Investigation of H2O2 Decomposition in Heterogeneous Catalysts

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

Conferencia; The 9th International Conference on Magnetic Resonance Microscopy (ICMRM); 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 T 1  or T 2  change over the time. On the other hand, using NMR  Imaging  it  is  possible  determine  T 1 ,  T 2   in  different  regions  inside  and  outside  the pellets. We will present some results obtained with High Resolution NMR.