Ba- and Mg- -promoted Ni/g-C3N4 for efficient CO2 methanation using in-house data machine learning

Methanation is one of the solutions for decreasing carbon emissions and synthetic green fuels production, being a potentially large-scale option for CO 2 utilization. However, thermodynamics in conventional reaction conditions limit the CO 2 conversion. In this work, alkaline-earth-metal modified nickel nanoparticles supported on graphitic carbon nitride (g-C 3 N 4 ) were tested in a fixed-bed reactor, being a compelling route for atmospheric CO 2 methanation. The use of g-C 3 N 4 as a support offers a more sustainable and environmentally friendly alternative to conventional oxide materials, while maintaining competitive catalytic performance with lower Ni loading and milder activation conditions. The catalysts were synthesized using two synthesis approaches: pre-incorporation and co-impregnation with Ni and Ba or Mg into the g-C₃N₄ lattice. The catalysts were characterised using N 2 physisorption, XRD, H 2 -TPR, CO 2 -TPD, TPR, Raman and XPS. The catalyst, where first incorporating the alkaline-earth metal into the g-C 3 N 4 support (Ni/g-C 3 N 4 -Mg), exhibited the best overall performance, achieving a 17-fold increase in CO 2 conversion (from 3 to 51%) and a substantial enhancement in CH 4 selectivity (from 13 to 94%) compared to the undoped catalyst at 325 °C. The Mg-promoted catalyst has better-dispersed Ni nanoparticles, stronger metal-support interaction, greater proportion of metallic Ni species and more moderate basic sites than Ni/g-C 3 N 4 . Mg incorporation also creates Lewis basic sites and increases overall CO 2 adsorption sites on the eg-C 3 N 4 support. The linked Mg dopant and eg-C 3 N 4 synergy drives high activity and stability for low-temperature CO 2 methanation. The catalyst prepared by pre-incorporating the alkaline-earth Ba into the g-C 3 N 4 (Ni/g-C 3 N 4 -Ba) exhibits degraded textural properties and basicity. The presence of barium promotes the stabilization of ferromagnetic Ni clusters. However, the intense signals from Ni + single sites justify the catalyst's performance. Overall, the increased surface nitrogen concentration and the higher proportion of metallic Ni species are clearly beneficial for CO₂ activation and hydrogenation, underscoring their pivotal role in enhancing catalytic activity. In summary, these results highlight the crucial role of promoter choice in CO₂ hydrogenation catalysts, with Mg or Ba-doped Ni/g-C₃N₄ presenting an effective route for methane synthesis. An in-house dataset (71 CO 2 methanation runs) was used to train a Random Forest model to predict CH 4 yield from six controllable descriptors. The model ranked temperature, metal identity, and support-promoter loading as the main drivers and, through surrogate-based screening, identified an optimum at 350 °C over Ni/g-C 3 N 4 promoted with Mg on the support (CSP = 1.0 mmol) without a metal promoter.