Platinum Metals Rev., 1972, 16, (4), 138
Fifth International Congress on Catalysis
Dominant Role of the Platinum Metals
The four-yearly International Congress on Catalysis provides a welcome opportunity to review the state of the art and to exchange ideas and experiences. A review of the papers relevant to the platinum metals is given in this article.
The Fifth International Congress on Catalysis was held at the Breakers Hotel, Palm Beach, Florida, in the last week of August. It was attended by over five hundred participants from thirty countries—the great majority, however, coming from the United States. The papers presented at this Congress show the dominant role that the platinum group metals play in catalysis, for of the 110 papers at least thirty-six were concerned wholly or in part with these metals. The more significant of these papers are reviewed below.
The adsorptive properties of very clean platinum surfaces are proving to be very different from those of slightly contaminated surfaces; thus the sticking probability of oxygen on the (111) face of clean platinum has the remarkably low value of about 10−6, which is much lower than previously reported values (Weinberg, Lambert, Comrie, and Linnett). Three papers treat adsorption on platinum/alumina catalysts. Thermochemical studies (Basset, Theolier, Primet, and Prettre) demonstrate strong and weak forms of adsorbed hydrogen, while temperature-programmed desorption of adsorbed hydrogen (Aben, van den Eijk and Oelderik) suggests three forms, the amount adsorbed in the weakest state correlating with catalytic activity for benzene hydrogenation. The enhancement of hydrogen adsorption caused by preadsorption of oxygen is shown to be due to a surface area increase (Darensbourg and Eischens). The generality of “hydrogen spillover” is demonstrated by work with mechanical mixtures of platinum black with CeY zeolite and various oxides including ruthenium dioxide (Neikam and Vannice).
Reactions of Hydrocarbons
We have witnessed in recent years a revolution in our thinking concerning the kinds of hydrocarbon transformation which platinum can bring about; at least fourteen papers discussed various aspects of hydrocarbon reactions on the platinum metals.
It is now well established that unsupported platinum can catalyse the skeletal isomerisation of hydrocarbons; this was, of course, formerly thought to be the prerogative of dual-function catalysts, and that there was a carbonium ion intermediate. On platinum films at 275 to 300°C multiple isomerisation occurs in one residence; thus 2,3-dimethylbutane yields 3-methylpentane, n -hexane, methylcyclopentane, and benzene as primary products (Muller and Gault). Hydrogenolysis to smaller molecules occurs simultaneously, and the proportion of hydrogenolysis products decreases with increasing particle size (Anderson and Shimoyama). Reactions on dual-function catalysts continue to be of interest. The direct dehydrocyclisation of linear alkanes to aromatics, a reaction which has long been mechanistically obscure, has now been shown to proceed independently on platinum and on alumina (Callender, Brandenberger and Meerbott). Over platinum/CaY zeolite, methylcyclopentane suffers cationic ring-opening at 275°C, yielding products of higher molecular weight (Schulz, Weitkamp and Eberth).
Some quite novel hydrocarbon reactions were reported. The disproportionation of alkanes and alkyl benzenes is achieved by use of a mechanical mixture of a dehydrogenation catalyst such as platinum/alumina and an olefin disproportionation catalyst (tungsten trioxide on silica) (Hughes, Burnett and Wall). Linear mono-olefins have been observed to disproportionate into the alkane and diolefin over supported palladium catalysts between 100 and 200°C, although yields are low (Bhasin).
Palladium foil at 0°C rapidly loses activity for ethylene hydrogenation but the activity is stabilised by pre-adsorption of acetylene (Yasumori, Shinohara and Inoue). The addition of deuterium to 4-tBu-cyclohexanone results in simple addition over ruthenium, osmium, iridium, and platinum, but over rhodium and palladium substitution occurs at the 2- and 6-positions (Takagi, Teratani and Tanaka). The reactions of deuterium with norbornadiene and norbornane do not exhibit structure sensitivity (Clarke, McMahon and O’Cinneide). The ruthenium-catalysed hydrogenolysis of isopentane and of neopentane gives methane as the primary product in both cases, but the former is much more reactive (Kempling and Anderson).
In a further search for structure-sensitive reactions, the oxidation of carbon monoxide on the low index planes of a palladium monocrystal has been examined by LEED and Auger spectroscopy (Ertl and Koch); however, strengths of adsorption and activity were found to be similar on all planes. The formally simple hydrogen-oxygen reaction on platinum, when studied by field-emission microscopy, proves to be rather complex (Gorodetski and Savchenko).
Interest continues in the ability of palladium and its salts to oxidise olefins selectively to useful products. Various palladium salts when adsorbed on active carbon oxidise ethylene to acetaldehyde at 100°C with high selectivity, and without the necessity of a redox component (Fujimoto and Kunugi). Propylene oxidation on palladium/carbon gives acrolein and acrylic acid, the reaction proceeding in the presence of water whose oxygen atom becomes incorporated in the product (Moro-oka and Takita).
The heterogenisation of homogeneous catalysts is now an active field of research. The idea is to mount organometallic complexes on supports so that they may be used as heterogeneous catalysts. Rhodium complexes may be linked to phosphorus atoms incorporated into polymers such as polystyrene and polyvinyl chloride, and through a reactive silaphosphine also to silica (Allum, Hancock, McKenzie, and Pitkethly). These catalysts are active for hydrogenation and hydroformylation of olefins. A further paper (Haag and Whitehurst) treated other aspects of polymer-supported catalysts.
High dispersion of metals in zeolites has been claimed before, but it has now been shown conclusively that careful preparation through ion exchange of into Y zeolite gives particles containing less than six platinum atoms (Dalla Betta and Boudart): the catalyst shows high activity and the particles are thought to be electron-deficient. The use of palladium alloys as hydrogenpermeable membrane catalysts has been developed and product yields from dehydrogenation reactions are shown to be significantly better than thermodynamics would suggest (Gryaznov, Smirnov and Slinko).
Although the activity of ruthenium in ammonia synthesis has long been known, it is now shown that this activity can be powerfully promoted by alkali metals (Ozaki, Aika and Morikawa). A 5 per cent ruthenium/carbon catalyst promoted with 22 per cent potassium is some four times more active than a conventional doubly-promoted iron catalyst, and it possesses the advantage of not being retarded by the product ammonia.