PLATINUM METALS REVIEW A quarterly survey of research on the platinum metals and of developments in their applications in industry VOL. 9 JULY 1965 NO. 3 Contents Supported Platinum Metal Catalysts 74 Bright Palladium Plating 80 High-Stability Fixed Resistors 82 Pallador I1 83 Field Ion Microscopy of the Platinum Metals 84 Disposal of Radioactive Residues 89 Soldering Palladium Plated Electronic Components 90 Continuous Temperature Control in the Blast Furnace 91 The Wetting of Platinum and Its Alloys by Glass 92 Anodic Protection for Sulphuric Acid Tank Car 99 Abstracts I00 New Patents 107 Communications should be addressed to The Editor, Platinum Metals Revim Johnson, Matthey & Co., Limited, Hatton Garden, London, E.C.1 Supported Platinum Metal Catalysts THEIR SELECTION AND METHODS OF USE IN INDUSTRIAL PROCESSES By G. C. Bond, Pm., F.R.I.C. and E. J. Sercombe, BSC. Research Laboratories, Johnson Matthey & Co Limited A catalyst has two main functions: bond, where this is possible, is most marked (I) By its presence to enable a chemical with palladium catalysts and least with reaction to proceed much more rapidly, platinum and iridium catalysts. or under milder conditions of tempera- Rhodium, palladium and platinum readily ture and pressure, than would otherwise catalyse the hydrogenation of the acetylenic be possible. triple bond; the corresponding olefin is (2) To direct the course of a chemical formed most selectively (and most stereo- reaction into the commercially desirable specifically) over palladium, and least selec- route when two or more routes are tively over platinum. possible. The rate of hydrogenation of the benzene Of the six platinum metals-platinum, ring catalysed by any of the platinum metals palladium, rhodium, ruthenium, iridium and varies greatly with the number and nature of osmium-all except osmium show remarkable substituent groups. Thus, for example, catalytic properties and are widely employed. toluene and other alkyl benzenes are more Each of them has some particular catalytic easily reduced than benzene. The nature of properties that distinguish it from its neigh- the solvent used is also very important, acetic bours. Some of these properties are truly acid, methanol or, where possible, water being characteristic of the metal and remain un- generally preferred. Reduction of the aro- affected by variation of the support or solvent. matic hydrocarbons does not usually proceed Other properties are modified substantially by rapidly at room temperature and atmospheric the method of preparation of the catalyst. It is pressure with supported platinum metal not possible, therefore, to state with precision catalysts, although Adams platinum oxide is the metal of choice for any particular applica- sometimes satisfactory. At high pressures tion, but there are several factors that may be rhodium and ruthenium are the preferred outlined to assist the research or development metals. worker to select the metal most likely to suit Ruthenium, rhodium, palladium and plati- his needs. num have all been reported as being active for the hydrogenation of heterocyclic com- The Choice of Metal pounds such as pyridine and furan, but Palladium, platinum and rhodium are com- elevated pressure is usually required. parably efficient metals for the hydrogenation The hydrogenation of aromatic nitro- of carbon-carbon multiple bonds; ruthenium compounds to the corresponding amine pro- is generally less efficient. The hydrogenation ceeds readily at room temperature and pressure of olefinic bonds is readily achieved over in the presence of supported palladium cata- rhodium, palladium and platinum catalysts, lysts. Platinum and rhodium catalysts are the reaction usually proceeding faster in the somewhat less effective, while ruthenium presence of solvents such as the lower alcohols catalysts are almost without activity. Aliphatic or glacial acetic acid. Migration of the double nitro-compounds are, however, reduced with Platinum Metals Rev., 1965, 9, (3),74-79 74 I The phenomenal development of the petroleum, chemical and pharmaceutical industries during the past two or three decades has largely resulted from the intro- duction of a wide range of catalysts designed to meet specijc requirements. Among such catalysts are many containing one or more of the platinum metals, the high intrinsic value of these metals in no way inhibiting their use on a large scale. The successful use of a platinum metal catalyst must necessarily depend upon the very high proportion of its initial cost that may be reclaimed following its almost complete recovery from the spent catalyst. On this basis, the net cost of catalyst required to yield a unit increase in the value of the raw material is remarkably low in most industrial processes. This paper reviews the use of the platinum metals as sup- ported catalysts and discusses the factors governing the choice of metal and of support for a variety of industrial reactions. greater difficulty to the corresponding amine, to toluene, phenol to cyclohexane, alkyl but the intermediate hydroxylamine can halides to paraffins and halobenzenes to