END-ON SURFACE COORDINATED (ADSORBED) CO-2 - A SPECIFIC LIGAND FOR SURFACE LEWIS ACIDIC CENTERS

Carbon dioxide interacts at ambient temperature at the surface of most solids, yielding several families of chemisorbed species. One such family corresponds to the end-on sigma-coordination of CO2 to coordinatively unsaturated surface cationic centres, and in this coordination the molecule preserves the linear shape and loses the center of symmetry (D (infinity h) --> C (infinity v)). End-on coordinated CO2, which does not seem to be so common as a ligand in coordination compounds, turns out to be a probe adspecies very sensitive to the features of surface Lewis acidic sites: a) in systems towards which other probe molecules, like CO, are not active at all (e.g., Al hydroxides, Al fluoride); b) in systems where the presence of surface adsorbate-adsorbate interactions produces differences of acidic strength, towards which other test molecules are not so sensitive (e.g., sulfate-doped ZrO2).