Evaluation of V2O5–WO3–TiO2 and alternative SCR catalysts in the abatement of VOCs

Two different commercial SCR catalysts belonging to the V₂O₅–WO₃–TiO₂ system, and different alternative catalysts based on Mn, Fe, Cr, Al and Ti oxides have been tested in the conversion of VOCs in excess oxygen in a temperature range typical of the SCR process (500–700 K). Propane, propene, isopropanol, acetone, 2-chloropropane and 1,2-dichlorobenzene have been fed with excess oxygen and helium. The industrial catalysts are poorly active in the conversion of propane, giving mainly rise to propene by oxy-dehydrogenation. The conversion of propene is higher with CO as the predominant product. In any case, the oxidation activity depends on the vanadium content of the catalyst. Isopropanol is mainly converted into acetone and propene, while acetone is burnt predominantly to CO. Mn- and Fe- containing systems are definitely more active in the conversion of hydrocarbons and oxygenates, giving rise almost exclusively to CO₂. 2-Chloropropane is selectively dehydrochlorinated to propene and HCl starting from 350 K, propene being later burnt to CO on the industrial V₂O₅–WO₃–TiO₂ catalysts, whose combustion activity is, apparently, not affected by chlorine. On the contrary, chlorine strongly affects the behavior of Mn-based catalysts, that are active in the dehydrochlorination of 2-chloropropane, but are simultaneously deactivated with respect to their combustion catalytic activity. The conversion of 1,2-dichlorobenzene gives rise to important amounts of heavy products in our experimental conditions with relatively high reactant concentration.     

Sulfuric acid formation over ammonium sulfate loaded V2O5–WO3/TiO2 catalysts by DeNOx reaction with NOx

Operating the SCR DeNO_x reactor at temperatures below 200 °C results in a considerable saving in operating costs. Plant experience shows that on the catalysts in these second generation DeNO_x plants, even for flue gases with SO₂ concentration below 10 mg/m³, over 1–2 years operating time sizeable quantities of ammonium sulfates accumulate. Ammonium sulfates deposited on V₂O₅–WO₃/TiO₂ catalysts react with NO_x to nitrogen and sulfuric acid. Second-order rate constants of this reaction for temperatures of 170 °C have been derived. It could be shown that the sulfuric acid formed on the catalyst is displaced by water vapour and desorbs resulting in gas phase concentrations of up to 6.5 mg acid/m³ flue gas. Plant equipment downstream of the ammonium sulfate containing low temperature DeNO_x catalysts has to be protected against the corrosive action of the sulfuric acid in the flue gases leaving the DeNO_x reactor.     

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.     

Characterization and catalytic activity of Pd/V2O5/Al2O3 catalysts on benzene total oxidation

The role of vanadium oxide and palladium on the benzene oxidation reaction over Pd/V₂O₅/Al₂O₃ catalysts was investigated. The Pd/V₂O₅/Al₂O₃ catalysts were more active than V₂O₅/Al₂O₃ and Pd/Al₂O₃ catalysts. The increase of vanadium oxide content decreased the Pd dispersion and increased the benzene conversion. A strong Pd particle size effect on benzene oxidation reaction was observed. Although the catalysts containing high amount of V⁴⁺ species were more active, the Pd particle size effect was responsible for the higher activity.     

The influence of sulphate on the catalytic properties of V2O5-TiO2 and WO3-TiO2 in the reduction of nitric oxide with ammonia

The effect of sulphate on the catalytic properties of V₂O₅/TiO₂ and WO₃/TiO₂ in the selective reduction of NO with NH₃ has been investigated. For both catalytic systems, the presence of sulphate results in the enhancement of catalytic activity without reduction of selectivity to nitrogen. The rate of NO reduction depends on the sulphate content, which is affected by the original composition of titania, the method of catalyst preparation and the metal oxide loading.     

