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.