The mechanism of the protonation of metal (hydr)oxides in aqueous solutions studied for various interfacial/surface ionization models and physicochemical parameters: A critical review and a novel approach
Kyriakos Bourikasa, b, Christos Kordulisa, c and Alexis Lycourghiotisa, ,
aDepartment of Chemistry, University of Patras, GR — 26500 Patras, Greece bSchool of Science and Technology, Hellenic Open University, Sahtouri 16, GR — 26222 Patras, Greece cInstitute of Chemical Engineering and High Temperature Chemical Processes, FORTH/ICE–HT, P.O. Box 1414, GR — 26500 Patras, Greece
Available online 27 July 2006.
The mechanism of the protonation of solid metal (hydr)oxides in aqueous media was investigated using simulation and experimental work. It was found that the apparent acidity/basicity of each kind of surface sites of metal (hydr)oxides in aqueous suspensions is strongly influenced by the overall surface charge of the (hydr)oxide and thus by the electrical potential smeared out at the interfacial region. Depending on its sign this increases or decreases the hydrogen ion concentration on the surface, thus promoting or hindering protonation. This is manifested by the shifts of the protonation peaks of the various kinds of sites with respect to the − pK values of the corresponding intrinsic protonation constants and the appearance of an extra peak in the d[H+cons,surf] / dpH vs. pH curves. Potentiometric titrations experiments performed for four technologically important oxides showed that the proposed protonation mechanism describes indeed the protonation of polycrystalline (hydr)oxides in aqueous media.