Study of MoO3 (0 1 0) surface clusters by ab initio HF approaches

The clusters Mo₂O₁₁H₁₀, Mo₃O₁₆H₁₄ and Mo₇O₃₂H₂₂ of MoO₃ (0 1 0) surface were studied by using ab initio HF approaches. The geometry of each cluster was fully optimized at RHF or UHF/MINI/ECP-SBK level of theory. The bonding properties, electronic structures and orbital populations of structurally different oxygens were obtained. It was shown that there exist some differences in the bonding properties between Mo atoms and the different oxygens. The symmetrically bridging oxygens exhibit more ionic feature while the terminal oxygens are more covalent. Moreover, information on explaining the active sites for the insertion of oxygen into allylic species in the second step of the oxidation processes of propylene to acrolein could be inferred from the frontier orbital populations of MoO₃ surface cluster.     

Quantum-chemical description of the active sites for the selective oxidation of hydrocarbons

Different aspects of quantum chemical modelling of catalytic systems are discussed. Further, the electronic structure of various V_nO_m clusters representing the vanadium pentoxide (010) surface is studied and the reactivity of different surface oxygen sites with respect to adsorption of hydrogen as well as propylene is examined. Results of ab initio HF and DFT calculations are compared with those of the semiempirical INDO-type method. It is found that the different quantum chemical methods lead to the same qualitative results. Satisfactory convergence in cluster properties is achieved for clusters of ten vanadium atoms. The saturation of dangling bonds by hydrogen atoms does not influence the properties of the oxygen centers significantly. Among different oxygen centers the oxygens bridging two bare vanadium atoms are most negatively charged. Hydrogen binds to all inequivalent oxygen sites with the strongest binding occurring for oxygens bridging two vanadium atoms. INDO cluster studies for propylene adsorption/reaction on V₂O₅ (010) show that different approaches of the molecule to the surface yield different surface complexes which can then react to form different products.     

A comparative study of the partial oxidation of methane to formaldehyde on bulk and silica supported MoO3 and V2O5 catalysts

The mechanism of the partial oxidation of methane to formaldehyde with O₂ has been investigated on bulk and differently loaded silica supported (4–7 wt%) MoO₃ and (5–50 wt%) V₂O₅ catalysts at 600–650°C in a pulse reactor connected to a quadrupole mass spectrometer. The reaction rate and product distribution in the presence and in the absence of gas-phase O₂ have been evaluated. On bare SiO₂, low and medium loaded silica supported MoO₃ and V₂O₅ catalysts the reaction proceeds via a concerted mechanism involving the activation of gas-phase oxygen on the reduced sites of the catalyst surface as proved by the direct correlation between catalytic activity and density of reduced sites evaluated in steady-state conditions, while on highly loaded catalysts as well as on bulk MoO₃ and V₂O₅ the reaction rate drops dramatically and the reaction pathway via redox mechanism becomes predominant. The results indicate that the surface mechanism is essentially more effective than the redox mechanism enabling also a higher selectivity to HCHO.     

Density functional studies of the electronic structure and adsorption at molybdenum oxide surfaces

The electronic structure and bonding at different oxygen sites of MoO₃(0 1 0) and (1 0 0) surfaces is reviewed on the basis of ab initio density functional theory (DFT-LCGTO) cluster calculations. The clusters are chosen as finite sections of the ideal MoO₃ surface where cluster embedding is achieved by bond saturation with hydrogen terminator atoms yielding clusters up to Mo₇O₃₀H₁₈. Resulting charge density distributions and binding properties are analyzed by populations, bond orders, and electrostatic potential maps. Interatomic binding at the surface is determined by both ionic and covalent contributions with a clear distinction between terminal oxygens and different bridging surface oxygens. Electronic differences between the MoO₃ (0 1 0) and (1 0 0) surfaces are found to be mainly due to the different atom arrangement while local atom charging and binding properties seem surface independent. The electronic surface parameters influence the behavior and reactions of adsorbed molecules as will be shown for H, OH, and C₃H₅ adsorbates.     

