Cu-exchanged SSZ-13 zeolite catalyst (Cu/SSZ-13) has been extensively studied for selective catalytic reduction (SCR) of NOx in diesel engine exhausts. 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). 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. To obtain atomic insights into the poisoning effects of Pd on Cu/SSZ-13 catalysts, two Cu/SSZ-13 samples with Cu loadings of 2 and 3 wt%, were employed here. Following which, both samples were loaded with 1wt% Pd via impregnation (abbreviated as 1Pd2Cu and 1Pd3Cu, respectively). 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. 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. 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).

Insights into the impacts of palladium on the SCR and AMOX performance over Cu/SSZ-13 catalyst

ACS Spring 2022

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