DRIFT spectroscopy technique

Infrared microscopy, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and photoacoustic spectroscopy (PAS) techniques may be suitable for some types of sample but the use of... 

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been proven to be an excellent means of characterizing coals and related materials. This report is devoted to the evaluation of the technique as a method for situ monitoring of the chemical structural changes wrought in reactions of coal with fluid phases. This technique does not require a supporting medium (matrix) which can contain chemical artifacts which inherently serve as a barrier for access to the solid coal. The rapid response of the Fourier transform infrared technique is further beneficial for kinetic studies related to combustion, liquefaction, gasification, pyrolyses, etc. Experimental equipment and techniques are described for studies over wide ranges of pressure (10 5 Pa to ca 1.5 x 10 kPa) and temperature (298 K to 800 K). 

Our intention in this report is to demonstrate the utility of diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy for coal analysis, particularly in relation to monitoring the in situ oxidation of coal, and to compare its relative merits to those of the KBr pellet and PA sampling techniques. 

A number of distinct advantages over the standard KBr pellet technique has been demonstrated for the analysis of coal using DRIFT spectroscopy. The principle advantage is the ability to monitor various reaction processes iri situ where temperature and fluid phase environment can be accurately controlled. 

Specifically, data was presented describing moisture desorption and intermediate temperature air oxidation of a powdered sub-bituminous coal. In comparison to its companion method, PA-IR, DRIFT spectroscopy would appear to be the technique of choice for the study of such reaction processes involving powdered samples since the temperature and environment of the sample are more conveniently controlled. Also PA-IR in general requires longer data acquisition times than DRIFT to produce a similar quality S/N ratio (34), No effort has been made in this report to treat in any way the quantitative aspects which most surely at some point must be considered. Most quantitative work involving DR spectra has utilized the Kubelka-Munk Equation to mathematically treat the data. This Equation seems to apply mainly to species in highly reflecting matrices at low dilution. Therefore, it remains to be determined what treatment may be required for DR spectral data obtained from neat materials such as coal. 

Accessories necessary for DRIFT measurements are described in the literature and in part are commercially available [174-176]). However, reflectance equipment maybe also conveniently made in the laboratory using commercial lenses and mirrors [177]. An interesting new combination of DRIFT spectroscopy with the frequency response technique was recently developed and tested by Harkness et al. [178]. This enables one to measure simultaneously the dynamic responses of both the gas and adsorbate, which should be of great potential for the study of heterogeneous catalysis. A cell for fast response DRIFT spectroscopy is described in Ref. [178]. 

Detailed information on oxyanion bonding mechanism and adsorbed oxide surface species can be obtained by utilizing a combination of spectroscopic techniques and macroscopic measurements, particularly, EM and measurements of net OH change/anion adsorption. ATR-FTIR spectroscopy in suspensions is complemented by DRIFT spectroscopy under conditions where small quantities of adsorbed water are still present. Neither FTIR nor EXAFS provide complete information on surface speciation and bonding. FTIR may not always be... 

Spectra of insoluble solid phenolic prepolymers or crosslinked products can be recorded by diffuse reflectance (DRIFT) and attenual total reflection spectroscopy (ATR). Reflection spectroscopy techniques are qualitative methods for analysis of insoluble materials [228], which work by reflecting the infrared light off the surface of the material to be analyzed. Table 7 gives a summary of wavenumbers of absorbance bands in modified phenolic materials. 

Photoacoustic IR spectroscopy has similar advantages to DRIFT spectroscopy in its ability to handle solids with the minimum of preparation. The principle of this technique is that when a modulated beam of IR radiation is absorbed by a sample, temperature oscillations set up thermal waves. If the sample is sealed in a cell and surrounded by gas, then a microphone can pick up the sound waves in the gas and an IR absorption spectrum generated. 

Different characterization techniques are used to get an insight into the location of transition metal ions in an aluminophosphate framework. Generally, the data on the cation location are collected with difficulty since the metal concentration is low. In this regard, it is necessary to use more than one method if a reliable conclusion is to be reached (ie, the simultaneous application of several physical techniques is recommended). The following characterization methods are commonly applied diffuse reflectance UV-vis spectroscopy (DRS), electron spin resonance (ESR), electron spin echo modulation (ESEM), infrared (IR), and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopies, as well as the nuclear magnetic resonance spectroscopy (NMR), Mossbauer spectroscopy and the X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption spectroscopy for fine structure (EXAFS) (167,168) and references therein). 

Diffuse reflection spectroscopy, also known as diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, enables the analysis of many samples for which traditional techniques fail, to be made with little or no sample preparation. Many substances in their natural. state. e.spe-cially powders but also any solid with a rough surface, such as dyed textiles and printed papers, exhibit diffuse reflection, i.e., incident light is... 

SSITKA. The idea behind this approach was to compare the time response of isotopically labelled surface and gas-phase species by DRIFT spectroscopy and MS analysis. This novel technique was applied for mechanistic analysis water-gas shift (WGS) and reversed WGS reactions over F t- and Au-based catalysts with the aim of identifying true surface intermediates. These authors found that both formate and carbonate species labelled with C were formed on the catalyst surface after switching from C0/H20/Ar = 2/7/91 to C0/H20/A = 2/7/91. In order to determine which surface species actively participated in CO2 formation, they compared temporal changes in the IR bands of these surface species with those of gas-phase C02. Since the rate of CO2 formation was ca. 60 times higher than the rate of the exchange of formate species it was concluded that the formates detected by DRIFT spectroscopy could not be the main surface intermediates of gas-phase CO2. However, the role of surface formates in CO2 production may change with rising temperature as demonstrated in Ref. 129 where the formate species were spectators at 160°C but became main reaction intermediates at 220 C. 

Several properties of the filler are important to the compounder (279). Properties that are frequentiy reported by fumed sihca manufacturers include the acidity of the filler, nitrogen adsorption, oil absorption, and particle size distribution (280,281). The adsorption techniques provide a measure of the surface area of the filler, whereas oil absorption is an indication of the stmcture of the filler (282). Measurement of the sdanol concentration is critical, and some techniques that are commonly used in the industry to estimate this parameter are the methyl red absorption and methanol wettabihty (273,274,277) tests. Other techniques include various spectroscopies, such as diffuse reflectance infrared spectroscopy (drift), inverse gas chromatography (igc), photoacoustic ir, nmr, Raman, and surface forces apparatus (277,283—290). 

Diffuse Reflectance IR Fourier Transform Spectroscopy (DRIFTS) can be employed with high surface area catalytic samples that are not sufficiently transparent to be studied in transmission. In this technique, the diffusely scattered IR radiation from a sample is collected, refocussed, and analysed. Samples can be measured in the form of loose powders. 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

DRIFT-IR) spectroscopy was also used for polymorphic characterization. The authors detail the application of multivariate techniques, multivariate statistical process control (MSPC), PC A and PLS, to the spectroscopic data for a simple yet powerful, rapid evaluation of the given crystalhzation process. ... 

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