Crystal-plane-controlled selectivity and activity of copper catalysts in propylene oxidation with O2

Abstract

The direct epoxidation of propylene is commercially one of the most important selective oxidation reactions. Copper-based catalysts, with molecular oxygen as the oxidant, have drawn much attention but remain an extensive challenge in chemistry. The density functional method and microkinetic simulation were applied to explore the reaction mechanism over three pristine Cu surfaces, namely Cu(111), Cu(110) and Cu(100). A detailed reaction network was obtained, examining two possible mechanistic routes, including dehydrogenation and epoxidation processes. We show that the dominant Cu(111) facet was the most active surface while being only moderately selective for PO. The 100 facet was shown to exhibit the highest selectivity for PO and was the least active surface. As for the Cu(110) surface, it has moderate activity and selectivity for PO. The PO selectivity can be described in the order Cu(100) > Cu(110) > Cu(100), while the PO activity follows the order of Cu(111) > Cu(110) > Cu(100). The determining factor was quantitatively analyzed: the lower basicity of 110 and 100 facets favors metallacycle formation, thus PO selectivities were higher as compared to the 111 facet. On the other hand, the spin density of O atoms on Cu(111) was higher than on Cu(110) and Cu(100) surfaces, which makes it highly reactive toward the propylene double bond; thus, the 111 facet had the greatest PO activity. Our results demonstrate that the selectivity of PO is inversely proportional to its activity on copper catalysts, revealing the essential role of the Cu surface structure in catalytic performance, and providing insights into the mechanism of propylene oxidation over Cu-based catalysts.