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Physical Chemistry

Unusual Aluminum Bonding Geometry

Catalysis: Pentacoordinated Al joins Pt in supported catalysts

by Mitch Jacoby
September 28, 2009 | APPEARED IN VOLUME 87, ISSUE 39

Credit: PNNL
Pt atoms (gray) in PtO (a precursor to metallic Pt) bond to pentacoordinated Al atoms (green) via oxygen bridges (orange) and form thin raftlike structures.
Credit: PNNL
Pt atoms (gray) in PtO (a precursor to metallic Pt) bond to pentacoordinated Al atoms (green) via oxygen bridges (orange) and form thin raftlike structures.

Aluminum atoms with fivefold bonding coordination serve as atomic posts that anchor platinum in alumina-supported platinum catalysts, chemists at Pacific Northwest National Laboratory (PNNL) report in Science (2009, 325, 1670). The study provides a molecular-scale view of bonding interactions in one of the most common commercial catalysts and points to strategies for improving it.

Precious metals dispersed on solid oxides constitute a large fraction of industrial catalysts. Alumina-supported platinum, for example, facilitates a variety of hydrocarbon-conversion processes, such as the ones that purify engine emissions in automobile catalytic converters. To derive high activity from such catalysts, the metal must be dispersed finely because platinum atoms buried in the interior of catalyst particles cannot drive chemical reactions.

The extent of metal dispersion is related to the strength and nature of the interaction between the metal atoms and oxide support. Yet molecular-level details of the bonds that attach the metal to the support are difficult to discern and have been generally unknown.

PNNL chemists Ja Hun Kwak, Charles H. F. Peden, and Janos Szanyi and their coworkers have now determined that platinum atoms use oxygen bridges to bind to pentacoordinated Al3+ sites on γ-Al2O3, a common form of alumina. Based on high-field nuclear magnetic resonance spectroscopy, electron microscopy, and computational analysis, the study shows that platinum assembles into thin, two-dimensional "rafts" that "float" above the support at the point on which the raft is anchored.

The study also shows that the energetics of changing aluminum's coordination number from five to the more usual six drives formation of the 2-D structures. Importantly, the number of "penta" sites can be increased by raising the temperature during catalyst synthesis, the team notes.

Catalyst specialist Bert M. Weckhuysen of Utrecht University, in the Netherlands, finds this study to be "beautiful and highly relevant" because it addresses an important and widely used catalytic material and suggests new ways to improve it.



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