December 18, 2002
A Catalyst in Action: Structure and Bonding of Supported Iridium Nanoclusters During Propene Hydrogenation Catalysis Determined by EXAFS and Infrared Spectroscopies
A.M. Argo, J.F. Odzak, and B.C. Gates
Department of Chemical Engineering and Materials Science, University of California, Davis, CA
The structures of catalysts – ranging from molecular complexes and enzymes to clusters dispersed on supports – depend on the conditions under which they are used. So, the most incisive determinations of catalyst structures require techniques characterizing functioning catalysts. Scientists at the University of California, Davis, have used X-ray absorption spectroscopy in combination with infrared spectroscopy to investigate supported metal cluster catalysts as they functioned, revealing the structure of the catalyst, consisting of four iridium atoms (Ir4), and the roles of the ligands, including the support. This is the first characterization of a solid catalyst, elucidating the interplay of the support and reactant-derived ligands through their bonding with Ir4.
Metals in catalysts used in petroleum refining or automobile
exhaust conversion usually are in the form of clusters, each a few
billionths of a meter in size, attached to a support. Because the
clusters are so small, a large fraction of the atoms on the cluster
surface is exposed, thus increasing the rates of chemical reactions.
But such cluster-based catalysts are not well understood because
they are made of particles of varying sizes. Scientists have
developed structurally simple models, usually of single crystals,
but such models do not provide information about the support’s role
in a chemical reaction.
We prepared nearly uniform metal nanoclusters on supports, identified the catalytic clusters and their interactions with the support by using extended x-ray absorption fine structure (EXAFS) spectroscopy, and identified the reactant-derived ligands by applying infrared spectroscopy. The EXAFS spectroscopy techniques were developed at beamline X11A of Brookhaven National Laboratory’s National Synchrotron Light Source.
We investigated the reaction of propene with molecular hydrogen (H2) to form propane, catalyzed by clusters consisting of four iridium atoms (Ir4) dispersed on porous у-Al2O3 or MgO. The catalytic activity of Ir4 depends on the support. Replacement of the MgO support with у-Al2O3S boosts the catalytic activity tenfold.
When the catalyst attaches to propene or H2 alone, we observed no significant changes in the iridium-iridium or iridium-oxygen distances. But during a catalytic reaction, the iridium-iridium and the longer, non-bonding, iridium-oxygen distances exceeded the corresponding distances observed under non-catalytic conditions. We attribute the distance elongations to reaction intermediates bonded to Ir4.
The two supports have different characteristics that affect the electronic properties of the clusters, which in turn affect the interactions of the clusters with the ligands formed from the reactants. So, the support influences the reactivities of the reactant-derived ligands – determining which ones are observed during catalysis – and thus affects the rate of the catalytic reaction.
Our observations are consistent with the reaction mechanism shown schematically in the figure. In addition to electronic effects caused by the support, geometric properties of the clusters distinguish them from other metal catalysts. The smallness of the clusters limits the structures that can be bonded to them, thereby affecting the reactivity.
The electronic and geometric effects of the ligands are analogous to those known in molecular and enzymatic catalysis. Our results bolster theoretical cluster models that incorporate the support, and indicate new opportunities for tuning the catalytic properties of small-supported clusters by changing the nature of the support. None of the understanding emerging from this work would have been possible without the use X-ray absorption spectroscopy and its application to working catalysts.
BEAMLINE
X11A
FUNDING
U.S. Department of Energy
National Science Foundation
PUBLICATION
"Observation of Ligand Effects during Alkene Hydrogenation on
Supported Metal Clusters," A. M. Argo, J. F. Odzak, F. S. Lai, and
B. C. Gates, Nature, 415, 623 (2002).
FOR MORE INFORMATION
Bruce Gates
Department of Chemical Engineering and Materials Science
University of California
Davis, CA
Email: bcgates@ucdavis.edu
Web: http://www.chms.ucdavis.edu/faculty/gates.php