Platinum nanoparticle

From Wikipedia, the free encyclopedia

Platinum nanoparticles are usually in the form of a suspension or colloid of nanoparticles of platinum in a fluid, usually water. A colloid is technically defined as a stable dispersion of particles in a fluid medium (liquid or gas). 

Spherical platinum nanoparticles can be made with sizes between about 2 and 100 nanometres (nm), depending on reaction conditions. Platinum nanoparticles are suspended in the colloidal solution of brownish-red or black color. Nanoparticles come in wide variety of shapes including spheres, rods, cubes, and tetrahedra.

Platinum nanoparticles are the subject of substantial research, with potential applications in a wide variety of areas. These include catalysis, medicine, and the synthesis of novel materials with unique properties.

Synthesis

Platinum nanoparticles are typically synthesized either by the reduction of platinum ion precursors in solution with a stabilizing or capping agent to form colloidal nanoparticles, or by the impregnation and reduction of platinum ion precursors in a micro-porous support such as alumina.

Some common examples of platinum precursors include potassium hexachloroplatinate (K₂PtCl₆) or platinous chloride (PtCl₂) Different combinations of precursors, such as ruthenium chloride (RuCl₃) and chloroplatinic acid (H₂PtCl₆), have been used to synthesize mixed-metal nanoparticles Some common examples of reducing agents include hydrogen gas (H₂), sodium borohydride (NaBH₄) and ethylene glycol (C₂H₆O₂), although other alcohols and plant-derived compounds have also been used.

As the platinum metal precursor is reduced to neutral platinum metal (Pt⁰), the reaction mixture becomes supersaturated with platinum metal and the Pt⁰ begins to precipitate in the form of nanoscale particles. A capping agent or stabilizing agent such as sodium polyacrylic acid or sodium citrate is often used to stabilize the nanoparticle surfaces, and prevents the aggregation and coalescence of the nanoparticles. 

The size of nanoparticles synthesized colloidally may be controlled by changing the platinum precursor, the ratio of capping agent to precursor, and/or the reaction temperature. The size of the nanoparticles can also be controlled with small deviation by using a stepwise seed-mediated growth procedure as outlined by Bigall et al. (2008). The size of nanoparticles synthesized onto a substrate such as alumina depends on various parameters such as the pore size of the support.

Platinum nanoparticles can also be synthesized by decomposing Pt₂(dba)₃ (dba = dibenzylideneacetone) under a CO or H₂ atmosphere, in the presence of a capping agent. The size and shape distributions of the resulting nanoparticles depend on the solvent, the reaction atmosphere, the types of capping agents and their relative concentrations, the specific platinum ion precursor, as well at the temperature of the system and reaction time.

Shape and size control

Electron micrographs of Ostwald ripening in Pd nanoparticles dissolved in formaldehyde at 6 (a), 24 (b), 48 (c) and 72 hours (d). The small Pd particles are being consumed as the larger ones grow bigger.

 

Ramirez et al. reported the influence of ligand and solvent effects on the size and shape of platinum nanoparticles. Platinum nanoparticle seeds were prepared by the decomposition of Pt₂(dba)₃ in tetrahydrofuran (THF) under carbon monoxide (CO). These conditions produced Pt nanoparticles with weakly bound THF and CO ligands and an approximate diameter on 1.2 nm. Hexadecylamine (HDA) was added to the purified reaction mixture and allowed to displace the THF and CO ligands over the course of approximately seven days, producing monodispersed spherical crystalline Pt nanoparticles with an average diameter of 2.1 nm. After the seven-day period, an elongation of the Pt nanoparticles occurred. When the same procedure was followed using a stronger capping agent such as triphenyl phosphine or octanethiol, the nanoparticles remained spherical, suggesting that the HDA ligand affects particle shape. 

Oleylamine, oleic acid and platinum acetylacetonate [Pt(acac)₂] are also used in the synthesis of size/shape controlled platinum nanoparticles. Research showed that alkylamine can coordinate with Pt²⁺ ion and form tetrakis(amine)platinate precursor and replace the original acac⁻ ligand in Pt(acac)₂, and oleic acid can further exchange with acac⁻ and tune the formation kinetics of platinum nanoparticles.

When Pt₂(dba)₃ was decomposed in THF under hydrogen gas in the presence HDA, the reaction took much longer, and formed nanowires with diameters between 1.5 and 2 nm. Decomposition of Pt₂(dba)₃ under hydrogen gas in toluene yielded the formation of nanowires with 2–3 nm diameter independent of HDA concentration. The length of these nanowires was found to be inversely proportional to the concentration of HDA present in solution. When these nanowire syntheses were repeated using reduced concentrations of Pt₂(dba)₃, there was little effect on the size, length or distribution of the nanowires formed. 

Platinum nanoparticles of controlled shape and size have also been accessed through varying the ratio of polymer capping agent concentration to precursor concentration. Reductive colloidal syntheses as such have yielded tetrahedral, cubic, irregular-prismatic, icosahedral, and cubo-octahedral nanoparticles, whose dispersity is also dependent on the concentration ratio of capping agent to precursor, and which may be applicable to catalysis. The precise mechanism of shape-controlled colloidal synthesis is not yet known; however, it is known that the relative growth rate of crystal facets within the growing nanostructure determines its final shape. Polyol syntheses of platinum nanoparticles, in which chloroplatinic acid is reduced to PtCl₄²⁻ and Pt⁰ by ethylene glycol, have also been a means to shape-controlled fabrication. Addition of varying amounts of sodium nitrate to these reactions was shown to yield tetrahedra and octahedra at high concentration ratios of sodium nitrate to chloroplatinic acid. Spectroscopic studies suggest that nitrate is reduced to nitrite by PtCl₄²⁻ early in this reaction, and that the nitrite may then coordinate both Pt(II) and Pt(IV), greatly slowing the polyol reduction and altering the growth rates of distinct crystal facets within the nanoparticles, ultimately yielding morphological differentiation.

