The platinum group metals 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.
Platinum Group Metals: List
The group consists of the following six chemical elements arranged in increasing atomic weight:
- Ru - ruthenium.
- Rh is rhodium.
- Pd is palladium.
- Os - osmium.
- Ir is iridium.
- Pt is platinum.
The metals of the platinum group have a silver-white hue, with the exception of osmium, the color of which 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 greater hardness of ruthenium and osmium.
Discovery story
Although platinum-containing gold artifacts are dated 700 BC. e., the presence of this metal is rather an accident than a regularity. In the sixteenth century, the Jesuits referred to dense gray pebbles associated with alluvial gold deposits. These pebbles could not be melted, but they formed an alloy with gold, while the ingots became brittle, and it was already impossible to clean them. Pebbles became known as platina del Pinto - granules of silver material from the Pinto River, which flows into the San Juan River in Colombia.
Malleable platinum, which can only be obtained after complete metal cleaning, was isolated by the French physicist Shabano in 1789. From it was made a cup presented to Pope Pius VI. The discovery of palladium in 1802 was reported by the English chemist William Wollaston, who named the chem. an element of the platinum metal group in honor of the asteroid. Wollaston subsequently claimed the discovery of yet another substance present in platinum ore. He called it rhodium because of the pink color of metal salts. The discoveries of iridium (named after the rainbow goddess Irida because of the colorful coloring of its salts) and osmium (from the Greek word “smell” due to the chlorine smell of its volatile oxide) were made by the English chemist Smithson Tennant in 1803. French scientists Hippolytus-Victor Collet Descotci, Antoine-Francois Fourcroix, and Nicolas-Louis Vauclin isolated two metals at the same time. Ruthenium, the last isolated and identified element, got its name from the Latin name of Russia from the Russian chemist Karl Karlovich Klaus in 1844.
In contrast to such substances that are easily released in a relatively pure state by simple fire refining, such as gold, silver, platinum group metals require complex water-chemical treatment. These methods were not available until the end of the 19th century, therefore, 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. How precious metals of the platinum group began to be used in jewelry since 1900. Although this application remains relevant today, industrial has far surpassed it. Palladium became a very demanded material for contacts in telephone relays and other wire communication systems, providing a long service life and high reliability, and platinum, due to its resistance to spark erosion, during World War II began to be used in the spark plugs of military aircraft.
After the war, the expansion of molecular conversion methods in oil refining created a huge demand for the catalytic properties possessed by platinum group metals. By the 1970s, consumption had grown even more when automobile emission standards in the United States and other countries led to the use of these chemical elements in the catalytic conversion of exhaust gases.
Ores
With the exception of small alluvial deposits of platinum, palladium and osmoid iridium (an alloy of iridium and osmium), there is practically no ore in which the main component would be a chemical element - a platinum group metal. Minerals are usually found in sulfide ores, in particular in pentlandite (Ni, Fe) 9 S 8 . The most common are laurite RuS 2 , irarsite, (Ir, Ru, Rh, Pt) AsS, osmiridium (Ir, Os), cuperite, (PtS), and braggite (Pt, Pd) S.
The world's largest platinum group metal deposit is the Bushveld complex in South Africa. Large reserves of raw materials are concentrated in the fields of Sudbury in Canada and Norilsk-Talnakhsky in Siberia. In the United States, the largest deposits of platinum group minerals are located in Stillwater, Montana, but here they are much smaller than in South Africa and Russia. The largest platinum producers in the world are South Africa, Russia, Zimbabwe and Canada.
Mining and processing
The main South African and Canadian deposits are mined. Almost all platinum group metals are extracted from copper or nickel sulfide minerals using flotation separation. Melting the concentrate produces a mixture that is washed out of copper and nickel sulfides in an autoclave. The solid leach residue contains from 15 to 20% of platinum group metals.
Sometimes gravity separation is used before flotation. The result is a concentrate containing up to 50% platinum metals, which eliminates the need for smelting.
Mechanical properties
Platinum group metals are significantly different in mechanical properties. Platinum and palladium are quite soft and very malleable. These metals and their alloys can be used both in hot and in cold condition. Rhodium is first processed hot, and later it can be processed cold with fairly frequent annealing. Iridium and ruthenium must be heated; they cannot be cold worked.
Osmium is the hardest of the group and has the highest melting point, but its tendency to oxidize 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 platinum that is completely annealed is very soft, it is susceptible to scratches and spoilage. To increase hardness, it is fused with many other elements. Platinum jewelry is very popular in Japan, where it is called "hakkin" and "white gold". Jewelry alloys contain 90% Pt and 10% Pd, which is easy to process and solder. Adding ruthenium increases the hardness of the alloy while maintaining oxidation resistance. Alloys of platinum, palladium and copper are used in forged products, since they are harder than platinum-palladium and less expensive.
Crucibles used for the production of single crystals in the semiconductor industry require corrosion resistance and stability at high temperatures. Platinum, platinum-rhodium and iridium are best suited for this application. Platinum rhodium alloys are used in the manufacture of thermocouples, which are designed to measure elevated temperatures up to 1800 ° C. Palladium is used both in 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 automobile exhaust systems, where refractory granules or honeycomb structures with a platinum coating (as well as palladium and rhodium) contribute to the conversion of unburned hydrocarbons, carbon monoxide and nitrogen oxides into water, carbon dioxide and nitrogen.
An alloy of platinum and 10% rhodium in the form of a red-hot metal grid serves as a catalyst in the reaction between ammonia and air to produce nitrogen oxides and nitric acid. When fed with a mixture of ammonia methane, hydrocyanic acid can be obtained. In petroleum refining, platinum on the surface of alumina pellets in a reactor is a catalyst for the conversion of long chain oil molecules to branched isoparaffins, which are desirable in a mixture of high octane gasolines.
Electroplating
All platinum group metals can be applied galvanically. Due to the hardness and gloss of the resulting coating, rhodium is most commonly used. Although its cost is higher than platinum, the lower density allows the use of a lower mass of material with a comparable thickness.
Palladium is a platinum group metal that is 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 processing at low pressure.
Chemical compounds
Organic platinum group metal complexes, such as alkylplatinum complexes, are used as catalysts in the polymerization of olefins, in the production of polypropylene and polyethylene, and in the oxidation of ethylene to acetaldehyde.
Platinum salts are increasingly used in cancer chemotherapy. For example, they are part of drugs such as carboplatin and cisplatin. Ruthenium oxide coated electrodes are used in the production of chlorine and sodium chlorate. Rhodium sulfate and phosphate are used in rhodium plating baths.