Bonus material for Metals’ Hidden Strengths, by Roberta Baxter

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.. The

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

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 honeycomb-like 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)

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:

Au (catalyst) and light

Organic molecule + O2  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