Surface properties of catalytic aluminas modified by alkaline-earth metal cations: A microcalorimetric and IR-spectroscopic study

Lewis acidic properties of transition aluminas whose surfaces have been doped with alkaline-earth metal cations (Ca2+ and Ba2+) were studied by means of the room temperature adsorption of carbon monoxide. The vibrational features of CO adsorbed at the surface of doped aluminas were investigated by IR spectroscopy in comparison with pure parent aluminas, while the quantitative and energetic features were studied by adsorption microcalorimetry. Various CO adspecies were found to form at the surface of both pure and doped-alumina, owing to the structural heterogeneity of the Al2O3 surface and to the presence of alkaline-earth metal cations. The surface heterogeneity was revealed by different nu(CO) stretching frequencies, namely nu(CO) approximate to 2230, 2218 and 2205 cm(-1) for coordinatively unsaturated tetrahedral Al3+ cations in different crystallographic configurations, and nu(CO) approximate to 2186 and 2172 cm(-1) for coordinatively unsaturated Ca2+ and Ba2+ cations, respectively. Heats of adsorption of approximate to 80, 70 and 55 kJ/mol were assigned to the formation of Al3+/CO complexes, approximate to 45 KJ/mol for Ca2+/CO and approximate to 30 kJ/mol for Ba2+/CO complexes. The latter value was estimated through a correlation curve existing between nu(CO) stretching frequencies and adsorption enthalpies. This correlation, already proposed in the past for CO adsorbed on non-d/d(0)/d(10) metal cations, has been revisited and confirmed here, by including Al2O3 data for which an apparent lack of correlation between the two parameters was first observed. With respect to pure alumina, the population of Lewis acidic sites was found to be significantly depressed by the presence of alkaline-earth cus metal cations. These acidic sites are intrinsically weaker than tetrahedral cus Al3+ cations, as witnessed by smaller upward shifts of the nu(CO) stretching frequencies with respect to CO gas and lower heats of adsorption, in accordance with expectations from the charge/ionic radius ratios. Ca2+ cations were found to compete in adsorbing CO with Al3+ cations more efficiently than the larger Ba2+ cations. In the case of CaO/Al2O3 systems outgassed at 1023 K, a thin surface layer of calcium aluminate, not detected by XRD or HRTEM, was suggested to form.