Evidence of the Substrate Effect in Hydrogen Electroinsertion into

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Evidence of the Substrate Effect in Hydrogen Electroinsertion into Palladium Atomic Layers by Means of in Situ Surface X-ray Diffraction Chrystelle Lebouin,† Yvonne Soldo-Olivier,*,† Eric Sibert,† Maurizio De Santis,‡,§ Frederic Maillard,† and Rene Faure† † Laboratoire d’Electrochimie et de Physicochimie des Mat eriaux et des Interfaces, CNRS-Grenoble INP-UJF, 1130 rue de la Piscine, 38402 Saint Martin d’H eres, France, ‡Institut N eel, CNRS-UJF, 25 avenue :: des Martyrs, BP 166, 38042 Grenoble Cedex 9, France and §Institut fur Allgemeine Physik, Technische :: Universitat Wien, A-1040 Wien, Austria

Received November 26, 2008. Revised Manuscript Received February 12, 2009 In this work, we report an in situ surface X-ray diffraction study of the hydrogen electroinsertion in a two-monolayer equivalent palladium electrodeposit on Pt(111). The role of chloride in the deposition solution in favoring layer-by-layer film growth is evidenced. Three Pd layers are necessary to describe the deposit structure correctly, but the third-layer occupancy is quite low, equal to about 0.22. As a major result, resistance to hydriding of the two atomic Pd layers closest to the Pt interface is observed, which is linked to a strong effect of the Pt(111) substrate. As a consequence, we observe the lowering of the total hydride stoichiometry compared to bulk Pd. Our measurements also reveal good reversibility of the deposit structure, at least toward one hydrogen insertion-desorption cycle.

1.

Introduction

Thin palladium films can be very interesting as a protective barrier against oxidation and/or as a hydrogen-insertion promoter for hydrogen-inserting complex hydrides from light elements such as Mg2Ni, LaNi5, NaAlH4, and LiAlH4.1-4 Well-defined epitaxial deposits can be obtained using singlecrystalline supports.5-17 A platinum single crystal is a particularly well suited support for palladium deposition. Because platinum has a lattice parameter (3.924 A˚) very close to that of palladium (3.891 A˚), it is likely that palladium deposits have crystallographic properties close to those of bulk palladium single crystal. Electrochemical palladium deposition on Pt (111) in a sulfate containing deposition electrolyte has been studied using cyclic voltammetry,5-17 in situ STM,9 and in situ *Corresponding author. E-mail: [email protected]. (1) Barsellini, D.; Visintin, A.; Triaca, W. E.; Soriaga, M. J. Power Sources 2003, 124, 309. (2) Kumar, P.; Malhotra, L. K. Electrochim. Acta 2004, 49, 3355. (3) Ambrosio, R. C.; Ticianelli, E. A. Surf. Coat. Technol. 2005, 197, 215.  (4) Pitt, M. P.; Blanchard, D.; Hauback, B. C.; Fjellvag, H.; Marshall, W. G. Phys. Rev. B 2005, 72, 214113. (5) Attard, G. A.; Price, R.; Al-Akl, A. Electrochim. Acta 1994, 39, 1525. (6) Attard, G. A.; Bannister, A. J. Electroanal. Chem. 1991, 300, 467. (7) Clavilier, J.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1993, 351, 299. (8) Baldauf, M.; Kolb, D. M. J. Phys. Chem. 1996, 100, 11375. (9) Hoyer, R.; Kibler, L. A.; Kolb, D. M. Electrochim. Acta 2003, 49, 63. (10) Alvarez, B.; Climent, V.; Rodes, A.; Feliu, J. M. J. Electroanal. Chem. 2001, 497, 125. (11) Alvarez, B.; Climent, V.; Rodes, A.; Feliu, J. M. Phys. Chem. Chem. Phys. 2001, 3, 326. (12) Alvarez, B.; Rodes, A.; Perez, J. M.; Feliu, J. M. J. Phys. Chem. 2003, B 107, 2018. (13) Markovic, N. M.; Lucas, C. A.; Climent, V.; Stamenkovic, V.; Ross, P. N. Surf. Sci. 2000, 465, 103. (14) Ball, M. J.; Lucas, C. A.; Markovic, N. M.; Stamenkovic, V.; Ross, P. N. Surf. Sci. 2002, 518, 201. (15) Llorca, M. J.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1993, 351, 299. (16) Climent, V.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. 2000, 104, 3116. (17) Llorca, M. J.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1994, 376, 151.

