Article pubs.acs.org/JPCC
H2 Isotherms and Diffusion Parameters of H in fcc Pd−Mg Alloys Da Wang and Ted B. Flanagan* Chemistry Department, University of Vermont, Burlington, Vermont 05405, United States ABSTRACT: Hydrogen isotherms, pH2-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 pH2 the H2 solubility increases with atom fraction Mg, XMg, which is expected for alloys with expanded lattices relative to Pd. Thermodynamic parameters for H2 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, DH, on the H content at the upstream side of the membrane has been determined. The diffusion constants decrease with increase in XMg. The activation energies for diffusion have been determined for three membranes with different Mg contents and these increase with XMg. Internal oxidation of Pd−Mg alloys results in nanosized MgO precipitates within a Pd matrix. H2 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.
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= Y, Rh, Al.8,9,12 From the hydrogen isotherms for these IOed alloys, there is strong evidence of H trapping at or near the Pd/ oxide interfaces, that is, regions of the isotherms where pH2 ≈ 0. There has been considerable interest in the IO of Pd−Mg alloys; for example, TEM has been carried out on these IOed alloys.8,13,14 The oxide precipitates are found to be octahedra and, after IO at 1273 K, it was found that there is a spread of oxide sizes with the mean size ∼30 nm,8 which is smaller than the oxide precipitates formed by IO of most of the Pd alloys.8−12 Huang et al.8 have determined H2 isotherms electrochemically at 295 K for an internally oxidized (IOed) XMg = 0.019 alloy.8 Initial regions of the isotherms where pH2, or its equivalent in electrochemical potential,8 are nearly zero indicate trapping of H at or near the Pd/MgO interfaces. They found that this strongly trapped H is not removed by anodic treatment and, consequently, they characterized it as strongly trapped H. This strongly trapped H can be removed by evacuation at elevated temperatures, that is, ≥573 K.8,9 Huang et al. found that the strongly trapped H corresponded to about r = 0.0016 for a Pd0.981Mg0.019 alloy IOed at 1273 K, where r = H-to-Pd, atom ratio. The characterization of metal/oxide interfaces is of technological interest and the trapping of H at or near the oxide interfaces can be considered to be an important part of this characterization. Hydride formation and subsequent decomposition (cycling) at moderate temperatures leads to high dislocation densities in
INTRODUCTION There have been many recent publications concerning Mg as a H storage system because of its low cost and large H capacity, for example, refs 1 and 2; however, the slow kinetics of H2 uptake and release are a problem for the utilization of this storage system. One approach has been to employ thin coatings of Pd on the Mg to enhance the kinetics, for example, ref 3. Because of the possibility that Pd−Mg alloys may form in the interfacial region, the behavior of Pd−Mg alloys toward H2 is relevant. The Pd-rich side of the Pd−Mg phase diagram is rather uncertain, but it is estimated that solid solution fcc alloys form up to about Mg atom fraction, XMg = 0.18, at ambient temperatures.4 Solid solution Pd−Mg alloys have been prepared in this research, and the solubility of H2 and corresponding thermodynamic parameters has been determined for these alloys from p-c (pH2-composition) isotherms. There are no detailed data available in the literature concerning the thermodynamics of H2 in fcc Pd−Mg alloys.5 Lattice parameters for these Pd-rich solid solution alloys have not been reported except that Westin6 indicated that Mg expands the Pd lattice. The fcc lattice parameters of the H-free alloys will be determined in this research. Internal oxidation (IO) of alloys is a simple technique employed to produce nanosized oxide precipitates dispersed within a metallic matrix such as Pd.7 IO of Pd alloys containing readily oxidizable solutes such as Al, Zr, Y, and Fe has been carried out by several investigators8−12 and the subsequent H2 solubilities in these IOed alloys have been reported.8−12 The present workers have determined H2 solubilities by gas-phase measurements for several different IOed Pd-M alloys, where M © 2012 American Chemical Society
Received: November 6, 2012 Revised: December 18, 2012 Published: December 18, 2012 1071
dx.doi.org/10.1021/jp310978y | J. Phys. Chem. C 2013, 117, 1071−1080
The Journal of Physical Chemistry C
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Pd,15 and such cycled Pd leads to enhanced H2 solubilities in the dilute phase due to the segregation of H to the stress fields of the dislocations.16 Cycling an internally oxidized (IOed) Pd−Al alloy has been found to lead to an even greater solubility enhancement9 than cycling pure Pd.17 Because the MgO precipitates are smaller than the other oxides formed by IO of Pd−M alloys, it will be of interest to determine whether or not cycling Pd/MgO composites leads to an even greater solubility enhancement. It has been found for several Pd alloys containing oxidizable solutes, for example, Pd−Al, that H permeation membranes, which have been partially IOed, have a greater resistance to poisoning by gaseous CO than the unoxidized alloys or Pd membranes,18 and partially IOed Pd−Mg alloy membranes will be examined in this regard.
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initiate the H permeation, we opened the valve separating the small volume containing the CO, which is adjacent to the membrane, and the large volume containing only H2 was opened. Before any permeation occurred, the total amount of CO in the gas stream is 0.03. ΔS°H values with β = 1.0 become more negative with XMg, indicative of blocking of interstices by Mg atom neighbors. With regard to coating Mg with thin layers of Pd to promote the H2 chemisorption the present research sheds some light on the behavior of possible Pd−Mg alloys that may form in the interfacial region from heat treatment and consequent metal atom diffusion. The present results indicate that any Pd−Mg alloys that form with XMg ≤ 0.10 should absorb H and allow it to diffuse to the inner Mg. Pd/MgO composites are produced from IO of Pd−Mg alloys. H2 isotherms have been measured for these IOed alloys with initial regions, where pH2 ≈ 0 indicative of strong trapping at or near the Pd/MgO interfaces. There is an increased H2 solubility in the dilute phase in the IOed alloys as compared with pure Pd due to dislocation generation during the IO. IOed Pd−Mg are of special interest because the MgO precipitates are very small, ∼30 nm,8 and it would be expected that the H trapping would be extensive; however, it is not significantly greater than that for other IOed Pd alloys, where the precipitates are significantly larger.8−12 Diffusion parameters have been determined and the concentration-independent values of D*H and E*D decrease and increase, respectively, with XMg. It is known that the diffusion parameters are affected by the H content of Pd32 and Pd−Ag membranes,20 and it is found here for Pd−Mg alloy membranes that DH decreases and ED increases with increase in r.
Figure 16. Plot of ED against r for the XMg = 0.08 alloy, where ED* for the plot has been determined from extrapolation of DH values to r = 0.
Figure 17. Plot of ED* against XMg.
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more resistant to the CO than Pd. The CO resistance is similar
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
to partially IOed Pd−Al alloy membranes.18 1079
dx.doi.org/10.1021/jp310978y | J. Phys. Chem. C 2013, 117, 1071−1080
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