Why Are Noble Metals Important in Chemistry and Industry?
Noble metals meaning refers to the selection of the periodic table's transition metal group. The list includes Silver, Gold, Platinum, Iridium, Palladium, Rhodium, Ruthenium, and Osmium. These metals are used to make noble alloys and various valuable elements. The noble metal prices vary hugely depending on their properties and various other factors like the ease to find and extract them and the noble metal refining process.
This section contains a brief description of the noble metals and popular 9 noble metals along with the others.
A Noble Metal: Brief Explanation
Platinum also refers to a subset of some of these elements, famous as the Platinum group and Ruthenium, Rhodium, Palladium, Platinum, Iridium and Osmium. Furthermore, a noble metal can also be referred to as an inert metal, and it can be a more or a less noble metal. Ruthenium, Rhodium, Iridium and Osmium are also famous as refractory metals defined by having melting points above 2000°C.
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Most noble metals crystallise in the fcc (face-centred cubic) structure, except Ruthenium and Osmium. These three metals have an hcp (hexagonal close-packed) structure. A noble metal includes the properties at the nanoscale, and its applications are mainly in the fields of biomedicine and catalysis.
Silver (Ag)
Silver has excellent electrical conductivity and higher chemical stability. Bulk Silver acts as a common element for making the higher-quality reflectors of electromagnetic radiations in a visible region, thus superseding the cheaper non-noble metals. Ag NPs have various applications like antimicrobial actions and catalytic properties, including enhancing the drinking water's microbial quality. Ag NPs represent ferromagnetic behaviour, and the percentage concentration of its surface atoms increases with the decrease in its particle size.
Gold (Au)
Due to exceptional chemical stability and good electrical conductivity, this noble metal has various significant uses. It is one of the best optical reflectors for the infrared region of the electromagnetic spectrum. Gold NPs have a more substantial size-dependent position of localised Plasmon resonance and have acquired attention in biomedicine, electronics and catalysis. Gold NPs also have a size-dependent magnetic behaviour that originates from the electron transfers between Gold's surface atoms and capping agents.
Platinum (Pt)
It is also ductile and malleable but is denser than the other ones. This inert metal is also chemically unreactive, but it is known to have a more significant density of free electrons, making it a suitable chemical catalyst. Platinum is widely used in catalytic converters for oxidising carbon monoxide that generates in combustion engines. Platinum also serves as a versatile electrode for electrochemical experiments. Platinum NPs are mainly engineered for catalytic applications and is also known to have been helpful in cancer therapies.
Palladium (Pd)
Palladium is a silvery-white metal found in deposits with Platinum, Nickel, and Copper. It has corrosion resistance, and its noble alloys are used for making jewellery as "white gold." It has an extensive capacity for absorbing hydrogen in a ration 900:1 by volume, and thus it serves as an excellent catalyst for hydrogenation and dehydrogenation reactions. There are various ways to synthesise Palladium NPs, and they have various applications like antimicrobial actions and enhanced Raman scattering.
Iridium (Ir)
It is also a silvery-white metal with higher corrosion resistance and higher density. It is an unworkable metal, but it is used in space components and spark plugs when alloyed with Platinum. Iridium NPs are synthesised using various chemical methods and tested as catalysts and sensors. It also helps as a biosensor to detect glucose.
Osmium (Os)
Osmium is a member of the Platinum group and is found in its ores. It has a higher density and melting point. The alloys of osmium and Platinum are harder than Platinum and are helped for speciality equipment. Osmium oxide is toxic to the respiratory system. Osmium NPs are used for hydrogenation reactions, CO oxidation, and electrocatalysis for PEM fuel cells.
Ruthenium (Ru)
Ruthenium is chemically inert and has a silvery colour. With less than 1% concentration, Ru can raise Palladium and Platinum alloys' hardness and increases the corrosion resistance in Titanium. Ruthenates also appear in electronics as the thicker film resistors, and some appear in the explorations of magnetism, superconductivity, and multiferroic prototypes.
Rhodium (Rh)
Rhodium is a harder and silvery-white transition metal with corrosion resistance and chemically inert action. It is extensively used in the automotive industry and has other applications, like neutron flux detectors and electrical contracts. It also serves biological needs and heterogeneous catalysis.
