Characterization of vacant coordination sites of cations on the surfaces of oxides and zeolites using infrared spectroscopy of adsorbed probe molecules

A critical review is given of the application of IR spectroscopy of small adsorbed probe molecules such as CO and N2 for the determination of the coordination state of surface cations on oxides and zeolites. Special emphasis is devoted to the different kinds of geminal species. So-called complex-specified geminal species are formed as a result of stabilization of the structure by different factors (stable electron configuration, strong ligand-ligand interaction, etc.). During the decomposition of these species linear intermediates are not produced. When complex-specified geminal species are formed, direct conclusions on the original coordination state of the coordinatively unsaturated adsorption site can not be drawn. Site-specified geminal species are produced as a result of low coordination of the adsorption site. These species loose their ligands stepwise and show a tendency to form mixed-ligand species. Site-specified geminal complexes are typically formed with cation-exchanged zeolites, particularly with bulky cations having large radii. Analysis of adsorption isotherms of surface complexes bearing either one or more adsorbate molecules is performed and practical hints for their use are proposed. Finally, possible future directions of the research on this topic are summarized.