有序多孔过渡金属氧化物及其负载贵金属催化剂对挥发性有机物氧化的催化性能

大部分的挥发性有机物(VOCs)污染环境,危害人身健康.目前,我国虽然已开展了治理 VOCs污染的工作,但还缺乏有效的、拥有自主知识产权的 VOCs治理技术,因此研发新型高效 VOCs处理技术迫在眉睫.催化氧化法是公认的最有效消除 VOCs的途径之一,而高性能催化剂的研发是实现该过程的关键.近年来,人们围绕消除 VOCs的高效且价廉的催化剂的研发开展了卓有成效的工作,许多过渡金属氧化物、混合或复合金属氧化物及其负载贵金属催化剂均被认为是有效的催化氧化材料.与体相材料相比,多孔材料具有发达的孔道结构和高的比表面积,一方面有利于反应物的扩散、吸附和脱附,因而具有更高的催化活性和选择性;另一方面有利于活性组分(如贵金属等)在多孔材料表面的高分散,抑制活性组分的烧结,因而具有更好的催化稳定性.本文简述了近年来多孔金属氧化物在环境污染物消除领域的研究进展,阐述了以有序介孔或大孔过渡金属氧化物、钙钛矿型氧化物和负载贵金属催化剂的制备及其对典型 VOCs(如苯系物、醇类、醛类及酮类等)氧化的催化性能,重点介绍了四类催化材料,包括有序介孔过渡金属氧化物或复合氧化物(Co3O4, MnO2, Fe2O3, Cr2O3和 LaFeO3等)催化剂,有序介孔金属氧化物负载贵金属(Au/Co3O4, Au/MnO2和 Pd/Co3O4等)催化剂,三维有序大孔过渡金属氧化物或复合氧化物(Fe2O3, LaMnO3, La0.6Sr0.4MnO3和 La2CuO4等)催化剂,以及三维有序大孔金属氧化物负载贵金属(Au/Co3O4, Au/LaCoO3, Au/La0.6Sr0.4MnO3和 AuPd/Co3O4等)催化剂的制备及其物化性质与对苯、甲苯、二甲苯、乙醇、丙酮、甲醛、甲烷或氯甲烷等 VOCs氧化的催化性能之间的相关性.借助二氧化硅或聚甲基丙烯酸甲酯微球等硬模板,采用纳米浇铸法可制备出二维或三维的有序单一或多级孔道结构的金属氧化物.研究表明,多孔金属氧化物的催化性能远优于其体相甚至纳米催化剂的.有序多孔材料的优异催化性能与其拥有大的比表面积、高的吸附氧物种浓度、优良的低温还原性、独特的孔道结构、活性组分的高分散以及贵金属与氧化物载体之间的强相互作用等有关.探明影响催化剂活性的因素有利于从原子水平上认识催化过程,为新型高效催化剂的设计与制备奠定基础.本文还指出了此类研究中存在的一些问题,例如利用硬模板法制备多孔材料的缺点是目标催化剂的收率低,硬模板浪费严重,大规模制备多孔催化剂势必增加制备成本,这些问题有待于妥善解决.与此同时,还展望了 VOCs消除技术的未来发展趋势,采用多种技术联用的方法有望最大程度地提高 VOCs的消除效率.     

Kinetics of lactose and rhamnose oxidation over supported metal catalysts

Several mono- and bimetallic Pd, Pt, Rh and Ru supported on alumina and active carbon catalysts were characterized by CO chemisorption, nitrogen adsorption, XPS and XRD and acidity titrations were performed for active carbon supported catalysts. These catalysts were tested in oxidation of two sugars, namely lactose and rhamnose, at 60 °C and at 70 °C under slightly alkaline conditions (pH 8) with molecular oxygen. The results revealed that there is an optimum metal particle size in a range of 3-10 nm giving the highest initial TOFs for both oxidations. Furthermore, the catalytic activities and conversions were related to other catalyst properties, such as the type and amount of promoters and the presence of different phases. In situ catalyst potential measurements revealed that there is an inverse correlation between the increase of catalyst potential as a function of sugar conversion and the catalyst activity after prolonged reaction times. This method is a valuable tool for in situ characterization of catalysts correlating well with their activities.     

Oxidative dehydrogenation of isobutane to isobutylene over supported transition metal oxide catalysts

Para-selectivity of ZSM-5 zeolites with similar bulk Si/Al ratio, but different particle size and surface Al concentration has been investigated in toluene disproportionation. Results showed that enhancedpara-selectivity is a consequence not only of the particle size but also of the external surface aluminium concentration in the particles.     

