IR study of CO and H2O coadsorption on Ptn+/TiO2 and Pt/TiO2 samples

Abstract

The adsorption of CO on a Pt/TiO₂ sample has been studied by IR spectroscopy. On the unreduced sample, CO is adsorbed reversibly on Ti⁴⁺ cations (bands at 2188 and 2206 cm⁻¹) and irreversibly on Pt⁴⁺ and Pt²⁺ sites (bands at 2186 and 2138 cm⁻¹, respectively). Water replaces CO adsorbed on Pt⁴⁺ sites and interacts with the Pt²⁺–CO carbonyls producing a doublet with maxima at 2114 and 2088 cm⁻¹. The 2114 cm⁻¹ band is assigned to Pt²⁺–CO carbonyls interacting with water adsorbed on the support, whereas the 2088 cm⁻¹ band is attributed to Pt²⁺(H₂O)CO species. CO adsorption on reduced Pt/TiO₂ samples causes the appearance of bands due to Ti⁴⁺–CO complexes and to linear and bridged carbonyls of P (bands at 2084 and 1860 cm⁻¹, respectively). Subsequent water adsorption produces a tailed P–CO carbonyl band. It is demonstrated that, as a result of the strong CO adsorption on both, Pt²⁺ and P sites, CO can be used as a probe molecule to test inactivated samples, thus providing information about the coexistence of oxidized and reduced platinum on the surface. The stability of the different carbonyls is discussed in terms of the σ- and π-components of Pt–CO bond.