FTIR Study of Low-Temperature CO Adsorption on Pure and Ammonia-Precovered TiO2 (Anatase)

Abstract

Low-temperature CO adsorption on TiO 2 (anatase) has been investigated by FTIR spectroscopy on (i) a pure sample, (ii) a sample on which the sites for CO adsorption at room temperature are blocked by ammonia, and (iii) anatase whose surface is covered by ammonia. Adsorption of small amounts of CO at 100 K on anatase leads to the appearance of two bands at 2210 and 2192 cm - 1 due to CO adsorbed on two kinds (α and β‘, respectively) of Ti 4+ sites (the same species are also observed when adsorbing CO at room temperature). The increase of the introduced CO amount involves sites (β‘‘-sites) that are inert at room temperature. The CO molecules adsorbed on β‘‘-sites interact with CO molecules preadsorbed on β‘ sites, as a result of which the two adsorption 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 (γ) of Ti 4+ site with very weak acidity is detected, the corresponding absorption band being at 2165 cm - 1 . Under CO equilibrium pressure two additional reversible adsorption forms appear: CO H-bonded to surface hydroxyl groups (ν(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 broadening and shift by about −115 cm - 1 of the bands for the surface hydroxyl groups occur. Adsorption of a 13 CO shows that the shifts of 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 β-carbonyls and not measurable for the α-carbonyls. CO adsorption on reduced anatase indicates the formation of the same types of carbonyls. However, in this case, part of the introduced CO probably dissociates and oxidizes the Ti 3+ ions into Ti 4+ . Low-temperature CO adsorption on anatase on which the sites for CO adsorption at room temperature (the α and β‘ sites) are preliminary blocked by ammonia leads to the formation of carbonyls on β‘‘ and γ sites (ν(CO) at 2177 and 2156 cm - 1, respectively), as well as of CO adsorbed 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 adsorption on anatase fully precovered with ammonia shows the appearance of weakly bound forms only: a part of the carbonyls on γ-sites (ν(CO) shifted to 2151 cm - 1 ), CO adsorbed on hydroxyl groups, and physically adsorbed CO. However, in this case the shift of the ν(OH) stretching modes is only −65 cm - 1 . The nature of the different kinds of active sites for adsorption of ammonia and CO on anatase is discussed.