The Adsorption of Nitrogen on Precipitated and Pyrogenic Silicas

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Abstract
Adsorption isotherms of nitrogen have been determined on three pyrogenic silicas (TK800, TK900, and Aerosil 200) and two precipitated silicas (HiSil 233 and VN3). Application of the αs-method reveals that the precipitated silicas were, to a small extent, microporous, the micropore volume of VN3 becoming appreciable when the outgassing temperature was raised to 383 K. It is concluded that the BET method does not provide an accurate assessment of the surface area of these materials, but that the αs-method can be used to evaluate the external area and the micropore volume. Analysis of the isotherms on the pyrogenic silicas confirms their non-microporous nature but indicates the presence of interparticle capillary condensation. The TK800, which was previously recommended as a non-porous standard, has undergone slight ageing over a ten-year period with the loss of surface area and the development of interparticle mesoporosity.

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