Modeling the active centers of V<Subscript>2</Subscript>O<Subscript>5</Subscript>/SiO<Subscript>2</Subscript> and V<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript> supported catalysts. DFT theoretical analysis of optical properties

Within the framework of the density functional theory (DFT), the electronic structure of monooxodioxovanadium functional groups in tetrahedral coordination, which model the active centers (ACs) of fine supported catalysts V₂O₅/SiO₂ and V₂O₅/TiO₂, has been analyzed. The optimal structures of three ACs as possible models of monomeric and polymeric oxovanadium forms on the carriers with low vanadium content were determined. The modified DFT method involving the time dependence of Kohn-Sham equation (TDDFT) was used for the adopted AC models to calculate the energies of the excited states, and optical spectra of the absorption in 25000–60000 cm^?1 region were reconstructed on their base. The spectrum in this region is due to O → V charge transfer. The features of electronic spectra with the charge transfer for V₂O₅/SiO₂ and V₂O₅/TiO₂ catalysts and the vibrational spectra of three AC models corresponding to the monomeric and dimeric oxovanadium forms of the supported catalysts V₂O₅/SiO₂ and V₂O₅/TiO₂ were defined. The detailed interpretation of normal vibration frequencies is given. The frequencies typical of the monomeric and dimeric oxovanadium forms on the carrier surface were identified.     

Models of finely dispersed MgO and V<Subscript>2</Subscript>O<Subscript>5</Subscript> on silica. Theoretical analysis of optical properties using TDDFT

The results of Density Functional Theory (DFT) calculations on optical properties of vanadium complexes VOCl₃, VOCl₄ ^-, VOCl₅ ^2-, as well as the VO₄ ^3- ion, are presented. The spectra of excited states in the range 25000-60000 cm^-1 have been analyzed using the time-dependent DFT method (TDDFT). Spectroscopic features of structural defects (low-coordinated (LC) oxygen ions), as well as surface point defects (F⁺ and F sites) in MgO, have been studied within the cluster approach. The charge-transfer spectra and frequencies of normal vibrations for a number of active site models of finely dispersed oxides MgO and V₂O₅ on silica have been calculated. Comparison of the obtained results with experimental electronic diffuse reflectance spectra and fundamental frequencies confirms a hypothesis about the structure of active centers of finely dispersed oxide V₂O₅ on silica as monomeric forms, (O=V-O _n ).     

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.     

Dispersion state and catalytic properties of vanadia species on the surface of V<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript> catalysts

The dispersion state and catalytic properties of anatase-supported vanadia species are studied by means of X-ray diffraction (XRD), laser Raman spectroscopy (LRS), H₂ temperature-programmed reduction (TPR) and the selective oxidation of o-xylene to phthalic anhydride. The almost identical values of the experimental dispersion capacity of V₂O₅ on anatase and the surface vacant sites available on the preferentially exposed (001) plane of anatase suggest that the highly dispersed vanadium cations are bonded to the vacant sites on the surface of anatase as derived by the incorporation model. When the loading amount of V₂O₅ is far below its dispersion capacity, the dispersed vanadia species might mainly consist of isolated VO_x species bridging to the surface through V-O-Ti bonds. With the increase of V₂O₅ loading the isolated vanadia species interact with their nearest neighbors (either isolated or polymerized vanadia) through bridging V-O-V at the expenses of V-O-Ti bonds, resulting in the increase of the ratio of polymerized to isolated vanadia species and the decrease of the reactivity of the associated surface oxygen anions and, consequently, although the activity increases with loading to reach a maximum value, the turn over number (TON) of the V₂O₅/TiO₂ catalyst decreases linearly. When the loading amount of V₂O₅ is higher than its dispersion capacity, the turn over number decreases more rapidly with the increase of V₂O₅ loading due to the formation of V₂O₅ crystallites in which the oxygen anions associated with V-O-V bonds are less reactive and only partially exposed on the surface.     

