Spray a way to better catalysts

Using a technique called ultrasonic spray pyrolysis, researchers at the

have created an improved catalyst for removing smelly sulphur-containing compounds from gasoline and other fossil fuels.

The improved catalyst is a form of molybdenum disulphide, most commonly recognised as the black lubricant used to grease automobiles and machinery.

Molybdenum disulphide is made of long flat layers of molybdenum metal atoms sandwiched above and below by single atomic layers of sulphur. The interactions between sulphur-sulphur planes are weak, so they can easily slide past one another, providing excellent high-temperature lubrication.

Molybdenum disulphide's other important commercial application is as a catalyst used by the petroleum industry to remove ecologically damaging sulphur-containing compounds in gasoline. When burned, these sulphur compounds cause the formation of acid rain.

"The flat planes of molybdenum disulphide that make it a good lubricant also decrease its ability to react with fuels to remove sulphur," said Ken Suslick, the Marvin T. Schmidt Professor of Chemistry at Illinois and a researcher at the Beckman Institute for Advanced Science and Technology. "This is because all the reactions necessary for sulphur removal occur on the edges of the long planes, and the bigger the planes, the less relative edge there is."

Using ultrasonic spray pyrolysis, Suslick and graduate student Sara Skrabalak discovered a way to make a highly porous network of molybdenum disulphide that preferentially exposes the catalytic edges.

Using an ordinary household ultrasonic humidifier, Suslick and Skrabalak spray small droplets of precursor solutions into micron-sized droplets. The droplets are then carried by a gas stream into a furnace, where the solvent evaporates and dissolved substances react to form a product.

This spray-synthesis technique has allowed for the continuous, inexpensive production of spherical powders of varying composition. Research efforts are expanding this technique to the production of nanoparticles and industrially important catalysts.

The new form of molybdenum disulphide is made by spraying droplets of a water solution of ammonium tetrathiomolybdate (a molybdenum disulfide precursor) and colloidal silica (very fine sand).

As the droplets are heated in the furnace, water evaporates and a molybdenum disulfide/silica composite is formed. The composite is then treated with hydrofluoric acid, which etches away the silica and leaves a network of molybdenum disulphide behind.

"This treatment leaves pores where the silica used to be and exposes the catalytically active edge of molybdenum disulphide," Suslick said.

"Molybdenum disulphide is the standard industrial catalyst for hydrodesulphurisation (the removal of sulphur from fuels using hydrogen), but this unique form of molybdenum disulphide has superior catalytic properties when compared to conventionally synthesized molybdenum disulphide."

In addition, the new spray-synthesis route to catalytic materials is simple, easily scaled-up, and can be adapted to other industrial materials.