Analysis of oxidation states of vanadium in vanadia–titania catalysts by the IR spectra of adsorbed NO

Adsorption of NO on vanadia–titania samples pre-subjected to different reduction treatments has been studied by FTIR spectroscopy. When the NO adsorption is performed at 85 K on oxidized samples, antisymmetric NONO species, typical for V⁵⁺ sites, are detected and characterized by bands at 1779 and 1686 cm⁻¹. At ambient temperature, however, adsorption is negligible and only with time reactive adsorption occurs producing NO⁺ (2120 cm⁻¹), nitro/nitrato species (bands in the 1650–1100 cm⁻¹ region) and weakly adsorbed NO (broad band at 1915 cm⁻¹). Adsorption of NO at ambient temperature on reduced samples results in the formation of two types of species: (i) V⁴⁺(NO)₂ dinitrosyls characterized by ν_s(NO) and ν_as(NO) at 1903–1880 and 1769–1753 cm⁻¹, respectively, and (ii) V³⁺(NO)₂ complexes, which give rise to ν_s(NO) at 1834–1822 cm⁻¹ and ν_as(NO) at 1697–1685 cm⁻¹. At low temperature the dinitrosyls are transformed into species in which more than one (NO)₂ dimer is attached to one cationic site. Addition of O₂ to NO, preadsorbed on reduced vanadia–titania samples, results in a fast oxidation of the V³⁺(NO)₂ species, whereas the V⁴⁺(NO)₂ complexes are more stable and do not disappear completely in the presence of oxygen. The results obtained suggest that NO is a convenient probe molecule for the analysis of the oxidation state of vanadium in vanadia–titania catalysts. To prevent oxidation of reduced vanadium sites, low equilibrium pressures of NO and registration of the IR spectrum soon after the NO admission are recommended. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cu–ZrO2 Catalysts for Water-gas-shift Reaction at Low Temperatures

A series of Cu–ZrO₂ catalysts with Cu content in the range of 10–70 at.% Cu (=100×Cu/(Cu+Zr)) were prepared by coprecipitation, and their performances were tested for the water-gas-shift (WGS) reaction. The activity of the catalyst increased with Cu loading and, depending on the loading, the activity was comparable to or better than the activity of a conventional Cu–ZnO–Al₂O₃ catalyst at low temperatures below 473 K. Characterization of the catalysts revealed that the amount of Cu⁺ present on the catalyst surface, after being reduced by a H₂ mixture at 573 K, was well correlated with the activity of the catalyst, indicating that the Cu⁺ species were the active sites of the WGS reaction. The easy redox between Cu²⁺ and Cu⁺ during the WGS reaction was considered to be responsible for the high activity of Cu–ZrO₂ at low temperatures. A reaction mechanism based on the redox was proposed.     

Characterization of Au/MnO x /TiO2 for Photocatalytic Oxidation of Carbon Monoxide

The Au/MnO _x /TiO₂ catalyst was used for the photocatalytic oxidation of carbon monoxide. The catalytic activity of Au/MnO _x /TiO₂ with low concentration of manganese (3–7 mol%) was much higher than that of Au/TiO₂. The surface of Au/MnO _x /TiO₂ was characterized by XPS and Raman spectroscopy. While the main state of manganese in 13.8 mol% MnO _x /TiO₂ was Mn⁴⁺ species, Mn³⁺ was the dominant species in the samples with below 6.5 mol% manganese. Raman spectroscopy revealed that the interaction between the MnO _x and TiO₂ form Mn–O–Ti species in which the state of manganese was Mn³⁺. The Au particles also interacted with both MnO _x and TiO₂ to modify the surface of them. In the case of the Au species, low loading of manganese produced the metallic Au⁰ and perimeter interfacial Au^δ+, whereas high loading showed the coexistence of three components which were metallic Au⁰, perimeter interfacial Au^δ+, and oxidic Au³⁺. The catalytic active component was the metallic Au⁰ and perimeter interfacial Au^δ+ species, which were dispersed on TiO₂ and Mn³⁺/TiO₂.     

Synergistic effects of doped Fe3+ and deposited Au on improving the photocatalytic activity of TiO2

Fe³⁺ doped together with Au deposited TiO₂ (Au/Fe³⁺–TiO₂) was successfully prepared, which shows excellent photocatalytic activity for degradation of methyl orange (MO) under both UV and visible light (λ > 420 nm) illumination. Fe³⁺ has been confirmed by EPR to substitute for Ti⁴⁺ in the TiO₂ lattice, and Au exists as Au⁰ on the surface of the photocatalyst indicated by the results of XRD. Fe³⁺ and Au have synergistic effects on improving the photocatalytic activity of TiO₂. A proposed mechanism concerning the synergistic effects is discussed to explain the improvement of the photocatalytic activities.     

