Page 104 - Principles of Catalyst Development
P. 104

DESIGN  OF  CATALYSTS                                            91
                Oxygen insertion activity in oxides is  essentially mild oxidation. Clues
            for potential  materials are  found  from  three sources. The first  is  a  pattern
            of behavior for CO oxidation, giving(l5J)

                         'd'
                Strong OXI  atlOn:   C  2+  N· 2 +  C  2+  Pd 4 +  A  1+  Cd l +  M  2+
                                                                 ,n
                                             u,
                                                     ,  g,
                                     ,
                                        1  ,
                                  0
                Moderate oxidation:  Rh +,  InH, Sn H  ,  Pb +
                                                      1
                                      4
                                      4              2
                                                                            6
                Mild oxidation:  Sc H , Ti +,  y3+,  Cr H ,  Fe +,  Zn2+,  Zr1+,  Nb1+,  Mo +
            Second,  oxygen  mobility  correlates  well  with  oxidation.  IIS2 )  Available
            measurements give  the sequence
                CO,04> Mn01  >  NiO >  CuO >  Fe2 0J >  Cr 20, >  Y20S >  MoO,
            Third,  activity  for  strong  oxidation  is  found  in  oxides that  reduce  easily,
            suggesting  that  reduction  followed  by  oxidation  may  be  a  mechanism. 14S !
            Oxides that are  hard to  reduce include
                Aluminates:  CoAI 2 0 4 ,  NiAI 20 4
                Titanates:  ZnTi0 4
            5.2.8.  Proposed  Catalyst
                By excluding strong oxidation ions from the mild dehydrogenation list,
            we  arrive  at the  following  listing  in  order of decreasing activity:

                        Mild dehydrogenation      Oxygen insertion
                                Fe H                  SC 3 +yH
                                y3+                     Ti 4 +
                                V'"~                    yH
                                Ti 4 +                  Fe 2 +
                                                       Zn2+
                                                        Zr 3 +
                                                       Nb3+
                                                       Mo6+

           Candidates that are  consistent with  all  these  properties are
                Single oxides:  TiO l ,  Y203
                Mixed oxides:  TiO l  + MoO], ZnO + Y203
               Complex  oxides:  FeJ04,  Fe  molybdate, Zn TiO]
           Dispersion  will  not be  necessary, since  high  areas only facilitate  complete
           oxidation. Therefore, simple methods of preparation will be sufficient. These
           materials are suggestions only.  No priorities can be assigned and each must
           be  tested.  Only  then  may  the  next  task  of optimization  and  modification
           proceed.
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