The reduction of NO by CO under oxygen-rich conditions in a fixed-bed catalytic reactor: A mathematical model that can explain the peculiar behavior

A kinetic model of the NO + CO + O-2 catalytic reaction in the isothermal continuous stirred tank reactor (CSTR) has been introduced previously in [A.G. Makeev, N.V. Peskov, H. Yanagihara, Appl. Catal. B: Environ. 119-120 (2012) 273]. The present study implements the same kinetic mechanism but now it is combined with a more realistic model of the catalytic flow reactor - the axially dispersed plug flow reactor model. The resulting new model is compared with the CSTR-based model. The calculations demonstrate that back-mixing in the isothermal reactor can lead to the following effect. At sufficiently high temperatures, when the conversion of CO is close to 100%, an increase in NO-to-N-2 conversion can be induced by: (1) the decrease of the active catalyst surface area and (2) the increase of the space velocity of the feed gas. These items just represent the peculiar behavior, which seems to contradict with the physical intuition. In fact, such results have been observed experimentally, but probably they have not been properly explained. We demonstrate that these peculiar effects can be caused by the kinetics of competing reactions in a catalytic flow reactor. The vital step in the reaction mechanism appears to be the reversible dissociation of the adsorbed NO species. In particular, the calculations show that the overall reactor efficiency can decrease significantly when the catalyst surface area is increasing. This effect disappears if the ideal plug flow model (i.e., without dispersion/diffusion) is applied. (C) 2012 Elsevier B.V. All rights reserved.