Fig. 4: The catalytic performance of CeZrO_x/Ni/Ni-foam inverse catalysts over direct biogas methanation.

a Temperature-dependent activities of Ni-based catalysts in simulated biogas atmosphere under different pressure (Reaction conditions: GHSV = 10,000 h⁻¹, 180-380 °C, CO₂:CH₄ = 50%:50%, CO₂:H₂ = 1:4, P = 1-15 bar), methane concentration in dehumidified post-reaction outlet gas at CO₂ conversion rates of 92%–98% (upper panel); b Influence of gas composition of simulated biogas on the catalytic performance of inverse and conventional catalysts; c The long-term stability of CeZrO_x/Ni/Ni-foam catalyst under varied operating conditions (P = 15 bar). d thermodynamic equilibrium of biogas direct methanation of 50% CO₂/CH₄ and CO₂ methanation performance of state-of-the-art Ni-based catalysts. The gray dot line refers to the minimum CO₂ conversion (97.8%) corresponding to the purity of outlet methane of 95% in the biogas direct methanation process (50% CO₂-50% CH₄). The color-filled region is defined by thermodynamic equilibrium and the minimum CO₂ conversion is a “sweet area” for qualified single-stage biogas methanation catalysts. ● with number inside indicates pure CO₂ methanation performance of reported Ni-based catalysts; open ◯ represents pure CO₂ methanation performance for CeZrO_x/Ni/Ni-foam catalyst; presents the direct biogas methanation performance of CeZrO_x/Ni/Ni-foam catalyst (50% CO₂/CH₄). The ratio of CO₂:H₂ fixed at 1:4 for catalytic evaluation.