Computational and Variable‐Temperature Infrared Spectroscopic Studies on Carbon Monoxide Adsorption on Zeolite Ca‐A

Abstract Bridged Ca 2+ ⋅⋅⋅CO⋅⋅⋅Ca 2+ complexes are formed on dual‐cation sites, constituted by a pair of nearby Ca 2+ cations, when CO is adsorbed on zeolite Ca‐A. Two types of such species can be formed, designated S2–S1 and S1–S1 (see picture). Ca 2+ ⋅⋅⋅CO monocarbonyl species are also identified, and at a relatively high CO equilibrium pressure, dicarbonyl complexes can also form. magnified image Carbon monoxide adsorption on LTA (Linde type 5A) zeolite Ca‐A is studied by using a combination of variable‐temperature infrared spectroscopy and computational methods involving periodic density functional calculations and the correlation between stretching frequency and bond length of adsorbed CO species ( $\tilde \nu $ CO / r CO correlation). Based on the agreement between calculated and experimental results, the main adsorption species can be identified as bridged Ca 2+ ⋅⋅⋅CO⋅⋅⋅Ca 2+ complexes formed on dual‐cation sites constituted by a pair of nearby Ca 2+ cations. Two types of such species can be formed: One of them has the two Ca 2+ ions located on six‐membered rings of the zeolite framework and is characterized by a CO stretching frequency in the range of 2174–2179 cm −1 and an adsorption enthalpy of −31 to −33 kJ mol −1 , whereas the other bridged CO species is formed between a Ca 2+ ion located on an eight‐membered ring and another one on a nearby six‐membered ring and is characterized by $\tilde \nu $ CO in the range 2183–2188 cm −1 and an adsorption enthalpy of −46 to −50 kJ mol −1 . Ca 2+ ⋅⋅⋅CO monocarbonyl complexes are also identified, and at a relatively high CO equilibrium pressure, dicarbonyl species can also be formed.