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 In2O3 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.