  Catalysis has entered everyday life through a number of technological
processes relying on different catalytic systems. The increasing demand for
such systems requires rationalization of the use of their expensive components,
like noble metal catalysts. As such, a catalyst with low noble metal
concentration, in which each one of the noble atoms is active, would reach the
lowest price possible. Nevertheless, there are no reactivity descriptors
outlined for this type of low coordinated supported atoms. Using DFT
calculations, we consider three diverse systems as models of single atom
catalysts. We investigate monomers and bimetallic dimers of Ru, Rh, Pd, Ir and
Pt on MgO(001), Cu adatom on thin Mo(001)-supported films (NaF, MgO and ScN)
and single Pt adatoms on oxidized graphene surfaces. Reactivity of these metal
atoms was probed by CO. In each case we see the interaction through the
donation-backdonation mechanism. In some cases the CO adsorption energies can
be linked to the position of the d-band center and the charge of the adatom.
Higher positioned d-band center and less charged supported single atoms bind CO
weaker. Also, in some cases metal atoms less strongly bonded to the substrate
bind CO more strongly. The results suggest that the identification of common
activity descriptor(s) for single metal atoms on foreign supports is a
difficult task with no unique solution. However, it is also suggested that the
stability of adatoms and strong anchoring to the support are prerequisites for
the application of descriptor-based search for novel single atom catalysts.
