There is a known problem with using the unmodified Tarazona prescription to construct models for mesoporous materials: most real solids are energetically and geometrically heterogeneous, while the model assumes surface homogeneity. This often forces a "false positive" indication of micropores and small mesopores where none actually exist. Also, the "fit" of the model to the actual isotherm is not usually impressive. If you are using Micromeritics software, I suggest using the model called "cylindrical pores in oxide surfaces", which anticipates a heterogeneous surface. Otherwise, the errors can be somewhat reduced by not including adsorption data below about 0.2 - 0.25 relative pressure in the data reduction.

Hi James,

Thanks for your answer, I really appreciate your feedback,

I am using this model because a researcher form Micromeritics advised me to use this one. Olivine nano-silica is microporous (determined using annihilation spectroscopy) mesoporous (determined by DFT tarazona and BJH) and macroporous (determined by BJH and Hg porosimetry).

The most problematic pore size determination is in the range 2 nm to 10 nm, since the BJH method does not work properly and I am not experienced with the DFT determination. Should I use the micromeretics DFT model of cylindrical pores in oxides surfaces as you have suggested earlier? Another problem in the interpretation is that the gas physisorption equipment that we have does not work below 0.01 relative pressure.

Best regards,

Hi Alberto,

The "pores in an oxide surface" model would definitely be a better choice. But the PSD data show a problem or two: the first, and most easily solved, arises because of the requirements for deconvolution of the data vs the model. [Journal of Porous Materials 2 (1), 9-17, 1995] The kernel isotherms are all calculated using a pressure vector optimized to equally weight each pore size class. The algorithm conditions the data by interpolating the experimental data onto this common pressure vector using a spline technique. If the experimental points are too widely spaced, the interpolated points generated can differ significantly from the "true" isotherm. This leads to the typical signature of a quasi-periodic, or cycling, PSD. (In short, systematic noise). So the cure is simple... lots more data points. I know you don't want to hear that, but a hundred points in the region of interest is not too many. An equilibration time of 20 - 30 seconds should be ok.

The second possible problem is more purely physical. Micropores fill and saturate at very low pressures, so in order to characterize a micropore distribution, you would need lots of low pressure points between say 10e-6 and 10e-2 relative pressure. (I'm pretty sure there are a few ASAP2020 at Eindhoven) However, if you can be satisfied with a value of total micropore volume, then you are OK with your instrument and the DFT method. On the DFT report heading, you will see "Volume of pores less than ... " On recent releases of the software, this is also shown on the PSD graph.

Let me know of your progress...

Best Regards,

Jim Olivier

[email protected]