Theoretical and experimental study of CO adsorption on Ca-FER zeolite

• Eint(CO) and ν(CO) depend primarily on distance of Ca2+ cation from the channel wall.
• The geometry of the Al pair in particulate site also affects Eint(CO) and ν(CO).
• Polycarbonyls require enough space around cation—it depends on cation location.
• Difference in specific effects from the bottom are much smaller compared with Mg-FER.

The interaction of CO with Ca-FER was investigated by a combination of IR spectroscopy and microcalorimetry and by periodic DFT calculations employing the ω/r correlation for calculations of CO stretching frequencies in order to gain more insight on Ca2+ 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 Ca2+ 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−1 gradually shifting to 2194 cm−1 with increasing coverage represents monocarbonyls at all Ca2+ sites and most stable dicarbonyls formed on Ca2+ cations at the intersection of channels. The band at 2188 cm−1 is assigned to the dicarbonyl complexes on P6, M5, and M7 sites, whereas the band at 2184 cm−1 belongs to the vibration 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 for 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 Ca2+ cations than in the case of Mg2+ cations due to the fact that large Ca2+ cations interacts with more framework oxygen atoms and they are farther from the zeolite framework than Mg2+ cations.

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