Origin of rate bistability in Mn-O/Al2O3 catalysts for carbon monoxide oxidation: role of the Jahn-Teller effect

Peculiarities in catalytic activity in carbon monoxide oxidation as well as some structure, electronic and magnetic properties of the three oxide cata lysts, Mn3+-O/Al2O3 (1), Mn3+-O-Fe/Al2O3 (Mn-substituted spinel, 2) and gam ma -Fe2O3/Al2O3 (3), were studied by kinetic measurements and by Mossbauer spectroscopy. The catalysts 1 and 2 showed a kinetic bistability with a sha rp transition towards more reactive state at similar to 200 degreesC (ignit ion point). In contrast, for catalyst 3, at 200-250 degreesC, the behavior of reaction rate against temperature did not display noticeable hysteresis. On cooling the catalysts 1 and 2, extinction was observed at about 170 and 120 degreesC, respectively, i.e., at 30-80 degreesC lower than the corresp onding ignition points. Proximity of activation energy for the high and low activity (similar to 15-19 kJ/mol) for both Mn-containing catalysts sugges ts an increase in the number of active sites at high temperature with no ch anges in the reaction mechanism. The considerable difference between Mn-con taining catalysts 1, 2 and Fe-containing catalyst 3 may be caused by Jahn-T eller (JT) type distortions of the oxygen polyhedron around Mn3+. A signifi cant spontaneous axial bond stretching within the local polyhedron seems to diminish Mn-O binding energy, facilitate the participation of surface oxyg en species, O-S, in the oxidation of CO by a redox mechanism and promote ox ygen vacancies at the surface that would cause considerable effect on the a ctivity. An increase in the width of the counterclockwise hysteresis loop f or the catalyst 2 compared to the catalyst 1 indicates that clusters of mix ed spinel provide more active sites and more labile O-S species than cluste rs of the binary Mn oxide.