Kinetics of Solid-State Reactions in Heterogeneous Catalysis from Time-Resolved X-Ray Absorption Spectroscopy

The potentials of X-ray absorption spectroscopy (XAS) (quantitative phase composition and average valence together with a short-range order structure analysis) combined with a time-resolution in the second range make time-resolved (TR-) XAS a powerful tool for investigating the reactivity of solids in catalysis and solid-state chemistry. General aspects of TR-XAS investigations are discussed (i.e., instrumentation, data analysis). In addition, some experiments illustrate how the kinetics of solid-state reactions in heterogeneous catalysis can be elucidated from TR-XAS studies.     

Lab Scale Fixed-Bed Reactor for Operando X-Ray Absorption Spectroscopy for Structure Activity Studies of Supported Metal Oxide Catalysts

Lab scale fixed-bed reactor is applied for operando transmission X-ray absorption spectroscopy (XAS) for structure–activity studies of supported metal oxide catalysts under real reaction conditions. This setup includes many properties of an optimal fixed-bed reactor for operando transmission XAS studies. For instance, it is usable in a wide range of temperature (up to 1,000 °C), pressure and space velocity. Besides, this operando setup can be used for transmission XAS measurements in a wide edge energy range. The potential of this reactor for operando transmission XAS is demonstrated by, as examples, the three-way catalytic performance of Pd/Al₂O₃/CeZrO₂ and Rh/Al₂O₃.     

X-Ray Photoelectron Spectroscopy Investigation of Pd-Beta Zeolite Catalysts with Different Acidities

Three different H-Beta zeolites with SiO₂/Al₂O₃ molar ratios of 300, 150 and 25 were loaded with palladium by evaporation–impregnation method using Pd(NO₃)₂ as a precursor. Dependence between the electronic states of Pd as a function of the acidity of Beta zeolite was studied by XPS for calcined and reduced samples. The dependence of the electronic states of Pd on the support was detected to have a complicated character and to be influenced by various factors such as metal loading, density of the imperfections in the zeolitic framework and proton localization. In calcined zeolites mainly agglomerated PdO and AlPd _x O crystallites were detected referring to interaction of palladium with the support. Reduction of PdO led to formation of metallic Pd_n clusters, which further reduced to [Pd_n–H_m]^m+ complexes and AlPd _x O complexes. The increase of the reduction temperature seems to enhance interactions of Pd_n clusters with imperfect [AlO _x ⁻] components instead of protons, especially in less acidic samples due to enhanced agglomeration of palladium particles outside the pores of the zeolite. The fraction, size and shape of the formed species were detected to be sensitive to the acidity of the support.     

Surface Structure of Heterogeneous Catalysts: Cinchona and Tartaric Acid on Solid Surface

Enantioselective hydrogenation by heterogeneous catalysts has gained great importance in organic synthesis, chemical engineering and pharmaceutical industry. However, the mechanism of enantioselectivity is not well understood due to the absence of evidence at the molecular level. Recently, the great advance on this topic has been made by applying scanning tunneling microscopy (STM) and other surface characterization techniques to get direct evidence about the enantioselective mechanism of catalysts. Here, the progress on the adsorption mode of chiral modifiers such as cinchona and tartaric acid and the related enantioselective mechanism is reviewed from the viewpoint of surface science. Particularly, some STM works on this topic are summarized. Finally, several key issues for further investigation are outlined.     