Understanding the atomic force microscopy image of the V2O5 and Li0.03V2O5(0 0 1) surface using ab initio calculations

Single crystals of V₂O₅ and Li_0.03V₂O₅ were imaged in ambient conditions by atomic force microscopy (AFM). Atomic-scale resolution images are compared with total electron-density plots of the surface calculated using the ab initio HartreeFock method. The calculated oxygen charge at the V₂O₅(0 1 0) surface suggests an increased local reactivity of the bridging oxygens with respect to electrophilic attacks by adsorbate molecules. The intercalation of lithium has no consequence on the reactivity of the surface. This is supported by results from electrostatic potentials calculated from the cluster charge distributions.     

Structural and optical properties of MoO3 and V2O5 thin films prepared by Spray Pyrolysis

MoO₃ and V₂O₅ thin films were prepared on glass substrates by Spray Pyrolysis technique at a substrate temperature of 423 K. The precursor solutions were obtained by varying the concentrations of MoCl₅ and VCl₃ in bi-distilled water. The structural investigation conducted by X-ray diffraction showed that MoO₃ and V₂O₅ thin films were polycrystalline with orthorhombic structure. The optical properties studied in the 300–2500 nm range suggest that the thin film behaviours are related to bound electronic states. The optical band gaps have been estimated from slopes of ln(hν) versus hν plots of MoO₃ and V₂O₅ films were 3.35 and 2.44 eV, respectively. The electrical conductivity was measured using four probes method.     

Catalytic performance of a novel ceramic-supported vanadium oxide catalyst for NO reduction with NH3

A novel TiO₂/Al₂O₃/cordierite honeycomb-supported V₂O₅–MoO₃–WO₃ monolithic catalyst was studied for the selective reduction of NO with NH₃. The effects of reaction temperature, space velocity, NH₃/NO ratio and oxygen content on SCR activity were evaluated. Two other V₂O₅–MoO₃–WO₃ monolithic catalysts supported on Al₂O₃/cordierite honeycomb or TiO₂/cordierite honeycomb support, two types of pellet catalysts supported on TiO₂/Al₂O₃ or Al₂O₃, as well as three types of pellet catalysts V₂O₅–MoO₃–WO₃–Al₂O₃ and V₂O₅–MoO₃–WO₃–TiO₂ were tested for comparison. The experiment results show that this catalyst has a higher catalytic activity for SCR with comparison to others. The results of characterization show, the preparation method of this catalyst can give rise to a higher BET surface area and pore volume, which is strongly related with the highly active performance of this catalyst. At the same time, the function of the combined carrier of TiO₂/Al₂O₃ cannot be excluded.     

Synergistic catalysis of carbon black oxidation by Pt with MoO3 or V2O5

A series of SiO₂-supported MoO₃, V₂O₅, and Pt catalysts were prepared for the study of model soot oxidation with simulated diesel exhaust gas. Composite samples of Pt with the metal oxides demonstrated higher oxidation activities than the single-component SiO₂-supported MoO₃, V₂O₅ or Pt catalysts in the absence of SO₂ in the reactant gas. Based on the effects of NO₂ on carbon oxidation, a synergistic reaction mechanism was suggested to explain the effects of combining Pt with the oxides: Pt catalyzes the oxidation of NO with gas phase O₂ to NO₂, while MoO₃ and V₂O₅ catalyze the oxidation of carbon with NO₂. Finally, the effects of SO₂ on the carbon oxidation reaction were examined and discussed.     

Photocatalytic degradation of 4-nitrophenol in aqueous suspension by using polycrystalline TiO2 samples impregnated with Cu(II)-phthalocyanine

