Hydride phase formation

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. 13. Arrhenius plots of the kinetics of H atom recombination on a Ni77Cu23 alloy film catalyst. Above room temperature—active NiCu film with low activation energy. Below room temperature—film deactivated owing to a 0-hydride phase formation activation energy markedly increased. After Karpinski el al. (65).

The preference for isomerization can be attributed to the moderate hydrogen sorption capacity due to contaminants to the acidic medium and to the high dispersity of the catalyst. Under these conditions, p-phase palladium hydride, which can be active in C=C double bond hydrogenation, may be absent. Saturation by hydrogen of dispersed palladium in an acidic medium at 1 bar results in a-hydride phase formation only (8). An a-p phase transformation in 0.05 mol dm- Na2S04 demands a higher pressure than predicted by the Pd-H solubility diagram (4) or a considerable increase in the saturation time up to 15-20 days, especially in the presence of contaminants. 

C. Hydride Phase Formation in the Ti-Ct2-H2 System , Int. J. Hydrogen Energy, 24,149-152... 

These transitions do not take place as a change of the lattice stmcture, but as a lattice dilatation. The ]S-hydride phase formation is represented as a clustering of hydrogen atoms, whose energy of attraction, being associated with the lattice, strains around the dissolved hydrogen atom [41]. 

Rose, A., Maniguet, S., Mathew, R.J. et al. (2003) Hydride phase formation in carhon supported palladium nanoparticle electrodes investigated using in situ EXAFS and XRD. Physical Chemistry Chemical Physics, 5, 3220-3225. 

The other mode of failure for titanium alloys in the presence of hydrogen predominates rmder slow strain rate loading. The low strain rate embrittlement is related to hydride formation caused by strain-enhanced precipitation, but embrittlement imder impact is caused by hydride-phase formation after fabrication or heat treatment. Unlike many hydrideforming systems, titanium forms a stable hydride, but the kinetics of precipitation are slow compared to the Group Vb metals. Therefore, embrittlement is more prone to occur at low strain rates at which precipitation can proceed at a rate that is sufficient to provide a brittle crack path. 

All TPR profiles, see Fig. le and Fig. 2ed, show two main features (i) a complex H2 consumption formed by a peak plus a shoulder in the 260-330 K region, due to the Pd" Pd° reduction and to the surface PdHx and bulk Pd-hydride phases formation and (ii) a negative peak (due to the Pd-hydride deeomposition). 

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 ... 

As mentioned previously in the introduction to the present review the ability to form the hydride phase is not characteristic solely of palladium or nickel. It would be of interest, therefore, to verify the results on the poisoning effect of hydride formation in the case of nickel or palladium by comparing with the other transition 3d, 4d, and 5d metals and the rare earth (4f) metals. 

So far ignored, but perhaps the most important factor in catalysis by metals able to form hydrides, are the dynamical conditions of formation and decomposition of hydride phases. 

Shao et al. [62] used Mg and Co nanoparticles with the size of 200-300 and 5-60 nm, respectively, for the synthesis of nanoparticulate Mg CoH (particle size 50-300 nm range) by pressing 2 1 powder mixtures of Mg and Co into pellets and then very complicated sintering under a hydrogen scheme. They observed the formation of two hydride phases Mg CoH and MgjCoH. From the Van t Hoff plot the formation enthalpy and enttopy were estimated as -82.3 kJ/molH and -138.8 kJ/molH K for Mg CoH, and -73.2 kJ/molH and -123.0 kJ/molH K for MgjCoHj, respectively. 

Figure 3.32 shows XRD patterns of (MgH -i-LiAlH ) composites after DSC testing up to 500°C. The primary phases present are Mg and Al. Peaks of MgO and (LiOH) HjO arise from the exposure of Mg and Li (or possibly even some retained LiH) to the environment during XRD tests. Apparently, XRD phase analysis indicates that a nearly full decomposition of original MgH and LiAlH hydride phases has occurred to the elements during a DSC experiment. In addition, no diffraction peaks of any intermetallic compound are observed in those XRD patterns. That means that no intermetallic compound was formed upon thermal decomposition of composites in DSC. Therefore, the mechanism of destabilization through the formation of an intermediate intermetallic phases proposed by Vajo et al. [196-198] and discussed in the beginning of this section seems to be ruled out of hand. 

T.N. Dymova, V.N. Konoplev, A.S. Sizareva, D.P. Alexandrov, Magnesium tetrahydroalu-minate solid-phase formation with mechanochemical activation of a mixture of aluminum and magnesium hydrides , Russ. J. Coord. Chem. 25 (1999) 312-315. 

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...

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 ... 

Several repetitions of this experiment have shown that absorption was sometimes initiated at temperatures as low as 70 and 80°K. if more time is allowed. The increase observed with lower temperatures seems to be due to the higher surface concentration of molecules in the van der Waal s adsorption layer. Once the formation of the hydride phase was initiated at higher temperatures the growth of this new phase, even at temperatures below the initiation temperature, is probably due to the assistance of nuclei of the new phase which have opened up the passage into the interior. 

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