Acidity of SiO2-Supported Metal Oxides in the Presence of H2O Using the AEIR Method: 2. Adsorption and Coadsorption of NH3 and H2O on TiO2/SiO2 Catalysts.

ABSTRACT

Two different TiO2/SiO2 compounds containing TiO2 nanodomains dispersed over SiO2 were investigated applying the AEIR method at the adsorption equilibrium of NH3 and H2O from 300 to 723 K, particularly for the measurement of the individual heats of adsorption of the different species on Lewis acidic sites (LAS) and Brønsted acidic sites (BAS) as evaluation of the strength of the sites. It revealed two types of NH3 adsorption sites the first ones could correspond either to NH3 species H-bonded to free OH groups or to coordinated weak LAS (named L1). The second ones (L2) were attributed to strongest LAS similar to those present at the surface of TiO2 nanocrystallites. They also correspond to the stronger adsorption sites of H2O. Two types of Brønsted acid sites (BAS) were additionally evidenced by the AEIR method and proposed to be specifically located on the Si-O-Ti bridging bonds at the TiO2/SiO2 interface. The heats of adsorption of the different adsorbed species provided by the AEIR method were consistent with literature data on average values of the heats of adsorption of NH3 and H2O from microcalorimetry measurements. The surface acidity of the two compounds in the presence of H2O was determined using NH3-H2O coadsorption. At T ≥ 473 K, the NH3 species on the L2 sites were not significantly displaced from the surface whatever the partial pressure of H2O studied in agreement with the Temkin competitive model using the individual heats of adsorption of the NH3 and H2O species. This model also revealed the presence of a small amount of H2O species adsorbed on L2 sites allowing H2O dissociation or/and hydrolysis of SiOTi or TiOTi bridges, leading to the formation of a much higher amount of BAS. Therefore, this original work combining the AEIR method and the Temkin competitive model provided new insights for understanding water effects on acidic oxide catalysts.