Infrared and Raman imaging of biological and biomimetic samples

Established methods for imaging of biological or biomimetic samples, such as fluorescence and optical microscopy, magnetic resonance imaging (MRI), X-ray tomography or positron emission tomography (PET) are currently complemented by infrared (both near-IR and mid-IR) as well as Raman spectroscopic imaging, whether it be on a microscopic or macroscopic scale. These vibrational spectroscopic techniques provide a wealth of information without a priori knowledge of either the spectral data or the composition of the sample. Infrared radiation does not harm the organism, no electric potential needs to be applied, and the measurements are not influenced by electromagnetic fields. In addition, no extrinsic labeling or staining, which may perturb the system under investigation, has to be added. The immense volume of information contained in spectroscopic images requires multivariate analysis methodologies in order to effectively mine the chemical and spatial information contained within the data as well as to analyze a time-series of images in order to reveal the origin of a chemical or biochemical process. The promise and limitations of this new analytical tool are surveyed in this review.     

Resonance Raman and surface- and tip-enhanced Raman spectroscopy methods to study solid catalysts and heterogeneous catalytic reactions

Resonance Raman (RR) spectroscopy has several advantages over the normal Raman spectroscopy (RS) widely used for in situ characterization of solid catalysts and catalytic reactions. Compared with RS, RR can provide much higher sensitivity and selectivity in detecting catalytically-significant surface metal oxides. RR can potentially give useful information on the nature of excited states relevant to photocatalysis and on the anharmonic potential of the ground state. In this critical review a detailed discussion is presented on several types of RR experimental systems, three distinct sources of so-called Raman (fluorescence) background, detection limits for RR compared to other techniques (EXAFS, PM-IRAS, SFG), and three well-known methods to assign UV-vis absorption bands and a band-specific unified method that is derived mainly from RR results. In addition, the virtues and challenges of surface-enhanced Raman spectroscopy (SERS) are discussed for detecting molecular adsorbates at catalytically relevant interfaces. Tip-enhanced Raman spectroscopy (TERS), which is a combination of SERS and near-field scanning probe microscopy and has the capability of probing molecular adsorbates at specific catalytic sites with an enormous surface sensitivity and nanometre spatial resolution, is also reviewed (300 references).     

Single-site heterogeneous catalysts

Intellectually, the advantages that flow from the availability of single-site heterogeneous catalysts (SSHC) are many. They facilitate the determination of the kinetics and mechanism of catalytic turnover-both experimentally and computationally-and make accessible the energetics of various intermediates (including short-lived transition states). These facts in turn offer a rational strategic principle for the design of new catalysts and the improvement of existing ones. It is generally possible to prepare soluble molecular fragments that circumscribe the single-site, thus enabling a direct comparison to be made, experimentally, between the catalytic performance of the same active site when functioning as a heterogeneous (continuous solid) as well as a homogeneous (dispersed molecular) catalyst. This approach also makes it possible to modify the immediate atomic environment as well as the central atomic structure of the active site. From the practical standpoint, SSHC exhibit very high selectivities leading to the production of sharply defined molecular products, just as do their homogeneous analogues. Given that mesoporous silicas with very large internal surface areas are ideal supports for SSHC, and that more than a quarter of the elements of the Periodic Table may be grafted as active sites onto such silicas, there is abundant scope for creating new catalytic opportunities.     

Preparation of heterogeneous catalysts

The preparation of heterogeneous catalysts has been for many years a dynamic field of sub-nanotechnology and remains so nowadays. The approach to preparation must be examined in function of the specific demands concerning (i) activity and (ii) selectivity, that both depend on the arrangement of atoms at a scale smaller than 0.02 nm. Adequate access of reactants to the surface must be provided. Most catalysts are used in the form of pellets or cylinders obtained by pressing, extrusion or other techniques. This implies a control of texture at dimension scales extending from a fraction of a nanometer to several millimetres (and sometimes more). A third demand (iii) is resistance to ageing. In particular, stability at relatively high temperatures is necessary.The strategy adopted in the majority of cases is to start from a material that is homogeneous in composition at the Angström scale, generally a liquid or a solid of complex composition, frequently amorphous. A general objective is to locate the different constituting atoms at precise positions. The main difficulty is to transform the starting precursor into a highly porous solid without segregation of different elements that would produce tiny parts with different properties.The specific approach to catalyst preparation is based on the general concepts used for controlling the reactivity of solids. Typical methods of general use will be examined. Chemical bonds of practically any kind can retain the elements constituting the future catalyst at the position they have in the precursor. The so-called ‘citrate process’ and its variants are of wide application. More elaborate approaches start from molecules or polymers associating the necessary elements.     

