Cr2O3 Composites Prepared by Internal Oxidation

Apr 20, 2009 - ... |ΔHH| for 1/2H2(g) solution in Pd. This strongly trapped H cannot be removed by evacuation at 303 K but most can be removed by eva...
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J. Phys. Chem. C 2009, 113, 8220–8227

H2 Solubility in Pd/Cr2O3 Composites Prepared by Internal Oxidation of Pd-Cr Alloys. Calorimetric Determination of Enthalpies S. Luo and Ted B. Flanagan* Chemistry Department, UniVersity of Vermont, Burlington, Vermont 05405 ReceiVed: January 17, 2009; ReVised Manuscript ReceiVed: March 17, 2009

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 H2 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 |∆HH| decreases continuously from ≈ 64 to 14.5 kJ/mol H as the traps at the internal interfaces are filled by H where ∆HH refers to reaction with 1/2H2(g). The partial enthalpies for H segregation can be determined from these values plus |∆HH| for 1/2H2(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. |∆HH| and |∆SH| 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, |∆HH| is constant, 14.5 kJ/mol H which is about 4 kJ/mol H greater than that for the solution of 1/2H2(g) in the dilute phase of pure, annealed Pd. It is shown from |∆HH| and isotherm measurements during absorption/desorption that the solubility is reversible in the extensive dilute phase region of an IOed Pd0.095Cr0.05 alloy. Introduction Metal/oxide interfaces are employed in many technological applications. For example, the capacitance-voltage characteristics of Pd/SiO2 interfaces are employed for H2(g) detectors.1,2 The work to be described here is part of an effort to thermodynamically characterize the behavior of solutes dissolved in composites with nanosized precipitates having an appreciable internal metal/oxide interfacial area. Partial molar enthalpies for trapping a solute, H, at internal metal/oxide interfaces, Pd/ chromia, will be determined calorimetrically as a function of H content. Since it diffuses rapidly, H has the advantage that it is the only solute that can segregate to internal interfaces at moderate temperatures; this permits a calorimeter designed for moderate-temperatures to be employed.3 Internal oxidation, IO, is used for the preparation of the Pd/ chromia composites. Pd-M alloys can be internally oxidized, IOed, if M is more oxidizable than the Pd matrix. The size of the internal oxide precipitates depends on the IO temperature and the stability of the oxide.4 The interfacial areas of IOed Pd alloys can be several m2 per cm3 of alloy with the actual area depending on the precipitate size and number density.5 Strong trapping of H by metal/oxide interfaces is shown by regions in the dilute phase solubility isotherms of pH1/22 versus (H/Pd, atom ratio) ) r where pH2 ≈ 0, or its equivalent in electrode potential.6-9 Kirchheim and coworkers9 have given experimental support for the model of strong trapping by Pd/oxide internal interfaces, e.g., abrupt increases of electrical resistivity and H diffusion constants when the traps are filled. In an earlier paper10 concerning H trapping in IOed Pd-Cr alloys, it was shown that the extent of strong trapping is larger * To whom correspondence should be addressed.

than for other IOed Pd alloys which have been investigated and that the dilute phase solubility enhancements, after the traps are filled, are also greater. Thermodynamic parameters for H2 solution were determined from equilibrium pH2-r-T measurements but they could not be determined in the strong trapping region because of the low equilibrium pH2. In this work the enthalpies for strong trapping will be determined calorimetrically. After IO, the chromia precipitate sizes were estimated from TEM photomicrographs as 5-15 nm in the bulk and larger near grain boundaries.11 It has been shown from TEM photomicrographs that IO of Pd-Al alloys with XAl e 3 at % introduces negligible dislocation densities.12,13 By contrast, after IO Pd-Cr alloys have large dislocation densities arranged in loops in greater numbers than the oxide precipitates.11 As noted above, Eriksson et al. have investigated internal Pd-SiO2 interfaces for use as H detectors.1,2 These detectors operate by monitoring changes induced by adsorbed H in the capacitance-voltage relations of the Pd/SiO2 interfaces where ∆V is the voltage shift at a given capacity. These capacitancevoltage shifts can be employed to sensitively detect. H2 isotherms were derived from these shifts of V which were directly proportion to ln pH2 which implies that the heat of adsorption at the interface is directly proportional to the coverage. The interface coverage was assumed to be given by ∆V ) θi∆Vmax where θi is the interface coverage. In order to fit their isotherms, they chose the initial heat at zero coverage as about -76 ( 10 kJ/ mol H, however, a ∆HH-r plot was not given. They concluded that the fall is consistent with an electrostatic model of interacting dipoles at the interface. In this research, detailed measurements of partial enthalpies of H2 absorption, ∆HH, will be made for the strong trapping region using reaction calorimetry. To our knowledge, aside from

