Effects of sulfation level on the Desulfation behavior of presulfated Pt-BaO/Al ₂O ₃ lean NO _x trap catalysts: A combined H ₂ temperature-programmed reaction, in situ sulfur K-edge X-ray absorption near-edge spectroscopy, X-ray photoelectron spectroscopy, and time-resolved X-ray diffraction study

Desulfation by hydrogen of presulfated Pt (2 wt %)-BaO (20 wt %)/Al ₂O ₃ with various sulfur loading (S/Ba = 0.12, 0.31, and 0.62) were investigated by combining H2 temperature programmed reaction (TPRX), X-ray photoelectron spectroscopy (XPS), in situ sulfur K-edge X-ray absorption near-edge spectroscopy (XANES), and synchrotron time-resolved X-ray diffraction (TR-XRD) techniques. We find that the amount of H2S desorbed during the desulfation in the H2 TPRX experiments is not proportional to the amount of initial sulfur loading. The results of both in situ sulfur K-edge XANES and TR-XRD show that at low sulfur loadings, sulfates were transformed to a BaS phase and remained in the catalyst rather than being removed as H2S. On the other hand, when the deposited sulfur level exceeded a certain threshold (at least S/Ba = 0.31) sulfates were reduced to form H ₂S, and the relative amount of the residual sulfide species in the catalyst was much less than at low sulfur loading. Unlike samples with high sulfur loading (e.g., S/Ba = 0.62), H ₂O did not promote the desulfation for the sample with S/Ba of 0.12, implying that the formed BaS species originating from the reduction of sulfates at low sulfur loading are more stable to hydrolysis. The results of this combined spectroscopy investigation provide clear evidence to show that sulfates at low sulfur loadings are less likely to be removed as H ₂S and have a greater tendency to be transformed to BaS on the material, leading to the conclusion that desulfation behavior of Pt-BaO/Al ₂O ₃ lean NO _X trap catalysts is markedly dependent on the sulfation levels.