PARTIAL OXIDATION OF METHANE TO SYNTHESIS GAS-USING LNCOO(3) PEROVSKITES AS CATALYST PRECURSORS

In this work a series of cobalt-containing perovskites LnCoO(3) (Ln = La, Pr, Nd, Sm, and Gd) has been studied as catalyst precursors for th e partial oxidation of methane to synthesis gas. All the perovskite pr ecursors were prereduced in situ, producing cobalt metal finely disper sed 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 H-2 production (with methane conversion of 73% and selectivities of 79 and 81.% for CO and H-2, respectively, at 100 9 K). The production of synthesis gas over the other catalysts decreas ed 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 11 2 were observed under the reaction conditions. XRD and XPS analyses of the catalyst La-Co-O showed that under the reaction conditions the co balt metal is completely reoxidized, regenerating the original LnCoO(3 ) perovskite structure. For the reaction over Nd-Co-O the cobalt is on ly partially reoxidized to NdCoO3. For Gd-Co-O and Sm-Co-O, the most s table and active catalysts for the partial oxidation of methane no reo xidation to LnCoO(3) was observed. TPR and XRD studies showed that the perovskite NdCoO3 is reduced in two steps, first to NdCoO2.5 and furt her to Co degrees/Nd2O3 and in both stages it was demonstrated that th e reoxidation with O-2 is capable of recovering the perovskite structu re. 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 t wo steps: first by oxidation of the supported Co degrees to the spinel Co3O4 (Co2+Co23+O4) followed by a further oxidation of the Co2+ to Co 3+ with a simultaneous solid state reaction with Ln(2)O(3), regenerati ng the perovskite structure. It was observed that the temperature for the second oxidation step is strongly dependent on the nature of the l anthanide. 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 al so present evidence that hydroxyl groups on the rare earth oxide, spec ially in the La-Co-O system, might make some contribution to the reoxi dation of cobalt metal during the reaction via a reverse spillover pro cess. (C) 1997 Academic Press.