Asymmetric pair distribution functions in catalysts

The structural parameters, i.e., coordination numbers, bond distances and disorder obtained from the analysis of EXAFS spectra may sometimes be significantly influenced by errors introduced due to the inadequacy of the analysis method applied. Especially in the case of heterogeneous catalysts it has been realized that often there is a need to use an improved EXAFS data analysis compared to the simple harmonic approach which works well for well‐defined bulk structures. This is due to the fact that catalysts contain highly dispersed or disordered structures with pair distribution functions quite different from well‐crystalline bulk materials. In addition, the increased interest for in situ studies, which typically imply high temperatures, makes the influence from anharmonic vibrations on the shape of the pair distribution a significant factor. In this paper, we discuss the importance of asymmetric pair distribution functions for nano‐sized particles and how they influence the structural parameters obtained from the standard data analysis. An alternative method, which takes into account deviations from the Gaussian pair distribution function typically used in the analysis of EXAFS spectra, will be described. The method is based on an analysis of the pair distribution functions derived from molecular dynamics simulations of small metal particles and its reliability is demonstrated by comparing structural parameters obtained from independent X‐ray diffraction experiments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Combined EXAFS and XRD for the in situ structural elucidation of solid catalysts under operating conditions

Two distinct kinds of experimental arrangement permit X-ray absorption and X-ray diffraction data to be recorded in a combined manner under in situ conditions. Results are shown for the conversion of a copper-zinc hydroxycarbonate precursor to a live water-gas shift catalyst and for a zinc oxide and a zeolite model catalyst heated to ca. 1000 ° C. The short-range and long-range structural changes accompanying the conversion of the zeolite into cordierite are charted at high temperature. EXAFS and XANES data can be recordable both in fluorescence or transmission mode with one of the arrangements, which also permits successive measurements (around more than one absorption edge) within a short space of time.     

In situ IR,Raman, and UV-Vis DRS spectroscopy of supported vanadium oxide catalysts during methanol oxidation

The application of in situ Raman, IR, and UV-Vis DRS spectroscopies during steady-state methanol oxidation has demonstrated that the molecular structures of surface vanadium oxide species supported on metal oxides are very sensitive to the coordination and H-bonding effects of adsorbed methoxy surface species. Specifically, a decrease in the intensity of spectral bands associated with the fully oxidized surface (V⁵⁺) vanadia active phase occurred in all three studied spectroscopies during methanol oxidation. The terminal V = O (∼1030 cm⁻¹) and bridging V–O–V (∼900–940 cm⁻¹) vibrational bands also shifted toward lower frequency, while the in situ UV-Vis DRS spectra exhibited shifts in the surface V⁵⁺ LMCT band (>25,000 cm⁻¹) to higher edge energies. The magnitude of these distortions correlates with the concentration of adsorbed methoxy intermediates and is most severe at lower temperatures and higher methanol partial pressures, where the surface methoxy concentrations are greatest. Conversely, spectral changes caused by actual reductions in surface vanadia (V⁵⁺) species to reduced phases (V³⁺/V⁴⁺) would have been more severe at higher temperatures. Moreover, the catalyst (vanadia/silica) exhibiting the greatest shift in UV-Vis DRS edge energy did not exhibit any bands from reduced V³⁺/V⁴⁺ phases in the d–d transition region (10,000–30,000 cm⁻¹), even though d–d transitions were detected in vanadia/alumina and vanadia/zirconia catalysts. Therefore, V⁵⁺ spectral signals are generally not representative of the percent vanadia reduction during the methanol oxidation redox cycle, although estimates made from the high temperature, low methoxy surface coverage IR spectra suggest that the catalyst surfaces remain mostly oxidized during steady-state methanol oxidation (15–25% vanadia reduction). Finally, adsorbed surface methoxy intermediate species were easily detected with in situ IR spectroscopy during methanol oxidation in the C–H stretching region (2800–3000 cm⁻¹) for all studied catalysts, the vibrations occurring at different frequencies depending on the specific metal oxide upon which they chemisorb. However, methoxy bands were only found in a few cases using in situ Raman spectroscopy due to the sensitivity of the Raman scattering cross-sections to the specific substrate onto which the surface methoxy species are adsorbed. This revised version was published online in August 2006 with corrections to the Cover Date.     

