Spectroscopy background spectrum

Catalyst Activation Gas phase activation of supported DENs was examined using in-situ FTIR spectroscopy and FTIR spectroscopy of adsorbed CO. For in-situ dendrimer decomposition studies, the spectra were collected under a gas flow composed of 20% 02/He or 20% H2/He. The supported DEN sample was pressed into a self-supporting wafer, loaded into a controlled atmosphere IR cell, and collected as the sample background. The temperature was raised stepwise and spectra were collected at each temperature until little or no change was observed. After oxidation, the sample was reduced in 20% H2/He flow with various time/temperature combinations. The sample was then flushed with He for lhr at the reduction temperature. After cooling under He flow, a background spectrum was collected at room temperature. A 5% CO/He mixture was flowed over the sample for 15 minutes, followed by pure He. IR spectra of CO adsorbed on the catalyst surface were collected after the gas phase CO had been purged from the cell. 

Infrared spectroscopy has been the most useful method, especially when the attached species incorporate carbonyl ligands. FT analysis is useful for substractlng the background spectrum of the support and for allowing identification of species present in low concentrations. Membrane supports about 10 ym thick, described above, are optimal. Many examples are given in the literature (27), and the technique has been used to characterize working catalysts in the presence of vapor- and liquid-phase reactants. 

For FTIR spectroscopy of adsorbed CO the calcined samples (thin discs of known weight) were degassed at 773 K for 1 hour and the background spectrum was recorded after cooling at 300K. After introduction of CO (around 50 Torr) at 300 K as well as after evacuation at 300 K the spectra were recorded as a function of time, as already described (10, 11, 12). Spectra were recorded on a Nicolet 550 spectrometer (2 cm resolution). The optical densities of the bands were normalized by taking into account the amount of copper. 

The experimental setup consists of a gas dosing system and the DRIFT spectroscopy apparatus. For the pyrolysis experiments KBr was selected as matrix, different to the laser-induced decomposition experiments [141], where SiC was used. KBr was chosen because the emissivity did not increase drastically, as in the case of SiC, where it interfered with the measurements. The Kapton-KBr mixtures are placed in the sample holder of the DRIFT cell and packed using a pressure of 1 MPa as described elsewhere [288, 306]. The sample is heated in an inert gas atmosphere to the desired temperature using a heating rate of 10 K min-1. The spectrum of the Kapton-KBr mixture at a given temperature is collected and used as background spectrum. The following experiments were carried out. 

The molecules in the sample absorb at their characteristic frequencies, and hence the radiation intensity / (x) that reaches the detector is modified by the presence of the sample. The Fourier transform of /a(x) is the absorption spectrum of the sample, and this transform yields percent transmission versus wavenumber (cm ). This type of spectroscopy is single-beam, so a background spectrum is required in most cases. Also it requires the use of a digital computer to calculate the Fourier transform of /a(x). 

X-ray photons result from electronic transitions between the inner energy levels of atoms. When high-energy electrons are absorbed by matter, an x-ray line spectrum results. The structure depends on the substance and is thus used in x-ray spectroscopy. The line spectrum is always formed in conjunction with a continuous background spectrum. The minimum (cutoff) wavelength Xq corresponds to the maximum x-ray energy, This equals the... 

FTIR spectroscopy has been used to study hydrogen bonding in nylon as suggested in the literature (3). In each of our experiments 200-400 scans were averaged, processed, and compared against a previously ran background spectrum of the empty cell at the same temperature. The spectral resolution was set at 1.0 cm". Spectra were recorded at a number of temperatures between 23 and 280 °C. Prior to data collection, the cell was equilibrated for at least 5 minutes at each temperature. CO2 saturation time in the first measurement was 60 minutes to reach sorption equilibrium. Subsequently, once the test temperature was reached, an equilibration time of... 

With modern detectors and electronics most Enei -Dispersive X-Ray Spectroscopy (EDS) systems can detect X rays from all the elements in the periodic table above beryllium, Z= 4, if present in sufficient quantity. The minimum detection limit (MDL) for elements with atomic numbers greater than Z = 11 is as low as 0.02% wt., if the peaks are isolated and the spectrum has a total of at least 2.5 X 10 counts. In practice, however, with EDS on an electron microscope, the MDL is about 0.1% wt. because of a high background count and broad peaks. Under conditions in which the peaks are severely overlapped, the MDL may be only 1—2% wt. For elements with Z < 10, the MDL is usually around 1—2% wt. under the best conditions, especially in electron-beam instruments. 

Although the purpose here is not to give a full understanding of photoeleciron spectroscopy, it can be useful to discuss some of the specific features in a photoelectron spectrum which can be helpful for the understanding of the different examples discussed in this chapter. The main emphasis in the background to PES will be focused on the molecular solids aspect since this chapter deals with condensed conjugated systems. The interested reader can find a more in-depth discussion on the technique, relative to organic polymeric systems, in Refs. [4, 9, 10]. 

In situ FTIR spectroscopy was used to study the adsorbed species generated on the catalyst surface in the presence of Hj and Oj. Before the experiment, the catalyst wafer was pretreated by O, (5.3 kPa) at 723 K for 1 h followed by evacuation at the same temperature in vacuum ca. 6x10 Pa) for 2 h. After the pretreatment, the temperature was decreased to a desired one in vacuum and IR spectrum was recorded at that temperature. The spectra of the catalyst wafer recorded at different temperatures were used as the background ones for the adsorption studies described below. 

Figure 9. Data reduction and data analysis in EXAFS spectroscopy. (A) EXAFS spectrum x(k) versus k after background removal. (B) The solid curve is the weighted EXAFS spectrum k3x(k) versus k (after multiplying (k) by k3). The dashed curve represents an attempt to fit the data with a two-distance model by the curve-fitting (CF) technique. (C) Fourier transformation (FT) of the weighted EXAFS spectrum in momentum (k) space into the radial distribution function p3(r ) versus r in distance space. The dashed curve is the window function used to filter the major peak in Fourier filtering (FF). (D) Fourier-filtered EXAFS spectrum k3x (k) versus k (solid curve) of the major peak in (C) after back-transforming into k space. The dashed curve attempts to fit the filtered data with a single-distance model. (From Ref. 25, with permission.)...

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