Page 102 - Principles of Catalyst Development
P. 102

DESIGN  OF  CATALYSTS                                             89
           gaps in existing data become apparent and lead to ideas for future research.
            For this example, the mechanism shown in  Fig.  5.3  was adopted as a "best
           guess."  The  reader should appreciate that others are  possible.
                Both  CH 4  and  O 2  are  chemisorbed  and  dissociated.  It is  known  that
           CH 4  may adsorb as CH, with x  from 0 to 3.(150)  Excessive dehydrogenation
           is  undesirable since CH 2  is  much  more  reactive  than  CH 3  and  is  likely to
           lead  to  oxidative  products.  Also,  strong  dehydrogenation  could  also  lead
           to decomposition of the  product CH 20, as  it  forms.
                Oxygen adsorbs as  molecular ionic and atomic speciesYSl) The latter
           are preferred since the product contains only one oxygen.  However, adsor-
           bed oxygen atoms are reactive and lead to oxidation. There are correlations
           between  combustion  activity and mobility of these surface speciesY 52  )  To
           avoid oxidation, we require that oxygen dissociation leads to chemisorbed
           bonds that are too strong to allow migration. This means that the adsorbed
           CH 3  must be sufficiently mobile to diffuse to oxygen sites. Reaction between
           o and CH 3  fo\1ows,  and the intermediate  dehydrogenated to  give  formal-
           dehyde.


           5.2.5.  Reaction  Path  Identification

               The necessary reaction  paths have now been established. The catalyst
           must promote oxygen insertion and mild dehydrogenation but inhibit strong
           oxidation  and dehydrogenation.













                                                     j





                                                    o
                                    H
                             ?@/w-              ~



                       Figure S.3.  Surface mechanism  for formaldehyde  formation.
   97   98   99   100   101   102   103   104   105   106   107