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

Abstract

The Lewis and Brønsted 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 (ν_CO = 2210–2196 cm⁻¹), (ii) Sn⁴⁺_II (ν_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 (ν_OH = 3445–3435 cm⁻¹), (ii) non-acidic OH groups (ν_OH = 3740–3725 and 3660 cm⁻¹) which give rise to a ν_CO band at 2155 cm⁻¹ and ν_OH bands respectively at 3640 and 3570 cm⁻¹ (Δν_OH = 90 cm⁻¹) as CO adsorption occurs, (iii) slightly Brønsted acidic OH groups (ν_OH = 3625 cm⁻¹) which lead to a ν_CO band at 2164 cm⁻¹ and a ν_OH band at 3740 cm⁻¹ (Δν_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.