Spectroscopic technique

Various spectroscopic techniques can also be used to measure gas solubilities in ILs. For instance, Welton and coworkers have used proton NMR spectroscopy [16] to determine the solubility of hydrogen in a series of ILs. Since hydrogen exhibits low solubility in ILs and has such a low molecular weight, it is difficult to measure gravimetrically or by any of the stoichiometric techniques. As a result, it is particularly well suited to determination by spectroscopy. In addition, Kazarian and coworkers have measured CO2 solubility by infrared spectroscopy [17]. In general, spectroscopic techniques are quite attractive, as long as extinction coefficients do 

Number of times characterization techniques were used at the 11 1 ICC Baltimore 1996 

Characterization techniques become surface sensitive if the particles or radiation to be detected come from the outer layers of the sample. Low energy electrons, ions and neutrals can only travel over distances between one and ten interatomic spacings in the solid state, implying that such particles coming off a catalyst reveal surface-specific information. The inherent disadvantage of the small mean free path is that measurements need to be carried out in vacuum, which conflicts with the wish to investigate catalysts under reaction conditions. 

In this book we describe some the most often used techniques in catalyst characterization (see Fig. 1.5). We will highlight those methods that have been particularly useful in the study of metal, oxide and sulfide catalysts, and related model systems. Zeolites and techniques such as nuclear magnetic resonance [2,3,16] fall outside the scope of this book. A number of books on catalyst characterization are listed in the references [3, 16-22], 

Since the mid-1990s, there has been plenty of activity regarding the use of spectroscopic techniques for on-line evaluation of polymer properties [143-146]. This has been possible due to the recent development of fiber-optic probes, which allow in-situ measurements in remote and harsh environments (high temperatures, pressures, toxic environments, and so on). An additional advantage is that a fiber-optic probe can be installed in an existing reactor within a short time without expensive modifications. Fluorescent, ultraviolet (UV), infrared (IR), near-infrared (NIR), mid-infrared (MIR) and Raman spectroscopic techniques can be used for polymerization reaction monitoring. These can be divided between absorption- and emission-based techniques. IR, NIR, and MIR are absorption-based. 

IR is not well suited to monitor polymerizations in dispersed media because water gives a strong absorption, and hence important bands are overlapped or hidden by that of the water. In addition, transmission through fiber-optics is still relatively poor in the infrared region, which makes IR not as suitable as other techniques for remote monitoring. 

The MIR spectral region is from 4000 to 400 cm k This region is very rich in fundamental absorptions and hence the potential of this technique is high. However, the use of MIR spectroscopy to monitor emulsion polymerization reactors is scarce, mainly because remote monitoring is not possible at low cost as currently only a few exotic materials are known to be able to transmit in this region. In addition, water is absorbed in this region and it may hide bands that are important for further analysis. 

Raman spectroscopy is an emission-based technique. Although conventional dispersive Raman spectroscopy (laser wavelengths between 500 and 700 nm) has not been successfully used to monitor polymerization reactions due to the tremendous effect of fluorescence on the spectra, FT-Raman (laser wavelength in the NIR region, 1034 nm) or modem dispersive Raman equipments (laser wavelengths over 800 nm) overcome this difficulty. Currently, Raman spectroscopy can be considered as the spectroscopic technique with the greater potential to monitor polymerization reactors, and especially emulsion polymerization reactors, in situ. Raman spectroscopy presents several advantages over the absorption techniques (MIR and NIR). The most important ones are  

Water is a weak scatterer and hence Raman is well suited to monitor polymerization in dispersed media. 

The industrial view on catalyst characterization is different, however. Here, the emphasis is placed mainly on developing an active, selective, stable and mechanically robust catalyst. In order to accomplish this, tools are needed which identify those structural properties that discriminate efficient from less-efficient catalysts, and all spectroscopic information that helps to achieve this is welcome. The establishment of empirical relationships between the factors that govern catalyst composition, particle size and shape and pore dimensions on the one hand, and catalytic performance on the other hand, are extremely useful in the process of catalyst development. However, such relationships may not provide much fundamental insight into how the catalyst operates in molecular detail. 

Van Santen [14] identifies three levels of research in catalysis  

In simplifying, one could say that catalyst characterization in industrial research deals with the materials science of catalysts on a more or less mesoscopic scale, whereas the ultimate goal of fundamental catalytic research is to characterize the surface of a catalyst at the microscopic level - that is, on the atomic scale. 

Catalyst characterization is a lively and highly relevant discipline in catalysis, with the literature revealing a clear desire to work with defined materials. For example, about 80% of the 143 oral reports at the 11th International Congress on Catalysis [15] contained at least some results on the catalyst(s) obtained by characterization techniques, whereas only 20% of these reports dealt with catalytic reactions over uncharacterized catalysts. Another remarkable fact obtained from these statistics is that about 10% of the reports included the results of theoretical calculations. Clearly, the modern trend is to approach catalysis from many different viewpoints, using a combination of sophisticated experimental and theoretical tools. 

