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 kH/kD 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, CHxO (x=1 or 2) decomposition is considered to be a rate-determining step.
Scopus Subject Areas
- Condensed Matter Physics
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry