Sieverts palladium

Fig. 2. Example of a hysteresis loop on the isotherm pHj = /(H/Pd) obtained during absorption (upper curve) and desorption (lower curve) of hydrogen from palladium black at 100°C. After Sieverts and Danz (19).

Palladium hydride is not a stoichiometric chemical compound but simply a metal in which hydrogen is dissolved and stored in solid state, in space between Pd atoms of crystal lattice of the host metal. Relatively high solubility and mobility of H in the FCC (face-centered-cubic) Pd lattice made the Pd H system one of the most transparent, and hence most studied from microstructural, thermodynamic, and kinetic points of view. Over the century that followed many metal-hydrogen systems were investigated while those studies were driven mostly by scientific curiosity. Researchers were interested in the interaction of hydrogen molecule with metal surfaces adsorption and diffusion into metals. Many reports on absorption of in Ni, Fe, Ni, Co, Cu, Pd, Pt, Rh, Pd-Pt, Pd-Rh, Mo-Fe, Ag-Cu, Au-Cu, Cu-Ni, Cu-Pt, Cu-Sn, and lack of absorption in Ag, Au, Cd, Pb, Sn, Zn came from Sieverts et al. [30-33]. 

A. Sieverts, E. Jurish, A. Metz, Solubility of hydrogen in sohd alloys of palladium with gold, silver, platinum, Z. Anorg. Chem. 91 (1915) 1 5. 

Sieverts has studied the absorption of hydrogen from 138° to 821° C. and with varying pressures of gas ranging from 1 mm. to 760 mm. He observed that the amount of hydrogen absorbed per unit weight of palladium is nearly proportional to the square root of the hydrogen pressure. 

In further support of this may be cited the observation of Sieverts,1 that the quantity of hydrogen absorbed by unit -weight of palladium is a function of the pressure and temperature only, and is quite independent of the superficial area of the metal. This would indicate that the absorption of nydrogen is an example of true solution rather than of definite chemical combination. 

In the specific case of a hydrogen-palladium system, the absorption mechanism involves the surface dissociation of hydrogen, and the concentration of hydrogen atoms in palladium can be related to hydrogen partial pressure by the Sievert s equation ... 

In palladium-silver alloys, larger amounts of hydrogen can be solubilised because the metal lattice has already been expanded by the silver atoms, and thus it is less brittle than the pure Pd lattice. Based on these considerations, the ideal silver content of the Pd cathode is between 20 wt% and 30 wt%. In order to obtain high flow rates of pure hydrogen, the maximum permeability of the Pd-Ag cathode must be realized. The hydrogen permeability P through the lattice of the Pd-Ag tubular electrode follows Sieverts law. The H2 permeating flow rate Qp (mol s ) can be derived from Equation [16.2] ... 

The solubility of the isotopes in palladium has been made the subject of an experimental study by Sieverts and his co-workers (2i), and shows that in an equilibrium system there are considerable solubility differences. Sieverts and Danz s (2i) results are shown in Fig. 40. The higher temperature results, where Nernst s distribution law, 8 = k p, may be expected to hold (>S denotes the solubility and i is a constant), lead to the following solubility ratios at one atmosphere pressure ... 

Lacher found he could express Sieverts and Zapf s(2i) data on the solubility of the isotopes in palladium at high tempera-tures as... 

The hydrogen flux through the membrane is proportional to the diffusion coefficient Dh of hydrogen in palladium and Sievert s solubility constant Ks of the hydrogen/palladium system ... 

A membrane separation device was prepared by Wilhite et al. by micro-electromechanical techniques [526]. The palladium/silver membrane deposited onto a silicon oxide support was only 20-nm thick, which was possibly the lowest membrane thickness ever reported for hydrogen separative purposes. A lanfhanum/nickel/cobalt oxide catalyst (LaNi0.95Co0.05O3) catalyst for partial oxidation of methanol was deposited onto the membrane. At a O/C ratio of 0.86 and 475 °C reaction temperature, up to 64% methanol conversion and more than 90% hydrogen selectivity could be achieved. These workers claimed a deviation from Sievert s law (see Section 5.2.4) for their membrane. N amely, the hydrogen flux did not depend by a power of 0.5 of the retenate and permeate pressure but rather by a power of 0.97, which they attributed to the absence of internal solid-state diffusion limitations in their ultra-thin membrane. 

Hydrogen is to be recovered from a gas mixture by permeation through a thin film (thickness S ) of palladium (Pd) membrane into an aqueous solution prior to chemical reaction in the aqueous solution. The diatomic gas H2 is present in the feed gas at a partial pressure Pnzg/ - hydrogen concentration in the bulk water is the mass-transfer coefficient of H2 in the aqueous film next to the palladium membrane is kn w The permeability coefficient of H2 through the Pd membrane via Sievert s law is... 

The H2 transport through a dense palladium film is an activated process and, in many practical cases, Sieverts law defines the pressure dependence of the Hj permeation at constant temperature. Generally, it is implicitly assumed that n does not depend on temperature. 

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