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.
UNUSUAL ALUMINUM BONDING GEOMETRY CATALYSIS: Pentacoordinated Al
joins Pt in supported catalysts
A
LUMINUM ATOMS with fivefold bonding coor-
dination 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. Aluminasupported 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.
HEADS BUTT OVER DEADLY EXPLOSION DUST ACCIDENT: Safety board seeks better maintenance; unions want tougher regulations
A Clouds of airborne dust fueled a deadly blast at the Imperial Sugar plant in Georgia.
N ACCUMULATION of airborne sugar dust led
to a 2008 explosion and fire that killed 14 and injured 36 workers at the Imperial Sugar plant outside Savannah, Ga., says the Chemical Safety & Hazard Investigation Board (CSB) in a report released last week. On the circumstances leading to the Feb. 7, 2008, accident, there is little disagreement between CSB and its frequent ally—labor unions that represent plant workers. But the two are deeply split over what should be done to avoid similar accidents. The accident’s cause was ignition of airborne sugar dust that had accumulated for years due to lousy maintenance and inadequate equipment design. A spark from an overheated conveyor bearing likely triggered the primary blast, which was quickly followed by more powerful secondary explosions and fires that ripped through the plant. CSB
PNNL
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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.—MITCH JACOBY
Looking at company documents, the board finds a 40-year failure by Imperial to address dust problems, despite management’s knowledge that they posed a threat. The board recommends that Imperial and various trade associations implement better training and housekeeping programs. The tragedy of industrial dust explosions is not new to the board. In 2006, a CSB study discovered 281 industrial dust accidents—many in chemical plants—over a 25year period, resulting in 119 worker deaths and 718 injuries. Consequently, CSB recommended that year that the Occupational Safety & Health Administration develop a comprehensive dust regulatory standard. Three years passed, and last April, OSHA said it would develop the regulation. But it has yet to take the first step: an advanced notice of proposed rule-making. Labor unions say the regulatory process will take years, and they want OSHA to implement a dust standard quickly, on an emergency basis, while it draws up a regulation. They had wanted CSB’s support in that recommendation. Because of the lack of support, Jackie Nowell, director of occupational safety and health at United Food & Commercial Workers, a union that represents chemical, sugar, and food workers, is now calling for new leadership at the board. For its part, CSB Chairman John Bresland says, “the board decided that the best way was to allow OSHA to continue and develop a regulatory program.” He says he was “disappointed” in the three-year delay, “but my hope is that OSHA will give the regulation priority and develop it as soon as possible.”—JEFF JOHNSON
SEPTEMBER 28, 2009
