Kinetics investigation of NO + CO reaction on La–Sr–Mn–O perovskite-type mixed oxides Original Research Article

Substituted perovskite-type mixed oxides of the general formula La1−xSrx(MnIII/MnIV)O3±δ prepared by the nitrate method were examined by X-ray diffractometry (XRD) and the diffraction data were analyzed using the Rietveld refinement method. The amount of desorbed oxygen from the samples when heating in a He atmosphere was estimated with thermogravimetric O2/temperature programmed desorption experiments while the amount of MnIII and MnIV in the samples was determined iodometrically. The substitution of La3+ by Sr2+ leads to a gradual increase of MnIV in the solids and to the transition from an oxygen excess state to an oxygen deficient one. The samples were checked as catalysts for the reaction NO+CO→products in a lab-scale plug–flow reactor between 220 and 560 °C. The reaction rate profiles were simulated using a relationship of the form R=w1Rlow T+w2Rhigh T, where w1, w2 the weighing coefficients for the corresponding R at low Rlow T and high temperatures Rhigh T. From the fitting data the true activation energies E1 and E2 were estimated for Rlow T and Rhigh T as well as the heats of adsorption ΔHads(NO) and ΔHads(CO) for the NO and CO species. Finally, the values of Eapp/low T estimated from typical Arrhenius plots ln R=f(1/T) at the low temperature region are in reasonable agreement with the difference E1−ΔHads(NO), a fact validating the results of simulation/fitting and indicating its usefulness for describing the reaction rates and estimating the kinetic and thermodynamic parameters controlling them. This methodology of simulation provides easily kinetic (activation energies) as well as thermodynamic (heats of adsorption) parameters which otherwise would necessitate differentiated experimental set-ups.