Page 132 - Principles of Catalyst Development
P. 132

120                                                      CHAPTER  6
            out  reduction  during  preparation,  where  conditions  are  more  uniform.
            Pre-reduction for commercial operations has many advantages. Nonunifor-
            mity  due  to  impure  hydrogen  and  poor  flow  control  is  avoided,  start-up
            time  is  decreased,  process  hydrogen  is  not  needed,  and  catalyst  damage
            during reduction is  eliminated. There are also  benefits,  even  in laboratory
            use.  If a  catalyst charge  is  reduced  in  a  large  batch and  used  for separate
           testing  and  characterization,  uniformity  of the  metal  dispersion  is  more
           dependable. Unfortunately, high dispersions of metals are often pyrophoric
           and burn when exposed to  air.  They cannot be  handled or shipped in  the
            reduced  state  but  must first  be  passivated.  Passivation is  accomplished  in
            two  ways.  In  the  first  method,  the  reduced  catalyst  charg~ is  cooled  and
            remaining  hydrogen  removed  with  a  flow  of inert  gas.  Oxygen  or  air  is
           introduced into the inert stream at low concentration (less than 1 %) slowly
           enough  that the  exothermic oxidation  reaction  does  not  cause  hot bands.
           Oxidation is  restricted to the first few  layers of the metal surface, effectively
           protecting the bulk from further reaction. The catalyst can be handled safely.
           Only a small amount of surface reduction is  necessary after loading in  the
           reactor. Tests  show  that the  original  metal  dispersions are  retained. ([ 781
               The second method is based on observations that the cause of pyrophor-
           icity is not the reaction of oxygen with nickel but with adsorbed and occluded
           hydrogen atoms. (178)  Residual  hydrogen  is  removed  by  heating in  an inert
           gas  at  temperatures  lO-20°C  above  the  reducing  conditions.  The  catalyst
           may then be exposed directly to the atmosphere without burning. Passivation
           takes  place  through  adsorption  but  without  the  painstaking steps  used  in
           the  first  method.
               Another example of activation is found in hydrodesulfurization proces-
           ses. Prepared as dispersed molybdena on alumina and promoted with cobalt
           or nickel,  these  catalysts are sulfided before use.  In the plant, this  is  done
           either with  a  sulfur-containing  feed  or  by  pretreatment  with  CS 2  or  H 2S.
           For laboratory operations, sulfiding with 10%  H2S/ H2 mixtures is sufficient.
           Care must be taken not to reduce in  H2  prior to sulfiding since the reduced
           state is  less  easily sulfided.  Exact conditions, such as  temperature, flow,  or
           time, depend on the properties of the catalyst and the method of preparation,
           so that  the sulfiding sequence is  a  factor in establishing activity. (179)  Argu-
           ments  for  presulfiding  before  handling  and  shipment  are  also  valid,  and
           this  practice  is  now  becoming common in the industry.



           6.5.  EXTRACTION

               Each method discussed so far uses some form of deposition to produce
           dispersion. It is also possible to generate porosity by extracting a component
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