Structure, reaction mechanism and resistance of Ni catalyst grafted on Ca-promoted hydroxyapatite for CO₂ methanation under intermittent operation

CO 2 methanation represents a promising strategy to couple CO 2 utilization with green hydrogen (GH2), enabling the production of synthetic methane as a renewable energy carrier. In this work, Ni catalysts (0.6–3.6 wt.%) supported on Ca-modified hydroxyapatite (HAP) were synthesized via a grafting method and tested in CO 2 methanation. Upon activation, well-dispersed Ni nanoparticles (4.6–7.3 nm) were formed on the Ca-HAP surface. The optimized sample exhibits an excellent performance in terms of activity and selectivity to CH 4 , achieving 50% CO 2 conversion at low temperatures (T 50 = 280°C), maximum conversion (85%) at 375°C and S CH4 of 100%. A comparison of turnover frequency (TOF) values confirmed the structure-sensitive nature of CO 2 methanation. A large density of surface Ni favored methanation, whereas the sample with the lowest density is more selective towards CO formation. In addition, stability test was performed at 325°C for 220 h time-on-stream (TOS), including periods with H 2 O (8.3-16.7%) and/or O 2 (8.3-16.7%). Though the effect is reversible, oxygen addition induces a significant decrease in methanation activity, due to its competition with CO 2 for the active sites. By contrast, water alone provokes a smaller decrease in the catalyst activity. To further probe the resistance of the catalyst, complementary experiments were conducted under alternating exposures to oxidative and humid atmospheres during cooling/heating stages, followed by CO 2 methanation (start-up/shut-down cycles). In this case, condensed water provoked a stronger inhibitory effect on subsequent CO 2 methanation activity. This effect is even more pronounced under a simultaneous presence of 17% O 2 and 17% H 2 O, since it nearly suppressed the initial activity. Nevertheless, this was largely reversible, as the catalyst tended to recover its activity after a regeneration process at 500°C for 60 min under CO 2 methanation reaction mixtures.