Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

A Single-Molecule Approach for Understanding and Utilizing Surface and
Subsurface Adsorption to Control Chemical Reactivity and Selectivity
E. Charles H. Sykes (charles.sykes@tufts.edu)
Department of Chemistry, Tufts University, 62 Talbot Ave, Medford, MA 02155
The second year research program has been focused on understanding the ability of single
palladium atoms in and under the surface of industrially important alloys to promote
hydrogenation reactions and using novel surface supported cobalt nanoparticles to understand
cobalt’s surface and subsurface chemistry [1-5]. Well-defined Pd, Co, Au and Cu systems are
designed to be amenable to high resolution scanning probe studies, X-ray photoelectron
spectroscopy, and chemical analysis of adsorbate binding, diffusion and reaction. In this way we
can relate the atomic scale structure of these catalytically important systems to their electronic
properties and surface chemistry. In terms of equipment, a state of the art temperature
programmed reaction system built in year one is enabling us to relate the activity and selectivity
of hydrogenation reactions on these model systems to their atomic scale structure.
Single Atom Alloys as a Strategy for Selective Heterogeneous Hydrogenations
Hydrogenation reactions are central to the petrochemical, fine chemical, pharmaceutical, and
food industries and are of increasing interest in energy production and storage technologies.
Typical heterogeneous catalysts often involve noble metals and alloys based on platinum,
palladium, rhodium and ruthenium. While these metals are active at modest temperature and
pressure, they are not always completely selective and are expensive. Facile dissociation of
reactants and weak binding of intermediates are key requirements for efficient and selective
catalysis. However, these two variables are intimately linked in a way that does not generally
allow the optimization of both properties simultaneously.
We are interested in how the reactivity of a catalytically active metal is altered when it is
atomically dispersed in a more inert host metal. In a series of low temperature scanning tunneling
microscopy (LT STM), temperature programmed desorption/reaction (TPD/R) studies, we have
investigated the formation of a class of bimetallic alloy systems, which we term single atom
alloys, and their interaction with hydrogen (Figure 1). Two key characteristics of single atom
alloys are: (i) one of the two components (in our case Pd) is present in the surface of the host
metal at very low concentrations (0.01 monolayers) and (ii) the Pd atoms are thermodynamically
more stable when surrounded by atoms of the host surface (i.e. no Pd dimers or trimers are
present). Using desorption measurements we demonstrate that single atom alloys can act as very
selective hydrogenation model catalysts. The mode of action involves the facile dissociation of
hydrogen on individual Pd atoms and subsequent spillover of H atoms onto the Cu(111) surface.
The selective hydrogenation of alkenes and alkynes takes place on the bare Cu(111) surface
where the H atoms are weakly bound [1].
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