Nanotechnology points the way to greener pastures

A research team led by Miho Yamauchi and Masaki Takata from the RIKEN SPring-8 Center in Harima has now discovered an almost ideal way to detoxify the effects of ammonia fertilizers. By synthesizing photoactive bimetallic nanocatalysts that generate hydrogen gas from water using solar energy, the team can catalytically convert NO₃– back into NH3 through an efficient route free from carbon dioxide emissions.

Replacing the oxygen atoms of NO₃– with hydrogen is a difficult chemical trick, but chemists can achieve this feat by using nanoparticles of copper–palladium (CuPd) alloys to immobilize nitrates at their surfaces and catalyzing a reduction reaction with dissolved hydrogen atoms. However, the atomic distribution at the ‘nanoalloy’ surface affects the outcome of this procedure: regions with large domains of Pd atoms tend to create nitrogen gas, while well-mixed alloys preferentially produce ammonia.

According to Yamauchi, the challenge in synthesizing homogenously mixed CuPd alloys is getting the timing right—the two metal ions transform into atomic states at different rates, causing phase separation. Yamauchi and her team used the powerful x-rays of the SPring-8 Center’s synchrotron to characterize the atomic structure of CuPd synthesized with harsh or mild reagents. Their experiments revealed that a relatively strong reducing reagent called sodium borohydride gave alloys with near-perfect mixing down to nanoscale dimensions.

Most ammonia syntheses use hydrogen gas produced from fossil fuels, but the use of solar energy by the researchers avoids this. They found that depositing the nanoalloy onto photosensitive titanium dioxide (TiO₂) yielded a material able to convert ultraviolet radiation into energetic electrons; in turn, these electrons stimulated hydrogen gas generation from a simple water/methanol solution. When they added nitrate ions to this mixture, the CuPd/TiO₂ catalyst converted nearly 80% into ammonia—a remarkable chemical selectivity that the researchers attribute to high concentrations of reactive hydrogen photocatalytically produced near the CuPd surface.

Yamauchi is confident that this approach can help reduce the ecological impact of many classical chemical hydrogenation reactions. “Considering the environmental problems we face, we have to switch from chemical synthesis using fossil-based hydrogen to other clean processes,” she says.

More information: Yamauchi, M., Abe, R., Tsukuda, T., Kato. K. & Takata, M. Highly selective ammonia synthesis from nitrate with photocatalytically generated hydrogen on CuPd/TiO2. Journal of the American Chemical Society 133, 1150–1152 (2011). http://pubs.acs.or … 21/ja106285p

Provided by RIKEN