PROMOTING EFFECT OF PT ON COZEOLITES UPON THE SCR OF NOX

The promoting effect of Pt incorporated to Cozeolites (viz., mordenite , ferrierite, ZSM-5, and Y-zeolites) on the selective catalytic reduct ion (SCR) of NOx with CH4 was studied. The PtCo bimetallic zeolites sh owed higher NO to N-2 conversion and selectivity than the monometallic ones under all reaction conditions tested. The Y-zeolite based sample s which were inactive both in the mono and bimetallic samples constitu ted an exception. The promoting effect was a function of the zeolite t ype and, for a given zeolite, of the pretreatment of the catalysts and Pt loading. The best results were obtained for a 0.5% Pt loading afte r reduction of the samples in H-2 flow for 1 h at 350 degrees C. Under dry conditions Pt0.5Co2.0 mordenite converted 60% of NO to N-2 at GHS V of 100,000 h(-1) with a CH4/NO ratio of 3 at 500 degrees C and 2% ox ygen in the feed stream. The specific activities given as molecules of NO converted per total metal atoms per second at 500 degrees C were 1 6.5 10(-4) s(-1) for Pt(0.5)Co(2.0)Fer, 13 10(-4) s(-1) for Pt(0.5)Co( 2.0)Mor, 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 un der wet reaction conditions. The XPS results revealed that in the calc ined samples most of the Co2+ and Pt2+ was located at exchange positio ns. After reduction no changes in the binding energies (B.E.) were obs erved in samples with low Pt loading. For higher contents (viz., 1 to 5 wt%) the B.E. characteristics of Co-o and Pt-o, as well as Co2+ and Pt2+, were detected. No evidence of alloy or intermetallic compounds a t the surface level was observed. The incorporation of Pt also promote d the Co reducibility in the zeolitic matrix. The greater reducibility and a shift of the maxima in the temperature-programmed reduction pro files suggest a Pt-Co interaction inside of the zeolitic channels. The solid that the reacting mixture initially ''sees'' has Co-o and Pt-o metallic particles highly dispersed in the zeolitic matrix, together w ith Co2+. Some Pt2+ and protons generated during reduction also appear ed. Such species remained after reaction but some reorganization occur red. In order to get an efficient catalysts for nitric oxide abatement it is necessary for such species to be in intimate contact. A synerge tic effect among the different sites could be responsible for the high activity of the bimetallic zeolites. (C) 1998 Academic Press.