Bringing Molecules Together: Synergistic Coadsorption at Dopant Sites of Single Atom Alloys

Bringing molecules together on a catalytic surface is
a prerequisite
for bimolecular and recombination reactions. However, in the absence
of attractive interactions between reactants, such as hydrogen bonds,
this poses a challenge. In contrast, based on density functional theory,
we show that coadsorption at active sites of single-atom alloys (SAAs)
is favored and that coadsorption is a general phenomenon observed
for catalytically relevant adsorbates on a broad range of SAAs under
temperature and pressure conditions commonly employed for catalysis.
Dopants located in both terrace sites and in step edge defects exhibit
a preference for coadsorption, displaying similar periodic trends.
Using kinetic Monte Carlo simulations, we compare the reactivity of
a model reaction on both a pure metal and an SAA and show that the
preference for coadsorption significantly alters the overall reaction
energy profile, even when the barriers for the rate-determining elementary
step are identical. In our models, the coadsorption preference enhances
the catalytic activity of the SAA surface by several orders of magnitude
compared to the pure metal. We also report infrared (IR) spectroscopic
signatures of coadsorption, which facilitate experimental detection.
Analysis reveals that in these systems repulsive lateral interactions
between nearby molecules are more than compensated for by the enhanced
binding at dopant sites. Among the broad range of systems considered,
SAAs containing early transition metals (TMs) exhibit the strongest
coadsorption preference, which can be rationalized by assuming the
existence of an optimal number of electrons involved in binding. The
strong coadsorption preference, together with facile product desorption
from early TMs, renders these systems attractive candidates for catalysis.
Moreover, these SAAs could open new routes for reduction reactions
because coadsorption with hydrogen is favored.