The Kinetics of NOxDecomposition and NO Reduction by CH4overLa2O3and Sr/La2O3

Abstract

The kinetics of four different reactions over two rare earthoxide catalysts have been determined at 923 K—N2Odecomposition, NO decomposition, NO reduction by CH4in theabsence of O2, and NO reduction byCH4with excess O2present.After examination of many different reaction mechanisms and therate expressions derived from each of them, the sequence ofelementary steps and the resulting rate equation which bestdescribe the experimental results for each are reported here.The Langmuir–Hinshelwood-type expressions indicate that N2Oproceeds via unimolecular decomposition, whereas the ratedetermining step for NO decomposition involves a surfacereaction between two adsorbed NO molecules. The reduction of NOby CH4in the absence of O2proceeds with methyl radicalformation via hydride abstraction by NO implied as the slowstep, and the rate determining step for NO reduction when bothCH4and O2are present again appears to be methyl radicalformation by reaction betweenadsorbed CH4and NO2. The rateenhancements for NO reductionto N2and O2when CH4, or CH4plus O2, is utilized are attributed to the formation of methylradicals and subsequent C-containing surface intermediates whichrapidly interact with NO. The proposed chemistry is consistentwith both that associated with methane oxidative coupling overrare earth oxides and that describing homogeneous free-radicalreactions involving these intermediates. Consequently, acomplete catalytic cycle of elementary steps is proposed foreach reaction to demonstrate the possible presence of thesevarious intermediates and to allow the testing of these modelsin the future.