On the coke deposition in dry reforming of methane at elevated pressures

The reaction pathways leading to coke formation in dry reforming on Ni and Pt-based catalysts were investigated. Using isotope-labeled reactants (i.e., CH₄ + ¹³CO₂ ⇌ 2CO + 2H₂) showed that initially ¹³CO₂ is converted faster than CH₄ and that this higher activity leads to a higher concentration of ¹³CO compared with ¹²CO in the product stream, suggesting little isotope scrambling among products at this stage. Gasification of carbon deposits was found to be an important pathway enhancing the catalyst stability. Analysis of the pathways leading to carbon deposits suggests that coke is formed predominantly via reverse Boudouard reaction on Ni, while both metals contribute to CH₄ dissociation. The pronounced reversibility of the C-H and C-O bond formation and cleavage was also shown by the presence of ¹³CH₄ formed from ¹³CO₂ and ¹²CO formed from ¹²CH₄. Numeric calculations complementing the experimental results led to the proposal of a reaction pathway for the surface reactions, accounting for the differences between Ni and Pt.