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.     

Effect of the surface vanadium valence state on activity and selectivity properties of (VO)2P2O7 used as catalyst in the ammoxidation of toluene

The effect of the surface vanadium valence state of a (VO)₂P₂O₇ catalyst, which was changed by reductive as well as oxidative pretreatment, on its catalytic properties in toluene ammoxidation to benzonitrile is reported.     

载体对钒铬氧化物催化剂结构及催化3-甲基吡啶氨氧化反应活性的影响

通过等体积浸渍法制备了Nb₂O₅,MgF₂,TiO₂和SiO₂负载和未负载的钒铬氧化物催化剂,并应用X射线衍射、N₂吸附-脱附、H₂程序升温还原和NH₃程序升温脱附对催化剂进行了表征. 结果表明,催化剂比表面积较大时有利于CrVO₄-I(单斜)的生成,比表面积较小则有利于CrVO₄-III(斜方)的生成. 在310-400℃下3-甲基吡啶氨氧化制备3-氰基吡啶反应中,具有较高催化活性的催化剂与V物种还原性较高和表面活性位数量较多有关; 而高的3-氰基吡啶选择性则与催化剂表面较低的酸性密切相关. 升高反应温度可大幅度提高催化剂的活性,且其选择性基本不变.     

Vanadium oxide-porous phosphate heterostructure catalysts for the selective oxidation of H2S to sulphur

Vanadium oxide–containing mesoporous materials, based on a surfactant expanded zirconium phosphate with silica galleries into the interlayer space, named porous phosphate heterostructure (PPH), were prepared by using TEOS and vanadium oxytripropoxide in n-propanol as sources of Si and V, respectively; with different Si/V molar ratios of 1, 2, 5 and 25; and calcining at 550 °C for 6 h. Using this method, vanadium can be partially incorporated to the structure of the gallery, but the surface area strongly decreases and the appearance of V₂O₅ crystallites increases when increasing the vanadium content. The catalysts were characterized by XRD, XPS, TEM, and Raman, and tested in the selective catalytic oxidation of H₂S using a fixed bed reactor, at atmospheric pressure, at 180–260 °C. The catalysts with high contents of vanadium are very active at 200 °C, showing H₂S conversions of 85–99%, with a high selectivity to elemental sulphur and with a low formation of SO₂. Accordingly V₂O₅ crystallites can be proposed as active and selective although the catalytic behavior is related to the number of accessible V-sites in the surface of the catalyst.     

Cluster Models of the VOx/TiO2 Supported Catalyst

Molecular structures of the active vanadium phase of the VO _x /TiO₂ supported catalyst are calculated in the framework of the cluster approximation of density functional theory (DFT). It is shown that vanadium can be stabilized on the anatase (001) surface both in the tetrahedral and octahedral coordinations with the formation of monoxo- and dioxovanadyl structures. The energy of the dioxovanadyl structure binding to the support surface is 600–800 kJ/mol. The formation of dioxovanadyl structures from monoxovanadyl ones with the formation of water molecules is energetically favorable. The effect of support on the electronic state and acidic properties of the supported vanadium phase is discussed.     

Selective photocatalytic oxidation of alcohols to aldehydes in water by TiO2 partially coated with WO3

Semiconductor TiO₂ particles loaded with WO₃ (WO₃/TiO₂), synthesized by impregnation of tungstic acid followed by calcination, were used for photocatalytic oxidation of alcohols in water with molecular oxygen under irradiation at λ>350 nm. The WO₃/TiO₂ catalysts promote selective oxidation of alcohols to aldehydes and show higher catalytic activity than pure TiO₂. In particular, a catalyst loading 7.6 wt % WO₃ led to higher aldehyde selectivity than previously reported photocatalytic systems. The high aldehyde selectivity arises because subsequent photocatalytic decomposition of the formed aldehyde is suppressed on the catalyst. The TiO₂ surface of the catalyst, which is active for oxidation, is partially coated by the WO₃ layer, which leads to a decrease in the amount of formed aldehyde adsorbed on the TiO₂ surface. This suppresses subsequent decomposition of the aldehyde on the TiO₂ surface and results in high aldehyde selectivity. The WO₃/TiO₂ catalyst can selectively oxidize various aromatic alcohols and is reusable without loss of catalytic activity or selectivity.     