A number of supported metal oxide catalysts were screened for their catalytic performance for the oxidation of carbon black (CB; a model diesel soot) using NO₂ as the main oxidant. It was found that contact between the carbon and catalyst was a key factor in determining the rate of oxidation by NO₂. Oxides with low melting points, such as Re₂O₇, MoO₃ and V₂O₅ showed higher activities than did Fe₃O₄ and Co₃O₄. The activities of MoO₃ and V₂O₅ on various supporting materials were also examined. MoO₃/SiO₂ was the most active catalyst among the supported MoO₃ examined, whereas, V₂O₅/MCM-41 showed the highest activity among the supported V₂O₅. Different performances of the supported MoO₃ catalysts were explained by the interaction of MoO₃ with the supports: a strong MoO₃/support interaction may result in a poor mobility of MoO₃ and a poor activity for oxidation of carbon by NO₂. The high activity of V₂O₅/MCM-41 was associated with its catalysis of the oxidation of SO₂ by NO₂ to form SO₃, which substantially promotes oxidation of carbon by NO₂. Addition of transition metal oxides or sulfates to supported MoO₃ and V₂O₅ was also investigated. Combining MoO₃ or V₂O₅ with CuO on SiO₂, adding VOSO₄ to MoO₃/SiO₂ or MoO₃/Al₂O₃ and adding TiOSO₄ or CuSO₄ to V₂O₅/Al₂O₃ improved the catalytic performance.     

Reactivity of V2O5&z.sbnd;WO3TiO2 de-NOx catalysts by transient methods

The reactivity of ternary V₂O₅&z.sbnd;WO₃TiO₂ de-NO_xing catalysts with compositions similar to those of commercial catalysts (WO₃ ca. 9% w/w, V₂O₅ < 2% w/w) is investigated by transient techniques (temperature programmed desorption, TPD; temperature programmed surface reaction, TPSR; and temperature programmed reaction, TPR). The results indicate that the reactivity of the ternary catalysts in the SCR reaction increases on increasing the vanadia loading, and that the ternary catalysts are more active than the corresponding binary vanadia-titania samples with the same V₂O₅ loading. Indeed the SCR reaction is monitored at lower temperatures and high NO conversions are also preserved at high temperatures. TPSR and TPR data show that at low temperatures the SCR reaction occurs via a redox mechanism that involves at first the participation of the catalyst lattice oxygen and then the reoxidation of the reduced sites by gas-phase oxygen. Based on TPSR and TPR data, the higher reactivity of the ternary catalysts has been related to their superior redox properties, in line with previous chemico-physical characterisation studies. The catalyst redox properties thus appear as a key-factor in controlling the reactivity of V₂O₅&z.sbnd;WO₃TiO₂ de-NO_xing catalysts at low temperatures. The results also show that at high temperatures the surface acidity plays an important role in the adsorption and activation of ammonia.     

Inhibition effect of H2O on V2O5/AC catalyst for catalytic reduction of NO with NH3 at low temperature

The inhibition effect of H₂O on V₂O₅/AC catalyst for NO reduction with NH₃ is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H₂O does not reduce NO and NH₃ adsorption on V₂O₅/AC catalyst surface, but promotes NH₃ adsorption due to increases in Brønsted acid sites. Many kinds of NH₃ forms present on the catalyst surface, but only NH₄⁺ on Brønsted acid sites and a small portion of NH₃ on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH₃ on Lewis acid sites does not react with NO, regardless the presence of H₂O in the feed gas. H₂O inhibits the SCR reaction between the NH₃ on the Lewis acid sites and NO, and the inhibition effect increases with increasing H₂O content. The inhibition effect is reversible and H₂O does not poison the V₂O₅/AC catalyst.     

Factors controlling the selectivity of V2O5 supported catalysts in the oxidative dehydrogenation of propane

The surface properties of a series of V₂O₅ catalysts supported on different oxides (Al₂O₃, H–Na/Y zeolite, MgO, SiO₂, TiO₂ and ZrO₂) were investigated by transmission electron microscopy and FTIR spectroscopy augmented by CO and NH₃ adsorption. In the case of the V₂O₅/SiO₂ system TEM images evidenced the presence of V₂O₅ crystallites, whereas such segregated phase was not observed for the other samples. VO_x species resulted widely spread on the surface of Al₂O₃, H–Na/Y zeolite, MgO and SiO₂, whereas on TiO₂ and ZrO₂ they are assembled in a layer covering almost completely the support. Furthermore, evidences for the presence in this layer of V–OH Brønsted acid sites close to the active centres were found. It is proposed that propene molecules primarily produced by oxydehydrogenation of propane can be adsorbed on this acid centres and then undergo an overoxidation by reaction with redox centres in the neighbourhood. This features could account for the low selectivity of V₂O₅/TiO₂ and V₂O₅/ZrO₂ catalysts.     