Hydrodechlorination of chlorobenzene over supported metal catalysts

The use of supported Pd catalysts, with low and high metal content, for the hydrodechlorination of chlorobenzene is presented in this article. Application of microwave irradiation during preparation of catalysts resulted in the synthesis of large Pd particles at moderate temperatures. The nature of the support played a key role in the formation of cationic Pd species. The extent of interaction of the Pd species with the support, the nature of metal precursor, particularly the residual chlorine on the surface were found to significantly affect the activity of the catalysts. In the case of bimetallic catalysts also microwave heating resulted in creation of bigger particles of Pd compared to those observed in conventionally heated catalysts. Besides, it minimized alloy formation as a result of which the activity of the catalysts in hydrodechlorination was found to be higher compared to that observed on conventionally prepared catalysts. Contrary to the general observation that low dispersed Pd catalysts are preferable for high stability, by means of the deposition-precipitation method adopted for catalyst preparation it was demonstrated that even highly dispersed (low Pd containing) catalysts can exhibit comparable activity and stability. An analysis of the nature of Pd species and its role in the stability of the catalysts is presented.IICT communication No. 051224     

Selective oxidation of propane to acrylic acid over mixed metal oxide catalysts

The effects of metal atomic ratio, water content, oxygen content, and calcination temperature on the catalytic performances of MoVTeNbO mixed oxide catalyst system for the selective oxidation of propane to acrylic acid have been investigated and discussed. Among the catalysts studied, it was found that the MoVTeNbO catalyst calcined at a temperature of 600 ℃ showed the best performance in terms of propane conversion and selectivity for acrylic acid under an atmosphere of nitrogen. An effective MoVTeNbO oxide catalyst for propane selective oxidation to acrylic acid was obtained with a combination of a preferred metal atomic ratio （Mo1V0.31Te0.23Nb0.12）. The optimum reaction condition for the selective oxidation of propane was the molar ratio of C3H8 ：O2 ： H2O ： N2 = 4.4： 12.8 ： 15.3 ： 36.9. Under such conditions, the conversion of propane and the maximum yield of acrylic acid reached about 50% and 21%, respectively.     

Propylene oxidation over tin-molybdenum oxide catalysts

The influence of preparation of tin-molybdenum catalysts on their phase composition and activity has been elucidated. The mutual dissolution of Sn and Mo oxides leads to a considerable increase in their activities and selectivities in the partial oxidation of propylene to acetone.

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Effect of ammonia on toluene ammoxidation over oxide catalysts

The increased activity of the catalyst for ammoxidation of toluene observed in the presence of ammonia is ascribed to its binding with the acidic centers. which leads to the formation of new active basic sites on the catalyst surface.

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Selective oxidation of methanol over supported vanadium oxide catalysts as studied by solid-state NMR spectroscopy

Methanol catalytic oxidation over VO_x/Al₂O₃, VO_x/ZrO₂ and VO_x/MgO catalysts has been studied by solid-state nuclear magnetic resonance (NMR) spectroscopy. It was found that stronger acid sites in VO_x/Al₂O₃ result in almost the same selectivities for dimethoxymethane, paraformaldehyde and formic acid, and weaker acid sites in VO_x/ZrO₂ favor the formation of paraformaldehyde, while the VO_x/MgO catalyst with the base support shows high selectivity for formate. Supporting VO_x species on γ-Al₂O₃ and ZrO₂ leads to the formation of Brønsted acid sites as revealed by the adsorption of probe molecules. The acid strength of Brønsted acid sites on the VO_x/Al₂O₃ catalyst is found to be stronger than that of the VO_x/ZrO₂ catalyst which has the acid strength similar to zeolite HZSM-5's. The proposed bridging hydroxyl models accounting for the Brønsted acid sites formation were also confirmed by quantum chemical calculation.     

Optimization of silica support pore size for Pt catalysts in the complete oxidation of volatile organic compounds

Highest catalytic performance for complete oxidation of propylene and toluene was obtained by using a Pt catalyst on a mesoporous silica support with pore sizes from 20 to 50 nm and 10 to 40 nm, respectively.     

The vapor phase oxidation of 2-ethylhexanal over oxide catalysts

The vapor phase oxidation of 2-ethylhexanal over a series of oxide catalysts has been studied at 373–623 K. Monolayer vanadia supported on SiO₂ and TiO₂, molybdena and tungstenia supported on SiO₂, and SnO₂ were used as the catalysts. In contrast to the liquid phase process, resulting in 2-ethylhexanoic acid, the main observed reaction was reactant combustion. The partial oxidation products were 3-heptanone, 3-heptyl formate, and heptene. No traces of 2-ethylhexanoic acid were detected in any of the transformations performed.     