10.1021/jp900506u CCC: $40.75  2009 American Chemical Society Published on Web 04/20/2009

H2 Solubility in Pd/Cr2O3 Composites

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some earlier work on Pd/alumina and Pd/yttria composites from this laboratory14,15 and the results of Eriksson et al.,1,2 who determined the heats indirectly for a quite different system, there have been no detailed measurements of enthalpies for solute segregation to internal interfaces. In conjunction with the calorimetric measurements, isotherms (303 K) will be measured in the regions where pH2 becomes measurable which will allow ∆SH values to be determined. The enthalpies to be determined here will be more precise than the earlier ones from this laboratory for other Pd/oxide systems14,15 because of an improved calorimeter and because of the greater extent of the strong trapping for the Pd/Cr oxide system. Calorimetry will also be employed in the dilute phase region to characterize the large solubility enhancement for these IOed Pd-Cr alloys.10 Experimental Section Pd-Cr alloys were prepared by arc-melting the pure components and then annealing the buttons at 1173 K for 72 h. The buttons were then rolled into thin foil (120 µ) and reannealed. A Pd0.095Cr0.05 alloy was completely IOed in a tube furnace in the atmosphere for 96 h at 1053 K and then for 5 h at 1113 K. The lower temperature, 1053 K, should largely determine the size of the majority of the precipitates because most of the IO occurs at that temperature. This IOed alloy was quenched into water from the IO temperature. The XRD patterns of IOed Pd-Cr alloys show broadened reflections with the maxima of each corresponding exactly with the angles of bulk Pd, to be indicated as Pd(b), proving that the Pd matrix is nearly chemically pure and any differences in H2 solubilities between an IOed alloy and Pd(b) must arise from microstructural rather than chemical differences. The twin-cell, heat leak calorimeter has been described elsewhere;3 its sensitivity has been improved for this investigation. Simultaneously with the calorimetric measurements, pH1/22 - r()H/Pd) isotherms are measured (303 K). It should be noted that each point in the plots of ∆HH versus r shown below corresponds to a separate determination of the heat over a small increment of r and the average r for each increment is shown in the figures. It should be noted that the interaction of H with the Pd/chromia interfaces is expected to depend on the activity of O2(g) during any subsequent treatments at elevated temperatures following the IO. For example, after O2(g) exposure, Huang et al. 6 found for Pd/alumina composites that excess O is incorporated in the outer layer of the oxide precipitates within the Pd matrix. Maximum H trapping followed such an exposure to O2(g) but, after heating in Al(g), which causes the interface to be Al-rich, the strong trapping was nil. The strong trapping was restored by heating in air.6 Most of the present experiments have been carried out after IO without any additional high temperature treatment, i.e., with O-rich interfaces. Results and Discussion First, results of isotherm measurements will be described, and then the accompanying calorimetric results. Even though some isotherms for IOed Pd-Cr alloys were shown earlier for a range of Cr compositions,10 it seems useful to show them here for a Pd0.095Cr0.05 alloy IOed at a different temperature and employing a different apparatus and some of the H2 solubility experiments and procedures differed from those previously carried out. In addition, there is a need to relate the calorimetric data to the isotherms. H2 Solubilities (303 K) in an IOed Pd0.95Cr0.05 Alloy. 1. Strongly Trapped H. Figure 1 shows dilute phase H2 solubilities (303 K) for an IOed Pd0.095Cr0.05 alloy along with

Figure 1. Dilute phase solubilities (303 K) for an IOed (1053 K, 96 h + 1113 K, 5 h) Pd0.95Cr0.05 alloy and Pd(b). O, Pd(b); ∆, first absorption IOed alloy; 2, after evacuation at 303 K (2 h), b, after evacuation at 573 K (2 h); 0, after cycling at 303 K and evacuation at 303 K (12 h).

