Low temperature complete combustion of dilute methane over Mn-doped ZrO2 catalysts:: factors influencing the reactivity of lattice oxygen and methane combustion activity of the catalyst

Complete combustion of methane (1.0 vol.% in air) over Mn-dopedZrO(2) catalysts with different Mn/Zr mole ratios (0-1.0) at different temperatures (400-600degreesC) and space velocities (51,000-150,000 cm(3) g(-1) h(-1)) and also over Mn-impregnated ZrO2 have been thoroughly investigated. Reactivity of the lattice oxygen of the catalysts was studied by temperature-programmed reduction (TPR) with H-2 and also by the temperature-programmed reaction of lattice oxygen with pure methane (TPRLOM), both from 100 to 600degreesC. Such reactivity is drastically increased due to the doping of Mn in ZrO2, particularly when the resulting catalyst is in cubic form. The methane combustion activity of the catalyst is first increased, passes through a maximum and then decreases with increasing the Mn/Zr ratio from 0 to 1.0, the maximum activity was observed for the Mn/Zr ratio of about 0.25. Similar trends were also observed for the variation of the catalyst surface area, stabilization of ZrO2 in its cubic form and also for the shift in the TPR peak (at higher temperatures) towards the lower temperature side, indicating higher reactivity and/or mobility of the lattice oxygen. However, when the calcination temperature of the Mn-doped ZrO2 (cubic, Mn/Zr = 0.25) is increased from 500 to 800degreesC, its surface area is decreased continuously, but its methane combustion activity passed through a maximum at the calcination temperature of 600degreesC, its cubic form is also converted into a mixed monoclinic and cubic form at 800degreesC. (C) 2002 Elsevier Science B.V. All rights reserved.