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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 × 1018 molecules/m2 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 O2. 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 O2. The specific activities for NO reduction to N2 by CH4 were higher than those for NO decomposition, and CH4 reduction of NO gave selectivities to N2 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 O2 promoted the rate of NO conversion to N2. Overall, Sr/La2O3 had the highest specific activity for NO reduction by CH4 in either the absence or presence of O2, with respective values of 4.6 × 10−3 and 13 × 10−3 μ mole N2/s · m2 at 773 K. Turnover frequencies under these two sets of conditions, based on NO adsorption, were 0.78 × 10−3 and 2.3 × 10−3 s−1, respectively. Activation energies fell between 22 and 32 kcal/mole for all the REOs. The highest specific activities for NO decomposition to N2 occurred on Sm2O3 and Nd2O3 and were 1.6 × 10−3 and 1.1 × 10−3 μmol N2/s · m2 at 773 K; these correspond to TOFs of 3.6 × 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. |
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