Nature and spatial distribution of sulfur species in a sulfated barium-based commercial lean NOx trap catalyst

We report observations of the nature and spatial distribution of sulfur species on a sulfated Ba-based commercial lean NOx trap (LNT) catalyst. The monolithic catalyst was sulfated in a bench flow reactor during 60/5-s NOx-storage/reduction cycling to achieve a total sulfur loading of 3.4 g L−1 of catalyst. Washcoat composition, structure and sulfur distribution were analyzed with electron probe microanalysis, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction. The most significant washcoat elements of catalytic relevance were Pt, Pd, Rh, Ba, Ce, Zr, Mg, Al, and these were present mainly in four distinct domains: (i) Mg/Al mixed oxide with Pt, Ce; (ii) Al oxide with Rh, Pd; (iii) Ce/Zr mixed oxide with Pt, Pd, Ba (high Ba content); (iv) Ce/Zr mixed oxide with Pt, Pd, Ba (low Ba content). Sulfur was present in the form of sulfates that decreased in concentration along the LNT axis from front to back. Barium showed the highest sulfur affinity leading to a plug-like axial progression of its sulfation. The sulfation of Al, Mg/Al, and Ce/Zr oxides was less vigorous with a more axially dispersed and less penetrating front.