Insights into the impacts of palladium on the SCR and AMOX performance over Cu/SSZ-13 catalyst
Yiqing Wu, Pacific Northwest National Laboratory
Cu/SSZ-13 catalysts have been widely studied for NOx abatement in lean-burn engine exhausts, due to their superior activity, selectivity, and long-term stability in selective catalytic reduction (SCR). However, platinum group metals (PGMs) from the upper stream units, such as diesel oxidation (DOC) and passive NOx adsorbers (PNA) units, can migrate to the SCR unit, particularly under high-temperature regeneration events of DOC, PNA, and diesel particulate filter (DPF). Due to the strong NH3 oxidation capacity of PGMs, this can deteriorate performance and long-term stability of the SCR unit. To eliminate the adverse environmental impact of ammonia slip, some diesel aftertreatment systems also incorporate ammonia oxidation catalyst (AMOX) units. State-of-the-art AMOX catalysts adopt a layered structure where a Cu/SSZ-13 catalyst layer is underneath of an alumina supported PGM catalyst layer. Such a design can also lead to PGM penetration to the Cu/SSZ-13 catalyst layer during high-temperature treatments. Pt and Pd are the most employed PGMs in the aftertreatment units described above. Due to the much higher volatility in oxidizing environments [1, 2], Pt has a much higher probability than Pd in contaminating Cu/SSZ-13. However, given the extensive employment of Pd in current emission control systems, it is of great practical importance to study the impacts of Pd on the performance of Cu/SSZ-13 catalysts in SCR and AMOX applications, in terms of activity, selectivity and hydrothermal stability.
To obtain atomic insights into the poisoning effects of Pd on Cu/SSZ-13 catalysts, two Cu/SSZ-13 samples (Si/Al =12) were employed here: one prepared by solution exchange that contains 2 wt% Cu (as isolated Cu(II) cations), and one prepared by impregnation that contains 3 wt% Cu (2 wt% as isolated Cu(II) cations and 1 wt% as CuOx clusters). Following which, both samples were loaded with 1wt% Pd via impregnation (abbreviated as 1Pd2Cu and 1Pd3Cu, respectively). In addition to SCR and AMOX tests, we employed characterizations with electron paramagnetic resonance (EPR), transmission electron microcopy (TEM), surface area/porosity measurement, H2 temperature-programmed reduction (H2-TPR), ammonia temperature-programmed desorption (NH3-TPD) and NO/CO titrations to study the Pd poisoning effects. In SCR tests, we found that the introduction of Pd led to a substantial drop in deNOx efficiency, particularly at high temperatures, for the 1Pd2Cu catalyst. In contrast, the 1Pd3Cu catalyst showed the opposite trend, exhibiting slightly higher NOx conversions as compared to the 3Cu/SSZ-13 catalyst. In AMOX tests, 1Pd2Cu and 1Pd3Cu also behave very differently in terms of selectivities, particularly after hydrothermal aging. To summarize, such preliminary studies demonstrate that (1) Pd stays as both isolated cations and PdO in these catalysts; (2) Pd(II) cations do not compete with Cu(II) cations for ion-exchange positions; (3) PdO and CuOx appear to form solid solutions to lower oxidation capacity; and (4) Pd introduction has no effects on support integrity (surface area /porosity, XRD).
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