an isotherm for Pd(b).16 The IO temperature is low enough to expect a large H solubility enhancement.10 The initial isotherm for the IOed alloy has a region of strong trapping from r ) 0 to ≈0.012. The extent of the strong trapping is greater here than that found for other IOed Pd alloys such as an IOed (973 K) Pd0.926Fe0.074 alloy where the intercept characterizing the strong-trapping is r ) 0.005.17 Before any hydride phase had formed, the sample was evacuated at 303 K for 2 h, and its redetermined pH1/22 -r plot intersected the origin (Figure 1) indicating that the strongly trapped H was not removed by this evacuation. The intersection at the origin by the redetermined solubility plot agrees with that found earlier for IOed Pd-Cr alloys10 and also for IOed Pd-Al alloys.14 This alloy, which still contains H in its strong traps, was then evacuated at 573 K (2 h) and its intercept indicates that more than half of the strongly trapped H is removed by this higher temperature evacuation (Figure 1). The IOed Pd0.095Cr0.05 alloy was completely hydrided and dehydrided, i.e., cycled, and then evacuated at 303 K. Its dilute phase solubility was redetermined (Figure 1) and the intercept is now r ) 0.004, showing that about 65% of the H is removed after this cycling and evacuation at 303 K (2 h). It should be recalled that none of the H is removed by evacuation of this uncycled alloy at 303 K. After evacuation of an IOed cycled Pd-Cr alloy at a slightly higher temperature, 323 K, however, almost all of its trapped H is removed.10 2. Enhanced Dilute Phase H Solubilities. As found earlier,10 the dilute phase H solubility (303 K) in IOed Pd-Cr alloys extends over a greater range of H contents than it does for wellannealed Pd(b) (Figure 1). The hydride phase forms for Pd(b) at r ) 0.015 which is the terminal H solubility or THS16 whereas at this temperature the hydride phase forms at a much higher H content for the IOed alloy (Figure 1). THS can be important, e.g., Zr-alloys, e.g., zircaloy, are used as pressure tubes in some nuclear reactors and hydride formation should be avoided in them because it can give rise to stress-enhanced cracking and leaking 18

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Figure 3. ∆HH as a function of r at 303 K for an IOed Pd0.095Cr0.05 alloy. O, ∆HH; ∆, pH1/22 ; 2, ∆HH for Pd(b).16

Figure 2. Dilute phase solubilities (323 K) for the Pd0.93Cr0.07 alloy after at 1098 K, 72 h. 0, Pd(b); b, after IO of the Pd0.93Cr0.07 alloy; O, repeat solubility after evacuation at 323 K; ∆, solubility after evacuation at 573 K; 2, after cycling (303 K).

The solubility is compared to Pd(b) in Figure 1 because the large negative free energy for chromia formation, leads to the matrix of the composite being essentially free of unoxidized Cr atoms. At pH2 ) 0.53 kPa the solubility enhancement for the IOed alloy is (r′IOed/rPd(b)) ) 3.25 and for an IOed Pd0.94Al0.06 alloy it is much smaller, 1.95.14 After evacuation of the IOed alloy at 303 K, the pH1/22 -r relation intersects the origin and is linear over a large range, i.e., greater than the range of the initial ideal solubility in Pd(b)-H (Figure 1). As found earlier,10 alloys with XCr > 0.02 do not exhibit supersaturation of the dilute phase in their initial isotherm following IO at |∆HH| (Pd(b)) is due to thermal residual stress because the coefficient of thermal expansion of chromia is similar to that of alumina but the enhanced solubility for Pd/chromia is much greater than for Pd/alumina composites. In view of the observed large dislocation densities in IOed Pd-Cr alloys,11 it seems that the |∆HH| values are affected by stress, |σtensileVH|, arising from the dislocation arrays, e.g., dislocation forests and the many loops. The following equation holds in the single phase regions at any H content:

ln p1⁄2 ) ∆HH ⁄ RT - ∆SH ⁄ R

(4)

