Dynamic Methods in Catalytic Reaction Engineering: Applications to the Investigation of the NH3 Selective Catalytic Reduction Reactions for Diesel Emission Control

Abstract

For the case of the NH3 selective catalytic reduction (NH3-SCR) reactions over V-based and metal-exchanged zeolite catalysts, we have used step response and temperature-programmed experiments in a fixed-bed flow microreactor in order to accomplish the following. (i) Identify the adspecies and evaluate their storage capacities: it was found that, in addition to ammonia, NO2 as well is adsorbed in comparable amounts onto SCR catalysts in the form of nitrites and nitrates. (ii) Investigate the individual steps of mechanistic schemes: the key role of surface nitrates in the NO2-related SCR chemistry was demonstrated by dedicated transient experiments. (iii) Discriminate rival mechanistic hypotheses: the differences in the temporal evolutions of NO and NO2 during transient runs unambiguously indicated that they participate in different steps of the fast SCR mechanism. (iv) Analyze the red-ox features of the catalytic mechanism: specifically designed transient tests highlighted the superior efficiency of surface nitrates in reoxidizing the V-related redox sites. (v) Evaluate surface kinetics: chemically consistent transient kinetic models were developed from systematic sets of transient runs and applied to design and optimization of commercial SCR converters for vehicles. The results of the overall SCR investigation prove that transient response techniques can yield a superior level of mechanistic and kinetic information under process relevant conditions with a limited experimental effort.