Subsolidus relations in the BaO–La2O3–V2O5 phase diagram

The subsolidus region of the BaO–La₂O₃–V₂O₅ phase diagram has been redetermined. Previously reported binary phases of LaVO₄, La₈V₂O₁₇, La₃VO₇, BaLa₂O₄, Ba₃V₂O₈, Ba₂V₂O₇, Ba₃V₄O₁₃ and BaV₂O₆ have all been confirmed. The ternary phases Ba₂LaV₃O₁₁, Ba₃LaV₃O₁₂ and Ba₃La₄₀V₁₂O₉₃ have also been confirmed. The new phase “BaLa₁₀V₄O₂₆” has been synthesised for the first time and is reported here along with the amended phase diagram. The previously omitted phase of La_1.42V_0.58O_3.58 has also been included and reasons for this are cited. The diagram is shown for two temperatures (1000 and 600 °C) due to the low melting point of V₂O₅-rich compounds.     

WO3–TiO2 monolithic catalysts for high temperature SCR of NO by NH3: Influence of preparation method on structural and physico-chemical properties, activity and durability

The WO₃–TiO₂ catalysts with different WO₃ loadings prepared by the coprecipitation method were investigated in comparison with those prepared by the conventional impregnation method for the activity and durability in the high temperature SCR of NO by NH₃ and the structural and physico-chemical properties which were characterized by BET and XRD measurements, IR, Raman and XPS spectroscopies. The catalyst prepared by coprecipitation, as compared with that prepared by impregnation, was found to exhibit a higher SCR activity at high temperatures and also to possess a larger surface area, higher Brønsted acidity and larger monolayer capacity of the support with WO₃. Increasing the WO₃ loading of the catalysts enhances the SCR activity and simultaneously increases the Brønsted acidity. The observed improvement of SCR activity for the catalyst prepared by coprecipitation is mainly attributed to the higher Brønsted acidity and the presence of the more highly dispersed WO₃ species which is suggested by the larger monolayer capacity of ca. 13 μmol(W)/m² and no crystalline WO₃ on TiO₂ detected with XRD at the high WO₃ loading up to 40 wt.%. The catalyst with 20 wt.% WO₃, as compared with that prepared by impregnation, was found to exhibit a better thermal durability at high temperatures from 550 to 600 °C. The better durability is attributed to that the reduction of the surface area and the formation and subsequent growth of crystalline WO₃ upon aging are more remarkably inhibited.     

Sub-monolayer V2O5–anatase TiO2 and Eurocat catalysts: IR, Raman and XPS characterisation of VOx dispersion

VO_x–TiO₂ catalysts with vanadium loading less than that of a monolayer have been prepared either by impregnation in aqueous media from solutions of V(V) or V(IV) at different concentrations and pH, or by grafting in anhydrous media on anatase supports with surface areas of 10, 150 and 350 m² g⁻¹. Their characterisation by XPS, DRIFT and Raman spectroscopy, compared to that of EUROCAT EL10V1 and V8 reference catalysts, shows that the dispersion of the load depends on the mode of preparation and is not necessarily equal to 1.     

载体对V_2O_5-MoO_3/TiO_2催化剂表面性质及其选择性催化还原脱硝性能的影响

采用比表面积分别为101.86 m2·g-1(A)、86.37 m2·g-1(B)和7.78(C)m2·g-1(C)的TiO_2载体,通过分步浸渍法制备V2O5-Mo O3/TiO_2(A,B,C)选择性催化还原脱硝催化剂。在空速为10 000 h-1和氨氮体积比1.0条件下,以TiO_2(A)与TiO_2(B)为载体制备的催化剂脱硝活性在反应温度窗口(350~450)℃超过90%,且具有良好的高温抗硫中毒性能和相对较小的氨气氧化率。而以TiO_2(C)为载体制备的脱硝催化剂活性温度窗口窄,在350℃时获得的最高脱硝活性仅为73%,且对NH3的氧化作用较强。利用X射线衍射、低温N2吸附-脱附、紫外-可见漫反射光谱、H2程序升温还原和NH3程序升温脱附等对载体和催化剂进行表征。结果表明,活性组分V2O5在载体TiO_2(A)上分散性良好,主要以孤立态钒氧物种形式存在,因此,以TiO_2(A)为载体制备的催化剂比表面积、氧化还原性和表面酸性等性能更优。     

Isomerization of n-hexane over mono- and bimetallic Pd–Pt catalysts supported on ZrO2–Al2O3–WOx prepared by sol–gel

