Acidity of SiO-Supported Metal Oxides in the Presence of HO Using the AEIR Method: 2. Adsorption and Coadsorption of NH and HO on TiO/SiO Catalysts.

Two different TiO/SiO compounds containing TiO nanodomains dispersed over SiO were investigated applying the AEIR method at the adsorption equilibrium of NH and HO 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 NH adsorption sites: the first ones could correspond either to NH 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 TiO nanocrystallites. They also correspond to the stronger adsorption sites of HO. 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 TiO/SiO 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 NH and HO from microcalorimetry measurements. The surface acidity of the two compounds in the presence of HO was determined using NH-HO coadsorption. At ≥ 473 K, the NH species on the L2 sites were not significantly displaced from the surface whatever the partial pressure of HO studied in agreement with the Temkin competitive model using the individual heats of adsorption of the NH and HO species. This model also revealed the presence of a small amount of HO species adsorbed on L2 sites allowing HO 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.