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A common expectation in heterogeneous
catalysis is that the optimal
activity will occur for the particle size with the highest concentration
of undercoordinated step, edge, or corner sites, expectedly in the
<5 nm range. However, many metal-catalyzed reactions follow a different
trend, where the turnover frequency (TOF, here rate per surface atom)
is instead lower for these smaller particles and increases strongly
with increasing size toward a stabilized level with a size-independent
TOF. Here, we use one of these reactions, the Rh-catalyzed CO hydrogenation
to hydrocarbons and C2-oxygenates, to illuminate the origin
of this effect. Studying Rh/SiO2 catalysts, we show that
smaller (<4 nm) Rh particles are richer in undercoordinated edge,
corner, and step sites, but are nevertheless of lower activity because
the entire surface, including the planar facets, is shifted to a prohibitively
high adsorbate coveragein this case of CO. In transient experiments,
where the inhibiting adsorbates are allowed to desorb, smaller 1.7
nm Rh particles and larger 3.7 nm Rh particles reach similar rates
of CO activation despite the steady-state TOF being an order of magnitude
higher on the larger particles. This shows that it is a prohibitive
adsorbate coverage under reaction conditions rather than a lower number
of active sites or a lower intrinsic activity of the sites that causes
the lower activity of the smaller particles. In steady-state experiments
at 20 bar, the TOF for CO hydrogenation increases by 55% from 3.7
nm Rh particles to 5.3 nm Rh particles even though the measured concentration
of step sites decreases by 30% in this size range. This indicates
that such undercoordinated sites are not necessarily the primary active
centers and that the reaction is instead focused on the planar facets.
The reaction kinetics show that the reaction becomes increasingly
pressure-dependent with increasing particle size, implying that the
surface becomes increasingly free of adsorbates on larger particles.
Taken together with the indications that the reaction may be focused
on the planar facets, this leads to the new insight that it is a prohibitively
high adsorbate coverage on the entire surface (and not just on a minority
of undercoordinated sites) that is the primary reason for the low
activity of small nanoparticles. The identification of a detrimental
high-coverage state for small particles is expected to be of general
relevance to the many industrially important reactions sharing the
same behavior. The high-coverage state is not exclusively negative,
but can also facilitate different reaction pathways. It is the higher
CO coverage on small particles that drives the C2-oxygenate
formation and is the reason for the high selectivity of rhodium to
such complex products, which is at its highest for the smallest (∼2
nm) investigated particles.
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