An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa. On Cu/SiO₂ catalyst formate species were initially formed which were subsequently hydrogenated to methanol. During the reaction a steady state concentration of formate species persisted on the copper. Additionally, a small quantity of gaseous methane was produced. In contrast, the reaction of CO₂ and H₂ on ZnO/SiO₂ catalyst only resulted in the formation of zinc formate species: no methanol was detected. The interaction of CO₂ and H₂ with Cu/ZnO/SiO₂ catalyst gave formate species on both copper and zinc oxide. Methanol was again formed by the hydrogenation of copper formate species. Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis. Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism.     

The role of zinc oxide in Cu/ZnO catalysts for methanol synthesis and the water–gas shift reaction

All commercial catalysts for methanol synthesis and for the water–gas shift reaction in the low temperature region contain zinc oxide in addition to the main active component, copper. The varied benefits of zinc oxide are analysed here. The formation of zincian malachite and other copper/zinc hydroxy carbonates is essential in the production of small, stable copper crystallites in the final catalyst. Further, the regular distribution of copper crystallites on the zinc oxide phase ensures long catalyst life. Zinc oxide also increases catalyst life in the water–gas shift process by absorbing sulphur poisons but it is not effective against chloride poisons. In methanol synthesis, zinc oxide (as a base) removes acidic sites on the alumina phase which would otherwise convert methanol to dimethyl ether. Although bulk reduction of zinc oxide to metallic zinc does not take place, reduction to copper–zinc alloy (brass) can occur, sometimes as a surface phase only. A new interpretation of conflicting measurements of adsorbed oxygen on the copper surfaces of methanol synthesis catalysts is based on the formation of Cu–O–Zn sites, in addition to oxygen adsorbed on copper alone. The possible role of zinc oxide as well as copper in the mechanisms of methanol synthesis is still the subject of controversy. It is proposed that, only under conditions of deficiency of adsorbed hydrogen on the copper phase, hydrogen dissociation on zinc oxide, followed by hydrogen spillover to copper, is significant. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mechanisms for activation of hydrogen and hydrogenation of acetone to isopropanol and of carbon oxides to methanol over copper-containing oxide catalysts

The present paper summarizes the results of comprehensive studies by a set of methods of the processes of reduction of copper chromite and of solid solutions of copper ions in zinc oxide. The composition and structure of copper-containing oxide catalysts in hydrogen medium and the nature of activated hydrogen species were investigated. Quantitative data on the interaction between hydrogen and the oxide catalysts and on the interaction of molecules to be hydrogenated with the reduced state of the catalysts are presented. The mechanisms for the synthesis of methanol from CO and CO₂ and for the hydrogenation of acetone to isopropanol are discussed.     

吸附相反应技术制备Cu基催化剂在甲醇合成第一步反应中的活性

采用吸附相反应技术(APRT)制备了Cu基催化剂,并用XRD、HRTEM、H₂-TPR等表征手段进行了分析。结果表明催化剂中的Cu良好分散于载体表面,粒径在5～10 nm。在液相乙醇体系合成气制甲醇的反应中,该Cu基催化剂对第一步形成中间产物甲酸乙酯的催化活性远高于工业催化剂。APRT制备的催化剂与其他催化剂(包括工业催化剂)在液相合成气制甲醇的两步反应中表现出的显著差异,不仅说明APRT催化剂具有不同的结构特点,也表明甲酸乙酯的形成和进一步的加氢的活性位是不同的。     

Photo-Assisted Synthesis of V-MCM-41 Under UV Light Irradiation

New preparation routes are applied to synthesize novel Cu/ZnO catalysts exhibiting high catalytic activity in methanol synthesis. In particular, the deposition–precipitation of copper onto high specific surface area (SSA) zinc oxide particles and the chemical vapor deposition of diethyl zinc turned out to be effective techniques, leading to promising methanol synthesis catalysts due to the enlargement of the Cu-ZnO interface.     

Precursors of copper/zinc oxide catalysts

Recent results on hydroxycarbonate precursors of copper/zinc oxide catalysts for methanol synthesis are reinterpreted, taking into account earlier work on these systems. This revised version was published online in July 2006 with corrections to the Cover Date.     

