The inhibiting effect of extra-framework Al on monolithic Co-ZSM5 catalysts used for NOx SCR

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 Co-ZSM5/cordierite monolith system led to a sharp decrease of the N2 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 SiO2) and co-impregnating Al(NO3)3 in varying proportions. It was found that the simultaneous presence of Al3+ and Co2+ 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 550 °C. 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 Al3+ 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 Al3+ added in the slurry showed a strong band at 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.