氯酚红探针可见吸收光谱法测定乌拉地尔

建立了测定药物中乌拉地尔的可见吸收光谱法。在pH 7.93的Tris-盐酸缓冲介质中,氯酚红与乌拉地尔反应生成具有2个明显负吸收峰的二元离子缔合物,最大负吸收峰位于600nm波长处,表观摩尔吸光系数(κ)为1.44×10⁴ L·mol^-1·cm^-1,乌拉地尔的质量浓度在0～3.7mg/L范围内与吸光度(A)呈线性关系并服从朗伯-比尔定律。探讨了吸收光谱特征、反应条件及共存物质的影响。方法用于市售乌拉地尔药物中乌拉地尔含量分析,加标回收率和相对标准偏差(RSD)(n=6)分别为98.35%～102.6%和2.7%～2.8%。     

Surface Chemical Composition of Size-Fractionated Urban Walkway Aerosols Determined by X-Ray Photoelectron Spectroscopy

Most aerosol chemical characterization studies to date involve bulk particle analysis. The surface chemical and physical properties of aerosol particles have rarely been analyzed, despite the particles’ potential health impacts and interactions with gas in the atmosphere. Aerosol particles ranging from 0.056 to 10 μm in size collected using a 10-stage impactor sampler from a busy walkway in a downtown area of Hong Kong were analyzed using X-ray photoelectron spectroscopy (XPS), a technique providing both elemental and chemical state information about the particle surfaces. Six key elements were detected: nitrogen (N), sulfur (S), calcium (Ca), silicon (Si), oxygen (O), and carbon (C). Carbon was the dominant species on the surfaces of all particles regardless of their sizes. A higher carbon concentration was found on the surfaces of the 0.056–0.32 μm particles. The N, Si, Ca, and O concentrations were higher on the surface of the 3.2–10 μm particles than in the smaller size fractions. Sulfur was mainly found on the surface of the 0.32–1.8 μm particles. High-resolution scans of C, N, and S were obtained to provide chemical state information about these elements. Aromatic C-H and aliphatic C-H were found to be the major carbon chemical states. Fullerenic carbon was detected on the surfaces of the finest (0.056–0.32 μm) particles. Oxygen- and nitrogen-containing organics were found on the surfaces of the 0.32–1.8 μm particles. Sulfur was present in the form of sulfates as expected. Ammonium salts, amide, and nitrate were found to form especially on the surfaces of aerosol particles in the nucleation, accumulation, and coarse modes, respectively. Silicates and carbonates were only discovered on the surfaces of coarse-mode particles (3.2–10 μm). The results suggest that both the chemical elements and their chemical states were significantly dependent on the size of the aerosol particles.

Copyright 2013 American Association for Aerosol Research     

制备生物柴油的固体催化剂研究进展

生物柴油是一种清洁、可再生能源。对催化油脂酯交换反应制备生物柴油的固体催化剂的研究进展进行综述,分析了各种固体催化剂的特性,并对催化油脂酯交换反应的固体催化剂今后研究方向进行讨论。     

Identification of Neutral and Charged NxOy Surface Species by IR Spectroscopy

The infrared spectral performance of the N_xO_y species observed on oxide surfaces [N₂O, NO^-, NO, (NO)₂, N₂O₃, NO⁺, NO₂^- (different nitro and nitrito anions), NO₂, N₂O₄, N₂O₅, NO₂, and NO₃^- (bridged, bidentate, and monodentate nitrates)] is considered. The spectra of related compounds (N₂, H-, and C-containing nitrogen oxo species, C─N species, NH_x species) are also briefly discussed. Some guidelines for spectral identification of N_xO_y adspecies are proposed and the transformation of the nitrogen oxo species on catalyst surfaces are regarded.     

Surface Science Approach to Modeling Supported Catalysts

Nanoscale structural information underlies research aimed at fabricating catalysts in a more controlled way. Surface science methods can provide that information, but the complexity of heterogeneous systems in general hinders the application of these methods to their full potential. In the last decades, a solution to this problem has been found in the use of model systems, ranging from well-defined single crystals of the supported phase to films or particles of that phase on flat or spherical model supports. In this paper, we review the literature on the latter model systems, that is, particles on a model support. Attention is payed to both preparation and use of such model systems.     

