Speaker
Description
The Sabatier reaction (CO2 + 4 H2 ⟶ CH4 + 2 H2O) is of growing interest in the context of limiting anthropogenic CO2 emissions. Despite its low reaction temperature (200-400°C), the activation of CO2 remains difficult to achieve and requires the formulation of highly active catalysts. Among them, Ni, Co or Ru based catalysts supported on CeO2 are often considered for the reaction but still suffer from low activity at low temperature and/or rapid deactivation [1]. An amelioration of the catalyst formulation is therefore needed to envisage industrialization of the CO2 methanation process.
We have recently developed a synthesis method to produce Ni-doped CeO2 nanoparticles (NPs) based on Schiff base metal complexes [2,3]. This method produces nanoparticles with well-defined composition of Ni0.04Ce0.96O2, a particle size distribution ranging between 5–7 nm and a considerably higher methanation activity in comparison to classical Ni/CeO2 prepared with same nominal Ni loading. A first NAP-XPS study conducted at the TEMPO beamline of SOLEIL synchrotron (France) showed a better reducibility of cerium (Ce4+ → Ce3+) between 200 and 700 °C under different gases encountered in methanation conditions (H2, H2O, CO2) for the doped sample in comparison to pure CeO2 prepared via the same synthesis method. Ce3+ species is accompanied by the formation of oxygen vacancies in ceria lattice and better ability to activate CO2 for its gradual hydrogenation [4].
To gain insight on the Ni oxidation state evolution, the surface state of the NPs was studied under reducing conditions (1 bar of H2) by NEXAFS at the APE-HE beamline of ELETTRA synchrotron (Italy). It was found that the reduction of Ni2+ ions in 1 bar H2 is hindered while reduction of Ce4+ is promoted on Ni0.04Ce0.96O2 NPs, as compared to pure NiO and CeO2 reference samples. Interestingly, reduction of Ce4+ is accompanied by further oxidation of Ni2+ into Niδ+ (2<δ<3). This is quite unexpected observation, for nickel oxide treated in 1 bar H2, and indicates an electronic interaction between Ni and Ce ions which was also observed under reaction conditions (CO2/H2 = 1:4). Theoretical simulation of Ni L-edge spectra suggested that Ni atoms are tetrahedrally coordinated into ceria lattice, in contrast to the familiar octahedral symmetry of bulk NiO.
References
[1] H. Yang, C. Zhang, P. Gao, H. Wang, X. Li, L. Zhong, W. Wei, Y. Sun, Catal. Sci. Technol., 2017, 7, 4580-4598
[2] W. Derafa, F. Paloukis, B. Mewafy, W. Baaziz, O. Ersen, C. Petit, G. Corbel, S. Zafeiratos, RSC Adv., 2018, 8, 40712-40719
[3] M. Barreau, D. Chen, J. Zhang, V. Papaefthimiou, C. Petit, D. Salusso, E. Borfecchia, S. Turczyniak, K. Sobczak, S. Mauri, L. Braglia, P. Torelli, S. Zafeiratos, Mater. Today Chem., 2022, 26, 101011
[4] M. Boaro, S. Colussi, A. Trovarelli, Front. Chem., 2019, 7 , 28
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