  An atomic layer deposition process is used to modify size-selected
Pt7/alumina model catalysts by Sn addition, both before and after Pt7 cluster
deposition. Surface science methods are used to probe the effects of
Sn-modification on the electronic properties, reactivity, and morphology of the
clusters. Sn addition, either before or after cluster deposition, is found to
strongly affect the binding properties of a model alkene, ethylene, changing
the number and type of binding sites, and suppressing decomposition leading to
carbon deposition and poisoning of the catalyst. Density functional theory on a
model system, Pt4Sn3/alumina, shows that the Sn and Pt atoms are mixed, forming
alloy clusters with substantial electron transfer from Sn to Pt. The presence
of Sn also makes all the thermally accessible structures closed shell, such
that ethylene binds only by {\pi}-bonding to a single Pt atom. The Sn-modified
catalysts are quite stable in repeated ethylene temperature programmed reaction
experiments, suggesting that the presence of Sn also reduces the tendency of
the sub-nano-clusters to undergo thermal sintering.
