Recent topics of research and development of catalysis by niobium and tantalum oxides

Current topics of catalysts containing niobium and tantalum, especially in the field of solid acid catalysis and selective oxidation of hydrocarbons are reviewed. Hydrated niobium oxide and hydrated tantalum oxide are highly acidic. Hydrated niobium oxide is active for the hydration of ethene to ethanol, and Nb–W mixed metal oxide is more active for the reaction. Acid properties of tantalum oxide are changed by being supported on SiO₂. Ta oxide/SiO₂, prepared by the chemical reaction between tantalum alkoxide and surface hydroxyl groups of SiO₂, is active and selective for the gas phase Beckmann rearrangement of cyclohexanoneoxime to caprolactam. Niobium oxide and tantalum oxide easily react with many other oxides to form mixed metal oxide phases with complex structure. Mixed metal oxide catalysts, containing molybdenum, vanadium, certain elements together with niobium are active for the selective oxidation of hydrocarbons. Especially, the selective oxidation of propane by such mixed metal oxide catalysts has been paid attention. Additionally, recent progress of environmental catalysts, promoted by niobium and tantalum compounds, namely catalysts for the pollution abatement is reviewed.     

Activation of propane and butanes over niobium- and tantalum-based oxide catalysts

Niobium and tantalum are important elements for the activation of alkanes in the viewpoints of acidic property and the formation of unique mixed metal oxides. And the difference of the ability of alkane activation between niobium- and tantalum-based oxide catalysts is studied. Although hydrated niobium and tantalum oxides show strong acid property, only hydrated tantalum oxide is activated to a solid superacid by the treatment with sulfuric acid, and isomerizes n-butane to isobutane at room temperature. The sulfuric acid treated tantalum oxide activates P–Mo–V heteropolyacid compounds for the selective oxidation of isobutane to methacrolein (MAL) and methacrylic acid (MAA). The difference of ability of alkanes activation between niobium and tantalum is studied by using surface science technique. Mo–V–Nb–Te mixed metal oxide catalysts are active for the ammoxidation of propane to acrylonitrile (AN). However, Mo–V–Ta–Te mixed metal oxide is less active. The effect of catalyst preparation condition is studied. Mo–V–Nb–Te mixed metal oxide catalysts are also active for the oxidation of propane to acrylic acid (AA).     

Liquid-phase catalytic oxidation of unsaturated fatty acids

Liquid-phase catalytic oxidation of oleic acid with hydrogen peroxide in the presence of various transition metal/metal oxide catalysts was studied in a batch autoclave reactor. Azelaic and pelargonic acids are the major reaction products. Tungsten and tantalum and their oxides in supported and unsupported forms were used as catalysts. Alumina pellets and Kieselguhr powder were used as supports for the catalysts. Tungsten, tantalum, molybdenum, zirconium, and niobium were also examined as catalysts. Tertiary butanol was used as solvent. Experimental results concluded that tungsten and tungstic oxide are more suitable catalysts in terms of their activity and selectivity. The rate of reaction observed in the case of supported catalysts appears to be comparable or superior to that of unsupported catalysts. In pure form, tungsten, tantalum, and molybdenum showed strong catalytic activity in the oxidation reaction; however, except for tantalum the other two were determined to be economically unfeasible. Zirconium and niobium showed very little catalytic activity. Based on the experimental observations, tungstic oxide supported on silica is the most suitable catalyst for the oxidation of oleic acid with 85% of the starting oleic acid converted to the oxidation products in 60 min of reaction with high selectivity for azelaic acid.     

金属氧化物在OX-ZEO催化剂中催化CO x 加氢制低碳烯烃研究进展

金属氧化物-分子筛（OX-ZEO）双功能催化剂可实现CO _x 加氢制低碳烯烃的高选择性转化。本文概述了OX-ZEO催化CO _x 加氢制低碳烯烃反应中金属氧化物的研究进展,通过对CO _x 加氢制甲醇/乙烯反应热力学分析指出了“接力催化”的优势,重点讨论了金属氧化物的种类和组成、制备方法及金属氧化物和分子筛的“亲密度”对催化性能的影响,探讨了催化反应机理、氧空位的作用及抑制副反应的策略。分析了OX-ZEO催化反应面临的问题和挑战,展望了OX-ZEO催化体系的发展趋势,认为通过元素掺杂、助剂修饰、优化制备条件等可提高金属氧化物的氧空位含量,进而可提高催化活性,也可通过对金属氧化物进行表面疏水改性抑制副产物CO₂,提高C原子利用率。     

Rate measurement by pulse technique for individual steps involved in CO hydrogenation and its application to catalyst design

By using pulse reaction technique, the rates were measured for the individual steps involved in CO hydrogenation over transition metal catalysts with and without promoters. On all the transition metals other than Rh, the rate constant for the dissociation of C-O bond (k ₁) was much smaller than that for the hydrogenation of surface carbon species (k ₂). The oxides of V, Nb, Mo, and W added to Ru/Al₂O₃ increasedk ₁ and decreasedk ₃. The Rh catalyst was unique in the sense that there was not much difference betweenk ₁ andk ₂. The characteristic feature observed was found to be useful in designing a catalyst for the production of liquid fuel or C₂-oxygenates from syngas.     