Green synthesis

An ecologically-friendly synthesis of platinum nanoparticles from chloroplatinic acid was achieved through the use of a leaf extract of Diospyros kaki as a reducing agent. Nanoparticles synthesized as such were spherical with an average diameter ranging from 212 nm depending on reaction temperature and concentration of leaf extract used. Spectroscopic analysis suggests that this reaction is not enzyme-mediated and proceeds instead through plant-derived reductive small molecules. Another eco-friendly synthesis from chloroplatinic acid was reported using leaf extract from Ocimum sanctum and tulsi as reducing agents. Spectroscopic analysis suggested that ascorbic acid, gallic acid, various terpenes, and certain amino acids were active in the reduction. Particles synthesized as such were shown through scanning electron microscopy to consist in aggregates with irregular shape. It has been shown that tea extracts with high polyphenol content may be used both as reducing agents and capping agents for platinum nanoparticle synthesis.

Properties

The chemical and physical properties of platinum nanoparticles (NP) make them applicable for a wide variety of research applications. Extensive experimentation has been done to create new species of platinum NPs, and study their properties. Platinum NP applications include electronics, optics, catalysts, and enzyme immobilization.

Catalytic properties

Platinum NPs are used as catalysts for proton exchange membrane fuel cell (PEMFC), for industrial synthesis of nitric acid, reduction of exhaust gases from vehicles and as catalytic nucleating agents for synthesis of magnetic NPs. NPs can act as catalysts in homogeneous colloidal solution or as gas-phase catalysts while supported on solid state material. The catalytic reactivity of the NP is dependent on the shape, size and morphology of the particle.

One type of platinum NPs that have been researched on are colloidal platinum NPs. Monometallic and bimetallic colloids have been used as catalysts in a wide range of organic chemistry, including, oxidation of carbon monoxide in aqueous solutions, hydrogenation of alkenes in organic or biphasic solutions and hydrosilylation of olefins in organic solutions. Collodial platinum NPs protected by Poly(N-isopropylacrylamide) were synthesised and their catalytic properties measured. It was determined that they were more active in solution and inactive when phase separated due to its solubility being inversely proportional to temperature.

Optical properties

Platinum NPs exhibit fascinating optical properties. Being a free electron metal NP like silver and gold, its linear optical response is mainly controlled by the surface plasmon resonance. Surface plasmon resonance occurs when the electrons in the metal surface are subject to an electromagnetic field that exerts a force on the electrons and cause them to displace from their original positions. The nuclei then exert a restoring force that results in oscillation of the electrons, which increase in strength when frequency of oscillations is in resonance with the incident electromagnetic wave.

The SPR of platinum nanoparticles is found in the ultraviolet range (215 nm), unlike the other noble metal nanoparticles which display SPR in the visible range Experiments were done and the spectra obtained are similar for most platinum particles regardless of size. However, there is an exception. Platinum NPs synthesized via citrate reduction do not have a surface plasmon resonance peak around 215 nm. Through experimentation, the resonance peak only showed slight variations with the change of size and synthetic method (while maintaining the same shape), with the exception of those nanoparticles synthesized by citrate reduction, which did not exhibit and SPR peak in this region..

Through the control of percent composition of 2–5 nm platinum nanoparticles on SiO₂, Zhang et al. modeled distinct absorption peaks attributed to platinum in the visible range, distinct from the conventional SPR absorption. This research attributed these absorption features to the generation and transfer of hot electrons from the platinum nanoparticles to the semiconductive material. The addition of small platinum nanoparticles on semiconductors such as TiO₂ increases the photocatalytic oxidation activity under visible light irradiation. These concepts suggest the possible role of platinum nanoparticles in the development of solar energy conversion using metal nanoparticles. By changing the size, shape and environment of metal nanoparticles, their optical properties can be used for electrontic, catalytic, sensing, and photovoltaic applications.

Applications

Fuel cells application

Hydrogen fuel cells

Among the precious metals, platinum is the most active toward the hydrogen oxidation reaction that occurs at the anode in hydrogen fuel cells. In order to meet cost reductions of this magnitude, the Pt catalyst loading must be decreased. Two strategies have been investigated for reducing the Pt loading: the binary and ternary Pt-based alloyed nanomaterials and the dispersion of Pt-based nanomaterials onto high surface area substrates.

Methanol fuel cells

The methanol oxidation reaction occurs at the anode in direct methanol fuel cells (DMFCs). Platinum is the most promising candidate among pure metals for application in DMFCs. Platinum has the highest activity toward the dissociative adsorption of methanol. However, pure Pt surfaces are poisoned by carbon monoxide, a byproduct of methanol oxidation. Researchers have focused on dispersing nanostructured catalysts on high surface area supporting materials and the development of Pt-based nanomaterials with high electrocatalytic activity toward MOR to overcome the poisoning effect of CO.

Electrochemical oxidation of formic acid

Formic acid is another attractive fuel for use in PEM-based fuel cells. The dehydration pathway produces adsorbed carbon monoxide. A number of binary Pt-based nanomaterial electrocatalysts have been investigated for enhanced electrocatalytic activity toward formic acid oxidation.

Modifying conductivity of zinc oxide materials

Platinum NPs can be used to dope zinc oxide (ZnO) materials to improve their conductivity. ZnO has several characteristics that allow it to be used in several novel devices such as development of light-emitting assemblies and solar cells. However, because ZnO is of slightly lower conductivity than metal and indium tin oxide (ITO), it can be doped and hybridized with metal NPs like platinum to improve its conductivity. A method to do so would be to synthesize ZnO NPs using methanol reduction and incorporate at 0.25 at.% platinum NPs. This boosts the electrical properties of ZnO films while preserving its transmittance for application in transparent conducting oxides.

