  The field of single atom catalysis (SAC) has expanded greatly in recent
years. While there has been much success developing new synthesis methods, a
fundamental disconnect exists between most experiments and the theoretical
computations used to model them. The real catalysts are based on powder
supports, which inevitably contain a multitude of different facets, different
surface sites, defects, hydroxyl groups, and other contaminants due to the
environment. This makes it extremely difficult to determine the structure of
the active SAC site using current techniques. To be tractable, computations
aimed at modelling SAC utilize periodic boundary conditions and low index
facets of an idealized support. Thus the reaction barriers and mechanisms
determined computationally represent, at best, a plausibility argument, and
there is a strong chance that some critical aspect is omitted. One way to
better understand what is plausible is by experimental modelling, i.e.
comparing the results of computations to experiments based on precisely defined
single-crystalline supports prepared in an ultrahigh vacuum (UHV) environment.
In this article, we review the status of the surface science literature as it
pertains to SAC. We focus on experimental work on supports where the site of
the metal atom are unambiguously determined from experiment, in particular the
surfaces of rutile and anatase TiO2, the iron oxides Fe2O3 and Fe3O4, as well
as CeO2 and MgO. Much of this work is based on scanning probe microscopy in
conjunction with spectroscopy, and we highlight the remarkably few studies in
which metal atoms are stable on low index surfaces of typical supports. In a
perspective, we discuss the possibility for expanding such studies into other
relevant supports.