C,CO and CO2 hydrogenation on cobalt foil model catalysts: evidence for the need of CoO reduction

The hydrogenation of C, CO, and CO₂ has been studied on polycrystalline cobalt foils using a combination of UHV studies and atmospheric pressure reactions in temperature range from 475 to 575 K at 101 kPa total pressure. The reactions produce mainly methane but with selectivities of 98, 80, and 99 wt% at 525 K for C, CO, and CO₂, respectively. In the C and CO₂ hydrogenation the rest is ethane, whereas in CO hydrogenation hydrocarbons up to C₄ were detected. The activation energies of methane formation are 57, 86, and 158 kJ/mol from C, CO, and CO₂, respectively. The partial pressure dependencies of the CO and CO₂ hydrogenation indicate roughly first order dependence on hydrogen pressure (1.5 and 0.9), negative first order on CO (–0.75) and zero order on CO₂ (–0.05). Post reaction spectroscopy revealed carbon deposition from CO and oxygen deposition from CO₂ on the surface above 540 K. The reduction of cobalt oxide formed after dissociation of C-O bonds on the surface is proposed to be the rate limiting step in CO and CO₂ hydrogenation.     

Effect of promoters on catalytic performance of Cr/SiO2 catalysts in oxidative dehydrogenation of ethane with carbon dioxide

Several acidic and basic oxide promoted Cr/SiO₂ catalysts were prepared and investigated in oxidative dehydrogenation of ethane in the presence of carbon dioxide. The effects of SO₄ ^2–, WO₃ and alkali metal oxides (Li₂O, Na₂O, and K₂O) on the catalytic activity were studied. It is found that sulfation of silica produces a positive effect on ethane conversion and ethylene yield while tungstation and addition of strong basic promoters (alkali metal oxides) suppress the catalytic activity. Characterization indicates that the varying activity of the promoted catalysts can be attributed to the difference in acid/base property and redox potential.     

Gold dispersed on TiO2 and SiO2: adsorption properties and catalytic behavior in hydrogenation reactions

Titania-supported Au catalysts were given both low temperature reduction and high temperature reduction at 473 and 773 K, respectively, and their adsorption and catalytic properties were compared to identically pretreated Pt/TiO₂ catalysts and pure TiO₂ samples as well as Au/SiO₂ catalysts. This was done to determine whether a reaction model proposed for methanol synthesis over metals dispersed on Zn, Sr and Th oxides could also explain the high activities observed in hydrogenation reactions over MSI (Metal-Support Interaction) catalysts such as Pt/TiO₂. This model invokes O vacancies on the oxide support surface, formed by electron transfer from the oxide to the metal across Schottky junctions established at the metal-support interface, as the active sites in this reaction. The similar work functions of Pt and Au should establish similar vacancy concentrations, and O₂ chemisorption indicated their presence. However, these Au catalysts were completely inactive for CO and acetone hydrogenation, and ethylene hydrogenation rates were lower on the supported Au catalysts than on the supports alone. Consequently, this model cannot explain the high rate of the two former reactions over TiO₂-supported Pt although it does not contradict models invoking specialinterfacial sites.     

The anodic oxidation of superimposed niobium and tantalum layers: theory

Perriere, Rigo and Siejka have investigated the anodization of superimposed layers of niobium and tantalum, and their results are interpreted here in terms of known processes for the individual metals. Niobium oxide is less resistive to the passage of anodizing current than tantalum oxide, and it can then be shown that, when the niobium layer is superimposed, the metal order will be conserved during anodization. When the tantalum layer is superimposed, however, a partial inversion occurs, with fingers of the substrate niobium oxide pushing their way through the tantalum oxide. The latter is thus converted to a mesh, whose holes are filled with niobium oxide; because the current flows preferentially through this niobium oxide, the tantalum oxide in the mesh is almost completely divorced from the anodizing process. It then acts as a good inert, immobile marker for the purpose of measuring the transport numbers of metal and oxygen, and thus confirms the immobility of the noble gas markers used previously. The tantalum oxide originally within the holes mixes with the niobium atoms pushing their way through; these tantalum atoms then migrate outwards with the niobium atoms, but at approximately two-thirds the rate. The observed movement of the tantalum atoms thus depends on their state of aggregation within the anodic film. The experiments of Perriere et al. with oxygen isotopes can be interpreted on the basis that the oxygen order is conserved throughout the migration process     