酸性助剂对V2O5/TiO2催化剂甲醇选择氧化为甲缩醛的影响

研究了酸性助剂对TiO₂纳米管(TNT)负载的V₂O₅催化剂(V₂O₅/TNT)性能的影响, 发现经硫酸、磷酸或磷钨酸处理后, TNT的结构稳定, 但表面酸性和氧化-还原性发生了变化, 从而改变了甲醇选择氧化为甲缩醛的催化性能. 实验结果表明, V₂O₅/TNT催化剂经硫酸修饰和673 K焙烧, 其甲缩醛选择性显著提高, 且维持了较高的甲醇转化率. 催化剂表征表明, 高温焙烧促进了硫酸根与钒物种之间的强相互作用, 从而提高了催化剂的表面酸性而没有降低钒的氧化-还原性. 磷酸和磷钨酸修饰虽然也提高了V₂O₅/TNT催化剂的表面酸性, 但降低了其中钒氧化物的氧化-还原能力, 反而降低了催化剂的活性.     

Selective oxidation of hydrogen sulfide to ammonium thiosulfate and sulfur over vanadium-bismuth oxide catalysts

The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium-bismuth mixed oxide catalysts. The investigation was focused on understanding the complex reaction steps and the roles of each metal oxide. Therefore, supported V₂O₅/TiO₂, V-Bi-O/TiO₂ catalysts and a mechanical mixture of V₂O₅ + Bi₂O₃ were tested in the reaction. Ammonia reacted either with H₂S or SO₂, produced from the oxidation of H₂S. Water vapor promoted the reaction of ammonia and SO₂. Strong synergistic phenomena in catalytic activity were observed for the mechanically mixed catalyst of V₂O₅ and Bi₂O₃. V-Bi-O/TiO₂ catalyst showed very high H₂S conversion without any considerable emission of SO₂. Temperature-programmed studies (TPR and TPO), XRD and Raman analyses revealed that the high catalytic performance of V-BiO/TiO₂ catalyst originated from the high redox capacity of the bismuth vanadate phase.     

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     

Chemistry of O‐ and H‐Containing Species on the (001) Surface of Anatase TiO2: A DFT Study

The chemistry of oxygen, hydrogen, water, and other species containing both oxygen and hydrogen atoms on the anatase TiO₂ (001) surface is investigated by DFT. The adsorption energy of atoms and radicals depends appreciably on the position and mode of adsorption, and on the coverage. Molecular hydrogen and oxygen interact weakly with the clean surface. However, H₂O dissociates spontaneously to give two nonidentical hydroxyl groups, and this provides a model for hydroxylation of TiO₂ surfaces by water. The mobility of the hydroxyl groups created by water splitting is initially impeded by a diffusion barrier close to 1 eV. The O₂ adsorption energy increases significantly in the presence of H atoms. Hydroperoxy (OOH) formation is feasible if at least two H atoms are present in the direct vicinity of O₂. In the adsorbed OOH, the O? O bond is considerably lengthened and thus weakened.     

掺杂钒和硅对TiO2薄膜超亲水性的影响

0引言 TiO2薄膜是众多氧化物半导体薄膜中研究最为广泛的一种材料．其表面的超亲水性和表面自清洁效应开辟了光催化薄膜功能材料的新的研究领域，已成为众多研究者研究的对象。但是如果薄膜仅由TiO2组成，当光照停止，水在TiO2薄膜表面的润湿角逐渐升高．并恢复原始状态。TiO2的禁带较宽，普通光线如太阳光等都不能将其激发．限制了其实际应用。因此如何使TiO2材料的光谱响应范围由紫外光反扩展到可见．光区一日如何更长时间地保持薄膜良好的亲水性是目前研究的重点。     

V2O5-TiO2复合半导体光催化材料结构及光响应性能研究

采用溶胶凝胶法制备了V₂O₅-TiO₂复合半导体材料，通过Raman、XRD及UV-Vis DRS等实验方法研究了V₂O₅与TiO₂复合对材料表面组成、晶体结构以及光响应性能的影响。结果表明：钒加入后优先与TiO₂作用形成较为稳定的金红石型TiVO₄晶相，其中V⁴⁺是促进TiO₂发生相变的关键；随着钒加入量的增加，V₂O₅由表面高分散状态逐渐聚集形成晶相，并释放部分Ti⁴⁺使之形成锐钛矿型TiO₂晶相，使得体相中金红石型TiO₂的含量有所下降；复合后形成的TiVO₄晶相显著提高了材料对可见光的吸收率，并使其吸光域红移至460 nm左右。     

HREM Study of Nanostructured Molybdenum and Vanadium Doped Titania Catalysts

MoO₃/V₂O₅/TiO₂ (anatase) catalyst with 5 and 20 wt.% loadings of MoO₃ and V₂O₅ has been studied, using high resolution electron microscopy. Main structural peculiarity of this system was found to be the presence of nanometer size layers of a complex Mo-V-O phase on the surface of the titania support. The observed structure seems to be a metastable and may exist only on the surface of TiO₂ particles.     

