Tuning the Electronic and Chemical Properties of Bimetallic Surfaces
Professor Jingguang G. Chen
Center for Catalytic Science and Technology (CCST)
Departments of Chemical Engineering
jgchen@udel.edu
It is well
known that bimetallic surfaces often show novel properties that are not present
on either of the parent metal surfaces. However, it is difficult to know a priori how the chemical properties of
a particular bimetallic surface will be modified relative to the parent metals.
There are two critical factors that contribute to the modification of the
chemical properties of a metal in a bimetallic surface. First, the geometry of
the bimetallic structure is typically different from that of the parent metals,
e.g. the average metal-metal bond lengths change. This gives rise to strain
effects that are known to modify the electronic structure of the metal through
changes in orbital overlap. Second, the presence of other metals around a metal
atom also changes its electronic environment, giving rise to further
modifications of its electronic structure through the ligand effect. We have
investigated the electronic and chemical properties of model bimetallic surface
structures, epitaxial monolayers and subsurface epitaxial monolayers, using a
combination of experimental and theoretical modeling to gain further insights
into these factors [1-6]. In the current presentation we will first utilize the
adsorption and desorption of hydrogen to demonstrate the correlation between
the hydrogen binding energy and the center of the d-band in various bimetallic
surfaces. We will also provide a general equation that allows one to predict
how the electronic properties, especially the d-band center, will be affected
in bimetallic systems. We will then use the hydrogenation of cyclohexene to
demonstrate the effect of weakly-bonded hydrogen on the novel low-temperature
hydrogenation activities on the bimetallic surfaces. Finally, we will use the
results from the hydrogenation of cyclohexene to demonstrate a strong
correlation between UHV studies on model bimetallic surfaces and reactor
studies on corresponding supported bimetallic catalysts.
[1] J. Am.
Chem. Soc. 124 (2002) 702
[2] J.
Catalysis, 205 (2002) 259
[3] Chemical
& Engineering News,
[4] Surface Science, 544 (2003) 295.
[5] In “Nanotechnology in Catalysis”, ed. by B. Zhou, S. Hermans, and G.A.
Somorjai, Kluwer Academic/Plenum,
New York, (2003) 17.
[6] J. Chem. Phys. 120 (2004) 10240