CeO2 - ZrO2 Catalysts for the Use of Biogas in IT-SOFC

SOFC technology is a valid approach to promote the transition from an oil based world economy to a carbon free society. For this purpose is demanding to develop SOFC anodes that operate at intermediates temperatures (550-700°C) and with renewable resources, such as biogas (60% CH₄, 40% CO₂) coming from fermentation of biomasses and agricultural wastes [1]. Biogas can be directly reformed into the anodic compartment or in an external reformer using appropriate catalysts, being active in the dry (DR) and oxidative dry (ODR) reforming [2,3]. As preliminary study, this work investigated the reactivity of defined ceria-zirconia compositions towards the dry reforming reaction under IT_SOFC conditions, with the aim to design a suitable anode directly fed with biogas. This requires to develop compositions able to reduce side reactions such as the reverse water gas shift, which consumes part of produced H₂, methane cracking or Boudouard reactions that form carbon with a consequent deactivation of the catalyst. Considering this purpose mesoporous compositions, Ce_0.8Zr_0.2O₂ (CZ80) and Ce_0.8Zr_0.13La_0.5Nd_0.2O_2-x (LN_CZ80), were prepared with a proprietary surfactant assisted method [4]. The materials were used to prepare nickel-based catalysts with two different metal loadings. The effects of dopants and nickel content on the chemical-physical and catalytic properties of the materials were investigated.

Fresh powders (500°C/4 h) were calcined at 800°C/3 h, then impregnated with a nickel nitrate solution up to obtain a metal loading of 3.5 and 7 wt% respectively, and the final catalysts were calcined at 800°C/3 h. All materials were extensively characterized by conventional techniques (X-Ray Diffraction, Temperature Programmed Reduction, B.E.T. and BJH methods). The dry reforming tests were performed in a fixed-bed quartz reactor at atmospheric pressure. The catalysts were diluted with quartz, and previously reduced at 800°C/1 h in a pure H₂ flow. The reaction feed consisted of CH₄/CO₂ mixtures; small amount of N₂ were used as internal standard. The reaction was studied in the temperature range between 600-700°C at 12000 h^-1GHSV. Reactants and products were analyzed with a microgas-chromatograph equipped with a TCD, a molecular sieve and a polar PLOTQ columns.

The catalytic tests were performed with two different CO₂/CH₄ ratios: 50/50 and 40/60. Figure below shows results obtained testing materials with the former mixture. It is possible to observe that in the range of temperature investigated the conversion of CO₂ is always higher than the conversion of CH₄. Ni-CZ80 is not active at low temperature even using a 7 wt.% metal loading, while the doped material shows an appreciable conversion also with a 3.5 wt% Ni amount. The presence of dopants contributes to increase the conversion of both reactants, obtaining conversion of 42% and 60% at 670°C respectively for CH₄ and CO₂ for supports loaded with 7 wt.% of nickel. Results obtained with a 40/60 CO₂/CH₄ ratio showed a similar trend, but we observed a large formation of carbon.

These results suggest that the addition of La and Nd in the CZ80 lattice not only improves the surface basicity of support, activating CO₂, but also enhances the nickel dispersion, thus favoring CH₄ conversion. The modifications induced on the surface by dopants strongly influence the interplay between support, metal and gas: a lower Ni loading is necessary to obtain CH₄ conversion at low temperatures. Further investigations are undergoing to investigate the behavior of these materials under ODR conditions [2,5]. The effects on carbon formation and on the endothermicity of DR will be studied in order to fully evaluate the potential application of these materials as anodic catalysts in IT-SOFC.

References

[1] D. Pakhare, J. Spivey, Chem. Soc. Rev., 43 (2014) 7813-7837

[2] C. Gaudillère, P. Vernoux, C. Mirodatos, G. Caboche, D. Farrusseng, Catal. Today, 157 (2010) 263-269

[3] S. Assabumrungrat, N. Laosiripojana, J. Power Sources, 159 (2006) 1274-1282

[4] A. Pappacena, E. Aneggi, K. Schermanz, A. Sagar, A. Trovarelli, Stud. Surf. Sci.  Catal.,175 (2010) 835-838

[5] K. Tomishige, M. Nurunnabi, K. Maruyama, K. Kunimori, Fuel process. technol. 85 (2004) 1103-1120