The inhibiting effect of extra-framework Al on monolithic Co-ZSM5 catalysts

BOIX, ALICIA V.; MIRÓ, EDUARDO E.; LOMBARDO, EDUARDO A.; GARCÍA FIERRO, JOSÉ LUIS

CATALYSIS TODAY

Elsevier

Año: 2008 vol. 133 p. 428 - 434

0920-5861

Co-zeolite catalysts are active for the NOx selective reduction with CH4. 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(NO3)3 as a binder to the CoZSM5/cordierite monolith system led to a sharp decrease of the N2 selectivity compared to the original CoZSM5 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 (NaZSM5, HZSM5, and SiO2) and co-impregnating Al(NO3)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 Co1+yAl2-xO4 species after calcining the solid at 550ºC. The Raman spectra of impregnated samples showed the band at 755-770 cm-1 characteristic of the CoAl2O4 spinel together with the other lines (690, 619, 523 and 482 cm-1), which coincided with those of Co3O4. The modified Auger parameter revealed the presence of octahedrally coordinated Al+3 when SiO2 was the support. This was also consistent with the appearance of the XRD reflections of Co2AlO4 and CoAl2O4. The Raman spectrum of Co-exchanged samples plus Al+3 added in the slurry showed a strong band al 595 cm-1 corresponding to CoxOy. The band at 690 cm-1 was weak indicating that the Co3O4 and CoAl2O4 phases were not well developed. These results, together with the TPR patterns support the hypothesis that a Co1+yAl2-xO4 non- stoichiometric spinel is responsible for its poor activity in the NOx SCR with methane.