Temperature-programmed reduction of V2O5 and coprecipitated V2O5−TiO2 by hydrogen

Temperature-programmed reductions (TPR) with H₂ of both pure V₂O₅ and coprecipitated V₂O₅?TiO₂ systems with different titanium concentrations was performed. The original and the reduced samples following each TPR step were characterized by X-ray diffraction, Fourier transform infrared analysis and scanning electron microscopy. Within the temperature range in which TPR analysis was carried out (100–600°C) the V₂O₅ phase was reduced in two or three steps, while no variation in the TiO₂ phase (anatase or rutile) was observed. In the first reduction step only superficial reduction of the oxides was detected. In the following steps, the H₂ reacted with oxygen atoms of the V=O and V?O?V bonds. This led to important changes in the structure and morphology of the system. The experimental evidence allowed the conclusion that titanium stabilizes certain phases of vanadium oxides in which vanadium appears as V(+4) or as a mixture of V(+4) and V(+5). Moreover, when moderate and high titanium concentrations were used, the reduction temperature of the bulk V₂O₅ decreased markedly.     

17O MAS and 3QMAS NMR investigation of crystalline V(2)O(5) and layered V(2)O(5).nH(2)O gels

The environments for oxygen sites in crystalline V(2)O(5) and in layered vanadia gels produced via sol-gel synthesis have been investigated using (17)O MAS and 3QMAS NMR. For crystalline V(2)O(5), three structural oxygen sites were observed: V=O (vanadyl), V(2)O (doubly coordinated), and V(3)O (triply coordinated). Line-shape parameters for these sites were determined from numerical simulations of the MAS spectra. For the vanadia gels at various stages of dehydration, assignments have been proposed for numerous vanadyl, doubly coordinated, and triply coordinated oxygen sites. In addition, by correlating the (17)O MAS and 3QMAS NMR, (51)V MAS NMR, and thermogravimetric analysis data, the coordination of water sites has been established. On the basis of these results, the gel structure and its evolution at various stages of hydration have been detailed. Upon rehydration of the layered gel, we observed a preferred site for initial water readsorption. The oxygen atoms of these readsorbed water molecules readily exchanged into all types of oxygen sites even at room temperature.     

Theoretical and experimental determination of the electronic structure of V(2)O(5), reduced V(2)O(5-x) and sodium intercalated NaV(2)O(5)

In this work the electronic structure of V(2)O(5), reduced V(2)O(5-x) (V(16)O(39)) and sodium intercalated NaV(2)O(5) has been studied by both theoretical and experimental methods. Theoretical band structure calculations have been performed using density functional methods (DFT). We have investigated the electron density distribution of the valence states, the total density of states (total DOS) and the partial valence band density of states (PVBDOS). Experimentally, amorphous V(2)O(5) thin films have been prepared by physical vapour deposition (PVD) on freshly cleaved highly oriented pyrolytic graphite (HOPG) substrates at room temperature with an initial oxygen understoichiometry of about 4%, resulting in a net stoichiometry of V(2)O(4.8). These films have been intercalated by sodium using vacuum deposition with subsequent spontaneous intercalation (NaV(2)O(5)) at room temperature. Resonant V3p-V3d photoelectron spectroscopy (ResPES) experiments have been performed to determine the PVBDOS focusing on the calculation of occupation numbers and the determination of effective oxidation state, reflecting ionicity and covalency of the V-O bonds. Using X-ray absorption near edge spectra (XANES) an attempt is made to visualize the changes in the unoccupied DOS due to sodium intercalation. For comparison measurements on nearly stoichiometric V(2)O(5) single crystals have been performed. The experimental data for the freshly cleaved and only marginally reduced V(2)O(5) single crystals and the NaV(2)O(5) results are in good agreement with the calculated values. The ResPES results for V(2)O(4.8) agree in principle with the calculations, but the trends in the change of the ionicity differ between experiment and theory. Experimentally we find partly occupied V 3d states above the oxygen 2p-like states and a band gap between these and the unoccupied states. In theory one finds this occupation scheme assuming oxygen vacancies in V(2)O(5) and by performing a spin-polarized calculation of an antiferromagnetic ordered NaV(2)O(5.).     

