Promotion Effect of Methane Activation on Cu(111) by the Surface-Active Oxygen Species: A Combination of DFT and ReaxFF Study

Inspired by the recent experiment results that the methane can be activated on a copper surface in the present of molecular oxygen (J. Phys. Chem. B., 2018, 122, 855-, DOI : 10.1021/acs.jpcb.7b06956), the effect of adsorbed oxygen species like O*, O-2*, OH*, CH3O*, as well as OOH* on the C-H bond activation involved in CH4 over Cu(111) surface was systematically studied by the density functional theory (DFT) calculations in the present work. The results show that compared with the Cu(111) surface without any adsorbed oxygen species, oxygen atom of oxygen species can activate the C-H bond of methane, capture hydrogen atom, and promote the C-H cleavage. In addition, the activation energy of the decomposition of methane on the Cu(111) surface containing one of O*, O-2*, OH*, OOH*, and OCH3* is generally reduced, and the order of the activation energy is O-2* < OOH* [or O*(OH*)] < O* < OCH3* approximate to OH*, namely, the adsorbed O-2* has the best ability to activate the C-H bond of methane. The calculated energy barrier of C-H bond activation is 0.82 eV in the presence of O-2*, which is comparable to the experimental observation of C-H bond activation at 300 K. The high activity of O-2* on the C-H bond activation has been analyzed by the some key properties of O-2*: strong Bronsted base, short distance with dissociated H atom, relatively weak adsorption strength of O-2* species, strong dipole-dipole interaction at transition states, and small strain energy of methane during the activation process. Finally, we have also studied the dynamic process of the activation of methane in the gas phase and on the Cu(111) surface by the reactive force field model (ReaxFF) and obtained the dynamic information in the activation process of the C-H bond, which improved the static DFT calculation results further.