On the Mechanism Underlying the Distinct Activities of Rutile-Type Metal Oxides Toward Methane Activation

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We report herein a density functional theory study on the C–H bond activation of CH4 over several rutile-type metal oxide catalysts, namely IrO2(110), TiO2(110), and β-MnO2(110) surfaces. We find that CH4 is strongly chemisorbed on the IrO2(110) surface, which distorts the CH4 geometry. Together with a strong thermodynamic driving force derived from the formation of Ir–CH3 bond, the H–CH3 bond activation proceeds with a negative barrier. In contrast, a weakly chemisorbed CH4 molecule on the TiO2(110) surface cannot proceed to the C–H bond cleavage due to a high activation barrier and a low thermodynamic driving force. The reaction on the β-MnO2(110) surface, on the other hand, is found to begin with a weak CH4 physisorption, followed by the C–H bond scission with a low activation barrier. However, here, the formation of •CH3 radical is more preferred than the Mn–CH3 bond formation, most possibly due to the electrophilic nature of MnO that suitably renders the catalyst as a perfect electron acceptor for the H-atom abstraction of CH4. With such low barrier and stability of the •CH3 formation, we suggest β-MnO2(110) as a potential catalyst that is good not only for the H–CH3 bond activation but also for the methanol formation.