Combined catalytic systems for enhanced low-temperature NOx abatement

• Strong synergistic effect is found for [(Mn–Ce)/CeO2–ZrO2 + zeolite] mixture.
• Fast SCR activity of zeolite is a prerequisite of the synergistic effect.
• Standard SCR activity of zeolite plays a minor role in the observed synergy.
• Two-stage bifunctional mechanism is proposed to explain the observed effect.
• However further development of the mechanism is required.

NH3-DeNOx performances of the combined catalysts (CombiCats) prepared by mechanical mixing of RedOx component (CeO2–ZrO2 or Mn–Ce/CeO2–ZrO2) and zeolite component (Fe-Beta, H-Beta, USY, etc.) were studied in details. By comparing DeNOx performances of the CombiCats and their individual components a pronounced synergistic effect was revealed, as evidenced by the improvement of DeNOx activities and selectivities far above performances of the individual components. Significant improvement was observed within 150–250 °C temperature range. Detailed study of a possible origin of the synergistic effect suggested that the improvement of NH3-DeNOx activity can be attributed to the bifunctional mechanism comprising two main stages:equation(1)2NO+O2↔2NO2 over   RedOx   component2NO+O2↔2NO2 over   RedOx   componentequation(2)NO+NO2+2NH3→2N2+3H2O over   zeolite   componentNO+NO2+2NH3→2N2+3H2O over   zeolite   componentRemarkably, the synergistic effect was attained by mixing RedOx component with H-Beta possessing negligible activity in Standard SCR but active in Fast SCR. Nevertheless, further research is required for revealing overall reaction network and understanding reaction mechanism responsible for the observed synergy.

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