Hybrid chemoenzymatic heterogeneous catalysts

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Infrared and Raman study of thiobisamines

The i.r. and Raman spectra of various N,N′-thiobisamines were investigated. By comparing the spectra with those of the corresponding amines, an assignation of bands characteristic of NSz.sbnd;N structures is proposed. From the estimation of the force constants of the SN bonds followed by their correlation with the interatomic distances, the SN bond orders were evaluated. These bond orders, which depend only slightly on the nature of the amine, indicate some double bond character for the SN linkage.     

Infrared and Raman spectra of N-n-alkylthioureas

The i.r. and Raman spectra of solid alkyl thioureas H₂NCSNH(CH₂)nCH₃ (n = 0–15), TUR′, and their urea analogue H₂NCONH(CH₂)nCH₃ (n = 0–9), UR′, are presented. The vibrational properties of N-monosubstituted TUR′and UR′are studied on the basis of the spectral comparison between the members of the series and compared with the properties of N,N′-di-substituted MTUR′ and MUR′, correspondingly. Some features of the methylene ν₄ and ν₇ band progressions were found to shift from the corresponding features of solid n-alkanes by vibrational coupling. The observed wavenumber of the longitudinal accordion-like skeletal motion (LAM-1) of TUR′ was found to correspond to that of the n-alkane with the same number of carbon atoms as TUR′ has skeletal atoms.     

UV-VIS-NIR spectroscopy and microscopy of heterogeneous catalysts

This critical review article discusses the characterization of heterogeneous catalysts by UV-VIS-NIR spectroscopy and microscopy with special emphasis on transition metal ion containing catalysts. A review is given of the transitions, that can be observed in the UV-VIS-NIR region and the peculiarities of catalytic solids that have to be taken into account. This is followed by a short discussion of the techniques that have been developed over the years: diffuse reflectance spectroscopy, UV-VIS microscopy, in situ or operando spectroscopy, the combination of UV-VIS spectroscopy with other spectroscopic techniques, with chemometrics and with quantum chemistry. In the third part of this paper four successes of UV-VIS-NIR spectroscopy and microscopy are discussed; (1) coordination of transition metal ions to surface oxygens; (2) quantitative determination of the oxidation states of transition metal ions; (3) characterization of active sites and (4) study of the distribution of transition metal ions and carbocations in catalytic bodies, particles and crystals (104 references).     

High-throughput selection for heterogeneous catalysts

A catalyst library of 80 samples with different mass ratios of rare earth elements, cobalt (Co), cerium (Ce), and indium (In), was prepared by impregnation of a fresh HZSM-5 zeolite support. A high-throughput detection setup, based on UV absorption spectroscopy, was developed for heterogeneous catalyst selection. The catalytic properties of the library were tested in the selective catalytic reduction of NO by methane at 673 K. Among the Co/Ce series, the catalyst with Co/Ce = 2:1 and Co/H-ZSM5 = 2.5% has shown remarkable efficiency (up to 78%). In the Ce/In series, the reactivity of the catalyst with the support composition of Ce/In = 1:1 and Ce/H-ZSM5 = 2.0% was < or = 88%. Our initial experiments definitely indicated that this simple and inexpensive multichannel setup can be applied for the selection of other heterogeneous catalysts. According to the variation of the UV light intensity, resulting from the absorption of the reactant or product, it was possible to monitor the relative quantity of reactants or products during a catalytic reaction.     

非均相金属卟啉催化剂的研究进展

本文综述了近年来非均相金属卟啉催化剂的最新研究成果,对不同类型载体的结构特点、负载方法、及催化活性进行了详细阐述。负载后的金属卟啉配合物不仅避免了二聚,提高了稳定性和活性,而且表现出较高的形状和尺寸选择性。     

Toward three-dimensional nanoengineering of heterogeneous catalysts

Cobalt-based Fischer-Tropsch systems are widely used to convert synthesis gas to clean hydrocarbon fuel. However, surprisingly little is known about the morphology of the catalysts on the nanoscale. Here we show that scanning transmission electron tomography reveals their true 3-D morphology and provides direct evidence that the support controls the final morphology of the catalyst. Such direct local three-dimensional measurements provide unprecedented insight into catalysis, and can henceforth transform our understanding of these complex materials.     

Thermal irradiation of deactivated heterogeneous catalysts

Radiation heating decreases 10–100 — fold the regeneration time of deactivated catalysts, accelerates the removal of carbon and sulfur, affects on phase composition, pore structure and mechanical properties of catalysts and sorbents.     

Surface analysis of supported heterogeneous catalysts

During the past decade, X-ray photoelectron spectroscopy (XPS or ESCA) has become a common tool for characterizing heterogeneous catalysts. ESCA is able to provide both qualitative and quantitative information about the nature of chemical species on catalyst surfaces. It is both surface sensitive and chemically specific.