Redox reaction process between hydrocarbon and adsorbed NOx over lean NOx trap catalyst

Abstract

In this study, we investigated the redox reaction between a hydrocarbon (HC) and adsorbed NOx over CuPtBa/Al2O3 lean NOx trap (LNT) catalyst. Experimental characterizations included temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), in situ diffuse reflectance Fourier transform spectroscopy (in situ DRIFTs) and Raman spectroscopy. Density functional theory (DFT) calculations were also performed. The model C3H6 first adsorbed to the catalyst surface, and the resulting C3H6-adsorbed species reacted with adsorbed NOx species to form various intermediates such as: carboxylates, enolic compounds, carbonato complexes, Pt-carbonyls, acid anhydrides, basic hydroxyls, nitrogen-containing organic species, acyl chlorides, and ammonium salts. These intermediates participated in subsequent reactions and finally produced the effluent gases CO2, N2, NH3, and H2O. The decomposition ability of adsorbed NOx species indicated that the NOx species participated in the C3H6 oxidation in the order: adsorbed N2O4 > monodentate nitrates > free ionic nitrates > bulk free ionic nitrates. Carbonaceous materials were generated during the C3H6 oxidation process and were consumed as intermediates by gaseous NOx released upon decomposition of the adsorbed NOx species. In summary, the redox reaction between C3H6 and adsorbed NOx followed the Langmuir–Hinshelwood mechanism, and three reaction routes were proposed for the redox process over the studied catalyst. The activation energy barriers determined by DFT + U calculations for the three routes indicated that the initial dissociation of NO3− species in R(2) occurred more easily than the oxidation of C3H6 species in R(1), delivering the active oxygen species that participated in R(1) and R(3). As the reactions proceeded, the higher energy barrier for the complete dissociation of NO2* indicated that the temperature determined the further decomposition of adsorbed NOx species, and R(1) and R(3) were constrained owing to the limited surface active oxygen species from R(2).