FTIR study of CO and NH3 co-adsorption on TiO2 (rutile)

FTIR spectroscopy has been used to study CO and ammonia adsorption and co-adsorption on rutile. Ammonia is adsorbed coordinatively, but with time, different dissociative ammonia forms appear. At room temperature CO is adsorbed forming one type of Ti4+–CO carbonyls alone (absorption band at 2191 cm−1) and is oxidized to bicarbonates. At 100 K CO occupies, at first, the sites detected at room temperature, whereas the increase in amount of adsorbed CO results in new adsorption forms: (i) CO adsorbed on titanium cations inert at room temperature (monitored by an intense band with a maximum at 2183 cm−1), (ii) CO adsorbed on hydroxyl groups (band at 2150 cm−1), and (iii) physically adsorbed CO (band at 2140 cm−1). Pre-adsorbed ammonia decreases the acidity of both, the Lewis acid sites and the surface Ti4+–OH groups. Low-temperature CO adsorption on rutile partly poisoned by ammonia allows to detect only the sites inert towards CO adsorption at room temperature. On the basis of the results obtained, the active sites for adsorption of ammonia and CO are discussed and comparison between the surface properties of anatase and rutile is made.