Glucose detection applications

Enzymatic glucose sensors have drawbacks that originate from the nature of the enzyme. Nonenzymatic glucose sensors with Pt-based electrocatalysts offer several advantages, including high stability and ease of fabrication. Many novel Pt and binary Pt-based nanomaterials have been developed to overcome the challenges of glucose oxidation on Pt surfaces, such as low selectivity, poor sensitivity, and poisoning from interfering species.

Other applications

Platinum catalysts are alternatives of automotive catalytic converters, carbon monoxide gas sensors, petroleum refining, hydrogen production, and anticancer drugs. These applications utilize platinum nanomaterials due to their catalytic ability to oxidize CO and NOx, dehydrogenate hydrocarbons, and electrolyze water and their ability to inhibit the division of living cells.

Biological interactions

The increased reactivity of nanoparticles is one of their most useful properties and is leveraged in fields such as catalysis, consumer products, and energy storage. However, this high reactivity also means that a nanoparticle in a biological environment may have unintended impacts. For example, many nanoparticles such as silver, copper, and ceria interact with cells to produce reactive oxygen species or ROS which can cause premature cell death through apoptosis. Determining the toxicity of a specific nanoparticle requires knowledge of the particle’s chemical composition, shape, size and is a field that is growing alongside advances in nanoparticle research.

Determining the impact of a nanoparticle on a living system is not straightforward. A multitude of in vivo and in vitro studies must be done to fully characterize reactivity. In vivo studies often use whole organisms such as mice or zebrafish to infer the interaction of the nanoparticle on a healthy human body. In vitro studies look at how nanoparticles interact with specific cell colonies, typically of human origin. Both types of experiments are needed for a complete understanding of nanoparticle toxicity, especially human toxicity, since no one model has complete human relevance.

Drug delivery

A topic of research within the field of nanoparticles is how to use these small particles for drug delivery. Depending on particle properties, nanoparticle may move throughout the human body are promising as site-specific vehicles for the transport of medicine. Current research using platinum nanoparticles in drug delivery uses platinum-based carries to move antitumor medicine. In one study, platinum nanoparticles of diameter 58.3 nm were used to transport an anticancer drug to human colon carcinoma cells, HT-29. Uptake of the nanoparticles by the cell involves compartmentalization of the nanoparticles within lysosomes. The high acidity environment enables leaching of platinum ions from the nanoparticle, which the researchers identified as causing the increased effectiveness of the drug. In another study, a Pt nanoparticle of diameter 140 nm was encapsulated within a PEG nanoparticle to move an antitumor drug, Cisplatin, within a prostate cancer cell (LNCaP/PC3) population. Use of platinum in drug delivery hinges on its ability to not interact in a harmful manner in healthy portions of the body while also being able to release its contents when in the correct environment.

Toxicology

Toxicity stemming from platinum nanoparticles can take multiple forms. One possible interaction is cytotoxicity or the ability of the nanoparticle to cause cell death. A nanoparticle can also interact with the cell’s DNA or genome to cause genotoxicity. These effects are seen in different levels of gene expression measured through protein levels. Last is the developmental toxicity that can occur as an organism grows. Developmental toxicity looks at the impact the nanoparticle has on the growth of an organism from an embryonic stage to a later set point. Most nanotoxicology research is done on cyto- and genotoxicity as both can easily be done in a cell culture lab.

Platinum nanoparticles have the potential to be toxic to living cells. In one case, 2 nm platinum nanoparticles were exposed to two different types of algae in order to understand how these nanoparticles interact with a living system. In both species of algae tested, the platinum nanoparticles inhibited growth, induced small amounts of membrane damage, and created a large amount of oxidative stress. In another study, researcher tested the effects of differently sized platinum nanoparticles on primary human keratinocytes. The authors tested 5.8 and 57.0 nm Pt nanoparticles. The 57 nm nanoparticles had some hazardous effects including decreased cell metabolism, but the effect of the smaller nanoparticles was much more damaging. The 5.8 nm nanoparticles exhibited a more deleterious effect on the DNA stability of the primary keratincoytes than did the larger nanoparticles. The damage to the DNA was measured for individual cells using single-gel electrophoresis via the comet assay. 

Researchers have also compared the toxicity of Pt nanoparticles to other commonly used metallic nanoparticles. In one study, the authors compared the impact of different nanoparticle compositions on the red blood cells found in the human bloodstream. The study showed that 5–10 nm platinum nanoparticles and 20–35 nm gold nanoparticles have very little effect on the red blood cells. In the same study it was found that 5–30 nm silver nanoparticles caused membrane damage, detrimental morphological variation, and haemagglutination to the red blood cells.

In a recent paper published in Nanotoxicology, the authors found that between silver (Ag-NP, d = 5–35 nm), gold (Au-NP, d = 15–35 nm), and Pt (Pt-NP, d = 3–10 nm) nanoparticles, the Pt nanoparticles were the second most toxic in developing zebrafish embryos, behind only the Ag-NPs. However, this work did not examine the size dependence of the nanoparticles on their toxicity or biocompatibility. Size-dependent toxicity was determined by researchers at the National Sun Yat – Sen University in Kaohsiung, Taiwan. This group’s work showed that the toxicity of platinum nanoparticles in bacterial cells is strongly dependent on nanoparticle size and shape/morphology. Their conclusions were based on two major observations. First, the authors found that platinum nanoparticles with spherical morphologies and sizes less than 3 nm showed biologically toxic properties; measured in terms of mortality, hatching delay, phenotypic defects and metal accumulation. While those nanoparticles with alternative shapes—such as cuboidal, oval, or floral—and sizes of 5–18 nm showed biocompatibility and no biologically toxic properties. Secondly, out of the three varieties of platinum nanoparticles which exhibited biocompatibility, two showed an increase in bacterial cell growth.

