Periodic density functional and FTIR spectroscopic studies on CO adsorption on the zeolite Na-FER

摘要

Carbon monoxide adsorption on Na-FER was studied by a combination of variable-temperature IR spectroscopy and computational methods involving,mainly,periodic density functional theory and omega_(CO)/r_(CO)correlation.Both,C-O stretching frequency and interaction energy of adsorption complexes were investigated.Interplay between calculated and experimental results led to identification of the following adsorbed species:(i)C-down monocarbonyls formed on single Na~+ sites,characterized by an infrared absorption band at 2175 cm~(-1);(ii)O-down isocarbonyls showing a characteristic IR absorption band at 2113 cm~(-1);and(iii)bridged Na~+...CO...Na~+ complexes formed on dual cation sites constituted by a pair of nearby Na~+ ions,and characterized by an IR absorption band at 2158 cm~(-1).The bridged carbonyl complex was found to be about 4 kJ mol~(-1)more stable than the C-down monocarbonyl,despite its smaller C-O stretching frequency.The least stable adsorption complex is the O-down(isocarbonyl)species formed on single Na~+ sites.Besides carrying out a detailed characterization of the CO/Na-FER system,investigation of the bridged CO complex on dual cation sites was one of the main aims pursued;such a novel type of CO adsorption complex was described very recently,for the first time,for the case of CO adsorbed on K-FER [J.Phys.Chem.B 110(2006)22542].