Editorial Feature

Gold Applications in Reducing Pollution

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Gold has a significant role to play in future technologies, with the goal of minimizing energy consumption and pollution, thanks to its exclusive metallurgical and chemical properties. This article discusses a few examples.

Water Purification

Chlorinated hydrocarbons are one of the leading pollutants of groundwater. For instance, trichloroethene (TCE) is used for degreasing electronic parts and metals in the electronics, metals, and automotive industries. It is also used in consumer products, textile cleaning, and production of chemicals.

New research at Rice University’s Centre for Biological and Environmental Nanotechnology has shown that bimetallic gold-palladium nanoparticles offer an active catalyst to disintegrate TCE, one of the most prevalent and poisonous groundwater pollutants.

TCE causes impaired pregnancy, liver damage, and cancer. The function of the new catalyst is better than that of the carbon filters used at present because it turns the TCE into non-toxic components rather than just trapping it in the filter. It also works better than iron as it is not consumed in the reaction; thus, it can be used repeatedly. On the other hand, iron catalysts create toxic intermediate chemicals such as vinyl chloride.

It has been previously demonstrated that palladium catalysts effectively eliminate TCE and other chlorinated compounds from water at room temperature, using hydrogen. However, catalyst cost is a major barrier to the extensive use of palladium catalysts. To minimize the use of palladium metal, Dr. Wong’s team at Rice, coated small quantities of palladium atoms onto gold nanoparticles.

As a result, catalytic activity increased considerably. Although gold is costlier than palladium, as the nanoparticles are so much more active, they are more economical. This nanomaterial paves the way to great opportunities in groundwater cleanup.

In another research, scientists from the Indian Institute of Technology have shown that gold nanoparticles, added into a point-of-use water purification device, can be successful in capturing and eliminating halocarbon-based pesticides from drinking water.

Mercury Control

The United States is increasingly dependent on coal to generate electrical power, and substantial levels of mercury are found in the effluent from these power plants. It is expected that control over the levels of mercury, which has been known to cause Alzheimer’s disease and autism, will be realized in the United States, by imposing restrictions on mercury emissions from coal-fired boilers in the utility sector.

One technique that is turning out to be highly potential in improving mercury removal is the addition of a catalyst to optimize the oxidation of mercury and gold. Full-scale experiments are presently in progress.

Diesel Emission Control

A huge step forward in economical emission control is the latest announcement by U.S. company Nanostellar, that it has created an automotive pollution control catalyst for diesel engines that has gold, as well as the conventional palladium and platinum ingredients.

Platinum group metals are used in the nanoparticulate form. Their application has increased from when they were first launched in the mid-1970s to over 260 tons per year at present. With an inadequate newly mined supply of platinum group metals, the cost of these catalyst systems has become a major problem for automotive manufacturers. The decrease in the cost of precious metal used is a target yet to be achieved.

In the last few years, manufacturers of catalyst materials have changed the relative amounts of platinum and palladium, based on the price of the individual metals. Independent testing of Nanostellar’s NS Gold™ has revealed that NS Gold™ boosts hydrocarbon oxidation activity by 15%–20% at equal precious-metal cost.

NS Gold™ is a tri-metal formulation of platinum, gold, and palladium. It enables the proportions of each metal to be tweaked to help catalyst systems engineers to meet engine-specific performance targets. It stabilizes the total cost of diesel catalysts, in spite of instabilities in the price of precious metals.

NS Gold™ is potentially ideal for treating all lean-stream exhaust, where the air is in surplus of fuel-borne hydrocarbon gases. Applications include, but are not restricted to, the treatment of stationary-source volatile organic compound (VOC) emissions, particulates and hydrocarbons in soot filters, and ammonia slip in selective catalytic reduction (SCR) systems.

“Green” Chemistry

Green chemistry, also called sustainable chemistry, is a chemical philosophy that promotes the design of industrial chemicals and processes that decrease or prevent the use and creation of dangerous substances. The use of gold as a catalyst has a huge role to play in green chemistry.

For instance, a majority of the industrial oxidation processes involve the use of chlorine or organic peroxides. The chlorine processes generate large quantities of chloride salts and substantial amounts of chlorinated organic by-products. The drawback of organic peroxides is their cost. It can be said that the chemical industry would be revolutionized if selective oxidation of hydrocarbons could be efficiently realized using inexpensive and clean oxygen from the air.

In recent times, a research team headed by Graham J. Hutchings, professor of physical chemistry at Cardiff University, in Wales, has demonstrated that gold nanoparticles can epoxidize alkenes when hydrogen is used to activate the molecular oxygen, at atmospheric pressure and temperatures of 60 °C to 80 °C (Nature 2005, 437, 1132). Such an advance of “greener” techniques for oxidation catalysis using gold is a highly crucial development.

With the continuous increase in international demand and prices for petroleum-based feedstocks, chemists have been facing the challenge of developing processes that involve the use of biomass-derived feedstocks.

Recently, researchers at the Center for Sustainable & Green Chemistry at the Technical University of Denmark, in Lyngby, have formulated a gold-catalyzed process for the selective oxidation of furfural and hydroxymethylfurfural—biomass-derived platform chemicals—to produce their respective methyl esters. These chemicals are used for fragrance and flavor applications, in plastics, and potentially as industrial solvents.

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