NO REDUCTION BY CH4 OVER RARE-EARTH-OXIDES

NO reduction by methane was studied between 773 and 973 K. All the REO catalysts tested were active for this reaction in both the absence an d presence of O-2. Activities increased continuously with reaction tem perature and no deactivation or bend-over was observed at high tempera tures except for Sm2O3, over which complete combustion of CH4 occurred in the presence of O-2. The specific activities for NO reduction to N -2 by CH4 were higher than those for NO decomposition, showing that CH 4 enhances NO conversion. CH4 reduction of NO gave selectivities to N- 2 that were near 100% for all the catalysts except Sr/La2O3, Sm2O3 and Sr/Sm2O3, over which 5-20% N2O was formed. Except for CeO2, the prese nce of O-2 promoted the rate of NO conversion to N-2. Overall, Sr/La2O 3 had the highest specific activity for NO reduction by CH4 in either the absence or presence of O-2, with respective values of 4.6 X 10(-3) and 13 X 10(-3) mu mole N-2/s/m(2) at 773 K. Turnover frequencies (TO Fs) under these two sets of conditions, based on NO adsorption at 300 K, were 0.78 X 10(-3) s(-1) and 2.3 X 10(-3) s(-1), respectively. Acti vation energies fell between 22-32 kcal/mole for all the REOs. The bes t REO catalysts correlated with those best for the oxidative coupling of methane. On either a specific activity or a TOF basis, the best REO catalysts were comparable to Co-ZSM-5.