oxidation of propane to acrylic acid

The performance of Mo 1.0V 0.3Te 0.23Nb 0.23O x catalyst in the partial oxidation of propane to acrylic acid was improved when the catalyst was modified with an optimal amount of tungsten oxide (WO x ), which was present largely in an amorphous phase. The added WO x increased the catalyst activity by promoting the dehydration of the oxygenate intermediates in the oxidation process, which eventually accelerated the production of propylene. An increase in the dehydration rates on the W-added catalysts was verified by separate experiments using 1- and 2-propanol as the reactants. The selectivity for the production of acrylic acid was also improved for the W-added catalysts because the dehydration step was promoted to a greater extent than the dehydrogenation step and the number of acidic sites in the catalysts was decreased. The reaction results obtained using the W-added catalysts can be explained based on the reaction paths involved in the propane-oxidation process and on the surface properties of the catalysts analyzed using X-ray diffraction (XRD), infrared (IR) spectroscopy, and the temperature-programmed desorption of ammonia (NH 3-TPD).

A new approach to achieving a pure M1 phase catalyst for the selective oxidation of propane

A pure M1 phase catalyst was originally obtained by treatment of the multi-phase Mo–V–Te–Nb–O catalyst in the reaction atmosphere and was systematically studied by some physico-chemical techniques. The results showed that the reaction treatment played an important role in improving the performance of the catalyst.

Performance of Pd-promoted Mo–V–Te–Nb–O catalysts in the partial oxidation of propane to acrylic acid

The performance of a Mo 1.0V 0.3Te 0.23Nb 0.23O x catalyst in the partial oxidation of propane to obtain acrylic acid (AA) was improved when the catalyst was modified with an optimal amount of Pd oxide. The activity was increased due to an increase in the number of sites for the adsorption of propane and for the conversion of the adsorbed propane to propylene via oxidative dehydrogenation, which is the rate-determining step (RDS) in the process. An increase in the number of acidic sites was also responsible for the activity improvement. The selectivity for AA production was increased due to the enhancement of the RDS, accompanied by the suppression of the side-reaction steps. Added Pd oxide reduced the amount of propane, which was strongly adsorbed on the catalyst, such that propane was rapidly desorbed from the surface without being over-oxidized to CO x . The suppression of AA oxidation also contributed to the improved selectivity. Both activity and selectivity of the catalyst were eventually lowered when the Pd content was greater than 10 −2 wt% because the catalyst surface was covered with an excess amount of Pd oxide.

Acrylic acid and electric power cogeneration in an SOFC reactor

A highly efficient catalyst, MoV(0.3)Te(0.17)Nb(0.12)O, used for acrylic acid (AA) production from propane, was used as an anodic catalyst in an SOFC reactor, from which AA and electric power were cogenerated at 400-450 degrees C.

Selective oxidation of ethane: Developing an orthorhombic phase in Mo–V–X (X = Nb, Sb, Te) mixed oxides

Mo–V–X (X = Nb, Sb and/or Te) mixed oxides have been prepared by hydrothermal synthesis and heat-treated in N 2 at 450 °C or 600 °C for 2 h. The calcination temperature and the presence or absence of Nb determines the nature of crystalline phases in the catalyst. Nb-containing catalysts heat-treated at 450 °C are mostly amorphous solids, while Nb-free catalysts heat-treated at 450 °C and samples treated at 600 °C clearly contain crystalline phases. TPR-H 2 experiments show higher H 2-consumption on catalysts with amorphous phases. Catalytic results in the oxidative dehydrogenation of ethane indicate that the selective production of the olefin is strongly related to the development of the orthorhombic Te 2M 20O 57 or (SbO) 2M 20O 56 (M = Mo, V, Nb) phase (the so-called M1 phase), which is mainly formed at 600 °C. This active and selective crystalline phase is characterized to show moderate reducibility and active centers enough for the selective oxidative activation of ethane with the minimum quantity possible of active centers for ethylene activation. In this sense, the best yield to ethylene has been achieved on a Mo–V–Te–Nb mixed oxide.

