Page 80 - Principles of Catalyst Development
P. 80

CATALYTIC  MATERIALS                                              67

                                   TABLE 4.5.  (continued)
                   Group    Element    Compound       Type      Eg,eY

                              Nb        NbO             P
                                        Nb 20 S         n        3.3
                             Ta         Ta20S           n        3.6
                    (YB       Ti        TiO             P
                                        Ti02            n        3.1
                                        Ti 2O)          P        2.2
                              Zr        Zr0 2           n        2.3
                              Hf        Hf0 2          n,p       4.4
                    IIIB      Sc        Sc 2O)         p,n       2.2
                              y         Y20)            n        2.9
                              La        La20)           n        2.6
                              Al        I'-AI 2O)       n        7.3
                              Zn        ZnO             n        3.3
                                        ZnS             n        3.6
                              Mg        MgO            n,p       8.7
                              Ca        CaO            n,p       7.5
                              Ce        Ce02            n
                              Th        Th0 2           n        3.5

           overall trend does not follow any specific quantitative correlation. Certainly,
           other factors  such as  dispersion and impurities must influence the  results,
           but  dimensions  and  symmetry  at  adsorptive  sites  are  also  important.
           Refinement beyond mere type  of semiconductivity is  necessary.
               Third, for a given crystal structure, the position of the Fermi level plays
           an important role.  For example, in three  NiO-containing samples in  Table
           4.6, the  influence  of doping is  clearly evident.  Addition  of Li+  ions to the
           Ni2+  lattice  creates  more  acceptor  sites,  decreasing  the  Fermi  level  and
           increasing activity. With Cr +  incorporation, donor levels are produced, the
                                    3
           Fermi level  increases, and activity decreases.


                         !     ......................       /.  !   M  +
                                                        /...
                            ~ CONDUCTION  BAND
                         e  __________________________ ;  e
                     M                      E                 11
                      -D-                    F               0
                             'Q                         o
                            ~-o.-o-o-o-o  _0_0.01:)'          I
                                                       I      I
                                                       ,      I
                                       VALENCE  BAND          I
                                                          3   I
                                                        10  nm  I
                       Figure 4.16.  Electron  transfer at  the  semiconductor surface.
   75   76   77   78   79   80   81   82   83   84   85