New technology for selective catalytic oxidation of ammonia to nitrogen

The selective catalytic oxidation (SCO) of ammonia to N₂ was studied by using a series of noble metal-V₂O₅-WO₃ catalysts supported on titania-silica (TS) prepared by coprecipitation method. In the V₂O₅-WO₃ catalyst system, the use of TS as a support was very effective to enhance catalytic activity compared with TiO₂ or SiO₂ alone. The addition of a slight amount of Pd and Ir to V₂O₅-WO₃/TS catalyst caused also remarkable enhancement of the catalytic activity without decreasing the selectivity to N₂. The present catalysts provide remarkably high catalytic performance for SCO of ammonia to N₂ under the practical reaction conditions for an industrial application.     

NO-NH3-O2 reaction catalyzed by V2O5/AC at low temperatures

The effects of SO₂, V₂O₅ loading and reaction temperature on the activity of activated carbon supported vanadium oxide catalyst have been studied for the reduction of NO with NH₃ at low temperatures (150—250°C). It is found that SO₂ significantly promotes the catalyst activity. Both V₂O₅ loading and reaction temperature are vital to the promoting effect of SO₂. The catalysts with V₂O₅ loadings of 1—5 weight percent have a positive effect on the promotion of SO₂, while the catalysts with V₂O₅ loadings of above 7 weight percent have not such an effect or show a negative effect. At lower temperatures (<180°C) SO₂ poisons the catalyst but at higher temperatures promotes it. The reason of the SO₂ promotion was also discussed; it may results from the formation of SO₄ ^2? on the catalyst surface, which increases the surface acidity and hence the catalytic activity.     

Relationship between sulfur dioxide oxidation and selective catalytic NO reduction by ammonia on V2O5?TiO2 catalysts doped with WO3 and Nb2O5

The performance of V₂O₅–TiO₂ catalysts doped by WO₃ and Nb₂O₅ in sulfur dioxide oxidation, and in selective catalytic reduction (SCR) of NO by ammonia has been studied. Addition of tungsten and niobium oxides was found to suppresses sulfur dioxide oxidation thus increasing the catalysts resistance to SO₂ poisoning and their activity in SCR.     

Study of Fe-doped V<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript> catalyst for an enhanced NH<Subscript>3</Subscript>-SCR in diesel exhaust aftertreatment

Selective catalytic reduction (SCR) with ammonia has been considered as the most promising technology, as its effect deals with the NO_X. Novel Fe-doped V₂O₅/TiO₂ catalysts were prepared by sol–gel and impregnation methods. The effects of iron content and reaction temperature on the catalyst SCR reaction activity were explored by a test device, the results of which revealed that catalysts could exhibit the best catalytic activity when the iron mass ratio was 0.05%. It further proved that the VTiFe (0.05%) catalyst performed the best in denitration and its NO_X conversion reached 99.5% at 270 °C. The outcome of experimental procedures: Brunauer–Emmett–Teller surface area, X-ray powder diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction and adsorption (H₂-TPR, NH₃-TPD) techniques showed that the iron existed in the form of Fe³⁺ and Fe²⁺ and the superior catalytic performance was attributed to the highly dispersed active species, lots of surface acid sites and absorbed oxygen. The modified Fe-doped catalysts do not only have terrific SCR activities, but also a rather broad range of active temperature which also enhances the resistance to SO₂ and H₂O.     

NO-NH3-O2 reaction catalyzed by V2O5/AC at low temperatures —Effects of SO2, V2O5 loading and reaction temperature

The effects of SO₂, V₂O₅ loading and reaction temperature on the activity of activated carbon supported vanadium oxide catalyst have been studied for the reduction of NO with NH₃ at low temperatures (150—250°C). It is found that SO₂ significantly promotes the catalyst activity. Both V₂O₅ loading and reaction temperature are vital to the promoting effect of SO₂. The catalysts with V₂O₅ loadings of 1—5 weight percent have a positive effect on the promotion of SO₂, while the catalysts with V₂O₅ loadings of above 7 weight percent have not such an effect or show a negative effect. At lower temperatures (<180°C) SO₂ poisons the catalyst but at higher temperatures promotes it. The reason of the SO₂ promotion was also discussed; it may results from the formation of SO₄ ²⁻ on the catalyst surface, which increases the surface acidity and hence the catalytic activity.     