ZnO对CuO-ZnO/Al2O3催化剂催化甘油氢解性能的影响

以氧化铝为载体,采用浸渍法制备了负载型CuO-ZnO/Al2O3催化剂,通过XRD、XPS、TPR手段表征催化剂上CuO和ZnO的分布和化学形态。结果表明,CuO-ZnO/Al2O3催化剂催化甘油氢解反应中,CuO是活性组分,ZnO的引入可以降低CuO与载体氧化铝的相互作用强度,有利于CuO的还原,提高催化剂甘油氢解活性;催化剂表面呈缺电子状态的Cu物种是甘油氢解的活性中心。当活性组分CuO质量分数为12%,n(Cu)∶n(Zn)=1∶1.5时,CuO-ZnO/Al2O3催化剂催化甘油氢解活性最高,甘油转化率可达97.82%,对1,2-丙二醇选择性达94%。     

A new low-temperature methanol synthesis method: Mechanistic and kinetics study of catalytic process

Mechanism and kinetics of catalytic process for a new low-temperature methanol synthesis on Cu/ZnO catalysts from syngas (CO/CO₂/H₂) using catalytically active alcohol promoters were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Two intermediate species, adsorbed formate species and alkyl formate species, were formed in this synthesis process. The adsorbed formate species easily reacted with ethanol or 2-propanol at 443 K and atmospheric pressure, and the reaction rate with 2-propanol was faster than that with ethanol. Alkyl formate was readily reduced to form methanol at 443 K and 1.0 MPa, and the hydrogenation rate of 2-propyl formate was found to be quicker than that of ethyl formate. As a promoter, 2-propanol exhibited a higher activity than ethanol in the reaction of the low-temperature methanol synthesis.     

不同制备工艺的铜基合成甲醇催化剂的性能及结构

探讨不同制备工艺条件对Cu基合成甲醇催化剂性能的影响。分别采用反加共沉淀法、反加共沉淀法+助剂、并流法和并流法+助剂制备了4种催化剂样品,考察催化剂性能的变化,并对助剂在催化剂合成过程中所起的作用进行推测,采用X射线衍射、低温N₂吸附和扫描电子显微镜等对催化剂进行表征。结果表明,Cu基甲醇合成催化剂制备过程中加入助剂可以调整催化剂的孔径分布和表面组分分布,催化剂组成确定后,催化剂制备过程采用并流法+助剂制备的样品,孔径分布更合理,ZnO组分晶粒尺寸较小,Cu和Zn组分间相互接触的几率更大,催化剂性能更好。     

Active sites in Cu/ZnO/ZrO2 catalysts for methanol synthesis from CO/H2

Among various Cu/ZnO/ZrO₂ catalysts with the Cu/Zn ratio of 3/7, the one with 15 wt.% of ZrO₂ obtains the best activity for methanol synthesis by hydrogenation of CO. The TPR, TPO and XPS analyses reveal that a new copper oxide phase is formed in the calcined Cu/ZnO/ZrO₂ catalysts by the dissolution of zirconium ions in copper oxide. In addition, the Cu/ZnO/ZrO₂ catalyst with 15 wt.% of ZrO₂ turns out to contain the largest amount of the new copper oxide phase. When the Cu/ZnO/ZrO₂ catalysts is reduced, the Cu²⁺ species present in the ZrO₂ lattice is transformed to Cu⁺ species. This leads to the speculation that the addition of ZrO₂ to Cu/ZnO catalysts gives rise to the formation of Cu⁺ species, which is related to the methanol synthesis activity of Cu/ZnO/ZrO₂ catalyst in addition to Cu metal particles. Consequently, the ratio of Cu⁺/Cu⁰ is an important factor for the specific activity of Cu/ZnO/ZrO₂ catalyst for methanol synthesis.     

负载型铟基催化剂二氧化碳加氢动力学研究

探讨了载体对铟基催化剂上CO₂加氢动力学的影响。通过浸渍法制备了不同载体的负载型In基催化剂,仅ⅣB族元素（Ti,Zr,Hf）氧化物负载的In基催化剂表现出明显的CO₂加氢活性,其中In₁/HfO₂和In₁/ZrO₂催化剂具有较高的甲醇选择性,而In₁/TiO₂催化剂主要起催化逆水气变换反应的作用。通过稳态动力学、高压原位漫反射红外和程序升温实验等动力学手段,证明反应条件下In₁/ZrO₂和In₁/HfO₂上的关键表面反应中间体是甲酸盐与甲氧基,甲醇主要通过表面甲酸盐的逐步加氢生成。In₁/HfO₂具有最强的氢解离与加氢能力,因此最有利于甲醇合成。In₁/TiO₂在CO₂加氢中表面无明显含碳中间物种,高CO选择性可能与界面氧空缺位点促进redox循环以及甲酸盐中间体分解相关。     

Planar defect of the nano-structured zinc oxide as the site for stabilization of the copper active species in Cu/ZnO catalysts

The catalytic activity of CuZn catalysts in the synthesis of methanol is related to those reduced Cu species, which originate from the Cu_xZn_1−xO solid solution of wurtzite-like structure. Copper cations in the Cu_xZn_1−xO solid solution are localized in the extended stacking faults of the ZnO lattice. Copper sites could be supposedly described as the product of introducing (OH)Cu(OH) to the planar defects of zinc oxide structure. Hydroxyl groups stabilize the planar defects of ZnO. The process of the samples reduction leads to the formation of flat Cu⁰ particles over the surface of zinc oxide. The planar defects of ZnO structure are preserved in the reduced state. During the reoxidation, copper atoms return back to the extended stacking faults of ZnO as the tape-like clusters of flat-square coordinated copper cations.     

