NO decomposition over Mn2O3 and Mn3O4

Mn2O3 is better than Mn3O4 for catalytic NO decomposition; regardless, appreciable activities were not observed below 773 K. For example, at 0.040 atm NO and 773 K, Mn2O3 had a specific activity of 3.5 x 10(-4) mu mole N-2/s/m(2) and an activation energy of 11 kcal/mole, while for Mn3O4 comparable values were 6.5 x 10(-5) mu mole N-2/s/m(2) and 15 kcal/mole, respectively. Based on NO chemisorption at 300 K, these specific activities correspond to turnover frequencies of 7.5 x 10(-5) s(-1) and 1.6 x 10(-5) s(-1) for Mn2O3 and Mn3O4, respectively. Pretreatment in He at 873 K produced more O-2 desorption from Mn2O3 (6.1 molecules O-2/m(2)) than from Mn3O4 (2.2 molecules O-2/m(2)), but no changes in either XRD pattern were detected. Consequently, oxygen vacancies were formed and, as in previous studies, activity is associated with these vacancies. Reaction orders on NO ranged from 1.4 to 1.9 between 833 and 873 K, with the higher values occurring with O-2 in the feed. Negative reaction orders on O-2 were around -0.3 and not strongly dependent on temperature. A Langmuir-Hinshelwood model involving a surface reaction between two adsorbed NO molecules fit the data well and gave kinetic parameters which provided the following values: for the reaction between two adsorbed NO molecules, E(a) = 46 kcal/mole; for NO adsorption, Delta(ad)(0) = -25 kcal/mole and Delta S-ad(0) = -25 cal/mole/K; for O-2 adsorption, Delta(ad)(0) = -35 cal/mole and Delta S-ad(0) = -31 cal/mole/K. A comparison to specific activities obtained from the literature for oxides showed the following order at 773 K: Cu/ZSM-5 much greater than Co3O4 > La2O3 similar or equal to Mn2O3 > CuO > NiO, Fe2O3. (C) 1996 Academic Press, Inc.