Water-vapor adsorption and surface area measurement of poorly crystalline boehmite

Water-vapor adsorption on poorly crystalline boehmite (PCB) was studied using a gravimetric FTIR apparatus that measured FTIR spectra and water adsorption isotherms simultaneously. The intensity of the <f>δ</f>(HOH) band of adsorbed water changed linearly with water content and this linear relationship was used to determine the dry mass of the sample. Adsorption and desorption isotherms of PCB showed a Type IV isotherm. The BETH2O surface area of PCB was <f>514±36</f> m2<f>/</f>g. The mean crystallite dimensions of PCB were estimated to be <f>4.5×2.2×10.0</f> nm (dimensions along the <f>a</f>, <f>b</f>, and <f>c</f> axes, respectively) based on application of the Scherrer equation to powder diffraction data of PCB. A surface area value of <f>504±45</f> m2<f>/</f>g calculated using the mean crystallite dimensions was in good agreement with the BETH2O surface area. This work also demonstrated a method to determine surface areas for materials with minimal perturbation of their surface structure. In addition, the FTIR spectra of PCB were influenced by changes in water content. The <f>δ</f>(AlOH) band at 835 cm<f>−1</f> observed under dry conditions was assigned to the non-H-bonded surface OH groups. As the amount of adsorbed water increased, the intensity at 835 cm<f>−1</f> decreased and that at 890 and 965 cm<f>−1</f> increased. The 890- and 965-cm<f>−1</f> bands are assigned to surface OH groups H-bonded with adsorbed water. [Copyright &y& Elsevier]