Characterization of LaRhO3 perovskites for dry (CO2) reforming of methane (DRM)

This work reports on the characterization of LaRhO3 perovskite as a catalyst for dry reforming of methane. The catalyst was studied using CH4-temperature programmed reduction (TPR), H-2-TPR, and temperature programmed surface reaction (TPSR), and the changes in the crystal structure of the catalyst due to these treatments were studied by X-ray diffraction (XRD). XRD pattern of the freshly calcined perovskites showed the formation of highly crystalline LaRhO3 and La2O3 phases. H-2-TPR of the fresh calcined catalyst showed a shoulder at 342A degrees C and a broad peak at 448A degrees C, suggesting that the reduction of Rh in perovskite occurs in multiple steps. XRD pattern of the reduced catalyst suggests complete reduction of the LaRhO3 phase and the formation of metallic Rh and minor amounts of La(OH)(3). The CH4-TPR data show qualitatively similar results as H-2-TPR, with a shoulder and a broad peak in the same temperature range. Following the H-2-TPR up to 950A degrees C, the same batch of catalyst was oxidized by flowing 5 vol. % O-2/He up to 500A degrees C and a second H-2-TPR (also up to 950A degrees C) was conducted. This second H-2-TPR differed significantly from that of the fresh calcined catalyst. The single sharp peak at 163A degrees C in the second H-2-TPR suggests a significant change in the catalyst, probably causedby the transformation of about 90 % of the perovskite into Rh/La2O3. This was confirmed by the XRD studies of the catalyst reduced after the oxidation at 500A degrees C. TPSR of the dry reforming reaction on the fresh calcined catalyst showed CO and H-2 formation starting at 400A degrees C, with complete consumption of the reactants at 650A degrees C. The uneven consumption of reactants between 400A degrees C and 650A degrees C suggests that reactions other than DRM occur, including reverse water gas shift (RWGS) and the Boudouard reaction (BR), probably as a result of in-situ changes in the catalyst, consistent with the H-2-TPR results. TPSR, after a H-2-TPR up to 950A degrees C, showed that the dry reforming reaction did not light off until 570A degrees C, which is much higher temperature than the one observed using fresh calcined catalyst. This shows that the uniform sites produced during the 950A degrees C H-2-TPR are catalytically less active than those of the fresh calcined catalyst, and that no significant side reactions such as RWGS or the Boudouard reaction occur. This suggests that reduction leads to the formation of a single type of sites which do not catalyze simultaneous side reactions.