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.