Speaker
Description
At temperatures of 220-280 °C silver catalyzes the conversion of ethylene and oxygen gas into ethylene oxide (EO), and with the addition of promoters it does so with high epoxide selectivity. Why and how silver has the unique ability to catalyze this reaction with such efficiency has been a subject of studies for decades. Under reaction conditions silver has been shown to be covered by two broad classes of oxygen: nucleophilic (O$_{nuc}$) and electrophilic (O$_{elec}$) oxygen, which can be distinguished by their O1s binding energies (BE) of ca. 528 eV and 530 eV, respectively (1, 2). It has been shown that there is a correlation between the surface concentration of O$_{elec}$ and the amount of EO produced (3, 4) and that this Oelec species with a BE of ca. 530 eV is the oxygen in adsorbed O-SO$_{3}$, O-SO$_{3,ads}$, on the unreconstructed silver surface (5). During ethylene epoxidation, the formation of the selective O$_{elec}$ requires high near-surface oxygen concentrations (5, 6, 7), which are difficult to achieve during direct epoxidation of propylene to propylene oxide (PO) under O$_{2}$. Thus, the lack of O$_{elec}$ has been thought to be a limitation in the direct epoxidation of propylene on silver (8). It has been recently demonstrated that O-SO$_{3,ads}$ is not spontaneously formed under propylene epoxidation conditions unless high O2:C$_{3}$H$_{6}$ ratios are used. Externally dosing SOx species however, was shown to result in an increase in PO selectivity (7). The absence of spontaneous O$_{elec}$ formation under direct propylene epoxidation may be due to the low near-surface oxygen concentrations which prevents the dissolution of oxygen into silver. Here, using near ambient pressure X-Ray photoelectron spectroscopy (NAP-XPS) combined with density functional theory (DFT), we provide the characterization of the dissolved and sub-surface SO$_{x}$ species in silver that act as a reservoir for O$_{elec}$, the species active in EO production.
References
1. T. C. R. Rocha et al., Physical Chemistry Chemical Physics 14, 4554-456 (2012).
2. T. E. Jones et al., J Phys Chem C 120, 28630-28638 (2016).
3. V. I. Bukhtiyarov et al., J Catal 238, 260-269 (2006).
4. T. C. R. Rocha et al., J Catal 312, 12-16 (2014).
5. T. E. Jones et al., ACS Catal 8, 3844-3852 (2018).
6. V. I. Bukhtiyarov et al., Surf Sci 320, L47-L50 (1994).
7. E.A. Carbonio et al., (Submitted).
8. A. Palermo et al., J Catal 207, 331-340 (2002).
| Abstract Number (department-wise) | AC 2.4 |
|---|---|
| Department | AC (Schlögl) |