In situ Raman spectroscopy studies of catalysts

In situ Raman spectroscopy is rapidly becoming a very popular catalyst characterization method because Raman cells are being designed that can combine in situ molecular characterization studies with simultaneous fundamental quantitative kinetic studies. The dynamic nature of catalyst surfaces requires that both sets of information be obtained for a complete fundamental understanding of catalytic phenomena under practical reaction conditions. Several examples are chosen to highlight the capabilities of in situ Raman spectroscopy to problems in heterogeneous catalysis: the structural determination of the number of terminal M=O bonds in surface metal oxide species that are present in supported metal oxide catalysts; structural transformations of the MoO₃/SiO₂ and MoO₃/TiO₂ supported metal oxide catalysts under various environmental conditions, which contrast the markedly different oxide–oxide interactions in these two catalytic systems; the location and relative reactivity of the different surface M–OCH₃ intermediates present during CH₃OH oxidation over V₂O₅/SiO₂ catalysts; the different types of atomic oxygen species present in metallic silver catalysts and their role during CH₃OH oxidation to H₂CO and C₂H₄ epoxidation to C₂H₄O; and information about the oxidized and reduced surface metal oxide species, isolated as well as polymerized species, present in supported metal oxide catalysts during reaction conditions. In summary, in situ Raman spectroscopy is a very powerful catalyst characterization technique because it can provide fundamental molecular‐level information about catalyst surface structure and reactive surface intermediates under practical reaction conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Strong metal-support interaction in Ni/TiO2 catalysts: in situ EXAFS and related studies

In situ EXAFS and X-ray diffraction investigations of Ni/TiO₂ catalysts show that NiTiO₃ is formed as an intermediate during calcination of catalyst precursors prepared by the wet-impregnation method; the intermediate is not formed when ion-exchange method is used for the preparation. On hydrogen reduction, NiTiO₃ gives rise to Ni particles dispersed in the TiO₂(rutile) matrix. The occurrence of the anatase-rutile transformation of the TiO₂ support, the formation and subsequent decomposition/reduction of NiTiO₃ as well as the unique interface properties of the Ni particles are all factors of importance in giving rise to metal-support interaction. Active TiO₂(anatase) prepared from gel route gives an additional species involving Ni³⁺.     

An EXAFS study of Cu/ZnO and Cu/ZnO-Al2O3 methanol synthesis catalysts

Cu K-absorption edge and EXAFS measurements on binary Cu/ZnO and ternary Cu/ ZnO-Al₂O₃ catalysts of varying compositions on reduction with hydrogen at 523 K, show the presence of Cu microclusters and a species of Cu¹⁺ dissolved in ZnO apart from metallic Cu and Cu₂O. The proportions of different phases critically depend on the heating rate especially for catalysts of higher Cu content. Accordingly, hydrogen reduction with a heating rate of 10 K/min predominantly yields the metal species (>50%), while a slower heating rate of 0.8 K/min enhances the proportion of the Cu¹⁺ species ( 60%). Reduced Cu/ZnO-Al₂O₃ catalysts show the presence of metallic Cu (upto 20%) mostly in the form of microclusters and Cu¹⁺ in ZnO as the major phase ( 60%). The addition of alumina to the Cu/ZnO catalyst seems to favour the formation of Cu¹⁺/ZnO species.     