M1 to M2 Phase Transformation and Phase Cooperation in Bulk Mixed Metal Mo–V–M–O (M=Te, Nb) Catalysts for Selective Ammoxidation of Propane

In the present study we systematically explored hydrothermal synthesis of bulk mixed metal Mo–V–(Te–Nb)–O catalysts and investigated the bulk characteristics of the resulting M1 and M2 phases as well as their roles in the selective ammoxidation of propane. It was found that unlike Mo–V–Te–Nb–O M1 phases, the Mo–V–Te–O M1 phases may be quantitatively transformed into M2 phases of the same chemical composition indicating that Nb stabilizes the M1 structure. The stabilizing role of Nb and Te was further observed in high resolution TEM studies of Mo–V–(Te–Nb)–O catalysts which indicated that structural order and the M1 phase domain size progressively decreased in this order: Mo–V–Te–Nb–O > Mo–V–Te–O > Mo–V–O. The cooperation between the M1 and M2 phases in propane ammoxidation to acrylonitrile was observed only at low propane conversions suggesting that the M1 phase is the only crystalline phase required for the activity and selectivity of the Mo–V–Te–Nb–O catalysts in propane ammoxidation to acrylonitrile at practical propane conversions.

Effect of preparation and reaction condition on the catalytic performance of Mo–V–Te–Nb catalysts for selective oxidation of propane to acrylic acid by high-throughput methodology

Combinatorial screening technique has been applied to investigate the catalytic activity and selectivity of quaternary Mo–V–Te–Nb mixed oxide catalysts treated with various chemicals during preparation for selective oxidation of propane to acrylic acid. The catalyst libraries were prepared by the slurry method and catalytic activities were examined in 32-channel high-throughput screening reactor system coupled with a mass spectrometer and/or gas chromatograph. The obtained results provided substantial evidence that the sample preparation condition would have strong effect on the catalytic performance for propane selective oxidation. Among screened samples, Mo–V–Te–Nb treated with HIO 3 solution presented a better performance. The reaction results of promising catalysts selected from the libraries were applied to further scale-up of the system and confirmed catalytic performance. Quantification of the result of Mo–V–Te–Nb treated with HIO 3 solution was realized by combination of GC and MS and relationship between the MS data and the GC results can be established.

Characterization and evaluation of Mo [sbnd]V [sbnd]Te [sbnd]Nb mixed metal oxide catalysts fabricated via hydrothermal process with ultrasonic pretreatment for propane partial oxidation

Mo 1.00V x Te 0.20Nb 0.16O n ( x = 0.35 – 0.50 ) mixed-metal oxide catalysts were synthesized through ultrasonic and hydrothermal treatments. Both TeO 2 and H 6TeO 6 were used as tellurium sources. The enhanced dispersion of TeO 2 by ultrasonic treatment is crucial for obtaining an active and selective Mo V Te Nb O catalyst for acrylic acid (AA) formation from propane oxidation. The TeO 2-derived Mo 1.00V x Te 0.20Nb 0.16O n ( x = 0.35 – 0.41 ) are superior to their H 6TeO 6-derived counterparts; propane conversion and AA selectivity over the Mo 1.00V 0.41Te 0.20Nb 0.16O n is 55% and 60 mol% at 380 °C, respectively, giving an AA formation rate of 22.3 μmol g −1 min −1. Based on the physicochemical properties of the catalysts, we propose that the ultrasonic treatment can give rise to (i) enhanced presence of the orthorhombic Te 2 M 20 O 57 ( M = Mo , V, Nb) and hexagonal Te 0.33 MO 3.33 ( M = Mo , V, Nb) phases, (ii) surface enrichment of Te, (iii) enhanced reactivity of lattice oxygen, (iv) an increase in Mo O Te and V O Te entities, and (v) better isolation of active sites.