Methane CH4 as an Adsorbate

Methane molecules do not possess a permanent dipole moment, and also the quadrupole moment is zero. CH4 exhibits four fundamental vibration modes, the modes Vj and V2, which are in the gas phase IR-forbidden and only Raman-allowed, and the IR- and Raman-active V3 and V4 modes. The corresponding wavenumbers are Vi,V2 3 and V4,i.e., 2914,1526,3020 and 1306 cm respectively. In fact, V2 of adsorbed CH4 was never observed, most probably because the intensity is too low. The spectra after adsorption on Na-A shown in Fig. 40 were taken at 203-273 K (cf. [216,642). 

A molecular-dynamics study of methane (and ethane, propane, ethene as well as ethyne) adsorbed in silicahte-1 and (purely siliceous) zeolite A was performed by Dumont and Bougeard [643] using flexible molecules in rigid frameworks. The reproduction of the experimental IR and Raman features was satisfactory. The agreement was good in the case of silicalite-1, whereas some discrepancies appeared with zeolite A as the adsorbent, due to the role of the cations in the experimentally used, Al-containing zeolite A. 

A combined NMR and DRIFT study of adsorption of CH4 and trimethylgal-lium on H-Y was conducted by Seidel et al. [644]. In the IR spectra of adsorbed CH4, besides the IR-allowed V3 (3007 cm ) and V4 modes the forbidden Vj transition appeared and, furthermore, after adsorption of trimethylgallium a corresponding band at 3005 cm suggesting the formation of methane. 

As important examples, IR investigations on SOj, H2S and HjO molecules adsorbed on zeolites should be mentioned. All three of them proved to be non-hnear, exhibiting the fundamental modes displayed in Fig. 41 (cf. [645]). 

Sulfur dioxide, SO2, adsorbed on zeohtes was first investigated by Deo et al. [646] and Foerster and Seelemann [649]. Deo et al. used as adsorbents Na-Y and H-Y zeolites, and their investigations were related to the modified Claus reaction which occurs according to the overall reaction formula of Eq. (29). Also, studies by Karge et al. [227,228], using combined IR and UV-Vis spectroscopy for measurements of SO2 adsorption on faujasite-type zeolites, were related to the modified Claus reaction  

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