Adsorbed species, spectra

Differential spectra of diffused reflection around 230-700 nm were measured with a two-beam recording spectrophotometer with corrected zero line it was constructed in our laboratory to measure the spectra of adsorbed species. Spectra were recorded before and after irradiation with a PRK-2 mercury lamp for 15 minutes of the zeolite sample containing adsorbed amine. 

Previous IR studies of metal oxides sulfation have evidenced two types of sulfate species, surface species characterized by one or more bands in the 1410-1370 cm l frequency range, and bulklike species leading to wide bands in the 1200-1000 cm l range (3, 9-12). In a recent paper (4), it has been shown that Pt does not affect the sulfate formation nor the nature of adsorbed species. Spectra reported in Fig.l show that only surface sulfate species are present on VxJZrOj (Fig.la) while both species are formed on Pt/Ce02 (Fig.lb) and Pt/Ce02-Zr02 (Fig.lc). 

HREELS High-resolution electron energy-loss spectroscopy [129, 130] Same as EELS Identification of adsorbed species through their vibrational energy spectrum... 

The Raman spectrum of an oxide sample after adsorption may be considered to consist of the spectrum of the adsorbed species superimposed on the spectrum due to the oxide adsorbent. In general, the Raman spectra of oxide adsorbents are sufficiently weak or sufficiently simple that they allow the detection of Raman lines due to the adsorbed species. This is one major advantage of Raman scattering over infrared absorption spectroscopy. The infrared spectra of most oxide adsorbents show strong absorptions which may obscure those arising from the adsorbates (Figs. 13,14). 

Angell (1) has investigated the Raman spectra of acetonitrile, propylene, and acrolein on a number of zeolites and found that physical adsorption occurred. There are sufficient differences between the spectrum of the liquid and of the adsorbed species (e.g. the carbon-carbon double bond stretching in the case of propylene and the carbon-nitrogen triple bond stretching in the case of acetonitrile) to make it quite clear that it was not merely a case of condensation in the pores of the solid adsorbent. 

The fact that substrates do not substantially interfere with the spectrum of the adsorbed molecule itself makes Raman spectroscopy a most valuable method for examining vibrations of adsorbed species. 

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. 

Fig. 1. Ft-ER spectra of the adsorbed species arising from the interaction of C03O4+X with propene (upper spectrum) and acrylic acid (lower spectrum) both at 373 K.

Figure 11(a) shows the spectrum of adsorbed species on an active catalyst in a hydrogen-ethylene stream. This spectrum appears and stabilizes within minutes after hydrogen is blended into the ethylene stream. Three new bands appear in the presence of hydrogen at 2892, 2860, and 2812 cm-1. The appearance and location of these bands were verified by expanded scale spectra. Experiments at lower ethylene pressures reveal that there is an additional band at about 2940 cm-1 partially obscured in Fig. 11 by overlap of the ethylene spectrum. On a poisoned catalyst, which does not show the ZnH and OH bands, only the bands characteristic of chemisorbed ethylene are seen. 

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

A variety of radical ions have been produced by photochemical reactions involving adsorbed species. One of the simplest of these is the radical ion which is formed upon y irradiation of ethylene adsorbed on silica gel (92). The resulting nine-line spectrum has been attributed to (CH2 = CHi-) + however, it may be due to the corresponding negative ion. The neutral ethyl radical is also formed under these conditions. 

Some idea of the problem can be gleaned from Figure 2.37, which shows a transmittance spectrum of a c. 5 pm thick layer of water. On the same scale the IR absorptions expected from a monolayer of an adsorbed species may be of the same order as the thickness of the pen trace. 

The band at 1120cm"1 in the PySH solution spectrum is the substituent-sensitive band which shifts to 1100cm "1 in the SERS spectra of the two adsorbed species and increases in intensity. 

Compounds known to undergo changes in their absorption spectra upon sorption onto a solid surface are termed adsorptiochromic, and such effects would be ideally studied by means of diffuse reflectance spectroscopy. In one such study, the absorption of various spiropyrans onto many different solids was investigated [35]. For the compounds studied, the reflectance spectra were dominated by bands at 550 nm and in the range of 400-500 nm (most often at 472 nm). As an example, the reflectance spectra obtained for 6-nitrobenxospir-opyran are shown in Fig. 5. When the difference spectrum was taken between the spectrum of the pure compound and that obtained after sorption onto silicic acid, the bands characteristic of the adsorbed species were clearly evident. 

Transmission infrared spectra of species adsorbed on the catalyst were taken with a Digilab FTS-10M Fourier-transform infrared spectrometer, using a resolution of 4 cm-l. To improve the signal-to-noise ratio, between 10 and 100 interferograms were co-added. Spectra of the catalyst taken following reduction in H2 were subtracted from spectra taken in the presence of NO to eliminate the spectrum of the support. Because of the very short optical path through the gas in the reactor and the low NO partial pressures used in these studies, the spectrum of gas-phase NO was extremely weak and did not interfere with the observation of the spectrum of adsorbed species. 

As noted in the introduction, vibrations in molecules can be excited by interaction with waves and with particles. In electron energy loss spectroscopy (EELS, sometimes HREELS for high resolution EELS) a beam of monochromatic, low energy electrons falls on the surface, where it excites lattice vibrations of the substrate, molecular vibrations of adsorbed species and even electronic transitions. An energy spectrum of the scattered electrons reveals how much energy the electrons have lost to vibrations, according to the formula ... 

Figure 7 shows SNIFTIRS spectra for isoquinoline molecules adsorbed on mercury. The reference spectrum in each case was obtained at 0.0V vs. a SCE reference electrode at this potential the molecules are believed to be oriented flat on the metal surface. The vibrational frequencies of the band structure (positive values of absorbance) are easily assigned since they are essentially the same as those reported by Wait et al. (22) for pure isoquinoline. The differences in the spectra are that the bands for the adsorbed species exhibit blue shifting of 3-4 cm" relative to those of the neat material, and the relative intensities of the bands for the adsorbed species are markedly changed. 

By the potential corresponding to about 0.6 V, the -C 0 spectrum has transformed to one corresponding to about 1750 wave numbers, which is typical of JC-0. Willsau et al (18) contends that COH is the intermediate in this reaction, however C-H vibrations were not detected for the adsorbed species. Further, three... 

Depending on the source of the graphite, one obtains distinctly different IR/PA spectra (frequently caused by adsorbed species) and the response of the DTGS detector of an IR spectrometer turns out to be a more accurate measure of variable source intensity (12). A normalization technique (13) requiring measurement of the spectrum at two different mirror velocities and corrected by black body spectra taken at the same two velocities appears to be the best normalization method reported thus far. 

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