Increased Back-Bonding Explains Step-Edge Reactivity and Particle Size Effect for CO Activation on Ru Nanoparticles

Carbon monoxide is
a ubiquitous molecule, a key feedstock and intermediate
in chemical processes. Its adsorption and activation, typically carried
out on metallic nanoparticles (NPs), are strongly dependent on the
particle size. In particular, small NPs, which in principle contain
more corner and step-edge atoms, are surprisingly less reactive than
larger ones. Hereby, first-principles calculations on explicit Ru
NP models (1–2 nm) show that both small and large NPs can present
step-edge sites (e.g., B5 and B6 sites). However,
such sites display strong particle-size-dependent reactivity because
of very subtle differences in local chemical bonding. State-of-the-art
crystal orbital Hamilton population analysis allows a detailed molecular
orbital picture of adsorbed CO on step-edges, which can be classified
as flat (η1 coordination) and concave (η2 coordination) sites. Our analysis
shows that the CO π-metal dπ hybrid band responsible for the electron back-donation is better
represented by an oxygen lone pair on flat sites, whereas it is delocalized
on both C and O atoms on concave sites, increasing the back-bonding
on these sites compared to flat step-edges or low-index surface sites.
The bonding analysis also rationalizes why CO cleavage is easier on
step-edge sites of large NPs compared to small ones irrespective of
the site geometry. The lower reactivity of small NPs is due to the
smaller extent of the Ru–O interaction in the η2 adsorption mode, which destabilizes the η2 transition-state
structure for CO direct cleavage. Our findings provide a molecular
understanding of the reactivity of CO on NPs, which is consistent
with the observed particle size effect.