Effect of calcination temperature on the properties of industrial V2O5-TiO2 (anatase) catalysts in methanol oxidation

The properties of the catalysts for partial oxidation of o-xylene depend on the structure of the supported vanadium sites. The structure itself is strongly dependent on the calcination temperature of the catalyst at which thermal deposition of the metal oxide on the oxide support takes place. We have investigated the effect of calcination temperature on the activity and selectivity of industrial V₂O₅-TiO₂ (anatase) supported catalysts designed for partial oxidation of o-xylene in their application to methanol oxidation.This revised version was published online in December 2005 with corrections to the Cover Date.     

Ethylene polymerization with a supported vanadium catalyst obtained by the molecular deposition method

An active-phase monolayer has been deposited on SiO₂ using replacement of the surface OH groups by VOCl₃ vapour. The amount of vanadium fixed on the SiO₂ surface depends on the initial concentration of the silanol groups and ranges from 3.36 to 1.43%. In combination with diethyl aluminium chloride, the products are active catalysts for ethylene polymerization. The effects of the reaction conditions (the time of catalyst-complex formation, the catalyst life time and the temperature of polymerization) as well as the effect of the vanadium content, the A1:V ratio and the presence of diphenyl magnesium on the activity of the catalyst system have been investigated. The catalyst activity was found to depend strongly on the amount of vanadium fixed on the support surface. The maximum productivity obtained is about 22,000 gPE/g vanadium. Some basic characteristics of the synthesized polymer such as tensile strength, elongation at break, density and crystallization degree are given.     

Correlating Oxidation State and Surface Ligand Motifs with the Selectivity of CO2 Photoreduction to C2 Products

The design for non-Cu-based catalysts with the function of producing C₂₊ products requires systematic knowledge of the intrinsic connection between the surface state as well as the catalytic activity and selectivity. In this work, photochemical in situ spectral surface characterization techniques combined with the first principle calculations (DFT) were applied to investigate the relationships between the composition of surface states, coordinated motifs, and catalytic selectivity of a titanium oxynitride catalyst. When the catalyst mediates CO₂ photoreduction, C₂ product selectivity is positively correlated with the surface Ti²⁺/Ti³⁺ ratio and the surface oxidation state is regulated and controlled by coordinated motifs of N−Ti-O/V[O], which can reduce the potential dimerization energy barriers of *CO−CO* and promote spontaneous formation of the subsequent *CO−CH₂* intermediate. This phenomenon provides a new perspective for the design of heterogeneous catalysts for photoreduction of CO₂ into useful products.     

Effect of methane co-feeding on the selectivity of ethylene produced from oxidative dehydrogenation of ethane with CO2 over a Ni-La/SiO2 catalyst

A Ni-La/SiO₂ catalyst was prepared through the incipient wetness impregnation method and tested in the oxidative dehydrogenation of ethane (ODHE) with CO₂. The fresh and used catalysts were characterized by XRD and SEM techniques. The Ni-La/SiO₂ catalyst exhibited catalytic activity for the oxidative dehydrogenation of ethane, but with low ethylene selectivity in the absence of methane. The selectivity to ethylene increased with increasing molar ratio of methane in the feed. The carbon deposited on the catalyst surface in the sole ODHE with CO₂ was mainly inert carbon, while much more filamentous carbon was formed in the presence of methane. The filamentous carbon was easy to be removed by CO₂, which might play a role in improving the conversion of ethane to ethylene. The introduction of methane might affect the equilibrium of the CO₂ reforming of ethane and the ODHE with CO₂. As a consequence, the synthesis gas produced from CO₂ reforming of methane partly inhibited the reaction of ethane and promoted the ODHE with CO₂, thus increasing the selectivity of ethylene.     

Application of EPR spectroscopy for elucidation of vanadium speciation in VO x /ZrO2 catalysts subject to redox treatment

Application of EPR spectroscopy corroborated by spectra simulation in speciation studies of the tetravalent vanadium in supported VO _x /ZrO₂ catalyst has been discussed. Implementation of genetic algorithms into automated analysis of the EPR spectra has greatly improved the simulation efficiency. The performance of the new procedure has been benchmarked against common simplex method using the multi-component model and real EPR spectra of tetravalent vanadium in VO _x /ZrO₂ catalysts. The analysis has revealed speciation of vanadium into surface isolated and clustered vanadyl entities and isolated bulk V _Zr ^x ions due to formation of Zr_1?x V _x O₂ solid solution in the near to surface region. The structural heterogeneity of vanadium can be controlled by the calcination temperature and the redox treatment.     

