Platinum Group Metals: Overview, List, Properties and Applications

Metals of the platinum group are six noble precious chemical elements that are located side by side in the periodic table. All of them are transition metals of 8-10 groups of the 5-6 period.

Metals of the platinum group: list

The group consists of the following six chemical elements arranged in order of increasing atomic weight:

• Ru is ruthenium.
• Rh is rhodium.
• Pd is palladium.
• Os is osmium.
• Ir is iridium.
• Pt is platinum.

Metals of the platinum group have a silvery white hue, with the exception of osmium, whose color is bluish-white. Their chemical behavior is paradoxical in that they are highly resistant to most reagents, but are used as catalysts that easily accelerate or control the rate of oxidation, reduction, and hydrogenation reactions.

Ruthenium and osmium crystallize into a hexagonal close-packed system, while others have a face-centered cubic structure. This is expressed in the greater hardness of ruthenium and osmium.

History of the discovery

Although platinum-containing gold artifacts are dated to 700 BC. E., The presence of this metal is more likely an accident than a regularity. Jesuits in the XVI century mentioned gray dense pebbles associated with alluvial gold deposits. These stones could not be melted, but they formed an alloy with gold, while the ingots became brittle, and they were already impossible to clean. The pebbles became known as platina del Pinto, a pellet of silvery material from the Pinto River, which flows into the San Juan River in Colombia.

The forging platinum, which can only be obtained after a complete purification of the metal, was isolated by the French physicist Shabano in 1789. From it was made a goblet, presented to Pope Pius VI. The discovery of palladium in 1802 was reported by the English chemist William Wollaston, who named him. Element of the platinum metal group in honor of the asteroid. Wollaston subsequently announced the discovery of another substance present in the platinum ore. He called it rhodium because of the pink color of the metal salts. The discoveries of iridium (after the name of the Iridi rainbow goddess because of the variegated color of its salts) and osmium (from the Greek word "smell" due to the chlorine smell of its volatile oxide) were made by English chemist Smithson Tennant in 1803. French scientists Ippolit-Victor Kolle-Descoty, Antoine-Francois Fourcroix, and Nicolas-Louis Vauquelin singled out two metals at the same time. Ruthenium, the last isolated and identified element, received its name from the Latin name of Russia from the Russian chemist Karl Karlovich Klaus in 1844.

Unlike such easily isolated substances in a relatively pure state by simple fire refining, like gold, silver, platinum group metals require complex water-chemical treatment. These methods were not available until the end of the 19th century, so the identification and isolation of the platinum group lagged behind silver and gold for thousands of years. In addition, the high melting point of these metals limited their use until researchers in Britain, France, Germany and Russia developed methods for converting platinum into a form suitable for processing. As precious metals of the platinum group began to be used in jewelry since 1900. Although such an application remains relevant today, the industry has far surpassed it. Palladium has become a highly sought-after material for contacts in telephone relays and other wire communication systems, providing long life and high reliability, and platinum, because of its resistance to spark erosion, was used during the Second World War in the spark plugs of combat aircraft.

After the war, the expansion of the methods of molecular conversion during the processing of oil created a huge demand for the catalytic properties that metals of the platinum group possess. By the 1970s, consumption increased even further when automobile emission standards in the US and other countries led to the use of these chemical elements in the catalytic conversion of exhaust gases.

Ore

With the exception of small alluvial deposits of platinum, palladium and osmium iridium (an iridium-osmium alloy), there is practically no ore in which the chemical element, the metal of the platinum group, would be the main component. Minerals are generally found in sulphide ores, in particular in pentlandite (Ni, Fe) ₉ S ₈ . The most common are laurite RuS ₂ , irapsite, (Ir, Ru, Rh, Pt) AsS, osmiridium (Ir, Os), cuperite, (PtS) and braggite (Pt, Pd) S.

