Thermodynamics of hydrogen absorption (desorption) in unoxidized and internally oxidized Pd–Ti alloys

Abstract

Hydrogen isotherms have been measured from 273 K to 323 K in solid solution, low Ti content fcc Pd-Ti alloys over H content ranges available up to p H 2 ≈0.10 MPa. ∆H H and ∆S H have been determined from the H 2 isotherms in the dilute phase region as a function of the H content of the alloys. Thermodynamic parameters have been determined for hydride formation and decomposition reactions from the temperature dependence of the plateau pressures. Hysteresis has been determined for the alloys up to Pd 0.94 Ti 0.06 ; the Pd 0.92 Ti 0.08 alloy does not form a hydride phase at T≥273 K. Alloys were internally oxidized to form Pd/TiO 2 composites and the H 2 solubilities re-determined. The isotherms indicate H trapping at the Pd/TiO 2 interfaces. The enthalpies for H trapping have been determined by reaction calorimetry for the internally oxidized Pd 0.98 Ti 0.02 alloy and the relative partial enthalpies for absorption are quite negative compared to H 2 solution in the dilute phase of pure Pd. These results probe the properties of metal/oxide interfaces which are important for a number of technological applications.

Assessment of Sieverts Law Assumptions and ‘n’ Values in Palladium Membranes: Experimental and Theoretical Analyses

“…Additionally, some theoretical case studies reported a different Sieverts' pressure exponent n from the ideal value even in the presence of pure hydrogen feed [27,30,31]. Flanagan and Wang [32][33][34] presented a common approach to evaluate the Einstein hydrogen diffusivity (referred to as "intrinsic" or "ideal" diffusivity) and the nonideal correction from the experimental isotherms of the thermodynamic factor versus the atom ratio, in addition to the case where the excess of chemical potential is estimated by a linear function of the atom ratio. Another approach is proposed by Hara and coworkers, who established a methodology to assess the intrinsic concentration-independent hydrogen permeability and solubility by means of polynomial functions of the square root of the hydrogen partial pressure [27,35].…”