Hydrogenation of CO2, acetone,and CO on a Rh foil promoted by titania overlayers

The effects of submonolayer deposits of titania on the hydrogenation of CO₂, acetone, and CO on a Rh foil have been investigated. Titania has been found to promote all three of the hydrogenation reactions, with each reaction exhibiting a maximum rate at a titania coverage of 0.5 ML. The maximum rate for CO₂ hydrogenation is 15 times that of the bare Rh surface. Acetone hydrogenation shows a 6-fold rate enhancement, while CO displays a 3-fold increase. Changes in the selectivities for each reaction are also observed upon titania promotion. The effects of titania on these reactions are attributed to an interaction between C-O bonds and Ti³⁺ ions located at the perimeter of titania islands.     

Selective hydrogenation of acetylene on Pd/SiO2 catalysts promoted with Ti, Nb and Ce oxides

Transition-metal oxides added to Pd/SiO₂ improve significantly the activity and the ethylene selectivity of the catalyst in acetylene hydrogenation, which is caused by the interaction between the oxides and the Pd surface similar to the case of the oxide-supported catalysts. It has been confirmed through experiments that metal oxides spread on and modify both geometrically and electronically the Pd surface after the catalyst is reduced at 500°C. Such a behavior of metal oxides in the catalyst is correlated well with their promotional effect on the catalyst performance. That is, the oxides on the Pd surface retard the sintering of the dispersed Pd particles, suppress the adsorption of ethylene in the multiply-bound mode, and facilitate the desorption of ethylene produced by acetylene hydrogenation. Among the three metal oxides examined in this study, Ti oxide is found to have the most promotional effect.     

Promoted NiO Catalysts for the Oxidative Dehydrogenation of Ethane

Metal oxide promoted NiO catalysts with a Ni/(Me + Ni) atomic ratio 0.92 have been investigated for the oxidative dehydrogenation of ethane. These materials have been characterized by several techniques (N₂-adsorption, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transformed infrared spectroscopy of adsorbed CO and ethylene). The nature of surface sites is strongly influenced by the valence and the acid/base characteristics of the metal oxide promoters, which have a great impact on the selectivity to ethylene. Accordingly, a clear correlation between selectivity to ethylene and the valence of the promoter has been observed in the present work. Additionally, the acidity of the catalyst also enhances the selectivity to ethylene.     

CO hydrogenation on alumina pre-treated with sodium acetate aqueous solution

CO hydrogenation was studied over various alumina catalysts, untreated, pre-treated with sodium acetate (NaOAC) and calcined at 773 K after treatment. The catalysts were characterized by means of XRD and IR spectroscopy after pyridine adsorption. An apparent increase of ethylene selectivity over NaOAC pre-treated catalyst was observed. Both CO conversion and ethylene selectivity depended on the temperature of CO hydrogenation and a new -Al₂O₃ layer formed by surface boehmite transformation at elevated temperature on the original alumina surface. Several probe molecules, including HC1, NH₃ and pyridine, were used to detect the active sites for CO hydrogenation. There are two kinds of active sites (e.g. Lewis acidic and basic sites) which seem to be involved in the heterolytic dissociation of hydrogen to form a hydroxyl. Thus, the hydroxyl possibly plays a very important role in the formation of formyl, which may be the intermediate of the methoxide, and the methoxide seems to be the intermediate for hydrocarbons.     

Effect of palladium and nickel on the temperature programmed reduction of metal oxides and metal oxide layers

A new binary oxide support was suggested as being useful in many commercial reactions. Our study was focused on the reduction effect of metal oxide layer on alumina during reaction. Hence temperature programmed reduction of both bulk metal oxide and metal oxide layer on alumina was studied first and the effect of palladium and nickel on the reduction of the oxide support was also investigated. Vanadium oxide was mainly studied and niobium oxide, tantalum oxide, titanium oxide and zirconium oxide were also compared. Some metal oxides were reduced in a hydrogen stream at elevated temperature. In these cases both the bulk metal oxide and metal oxide layer were reduced. A tiny amount of palladium or nickel affected the reduction by decreasing the reduction temperature. The decrease of the reduction temperature was explained by means of increased adsorption of hydrogen on the transition metal and ability of the metal to spillover of the hydrogen to the oxides.     