The paper introduces many hypotheses for why these observations were made, but based on other works and basic knowledge of bacterial cell membranes, the reasoning behind the size dependent toxicity observation seems to be twofold. One: The smaller, spherically shaped nanoparticles are able to pass through cell membranes simply due to their reduced size, as well as their shape-compatibility with the typically spherical pores of most cell membranes. Although this hypothesis needs to be further supported by future work, the authors did cite another paper which tracked the respiratory intake of platinum nanoparticles. This group found that 10 µm platinum nanoparticles are absorbed by the mucus of the bronchi and trachea, and can travel no further through the respiratory tract. However, 2.5 µm particles showed an ability to pass through this mucus layer, and reach much deeper into the respiratory tract. Also the larger, uniquely shaped nanoparticles are too large to pass through the pores of the cell membrane, and/or have shapes which are incompatible with the more spherically shaped pores of the cellular membrane. In regards to the observation that the two largest platinum nanoparticles (6–8 nm oval, and 16–18 nm floral) actually increase bacterial cell growth, the explanation could originate in the findings of other works which have shown that platinum nanoparticles have demonstrated significant antioxidative capacity. However, it must be noted that in order for these antioxidative properties to be exploited, the platinum nanoparticles must first enter the cells, so perhaps there is another explanation for this observation of increased bacterial cell growth. 

Most studies so far have been size based using an in vivo mouse model. In one study, researchers compared the effects of sun 1 nm and 15 nm platinum nanoparticles on mice. The 15 mg/kg dose of sub 1 nm platinum nanoparticles were found to cause liver damage while the larger particles had no effect. A similar study using a singular injection as an exposure source of platinum nanoparticles into the mouse model found necrosis of tubular epithelial cells for particles under 1 nm, but no effect with those particles of 8 nm. These in vivo studies show a trend that the toxicity of the platinum nanoparticles is size dependent, most likely due to the ability of the nanoparticle to get into a high impactful region within the body. A complete study analyzing the effect of varying sized platinum nanoparticles used both in vivo and in vitro models is used to gain a better understanding what impact these nanoparticles could have. Using mice as a model, they found retention of the platinum nanoparticles by the respiratory tract of the mouse. This was accompanied by a minor to mild inflammation of the surrounding lung tissue. However, their in vitro tests using human and lung epithelial cells found no cytotoxic or oxidative stress effects caused by the platinum nanoparticles despite clear evidence of cellular uptake.

Platinum

From Wikipedia, the free encyclopedia

Platinum,  ₇₈Pt

Platinum
Pronunciation	/ˈplætɪnəm/ ​(PLAT-ə-nəm)
Appearance	silvery white
Standard atomic weight Ar, std(Pt)	195.084(9)
Platinum in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Pd ↑ Pt ↓ Ds iridium ← platinum → gold
Atomic number (Z)	78
Group	group 10
Period	period 6
Block	d-block
Element category	transition metal
Electron configuration	[Xe] 4f14 5d9 6s1
Electrons per shell	2, 8, 18, 32, 17, 1
Physical properties
Phase at STP	solid
Melting point	2041.4 K ​(1768.3 °C, ​3214.9 °F)
Boiling point	4098 K ​(3825 °C, ​6917 °F)
Density (near r.t.)	21.45 g/cm3
when liquid (at m.p.)	19.77 g/cm3
Heat of fusion	22.17 kJ/mol
Heat of vaporization	510 kJ/mol
Molar heat capacity	25.86 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2330 (2550) 2815 3143 3556 4094
Atomic properties
Oxidation states	−3, −2, −1, +1, +2, +3, +4, +5, +6 (a mildly basic oxide)
Electronegativity	Pauling scale: 2.28
Ionization energies	1st: 870 kJ/mol 2nd: 1791 kJ/mol
Atomic radius	empirical: 139 pm
Covalent radius	136±5 pm
Van der Waals radius	175 pm
Spectral lines of platinum
Other properties
Natural occurrence	primordial
Crystal structure	​face-centered cubic (fcc)
Speed of sound thin rod	2800 m/s (at r.t.)
Thermal expansion	8.8 µm/(m·K) (at 25 °C)
Thermal conductivity	71.6 W/(m·K)
Electrical resistivity	105 nΩ·m (at 20 °C)
Magnetic ordering	paramagnetic
Magnetic susceptibility	+201.9·10−6 cm3/mol (290 K)[2]
Tensile strength	125–240 MPa
Young's modulus	168 GPa
Shear modulus	61 GPa
Bulk modulus	230 GPa
Poisson ratio	0.38
Mohs hardness	3.5
Vickers hardness	400–550 MPa
Brinell hardness	300–500 MPa
CAS Number	7440-06-4
History
Discovery	Antonio de Ulloa (1735)
Main isotopes of platinum
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct 190Pt 0.012% 6.5×1011 y α 186Os 192Pt 0.782% stable 193Pt syn 50 y ε 193Ir 194Pt 32.864% stable 195Pt 33.775% stable 196Pt 25.211% stable 198Pt 7.356% stable

Platinum is a chemical element with symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name is derived from the Spanish term platino, meaning "little silver".

