Nb2O5-supported WO3: a comparative study with WO3/Al2O3

WO₃/Nb₂O₅-supported samples prepared by impregnation are characterised by X-ray diffraction (XRD), Raman spectroscopy and X-ray absorption spectroscopy (XAS) at the W–L₃ absorption edge, as well as temperature programmed reduction (TPR) and FT-IR monitoring of pyridine adsorption. Results are compared with those obtained for WO₃/Al₂O₃ samples prepared in the same conditions, showing that niobia is able to disperse tungsta better than alumina does. Formation of a crystalline WO₃ needs larger tungsten contents on niobia than on alumina, since tungsten solution into niobia is easier than into alumina. Raman and XAS spectra recorded under ambient conditions suggest that similar WO_x species are formed on both supports at tungsten contents 0.5–1 theoretical monolayers; however, TPR results for the low tungsten loaded samples indicate that, when reduction starts (always at temperatures higher than 700 K under H₂/Ar flow) there is a larger concentration of tetrahedral [WO₄] species on alumina, than on niobia. Samples with low tungsten loading have been tested in isopropanol decomposition and ethylene oxidation, following both processes by FT-IR of adsorbed species up to 673 K. Results show that adsorption of ethylene on WO₃/Nb₂O₅ yields acetaldehyde and acetate at 473 K, while this adsorption is non-reactive either on the supports or on WO₃/Al₂O₃. Isopropanol adsorbs dissociatively on both supports, leading to acetone and propene formation on tungsta–niobia, but only propene on tungsta–alumina, probably due to the larger reducibility of the tungsten-containing phases.     

Surface properties and reactivity of Cu/γ-Al2O3 catalysts for NO reduction by C3H6: Influences of calcination temperatures and additives

The influences of calcination temperatures and additives for 10 wt.% Cu/γ-Al₂O₃ catalysts on the surface properties and reactivity for NO reduction by C₃H₆ in the presence of excess oxygen were investigated. The results of XRD and XPS show that the 10 wt.% Cu/γ-Al₂O₃ catalysts calcined below 973 K possess highly dispersed surface and bulk CuO phases. The 10 wt.% Cu/γ-Al₂O₃ and 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalysts calcined at 1073 K possess a CuAl₂O₄ phase with a spinel-type structure. In addition, the 10 wt.% La–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K possesses a bulk CuO phase. The result of NO reduction by C₃H₆ shows that the CuAl₂O₄ is a more active phase than the highly dispersed and bulk CuO phase. However, the 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K possesses significantly lower reactivity for NO reduction than the 10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K, although these catalysts possess the same CuAl₂O₄ phase. The low reactivity for NO reduction for 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K is attributed to the formation of less active CuAl₂O₄ phase with high aggregation and preferential promotion of C₃H₆ combustion to CO_x by MnO₂. The engine dynamometer test for NO reduction shows that the C₃H₆ is a more effective reducing agent for NO reduction than the C₂H₅OH. The maximum reactivity for NO reduction by C₃H₆ is reached when the NO/C₃H₆ ratio is one.     

Quantified NOx adsorption on Pt/K/gamma-Al2O3 and the effects of CO2 and H2O

A multi-component NO_x-trap catalyst consisting of Pt and K supported on γ-Al₂O₃ was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H₂O and CO₂ on NO_x storage. The Al₂O₃ support was shown to have NO_x trapping capability with and without Pt present (at 250 °C Pt/Al₂O₃ adsorbs 2.3 μmols NO_x/m²). NO_x is primarily trapped on Al₂O₃ in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO_x/m², and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al₂O₃ catalyst are coordinated on the Al₂O₃ support at saturation.

When 5% CO₂ was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H₂O was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al₂O₃-based nitrates decreased by 92%. Interestingly, with both 5% CO₂ and 5% H₂O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al₂O₃ destabilized the K–CO₂ bond; specifically, H₂O mitigates the NO_x storage capacity losses associated with carboxylate competition.     

