Mechanisms of Formaldehyde and C₂ Formation from Methylene Reacting with CO₂ Adsorbed on Ni(110)

Abstract

Methylene (CH₂) is thought to play a significant role as a reaction intermediate in the catalysis of methane dry reforming as well as in converting synthesis gas to light olefins via Fischer–Tropsch synthesis. Here, we report high quality Born–Oppenheimer molecular dynamics (BOMD) simulations of the reaction mechanisms associated with CH₂ impinging on a Ni(110) surface with CO₂ adsorbed at 0.33 ML coverage. The results show the formation of formaldehyde, carbon monoxide, C₂ species such as H₂C–CO₂, and others. Furthermore, we provide real-time demonstration of both Eley–Rideal (ER) and hot atom (HA) reaction mechanisms. The ER mechanism mostly happens when CH₂ directly collides with an oxygen of CO₂, while CH₂ attacks the carbon of CO₂, dominantly following the HA mechanism. If CH₂ reaches the Ni surface, it can easily break one C–H bond to form CH and H on the surface. The mechanistic details of H₂CO, H/CO, C₂, and H/CH formation are illuminated through the study of bond breaking/formation, charge transfer, and spin density of the reactants and catalytic surface. This illuminates the key contribution of geometry and electronic structure of catalytic surface to the reaction selectivity. Moreover, we find that ³CH₂ switches to surfaces of ¹CH₂ character as soon as the methylene and nickel/CO₂ orbitals show significant interaction, and as a result the reactivity is dominated by low barrier mechanisms. Overall, the BOMD simulations provide dynamical information that allows us to monitor details of the reaction mechanisms, confirming and extending current understanding of CH₂ radical chemistry in the dry reforming of methane and Fischer–Tropsch synthesis