机械研磨燃烧法制备Cu/ZnO/Al2O3甲醇合成催化剂

引言Cu/ZnO/Al2O3催化剂近年来广泛应用于低压甲醇合成、二甲醚合成和水煤气变换等领域[1-2],该催化体系具有活性高、使用寿命长、反应温度及     

Development of high performance Raney Cu-based catalysts for methanol synthesis from CO2 and H2

Catalytic hydrogenation of CO₂ into methanol has been investigated over Raney Cu-based catalysts. The Raney catalysts leached in NaOH/ZnO solutions showed high activities and selectivities for methanol synthesis. The deposition of Zn on the surface of Cu particles increased the surface area and the specific activity of Raney Cu–M. Raney Cu–Zr developed was significantly more active than a commercial catalyst.     

Combining XRD and EXAFS with on-Line Catalytic Studies for in situ Characterization of Catalysts

Advances in the study of catalysts by in situ structural characterization, using X-ray absorption spectroscopy (XAFS) and X-ray diffraction (XRD), have recently been achieved and they are illustrated by reference to several examples. Emphasis in the present overview is laid on the study of catalysts under realistic working conditions and therefore in particular on-line gas analysis during activation and/or during catalysis. The examples encompass (a) activation of copper-based methanol catalysts, (b) dynamic changes of copper-based catalysts during methanol synthesis conditions, (c) catalytic partial oxidation on Rh/Al₂O₃ catalysts, and (d) reduction (activation) of copper-promoted high temperature shift catalysts.     

改进的二步共沉淀法制备Cu/ZnO/Al2O3甲醇合成催化剂

采用改进的二步共沉淀法制备了Cu/ZnO/Al₂O₃甲醇合成催化剂。用XRD、SEM和BET等手段对催化剂结构和形貌进行了表征。采用流动固定床微型反应器在5.0 MPa和空速5 000 h^-1条件下,考察了其催化合成气合成甲醇的活性,并在同一条件下用一种工业催化剂做对比测试。结果表明,改进的二步法制备的甲醇合成催化剂结晶度较低,基本以无定形状态的固溶体形式存在,铜锌之间的协同作用大,催化剂粒度较小,尺寸分布较均匀,比表面积较大,催化剂单位面积活性达98.54 mg·m^-2。     

A technique for simultaneous in situ MAS NMR and on-line gas chromatographic studies of hydrocarbon conversions on solid catalysts under flow conditions

A new technique is introduced allowing simultaneous in situ MAS NMR investigations of hydrocarbon conversions on solids under flow conditions and on-line gas chromatography. For adsorption of methanol on zeolite HBeta, equal amounts of adsorbed molecules were determined by both analytical methods. Studying the synthesis of methyl tertiary-butyl ether (MTBE) on zeolite HBeta using an MAS NMR rotor reactor, a constant yield of MTBE of Y_mtbe= 27% was obtained up to a weight hourly space velocity of 1.4 h^-1. The variation of the reaction temperature led to a simultaneous change of the ¹³C MAS NMR signals of isobutoxy species and of the yield of MTBE determined by on-line gas chromatography which indicates that isobutoxy species act as chemically active compounds. In this first application, the new in situ technique has demonstrated its advantage for a simultaneous investigation of compounds with a long residence time on the catalyst surface and of compounds rapidly leaving the catalyst surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

On the nature of surface structural changes in Cu/ZnO methanol synthesis catalysts

Infrared spectroscopic studies of CO adsorption have been used to elucidate the surface structure of Cu/ZnO methanol synthesis catalysts. Significant frequency shifts are observed as functions of the severity of the reduction treatment. The changes, which are unlike those observed in other supported Cu catalysts, cannot be interpreted solely by changes in morphology and abundance of different Cu surface planes. Rather, the band shifts are suggested to be related to the formation of Cu–Zn surface alloys under more severe reduction conditions. The formation/destruction of the surface alloys is apparently reversible and it is proposed that such processes may be the origin of many of the unusual steady state and transient kinetic behaviors observed for Cu/ZnO catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mechanism of the formation of precursors for the Cu/ZnO methanol synthesis catalysts by a coprecipitation method

Precursors of Cu/ZnO catalysts with various Cu/Zn molar ratios were prepared by a coprecipitation method. It was found that amorphous copper hydroxycarbonate and sodium zinc carbonate were intermediates for the formation of various precursors such as hydrozincite, malachite and aurichalcite. Aurichalcite having a high Cu/Zn ratio was prepared from a mechanical mixture of these intermediates.     

