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J. Phys. Chem. 1991,95,6603-6610 6603 Growth/Restructuring of Pd Clusters Induced by CO Adsorption

Summary: J. Phys. Chem. 1991,95,6603-6610 6603
Growth/Restructuring of Pd Clusters Induced by CO Adsorption
Scott L. Anderson,**+Takanori Mizushima, and Yasuo Udagawa*
Institutefor Molecular Science, Okazaki, Aichi 444, Japan (Received: March 5. 1990;
In Final Form: April 1, 1991)
Highly dispersed palladium catalysts supportedon y-alumina and silica were prepared and studied by EXAFS spectroscopy,
IR spectroscopy, X-ray diffraction, and CO uptake. The Pd-Pd EXAFS oscillationsfor the freshly reduced catalysts are
only about '/,th as strong as thoseobservedfor bulk Pd, indicatingthat the Pd particlesin the catalysts areeither very small,
highlydisordered,or both. Exposureto CO at room temperature causesthe Pd-Pd EXAFSintensityto increasedramatically,
in some cases to nearly the bulk Pd value. This increase is irreversiblewith respect to CO removal. IR spectra show CO
binding only in the expected 2-fold bridging and linear sites. The increase was observed on both SiOl and A1203supports
but varied in magnitude with support and support condition. Exposure to water vapor did not affect the Pd-Pd EXAFS.
Severalpossibleexplanationsfor the EXAFSintensityincreasesarediscussed. The most likelyappearsto be that COincreases
the mobility of Pd atoms over the support, and allows growthof the Pd clusters present after reduction. These results have
implications for the interpretation of the commonly used CO adsorption method for measuring catalyst dispersion.
An important issue in understanding surface chemistry, par-
ticularlyon highly dispersedcatalysts, is the effectof chemisorption
of an adsorbateon the structure of the catalyst. For single-crystal
surfaces,structural changes may be observed more or less directly


Source: Anderson, Scott L. - Department of Chemistry, University of Utah


Collections: Energy Storage, Conversion and Utilization; Materials Science; Chemistry