Platinum, one of the rarest and most expensive metals on Earth, may soon find itself out of a job.
Known for its allure in engagement rings, platinum is also treasured for its ability to jump-start chemical reactions. It’s an excellent catalyst, able to turn standoffish molecules into fast friends. But Earth’s supply of the metal is limited, so scientists are trying to coax materials that aren’t platinum — aren’t even metals — into acting like they are.For years, platinum has been offering behind-the-scenes hustle in catalytic converters, which remove harmful pollutants from auto exhaust. It’s also one of a handful of rare metals that move along chemical reactions in many well-established industries. And now, clean energy technology opens a new and growing market for the metal. Energy-converting devices like fuel cells being developed to power some types of electric vehicles rely on platinum’s catalytic properties to transform hydrogen into electricity. Even generating the hydrogen fuel itself depends on platinum.
Without a cheaper substitute for platinum, these clean energy technologies won’t be able to compete against fossil fuels, says Liming Dai, a materials scientist at Case Western Reserve University in Cleveland.
To reduce the pressure on platinum, Dai and others are engineering new materials that have the same catalytic powers as platinum and other metals — without the high price tag. Some researchers are replacing expensive metals with cheaper, more abundant building blocks, like carbon. Others are turning to biology, using catalysts perfected by years of evolution as inspiration. And when platinum really is best for a job, researchers are retooling how it is used to get more bang for the buck.
Moving right along
Catalysts are the unsung heroes of the chemical reactions that make human society tick. These molecular matchmakers are used in manufacturing plastics and pharmaceuticals, petroleum and coal processing and now clean energy technology. Catalysts are even inside our bodies, in the form of enzymes that break food into nutrients and help cells make energy.
Catalysts lower the amount of energy needed to make a chemical reaction run. The starting materials and ending products are the same, but the catalyst offers an easier route to get between the two.
During any chemical reaction, molecules break chemical bonds between their atomic building blocks and then make new bonds with different atoms — like swapping partners at a square dance. Sometimes, those partnerships are easy to break: A molecule has certain properties that let it lure away atoms from another molecule. But in stable partnerships, the molecules are content as they are. Left together for a very long period of time, a few might eventually switch partners. But there’s no mass frenzy of bond breaking and rebuilding.
Catalysts make this breaking and rebuilding happen more efficiently by lowering the activation energy — the threshold amount of energy needed to make a chemical reaction go. Starting and ending products stay the same; the catalyst just changes the path, building a paved highway to bypass a bumpy dirt road. With an easier route, molecules that might take years to react can do so in seconds instead. A catalyst doesn’t get used up in the reaction, though. Like a wingman, it incentivizes other molecules to react, and then it bows out.
A hydrogen fuel cell, for example, works by reacting hydrogen gas (H2) with oxygen gas (O2) to make water (H2O) and electricity. The fuel cell needs to break apart the atoms of the hydrogen and oxygen molecules and reshuffle them into new molecules. Without some assistance, the reshuffling happens very slowly. Platinum propels those reactions along.
In a fuel cell, catalysts help hydrogen and oxygen atoms reshuffle, forming water. In the process shown, hydrogen ions move through an electrolyte while hydrogen’s electrons generate current. Hydrogen and oxygen meet at the cathode.
Platinum works well in fuel cell reactions because it interacts just the right amount with both hydrogen and oxygen. That is, the platinum surface attracts the gas molecules, pulling them close together to speed along the reaction. But then it lets its handiwork float free. Chemists call that “turnover” — how efficiently a catalyst can draw in molecules, help them react, then send them back out into the world.
Platinum isn’t the only superstar catalyst. Other metals with similar chemical properties also get the job done — palladium, ruthenium and iridium, for example. But those elements are also expensive and hard to get. They are so good at what they do that it’s hard to find a substitute. But promising new options are in the works.
Carbon is key
Carbon is a particularly attractive alternative to precious metals like platinum because it’s cheap, abundant and can be assembled into many different structures.
Carbon atoms can arrange themselves into flat sheets of orderly hexagonal rings, like chicken wire. Rolling these chicken wire sheets — known as graphene — into hollow tubes makes carbon nanotubes, which are stronger than steel for their weight. But carbon-only structures don’t make great catalysts.
“Really pure graphene isn’t catalytically active,” says Huixin He, a chemist at Rutgers University in Newark, N.J. But replacing some of the carbon atoms in the framework with nitrogen, phosphorus or other atoms changes the way electric charge is distributed throughout the material. And that can make carbon behave more like a metal. For example, nitrogen atoms sprinkled like chocolate chips into the carbon structure draw negatively charged electrons away from the carbon atoms. The carbon atoms are left with a more positive charge, making them more attractive to the reaction that needs a nudge.
That movement of electrical charge is a prerequisite for a material to act as a catalyst, says Dai, who has pioneered the development of carbon-based, metal-free catalysts. His lab group demonstrated in 2009 in Science that clumps of nitrogen-containing carbon nanotubes aligned vertically — like a fistful of uncooked spaghetti — could stand in for platinum to help break apart oxygen inside fuel cells.
By itself, carbon is not a great catalyst. But mixing in other elements (left) and changing its three-dimensional structure (right) give it new powers. Scientists can vary these parameters to design carbon catalysts suited to different situations.
To perfect the technology, which he has patented, Dai has been swapping in different atoms in different combinations and experimenting with various carbon structures. Should the catalyst be a flat sheet of graphene or a forest…
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