MC尼龙6/纳米ZnO复合材料热稳定性的原位XRD法研究

采用原位聚合反应制备MC尼龙6/纳米ZnO复合材料. 动态高温XRD(20~210℃)分析表明,随着温度的升高,MC尼龙6的α1(200)晶面和α2(002+202)晶面分别发生了收缩和膨胀,纯MC尼龙6和MC尼龙6/纳米ZnO复合材料的α1和α2晶面热膨胀系数分别为aTα1=-8.8×10-5 ℃-1, aTα2=1.6×10-4 ℃-1和aTα1=-1.7×10-4 ℃-1, aTα2=3.4×10-4 ℃-1. 随着温度的升高,MC尼龙6/纳米ZnO复合材料相对结晶度逐渐降低,在熔融温度附近结晶结构基本被破坏,在达到升温高点后的降温过程中产生了重结晶,在20~210℃之间的结构变化基本是可逆的. 加入纳米ZnO提高了MC尼龙6的热稳定性,随着纳米材料含量的增加,复合材料的热稳定性呈上升趋势,分散得越好,热稳定性越强;加入纳米ZnO使MC尼龙6的起始降解温度提高1~9℃,最大失重速率时的温度提高.     

Cu/Zn比对CuO/ZnO/ZrO_2催化CO_2加氢合成甲醇性能的影响

研究了不同Cu/Zn摩尔比对CO2加氢合成甲醇催化性能的影响。采用草酸凝胶共沉淀法制备了一系列不同Cu/Zn摩尔比的Cu O/Zn O/Zr O2催化剂,考察不同温度及Cu/Zn摩尔比对催化性能的影响,并结合X射线衍射(XRD)、N2物理吸附、程序升温还原(H2-TPR)和程序升温脱附(H2/CO2-TPD)技术对催化剂的结构和性质进行表征。结果表明:适宜的Cu/Zn摩尔比可以提高催化剂的反应性能。在513 K,2.0 MPa,n(H2)/n(CO2)=3/1和GHSV=4 800 h-1反应条件下,当R(Cu/Zn)=4时,Cu O/Zn O/Zr O2催化剂反应性能最好,CO2转化率高达17.8%,甲醇选择性高达67.8%。     

Heterogeneous catalysts,reaction kinetics,and reactor designs for methanol production from carbon dioxide: A critical review

The process of preparing methanol from carbon dioxide is one of the ways to solve the environmental problems caused by greenhouse gases, the problem of fossil energy depletion, and the problem of fuel exhaust emissions. However, after decades of development, the process of preparing methanol from carbon dioxide is still unable to achieve large-scale industrialization. This critical review sharply points out the problems and obstacles that need to be solved urgently on the industrialization road of carbon dioxide methanol preparation process and summarizes its progress. The problems faced by the methanol production industry from carbon dioxide are first the thermodynamic constraints, second the lower reaction rate and conversion effect, and finally the energy loss. In order to solve these problems, this paper first introduces the active sites, structural effects, and dynamic changes of copper-based catalysts, as well as the unique reaction mechanism and doping modification of indium-based catalysts. Zinc and zirconium promoters and some metal oxide supports can form unique interactions with active components to improve the catalytic performance of the catalyst. Next, various kinetic models and applications of methanol production from carbon dioxide are summarized, which is an important bridge linking laboratory and industrialization; the advantages and disadvantages of fixed bed reactor and paddle reactor were compared. Finally, the full text is summarized and prospected.