Atomic, Molecular and Hybrid Oxygen Structures on Silver

Interactions between oxygen and silver are important in many areas of science and technology, including materials science, medical, biomedical and environmental applications, spectroscopy, photonics, and physics. In the chemical industry, identification of oxygen structures on Ag catalysts is important in the development of environmentally friendly and sustainable technologies that utilize gas-phase oxygen as the oxidizing reagent without generating byproducts. Gas-phase oxygen adsorbs on Ag atomically by breaking the O-O bond and molecularly by preserving the O-O bond. Atomic O structures have Ag-O vibrations at 240-500 cm^-1. Molecular O₂ structures have O-O vibrations at significantly higher values of 870-1150 cm^-1. In this work, we identify hybrid atomic-molecular oxygen structures, which form when one adsorbed O atom reacts with one lattice O atom on the surface or in the subsurface of Ag. Thus, these hybrid structures require dissociation of adsorbed molecular oxygen into O atoms but still possess the O-O bond. The hybrid structures have O-O vibrations at 600-810 cm^-1, intermediate between the Ag-O vibrations of atomic oxygen and the O-O vibrations of molecular oxygen. The hybrid O-O structures do not form by a recombination of two adsorbed O atoms because one of the O atoms in the hybrid structure must be embedded into the Ag lattice. The hybrid oxygen structures are metastable and, therefore, serve as active species in selective oxidation reactions on Ag catalysts.