First-principles kinetic Monte Carlo modeling of propylene epoxidation on the Ti2CuO6/Cu(111) and Cu2O(111) surfaces

Abstract

Direct propylene epoxidation by molecular oxygen is one of the most important commercial selective oxidation reactions. Although the mixed metal oxide (Ti2CuO6/Cu(111)) with highly dispersed and stabilized Cu+ has high selectivity toward propylene oxide (PO), the reaction mechanism and the physical nature of stable Cu+ involved in Ti2CuO6/Cu(111) remain unclear. To better understand the key role of stabilized Cu+ active sites, the systematic kinetic Monte Carlo (kMC) studies of the propylene epoxidation reaction on Ti2CuO6/Cu(111) and Cu2O(111) surfaces are carried out. It was found that O2 is weakly bound to Ti2CuO6/Cu(111) due to the less active of Cu+ ions compared to that of Cu+ ions in Cu2O(111), which can exist stably. The current simulation demonstrates that the relatively stable Ti2CuO6/Cu(111) mixed oxide exhibits better PO selectivity and activity than Cu2O(111). The reasons for the different catalytic performance are revealed: For the primary chemistry of propylene epoxidation mechanism, Cu2O(111) favors the first allylic hydrogen stripping, while Ti2CuO6/Cu(111) has an excellent open-shell electronic structure character, which makes it more reactive toward the propylene double bond to form the oxametallacycle (OOMMP or OMMP). For the secondary chemistry of propylene epoxidation mechanism, the energy barrier of PO formation is smaller than that of propanal(PA)/acetone(AC) due to the larger difference in bonding affinity toward the O and C atoms. In contrast, the opposite trend holds for Cu2O(111). In addition, the surface species can be balanced quickly on Ti2CuO6/Cu(111). The catalyst surface is not poisoned because the surface species coverage is moderate, which means that Ti2CuO6/Cu(111) remains highly stable during the reaction processes. The Cu2O(111) has fewer active sites due to higher surface species coverage (i.e., catalyst poisoning). One may rationally design an efficient propylene epoxidation catalyst based on the present work, i.e., doping Cu into the stable spinels or perovskites such as Cu-modified LaCoO3 perovskites (LaCoxCu1−xO3−δ) may show the similar catalytic properties like that of Ti2CuO6/Cu(111).