Platinum Metals Rev., 1989, 33, (1), 17
Symposium on Hydrogen in Metals
Palladium Continues to Find Application
The International Symposium on Metal-Hydrogen Systems, Fundamentals and Applications (I.S.M.H.S.) was held at the Max-Planck Institute for Metal Research, Stuttgart, West Germany from 4th to 9th September 1988. Over 300 papers and posters were presented covering all aspects of metal-hydrogen behaviour. The symposium represented the unification of two, previously separate, conference series and the end of both the Hydrogen in Metals and the Metal-Hydride series of meetings. This review outlines some of the 30 to 40 papers that were concerned with the interaction of hydrogen with the platinum group metals.
The reported work on amorphous materials included a study by U.S. investigators of the palladium-containing Zr2Pd alloy which absorbs hydrogen to form either an amorphous Zr2PdHx hydride or a separate crystalline phase (R. C. Bowman, D. R. Torgeson, R. G. Barnes, A. J. Maeland and J. J. Rush). The experimental data showed that the diffusion rates followed simple Arrhenius behaviour when the hydrogen atoms occupy only one specific interstitial site, for example the Zr4 tetrahedra in Zr2PdH1.84. However, at higher hydrogen concentrations, where different interstitial sites are filled, considerably more complex behaviour was observed for both the crystalline and glassy hydrides. A thorough analysis of diffusion in both forms of the hydride has been carried out.
A paper by G. Driessen and K. W. Kehr of the Institut für Festkorperforschung der Kernforschungsanlage, Jülich, West Germany, and D. Richter, Institut Lave-Langevin, Grenoble, France, questioned the accepted view that the motion of hydrogen in amorphous Pd1−xSix alloy, as investigated by quasi-elastic neutron scattering (Q.N.S.), could be described by considering the hydrogen in interstitial sites of energy given by a simple Gaussian distribution. In such treatments the saddle point energies are considered to be constant, but Driessen considers this to be inappropriate in view of the glassy structure. Instead he employed a modified model in which the PdSi alloy was described by a Gaussian distribution of saddle point energies. In a second paper on the Pd1−xSix alloy, G. Driessen, R. Hempelmann and D. Richter employed Q.N.S. for the direct observation of hydrogen motions on the time scale 10−8<t<10−11 seconds, and in the spatial range <50 Å. This work revealed two distinct jump processes, slow and fast. Using an appropriate model, they proposed that these two regimes suggested the occupation of two types of interstice, these being the octahedral and tetrahedral sites.
A paper by B. Bogdanovic, S. Huckett, B. Spliethoff and U. Wilczok of the Max-Planck Institute reported the reaction of catalytically prepared MgH2 or Mg(CH2CH3)2 with certain palladium complexes to produce an amorphous compound which analyses as MgPd. So far this has been obtained only as a crystalline solid. The paper points to the interesting possibility of being able to prepare previously unknown amorphous alloys via a metal-hydrogen route, and also reports the existence of a previously unknown hydride Mg2PdH2 and the new metallic phase Mg2Pd.
Another popular study which featured the platinum group metals was one detailing the influence of order-disorder transformations on the equilibrium and the kinetic behaviour of dissolved hydrogen.
In a paper by P. Ahlzen, Y. Andersson and R. Tellgren of the University of Uppsala, Sweden and Professor T. B. Flanagan of the University of Vermont, U.S.A., the Pd3Mn alloy was studied. This orders to the Ag3Mg type structure at 803 K, and the ordered alloy was observed to absorb more hydrogen than the disordered alloy, a fact attributed to the alteration in the relative populations of different sites during the ordering process. In addition, hydrogenated Pd3Mn was observed to undergo a hydrogen induced phase change, whereby an ordered structure of the type Cu3Au formed at temperatures between 573 and 793 K.
Two platinum group metals were featured in a paper on ordering by B. Coluzzi, C. Costa, P. Marzola and F. M. Mazzolai of the University of Perugia, Italy. This paper concentrated on the stress induced ordering process in a single crystal of Pd0.85Pt0.15H0.29 as observed through ultrasonic elastic measurements.
The order-disorder systems Pd3Fe and PdFe were prepared and electrolytically charged with hydrogen by C. U. Maier, M. Hirscher and Professor H. Kronmuller of the Max-Planck Institute. This group used magnetic after-effect measurements to follow single jumps in the hydrogen diffusion process. They found relaxation spectra composed of characteristic maxima and minima, the extent of the maximum being highly dependent on the degree of order within the alloy. Using the data obtained about the specific jumps in conjunction with a statistical model of the site distribution, they assigned the various jump processes to particular cell configurations, such as 6Pd, 5PdFe and 4Pd2Fe, and thus fitted the hydrogen behaviour to the degree of order in the alloy.
Two papers from Professor I. R. Harris, M. L. Doyle and R. Wileman of the University of Birmingham, U.K., reported the marked influence of lattice order in palladium-12.5 per cent yttrium and palladium-8 per cent yttrium systems on their hydrogen dissolving and diffusing properties, respectively. This work showed that the formation of the ordered Pd7 Y superlattice, from the disordered solid solution, caused a significant decrease in hydrogen solubility, a fact which was ascribed to a rearrangement in site population and to a small change in lattice parameter. The second paper reported the extremely high permeation rates which could be obtained using a palladium-8 per cent yttrium alloy, and associated the sensitivity of the permeation process with the thermal history of the sample. Interesting isotope effects were also presented.
A poster presentation from Professor V. A Somenkov, V. Glazkow, A. Iroova, I. Kurchatov (Institute of Atomic Energy), V. Antonov, and E. Ponyatovskii (Institute of Solid State Physics), U.S.S.R., detailed a hydrogen induced ordering process in a palladium-silver alloy, whereby the injection of hydrogen at high pressures caused the ordering of the metal atoms at temperatures around 420 K. The work also raised the possibility of producing ordered binary alloys which might not be available by normal routes.
