NO ADSORPTION, DECOMPOSITION, AND REDUCTION BY METHANE OVER RARE-EARTH-OXIDES

Adsorption and decomposition of NO and its reduction by methane have been conducted over La2O3, CeO2, Nd2O3, Sm2O3, Tm2O3, and Lu2O3 as well as Sr-promoted LaO3 and Sm2O3. NO was irreversibly adsorbed on all these rare earth oxide (REO) catalysts at 300 K, and the uptakes per unit surface area were 2.5 +/- 1.0 x 10(18) molecules/m(2) except on CeO2, which had a lower uptake. On La2O3 and Sm2O3, no significant CH4 chemisorption occurred at 300 or 573 K, while oxygen did not chemisorb at 300 K and only small irreversible uptakes were detected at 573 K. Both NO decomposition in He and NO reduction by CH4 were conducted in a quartz microreactor between 773 and 973 K; all the REO catalysts were active for either reaction in both the absence and presence of O-2. Activities increased continuously with increasing temperature and no deactivation or bendover was observed except for Sm2O3, over which complete combustion of CH4 occurred at high temperature in the presence of O-2. The specific activities for NO reduction to N-2 by CH4 were higher than those for NO decomposition, and 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 presence of O-2 promoted the rate of NO conversion to N-2. Overall, Sr/La2O3 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 under these two sets of conditions, based on NO adsorption, were 0.78 x 10(-3) and 2.3 x 10(-3) s(-1), respectively, Activation energies fell between 22 and 32 kcal/mole for ail the REOs. The highest specific activities for NO decomposition to N-2 occurred on Sm2O3 and Nd2O3 and were 1.6 x 10(-3) and 1.1 x 10(-3) mu mol N-2/s . m(2) at 773 K; these correspond to TOFs of 3.6 x 10(-4) and 4.3 x 10(-4) s(-1), respectively. Activation energies for NO decomposition ranged from 21 to 29 kcal/mole. The best REO catalysts correlated with those best for the oxidative coupling of methane. On a TOF basis, the best REO catalysts were comparable to Co/ZSM-5. (C) 1995 Academic Press, Inc.