where p is the equilibrium pH2 at a given r. Since ∆HH/RT is found to be constant in this linear, dilute region, we can equate it to ln B where B is a constant. Since ∆SH ≈ ∆S °H - R ln r/(1 - r),19 it follows from eq 4 that at small r, p1/2 ≈ B × r, i.e., p1/2 depends linearly on r as observed (Figure 1). This idealtype behavior in this region suggests that the H-H interaction is negligible in this region. The attractive H-H interaction is believed to be due to an indirect elastic interaction caused by the insertion of the H atoms in the lattice21 and to short-range chemical-type interactions.22 It is possible that the entry of H atoms into the strained interstices in this region do not expand the lattice as they do for annealed Pd(b) and also that the chemical interactions are modified. The greater terminal hydrogen solubility, THS, of the IOed alloy (Figure 1) is consistent with the plateau pf being not very different from that of Pd(b)-H while ∆HH for absorption is more negative, and therefore pH2 are lower in the dilute phase as compared to Pd(b)-H and, consequently, greater values of r are needed to reach pf. The THS can be expressed thermodynamically by -RT ln(THS) ) ∆Hsolv - T∆Ssolv and ∆Hsolv ≈ ∆H °H - ∆Hplat and ∆Ssolv ≈ ∆S °H - ∆Splat.23 Since ∆H °H is more negative for the IOed alloy than for Pd(b) but ∆Hplat is similar (see below), it follows from this equation that the THS must be greater for the IOed alloy assuming that the ∆Ssolv terms are similar for Pd(b) and the IOed alloy which is expected since this is found for different alloys of Pd.23

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Figure 5. |∆SH| as a function of r at 303 K for an IOed Pd0.095Cr0.05 alloy where the results are given in sequence. O, initial determination for IOed alloy; ∆, after evacuation at 303 K, 2 h; 9, Pd(b) 303 K, 2 h.16

After the various annealing treatments of the IOed Pd0.095Cr0.05 alloy, in the region where p1/2 H2 increases linearly, the |∆HH| values are all similar, 14.5 kJ/mol H. As the H content is increased to exceed that in the linear region, the |∆HH| increases to a plateau value, |∆Hplat|, a value which is similar to Pd(b),16 but, unlike Pd(b), they do not increase abruptly16 but gradually (Figure 4). 1/2 for the ∆SH Values (303 K) Derived from ∆HH and pH 2 IOed Pd0.95Cr0.05 Alloy. Equation 5 can be employed to calculate, ∆SH, in single phase regions

∆SH ) ∆HH ⁄ T - R ln pH1⁄22

(5)

where ∆SH is the relative partial molar entropy and is relative to 1/2H2 (1 bar); values of ∆SH depend on the equilibrium pH2 which could not be measured in the strong trapping region with the pressure gauges available but could be measured in the dilute phase region. If the |∆SH|-r relation for absorption by a virgin IOed alloy is examined (Figure 5), |∆SH| appears to decrease slightly with r in the region from r ) 0.017 to 0.025 and then increase in direct proportion to r from 0.030 to 0.045, i.e., where pH2 also increases linearly (Figure 1). The small decrease of |∆SH| may indicate a region where H enters the last of the limited number of strong traps and simultaneously enters the strained interstices where, it is believed, the dilute solubility occurs. After evacuating the alloy at 303 K (2 h), the redetermined data no longer has a region where |∆SH| decreases with r but it only increases (Figure 5). If the linear increase of |∆SH| with r for this re-evacuated alloy is shifted to smaller r along the r axis, it is found to have a similar slope as that of the virgin IOed alloy. After evacuation of the IOed alloy at a higher temperature, 573 K, the redetermined |∆SH|-r relation at 303 K has a small region of r where |∆SH| decreases, however, it is less extensive than for the virgin IOed alloy. After this small decrease, |∆SH| slowly increases with r and then increases more strongly to the plateau value. After cycling, there is a small region where |∆SH| appears to decrease with r and then increases regularly without any indication of a steep increase as for the initial determinations before cycling (Figure 5). Reversibility/Irreversibility and Hysteresis Scans for the IOed Pd0.95Cr0.05 Alloy. Isotherms in the dilute phase near the dilute/(dilute + hydride) phase boundary of cycled Pd, which

