H2 Solubility in Pd/Cr2O3 Composites Prepared by Internal Oxidation of Pd−Cr Alloys: Equilibrium Pressure-Composition-Temperature Data

2009

J. Phys. Chem. C

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Internal oxidation of Pd-Cr alloys results in composites containing nanosized oxide precipitates within a Pd matrix. The Pd/chromia interfacial areas are appreciable. The Pd matrix is essentially pure and any differences between H 2 solution in the composite and in pure Pd must be due to perturbations introduced nanometersized oxide precipitates. Relative partial molar enthalpies for trapping of H by the internal Pd/chromia interfaces have been determined calorimetrically as a function of H content. The result of most interest is that |∆H H | decreases continuously from ≈ 64 to 14.5 kJ/mol H as the traps at the internal interfaces are filled by H where ∆H H refers to reaction with 1 / 2 H 2 (g). The partial enthalpies for H segregation can be determined from these values plus |∆H H | for 1 / 2 H 2 (g) solution in Pd. This strongly trapped H cannot be removed by evacuation at 303 K but most can be removed by evacuation at T g 573 K. |∆H H | and |∆S H | have also been determined in the dilute phase after the traps are filled with H. The dilute phase solubility extends over a large range compared to either pure Pd or to other IOed alloys. In this dilute region from about r ) (H/Pd, atom ratio) ) 0.02-0.045, |∆H H | is constant, 14.5 kJ/mol H which is about 4 kJ/mol H greater than that for the solution of 1 / 2 H 2 (g) in the dilute phase of pure, annealed Pd. It is shown from |∆H H | and isotherm measurements during absorption/desorption that the solubility is reversible in the extensive dilute phase region of an IOed Pd 0.095 Cr 0.05 alloy.

Section: Resultssupporting

confidence: 87%

Section: Resultsmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 96%

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H₂ Solubility in Pd/Cr₂O₃ Composites Prepared by Internal Oxidation of Pd−Cr Alloys. Calorimetric Determination of Enthalpies

Luo

2009

J. Phys. Chem. C

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“…Figure shows the dilute phase H 2 isotherms (323 K) for the resulting Pd/MgO composite compared with the unoxidized alloy, which has a greater dilute phase solubility than Pd at a given low p H 2 . After IO, the dilute phase solubility intersects the r axis at ∼0.0035 (Figure ), which reflects the amount of strongly trapped H; , this is similar to that found for IOed Pd 0.97 M 0.03 alloys with M = Fe or Cr but slightly greater than that for M = Al . After allowing for the greater amount of Mg in the Pd 0.97 Mg 0.03 alloy, this intercept is greater than that found by Huang et al for their IOed Pd 0.981 Mg 0.019 , which is not unexpected because of the lower IO temperature employed for the present alloy.…”

Section: Resultssupporting

confidence: 77%

H₂ Isotherms and Diffusion Parameters of H in fcc Pd–Mg Alloys

2013

J. Phys. Chem. C

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Hydrogen isotherms, p H 2 -H content relationships, have been obtained for fcc Pd−Mg alloys. One set of alloy compositions was employed for isotherm measurements from 273 to 343 K and another set of compositions was employed for isotherms from 423 to 523 K. It is shown in this work from X-ray diffraction that Mg expands the fcc Pd lattice. In the dilute phase at a given low p H 2 the H 2 solubility increases with atom fraction Mg, X Mg , which is expected for alloys with expanded lattices relative to Pd. Thermodynamic parameters for H 2 solution have been obtained from the isotherms. Diffusion parameters have been determined from the H fluxes through Pd−Mg alloy membranes. The dependence of Fick's diffusion constants, D H , on the H content at the upstream side of the membrane has been determined. The diffusion constants decrease with increase in X Mg . The activation energies for diffusion have been determined for three membranes with different Mg contents and these increase with X Mg . Internal oxidation of Pd−Mg alloys results in nanosized MgO precipitates within a Pd matrix. H 2 solubilities (323 K) have been measured for several internally oxidized alloys, and these indicate trapping at or near the Pd/MgO interfaces. Partially internally oxidized Pd−Mg alloy membranes are shown to have a greater resistance to CO poisoning than Pd membranes.

“…Kirchheim and co-workers [3] investigated such trapping in IOed Pd-alloys employing electrochemically measured isotherms. More recently Flanagan and co-workers, e.g., [4,5,6], determined the strongly trapped H from gas phase isotherms. Small angle neutron scattering, SANS, will be employed here to characterize the TiO 2 precipitates resulting from IO.…”

Section: Introductionmentioning

confidence: 99%

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

Journal of Alloys and Compounds

Wang

Luo

2017

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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.