The inhibiting effect of extra-framework Al on monolithic Co-ZSM5 catalysts used for NOx SCR
✍ Scribed by A.V. Boix; E.E. Miró; E.A. Lombardo; J.L.G. Fierro
- Publisher
- Elsevier Science
- Year
- 2008
- Tongue
- English
- Weight
- 557 KB
- Volume
- 133-135
- Category
- Article
- ISSN
- 0920-5861
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✦ Synopsis
Co-zeolite catalysts are active for the NO x selective reduction with CH 4 . For practical purposes, they should be in the form of monoliths and a binder should be added to firmly attach the Co-zeolite powders to the monoliths. The incorporation of Al(NO 3 ) 3 as a binder to the Co-ZSM5/ cordierite monolith system led to a sharp decrease of the N 2 selectivity compared to the original Co-ZSM5 powder. A systematic study was conducted to identify the species formed during the preparation of samples obtained by either ionic exchange or impregnation of the cobalt acetate using different supports (Na-ZSM5, H-ZSM5, and SiO 2 ) and co-impregnating Al(NO 3 ) 3 in varying proportions. It was found that the simultaneous presence of Al 3+ and Co 2+ in washcoating suspension led to a strong interaction that impaired the incorporation of the latter to the zeolite exchange sites, generating a new Co 1+y Al 2Àx O 4 species after calcining the solid at 550 8C. The Raman spectra of impregnated samples showed the band at 755-770 cm À1 characteristic of the CoAl 2 O 4 spinel together with the other lines (690, 619, 523 and 482 cm À1 ), which coincided with those of Co 3 O 4 . The modified Auger parameter revealed the presence of octahedrally coordinated Al 3+ when SiO 2 was the support. This was also consistent with the appearance of the XRD reflections of Co 2 AlO 4 and CoAl 2 O 4 . The Raman spectrum of Co-exchanged samples plus Al 3+ added in the slurry showed a strong band at 595 cm À1 corresponding to Co x O y . The band at 690 cm À1 was weak indicating that the Co 3 O 4 and CoAl 2 O 4 phases were not well developed. These results, together with the TPR patterns support the hypothesis that a Co 1+y Al 2Àx O 4 non-stoichiometric spinel is responsible for its poor activity in the NO x SCR with methane.