CO₂/CH₄ reforming over Ni-La₂O₃/5A: An investigation on carbon deposition and reaction steps

CO₂/CH₄ reforming over Ni-La₂O₃/5A was studied using XRD, in situ thermal gravimeter (TG), pulse experiments, chemical trapping, transmission electron microscopy (TEM), and electron paramagnetic resonance. The small size (ca. 9 nm) Ni⁰ crystallites formed in H₂ reduction remained unsintered during 48 hr of on-stream reaction at 800°C because of the formation of perovskite-like La₂NiO₄ phase in Ni-La₂O₃/5A. The main cause of the deactivation of Ni-La₂O₃/5A was the accumulation of carbon in the active sites. The contribution of CO₂ towards deposited carbon was greater than that of CH₄ at higher temperatures. Based on the facts that the TG profiles and TEM images of carbon formed in CO and CO₂/CH₄ atmospheres were very similar, the carbon deposition was mainly via CO disproportionation. Due to the mutual activation of CO₂ and CH₄, the existence of H (or O) species on the catalyst surface could promote the activation of CO₂ (or CH₄) significantly. A reaction pathway for the reforming reaction was proposed and suggested that CH_xO (x = 1 or 2) decomposition is a rate-determining step.