Temperature-programmed desorption of NO adsorbed on Mn 2O 3 and Mn 3O 4

Abstract

When heated to 873 K, large amounts of H 2O along with lesser amounts of CO 2 and O 2 desorbed from single-phase samples of Mn 2O 3 and Mn 3O 4 to form O vacancies and create coordinatively unsaturated (CUS) cations. Exposure of these surfaces to NO at 300 K produced a certain amount of N 2O, but no other detectable gas-phase products, with more formation over Mn 2O 3 (0.72 × 10 18 molecule N 2O/m 2) than that over Mn 3O 4 (0.10 × 10 18 molecule N 2O/m 2). Temperature-programmed desorption (TPD) spectra with Mn 2O 3 after this exposure gave three principal NO peaks at 390 (α), 470 (β) and 650 K (γ), while only one O 2 peak occurred at 650 K concomitant with the γ NO peak. A weak N 2 peak at 650 K was also observed. This O 2 peak was not obtained with Mn 3O 4. The total amount of NO desorbed from either surface was typically 1.6 × 10 18 − 2.0 × 10 18 molecule/m 2 and the NO/O 2 ratio of the concomitant γ NO and O 2 peaks was routinely near unity. Based on these results and earlier studies, the 650 K peak appears to be the decomposition of surface nitrate groups whereas the α and β NO peaks are assigned to dinitrosyl and mononitrosyl species, respectively, on CUS sites. The nitrate groups do not seem to be involved in the catalytic cycle for NO decomposition above 773 K, whereas the latter two species may well be. A reaction intermediate containing two NO molecules is postulated, and three proposed models provide rate expressions which fit the data well, with the model invoking the decomposition of an (NO) 2 species on a CUS single site giving the best fit.