Thermodynamics of Hydrogen Solution and Diffusion in Oxidized and

Mar 23, 2015 - Thermodynamics of Hydrogen Solution and Diffusion in Oxidized and Unoxidized Pd–W Alloys. Ted B. Flanagan and D. Wang. Material Scien...
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Thermodynamics of Hydrogen Solution and Diffusion in Oxidized and Unoxidized Pd−W Alloys Ted B. Flanagan* and D. Wang Material Science Program and Department of Chemistry, University of Vermont, Burlington, Vermont 05405, United States ABSTRACT: Equilibrium hydrogen isotherms have been measured from 423 to 523 K for two fcc Pd−W alloys, Pd0.98W0.02 and Pd0.96W0.04, over H content ranges available up to pH2 ≈ 0.10 MPa. ΔHH and ΔSH values have been determined from the H2 isotherms as a function of the H content of the alloys. After measurement of the isotherms, the Pd0.98W0.02 and Pd0.96W0.04 alloys were internally oxidized. The H2 solubilities in the internally oxidized alloys were measured and evidence is given for the formation of Hbronzes, HxWO3, from the nanosized WO3 precipitates within the Pd matrix. Pd−W alloy membranes prepared for H diffusion measurements were partially internally oxidized in order to improve their resistance toward gaseous contaminants. An equation has been derived to obtain the H diffusion constant for the inner, unoxidized alloy layer within the three layer membrane resulting from internal oxidation. This equation has been applied to data for the two internally oxidized Pd−W alloys. Activation energies and pre-exponential factors have been derived for H diffusion constants through the inner, unoxidized Pd−W alloy layers.

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measurements to characterize the H-trapping.12 H-trapping in Pd/oxide composites, such as Pd/Al2O3, can be determined from the extent of the initial region where pH2 ≈ 0 in isotherm plots of p1/2 H2 against r = H/Pd, atom ratio. Following the region of such strong H trapping in the IOed alloy where pH2 ≈ 0, the dilute phase isotherm becomes similar to that of pure Pd−H. The Pd/oxide interfaces are the origin of H trapping in IOed alloys such as Pd−Al, Pd−Fe, and so on. Aside from trapping at or near the Pd/oxide interfaces, another possibility for H trapping is the formation of H-bronzes within the composite. This can occur for certain oxide precipitates which are known to form H-bronzes, for example, after IO Pd−Mo alloys form MoO3 precipitates within the Pd matrix and, upon exposure to H, intercalate H forming H-bronzes, HxMoO3, within the Pd matrix.10 It should be noted that H-bronzes can only be prepared by reaction of an oxide such as MoO3, with atomic H. Methods of preparation which have been successfully employed are (i) H spillover,13 where the oxide is in intimate contact with an H2 dissociating metal such as Pt, (ii) electrochemical reduction,14 or (iii) chemical reduction in a solution containing the oxide.15 It was first found by Flanagan et al.10 that H-bronzes can also be formed by reaction of H2 with an internally oxidized Pd alloy, such as Pd−Mo, where the H bronzes form by intercalation of H in the nanosized oxide precipitates within the Pd matrix. H2(g) dissociates at the catalytically active Pd surface and the H atoms enter the composites and diffuse internally to the oxide precipitates to form H-bronzes. Evidence

INTRODUCTION Pd−W alloys have been used as electronic barriers on Si and nanoparticle Pd−W alloys as catalysts for oxygen reduction;1,2 their properties are therefore of some practical interest. W forms fcc solid solutions with Pd up to XW = 0.1463 and the fcc lattice parameter does not change appreciably according to ref 4; however, Wang et al.5 found that there was a small contraction of the fcc lattice from Pd to the Pd0.98W0.02 alloy. For Pd alloys whose fcc lattice parameters are greater than that of Pd, such as Pd−Ag and Pd−Au, the enthalpies of H2 solution at infinite dilution are more exothermic than for Pd, and for contracted Pd alloys, such as Pd−Cu and Pd−V, H2 solution is more endothermic than in Pd.6 It has been suggested recently7 that the enthalpy at infinite dilution, ΔHH° , is a good reflection of the expanded/contracted Pd alloy classification, that is, an expanded alloys should have ΔH°H values more negative and contracted alloys should have values more positive than Pd. Aside from several hydrogen isotherms measured for the Pd0.98W0.02 alloy for comparison with isotherms for the Pd0.98Cr0.02 and Pd0.98Mo0.02 alloys,5 there have been no investigations of the thermodynamics of H in fcc Pd−W alloys. Equilibrium H2 isotherms can be employed to determine partial molar enthalpies and entropies of H2 solution from plots of ln pH2 against 1/T at constant H contents.8 Internal oxidation (IO) of Pd alloys, where the metal solute is more readily oxidizable than Pd, results in Pd/oxide composites consisting of nanosized oxide precipitates within Pd matrices.9 For example, IOed Pd0.98Mo0.02 and Pd0.97Mo0.03 alloys form oxide precipitates whose radii are 7.6 and 10.6 nm, respectively, according to SANS.10 Kirchheim and co-workers first characterized H trapping in IOed Pd alloys using an electrochemical technique11 and Flanagan and co-workers subsequently employed gas phase © 2015 American Chemical Society

