RAIRS cyclohexane

We then designed model studies by adsorbing cinchonidine from CCU solution onto a polycrystalline platinum disk, and then rinsing the platinum surface with a solvent. The fate of the adsorbed cinchonidine was monitored by reflection-absorption infrared spectroscopy (RAIRS) that probes the adsorbed cinchonidine on the surface. By trying 54 different solvents, we are able to identify two broad trends (Figure 17) [66]. For the first trend, the cinchonidine initially adsorbed at the CCR-Pt interface is not easily removed by the second solvent such as cyclohexane, n-pentane, n-hexane, carbon tetrachloride, carbon disulfide, toluene, benzene, ethyl ether, chlorobenzene, and formamide. For the second trend, the initially established adsorption-desorption equilibrium at the CCR-Pt interface is obviously perturbed by flushing the system with another solvent such as dichloromethane, ethyl acetate, methanol, ethanol, and acetic acid. These trends can already explain the above-mentioned observations made by catalysis researchers, in the sense that the perturbation of initially established adsorption-desorption equilibrium is related to the nature of the solvent. 

The VEEL spectra of the species formed from cyclohexane on Pt(lll) show that at least two intermediate species occur along the decomposition pathway to benzene. These spectra are discussed in Sections VI.A and VI.C, in the context of spectra of species formed from adsorbed cyclohexene (239) and cyclo-l,3-hexadiene (240) on the same surface. On Pt(100) hex, in contrast to Pt(lll), most of the cyclohexane molecules desorb before conversion to benzene, but the latter was formed after adsorption at 300 K. An intermediate in the conversion of cyclohexane into benzene on Pt(100) (1 X 1), stable between ca. 200 and 300 K, was recognized spectroscopically, but not structurally identified, by RAIRS (230) and by VEELS (224). It seems that there is a smooth transition from the spectrum of adsorbed cyclohexane on Pd(100) to that of benzene at temperatures exceeding 250 K without the detection of intermediate spectra (220). 

Since both HREELS and RAIRS are vibrational spectroscopies, and the same selection rules apply, their information contents must overlap. This is demonstrated in Fig. 8 [2], in which the HREELS and RAIRS spectra from a Cu(l 11) surface covered with about 10 molecular layers of cyclohexane at low temperature are shown. The vibrational spectra appear at the same energetic positions in both techniques, but it should be noted that whereas RAIRS has the advantage of better energy resolution HREELS is able to record spectra down to losses close to 0 cm. For reasons of IR transmission of window materials, the cutoff in RAIRS is in the region 400-800 cm". ... 

Figure 8. Comparison of the HREELS (upper) and RAIRS (lower) vibrational spectra from a multilayer of cyclohexane condensed on Cu(lll) at low temperature. Energy resolution was 8.0 meV. RAIRS has superior energy resolution but suffers a frequency cutoff in the range 400-800 cm. Note the difference in energy resolution between the HREELS spectra in this figure and in Fig. 7. demonstrating the progressive improvement in the HREELS capability with time. (From Ref. 2.)...

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