Hydrides phase

The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and difRision as a function of grain boundary orientation. REELS is not nearly as commonly used as AES orXPS. 

There is a lively controversy concerning the interpretation of these and other properties, and cogent arguments have been advanced both for the presence of hydride ions H" and for the presence of protons H+ in the d-block and f-block hydride phases.These difficulties emphasize again the problems attending any classification based on presumed bond type, and a phenomenological approach which describes the observed properties is a sounder initial basis for discussion. Thus the predominantly ionic nature of a phase cannot safely be inferred either from crystal structure or from calculated lattice energies since many metallic alloys adopt the NaCl-type or CsCl-type structures (e.g. LaBi, )S-brass) and enthalpy calculations are notoriously insensitive to bond type. 

Figure 1. Ideal pressure-composition isotherms showing the hydrogen solid-solution phase, a, and the hydride phase, j3. The plateau marks the region of coexistence of the a and fl phases. As the temperature is increased the plateau narrows and eventually disappears at some consolule temperature...

Switendick was the first to apply modem electronic band theory to metal hydrides [5]. He compared the measured density of electronic states with theoretical results derived from energy band calculations in binary and pseudo-binary systems. Recently, the band structures of intermetallic hydrides including LaNi5Ht and FeTiH v have been summarized in a review article by Gupta and Schlapbach [6], All exhibit certain common features upon the absorption of hydrogen and formation of a distinct hydride phase. They are ... 

There have been numerous studies with the objective of gaining an understanding of the factors that influence the stability, stoichiometry, and H-site occupation in hydride phases. Stability has been correlated with cell volume [7] or the size of the interstitial hole in the metal lattice [8] and the free energy of the a p phase conversion. This has been widely exploited to modulate hydride phase stability, as discussed in Sec. 7.2.2.1. 

Westlake developed a geometric model which is fairly successful in predicting site occupation in ABs and AB2 hydride phases [9], It involves two structural constraints ... 

In order for an intermetallic compound to react directly and reversibly with hydrogen to form a distinct hydride phase, it is necessary that at least one of the metal components be capable of reacting directly and reversibly with hydrogen to form a stable binary hydride. 

If the metal atoms are not mobile (as is the case in low—temperature reactions) only hydride phases can result in which the metal lattice is structurally very similar to the starting intermetallic compound because the metal atoms are essentially frozen in place. In effect the system may be considered to be pseudo-binary as the metal atoms behave as a single component. 

Deterioration of electrode performance due to corrosion of electrode components is a critical problem. The susceptibility of MHt electrodes to corrosion is essentially determined by two factors surface passivation due to the presence of surface oxides or hydroxides, and the molar volume of hydrogen, VH, in the hydride phase. As pointed out by Willems and Buschow [40], VH is important since it governs alloy expansion and contraction during the charge-discharge cycle. Large volume changes... 

It is of interest to note that VH in the hydride phase is significantly less than in AB5 hydrides. Consequently, lattice expansion is also significantly reduced. However, the corrosion rate of the electrodes in Table 9 is still appreciable. Indeed, for the electrode with x - 0.25 the... 

This survey presents an overview of the chemistry of metal-hydrogen systems which form hydride phases by the reversible reaction with hydrogen. The discussion then focuses on the AB5 class and, to a lesser extent, the AB2 class of metal hydrides, both of which are of interest for battery applications. 

Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases... 

III. Catalytic Activity of Hydride Phases of Palladium and Its Alloys with... 

Among the three commonly used metal catalysts mentioned above which activate hydrogen, nickel and palladium form hydride phases of essentially the same type. The existence of a platinum hydride has not so far been proved. 

Neutron diffraction studies have shown that in both systems Pd-H (17) and Ni-H (18) the hydrogen atoms during the process of hydride phase formation occupy octahedral positions inside the metal lattice. It is a process of ordering of the dissolved hydrogen in the a-solid solution leading to a hydride precipitation. In common with all other transition metal hydrides these also are of nonstoichiometric composition. As the respective atomic ratios of the components amount to approximately H/Me = 0.6, the hydrogen atoms thus occupy only some of the crystallographic positions available to them. 

Fig. 4. Lattice parameter changes of Ni-Cu alloys and of Ni-Cu hydrides from 100% by weight of Ni to 100% wt. Cu. O, Ni-Cu +, (3-Ni-Cu hydride phases of alloys with different Ni content. After Baranowski and Majchrzak (26, 25a).

In this part of the review palladium alloys refers to those containing more than 40% Pd, i.e. such alloys which are able to form a (3-hydride phase. 

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 authors stated at the beginning of their work that to understand the mechanism of the reactions studied required an unambiguous determination of the influence of the hydrogen pressure on the rate of conversion or equilibration reactions. This could be possible only when dealing with a palladium catalyst incapable of absorbing hydrogen, i.e. with the palladium samples previously fully transformed into the /3-hydride phase, in which the H/Pd ratiq would be constant, almost independent of the hydrogen pressure. Then, for example, at room temperature under p = 1 atm, H/Pd = 0.68 when under p = 10 atm, H/Pd = 0.70 and under p = 1000 atm, H/Pd = 0.80 only. 

A ten to hundredfold decrease in the velocity of the reaction, seen as a break down of the Arrhenius plot, is observed at a temperature which, for any given pressure, is always higher than that thermodynamically foreseen for the beginning of the a-/3 transition (this discrepancy is smallest at 265 mm Hg pressure). The marked decrease of the rate of reaction is characteristic of the appearance of the /3-hydride phase. The kinetics of reaction on the hydride follows the Arrhenius law but with different values of its parameters than in the case of the a-phase. 

Table III lists the kinetic equations for the reactions studied by Scholten and Konvalinka when the hydride was the catalyst involved. Uncracked samples of the hydride exhibit far greater activation energy than does the a-phase, i.e. 12.5 kcal/mole, in good accord with 11 kcal/mole obtained by Couper and Eley for a wire preexposed to the atomic hydrogen. The exponent of the power at p amounts to 0.64 no matter which one of the reactions was studied and under what conditions of p and T the kinetic experiments were carried out. According to Scholten and Konvalinka this is a unique quantitative factor common to the reactions studied on palladium hydride as catalyst. It constitutes a point of departure for the authors proposal for the mechanism of the para-hydrogen conversion reaction catalyzed by the hydride phase.

Assuming the composition of the hydride to be expressed by PdjH2 (which corresponds to PdH0. 7) and bearing in mind the interstitial positioning of the hydrogen in the palladium lattice, the authors postulate the existence of the following equilibrium at the surface of the j8-hydride phase... 

Fig. 8. Arrhenius plots for the formic acid decomposition on palladium foil (1) and small pieces of this foil (2) at a higher temperature range, when hydrogen evolving as a product of the reaction was absorbed by Pd and transformed into the /3-Pd-H hydride phase. At the lower temperature range the reaction proceeds on the a-Pd-H phase, with a higher activation energy when the foil was hydrogen pretreated (2a), and a lower activation energy for a degassed Pd foil (3a). After Brill and Watson (57).

It seems justified to supplement the authors conclusions by adding that in the case of samples pretreated with hydrogen their higher energy of activation (12.3 kcal/mole) may result from the presence of a certain content of the /8-hydride phase in the a-solution phase. 

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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