Received: December 9, 2014 Revised: March 21, 2015 Published: March 23, 2015 8124

DOI: 10.1021/jp512271j J. Phys. Chem. C 2015, 119, 8124−8130

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The Journal of Physical Chemistry C that the IOed Pd−Mo alloys form H-bronzes within the Pd matrix is based on the large H uptake which greatly exceeds that of trapping at Pd/oxide interfaces as in Pd/Al2O3 composites formed by IO of Pd−Al alloys.16 It was found that the IOed Pd0.97Mo0.03 alloy after exposure to H2 forms an H-bronze corresponding to H1.7MoO3,10 which closely agrees with the stoichiometry found by Sermon and Bond17 who prepared HxMoO3 by H-spillover. WO3 is known to form a H-bronze by conventional means, for example, H-spillover,18 although the amount of occluded H is less than that of MoO3, for example, Tinet et al.19 give the stoichiometries as H0.35WO3 when the H bronze is prepared by H-spillover, whereas MoO3 forms an H-bronze of stoichiometry H1.7MoO3. Since H-bronze formation from IOed Pd−V7 and Pd−Mo10 alloys have been investigated, it seems reasonable to investigate H trapping within a Pd composite by the other well-characterized H-bronze, that is, HXWO3. Pd/ WO3 composites will be prepared by the IO of the Pd0.98W0.02 and Pd0.96W0.04 alloys. There have been no investigations of the diffusion of H in Pd−W alloys as far as the authors are aware. For this reason some diffusion studies will be carried out here employing H permeation through partially IOed Pd−W membranes. Partially IOed membranes have been shown to have a greater resistance to gaseous poisons than the unoxidized alloys or Pd.20

Figure 2. Dilute phase hydrogen isotherms for the Pd0.96W0.04 alloy: □, 423 K; ◇, 453 K; ○, 473 K; △, 523 K.

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EXPERIMENTAL SECTION The alloys were prepared by arc-melting the component pure metals under argon. The arc-melted buttons were annealed in vacuo for 3 days at 1100 K and then cold-rolled into foil of about 100 μm in thickness and then reannealed. Some of the Pd−W alloys were IOed in the atmosphere at 1098 K for 72 h and the progress of the IO was followed by their weight increases. Membranes about 100 μm thick were prepared from these alloys and the H permeation studies were carried out in the apparatus employed for earlier diffusion studies employing Pd membranes.21

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RESULTS AND DISCUSSION Hydrogen Isotherms for Unoxidized fcc Pd−W Alloys. Dilute phase absorption isotherms in the range from 423 to 523 K are shown in Figures 1 and 2 for well-annealed, unoxidized Pd0.98W0.02 and Pd0.96W0.04 alloys. Figure 3 shows isotherms at

Figure 3. Dilute phase isotherms at 473 K: □, Pd; ○, Pd0.98W0.02; Pd0.96W0.04.

△,

473 K for the two alloys where they are compared with an isotherm for pure Pd. It can be seen that the presence of W decreases the solubility at a given low pH2 in agreement with the finding that there is a small contraction of the Pd lattice for the Pd0.98W0.02 alloy.5 Thermodynamic parameters in the dilute phase have been determined from plots of ln p1/2 H2 versus 1/T at a series of H contents where the slopes give ΔHH/R and the intercepts, −ΔSH/R. Results are shown in Table 1. ΔSH′,° is defined as ⎛ r ⎞ ⎟ ΔSH′ , ° = ΔSH + R ln⎜ ⎝1 − r ⎠

(1)

ΔS′H,°

that is, refers to the relative partial standard entropy at a finite value of r where r = H-to-metal atom ratio and reflects any nonideality in the entropy, whereas ΔSH° refers to ΔSH′,° at infinite dilution, r → 0 and ΔSH refers to the experimental relative partial entropy, that is, Figure 1. Dilute phase hydrogen isotherms for the Pd0.98W0.02 alloy: □, 523 K; +, 503 K; ○, 473 K; ◇, 453 K; △, 423 K.

