CeO2 - ZrO2 Catalysts for the Use of Biogas in IT-SOFC
Authors: Pappacena Alfonsina , Graziutti Rafael , Boaro Marta , Trovarelli Alessandro 
Published: February 2020

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, Ce0.8Zr0.2O2 (CZ80) and Ce0.8Zr0.13La0.5Nd0.2O2-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

Figure 1