Platinum Metals Rev., 1967, 11, (4), 146
The Platinum Metals in Catalysis
Papers at the Second Canadian Symposium
The second Symposium on Catalysis organised by the Canadian Institute of Chemistry was held in June at McMaster University, Hamilton, Ontario, and was well attended by workers mainly from Canadian industries and universities. Of the twenty-eight papers presented, covering a very wide range of subjects, some eight or nine had relevance to the use of platinum metals in heterogeneous and homogeneous catalysis, including two on electrocatalytic phenomena.
The electrochemical behaviour of gold-palladium alloys was described in a paper by T. J. Gray, R. Rozelle, A. Schneider and M. L. Soeder (Alfred University, New York State); by studying alloys containing about 12, 26, 44, 62 and 68 per cent gold, the authors established that the maximum rate of hydrogen occlusion occurred with the 26 per cent gold alloy, for which the H/Pd ratio at the rest potential (32 mV) was 0.042. Alloys containing 12 and 44 per cent gold did not achieve rest potentials (indicating lower rates of occlusion), while the alloy having 68 per cent gold behaved similarly to pure gold. The observations reported by D. J. G. Ives, F. R. Smith, P. D. Marsden and J. B. Senior, of Birkbeck College, on the cathodic activation of mercury-poisoned platinum and of gold strongly suggested that the desorption of hydrogen atoms is retarded on these inactive surfaces.
The mechanism of the exchange of liquid saturated hydrocarbons with deuterium catalysed by supported platinum metals differs substantially from the corresponding gas phase processes. J. G. Atkinson, M. O. Luke and R. S. Stuart, of Merck, Sharp and Dohme, Montreal, disclosed that in the liquid phase systems the exchange is predominantly stepwise, and by continually passing pure deuterium through the liquid hydrocarbon in which the catalyst was suspended they were able to achieve complete substitution of all the hydrogen atoms.
R. J. Harper and C. Kemball, of Queen’s University, Belfast, compared the behaviours of palladium and platinum with those of nickel and tungsten in the exchange of a series of mono-halogenated benzenes with deuterium. The exchange rates, which decreased in the sequence of increasing atomic number of the halogen (iodobenzene could not however be studied), were all slower than for benzene itself. The noble metals were less poisoned by the small amount of halogen cleaved from the ring than were the base metals.
The development of a new platinum-on-alumina catalyst having high activity and selectivity for the isomerisation of n -hexane was described by W. J. M. Pieters and G. C. A. Schuit, of the Technical University, Eindhoven. It is well established that treatment of platinum on alumina with carbon tetrachloride at elevated temperatures forms a surface layer of aluminium chloride which greatly increases the activity of the catalyst for isomerisation. However, Pieters and Schuit were able to show that selectivity could also be improved by controlled poisoning of the platinum by thiophene.
G. C. Bond (Johnson Matthey) reviewed the hydrogenation of acetylene catalysed by the platinum group metals. The ability of palladium to hydrogenate acetylene selectively to ethylene in the presence of a large excess of ethylene was attributed to its ability to become rapidly and selectively poisoned for ethylene hydrogenation. The addition of deuterium to acetylene over palladium and platinum catalysts gives about 80 per cent of cis -C2H2D2, rhodium and iridium giving a broader distribution of deuterated ethylenes.
Two of the contributions dealt with homogeneous catalysis by platinum metal compounds. P. R. Rony, of Monsanto, St Louis, gave a theoretical treatment of supported catalytic solutions, and showed that there should exist an optimum liquid loading for efficient catalysis. The system had been discovered independently by workers in both the Monsanto Company and the Johnson Matthey Research Laboratories (G. J. K. Acres, G. C. Bond, B. J. Cooper and J. A. Dawson, J. Catalysis, 1966, 6, 139).
The various products obtained from the reaction of disubstituted acetylenes with palladous chloride were listed by P. M. Maitlis of McMaster University; in non-hydroxylic solvents, hexaphenyl-benzene is obtained almost quantitatively from diphenylacetylene. Dimethylacetylene in methylene chloride solution on the other hand reacts with palladium chloride to give only about 10 per cent of hexamethylbenzene, the remainder of the product being polymeric in nature.