ΔSH = ΔHH/T − R ln p1/2 8125

(2) DOI: 10.1021/jp512271j J. Phys. Chem. C 2015, 119, 8124−8130

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The Journal of Physical Chemistry C Table 1. Dilute Phase Thermodynamic Parameters for the Pd0.98W0.02 and Pd0.96W0.04 Alloys and Pd (423−523 K)a

Table 2. Plateau Thermodynamic Parameters for the Pd0.98W0.02 Alloy (273−343 K)a

Pd r

0

ΔHH (kJ/mol·H) ΔSH′,° (J/K mol·H)

0.005

0.010

0.015

−9.5 −10.1 −52.7 −53.5 Pd0.98W0.02

−10.3 −53.7

−10.5 −53.7

r

0

ΔHH (kJ/mol·H) ΔS′H,° (J/K mol·H)

a

0.020

0.005

0.010

0.015

0.020

−8.69 −8.87 −52.4 −52.4 Pd0.96W0.04

−8.52 −51.1

−8.57 −50.8

−8.88 −51.0

r

0

0.004

0.006

0.008

0.010

0.012

ΔHH (kJ/mol·H) ΔS′H,° (J/K·mol H)

−8.59 −54.3

−8.98 −55.4

−9.17 −55.5

−9.02 −55.5

−8.82 −54.8

−9.03 −55.0

(3)

where g1 is the first order term in a polynomial expansion of the excess chemical potential and ΔμH° = ΔHH° − TΔSH° . Rearrangement of eq 2 gives RT ln[p1/2 (1 − r )/r ] = ΔμH° + g1r

ΔHdplat

ΔSfplat

ΔSdplat

x=0 x = 0.02

−18.7 −16.3

19.5 19.1

−46.3 −44.2

46.2 51.2

dilute phase thermodynamic values. Hysteresis is smaller than for Pd, for example, at 323 K, RT ln(pf/pd)1/2 = 0.92 kJ/mol·H and 0.62 kJ/mol·H for Pd and the Pd0.98W0.02 alloy, respectively. Hydrogen Solubility in Internally Oxidized Pd−W Alloys. Internal oxidation (IO) of Pd−W alloys results in nanoprecipitates of WO3 within a Pd matrix as based on the weight gains after IO. For example, after IO of the Pd0.98W0.02 alloy at 1093 K for 216 h, a weight increase corresponding to a composition of 98.6% WO3 for the IOed Pd0.98W0.02 alloy was found indicating that the stoichiometry of the oxide is WO3, because otherwise, the % would not be as close to 100%, as found. The H2 solubility in the IOed Pd0.98W0.02 alloy is shown in Figure 5 where there is a shoulder region from about r =

where p is the equilibrium H2 pressure at the given H content. The mean field model for H in Pd is given as22 2

ΔHfplat

a Values for Pd from Lässer;23 ΔHplat in kJ/mol·H and ΔSplat in J/K mol·H. The superscripts f and d refer to hydride formation and decomposition plateau enthalpies, respectively.

The values for r = 0 are obtained from plots of ΔμH° /T against 1/T.

RT ln pH1/2 = ΔμH° + RT ln[r /(1 − r )] + g1r

Pd1−xWx

(4)

It can be seen from eq 3 that a plot of RT ln[p (1 − r)/r] against r gives g1 from the slope and ΔμH° from the intercept. The values of g1, which have been found from such plots at 473 K, are −44.0 and −37.4 kJ/mol H for the Pd0.98W0.02 and Pd0.96W0.04 alloys, respectively, which is more negative than that for Pd−H, −33.9 kJ/mol H at 473 K. A plot of Δμ°H/T against 1/T gives ΔHH° and ΔSH° from the slope and intercept, respectively. Figure 4 shows isotherms at different temperatures for the Pd0.98W0.02 alloy where plateaux can be seen from 273 to 343 K. Hysteresis is also apparent from the difference between the absorption and desorption isotherms in the plateau regions. The thermodynamic values for the plateau region of the Pd0.98W0.02 alloy are shown in Table 2 where the ΔHplat values are smaller in magnitude than those for Pd as expected from the 1/2

Figure 5. Dilute phase isotherms (323 K) for an IOed Pd0.98W0.02 alloy: ○, initial isotherm after IO; △, repeat isotherm after evacuation at 323 K; □, after cycling at 323 K; curves without data points are dilute phase solubilities for annealed and cycled Pd, as shown by the labels on the curves.

