Development of a transient kinetic model for the simultaneous adsorption of SO2-NOx over Na/γ-Al2O3 sorbent

A transient kinetic model has been developed for the simultaneous adsorption of SO2-NOx on Na/gamma -Al2O3 sorbent, using step response experimental data from a fixed-bed microreactor. The reactor temperature ranges from 367 to 407 K, and the molar SO2/NO ratio from 2 to 8 with and without NO2. SO2 and NO2 readily adsorb on the sorbent surface, producing SO2* and NO2* species. However, NO and O-2 only adsorb simultaneously and in the presence of sufficient SO2* on the surface. The proposed mechanism consists of chemisorption steps for SO2 and NO2, whereas NO and O-2 are adsorbed simultaneously via an Eley-Rideal step involving a surface species derived from SO2*. The latter is followed by several consecutive steps involving more SO2* species as well as O-2, leading to the formation of a complex with a stoichiometry of SO2/O-2/NO of 10/5.5/1. The enhanced SO2 sorption capacity in the presence of NO2 is described adequately by considering that the adsorbed NO2* opens a new reaction path for SO2 adsorption. Both the Langmuir ideal surface and the Elovich nonuniform surface kinetics were considered for the steps involving free sites. The latter were found to simulate the experimental data more closely, indicating that the interaction among various species and intrinsic surface nonuniformity are important.