Platinum is a member of the platinum group of elements and group 10 of the periodic table of elements. It has six naturally occurring isotopes. It is one of the rarer elements in Earth's crust, with an average abundance of approximately 5 μg/kg. It occurs in some nickel and copper ores along with some native deposits, mostly in South Africa, which accounts for 80% of the world production. Because of its scarcity in Earth's crust, only a few hundred tonnes are produced annually, and given its important uses, it is highly valuable and is a major precious metal commodity.

Platinum is one of the least reactive metals. It has remarkable resistance to corrosion, even at high temperatures, and is therefore considered a noble metal. Consequently, platinum is often found chemically uncombined as native platinum. Because it occurs naturally in the alluvial sands of various rivers, it was first used by pre-Columbian South American natives to produce artifacts. It was referenced in European writings as early as 16th century, but it was not until Antonio de Ulloa published a report on a new metal of Colombian origin in 1748 that it began to be investigated by scientists.

Platinum is used in catalytic converters, laboratory equipment, electrical contacts and electrodes, platinum resistance thermometers, dentistry equipment, and jewelry. Being a heavy metal, it leads to health problems upon exposure to its salts; but due to its corrosion resistance, metallic platinum has not been linked to adverse health effects. Compounds containing platinum, such as cisplatin, oxaliplatin and carboplatin, are applied in chemotherapy against certain types of cancer.

As of 2018, the value of platinum is $833.00 per ounce.

Characteristics

Physical

Pure platinum is a lustrous, ductile, and malleable, silver-white metal. Platinum is more ductile than gold, silver or copper, thus being the most ductile of pure metals, but it is less malleable than gold. The metal has excellent resistance to corrosion, is stable at high temperatures and has stable electrical properties. Platinum does oxidize, forming PtO₂, at 500 °C; this oxide can be easily removed thermally. It reacts vigorously with fluorine at 500 °C (932 °F) to form platinum tetrafluoride. It is also attacked by chlorine, bromine, iodine, and sulfur. Platinum is insoluble in hydrochloric and nitric acid, but dissolves in hot aqua regia (A mixture of nitric and hydrochloric acids), to form chloroplatinic acid, H₂PtCl₆.

Its physical characteristics and chemical stability make it useful for industrial applications. Its resistance to wear and tarnish is well suited to use in fine jewellery.

Chemical

Platinum being dissolved in hot aqua regia

 

The most common oxidation states of platinum are +2 and +4. The +1 and +3 oxidation states are less common, and are often stabilized by metal bonding in bimetallic (or polymetallic) species. As is expected, tetracoordinate platinum(II) compounds tend to adopt 16-electron square planar geometries. Although elemental platinum is generally unreactive, it dissolves in hot aqua regia to give aqueous chloroplatinic acid (H₂PtCl₆):

Pt + 4 HNO₃ + 6 HCl → H₂PtCl₆ + 4 NO₂ + 4 H₂O

As a soft acid, platinum has a great affinity for sulfur, such as on dimethyl sulfoxide (DMSO); numerous DMSO complexes have been reported and care should be taken in the choice of reaction solvent.

In 2007, Gerhard Ertl won the Nobel Prize in Chemistry for determining the detailed molecular mechanisms of the catalytic oxidation of carbon monoxide over platinum (catalytic converter).

Isotopes

Platinum has six naturally occurring isotopes: ¹⁹⁰Pt, ¹⁹²Pt, ¹⁹⁴Pt, ¹⁹⁵Pt, ¹⁹⁶Pt, and ¹⁹⁸Pt. The most abundant of these is ¹⁹⁵Pt, comprising 33.83% of all platinum. It is the only stable isotope with a non-zero spin; with a spin of ¹/₂, ¹⁹⁵Pt satellite peaks are often observed in ¹H and ³¹P NMR spectroscopy (i.e., Pt-phosphine and Pt-alkyl complexes). ¹⁹⁰Pt is the least abundant at only 0.01%. Of the naturally occurring isotopes, only ¹⁹⁰Pt is unstable, though it decays with a half-life of 6.5×10¹¹ years, causing an activity of 15 Bq/kg of natural platinum. ¹⁹⁸Pt can undergo alpha decay, but its decay has never been observed (the half-life is known to be longer than 3.2×10¹⁴ years); therefore, it is considered stable. Platinum also has 31 synthetic isotopes ranging in atomic mass from 166 to 204, making the total number of known isotopes 39. The least stable of these is ¹⁶⁶Pt, with a half-life of 300 µs, whereas the most stable is ¹⁹³Pt with a half-life of 50 years. Most platinum isotopes decay by some combination of beta decay and alpha decay. ¹⁸⁸Pt, ¹⁹¹Pt, and ¹⁹³Pt decay primarily by electron capture. ¹⁹⁰Pt and ¹⁹⁸Pt are predicted to have energetically favorable double beta decay paths.

Occurrence

A native platinum nugget, Kondyor mine, Khabarovsk Krai

 

Platinum is an extremely rare metal, occurring at a concentration of only 0.005 ppm in Earth's crust. It is sometimes mistaken for silver. Platinum is often found chemically uncombined as native platinum and as alloy with the other platinum-group metals and iron mostly. Most often the native platinum is found in secondary deposits in alluvial deposits. The alluvial deposits used by pre-Columbian people in the Chocó Department, Colombia are still a source for platinum-group metals. Another large alluvial deposit is in the Ural Mountains, Russia, and it is still mined.

In nickel and copper deposits, platinum-group metals occur as sulfides (e.g. (Pt,Pd)S), tellurides (e.g. PtBiTe), antimonides (PdSb), and arsenides (e.g. PtAs₂), and as end alloys with nickel or copper. Platinum arsenide, sperrylite (PtAs₂), is a major source of platinum associated with nickel ores in the Sudbury Basin deposit in Ontario, Canada. At Platinum, Alaska, about 17,000 kg (550,000 ozt) was mined between 1927 and 1975. The mine ceased operations in 1990. The rare sulfide mineral cooperite, (Pt,Pd,Ni)S, contains platinum along with palladium and nickel. Cooperite occurs in the Merensky Reef within the Bushveld complex, Gauteng, South Africa.