The catalytic performance of mono- and bimetallic Pd (0.6, 1.0 wt.%)–Pt (0.3 wt.%) catalysts supported on ZrO₂ (70, 85 wt.%)–Al₂O₃ (15, 0 wt.%)–WO_x (15 wt.%) prepared by sol–gel was studied in the hydroisomerization of n-hexane. The catalysts were characterized by N₂ physisorption, XRD, TPR, XPS, Raman, NMR, and FT-IR of adsorbed pyridine. The preparation of ZrW and ZrAlW mixed oxides by sol–gel favored the high dispersion of WO_x and the stabilization of zirconia in the tetragonal phase. The Al incorporation avoided the formation of monoclinic-WO₃ bulk phase. The catalysts increased their S_BET for about 15% promoted by Al₂O₃ addition. Various oxidation states of WO_x species coexist on the surface of the catalysts after calcination. The structure of the highly dispersed surface WO_x species is constituted mainly of isolated monotungstate and two-dimensional mono-oxotungstate species in tetrahedral coordination. The activity of Pd/ZrW catalysts in the hydroisomerization of n-hexane is promoted both with the addition of Al to the ZrW mixed oxide and the addition of Pt to Pd/ZrAlW catalysts. The improvement in the activity of Pd/ZrAlW catalysts is ascribed to a moderated acid strength and acidity, which can be correlated to the coexistence of W⁶⁺ and reduced-state WO_x species (either W⁴⁺ or W⁰). The addition of Pt to the Pd/ZrAlW catalyst does not modify significantly its acidic character. Selectivity results showed that the catalyst produced 2MP, 3MP and the high octane 2,3-dimethylbutane (2,3-DMB) and 2,2-dimethylbutane (2,2-DMB) isomers.     

TiO2-SiO2 and V2O5/TiO2-SiO2 catalyst: Physico-chemical characteristics and catalytic behavior in selective catalytic reduction of NO by NH3

TiO₂-SiO₂ with various compositions prepared by the coprecipitation method and vanadia loaded on TiO₂-SiO₂ were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH₃ and in the undesired oxidation of SO₂ to SO₃, using BET, XRD, XPS, NH₃-TPD, acidity measurement by the titration method and activity test. TiO₂-SiO₂, compared with pure TiO₂, exhibits a remarkably stronger acidity, a higher BET surface area, a lower crystallinity of anatase titania and results in allowing a good thermal stability and a higher vanadia dispersion on the support up to high loadings of 15 wt% V₂O₅. The SCR activity and N₂ selectivity are found to be more excellent over vanadia loaded on TiO₂-SiO₂ with 10–20 mol% of SiO₂ than over that on pure TiO₂, and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH₃ adsorbed on the catalysts. With increasing SiO₂ content, the remarkable activity decrease in the oxidation of SO₂ to SO₃, favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V⁴⁺ on TiO₂-SiO₂, while V⁵⁺ exists on TiO₂, according to XPS. It is concluded that vanadia loaded on Ti-rich TiO₂-SiO₂ with low SiO₂ content is suitable as SCR catalysts for sulfur-containing exhaust gases due to showing not only the excellent de-NO_x activity but also the low SO₂ oxidation performance.     

V2O5 catalysts supported on Al2O3–SiO2 mixed oxide: V, H MAS solid-state NMR, DRIFTS and methanol oxidation studies

Alumina–silica mixed oxide, synthesized by the sol–gel technique, was used as a support for dispersing and stabilizing the active vanadia phase. The catalysts were characterized employing ⁵¹V and ¹H solid-state MAS NMR, diffuse reflectance FT-IR, BET surface area measurements. The partial oxidation activities of the catalysts were tested using methanol oxidation as a model reaction. ⁵¹V solid-state NMR studies on the calcined catalysts showed the peaks corresponding to the presence of both tetrahedral and distorted octahedral vanadia species at low vanadia loadings and with an increase in V₂O₅ content, the ⁵¹V chemical shifts corresponding to amorphous V₂O₅ like phases were observed. DRIFTS studies of the catalysts indicated the vibrations corresponding tetrahedral vanadia species at low and medium loadings and at high V₂O₅ contents the vibrations corresponding V=O bonds of V₂O₅ agglomerates were observed. The V/Al–Si catalysts exhibited high selectivity for the dehydration product dimethyl ether in the methanol partial oxidation studies showing the predominance of the acidic nature of the alumina–silica support over the redox properties of the active vanadia phase.     

