Anisotropic Reactivity of CaAg under Ethylene Epoxidation Conditions

Abstract

The chemical behavior of CaAg as catalyst for ethylene epoxidation was studied using a combination of experimental (X-ray powder diffraction, scanning electron microscopy, thermal analysis and infrared spectroscopy), and quantum chemical techniques as well as real-space chemical bonding analysis. Under oxidative ethylene epoxidation conditions, the CaAg (010) surface possesses an outstanding stability during long-term experiments. It is caused by the formation of an ordered, stable and dense CaO passivation layer with a small amount of embedded Ag atoms. On this way, the (010) surface constitutes a kinetic barrier for further incorporation of oxygen into the subsurface region and thereby prevents further oxidative decomposition of CaAg. The calculated adsorption energies of the reaction species show strong adsorption of the reaction products that may explain the observed low conversion of ethylene toward ethylene oxide using CaAg as catalyst.

Ca–Ag compounds in ethylene epoxidation reaction

“…In terms of cleavage energies, the Ag-rich compounds behave differently from others (Table 1). Two [34]. On the other hand, in CaAg, Ca 5 Ag 3 , and Ca 3 Ag there are clearly preferred cleavage planes implying anisotropic behaviour for these compounds.…”

“…They crystallize with unique crystal structures ( Figure 1, [59] with a 3D framework of planar honeycomb nets of Ag atoms (Ag-Ag contacts in orange, Figure 1), linked via longer Ag-Ag contacts (in green, Figure 1) along [010]. Calcium atoms are located in the cavities of this atomic arrangement [34]. In the equiatomic compound CaAg (α-TlI type), the zig-zag chains of Ag atoms along [001] are separated by Ca atoms [35,59,60].…”

“…Calcium atoms are located in the cavities of this atomic arrangement [34]. In the equiatomic compound CaAg (α-TlI type), the zig-zag chains of Ag atoms along [001] are separated by Ca atoms [35,59,60]. The silver atoms are surrounded by seven calcium and two silver atoms in the form of trigonal prisms with three additional vertices in front of side faces (CN = 9).…”

“…Apart from CaAg, all Ca-Ag binaries have a Caterminated surface as the preferred surface in Ca-rich environments (covering a wider range of µ Ag values) and either Ag-or mixed-terminated surfaces in Ag-rich environments. The surfaces with the lowest surface energies calculated for Ca 2 Ag 7 , Ca 5 Ag 3 and Ca 3 Ag are shown in Figure 6, whereas such information for CaAg 2 and CaAg compounds was presented elsewhere [34,35].…”

“…In the context of ethylene epoxidation process, our previous studies clearly show that the reactivity of CaAg 2 and CaAg to oxygen strongly depends on the crystal structure and chemical bonding features [34,35]. Here, a comprehensive overview of the reactivity of the binary compounds in the Ca-Ag system under ethylene epoxidation conditions is presented.…”