Abstract
The adsorption of nitrogen, carbon tetrachloride and water vapour was studied on various well-characterized chromium oxide gels in an attempt to relate the adsorptive properties with the texture and surface chemistry of the gels. Standard adsorption data obtained on non-porous chromium oxides provided a basis for the analysis of the isotherms on the porous gels by the application of the αs-method. The pore structures identified were within both the micropore and the mesopore range. For the microporous gels it was possible to use the nitrogen αs-plot to assess the pore volume and the external surface area; comparison of the micropore volumes obtained with the three adsorbates gave an indication of the micropore size distribution.Certain of the gels outgassed at temperatures below 200°C exhibited molecular sieve properties; in these cases, the water sorption capacity was high, whereas the pore volume available for nitrogen and carbon tetrachloride sorption was small. It is suggested that cavities were formed in the gel in the vicinity of the Cr3+ ions through the removal of water ligands under conditions where the hydroxide, oxide-hydroxide or oxide structures were slow to develop. The molecular sieve character was not found with gels prepared at high pH, presumably because of replacement of H2O ligands by OH– ions. Heat treatment of gels in air and nitrogen at temperatures between 200 and 600°C led in some cases to the closure of micropores and in others to pore widening with the development of mesopores. The results show that the pore structure may be stabilized to higher temperatures if the formation of the thermally unstable CrO3 is prevented. Crystalline α-Cr2O3 was non-porous, but the water sorption results indicate that the surface structure was not perfected until the calcination temperature was above 900°C.
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