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IR spectroscopy.10-12 They reveal a Stransky-Krastanov growth mode. In situ SXRD measurements13,14 confirm the presence of 3D islands on top of the pseudomorphic first monolayer film. Hoyer et al.9 showed from in situ STM and electrochemical experiments that the layer-by-layer growth of the Pd deposit is favored by the addition of chloride ions into the deposition electrolyte. Hydrogen electrochemical insertion into low-dimensional and well-defined palladium films can present unusual properties because surface effects are no longer negligible in nanosized systems. This is the case for palladium nanoparticules, where an increase in the R-PdH phase domain and a reduction in the maximum solubility, compared to that of bulk palladium, are observed.18-21 Whereas the electrocatalytic properties of Pd/Pt(111) nanofilms have been intensively studied,8-11,13,16,17 only a few reports deal with their behavior toward hydrogen electroinsertion phenomena.22-26 Hydrogen insertion in bulk Pd27,28 is characterized by three domains. The solid solution at low hydrogen concentration (R PdH phase) is followed at intermediate concentration values by a two-phase region, where the substoichiometric hydride phase (β-PdH) coexists with :: (18) Pundt, A.; Suleiman, M.; Bahtz, C.; Reetz, M. T.; Kirchheim, R.; Jisrawi, N. M. Mater. Sci. Eng. 2004, B108, 19. (19) Pundt, A. Adv. Eng. Mater. 2004, 6, 11. :: :: (20) Zuttel, A.; Nutzenadel, Ch.; Schmidt, G.; Emmenegger, Ch.; Sudan, P.; Schlapbach, L. Appl. Surf. Sci. 2000, 162-163, 571. (21) Kishore, S.; Nelson, J. A.; Adair, J. H.; Eklund, P. C. J. Alloys Compd. 2005, 389, 234. (22) Gabrielli, C.; Grand, P. P.; Lasia, A.; Perrot, H. J. Electrochem. Soc. 2004, 151, A1937. (23) Birry, L.; Lasia, A. Electrochim. Acta 2006, 51, 3356. (24) Duncan, H.; Lasia, A. Electrochim. Acta 2007, 52, 6195. (25) Duncan, H.; Lasia, A. J. Electroanal. Chem. 2008, 621, 62. (26) Lebouin, C.; Soldo-Olivier, Y.; Sibert, E.; Millet, P.; Maret, M.; Faure, R. J. Electroanal. Chem. 2009, 626, 59. (27) Lewis, F. A. The Palladium-Hydrogen System; Academic Press: London, NY, 1967. (28) Wicke, E.; Brodowsky, H. Hydrogen in Palladium and Palladium Alloys. In Hydrogen in Metals II; Topics in Applied Physics;Alefeld, G., :: Volkl, J., Eds.; Springer-Verlag: Berlin, 1978; Vol. 29.

Published on Web 3/10/2009

DOI: 10.1021/la803913e

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saturated R-PdH. At high hydrogen concentrations, a solid solution of hydrogen in β-PdH is formed. Usually, significant hysteresis between absorption and desorption paths is also observed over a wide potential (pressure) domain, mostly in the two-phase domain. Compared to bulk Pd, it was shown from electrochemical measurements26 that the insertion branch of the isotherm for Pd/Pt(111) films presents (i) an increase in the solid solution R maximal solubility (Rmax), (ii) a reduced total hydrogen solubility (H/Pdmax), and (iii) a steep plateau in the two-phase domain. Moreover, H/Pdmax decreases with the thickness of the deposited palladium film, even if the origin of this evolution was not elucidated. Structural information is needed for the comprehension of the mechanisms underlying hydrogen electroinsertion phenomena in Pd thin films. In the present study, we carried out for the first time in situ surface X-ray diffraction (SXRD) measurements of the hydrogen electroinsertion into a supported Pd ultrathin film. The effects of hydrogen insertion and desorption on the film structure have been observed.

2.

