Enhanced CO2 methanation over Ni-based catalysts: a comparative study on silica and alumino-silicate supports

Various catalysts were prepared through wet impregnation of a Ni precursor. Different supports (SiO2, SiO2 Fumed and ZSM-5) and metal loadings were used, yielding catalysts with different specific surface area, metal support interaction and Ni dispersion. All catalysts showed the presence of a crystalline phase referable to NiO as precursor of the Ni active phase. SEM-EDX analysis confirmed a uniform distribution of the active phase on the support. H2-TPR analysis allowed to discriminate between two metal oxide reduction temperatures, indicative of metal dispersion and metal-support interaction strength. Provided that the overall peaks intensity increased with raising Ni loading, raising the Ni content led to a higher increase in intensity of a low reduction temperature peak with respect to higher temperature ones. This was attributed to weak interaction between the NiO and the support and bulker metal particles at high metal concentration. The reduction feature separated in two clearly identifiable reduction peaks for the samples with 45 wt% Ni loading. Experimental tests conducted at atmospheric pressure showed very promising results for the catalysts with the highest Ni-loading with the Silica support. Significant differences between the use of SiO2 and SiO2 Fumed were found. SiO2 Fumed supported materials showed a very high SSA, higher pore volume and bigger pore width compared to the other catalysts. A higher NiO reducibility was correlated for all tests with a higher activity towards CO2 methanation. The bests results have been obtained with 36 wt% and 45 wt% Ni/SiO2 Fumed. These materials showed the highest CO2 conversion at 413 °C and 390 °C respectively, with 99 % selectivity towards methane in both cases. Of all of them, the 36 % Ni/SiO2 Fumed allowed to achieve complete selectivity toward methane at about 390 °C. Ni/SiO2 Fumed material showed extremely promising results when compared to catalysts reported in literature. Furthermore, the activity and selectivity obtained in the present work referred to a relatively high time factor (Qt/W of 60000 mL h−1 gcat−1), demonstrating the achievement of CO2 conversions close to thermodynamic values at low temperatures, with full selectivity to methane under conditions where very high productivity can be obtained with small reactor volume. This leaves considerable room for optimizing operating conditions, to further improve the catalytic performances.