'Shape effects' in metal oxide supported nanoscale gold catalysts

We report the activity of shape-controlled metal oxide (CeO(2), ZnO and Fe(3)O(4)) supported gold catalysts for the steam reforming of methanol (SRM) and the water gas shift (WGS) reactions. Metal oxide nanoshapes, prepared by controlled hydrolysis and thermolysis methods, expose different crystal surfaces, and consequently disperse and stabilize gold differently. We observe that similar to gold supported on CeO(2) shapes exposing the {110} and {111} surfaces, gold supported on the oxygen-rich ZnO {0001} and Fe(3)O(4) {111} surfaces shows higher activity for the SRM and WGS reactions. While the reaction rates vary among the Au-CeO(2), Au-ZnO and Au-Fe(3)O(4) shapes, the apparent activation energies are similar, indicating a common active site. TPR data further indicate that the reaction lightoff coincides with the activation of Au-O-M species on the surface of all three oxide supports evaluated here. Different shapes contain a different number of binding sites for the gold, imparting different overall activity.     

Catalytic oxidation of volatile organic compounds (VOCs). Oxidation of o-xylene over Pd and Pt/HFAU catalysts

The transformation of o-xylene in low concentration (1 700 ppmv) into air was investigated over Pd and Pt/HFAU catalysts (framework Si/Al ratio equal to 17 and 100). Whatever the catalyst, o-xylene oxidation into CO₂ and H₂O is accompanied by the retention within the zeolite pores of heavy compounds (‘coke’). The relative significance of these reactions depends on the operating conditions (temperature, time-on-stream) and on the catalyst characteristics (Pd or Pt, Si/Al ratio). Over Pt and Pd/HFAU(17), time-on-stream has a positive effect on the xylene oxidation apparently related to the reducibility of Pd and Pt species during the reaction. The higher activity of Pt/HFAU catalysts can be attributed to its greater number of active species (especially Pt⁰). Those active species can be more rapidly formed than Pd⁰ by auto reduction during the calcination of Pt precursor. Whatever the metal, the higher the Si/Al ratio of the support, the faster the xylene oxidation and the lower the coke formation. This can be related to the higher proportion of reduced species (Pd⁰ and Pt⁰) formed on the more dealuminated catalyst but also to the hydrophobicity of the support. Indeed, the hydrophobicity of the zeolite play a positive role in the oxidation activity in presence of steam; the higher the Si/Al ratio of the zeolite, the faster the o-xylene oxidation. Thus a catalyst with a low platinum content supported on a hydrophobic zeolite (0.10 Pt/HFAU(100)) allows to oxidising totally o-xylene at 210 °C in presence of steam.     

MCM-41 supported Cu-Mn catalysts for catalytic oxidation of toluene at low temperatures

Complete catalytic oxidation of toluene was investigated on Cu-Mn doped mesoporous and microporous catalysts, i.e., Cu-Mn/MCM-41, Cu-Mn/beta-zeolite, Cu-Mn/ZSM-5 (where SiO2/Al2O3 is either 25 or 38), and Cu-Mn/porous silica, in the presence of excess oxygen. The result shows that mesoporous catalysts have exhibited the highest catalytic activity among these catalysts above. The less amount of coke formation due to the unique mesoporous structures could play a key role in the high activity on the mesoporous catalyst. In addition, the bimetallic Cu-Mn-MCM-41 supported catalyst shows higher oxidation activity than either single metal catalyst, i.e., Cu-MCM-41 and Mn-MCM-41. The highly dispersed Cu-Mn mixed oxides on mesoporous structures probably provide active sites for the complete oxidation of toluene on these mesoporous catalysts.     

Modelling reduction of nitric oxide by hydrogen over supported catalysts

A mean field model for NO oxidation with H₂ over supported catalysts is proposed and solved numerically. The model is composed of a system of PDEs subject to nonclassical conjugate conditions at the catalyst–support interface and includes the bulk diffusion of reactants and reaction products and surface diffusion of all intermediate products. The influence of the particle jumping rate constants via the catalyst–support interface and reaction rate constants on the evolution of the reactivity of the catalyst surface is investigated. It is shown that the conversion rates (turnover frequencies) of NO and H₂ into products, N₂, H₂O, NH₃, and N₂O, are nonmonotonous functions of time. The conversion rates of NO and H₂ into N₂ and N₂O can have one or two local maxima, while their conversion rates into H₂O and NH₃ can possess one, two, or three local maxima. The mechanism and conditions for arising of the second maximum are discussed and reaction steps that essentially increase the surface reactivity are indicated.