Effect of oxygen concentration on the NOx reduction with ammonia over V2O5–WO3/TiO2 catalyst

The catalytic reduction of NO_x in the typical operation temperatures and oxygen concentrations of diesel engines has been studied in the presence of V3W9Ti in a tubular flow reactor. The results have shown that the selective catalytic reduction is strongly affected by the oxygen concentration in low temperature range (150–275 °C). At higher temperatures, the reaction becomes independent of the O₂ concentration. The rate of the selective catalytic reduction of NO with ammonia may be considerably enhanced by converting part of the NO into NO₂. DRIFT measurements have shown that NH₃ and NO₂ are adsorbed on the catalyst surface on the contrary of NO. The experiments have shown that the decrease in N₂ selectivity of the SCR reaction is mainly due to the SCO of ammonia and to the formation of nitrous oxide.     

Microwave dielectric ceramics in the system BaO–Li2O–Nd2O3–TiO2

Ceramics in the system BaO-Li₂O–Nd₂O₃–TiO₂ (BNT–LNT) were prepared by the mixed oxide route. Powders were mixed, milled, calcined and sintered at 1475°C for 4 h. Fired densities decreased steadily along the series from BNT to LNT. The microstructures of samples rich in BNT were dominated by small needle-like grains; the LNT samples comprised larger (6 μm) cubic grains. X-ray diffraction showed that there was a transition from orthorhombic BNT to cubic LNT; small amounts of LNT could be accommodated in BNT, but between 10–20% LNT there was the development of the second phase. Small additions of LNT led to a small increase in relative permittivity, but decreased the dielectric Q-value (from the maximum of 1819 at 4 GHz). As BNT and LNT exhibit negative and positive temperature dependencies of permittivity respectively, the addition of 10–20% LNT to BNT should yield samples with zero temperature dependence of _r Impedance spectroscopy showed that data could only be acquired at elevated temperatures for BNT rich samples (above 500°C), but at modest temperatures (less than 100°C) for the more conductive LNT.     

Single and combined deactivating effect of alkali metals and HCl on commercial SCR catalysts

Two commercial SCR catalysts, with a nominal W content of about 9 wt.% and a V nominal content of 0.55 and 1.8 wt.%, respectively, were contacted with different amounts of Na and K and with HCl vapours in order to simulate poisoning by species more specifically contained in exhaust gases from MSW combustion. Catalysts were characterised using XRD analysis, SEM/EDX analysis, BET and pore size distribution measurements, NH₃ TPD, TG analysis. Poisoning agents do not cause loss of surface area nor pore occlusion. A significant loss of surface acidity was observed upon alkali metals poisoning whereas a decreasing of vanadium content was observed for the more concentrated catalysts upon HCl poisoning. Catalysts deactivation is proportional to the number of acid sites neutralised by alkali metals adsorbing ammonia in the temperature range typical of SCR process. HCl promotes the formation of new acid sites showing a lower activity compared to the original one.     

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.     

Hydrodesulfurization activity of CoMo and NiMo supported on Al2O3–TiO2 for some model compounds and gas oils

Catalytic activities of Al₂O₃–TiO₂ supporting CoMo and NiMo sulfides (CoMoS and NiMoS) catalysts were examined in the transalkylation of isopropylbenzene and hydrogenation of naphthalene as well as the hydrodesulfurization (HDS) of model sulfur compounds, conventional gas oil (GO), and light cycle oil (LCO). Al₂O₃–TiO₂ supporting catalysts exhibited higher activities for these reactions except for the HDS of the gas oil than a reference Al₂O₃ supporting catalyst, indicating the correlation of these activities. Generally, more content of TiO₂ promoted the activities. Inferior activity of the catalyst for HDS of the gas oil is ascribed to its inferior activity for HDS of dibenzothiophene (DBT) in gas oil as well as in model solvent decane, while the refractory 4,6-dimethyldibenzothiophene (4,6-DMDBT) in gas oil as well as in decane was more desulfurized on the catalyst. Characteristic features of Al₂O₃–TiO₂ catalyst are discussed based on the paper results.     