Effects of microwaves on selective oxidation of toluene to benzoic acid over a V2O5/TiO2 system

Two series of catalysts, V₂O₅/TiO₂ and modified V₂O₅/TiO₂, were prepared with a conventional impregnation method. They were tested in the selective oxidation of toluene to benzoic acid under microwave irradiation. The reaction conditions were optimized over V₂O₅/TiO₂. It was found that in the microwave catalytic process the optimum reactor bed temperature of the titled reaction decreases to 500 K (600 K in the conventional process). The modification of V₂O₅/TiO₂ with MoO₃, WO₃, Nb₂O₅ or Ta₂O₅, which has no negative influence on the reaction in the conventional catalytic process, can greatly promote the catalytic activities in the microwave process, leading to a high yield of benzoic acid (41%). The effects of microwave electromagnetic field on the catalysts are discussed.     

Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils and urea solution: Part II. Characterization study of the effect of alkali and alkaline earth metals

A characterization study on a practice-oriented V₂O₅/WO₃–TiO₂ SCR catalyst deactivated by Ca and K, respectively, was carried out using NH₃-TPD, DRIFT spectroscopy, and XPS as well as theoretical DFT calculations. It was found from NH₃-TPD experiments that strongly basic elements like K or Ca drastically affect the acidity of the catalysts. Detailed DRIFT spectroscopy experiments revealed that these poisoning agents mostly interact with the Brønsted acid sites of the V₂O₅ active phase, thus affecting the NH₃ adsorption. Moreover, these experiments also indicated that the V⁵⁺ = O sites are much less reactive on the poisoned catalysts. XPS investigations of the O 1s binding energies showed that the oxygen atoms of the V⁵⁺ = O sites are affected by the presence of the poisoning agents. Based on these results and on DFT calculations with model clusters of the vanadia surface, the poisoning mechanism is explained by the stabilization of the non atomic holes of the (0 1 0) V₂O₅ phase as a result of the deactivation element. Consequently, V–OH Brønsted acid sites and V⁵⁺ = O sites are inhibited, which are both of crucial importance in the SCR process. The deactivation model also gives an explanation to the very low concentrations of potassium needed to deactivate the SCR catalyst, since one metal atom sitting on such a non-atomic hole site deactivates up to four active vanadium centers.     

Impact of surface mobility in selective oxidation. Isotopic exchange of O2 with O2 on various oxides: MoO3, SnO2 and Sb2O4. Effect of a reducer gas

Isotopic oxygen exchange experiments were carried out on simple oxides (Sb₂O₄, MoO₃ and SnO₂) and mechanical mixtures of MoO₃ or SnO₂(acceptor phase) with Rh//Al₂O₃ or Sb₂O₄ (donor phase). With Rh//Al₂O₃ a synergy effect is found: the amount of oxygen exchanged on the mixture is higher than the sum of oxygen exchanged on the two separate phases. This is a prove of oxygen mobility between donor and acceptor phases. With Sb₂O₄ this effect is less marked suggesting that the presence of a reducer gas is required. In presence of hydrocarbon, no isotopic exchange is observed: only total oxidation occurs. The presence of Sb₂O₄ decreases the rate of total oxidation by blocking, by oxygen spillover, the most active sites for oxidation.     

Promoting effect of calcium on reduction of NO by C3H6 over V2O5/ZrO2 catalysts

The reduction of nitrogen monoxide by propene on V₂O₅/ZrO₂ doped with or without calcium has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Considerable promoting effect of calcium doping on the reduction of nitrogen monoxide by propene was observed on the V₂O₅/ZrO₂ catalysts. For the reaction of a mixture of NO+C₃H₆, carbonyl and carboxylate species were observed above 373 K, although nitrate species formed at room temperature on V₂O₅/ZrO₂ doped with calcium. No bands due to a compound including both carbon and nitrogen atoms were observed. Thus, the redox mechanism, i.e. propene reduces the catalyst and nitrogen monoxide oxidizes the catalyst, is confirmed on V₂O₅/ZrO₂ catalysts doped with or without calcium. The analysis of the V=O band in the region of 1100–900 cm⁻¹ indicates that this promotion is mainly due to new V=O species formed by the addition of calcium onto the catalyst. This species is easily reproduced in comparison with the other V=O species on the surface in the reoxidation process of the catalyst.     