Experimental Setup and Procedure

A platinum (111) single crystal with a mosaic spread of about 0.1° and a diameter of ca. 10 mm has been used for the Pd electrochemical deposition. Before each experiment, the Pt(111) surface was flame annealed (H2/O2) and cooled down in a reducing atmosphere (Ar + H2 10%), as described previously.29 The quality and cleanliness of the surface were checked by cyclic voltammetry in 0.1 M H2SO4.29 We used the saturated calomel electrode (SCE) as a reference. In the following text, all potentials are expressed versus the reversible hydrogen electrode (RHE) potential (0 VSCE = +0.301 VRHE). Pd deposition was carried out in a 10-4 M PdCl2 + 3  10-3 M HCl + 0.1 M H2SO4 electrolyte using a VMP2 Biologic potentiostat. The Pt(111) electrode was introduced at 1.05 V vs RHE, scanned negatively to 0.76 V vs RHE in the Pd bulk deposition region and maintained at this potential value until the equivalent deposition of two monolayers was obtained. The amount of deposited Pd was monitored through the coulometric charge, assuming one palladium atom for each surface platinum atom and a two-electron charge transfer. In the following discussion, the Pd deposit will be referred to as Pd2 ML/Pt(111). Before SXRD experiments, deposits were checked by cyclic voltammetry in 0.1 M H2SO4 7 in a cell free of any trace of palladium ions. In situ SXRD experiments were carried out in a dedicated electrochemical polytetrafluoroethylene (PTFE) cell. Its thin layer configuration is similar to that used in previous XAS experiments30 and allows an electrolyte thickness of only a few tens of micrometers in front of the crystal surface. Data were collected using the five-circle diffractometer on the D2AM French CRG beamline at the European Synchrotron Radiation Facility (ESRF, France), with a monochromatic incident beam of 23.5 keV. A unit cell was chosen to index the reflections with A1 and A2 defining a hexagonal mesh in the surface plane. A3 is perpendicular to the surface, and its length equals the period of the fcc close-packing stacking. A1 = (aPt/2)  [110], A2 = (aPt/2)  [011], and A3 = aPt  [111], with aPt being the bulk lattice constant of Pt.31 The diffraction intensity was measured by fixing the (H, K) Miller indices and scanning the momentum transfer perpendicular to the surface, describing the so-called crystal truncation rod (CTR).32 Far from the Bragg peaks, located at L= 3  n + H - K (29) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (30) Soldo, Y.; Sibert, E.; Tourillon, G.; Hazemann, J. L.; Levy, J. P.; Aberdam, D.; Faure, R.; Durand, R. Electrochim. Acta 2002, 47, 3081–3091. :: (31) Grubel, G.; Huang, K. G.; Gibbs, D. Phys. Rev. B 1993, 48, 18119. (32) Robinson, I. K. Phys. Rev. B 1986, 33, 3830.

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(n is an integer), the diffraction intensity originates from the presence of the surface. CTRs are sensitive to the structure and composition of the surface layers. For each L value, the intensity was recorded with rocking scans: the (H, K) rod was completely crossed by rotating the crystal around the direction normal to its surface. One of these azimuthal scans is shown in the inset of Figure 1. The modulus of the structure factor was then extracted from the integrated intensity after applying standard correction factors, adapted to the diffractometer geometry.33 SXRD measurements were carried out at different stages under potential control: before insertion (0.2 V vs RHE), after insertion (-0.02 V vs RHE), and after desorption (0.2 V vs RHE) of hydrogen. We strictly controlled the potential applied to the crystal during the data acquisition. For each potential value, we measured (1, 0), (0, 1), and (1, 1) CTRs as well as the equivalent (2, -1) and (1, -1) CTRs. Error bars for the structure factors are based on the agreement factor between equivalent reflections.34 The data analysis was performed with the ROD package.35 The structural parameters were optimized through a fit of the structure factors, employing a χ2 minimization for the refinement. All electrolytes were prepared from H2SO4 (Merck, suprapur), HCl (Merck, suprapur), PdCl2 (Alfa Aesar, 99.99%), and Milli-Q grade ultrapure water (18.2 MΩcm, 3 ppb TOC). The solutions were deaerated under an argon flow (Air Liquide N45). All experiments were carried out at room temperature (25 °C).