In 1865, chromites were identified in the Bushveld region of South Africa, followed by the discovery of platinum in 1906. In 1924, the geologist Hans Merensky discovered a large supply of platinum in the Bushveld Igneous Complex in South Africa. The specific layer he found, named the Merensky Reef, contains around 75% of the world's known platinum. The large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin, Canada, are the two other large deposits. In the Sudbury Basin, the huge quantities of nickel ore processed make up for the fact platinum is present as only 0.5 ppm in the ore. Smaller reserves can be found in the United States, for example in the Absaroka Range in Montana. In 2010, South Africa was the top producer of platinum, with an almost 77% share, followed by Russia at 13%; world production in 2010 was 192,000 kg (423,000 lb).

Large platinum deposits are present in the state of Tamil Nadu, India.

Platinum exists in higher abundances on the Moon and in meteorites. Correspondingly, platinum is found in slightly higher abundances at sites of bolide impact on Earth that are associated with resulting post-impact volcanism, and can be mined economically; the Sudbury Basin is one such example.

Compounds

Halides

Hexachloroplatinic acid mentioned above is probably the most important platinum compound, as it serves as the precursor for many other platinum compounds. By itself, it has various applications in photography, zinc etchings, indelible ink, plating, mirrors, porcelain coloring, and as a catalyst.

Treatment of hexachloroplatinic acid with an ammonium salt, such as ammonium chloride, gives ammonium hexachloroplatinate, which is relatively insoluble in ammonium solutions. Heating this ammonium salt in the presence of hydrogen reduces it to elemental platinum. Potassium hexachloroplatinate is similarly insoluble, and hexachloroplatinic acid has been used in the determination of potassium ions by gravimetry.

When hexachloroplatinic acid is heated, it decomposes through platinum(IV) chloride and platinum(II) chloride to elemental platinum, although the reactions do not occur stepwise:

(H₃O)₂PtCl₆·nH₂O ⇌ PtCl₄ + 2 HCl + (n + 2) H₂O

PtCl₄ ⇌ PtCl₂ + Cl₂

PtCl₂ ⇌ Pt + Cl₂

All three reactions are reversible. Platinum(II) and platinum(IV) bromides are known as well. Platinum hexafluoride is a strong oxidizer capable of oxidizing oxygen.

Oxides

Platinum(IV) oxide, PtO₂, also known as 'Adams' catalyst', is a black powder that is soluble in potassium hydroxide (KOH) solutions and concentrated acids. PtO₂ and the less common PtO both decompose upon heating. Platinum(II,IV) oxide, Pt₃O₄, is formed in the following reaction:

2 Pt²⁺ + Pt⁴⁺ + 4 O²⁻ → Pt₃O₄

Other compounds

Unlike palladium acetate, platinum(II) acetate is not commercially available. Where a base is desired, the halides have been used in conjunction with sodium acetate. The use of platinum(II) acetylacetonate has also been reported.

Several barium platinides have been synthesized in which platinum exhibits negative oxidation states ranging from −1 to −2. These include BaPt, Ba
₃Pt
₂, and Ba
₂Pt. Caesium platinide, Cs
₂Pt, a dark-red transparent crystalline compound has been shown to contain Pt²⁻ anions. Platinum also exhibits negative oxidation states at surfaces reduced electrochemically. The negative oxidation states exhibited by platinum are unusual for metallic elements, and they are attributed to the relativistic stabilization of the 6s orbitals.

Zeise's salt, containing an ethylene ligand, was one of the first organometallic compounds discovered. Dichloro(cycloocta-1,5-diene)platinum(II) is a commercially available olefin complex, which contains easily displaceable cod ligands ("cod" being an abbreviation of 1,5-cyclooctadiene). The cod complex and the halides are convenient starting points to platinum chemistry.

Cisplatin, or cis-diamminedichloroplatinum(II) is the first of a series of square planar platinum(II)-containing chemotherapy drugs. Others include carboplatin and oxaliplatin. These compounds are capable of crosslinking DNA, and kill cells by similar pathways to alkylating chemotherapeutic agents. (Side effects of cisplatin include nausea and vomiting, hair loss, tinnitus, hearing loss, and nephrotoxicity.)

• 

  The hexachloroplatinate ion
  
• 

  The anion of Zeise's salt
  
• 

  Dichloro(cycloocta-1,5-diene)platinum(II)
  
• 

  Cisplatin
  

History

Early uses

Archaeologists have discovered traces of platinum in the gold used in ancient Egyptian burials as early as 1200 BC. However, the extent of early Egyptians' knowledge of the metal is unclear. It is quite possible they did not recognize there was platinum in their gold.

The metal was used by pre-Columbian Americans near modern-day Esmeraldas, Ecuador to produce artifacts of a white gold-platinum alloy. Archeologists usually associate the tradition of platinum-working in South America with the La Tolita Culture (circa 600 BC - AD 200), but precise dates and location is difficult, as most platinum artifacts from the area were bought secondhand through the antiquities trade rather than obtained by direct archeological excavation. To work the metal, they employed a relatively sophisticated system of powder metallurgy. The platinum used in such objects was not the pure element, but rather a naturally occurring mixture of the platinum group metals, with small amounts of palladium, rhodium, and iridium.

