V.Yu. Bychkov

Physico-chemical properties of V-Sb-oxide systems and their catalytic behaviour in oxidative dehydrogenation of light paraffins

Catalysts prepared as bulk VSb0.1Ox and supported V2O5/Al2O3. V2O5-Sb2O3/Al2O3 and Sb2O3/Al2O3 (containing 0.5, 1 or 2 theoretical monolayers of V2O5 or Sb2O3) were tested in the oxidative dehydrogenation of iso-butane at 550 degrees C in i-C4H10:O-2 :He=20:10:70 gas mixture. Fresh and used catalysts were characterised by BET, XRD and XPS, Reactivity and thermochemistry of active oxygen taking part in the redox cycle with ethane and hydrogene were studied using in situ differential scanning calorimetry. Temperature-programmed desorption of O-2 in He Row was also investigated and in situ DRIFT was applied to investigate surface species of the catalysts in flows of i-C4H10, O-2 and i-C4H10/O-2 mixture. Supported VSbyOx catalysts are more active and selective than bulk one. V-only supported catalysts display a high efficiency due to the high reactivity of VOX-species. In bulk catalyst, the surface is enriched with antimony. In supported samples, the surfaces V/Sb are close to the calculated ones. In the presence of antimony, the amount of active oxygen species and their reactivity in redox transformation is improved. The rates of vanadium reduction and reoxidation are also higher. Compared to V-only catalysts, supported V-Sb-catalysts display a lower coking activity and higher on-stream stability

Thermochemistry and Reactivity of Lattice Oxygen in V–Sb Oxide Catalysts for the Oxidative Dehydrogenation of Light Paraffins

Differential scanning calorimetry is used to study in situthe properties of strongly bound (lattice) oxygen in vanadium-containing supported catalysts for the oxidative dehydrogenation of paraffins C2–C4. Evidence is found that the process occurs via a stepwise redox mechanism with the participation of lattice oxygen from the catalyst. When the supported component is modified by an antimony additive, the amount of reactive oxygen increases and redox processes accelerate. Simultaneously, the rate of coke formation decreases. Additional modification by Bi and Ba leads to a further increase in the amount of reactive oxygen. In all cases, oxygen bound to vanadium atoms is responsible for the redox properties of the systems. The observed effects are analyzed from the standpoint of the ratio between different forms of active oxygen.

Free radicals as intermediates in oxidative transformations of lower alkanes

Publisher Summary Experimental facts and theoretical concepts, existing in the literature, indicate that the formation of free radicals play an important role in a number of catalytic oxidation reactions. This chapter discusses several studies analyzing the contribution of free radicals to several oxidative transformations of lower alkanes over oxide catalysts. It has been showed that the formation of free radicals in the interaction of alkane molecules with the surface of oxides may prove to be energetically preferable, as compared to any other mechanisms of their activation. The fractions of radicals transformed into different final products depend on the reaction conditions (temperature and oxygen pressure) and on the number of carbon atoms in the alkane molecule. The fate of radicals captured by the surface sites with the formation of the alkoxy groups depends on the number of carbon atoms in the alkane molecule, as well as on the properties of the catalyst surface. The most energetically favorable process of lower alkanes activation over oxide catalysts is a homolytic C–H bond dissociation with the formation of free radicals. The difference in energy expenditures for the formation of free alkyl radicals causes the difference in reactivities between C1–C4 alkanes. The main factors determining the efficiency of different oxides, as catalysts for lower alkanes oxidation, are the H-atom affinity of strong oxidizing surface sites and the oxygen binding energy. These thermochemical factors cause the rates and directions of free-radical reactions and, as a result, the catalytic activity and selectivity to certain products.

Oxidative conversion of isobutane to isobutene over V-Sb-Ni oxide catalysts

Insertion of proper amounts of nickel oxide into alumina-supported V-Sb oxide catalyst for the oxidative dehydrogenation of isobutane substantially increases isobutane conversion (from 36 to 42-44%) at selectivity to isobutene similar to70%. Fresh and used catalysts (including reference bulk V-Sb-O and V-Sb-Ni-O systems) were characterised by BET, XRD, XPS and H-2-TPR. Formation of new phase of nickel vanadate NiV2O6 at the expense of free VOx-phase leads to more reducible catalyst with increased amounts of mobile lattice oxygen. The facile redox cycle of vanadium species is considered to improve the catalyst activity

The Mechanistic Study of Methane Reforming with Carbon Dioxide on Ni/α-Al2O3

The reactions of oxidized and reduced 6 wt % NiO/α-Al2O3 with H2, CH4, CO2, O2, and their mixtures are studied in flow and pulse regimes using a setup equipped with a differential scanning calorimeter DSC-111 and a system for chromatographic analysis. It is shown that treatment with hydrogen at 700°С results in the partial reduction of NiO to Ni. Methane practically does not react with oxidized Ni/α-Al2O3 but it does react actively with the reduced catalyst to form H2 and surface carbon. The latter is capable of reacting with lattice oxygen of Ni/α-Al2O3 (slowly) and with adsorbed oxygen (rapidly). Carbon dioxide also reacts with surface carbon to form CO (rapidly) and with metallic Ni to yield CO and NiO (slowly). Thus, the main route of methane reforming with carbon dioxide on Ni/α-Al2O3 is the dissociative adsorption of CH4 to form surface carbon and H2 and the reaction of this carbon with CO2 resulting in the formation of CO by the reverse Boudouard reaction. Side routes are the interaction of the products of methane chemisorption with catalyst oxygen and the dissociative adsorption of CO2 on metallic nickel. A competitive reaction of surface carbon with adsorbed oxygen results in a decrease in the CO2 conversion in methane reforming with carbon dioxide. Therefore, the presence of gaseous oxygen in the reacting mixture decelerates methane reforming (catalyst poisoning by oxygen).