3. Results and Discussion 3.1. Characterization of Pd2ML/Pt(111) before Hydrogen Insertion. We first performed SXRD measurements in the (H, K) plane: the absence of diffraction peaks at positions different from the Pt rods indicates the pseudomorphic growth of the Pd film. The structure factors of (1, 0) and (1, 1) CTRs, corresponding to the palladium deposit before hydrogen insertion (0.2 V vs RHE), are presented in Figure 1a,b, respectively. We recall that by symmetry the (0, 1, L) reflections are equivalent to the (1, 0, -L) ones. As can be seen from the comparison with the theoretical CTR for a clean Pt(111) surface (open circles), Pd deposition greatly changes the CTRs shape. The need for at least three layers to model the experimental data correctly is seen at first sight because our CTRs do not show a well-defined bump midway between each pair of Bragg peaks, characteristic of a 2 ML flat Pd deposit.14 The Pd film was modeled with three atomic layers (Pd1, Pd2, and Pd3) of different occupancies, following the same ABC stacking of the underlying platinum layers. The relaxation of Pt interlayer distances, interdiffusion at the interface, stacking faults, and occupancy in the next layers were found to be negligible in the fit. The first deposited layer was considered to be complete (i.e., with occupancy τ1 = 1), as the electrochemical characterization of the deposit shows no Pt(111) voltammetric signal.26 The occupancy of the second (τ2) and third (τ3) Pd atomic layers, the interlayer distances between each Pd layer and at the interface with Pt, and the Pd and interfacial Pt Debye-Waller factors were refined. A separate Debye-Waller factor for the first Pd layer (Pd1) was optimized at an intermediate stage of the analysis, without affecting the values of the other structural parameters. Coverage and Debye-Waller factors are weakly correlated, thanks to the large range of the perpendicular momentum (33) Vlieg, E. J. Appl. Crystallogr. 1997, 30, 532. (34) Robinson, I. K. In Handbook on Synchrotron Radiation; Brown, G. S., Moncton, D. E., Eds.; North-Holland: Amsterdam, 1991, Vol. 3, p 221. (35) Vlieg, E. J. Appl. Crystallogr. 2000, 33, 401.

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Figure 1. (1, 0) and (1, 1) CTRs (b) measured on Pd2ML/Pt(111) in situ (H2SO4 0.1 M) (a, b) at 0.2 V vs RHE before hydrogen insertion and (c, d) at -0.02 V vs RHE after hydrogen insertion. The continuous line shows the best fit. For comparison, we also show the simulated CTRs of the Pt (111) substrate (O). The rocking scan measured for (1.0, 2.5) before hydrogen insertion is shown as the inset.

transfer available in the fitting procedure. Figure 2 shows a scheme of the film structure derived from the best-fit results, summarized in Table 1. The total amount found for deposited Pd, 2.0 ( 0.1 ML, is in very good agreement with the evaluation obtained from coulometric measurements during the deposition. The interlayer distance dPt-Pd1 between the last platinum layer and Pd1 is 2.26 ( 0.02 A˚, the same as for bulk Pt(111). The smaller interplanar distance between the palladium layers, 2.22 ( 0.02 A˚, allows to preserve the unit cell volume in the Langmuir 2009, 25(8), 4251–4255

pseudomorphic growth (where interplanar compression is compensated for by lateral expansion). The second atomic layer has a quite high occupancy, τ2 ≈ 0.75, whereas for the third layer we find only τ3 ≈ 0.22. These occupancy values imply that ca. 25% of the first palladium layer is uncovered. This is consistent with the coulometric evaluation of the uncovered area of the first layer deduced from our electrochemical characterization of the deposit. Ball et al.,14 working on the same equivalent deposited thickness elaborated in a chloride-free deposition solution DOI: 10.1021/la803913e

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Figure 2. Schematic representation of the Pd2ML/Pt(111) structure before hydrogen insertion. Table 1. Best-Fit Parameters Obtained for the Pd2 ML/Pt(111) Deposit before Insertion, after Insertion, and after the Desorption of Hydrogena before insertion after insertion

after desorption

working potential +0.2 V vs RHE -0.02 V vs RHE +0.2 V vs RHE dPt-Pd1 (A˚) 2.26 ( 0.02 2.27 ( 0.01 2.27 ( 0.01 2.22 ( 0.02 2.24 ( 0.01 2.22 ( 0.01 dPd1-Pd2 (A˚) 2.22 ( 0.02 2.305 ( 0.02 2.25 ( 0.02 dPd2-Pd3 (A˚) 1 1 1 τ1 0.76 ( 0.05 0.68 ( 0.05 0.67 ( 0.05 τ2 0.22 ( 0.05 0.19 ( 0.05 0.17 ( 0.05 τ3 0.53 ( 0.07 0.47 ( 0.03 0.51 ( 0.08 DWPt (A˚2) 2 0.8 ( 0.8 0.7 ( 0.8 0.6 ( 0.8 DWPd (A˚ ) 2 4.8 2.7 2.8 χ a We report interplanar distances between the last Pt(111) layer and the first Pd layer (dPt-Pd1) and between the first and the second and the second and the third Pd layers (dPd1-Pd2 and dPd2-Pd3), the occupancy of the first (τ1), second (τ2), and third (τ3) Pd layers, and the Debye-Waller parameters for the interface Pt layer (DWPt) and for Pd (DWPd) and the χ2 factors.

