Near-Ambient Pressure\nOxidation of Silver in the Presence\nof Steps: Electrophilic Oxygen and Sulfur Impurities

The oxidation of Ag crystal surfaces has recently triggered\nstrong\ncontroversies around the presence of sulfur impurities that may catalyze\nreactions, such as the alkene epoxidations, especially the ethylene\nepoxidation. A fundamental challenge to achieve a clear understanding\nis the variety of procedures and setups involved as well as the particular\nhistory of each sample. Especially, for the often-used X-ray photoemission\ntechnique, product detection, or photoemission peak position overlap\nare problematic. Here we investigate the oxidation of the Ag(111)\nsurface and its vicinal crystal planes simultaneously, using a curved\ncrystal sample and in situ X-ray photoelectron spectroscopy at 1 mbar\nO<sub>2</sub> near-ambient pressure conditions to further investigate\nsurface species. The curved geometry allows a straightforward comparative\nanalysis of the surface oxidation kinetics at different crystal facets,\nso as to precisely correlate the evolution of different oxygen species,\nnamely nucleophilic and electrophilic oxygen, and the buildup of sulfur\nas a function of the crystal orientation. We observed that emission\nfrom both surface and bulk oxide contributes to the characteristic\nnucleophilic oxygen core-level peak, which arises during oxygen dosing\nand rapidly saturates below temperatures of 180 °C. The electrophilic\noxygen peak appears later, growing at a slower but constant rate,\nat the expenses of the surface oxide. Electrophilic oxygen and sulfur\ncore-levels evolve in parallel in all crystal facets, although faster\nand stronger at vicinal surfaces featuring B-type steps with {111}\nmicrofacets. Our study confirms the intimate connection of the electrophilic\nspecies with the formation of adsorbed SO<sub>4</sub>, and points\nto a higher catalytic activity of B-type stepped silver surfaces for\nalkene epoxidation or methane to formaldehyde conversion.