0.002 to about 0.012 after which the hydrogen pressure increases more steeply. Such shoulder regions also appear in the H2 isotherms of the other IOed Pd alloys where the oxide precipitates form H-bronzes, that is, Pd−Mo alloys10 and Pd−V alloys.7 For oxide precipitates which do not form H-bronzes, for example, IOed Pd−Al16 or IOed Pd−Fe24 alloys, there are no shoulder regions and for such IOed alloys there is an initial region where pH2 ≈ 0 before pH2 increases and then the isotherm resembles the dilute solubility isotherm characteristic of Pd.16,24 It is of interest that the magnitudes of the enthalpies of H2 intercalation into H-bronze forming oxides is in the order:

Figure 4. Isotherms for the Pd0.98W0.02 alloy: ◇ 273 K; △, 303 K; ○, 323 K; □, 343 K, where the unfilled and filled symbols refer to hydrogen absorption and desorption, respectively. 8126

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The Journal of Physical Chemistry C H3.8V2O5 > H1.6MoO3 > H0.35WO325 and the shoulder regions for these IOed Pd alloys decrease in importance as the exothermicity increases, suggesting that the shoulder regions are due to relatively weakly bonded H within the H-bronzes. The r value, where pH2 starts to strongly increase, will be taken as an estimate of the stoichiometry of the H-bronze. For the IOed Pd0.98W0.02 alloy the initial isotherm intersects the abscissa at r = 0.0075 or H0.375WO3 (Figure 5), which is close to the stoichiometry found by other workers where the H-bronze was prepared by a more conventional method, that is, H spillover.25 After measurement of the H solubility of the IOed Pd0.98W0.02 alloy, it was evacuated at 323 K and the H solubility was remeasured (Figure 5). The remeasured solubility indicates that some H has been removed by this evacuation. The ease of H removal by evacuation at 323 K from those IOed Pd alloys that form H-bronzes is in the order Pd−W > Pd−Mo > Pd−V alloys,10 which is the opposite order of the exothermicity of Hbronze formation, as would be expected. For IOed Pd−Al alloys, which do not form H-bronzes, H is not removed by evacuation after the initial absorption following IO; for example, see ref 16. The IOed Pd0.98W0.02 alloy containing H was then evacuated at 823 K for 12 h, and its subsequent intercept upon H solution was r = 0.0055, indicating that about 73% of the H had been removed. The IOed Pd0.98W0.02 alloy was completely hydrided and then evacuated at 323 K, that is, it was cycled through the dilute ↔ hydride phase changes. The subsequent dilute phase H2 isotherm was found to lie between the initial one and that after the 323 K evacuation. The dilute phase isotherm of the cycled alloy is much more sloping that of the uncycled one due to the solubility enhancement from dislocations introduced during the cycling as in Pd; for example, see ref 26. The r axis intercept of this cycled alloy is about 0.0045, which is somewhat greater than that found after the initial isotherm measurement. Figure 6 shows the H solubility (323 K) of a Pd0.96W0.04 alloy after IO at 1003 K for 216 h. From the weight gain, this alloy

from the subsequent H2 absorption isotherm which is similar to the behavior of the IOed Pd0.98W0.02 alloy (Figure 5) and is therefore not shown in Figure 6. After evacuation of the IOed Pd0.96W0.04 alloy at 823 K for 24 h, the remeasured isotherm is nearly the same as the initial one, that is, all of the H has been removed. After cycling this IOed alloy at 323 K, the solubility curve, which is not shown in Figure 6, exhibited a shoulder initially, and its intercept was about the same as that after evacuation and before the cycling (323 K), indicating that cycling itself does not facilitate H removal from within the Hbronze. It is of interest to subtract the contribution of the Pd solubility from the overall solubility isotherm for the IOed Pd0.96W0.04 alloy. The isotherm chosen for removal of the Pd contribution is that after evacuation at 823 K (Figure 6) because it is quite similar to the initial isotherm. The Pd matrix should exhibit a given pH2 with a corresponding r characteristic of pure Pd, independent of the presence of H x WO 3 precipitates. When this Pd contribution is subtracted from the overall solubility isotherm, the results shown in Figure 6 are obtained. The shoulder region is not affected appreciably, but the more steeply rising portion is now much steeper, indicating that H-bronze formation is nearly complete at the end of the shoulder region. The steeply rising portion differs slightly from an isotherm for pure, annealed Pd, and can be attributed to some H trapping by dislocations which form from the IO. This result indicates that the shoulder region, which remains after subtraction of the Pd contribution, arises from formation of HxWO3 precipitates where the H bonding is weak enough so that there is a measurable equilibrium pH2 (Figure 6). Such shoulder regions are much smaller than the IOed Pd−Mo alloys,10 indicating that the bonding of the H in the HxWO3 bronze is weaker than that in the IOed Pd−Mo alloys, which, as pointed out above, is consistent with direct measurements of the heats of formation of these bronzes.19,27 Initial, more complete, isotherms measured up to large H contents for both of the IOed Pd−W alloys are shown in Figure 7, where it can be seen that there are small shifts to higher H contents in the dilute phase corresponding to the formation of the H-bronzes, which is reflected by the greater H contents in the hydride phase region, but otherwise, the isotherms are similar to those of Pd showing that the alloys have been completely IOed.

