Carbon deposition and reaction steps in CO_2/CH_4 reforming over Ni-La_2O_3/5A catalyst

摘要

The cause of carbon deposition and the reaction pathways for CO_2/CH_4 reforming over Ni-La_2O_3/5A have been investigated. XRD results revealed that due to the formation of perovskite-like La_2NiO_4 in Ni-La_2O_3/5A, the small-size (ca 9 nm) Ni~0 crystallites formed in H_2 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 accumulation of ~(13)CO_2 and CO_2 in O_2 pulsing onto a sample pretreated with ~(13)CH_4/CO_2 confirmed that the deposited carbon was from both CH_4 and CO_2. In CO and CO_2/CH_4 atmophseres, 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 CD_3 COOH and CD_3CHO in CD_3I chemical trapping experiments and the detection of formate/formyl bands in DRIFT suggested that HCOO and HCO were intermediates. The amount of CO_2 converted was roughly proportional to the amount of H present on the catalyst. These results indicated that CO_2 activation could be H-assisted. Pulsing CH_4 onto a H_2-reduced sample and a similar sample pre-treated with CO_2, we found that CH_4 conversion was higher in the latter case. Hence, the idea of oxygenl-assisted CH_4 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 700degC, respectively, implying that C-H cleavages are slow kinetic steps. Based on these experimental results, we have derived reaction pathways for CO_2/CH_4 reforming. In the proposed mechanistic model, CH_xO (x=1 or 2) decompsoition is considered to be a rate-determining step.