Reactivity of mixed iron-cobalt spinels in the lean methane combustion

In this work a series of mixed iron-cobalt spinel catalysts was synthesized by solution combustion synthesis (SCS) and doped with 3 wt.% of palladium via incipient wetness impregnation (IWI). The catalysts were synthesized with different Co:Fe ratios, characterized from the structural and surface point of view (XRD, XRF, μRS, BET, FESEM), and tested toward the oxidation of methane in lean conditions, in gas mixtures containing from 0.5 to 2 vol.% of CH4. The temperature window for the catalytic activity of the undoped mixed iron-cobalt spinels spread from 350 to 650 °C. While lower iron substitution levels lead to a decrease in the spinel’s activity, the catalyst with 2/3 of the cobalt in the Co3O4 substituted with iron shows similar activity to the parent Co3O4. Although the materials were proved to be heterogeneous from the crystal phase point of view (a mixture of cobalt-iron spinels), the observed reactivity still depends on the amount of iron introduced during the synthesis. The addition of Pd lowered the temperature window for the catalytic activity to the range of 250–550 °C. The promoting effect of Pd is most evident for the pure Co3O4 catalyst. In contrast to the Pd-free catalysts, the best activity of Pd-promoted samples was observed for the catalysts with the highest iron content.