Figure 6. Dilute phase isotherms for an IOed Pd0.96W0.04 alloy (323 K): −○−, initial isotherm after IO; −△−, after evacuation at 823 K for 24 h; −▲−, isotherm after removing the solubility data for pure Pd from the isotherm for the IOed alloy evacuated at 823 K; ·, Pd.

had been IOed 95.6% to WO3. The effects of IO on the subsequent solubility are greater for this alloy than for the Pd0.98W0.02 alloy because of its greater W content and increased number of WO 3 precipitates.9 The shoulder is more pronounced for the IOed Pd0.96W0.04 alloy (Figure 6) than for the IOed Pd0.98W0.02 alloy (Figure 5). After evacuation of the IOed Pd0.96W0.04 alloy at 323 K, some H is removed judging

Figure 7. Initial, complete absorption isotherms for the IOed Pd0.98W0.02 and Pd0.96W0.04 alloys (323 K) including the concentrated phase: △, IOed Pd0.98W0.02; ○, IOed Pd0.96W0.04; , Pd. 8127

DOI: 10.1021/jp512271j J. Phys. Chem. C 2015, 119, 8124−8130

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The Journal of Physical Chemistry C

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H DIFFUSION THROUGH THREE-LAYER MEMBRANES Deivation of an Equation for DH,alloy Corresponding to the Inner Layer of a Three-Layer Membrane. It has been shown that IO proceeds from the surface inwardly,9 and therefore, the two outer layers of a Pd−W foil will be internally oxidized to the same extent after partial IO. After IO a membrane such as Pd−W will consist of three layers, an upstream Pd/oxide composite, an inner unoxidized alloy, and a downstream Pd/oxide composite layer. The motive is to derive an equation to determine the diffusion parameters of the inner pure alloy membrane from the H flux through the three layers. It has been shown by Huang et al.11 that, after some initial strong trapping of dissolved H, the subsequent diffusion of H through IOed Pd alloys, for example, Pd−Al, corresponds to diffusion through pure Pd. Therefore, it can be assumed that the flux through the outer two IOed layers of the three-layer membrane corresponds to diffusion through pure Pd. It should be noted that steady state H fluxes, J, are established very rapidly through these alloy membranes. Partially IOed Pd−W alloys have been employed here as diffusion membranes to determine diffusion parameters because it has been found that IO leads to membrane surfaces which are more resistant to gaseous poisons such as CO than unoxidized alloy membranes.20 A partially IOed Pd alloy membrane is

eq 5 that the concentration ratios at the two interfaces, K1 and K2, are both approximately equal, K, as will be discussed below. In the steady state, μH in each of the two coexisting phases at the two internal boundaries are equal, which means that the equilibrium pH2 for the two coexisting phases must be the same at these interface boundaries. The equality of the H chemical potentials should hold since the two phases are in contact and the H is mobile. From the isotherms for Pd and the alloy, it can be shown that there is no clear trend in the value of K = (rPd/ ralloy)pH2 with either pH2 or temperature. The reason for this is that, at the relatively high temperatures employed for these permeation studies, the isotherms of both coexisting phases are nearly linear (Figures 1 and 2) and the small degree of nonlinearity is rather similar for Pd and the unoxidized alloys. This demonstrates that K1 = K2 = K, at least for the two alloys employed here at the temperatures of the permeation studies, ≥423 K. The K values for Pd and the Pd0.98W0.02 alloy are consistently smaller than those for Pd and the Pd0.96W0.04 alloy, which is expected since the dilute phase isotherm is steeper for the Pd0.96W0.04 alloy. A plot of 1/J against 2dPd should be linear, according to eq 4, with intercepts at 2dPd = 0 and do equal to DH,alloy and DH,Pd, respectively. Such experimental plots have been shown to be quite linear over a large range of dPd values for both a partially IOed Pd0.096Al0.04 alloy28 and a partially IOed Pd0.926Fe0.074 alloy.29 In contrast to the Pd−Fe alloys,29 where data are available for a series of different degrees of partial internal oxidation and corresponding J values,28,29 in the present case, data are available only for one partially IOed Pd0.98W0.02 alloy and for one partially IOed Pd0.96W0.04 alloy, and it was desired to learn whether this would be sufficient to determine DH,alloy for each of these alloys. The flux through the inner unoxidized alloy layer of a partially IOed alloy is given by

Figure 8. Schematic illustration of the three layers within a membrane resulting from partial internal oxidation. The H concentrations at the interfaces are indicated as co and so on. The two end portions labeled as Pd/WO3 are the internally oxidized outer layers.

