FTIR and NMR study of the adsorbed water on nanocrystalline anatase

A hydrated anatase sample with crystal size of 11 nm, prepared by a method involving a calcination treatment, has been studied by FTIR, H-1 MAS NMR, and TPD techniques, to determine the characteristics of the hydroxyls and water adsorbed on its surface. The H-1 NMR and FTIR spectra of the sample, hydrated and evacuated at increasing temperature, show that most of the hydrated anatase water, weakly adsorbed and highly mobile, is forming the multilayer of water molecular arrangements (MAs). The easy desorption of this water at room-temperature facilitates the observation of less mobile adsorbed water, whose removal originates two overlapped water desorption peaks, with maxima at 353 and 373 K, observed in the TPD profile. Most of this less weakly adsorbed water is desorbed from the MAs second layer, originating the low-temperature maximum. On the other hand, the removal of some water bound to bridging O2- anions, interacting with terminal hydroxyls, contributes to the maximum at 373 K. The TPD peak with maximum at 430 K is mainly formed by the desorption of water interacting with bridging hydroxyls. The TPD shoulder observed at 540 K is originated by the desorption of water coordinated to Ti4+ cations, forming the first layer of the water MAs. The anatase H-1 MAS NMR spectrum obtained after evacuation at 473 K is mainly formed by the signals of bridging and terminal hydroxyls.