and in the same HUPD potential region, find a second layer occupancy equal to only 0.5; they also find that four atomic layers are needed to describe the deposit’s structure. The comparison with our results quantitatively proves the role of chloride in favoring the layer-by-layer growth of the palladium electrodeposition on Pt(111). In the palladium islands, they find an interlayer distance that is larger by about 0.04 A˚, which no longer corresponds to the conservation of the unit cell volume. More surprisingly, the value they report for the Pt(111)-Pd first-layer spacing, 2.32 ( 0.01 A˚, is not only quite larger than our result (2.26 A˚) but is also larger than the values found for Pd nanoparticles (2.25 A˚),36 bulk Pd(111) (2.246 A˚), and bulk Pt(111) (2.266 A˚). Comparison with Ball et al.’s results would lead to the conclusion that the presence of chloride in the deposition electrolyte modifies the structure of deposited thin palladium films. Nevertheless, we remain cautious and believe that further investigations on chloride-free Pd deposition are necessary to confirm such results. 3.2. Characterization of Pd2ML/Pt(111) after the Insertion and after the Desorption of Hydrogen. The same CTRs were acquired after hydrogen insertion into the Pd2ML/Pt (111) deposit at -0.02 V vs RHE, a sufficiently low potential value to ensure that any further insertion would be negligible. Figure 3 shows the difference between the structure factors measured after and before the insertion of hydrogen. A derivative shape centered on the Bragg peaks is observed, especially at large |L| values, which is the signature of an expansion of the interlayer distance. The same structural (36) Di Vece, M; Grandjean, D; Van Bael, M. J.; Romero, C. P.; Wang, X.; Decoster, S.; Vantomme, A.; Lievens, P. Phys. Rev. Lett. 2008, 100, 236105.

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model used to fit the data before hydrogen insertion allows data modeling (Figure 1c,d), implying that pseudomorphism is maintained after hydrogen insertion. The best-fit values of the parameters are reported in Table 1. For the sake of clarity, Figure 4 shows the comparison of the interlayer distances values before and after hydrogen insertion. The fit quality is even better than that obtained for the measurements before insertion (χ2 = 2.7). As a major result, we do not observe any significant evolution of either the distance at the Pt-Pd1 interface or the distance between Pd1 and Pd2 layers, contrary to what would be expected in the case of hydride formation. In the β phase, hydrogen is localized at the octahedral sites of the palladium lattice (i.e., in a plane that is midway between two consecutive Pd(111) planes), yet the formation of hydride β is accompanied by an increase in the lattice parameter, and this expansion is related to the PdHx stoichiometry.37 Because no significant expansion is revealed after hydrogen insertion for the interplanar distances Pt-Pd1 and Pd1-Pd2, we can conclude that no hydride is formed in the two atomic Pd layers closest to the Pt interface. However, our measurements cannot exclude the presence of interstitial hydrogen with lower concentration (R phase), inducing no interplanar distance expansion. This conclusion was cross-checked by imposing an expansion of 3.8% to the Pd1-Pd2 distance (the same relative expansion that we find for dPd2-Pd3, see below). In that case, the best-fit χ2 significantly increases from 2.7 to 4.2. This resistance to hydriding is certainly due to an effect of the Pt(111) substrate. Indeed, it has been shown that mechanical stress generated by the presence of a substrate should be taken into account in the thermodynamic treatment of hydrogen insertion in thin films.19,38 Variations in the surface electronic structure for pseudomorphic metallic overlayers have also been pointed out with density functional calculations.39,40 It should be also noted that the pseudomorphic growth of the deposit is evidence of the substrate effect on the palladium prior to any hydrogen insertion. We observe hydride formation between the second and the third Pd layers, with the increase in their interplanar distance (dPd2-Pd3 = 2.305 ( 0.02A˚) after hydrogen insertion. Compared to its value before insertion, we observe an expansion of dPd2-Pd3 by 3.8 ( 2.2%, associated to hydriding. However, this expansion reduces to 2.6 ( 0.9% when calculated (37) Rose, A.; Maniguet, S.; Mathew, J. R.; Slater, C.; Yao, J.; Russel, A. Phys. Chem. Chem. Phys. 2003, 5, 3220. (38) Suleiman, M.; Faupel, J.; Borchers, C.; Krebs, H. U.; Kircheim, R.; Pundt, A. J. Alloys Compd. 2005, 404, 523. (39) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Norskov, J. K. J. Mol. Catal. 1997, 115, 421. (40) Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Phys. Rev. Lett. 2004, 93, 156801.

