Adsorption on metal oxides: Studies with the metastable impact electron spectroscopy

An overview is given on the application of metastable impact electron spectroscopy, in combination with UPS, to the study of clean magnesia and titania surfaces and their interaction with metal atoms and small molecules. The mechanisms for metal adsorption on reducible (titania) and non-reducible (magnesia) substrates are different: while on titania the metal atom often bonds by electron transfer to Ti3d states, it is hybridization of the adsorbate and anion wavefunctions which accounts for the bonding on MgO. In the case of H2O, molecular adsorption takes place both on MgO and TiO2; on the other hand, water-alkali coadsorption leads to hydroxide formation. In the case of CO2, chemisorption takes place in form of carbonate (CO3) species. These originate from the CO2 interaction with O2- surface anions. While for CaO chemisorption takes place at regular oxygen sites, for MgO this occurs at low-coordinated oxygen ions only; for TiO2 chemisorption requires alkali coadsorption