Carbon deposition and reaction steps in CO₂/CH₄ reforming over Ni-La₂O₃/5A catalyst

The cause of carbon deposition and the reaction pathways for CO₂/CH₄ reforming over Ni-La₂O₃/5A have been investigated. XRD results revealed that due to the formation of perovskite-like La₂NiO₄ in Ni-La₂O₃/5A, the small-size (ca 9 nm) Ni⁰ crystallites formed in H₂ reduction remained unsintered during 48 h of on stream reaction. The accumulation of carbon on the active sites is the main reason for catalyst deactivation. The detection of ¹³CO₂ and CO₂ in O₂ pulsing onto a sample pretreated with ¹³CH₄/CO₂ confirmed that the deposited carbon was from both CH₄ and CO₂. In CO and CO₂/CH₄ atmospheres, we observed similar TGA patterns and identical TEM images (carbon nanotubes) of deposited carbon; we propose that carbon deposition is mainly via CO disproportionation. The observation of CD₃COOH and CD₃CHO in CD₃I chemical trapping experiments and the detection of formate/formyl bands in DRIFT suggested that HCOO and HCO were intermediates. The amount of CO₂ converted was roughly proportional to the amount of H present on the catalyst. These results indicated that CO₂ activation could be H-assisted. Pulsing CH₄ onto a H₂-reduced sample and a similar sample pre-treated with CO₂, we found that CH₄ conversion was higher in the latter case. Hence, the idea of oxygen-assisted CH₄ dissociation is plausible. As for the rate of methane conversion, a k_H/k_D ratio of 1.2 and 1.1 was observed at 600 and 700°C, respectively, implying that C-H cleavages are slow kinetic steps. Based on these experimental results, we have derived reaction pathways for CO₂/CH₄ reforming. In the proposed mechanistic model, CH_xO (x=1 or 2) decomposition is considered to be a rate-determining step.