Deactivation mechanism of hydrotalcite-derived Ni--AlO x catalysts during low-temperature CO 2 methanation via Ni-hydroxide formation and the role of Fe in limiting this effect

When using undiluted H2 + CO2 feeds in stoichiometric ratios, it is necessary to use a reactor outlet temperature below about 320 °C in CO2 methanation in order to achieve equilibrium conditions which allow the specifications of natural gas to be met. Under these conditions, a deactivation mechanism different from that reported for higher temperature studies is observed in Ni/AlOx catalysts. The water which is formed in situ in the CO2 methanation reaction induces the formation of Ni-hydroxide, which at these low temperatures is not completely reduced to metallic nickel with a consequent decrease of the active phase (metallic Ni), easier sintering and formation of inactive side species. By using a catalyst with a nanosheet structure, prepared from hydrotalcite precursors containing low amounts of Fe, it is possible not only to obtain higher activity and selectivity, but also to enhance the reducibility of Ni-hydroxide during the catalytic reaction, obtaining stable catalysts under extended reaction tests. Under these reaction conditions, no deactivation by coke formation is observed.