Locating Catalytically Active Oxygen on Ag(1 1 1)—A Spectromicroscopy Study

Abstract

The loading of an Ag(1 1 1) sample with oxygen was monitored by in situ low‐energy electron microscopy and X‐ray photoemission electron microscopy during NO2 dosing at T≥480 K. At first, adsorbed oxygen populates the Ag(1 1 1) surface, which initiates the (4×4) reconstruction leading to the characteristic O 1s core level at 528.30 eV. The formation of this phase proceeds on a mesoscopic length scale by traveling fronts separating reconstructed from non‐reconstructed surface areas. Continued NO2 dosing leads to the accumulation of a new oxygen species mainly at steps and step bunches. Characterized by an O 1s peak with two components at 530.20 eV and 530.75 eV, this species represents the active oxygen during the ethylene epoxidation reaction over Ag. The 530.20 eV component is attributed to surface oxygen, the 530.75 eV species to subsurface oxygen. This inhomogeneous accumulation of the active oxygen occurs at a very low rate. However, the preparation route can be changed, which strongly accelerates the population of the catalytically active oxygen species and leads to a homogeneous distribution of oxygen on the surface. This route involves the complete formation of the O(4×4) reconstruction by NO2 dosing, followed by a complete de‐reconstruction of the surface by desorption of the oxygen adlayer. The faster population kinetics is related to the Ag adatom transport during such a reconstruction/de‐reconstruction cycle.

Oxidation of Epitaxial Silicene on Ag(111)

“…The AgO peak can be fitted with only one component at a binding energy close to polycrystalline Ag 2 O (529.2–529.4 eV) . Similar peak position can also be found for the well‐known p‐(4 × 4)‐O/Ag(111) reconstruction (528.2 eV) and for active oxygen surface states on Ag(111) (530 eV) . Fitting of the SiO contribution reveals two components found at 531.9 and 532.7 eV, respectively.…”

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

“…Signals in the region between 530 and 531 eV were assigned to atomic oxygen either on the surface (O a ) or in the subsurface (O sub ), whereas surface oxide (O ox ) signals were found between 528.1 and 528.5 eV. [26][27][28][29][30][38][39][40]44,50,57 As mentioned above, ordered surface oxide (O ox ) species were not detectable at 140 K according to LEED experiments. Schlogl and co-workers previously reported that the O 1s signal of bulk silver oxide appeared at 529.0 eV.…”

“…24,44 Thus, in most of the former studies, high-pressure O 2 exposures were used to investigate the nature of oxygen on a Ag(111) model catalyst surface. [25][26][27][28][29]32 Other methods were also utilized for atomic oxygen delivery onto the Ag(111) surface under UHV conditions, such as NO 2 decomposition 31,36,38,40,47,50 and thermal gas cracking. 41,42,45 Ozone decomposition is an extremely efficient method for dosing high concentrations of oxygen atoms on metal surfaces under UHV conditions (i.e., in the absence of elevated-pressure exposures).…”

Desorption kinetics from a surface derived from direct imaging of the adsorbate layer

“…An alternative explanation could be an impurity at the Ag(111) surface, which binds oxygen ARTICLE more strongly by somehow modifying the electronic structure of the surface, given this impurity remains below spectroscopic detection limits. However, we found a way to greatly accumulate the postulated oxygen subsurface species, so that it became spectroscopically detectable 30 , whereas other impurities were still absent. Hence, although we initially considered foreign elements and intensively searched for such a possibility 31 , we now consider it as highly unlikely.…”