Investigation into the coking-related key reaction steps in dry reforming of methane over NiMgO_x catalyst

With pre-reduced NiMgO_x catalyst, the structure evolution, catalytic performance, surface species and coking-related reaction steps are investigated in methane dry reforming (DRM) at selected temperatures. Satisfactory conversion of CH₄ and CO₂, H₂/CO ratio, and anti-coke stability are achieved in long-term (78 h) reaction at 650 °C as a proper temperature. The characterizations evidence co-existence of metallic Ni⁰ and solid solution phases in the working catalysts. Combined catalytic evaluation and coke analysis reveal that, at 650 °C, endothermic CO₂-free CH₄ decomposition shows a high initial rate for H₂ production but severe coke deposition for rapid deactivation, serving as the primary coking process. In contrast, exothermic CO disproportionation exhibits a lower reaction rate and a higher coke graphitization degree. The in-situ DRIFT spectra demonstrate that carbonates and bicarbonates serve as the primary adsorbates during CO₂ dissociation process, whereas formate intermediates dominate in DRM reactions. Such evolution of the surface species verifies facile dissociation of both CH₄ and CO₂ in well-matched rates, promoting coke-resistance DRM reaction over NiMgO_x catalyst.