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.     

Preparation of carbon-coated W18O49 and its photoactivity under visible light

Carbon-coated W₁₈O₄₉ powders were prepared from the mixture of para-ammonium tungstate with poly(vinyl alcohol) by heat treatment in inert atmosphere at a temperature between 750 and 900 °C for 1 h. The synthesized W₁₈O₄₉ crystals had prismatic morphology in small size, less than 0.5 μm in diameter and about 1 μm in length. Carbon-coated W₁₈O₄₉ was shown to have photoactivity under visible light irradiation by comparing the concentration changes of methylene blue, phenol and dimethylsulfoxide with time under the irradiation of visible light to that in the dark. Photoactivity of W₁₈O₄₉ was supposed to be due to the formation of OH radicals on the basis of the degradation of dimethylsulfoxide, its quantitative transformation to methanesulfonic acid. Carbon coating seemed to have various roles: to reduce WO₃ to W₁₈O₄₉, to inhibit the sintering and crystal growth of W₁₈O₄₉ to keep them small size, and also to concentrate pollutants around W₁₈O₄₉ crystal by adsorption.     

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.     

Lean NOx reduction with CO + H2 mixtures over Pt/Al2O3 and Pd/Al2O3 catalysts

The reduction of NO_x by hydrogen under lean burn conditions over Pt/Al₂O₃ is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NO_x conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al₂O₃. In this case, although pure H₂ and pure CO are ineffective for NO_x reduction under lean burn conditions, H₂/CO mixtures are very effective. With a realistic (1:3) H₂:CO ratio, typical of actual exhaust gas, Pd/Al₂O₃ is significantly more active than Pt/Al₂O₃, delivering 45% NO_x conversion at 160 °C, compared to >15% for Pt/Al₂O₃ under identical conditions. The nature of the support is also critically important, with Pd/Al₂O₃ being much more active than Pd/SiO₂. Possible mechanisms for the improved performance of Pd/Al₂O₃ in the presence of H₂+CO are discussed.     

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.     

Hydrogen-storage properties of a Mg-based mixture prepared by reactive mechanical grinding with ultrafine Fe2O3 particles and Ni

The sample Mg-10 wt.% (Fe₂O₃, Ni) was prepared by grinding Mg mechanically under H₂ (reactive mechanical grinding) with ultrafine Fe₂O₃ particles and Ni particles. The sample Mg-10 wt.% (Fe₂O₃, Ni) as milled absorbed 4.24 wt.% hydrogen at 593 K under 12 bar H₂ for 60 min. Its activation was accomplished only by one hydriding–dehydriding cycle. The activated sample absorbed 4.05 wt.% hydrogen at 593K, 12 bar H₂ for 60 min and desorbed 3.05 wt.% hydrogen at 593 K, 1.0 bar H₂ for 60 min. After hydriding–dehydriding cycling, Fe₂O₃ was reduced, Mg₂Ni was formed by the reaction of Mg with Ni, and a small fraction of Mg was oxidized.     

Study of n-hexane isomerization on mixed Al2O3–ZrO2/SO4 catalysts

Mixed oxides of alumina and zirconia having a relative composition of 50, 80 and 100% Zr₂O were synthesized by means of sol–gel methods. The catalysts were sulfated with H₂SO₄ 1N, and were loaded with 0.3% Pt metal using the incipient wetness technique. The characterization of the physicochemical properties was carried out using XRD, N₂-adsorption at 78 K, and SEM. The catalytic properties of the Al₂O₃–ZrO₂ series were studied by means of dehydration of 2-propanol at 180°C and isomerization of n-hexane at 250°C, 1 atm. The sulfated solids presented a high surface acidity and a limited crystallinity, together with high activity for alcohol dehydration (i.e. 2-propanol). On the other hand, the Al₂O₃–ZrO₂ solid solutions (i.e. those having a 20–80% composition) turned out to be the most active ones for the isomerization of n-hexane.     

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.     