Figure 6. p1/2 as a function of r at 303 K for Pd(b) and an IOed Pd0.095Cr0.05 alloy. O, initial isotherm for Pd(b) showing supersaturation; 0, Pd(b) after cycling; 9, desorption of Pd (cycled) from r ≈ 0.01. IOed alloy, open and filled symbols represent absorption and subsequent desorption scans along the dilute phase region. 4, absorption to r ≈ 0.06 followed by reversible desorption as shown by 2; ], measurements to r ≈ 0.07 which show irreversibility during desorption as shown by corresponding black symbols.

contains a large dislocation density, are irreversible.24 The corresponding ∆HH values are found to reflect this irreversibility more dramatically than the isotherms.16 Such irreversibility, which is not found in annealed Pd(b), has been attributed to some hydride formation in the stress fields of the dislocation arrays introduced by the cycling.16,24 The extent of irreversibility of H2 solution in the IOed Pd0.095Cr0.05 alloy will be examined using p1/2 H2 - and ∆HH-r relations. Irreversibility is characteristic of hydride formation and decomposition and would account for the enhanced solubility in the IOed Pd-Cr alloys in the region before the plateau. It was shown earlier for a cycled IOed Pd0.095Cr0.05 alloy that H2 solution was reversible in the dilute phase to r ≈ 0.05 (323 K).10 This was surprising in view of the large solubilities and will be re-examined here using H2 solubilities to somewhat larger H contents and, the ∆HH-r relations. An absorption isotherm for a cycled, IOed alloy was reversed at several, increasing values of r and, even for the region where the isotherm is no longer linear at r > 0.05, the p1/2 H2 -r isotherms exhibit no significant irreversibility (Figure 6). Desorption from r ) 0.07 does show irreversibility; however, this is clearly within the two phase region since it has a pH2 corresponding to the plateau value. The irreversibility for cycled Pd is shown for comparison in Figure 6,16 and although the irreversibility does not appear to be very large on the scale of the figure, it is significant in terms 1/2 ) of the total H solubility in cycled Pd, e.g., at r ) 0.008, pabs

H2 Solubility in Pd/Cr2O3 Composites

Figure 7. Plots of ∆HH and p1/2 as a function of r (303 K) for the IOed Pd0.095Cr0.05 alloy after several cycles and evacuation at 303 K. The open and filled symbols represent absorption and desorption, respectively. The desorption determinations start from increasing r values and they can be seen to give the same |∆HH| and p1/2 as absorption except for the desorption from r ) 0.073 which is on the plateau. In sequence: ], after evacuation of IOed alloy for 12 h at 303 K; ∆, after evacuation at 303 K, 12 h; 0, after evacuation at 303 K, 12 h. 1/2 27.2 Pa1/2 and pdes ) 24.1 Pa1/2 whereas in the same pH2 range, 1/2 1/2 and pdes are very similar (Figure 6). For for the IOed alloy, pabs cycled Pd and the IOed alloy, 100% × (∆r/r) ≈ 21% and 3%, respectively, at 0.9 kPa where ∆r refers to the change in r between the desorption and absorption isotherms and the denominator is the H content of the uncycled alloy and cycled Pd. For absorption by annealed Pd(b) there is an abrupt transition of |∆HH| from values characteristic of the dilute phase, ≈10 kJ/mol H, to those for the two-phase region, 19.2 kJ/mol H3 as shown in Figure 4. For cycled Pd in the dilute region, however, there is a continuous range of |∆HH| values from 10 to 19 kJ/ mol H with increase of r.16 If desorption is initiated from r ) 0.017, the ∆HH values for cycled Pd(b) decrease markedly with decrease of r and then increase and decrease again.16 These dramatic changes have been discussed elsewhere.16 The relevant point here is that dramatic changes of the ∆HH with r relations for Pd accompany the irreversibility due to the dislocations introduced by its prior cycling. In the present work a desorption scan for the IOed alloy was initiated at r ≈ 0.05 (Figure 7) and the |∆HH| values are basically the same as for the absorption in marked contrast to cycled Pd. Although there are many dislocations in the Pd0.095Cr0.05 alloy both before and especially after cycling,11 and despite the large solubility enhancement, no evidence is found for any significant irreversibility in the dilute phase. The ∆HH values for the IOed alloy are more scattered for desorption than for absorption