The world's largest deposit of platinum group metals is the Bushveld complex in South Africa. Large reserves of raw materials are concentrated in Sudbury deposits in Canada and Norilsk-Talnakh in Siberia. In the US, the largest deposits of platinum group minerals are located in Stillwater, Montana, but here they are much less than in South Africa and Russia. The world's largest producers of platinum are South Africa, Russia, Zimbabwe and Canada.

Extraction and processing

The main South African and Canadian deposits are operated by the mine method. Virtually all metals of the platinum group are extracted from copper or nickel sulfide minerals by flotation separation. The melting of the concentrate produces a mixture that is washed out of copper and nickel sulphides in an autoclave. The solid leach residue contains from 15 to 20% of the metals of the platinum group.

Sometimes gravity separation is used before flotation. As a result, a concentrate containing up to 50% platinum metals is obtained, which eliminates the need for smelting.

Mechanical properties

Metals of the platinum group are significantly different in mechanical properties. Platinum and palladium are rather soft and very malleable. With these metals and their alloys it is possible to work both in hot and in cold condition. Rhodium is first treated hot, and later it can be treated cold with quite frequent annealing. Iridium and ruthenium must be heated, cold processing, they do not lend themselves.

Osmium is the hardest of the group and has the highest melting point, but its tendency to oxidation imposes its limitations. Iridium is the most corrosion-resistant of platinum metals, and rhodium is valued for maintaining its properties at high temperatures.

Structural applications

Since the annealed platinum is very soft, it is susceptible to scratches and spoilage. To increase the hardness, it is fused with many other elements. Platinum jewelry is very popular in Japan, where it is called "hakkin" and "white gold". Alloys for jewelry contain 90% Pt and 10% Pd, which is easy to handle and solder. The addition of ruthenium increases the hardness of the alloy, while maintaining its resistance to oxidation. Alloys of platinum, palladium and copper are used in forged products, since they are harder than platinum-palladium and less costly.

Crucibles used for the production of single crystals in the semiconductor industry require corrosion resistance and stability at high temperatures. For this application, platinum, platinum-rhodium and iridium are best suited. Platinum-rhodium alloys are used in the production of thermocouples, which are designed to measure elevated temperatures up to 1800 ° C. Palladium is used in both pure and mixed form in electrical devices (50% of consumption), in dental alloys (30%). Rhodium, ruthenium and osmium are rarely used in pure form - they serve as an alloying additive for other metals of the platinum group.

Catalysts

About 42% of all platinum produced in the West is used as a catalyst. Of these, 90% is used in automotive exhaust systems, where refractory pellets or platinum-plated honeycomb structures (as well as palladium and rhodium) assist in the conversion of unburned hydrocarbons, carbon monoxide and nitrogen oxides into water, carbon dioxide and nitrogen.

The alloy of platinum and 10% of rhodium in the form of red-hot metal mesh serves as a catalyst in the reaction between ammonia and air to produce nitrogen oxides and nitric acid. When supplied with a mixture of ammonia, methane can produce cyanide. In oil refining, platinum on the surface of the aluminum oxide pellets in the reactor is a catalyst for the conversion of long chain oil molecules into branched isoparaffins, which are desirable in a mixture of high-octane gasolines.

Electroplating

All metals of the platinum group can be applied galvanically. Due to the hardness and luster of the coating obtained, rhodium is most often used. Although its cost is higher than that of platinum, a lower density makes it possible to use a smaller mass of material at comparable thickness.

Palladium is a metal of the platinum group, which is the easiest to use for coating. Due to this, the strength of the material is significantly increased. Ruthenium has found application in tools designed for friction treatment at low pressure.

Chemical compounds

Organic complexes of platinum group metals, such as alkylplatinum complexes, are used as catalysts in the process of olefin polymerization, production of polypropylene and polyethylene, and also in the oxidation of ethylene to acetaldehyde.

Platinum salts are increasingly used in cancer chemotherapy. For example, they are part of such drugs as "Carboplatinum" and "Cisplatinum." Covered with ruthenium oxide electrodes are used in the production of chlorine and sodium chlorate. Rhodium sulphate and phosphate are used in rhodium plating baths.