Page 76 - Principles of Catalyst Development
P. 76

CATALYTIC  MATERIALS                                              63
                                   TABLE  4.4.  (continued)

            Group   Metal                        Reaction

                   Ru      NH, synthesis
                          Hydrogenation  of CO, CO 2  (Fischer-Tropsch), olefins,  oximes,  nitriles,
                            phenols, benzyl alcohols, aryl amines, aromatic heterocycles, cyclopen-
                            tane, aliphatic ketones,  nitrobenzenes, oxidation of hydrocarbons
                   Os     Ammonia synthesis
            VIIB   Mn
                   Tc
                   Re      Hydrogenation  of olefins,  aromatic,  carboxylic acids,  amides,  nitrogen-
                            zenes,  ketones, dehydrogenation of cyclohexanes, alcohols
           "  Note:  Reverse  reactions are also catalyzed.  Similarities  within groups  are expected,


            from a fundamental point of view, the role of these phenomena in practical
            catalysis  remains to be demonstrated.
                Another emerging  area  of investigation  is  the  behavior of supported
            metal  cluster  compounds. (I t3.114)  For  research  purposes,  these  materials
            provide well-defined models.  However, they have not yet led to any major
            developments in  industrial applications.
                We leave the topic the metal catalysis with a listing of observed activities
            (Table 4.4).  Patterns of behavior are also important. Examples of these are
            given  in  Chapter  5,  and  the  reader  should  also  refer  to  many  excellent
            sources in  the literature.(115)


           4.3.  SEMICONDUCTORS

                Catalytically  interesting  semiconductors  are  oxides  and  sulfides  of
           transition elements. (45.116.117)  Unlike metals, electronic bands do not overlap
           but form  separated regions,  shown in  Fig.  4.14.
                The  lower  band, the  valence  band, contains  levels  normally  filled  by
           electrons participating in valence bonds of the  crystal. (99)  Separated by an
           energy E g ,  the second band is empty unless electrons are promoted by heat
           or  radiation  from  their  valence  positions.  The  upper  band  is  called  the
           conduction band, since electrons with these energies are sufficiently free to
           migrate  with  the  application  of an  electric  field,  i.e.,  conduction.  Simul-
           taneously,  the  "hole"  left  by  the  promoted  electron  conducts  electrical
           energy in the opposite direction. Two types of current carriers are involved.
           Semiconductors of this  type  are  called  intrinsic  but are  not  important  in
           catalysis,  since  the  temperature  range,  300-700°C,  is  usually  too  low  to
           generate a  sufficient number of promoted electrons.
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