The Effects of Platinum and Rhodium on the Functional Properties of a Lean NOx Trap

A laboratory study was performed to assess the contributions of platinum and rhodium to the emissions performance of a lean NO_x 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 NO_x storage performance of the trap at low temperatures (e.g., 250°C), whereas rhodium is needed for the NO_x reduction capability and consequently the purgability of the trap at low temperatures. As a result, the bimetallic Pt/Rh samples provided the best overall NO_x 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 NO_x from the LNT during the purges at high temperatures. Therefore, the platinum-only sample typically provided the best overall NO_x conversion at high temperatures (e.g., 500-550°C) fresh and after aging.