Role of Gold in Reducing Pollution

Topics Covered

Overview
Water Purification
Mercury Control
Diesel Emission Control
'Green' Chemistry

Overview

Due to its unique chemical and metallurgical properties gold is has a vital role to play in future technologies aimed at reducing pollution and energy consumption. Examples include:

Water Purification

Chlorinated hydrocarbons are major pollutants of groundwater. For example, trichloroethene is used to degrease metals and electronic parts in the automotive, metals and electronic industries and also in chemicals production, textile cleaning and consumer products. Recent research at Rice University's Centre for Biological and Environmental Nanotechnology has revealed that bimetallic gold-palladium nanoparticles provide an active catalyst to break down trichlorethene (TCE), one of the most common and poisonous groundwater pollutants. TCE has been linked to liver damage, impaired pregnancy and cancer. The new catalyst works better than the carbon filters currently in use because it converts the TCE to non-toxic components instead of just trapping it in the filter. It also performs better than iron because it is not consumed in the reaction and, thus, can be used repeatedly; in contrast, iron catalysts produce toxic intermediate chemicals such as vinyl chloride.

Palladium catalysts have previously been shown to remove trichloroethene and other chlorinated compounds from water effectively at room temperature using hydrogen, but catalyst cost is a significant barrier to widespread adoption. In order to use less metal, Dr Wong’s team at Rice coated small amounts of palladium atoms onto gold nanoparticles. The increase in catalytic activity was exciting. Gold is more expensive than palladium but, since the nanoparticles are so much more active they are more cost effective. This nanomaterial opens up tremendous opportunities in groundwater clean-up. In other work, researchers from the Indian Institute of Technology, have proven that gold nanoparticles, incorporated into a point-of-use water purification device, can be effective in the capture and removal of halocarbon-based pesticides from drinking water

Mercury Control

The US is relying increasingly on the use of coal to produce electrical power and significant levels of mercury occur in the effluent from these power plants. Control of mercury, which has been linked to Alzheimer’s disease and autism, is expected to be achieved in the US by imposed limits on mercury emissions from coal-fired boilers in the utilities industry. One method to increase mercury removal is to introduce a catalyst to enhance the oxidation of mercury and gold catalysts are proving to be very promising. Full scale trials are currently underway, see National Energy Technology Laboratory for more information.

Diesel Emission Control

The recent announcement by U.S. company Nanostellar that they have developed an automotive pollution control catalyst for diesel engines that contains gold, as well as the traditional platinum and palladium ingredients, is a major step-forward in cost effective emission control.

Used in nanoparticulate form, the use of platinum group metals in this application has soared from when they were first introduced in the mid-70’s to currently over 260 tonnes annually. With limited newly mined supply of platinum group metals, the cost of these catalyst systems is a major issue for automotive manufacturers and reductions in the cost of precious metal used is an on-going target.

In recent years, producers of catalyst materials have varied the relative amounts of palladium and platinum, depending on the price of the respective metals. Independent testing of Nanostellar’s NS Gold™, has shown that NS Gold™ increases hydrocarbon oxidation activity by 15-20 percent at equal precious-metal cost. A tri-metal formulation of gold, platinum, and palladium, NS Gold™ allows the proportions of each metal to be adjusted to help catalyst systems engineers meet engine-specific performance targets and stabilize the overall cost of diesel catalysts, despite fluctuations in the price of precious metals.

NS Gold™ is potentially suitable for treating all lean-stream exhaust, where air is in excess of fuel-borne hydrocarbon gases. Applications include, but are not limited to, treating particulates and hydrocarbons in soot filters, stationary-source volatile organic compound (VOC) emissions, and ammonia slip in selective catalytic reduction (SCR) systems. For more information visit www.nanostellar.com.

'Green' Chemistry

Green chemistry, also called sustainable chemistry, is a chemical philosophy encouraging the design of industrial chemicals and processes that reduce or eliminate the use and generation of hazardous substances. The use of gold as a catalyst has a major role to play in green chemistry.

For example, most industrial oxidation processes tend to use chlorine or organic peroxides. The chlorine processes produce large amounts of chloride salts and significant amounts of chlorinated organic by-products. The disadvantage of organic peroxides is their expense. It is fair to say that the chemical industry would be transformed if selective oxidation of hydrocarbons could be achieved efficiently using cheap and clean oxygen from the air. Recently a team led by Graham J. Hutchings, professor of physical chemistry at Cardiff University, in Wales, has shown that gold nanoparticles supported on carbon activate molecular oxygen in air to convert alkenes to partial oxidation products such as epoxides at atmospheric pressure and temperatures of 60–80 °C (Nature 2005, 437, 1132). This advance of ‘greener’ methods for oxidation catalysis using gold is a very important development.

As global demand and prices for petroleum-based feedstocks continue to rise, chemists are being challenged to devise processes that use biomass-derived feedstocks. In one of the latest developments, workers of the Center for Sustainable & Green Chemistry at the Technical University of Denmark, in Lyngby, have come up with a gold-catalyzed procedure for selective oxidation of the biomass-derived platform chemicals furfural and hydroxymethylfurfural to form their respective methyl esters. These chemicals are used for flavour and fragrance applications, in plastics and potentially as industrial solvents.

Source: World Gold Council

For more information on this source please visit World Gold Council