What Is Rhodium?
Rhodium is a rare and precious transition metal belonging to the platinum group. It is characterized by its unique electronic structure, which imparts excellent catalytic properties. Rhodium-based catalysts find extensive applications in various fields, including environmental protection, petrochemical processing, fine chemical industry, and biopharmaceuticals.
Properties of Rhodium
Physical Properties
- Silvery-white color with a high reflectivity
- High melting point (1966°C) and boiling point (3727°C)
- High density (12.41 g/cm³)
- Excellent corrosion resistance and chemical stability
- High elastic modulus and hardness
Chemical Properties
- Excellent catalytic activity, particularly for reduction reactions
- Forms stable complexes with various ligands
- Resistant to oxidation and tarnishing
- Exhibits multiple oxidation states (0 to +6)
Production of Rhodium
- Aqueous Reduction Method: This involves providing an aqueous suspension of diethylenetriammonium hexahalorhodate adjusted to a pH between -1 and +2, adding a reducing agent to convert it to elemental rhodium, and separating the rhodium product from the aqueous acid solution.
- Molten Salt Dissolution: The process separates rhodium from other metals/substrates, dissolves it in molten salt, and selectively precipitates contaminants.The rhodium solution is then purified via liquid-liquid extraction and treated with hydrogen peroxide to produce rhodium powder or electrodeposit rhodium films.
- Hydroxide Dissolution: The process dissolves rhodium black in sulfuric acid, neutralizes it to precipitate rhodium hydroxide, and then dissolves it in nitric acid to produce a stable rhodium nitrate solution suitable for catalyst preparation.
Applications of Rhodium
Catalytic Applications
- Automotive Catalytic Converters: Rhodium is a crucial component in catalytic converters used to reduce the toxicity of exhaust gases from vehicles. Each converter typically contains around grams of rhodium and other noble metals coated on a ceramic support. This accounts for over 80% of the global rhodium demand.
- Industrial Emission Control: Industries employ rhodium catalysts to treat gaseous emissions from power plants and waste gases contaminated with harmful organic compounds.
- Chemical and Pharmaceutical Synthesis: Chemical and pharmaceutical companies use rhodium as a catalyst in various processes, leveraging its unique catalytic properties.
Electrochemical Applications
- Electrochemical Cells and Batteries: Electrochemical cells and batteries use rhodium compositions as electrodes, including cathodes.
- Electroplating and Coatings: Industries use rhodium for electroplating and forming rhodium films on substrates, providing hardness, wear resistance, and attractive finishes.
Alloy and Material Applications
- Spark Plug Electrodes: Manufacturers use rhodium alloys containing chromium and yttrium as spark plug electrodes due to their high melting point and durability.
- Sensors and Lead-ins: Industries form rhodium alloys into wires, tubes, or sheets for sensors, lead-ins, and applications needing corrosion resistance and high-temperature stability.
- Aerospace and High-Temperature Materials: Researchers are exploring rhodium alloys for potential applications in aerospace and other high-temperature environments.
Emerging Applications
- Anticancer Drugs: Recent research has explored the potential of rhodium-based compounds as anticancer agents, leveraging their unique structural and reactivity properties at the chemistry-biology interface.
- Electronics and Optics: Rhodium nanoparticles and films are being investigated for novel applications in electronics, optics, and other emerging fields, taking advantage of their unique properties.
Application Cases
| Product/Project | Technical Outcomes | Application Scenarios |
|---|---|---|
| Rhodium Catalytic Converters | Rhodium catalysts enable efficient reduction of harmful emissions like carbon monoxide, hydrocarbons, and nitrogen oxides in automotive exhaust gases, meeting stringent environmental regulations. | Automotive industry, reducing vehicular emissions to improve air quality in urban areas. |
| Rhodium Electroplating | Rhodium coatings provide exceptional hardness, corrosion resistance, and attractive finishes, enhancing durability and aesthetics of various products. | Decorative and functional coatings for jewellery, watches, electrical contacts, and optical instruments. |
| Rhodium Alloys | Rhodium alloys exhibit high melting points, resistance to oxidation and corrosion, and excellent mechanical properties, enabling their use in harsh environments. | Aerospace components, high-temperature applications, and chemical processing equipment. |
| Rhodium Fuel Cells | Rhodium-based catalysts facilitate efficient electrochemical reactions in fuel cells, enabling clean energy generation with high power density and low emissions. | Stationary and portable power generation, transportation, and backup power systems. |
| Rhodium Pharmaceuticals | Rhodium complexes exhibit unique catalytic properties and biological activities, enabling their use in drug synthesis and as potential anticancer agents. | Pharmaceutical industry, drug development, and targeted cancer therapies. |
Latest innovations of Rhodium
Rhodium Alloy Compositions and Properties
Novel rhodium alloys, developed for spark plug electrodes, feature enhanced wear resistance, oxidation resistance, and improved formability. These alloys primarily contain rhodium, iridium, platinum, palladium, and other elements like yttrium, zirconium, and samarium. The addition of at least 5 wt% chromium to rhodium-nickel alloys enhances oxidation and spark erosion resistance.
Rhodium Film Deposition and Synthesis
Researchers developed low-temperature atomic layer deposition (ALD) techniques to form smooth, continuous rhodium films with good step coverage and reduced carbon content. These techniques involve using a rhodium precursor gas followed by oxygen exposure.
Novel rhodium nitrate solutions have been prepared by reacting rhodium hydroxide with nitric acid and concentrating the mixture under heating. These solutions exhibit high purity and improved supporting efficiency (up to 99%) on inorganic catalyst supports.
Rhodium Recovery and Purification
Improved processes have been developed for recovering and purifying rhodium from other metals or substrates. These processes dissolve rhodium in molten salt, followed by selective precipitation of contaminants and extraction of impurities. The impurities undergo liquid-liquid extraction, and treatment with hydrogen peroxide yields purified rhodium solutions. These solutions are ideal for electrochemical baths or chemical reduction, producing high-quality rhodium powder.
New methods have been proposed for producing rhodium with improved productivity by increasing the dissolution and recovery rate. These involve dissolving a sodium compound in a rhodium-containing raw material, heating the mixture, filtering, and dissolving the sludge in aqua regia to obtain a rhodium-containing filtrate.
Technical challenges
| Rhodium Alloy Development | Developing novel rhodium alloys with enhanced properties such as wear resistance, oxidation resistance, and formability for applications like spark plug electrodes and ignition devices. |
| Rhodium Film Deposition | Developing low-temperature atomic layer deposition techniques for forming smooth, continuous rhodium films with good step coverage and reduced carbon content. |
| Rhodium Nitrate Solution Synthesis | Preparing high-purity rhodium nitrate solutions with improved heat stability and long-term storage capability for various applications. |
| Rhodium Recovery and Purification | Developing processes for efficient recovery and purification of rhodium from various sources, including molten salt dissolution, selective precipitation, and liquid-liquid extraction. |
| Rhodium Nanostructure Synthesis | Synthesizing rhodium nanostructures, such as nanoparticles and two-dimensional nanomaterials, for catalytic applications in various chemical reactions. |
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