Control of spatial nickel distribution inside pellet catalysts for reforming of biogas with additional steam

Abstract

Growing concerns over climate change and the increasing demand for renewable energy have intensified interest in utilizing biogas. Among various strategies, the combined reforming of methane (CRM), which utilizes CH₄ and CO₂ from biogas and H₂O, has attracted attention. Nickel-based catalysts are extensively studied for CRM due to their high activity; however, they suffer from rapid deactivation caused by coking. In this work, Ni/Al₂O₃ egg-shell type pellet catalyst was investigated to address these challenges. To simplify synthesis, a novel developed wetness impregnation (DWI) method was introduced, allowing the concentration of the active phase near the outer region of the pellet. Characterization results revealed that the egg-shell catalysts exhibited higher surface nickel concentrations than their actual nickel content. Although this localization of the active phase led to lower nickel dispersion compared to homo type catalysts, it suggested improved accessibility of reactants to active sites and enhanced reducibility at lower temperatures. Among catalysts, the 2E-T_0.1 catalyst (2 wt% Ni, 0.10 mm egg-shell thickness) showed the highest CH₄ and CO₂ conversions, improved long-term stability, and superior coking resistance at 700 °C, compared to the homo type catalysts. Additionally, controlling the steam ratio indicated the possibility of producing H₂-rich syngas (H₂/CO). This indicates the role of spatial nickel distribution in optimizing catalytic performance and coking resistance. In particular, egg-shell catalyst with thin shell offers a promising strategy for efficient biogas reforming applications.