Size- and Morphology-Dependent Kinetics and Thermodynamics of Adsorptions of Basic Fuchsin on Nano-TiO2

Abstract

Nanoparticles have superior adsorption properties compared with corresponding bulk materials, which depends on their particle size and morphology. Herein, by introducing morphology factors, we derived the general relations of adsorption kinetic parameters and thermodynamic properties with an equivalent particle diameter of nanoparticles with different morphologies. Experimentally, we researched the adsorption kinetics and thermodynamics of basic fuchsin on nano-TiO2 with different equivalent particle diameters and morphologies. The results show that the equivalent particle diameter and morphology have significant effects on the adsorption kinetics and the adsorption thermodynamics. Within the range of the experiment, the adsorption rate constant and the equilibrium constant increase, while the activation energy, the pre-exponential factor, the Gibbs free energy, the enthalpy, and the entropy of adsorption decrease with the decrease in the equivalent particle diameter, and the logarithm of rate constant, activation energy, the logarithm of pre-exponential factor, the logarithm of equilibrium constant, molar Gibbs free energy, molar enthalpy, and molar entropy of adsorption are linearly related to the reciprocal of equivalent particle diameter, respectively. For this adsorption system, with the decrease of equivalent particle diameter, the effect of morphology on the adsorption rate constant decreases and the activation energy and the pre-exponential factor approximately remain unchanged, while the logarithm of equilibrium constant, molar Gibbs free energy, molar enthalpy, and molar entropy of adsorption increase. The experimental results are consistent with the theoretical relations. In addition, we are surprised to find a phenomenon that is contrary to the traditional concept of dynamics: compared with the three morphologies, when the adsorption rate constant of nanoparticles is larger, the adsorption activation energy is also larger and when the adsorption rate constant of nanoparticles is smaller, the adsorption activation energy is also smaller, and these can be attributed to the fact that the morphology of nanoparticles has a greater influence on pre-exponential factor than on activation energy. The theory can quantitatively describe the effect of the equivalent particle diameter and morphology on adsorption kinetics and thermodynamics of nanoparticles and better predict the adsorption behaviors of nanoparticles.