Quantification of the in situ DRIFT spectra of Pt/K/γ-A12O3 NOx adsorber catalysts

A method to quantify DRIFT spectral features associated with the in situ adsorption of gases on a NO_x adsorber catalyst, Pt/K/Al₂O₃, is described. To implement this method, the multicomponent catalyst is analysed with DRIFT and chemisorption to determine that under operating conditions the surface comprised a Pt phase, a pure γ-Al₂O₃ phase with associated hydroxyl groups at the surface, and an alkalized-Al₂O₃ phase where the surface –OH groups are replaced by –OK groups. Both DRIFTS and chemisorption experiments show that 93–97% of the potassium exists in this form. The phases have a fractional surface area of 1.1% for the 1.7 nm-sized Pt, 34% for pure Al₂O₃ and 65% for the alkalized-Al₂O₃. NO₂ and CO₂ chemisorption at 250 °C is implemented to determine the saturation uptake value, which is observed with DRIFTS at 250 °C. Pt/Al₂O₃ adsorbs 0.087 μmol CO₂/m²and 2.0 μmol NO₂/m², and Pt/K/Al₂O₃ adsorbs 2.0 μmol CO₂/m²and 6.4 μmol NO₂/m². This method can be implemented to quantitatively monitor the formation of carboxylates and nitrates on Pt/K/Al₂O₃ during both lean and rich periods of the NO_x adsorber catalyst cycle.     

Preparation of CexZr1−xO2 (x = 0.75, 0.62) solid solution and its application in Pd-only three-way catalysts

Nanoparticles of Ce_xZr_1−xO₂ (x = 0.75, 0.62) were prepared by the oxidation-coprecipitation method using H₂O₂ as an oxidant, and characterized by N₂ adsorption, XRD and H₂-TPR. Ce_xZr_1−xO₂ prepared had single fluorite cubic structure, good thermal stability and reduction property. With the increasing of Ce/Zr ratio, the surface area of Ce_xZr_1−xO₂ increased, but thermal stability of Ce_xZr_1−xO₂ decreased. The surface area of Ce_0.62Zr_0.38O₂ was 41.2 m²/g after calcination in air at 900 °C for 6 h. TPR results showed the formation of solid solution promoted the reduction of CeO₂, and the reduction properties of Ce_xZr_1−xO₂ were enhanced by the cycle of TPR-reoxidation. The Pd-only three-way catalysts (TWC) were prepared by the impregnation method, in which Ce_0.75Zr_0.25O₂ was used as the active washcoat and Pd loading was 0.7 g/L. In the test of Air/Fuel, the conversion of C₃H₈ was close to 100% and NO was completely converted at λ < 1.025. The high conversion of C₃H₈ was induced by the steam reform and dissociation adsorption reaction of C₃H₈. Pd-only catalyst using Ce_0.75Zr_0.25O₂ as active washcoat showed high light off activity, the reaction temperatures (T₅₀) of 50% conversion of CO, C₃H₈ and NO were 180, 200 and 205 °C, respectively. However, the conversions of C₃H₈ and NO showed oscillation with continuously increasing the reaction temperature. The presence of La₂O₃ in washcoat decreased the light off activity and suppressed the oscillation of C₃H₈ and NO conversion. After being aged at 900 °C for 4 h, the operation windows of catalysts shifted slightly to rich burn. The presence of La₂O₃ in active washcoat can enhance the thermal stability of catalyst significantly.     

