ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

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

Ted Johansson, Devendra Pakhare, Daniel Haynes, Victor Abdelsayed, Dushyant Shekhawat, and James Spivey

School of Chemical Sciences and Engineering, Royal Institute of Technology, Teknikringen 42, SE-100 44, Stockholm, Sweden

E-mail: jjspivey@lsu.edu

Abstract: 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), H₂-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. H₂-TPR of the fresh calcined catalyst showed a shoulder at 342°C and a broad peak at 448°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 H₂-TPR, with a shoulder and a broad peak in the same temperature range. Following the H₂-TPR up to 950°C, the same batch of catalyst was oxidized by flowing 5 vol. % O₂/He up to 500°C and a second H₂-TPR (also up to 950°C) was conducted. This second H₂-TPR differed significantly from that of the fresh calcined catalyst. The single sharp peak at 163°C in the second H₂-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 500°C. TPSR of the dry reforming reaction on the fresh calcined catalyst showed CO and H₂ formation starting at 400°C, with complete consumption of the reactants at 650°C. The uneven consumption of reactants between 400°C and 650°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 H₂-TPR results. TPSR, after a H₂-TPR up to 950°C, showed that the dry reforming reaction did not light off until 570°C, which is much higher temperature than the one observed using fresh calcined catalyst. This shows that the uniform sites produced during the 950°C H₂-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.

Keywords: perovskite – LaRhO3 – dry reforming of methane – temperature programmed reduction – temperature programmed surface reaction

Full paper is available at www.springerlink.com.

DOI: 10.2478/s11696-014-0566-2

Chemical Papers 68 (9) 1240–1247 (2014)