CO₂ reforming of CH₄ on Ni(111): A density functional theory calculation

CO₂ reforming of CH₄ on Ni(111) was investigated by using density functional theory. On the basis of thermodynamic analyses, the first step is CH₄ sequential dissociation into surface CH (CH ₄ → CH₃ → CH₂ → CH) and hydrogen, and CO₂ dissociation into surface CO and O (CO₂ → CO + O). The second step is CH oxygenation into CHO (CH + O → CHO), which is more favored than dissociation into C and hydrogen (CH → C + H). The third step is the dissociation of CHO into surface CO and H (CHO → CO + H). This can explain the enhanced selectivity toward the formation of CO and H₂ on Ni catalysts. It is found that surface carbon formation by the Bouduard back reaction (2CO = C_(ads) + CO₂) is more favored than by CH₄ sequential dehydrogenation. The major problem of CO₂ reforming of CH₄ is the very strong CO adsorption on Ni(111), which results in the accumulation of CO on the surface and hinders the subsequent reactions and promotes carbon deposition. Therefore, promoting CO desorption should maintain the reactivity and stability of Ni catalysts. The computed energy barriers of the most favorable elementary reaction identify the CH₄ activation into CH₃ and H as the rate-determining step of CO₂ reforming of CH₄ on Ni(111), in agreement with the isotopic experimental results.