Reactivity of V2O5-WO3/TiO2 catalysts in the selective catalytic reduction of nitric oxide by ammonia

The physico-chemical characteristics and the reactivity of sub-monolayer V₂O₅-WO₃/TiO₂ deNO_x catalysts is investigated in this work by EPR, FT-IR and reactivity tests under transient conditions. EPR indicates that tetravalent vanadium ions both in magnetically isolated form and in clustered, magnetically interacting form are present over the TiO₂ surface. The presence of tungsten oxide stabilizes the surface V^IV and modifies the redox properties of V₂O₅/TiO₂ samples. Ammonia adsorbs on the catalysts surface in the form of molecularly coordinated species and of ammonium ions. Upon heating, activation of ammonia via an amide species is apparent. V₂O₅-WO₃/TiO₂ catalysts exhibits higher activity than the binary V₂O₅/TiO₂ and WO₃/TiO₂ reference sample. This is related to both higher redox properties and higher surface acidity of the ternary catalysts. Results suggest that the catalyst redox properties control the reactivity of the samples at low temperatures whereas the surface acidity plays an important role in the adsorption and activation of ammonia at high temperatures.     

SO2 resistant antimony promoted V2O5/TiO2 catalyst for NH3-SCR of NOx at low temperatures

To get the low temperature sulfur resistant V₂O₅/TiO₂ catalysts quantum chemical calculation study was carried out. After selecting suitable promoters (Se, Sb, Cu, S, B, Bi, Pb and P), respective metal promoted V₂O₅/TiO₂ catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD) and Brunner Emmett Teller surface area (BET-SA). Se, Sb, Cu, S promoted V₂O₅/TiO₂ catalysts showed high catalytic activity for NH₃ selective catalytic reduction (NH₃-SCR) of NO_x carried at temperatures between 150 and 400 °C. The conversion efficiency followed in the order of Se > Sb > S > V₂O₅/TiO₂ > Cu but Se was excluded because of its high vapor pressure. An optimal 2 wt% ‘Sb’ loading was found over V₂O₅/TiO₂ for maximum NO_x conversion, which also showed high resistance to SO₂ in presence of water when compared to other metal promoters. In situ electrical conductivity measurement was carried out for Sb(2%)/V₂O₅/TiO₂ and compared with commercial W(10%)V₂O₅/TiO₂ catalyst. High electrical conductivity difference (ΔG) for Sb(2%)/V₂O₅/TiO₂ catalyst with temperature was observed. SO₂ deactivation experiments were carried out for Sb(2%)/V₂O₅/TiO₂ and W(10%)/V₂O₅/TiO₂ at a temperature of 230 °C for 90 h, resulted Sb(2%)/V₂O₅/TiO₂ was efficient catalyst. BET-SA, X-ray photoelectron spectroscopy (XPS) and carbon, hydrogen, nitrogen and sulfur (CHNS) elemental analysis of spent catalysts well proved the presence of high ammonium sulfate salts over W(10%)/V₂O₅/TiO₂ than Sb(2%)/V₂O₅/TiO₂ catalyst.     

Effect of V2O5 on the catalytic activity of Pt-based diesel oxidation catalyst

We investigated the suppression of SO₂ oxidation activity by vanadium oxide in Pt-based diesel oxidation catalyst using reaction experiments, temperature programmed desorption (TPD), infrared (IR) and X-ray photoelectron spectroscopy (XPS). There was no interaction between Pt and S indicated by the XPS results. SO₂ was not adsorbed on Pt at room temperature indicated by the absence of peak arising from SO₂ in SO₂ TPD spectra. SO₂ molecules were adsorbed on the hydroxyl groups of TiO₂ and migrated to Pt particles to react with oxygen adsorbed on it. V₂O₅ decreased the adsorption of SO₂ on TiO₂ by the blockage of V₂O₅ on TiO₂.