Structure and reducibility of the mixed metal oxides obtained from Ni-Al layered double hydroxides. Catalytic activity in CO 2 methanation reaction

The effect of nickel amount on the structure, reducibility and activity of the mixed metal oxides generated by controlled thermal treatment of co-precipitated Ni-Al layered double hydroxides as catalyst precursors for CO2 removal by methanation reaction, was examined by variation of the Ni2+/Al3+ molar ratio (Ni2+/Al3+ = 0.5, 1.5, and 3.0), the reduction and reaction temperatures. The powder X-ray diffraction of the thermally treated samples (200−1000 °C) established the formation of nano-sized NiOand spinel NiAl2O4-like phases in different proportion and degree of crystallinity. The calcination temperature of 500 °C was selected as most convenient one because of the high dispersion of the mixed oxide phases predetermining the high dispersion of the metallic nickel. It was revealed that after preliminary ‛in situ’ reduction at 400, 450, 530 and 600 °C, all studied catalysts hydrogenated CO2 effectively at reaction temperatures from 400 to 280 °C and space velocities between 3000 and 22000 h–1. All three catalysts demonstrated similar activity at lower reduction and reaction temperatures due to the formation of readily reducible Ni2+–O species which generated sufficient number of accessible Ni0 surface active sites. Partial regeneration of the original layered structure was registered in the higher nickel containing solids after finishing of the catalytic test. The advantage of the catalyst with the lowest nickel amount is disclosed at low reaction and reduction temperatures. Its performance dominates after reduction at higher temperatures because of the role of NiAl2O4 spinel-like phase to act as a “reservoir” generating fresh Ni0. The decrease of activity in the rest of the catalysts is attributed mainly to the Ni0 sintering.