The ability of Nb2O5 and Ta2O5 to generate active oxygen in contact with hydrogen peroxide

Amorphous and crystalline niobium(V) and tantalum(V) oxides were treated with hydrogen peroxide and studied by XRD, UV–vis, FTIR and ESR techniques to identify changes on their surface upon interaction with hydrogen peroxide. Differences between amorphous and crystalline materials in the interaction with H₂O₂ depending on the hydroxylation of the surface and the nature of OH groups were evident. The type of radical species formed on hydroxylated amorphous materials treated with H₂O₂ depended on the nature of metal oxide. It was proved that peroxo radical species formed in the interaction of H₂O₂ with amorphous Nb₂O₅ were the active intermediates in the oxidation of glycerol to glycolic acid with hydrogen peroxide. The radicals formed on amorphous Ta₂O₅ surface treated with hydrogen peroxide were poorly active in the oxidation of glycerol. Detailed study of the above mentioned radicals is in progress and will be a subject of a separate paper.     

Supported copper–ceria catalysts for low temperature CO oxidation

In this investigation, CuO/CeO₂–M_xO_y (M_xO_y = Al₂O₃, ZrO₂ and SiO₂) nanocomposite oxide catalysts were prepared by deposition-precipitation and wet impregnation methods, and evaluated for CO oxidation. Catalysts were characterized by XRD, TEM, UV–vis DRS, BET surface area and H₂-TPR techniques. The synthesized catalysts exhibited high specific surface area, and uniform particle size distribution over the supports. The nanocrystalline texture of mixed metal oxides is clearly evidenced by TEM analysis. TPR and XRD results revealed synergetic interactions between copper oxide and ceria. Among various catalysts investigated, the CuO/CeO₂–Al₂O₃ combination exhibited excellent CO oxidation activity with T_1/2 = 374 K and 100% CO conversion at below 420 K.     

钯基乙炔选择性加氢催化剂研究进展

催化选择加氢去除乙烯中微量乙炔是石化工业重要的反应过程,工业钯基催化剂选择性低、催化剂使用寿命较短。本文综述了近年来国内外乙炔选择性加氢钯基催化剂的研究进展。主要探讨了过渡金属、金属氧化物与非金属配体助剂能调变钯粒子空间结构,隔离分散钯粒子并与钯粒子产生电子效应;阐明了钯粒子尺寸与织构形貌的调控能改变钯的晶面结构,影响钯对乙烯的吸脱附和对氢气的活化与解离性能;评述了单一氧化物、复合金属氧化物、碳材料等载体为催化剂提供合适的表面酸碱性并加强了与活性中心之间的相互作用,稳定钯粒子抑制其发生迁移与团聚。提高乙烯选择性与催化剂稳定性是该研究的重点与难点,负载型钯基催化剂的发展方向是构建高分散钯粒子,并在反应过程中保持稳定。     

分子筛在双功能催化剂中催化CO/CO2加氢研究进展

双功能催化剂可将CO/CO_2加氢直接合成低碳烯烃、芳烃和汽油等,具有工艺流程短、能耗低的优势。双功能催化剂由金属氧化物和分子筛两部分组成,分子筛特定的腔体结构、酸性质及与金属氧化物的结合方式等均可显著影响其催化性能。该文综述了近年来分子筛在双功能催化剂中用于CO/CO_2加氢反应的研究进展,简述了分子筛的类型、酸性质、形貌及颗粒尺寸、金属改性、分子筛与金属氧化物的结合方式等对双功能催化剂催化性能的影响规律,展望了双功能催化剂中分子筛的发展趋势。     

Nanocatalysis on Supported Oxides for CO Oxidation

Active gold and palladium nanoparticles supported on a variety of oxides (CeO₂, ZrO₂, Al₂O₃, SiO₂, MgO and ZnO) were synthesized using laser vaporization and microwave irradiation methods. The catalytic activities for CO oxidation on the nanoparticle catalysts were evaluated and compared among different oxide supports. The effect of shape on the catalytic activity is demonstrated by comparing the activities of the Au and Pd catalysts deposited on MgO nanocubes and ZnO nanobelts. The Au/CeO₂ nanoparticles deposited on MgO nanocubes exhibit high catalytic activity and stability. The enhanced catalytic activity is attributed to the presence of a significant concentration of the corner and edge sites in MgO nanocubes. The Au- and Pd-doped Mn₂O₃ nanoparticles show promising results for the low temperature CO oxidation. Several approaches for incorporating the Au and Pd nanocatalysts within mesoporous oxide supports are presented and discussed.     

High-temperature electrochemical synthesis of oxide thin films and nanopowders of some metal oxides

An electrochemical method is proposed for preparing metal (aluminum, titanium, tantalum, zirconium) oxides with a high affinity to oxygen in a chloride-nitrate melt at temperatures above 830 K in an argon atmosphere. It is demonstrated that either dense metal oxide layers firmly bonded to the metal base can be produced or large amounts of nanosized oxides can be formed in the melt bulk depending on the anodic oxidation conditions (electrolyte composition, oxidation temperature).