Sponge-Like Aerogel Soaks Up Hg, Separates H^sub 2^, and Improves HDS

Northwestern Univ. chemists have designed a brittle, freeze-dried, sponge-like material that can remove mercury from polluted water, easily separate hydrogen from other gases, and is a more-effective hydrodesulfurization (HDS) catalyst than the one currently used to pull sulfur out of crude oil.

The material is a new class of chalcogels - random networks of metal and sulfur atoms with very high surface areas discovered only a few years ago at Northwestern. The new chalcogel is made from common elements, is stable when exposed to air or water, and can be used as a powder.

Mercouri G. Kanatzidis, professor of chemistry in Northwestern 's Weinberg College of Arts and Sciences, and doctoral student Santanu Bag make their catalyst using a method different from that of conventional catalysts.

The Northwestern material is an aerogel made of cobalt, nickel, molybdenum and sulfur (in nominal stoichiometries of CoMoS^sub 4^ and NiMoS^sub 4^) that is freeze-dried to produce a sponge-like material with a very high surface area. (One cubic centimeter has approximately 10,000 H^sub 2^ of surface area, or about half the area of a football field.) This high surface area and the material's stability under catalytic conditions are responsible for the chalcogel's high, sustainable activity.

The researchers demonstrated that the new chalcogel soaks up toxic heavy metals from contaminated water better than other materials. The chalcogel removed nearly 99% of the mercury from water containing several parts per million of the pollutant. Mercury has an affinity for binding to sulfur, and the chalcogel contains many sulfur atoms that serve as binding sites.

Two years ago, Kanatzidis and Bag reported a chalcogel that could remove mercury from liquid, but that chalcogel contained expensive platinum. The new material contains only inexpensive elements, with cobalt and nickel taking the place of the platinum. The cobalt and nickel link through the sulfur atoms of the thiomolybdate anions to create a three-dimensional porous network.

"We succeeded in doing something very difficult: eliminating the platinum and only using common materials to create a gel," says Kanatzidis. "We found the proper conditions to get the properties we wanted. The key was changing the solvent from water to formamide. It is important to stabilize a gel. If a stable gel cannot form, no high surface or porous material is obtained."

Hydrodesulfurization is a widely used, highly optimized catalytic process that removes sulfur from natural gas and refined petroleum products, such as gasoline, diesel and jet fuels. It is the largest scale catalytic reaction. Scientists have tried to improve HDS, but have not made much progress - many consider it to be optimized already. The Northwestern researchers, in collaboration with colleagues at Western Washington Univ., also report that their catalyst is twice as active as the conventional HDS catalyst, even though it is composed of the same elements.

"I was surprised at the impressive activity of our catalyst, given how difficult it has been to improve HDS," says Kanatzidis. "In principle, our catalyst could process and desulfurize twice as much crude oil as the same amount of conventional catalyst. We are currently conducting studies to see how the catalyst operates under more commercial conditions."

In addition to being a mercury sponge and a better HDS catalyst, the chalcogel also is very effective at gas separation. The researchers showed that the material easily removes carbon dioxide from hydrogen, an application that could be useful in the hydrogen economy. This is the first published research to report chalcogels being used for catalysis and gas separation.

The gas separation process takes advantage of the soft sulfur atoms of the chalcogel's surface along with the very high surface area. These atoms interact with other soft moving molecules, slowing them down as they pass through. Hydrogen, the smallest element, is a hard molecule - it quickly transfers through while softer molecules like carbon dioxide take more time.

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Northwestern Univ. researchers have created a chalcogel that separates H^sub 2^ from CO2 (top), plays a role in hydrodesulfurization (middle) and removes Hg from water (bottom). The images at the right show the chalogel on centimeter, micron and nanometer scales. Image courtesy of S. Bag.