Investigation of the oxygen coverage effect on the direct epoxidation of propylene over copper through DFT calculations

The direct epoxidation of propylene is one of the most important selective oxidation reactions in industry. The development of high-performance copper-based catalysts is the key to the selective oxidation technology and scientific research of propylene. The mechanism of propylene's partial oxidation catalyzed by Cu(111) under different oxygen coverage conditions was studied using density functional theory calculations and microkinetic modeling. We report here in detail two parallel reaction pathways: dehydrogenation and epoxidation. The transition states and energy distributions of the intermediates and products were calculated. The present results showed that propylene oxide (PO) selectivity was high under low oxygen coverage, and increasing the oxygen coverage would decrease the PO selectivity but increase the PO activity, and there was an inverse relationship between PO selectivity and activity. Increasing oxygen coverage would reduce the energy barrier for the C-O bond formation of C3H5O due to the weaker adsorption strength of C3H5, thus decreasing the PO formation selectivity. On the other hand, increasing oxygen coverage would reduce the energy barrier for the possible reaction steps of propylene epoxidation in general, and thus increasing the catalytic activity. It might be proposed that the active site for propylene epoxidation is the metallic copper or partially oxidized copper in terms of the change of PO formation selectivity with oxygen coverage.

Enhanced Selectivity of the Propylene Epoxidation Reaction on a Cu Monolayer Surface via Eley-Rideal Mechanism

The aerobic oxidation of propylene to selectively achieve propylene oxide (PO) is a challenging reaction in catalysis. Therefore, an active catalyst which shows enhanced PO selectivity is extremely desired. In the present investigation, an attempt has been made to explore the catalytic activity of a mono-atomically thin two-dimensional (2D) hexagonal (HX) Cu layer for selective propylene epoxidation using molecular O₂ with the aid of density functional theory calculations. The results reveal that the conversion of propylene to PO via Eley-Rideal mechanism is an exoergic and barrierless reaction on the O₂ pre-adsorbed Cu monolayer. The Pauli energy component plays a decisive role for barrierless reaction whereas the electrostatic and orbital contribution governs the energetic stability of PO. Car-Parrinello molecular dynamics (CPMD) simulation reinforces the outcomes of climbing image nudged elastic band (CI-NEB) calculation. Further, the formation of oxametallacycle OMC-2 (0.47 eV) is kinetically favourable over OMC-1 (0.87 eV) and AHS (0.50 eV) on O pre-adsorbed 2D HX Cu. Interestingly, the energy barrier for the conversion of OMC-2 to PO (0.70 eV) is considerably low in comparison with the acetone formation (0.90 eV). Therefore, it is worth to mention that the 2D HX Cu surface provides a promising platform for selective propylene epoxidation.

Effect of Potassium for Propylene Epoxidation on Cu2O(111) by Molecular Oxygen: A DFT Study

In the present work, spin-polarized density functional theory (DFT) calculations with a Hubbard U correction were employed to investigate the effect of potassium for propylene epoxidation on Cu2O(111) surface by...