Partial Oxidation of Methane to Synthesis Gas Using LnCoO3Perovskites as Catalyst Precursors

In this work a series of cobalt-containing perovskites LnCoO3(Ln=La, Pr, Nd, Sm, and Gd) has been studied as catalyst precursors for the partial oxidation of methane to synthesis gas. All the perovskite precursors were prereducedin situ, producing cobalt metal finely dispersed over the rare earth sesquioxide support described here as Ln–Co–O. Of the catalyst tested the system Gd–Co–O showed exceptionally better performance for CO and H2production (with methane conversion of 73% and selectivities of 79 and 81% for CO and H2, respectively, at 1009 K). The production of synthesis gas over the other catalysts decreased in the following order: Sm–Co–O >> Nd–Co–O > Pr–Co–O. The catalyst La–Co–O was active for methane combustion and only traces of CO and H2were observed under the reaction conditions. XRD and XPS analyses of the catalyst La–Co–O showed that under the reaction conditions the cobalt metal is completely reoxidized, regenerating the original LnCoO3perovskite structure. For the reaction over Nd–Co–O the cobalt is only partially reoxidized to NdCoO3. For Gd–Co–O and Sm–Co–O, the most stable and active catalysts for the partial oxidation of methane no reoxidation to LnCoO3was observed. TPR and XRD studies showed that the perovskite NdCoO3is reduced in two steps, first to NdCoO2.5and further to Co°/Nd2O3and in both stages it was demonstrated that the reoxidation with O2is capable of recovering the perovskite structure. TPO experiments with reduced La–Co–O, Nd–Co–O, Sm–Co–O, and Gd–Co–O catalysts indicated that reoxidation of cobalt also takes place in two steps: first by oxidation of the supported Co° to the spinel Co3O4(Co2+Co3+2O4) followed by a further oxidation of the Co2+to Co3+with a simultaneous solid state reaction with Ln2O3, regenerating the perovskite structure. It was observed that the temperature for the second oxidation step is strongly dependent on the nature of the lanthanide. Based on these results it is proposed that the deactivation of the catalysts Ln–Co–O by reoxidation of cobalt metal is related to the thermodynamic stability of the parent perovskite structure. We also present evidence that hydroxyl groups on the rare earth oxide, specially in the La–Co–O system, might make some contribution to the reoxidation of cobalt metal during the reaction via a reverse spillover process.