Theoretical insights into the effect of metal co-substituted CeO2(111) surfaces on oxygen vacancy formation and chemical looping CO2 assisted CH4 conversion to synthesis gas

Abstract

The density functional theory (DFT) method is used to investigate the effect of low oxygen vacancy formation energy on the catalytic performance of chemical looping dry reforming of methane (CL-DRM) when metal ions are co-substituted on CeO₂(111) surfaces. The results show that the oxygen vacancy formation energy is extremely low with a value of −2.05 eV when Zn and Nd are co-substituted on the CeO₂(111) surface. For the CH₄ conversion process in CL-DRM, the reaction paths are found to be CH₄ → CH₃ → CH₂ → CH → C → CO paths on the pristine as well as on the Zn and Nd co-substituted surfaces. The critical rate-limiting step for both pristine and co-substituted surfaces is the dehydrogenation of CH₂ to form CH and H with activation energies of 1.62 and 1.00 eV, respectively. This indicates that the surface co-substituted with Zn and Nd promotes the CH₄ conversion process more effectively than the clean surface. However, the desorption process of syngas on the co-substituted surface requires high energy, and CO is easily peroxidized to CO₂ before desorption, reducing the selectivity of CO to the detriment of syngas production. For the CO₂ cleavage process in CL-DRM, it is difficult for CO₂ to generate enough energy on the co-substituted surfaces to overcome the activation energy of the reaction. The formation of oxygen vacancies is facilitated by an extremely low oxygen vacancy formation energy, which in turn enhances the adsorption of reaction intermediates in the CL-DRM process onto the oxygen carrier. Nevertheless, an excessive accumulation of oxygen vacancies can drive the oxygen carrier into a hyperactivated condition, which may inhibit the desired reaction pathways and reduce the efficiency and selectivity of the CL-DRM process. The present study is of great importance for the design concept of oxygen carriers in CL-DRM and the application potential of oxygen vacancy regulation.