Implication of the microstructure of binary Cu/ZnO catalysts for their catalytic activity in methanol synthesis

Binary Cu/ZnO catalysts with varying molar ratios (90/10 through 10/90) were studied under methanol synthesis conditions at 493 K and at atmospheric pressure. The methanol synthesis activity of the catalysts was correlated to their specific Cu surface area (N₂O reactive frontal chromatography, N₂O RFC) after reduction in 2 vol% H₂ at 513 K. Activity data were supplemented with a detailed analysis of the microstructure, i.e., crystallite size and strain of the reduced Cu and the ZnO phases after reduction using X-ray diffraction line profile analysis. The estimated copper surface area based on a spherical shape of the copper crystallites is in good agreement with data determined by N₂O RFC. A positive correlation of the turnover frequency for methanol production with the observed microstrain of copper in the Cu/ZnO system was found. The results indicate a mutual structural interaction of both components (copper and zinc oxide) in the sense that strained copper particles are stabilized by the unstrained state of the zinc oxide microcrystallites. The observed structural deformation of ZnO in samples with higher Cu loading can originate, for instance, from epitaxial bonding of the oxide lattice to the copper metal, insufficient reduction or residual carbonate due to incomplete thermal decomposition during reduction. Additional EXAFS measurements at the Cu K and the Zn K edge show that about 5% ZnO are dissolved in the CuO matrix of the calcined precursors. Furthermore, it is shown that the microstructural changes (e.g., size and strain) of copper can be traced back to the phase composition of the corresponding hydroxycarbonate precursors.     

Oxides of copper, ceria promoted copper, manganese and copper manganese on Al2O3 for the combustion of CO, ethyl acetate and ethanol

Combustion of CO, ethyl acetate and ethanol was studied over CuO_x/Al₂O₃, CuO_x–CeO₂/Al₂O₃, CuMn₂O₄/Al₂O₃ and Mn₂O₃/Al₂O₃ catalysts. It was found that modification of the alumina with ceria before subsequent copper oxide deposition increases the activity for combustion of CO substantially, but the effect of ceria was small on the combustion of ethyl acetate and ethanol. The activity increases with the CuO_x loading until crystalline CuO particles are formed, which contribute little to the total active surface. The CuO_x–CeO₂/Al₂O₃ catalyst is more active than the CuMn₂O₄/Al₂O₃ catalyst for the oxidation of CO but the CuMn₂O₄/Al₂O₃ catalyst is more active for the combustion of ethyl acetate and ethanol.

Thermal ageing and water vapour in the feed caused a modest decrease in activity and did not affect the CuO_x–CeO₂/Al₂O₃ and CuMn₂O₄/Al₂O₃ catalysts differently. In addition, no difference in intermediates formed over the two catalysts was observed.

Characterisation with XRD, FT-Raman and TPR indicates that the copper oxide is present as a copper aluminate surface phase on alumina at low loading. At high loading, bulk CuO crystallites are present as well. Modification of the alumina with ceria before the copper oxide deposition gives well dispersed copper oxide species and bulk CuO crystallites associated to the ceria, in addition to the two copper oxide species on the bare alumina. The distribution of copper species depends on the ceria and copper oxide loading. The alumina supported copper manganese oxide and manganese oxide catalysts consist mainly of crystalline CuMn₂O₄ and Mn₂O₃, respectively, on Al₂O₃.     

Methanol Synthesis from Carbon Monoxide and Hydrogen over Ceria-Supported Copper Catalyst Prepared by a Coprecipitation Method

High catalytic activity in the synthesis of methanol from carbon monoxide and hydrogen can be produced with ceria-supported copper catalysts prepared by a coprecipitation method. The activity at 468 K is comparable with that produced with commercial copper-zinc catalysts at 503-523 K, while it is still unstable. Although the reaction atmosphere is reductive, metallic copper particles on cerium oxide are oxidized during the reaction and the catalyst is activated. Hence, formation of the copper oxide species is indispensable for the appearance of the high catalytic activity.     