FAQs on What Are Noble Metals? Definition, Properties & Examples
1. What are noble metals and which elements are commonly included in this group?
Noble metals are a group of metals that are highly resistant to corrosion and oxidation in moist air. Unlike most base metals, they do not easily react with non-oxidising acids. The core list of noble metals includes:
- Ruthenium (Ru)
- Rhodium (Rh)
- Palladium (Pd)
- Silver (Ag)
- Osmium (Os)
- Iridium (Ir)
- Platinum (Pt)
- Gold (Au)
2. Why are these specific metals referred to as 'noble'?
The term 'noble' is an analogy to nobility, suggesting that these metals are superior or stand apart from others. Chemically, this 'nobility' refers to their low reactivity. This is because they have a full or nearly full 'd' electron subshell, which makes them chemically stable and reluctant to lose electrons to form compounds. This inherent stability is what gives them their excellent resistance to tarnish, corrosion, and acid attacks.
3. What are the most important shared properties of noble metals?
While each noble metal is unique, they share several key characteristics that make them valuable:
- High Resistance to Corrosion: They do not rust or tarnish easily when exposed to air and moisture.
- Catalytic Activity: Many noble metals, especially platinum and palladium, are excellent catalysts, speeding up chemical reactions without being consumed.
- Ductility and Malleability: Metals like gold and silver are extremely malleable and ductile, meaning they can be easily shaped into wires or thin sheets.
- High Lustre: They have a characteristic metallic shine, making them desirable for jewellery and decorative items.
- High Density and Melting Points: Most noble metals are very dense and have high melting points.
4. How do the properties of noble metals differ from those of base metals?
The primary difference lies in their reactivity. Noble metals are unreactive and resist oxidation and corrosion. They are often found in their pure, metallic form in nature. In contrast, base metals like iron, lead, and zinc are highly reactive. They corrode or oxidise easily and are typically found in nature as ore compounds (e.g., oxides, sulphides) from which the pure metal must be extracted.
5. What are some common real-world applications of noble metals?
Noble metals are used across various industries due to their unique properties:
- Jewellery and Coinage: Gold, silver, and platinum are widely used for making jewellery and investment coins (bullion) because of their beauty and resistance to tarnish.
- Automotive Industry: Platinum, palladium, and rhodium are essential components in catalytic converters, which convert toxic gases from engine exhaust into less harmful substances.
- Electronics: Gold and silver are used for high-quality electrical contacts and connectors due to their excellent conductivity and resistance to corrosion.
- Dentistry and Medicine: Gold and palladium alloys are used for dental fillings and crowns. Silver has antimicrobial properties, and platinum-based drugs are used in cancer therapy.
- Alloys: Iridium and ruthenium are often added to other metals like platinum to increase their hardness and durability for items like spark plugs and fountain pen nibs.
6. Why do gold and platinum dissolve in aqua regia but not in single acids?
Gold and platinum are extremely unreactive and will not dissolve in individual acids like nitric acid or hydrochloric acid. However, they dissolve in aqua regia (a mixture of one part concentrated nitric acid and three parts concentrated hydrochloric acid). This is because the two acids perform different tasks: the nitric acid is a powerful oxidising agent that converts a tiny amount of gold into gold ions (Au³⁺). The hydrochloric acid then provides chloride ions (Cl⁻) which react with the gold ions to form a stable complex ion, tetrachloroaurate(III) ([AuCl₄]⁻). This second reaction removes gold ions from the solution, allowing more gold metal to oxidise and dissolve.
7. Is copper considered a noble metal?
Copper's classification is often debated. It is in the same group of the periodic table as the noble metals silver and gold and shows more resistance to corrosion than many base metals. For this reason, it is sometimes classified as a semi-noble metal. However, copper oxidises more readily than true noble metals like gold and platinum (as seen when it turns green), so it is not typically included in the core list of noble metals.
8. How does the behaviour of noble metals, like gold and silver, change at the nanoscale?
At the nanoscale, the properties of noble metals can change dramatically. When broken down into nanoparticles (NPs), their surface area to volume ratio increases, leading to unique behaviours:
- Optical Properties: Bulk gold is yellow, but gold nanoparticles can appear deep red or blue depending on their size. This is due to a phenomenon called Localised Surface Plasmon Resonance (LSPR), which has applications in medical diagnostics and sensors.
- Catalytic Activity: The catalytic efficiency of metals like gold and platinum can be significantly enhanced at the nanoscale, making them even more effective in chemical reactions.
- Antimicrobial Action: Silver nanoparticles (Ag NPs) exhibit powerful antimicrobial properties far greater than bulk silver, making them useful in water purification and medical coatings.