Surface Science Approach to Modeling Supported Catalysts

Nanoscale structural information underlies research aimed at fabricating catalysts in a more controlled way. Surface science methods can provide that information, but the complexity of heterogeneous systems in general hinders the application of these methods to their full potential. In the last decades, a solution to this problem has been found in the use of model systems, ranging from well-defined single crystals of the supported phase to films or particles of that phase on flat or spherical model supports. In this paper, we review the literature on the latter model systems, that is, particles on a model support. Attention is payed to both preparation and use of such model systems.     

Model Studies on Heterogeneous Catalysts at the Atomic Scale

Thin single crystalline oxide films comprise perfect supports to grow nanoparticles of metals and other catalytically relevant materials. The model systems thus created can be thoroughly investigated with respect to structure and/or chemical activity applying both, the techniques of surface science under ultrahigh vacuum conditions as well as the traditional techniques applied in catalysis to study chemical kinetics under ambient conditions. We discuss here in particular the oxidation of methanol to formaldehyde on ceria supported vanadia nanoclusters as well as the effect of transition-metal dopants in single crystalline oxides, such as CaO, on the activation of molecular oxygen.     

A New Approach to the Design of Heterogeneous Single-Site Enantioselective Catalysts

We analyze the merits of an approach to the design of enantioselective catalysts that relies on constraining an anchored organometallic complex at the inner walls of mesoporous silica. Two distinct metal centers {Rh(I) and Pd(II)}, three distinct diamino ligands {(S)-(-)-2-aminomethyl-1-ethylpyrrolidine, (AMP), (S)-(+)-(2-pyrrolidinyl-methyl)pyrrolidine, (PMP), and (1R,2R)-(+)-1,2-diphenylethylenediamine, (DED)}, two distinct methods of anchoring (a covalent and a noncovalent one), and two specific examples of enantioselective hydrogenation (the asymmetric hydrogenation of the C=C bond in E-α-phenylcinnamic acid and the asymmetric hydrogenation of the C=O bond in methyl benzoylformate) have been investigated. Values of enantiomeric excess (ee) ranging from a low of 66 to a high of 93 have been achieved by this approach. Moreover, the catalysts are robust and stand up well to recycling.     

Cinchona Alkaloids Tethered on Porous Silica as Enantioselective Heterogeneous Catalysts

Cinchonidine, a naturally occurring cinchona alkaloid, was tethered to a high-surface-area silica substrate in order to create a new solid chiral catalyst. Two synthetic routes were explored for this grafting, relying on the use of an intermediate linker and so-called ??click?? chemistry. Both routes proved viable, but the procedure where cinchonidine is first derivatized with 3-isocyanatopropyltriethoxysilane (ICPTEOS) at the alcohol position and the resulting product then anchored to the silica surface was deemed the most efficient. Bonding to the surface occurs via the formation of Si?CO?CSi bonds, on average two out of the three possible per cinchonidine, and takes place preferentially at silica surface sites with two geminal hydroxyl groups on the same silicon atom. Approximately 10% of the available OH surface groups are derivatized in this procedure. The resulting catalyst was successfully tested for the addition of aromatic thiols to unsaturated ketones, a reaction promoted by amines (the tertiary quinuclidine nitrogen atom in the case of cinchonidine). The activity of the supported cinchonidine proved comparable to that of the free molecule, but tethering does lead to a significant loss in enantioselectivity.     

基于废弃生物质构建多相催化材料的研究进展

综述了以稻壳、果壳、秸秆、动物壳和废皮渣等废弃生物质原料为碳源、载体或模板制备出高选择性、形貌多样的多相高效催化剂的研究进展及其在催化合成领域中的应用现状,同时讨论了利用现有生物质废弃物构建多相催化剂存在的关键科学问题和应用前景,为废弃生物质材料的功能化转化和高值化利用提供了新途径和新思路。     

Application of Electron Energy-Loss Spectroscopy to the Study of Solid Catalysts

The addition of an electron spectrometer to a transmission electron microscope provides electron energy-loss spectra and energy-filtered images, from which thickness and chemical information can be extracted. This article reviews the techniques and instrumentation required, together with recent applications involving heterogeneous catalysts.