[Introduction] Silver (Ag) is an established industrial catalyst to produce ethylene oxide (EO) through direct oxidation of ethylene with gaseous oxygen. Ethylene epoxidation occurs through insertion of an oxygen atom into a π-bond of ethylene on Ag surfaces. In previous studies, it was revealed that atomic oxygen species at Ag surfaces extract hydrogen from ethylene resulting in full oxidation to carbon dioxide (CO₂) and water (H₂O). On the other hand, covalent oxygen species such as a sulfate (SO₄) was proposed to provide an oxygen atom to the p-bond of ethylene to produce EO [1]. Recently, a carbonate (CO₃) species was also suggested as a candidate of such oxygen-donor species [2]. In this study, we report new insights into behavior of the covalent oxygen species contributing to ethylene epoxidation on Ag(110) obtained from in situ measurements of near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS).

[Methods] We measured the surface species on a Ag(110) model catalyst under ethylene epoxidation conditions (total pressure, P = 0.2 Torr or 1 Torr, sample temperature, T = 370 ~ 520 K) using NAP-XPS and PM-IRRAS. The catalytic activity was also monitored using a quadrupole mass spectrometer (QMS) at the same time. NAP-XPS measurements were conducted at BL-13B of the Photon Factory (PF) at High Energy Accelerator Research Organization (KEK) [3]. In order to obtain information from the outermost surfaces, we tuned the incident X-ray energy as the kinetic energy of photoelectron to be about 100 eV. Second, PM-IRRAS measurements were carried out with a FTIR spectrometer (Nicolet: iS50) and a photo elastic modulator (Hinds: PEM-100). All PM-IRRA spectra were measured with 500 scans at a resolution of 4 cm^-1. To distinguish EO (m/z = 44) from CO₂ (m/z = 44) by QMS, we used deuterated ethylene (C₂D₄) instead of normal ethylene (C₂H₄), where deuterated EO (C₂D₄O) is detected as m/z = 48.

[Results and Discussion] NAP-XP spectra of O 1s, C 1s and S 2p regions for Ag (110) surfaces in the presence of reactant gases (0.16 Torr O₂ and 0.04 Torr C₂D₄) at 470 K indicate formation of CO₃ and SO₄ (Figure (a) and (b)) accompanied by oxidation of Ag atoms. In this case, the CO₃ species is formed from a reaction between O₂ and C₂D₄ on the Ag surface, on the other hand, the sulfur (S) of SO₄ is assumed to segregate from the Ag bulk followed by oxidation. To obtain further information, we measured PM-IRRA spectra simultaneously with catalytic activity using QMS where production of EO and CO₂ (total oxidation product) were clearly detected above 450 K. Under this reaction condition, PM-IRRA spectra exhibit peaks associated with the CO₃ and SO₄, which is consistent with the XPS results. In situ measurements to investigate correlation between behavior of the IRRAS peaks and the EO production were conducted as a function of temperature, which showed clear correlation. Detailed analyses results on the in situ observations are presented in the talk.

[References] [1] (a) T. E. Jones et al. ACS Catal. 2015, 5, 5846. (b) T. E. Jones et al. ACS Catal. 2018, 8, 3844. [2] K. Isegawa et al. J. Phys. Chem. C 2021, 125, 17, 9032. [3] R. Toyoshima et al. J. Phys.: Condens. Matter. 2015, 27, 083003.