European discovery

The first European reference to platinum appears in 1557 in the writings of the Italian humanist Julius Caesar Scaliger as a description of an unknown noble metal found between Darién and Mexico, "which no fire nor any Spanish artifice has yet been able to liquefy". From their first encounters with platinum, the Spanish generally saw the metal as a kind of impurity in gold, and it was treated as such. It was often simply thrown away, and there was an official decree forbidding the adulteration of gold with platinum impurities.

This alchemical symbol for platinum was made by joining the symbols of silver (moon) and gold (sun).

 

Antonio de Ulloa is credited in European history with the discovery of platinum.

 

In 1735, Antonio de Ulloa and Jorge Juan y Santacilia saw Native Americans mining platinum while the Spaniards were travelling through Colombia and Peru for eight years. Ulloa and Juan found mines with the whitish metal nuggets and took them home to Spain. Antonio de Ulloa returned to Spain and established the first mineralogy lab in Spain and was the first to systematically study platinum, which was in 1748. His historical account of the expedition included a description of platinum as being neither separable nor calcinable. Ulloa also anticipated the discovery of platinum mines. After publishing the report in 1748, Ulloa did not continue to investigate the new metal. In 1758, he was sent to superintend mercury mining operations in Huancavelica.

In 1741, Charles Wood, a British metallurgist, found various samples of Colombian platinum in Jamaica, which he sent to William Brownrigg for further investigation. 

In 1750, after studying the platinum sent to him by Wood, Brownrigg presented a detailed account of the metal to the Royal Society, stating that he had seen no mention of it in any previous accounts of known minerals. Brownrigg also made note of platinum's extremely high melting point and refractoriness toward borax. Other chemists across Europe soon began studying platinum, including Andreas Sigismund Marggraf, Torbern Bergman, Jöns Jakob Berzelius, William Lewis, and Pierre Macquer. In 1752, Henrik Scheffer published a detailed scientific description of the metal, which he referred to as "white gold", including an account of how he succeeded in fusing platinum ore with the aid of arsenic. Scheffer described platinum as being less pliable than gold, but with similar resistance to corrosion.

Means of malleability

Carl von Sickingen researched platinum extensively in 1772. He succeeded in making malleable platinum by alloying it with gold, dissolving the alloy in hot aqua regia, precipitating the platinum with ammonium chloride, igniting the ammonium chloroplatinate, and hammering the resulting finely divided platinum to make it cohere. Franz Karl Achard made the first platinum crucible in 1784. He worked with the platinum by fusing it with arsenic, then later volatilizing the arsenic.

Because the other platinum-family members were not discovered yet (platinum was the first in the list), Scheffer and Sickingen made the false assumption that due to its hardness—which is slightly more than for pure iron—platinum would be a relatively non-pliable material, even brittle at times, when in fact its ductility and malleability are close to that of gold. Their assumptions could not be avoided because the platinum they experimented with was highly contaminated with minute amounts of platinum-family elements such as osmium and iridium, amongst others, which embrittled the platinum alloy. Alloying this impure platinum residue called "plyoxen" with gold was the only solution at the time to obtain a pliable compound, but nowadays, very pure platinum is available and extremely long wires can be drawn from pure platinum, very easily, due to its crystalline structure, which is similar to that of many soft metals.

In 1786, Charles III of Spain provided a library and laboratory to Pierre-François Chabaneau to aid in his research of platinum. Chabaneau succeeded in removing various impurities from the ore, including gold, mercury, lead, copper, and iron. This led him to believe he was working with a single metal, but in truth the ore still contained the yet-undiscovered platinum-group metals. This led to inconsistent results in his experiments. At times, the platinum seemed malleable, but when it was alloyed with iridium, it would be much more brittle. Sometimes the metal was entirely incombustible, but when alloyed with osmium, it would volatilize. After several months, Chabaneau succeeded in producing 23 kilograms of pure, malleable platinum by hammering and compressing the sponge form while white-hot. Chabeneau realized the infusibility of platinum would lend value to objects made of it, and so started a business with Joaquín Cabezas producing platinum ingots and utensils. This started what is known as the "platinum age" in Spain.

Production

An aerial photograph of a platinum mine in South Africa. South Africa produces 80% of the world production and has most of the world's known platinum deposits.

 

Time trend of platinum production

 

Platinum, along with the rest of the platinum-group metals, is obtained commercially as a by-product from nickel and copper mining and processing. During electrorefining of copper, noble metals such as silver, gold and the platinum-group metals as well as selenium and tellurium settle to the bottom of the cell as "anode mud", which forms the starting point for the extraction of the platinum-group metals.

If pure platinum is found in placer deposits or other ores, it is isolated from them by various methods of subtracting impurities. Because platinum is significantly denser than many of its impurities, the lighter impurities can be removed by simply floating them away in a liquid. Platinum is paramagnetic, whereas nickel and iron are both ferromagnetic. These two impurities are thus removed by running an electromagnet over the mixture. Because platinum has a higher melting point than most other substances, many impurities can be burned or melted away without melting the platinum. Finally, platinum is resistant to hydrochloric and sulfuric acids, whereas other substances are readily attacked by them. Metal impurities can be removed by stirring the mixture in either of the two acids and recovering the remaining platinum.

One suitable method for purification for the raw platinum, which contains platinum, gold, and the other platinum-group metals, is to process it with aqua regia, in which palladium, gold and platinum are dissolved, whereas osmium, iridium, ruthenium and rhodium stay unreacted. The gold is precipitated by the addition of iron(II) chloride and after filtering off the gold, the platinum is precipitated as ammonium chloroplatinate by the addition of ammonium chloride. Ammonium chloroplatinate can be converted to platinum by heating. Unprecipitated hexachloroplatinate(IV) may be reduced with elemental zinc, and a similar method is suitable for small scale recovery of platinum from laboratory residues. Mining and refining platinum has environmental impacts.

