Materials that can clean up the environment and improve efficiency of industrial processes are always in demand. So prospects are promising for a new material that can do this in three ways: it strips sulphur from fuel, removes mercury from polluted water, and separates gases in industrial processes.
The material is an 鈥� a gel from which the liquid component has been removed to leave gas-filled voids. Aerogels have the lowest density of any known solid-state material.
While typically aerogels are composed of silicon or aluminium oxides, 鈥榮 team at Northwestern University in Evanston, Illinois, has been experimenting with 鈥渃halcogels鈥�. These are aerogels composed of 鈥渃halcogenides鈥� such as sulphides and selenides, linked by platinum atoms.
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Less expensive
Now the team has begun investigating the properties of chalcogels in which cobalt or nickel has replaced the far more expensive platinum. The chalcogels are formed by mixing the reactants in a container filled with until a gel forms, then freeze-drying the gel to remove the solvent molecules and allow air molecules to replace them.
Chalcogels have a huge surface area. This makes them especially suitable for use as catalysts, or to bind molecules in order to remove them from a solution. When Kanatzidis鈥檚 team placed a 10-milligram sample of chalcogel into 10 millilitres of mercury-contaminated water, the mercury concentration fell from 755 parts per million to 0.1 ppm within 48 hours.
鈥淭he explanation is the strong affinity of mercury ions for sulphur atoms,鈥� says Kanatzidis. 鈥淢ercury ions entering the chalcogel structure face a surface of sulphur atoms and bind to it strongly.鈥�
The team also tested the new gel鈥檚 ability to bind to CO2 or hydrogen gas. At -15 掳C and atmospheric pressure, 1 litre of chalcogel can hold 108 litres of CO2. At -196 掳C 鈥� the temperature of liquid nitrogen 鈥� the material switched its affinity to hydrogen, with 1 litre of gel holding 155 litres of the gas.
Sulphur scrubber
Unfortunately, the gels are not suitable for carbon capture and storage or storing hydrogen. Despite their low density 鈥渢hey are too heavy鈥� for these applications, Kanatzidis says.
One possible environmentally beneficial application for the new materials is as catalysts to remove sulphur from fossil fuels. Cobalt and nickel-rich catalysts are already known to bind to sulphur in crude oil. This was one reason the team chose these metals to make their new chalcogels.
At a typical reaction temperature of 370 掳C, the chalcogels performed twice as well as conventional sulphur-removing catalysts. This could be important, as environmental legislation continues to put limits on the sulphur content of fuel, says the team.
鈥淭hese are still preliminary results, and we have not done all the experiments needed to fully assess this,鈥� Kanatzidis says. But compared with existing catalysts, 鈥渙ur results showed 100 per cent improvement鈥�.
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