A DFT and microkinetic study of propylene oxide selectivity over copper-based catalysts: effects of copper valence states

Abstract

The development of high-performance copper-based catalysts is critical for the selective oxidation of propylene in both technology and scientific fields. To ably understand the effect of valence states between Cu and Cu₂O on the selectivity and activity of propylene oxide (PO), density functional theory (DFT) calculations were carried out to investigate the reaction mechanisms over Cu(111), Cu₂O(111) and Cu⁰/Cu⁺ interface catalysts, corresponding to Cu⁰, Cu⁺ and Cu⁰/Cu⁺ active sites. Herein, two reaction routes, namely, dehydrogenation (AHS) and epoxidation pathways (OMMP₁ and OMMP₂), are taken into account in the catalytic mechanism for propylene oxidation with atomic oxygen as the oxidant. According to the microkinetic model analysis, the Cu₂O(111) surface is least active and has only 6.56% selectivity for PO with a higher apparent activation energy of 1.89 eV. On the Cu(111) surface, it was found that the Cu⁰ site is favorable for propylene epoxidation, thus leading to a moderate selectivity for PO (52.3%), and the apparent activation energy is 1.47 eV. A comparison with the corresponding bulk experimental data shows good agreement with the modeling data for Cu(111) and Cu₂O(111). Compared with other two catalysts, the Cu⁰/Cu⁺ interface exhibits highest selectivity and activity for PO, and the selectivity for PO can reach up to 70.8% and at the same time the apparent activation energy is only 1.11 eV, which is lower than Cu(111) and Cu₂O(111) surfaces. The present work indicated that the Cu⁰/Cu⁺ site is the most favorable for propylene epoxidation and the PO formation selectivity and activity follow the trend of Cu⁰/Cu⁺ > Cu⁰ > Cu⁺. The relationship of selectivity and activity with the copper valence effect could supply a clue for designing Cu-based catalysts with high efficiency for propylene epoxidation.