Co-ZSM-5 catalysts in the decomposition of N2O and the SCR of NO with CH4: Influence of preparation method and cobalt loading

Co-ZSM-5 prepared via different methods with Co/Al ratios ranging from 0.03 to 0.83 are investigated in both the direct N2O decomposition and the selective catalytic reduction (SCR) of NO with CH4. UV-vis and H-2-TPR are used to get an insight in the active species in these reactions. It is observed that in catalysts with low Co loadings (Co/Al < 0.3) Co is predominantly present as mono-atomic Co species, located at ion exchange positions in ZSM-5. Higher Co loadings result in the formation of different kinds of Co-oxides, which constitute the majority of species in the over-exchanged catalysts (Co/Al > 0.5). The mono-atomic species show the highest activity in the direct decomposition of N2O, whereas the oxidic Co species do not seem to contribute much to the overall decomposition. In the SCR, the Co-oxide species catalyze the combustion of CH4 whereas the selectivity towards NO reduction is much increased at low Co loadings. Therefore, over-exchange of Co-ZSM-5 does not seem to be favorable for both the direct N2O decomposition and the SCR of NO with CH4. Co/Al ratios < 0.3 give the best results both in terms of conversion and activity per Co atom in both reactions. (C) 2008 Elsevier B.V. All rights reserved.