Page 81 - Principles of Catalyst Development
P. 81

68                                                       CHAPTER  4
                           TABLE 4.6.  Relative Activities  for  Nitrous
                                    Oxide  Decomposition
                    Oxide            Type      E,.eV      Relative activity

                 Cu 20                p         1.9          10.8
                 CoO                  P         0.8           7.91
                 NiO + 2%  Li,O                               3.78
                 NiO                  P         2.0           1.00
                 NiO + 2%  Cr20)                            3.02  x  10- 2
                 CuO                  n         1.4         7.28  x  10'
                 MgO                  n         8.7        2.10 x  10- 1
                 CaO                  n         7.5         1.10x 10- 1
                 CeO,                 n                     7.10 X  10- 2
                 Al,O,                n         7.3         2.75  x  10- 2
                ZoO                   n         3.3         1.25  x  10. 2
                TiO,                  n         3.1        9.48  x  10 3
                Cr,O,                 n         1.9        7.30 x  10- 3
                 Fe203                n         2.2        5.30x 10- 3


                The  most  successful  treatment  of semiconductor  catalysis  was  given
           by   Vol'kenstein,   and   the   serious   reader  is   referred   to   his
           publications. (118.119.120)  Although  phenomenological  and  qualitative  in
           nature, Vol'kenstein's model explains observed trends and predicts impor-
           tant  results.  For example,  in  C 2HsOH  decomposition,  dehydrogenation  is
           predicted  to  increase  with  increasing  Fermi  level,  whereas  dehydration
           decreases. Table 4.7 shows that this is true qualitatively, with dehydrogena-
           tion  increasing  and  dehydration  decreasing  as  the  oxide  becomes  more
           n-type.
                Another  prediction  of  Vol'kenstein's  theory  is  that  crystallite  size
           influences  semiconductor  electronics.  Figure  4.16  shows  the  bending  of
           energy  levels  at  the  surface,  the  so-called  Oebye  length.  Since  catalytic
           crystallites  are  much  smaller than  this  dimension,  the phenomenon  is  not
           found  in  these  materials.(119)  Catalysts may be treated as  two-dimensional
           semiconductors, a  fact  that greatly simplifies theoretical  models.
               Still  another  consequence  of the  Vol'kenstein  theory  is  photocataly-
           sis.ml.l m  It is  well  known that light quanta excite electrons out of and into
           impurity  levels.  The  effect  is  to  change  the  Fermi  level,  with  predicted
           influences  on  catalytic reactions.( 123.124)  In  recent years, a  large amount of
           attention  has  been  directed  toward  photocatalysis with  semiconductors as
           a  means  to  splitting water into hydrogen and oxygenY2S)
               From  a  practical  viewpoint,  the  most  significant  features  of the  elec-
           tronic  model  for  semiconducting catalysis  is  the framework  around which
           correlations may be organized. One such pattern is given in Table 4.7, others
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