改进的密度泛函理论在活性炭孔径分布的应用

为提高活性炭孔径分布模型的计算精度,以4种基于不同方法求解剩余自由能的密度泛函理论方法,计算孔径宽度在0.65~5 nm的狭缝孔对3种气体(CH_4,N_2和CO_2)在压力为0.2~2 MPa,温度为298 K下的过剩吸附量,发现4种方法对CH_4和N_2气体计算结果相近,但对于CO_2气体,平均场近似法和泛函展开法的结果比加权密度近似法的结果偏低,说明对于CO_2气体,在自由能的计算中相互作用项的影响不可忽略,并且,经矩阵条件数分析,确定加权密度近似法WDA(Yu)在计算的精度和稳定性上都更适用于孔径分布地建立。使用6种数值方法对3种活性炭的3种气体吸附等温线拟合,结果表明,平均偏差均小于5%,且所建立的活性炭孔径分布模型可以对样品的微观结构进行半定量地比较。

Application of Modified Density Functional Theory in Pore Size Distribution Model of Activated Carbon

To improve the accuracy of Pore Size Distribution ( PSD) for activated carbon, we adopt 4 kinds of Density Functional Theory ( DFT) with different methods to calculate excess Helmholtz free energy. The excess adsorption of 3 kinds of gases ( CH4 , N2 and CO2 ) in slit pore with the pore size in the range from 0. 65 to 5 nm was obtained using the pressure of 0. 2-2 MPa at 298 K. The results of methods are similar for CH4 and N2 , but for CO2 , the results of MF and FMSA are underestimated comparing with the results of two DFT methods based on Weight Density Approximation. This means that correlation effect shall not be neglected in the excess Helmholtz free energy. WDA ( Yu) with the smallest condition number of the kernel is more suitable for PSD study. Moreover, 6 numerical methods are used to fit 3 kinds of isotherms of 3 activated carbon samples with the average deviation between the fitting result and experiment data less than 5 %, and the required PSD models can be used to compare qualitatively for different samples.