Hydrogen in Catalysis
Two papers reported on the importance of hydrogen and platinum group metals as catalysts. In the first, Z. Paal of the Hungarian Academy of Sciences, examined the role of hydrogen in various hydrocarbon reactions which are promoted by a platinum catalyst. The effect of a hydrogen pre-treatment on the platinum catalyst showed that the size of crystallites was affected by the particular sintering temperatures, and he noted that a heat treatment in a hydrogen atmosphere may also influence the effectiveness of the catalyst, by changing the concentration of other elements present in the platinum. In addition to the role of hydrogen in pre-treating the catalyst, the paper reported that during the use of the catalyst, marked maxima were produced in the yields of various model hydrogenation reactions (such as n-heptane, n-hexane and alkylcyclopentanes) as a function of hydrogen pressure. This was interpreted as being due to the hydrogen influencing the dissociation of active intermediates during the catalytic process, so that at high hydrogen pressures less dehydrogenation species are produced. The role of hydrogen in the deactivation of the catalyst and the possible reactivation of a catalyst using hydrogen were also discussed.
A second paper also dealing with catalysis analysed the process of hydrogenation of cyclopolyolefines (5 to 12 carbon atoms) using a palladium-6 per cent ruthenium catalyst. This work by Professor V. M. Gryaznov, M. M. Ermilova and N. V. Orekhova of the Institute of Petrochemical Synthesis, Moscow, showed that if a mixture of hydrogen and cyclopolyene vapour was introduced above the surface of the alloy the reaction would only start, and be sustained, if a definite amount of hydrogen was absorbed by the catalyst. If the catalyst was employed in a membrane configuration then the hydrogen:palladium ratio critical to the continuing reaction could be made to remain constant, rather than decrease as the reaction proceeds; which was the case when the inert gas mixture was fed over the alloy.
A poster presented by I. T. Belash, V. E. Antonov, O. V. Zharikov, A. I. Latynin and A. V. Palnichenko of the Institute of Solid State Physics, Moscow, reported that developments in high pressure hydrogenation (pressures in the GPa range) enabled a number of superconducting hydride phases which are unstable under normal conditions to be synthesised. Among these phases were RuHx where the incorporation of hydrogen caused the superconducting transition temperature of the pure metal to fall from 0.495 to 0.455 K. The addition of hydrogen to rhodium, with a superconducting temperature of 3.25 × 10−4K, resulted in no superconductivity being detected within the limits of their determination.
The unique properties of palladium metal among the platinum group metals, and indeed within the whole periodic table, ensured a broad representation of papers on palladium-hydrogen and palladium alloy-hydrogen systems.
Joint work by Czechoslovakian and Belfast researchers examined the diffusion characteristics of the palladium-hydrogen system in the α -phase using the diffusion elastic technique. F. A. Lewis, Queen’s University, Belfast, analysed the effect of additions of titanium, zirconium, niobium and cerium on the plateau pressures of various palladium alloy hydrogen systems and concluded that there is a direct link between the expansion and contraction of the lattice and the corresponding shape of the hydrogen isotherms.
Professor H. Brodowsky and Y. Chen of the University of Kiel, West Germany, showed the isotherms of a series of palladium-indium alloys, containing between 3.5 and 14 atomic per cent indium. An interpretation of the shape of the isotherms, based on well understood principles of increasing lattice dilation and decreasing density of states at the Fermi level as the hydrogen content of the alloy increases, was successful in modelling this system.
A poster by Professor B. Baranowski (Polish Academy of Sciences), F. A. Lewis and S. G. McKee (Belfast) and K. Kandasamy (University of Jaffna, Sri Lanka), investigated the changes in pressure within tubular membranes of palladium alloy following abrupt changes in the hydrogen contents of the outer surfaces of the tube walls. These changes were attributed to a strain-induced Gorsky effect transfer of hydrogen within the tube walls as the membrane bends.
T. Greber and L. Schlapbach, the University of Fribourg, Switzerland, provided a poster on the possible photogeneration of hydrogen from water at a palladium-cerium interface, the reaction being driven by the cyclic oxidation and reduction of the Ce ion.
The newly discovered hydride Na2PdH2 was examined by D. Noreus of the University of Stockholm, Sweden, who showed that at 400°C the hydride became molten. This property of melting prior to disintegration of the hydride is unique among ternary metal hydrides. To date the efficiency of heat pumps has been limited by the poor thermal conductivity of conventional solid powder reaction beds, therefore this material offers the possibility of more efficient heat pumps.
Many other papers concerned with palladium were presented including two papers on hydrogen trapping at dislocations in pure palladium and on the isotopic effects associated with hydrogen, deuterium and tritium dissolved in palladium-silver alloys. The effects of hydrogen in palladium on Hall coefficients and thermoelectric power was also covered. In addition the behaviour of hydrogen in thin film and single crystal palladium was reported.
This survey reports briefly on only some of the platiniferous papers presented at the Stuttgart conference. The amount of work reported demonstrates continued interest in the interaction of platinum group metals with hydrogen, from both a fundamental and an applied viewpoint. The full conference proceedings will be published early in 1989 in the journal Zeitschrift für Physikalische Chemie Neue Folge.
The continuation of the I.S.M.H.S. conference series is to be held in the first week of September 1990 at Banff, Alberta, Canada. For details contact Professor F. D. Manchester, Department of Physics, University of Toronto, Canada. In the interim, a Gordon Conference on metal-hydrides is to be held 10th to 14th July 1989 at Tilton School, New Hampshire, U.S.A. A programme description, contact address and application details for this conference will appear in the March 1989 issue of the publication, Science.