A DFT Study and Microkinetic Simulation in Propylene Oxidation on the “29” Cu_xO/Cu(111) Surface

Abstract

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.

Propylene Epoxidation using Molecular Oxygen over Copper- and Silver-Based Catalysts: A Review

“…As both carbon atoms in ethylene are equivalent, one oxametallacycle can form with ethylene (OME: oxygen–metal–ethylene). In propylene, the sp 2 carbon atoms are inequivalent, giving rise to two possible oxametallacycles (OMP1 and OMP2) with atomic oxygen and two more (“dioxametallacycles”: OOMP1 or OOMP2) with molecular oxygen, which can form on oxides, metallic surfaces, or partially metallic surfaces . Analogously, an ethylene oxametallacycle with molecular oxygen (OOME) could also be possible, but it has not yet been observed.…”

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…”

The 2D or 3D morphology of sub-nanometer Cu5 and Cu8 clusters changes the mechanism of CO oxidation

“…Due to their ability to activate and dissociate molecular O 2 , copper catalysts have been applied to oxidation reactions like CO oxidation or propene epoxidation, with excellent results at the initial stages of the reaction. [19][20][21][22][23][24][25][26][27] However, the easy oxidation of metallic Cu 0 to cationic Cu + and Cu 2+ always leads to a clear decay in activity and/or selectivity, revealing that it is necessary to avoid the formation of oxide phases under reaction conditions in order to maintain the catalytic performance constant. Despite this being a challenging goal, we demonstrated both theoretically and experimentally that it is possible to stabilize metallic Cu 0 under oxidizing reaction conditions by adjusting the size and shape of electrochemically synthesized Cu clusters.…”