High catalytic activity of V2O5-ZrO2 modified with H2SO4 for propene partial oxidation

For V₂O₅–ZrO₂ catalysts, up to 10 mol% the crystalline structure of V₂O₅ was not observed, indicating a good dispersity the surface of ZrO₂. V₂O₅–ZrO₂ catalyst modified with H₂SO₄ exhibited much on higher catalytic activity for propene partial oxidation than unmodified catalysts due to the increased acidity and acid strength of modified catalyst.     

Kinetic Peculiarities in the Low-Temperature Oxidation of H2S on Vanadium Catalysts

H₂S oxidation by oxygen on catalysts V₂O₅/Al₂O₃, V₂O₅/TiO₂, V₂O₅/Al₂O₃/TiO₂ was studied at temperatures below the sulfur dew point. High activity and the oscillation character of the oxidation were demonstrated by catalysts with low contents of V₂O₅ (3–5 wt.%). The increase in the V₂O₅ concentration to 10–20 wt.% results in the reduction of the catalytic activity and oscillation ability. On a pure V₂O₅ catalyst, the oscillations were not detected. The difference between the catalysts with the high and low concentrations of V₂O₅ is explained in terms of the structures of the V⁵⁺ species formed in the catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Catalytic properties of oxides in vapor phase oxidation of acetone

The temperature dependences of the rate of oxidation of acetone vapors by oxygen on oxide surface catalysts: V₂O₅, Co₃O₄., MoO₃ and TiO₂ (rutile), and the industrial catalyst VKSh were determined. A series of the catalytic activity of the above surface catalysts was established. The relationship between the catalytic properties of the oxides in the oxidation of acetone and their redox and acid-base characteristics was analyzed. The catalytic activity of the oxides in the oxidation reaction of acetone and methanol were compared.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 114–117, January–February, 1988.     

Palladium Encapsulated by an Oxygen-Saturated TiO2 Overlayer for Low-Temperature SO2-Tolerant Catalysis during CO Oxidation

The development of oxidation catalysts that are resistant to sulfur poisoning is crucial for extending the lifespan of catalysts in real-working conditions. Herein, we describe the design and synthesis of oxide-metal interaction (OMI) catalyst under oxidative atmospheres. By using organic coated TiO₂, an oxide/metal inverse catalyst with non-classical oxygen-saturated TiO₂ overlayers were obtained at relatively low temperature. These catalysts were found to incorporate ultra-small Pd metal and support particles with exceptional reactivity and stability for CO oxidation (under 21 vol % O₂ and 10 vol % H₂O). In particular, the core (Pd)-shell (TiO₂) structured OMI catalyst exhibited excellent resistance to SO₂ poisoning, yielding robust CO oxidation performance at 120 °C for 240 h (at 100 ppm SO₂ and 10 vol % H₂O). The stability of this new OMI catalyst was explained through density functional theory (DFT) calculations that interfacial oxygen atoms at Pd−O−Ti sites (of oxygen-saturated overlayers) serve as non-metal active sites for low-temperature CO oxidation, and change the SO₂ adsorption from metal(d)-to-SO₂(π*) back-bonding to much weaker σ(Ti−S) bonding.     

Vapour phase synthesis of isobutyraldehyde from methanol and ethanol over mixed oxide supported vanadium oxide catalysts

The vapour phase synthesis of isobutyraldehyde from methanol and ethanol in one step was investigated over titania-silica, titania-alumina, titania-zirconia, titania-silica-zirconia, and magnesia supported vanadium oxide catalysts at 623 K and under normal atmospheric pressure. Among various catalysts the titania-silica binary oxide supported vanadia provided higher yields than the other single or mixed oxide supported catalysts. The high conversion and product selectivity of V₂O₅/TiO₂-SiO₂ catalyst (20 wt% V₂O₅) was related to the better dispersion of vanadium oxide over titania-silica mixed oxide support in addition to other acid-base and redox characteristics. A reaction path for the formation of isobutyraldehyde from methanol and ethanol mixtures over these catalysts was described.     

The Role of Lewis and Brønsted Acid Sites in NO Reduction with NH<Subscript>3</Subscript> on Sulfur Modified TiO<Subscript>2</Subscript>-Supported V<Subscript>2</Subscript>O<Subscript>5</Subscript> Catalyst

V₂O₅/S-doped TiO₂ was prepared by the sol-gel and impregnation methods. The adsorption of NO, NH₃, and O₂ over the catalyst was studied by in situ DRIFTS spectroscopy to elucidate the reaction mechanism of the low-temperature selective catalytic reduction of NO with NH₃. Exposing the catalyst to O₂ and NO, three types of nitrates species appeared on the surface. The introduction of S to TiO₂ could generate large amounts of acid sites for ammonia adsorption on the catalyst, which was believed to be an important role in the SCR reaction and hereby improved the catalytic activity. The results indicated two possible SCR reaction pathways for catalyst. One was that NO was absorbed to form nitrite species, which could react with NH₃ on Lewis acid sites, producing N₂ and H₂O. Another way was that NH₃ was adsorbed, then reacted with gas phase NO (E–R) and nitrite intermediates on the surface (L–H).     

