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

Abstract

Complete combustion of methane (1.0 vol.% in air) over Mn-doped ZrO₂ catalysts with different Mn/Zr mole ratios (0-1.0) at different temperatures (400-600°C) and space velocities (51,000-150,000 cm³ 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 H₂ and also by the temperature-programmed reaction of lattice oxygen with pure methane (TPRLOM), both from 100 to 600°C. 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 800°C, its surface area is decreased continuously, but its methane combustion activity passed through a maximum at the calcination temperature of 600 °C, its cubic form is also converted into a mixed monoclinic and cubic form at 800°C.