Alumni Relations
Faculty Highlights
|
Enrique Iglesia, professor of chemical engineering
|
How small is too small? A question posed
for decades in the context of heterogeneous catalysts is being answered
by several projects in the research group of chemical engineering professor
Enrique Iglesia. His group continues to probe the structure and
catalytic function of small domains of metals and metal oxides using emerging
spectroscopy methods and increasingly sophisticated chemical probes. “Changes
in the size and structure of minuscule domains, sometimes no larger than
a few metal atoms, alter the electronic properties of these materials
and hence their catalytic function,” he said.
A recent detailed study by the Iglesia group
explored the catalytic and electronic properties of protonated tungsten
oxide domains anchored on other oxides for isomerization and dehydration
reactions. They prepared a range of catalyst samples in which tungsten
oxide structures ranged from isolated monomers to polytungstate two-dimensional
monolayers to crystalline nano-meter-sized clusters.
Tungsten surface concentrations of about
10 atoms per nm2 gave much higher catalytic rates than those with higher
or lower tungsten densities. A range of spectroscopic methods showed that
the most active structures consist of two-dimensional monolayers containing
W-O-W bonds, absent in isolated tungstate monomers and different in reactivity
from those present in crystalline tungsten oxides.
“Isolated species are accessible to the reagents,
but they are unreactive because they are too small to delocalize the charge,
which allows them to stabilize the acidic protons needed for these reactions,”
he explained. Larger nanometer-sized clusters are ineffective as catalysts
because potential active centers lie at inaccessible position within crystallites
and because these crystallites become oxygen-deficient and unable to delocalize
the required negative charge.
The work was presented as a keynote
lecture in a special symposium dealing with nanotechnology applications
in catalysts at the ACS Meeting in Philadelphia this summer. These findings
were highlighted in a feature article covering this symposium in a recent
issue of Chemical and Engineering News.