负载型复合半导体V2O5-TiO2/SiO2表面V2O5和TiO2的相互修饰作用

 采用表面改性法制备了负载型复合半导体材料V2O5-TiO2/SiO2,并用X射线衍射、比表面积测定、拉曼光谱、程序升温还原和紫外-可见漫反射光谱等技术对固体材料的结构和光响应性能进行了表征. 结果表明,V2O5和TiO2在负载型复合半导体V2O5-TiO2/SiO2表面有相互修饰的作用. 一方面,V2O5能扩展TiO2的光响应范围,使TiO2的吸光区域由紫外光区拓宽至可见光区,从而提高了复合半导体对光能的利用率; 另一方面,TiO2则有助于提高V2O5在载体表面的分散程度,抑制VOx的聚合,减小V2O5的微晶尺寸,提高固体材料的能隙值和氧化还原能力.     

Investigations of V(2)O(5)-based LPG sensors

Sensor characteristics of V(2)O(5) dispersed on oxide supports such as Al(2)O(3), TiO(2) and ZrO(2) with respect to various gases and vapours including liquefied petroleum gas (LPG) have been investigated. Of all the systems studied, 20 mol% V(2)O(5) dispersed on ZrO(2) shows the highest sensitivity for LPG, the log sensitivity-log concentration (in ppm) plots being linear up to 1000 ppm or more. The sensitivity is not affected by humidity or recycling. Addition of P(2)O(5) to V(2)O(5) however destroys the sensitivity. Considering all aspects, 20 mol% V(2)O(5)/ZrO(2) is suggested for use as a practical LPG sensor. ESR spectroscopy indicates the formation of V(4+) species on exposure of V(2)O(5)/ZrO(2) or TiO(2) to LPG. In-situ high-temperature x-ray diffraction measurements show the formation of an unusual monoclinic form of VO(2) on exposure to LPG at 625 K which gets oxidized back to V(2)O(5) on exposure to air.     

V2O5/TiO2 Catalytic Xerogels Raman and EPR Studies

Raman spectroscopy and Electron Paramagnetic Resonance (EPR) studies were performed on a series of V₂O₅/TiO₂ catalysts prepared by a modified sol-gel method in order to identify the vanadium species. Two species of surface vanadium were identified by Raman measurements, monomeric vanadyls and polymeric vanadates. Monomeric vanadyls are characterized by a narrow Raman band at 1030 cm^–1 and polymeric vanadates by two broad bands in the region from 900 to 960 cm^–1 and 770 to 850 cm^–1. The Raman spectra do not exhibit characteristic peaks of crystalline V₂O₅. These results are in agreement with those of X-ray Diffractometry (XRD) and Fourier Transform Infrared (FT-IR) previously reported (C.B. Rodella et al., J. Sol-Gel Sci. Techn., submitted). At least three families of V⁴⁺ ions were identified by EPR investigations. The analysis of the EPR spectra suggests that isolated V⁴⁺ ions are located in sites with octahedral symmetry substituting for Ti⁴⁺ ions in the rutile structure. Magnetically interacting V⁴⁺ ions are also present as pairs or clusters giving rise to a broad and structureless EPR line. At higher concentration of V₂O₅, a partial oxidation of V⁴⁺ to V⁵⁺ is apparent from the EPR results.     

Enhancement of intercalation properties of V2O5 film by TiO2 addition

Although it is well-known that TiO2 incorporation can greatly improve the cyclic stability of V2O5, the influences of TiO2 addition on the Li+ intercalation properties of V2O5 remain an issue of debate in literature. In this paper, we report on a systematic investigation of the preparation and intercalation properties of V2O5-TiO2 mixture films. The present work demonstrates that high Li+ intercalation rates and capacity in V2O5 films are achievable with TiO2 addition. For example, the addition of 20 mol % Ti into V2O5 polycrystalline demonstrated an approximated 100% improvement in Li+ intercalation performance as compared to single V2O5 electrodes. Such enhancement in intercalation properties of V2O5 films with TiO2 addition was attributed to changes in microstructure, crystallinity, and also a possible lattice structure and interaction force between adjacent layers in V2O5.     

