High-pressure STM of the interaction of oxygen with Ag(111)

Abstract

To identify surface phases that could play a role for the epoxidation of ethylene on Ag catalysts we have studied the interaction of Ag(111) with O(2) at elevated pressures. Experiments were performed using high-pressure scanning tunneling microscopy (STM) at temperatures between 450 and 480 K and O(2) pressures in the mbar range. Below p(O(2)) approximately 1 mbar the surface largely showed the structure of bare Ag(111). At p(O(2)) above approximately 1 mbar the (4 x 4)O structure and the closely related (4 x 5 radical 3)rect structure were observed. The findings confirm theoretical predictions that the (4 x 4)O structure is thermodynamically stable at the oxygen partial pressure of the industrial ethylene oxide synthesis. However, in other experiments only a rough, disordered structure was observed. The difference is caused by the chemical state of the STM cell that depends on the pretreatment and on previous experiments. The surface was further analyzed by X-ray photoelectron spectroscopy (XPS). Although these measurements were performed after sample transfer to ultra-high vacuum (UHV), so that the surface composition was modified, the two surface states could still be identified by the presence of carbonate or a carbonaceous species, and by the absence or presence of a high-binding energy oxygen species, respectively. It turns out that the (4 x 4)O structure only forms under extremely clean conditions, indicating that the (4 x 4)O phase and similar oxygen-induced reconstructions of the Ag(111) surface are chemically unstable. Chemical reactions at the inner surfaces of the STM cell also complicate the detection of the catalytic formation of ethylene oxide.

Oxidation of Ethylene on Oxygen Reconstructed Silver Surfaces

“…Furthermore, there is scarce evidence that a measurable amount of the more ionic type of oxygen modeled by DFT is present under epoxidation conditions [15,36] because it forms surface reconstructions at oxygen chemical potentials relevant for ethylene epoxidation [10,19,21].…”

“…Thermodynamic considerations are often used to rule out the involvement of Ag 2 O, as the bulk oxide is not stable under epoxidation conditions 9]. Instead, at the oxygen chemical potentials required for epoxidation surface reconstructions with Ag δ+ sites available for ethylene adsorption are formed [6,10,11]. Such structures have been observed directly by scanning tunneling microscopy (STM) [10,11] and the presence of chemically similar oxygen species under near ambient pressure conditions can also be inferred by in situ X-ray photoelectron and near edge X-ray absorption spectroscopies (XPS and NEXAFS) [12][13][14][15].…”

Thermally Selective Formation of Subsurface Oxygen in Ag(111) and Consequent Surface Structure

“…In 2006, a model based on two Ag hexamer triangles separated by furrows containing oxygen atoms was proposed by the groups of Besenbacher (9) and Varga (31), as illustrated in Figure 1. Further studies over the past decade have further validated this model (8,39). Two other surface reconstructions, the c(3×5√3) and p(4×5√3), are minor structural rearrangements of the p(4×4) structure and have been observed with scanning tunneling microscopy (STM) to coexist on oxidized Ag(111) (9,32).…”

“…Oxygen induced reconstructions of Ag(111) surfaces have been widely studied and debated for many years (8). Ag(111) surfaces exhibit a variety of different surface reconstructions induced by adsorbed oxygen (O ad ) (5,9), along with several other oxide species (10)(11)(12)(13)(14)(15)(16).…”

Selective Catalytic Ammonia Oxidation to Nitrogen by Atomic Oxygen Species on Ag(111)

“…We believe that the current findings may also provide valuable insights regarding the nature of oxygen on Ag(111) which can also possibly be extrapolated to other crucial heterogeneous catalytic processes such as partial oxidation reactions of ethylene and methanol. 27,57,71 ■ EXPERIMENTAL SECTION Experiments were performed in a custom-made UHV chamber with a base pressure of 2 × 10 −10 Torr which is equipped with X-ray photoemission spectroscopy (XPS; Riber Mg/Al dual anode with a Riber EA150 electron energy analyzer), lowenergy electron diffraction (LEED; custom-made), infrared reflection absorption spectroscopy (IRAS; Bruker Tensor 37 with custom-design external IR optics and a tailor-made N 2 purge box, 4 cm −1 resolution, 100 scans, 10 kHz scanner velocity, mercury cadmium telluride (MCT) detector), temperature-programmed desorption (TPD), and temperature-programmed reaction spectroscopy (TPRS) capabilities. A quadrupole mass spectrometer (QMS; Ametek Dycor Dymaxion DM200) and a proportional−integral−derivative (PID)-controlled linear sample heater (Heatwave, model 101303) were used for the TPD/TPRS experiments.…”