Page 85 - Principles of Catalyst Development
P. 85

72                                                       CHAPTER  4
                                          •    A1 3 +

                                         0     0 2 -

                                         0     OH-






















                                                3

                       Figure 4.17.  Suggested scheme for  acidic sites  on  r-AI20,.,,'71

                Neutralization  of the  proton,  H',  by  a  base,  B,  occurs  through  the
            reaction()5)


                                                                          (4.6 )

            in which  the  acid  ionization constant is


                                                                          (4.7)


           with  y's  the  appropriate  activity  coefficients.  It  1S  convenient  to  express
           acid  strength  by
                                                                          ( 4.8)

           so  that  strong  acidity  shows  large  negative  values  of  pKa.  Methods  for
           measuring  pKa  are  discussed  in  Chapter  7.  Distributions  in  y-A1 20 1  are
           given in  Fig.  4.18, which also shows how acidity increases with calcination
           temperature as the surface becomes structured. Further heating above 400°C
           causes dehydration and a  decrease in acidity. (55)
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