My
research interest is nanoscale materials design and synthesis
for catalytic applications. The design and synthesis is then
complemented with reactivity studies and in-situ characterization
of the catalytic material. In particular, X-ray absorption spectroscopy
(XAS) experiments are periodically conducted at the National
Synchrotron Light Source at Brookhaven National Laboratory.
The combination of design, synthesis, and characterization allows
for the direct translation of variations in the material at
the atomic/molecular scale to expressed macroscopic properties
such as improved reactivity. Two areas of current research projects
involving nanoscale design and synthesis of catalytic materials
are pseudomorphic overlayer and encaged heteropolyacid catalysts.
Pseudomorphic
Overlayer Catalysts
Supported
metal catalysts have been the mainstay of heterogeneous catalysts
for many years. These catalysts are widely used since they combine
the desirable catalytic properties of the metal with the enhanced
activity resulting from their support on a high surface area
material. More complicated reactions often use bimetallic catalysts.
The drawback of bimetallic catalysts is that they are often
a simple alloy resulting from an equilibrated mixture of the
two metals. In particular, the bimetallic catalysts usually
have both elements randomly distributed throughout the particle
which therefore contains no atomic uniformity in the arrangement
of the two elements. Design of supported bimetallic catalysts
by synthesizing non-equilibrated arrangements could lead to
new compositions of matter and ultimately to more efficient
catalysts.
The
importance of materials design at the atomic level is demonstrated
through the use of computational quantum-chemical techniques.
It has been shown theoretically that pseudomorphic overlayers
of a metal on top of a different metal may provide lower intrinsic
barriers to reaction products than pure metals. Therefore, it
is desirable to extend this concept from the theoretical to
the practical by synthesizing actual supported metal catalysts
that could have industrial applications. This is an important
fundamental step in the evolution of bimetallic catalysts for
increasingly rigorous catalytic applications. Synthesis of metallic
catalysts with pseudomorphic overlayers is being carried out
using directed synthesis techniques. Subsequent in-situ characterization
will allow determination of conditions under which the pseudomorphic
overlayer catalysts remain in the desired structure.
Encaged
Heteropolyacids Catalysts
Heteropoly
acids (HPAs) are polymeric oxoanions with well-defined primary
structure. An example is H3PMo12O40 in which P, the heteroatom, is located at the center and Mo
addenda atoms are located around the heteroatom at the center
of MoO6 octahedra. A large variety of different elements
can be substituted for the hetero and addenda atoms and charge
balancing protons. Thus, a vast array of materials can be synthesized
in this manner which makes heteropolyacids ideal candidates
as molecular building blocks for material synthesis. In particular,
heteropoly acids can function as both acid catalysts and oxidation
catalysts in either homogeneous or heterogeneous catalysis applications
and are receiving significant industrial interest. However,
the low surface area of HPAs (typically only a few square meters
per gram) limits their use industrially.
One
approach to overcome this problem involves synthesizing HPAs
inside the cages of zeolites. In this way, the HPA molecule
is constrained and cannot diffuse out of the cage under reaction
conditions. Similar to HPAs, zeolites can be modified by numerous
methods to provide a wide variety of different materials. For
example, zeolites can be synthesized with different pore sizes
which are then used to perform shape selective catalysis on
large and small molecules. Additionally, zeolites allow for
a large number of other elements to be included in the cages
either by impregnation or ion exchange and provide for precise
positioning of individual atoms at different sites in the zeolite
framework. Therefore, the combination of zeolite supports and
HPA catalysts provides multiple synergistic opportunities to
design and synthesize a large variety of different structures
with nanoscale uniformity by properly selecting the desired
attributes of each system. We are currently designing and synthesizing
these materials as a way to control uniformity for potential
applications as both acid and selective oxidation catalysts.
In this way, the nanoscale changes designed into the structures
of both the zeolite and the HPA can be investigated through
changes in the expressed macroscopic properties
Selected
Publications
-
Dillon,
C.J., Holles, J.H., Davis, R.J., Labinger, J.A., and Davis,
M.E., "A Substrate-Versatile Catalyst for the Selective
Oxidation of Light Alkanes: II. Catalyst Characterization."
Submitted to Journal of Catalysis.
-
Holles,
J.H., Dillon, C.J., Labinger, J.A., and Davis, M.E., "A
Substrate-Versatile Catalyst for the Selective Oxidation
of Light Alkanes: I. Reactivity." Submitted to Journal
of Catalysis.
-
Dillon,
C.J., Holles, J.H., Davis, M.E., and Labinger, J.A., "Heteropolyacid-based
Catalysts for Selective Alkane Oxidation: Mechanism of Formation
of Maleic Acid from Propane." Catalysis Today, in press.
-
Davis,
M.E., Dillon, C.J., Holles, J.H., and Labinger, J., "A
New Catalyst for the Selective Oxidation of Butane and Propane."
Angewandte Chemie International edition 41, 858-860
(2002).
-
Holles,
J.H., and Davis, R.J., "Structure of Pd/CeOx/Al2O3 Catalysts for NOx Reduction Determined By In
Situ X-ray Absorption Spectroscopy." Journal of Physical
Chemistry, B, 104, 9653-9660 (2000).
-
Holles,
J.H., and Davis, R.J., Murray, T.M., and Howe, J.M., "Effects
of Pd Particle Size and Ceria Loading on NO Reduction with
CO." Journal of Catalysis 195, 193-206 (2000).
-
Holles,
J.H., Switzer, M.A., and Davis, R.J., "Influence of
Ceria and Lanthana Promoters on the Kinetics of NO and N2O
Reduction by CO over Alumina Supported Palladium and Rhodium."
Journal of Catalysis 190, 247-260 (2000).
Patent
-
Brait,
A., Davis, M.E., Dillon, C.J., Holles, J.H., and Labinger,
J., "Polyoxometalate Catalysts and Catalytic Process."
Provisional Patent Application, November 2001.
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