Factors determining the selective exposure of (010) plane of V2O5 Catalysts supported on various TiO2

V₂O₅ supported on various TiO₂ including anatase, rutile and mixtures of both have been investigated with various physicochemical measurements such as BET, NH₃-TPD, NARP, XRD and so on, and the effect of the crystal phase of the TiO₂ support on the structure of the supported V₂O₅ was discussed. It has been found that the V=O species on the (010) plane of V₂O₅ on the TiO₂ supports with large BET surface area are selectively exposed to the surface, though different crystal phases of TiO₂ result in different characteristics of exposure of the (010) plane. Anatase gives the maximum exposure of the surface V=O species at significantly lower surface concentration of V₂O₅ than that for rutile. For the mixture of both, two maxima are obtained at the surface concentration of V₂O₅ corresponding to those for anatase and rutile respectively. The chemical activity of the TiO₂ surface also seems to have an effect on the exposure of the (010) plane. That is, the higher surface area and the stronger acidic property resulted in the higher exposure of the (010) plane. On leave from: Department of Applied Chemical Engineering, South-China University of Science and Technology, Guangzhou 510641, P.R. China     

Thermal behaviour of high surface area V2O5/TiO2 catalysts

Thermal analysis (TG and DTA) was employed for the characterization of V₂O₅/TiO₂ catalysts supported on high surface area TiO₂. The results obtained are consistent with a uniform spreading of vanadium oxide on TiO₂ surface for V₂O₅ content less than 15% by weight.The presence of V₂O₅ on the surface of TiO₂ affects the anatase-rutile phase transition lowering the temperature at which it occurs.DTA measurements, performed on catalysts after many months from the preparation, show the appearance of an exothermic peak in the range 280°–340°C. This signal has been related to the oxidation of V(IV) to V(V) on the catalyst surface.Catalysts characterization, performed by chemical analysis and FT-IR spectroscopy, has confirmed this interpretation.It has been suggested that a slow modification of the catalyst occurs, leading to an increase of the V(IV) content during the time.

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Zusammenfassung Zur Charakterisierung von V₂O₅/TiO₂-Katalysatoren auf hochoberflächigem TiO₂ Trägermaterial wurde die Thermoanalyse (TG und DTA) angewendet. Für einen V₂O₅-Gehalt von weniger als 15 Gew.% entsprechen die erhaltenen Ergebnisse einer gleichmäßigen Verteilung des Vanadiumoxides an der TiO₂-Oberfläche.Die Gegenwart von V₂O₅ an der Oberfläche von TiO₂ beeinflußt die Anatas-Rutil-Phasenumwandlung, indem sie die zugehörige Temperatur verringert.DTA-Messungen an Katalysatoren mehrere Monate nach ihrer Herstellung zeigten das Auftreten eines exothermen Peaks im Bereich 280°–340°C. Dieses Signal wurde der Oxidation von V(IV) zu V(V) an der Katalysatoroberfläche zugeschrieben.Diese Interpretation konnte durch eine Charakterisierung des Katalysatoren durch chemische Analyse und FT-IR-Spektroskopie bestätigt werden.Es wurde angedeutet, daß der Katalysator mit der Zeit einer langsamen Modifikation unterliegt, die zu einem Ansteigen des V(IV)-Gehaltes führt.

     

Cr负载V2O5-WO3/TiO2催化剂的低温NH3-SCR脱硝活性

采用浸渍法制备了一系列不同Cr负载量的CrO_x-V₂O₅-WO₃/TiO₂催化剂, 研究了Cr改性对该催化剂脱硝活性及抗硫性能的影响. 结果表明, Cr的引入使V₂O₅-WO₃/TiO₂催化剂的低温NO_x去除效果和抗硫性能显著提升. Cr的负载量为3%(以Cr₂O₃计算质量分数)时, 催化剂具有最佳的低温脱硝活性和N₂选择性. 结构表征结果表明, 改性催化剂中Cr元素主要以Cr³⁺形式存在, Cr的引入使催化剂的氧化性能明显提升, 并使表面弱酸位点、 强酸位点和催化剂表面吸附氧数量增多. XPS分析结果表明, Cr能使催化剂中V⁵⁺/V⁴⁺比例降低, 促进催化剂中自由电子的迁移. In situ DRIFTs表征结果显示, Cr的引入能促进NO与吸附在Brönsted酸位点上的NH₃在低温条件下反应, 从而使催化剂具有更好的低温脱硝活性. UV-Vis光谱分析结果表明, CrO_x负载量提高会显著增加高价态Cr的烧结聚集. 同时, 提出了一种选择性催化还原(SCR)反应中Cr-V协同催化的机理.     