The Mechanism of Methane Reforming with Carbon Dioxide: Comparison of Supported Pt and Ni (Co) Catalysts

The interaction of the catalyst 5.16 wt % Pt/α-Al2O3 with СН4, СО2, О2, and СН4 + СО2 pulses is studied using a setup involving the differential scanning calorimeter DSC–111 and a system for chromatographic analysis. Comparison of the results obtained with analogous data on Ni/Al2O3 and Co/Al2O3 suggests that methane activation occurs via a common pathway via dissociative chemisorption on the metal surface with the formation of Н2 and carbon on all the catalysts studied. Carbon dioxide activation on Pt/Al2O3 differs from its activation on Ni(Со)/Al2O3. It follows from the enthalpy of formation that carbon on Pt/Al2O3 is graphite-like in contrast to carbide carbon on Ni(Co)/Al2O3. This graphite carbon is more stable and less reactive.

Methane Reforming with Carbon Dioxide on the Co/α-Al2O3 Catalyst: The Formation, State, and Transformations of Surface Carbon

The interactions of oxidized and reduced Co/α-Al2O3 (4 wt % CoO) with H2, CH4, CO2, and O2 and their mixtures are studied in flow and pulse regimes using a setup involving a DSC-111 differential scanning calorimeter and a system for chromatographic analyses. It is shown that treatment with hydrogen at 700°C results in the partial reduction of cobalt oxide to Co. Methane poorly reacts with the oxidized catalyst but readily reacts with the reduced catalyst to form H2 and surface carbon. The initial surface carbon transforms into other forms, which block the cobalt surface to different extents and differ in the heats of reaction with CO2. Carbon dioxide may react with the surface carbon to form CO (rapidly) and with metallic Co to form CO and CoO (slowly). Thus, the main route of methane reforming with carbon dioxide on Co/α-Al2O3 is the dissociative adsorption of CH4 to form surface carbon and H2 and the reaction of surface carbon with CO2 to form CO via the reverse Boudouard reaction.

Formation of the Ni–CrOx/MgO and Ni/MgO Catalysts for Carbon Dioxide Reforming of Methane

Temperature-programmed reduction by hydrogen, temperature-programmed desorption of O2, local X-ray spectral analysis, and scanning electron microscopy are used to study redox processes occurring on the Ni–Cr2O3/MgO and Ni/MgO catalysts for carbon dioxide reforming of methane. The reduction of Ni/MgO leads to the formation of nickel clusters distributed over the surface of MgO. During the reduction of NiO–Cr2O3/MgO, chromates are transformed into chromites, and then nickel is formed by the reduction of spinel NiCr2O4. Reoxidation leads to the oxidized structures NiO, NiCr2O4, and NiCrO4.

Study of the mobility of lattice oxygen in complex oxide catalysts for partial oxidation of propylene to acrolein

The amount of oxygen in the lattice of solids that participates in the elementary stages of partial propylene oxidation is determined for two types of Co-Mo-Bi-Fe-Sb-K-O catalysts (I, II) differing in the method of introduction of antimony and potassium. Two independent methods are used: (1) on the basis of the yield of the oxygen-containing products of propylene oxidation by oxygen of the catalyst in a pulse regime and (2) with the use of Möessbauer spectroscopy. Coincidence of the results obtained by both methods indicates that the active oxygen of the catalyst lattice is formed during redox transformations of iron(III) molybdate entering the composition of the catalysts. Data on the reduction of the catalysts in a pulse regime at various temperatures, which were processed in the framework of the diffusion model, allowed the estimation of the rate constants for diffusion of the lattice oxygen. An increase in the mobility of the lattice oxygen in catalyst I, which is modified with a small amount of antimony as compared to catalyst II, results in an increase in the overall productivity of the sample and in a decrease in the selectivity of propylene oxidation to acrolein. This correlates with the increase in the total amount of the lattice oxygen participating in the process.

Redox properties and catalytic performance of complex oxides in oxidative coupling of methane

Abstract The redox properties of model Mn-containing mixed oxide are studied. It is shown that water evolution can determine the rate of oxide reduction. The kinetic model of the redox process taking into account surface interactions and diffusion of oxygen, hydrogen and hydroxy ions in the oxide lattice can describe successfully the experimental data. The influence of slow water evolution on catalytic performance of the oxides in oxidative coupling of methane is discussed.