FTIR study of low-temperature CO adsorption on high surface area tin(IV) oxide: Probing Lewis and Bronsted acidity

The Lewis and Bronsted acidic properties of the surface sites of a high surface area tin(IV) oxide sample were investigated by the CO adsorption at low temperature using IR spectroscopy. Two distinct cus (coordinatively unsaturated sites) Sn4+ cation sites having different Lewis acidic strength have been evidenced: (i) Sn4+ I strongly bonding CO (nu(co) = 2210-2196 cm(-1)), (ii) Sn4+ (II) (nu(CO) = 2183=2177 cm(-1)), being less acidic. Several types of OH groups have been also evidenced: (i) OH occluded in the bulk and inaccessible to CO (nu(OH) = 3445- 3435 cm(-1)), (ii) non-acidic OH groups (nu(OH) = 3740-3725 and 3660 cm(-1)) which give rise to a nu(CO) band at 2155 cm(-1) and nu(OH) bands respectively at 3640 and 3570 cm(-1) (Deltanu(OH) = 90 cm(-1)) as CO adsorption occurs, (iii) slightly Bronsted acidic OH groups (nu(OH) = 3625 cm(-1)) which lead to a nu(CO) band at 2164 cm(-1) and a n band at 3740 cm(-1) (Deltanu(OH) = 155 cm(-1)). Depending on the treatment in vacuum (at room temperature or at 773 K) prior to CO adsorption, the populations of these different sites may vary. An outgassing in vacuum at 773 K leads to the partial dehydroxylation of the SnO2 surface, increasing the number of Sn4+ Lewis acidic sites.