The propylene epoxidation reaction in a rich variety of other industrially important catalytic processes was used for synthesis of many value-added products. As an attractive and environmentally friendly process for direct epoxidation of propylene by Cu-based catalysts, the active oxidation states of Cu (Cu0, Cu+, or Cu0/Cu+) in the epoxidation reaction were widely studied. Herein, “29” Cu x O/Cu­(111) was chosen to act as a precursor to bulk oxidation, as it can be observed in special oxide phases of Cu (Cu x+). In our work, the crucial competitive reactions of dehydrogenation versus the epoxidation using the lattice oxygen (Olatt*) and adsorbed molecular oxygen as the oxidants were investigated on “29” Cu x O/Cu­(111) surface by virtue of the periodic density functional theory computational method. Because of the higher energy barriers of the key steps for the lattice oxygen (Olatt*) as the oxidant, there are two active species in the whole reaction processes of propylene oxidation with O2 as oxidant: the molecular O2* species and the atomic O* species. It was found that the molecular oxygen is difficult to dissociate on the surface directly, while the atomic O* species can be produced via the C3H6* + Olatt* + OO* = C3H5* + Olatt* + OOH* = C3H5Olatt* + O* + OH* mechanism or by the mechanism of C3H6* + OO* = dioxametallacycle = oxametallacycle + O*. It was found that the O* mechanism is preferred for the PO formation as compared to that of O2* on pure “29” Cu x O/Cu­(111) surface. Promising results in terms of selectivity were achieved for PO formation, which can reach 49% with the apparent activation energy of only 0.62 eV by microkinetic simulation. Therefore, “29” Cu x O/Cu­(111) surface involving the Cu x+ active phase is expected to show an efficient PO selectivity, which is much higher that of Cu+ of Cu2O­(111) and similar to that of metallic Cu0. It is hoped that the present study could provide a theoretical guide for the further development of catalysts for propylene epoxidation.

The copper-catalyzed propylene epoxidation reaction is an important process to produce PO (propylene oxide), and the addition of Ru can enhance its selectivity significantly, so it is worthy to explore the physical nature behind the Ru promotion effect from a theoretical aspect. In the present work, the reaction of propylene-selective oxidation over Ru-doped Cu 2 O(111) (named Ru@Cu 2 O(111)) was studied by density functional theory calculations systematically. It is found that the addition of Ru has the ability to promote O−O bond activation, which might be beneficial to the propylene reaction. Our results show that when O* (O 2 *) bound to the unsaturated surface copper (Cu CUS ) atom connected to Ru(O*−Cu CUS −Ru), it shows the ability to inhibit the dehydrogenation reaction and to promote the epoxidation process, thereby leading to the high selectivity toward the PO formation compared to pure Cu 2 O(111). On the other hand, the too strong binding of O 2 * (O*) (usually binds to the Ru sites) is not beneficial for the PO formation because it is less active in the kinetic aspect, indicating that the active site toward the PO formation might be the Cu CUS adjacent to the Ru ions (Cu CUS −Ru), rather than the Ru site or the Cu CUS site that is far from the Ru site like that of pure Cu 2 O. The promotion effect of Ru is to affect the catalytic activity of the Cu site through the electronic effect by acting as the ligand, instead of acting as the active site to take part in the propylene epoxidation directly. Moreover, it was found that different oxygen species [lattice oxygen (O SUF ), adsorbed atomic oxygen (O*), or adsorbed molecular oxygen (O 2 *)] show different catalytic effects for propylene epoxidation, which follows the trend O* ≈ O 2 * > O SUF . Finally, the possible factors controlling the Ru promotion effect have been analyzed, and the stronger binding to OH hinders the dehydrogenation process and stronger binding to CH 3 CH 2 O is beneficial to the PO formation over Ru@Cu 2 O(111). It is hoped that the present results may be applied to other promoters of transition metals such as Rh or alkali metal such as Na and hence is useful for further development of promising catalysts for propylene epoxidation.