     

Hysteresis phenomena in sulfur dioxide oxidation over supported vanadium catalysts

Catalyst deactivation and hysteresis behavior in industrial SO₂-oxidation catalysts have been studied in the temperature region 350–480°C by combined in situ EPR spectroscopy and catalytic activity measurements. The feed gas compositions simulated sulfuric acid synthesis gas and wet/dry deNO_x'ed flue gas. The vanadium (IV) compound K₄(VO)₃(SO₄)₅ precipitated during all the investigated conditions hence causing catalyst deactivation. Hysteresis behavior of both the catalytic activity and the V(IV) content was observed during reheating.     

The study of the active surface for CO oxidation over supported Pd catalysts

CO oxidation was investigated on various powder oxide supported Pd catalysts by temperature-programmed reaction.The pre-reduced catalysts show significantly higher activities than the pre-oxidized ones.Model studies were performed to better understand the oxidation state,reactivities and stabilities of partially oxidized Pd surfaces under CO oxidation reaction conditions using an in situ infrared reflection absorption spectrometer(IRAS).Three O/Pd(100)model surfaces,chemisorbed oxygen covered surface,surface oxide and bulk-like surface oxide,were prepared and characterized by low-energy electron diffraction(LEED)and Auger electron spectroscopy(AES).The present work demonstrates that the oxidized palladium surface is less active for CO oxidation than the metallic surface,and is unstable under the reaction conditions with sufficient CO.     

Selective oxidation of light alkanes over Mo-based oxide catalysts

A highly reduced Keggin-type heteropolymolybdophosphate, H₃PMo₁₂O₄₀(Py), which was formed by the heat-treatment of pyridinium salt of H₃PMo₁₂O₄₀, can catalyze the propane oxidation to acrylic acid and acetic acid selectively. We propose a possible reaction mechanism for alkane oxidation, where protons and electrons on the reduced H₃PMo₁₂O₄₀ catalyst cooperate for activating molecular oxygen to form electrophilic oxygen species for alkane oxidation. It is also reported that Anderson-type heteropolycompounds linked with vanadyl cations VO²⁺ were able to be synthesized by hydrothermal reaction and showed good catalytic activity for the ethene oxidation to acetic acid.     

Cs-Fe-Co-Bi-Mn-Mo复合氧化物选择性催化氧化异丁烯

Mixed oxide catalyst Cs0.1 Fe2Co6BiMnMo12 Ox was prepared by the interprecipitation method, then the catalyst was calcined at different temperature. Selective oxidation of isobutene was carried out in a fixed-bed reactor. The results showed that the catalyst has high catalytic activity. Under the optimum reaction conditions ( n(-C4^= ) : n(O2) = 1:2-1:4, space velocity = 180h^-1, T = 360℃ ), the yield of methacmlein and methacrylic reached 80%, 8 %,respectively. The total yields of liquid products( methacrolein, methacrylic acid and acetic acid) reached about 90%.     

植物还原法制备负载型金纳米催化剂催化乙醇选择氧化反应

采用侧柏叶提取液还原氯金酸制备负载型金纳米催化剂,通过乙醇选择氧化反应,筛选出催化性能较好的TiO2载体。以TiO2载体为载体,考察了Au负载量、焙烧温度、催化剂用量、碳酸氢钠添加量及催化剂反应条件(时间、温度、压力)等因素对乙醇选择氧化反应的影响。结果表明,1.5%Au/TiO2催化剂(Au负载量为1.5%,质量分率,下同)催化乙醇选择氧化反应性能最佳,产物为乙醛、乙酸乙酯和缩醛,0.5%碳酸氢钠添加剂可抑制缩醛的生成,并可显著提高乙醇转化率和乙酸乙酯选择性。通过优化催化反应条件(1.5%Au/TiO2催化剂焙烧温度为400℃、用量为0.4 g、反应温度为100℃、氧气压力为3 MPa、反应时间为3 h时),乙醇转化率为47.9%,乙酸乙酯选择性为89.1%。     

Hydrogenation of acetophenone over bifunctional multi metal oxide catalysts

The vapor phase hydrogenation of acetophenone at atmospheric pressure is carried out for the synthesis of 1-phenylethanol in a fixed bed reactor using Cu-Zn-Al-O (LTS-1) and LTS-2, Zn-Cr-Al-O (HTS-1) and Zn-Cu-Cr-Al-O (HTS-2) metal oxide catalysts. Our studies clearly indicate that a catalyst of low temperature hydrogenation activity, when modified for its acidity with magnesium oxide, shows maximum selectivity of 1-phenylethanol. This revised version was published online in June 2006 with corrections to the Cover Date.