Effective MgO surface doping of Cu/Zn/Al oxides as water–gas shift catalysts

Trace amounts of MgO were doped on Cu/ZnO/Al₂O₃ catalysts with the Cu/Zn/Al molar ratio of 45/45/10 and tested for the water–gas shift (WGS) reaction. A mixture of Zn(Cu)–Al hydrotalcite (HT) and Cu/Zn aurichalcite was prepared by co-precipitation (cp) of the metal nitrates and calcined at 300 °C to form the catalyst precursor. When the precursor was dispersed in an aqueous solution of Mg(II) nitrate, HT was reconstituted by the “memory effect.” During this procedure, the catalyst particle surface was modified by MgO-doping, leading to a high sustainability. Contrarily, cp-Mg/Cu/Zn/Al prepared by Mg²⁺, Cu²⁺, Zn²⁺ and Al³⁺ co-precipitation as a control exhibited high activity but low sustainability. Mg²⁺ ions were enriched in the surface layer of m-Mg–Cu/Zn/Al, whereas Mg²⁺ ions were homogeneously distributed throughout the particles of cp-Mg/Cu/Zn/Al. CuO particles were significantly sintered on the m-catalyst during the dispersion, whereas CuO particles were highly dispersed on the cp-catalyst. However, the m-catalyst was more sustainable against sintering than the cp-catalyst. Judging from TOF, the surface doping of MgO more efficiently enhanced an intrinsic activity of the m-catalyst than the cp-catalyst. Trace amounts of MgO on the catalyst surface were enough to enhance both activity and sustainability of the m-catalyst by accelerating the reduction–oxidation between Cu⁰ and Cu⁺ and by suppressing Cu⁰ (or Cu⁺) oxidation to Cu²⁺.     

Pd/Pt on Ti-containing Mixed Oxides as Dearomatization Catalysts: Physico-chemical Characterization and Activity

Pd/Pt supported on pure and doped TiO₂ (TiO₂–WO₃ and TiO₂–WO₃–SiO₂) were prepared and characterized by different techniques (XPS, TEM, XRD, H₂-TPR and TPD of ammonia). These catalysts were investigated in the hydrogenation of tetralin at 6.0 MPa, checking also their thio-tolerance by feeding increasing amounts of dibenzothiophene (DBT, 300 and 1000 wt ppm). The catalytic activity followed the order: Pd/Pt–TiO₂ > Pd/Pt–TiO₂–WO₃–SiO₂ > Pd/Pt–TiO₂–WO₃, evidencing a negative role of a second oxide inside TiO₂. The Pd/Pt–TiO₂ catalyst showed high activity regardless of reaction conditions (temperature, contact time, H₂/tetralin ratio) together with a good thio-tolerance up to 300 wt ppm of DBT.     

Gas phase hydrogenation of maleic anhydride to tetrahydrofuran by Cu/ZnO/TiO2 catalysts in the presence of n-butanol

Cu/ZnO/TiO₂ catalysts were prepared via the coprecipitation method. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectrometry, temperature programmed reduction, and N₂ adsorption. The catalytic activity of Cu/ZnO/TiO₂ catalyst in gas phase hydrogenation of maleic anhydride in the presence of n-butanol was studied at 235–280 °C and 1 MPa. The conversion of maleic anhydride was more than 95.7% and the selectivity of tetrahydrofuran was up to 92.7%. At the same time, n-butanol was converted to butyraldehyde and butyl butyrate via reactions, namely, dehydrogenation, disproportionation, and esterification. There were two kinds of CuO species present in the calcined Cu/ZnO/TiO₂ catalysts. At a lower copper content, the CuO species strongly interacted with ZnO and TiO₂; at a higher copper content, both the surface-anchored and bulk CuO species were present. The metallic copper (CuO) produced by the reduction of the surface-anchored CuO species favored the deep hydrogenation of maleic anhydride to tetrahydrofuran. The deep hydrogenation activity of Cu/ZnO/TiO₂ catalyst increased with the decrease of crystallite sizes of CuO and the increase of microstrain values. Compensations of reaction heat and H₂ in the coupling reaction of maleic anhydride hydrogenation and n-butanol dehydrogenation were distinct.     

