Elucidation of the Reaction Mechanism on Low Temperature Dry Reforming of Methane with Suppressing Carbon Deposition in Electric Field

Dry reforming of methane (DRM) has recently attracted a significant interest, producing syngas with a low H₂:CO ratio, which is adequate for C1 chemistry such as Fischer-Tropsch reaction, hydroformylation and carbonylation reactions.
CH₄ + CO₂ → 2CO + 2H₂ ΔH⁰₂₉₈ = 247.2 kJ mol^-1 (1)
However, this reaction requires higher temperatures above 1073 K because of a large endothermic reaction, resulting in carbon deposition and requiring for sophisticated reactors.
Recently, we reported that DRM over Ni/La-ZrO₂ was significantly assisted at low temperature regions (423 K) by applying an electric field. The operation of DRM at low temperatures greatly suppresses carbon deposition on the catalyst and the catalytic activity is stably maintained. Furthermore, the kinetic analysis revealed that CH₄ conversion, which is the rate-determining step in conventional catalyst system, is enhanced by the electric field and the reaction rate was regulated by CO₂ dissociation.
To elucidate the reaction mechanism of DRM in the electric field, transient isotopic experiments using oxygen isotope (¹⁸O₂) was conducted. Results showed that redox mechanism using lattice oxygen is important for CO₂ activation in the electric field. It is assumed that lattice oxygen (Olat) will be consumed by hydrocarbon species (CH_x) which is stem from CH₄ and supplied CO₂ will re-oxidize the defect.
CH₄ → CH_x + (4-x)/2 H₂ (2)
CH_x + O_lat → CO + x/2 H₂ + V^∙∙_o (3)
CO₂ + V^∙∙_o → CO + O_lat (4)
Herein, the effects of surface defect on the catalytic activity of La-ZrO₂ were considered using DFT calculations. Results clarified that the formation of carbonate or bicarbonate, which are common intermediates formed during CO₂ conversion, occurs favorably at oxygen vacancy site. These results suggest that CO₂ conversion was assisted by promoting the formation of oxygen vacancy in the electric field.