sometimes be isolated. benzene. Conditions of acidity or alkalinity Aldehydes and ketones may be reduced in the solvent, and its polarity, are of great catalytically to primary and secondary alcohols importance to the rate of such processes. respectively. Ruthenium catalysts are par- Hydrogenolysis often occurs as an undesired ticularly suitable for these processes, and the side-reaction proceeding simultaneously with reduction of polysaccharides (with hydrolysis) another reduction process. Small quantities of to cyclitols using ruthenium catalysts is in alkali are especially effective in arresting the commercial operation. Palladium and plati- hydrogenolysis of activated hydroxyl groups. num are also effective in this reaction. In Palladium, platinum and rhodium catalysts acidic solutions there is sometimes a danger of are all in some measure effective for hydro- further reduction of the alcohol to the hydro- genolysis. Thus palladium on charcoal cata- carbon, especially when the hydroxyl group lysts will readily dehalogenate bromobenzene is activated, as for example in benzyl alcohol. and benzyl chloride in methanol solution. Such hydrogenolysis is minimised by the use Rhodium is the metal of choice for the hydro- of non-polar solvents or by small concentra- genation of the aromatic nucleus where hydro- tions of nitrogen bases such as aniline which genolysis of substituents such as the methoxy act as selective poisons. group is to be kept to a minimum. Hydrogenation of nitriles is performed in The disproportionation of cyclohexene into an acid solution when the corresponding cyclohexane and benzene and related reactions primary amine is the desired product, since are readily carried out under mild conditions substantial amounts of secondary and tertiary by palladium on charcoal catalysts. The amines are formed when hydrogenation is method of catalyst preparation is most import- carried out in neutral solution. For the hydro- ant here, some types being much more genation of ethyl or benzyl cyanides, or of efficient than others. Palladium on calcium benzonitrile, palladium catalysts are more carbonate is also effective for this reaction. effective than platinum catalysts in either Palladium catalysts also catalyse hydrogen acidic or neutral solution. transfer from a hydrogen donor such as cyclo- Hydrogenolysis is the term applied to any hexene to other hydrogen acceptors. This process in which the bond between two atoms sometimes forms a convenient basis for small- is cleaved by a molecule of hydrogen. Typical scale hydrogenations that do not involve examples are the reductions of benzyl alcohol molecular hydrogen. Platinum Metals Rev., 1965, 9, (3), 75 The preparation of palladium on calcium carbonate being carried out in a 300-gallon glass-lined vessel. The operator is adjusting the rate of addition of reagents during a critical stage of the process Supported platinum is active for the oxida- alumina and charcoal) are available in a wide tion of primary alcohol groups to the carb- variety of particle sizes, each having a range oxylic acid group under mild conditions. of surface areas and pore-size distributions. The proposed reaction conditions often im- The Choice of Support pose some limitations on the choice of Once the catalytically active metal has been support. For example, the support has to be chosen, it becomes necessary to decide the stable at the temperature used, and it must means by which it can be used most efficiently. not react chemically with any of the reactants, The prime function of the support is to extend products or solvent. the surface area of the metal. On a high sur- It is also particularly necessary, when con- face area support such as charcoal the metal sidering the form of support, to decide on the may be in the form of islands only one or two type of process envisaged, since this will deter- atoms thick, thus permitting very effective mine whether the support needs to be a pow- utilisation of the metal by providing a maxi- der or some coarser material such as granules mum surface area to weight ratio. or pellets. The factors governing the required The physical characteristics of the support, properties of the support in each case are so and in particular its pore structure, may also different as to warrant separate discussion. modify the role of the metal since the course of a reaction is often greatly influenced by the Powdered Supports rates of diffusion of reactants and products Certain materials, notably charcoal, have within the pore structure. Many of the com- been selected as catalyst supports by reason monly used catalyst supports (particularly of their high surface areas and their highly Platinum Metals Rev., 1965, 9, (3), 76 adsorptive natures. These can lead to losses Pure precipitated calcium carbonate, if of valuable product by adsorption from solu- carefully selected for particle size, is a very tion, but as most charcoals are fairly selective efficient catalyst support. While charcoal and this difficulty can often be overcome by alumina may both be used for all the platinum changing the grade of charcoal.