摘要
We have performed a comparative study of the acid–base characteristics of the surfaces of anatase TiO 2 and tetragonal ZrO 2 . To this end we performed DFT + U calculations on CO and CO 2 probe molecules adsorbed both on terraces and steps of the two oxides. For titania, CO adsorption results in a moderate adsorption energy (about − 0.3 eV) and in a positive shift of the [Formula presented] stretching frequency (about + 40 cm − 1 ), typical of Lewis acid sites, with no clear difference in the acidity between terraces or steps. For zirconia we found a similar CO binding energy as for titania, and a CO vibrational shift that depends on the location of the Zr cation: negligible on terraces, similar to TiO 2 on steps. We conclude that the acidic properties are similar in the two oxide surfaces. Things are different for CO 2 adsorption. On titania the interaction is weak and surface carbonates compete with physisorbed CO 2 , indicating a weak basic character. On the contrary, on zirconia three types of stable carbonates have been identified. Their vibrational frequencies are consistent with IR measurements reported in the literature. The most stable species forms on steps of the t-ZrO 2 surface and consists of a CO 3 2 − unit which lies flat on the surface with the O atoms pointing towards three Zr ions. The species forms spontaneously by extraction of a lattice oxygen by an incoming CO 2 molecule. The different reactivity points towards a much more pronounced basic character of zirconia compared to titania, at least if measured by CO 2 adsorption.