NO x decomposition and reduction by methane over La 2O 3

Abstract

Nitric oxide reduction by methane was conducted in a quartz microreactor from 773 to 973 K over La 2O 3, a good methane oxidative coupling catalyst. La 2O 3 is a better catalyst than MgO and Li/MgO for this reaction because: (1) it has a much higher specific activity, (2) the presence of oxygen enhances the rate of reduction, and (3) it gives a selectivity to nitrogen that is close to 100%. Both Li/MgO and La 2O 3 catalyst systems have rates that increase continuously with temperature and exhibit no bend over. The apparent activation energy for nitric oxide reduction by methane over La 2O 3 was 24.4 kcal/mol (26.0 kcal/mol with oxygen present), and the reaction orders in CH 4, NO and O 2 were 0.26, 0.98 and 0.50, respectively. Unlike Li/MgO, La 2O 3 is also active for direct nitric oxide decomposition, but the activity is noticeably lower than that for nitric oxide reduction by methane. Both the direct decomposition of nitrous oxide and nitrogen dioxide and their reduction by methane were also studied on La 2O 3. The rate of direct nitrous oxide decomposition was quite high and essentially unaltered by the presence of methane; in contrast, the rate of nitrogen dioxide decomposition was very low. However, the rate of nitrogen dioxide reduction to nitrogen was greatly enhanced by methane, and the rate became comparable to that for nitric oxide reduction by methane in the presence of 1.0% O 2. Methane oxidative coupling over this La 2O 3 catalyst was verified using oxygen and nitrous oxide as oxidants, whereas little or no coupling occurred when nitric oxide or nitrogen dioxide was used. At this time, the high NO x reductive activity of La 2O 3 and other coupling catalysts is attributed to the formation of surface methyl radicals.