J = DH,alloy [c1′ − c 2′] /dalloy = DH,alloy [c1 − c 2]/Kdalloy (6)

where c′1 = c1/K and c′2 = c2/K, and therefore, DH,alloy can be obtained if K, c1, and c2 are known. The H concentrations at the Pd/oxide composite sides of the interfaces can be obtained from the experimentally measured fluxes and the other known quantities for the Pd layers, that is,

illustrated schematically in Figure 8. It has been shown28,29 that the flux through such a three layer system is given by 1/J = Kdo/(coD H,alloy ) + (2dPd /co)[(1/DH,Pd ) − (K /DH,alloy )]

⎛ JdPd ⎞ JdPd ⎟⎟ and c 2 = c1 = ⎜⎜co − DH,Pd ⎠ DH,Pd ⎝

(5)

J is the flux, co is the upstream H concentration, DH,alloy, and DH,Pd are the diffusion constants in the inner and outer layers, respectively, and dPd and do are the thicknesses of each of the IOed layers and the overall thickness of the membrane, respectively, and the former is known from the percent IO. K1 = (c1/c′1) and K2 = (c2/c′2), where c1, c′1, c′2, and c2 are the H concentrations at the Pd side of the upstream Pd/alloy interface, at the alloy side of the same upstream interface, at the alloy side of the downstream alloy/Pd interface, and at the Pd side of the downstream interface (Figure 8). It is assumed in

(7)

where, in keeping with experimental conditions, pdown = 0. Substitution of these expressions for c1 and c2 into eq 5 gives DH,alloy from the measured flux and the other known quantities. This method of calculating DH,alloy from a single IOed alloy using eq 6 will first be applied to our previous experimental data at 473 K and 50.6 kPa29 for a partially, 46%, IOed Pd0.926Fe0.074 alloy 84 μm in thickness. This IOed alloy has been chosen since DH,alloy is available for this alloy determined from eq 4 using extensive data of 1/J versus 2dPd. Using values

Table 3. Interfacial Concentrations (mol H/cm3) at 473 K, pup = 50.6 kPa for a 10% IOed Pd0.98W0.02 and for a 42.4% IOed Pd0.96W0.04 Alloy alloy

co

c1

c1′

c2

c2′

Pd0.98W0.02 Pd0.96W0.04

1.64 × 10−3 1.64 × 10−3

1.57 × 10−3 1.47 × 10−3

1.40 × 10−3 9.24 × 10−4

6.47 × 10−5 1.5 × 10−4

4.25 × 10−5 8.5 × 10−5

8128

DOI: 10.1021/jp512271j J. Phys. Chem. C 2015, 119, 8124−8130

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The Journal of Physical Chemistry C derived from eq 6 and the known flux and membrane dimensions and DH,Pd = 13.3 × 10−6 cm2/s,21 we obtain DH,alloy = 6.4 × 10−6 cm2/s, which agrees reasonably well with the experimental value obtained for this alloy directly from eq 4, 7.7 × 10−6 cm2/s or the value at infinite dilution, 6.7 × 10−6 cm2/s obtained from extrapolation of DH to r = 0.29 This demonstrates that this approach for determining DH,alloy is valid. It will be employed here to obtain DH,alloy for the two Pd− W alloys investigated in this work. The fact that DH depends upon the H content21 should be considered. In the present case in the calculation of c1 using eq 6, DH should be taken as the value for r ≈ 1.5 × 10−2 because the H concentration in the upstream Pd/oxide composite layer does not change much from co (Table 3). For the downstream layer, the concentration-independent diffusion constant DH,Pd * is most appropriate since the H concentration in the downstream Pd layer is quite small, that is, c2 is quite small and appropriate for conditions of infinite dilution of H (Table 2). Steady state H concentrations at the two internal interfaces can be calculated from the equality of μH for the two coexisting phases at each interface. Results are shown for the two IOed alloys in Table 3 for 473 K, where it can be seen that for both alloys c1 is not much smaller than co, and therefore, diffusion in the upstream Pd composite layer takes place at H contents which change only slightly from co. In the inner, unoxidized, layers of the alloys, however, there are dramatic decreases in the H concentrations from c1′ at the first interface to c2′ at the second interface, that is, there is a large concentration gradient in the inner, unoxidized layer. It can be seen in Table 3 that c2 > c′2, that is, permeation at this interface takes place from a lower to a higher concentration, uphill diffusion, because according to the isotherms, the H concentration is greater in the Pd composite than in the alloy at a given μH. Diffusion Parameters for the Pd 0.98 W 0.02 and Pd0.96W0.04 Alloys. Arrhenius’ plots of DH,alloy for the Pd0.98W0.02 (IOed 10%) and Pd0.96W0.04 (IOed 42.4%) alloys along with that of Pd are shown in Figure 9, where it can be

results from these plots are given in Table 4. It can be seen that there is a significant increase in Ea with XW, but not much Table 4. Activation Energies and Pre-Exponential Factors (423−523 K) for the Pd0.98W0.02 and Pd0.96W0.04 Alloys Allowing for the Extent of IO Using Eqs 5 and 6 XW 0 0.02 0.04

