Effect of Pt on the water Resistance of Co-Zeolites upon the SCR of NOx with CH4

GUTIERREZ, LAURA; BOIX, ALICIA V.; PETUNCHI, JUAN

CATALYSIS TODAY

Elsevier Science B. V

Lugar: Amsterdam; Año: 1999 vol. 54 p. 451 - 460

0920-5861

The object of this work was to study the promotional effect of Pt on Co-zeolite (viz. mordenite, ferrierite, ZSM-5 and Y-zeolite) and Co/Al2O3 on the selective catalytic reduction (SCR) of NOx with CH4 under dry and wet reaction stream. After being reduced in H2 at 350°C, the PtCo bimetallic zeolites showed higher NO to N2 conversion and selectivity than the monometallic samples, as well as a combination of the latter samples such as mechanical mixtures or two-stage catalysts. After the same pre-treatment, under wet reaction stream, the bimetallic samples were also more active. Among the other catalysts studied with 5% of water in the feed, (NO = CH4 = 1000 PPM, O2 = 2%), the NO conversion dropped to zero over Co2.0Mor at 500°C and GHSV = 30,000 h-1, whereas it is 20% in Pt0.5Co2.0Mor.  In Pt/Co/Al2O3 the NOx conversion dropped below 5% with only 2% of water under the same reaction conditions. The specific activity given as molecules of NO converted per total metal atom per second were for Pt0.5Co2.0Fer > Pt0.5Co2.0Mor > Pt0.5Co2.0ZSM-5 > Pt/Co/Al2O3. The Y-zeolite-based samples were inactive in both mono and bimetallic samples. The species initially present in the solid were Pt° and Co°, together with Co2+ and Pt2+ at exchange positions. Co° seems not to participate as an active site in the SCR of NOx. Those species remained after the reaction but some reorganization occurred. A synergetic effect among the different species that enhances both the NO to NO2 reaction, the activation of CH4 and also the ability of the catalyst to adsorb NO, could be responsible for the high activity and selectivity of the bimetallic zeolites.