Sulfur and temperature effects on the spatial distribution of reactions inside a lean NO_x 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 NO_x 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 NO_x 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 NO_x 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 NO_x conversion. The sulfation impact on NO_x 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.