Reaction monitoring of hydrogen peroxide decomposition by NMR relaxometry

In this contribution we suggest the feasibility to use the NMR transverse relaxation rate, R-2, as an indicator of reaction evolution during the heterogeneous catalytic hydrogen peroxide decomposition: 2H(2)O(2)(liquid)-> 2H(2)O+O-2(gas). The influence of two-site intermediate chemical exchange between protons in water and hydrogen peroxide on the relaxation rate and its concentration dependence in bulk samples is pointed out. It is demonstrated that for the system under investigation-being representative for a heterogeneously catalyzed reaction-the sample peculiarities, such as shape, metal content, internal magnetic field gradients and susceptibility effects, are minor compared to the dominating contrast produced by chemical exchange, thus rendering R2 a suitable parameter to follow the decomposition in real-time. Transverse relaxation rates as a function of reaction time are measured and discussed that were obtained during the decomposition of aqueous hydrogen peroxide solutions in the presence of a commercial Al2O3 porous particle, impregnated with Cu as catalytically active sites. (C) 2009 Elsevier Ltd. All rights reserved.