Electrochemical behaviour of Cu-Zn-Al oxide catalysts in DMSO2 at 150°C

The electrochemical behaviour of Cu–Zn–Al oxide catalysts in a high-temperature solvent, dimethylsulphone (DMSO₂), at 150°C has been investigated. It has been shown that CuO electroreduction becomes easier and/or deeper in the Cu–Zn–Al oxide catalysts than in CuO alone. This electroreduction occurs at higher potentials, and the overall faradic yield decreases when CO₂ in dissolved in DMSO₂ on account of a strong interaction between the reduced species formed during the electronic transfer and CO₂. These results are consistent with the characterizations of the physical and chemical properties of these catalysts. They confirm that electrochemistry is a suitable method to determine the redox states of some copper industrial catalysts used in methanol synthesis.     

Spectroscopic and Kinetic Analysis of a New Low-Temperature Methanol Synthesis Reaction

The spectroscopy and kinetics of a new low-temperature methanol synthesis method were studied by using in situ DRIFTS on Cu/ZnO catalysts from syngas (CO/CO₂/H₂) using alcohol promoters. The adsorbed formate species easily reacted with ethanol or 2-propanol at 443 K and atmospheric pressure, and the reaction rate with 2-propanol was faster than that with ethanol. Alkyl formate was easily reduced to form methanol at 443 K and 1.0 MPa, and the hydrogenation rate of 2-propyl formate was found to be faster than that of ethyl formate. 2-Propanol used as promoter exhibited a higher activity than ethanol in the reaction of the low-temperature methanol synthesis.     

Deactivation of Copper Metal Catalysts for Methanol Decomposition, Methanol Steam Reforming and Methanol Synthesis

Laboratory and industrial results are reviewed to elucidate the general features of the deactivation of supported copper metal catalysts in various reactions involving methanol as reactant or product. Most catalyst types are based on Cu/ZnO formulations that contain stabilisers and promoters such as alumina, alkaline earth oxides and other oxides. These additional materials have several roles, including the inhibition of sintering and absorption of catalyst poisons. All copper catalysts are susceptible to thermal sintering via a surface migration process, and this is markedly accelerated by the presence of even traces of chloride. Care must be taken, therefore, to eliminate halides from copper catalysts during manufacture, and from reactants during use. Operating temperatures must be restricted, usually to below 300°C.In methanol synthesis involving modern promoted Cu/ZnO/Al₂ O₃ catalysts neither poisoning nor coking is normally a significant source of deactivation; thermal sintering is the main cause of deactivation. In contrast, catalyst poisoning and coking have been observed in methanol decomposition and methanol steam reforming reactions.     

A Fourier-transform infrared study of CO2 and CO2/H2 interactions with caesium-doped copper catalysts

FTIR spectra are reported of CO₂ and CO₂/H₂ on a silica-supported caesium-doped copper catalyst. Adsorption of CO₂ on a “caesium”/silica surface resulted in the formation of CO₂ ⁻ and complexed CO species. Exposure of CO₂ to a caesium-doped reduced copper catalyst produced not only these species but also two forms of adsorbed carboxylate giving bands at 1550, 1510, 1365 and 1345 cm⁻¹. Reaction of carboxylate species with hydrogen at 388 K gave formate species on copper and caesium oxide in addition to methoxy groups associated with caesium oxide. Methoxy species were not detected on undoped copper catalyst suggesting that caesium may be a promoter for the methanol synthesis reaction. Methanol decomposition on a caesium-doped copper catalyst produced a small number of formate species on copper and caesium oxide. Methoxy groups on caesium oxide decomposed to CO and H₂, and subsequent reaction between CO and adsorbed oxygen resulted in carboxylate formation. Methoxy species located at interfacial sites appeared to exhibit unusual adsorption properties.     

Spectroscopic evidence for adsorption sites located at Cu/ZnO interfaces

FTIR spectra are reported of CO and formic acid adsorption on a series of Cu/ZnO/SiO₂ catalysts. Peaks due to linear CO adsorbed on copper diminished in intensity as the loading of ZnO was increased. This behaviour was explained in terms of ZnO island growth on the copper surface. Similarly, reduction of the copper concentration while maintaining a constant ZnO loading also resulted in further attenuation in bands ascribed to CO chemisorbed on copper. Formic acid exposure to a Cu/SiO₂ sample produced a formate species displaying a _as(COO) mode at 1585 cm^–1. Addition of a small quantity of ZnO to the catalyst resulted in substantial promotion of formate growth, which was accompanied by a shift (and broadening) of the _as(COO) vibration to 1660–1600 cm^–1. Since further ZnO incorporation poisoned formate creation it was concluded that formate species bonded to Cu and Zn sites located at interfacial positions had been formed. The role of such species in methanol synthesis is discussed.