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.     

Mixed matrix vanadium oxide catalytic nanocomposite membrane for styrene oxidation

Mesoporous nanocomposite membranes with vanadium oxide–carbon nanotubes (V_xO_y-CNTs) embedded in γ-Al₂O₃ were successfully synthesized using the dip coating method. The membranes were evaluated for styrene oxidation to determine the optimum styrene conversion and benzaldehyde selectivity. Several factors that influence the preparation of defect-free coatings, such as the type of binder, the binder addition time and surface support treatments, were investigated. The physico-chemical permeation properties of the membranes were characterized using scanning electron microscope, transmission electron microscope (TEM), X-ray Diffraction XRD, Nitrogen adsorption (BET) and Thermogravimetric TGA. Response surface methodology (RSM) was used to investigate the effects of oxidant (H₂O₂) concentration, temperature, contact time and catalyst loading on styrene conversion and the selectivity of benzaldehyde. Based on the RSM analysis, the optimal oxidation conditions included a reaction temperature of 45 °C, a differential pressure of 1.5 bars, a molar ratio of H₂O₂: styrene of 1.5:1 and a catalyst loading of 30 %. These conditions resulted in the maximal styrene conversion of 25.6 and 84.9 % benzaldehyde selectivity.     

Theoretical and Experimental Studies of the Nature of the Catalytic Activity of VO x /TiO2 Systems

The states of supported vanadium and the nature of activation of ammonia adsorbed on vanadium sites of V _x /Ti₂ catalysts are studied by ⁵¹V NMR spectroscopy and diffuse-reflectance IR Fourier-transform (DRIFT) spectroscopy using cluster quantum chemical calculations of N₃ adsorption. We employ the V _x /Ti₂ catalyst of two types: the monolayer catalyst in which vanadium is located on the surface of well-crystallized anatase and the catalyst in which vanadium embedded in the anatase lattice at a rather great depth. It is shown that ammonia is predominantly adsorbed on Lewis acid sites of the monolayer catalyst, whereas most of N₃ adsorbed on the catalyst containing bulk vanadium is in the form of ammonium ions. Analysis of experimental and calculated data suggests that, in the monolayer catalyst, N₃ molecules in the selective reduction of nitrogen oxides are activated on Lewis acid sites. Ammonia activation involves the dissociation of the N–H bond in a coordinated molecule, which results in the formation of the amide V–N₂ group and a water molecule coordinated by a V⁵⁺ ion. It is likely that, in the case of the catalyst containing bulk vanadium, this reaction occurs with the predominant participation of ammonium ions.     

Gas Phase Selective Catalytic Oxidation of Toluene to Benzaldehyde on V2O5-Ag2O／η-Al2O3 Catalyst

Gas phase selective catalytic oxidation of toluene to benzaldehyde was studied on V2O5-Ag2O/η-Al2O3 catalyst prepared by impregnation. The catalyst was characterized by XRD, XPS, TEM,and FT-IR. The catalytic results showed that toluene conversion and selectivity for benzaldehyde on catalyst sample No.4 (V/(V Ag)=0.68) was higher than other catalysts with different V/Ag ratios. This was attributed to the higher surface area, larger pore volume and pore diameter of the catalyst sample No.4 than the other catalysts. The XRD patterns recorded from the catalyst before and after the oxidation reaction revealed that the new phases were developed, and this suggested that silver had entered the vanadium lattice. XPS results showed that the vanadium on the surface of No.4 and No.5 sample was more than that in the bulk, thus forming a vanadium rich layer on the surface. It was noted that when the catalyst was doped by potassium promoter, the toluene conversion and selectivity for benzalde hydewere higher than those on the undoped catalyst. This was attributed to the disordered structure of V2O5 lattice of the K-doped catalyst and a better interfacial contact between the particles.     

