| Popis: |
NO reduction by CH 4 was conducted in a quartz microreactor from 773 to 953 K over a series of nonmetallic catalysts typically used for CH 4 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 CH 4 which has not previously been investigated. All four catalysts were active for NO reduction in the absence of molecular O 2 , but the specific activities of N 2 formation (μmol N 2 /s · m 2 ) 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 CH 4 . The apparent activation energy for NO reduction to N 2 by CH 4 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 CH 4 and NO were approximately 0.73 ± 0.11 and 0.43 ± 0.05, respectively. Over all the samples, N 2 , CO 2 , and smaller amounts of N 2 O were observed as products with our GC column; little or no NO 2 formation occurred based on N 2 balances. Selectivity to N 2 increased and that to N 2 O decreased with increasing reaction temperature. These catalysts were also active with O 2 in the feed but NO conversion decreased and CH 4 conversion increased, although the inhibiting effect of O 2 on NO reduction to N 2 was less at a higher temperature. The apparent activation energy in the presence of 1.0% O 2 shifted to a higher value of 35.0 ± 1.0 kcal/mol with all four catalysts, and a negative reaction order in O 2 of −0.54 ± 0.13 was determined at 923 K. Over the three Li/MgO catalysts, the apparent activation energy for N 2 O formation was 19.7 ± 1.8 kcal/mol. |
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