CO2 methanation over Ni/Ce0.75Zr0.25O2 catalysts: Dependence on Ni particle size

Abstract

A series of supported catalysts Ni/Ce_0.75Zr_0.25O₂ with an equal Ni loading (9 wt%) was prepared via different routes. To obtain catalysts with higher metal dispersion, two methods were used: incipient wetness impregnation (IWI) of the support with an aqueous Ni(NO₃)₂ solution containing ethylene glycol (EG) and deposition–precipitation (DP) technique. Two catalysts were synthesized by conventional IWI using different Ni precursors (Ni(NO₃)₂ and [Ni(NH₃)₆](NO₃)₂). Diagnostics showed that the obtained catalysts differed in the dispersion of supported nanoparticles. Catalytic studies revealed the impact of Ni particle size in the range of 12–48 nm on the performance in the CO₂ methanation. The enlargement of Ni particles resulted in a decrease in selectivity and CO₂ methanation activity in terms of TOF. In situ synchrotron XRD and pseudo in situ XPS studies were used to detect alterations in the composition and structure of the catalysts that proceed during their activation by heating in a hydrogen-enriched atmosphere. The results showed that the activation treatment induces a partial reduction of the support material Ce_0.75Zr_0.25O₂ due to Ni-mediated hydrogen spillover. In situ DRIFTS results pointed out that the generation of surface Ce³⁺ and oxygen vacancies during the support reduction opens an associative formate pathway in the CO₂ methanation, which is widely believed to occur at the metal/support interfaces. The enhanced activity of the catalysts containing smaller Ni particles with developed contacts between the metal and support particles is most likely due to greater formate pathway contribution.