While most of the references tend nowadays to use FT-IR/FTIR (Fourier transform infrared) to denote the use of mid-infrared spectroscopy we mostly use the more precise phrasing of mid-infrared (mid-IR) spectroscopy. 

6 h at 145°C Cooled in air for 24 h to room temperature Carbonyl band when reheated to 145°C for 

As discussed earlier under Section 2.3, Carbonyl index, in one relatively recent comparison of the photo-oxidative and thermal (oven-aged) degradation behaviour of different polyethylenes, additive free grades of a metallocene (mPE), an HDPE and a linear low-density PE (LLDPE) were analysed by a combination of mid-IR spectroscopy, TGA and CL [13]. The mid-IR 

Coleman and Sivy also used an infrared transmission cell to undertake degradation studies under reduced pressure on a series of poly(acrylonitrile) (ACN) copolymers [30-33]. Thin films prepared from a polymer were mounted in the specially designed temperature-controlled cell mounted within the infrared spectrometer. The comparative studies were made on ACN copolymers containing vinyl acetate [30,32], methacrylic acid [30,31] and acrylamide [30,33]. The species monitored was the production of the cyclised pyridone structure. This was characterised in part by loss of C=N stretch (vC = N) intensity at 2,240 cm-1 accompanied by the appearance and increase in intensity of a doublet at 1,610/1,580 cm-1. 

Mid-IR spectroscopy, alongside gravimetric and molecular weight determinations, has also been used to analyse the biodegradation by a thermophilic bacterium (isolated from soil) of an LDPE film [44], The mid-IR studies were undertaken using the ATR sampling technique on control samples, samples that had been UV irradiated, and samples that had been UV irradiated then incubated with bacteria. The study showed that the particular bacterial strain was capable of utilising standard and photo-oxidised polyethylene as the sole carbon source. 

There are three important features of spectra which are of prime interest to the kineticist. 

The frequency and the fine structure of the lines give the identity of the molecule this is particularly important in detecting intermediates and minor products. 

The intensity of the lines gives the concentration this is useful for monitoring the concentrations of reactants and intermediates with time. 

The line width enables kinetic features of the transition and the excited state to be determined. 

In spectroscopic experiments, radiation is absorbed (absorption spectra) or emitted (emission spectra). The frequency of absorption, or emission, is a manifestation of transitions occurring within the molecule, and the frequency of a line in the spectrum is related to the energy change as the molecule moves from one energy state to another. 

The difficulties inherent in the use of absorption spectra have been dis- 

Automatic recording infrared apparatus can also be used in studying the kinetics of both complex reactions and intermediates if their concentrations are not less than 1 per cent. In this way, ketene, CH2CO, and acetaldehyde, CIIsCHO, have been identified as intermediates in the pyrolysis of ethylene oxide, C2H40.  

2 UV-Visible Absorbance Spectrometry UV-visible absorbance spectroscopy is used to characterize Pd nanoparticles in primarily two ways (i) to monitor the palladium precursor as it is reduced to the zero-valent state, indicating formation of palladium nanoparticles and/or (ii) to monitor the position of the palladium nanoparticles surface plasmon resonance (SPR) peak. 

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EXAFS Extended X-ray absorption fine structure spectroscopy. A spectroscopic technique which can determine interatomic distances very precisely. 

The importance of the solid-liquid interface in a host of applications has led to extensive study over the past 50 years. Certainly, the study of the solid-liquid interface is no easier than that of the solid-gas interface, and all the complexities noted in Section VIM are present. The surface structural and spectroscopic techniques presented in Chapter VIII are not generally applicable to liquids (note, however. Ref. 1). There is, perforce, some retreat to phenomenology, empirical rules, and semiempirical models. The central importance of the Young equation is evident even in its modification to treat surface heterogeneity or roughness. ... 

The cleaning process proceeds by one of three primary mechanisms solubilization, emulsification, and roll-up [229]. In solubilization the oily phase partitions into surfactant micelles that desorb from the solid surface and diffuse into the bulk. As mentioned above, there is a body of theoretical work on solubilization [146, 147] and numerous experimental studies by a variety of spectroscopic techniques [143-145,230]. Emulsification involves the formation and removal of an emulsion at the oil-water interface the removal step may involve hydrodynamic as well as surface chemical forces. Emulsion formation is covered in Chapter XIV. In roll-up the surfactant reduces the contact angle of the liquid soil or the surface free energy of a solid particle aiding its detachment and subsequent removal by hydrodynamic forces. Adam and Stevenson s beautiful photographs illustrate roll-up of lanoline on wood fibers [231]. In order to achieve roll-up, one requires the surface free energies for soil detachment illustrated in Fig. XIII-14 to obey... 

A beautiful and elegant example of the intricacies of surface science is the formation of transparent, thermodynamically stable microemulsions. Discovered about 50 years ago by Winsor [76] and characterized by Schulman [77, 78], microemulsions display a variety of useful and interesting properties that have generated much interest in the past decade. Early formulations, still under study today, involve the use of a long-chain alcohol as a cosurfactant to stabilize oil droplets 10-50 nm in diameter. Although transparent to the naked eye, microemulsions are readily characterized by a variety of scattering, microscopic, and spectroscopic techniques, described below. 

