Effect of Reductants and H2O on the NOx Storage Performance of Zeolite-Based Low Temperature NOx Adsorbers

Joseph  Theis, Ford Motor Company

A laboratory study was performed to assess the ability of zeolite-based low temperature NOx adsorbers (LTNA) to adsorb and release NOx during temperature ramp tests.  The monolithic samples were prepared in-house and contained 95% (by weight) of chabazite (CHA), beta (BEA) or ZSM5 zeolites along with 5% Al2O3for improved washcoat adhesion.  Each sample was catalyzed with 96 gpcf Pd and 24 gpcf Pt.  The temperature ramps were performed with different reductants (C2H4, CO/H2, a combination of C2H4 andCO/H2, H2alone, or none).  For each exhaust composition, the sample was oxidized at a high temperature (650oC for ZSM5, 750oC for BEA and CHA) and then evaluated on multiple consecutive tests where the bed temperature was ramped rapidly from 90oC to 470oC.  For all three zeolites, the presence of C2H4, CO/H2, and the combination of C2H4and CO/H2significantly increased the amount of NOx stored relative to the tests without reductants.   H2alone provided a small benefit in NOx storage with CHA but no benefit for BEA and ZSM5.   The NOx storage amounts for all three zeolites were very consistent on the consecutive tests with C2H4, H2alone, and no reductant, but the NOx storage amount decreased from test to test with CO/H2, both with and without C2H4.  After the multiple tests with one of the feedgas compositions, lean CH4oxidation was used to probe the average oxidation state of the Pd in the sample.   There was no reduction of the palladium following the multiple tests with C2H4, H2alone, and no reductants.  However, there was significant reduction of the palladium after the tests with CO/H2.  The loss of NOx storage performance on the tests with CO/H2was attributed to this reduction of the palladium by CO.   For all three zeolites, the NOx storage performance decreased significantly when the sample was dried and evaluated without H2O, but the NOx storage performance was restored when the H2O was resumed.  Thus, H2O is necessary for effective NOx storage on these zeolite-based LTNAs.