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Figure 3. Differences Δ|F| between experimental structure factors obtained after and before hydrogen insertion.

Figure 4. Evolution of interlayer distance d111 at the Pt-Pd inter-

face (Pt-Pd1) and for Pd1-Pd2 and Pd2-Pd3 layers of Pd2ML/Pt (111) (b) before hydrogen insertion, (9) after hydrogen insertion, and ()) after hydrogen desorption. Interlayer distance d111 values in PdH0.6 hydride ( 3 3 3 ), bulk platinum (- - -), and bulk palladium (-) are also shown.

with respect to the bulk Pd lattice constant. We recall that, considering bulk palladium (d111 = 2.246 A˚), the formation of nonstoichiometric hydride PdH0.6 is associated with a lattice parameter increase by 3.5% (d111 = 2.325 A˚). Unfortunately, we cannot determine in our case the exact stoichiometry of the hydride because, concerning the third incomplete Pd layer, we cannot establish if it completely retains its pseudomorphism or if it is lightly expanded in the surface plane. After hydriding, the large error bars do not allow us to make any statements about any decrease in the τ2 and τ3 values, however slight. The resistance to hydriding in the first Pd atomic layers is in accordance with a diminution of the total hydrogen solubility (H/Pdmax) compared to that of bulk palladium, as was inferred from the measured insertion branch of the electrochemical isotherm.26 Moreover, our results explain the observed reduction of H/Pdmax with the deposited films thickness.26 Indeed, the effect of resistance to hydriding via the substrate plays a role in the total hydrogen solubility, which is all the more important with the decrease of the thickness. Finally, SXRD data were acquired after hydrogen desorption, obtained by scanning the potential back to its initial value of 0.2 V vs RHE. As Figure 4 shows, each interplanar Langmuir 2009, 25(8), 4251–4255

distance reverts to its initial value, within experimental error. It seems that the structure of the Pd deposit is well preserved, at least after one complete cycle of hydrogen insertiondesorption. This could be an indication that the hysteresis process observed for bulk palladium is size-dependent and, in particular, disappears in extremely thin palladium films (a few layers). This should be confirmed by chemical or electrochemical isotherm measurements as well as by SXRD measurements on thicker deposits. Nevertheless, we must remark that the features present in the Δ|F| signal (Figure 3) are significantly reduced when calculated as the difference between the structure factors obtained “after hydrogen desorption” and “before hydrogen insertion”, but they do not completely disappear. Further investigations would be needed to draw conclusions on the reversibility or irreversibility of the Pd film structure over several insertion-desorption cycles.

4.

Conclusions

Hydrogen electroinsertion in a palladium equivalent electrodeposit of two monolayers was investigated with in situ surface X-ray diffraction. The measurements were carried out before insertion, after insertion, and after desorption of hydrogen. We quantitatively pointed out the role of chloride in the deposition solution in favoring layer-by-layer film growth. After hydrogen insertion, the resistance to hydriding by the two atomic Pd layers closest to the Pt interface is shown by the nonevolution of the corresponding interlayer distances. A strong effect of the underlying Pt(111) substrate is clearly demonstrated. Hydriding is observed only between the second and the third Pd layers, as revealed by the expansion of dPd2-Pd3. As a consequence, the total hydride stoichiometry of the Pd film is lower when compared to that of bulk Pd. Our results help to explain the origin of the reduction of H/Pdmax observed in nanosized Pd-supported films and its dependence on film thickness.26 Finally, measurements after the total desorption of hydrogen show that, within the error bars, interplanar distances revert to their initial values. This is an expression of good reversibility of the film structure, at least toward one complete hydrogen insertion-desorption cycle, which should be confirmed by chemical or electrochemical isotherm measurements. Acknowledgment. We are grateful to J-F. Berar for his help in making the SXRD measurements and to M. Chauveau for the construction of the X-ray electrochemical cell. The present work was financed by the French ANR, grant no. 06-JCJC-0111. DOI: 10.1021/la803913e

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