Sulfur and temperature effects on the spatial distribution of reactions inside a lean NOx trap and resulting changes in global performance

We experimentally studied the influence of temperature and sulfur loading on the axial distribution of reactions inside a commercial lean NOx trap (LNT) catalyst to better understand the global performance trends. Our measurements were made on a monolith core, bench-flow reactor under cycling conditions (60-s lean/5-s rich) at 200, 325, and 400 °C with intra-catalyst and reactor-outlet gas speciation. Postmortem elemental and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses of the catalyst also supplemented our gas species measurements. For the unsulfated catalyst, the NOx storage/reduction (NSR) reactions were localized in the front (upstream) portion of the monolith, whereas oxygen storage/reduction reactions were distributed more evenly along the entire catalyst length. As a result, two axially distinct reaction zones were developed inside the working catalyst: an upstream “NSR zone” where both NOx and oxygen storage/reduction took place and a downstream oxygen storage capacity (OSC)-only zone where the NSR reactions did not penetrate. The NSR zone involved less than half the LNT at 325 and 400 °C, but it included almost the entire length at 200 °C. Sulfation poisoned both the NSR and OSC reactions beginning at the catalyst upstream edge, with the NSR degradation occurring more rapidly and distinctly than the OSC. As sulfation proceeded, a third zone (the sulfated zone) developed and the NSR zone moved downstream, with a concomitant decrease in both the OSC-only zone and global NOx conversion. The sulfation impact on NOx conversion was greatest at 200 °C, when the NSR zone was largest. Ammonia selectivity increased with sulfation, which we attributed to a shortened OSC-only zone and resultantly reduced consumption of NH3, slipping from the NSR zone, by downstream OSC. Lower temperatures also increased NH3 selectivity. Nitrous oxide selectivity also increased with decreasing temperature but showed little dependence on sulfation. We proposed explanations for these trends in NH3 and N2O selectivity based on shifts in competing reaction rates in the three zones.