| Abstract
| - A detailed kinetic network for homogeneous CH4−O2−NOx reactions is used to estimate maximum attainableformaldehyde (and methanol) yields and to identify elementary steps that lead to the observed enhancementeffects of NOx on CH4 oxidation rates, to HCHO yield limits, and to NOx losses to unreactive N-compounds.NOx was shown previously to increase CH4 oxidation rates and HCHO yields in CH4−O2 reactions, butmaximum yields were low (<10%) and intrinsic kinetic limits were not rigorously examined. We show herethat the CH4 oxidation rate increases because NO2 reacts with CH4 during an initial induction period. NO andNO2 lead to similar effects, except that residence times required for a given yield are higher for NO feedsbecause NO−NO2 interconversion must first occur. CH4 leads to supra-equilibrium NO2 concentrations becauseHO2 formed during HCHO oxidation reacts with NO to form OH and NO2 faster than NO2 can decomposeto NO. Oxygenate selectivities decrease with increasing CH4 conversion, because weaker C−H bonds inHCHO and CH3OH relative to CH4 lead to their fast sequential oxidation to CO and CO2. Rate-of-formationanalyses show that NOx molecules introduce more effective elementary steps for the formation of CH3Ointermediates and for its conversion to HCHO, but H-abstraction from CH4 and HCHO remains the predominantstep in controlling rates and selectivities in the presence or absence of NOx. Without NOx, OH radicals accountfor all H-abstraction reactions from CH4, while HCHO reacts with OH but also with less reactive H and HO2radicals. NOx increases HCHO yields by converting these less reactive H and HO2 radicals to OH radicals,which become the predominant H-abstractor for both CH4 and HCHO and which react less selectively withHCHO than do H and HO2. Kinetic selectivity, based on C−H bond energy differences between CH4 andHCHO, becomes weaker with increasing radical reactivity and increasing reaction temperature. MaximumHCHO yields of 37% are theoretically possible for radicals that abstract H from CH4 and HCHO at equalrates, but radical species prevalent during CH4−O2−NOx reactions lead to maximum HCHO yields below10% under all conditions. Higher yields appear unlikely with more reactive radicals, because their reactivitywould lead to cascade reactions that form species with greater kinetic sensitivity to C−H bond energies.Maximum C1-oxygenate yields increase with increasing O2 pressure, suggesting that the O2 distribution alonga reactor will not improve HCHO yields but may prove useful to inhibit NOx losses to less reactiveN-compounds.
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