A comparative study of Pt/Ba/Al2O3 and Pt/Fe-Ba/Al2O3 NSR catalysts: New insights into the interaction of Pt–Ba and the function of Fe

The influence of the introduction of Pt and/or Fe on the structures, NOx storage property and sulfur removal performance of Ba/Al2O3 catalyst was studied. The techniques of TG/DTA, XRD, FT-IR, H2-TPR, EXAFS and DRIFTS were employed for the careful characterization of the catalysts. Two types of Ba species are identified, namely a well-spread monolayer of Ba species and a bulk BaAl2O4 phase. The addition of Fe inhibits the Ba dispersion by enhancing the bulk BaAl2O4 formation, thus slightly decreasing the SOx absorption and greatly suppressing the growth of the bulk BaSO4, and its addition also promotes the NOx storage by increasing the mobility of the stored NOx, contributing to the formation of bulk Ba(NO3)2. The introduction of Pt always re-disperses the bulk BaAl2O4 phase via a hydration process, and enhances both the NOx and SOx absorption capacity of the catalyst. Whereas the co-existence of Pt and Fe was detrimental for the NOx storage and sulfur removal as compared with Pt/Ba/Al2O3 catalyst, although it favors the reduction of BaSO4 phase. Based upon the EXAFS, in situ DRIFTS and repeated H2-TPR results, it is found that the interaction between Pt and Ba species is of great importance for NOx storage and sulfur removal. This Pt–Ba interaction not only accelerates the NOx spillover which is a key step during storage, but also facilitates the selective reduction of BaSO4 into H2S, favorable to sulfur removal and catalyst regeneration. The introduction of Fe to the Pt/Ba/Al2O3 catalyst decreases this Pt–Ba interaction by encapsulation of Pt in the matrix of Fe/FeOx lattice after repeated redox cycles, leading to the decrease of NOx storage capacity (NSC) of the catalyst, and making sulfur removal more difficult since Fe selectively catalyzes the reduction of BaSO4 into BaS.