Olive stone-derived hydrochars for CO2 methanation: synergistic and antagonistic effects of ZnCl2 and FeCl2 in Ni-based catalysts

The catalytic hydrogenation of CO2 into CH4 is a key reaction within emerging carbon utilization strategies, offering a direct route to synthetic natural gas. In this context, this study explores the synthesis of activated hydrochar supports derived from olive stones, focusing on their role in Ni-based catalytic systems for CO2 methanation. Activated hydrochars were synthesized through hydrothermal carbonization at 240°C followed by chemical activation using ZnCl2 and FeCl2 at biomass/agent ratios of 1:6 and 1:8 and activation temperatures of 700°C and 800°C. The influence of activation conditions on hydrochar composition, stability, and catalytic performance were systematically analyzed.Textural and chemical analyses revealed that Fe and Zn incorporation modified the reducibility and dispersion of Ni species, with XPS confirming the formation of Ni–Fe alloys through substitutional Fe incorporation into the Ni lattice. ZnCl2 activation produced exceptionally high surface areas (2301 m2 g-1) but did not yield catalytic activity. Conversely, FeCl2 activation generated lower surface area yet intrinsic catalytic functionality, enabling measurable CO2 conversion and CH4 selectivity even in absence of Ni. The combined Ni–Fe systems exhibited a synergistic effect, achieving XCO2 of 46% and SCH4 of 93% for the optimal catalyst (1:8 ratio, 700°C), attributed to enhanced reducibility, improved nanoparticle accessibility, and favorable structural properties.Overall, these results highlight olive stone-derived hydrochars as promising sustainable materials for CO2 methanation. While Zn activation enhances textural characteristics and Fe contributes limited activity, high catalytic performance arises from the cooperative Ni–Fe interaction enables efficient CO2 valorization.