The course of a surface reaction can in principle be followed directly with the use of various surface spectroscopic techniques plus equipment allowing the rapid transfer of the surface from reaction to high-vacuum conditions see Campbell [232]. More often, however, the experimental observables are the changes with time of the concentrations of reactants and products in the gas phase. The rate law in terms of surface concentrations might be called the true rate law and the one analogous to that for a homogeneous system. What is observed, however, is an apparent rate law giving the dependence of the rate on the various gas pressures. The true and the apparent rate laws can be related if one assumes that adsorption equilibrium is rapid compared to the surface reaction. 

Also, many of the stnictural and spectroscopic techniques are now being applied to new types of systems, such as those involving the physical adsorption of vapors. Such bridging of methodologies will surely help to keep surface chemistry a single, broad field with good intercommunication between the various subareas. 

Plieth W, Wilson G S and de la Fe C 1998 Spectroelectrochemistry a survey of in situ spectroscopic techniques Pure Appi. Chem. 70 1395... 

Beden B 1995 On the use of in situ UV-visible and infrared spectroscopic techniques for studying corrosion products and corrosion inhibitors Mater. Sc/. Forum 192-4 277... 

Structure elucidation systems that utilized information from several spectroscopic techniques were initiated in the late 1960s at Toyohashi, Japan, and at the University of Arizona. 

Amphiphiles often have a complex phase behaviour with several liquid crystalline phases These liquid crystalline phases are often characterised by long-range order in one directior together with the formation of a layer structure. The molecules may nevertheless be able tc move laterally within the layer and perpendicular to the surface of the layer. Structura information can be obtained using spectroscopic techniques including X-ray and neutror diffraction and NMR. The quadrupolar splitting in the deuterium NMR spectrum can be... 

Molecular chirality is most often observed experimentally through its optical activity, which is the elfect on polarized light. The spectroscopic techniques for measuring optical activity are optical rotary dispersion (ORD), circular di-chroism (CD), and vibrational circular dichroism (VCD). 

In the second broad class of spectroscopy, the electromagnetic radiation undergoes a change in amplitude, phase angle, polarization, or direction of propagation as a result of its refraction, reflection, scattering, diffraction, or dispersion by the sample. Several representative spectroscopic techniques are listed in Table 10.2. 

Spectroscopic techniques based on the absorption of UV or visible radiation depend on the excitation of an electron from one quantum state to another. References in physical and/or analytical chemistry should be consulted for additional details, but the present summary is sufficient for our purposes ... 

Inorganic flocculants are analyzed by the usual methods for compounds of this type. Residual metal ions in the effluent are measured by spectroscopic techniques such as atomic absorption. Polymeric aluminum species formed in solution have been characterized by Al-nmr (64). 

Air Monitoring. The atmosphere in work areas is monitored for worker safety. Volatile amines and related compounds can be detected at low concentrations in the air by a number of methods. Suitable methods include chemical, chromatographic, and spectroscopic techniques. For example, the NIOSH Manual of Analytical Methods has methods based on gas chromatography which are suitable for common aromatic and aHphatic amines as well as ethanolamines (67). Aromatic amines which diazotize readily can also be detected photometrically using a treated paper which changes color (68). Other methods based on infrared spectroscopy (69) and mass spectroscopy (70) have also been reported. 

Measuring Protein Sta.bihty, Protein stabihty is usually measured quantitatively as the difference in free energy between the folded and unfolded states of the protein. These states are most commonly measured using spectroscopic techniques, such as circular dichroic spectroscopy, fluorescence (generally tryptophan fluorescence) spectroscopy, nmr spectroscopy, and absorbance spectroscopy (10). For most monomeric proteins, the two-state model of protein folding can be invoked. This model states that under equihbrium conditions, the vast majority of the protein molecules in a solution exist in either the folded (native) or unfolded (denatured) state. Any kinetic intermediates that might exist on the pathway between folded and unfolded states do not accumulate to any significant extent under equihbrium conditions (39). In other words, under any set of solution conditions, at equihbrium the entire population of protein molecules can be accounted for by the mole fraction of denatured protein, and the mole fraction of native protein,, ie. 

Instrumental Methods. A variety of spectroscopic techniques are available for the characterization of siUcones. Descriptions of these techniques and Hterature references relevant to siUcone analysis are summarized in Table 12. 

The objective ia any analytical procedure is to determine the composition of the sample (speciation) and the amounts of different species present (quantification). Spectroscopic techniques can both identify and quantify ia a single measurement. A wide range of compounds can be detected with high specificity, even ia multicomponent mixtures. Many spectroscopic methods are noninvasive, involving no sample collection, pretreatment, or contamination (see Nondestructive evaluation). Because only optical access to the sample is needed, instmments can be remotely situated for environmental and process monitoring (see Analytical METHODS Process control). Spectroscopy provides rapid real-time results, and is easily adaptable to continuous long-term monitoring. Spectra also carry information on sample conditions such as temperature and pressure. 

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