Imaging of Metal Containing Catalyst Carriers for Visualizing H2O2 Decomposition

L. BULJUBASICH; T. OEHMICHEN; S. STAPF; L. B. DATSEVICH; B. BLÜMICH

Conferencia; Ampere NMR School; 2007

Heterogeneously  catalysed  reactions  mostly  take  place  in  the  presence  of  finely dispersed catalysts (i.e. metals such as Ni, Pt, Pd, …), these in turn are localized in materials of  large  internal  surfaces,  i.e.  porous  media  (typically,  pellets  of  Al2O3  of  several  mm  in size).  The  reaction  efficiency  then  depends  on  parameters  such  as  internal  surface  area; homogeneity  of  metal  distribution;  porosity  and  tortuosity  of  the  pellet;  transport  of  the reactants and products between the pellet (flow, diffusion) and inside the pellets (diffusion). It is known that the pore space of catalyst pellets is complex, usually bimodal, i.e. having pores in the nm and um range – it is also known that the presence of um scale pores (“macropores”) have  a  strong  influence  on  the  reaction  efficiency,  without  them  the  reaction  would  mostly take place at the outer edge of the pellet, and the core would remain useless.  In   most   technically   interesting   reactions,   gas   occurs   as   one   of   the   involved components; furthermore, steam may be generated in exothermic reactions. For not too small pores,  this  leads  to  the  generation  of  gas  or  steam  BUBBLES  (dependent  on  the  surface tension, for instance). Bubbles grow and eventually leave the pore; just like the dissolution of an  aspirin  tablet  in  water,  bubbles  form  more  or  less  at  regular  intervals  provided  that  the pores  have  the  same  size.  In  other  words,  each  pore  generates  bubbles  at  a  certain  rate  or frequency; large pores lead to large bubbles at a low frequency and vice versa.  On   the   other   hand,   pure   Hydrogen   Peroxide   solution   is   stable   with   weak decomposition,  but  when  it  comes  in  contact  with  heavy  metals,  produces  oxygen  gas  and decomposition heat.  The aim of this work is monitoring the Hydrogen Peroxide decomposition. During the reaction, some NMR parameters like T1  or T2  change over the time. On the other hand, using NMR Imaging it is possible determine T1, T2  in different regions inside pellets ( see Fig. ). We will present some results obtained with High Resolution NMR.