氧化铈在氢燃料电池中的催化作用

刘跃译

来自中国稀土http://www.cre.net/

美国能源部Brookhaven 国家实验室的研究人员最近公布了能改进燃料电池性能的稀土催化剂的重要细节。为了得到纯净的氢气，研究人员揭示了两种新型催化剂（组分中含金、铈、钛及氧）具有十分明显催化作用的原因。 　　燃料电池具有许多优异的性能，但面临的主要问题是参与反应的富氢材料在生成氢的过程中产生一氧化碳，一氧化碳可使昂贵的铂基催化剂中毒。而“水汽变 换”反应则是将一氧化碳与水进行反应，从而生成氢与二氧化碳。选用合适的辅助催化剂，则可将一氧化碳完全转换成二氧化碳。呈纳米粒度的金—钛氧化物则具有 上述功能。研究人员介绍，上述两种纳米材料是“水汽转换”反应的高效催化剂，令人奇怪的是块状的金或块状的氧化铈或氧化钛无此催化活性。

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原文链接

Experiments Reveal Unexpected Activity of Fuel Cell Catalysts

December 13, 2007

Written by Kendra Snyder

UPTON, NY - Researchers at the U.S. Department of Energy's Brookhaven National Laboratory have unveiled important details about a class of catalysts that could help improve the performance of fuel cells. With the goal of producing "clean" hydrogen for fuel cell reactions in mind, the researchers determined why two next-generation catalysts including gold, cerium, titanium, and oxygen nanomaterials exhibit very high activity. Their results will be published online in the December 14, 2007, edition of the journal Science.

Fuel cells combine hydrogen and oxygen without combustion to produce direct electrical power and water. They are attractive as a source of power for transportation applications because of their high energy efficiency, the potential for using a variety of fuel sources, and their zero emissions. However, a major problem facing this technology is that the hydrogen-rich materials feeding the reaction often contain carbon monoxide (CO), which is formed during hydrogen production. Within a fuel cell, CO "poisons" the expensive platinum-based catalysts that convert hydrogen into electricity, deteriorating their efficiency over time and requiring their replacement.

"Fuel cell reactions are very demanding processes that require very pure hydrogen," said Brookhaven chemist Jose Rodriguez. "You need to find some way to eliminate the impurities, and that's where the water-gas shift reaction comes into play."

The "water-gas shift" (WGS) reaction combines CO with water to produce additional hydrogen gas and carbon dioxide. With the assistance of proper catalysts, this process can convert nearly 100 percent of the CO into carbon dioxide. Rodriguez's group, which includes researchers from Brookhaven's chemistry department, the Center for Functional Nanomaterials (CFN), and the Central University of Venezuela, studied two "next-generation" WGS nanoscale catalysts: gold-cerium oxide and gold-titanium oxide.

"These nanomaterials have recently been reported as very efficient catalysts for the WGS reaction," said Brookhaven chemist Jan Hrbek. "This was a surprising finding because neither bulk gold nor bulk ceria and titania are active as catalysts."

To determine how these nanocatalysts work, the research team developed so-called "inverse model catalysts." The WGS catalysts usually consist of gold nanoparticles dispersed on a ceria or titania surface - a small amount of the expensive metal placed on the cheap oxide. But to get a better look at the surface interactions, the researchers placed ceria or titania nanoparticles on a pure gold surface.

"For the first time, we established that although pure gold is inert for the WGS reaction, if you put a small amount of ceria or titanium on it, it becomes extremely active," Rodriguez said. "So although these inverse catalysts are just models, they have catalytic activity comparable to, and sometimes better than, the real deal."

Using a technique called x-ray photoelectron spectroscopy at Brookhaven's National Synchrotron Light Source, as well as scanning tunneling microscopy and calculations, the researchers discovered that the catalysts' oxides are the reason for their high activity.

"The oxides have unique properties on the nanoscale and are able to break apart water molecules, which is the most difficult part of the WGS reaction," Hrbek said. Added Brookhaven physicist Ping Liu: "After you dissociate the water, the reaction continues on to eliminate CO. But if you don't have nanosized oxide particles, none of this will work."

The researchers plan to continue their study of these catalysts at the NSLS and CFN in order to further explore the reaction mechanism and optimize its performance. Funding for this research was provided by the Office of Basic Energy Sciences, within the U.S. Department of Energy's Office of Science.

The CFN is one of the five DOE Nanoscale Science Research Centers (NSRCs), premier national user facilities for interdisciplinary research at the nanoscale. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize, and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE's Brookhaven, Argonne, Lawrence Berkeley, Oak Ridge, and Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit http://nano.energy.gov/.

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