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.     

Impact of surface hydroxylation in MgO-/SnO-nanocluster modified TiO2 anatase (101) composites on visible light absorption,charge separation and reducibility

Anatase TiO₂ surfaces, whether oxidised or hydroxylated, can be modified by nanoclusters of SnO and MgO to give a red shift in light absorption, enhanced charge separation and high reducibility.     

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.     

Density Functional Theory Study of the Point Defects on KDP (100) and (101) Surfaces

Surface defects are usually associated with the formation of other forms of expansion defects in crystals, which have an impact on the crystals’ growth quality and optical properties. Thereby, the structure, stability, and electronic structure of the hydrogen and oxygen vacancy defects (V_H and V_O) on the (100) and (101) growth surfaces of KDP crystals were studied by using density functional theory. The effects of acidic and alkaline environments on the structure and properties of surface defects were also discussed. It has been found that the considered vacancy defects have different properties on the (100) and (101) surfaces, especially those that have been reported in the bulk KDP crystals. The (100) surface has a strong tolerance for surface V_H and V_O defects, while the V_O defect causes a large lattice relaxation on the (101) surface and introduces a deep defect level in the band gap, which damages the optical properties of KDP crystals. In addition, the results show that the acidic environment is conducive to the repair of the V_H defects on the surface and can eliminate the defect states introduced by the surface V_O defects, which is conducive to improving the quality of the crystal surface and reducing the defect density. Our study opens up a new way to understand the structure and properties of surface defects in KDP crystals, which are different from the bulk phase, and also provides a theoretical basis for experimentally regulating the surface defects in KDP crystals through an acidic environment.     

Defect formation and magnetic properties of carbon-doped ZnO nanowires by the first principles

Theoretical calculation based on density functional theory (DFT) and local density approximation (LDA) with Hubbard parameters has been carried out in studying defect formation energy, transition energy and ferromagnetism of carbon-doped ZnO nanowires (NW). The formation and ionization characteristics of the defects [C_O (B), C_O (S), C_Zn (B), V_O (B), V_Zn (B), I_O (oct) and I_Zn (oct)] in ZnO NW are analyzed in the text. Ferromagnetic (FM) and antiferromagnetic (AFM) coupling between C atoms are also investigated by 9 different configurations. The FM and AFM stability are explained by the interaction of C energy level. In addition, the vacancies [V_O (B) and V_Zn (B)] and interstitials [I_O (oct) and I_Zn (oct)] affecting the FM coupling are also investigated. It is found that magnetic moment of C 2p can be mediated by these defects.     

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.

     

Active Sites for Adsorption and Reaction of Molecules on Rutile TiO2(110) and Anatase TiO2(001) Surfaces (cited: 1)

The reactivity of specific sites on rutile TiO₂(110)-(1×1) surface and anatase TiO₂(001)-(1×4) surface has been comparably studied by means of high resolution scanning tunneling microscopy. At the rutile TiO₂(110)-(1×1) surface, we find the defects of oxygen vacancy provide distinct reactivity for O₂ and CO₂ adsorption, while the terminal fivefold-coordinated Ti sites dominate the photocatalytic reactivity for H₂O and CH₃OH dissociation. At the anatase TiO₂(001)-(1×4) surface, the sixfold-coordinated terminal Ti sites at the oxidized surface seem to be inert in both O₂ and H₂O reactions, but the Ti-rich defects which introduce the Ti³⁺ state into the reduced surface are found to provide high reactivity for the reactions of O₂ and H₂O. By comparing the reactions on both rutile and anatase surfaces under similar experimental conditions, we find the reactivity of anatase TiO₂(001) is actually lower than rutile TiO₂(110), which challenges the conventional knowledge that the anatase (001) is the most reactive TiO₂ surface. Our findings could provide atomic level insights into the mechanisms of TiO₂ based catalytic and photocatalytic chemical reactions.     

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.