Catalysis: Making light work of pollutants

Tungsten, gallium and copper help titania catalysts break down organic molecules under visible light.

One of the many uses of titania (TiO₂) is as a light-activated catalyst for the decomposition of organic molecules. This could potentially allow it to be used to clean pollutants or bacteria from wastewater or air, but the raw material’s catalytic function is activated only by ultraviolet light, limiting its usefulness in natural light applications.

Fig. 1: Organic pollutants in the air could be decomposed using sunlight and a cleverly designed titania-based catalyst© iStockphoto/GeorgeClerk

Huogen Yu and Kazuhito Hashimoto from the University of Tokyo and Hiroshi Irie from the University of Yamanashi in Japan¹ have now tweaked TiO₂ to produce a material which, in the presence of oxygen, is remarkably good at using the energy in visible light to decompose organic molecules by oxidation.

The team substituted some of the titanium atoms in the catalyst with tungsten and gallium atoms by a process known as doping. The electrons in the doped catalyst are more easily energized by visible light than those in bare titania, extending the catalyst’s activity from purely ultraviolet to visible wavelengths. They also impregnated the catalyst with copper ions to assist the transfer of electrons from the catalyst to oxygen molecules on the surface. Electrons mobilized by exposure of the catalyst to light reduce oxygen, while photogenerated ‘holes’ (electron vacancies) oxidize passing organic molecules, which causes them to degrade.

The scientists tested a fine powder of their catalyst in the oxidization of 2-propanol, a small organic molecule. By monitoring the amount of acetone and carbon dioxide produced as the 2-propanol was broken down, they found that their catalyst performed better than undoped TiO₂, with or without copper impregnation.

At doping levels equivalent to the replacement of about one in 30 titanium atoms, the catalyst’s efficiency was close to 13%, a dramatic improvement over the 4% achieved by exisiting commercial nitrogen-doped TiO₂ catalysts. With further improvements in efficiency, the material could one day be used as a practical light-driven catalyst for the purification of air.

The researchers are now investigating ways of introducing more tungsten and gallium into the material to further enhance its ability to absorb visible light. They have also recently found that impregnating with iron rather than copper provides efficiency improvements.