Page 78 - Principles of Catalyst Development
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CATALYTIC  MATERIALS                                             65
                Much  more  significant  are  the  impurity  "-type and  p-type  shown  in
            Fig.  4.15.  In  an  "-type  semiconductor,  for  example  ZnO,  the  structure  is
            nonstoichiometric,  and  an  excess  of Zn +  ions  exists.  The  small  cation  is
            easily accommodated in interstitial positions, generating energy levels that
            are filled  ( donor) with  energy values close to the conduction band. Only a
            small  amount  of thermal  or  radiation  energy  is  necessary  for  promotion
            and conduction. Conductivity occurs via electrons in the conduction band.
                With p-type semiconductors, there is an excess of anions and cationic
            vacancies. These produce empty levels (acceptor) close to the valence band,
            from  which  an  electron  is  easily  promoted.  Conductivity  occurs  through
            the resulting hole  in  the valence band.
                Donor and  acceptor  levels  are  also  created  by  introducing  (doping)
            altervalent  cations  into  interstital  or  cationic  lattice  positions.  Both  may
            coexist within the same crystal.
                The  Fermi level  is the electrochemical potential, midway between the
            highest filled and the lowest empty levels. Since the Fermi level also responds
            to doping (increases with more donor levels, decreases with acceptor levels)
            and is easily measured, it is a convenient index with which to follow changes
            in electronic structure.
                Table 4.5 gives the type of semiconductivity found in common transition
            oxides  and  sulfides.  Many  of  these  will  be  recognized  as  catalytically
            important materials.
                The  role  of these  electronic  structures  in  catalytic  reactions  is  best
            demonstrated with a much-studied probe reaction, decomposition of nitrous
           oxide(117):

                                                                          (4.2)

           The mechanism is  believed to  proceed as follows:

                                                                          (4.3 )
                                                                          (4.4)

           where  donation  of electrons  to  the  catalyst  is  the  rate-determining  step.
           Transfer of an  electron  from  adsorbed  0- only  takes  place  if the  Fermi
           level  of the  surface  is  below  the  ionization  potential  of 0-. This  is  most
           likely to occur in p-type semiconductors, where the Fermi level is sufficiently
           low as shown in  Fig.  4.16.
                Relative activities for reaction (4.2) over a wide range of semiconductors
           are  given  in  Table  4.6.  Several  important points  emerge  from  the  data  in
           Table 4.6.  First,  p-type oxides  are  more  active than  "-type oxides. This is
           consistent with  the  model  of electron transfer to  the catalyst.  Second, the
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