Platinum group metals, catalytic properties

The commercial applications of osmium are limited and considerably fewer than other platinum group metals. Its aUoys are very hard and are used to make tips of fountain-pen nibs, phonograph needles, and pivots. The metal also exhibits effective catalytic properties in hydrogenation and other organic reactions. Such catalytic apphcations, however, are limited and osmium fads to replace other noble metals, particularly paUadium and platinum, which are more effective as catalysts and cost less. 

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. 

The catalytic properties of supported gold catalysts appear to change dramatically when different metal oxide supports or different preparation methods are used. From this standpoint, supported gold catalysts behave differently from platinum-group metal catalysts. This paper attempts to summarize major factors which determine catalytic activity and selectivity in supported gold 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. 

Table 5-20 Modification of the catalytic properties of the platinum group metals by addition of other metals [T23]...

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. 

The typical example of a bifunctional catalyst is the ruthenium-modified platinum surfaces for the oxidation of methanol [51-58]. Of all the metals studied, platinum is the one that displays the best catalytic activity for methanol oxidation. However, its catalytic activity is still too low for practical purposes. The problem arises from the necessity of the adsorption of a second species able to transfer the oxygen group required for its complete oxidation to CO2. Ruthenium is known to have a better affinity for OH than platinum but its interaction with methanol is very poor [59]. Thus, a platinum surface, which has good interaction energy with methanol, modified with ruthenium, whose interaction with OH is better than that of platinum, exhibits better catalytic properties than platinum. 

Finally, attention has been drawn to currents of hydrogen evolution which in the presence of some substances occurs at more positive potentials than in their absence. Such substances catalyze hydrogen evolution and result in high currents which are denoted catalytic hydrogen waves. Such waves are observed in the presence either of platinum group metals (57,58), where the catalysis is attributed to clusters of metals deposited on mercury or of compounds which possess acid-base properties. Catalytic effects of the latter type in solutions of simple buffers have been observed for low molecular weight compounds (59,60), as well as for proteins (61,62). Similar catalytic effects in ammoniacal cobalt (III)-solutions (63) found utilization in Brdicka reaction (64-66), used in cancer diagnosis. 

The strong catalytic activity of platinum group metals has a significant impact on the properties of the solid products obtained in above described precipitation systems after longer hydrothermal treatment (9-72 h) at I60°C. In the course of hydrothermal treatment, the products of TMAH decomposition (methanol and trimethylamine) affect the reduction of metal cations and the formation of platinum group metal nanoparticles. These nanoparticles act as the catalyst for the reduction of Fe(lll) to Fe(ll) by the products of TMAH decomposition and the formation of characteristic Fe304 octahedra, which is followed by Mossbauer spectroscopy and FE-SEM (Fig. 23.19). 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

Rare, shiny, and lightest metal of the platinum group. Hardens platinum and palladium. The presence of 0.1 % of ruthenium in titanium improves its resistance to corrosion 100-fold. The spectacular catalytic properties of ruthenium are used on industrial scales (hydrogenations, sometimes enan-tioselective, and metathesis). Titanium electrodes coated with ruthenium oxide are applied in chlorine-alkaline electrolysis. Suitable for corrosion-resistant contacts and surgical instruments. 

The group 10 metals, such as palladium and platinum, are active for the conversion of methanol. However, they are much less selective than the copper-based catalysts, yielding primarily the decomposition products [123,124,133]. This catalytic property makes them less feasible for fuel cell applications. The only exception found is for Pd/ZnO, which showed selectivity close to that of a copper catalyst [105, 121]. 

---