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Figure 8. RT ln p1/2 ) ∆µH as a function of r (303 K) for an IOed Pd0.095Cr0.05 alloy after 10 previous cycles followed by evacuation at 303 K. O, absorption up to r ) 0.07; b, desorption scan from r ) 0.07; ∆, reabsorption starting from desorption scan at r ) 0.057; 2, desorption starting from r ) 0.18; 0, reabsorption from r ) 0.11; [, desorption starting from the fully hydrided alloy, r ) 0.73 (isotherm not shown to 0.73).

(Figure 7) which is partly due to the fact that at these relatively low pH2, only small increments of H can be removed for desorption for these heat measurements as compared to larger increments for absorption. It has been proposed on the basis of SANS that hydride phase forms about dislocation cores25 even at very low H concentrations. If some hydride phase forms about the dislocations, as indicated by the SANS results,25 a hydride phase might also be expected to form in the IOed Pd0.095Cr0.05 alloy. One explanation might be that the occupation of strained interstices in the dilute phase of the IOed alloy is energetically favorable to any growth of the hydride phase at pH2 < pf. In a desorption scan, H is first lost from the coexisting single phases resulting in a nearly linear p1/2-r or RT ln p1/2-r scan until some hydride phase itself starts to decompose causing nonlinearity, and if sufficient H is desorbed, the desorption plateau will be reached.26 An absorption scan from the desorption plateau is the reverse of this behavior. The slopes of the linear scan regions are determined from the slopes of ∆µH-r at their respective single phases weighted by their fractions.26 Some absorption and desorption scans for the IOed Pd0.095Cr0.05 alloy are shown in Figure 8. It can be seen that in the linear regions of the desorption scans the slope is greater when starting at r ) 0.07 than from 0.18 which means that the single phase hydride region must have a smaller slope than the coexisting dilute phase. The ∆µH-r slope of the desorption scan for the IOed alloy is not as steep as that for Pd(b) (Figure 9) as shown by comparing Figures 8 and 9. The slope of the desorption scan for the IOed alloy indicates that its dilute phase solubility is similar to the scan shown from r ≈ 0.045 to 0.055 and again demonstrates that the dilute phase is reversible. Irreversibility is found in the RT ln pH1/22 -r data when desorption is initiated from the absorption plateau at r ) 0.071

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Figure 9. RT ln p1/2 ) ∆µH as a function of r (303 K) for Pd(b). O, absorption to r ) 0.29 followed by desorption scan; b, desorption scan; O, absorption scan starting from r ) 0.23.

(Figure 8) and |∆HH| values decrease markedly during desorption from r ) 0.071 (Figure 7) reflecting desorption from single phases rather than any hydride decomposition. The value of pf is lower for the IOed alloy than for Pd(b) (Figures 8 and 9) which may be related to the stress fields of the dislocations. The ∆HH values shown in Figure 7 are somewhat greater than in Figure 4 which is due to the many cycles carried out on the alloy, e.g., see the changes in Figure 1 after one cycle, and supports the idea that dislocations are the cause of the enhanced solubilities and greater |∆HH|. Complete Isotherms and Calorimetric Enthalpies (303 K) for Absorption/Desorption of H2 by IOed Pd0.95Cr0.05. Consecutive isotherms have been shown earlier (323 K) for some IOed Pd-Cr alloys10 and the same general trend is found here for the IOed Pd0.095Cr0.05 alloy (303 K), i.e., pf and pd both decrease compared to Pd(b) but the change is not as great for pd. In this research both ∆HH and pH2 are shown for the IOed Pd0.095Cr0.05 alloy from r ) 0 to 0.2 (Figure 10). The absorption and desorption |∆Hplat| values are within experimental error of each other as found for Pd(b) and they are the same magnitude as found before, 19.2 kJ/mol H for Pd(b)-H.3 The desorption and absorption pH1/22 -r data do not match exactly in the dilute phase (Figure 10) due to cumulative errors arising from the small doses employed for absorbing/desorbing to/from r ≈ 0.7 and small uncertainties in the dosing and reaction volumes. The dilute phase ∆HH values for desorption show the same trend as for absorption with no evidence of irreversibility. For the IOed Pd0.095Cr0.05 alloy, both the absorption and desorption plateaux pH2 values are lower than for pure Pd(b), and therefore, it might be expected that the calorimetrically determined |∆Hplat| values would differ from Pd(b); however, any differences are within experimental error. The plateau enthalpies for Pd(b) from reaction calorimetry found earlier3 is 19.2 ( 0.1 kJ/ mol H and the present results for hydride formation/decomposition are within experimental error (Figure 11). It has been shown for Pd(b)3 that calorimetrically measured |∆Hplat| are the same for hydride formation and decomposition. f d | < |∆Hplat Although, |∆Hplat | from van’t Hoff plots, the average of the two is equal to the calorimetric value which is unaffected by hysteresis. |∆Splat| values for the IOed alloy are slightly smaller than for Pd(b), i.e., 46 ( 1 J/K mol H,3 because of the lower plateau pH2. For cycled IOed Pd0.93Cr0.07 the average values of hydride formation and hydride decomposition of the van’t Hoff-determined |∆Hplat| and |∆Splat| are 20.6 kJ/mol H and 48.5 J/K mol H, respectively,.10 These are somewhat greater than

