In DepthChemistry

New recipe produces ammonia from air, water, and sunlight

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Science  08 Aug 2014:
Vol. 345, Issue 6197, pp. 610
DOI: 10.1126/science.345.6197.610

The ability to turn the nitrogen in air into fertilizer has enabled farmers to feed billions more people than our planet could otherwise support. But it's costly. The massive chemical plants that produce ammonia—the starting material for fertilizer—consume up to 5% of the world's natural gas and belch out hundreds of millions of tonnes of carbon dioxide (CO2) annually. Now, chemists have come up with an alternative approach drawing on renewable energy. On page 637, they report using heat and electricity produced from sunlight to stitch together nitrogen from the air and hydrogen from water to make ammonia, all without emitting a molecule of CO2.

“It's an important scientific advance,” says Morris Bullock, a chemist at the Pacific Northwest National Laboratory in Richland, Washington. Still, says Ellen Stechel, a chemical physicist at Arizona State University, Tempe, the question is whether the process's “very respectable” efficiency in the lab can be scaled up to compete with the current ammonia industry.

Nitrogen molecules in air are inert, held together by triple bonds that aren't easily broken. In the early 1900s, the German chemists Fritz Haber and Carl Bosch figured out how to make nitrogen more biologically reactive. They used high pressures and temperatures to sever those bonds and weld nitrogen atoms to hydrogen to make ammonia, NH3. Today, that reaction produces hundreds of millions of tons of ammonia each year.

Yet the large amounts of energy required for this reaction have prompted a number of researchers to look for alternatives. One popular approach has been to search for catalysts that break nitrogen's triple bonds and make ammonia when fed electricity. So far, however, even the best such catalysts harness only about 1% of the electrons for forming ammonia's bonds.

Stuart Licht, a chemist at George Washington University in Washington, D.C., tackled the problem from the opposite direction. He spotted work on fuel cells that break down ammonia into nitrogen and hydrogen, generating electricity in the process. A new electrolyte, which helps charged ions move in the device, improved the efficiency of the fuel cell.

Licht and colleagues tried using the same electro lyte—a molten mixture of potassium and sodium hydroxide—in reverse to synthesize ammonia. It worked. In their reactor, they combined the electrolyte with catalytic nanoparticles made from iron oxide, then fed in water, air, heat, and electricity. The reactor split water, snapped nitrogen's strong bonds, and welded the components into ammonia and molecular hydrogen (H2)—itself a fuel. All told, 65% of the electricity wound up stored in chemical bonds: 35% in ammonia and 30% in H2 molecules.

Though impressive, the result “still has a long way to go” to replace the Haber-Bosch process, says James Miller, a chemist at Sandia National Laboratories in Albuquerque, New Mexico, who specializes in using solar energy to make chemical fuels. The reactor is most efficient when fed only a trickle of electricity. Licht and his team will need to boost the current 50-fold to match related industrial processes, Stechel says. Still, Miller adds, “he's on the right track.”

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