DH,alloy (473 K)/cm2/s −5

1.4 × 10 1.08 × 10−5 6.6 × 10−6

Ea (kJ/mol)

DH,alloy ° (cm2/s)

23.8 25.9 27.7

6.0 × 10−3 7.5 × 10−3 7.5 × 10−3

change in DH° . It has been found elsewhere that the latter does not change significantly upon alloying Pd, for example, Pd− Ag30 or Pd−Au.31 DH decreases nearly linearly from Pd to the Pd0.96W0.04 alloy to a greater degree than, for example, the Pd− Fe alloys,29 which is another alloy series where the dilute phase solubility decreases with XM. For alloys where the dilute phase solubility increases with XM, such as Pd−Ag, DH does not decrease as sharply with XM.30

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CONCLUSIONS It is shown that alloying Pd with W to form fcc solid solution Pd−W alloys markedly reduces the solubility of H in the Pd. The relative partial molar enthalpies of solution become less exothermic and the standard relative partial molar entropies become more negative with increase of atom fraction W. Hydrogen solubilities in completely internally oxidized (IOed) Pd−W alloys have been measured and evidence is given for the formation of H-bronzes, HxWO3, within the Pd matrix. An equation is derived to obtain the diffusion constant of H within an inner unoxidized alloy layer of a three layer membrane where the outer two layers are identical Pd/oxide composites. The equation is employed for partially IOed Pd− W alloy membranes. The diffusion constants for H in the Pd− W alloys decrease regularly with atom fraction W. From a practical viewpoint it is worth noting that in metals with W as an alloying agent, dissolved H may form an H-bronze which could lead to structural problems, for example, embrittlement.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: ted.fl[email protected]. Notes

The authors declare no competing financial interest.

Figure 9. Arrhenius plots of DH: □, Pd; Pd0.96W0.04 alloy.

△,

■ ■

ACKNOWLEDGMENTS The authors wish to thank Dr. J. D. Clewley for help in alloy preparation.

Pd0.98W0.02 alloy; ○,

REFERENCES

(1) Eizenber, M.; Tu, K. Formation of shallow silicide contacts of high Schottky barrier on Si; alloying with Pd and Pt with W versus alloying Pd and Pt with Si. J. Appl. Phys. 1982, 53, 1577−1585. (2) Sarkar, A.; Manugan, A.; Manthiram, A. Low cost Pd−W nanoalloy catalysts for oxygen reduction reaction for fuel cells. J. Mater. Chem. 2009, 19, 159−165. (3) Nemilov, V. A.; Rudnitsky, A. A.; Polyakova, R. S. The Pd−W phase diagram. Isvest. Sektora. Platiny 1949, 23, 101−103. (4) Greenfield, P.; Beck, P. A. Intermediate phases in binary system of certain transition elements. Trans. AIME 1956, 206, 265−276.

seen that the diffusion constants decrease with XW. These values of DH,alloy can be regarded as concentration-independent values, that is, Einstein diffusion constants, DH*, because the average H concentrations in the inner alloy layers at 473 K are relatively small, that is, r ≈ 0.007 (Pd0.98W0.02) and r ≈ 0.005 (Pd0.96W0.04). The activation energies and pre-exponential factors have been determined from Arrhenius’ plots of DH,alloy and the 8129

DOI: 10.1021/jp512271j J. Phys. Chem. C 2015, 119, 8124−8130

Article

The Journal of Physical Chemistry C

(28) Wang, D.; Flanagan, T. B.; Shanahan, K. Hydrogen diffusion through partially internally oxidized Pd-alloy membranes. Scr. Mater. 2006, 54, 1317−1320. (29) Flanagan, T. B.; Wang, D. Hydrogen solubilities and permabilities in un-oxidized and oxidized fcc Pd-Fe and Pd0.77Ag0.23)(1−x)Fex alloys. J. Alloys Compd. 2010, 488, 72−78. (30) Wang, D.; Flanagan, T. B.; Shanahan, K. L. Diffusion of hydrogen through Pd-Ag alloys. J. Phys. Chem. B 2008, 112, 1135− 1148. (31) Luo, S.; Wang, S.; Flanagan, T. B. Thermodynamics of hydrogen in fcc Pd-Au alloys. J. Phys. Chem. C 2010, 114, 6117−6125.