Preparation and Characterization of Multi-functional Titanium Dioxide Photocatalysts

Recent research trends of the preparation and characterization of highly efficient titanium oxide-based photocatalysts modified by different methods are reviewed on the basis of studies done in our laboratory. Special attention is focused on the preparation and characterization of TiO₂ photocatalysts prepared by the transitional metal doping and noble metal deposition method, especially combining above two methods. Fe³⁺ doped together with Au deposited TiO₂ (Au/Fe³⁺–TiO₂) was successfully prepared, which shows excellent photocatalytic activity for degradation of methyl orange (MO) under both UV and visible light (λ > 420 nm) illumination. Fe³⁺ has been confirmed by EPR to substitute for Ti⁴⁺ in the TiO₂ lattice, and Au exists as Au⁰ on the surface of the photocatalyst indicated by the results of XRD. Fe³⁺ and Au have synergistic effects on improving the photocatalytic activity of TiO₂. A proposed mechanism concerning the synergistic effects is discussed to explain the improvement of the photocatalytic activities.     

Experimental determination of the eutectic temperature in air of the CuO-TiO2 pseudobinary system

Eutectic temperature and composition in the CuO-TiO₂ pseudobinary system have been experimentally determined in air by means differential thermal analysis (DTA), thermogravimetry (TG) and hot-stage microscopy (HSM). Samples of the new eutectic composition treated at different temperatures have been characterized by X-ray diffraction (XRD) and X-ray absorption near-edge structural spectroscopy (XANES) to identify phases and to determine the Cu valence state, respectively. The results show that the eutectic temperature in air is higher by 100 °C (∼1000 °C) for a Ti-richer composition (XTiO₂=25 mol%) than the one calculated in the literature. The reduction of Cu²⁺ to Cu⁺ takes places at about 1030 °C. The existence of Cu₂TiO₃ and Cu₃TiO₄ has been confirmed by XRD in the temperature range between 1045 and 1200 °C.     

An in-situ Reduction/Oxidation XAS Study on the EL10V8 VO x /TiO2(Anatase) Powder Catalyst

The structural changes of the supported vanadium oxide in the V₂O₅/TiO₂(anatase) EUROCAT EL10V8 powder catalyst during reduction and oxidation at 420 and 490 °C were studied with in-situ X-ray absorption spectroscopy (XAS). The Vanadium K-edge XAS results are compared with pure bulk V₂O₅. For the reduction–oxidation cycle at 420 °C, similar structural changes as for bulk V₂O₅ were observed for the supported vanadium oxide: a reduction to the VO₂ structure and re-oxidation back to V₂O₅. After reduction at 490 °C however, a different structure was obtained: very regular “VO₆” octahedra with a V^2.8+ valence. This may point to a structural support effect.     

Preparation and characterization of SO42-/TiO2 and S2O82-/TiO2 catalysts

Nanosized solid superacids SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂, as well as MCM-41-supported SO₄ ²⁻/ZrO₂, were prepared. Their structures, acidities, and catalytic activities were investigated and compared using XRD, N₂ adsorption-desorption, and in situ FTIR-pyridine adsorption, as well as an evaluation reaction with pseudoionone cyclization. The results showed that SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ possess not only nanosized particles with diameters < 7.0 nm, a BET surface greater than 140 cm²/g and relatively regular mesostructures with pores around 4.0 nm, but also a pure anatase phase and strong acidity. Different from the Lewis acid nature of SO₄ ²⁻/ZrO₂/MCM-41, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibit mainly Bronsted acidities. The strongest Bronsted acid sites were produced on SO₄ ²⁻/TiO₂ promoted with H₂SO₄, while Lewis acid sites on S₂O₈ ²⁻/TiO₂ even stronger than those on SO₄ ²⁻/ZrO₂/MCM-41 were generated when persulfate solution was used as sulfating agent. Because of their distinct acid natures, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibited catalytic activities for the cyclization of pseudoionone that were much higher than that of SO₄ ²⁻/ZrO₂/MCM-41. It can be concluded that the existence of more Br?nsted acid sites was favorable for proton participation in the cyclization reaction. Translated from Journal of Chemical Engineering of Chinese Universities, 2006, 20(2): 239–244 [译自: 高校化学工程学报]     

The Effect of Additives on Cu/HZSM-5 Catalyst for DME Synthesis

The addition of ZnO or ZrO₂ into CuO/HZSM-5 was investigated for DME synthesis from syngas by using the reactive frontal chromatography method, TPR and in situ TPR. These promoters enhanced the catalytic activity of Cu/HZSM-5 and promotion with ZnO and ZrO₂ produced a maximum activity, which could be explained by the improvement of the dispersion of Cu and the promotion of CuO reduction. The Cu⁺ species existing during the reaction have been detected, based on which a Cu⁰ Cu⁺¹ redox cycle model was put forward.     

