Surface acidity of η-alumina. Part 2.—Interaction of pyridine with other adsorbates

Abstract

The interaction of Py with other adsorbates, whose behaviour was investigated previously, is studied here by i.r. spectroscopy and microgravimetry, both to confirm previous assignments and to gain new information on the nature of alumina surface sites. Py/CO interactions reveal that the strongest Lewis acidic sites (Al^IV _cus ions) are heterogeneous, whereas the adsorption of Py alone and the use of any simple surface model does not show this. Py/H₂O interactions indicate that Py adsorbed at Al^IV _cus ions is selectively and non reversibly removed, as these sites are the favoured centres for surface rehydration. Other weaker Lewis adsorbed Py species are, in contrast, involved in surface equilibria with adsorbing water. Py/CO₂ interactions indicate that Py Lewis coordination inhibits σ-coordination of CO₂ and that it induces an unusual basicity in surface oxygens, so that monodentate and bidenate carbonates are formed upon CO₂ uptake. However, two types of surface bicarbonates are inhibited, either through the coordination of Py at the proper acidic centre, or through strong H bonding between Lewis coordinated Py and the “reactive” hydroxyl. Chemical evidence is found for cooperative effects and lateral interactions involving coordinated Py species, which were previously postulated on the basis of the spectroscopic features of adsorbed species.