Partial oxidation of methane in Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor at high pressures

Oxygen permeation fluxes through dense disk-shaped Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membranes were investigated as a function of temperature (973–1123 K), pressure (2–10 atm), and membrane thickness (1–2 mm) under an air/helium gradient. A high oxygen permeation flux of 2.01 ml/cm² min was achieved at 1123 K and 10 atm under an air/He oxygen partial pressure gradient. Based on the dependence of the oxygen permeation flux on the oxygen partial pressure difference across the membrane and the membrane thickness, it is assumed that bulk diffusion of oxygen ions was the rate-controlling step in the oxygen transport across the BSCFO membrane disk under an air/He gradient. The partial oxidation of methane (POM) to syngas using LiLaNiO_x/γ-Al₂O₃ as catalyst in a BSCFO membrane reactor was successfully performed at high pressure (5 atm). Ninety-two percent methane conversion, 90% CO selectivity, and 15.5 ml/cm² min oxygen permeation flux were achieved in steady state at a temperature of 1123 K and a pressure of 5 atm. A syngas production rate of 79 ml/cm² min was obtained. Characterization of the membrane surface by SEM and XRD after reaction showed that the surface exposed to the air side preserved the Perovskite structure while the surface exposed to the reaction side was eroded.     

Water-induced bulk Ba(NO3)2 formation from NO2 exposed thermally aged BaO/Al2O3

Phase changes in high temperature treated (>900 °C) 8 or 20 wt% BaO supported on γ-Al₂O₃ model lean NO_x trap (LNT) catalysts, induced by NO₂ and/or H₂O adsorption, were investigated with powder X-ray diffraction (XRD), solid state ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and NO₂ temperature programmed desorption (TPD) experiments. After calcination in dry air at 1000 °C, the XRD and solid state ²⁷Al MAS NMR results confirm that stable surface BaO and bulk BaAl₂O₄ phases are formed for 8 and 20 wt% BaO/Al₂O₃, respectively. Following NO₂ adsorption over these thermally treated samples, some evidence for nanosized Ba(NO₃)₂ particles are observed in the XRD results, although this may represent a minority phase. However, when water was added to the thermally aged samples after NO₂ exposure, the formation of bulk crystalline Ba(NO₃)₂ particles was observed in both samples. Solid state ²⁷Al MAS NMR is shown to be a good technique for identifying the various Al species present in the materials during the processes studied here. NO₂ TPD results demonstrate a significant loss of uptake for the 20 wt% model catalysts upon thermal treatment. However, the described phase transformations upon subsequent water treatment gave rise to the partial recovery of NO_x uptake, demonstrating that such a water treatment of thermally aged catalysts can provide a potential method to regenerate LNT materials.     

Activity and acidity of Nb2O5-MoO3 and Nb2O5-WO3 in the Friedel-Crafts alkylation

Nb₂O₅ loaded on the supports and mixed with oxides was studied to investigate the activity and acidity for Friedel-Crafts benzylation of anisole. From the study on the loaded catalysts, a preliminary conclusion for the selection of metal oxide was obtained; namely, such an acidic oxide as silica was suitable for the support of Nb₂O₅. Then, MoO₃ and WO₃ were mixed with Nb₂O₅, and prominent high catalytic activity and acidities were observed. Both oxides of Nb₂O₅-MoO₃ and Nb₂O₅-WO₃ showed almost similar behavior with respect to characterization and catalytic activity. Surface area increased, X-ray diffraction (XRD) and Raman bands were lost, acid sites, both Brønsted and Lewis characters generated, and surface acid site density was as high as 2–4 nm⁻². The acid sites were generated on the amorphous metal oxides consisting of Nb and Mo or W oxides, different in nature from those of Nb₂O₅ calcined and un-calcined, and active for Friedel-Crafts benzylation.     

Ce1−xCuxO2−x/Al2O3/FeCrAl catalysts for catalytic combustion of methane

A series of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts (x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H₂-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts the surface phase structure were the Ce_1−xCu_xO_2−x solid solution, -Al₂O₃ and γ-Al₂O₃. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce_1−xCu_xO_2−x solid solution. The Cu component of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce_1−xCu_xO_2−x solid solution and the Al₂O₃ washcoats and the FeCrAl support.     

