Palladium-hydrogen system phases

X-ray analysis of the palladium— hydrogen systems gives results which are in harmony with the view that hydrogen and palladium form two crystalline solid phases, one of which is practically pure palladium and the other a saturated solution of hydrogen in palladium. ... 

Figure 3.1. Phase diagram for the palladium-hydrogen system (note the resemblance to the CO2 phase diagram).

The palladium-hydrogen system is a two-component system (Pd and H) whose degrees of freedom are determined by the number of existing phases at working temperature and pressure. 

The palladium-hydrogen system (Fig. 3.9) [16] shows the presence of two phases, a and a. In both the phases, hydrogen occupies, randomly, the interstitial octahedral sites of the f.c.c. Pd lattice. 

On palladium catalysts the main hydrogenating agent is hydrogen dissolved in the bulk of the metal. Specific adsorption of cations, the cadmium ion in particular, greatly increases the overpotential of the O phase transition [16]. The heat of the a transition in sulfuric acid and cadmium sulfate solutions (Table 2) was calculated from charging curves obtained at 20, 40, and 60°C in the region of the phase transition for the palladium — hydrogen system. 

One of the most useful features of metal-hydrogen systems are their pressure-composition-temperature data, P-C-T. Such relationships for palladium-hydrogen are shown in Figure 1. For compositions and temperatures within the envelope, two solid phases coexist, as required by the phase rule. The lower hydrogen-content a-phase represents solution of hydrogen into the metal, and the higher hydrogen-content jS-phase is the hydride. Both a and (3 are... 

When equilibrium is established between ions in solution and the gas phase, we have gaseous electrodes the intermediates of this equilibrium are usually adatoms Pt, H2IH+..., or Pt, Cl2 Cl ... Palladium, hydrogen-sorbing alloys, and intermetallic compounds pertain to gaseous electrodes. However, at the same time, these systems are example of intercalation processes operating under equilibrium conditions, which brings them close to electrodes of the first kind. 

Wicke E, Nemst GH (1964) Phase diagram and thermodynamic behaviour of the palladium-hydrogen and of the palladium-deuterium system at normal temperature H/D separation effect. Ber Bunsenges Phys Chem 68 224—235... 

Sakamoto Y, Takai K, Takashima I, Imada M (1996) Electrical resistance measurements as a function of composition of palladium-hydrogen(deuterium) systems by a gas phase method. J Phys Condens Matter 8 3399-3411 Schroeder K, Ecke W, WUlsch R (2009) Optical fibre Bragg grating hydrogen sensor based on evanescent-field interaction with palladium thin-film transducer. Opt Lasers Eng 47(10) 1018-1022 Sharma AK, Gupta BD (2007) On the performance of different bimetaUic combinations in surface plasmon resonance based fiber optic sensors. J Appl Phys 101(9) 093111... 

Another useful teehnique in kinetie studies is the measurement of the total pressure in an isothermal eonstant volume system. This method is employed to follow the eourse of homogeneous gas phase reaetions that involve a ehange in tlie total number of gaseous moleeules present in the reaetion system. An example is the hydrogenation of an alkene over a eatalyst (e.g., platinum, palladium, or niekel eatalyst) to yield an alkane ... 

This review aims to present an account of the catalytic properties of palladium and nickel hydrides as compared with the metals themselves (or their a-phase solid solutions with hydrogen). The palladium or nickel alloys with the group lb metals, known to form /8-phase hydrides, will be included. Any attempts at commenting on the conclusions derived from experimental work by invoking the electronic structure of the systems studied will of necessity be limited by our as yet inadequate knowledge concerning the electronic structure of the singular alloys, which the hydrides undoubtedly are. 

Fig. 1. Absorption isotherms of hydrogen in palladium within a large range of temperatures and pressures of hydrogen gas. Numbers denote temperature in °C. Hydrogen pressure is given in the logarithmic scale. Broken line closes the area of the two-phase o + region of the Pd-H system. Different shapes of experimental points denote different authors data, cited by Scholten and Konvalinka (9). After Scholten and Konva-linka (9).

As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni), the a-phase, and /3-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the /9-phase exhibits a significant expansion of the lattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko (8), Majchrzak (15), and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that at moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium at least up to about 100°C). 

Scholten and Konvalinka (9) in 1966 published the results of their studies on the kinetics and the mechanism of (a) the conversion of para-hydrogen and ortho-deuterium and (b) hydrogen-deuterium equilibration. At first the a-phase of the Pd-H system was used as catalyst, and then the results were compared with those obtained when the palladium had previously been transformed into its /3-hydride phase. 

The catalytic system studied by Rennard and Kokes was in fact very complex. It can be expected that the satisfactory prolongation of the reaction should, however, result in a deviation from the formulated kinetics. Unfortunately no investigation comparable to that of Scholten and Kon-valinka has been done in the case of olefin hydrogenation. Such a study of the catalytic activity of the pure /3-phase of palladium hydride in comparison with the a- or (a + /3)-phases would supplement our knowledge concerning catalytic hydrogenation on palladium. 

The results used for a subsequent comparison of catalytic activity of all group VIII metals are related by Mann and Lien to palladium studied at a temperature of 148°C. At this temperature the appearance of the hydride phase and of the poisoning effect due to it would require a hydrogen pressure of at least 1 atm. Although the respective direct experimental data are lacking, one can assume rather that the authors did not perform their experiments under such a high pressure (the sum of the partial pressures of both substrates would be equal to 2 atm). It can thus be assumed that their comparison of catalytic activities involves the a-phase of the Pd-H system instead of palladium itself, but not in the least the hydride. 

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