FTIR STUDY OF LOW-TEMPERATURE CO ADSORPTION ON PURE AND AMMONIA-PRECOVERED TIO2 (ANATASE)

Low-temperature CO adsorption on TiO2 (anatase) has been investigated by FTIR spectroscopy on (i) a pure sample, (ii) a sample on which the sites for CO adsorption at mom temperature are blocked by ammonia, and (iii) anatase whose surface is covered by ammonia. Adsorption of smal l amounts of CO at 100 K on anatase leads to the appearance of two ban ds at 2210 and 2192 cm(-1) due to CO adsorbed on two kinds (alpha and beta', respectively) of Ti4+ sites (the same species are also observed when adsorbing CO at room temperature). The increase of the introduce d CO amount involves sites (beta ''-sites) that are inert at room temp erature. The CO molecules adsorbed on beta ''-sites interact with CO m olecules preadsorbed on beta' sites, as a result of which the two adso rption forms produce a common absorption band whose maximum is shifted to 2179 cm(-1) at higher coverage. With increasing amount of adsorbed CO, one more kind (gamma) of Ti4+ site with very weak acidity is dete cted, the corresponding absorption band being at 2165 cm(-1). Under CO equilibrium pressure two additional reversible adsorption forms appea r: CO H-bonded to surface hydroxyl groups (nu(CO) at 2155 cm(-1)) and physically adsorbed CO (band at 2138 cm(-1)). Simultaneously with the appearance and increase in intensity of the band at 2155 cm(-1), a bro adening and shift by about -115 cm(-1) of the bands for the surface hy droxyl groups occur. Adsorption of a (CO)-C-13 shows that the shifts o f the bands at 2210 and 2192 cm(-1) are mainly of a static type(-4 and -17 cm(-1), respectively), the dynamic components being only +4 cm(-1 ) for the beta-carbonyls and not measurable for the alpha-carbonyls. C O adsorption on reduced anatase indicates the formation of the same ty pes of carbonyls. However, in this case, part of the introduced CO pro bably dissociates and oxidizes the Ti3+ ions into Ti4+. Low-temperatur e CO adsorption on anatase on which the sites for CO adsorption at roo m temperature (the alpha and beta' sites) are preliminary blocked by a mmonia leads to the formation of carbonyls on beta '' and gamma sites (nu(CO) at 2177 and 2156 cm(-1), respectively), as well as of CO adsor bed on OH groups and physically adsorbed CO. In this case the shift of the OH stretching modes is still -115 cm(-1). Low-temperature CO adso rption on anatase fully precovered with ammonia shows the appearance o f weakly bound forms only: a part of the carbonyls on gamma-sites (nu( CO) shifted to 2151 cm(-1)), CO adsorbed on hydroxyl groups, and physi cally adsorbed CO. However, in this case the shift of the nu(OH) stret ching modes is only -65 cm(-1). The nature of the different kinds of a ctive sites for adsorption of ammonia and CO on anatase is discussed.