Palladium and Rhodium

Wollaston worked on his assignment in accordance with the mutual agreement He dissolved natural platinum in aqua regia, removed the black residue by filtering and examined the filtrate. The excess of aqua regia was evaporated. When he added mercury cyanide, drop by drop, a yellow precipitate was formed. On calcination a metal was obtained that was not platinum. He named the new element palladium after the asteroid Pallas, discovered in 1802. The discovery year for palladium was 1803. Today we know that it was the almost insoluble palladium cyanide Pd(CN)j that Wollaston precipitated with mercury cyanide. 

Wollaston intended to do more experiments in order to reassure himself about the results. On the other hand he wanted to make his discovery public as soon as possible as he was afraid that the Frenchmen were on the same track. Now he used a very unconventional and obscure method of publication. He wrote a small handbill to members of London s scientific community, informing them anonymously about a new noble metal. Palladium or New Silver. It was announced that the metal could be obtained oiJy in the shop of a well-known collector and dealer in minerals, situated in London s Soho. A young Irish chemist and member of the Royal Society, Richard Chenevix, bought the whole quantity and examined the metal. He then submitted a presentation to Royal Society and asserted that the metal was not new at all but an alloy of platinum and mercury in the proportion of 2 to 1. 

Chenevix broadcast his results all over Europe. Well-known chemists such as V. Rose and M. H. Klaproth in Berlin also examined the same palladium sample but could not detect any mercury in it. VauqueHn in Paris checked the properties claimed in the handbill, and found them to be correct. 

Not until 1805 did Wollaston put an end to the mystery and reveal himself as the discoverer of palladium. This was an unpleasant surprise to Chenevix after his arguments against the existence of this new element. He remained, however, good friend with Wollaston and his family. He himself abandoned science and turned to writing novels and plays. Desmond Reilly has told the sad story of Richard Chenevix and his palladium adventure (32.6  

Wollaston continued his investigation of the aqua regia solution of platinum in 1803. He then discovered an additional metal in the following way. An aqua regia solution was partially neutralized with sodium hydroxide. Platinum was precipitated with ammonium chloride in the usual way and palladium with mercury cyanide. The common precipitates of chloroplatinate and palladium cyanide were removed by filtering. Hydrochloric add was added to the filtrate and the solution was evaporated to dryness. Wollaston tried to dissolve the residue in alcohol but a beautiful dark-red powder remained undissolved. It proved to be a double chloride of sodium and a new element Wollaston called the new metal rhodium because of the rose colors of its salts. The metal itself was prepared by hydrogen reduction and washing away the sodium chloride with water. 

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Table 8 shows the supply to the Western world of platinum, palladium, and rhodium. For each metal. supply kicreased significantly ki the latter... 

Selenium occurs in the slimes as intermetallic compounds such as copper silver selenide [12040-91 -4], CuAgSe disilver selenide [1302-09-6], Ag2Se and Cu2 Se [20405-64-5], where x < 1. The primary purpose of slimes treatment is the recovery of the precious metals gold, silver, platinum, palladium, and rhodium. The recovery of selenium is a secondary concern. Because of the complexity and variabiUty of slimes composition throughout the world, a number of processes have been developed to recover both the precious metals and selenium. More recently, the emphasis has switched to the development of processes which result in early recovery of the higher value precious metals. Selenium and tellurium are released in the later stages. Processes in use at the primary copper refineries are described in detail elsewhere (25—44). 

Rhodium-on-carbon has also been found to bring about the formation of 2,2 -biquinoline from quinoline, the yield and the percentage conversion being similar to that obtained with palladium-on-carbon. On the other hand, rhodium-on-carbon failed to produce 2,2 -bipyridine from pyridine, and it has not yet been tried with other bases. Experiments with carbon-supported catalysts prepared from ruthenium, osmium, iridium, and platinum have shown that none of these metals is capable of bringing about the formation of 2,2 -biquinoline from quinoline under the conditions used with palladium and rhodium. ... 

Platinum, palladium, and rhodium will function well under milder conditions and are especially useful when other reducible functions are present. Freifelder (23) considers rhodium-ammonia the system of choice when reducing -amino nitriles and certain )5-cyano ethers, compounds that undergo extensive hydrogenolysis under conditions necessary for base-metal catalysis. 

Probably the most significant control technology breakthrough came m 1977, when Volvo released a computer-controlled, fuel-mjected vehicle equipped with a three-way catalyst. The new catalytic converters employed platinum, palladium, and rhodium to simultaneously reduce NO and oxidize CO and HC emissions under carefully controlled oxygen conditions. The new Bosch fuel injection system on the vehicle provided the precise air/fuel control necessary for the new catalyst to perform effectively. The combined fuel control and three-way catalyst system served as the foundation for emissions control on the next generation of vehicles. 

