Study of reaction mechanism of methane conversion over Ni-based oxygen carrier in chemical looping reforming

First-principle calculations based on the density functional theory (DFT) were used to clarify the reaction mechanism of methane conversion over Ni-based oxygen carrier for chemical looping reforming. A systematic investigation about the methane sequential dehydrogenation process was firstly performed. Our calculation results showed that CH3 → CH2 + H is the rate-limiting step with the activation energy of 2.0 eV. Then the H2 formation was studied to understand the pathways of H2 production on NiO surface. It was revealed that the H atom on O-top site could facilely migrate to Ni-top site. We proposed two possible pathways for H2 formation, path (a) and path (b). The activation energy (2.79 eV) of path (a) is largely higher than that (0.54 eV) of path (b). For path (b), the H diffusion process is identified to the rate-limiting step with the activation energy of 0.59 eV. Finally, we also explored oxygen anion diffusion process in NiO crystal structure. The activation energy (2.23 eV) is very high and this process significantly limits the reaction rate between methane and Ni-based oxygen carrier in CLR process.