Composition-Dependent Performance of CexZr1-xO2 Mixed-Oxide-Supported WO3 Catalysts for the NOx Storage Reduction-Selective Catalytic Reduction Coupled Process
Résumé
WO3/CexZr1-xO2 materials were evaluated as a possible NH3-selective catalytic reduction (SCR) active catalyst in a NOx storage reduction (NSR) + SCR combined system. The effect of the support composition was investigated at a constant WO3 loading (9.1 wt % of WO3). The impact of WO3 promotion over textural, structural, acid-base, and redox properties of SCR samples was characterized by means of nitrogen adsorption-desorption isotherms, XRD, NOx storage capacity, NH3 temperature programmed desporption, pyridine adsorption followed by FTIR, and H2-TPR. Catalytic activities in NH3-SCR and NH3-SCO reactions as well as corresponding kinetics parameters are also discussed. All WO3/Ce-Zr materials are active and fully selective in N2 for NOx reduction by NH3 and ammonia oxidation by O2. For the SCR reaction, the rate of NO conversion is found approximately half-order with respect to NO, and negative to nearly zero-order with respect to NH3. Tungstated ceria-zirconia materials were then associated downstream with a model Pt-Ba/Al NSR catalyst. Whatever the WO3-Ce/Zr catalyst, the global NOx conversion and N2 yield are significantly enhanced by the addition of the SCR catalyst. Special attention is paid to the influence of the ceria content of the SCR catalyst on the ammonia reactivity in the combined NSR + SCR system. It is demonstrated that NH3 produced during the regeneration step of Pt-Ba/Al catalyst can react either with NOx (NH3-SCR) or with O2 (NH3-SCO). This reactivity depends both on the Ce/Zr ratio of support and on the temperature. Finally, it is demonstrated that at high temperature (i.e., 400 °C), the strength of the acid sites of the WO3-Ce/Zr catalysts is not sufficient to ensure no ammonia slip.