Abstract
The interaction of CO with Ca-FER was investigated by a combination of IR spectroscopy an microcalorimetry and by periodic DFT calculations employing the ω / r correlation for calculations of CO stretching frequencie in order to gain more insight on Ca 2+ cation coordination in the FER zeolite and site-specificity of CO stretching frequency in IR spectra of CO/Ca-FER system. Experimental data were interpreted on the basis of a good agreement between experimental and theoretical results.
Detailed analysis of both theoretical and experimental results reveals that no individual band can be assigned to specific type of CO-Ca complex or to a one type of Ca 2+ sites. On the contrary,several different type of (poly)carbonyl complexes contribute to each vibrational band discerned in the experimental IR spectra.The band at 2197 cm MINUS SIGN 1 gradually shifting to 2194 cm MINUS SIGN 1 with increasing coverage represents monocar- bonyl at all Ca 2+ sites and most stable dicarbonyls formed on Ca 2+ cations at the intersection of channels.
The band at 2188 cm MINUS SIGN 1 is assigned to the dicarbonyl complexes on P6, M5, and M7 sites, whereas the band at 2184 cm 1 belongs to thevibration of the less stable dicarbonyl complexes on P8 sites and to the tricarbonyl complexes on I2 sites. Comparison of results for Ca-FER with those reported previously fo Mg-FER zeolite (R.
Bulanek, I. Voleska, E.
Ivanova, K. Hadjiivanov, P.
Nachtigall, J. Phys.
Chem. C 113 (2009) 11066) clearly showed that the differences in specific "effect from the bottom" are much smaller in the case of Ca 2+ cations than in the case of Mg 2+ cations due to the fact that large Ca 2+ cationsinteractswithmore framework oxygen atoms and they are farther from the zeolite framework than Mg 2+ cations.