Selective catalytic reduction (SCR) of NO with methane over CoZSM-5 and HZSM-5 zeolites: On the role of free radicals and competitive oxidation reactions

The reactions of CH4 with NO, NO2, and O-2, and with the mixtures of NOx + O-2, were studied over CoZSM-5 and HZSM-5 zeolites. Based on the comparison of the ''light-off' temperatures as well as the activation energies of these reactions, a conclusion is drawn that over both catalysts at temperatures below 500 degrees C NO reduction into N-2 and CH4 oxidation into COx are coupled and initiated by the reaction of CH4 with NO2. The results demonstrate that this reaction may be considered as a limiting reaction step of NO2 reduction into Nz, and of the SCR reaction for the mixture of CH4 + NO + O-2 when O-2 is present in large amounts. With the latter mixture when O-2 concentration is relatively low (up to about 1.5%) another reaction step, namely, the catalytic oxidation of NO by O-2 into NO2 appears to control the rate of the SCR process. Thus, an important role of O-2 is to convert NO into NO2. At low temperatures (below 400 degrees C) O-2 does not compete with NOx for CH4, but even at these temperatures O-2 participates in oxidation of the intermediate reaction species decreasing in this way the selectivity of the SCR process. At higher temperatures (above 450 degrees C) a strong competition between O-2 and NOx for CH4 is observed and this results in a further decrease in the process selectivity. It is shown that the higher SCR selectivity of HZSM-5 catalyst in comparison with CoZSM-5 is due to the stronger competition between O-2 and NO2 for CH4 over the latter catalyst. The effects of space velocity and reactor size were briefly explored. These results show that the reaction pathways are the same for both catalysts studied in this work. Based on these, and literature results, the formation of CH3 . free radical is postulated and possible reactions of this radical with NOx and O-2, that form the pathways of the SCR process, are considered. Finally, the possible role of gas phase reactions in the SCR process is discussed. (C) 1996 Academic Press, Inc.