The chemisorption of H2O and CO2 on TiO2 surfaces: studies with MIES and UPS (HeI/II)

The interaction of H2O and CO2 with bare and alkali (Li, Na, K, Cs) pre-covered TiO2 surfaces was investigated (130 and 300 K). Metastable impact electron spectroscopy and ultraviolet photoelectron spectroscopy (HeI and II) spectra were collected in situ during the exposure procedure. For H2O at 130 K on bare TiO2 an eventual initial dissociative adsorption at defect sites is followed by molecular adsorption up to multi-layer adsorption. The interaction of H2O with the alkali pre-covered surfaces (130 and 300 K) leads to the formation of hydroxyl groups. At 130 K the formation of a mixed layer consisting of alkali atoms and OH species is followed by molecular adsorption on top of this layer. Annealing experiments show that the first layer remains intact when the second layer forms. For CO2 no chemisorption takes place on the bare surface. However, the alkali pre-covered surface was found to be highly reactive under CO2 exposure. The formation of carbonate (CO3) species is observed. Both for H2O and CO2 electron transfer between the alkali atoms and the molecules plays a decisive role in the chemisorption process.