  Density functional theory based global geometry optimization has been used to
demonstrate the crucial influence of the geometry of the catalytic cluster on
the energy barriers for the CO oxidation reaction over Pd-based bimetallic
nanoalloys. We show that dramatic geometry change between the reaction
intermediates can lead to very high energy barriers and thus be prohibitive for
the whole process. This introduces challenges for both the design of new
catalysts, and theoretical methods employed. On the theory side, a careful
choice of geometric configurations of all reaction intermediates is crucial for
an adequate description of a possible reaction path. From the point of view of
the catalyst design, the cluster geometry can be controlled by adjusting the
level of interaction between the cluster and the dopant metal, as well as
between the adsorbate molecules and the catalyst cluster by mixing different
metals in a single nanoalloy particle. We show that substitution of a Pd atom
in the Pd$_{5}$ cluster with a single Ag atom to form Pd$_{4}$Ag$_{1}$ leads to
a potential improvement of the catalytic properties of the cluster for the CO
oxidation reaction. On the other hand, a single Au atom does not enhance the
properties of the catalyst, which is attributed to a weaker hybridization
between the cluster's constituent metals and the adsorbate molecules. Such
flexibility of properties of bimetallic nanoalloy clusters illustrates the
possibility of fine-tuning, which might be used for design of novel efficient
catalytic materials.
