Solution combustion synthesis as an alternative synthesis route for novel Ni-Mg-Al mixed-oxide catalyst for CO2 methanation

Hydrotalcite-derived catalysts are reported to be active, selective, and stable in the CO2 methanation reaction. However, the synthesis of this kind of materials requires several energetic steps, and in addition, wastewater is produced. Solution combustion synthesis (SCS) was tested as an alternative pathway for the synthesis of mixed oxide with properties similar to those of hydrotalcite-derived catalysts. Series of Ni-Mg-Al oxide matrix containing 20 wt% of nickel were prepared with two different strategies. For this purpose, the influence of the final temperature and the impact of the fuel to nitrate ratio were studied. Thus, samples were prepared at 300 and 500 °C, with a urea content of 100% (stoichiometric) or 175% (above stoichiometric). Similarities with hydrotalcite-derived oxides were found for the catalyst prepared at 500 °C and with the urea content of 100%. It is worth to note that all prepared SCS-derived samples were more active and more selective than the reference hydrotalcite-based catalyst. Materials prepared with a stoichiometric amount of urea at both 300 and 500 °C were the most active among the tested series (83% and 89% of CO2 conversion at 300 °C, respectively). Both catalysts prepared with the above stoichiometric amount of urea gave ca. 70% of the CO2 conversion at 300 °C. Furthermore, this study confirmed that the increased urea ratio and synthesis temperature led to the formation of the spinel phase. Finally, SCS-derived samples showed an overall smaller specific surface area; however, the surface basicity was higher, resulting in improved catalytic activity compared to the reference hydrotalcite-derived catalyst.