Isotopic Studies on the Mechanism of Partial Oxidation of CH4 to Syngas over a Ni/Al2O3 Catalyst

Abstract

An isotopic transient technique and XPS were used to investigate the mechanism of partial oxidation of CH₄ to syngas over a Ni/Al₂O₃ catalyst at atmospheric pressure and 700°C. The experimental results show that CH₄ can decompose easily and quickly to H₂ and Ni _x C over the reduced catalyst, and Ni _x C can react rapidly with NiO from Ni oxidized by O₂ to CO or CO₂ depending on the relative concentration of Ni _x C around NiO on the catalyst surface. Both H₂ and CO are primary products in partial oxidation of CH₄. Isotopic tracing experiments prove that most of CO₂ produced during partial oxidation of CH₄ is from the surface reaction between Ni _x C and NiO, and it is impossible to mainly originate from the further oxidation of CO or the disproportionation of CO. The disproportionation of CO may happen at the experimental conditions, limited by thermodynamic equilibrium, however, the conversion of CO is very low. The pulse experiments of CH₄/O₂ and CH₄/CO₂ with stoichiometric ratio show that the rate of partial oxidation of CH₄ is faster than that of CH₄ reforming with CO₂, this implies that partial oxidation of CH₄ is unlikely to proceed via a combustion reforming mechanism. All the results support the direct oxidation mechanism: H₂ is from CH₄ decomposition and CO is the product of the surface reaction between Ni _x C and NiO.