New insights on the effect of the H₂/CO ratio for enhancement of CO methanation over metal-free fibrous silica ZSM-5: Thermodynamic and mechanistic studies

Thermodynamic and experimental facts of CO methanation were conducted by the Gibbs energy minimization method to produce substituted natural gas (SNG). The microemulsion technique was used to synthesized metal-free fibrous silica ZSM-5 (FSZSM-5) to compare with the catalytic activity results with the thermodynamic results. The catalysts were characterized by FESEM, TEM, XRD, BET, FTIR, and ESR. While spent catalysts were evaluated by TGA/DTA, TEM, EDX, and Raman spectra to study the change in morphology and carbon deposition. In-situ FTIR spectroscopy was used to investigate the possible reaction mechanism. The FSZSM-5 was proved the best catalyst than ZSM-5 due to unique fibrous morphology, high BET surface area (610 m²/g), high basicity, and extensive oxygen vacancies. The FSZSM-5 exhibited a higher CO methanation activity of 56% CO conversion and 78% CH₄ selectivity using H₂/CO = 5:1 at 450 °C. Increasing the H₂: CO ratio favored methanation activity to produce CH₄ and to resist carbon deposition in the following order: 5:1 > 3:1 > 2:1 > 1:1. Furthermore, the in-situ FTIR results supported the proposal that the dissociation of linear adsorbed CO over oxygen vacancies appeared to control the rate of CH₄ formation by hydrogenation of adsorbed C*. These results will be beneficial to understand better and optimize the CO methanation reaction process in the preparation of highly active catalysts for SNG production.