Spatially Resolved Monitoring of Catalytically Activated Hydrogen Peroxide Decomposition - A Test Case for Reaction Monitoring by NMR

L. BULJUBASICH; B. BLÜMICH; S. STAPF

Conferencia; Ampere NMR School 2008; 2008

Many reactions of technical and industrial significance include gas as one of several involved phases, either as a reactant or as a product. Even in reactions taking place entirely in the liquid state, localized temperature increases may lead to partial evaporation and therefore formation of steam at certain stages of the reaction process. Decomposition reactions  can  produce  gas  that may  remain  dissolved  in  the  liquid  phase,  but  also  may  accumulate  to  concentrations above the dissolution limit, thus forming bubbles within a reactor. The generation of gas and steam bubbles depends on the local geometry inside the reactor: frequently, such a device consists of a loose packing of individual pellets which, in turn, are microporous in order to accommodate the catalytically active metal at a sufficiently large accessible internal surface. Bubble formation occurs in dependence of the relative interfacial tension and is suppressed in nm-size pores, but does exist in the mesoporous network that is often found in commercial catalyst pellets. The reason for this choice of geometry is the observation that the presence of a well-connected macroporous network not only facilitates reactant and product transport to and from the internal surface, but also leads to bubble formation that can favourably influence the reactor performance: the continuous generation and release of bubbles generates an oscillatory behaviour that can greatly enhance fluid transport. Understanding the performance of a reactor, in order to achieve an optimized design strategy, thus requires a quantitative description of fluid transport in the presence of bubble formation in porous media. The reaction H2O2 (liquid) --> H2O (liquid)+1/2 O2 (gas) was investigated in this study; the experiments were performed  on  one  single,  metal-containing  catalyst  pellet immersed  in  an  aqueous  solution of H2O2,  and  different types  of  commercial  catalyst pellet  containing  Pd,  Ni  or Cu  were  compared.  During  the  reaction,  which  typically lasts several hours until no significant bubble formation is observed any longer, proton relaxation times and average diffusion coefficients were monitored for a defined volume containing  the  pellet  and  the  solution.  A  typical  result for  the  effective  diffusion coefficient  averaged  over  this volume  is  shown  in  the  figure.  While  the  value  of  the diffusion coefficient is influenced by the bubble formation and thus provides a measure of the integrated reaction rate, the  relaxation  times  provide  evidence  for the  change  of H2O2 concentration in the solution.  Furthermore, imaging experiments were performed in the interior of the pellet with and without the reaction taking place,  and  the  obtained  weighted  spin  density  maps  were  analyzed in terms of the three relevant NMR properties, i.e. signal intensity, relaxation, and diffusion, in order to provide a measure of the local reaction efficiency within the pellet.