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Catalyst Helps Control NOx Emissions And Resists Contamination

Materials Science: The catalyst traps alkali ions inside hollow nanorods, keeping it active

by Melissae Fellet
December 17, 2015

Credit: Xingfu Tang
A structural model shows tungsten oxide units (gray) forming hexagonal nanorods with tunnels that can hold potassium ions (purple).
Illustration of tungsten oxide nanorods holding potassium ions.
Credit: Xingfu Tang
A structural model shows tungsten oxide units (gray) forming hexagonal nanorods with tunnels that can hold potassium ions (purple).

A new study describes a catalyst that reduces nitrogen oxide emissions while resisting poisoning by sulfur and alkali metals (Environ. Sci. Technol. 2015, DOI: 10.1021/acs.est.5b03972). It is already being used to clean the emissions at two industrial plants in China, the researchers say.

Combustion—whether from power plants, vehicles, or factories—produces nitrogen oxides (NOx), health-harming gases that also generate ozone and particle pollution. Power plants remove NOx by reacting the exhaust gas with ammonia to produce nitrogen and water. However, the catalysts used for this selective catalytic reduction can be poisoned by other components of the exhaust, like alkali metals and sulfur dioxide, which render the catalysts ineffective.

Sulfur dioxide adsorbs onto the surface of the catalyst and reacts to form sulfates that block its active sites. Catalysts made of metal oxides with protonated hydroxyls on their surface, like vanadium oxide, resist sulfur deposition. However, this surface also makes it easier for alkali ions in the exhaust, such as potassium, that come from the fuel burned to replace vanadium in the catalyst. That alters the active sites and deactivates the catalyst.

Xingfu Tang of Fudan University and his colleagues wanted to build a catalyst that captured alkali ions inside it, rather than holding them on the material’s surface. The researchers attached vanadium oxide nanoparticles to the surface of hexagonal tungsten oxide nanorods. The rods contain tunnels that are slightly larger than the diameter of a potassium ion. Using synchrotron X-ray diffraction, the researchers saw potassium ions inside the channels of the nanorods, indicating that the material worked as hoped.

To test the catalyst’s performance, the researchers filled it with 350 μmol of potassium sulfate per gram of catalyst and passed a mixture of nitrogen oxide, ammonia, oxygen, and 1,300 mg m-3 sulfur dioxide through the catalyst, simulating high sulfur conditions in the exhaust treatment stage of a factory or power plant. The catalyst maintained its activity after four hours at 350 °C. However, a catalyst made from the conventional catalyst mixture of vanadium oxide, tungsten oxide, and titanium oxide exposed to the same conditions was deactivated after four hours, even with less than half the potassium loading.

The new catalyst is being used by two Chinese companies that burn biomass, Tang says. He adds that it might also be used to clean the emissions from boilers at glass and ceramic plants, where traditional catalysts suffer severe deactivation.

In industrial settings, catalysts are typically preloaded with alkali ions to reduce the impact of alkali poisoning. “But that’s not a solution,” says Pu-Xian Gao of the University of Connecticut. “The ultimate solution is to have an intrinsically stable catalyst to help with this problem. This kind of design gives that option.”

To use this catalyst to clean diesel emissions—another important source of NOx—Gao notes that it would have to be tested in the honeycomb structure of vehicle emissions control systems with realistic environmental tests.



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