Catalytic Reduction of NO by CH4 over Li-Promoted MgO

Abstract

NO reduction by CH4 was conducted in a quartz microreactor from 773 to 953 K over a series of nonmetallic catalysts typically used for CH4 oxidative coupling-MgO, 1 wt% Li/MgO, 4 wt% Li/MgO, and 16 wt% Li/MgO. This represents a new family of catalysts for NO reduction by CH4 which has not previously been investigated. All four catalysts were active for NO reduction in the absence of molecular O2, but the specific activities of N2 formation (μmol N2/s · m2) over the Li/MgO samples were almost five times higher than that over pure MgO, indicating that lithium promotes NO reduction; however, NO reduction was not a strong function of lithium loading. No direct NO decomposition occurred over any sample in the absence of CH4. The apparent activation energy for NO reduction to N2 by CH4 over all four catalysts was 29.0 ± 1.1 kcal/mol; thus, there was no correlation between activation energies and lithium loadings. For these four catalysts at 923 K in the absence of oxygen, reaction orders in CH4 and NO were approximately 0.73 ± 0.11 and 0.43 ± 0.05, respectively. Over all the samples, N2, CO2, and smaller amounts of N2O were observed as products with our GC column; little or no NO2 formation occurred based on N2 balances. Selectivity to N2 increased and that to N2O decreased with increasing reaction temperature. These catalysts were also active with O2 in the feed but NO conversion decreased and CH4 conversion increased, although the inhibiting effect of O2 on NO reduction to N2 was less at a higher temperature. The apparent activation energy in the presence of 1.0% O2 shifted to a higher value of 35.0 ± 1.0 kcal/mol with all four catalysts, and a negative reaction order in O2 of −0.54 ± 0.13 was determined at 923 K. Over the three Li/MgO catalysts, the apparent activation energy for N2O formation was 19.7 ± 1.8 kcal/mol.