Figure 10. Isotherm and ∆HH as a function of r for the IOed Pd0.095Cr0.05 alloy (303 K) shown only to r ) 0.20 although isotherms were carried out to r ≈ 0.73. ], p1/2 during absorption; [, p1/2 during desorption; ∆, ∆HH during absorption; 2, ∆HH during desorption.

Figure 11. ∆HH as a function of r for the IOed Pd0.95Cr0.05 alloy and Pd(b) (303 K) over a large range of H contents. ∆, absorption for IOed alloy; 2, desorption of IOed alloy; O, absorption for Pd(b).

found here calorimetrically for the IOed Pd0.095Cr0.05 alloy; however, the van’t Hoff-values are not as accurate as the present calorimetric ones. In the single hydride phase after the H-content of the two phase coexistence region is exceeded, there is an almost linear decline in |∆HH| values for Pd-H3 and such a region is also seen here for the IOed alloy in Figure 11. The fall in the |∆Hplat| values is seen to take place in the region where the absorption and desorption pH2 data merge which is at a pH2 greater than pf or pd; this indicates that some phase transformation still takes place during absorption even when pH2 > pf. Conclusions The observed continuous falloff of |∆HH| with H content in the strong trapping region of a IOed Pd0.095Cr0.05 alloy is the

H2 Solubility in Pd/Cr2O3 Composites most important finding of this work. It is likely that the binding energies of other solutes to metal/oxide interfaces also decrease with fraction of trapped solute. The results are consistent with those of Fogelberg et al.1 who employed a different metal/oxide system and an indirect technique where H segregated to Pd/ SiO2 interfaces. They postulated that H-dipoles at the interface surface interact such that |∆HH| decreases with coverage. This has some similarity to the decline of the heats of chemisorption of H2 on metals with coverage.27 The dilute phase solubility behaves ideally over a large range of H contents, e.g., as seen for the repeat solubility in Figure 1 for the IOed Pd0.095Cr0.05 alloy. In this range where r is directly proportional to pH1/22 , i.e., a modified Sieverts’ law of ideal solubility is obeyed such that the solubility does not pass through the origin. |∆HH| is found to be nearly constant in this ideal range at ≈14.5 kJ/mol H, which is larger than for Pd(b), i.e., 10.3 kJ/mol H as r f 0 and which increases to only about 11.0 kJ/mol H up to the THS (303 K) of Pd-H.19 The observed constant |∆HH| for the IOed alloy indicates that the H-H attractive interaction, which leads to hydride formation, cannot be a factor in this region of H contents. It seems that the occupation of the interstices with |∆HH| )14.5 kJ/mol H overrides any H-H interaction in this ideal range. Since the location of the upper, i.e., the (hydride + dilute)/ hydride, phase boundary is basically unchanged for the IOed Pd0.095Cr0.05 alloy (Figure 11), and ∆Hplat is the same as for Pd-H, the H-H interaction must become a factor after the extended linear region and it is nearly the same magnitude as for Pd(b)-H. The onset of the H-H attractive interaction is shown by the curvature of the p1/2-r plot at r > 0.03 (Figure 1). Hydride formation is detrimental in some practical situations because the hydride phase is more brittle than the metal and can lead to stressed-induced brittle fracture, e.g., ref 28. If the dilute phase solubility can be increased by IO, the possibility of such fracture would be reduced and the present results may be of interest in this regard. Acknowledgment. T.B.F. acknowledges financial support from Westinghouse Savannah River Co. The valuable conversa-

