Grafting of Ni(acac)2 complex on hydroxyapatite support for suitable speciation of Ni and intensification of the CO2 methanation reaction

Green hydrogen (GH2) is considered one of the most promising vectors capable of leading the energy transition, to achieve net-zero carbon emissions, and to reduce reliance on traditional energy raw materials. In this sense, to address the challenging storage and transport issues, its catalytic transformation through reaction with the unavoidable emitted CO₂ for synthetic natural gas (SNG) production appears to be an attractive strategy enabling the implementation of Power-to-Gas technology.

Here we report the high activity of a novel series of active materials composed of Ni(acac)₂ complex grafted on hydroxyapatite (HAP) support in the CO₂ methanation reaction. Optimal grafting conditions were achieved at 30 °C, yielding up to 4.6 % Ni loading (9.6 Ni at. nm⁻²). With reference to previously reported impregnated Ni/HAP samples, the characterization of the activated grafted catalysts shows that the grafting method seems to enhance the reducibility and improve the dispersion of supported Ni species exhibiting smaller average sizes (3.2–6.3 nm). The activated Ni(acac)₂/HAP grafted catalysts demonstrated a promising performance in the CO₂ methanation reaction, shedding light on their suitability and competitiveness when compared with reference materials. The optimized Ni(acac)₂/HAP-(5) sample exhibits a T₅₀ value close to 313 °C and its maximum conversion (84 %) can be achieved at 400 °C. On the other hand, it is found a clear correlation between turnover frequency (TOF) values and the relative abundance of easily reducible Ni species as well as the most reactive basic sites. Water addition temporarily reduces the activity; however, this effect is reversible since the activity is fully restored upon water removal.