Sulfated and Persulfated TiO2/MCM-41 Prepared by Grafting Method and their Acid-catalytic Activities for Cyclization of Pseudoionone

Solid acid catalysts of SO₄²⁻/TiO₂/MCM-41 and S₂O₈²⁻/TiO₂/MCM-41 were prepared via grafting method and sulfate/persulfate promotion. The catalysts exhibited desirable activity and better selectivity for cyclization reaction of pseudoionone compared to traditional SO₄²⁻/TiO₂. A combination of XRD, N₂ adsorption–desorption and FTIR spectroscopy indicated that the catalysts possess well-ordered mesostructure, and the grafted TiO₂ are in highly dispersed amorphous form rather than crystalline phase. For S₂O₈²⁻/TiO₂/MCM-41 higher S content and more Br?nsted acid sites can be achieved by persulfation, which is favorable for the protons participated cyclization reaction. The similar Si–O–Ti–O–S=O structure of all acid sites on pore surface of the catalysts is attributed to the improvement of selectivity in comparison with SO₄²⁻/TiO₂.     

Catalytic Activities of Al2O3-promoted NiSO4/TiO2 for Acid Catalysis

A series of catalysts, NiSO₄/Al₂O₃–TiO₂, for acid catalysis was prepared by the impregnation method, where support, Al₂O₃–TiO₂ was prepared by the coprecipitation method using a mixed aqueous solution of titanium tetrachloride and aluminum nitrate solution followed by adding an aqueous ammonia solution. The addition of nickel sulfate (or Al₂O₃) to TiO₂ shifted the phase transition of TiO₂ from amorphous to anatase to higher temperature because of the interaction between nickel sulfate (or Al₂O₃) and TiO₂. 15-NiSO₄/5-Al₂O₃–TiO₂ containing 15 wt% NiSO₄ and 5 mol% Al₂O₃, and calcined at 400°C exhibited maximum catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The charge transfer from Ti atoms to the neighboring Al atoms strengthens the Al–O bond between Al and the surface sulfate species. The addition of Al₂O₃ up to 5 mol% enhanced the acidity, thermal property, and catalytic activities of NiSO₄/Al₂O₃–TiO₂ gradually due to the interaction between Al₂O₃ and TiO₂ and consequent formation of Al–O–Ti bond.     

Characterization of Supported NiSO4 and Effect of TiO2–ZrO2 Composition on Catalytic Activity for Acid Catalysis

A series of catalysts, NiSO₄/TiO₂–ZrO₂ having different TiO₂–ZrO₂ composition, for acid catalysis was prepared by the impregnation method using an aqueous solution of nickel sulfate. The addition of TiO₂ to ZrO₂ improved the surface area of the catalyst and enhanced its acidity remarkably because of the formation of new acid sites through the charge imbalance of Ti–O–Zr bonding. The binary oxide, TiO₂–ZrO₂ calcined above 600 °C resulted in the formation of crystalline orthorhombic phase of ZrTiO₄. Therefore, NiSO₄/TiO₂–ZrO₂ calcined at 500 °C exhibited a maximum catalytic activity for acid catalysis, and then the catalytic activity decreased with the calcination temperature. The correlation between catalytic activity and acidity held for both reaction, 2-propanol dehydration and cumene dealkylation. NiSO₄ supported on 50TiO₂–50ZrO₂ (TiO₂/ZrO₂ ratio = 1) among TiO₂–ZrO₂ binary oxides exhibited the highest catalytic activity for acid catalysis.     

Effect of γ-ray Irradiation on the Wettability of TiO2 Single Crystals

The contact angles of the water droplets on TiO₂ single crystal surfaces decreased and became superhydrophilic state by γ-ray irradiation. It was found that these behaviors were dependent on γ-ray irradiation atmosphere, i.e., in air and in N₂ atmosphere (r. h.; 30%) as well as crystal faces of TiO₂ single crystals, i.e., TiO₂ (100) and (110) surfaces. It was also found from the results of UV–Vis and ESR measurements that γ-ray irradiation under N₂ atmosphere led to the oxygen vacancies and associated Ti³⁺, regardless of the presence of gaseous water. Moreover, it was suggested that the organic molecules adsorbed on TiO₂ single crystal surfaces decomposed by γ-ray irradiation.     