Improving the denitration performance and K-poisoning resistance of the V2O5-WO3/TiO2 catalyst by Ce4+ and Zr4+ co-doping

A series of V₂O₅-WO₃/TiO₂-ZrO₂, V₂O₅-WO₃/TiO₂-CeO₂, and V₂O₅-WO₃/TiO₂-CeO₂-ZrO₂ catalysts were synthesized to improve the selective catalytic reduction (SCR) performance and the K-poisoning resistance of a V₂O₅-WO₃/TiO₂ catalyst. The physicochemical properties were investigated by using XRD, BET, NH₃-TPD, H₂-TPR, and XPS, and the catalytic performance and K-poisoning resistance were evaluated via a NH₃-SCR model reaction. Ce⁴⁺ and Zr⁴⁺ co-doping were found to enhance the conversion of NO_x, and exhibit the best K-poisoning resistance owing to the largest BET-specific surface area, pore volume, and total acid site concentration, as well as the minimal effects on the surface acidity and redox ability from K poisoning. The V₂O₅-WO₃/TiO₂-CeO₂-ZrO₂ catalyst also presents outstanding H₂O + SO₂ tolerance. Finally, the in situ DRIFTS reveals that the NH₃-SCR reaction over the V₂O₅-WO₃/TiO₂-CeO₂-ZrO₂ catalyst follows an L-H mechanism, and that K poisoning does not change the reaction mechanism.     

Pd–Fe/TiO2 catalysts for phenol degradation with in situ generated H2O2

This study deals with the degradation of phenol over Pd–Fe/TiO₂ catalysts at mild conditions in the presence of in situ generated H₂O₂ from oxygen and formic acid. This catalytic system demonstrated interesting ability to oxidize phenol by Fenton process in a one-pot reaction without the addition of ferrous ion. Lower Pd content catalysts, despite producing a higher hydrogen peroxide amount for bulk purposes, did not reach the same efficiency as the 5Pd–5Fe catalyst in phenol degradation. A close interaction between Pd and iron oxide species is necessary to obtain high active catalysts. These results highlight the advantage of in situ generation of H₂O₂, for oxidation reactions with respect to conventional Fenton process.     

Vanadium oxide-porous phosphate heterostructure catalysts for the selective oxidation of H2S to sulphur

Vanadium oxide–containing mesoporous materials, based on a surfactant expanded zirconium phosphate with silica galleries into the interlayer space, named porous phosphate heterostructure (PPH), were prepared by using TEOS and vanadium oxytripropoxide in n-propanol as sources of Si and V, respectively; with different Si/V molar ratios of 1, 2, 5 and 25; and calcining at 550 °C for 6 h. Using this method, vanadium can be partially incorporated to the structure of the gallery, but the surface area strongly decreases and the appearance of V₂O₅ crystallites increases when increasing the vanadium content. The catalysts were characterized by XRD, XPS, TEM, and Raman, and tested in the selective catalytic oxidation of H₂S using a fixed bed reactor, at atmospheric pressure, at 180–260 °C. The catalysts with high contents of vanadium are very active at 200 °C, showing H₂S conversions of 85–99%, with a high selectivity to elemental sulphur and with a low formation of SO₂. Accordingly V₂O₅ crystallites can be proposed as active and selective although the catalytic behavior is related to the number of accessible V-sites in the surface of the catalyst.     

Active component of supported vanadium catalysts in the selective oxidation of methanol

The structure of catalysts based on vanadium oxide supported on different oxides (SiO₂, γ-Al₂O₃, ZrO₂, and TiO₂) was investigated. Their catalytic properties in the selective oxidation of methanol in a temperature range of 100–250°C were studied. It was shown that the nature of the support determines the structure of the oxide forms of vanadium. The supporting of vanadium on SiO₂ and γ-Al₂O₃ leads to the preferred formation of crystalline V₂O₅; the surface monomeric and polymeric forms of VO_x are additionally formed on ZrO₂ and TiO₂. It was established that the crystalline V₂O₅ oxide is least active in the selective oxidation of methanol; the polymeric forms are more active than monomeric ones. The mechanism of the selective oxidation of methanol to dimethoxymethane and methyl formate on the vanadium oxide catalysts is considered.     