The Use of Laser Induced Breakdown Spectroscopy (LIBS) to Study Catalyst Deactivation of V2O5/γ‐Al2O3 as Compared to Inductively Coupled Plasma Optical Emission Spectrometry

Laser Induced Breakdown Spectroscopy (LIBS) method is introduced as a novel approach in this work to study catalyst deactivation of V₂O₅/γ‐‐Al₂O₃ for gas‐phase dehydration of glycerol and producing acrolein. The LIBS results of V₂O₅/γ‐Al₂O₃ samples are compared with those data that are obtained by Inductively Coupled Plasma Optical Emission Spectrometry (ICP‐OES). Experimental data of LIBS data specify that line intensities of vanadium are decreased by deactivation of V₂O₅/γ‐Al₂O₃ catalyst. A comparison between the results of LIBS test as well as ICP‐OES analysis shows that the amount of vanadium is decreased in the catalyst. Moreover, coke formation changes the surface of the catalyst. The results of deactivation of V₂O₅/γ‐Al₂O₃ are also compared with Pd/C catalyst deactivation.     

Gas Phase Selective Catalytic Oxidation of Toluene to Benzaldehyde on V2O5-Ag2O/η-Al2O3 Catalyst

Gas phase selective catalytic oxidation of toluene to benzaldehyde was studied on V2O5-Ag2O/η-Al2O3 catalyst prepared by impregnation. The catalyst was characterized by XRD, XPS, TEM,and FT-IR. The catalytic results showed that toluene conversion and selectivity for benzaldehyde on catalyst sample No.4 (V/(V Ag)=0.68) was higher than other catalysts with different V/Ag ratios. This was attributed to the higher surface area, larger pore volume and pore diameter of the catalyst sample No.4 than the other catalysts. The XRD patterns recorded from the catalyst before and after the oxidation reaction revealed that the new phases were developed, and this suggested that silver had entered the vanadium lattice. XPS results showed that the vanadium on the surface of No.4 and No.5 sample was more than that in the bulk, thus forming a vanadium rich layer on the surface. It was noted that when the catalyst was doped by potassium promoter, the toluene conversion and selectivity for benzaldehyde were higher than those on the undoped catalyst. This was attributed to the disordered structure of V2O5 lattice of the K-doped catalyst and a better interfacial contact between the particles.     

Co-Precipitated Ni-Mg-Al Hydrotalcite-Derived Catalyst Promoted with Vanadium for CO2 Methanation

Co-precipitated Ni-Mg-Al hydrotalcite-derived catalyst promoted with vanadium were synthesized with different V loadings (0–4 wt%) and studied in CO₂ methanation. The promotion with V significantly changes textural properties (specific surface area and mesoporosity) and improves the dispersion of nickel. Moreover, the vanadium promotion strongly influences the surface basicity by increasing the total number of basic sites. An optimal loading of 2 wt% leads to the highest activity in CO₂ methanation, which is directly correlated with specific surface area, as well as the basic properties of the studied catalysts.     

Hydroxyl‐Mediated Non‐oxidative Propane Dehydrogenation over VOx/γ‐Al2O3 Catalysts with Improved Stability

Supported vanadium oxides are one of the most promising alternative catalysts for propane dehydrogenation (PDH) and efforts have been made to improve its catalytic performance. However, unlike Pt‐based catalysts, the nature of the active site and surface structure of the supported vanadium catalysts under reductive reaction conditions still remain elusive. This paper describes the surface structure and the important role of surface‐bound hydroxyl groups on VO_x / γ‐Al₂O₃ catalysts under reaction conditions employing in situ DRIFTS experiments and DFT calculations. It is shown that hydroxyl groups on the VO_x /Al₂O₃ catalyst (V?OH) are produced under H₂ pre‐reduction, and the catalytic performance for PDH is closely connected to the concentration of V?OH species on the catalyst. The hydroxyl groups are found to improve the catalyst that leads to better stability by suppressing the coke deposition.     

Organometallic vanadium-based heterogeneous catalysts for ethylene polymerization. Study of the deactivation process

Slurry polymerizations of ethylene over vanadium catalysts (based on VCl₄ and VOCl₃) and their MgCl₂(THF)₂-supported equivalents were studied. Unsupported vanadium catalysts were found to be unstable while the vanadium active sites deposited on the MgCl₂(THF)₂ complex are stable. A sharply outlined correlation was found between the concentration of vanadium(III) and catalyst productivity. The high activity and stability of the vanadium catalyst when supported on the magnesium complex is attributed to the increase of resistance to reduction of active vanadium(III) to inactive vanadium(II) by an organoaluminium co-catalyst.