Adsorption of Methylene Blue on TiO2/SiO2 Prepared by Chemical Vapor Deposition

TiO2/SiO2 was prepared by the chemical vapor deposition followed by characterization using XRD, Raman spectroscopy, UV-Vis spectroscopy and N-2-adsorption, respectively. Also, the adsorption of methylene blue (MB) on TiO2/SiO2 was studied including the optimization of adsorption conditions, kinetics, isotherms and thermodynamics which were investigated in detail. TiO2/SiO2 has a characteristic mesoporous pore size of 8.8 nm. More importantly, Ti species in TiO2/SiO2 exist as low coordinated frame Ti, extra frame Ti and even as bulk anatase and rutile. The adsorption ratio can exceed 74% under optimum conditions. The adsorption process is better fitted by both the pseudo-second-order kinetic model and the Langmuir isotherm model. All the thermodynamic parameters of the adsorption including Delta H-0, Delta S-0, and Delta G(0) are negative, indicative of an exothermic system, which is entropy-decreasing and in turn spontaneous. The adsorption mechanism suggests that TiO2/SiO2 performs better than SiO2 in the adsorption of MB resulting from all kinds of Ti species. Especially, Ti species with low coordination in TiO2/SiO2 prepared by the chemical vapor deposition are the prime responsible species for the adsorption of MB because of higher abilities to coordinate N and S atoms from MB ions. Consequently, the chemical vapor deposition of TiCl4 on SiO2 is a promising and prospective technique for preparing novel adsorbents, provided that improvements are employed such as increasing the specific surface area of SiO2 as well as adjusting Ti ion loading and pore structure.