P hydride phase

Binary addition elements, having in general Face-Centered-Cubic (FCC) crystal structures, stabilize the a-hydride phase against the P-hydride phase transition, reducing the problem of embrittlement, and also increase hydrogen permeability above that of pure palladium (see Table 14.2). 

The Pd Ag 90 10 co-deposited specimen was sealed effectively with minimal leaks. Though there is some variability in the data (Fig. 26 (right)), overall the H2 flux increases with increasing temperature, while the flux of both N2 and CO2 remain stable at a minimal level, essentially at the detection limit of the mass spectrometer. Up to 350 °C the p hydride phase dominates and these results show that above 350 °C there is a more rapad increase in H2 flux through the membrane with accelerated transport through the a phase. This increase in H2 flux above 350 °C is consistent with the results for the Pd only membrane (Fig. 26 (left)) indicating that increasing the Ag content to 30% (by EDX) has no detrimental effect on the H2 selectivity. 

Dispersions of the supported catalysts based on CO and H2 are very similar if equations 3.2 and 3.3 are used. There may be greater uncertainty about the stoichiometry for oxygen adsorption, but Figure 3.4(c) shows that the mono-layer coverage of H at 373 K, a temperature at which the p-hydride phase is not formed at these H2 pressures, is 100 pmole Hg, the same as that for O atoms at 300 K, thus the dispersions based on irreversible H uptakes at 300 K may be underestimated. Regardless, average dispersion values that vary by only + 10% can be obtained from these different adsorption methods. 

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. 

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. 

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.

The mechanism of the poisoning effect of nickel or palladium (and other metal) hydrides may be explained, generally, in terms of the electronic theory of catalysis on transition metals. Hydrogen when forming a hydride phase fills the empty energy levels in the nickel or palladium (or alloys) d band with its Is electron. In consequence the initially d transition metal transforms into an s-p metal and loses its great ability to chemisorb and properly activate catalytically the reactants involved. 

Fortunately, virtually everything said about hydrogenations is applicable to deuteriumations. Exceptions are certain rates and certain processes using metal and metal alloy membranes for hydrogenations (deuteriumations). Differences between the formation of the P-hydride and p-deuteride phases in these latter cases affect the reaction outcomes. Such differences between hydrogenations and deuteriumations are pointed out and discussed where appropriate. 

Figure 5.23 Pressure composition isotherms for critical temperature 7. The construction of the hydrogen absorption in atypical metal (left). The van t Hoff plot is shown on the right. The slope of solid solution (a-phase), the hydride phase the line is equal to the enthalpy of formation (p-phase) and the region ofthe coexistence ofthe divided by the gas constant and the intercept with two phases. The coexistence region is the axis is equal to the entropy of formation...

H-H interaction due to the lattice expansion becomes important and the hydride phase (P phase) nucleates and grows. The hydrogen concentration in the hydride phase is often found to be H M = 1. The volume expansion between the coexisting a- and P-phases corresponds in many cases to 10-20% ofthe metal lattice. Therefore, at the phase boundary high stress is built up and often leads to decrepitation of brittle host metals such as intermetaiiic compounds. The final hydride is a powder with a typical particle size of 10-100 pm (Figure 5.24). 

The thermodynamic aspects of hydride formation from gaseous hydrogen are described by means of pressure-composition isotherms in equilibrium (AG = 0). While the solid solution and hydride phase coexist, the isotherms show a flat plateau, the length of which determines the amount of H2 stored. In the pure P-phase, the H2 pressure rises steeply vfith increase in concentration. The two-phase region ends in a critical point T, above which the transition from the a- to the P-phase is continuous. The equilibrium pressure peq as a function of temperature is related to the changes AH° and AS° of enthalpy and entropy ... 

Of the steps listed in Table 1. some are encountered more frequently, while others are less common. Transition metal catalyzed processes usually begin with oxidative addition or coordination-addition as an Entry, which is commonly followed by transmetalation or insertion in the Attachment phase. The final Detachment step is either reductive elimination, or p-hydride elimination, depending on the nature of the intermediate. 

P-C-T data were obtained with a palladium sample ( 20 g) in the form of 1-mm rods (99.9% purity). Thus, the surface-to-volume ratio was quite small. The sample was not coated with palladium black but merely was cleaned mechanically after being annealed at 1100° K for several days. The sample had never been subjected to the hydride-phase transformation. 

The transition between two states of hydride phase takes place at the composition ZrM FLj. On the P-C diagram at 170°C (see Fig. 2) a small fold in... 

Abstract. The interaction of hydrogen with nonstoichiometric Tio.9Zro.iMnL3Vo.5 Laves phase compound at pressure up to 60 atm and in temperature range from 150 to 190°C has been studied by means of calorimetric and P-X isotherm methods. The obtained results allow us to propose the existence of one hydride phase, 0-hydride, in the Ti0 9Zr0 jMnj 3V0 5 - H2 system in the temperature range 150-170°C. It has been found that temperature 190°C is close to critical temperature (182°C) above which hydride phases does not exist. 

We will notice the following important circumstance in the area of disordered a P phases, initial IMC crystal structure in the most instances does not differ from the structure of the metallic matrix in the hydride phases in IMC-hydrogen systems. In this case chemical potential pH=GH/NH of the hydrogen component of the IMC hydride (that is, specific, per H atom, Gibbs energy GH) is as follows ... 

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