Site-specified and complex-specified formation of geminal species during adsorption of small molecules on cationic sites

The geminal complexes formed after adsorption of small molecules on cationi c sites may be divided into two main groups: site- and complex-specified. C omplex-specified formation always occurs during the interaction of definite cations with certain molecules. Typical examples are the Co2+(NO)(2) speci es. A peculiarity, in this case, is that the geminal complexes are more sta ble than the respective linear species. The site-specified formation of gem inal complexes is determined by the low co-ordination number of the cation serving as adsorption centre. Typical examples are the geminal species form ed on metal cations exchanged in ZSM-5 zeolite: Cu(CO)(2) and Na+(N-2)(2). In this case, the linear complexes are more stable than the geminal species . Mathematical expressions describing the adsorption isotherms as well as t he ratios between the linear and geminal species at different coverages are proposed. The co-adsorption of two gases is also considered. Simultaneous adsorption of three molecules on one centre is illustrated by the formation of Cu+(H2O)(2)CO adducts on Cu-ZSM-5. Finally, the possibility of formatio n of species that are site- and complex- specified is discussed. (C) 2001 E lsevier Science B.V. All rights reserved.