Simultaneous Activation of CH4 and CO2 for Concerted C-C Coupling at Oxide-Oxide Interfaces

The catalytic conversion of CO2 and CH4 to value-added platform chemicals via direct C-C coupling provides one of the most effective routes that not only addresses global climate change but also alleviates the dependency on traditional fossil fuels. Herein, three oxide-on-oxide model catalysts that can realize direct C-C coupling on the basis of simultaneous activation of CH4 and CO2 were investigated using density functional theory (DFT) calculations. The mean-field microkinetic modeling including active sites at the (ZnO)(3)-In2O3 interface and on the In2O3(110) surface were used to integrate the mechanistic and energetic information from the DFT calculations. The formation of oxide-on-oxide interfacial sites between the substrate (In2O3) and dispersed oxides [(ZnO)(3), (ZrO2)(3), or Ga2O3] enables CO2 activation at the defective site of In(2)O(3 )and CH4 activation at the M-O pair of the supported metal oxide. In contrast to the Eley-Rideal mechanism that the C-C coupling of CO2 and CH3 stabilized on Zn-doped ceria follows, the formation of a Zn-C-C-O transition state at the active centers originates from a Langmuir-Hinshelwood mechanism in which the activated CO2 also enhances the dissociative adsorption of CH4. Microkinetics analysis indicate that dissociative adsorption of CH4 plays a dominant role in the direct C-C coupling, whereas the adsorption and activation of CO2 is less significant. DFT calculation results of CH4 and CO2 conversion to acetic acid on the (ZnO)(3)/In2O3 catalyst surface indicates that the C-C coupling step is the kinetically most relevant step. Compared with Ga2O3/In2O3 and (ZrO2)(3)/In2O3 catalyst surfaces, (ZnO)(3/)In2O3(110) is more active for acetic acid formation. The present work provides mechanistic insights into the direct C-C coupling of CH4 and CO2, which could be useful in designing more-efficient catalysts.