Page 320 - Adsorption by Powders and Porous Solids
P. 320

CHAPTER 10. ADSORPTlON BY METAL OXIDES                         309






























    Figure 10.12.  Differential enthalpies of adsorption of argon and nitrogen on a mesoporous silical gel:
    effect of outgassing at different temperatures (Rouqueml er al., 1979b).


    energetic heterogeneity is a feature of the adsorption of both gases, with Ar the varia-
    tion of adsorption enthalpy is virtually unchanged  over the outgassing temperature
    range 150-900°C. The striking difference in the behaviour of N, can only be due to the
    weakening of the specific field gradient-quadruple interactions which is the result of
    the reduction in the number of surface hydroxyls. It is evident that the location of the
    N,  curve on the 900°C sample is not far removed from the common Ar curve. This is
    to be expected because of the similarity of the non-specific interactions for both Ar and
    N,  with the dehydroxylated surface. The remaining difference is probably due to the
     specific interaction of N, with the remaining isolated hydroxyls on the 900°C surface.
      As noted in Chapter 1, the specific interactions between polar molecules and silica
    are virtually eliminated by the removal of all the surface hydroxyls and therefore the
    effect of partial dehydroxylation is to drastically reduce the adsorption energies of
    certain molecules.  The polar  adsorptives studied by  Kiselev and his co-workers
    included alcohols, ketones, ethers and  amines (Kiselev,  1965,  1971): with  each
    adsorptive, the reduction in the adsorbent-adsorbate  interaction energy was accom-
    panied by a substantial change in the isotherm character.
      It might be expected that a dehydroxylated silica surface would be more energeti-
    cally homogeneous than the parent hydroxylated surface, and this is found in practice
    - as exemplified in Figure 10.12. However, the effect of outgassing a silica gel at high
     temperature may  lead to the development of  ultramicroporosity. To overcome this
    problem, much of  the later work by Kiselev's group was undertaken on hydrother-
   315   316   317   318   319   320   321   322   323   324   325