Catalytic reduction of NOx by hydrocarbons over Co/ZSM-5 catalysts prepared with different methods

Abstract Co/ZSM-5 catalysts were prepared by several methods, including wet ion exchange (WIE), its combination with impregnation (IMP), solid state ion exchange (SSI) and sublimation (SUB). FTIR results show that the zeolite protons in H-ZSM-5 are completely removed when CoCl 2 vapor is deposited. TPR shows peaks for Co 2+ ions at 695–705°C and for Co 3 O 4 at 385–390°C, but a peak in the 220–250°C region appears to indicate Co 2+ oxo-ions. The catalysts have been tested for the selective reduction of NO x with iso-C 4 H 10 under O 2 -rich conditions and in the absence of O 2 , both with dry and wet feeds. A bifunctional mechanism appears to operate at low temperature: oxo-ions or Co 3 O 4 clusters first oxidize NO to NO 2 , which is chemisorbed as NO y ( y ≥2) and reduced. In this modus operandi catalyst SUB shows the highest N 2 yield ∼90% near 390°C for dry and wet feeds. It is found to be quite stable in a 52 h run with a wet feed. In contrast, the WIE catalyst, which mainly contains isolated Co 2+ ions and has poor activity below 400°C, excels at T >430°C. This and the observation that, at high temperature, NO is reduced in O 2 -free feeds over Co/MFI catalysts, suggest that NO can be reduced over Co 2+ ions without intermediate formation of NO 2 . The bifunctional mechanism at low temperature is supported by the fact that a strongly enhanced performance is obtained by mixing WIE with Fe/FER, a catalyst known to promote NO 2 formation.