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

Abstract

The oxidation of Ag crystal surfaces has recently triggered strong controversies around the presence of sulfur impurities that may catalyze reactions, such as the alkene epoxidations, especially the ethylene epoxidation. A fundamental challenge to achieve a clear understanding is the variety of procedures and setups involved as well as the particular history of each sample. Especially, for the often-used X-ray photoemission technique, product detection, or photoemission peak position overlap are problematic. Here we investigate the oxidation of the Ag(111) surface and its vicinal crystal planes simultaneously, using a curved crystal sample and in situ X-ray photoelectron spectroscopy at 1 mbar O2 near-ambient pressure conditions to further investigate surface species. The curved geometry allows a straightforward comparative analysis of the surface oxidation kinetics at different crystal facets, so as to precisely correlate the evolution of different oxygen species, namely nucleophilic and electrophilic oxygen, and the buildup of sulfur as a function of the crystal orientation. We observed that emission from both surface and bulk oxide contributes to the characteristic nucleophilic oxygen core-level peak, which arises during oxygen dosing and rapidly saturates below temperatures of 180 °C. The electrophilic oxygen peak appears later, growing at a slower but constant rate, at the expenses of the surface oxide. Electrophilic oxygen and sulfur core-levels evolve in parallel in all crystal facets, although faster and stronger at vicinal surfaces featuring B-type steps with {111} microfacets. Our study confirms the intimate connection of the electrophilic species with the formation of adsorbed SO4, and points to a higher catalytic activity of B-type stepped silver surfaces for alkene epoxidation or methane to formaldehyde conversion.