Effects Of Size And Shape Of Nanoparticles On The Thermodynamics And Kinetics Of Adsorption

Owing to the excellent adsorption properties compared with the corresponding bulk materials,nanoparticles have been widely used in many fields.The properties depend on the thermodynamics of adsorption and the kinetics of adsorption,which depend on the nanoparticle size and morphology.However,the general influence regularities and the mechanisms of the particle size and the morphology of nanoparticles on the adsorption thermodynamics and adsorption kinetics have not been clear.In this paper,the effect regularities of the size and the morphology on the thermodynamics and the kinetics for nano-adsorption were researched in the theory and experiment.(1)The study of the theory of the nano-adsorptionIn the adsorption thermodynamics,the nano-adsorption process is thought to be a reaction,the chemical potentials of nano-adsorbent and the adsorption product were defined to derive the relations between the adsorption thermodynamic properties and the particle size of different morphologies,and the influence regularities and the mechanism of particle size on thermodynamic properties of adsorption were discussed.In the adsorption kinetics,with the assumption that the transition state theory for chemical reaction is applicable to nano-adsorption process,and the transition state is not affected by particle size,we derived the relations between the adsorption kinetic parameters and the particle size,respectively,and discussed the kinetic influence regularities and the mechanism.(2)The study of the experiment of nano-adsorptionIn the experiment,nano-Ce O2 with different morphologies(spherical,cubic and octahedral)was prepared by hydrothermal method.Then,we studied the size-dependent thermodynamics and kinetics of the nano-adsorption with the experimental system for the adsorption of methyl orange(MO)and basic fuchsine onto the nano-Ce O2.The theoretical and experimental results indicate that,The laws of the preparation of nano-Ce O2 are as follows.The spherical,cubic and octahedral nano-Ce O2 were prepared in hydrothermal method by using the cerium source Ce(NO3)3·6H2O and different precipitants of NH3·H2O,Na OH and Na3PO4·12H2O.The nano-Ce O2 with different morphologies of different particle sizes was prepared by controlling the experimental conditions.In the process of preparation,it was found that the concentration of cerium salt,the concentration of precipitant,the hydrothermal temperature,the surfactant and the calcination temperature have different effects on the particle size and morphology of the nanoparticles.The hydrothermal temperature has greater influence on the morphology of the nanoparticles,and the calcination temperature has the greatest influence on the particle size of the nanoparticles.The effect regularities of the size and the morphology on the thermodynamic properties of nano-adsorption are as follows.The adsorption capacity is high on the spherical nano-Ce O2,while it is little on the cubic and octahedral nano-Ce O2,and the nano-Ce O2 with different morphologies have good adsorption effects on basic fuchsine;the particle size of nano-Ce O2 has a significant effect on the adsorption thermodynamics of methyl orange and basic fuchsine;with the decrease of particle size,the equilibrium constant Ko-for adsorption increases,the molar Gibbs free energy of adsorption oads mG-(35),the molar adsorption entropyoads mS-(35)and the molar adsorption enthalpyoads mH-(35)decrease;oln K-,oads mG-(35),oads mS-(35)and oads mH-(35)are linearly related to the reciprocal of particle size respectively;the thermodynamic properties of the adsorption of basic fuchsine onto the nano-Ce O2 with the same particle size but different morphologies are determined by the surface area of nanoparticles and the adsorption capacity;these experimental results are consistent with the above thermodynamic theoretical relations.The mechanism of influence of particle size on thermodynamic properties of adsorption are that oads mG-(35)is influenced by the molar surface area and the difference between the surface tension before and after the adsorption,oads mS-(35)by the molar surface area and the difference in temperature coefficient of surface tension,oads mH-(35)by the molar surface area,the difference between the surface tension before and after adsorption and the difference between the temperature coefficient of surface tension before and after adsorption.The effect regularities of the size and the morphology on kinetic parameters of nano-adsorption are as follows.The particle size of nano-Ce O2 has a significant effect on the adsorption kinetic parameters of methyl orange onto the nano-Ce O2;with the particle size decreasing,the adsorption rate constant k increases,the adsorption activation energy Ea and the adsorption pre-exponential factor A decrease,and lnk,Ea and ln A are linearly related to the reciprocal of particle size,respectively.These experimental results are consistent with the kinetic theory relations of the above nano-adsorption kinetics.The mechanism of particle size influence on the kinetic parameters is that the activation energy is affected by the partial molar surface enthalpy of nano-adsorbent,the preexponential factor by the partial molar surface entropy,and the adsorption rate constant by the partial molar Gibbs energy.This paper built the nano-adsorption theory and it can well explain the mechanism and essence of the effect of particle size on the thermodynamic properties and kinetic parameters of nano-adsorption.These theories can quantitatively describe the adsorption behavior of nanoparticles,which is of great significance to solve the adsorption problems of nanoparticles in catalysis,sewage treatment and air purification.