电厂烟气脱硝催化剂V2O5-WO3/TiO2失活机理研究

分别以某电厂失活和新鲜的烟气选择性催化还原(SCR)脱硝催化剂为研究对象,模拟测试催化剂的脱硝效率,并采用扫描电子显微镜-能量色散X射线光谱(SEM-EDX)、X荧光光谱(XRF)、X光电子能谱(XPS)、氮气吸脱附、傅里叶红外光谱(FT-IR)、X射线衍射(XRD)和热重分析(TG)等手段对催化剂的微观结构和表面形态进行表征,探讨了SCR催化剂的失活机制。结果表明,380℃下失活催化剂的脱硝效率(35.0%)和比表面积(1.05 m²/g)明显低于新鲜催化剂(88.2%,72.50 m²/g)。与新鲜催化剂相比,失活催化剂中V⁵⁺的比例由17.4%升高到32.2%,并且表面出现了大量的Al₂(SO₄)₃。另外,SEM和XRD结果表明,失活催化剂出现了烧结。催化剂表面V价态的改变、高温烧结和表面Al₂(SO₄)₃相的大量生成是催化剂失活的主要原因。     

V2O5/TiO2 Catalyst Xerogels: Method of Preparation and Characterization

This work describes a modified sol-gel method for the preparation of V₂O₅/TiO₂ catalysts. The samples have been characterized by N₂ adsorption at 77 K, X-ray Diffractometry (XRD), Scanning Electronic Microscopy (SEM/EDX) and Fourier Transform Infrared Spectroscopy (FT-IR). The surface area increases with the vanadia loading from 24 m² g^–1 for pure TiO₂ to 87 m² g^–1 for 9 wt% of V₂O₅. The rutile form is predominant for pure TiO₂ but becomes enriched with anatase phase when vanadia loading is increased. No crystalline V₂O₅ phase was observed in the diffractograms of the catalysts. Analysis by SEM showed heterogeneous granulation of particles with high vanadium dispersion. Two species of surface vanadium were observed by FT-IR spectroscopy: a monomeric vanadyl and polymeric vanadates. The vanadyl/vanadate ratio remains practically constant. Ethanol oxidation was used as a catalytic test in a temperature range from 350 to 560 K. The catalytic activity starts around 380 K. For the sample with 9 wt% of vanadia, the conversion of ethanol into acetaldehyde as the main product was approximately 90% at 473 K.     

An Amine-Functionalized Titanium Metal-Organic Framework Photocatalyst with Visible-Light-Induced Activity for CO(2) Reduction

Let your light shine: The photocatalytic reduction of carbon dioxide to the formate anion under visible light irradiation is for the first time realized over a photoactive Ti-containing metal-organic framework, NH(2) -MIL-125(Ti), which is fabricated by a facile substitution of ligands in the UV-responsive MIL-125(Ti) material (see scheme; TEOA=triethanolamine).     

掺杂镧的TiO2纳米粒子的光致发光及其光催化性能

采用溶胶-凝胶法制备了纯的和掺杂La的TiO2纳米粒子,并利用XRD,TEM,XPS和荧光光谱(FS)等对样品进行表征,主要考察焙烧温度和La含量对TiO2纳米粒子的性质以及光催化降解苯酚活性的影响,并探讨了La的掺杂对TiO2相变的作用机制以及FS光谱与光催化活性的关系.结果表明,适量La掺杂能够提高TiO2纳米粒子FS光谱强度,这是因为La掺杂能够使表面氧空位和缺陷的浓度增加;600℃热处理的掺杂不同量La的TiO2样品的光催化活性顺序是:1%＞1.5%＞3%＞0.5%＞5%＞0%,这与它们的FS光谱强度的顺序是一致的,即FS光谱强度越高,其光催化活性越高.这是因为在光致发光过程中,FS信号主要来源于表面氧空位和缺陷,而在光催化反应过程中,表面氧空位和缺陷能够有利于光生电子被捕获.     

