Pt and Pd-promoted Ce-Zr Mixed Oxide Applied for Low-temperature NOx Adsorption
Yaying Ji, University of Kentucky Center for Applied Energy Research
The abatement of NOx emissions during engine cold start represents a significant challenge to the automotive industry. Recently, the use of a passive NOx adsorber (PNA) device combined with a downstream SCR catalyst has attracted interest as a means of mitigating cold start NOx emissions. The PNA adsorbs NOx emitted from the engine during cold starts, the stored NOx being thermally released when the exhaust gas becomes hotter (e.g., in the range 200-350 Â°C) and subsequently reduced by the SCR catalyst. In this study, we explored the use of a Pt and Pd-promoted Ce-Zr mixed oxide for PNA applications.
Pt and Pd-promoted Ce-Zr mixed oxide samples (total metal loading of 1 wt%) were prepared by incipient wetness co-impregnation of a Ce0.2Zr0.8O2 powder. Samples are hereafter denoted as xPt-yPd with x and y representing the Pt and Pd weight percentages. For comparison purposes, 1 wt% Pt- and 1 wt% Pd-promoted samples were also included in the study. A fixed bed reactor equipped with a mass spectrometer was employed for NOx adsorption and desorption measurements. NOx adsorption was conducted at 120 oC under lean conditions (300 ppm NO, 5% O2, 5% CO2, 3.5% H2O, He bal.). Temperature-programmed desorption (TPD) was subsequently performed under the same lean conditions but without NO in the feed gas.
For the fresh samples, NOx storage efficiency (NSE) increased with Pt loading, especially for the first 2 min of storage. After hydrothermal aging at 750 oC for 16 h under the same feed as used for NOx-TPD, all of the Pd-containing samples showed an increase in the initial NSE, the 0.5Pt-0.5Pd sample exhibiting the best NOx storage performance after aging.
Turning to the NOx desorption behavior, NOx-TPD profiles (not shown) contained two NOx desorption features: one below 300 oC and the other in the range 300 – 450 oC. After aging, both NOx desorption peaks shifted to lower temperatures. From Table 1, NOx desorption efficiency (NDE) below 350 oC ranked in the order Pd > 0.2Pt-0.8Pd ~ 0.5Pt-0.5Pd > Pt, being the reverse of the NSE order, i.e., Pd improved NOx desorption efficiency compared to Pt. Comparison of the overall performance based on the amount of NOx desorbed below 350 oC showed that the 0.5Pt-0.5Pd sample outperformed the others regardless of whether it was aged or not, which is explained by its superior NOx adsorption and desorption behavior. Hence, this study shows that the NOx storage and desorption behavior of Ce-Zr mixed oxides can be optimized by tuning the loading ratio of the Pt and Pd promoters.
Table 1. NDE and amount of NOx desorbed below 350 oC during NOx-TPD
Catalyst Fresh Aged
NDE(%) Amt.NOx.des.(Âµmol/g) NDE(%) Amt.NOx des.(Âµmol/g)
Pt 64.0 27.6 62.4 19.0
0.5Pt-0.5Pd 74.8 37.1 66.0 37.9
0.2Pt-0.8Pd 74.6 26.8 69.7 33.1
Pd 83.0 22.8 83.5 21.5
* NOx stored at 120 oC for 15 min.