Platinum-group metals chemical properties

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

Lambert reviews the role of alkali additives on metal films and nanoparticles in electrochemical and chemical behavior modihcations. Metal-support interactions is the subject of the chapter by Arico and coauthors for applications in low temperature fuel cell electrocatalysts, and Haruta and Tsubota look at the structure and size effect of supported noble metal catalysts in low temperature CO oxidation. Promotion of catalytic activity and the importance of spillover are discussed by Vayenas and coworkers in a very interesting chapter, followed by Verykios s examination of support effects and catalytic performance of nanoparticles. In situ infrared spectroscopy studies of platinum group metals at the electrode-electrolyte interface are reviewed by Sun. Watanabe discusses the design of electrocatalysts for fuel cells, and Coq and Figueras address the question of particle size and support effects on catalytic properties of metallic and bimetallic catalysts. 

Carbon-supported catalysts, especially of platinum group metals, are used industrially in hundreds of reactions, particularly for manufacture of pharmaceuticals, perfumes, and plastics. Most carbon supports are manufactured by pyrolysis of carbonaceous materials such as wood, charcoal, coal, or organic polymers. Chemical pretreatment is used to modify the surface chemistry to impart superior catalytic properties. 

Tab. 20.1 Chemical and physical properties of the platinum-group metals (PGM)... 

Earlier, it was difficult to produce a clean surface and to characterize its surface structure. However, with the development of electronic industry, techniques have been developed to produce clean surface with well-defined properties. It has been possible to investigate catalytic oxidation on metal surface in depth. Example of dynamic instability at gas-liquid interface is provided by such studies. Studies on chemical oscillations during oxidation of CO over surface of platinum group metals have attracted considerable interest [62-68]. 

The platinum group metals (Ru, Rh, Pd, Os, Ir, and Pt), Ag, and Au are called precious or noble metals. Nobility and catalytic activity are unique properties of precious metals, that result in a wide range of applications, such as catalysts in various industrial fields, in electronic industries, and in jewelry. The chemical and physical properties of each precious metal are shown in Table 1. The determination of precious metals attracted the interest of analysts and developed rapidly because these metals are valuable and rare, and also very important for many products. Their concentration levels are very low in many natural sources, metallurgical intermediates, and environmental samples. Furthermore, precious metals are collectively handled in the analytical chemistry field, because of the close resemblance of their chemical properties and behavior. Precious metals are the subproducts in copper, zinc, or lead smelting and refining, which is the most important source of precious metals. Whereas many analytical methods for the ultratrace determination of precious metals in environmental or biological samples were recently published with the development of high-sensitivity analytical instruments, the classical fire-assay has been widely applied for the accurate determination of expensive precious metals. 

Table 4.102. Selected physical and chemical properties of the six platinum-group metals (PGMs)... 

Electrochemists early on observed that noble and precious metals were stiff materials, with good tensile properties and machinability, high electronic conductivity, and exceptional chemical and electrochemical inertness in most corrosive media, all combined with intrinsic electrocatalytic properties/ Consequently, the first industrial anodes used in electrochemical processes requiring an excellent dimensional stabihty were made of the noble and precious metals (e.g., Au and Ag), the six platinum-group metals (PGMs) (e.g., Ru, Rh, Pd, Os, Ir, and Pt), or their alloys (e.g., Pt- lr and °Pt- °Rh) . Of these, the PGMs, especially platinum and iridium, occupied a particular place owing to their electrochemical inertness... 

THE NOBLE METALS, or precious metals, consist of gold, silver, and the platinum group metals (PGM)— platinum, palladium, iridium, rhodium, osmium, and ruthenium. These metals are known for their stability in corrosive environments, physical beauty, and unique physical and chemical properties. They command a premium price because of their low abundance in nature. The noble metals are used in many applications with great success and often with few, if any, substitutable materials. 

Iron (Fe), cobalt (Co) and nickel (Ni) are sometimes called iron group metals while ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt) are known as platinum group metals. Metals of each group have similar physical and chemical properties. Table 2.73 sununarizes some physical properties of group-VIII metals. 

Rhenium has good chemical resi stance due to its position in the periodic table nextto the noble metal s of the platinum group. However, it oxidizes readily. Its properties are summarized in Table 6.10. 

Electronic configuration 1. v22522/763 23/7 3 l04 24/764 /5.s 1. Ionic rad ius Ru4+ 0,60 A. Metallic radius 1,3251 A. First ionization potential 7.5 eV. Other physical properties of ruthenium will be found under Platinum and Platinum Group. See also Chemical Elements,... 

The crystal structures adopted by the binary carbides and nitrides are similar to those found in noble metals. The resemblance is not coincidental, and has been explained using Engel-Brewer valence bond theory [5]. Briefly, the main group elements C and N increase the metal s effective s-p electron count, so that structures and chemical properties of the early transition metals resemble those of the Group 8 metals. This idea was first introduced by Levy and Boudart [6] who noted that tungsten carbide had platinum-like properties. 

Osmium bears a close resemblance to ruthenium in many of its chemical properties in fact, in certain respects, such as the formation of tetroxides, these two elements are absolutely unique amongst the metals of the platinum group. 

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