Bonus material for Metals’ Hidden Strengths, by Roberta Baxter
Two additional discoveries may significantly impact our lives in the future. Scientists at
Cornell University, Ithaca, N.Y., have created a new type of material made from the
metal platinum that could help reduce the toxicity of car exhaust more efficiently. At
Queensland University of Technology, Brisbane, Australia, researchers have discovered
that gold, in combination with light, can help reduce air pollution.
More efficient catalytic converters
A catalytic converter is a device that removes pollutants from motor vehicle exhaust.
Located between a vehicle's engine and tailpipe, it converts exhaust pollutants such as
carbon monoxide, nitrogen oxides, and hydrocarbons into gases such as nitrogen, oxygen,
and carbon dioxide.
The chemical reactions involved consist of two oxidation reactions, in which oxygen is
captured by one of the reactants, and one reduction reaction, in which oxygen is released
by one of the reactants:
Oxidation of hydrocarbons (CxH2x+2): CxH2x+2 + 2x O2 → x CO2 + 2x H2O (where x is an
integer)
Oxidation of carbon monoxide: 2 CO + O2  2 CO2
Reduction of nitrogen oxides: 2 NOx  x O2 + N2 (where x = 1 or 2)
The catalytic converter contains a chemical called a catalyst that speeds up these
conversion reactions but is not involved in them. The catalyst is often a precious metal,
such as platinum, palladium, or rhodium.
But these metals are costly, so scientists and engineers usually try to make the most of
them. One way to do so is to use them in the form of a powder instead of a solid, because
the total surface area of the powder particles is larger than the total surface area of a solid
that would be made of these particles. This is a general principle in chemistry: Each time
you want to maximize use of a material in a chemical reaction, it is better to break it
down in smaller pieces to increase the amount of surface area available for the chemical
reaction (and when the surface area is increased, more reactants will bind to the metal
molecules, making the chemical reaction between these reactants more efficient).
Researchers at Cornell University have taken this principle a step further. They have
made a honeycomb-like structure made of platinum. The tiny spaces in that structure are
about 10 nanometers across (one nanometer is one billionth of a meter), thus providing
even more surface area for chemical reactions to occur. If converters were manufactured
with this type of catalyst, they would do more with less, that is, they would remove
pollutants faster with less amounts of platinum.
Earlier experiments had produced honeycomb-like metals with 2- to 4-nanometer pores,
but this size was too small for gases to flow through the honeycomb, which reduced the
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amount of pollution removed. This time, the scientists, led by Cornell professor of
materials science and engineering Uli Wiesner, tried a new method in which they mixed
tiny particles of platinum, also called nanoparticles and containing a few hundreds of
atoms each, with polymers—molecules made of long chains of mostly carbon and
hydrogen atoms.
The polymers directed the platinum nanoparticles into the desired honeycomb-like
structure. The resulting material was heated at high temperatures without air, which
converted the polymer into a carbon scaffold. This scaffold prevented the platinum
nanoparticles from fusing together, which would have destroyed the desired honeycomblike structure. The scaffold was subsequently allowed to cool, and the platinum
nanoparticles maintained their regular patterns. Then, the scaffold was removed with
acid, leaving a honeycomb-like structure of platinum nanoparticles.
Honeycomb-like platinum structure developed at Cornell University. [Courtesy of Scott
Warren & Uli Wiesner, Cornell University].
This new finding may usher in a new era of more efficient and cheaper catalysts in other
applications, such as fuel cells—devices that generate electricity by combining a fuel and
oxygen—and “plasmonic” surface structures that can carry more information across
microchips than conventional wires do.
“These new structures open a completely novel playground, because no one has been able
to structure metals in bulk ways using polymers,” Wiesner says. “In principle, if you can
do it with one metal you can do it with others or even mixtures of metals.”
Cleaner air with gold
Scientists have found a new use for gold. Huai Yong Zhu, professor of chemistry at
Queensland University, and colleagues found that very small clumps of gold atoms called
nanoparticles act as a catalyst to pull apart organic molecules and air pollutants called
volatile organic chemicals (VOCs).
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These gold particles—which are a few millionths of a millimeter in size—catalyze the
oxidation reaction of organic molecules, breaking them down into carbon dioxide (CO2)
and water (H2O), by absorbing visible and ultraviolet light. The generic chemical reaction
is:
Organic molecule + O2
Au (catalyst) and light

CO2 + H2O
In one experiment, a sample of an organic compound called formaldehyde (HCHO)—
which is one of many VOCs—was placed in a vessel along with gold nanoparticles
supported by zirconium dioxide (ZrO2). When the scientists directed blue light toward the
vessel, the concentration of formaldehyde decreased by 64% over 2 hours.
Zhu and colleagues found that formaldehyde had been broken into carbon dioxide by the
gold particles after they were activated by the blue light. (The zirconium compound did
not take part in the reaction; it acted only as a support material for the gold.) The
scientists did the same experiment with sunlight and found that the concentration of
formaldehyde decreased by 8% over 2 hours. “If gold and sunlight can be used to
catalyze reactions to purify the air, we can make the air cleaner without using energy,”
Zhu says.
This discovery could boost the use of gold in industrial applications seeking to purify the
air from pollutants, such as VOCs, both indoors and outside. Future technology may also
use gold and sunlight to drive chemical reactions that occur at ambient temperatures.
—Roberta Baxter
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