| Nanomaterials, based on their unique adsorption properties compared withthe corresponding bulk materials, have been widely used in environmentalprotection, heterogeneous catalysis, biological medicine, micro-electronics, et al.However, theory of adsorption of nanomaterials (nanoadsorption) has not beenreported, the rule and mechanism of the influence of particle size onthermodynamics and kinetics of adsorption are not clear. The research,development and application of nanoadsorption have been hampered. Therefore,theoretical and experimental researches on nanoadsorption were used to clarifythe nature of the effect of particle size on thermodynamics and kinetics ofnanoadsorption.(1) Theoretical research on nanoadsorptionTo study thermodynamics of nanoadsorption, chemical potentials ofnanoparticles before and after adsorption were defined and the relations ofadsorption equilibrium constant, adsorption Gibbs function, adsorption enthalpyand adsorption entropy, and particle size were deduced, respectively. The resultsshow that particle size has a significant influence on the thermodynamicproperties of nanoadsorption. The effect of particle size on the thermodynamic properties is due to the result of the particle size (special surface area) combinedwith surface tension difference. The adsorption equilibrium constant increasesand the adsorption Gibbs function decreases with the decrease of particle size,but the molar enthalpy and the molar entropy are also the effects of temperaturecoefficient difference of surface tension before and after adsorption.To study kinetics of nanoadsorption, the assumption that transition statetheory is also suitable for nanoadsorption was made and the theoretical relationsbetween kinetic parameters of nanoadsorption and particle size were derived.The results show that particle size has a significant influence on the adsorptionkinetics. With the decrease of particle size, the adsorption rate constant increases,and the adsorption activation energy and the adsorption pre-exponential factordecrease. The essence of influence of particle size on the nanoadsorptionkinetics is that the adsorption activation energy is influenced by the partial molarsurface enthalpy of nano materials, the adsorption rate constant by the partialmolar surface Gibbs function, and the adsorption pre-exponential factor by thepartial molar surface entropy.(2) The quantum chemical simulation of nanoadsorptionThe model of titanium dioxide cluster and the adsorption of silver ion ontitanium dioxide cluster were built to calculate thermodynamic properties ofnanoadsorption by using PM6method in Gaussian09. The results show thatparticle size has a significant influence on thermodynamic properties. With thedecrease of particle size, the standard molar Gibbs function of formation, the standard molar enthalpy of formation and the standard molar entropy of titaniumdioxide cluster increase. Particle size has also a significant effect onthermodynamic functions of adsorption process: The standard molar adsorptionGibbs function, the standard molar adsorption enthalpy and the standard molaradsorption entropy decrease,while the standard equilibrium constant increases asparticle size reduces, which is accordance with thermodynamic theory ofnanoadsorption above.(3) Experiments of nanoadsorptionThe adsorption of adsorption of copper ion on nano zinc oxide, adsorptionof silver ion on nano titanium dioxide and benzene on nano magnesium oxidewere systematically studied as adsorption systems to discuss the effect ofparticle size on thermodynamic properties and kinetic parameters. Nano zincoxide, nano titanium dioxide and nano magnesium oxide with different particlesizes were prepared by homogeneous precipitation method.The results of experiments of thermodynamics of nanoadsortpion show thatparticle size has a significant influence on thermodynamic properties ofnanoadsorption. The standard adsorption equilibrium constant increases and thestandard molar adsorption Gibbs function decreases with the decrease of particlesize. The standard molar enthalpy and the standard molar entropy ofadsorption of copper ion on nano zinc oxide increase as particle size decreases;the standard molar enthalpy and the standard molar entropy of silver ionadsorbed by nano titanium dioxide decrease. In addition, the standard molar adorption Gibbs function, the standard molar adsorption enthalpy, the standardmolar adsorption entropy and logarithm of the standard adsorption equilibriumconstant are linearly related with the reciprocal of particle size, respectively.Experimental results are accordance with thermodynamic theory ofnanoadsorption above, and the results of adsorption of silver ion on nanotitanium dioxide are identical to that simulated by PM6above. Further, theadsorption of copper ion on nano zinc oxide and the adsorption of silver ion onnano titanium dioxide conform to the Langmuir isotherm. The adsorptions arespontaneous and endothermic processes.The results of experiments of kinetics of nanoadsortpion show that particlesize has a significant influence on kinetic parameters. With the decrease ofparticle size, the adsorption rate constant increases, and the adsorption activationenergy and the adsorption pre-exponential factor decrease. Logarithm of theadsorption rate constant, the adsorption activation energy and logarithm of theadsorption pre-exponential factor have linear relation with the reciprocal ofparticle size, respectively. In addition, the adsorption of copper ion on nano zincoxide and the adsorption of benzene on nano magnesium oxide are welldescribed by pseudo-second-order model and particle size has no effect on theadsorption kinetic order.Theory of thermodynamics and kinetics of nanoadsorption was proposed.The effects of particle size on thermodynamic properties and kinetic parametersof nanoadsorption were revealed, and the nature and mechanism of nanoadsorption were clarified. It is crucial to the scientific and realisticsignificance to advance theory of adsorption and to solve the problems ofnanoadsorption in the field of nanaomaterials and others related. |
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