Selective oxidation of propylene to acrolein over supported V2O5/Nb2O5 catalysts: An in situ Raman, IR, TPSR and kinetic study

The vapor-phase selective oxidation of propylene (H₂CCHCH₃) to acrolein (H₂CCHCHO) was investigated over supported V₂O₅/Nb₂O₅ catalysts. The catalysts were synthesized by incipient wetness impregnation of V-isopropoxide/isopropanol solutions and calcination at 450 °C. The catalytic active vanadia component was shown by in situ Raman spectroscopy to be 100% dispersed as surface VO_x species on the Nb₂O₅ support in the sub-monolayer region (<8.4 V/nm²). Surface allyl species (H₂CCHCH_2*) were observed with in situ FT-IR to be the most abundant reaction intermediates. The acrolein formation kinetics and selectivity were strongly dependent on the surface VO_x coverage. Two surface VO_x sites were found to participate in the selective oxidation of propylene to acrolein. The reaction kinetics followed a Langmuir–Hinshelwood mechanism with first-order in propylene and half-order in O₂ partial pressures. C₃H₆-TPSR spectroscopy studies also revealed that the lattice oxygen from the catalyst was not capable of selectively oxidizing propylene to acrolein and that the presence of gas phase molecular O₂ was critical for maintaining the surface VO_x species in the fully oxidized state. The catalytic active site for this selective oxidation reaction involves the bridging VONb support bond.     

The homogeneity range and the microwave dielectric properties of the BaZn2Ti4O11 ceramics

Ceramics with a composition close to BaZn₂Ti₄O₁₁ were synthesized according to various substitutional mechanisms in order to verify an existence of a homogeneity range in the vicinity of this composition. Structural and microstructural investigations showed that the crystal structure of BaZn₂Ti₄O₁₁ was formed in the homogeneity range corresponding to the formula BaZn_2 − xTi₄O_11 − x (0 < x < 0.1). Densely sintered BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) ceramics exhibited a dielectric constant around 30, τ_f = −30 ppm/K and high Q × f values, which increased from 68,000 GHz at x = 0 to 83,000 GHz at x = 0.05. Structurally, the deficiency of Zn in BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) resulted in a slight decrease in the unit-cell volume. The influence of secondary phases in the BaZn₂Ti₄O₁₁-based materials on the microwave dielectric properties was also investigated. A presence of small amounts of ZnO, BaTiO₃, hollandite-type solid solutions (Ba_xZn_xTi_8 − xO₁₆) and BaTi₄O₉ caused a decrease in Q × f values.     

Effect of calcination temperature on the low-temperature oxidation of CO over CoOx/TiO2 catalysts

A 5 wt% CoO_x/TiO₂ catalyst has been used to study the effect of calcination temperature on the activity of this catalyst for CO oxidation at 100 °C under a net oxidizing condition in a continuous flow type fixed-bed reactor system, and the catalyst samples have been characterized using TPD, XPS and XRD measurements. The catalyst after calcination at 450 °C gave highest activity for this low-temperature CO oxidation, and XPS measurements yielded that a 780.2-eV Co 2p_3/2 main peak appeared with this catalyst sample and this binding energy was similar to that measured with pure Co₃O₄. After calcination at 570 °C, the catalyst, which had possessed practically no activity in the oxidation reaction, gave a Co 2p_3/2 main structure peak at 781.3 eV which was very similar to those obtained for synthesized Co_nTiO_n+2 compounds (CoTiO₃ and Co₂TiO₄), and this catalyst sample had relatively negligible CO chemisorption as observed by TPD spectra. XRD peaks indicating only the formation of Co₃O₄ particles on titania surface were developed in the catalyst samples after calcination at temperatures ≥350 °C. Based on these characterization results, five types of Co species could be modeled to exist with the catalyst calcined at different temperatures. Among these surface Co species, the Type A clean Co₃O₄ particles were predominant on a sample of the catalyst after calcination at 450 °C and highly active for CO oxidation at 100 °C, and the calcination at 570 °C gave the Type B Co₃O₄ particles with complete Co_nTiO_n+2 overlayers inactive for this oxidation reaction.