CONICET | Buscador de Institutos y Recursos Humanos

The promoting effect of Pt incorporated to CoZeolites (viz.: Mordenite, Ferrierite, ZSM-5 and Y-zeolites) on the selective catalytic reduction (SCR) of NOx with CH4 was studied. The PtCo bimetallic zeolites showed higher NO to N2 conversion and selectivity than the monometallic ones under all reaction conditions tested. The Y-zeolite based samples, which were inactive, both in the mono and bimetallic samples constituted an exception. The promoting effect was a function of the zeolite type and, for a given zeolite, of the pre-treatment of the catalysts and Pt loading. The best results were obtained for a 0.5% Pt loading after reduction of the samples in H2 flow for 1 hour at 350ºC. Under dry conditions Pt0.5Co2.0 Mordenite converted 60% of NO to N2 at GHSV of 100,000 h-1 with a CH4/NO ratio of 3 at 500ºC and 2% of oxygen in the feed stream. The specific activities given as molecules of NO converted per total metal atoms per second at 500ºC were 16.5 10-4 s-1 for Pt0.5Co2.0Fer, 13 10-4 s-1 for Pt0.5Co2.0Mor and 4.33 10-4 s-1 for Pt0.5Co2.0 ZSM-5 . The incorporation of Pt also shows a positive effect in the stability of the samples under wet reaction conditions. The XPS results revealed that in the calcined samples most of the Co2+ and Pt2+ was located at exchange positions. After reduction no changes in the binding energies (B.E.) were observed in samples with low Pt loading. For higher contents (viz. 1 to 5 wt %) B.E. characteristic of Co and Pt as well as Co2+ and Pt2+ were detected. No evidence of alloy or intermetallic compounds at surface level was observed. The incorporation of Pt also promoted the Co reducibility in the zeolitic matrix. The greater reducibility and a shift of the maxima in the temperature programmed-reduction profiles suggest a Pt-Co interaction inside of the zeolitic channels. The solid that the reacting mixture initially sees has Co and Pt metallic particles highly dispersed in the zeolitic matrix together with Co2+. Some Pt2+ and proton generated during reduction also appeared. Such species remained after reaction but some reorganisation occurred. In order to get an efficient catalyst for nitric oxides abatement it is necessary for such species to be in intimate contact. A synergetic effect among the different sites could be responsible for the high activity of the bimetallic zeolites.

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