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8227 tions with Professor R. Balasubramaniam are gratefully acknowledged. References and Notes (1) Fogelberg, J.; Eriksson, M.; Dannetun, H.; Petersson, L. J. Appl. Phys. 1995, 78, 988. (2) Eriksson, M.; Lundström, I.; Ekedahl, L.-G. J. Appl. Phys. 1997, 82, 3143. (3) Flanagan, T.; Luo, W.; Clewley, J. J. Alloys Compounds 1991, 172174, 42. (4) Meijering, J. In AdVances in Materials Research; Herman, H., Ed.; Wiley: New York, 1971; Vol. 5, p 1. (5) Gegner, J.; Hrz, G.; Kirchheim, R. Interfacial Sci. 1997, 5, 231. (6) Huang, X. Y.; Mader, W.; Eastman, J.; Kirchheim, R. Scripta Met. 1988, 22, 1114. (7) Huang, X. Ph.D. Thesis, University of Stuttgart, Stuttgart, 1989. (8) Kirchheim, R.; Huang, X. Y.; Mtschele, T. Hydrogen Effects on Material BehaVior; Moody, N., Thompson, A., Eds.; The Minerals, Metals and Materials Soc.: 1990; p 85. (9) Huang, X. Y.; Mader, W.; Kirchheim, R. Acta Metall. Mater. 1991, 39, 893. (10) Wang, D.; Flanagan, T.; Gouthama, T.; Balasubramaniam, R. J. Phys. Chem. C 2009, 113, 4078. (11) Gouthama, T.; Balasubramaniam, R.; Wang, D.; Flanagan, T. J. Alloys Compounds 2005, 404-406, 617. (12) Balasubramaniam, R.; Noh, H.; Fvlanagan, T.; Sakamoto, Y. Acta Metall. Mater. 1997, 39, 893. (13) Eastman, J.; Ru¨hle, M. Ceram. Eng. Sci. Proc. 1989, 10, 1515. (14) Wang, D.; Noh, H.; Luo, S.; Flanagan, T.; Clewley, J.; Balasubramaniam, R. J. Alloys Compd. 2002, 339, 76. (15) Balasubramaniam, R.; Zhang, W.; Luo, S.; Wang, D.; Flanagan, T. J. Alloys Compounds 2002, 330-332, 607. (16) Luo, S.; Flanagan, T. Scripta Mater. 2005, 53, 1269. (17) Wang, D.; Flanagan, T.; Balasubramaniam, R. J. Alloys Compd. 2003, 356-357, 3. (18) Katanian, D. J. Alloys Compounds 2005, 404-406, 297. (19) Flanagan, T.; Oates, W. Annu. ReV. Mater. Sci. 1991, 21, 269. (20) Tompkins, F. Chemisorption of Gases on Metals; Academic Press: London, 1978. (21) Alefeld, G. Ber. Bunsen-Ges. Phys. Chem. 1976, 74, 746. (22) Mohri, T.; Oates, W. Mat. Trans. 2001, 43, 2656. (23) Flanagan, T.; Oates, W.; Kishimoto, S. Acta Metall. 1983, 31, 199. (24) Flanagan, T.; Kuji, T. J. Less-Common Mets. 1984, 99, L5. (25) Maxelon, M.; Pundt, A.; Pyckhout-Hintzen, W.; Kirchheim, R. Scripta Mater. 2001, 44, 817. (26) Wang, D.; Flanagan, T. Phys. Chem. Chem. Phys. 2002, 4, 4244. (27) Samorjai, G. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994. (28) Thompson, A. Scripta Met. 1982, 16, 1189.

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