NO2 formation and its effect on the selective catalytic reduction of NO over Co/ZSM-5

Catalytic performance of Co/ZSM-5 with different metal loadings and of HZSM-5 was compared in the NO + O2, C3H8 + O2, and NO + C3H8 + O2 reactions. It was found that Co/ZSM-5 catalysts containing only isolated cobalt ions in cationic positions are inactive in NO2 formation. To achieve appreciable NO conversion in the SCR process over these catalysts higher reaction temperatures are required. These results make it possible to suggest that NO2 formation is not a prerequisite for the SCR of NO with hydrocarbons over Co/ZSM-5. With increasing Co loading, however, Co/ZSM-5 begins to exhibit activity in NO2 formation. This is explained by the formation of cobalt oxide particles on the zeolite carrier, which are active in the NO2 formation. Increase in NO2 formation strongly enhances catalytic activity in SCR of NO at lower reaction temperatures. Comparison of the C3H8 conversion in the C3H8 + O2 and C3H8 + O2 + NO reactions provides evidence that NO2 activates hydrocarbon molecules resulting in the formation of the reaction intermediates of the SCR process.