The Effects of Platinum and Rhodium on the Functional Properties of a Lean NOx Trap 2007-01-1055
A laboratory study was performed to assess the contributions of platinum and rhodium to the emissions performance of a lean NOx trap. Samples of a barium-only formulation were obtained with either 0.84 g/L of platinum, 0.51 g/L of rhodium, or 1.0 g/L of platinum and rhodium in ratios of 1/0/1 or 5/0/1. 60 s lean/5 s rich tests were performed on fresh samples and samples that were aged on high temperature durability cycles. The results indicate that platinum is necessary for the NOx storage performance of the trap at low temperatures (e.g., 250°C), whereas rhodium is needed for the NOx reduction capability and consequently the purgability of the trap at low temperatures. As a result, the bimetallic Pt/Rh samples provided the best overall NOx conversion at low temperatures fresh and after aging.
Relative to the platinum-only sample, the samples containing rhodium had significantly higher oxygen storage capacity (OSC), which promoted the release of NOx from the LNT during the purges at high temperatures. Therefore, the platinum-only sample typically provided the best overall NOx conversion at high temperatures (e.g., 500-550°C) fresh and after aging.