N2O decomposition over manganese oxides

The catalytic decomposition of N2O was studied over four different oxides of manganese-MnO, Mn3O4, Mn2O3, and MnO2. The respective specific activities were 6.3 x 10(-4), 2.5 x 10(-3), 4.8 x 10(-3), and 8.1 x 10(-4) mu mol/s . m(2) at 623 K and 0.066 atm of N2O. Apparent activation energies fell between 19 and 27 kcal/mol, and the reaction orders with respect to N2O were between 0.74 and 0.89. XRD patterns showed that pretreatment at 773 K under He converted MnO2 into Mn2O3, which gave a specific activity similar to that of the original Mn2O3 sample; therefore, MnO2 was pretreated under pure oxygen at 645 K prior to reaction to retain the bulk MnO2 phase. On the other hand, MnO was oxidized to Mn3O4 during the course of N2O decomposition. Thus Mn2O3 is the phase associated with the highest catalytic activity. A detailed kinetic study with the most active Mn2O3 sample found that the reaction order at 648 K with respect to N2O was near 0.8 over a wide pressure range, and the reaction rate was retarded by oxygen. The experimental data fit a simple Langmuir-Hinshelwood model well and both kinetic and thermodynamic parameters could be determined. The activation energy for the rate determining step-dissociation of adsorbed N2O into N-2(g) and adsorbed oxygen-was 31 kcal/mol. Weak N2O adsorption was indicated by heat of adsorption and entropy of adsorption values of 7 kcal/mol and -9 cal/mol . K, respectively, while for O-2 adsorption the respective values were 22 kcal/mol and -26 cal/mol . K. Independent Langmuir isotherms for N2O adsorption between 297 and 353 K revealed only reversible adsorption with a heat of adsorption of 5 kcal/mol. (C) 1996 Academic Press, Inc.