TiO 2 is widely used in OCM reaction because of its low cost, reducibility, high specific surface area and nontoxicity, but pure TiO 2 has poor activity on methane activation. Density functional theory (DFT) method was used to research the oxidative coupling of methane (OCM) reaction on doped Ir/TiO 2 (001) and Pt/TiO 2 (001) catalysts to explore catalysts with high activity and C 2 hydrocarbon selectivity. The results indicate that Ir and Pt doping can significantly reduce the activation energy of initial dissociation of methane, and subsequently improve the OCM activity of TiO 2 catalyst. Additionally, the evolution of surface oxygen species on doped Ir/TiO 2 (001) and Pt/TiO 2 (001) catalysts during the OCM reaction were also studied. The results show that on doped Ir/TiO 2 (001) and Pt/TiO 2 (001) surfaces the lattice oxygen (O 2- ) species and peroxide (O 2 2- ) species have the strongest activity to activate methane. However, the superoxide (O 2 - ) species, the OH species and the OOH species have the worst activity. Furthermore, the formation paths of ethane and ethylene were researched on doped Ir/TiO 2 (001) and Pt/TiO 2 (001) surfaces. Ethylene is generated through the competitive path of CH 4 →CH 3 →CH 2 →C 2 H 4 and CH 4 →CH 3 →C 2 H 6 →C 2 H 5 →C 2 H 4 , and the energy barriers of the rate determination step of two paths are much higher than that of the ethane generation path CH 4 →CH 3 →C 2 H 6 . Therefore, the main C 2 hydrocarbon product on Ir/TiO 2 (001) and Pt/TiO 2 (001) surfaces is ethane. And we provide important insights into the probable evolution of surface oxygen species and the detailed reaction network on doped Ir/TiO 2 (001) and Pt/TiO 2 (001) catalysts.
The adsorption and dissociation of H2 with different coverages over the Rh(100) surface have been systematically investigated to probe into the effect of coverage on H2 adsorption and dissociation. Here, the results are obtained using the density functional theory (DFT) method together with the periodic slab model. Both the parallel and vertical modes of H2 adsorption on the Rh(100) surface have been identified, and the detailed studies corresponding to H2 adsorption and dissociation at different coverages are presented. Our results show that the parallel mode of a single H2 adsorbed on the Rh(100) surface is more favorable than the vertical mode, in which the top site is the most stable adsorption site. However, the parallel modes of single H2 adsorbed at the bridge and 4F hollow sites, as well as the vertical mode of single H2 adsorbed at the 4F hollow site are all the dissociative adsorption. On the other hand, with the increasing of H2 coverage from low to high, the most stable adsorption configurations of H2 is the parallel adsorption mode at the top site, and the adsorption energies of these adsorbed H2 molecules will decrease gradually until the saturated adsorption with H2 coverage of 6/12 ML, further, the dissociation of these adsorbed H2 molecules is more favorable both kinetically and thermodynamically than their desorption, suggesting that the dissociation of the adsorbed H2 molecule is more favored than their desorption. Finally, considering the dissociative adsorption of the single H2 molecule with the parallel modes at the bridge and 4F hollow sites, as well as the vertical mode at the 4F hollow site, our results still show that the adsorptions of H2 with different coverages at these sites are still the dissociative adsorption with the dissociative H atoms adsorbed on the Rh surface. Therefore, H2 dominantly exists in the form of H atoms on Rh catalyst under realistic conditions.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)