Abstract

Since its publication, the Horvath—Kawazoe (H—K) equation has been rapidly and widely adopted for calculating the micropore size distribution from a single adsorption isotherm measured at a subcritical temperature (e.g. N 2 at 77 K or Ar at 87 K). In the H—K formulation, the ideal Henry's law (linearity) is assumed for the isotherm, even though the actual isotherms invariably follow the typical type I behavior, which is well represented by the Langmuir isotherm. The H—K formulation is modified by including the nonlinearity of the isotherm. Inclusion of nonlinearity results in sharpening of the pore size distribution and shifting of its peak position to a smaller size. Furthermore, the H—K equation is extended to spherical pores, and the improved H—K equation for spherical pores by including isotherm nonlinearity is also given. It is shown that the spherical-pore model is particularly useful for zeolites with cavities. Using the literature isotherm data, the improved H—K equations for three pore geometries (slit shape, cylinder and sphere) are compared with the original H—K equations. Clear improvements are seen in the calculated micropore size distributions by using the improved H—K equations.

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