Investigation of H2O2 Decomposition in Heterogeneous Catalysts

L. BULJUBASICH; B. BLÜMICH; S. STAPF

Conferencia; 9th International Bologna Conference, Magnetic Resonance in Porous Media (MRPM9); 2008

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.  The  reaction  efficiency  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  pellets  (flow,  diffusion)  and  inside  the pellets (diffusion).  In   most   technically   interesting   reactions,   gas   occurs   as   one   of   the   involved components.  For  not  too  small  pores,  this  leads  to  the  generation  of  gas  or  steam  bubbles, which  grow  and  eventually  leave  the  pore.  The  additional  dynamics  introduced  by  bubbles can greatly enhance mass transport and reactor efficiency.  The  decomposition  of  hydrogen  peroxide  represents  a  simple  gas-forming  reaction, H2O2  (liquid) --> H2O (liquid) + 1/2 O2  (gas). While a pure hydrogen peroxide solution is rather stable,  decomposition  is  catalyzed  by  the  presence  of  heavy  metals,  and  oxygen  gas  and decomposition heat are produced. In   this   contribution   we   present   results   obtained   monitoring   this   reaction   in commercial Al2O3  porous catalyst particles, with different metals as catalytically active sites. The motivation in this study is twofold: to follow the decomposition by means of properties easily  accessible  with  NMR  methods,  e.g.  by  monitoring  the  oxygen  concentration  and  the effective diffusion coefficient of the liquid in the vicinity of the pellet or in a defined closed volume;  and  to  visualize  the  heterogeneity  of  reaction  inside  the  pellet.  In  both  cases,  the change  of  relaxation  times  and  diffusion  coefficients  were  observed  in  real  time  and  were combined  with  imaging  of  the  spatial  distribution  of  these  parameters.  In  the  fluid  phase surrounding the pellet, the dependence of the liquid’s T2 on parameters such as the oxygen concentration allows the monitoring of the state of the system during the reaction. At the same time, the production and motion of gas bubbles produces a random change in the velocities of the liquid around the pellet. The increase of the effective diffusion coefficient (Deff ) therefore is indicative for the reaction rate in the system, as is shown in the figure. The dependence of T2  and Deff  inside the pellet is shown to correlate with the localized distribution of metal and the distance from the outer pellet surface.