Role of vanadium species in the selective oxidation of ethanol on V2O5/TiO2 catalysts

The catalytic properties of a sample of 20% V₂O₅/TiO₂ and its derivative, 12% V₂O₅/TiO₂, which was obtained by the treatment of the catalyst with nitric acid and did not contain bulk V₂O₅ species, were compared. In spite of a significant difference in the vanadium contents, the activity of both of the samples in the process of the gas-phase aerobic oxidation of ethanol to acetaldehyde and acetic acid was found to be the same. It was hypothesized that a monolayer of vanadium oxide on the surface of TiO₂ made the main contribution to the catalytic activity.     

Selective catalytic oxidation of ammonia into nitrogen and water vapour over transition metals modified Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, TiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript>

Copper or iron supported on commercially available oxides, such as γ-Al₂O₃, TiO₂ (anatase) and monoclinic tetragonal ZrO₂ (mt-ZrO₂) were tested as catalysts for selective catalytic oxidation of ammonia into nitrogen and water vapour (NH₃-SCO) in the low temperature range. Different commercial oxides were used in this study to determine the influence of the specific surface area, acidic nature of the support and crystalline phases as well as of the type of species and aggregation state of transition metals on the catalytic performance in selective ammonia oxidation. Copper modified oxide supports were found to be more active and selective to nitrogen than catalysts impregnated with iron. Activities of both transition metal modified samples decreased in the following order: mt-ZrO₂, TiO₂ (anatase), γ-Al₂O₃. Quantitative total ammonia conversion was achieved with the Cu/ZrO₂ catalytic system at 400°C. Characterisation techniques, e.g. H₂-temperature programmed reduction, UV-VIS-diffuse reflectance spectroscopy, suggest that easily reducible copper oxide species are important in achieving high catalytic performances at low temperatures.     

Ultra‐deep desulfurization of a model fuel using novel VOHPO4 0.5H2O/boehmite catalysts

A high‐surface‐area boehmite was used as the support for a series of vanadium phosphate catalysts. The catalysts were prepared by heating of V₂O₅ in an isobutyl alcohol and benzyl alcohol mixture at 140°C for 5 h to reduce V⁵⁺ to more active V⁴⁺ in the presence of phosphoric acid. Then a series of catalysts with various VPO loadings on boehmite were synthesized. The catalysts were characterized using various techniques. The catalysts were utilized for extraction combined with catalytic oxidation of dibenzothiophene. The important factors influencing the desulfurization process, including reaction time, temperature, H₂O₂, catalyst loading, catalyst amount and solvents, were systematically investigated. Under the optimized reaction conditions, i.e. 30 mg of catalyst loading at 50°C and in 60 min, sulfur removal reached 94%. The catalyst was recycled and reused five times.     

Synthesis, characterization and activity in cyclohexene epoxidation of V2O5–TiO2 anatase xerogel

The system of V₂O₅?CTiO₂ catalysts with V₂O₅ contents from 5 to 20 wt% were prepared by the sol?Cgel route and calcined at 500?°C. The mixed oxide series presented the crystalline structure of TiO₂ anatase phase. BET analysis showed a medium surface area decreasing from 73 to 19?m² g^?1 when V₂O₅ content rose from 5 to 20 wt%. The results of pyridine adsorption followed by FT-IR indicate that the catalysts display identical surface acid densities, independently of the V₂O₅ content, and both Br?nsted and Lewis acid sites are present on their surfaces. The V₂O₅ system presents an activity and selectivity during the cyclohexene oxidation reaction. The presence of V₂O₅ increases the catalyst efficiency and leads to a selectivity change from cyclohexenol (blank test) to epoxide, with a maximum for 15 wt% V₂O₅. The conversion of cyclohexene was 46?% while the selectivity to epoxide was higher (75?%).     

Oxidation of 3- and 4-methylpyridines on modified vanadium oxide catalysts

The partial oxidation of 3- and 4-methylpyridines on V₂O₅ and vanadium oxide catalysts doped with TiO₂, Al₂O₃, and ZrO₂ was studied. The catalytic activities of the studied catalysts were correlated with the calculated proton affinities of the vanadyl oxygen. A possible mechanism of the surface stages of the partial oxidation of 3- and 4-methylpyridines on the vanadium oxide catalysts was discussed.