钠盐对V_2O_5-WO_3/TiO_2催化剂NH_3-SCR反应活性的影响

采用浸渍法制备了V_2O_5-WO_3/TiO_2催化剂,并通过浸渍不同浓度的Na_2SO_4和NaCl,研究了不同的钠盐种类及含量对催化剂NH_3-SCR反应活性的影响,以考察催化剂的抗碱性,并采用BET、XRD、XPS、FT-IR和NH_3-TPD等测试技术对催化剂进行了结构与性能表征.研究结果表明钠盐降低了催化剂的比表面积,导致催化剂表面V~(5+)=O和V-OH酸性位点数量以及表面化学吸附氧含量降低,从而降低了催化剂的反应活性.催化剂钠盐中毒后NO转化率显著降低,脱硝温度窗口变窄.     

Structural and electrical characterisation of Li(2)O : TiO(2) : SnO(2) : P(2)O(5) electrolyte glass

Glasses of general formula 50Li(2)O : xSnO(2) : (10 -x)TiO(2) : 40P(2)O(5)(0.0 < or = x < or = 10) were investigated by differential scanning calorimetry, X-ray diffraction and ac impedance, (31)P solid-state NMR and IR spectroscopies. Three isotropic resonances can be identified in the (31)P NMR spectra, which have been assigned to various phosphate species. Analysis of the ratios of integrated intensities in the (31)P spectra leads to models for the Ti and Sn coordination environments. Both TiO(2) and SnO(2) are found to be predominantly network forming with Ti and Sn proposed to be in five- and four-coordinate environments respectively. Analysis of ac impedance spectra collected at low temperatures reveals two forms of permittivity dispersion, viz: high frequency conductivity dispersion and Cole-Cole type relaxation of permittivity. The activation energy of the relaxation frequency of the permittivity dispersion is equal to that of the dc conductivity, which is consistent with cooperative motion of lithium ions. The results also suggest that the observed increase in conductivity with temperature appears to be mainly due to an increase in mobility rather than increase in carrier concentration.     

ZnVSe2O7 and Cd6V2Se5O21: new d10 transition-metal selenites with V(IV) or V(V) cations

Two new metal selenites with a combination of vanadium(IV) or vanadium(V) cations, namely, ZnVSe 2O 7 and Cd 6V 2Se 5O 21, have been synthesized by hydrothermal and high-temperature solid-state reactions, respectively. The structure of ZnVSe 2O 7 features a 3D network of vanadium(IV) selenite with 1D tunnels occupied by zinc(II) ions. The 3D network of vanadium(IV) selenite is formed by corner-sharing V (IV)O 6 octahedral chains bridged by selenite groups. In Cd 6V 2Se 5O 21, the interconnection of cadmium(II) ions by bridging and chelating selenite groups led to a 3D framework with large tunnels along the b axis, and the 1D chains of corner-sharing V (V)O 4 tetrahedra are inserted in the above large tunnels and are bonded to the cadmium selenite framework via Cd-O-V bridges. Both compounds exhibit broad emission bands in the blue-light region. Results of magnetic property measurements show there is significant antiferromagnetic interaction between V (4+) centers in ZnVSe 2O 7. The electronic structure calculations for both compounds have been also performed.     

Visible-Light-Induced Photocatalytic Hydrogen Production over Binuclear Ru(II) -Bipyridyl Dye-Sensitized TiO(2) without Noble Metal Loading

Highly efficient, visible light induced photocatalytic H(2) production was achieved over a TiO(2) system sensitized by binuclear Ru(II) bipyridyl (bpy) complex [Ru(2) (bpy)(4) (BL)](ClO(4) )(2) (BL=bridging ligand) without Pt loading, which is almost unaffected by pH in aqueous solution in the wide range from pH?5.00 to 10.50, although the dye molecules can only be loosely attached to TiO(2) due to the absence of terminal carboxyl groups. The photocatalyst shows remarkable long-term stability and reproducibility of H(2) evolution even after exchanging the aqueous triethanolamine solution. The amount of H(2) evolved over 100?mg of photocatalyst in 27?h of irradiation corresponds to a turnover number of about 75?340, and the apparent quantum yields are estimated to be 16.8 and 7.3?% under 420 and 475?nm monochromatic light irradiation, respectively. A comparative study shows that the loosely attached dye [Ru(2) (bpy)(4) (BL)](ClO(4) )(2) has higher photosensitization efficiency than tightly linked dyes with terminal carboxyl groups, such as [Ru(2) (dcbpy)(4) (BL)](ClO(4) )(2) and N719. It can be rationalized by their different coordination, physicochemical, electron-injection, and back-transfer properties.