Enhanced activity of metal oxide-doped Ga2O3–Al2O3 for NO reduction by propene

Effect of additives, In₂O₃, SnO₂, CoO, CuO and Ag, on the catalytic performance of Ga₂O₃–Al₂O₃ prepared by sol–gel method for the selective reduction of NO with propene in the presence of oxygen was studied. As for the reaction in the absence of H₂O, CoO, CuO and Ag showed good additive effect. When H₂O was added to the reaction gas, the activity of CoO-, CuO- and Ag-doped Ga₂O₃–Al₂O₃ was depressed considerably, while an intensifying effect of H₂O was observed for In₂O₃- and SnO₂-doped Ga₂O₃–Al₂O₃. Of several metal oxide additives, In₂O₃-doped Ga₂O₃–Al₂O₃ showed the highest activity for NO reduction by propene in the presence of H₂O. Kinetic studies on NO reduction over In₂O₃–Ga₂O₃–Al₂O₃ revealed that the rate-determining step in the absence of H₂O is the reaction of NO₂ formed on Ga₂O₃–Al₂O₃ with C₃H₆-derived species, whereas that in the presence of H₂O is the formation of C₃H₆-derived species. We presumed the reason for the promotional effect of H₂O as follows: the rate for the formation of C₃H₆-derived species in the presence of H₂O is sufficiently fast compared with that for the reaction of NO₂ with C₃H₆-derived species in the absence of H₂O. Although the retarding effect of SO₂ on the activity was observed for all of the catalysts, SnO₂–Ga₂O₃–Al₂O₃ showed still relatively high activity in the lower temperature region.     

CoMo/MgO–Al2O3 supported catalysts: An alternative approach to prepare HDS catalysts

Three different supports were prepared with distinct magnesia–alumina ratio x = MgO/(MgO + Al₂O₃) = 0.01, 0.1 and 0.5. Synthesized supports were impregnated with Co and Mo salts by the incipient wetness method along with 1,2-cyclohexanediamine-N,N,N′,N′-tetraacetic acid (CyDTA) as chelating agent. Catalysts were characterized by BET surface area, Raman spectroscopy, SEM-EDX and HRTEM (STEM) spectroscopy techniques. The catalysts were evaluated for the thiophene hydrodesulfurization reaction and its activity results are discussed in terms of using chelating agent during the preparation of catalyst. A comparison of the activity between uncalcined and calcined catalysts was made and a higher activity was obtained with calcined MgO–Al₂O₃ supported catalysts. Two different MgO containing calcined catalysts were tested at micro-plant with industrial feedstocks of heavy Maya crude oil. The effect of support composition was observed for hydrodesulfurization (HDS), hydrodemetallization (HDM), hydrodeasphaltenization (HDAs) and hydrodenitrogenation (HDN) reactions, which were reported at temperature of 380 °C, pressure of 7 MPa and space-velocity of 1.0 h⁻¹ during 204 h of time-on-stream (TOS).     

Preparation, characterization and catalytic properties of Co–Nb2O5–SiO2 catalysts

Co–Nb₂O₅–SiO₂ catalysts were prepared using three different sol–gel procedures: (i) the colloidal sol–gel method using NbCl₅ and SiCl₄ as precursors; (ii) the polymeric sol–gel method using niobium ethoxide and tetraethyl-orthosilicate (TEOS); (iii) an intermediate procedure between the colloidal and polymeric sol–gel method in which the precursors were those utilized in the CSG but dissolved in a mixture of anhydrous ethanol and CCl₄. In all procedures, the elimination of the solvent carried out between 80 and 110°C was followed by a reduction in hydrogen flow (30 ml min⁻¹) at 773 K. Following these procedures, samples containing 10 wt.% Co and 15 wt.% niobium oxide (expressed as Nb₂O₅) were obtained. The characterization of the catalysts was performed using various techniques: N₂ adsorption and desorption curves at 77 K, NH₃- and H₂-chemisorption, TPO, XPS, XRD, and solid state ¹H MAS-NMR. Hydrogenolysis of butane was evaluated. The low reaction rates are assigned to the effect of the metal size, whereas the isobutane selectivity as well as the relatively high stability is due to the acidity of the support.