Applications

Cutaway view of a metal-core catalytic converter

 

Of the 218 tonnes of platinum sold in 2014, 98 tonnes were used for vehicle emissions control devices (45%), 74.7 tonnes for jewelry (34%), 20.0 tonnes for chemical production and petroleum refining (9.2%), and 5.85 tonnes for electrical applications such as hard disk drives (2.7%). The remaining 28.9 tonnes went to various other minor applications, such as medicine and biomedicine, glassmaking equipment, investment, electrodes, anticancer drugs, oxygen sensors, spark plugs and turbine engines.

Catalyst

The most common use of platinum is as a catalyst in chemical reactions, often as platinum black. It has been employed as a catalyst since the early 19th century, when platinum powder was used to catalyze the ignition of hydrogen. Its most important application is in automobiles as a catalytic converter, which allows the complete combustion of low concentrations of unburned hydrocarbons from the exhaust into carbon dioxide and water vapor. Platinum is also used in the petroleum industry as a catalyst in a number of separate processes, but especially in catalytic reforming of straight-run naphthas into higher-octane gasoline that becomes rich in aromatic compounds. PtO₂, also known as Adams' catalyst, is used as a hydrogenation catalyst, specifically for vegetable oils. Platinum also strongly catalyzes the decomposition of hydrogen peroxide into water and oxygen and it is used in fuel cells as a catalyst for the reduction of oxygen.

Standard

Prototype International Meter bar

 

From 1889 to 1960, the meter was defined as the length of a platinum-iridium (90:10) alloy bar, known as the International Prototype Meter bar. The previous bar was made of platinum in 1799. Until May 2019, the kilogram is defined by the International Prototype Kilogram; a cylinder of the same platinum-iridium alloy made in 1879.

The standard hydrogen electrode also uses a platinized platinum electrode due to its corrosion resistance, and other attributes.

As an investment

1,000 cubic centimeters of 99.9% pure platinum, worth about US$696,000 at 29 Jun 2016 prices

Platinum is a precious metal commodity; its bullion has the ISO currency code of XPT. Coins, bars, and ingots are traded or collected. Platinum finds use in jewellery, usually as a 90–95% alloy, due to its inertness. It is used for this purpose for its prestige and inherent bullion value. Jewellery trade publications advise jewellers to present minute surface scratches (which they term patina) as a desirable feature in attempt to enhance value of platinum products.

In watchmaking, Vacheron Constantin, Patek Philippe, Rolex, Breitling, and other companies use platinum for producing their limited edition watch series. Watchmakers appreciate the unique properties of platinum, as it neither tarnishes nor wears out (the latter quality relative to gold).

Average price of platinum from 1992 to 2012 in US$ per troy ounce (~$20/g)

 

The price of platinum, like other industrial commodities, is more volatile than that of gold. In 2008, the price of platinum dropped from $2,252 to $774 per oz, a loss of nearly 2/3 of its value. By contrast, the price of gold dropped from ~$1,000 to ~$700/oz during the same time frame, a loss of only 1/3 of its value. 

During periods of sustained economic stability and growth, the price of platinum tends to be as much as twice the price of gold, whereas during periods of economic uncertainty, the price of platinum tends to decrease due to reduced industrial demand, falling below the price of gold. Gold prices are more stable in slow economic times, as gold is considered a safe haven. Although gold is used in industrial applications, its demand is not so driven by industrial uses. In the 18th century, platinum's rarity made King Louis XV of France declare it the only metal fit for a king.

Other uses

In the laboratory, platinum wire is used for electrodes; platinum pans and supports are used in thermogravimetric analysis because of the stringent requirements of chemical inertness upon heating to high temperatures (~1000 °C). Platinum is used as an alloying agent for various metal products, including fine wires, noncorrosive laboratory containers, medical instruments, dental prostheses, electrical contacts, and thermocouples. Platinum-cobalt, an alloy of roughly three parts platinum and one part cobalt, is used to make relatively strong permanent magnets. Platinum-based anodes are used in ships, pipelines, and steel piers.

Symbol of prestige in marketing

An assortment of native platinum nuggets

 

Platinum's rarity as a metal has caused advertisers to associate it with exclusivity and wealth. "Platinum" debit and credit cards have greater privileges than "gold" cards. "Platinum awards" are the second highest possible, ranking above "gold", "silver" and "bronze", but below diamond. For example, in the United States, a musical album that has sold more than 1 million copies will be credited as "platinum", whereas an album that has sold more than 10 million copies will be certified as "diamond". Some products, such as blenders and vehicles, with a silvery-white color are identified as "platinum". Platinum is considered a precious metal, although its use is not as common as the use of gold or silver. The frame of the Crown of Queen Elizabeth The Queen Mother, manufactured for her coronation as Consort of King George VI, is made of platinum. It was the first British crown to be made of this particular metal.

Health problems

According to the Centers for Disease Control and Prevention, short-term exposure to platinum salts may cause irritation of the eyes, nose, and throat, and long-term exposure may cause both respiratory and skin allergies. The current OSHA standard is 2 micrograms per cubic meter of air averaged over an 8-hour work shift. The National Institute for Occupational Safety and Health has set a recommended exposure limit (REL) for platinum as 1 mg/m³ over an 8-hour workday.

Platinum-based antineoplastic agents are used in chemotherapy, and show good activity against some tumors. 

As platinum is a catalyst in the manufacture of the silicone rubber and gel components of several types of medical implants (breast implants, joint replacement prosthetics, artificial lumbar discs, vascular access ports, etc.), the possibility that platinum could enter the body and cause adverse effects has merited study. The Food and Drug Administration and other institutions have reviewed the issue and found no evidence to suggest toxicity in vivo.