Photochemical Degradation of 1- and 2-methylphenanthrenes in Acetonitrile and on Some Solids

In order to obtain basic information about photochemical degradability and degradation products for 1- and 2-methylphenanthrenes (MPs), photodegradation of these compounds in an organic solvent (acetonitrile) and on some solids (soil, silica gel [SiO₂], and titanium dioxide [TiO₂]) were carried out in a laboratory using solar simulators.

The results showed that the degradation rate (%) increases in the order of TiO₂ > SiO₂ > acetonitrile > soil. Products formed in acetonitrile included aromatic compounds with molecular ion [M⁺] peaks at m/z = 148, 162, 198, 206, 208, 210, 222, 224 and 238. In case of 1 MP, a compound with M⁺ 206, which was identified as 1-phenanthrene-carbaldehyde, was predominantly formed. However, 2MP showed no predominance for M⁺ 206 and also yielded M⁺ 210 and M⁺ 224 as major products. Degradation on soil and SiO₂ gave similar products to those detected in acetonitrile, while that on TiO₂ gave different products from the formers. Based on the results of product analysis, three degradation pathways which may occur in acetonitrile are proposed.     

Structural Study of Copper Oxide Supported on a Ceria-Modified Titania Catalyst System

A structural study of CuO supported on a CeO₂–TiO₂ system was undertaken using X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques. The results of XRD revealed the presence of only two phases, TiO₂ anatase and CeO₂ cerianite. A trend towards smaller TiO₂ crystallites was observed when cerium content increased. When the amount of cerium increased, Ti K-edge XANES analysis showed an increasing distortion of Ti sites. The results of Ce L_III-edge EXAFS showed that Ce atoms are coordinated by eight oxygen atoms at 2.32 Å. For the sample containing a small amount of cerium, the EXAFS analysis indicated that the local structure around Ce atoms was highly distorted. The catalysts presented quite different Cu K-edge XANES spectra compared to the spectra of the CuO and Cu₂O reference compounds. The Cu–O mean bond length was close to that of the CuO and the Cu atoms in the catalysts are surrounded by approximately four oxygen atoms in their first shell. Copper supported on the ceria-modified titania support catalysts displayed a better performance in the methanol dehydrogenation when compared to copper supported only on titania or on ceria.     

Low temperature selective catalytic reduction of NO by NH3 over V2O5 supported on TiO2–SiO2–MoO3

The V₂O₅ catalysts supported on TiO₂–SiO₂–MoO₃ (TSM) prepared by the coprecipitation method were investigated for the selective catalytic reduction (SCR) of NO by NH₃ at low temperatures. The V₂O₅/TSM catalyst with 7–13 wt% SiO₂ was found to exhibit a superior SCR activity and a good sulfur tolerance at low temperatures (<250 °C). The presence of highly active polymeric vanadates formed by the incorporation of MoO₃ to TiO₂–SiO₂ and superior redox properties seems to enhance SCR activity, and furthermore the very lower SO₂ oxidation activity due to the higher acidity leads to the remarkable improvement of sulfur tolerance.     

The effect of foreign ions on the reactivity of the CaO-SiO2-Al2O3-Fe2O3 system: Part II: Cations

The subject of this paper is the effect of foreign cations on the reactivity of the CaO-SiO₂-Al₂O₃-Fe₂O₃ system. One reference mixture and eighteen modified mixtures, prepared by mixing the reference sample with 1% w/w of chemical grade MnO₂, CuO, V₂O₅, PbO, CdO, ZrO₂, Li₂O, MoO₃, Co₂O₃, NiO, WO₃, ZnO, Nb₂O₅, CrO₃, Ta₂O₅, TiO₂, BaO₂ and H₃BO₃ were studied. The effect on the reactivity is evaluated on the basis of the free lime content in samples sintered at 1200 and 1450 °C. At 1200 °C, the reactivity of the mixture is greatly increased in the presence of Cu and Li oxides. Based on their effect at 1450 °C, the added elements can be divided into three groups. W, Ta, Cu, Ti and Mo show the most positive effect, decreasing the free CaO (fCaO) content by 30-60%, compared with the pure sample. Cr and B cause an increase of fCaO content, while the rest of the elements exhibit a marginal positive effect. According to their volatility at 1450 °C, the added compounds can be subdivided into three groups of low (Ti⁴⁺, Cu²⁺, Mo⁶⁺, W⁺⁶, V⁵⁺, Zn²⁺, Zr⁴⁺), moderate (Cr⁶⁺, Co³⁺, Ni²⁺, Mn⁴⁺) and high volatility (Cd²⁺, Pb²⁺). All burned samples, analyzed by means of X-ray diffraction, have a final mineralogical composition, which corresponds to the structure of a typical clinker.