For the noble metals used in oxidation, the loading is about 0.1 oz per car, with calls for a million ounces per year. The current world production rates of platinum, palladium, and rhodium are 1.9, 1.6, and 0.076 million ounces respectively the current U,S. demand for platinum, palladium, rhodium, and ruthenium are 0.52, 0.72, 0.045, and 0.017 million ounces respectively (72, 73). The supply problem would double if NO reduction requires an equal amount of noble metal. Pollution conscious Japan has adopted a set of automobile emission rules that are the same as the U.S., and Western Europe may follow this creates a demand for new car catalysts approaching the U.S. total. The bulk of world production and potential new mines are in the Soviet Union and South Africa. The importation of these metals, assuming the current price of platinum at 155/oz and palladium at 78/oz, would pose a balance of payment problem. The recovery of platinum contained in spent catalysts delivered to the door of precious metal refiners should be above 95% the value of platinum in spent catalysts is greater than the value of lead in old batteries, and should provide a sufficient incentive for scavengers. 

These complexes can be isolated in some cases in others they are generated in situ from appropriate precursors, of which diazo compounds are among the most important. These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, palladium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Ethyl a-diazoacetate reacts with styrene in the presence of bis(ferrocenyl) bis(imine), for example, to give ethyl 2-phenylcyclopropane-l-carboxylate. Optically active complexes have... 

A mixture of catalysts is needed to catalyze the variety of reactions that must be carried out in the converter. The most important catalyst is platinum metal, but palladium and rhodium are used as well, as are transition metal oxides such as CuO and Cr2 O3. ... 

As an introductory example we take one of the key reactions in cleaning automotive exhaust, the catalytic oxidation of CO on the surface of noble metals such as platinum, palladium and rhodium. To describe the process, we will assume that the metal surface consists of active sites, denoted as We define them properly later on. The catalytic reaction cycle begins with the adsorption of CO and O2 on the surface of platinum, whereby the O2 molecule dissociates into two O atoms (X indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site ) ... 

Table 9.5 shows the concentrations of pollutant parameters found in the precious metals subcategory raw waste streams. The major constituents are silver and gold, which are much more commonly used in metal finishing industry operations than palladium and rhodium. Because of their high cost, precious metals are of special interest to metal finishers. 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

Holies, J.H., Switzer, M.A. and Davis, R.J. (2000) Influence of ceria and lanthana promoters on the kinetics of NO and N20 reduction by CO over alumina-supported palladium and rhodium, J. Catal. 190, 247. 

Finally, with respect to successive evaporation, Pd-Rh films used for CO oxidation (34) are an example of preparing alloy films where a miscibility gap exists and interdiffusion rates are slow. These Pd-Rh films were prepared by depositing layers of palladium and rhodium at 0°C, followed by annealing in 50 Torr hydrogen at 400°C for 21 hr. The apparent surface compositions, evaluated from the CO oxidation rate as described in Section IV, and information on film structure obtained by X-ray diffraction (XRD) are recorded in Table II. 

Hence, the decision to use a heated substrate with simultaneous evaporation of the component metals as an aid to homogenization requires consideration of whether or not it might have an adverse effect, i.e., causing preferential nucleation of one component, which interdiffusion may not be able to remedy. It was believed (60) that in preparing Pd-Rh alloys by simultaneous deposition on a substrate at 400°C, rhodium nucleated preferentially and that crystallites grew by the addition of palladium (and rhodium) atoms. The diffusion of palladium atoms into this kernel formed a phase with 88 =t 5% Rh (phase II). The outer shell of the crystallite, phase I, was in effect a solid solution deficient in rhodium compared with the overall film composition, and the Rh content of phase I therefore increased as the Rh flux was increased. 

Beside these catalytically active metallophosphine dendrimers (see above), preliminary studies on the chemical properties of phoshorus-based dendrimers complexed to metals such as platinum, palladium and rhodium have been described by Majoral, Caminade and Chaudret [21], They showed that these macromolecules (see Scheme 13) could be useful for the (in situ) generation of metallodendrimer catalysts. 

Conventionally, organometallic chemistry and transition-metal catalysis are carried out under an inert gas atmosphere and the exclusion of moisture has been essential. In contrast, the catalytic actions of transition metals under ambient conditions of air and water have played a key role in various enzymatic reactions, which is in sharp contrast to most transition-metal-catalyzed reactions commonly used in the laboratory. Quasi-nature catalysis has now been developed using late transition metals in air and water, for instance copper-, palladium- and rhodium-catalyzed C-C bond formation, and ruthenium-catalyzed olefin isomerization, metathesis and C-H activation. Even a Grignard-type reaction could be realized in water using a bimetallic ruthenium-indium catalytic system [67]. 

Platinum, palladium and rhodium catalysts are non-pyrophoric as normally manufactured. Iridium and, more particularly, ruthenium catalysts may exhibit... 

Cabri, L.J., Sylvester, P., Tubrett, M., Peregoedova, a., Tubrett, M., LaFlamme, J.H.G. 2003. Comparison of LAM-ICP-MS and micro-PIXE results for palladium and rhodium in selected samples of Noril sk and Talnakh sulfides. The Canadian Mineralogist, 41, 321-329. 

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