Studies of Oxygen Species in Synthetic Todorokite-like Manganese Oxide Octahedral Molecular Sieves

Manganese oxide octahedral molecular sieves of 3 x 3 tunnel structure (OMS-1) doped with various cations possess high thermal stability and were studied by means of temperature-programmed desorption and reduction by H[sub 2] and CO. Different oxygen species can be discerned according to their peak positions in the temperature-programmed desorption and reduction and assigned to chemisorbed dioxygen, oxygen atoms bound to Mn[sup 2+], and those bound to Mn[sup 4+] ions in the framework. Differences in peak positions and availabilities of these species during TPD and TPR can be explained by creation of nascent Mn[sup 2+] ions during TPR. The effects of doping cations on the reactivity and availability of these oxygen species are demonstrated to be more pronounced in TPR in H[sub 2] or CO than in TPD. In some instances, the trends of changes in reactivity and availability of the oxygen species due to doping of Cu[sup 2+], Ni[sup 2+], Zn[sup 2+], and Mg[sup 2+] correlated with the changes in the heat of formation of oxides of these cations. Temperature-programmed reactions with methane show some reactivity of these doped OMS-1 materials. Pulse reactions with CO show higher reactivity of Cu-doped OM-1 than with butane. However, the recovery of Cu-doped OMS-1 by reoxidation with oxygen pulses seems rather incomplete at the same temperature. 27 refs., 9 figs.