Theoretical Investigation into the Key Role of Ru in the Epoxidation of Propylene over Cu₂O(111)

Abstract

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.

Exploring Stability of Transition-Metal Single Atoms on Cu2O Surfaces

“…In addition, it is also found that TMs doped on Cu 2 O(111) surfaces are much stronger than those on other two surfaces, indicating that the Cu 2 O(111) surface is a much better substrate to stabilize SACs than the other two surfaces. In particular, the second layer bicoordinated copper atom on the Cu 2 O(111) surface is a nice position for doped TMs to form SACs, which agrees well with the recent result 32 about the Ru dopant on the Cu 2 O(111) surface.…”

“…Copper oxide surfaces are important not only in the electrocatalytic reduction of CO 2 over oxide-derived copper surfaces, , environmental chemistry, and corrosion processes but also in technologies such as microelectronic and heterogeneous catalysis. − Due to the common usage of copper oxides as a catalyst support, the adsorption and doping of catalytically active late transition metals (TMs) have been extensively studied. − The role of SACs and SCCs, or even its location within the copper-oxide surface, is rarely studied. Considerably less effort has been invested studying TMs binding on Cu 2 O surfaces − despite the fact that they are known to promote catalytic reactions such as water–gas shift and propylene epoxidations.…”

Effects of potassium on propylene epoxidation by molecular oxygen on Cu2O (111): a DFT study

“…[28][29][30] Indeed, Wang and co-workers found that: (i) the K-promoted CuO x -SiO 2 catalyst has a superior catalytic nature for propylene epoxidation by O 2 , (ii) the selectivity of PO can be enhanced to 78% with K-promoted CuO x -SiO 2 catalyst under oxygenrich conditions, and (iii) the Cu I formed during the whole reaction is the active site for propylene epoxidation. 26,31 Based on the mechanism of propylene selective oxidation with molecular oxygen summarized in Scheme 1, 16,[32][33][34][35] propylene epoxidation on the Cu 2 O (111) surface with, and without, K modification by molecular oxygen was systematically studied in the present work with DFT calculations, to determine the reasons for such an increased PO selectivity, and this is a continuation of previous studies, and in addition the results are used to investigate the effect of the K-promoter. 24…”

Investigating the mechanism of propylene epoxidation over halogen (X = F, Cl, Br, I) modified Cu2O(110) surfaces: a theoretical study

“…Specifically, halogens such as fluorine (F), chlorine (Cl), bromine (Br), and iodine(I) ions have proven to be exceptionally effective modulators of catalyst behaviours for olefin epoxidation. [20][21][22][23][24] To our knowledge, density functional theory (DFT) investigations predominantly focus on the propylene oxidation mechanism on bare [25][26][27][28][29][30][31] or alkali metals, 32 or other metals [33][34][35] modified Cu-based surfaces, with lesser attention to the impact of individual halogens like F, Br, and I on DEP over Cu substrates.…”