Acidity of SiO-Supported Metal Oxides in the Presence of HO Using the Adsorption Equilibrium Infrared Spectroscopy Method: 1. Adsorption and Coadsorption of NH and HO on SiO.

The present study is dedicated to the characterization (identification, heats of adsorption, and coverages) of the adsorbed species formed by the adsorption and coadsorption of NH and HO on two SiO solids. Adsorption equilibrium infrared spectroscopy allowed us (a) to show that NH and HO are mostly adsorbed on free SiOH groups via H bonds and (b) to determine their individual heats of adsorption: 53 and 49 kJ/mol, whatever be their coverages (Langmuir adsorption model), for NH and HO, respectively. These values consistent with the microcalorimetry literature data explain that their coverages are decreased upon NH-HO coadsorption, considering a competitive Langmuir model. However, the temperature-programmed adsorption equilibrium procedure achieved from MS data indicated that a minor amount of other NH species (not detected using Fourier-transform infrared) is more strongly adsorbed and that hydrolysis of SiOSi siloxane by HO could occur in parallel. NH-HO coadsorption leads to the formation of NH species, which involves HO adsorbed species. Both NH and HO are not adsorbed above 450 K, which means that the SiO contribution to the characterization of the acidity of metal oxide catalysts supported on SiO using NH as the probe molecule in the presence of HO is negligible above this temperature.