(5) Wang, D.; Clewley, J. D.; Luo, S.; Flanagan, T. B. Thermodynamics of hydrogen absorption/desorption by Pd-group 6B alloys and some anomalous isotherms. J. Alloys Compd. 2001, 325, 151−159. (6) Sakamoto, Y.; Chen, F.; Ura, M.; Flanagan, T. B. Thermodynamic properties for sollution of hydrogen in palladium-based binary alloys. Ber. Bunsen-Ges. 1995, 99, 807−820. (7) Flanagan, T. B.; Wang, D.; Luo, S. Hydrogen solubility in Pd/ V2O5 composites formed by internal oxidation and hydrogen isotherms in un-oxidized Pd-V alloys. Metall. Mater. Trans. A 2013, 44, 2810−2818. (8) Oates, W. A.; Flanagan, T. B. The solubility of hydrogen in transition metals and their alloys. Prog. Solid State Chem. 1981, 13, 193−272. (9) Meijering, J. L. Internal oxidation in alloys. Adv. Mater. Res. 1971, 5, 1−81. (10) Noh, H.; Wang, D.; Luo, S.; Flanagan, T. B.; Balasubramaniam, R.; Sakamoto, Y. Hydrogen bronze formation within Pd/MoO3 composites. J. Phys. Chem. B 2004, 108, 310−319. (11) Huang, X. Y.; Mader, W.; Kirchheim, R. Hydrogen and oxygen at metal/oxide interfaces. Acta Metall. Mater. 1991, 39, 893−907. (12) Balasubramaniam, R.; Noh, H.; Flanagan, T. B.; Sakamoto, Y. Hydrogen solubility in Pd/ Al2O3 composites prepared by internal oxidation of Pd-Al alloys. Acta Mater. 1997, 45, 1725−1733. (13) Lin, X.; Lambert, J. F.; Fripiat, J. J.; Ancion, C. Thermodynamics of H2 dissociation into spilt-over hydrogen over Pt/transition metal oxide systems. J. Catal. 1989, 119, 215−229. (14) Schöllhorn, R.; Kuhlmann, R.; Besenhard, J. O. Topotactic redox reaction and ion exchange of layered MoO3 bronzes. Mater. Res. Bull. 1976, 11, 83−90. (15) Glemser, O.; Lutz, G. Ü ber molybdänblau. Z. Anorg. Allg. Chem. 1951, 264, 17−33. (16) Wang, D.; Noh, H.; Luo, S.; Flanagan, T. B.; Clewley, J. D.; Balasubramaniam, R. On hydrogen solubility in Pd/alumina composites prepared by internal oxidation of Pd/Al alloys. J. Alloys Compd. 2002, 339, 76−89. (17) Sermon, P. A.; Bond, G. C. Studies of hydrogen spillover. Part 1. Study of the rates, extents and products of hydrogen spillover from platinum to the trioxides of tungesten and molybdenum. J. Chem. Soc., Faraday Trans. I 1976, 72, 730−744. (18) Berzins, A. R.; Sermon, P. A. Reversible isothermal sorption of hydrogen by tungsten trioxide in the presence of platinum. Nature 1983, 303, 506−508. (19) Tinet, D.; Partyka, S.; Rouquerol, S.; Fripiat, J. J. Microcalorimetry applied to hydrogen bronzes. Mater. Res. Bull. 1982, 17, 561−565. (20) Flanagan, T. B.; Wang, E.; Shanahan, K. L. The effect of CO on hydrogen permeation through Pd and interally oxidized Pd alloy membranes. Sep. Purif. Technol. 2011, 79, 385−392. (21) Flanagan, T. B.; Wang, D.; Shanahan, K. L. Diffusion of H through Pd membranes: effects of non-ideality on DH ED and D°H. J. Membr. Sci. 2007, 306, 66−74. (22) Flanagan, T. B.; Oates, W. A. The palladium-hydrogen system. Annu. Rev. Mater. Sci. 1991, 21, 269−304. (23) Lässer, R. Tritium and Helium-3 in Metals; Springer-Verlag: Berlin, 1989. (24) Wang, D.; Flanagan, T. B.; Balasubramaniam, R. The interaction of hydrogen with internally oxidized Pd alloys as illustrated by Pd-Fe alloys. J. Alloys Compd. 2003, 356−357, 3−8. (25) Tinet, D.; Ancion, C.; Poncelet, G.; Fripiat, J. J. Chemical reactivity of hydrogen bronzes. J. Chim. Phys. 1986, 83, 809−817. (26) Flanagan, T. B.; Lynch, J. F.; Clewley, J. D.; von Turkovich, B. The effect of lattice defects on hydrogen solubility in palladium. I. Experimentally observed solubility enhancements and thermodynamic of absorption. J. Less-Common Met. 1976, 49, 13−24. (27) Birtill, J. J.; Dickens, P. G. Thermochemistry of hydrogen molybdenum bronze phases HxMoO3. J. Solid State Chem. 1979, 29, 367−372. 8130

DOI: 10.1021/jp512271j J. Phys. Chem. C 2015, 119, 8124−8130