Surface Science 608 (2013) 61–66

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Surface Science

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Oxygen-induced Y surface segregation in a CuPdY ternary alloy

D.N. Tafen a,c,⁎, J.B. Miller b,d, Ö.N. Doğan a, J.P. Baltrus b, P. Kondratyuk d

a National Energy Technology Laboratory, 1450 Queen Ave. SW, Albany, OR 97321, USAb National Energy Technology Laboratory, Pittsburgh, PA 15236, USAc URS Corporation, P.O. Box 1959, Albany, OR 97321-0521, USAd Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

⁎ Corresponding author at: National Energy TechnologSW, Albany, OR 97321, USA.

E-mail address: denyago.tafen@contr.netl.doe.gov (D

0039-6028/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.susc.2012.09.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 August 2012Accepted 21 September 2012Available online 29 September 2012

Keywords:Surface compositionSurface segregationCuPd alloysHydrogen purification alloysFirst-principles calculationsPalladium yttrium alloysAdsorbate-induced segregation

We present a comprehensive theoretical and experimental study of the segregation behavior of the ternaryalloy CuPdY in vacuum (i.e., the clean surface) and in the presence of oxygen. Theoretical prediction showsthat for clean surface, yttrium will substitute first for Cu and then for Pd at the subsurface lattice site beforesegregating to the surface where it substitutes for Cu. XRD characterization of the surface of CuPdY indicatesthe presence of two major phases, B2 CuPd and Pd3Y. In the presence of adsorbed oxygen, theory predictsthat Y preferentially occupies surface sites due to its stronger oxygen affinity compared to Cu and Pd. XPS ex-periments confirm the computational results in the adsorbed oxygen case, showing that surface segregationof yttrium is induced by the formation of Y-oxides at the top-surface of the alloy.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

To produce high purity hydrogen fuel from coal, hydrogenmust beseparated from synthesis gas (syngas), a product of coal gasification.Membrane technology can be used to achieve this goal. The U.S. De-partment of Energy has established performance targets for hydrogenseparation membranes for the syngas applications in its Hydrogenfrom Coal program [1]. Although a large number of hydrogen separa-tion membrane materials have been evaluated, none are suitable forlong-term use in contaminant-laden syngas at elevated temperatures.Surface poisoning and corrosion have been shown to be the dominantmechanisms by which membrane materials degrade at elevated tem-peratures in gasses containing carbon dioxide, carbon monoxide, watervapor, and impurities such as hydrogen sulfide, arsenic, selenium, andmercury.

Cu–Pd alloys have demonstrated significant potential for resis-tance to surface poisoning and bulk corrosion [2–4]. Furthermore,Cu–Pd compositions with ordered bcc (B2) crystal structure exhibithydrogen permeability comparable to pure palladium [5]. The superi-or hydrogen permeability of Cu–Pd membranes with primarily B2structure exists up through temperatures at which the alloy structuretransforms into the lower permeability fcc form.

When compared in pure hydrogen, PdY alloyswith up to 10% yttriumhave displayed permeabilities that are four times those of a commercial

y Laboratory, 1450 Queen Ave.

.N. Tafen).

rights reserved.

high-permeability Pd75Ag25 alloy [6]. In addition, our computationaland experimental work on a Pd50Cu50 alloy illustrates that substitutionof Y for Pd has a stabilizing effect on the high-permeability B2 orderedphase [7]. Improved performance may not be realized at practical oper-ating conditions, however, because of the reactivity of Y with oxygen,even in environments of very low oxygen partial pressure such as postwater–gas shift (WGS) syngas. Furthermore, syngas impurities such asH2S may also react with Y at the alloy surface and reduce its activityfor hydrogen dissociation. To provide a comprehensive understandingof the surface composition of CuPdY – and its response to oxygenexposure – we combine computational and experimental tools to char-acterize the surface and bulk of a Cu50Pd43.75Y6.25 alloy, both in the ab-sence and presence of oxygen. To illustrate general trends, we considera hypothetical configuration of Cu50Pd43.75Y6.25 alloy entirely composedof a B2 phase as the starting point of our calculations. Thenwe introducea second phase, Pd3Y, into our calculations to examine the effect of oxy-gen on the surface of a two-phase CuPdY alloy.

2. Computational details

Total energy calculations, based on density functional theory withthe generalized gradient approximation (GGA), were applied to thePerdew–Burke–Ernzerhof (PBE) [8] exchange-correlation energy func-tional as implemented in the VASP code [9,10]. A plane-wave basis setwas used to represent the solutions of the Kohn–Sham equations witha kinetic cutoff of 500 eV. Projector-augmented-wave (PAW) potentialswere used to describe the electron–ionic interactions [11,12]. The recip-rocal space integration was performed by sampling the Brillouin zone

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of the periodic supercells with 11×15×1 and 5×15×1 Monkhorst–Pack grid [13] for the 1×1 and 2×1 surface cells, respectively. Geo-metrical optimizations were done until all force components on allunconstrained atoms were less than 0.03 eV/Å.

The CuPdY (011) surfaces were represented by an eight-layer slabwith periodic boundary conditions in the two directions parallel tothe surface. The vacuum space above the slab was 14 Å thick, ensur-ing the decoupling of consecutive slabs. The lattice constant forthe surface was fixed to that of the bulk CuPd obtained from DFToptimizations. The DFT computed lattice constant for the bulk CuPdis a=3.02 Å, compared to the experimental value of 2.96 Å [14]. Dur-ing the geometry optimization, the coordinates of the atoms wereallowed to relax except for the bottom four layers of the slab, whichwere fixed to their calculated bulk positions.

The adsorption energy of oxygen, Eads, was calculated using theexpression

Eads ¼ EslabþO–Eslab–NOEO2=2� �

=NO

where Eslab+O is the total energy of the relaxed oxygen-surface sys-tem, and Eslab and EO2 are the total energy of the relaxed bare surfaceand the gas phase oxygen, respectively. NO is the number of adsorbedoxygen atoms per surface unit cell. Oxygen was placed on one side ofthe slab. The influence of the resulting electric dipole on the comput-ed energy values was estimated to be very small (~13 meV per sur-face atom) according to standard methods [15], and has thus beenneglected.

3. Experimental procedures

A 40 g button of a (nominal) Cu50Pd43.75Y6.25 alloy was preparedfrom high-purity elemental materials, using a water-cooled copperhearth in a vacuum arc furnace back-filled with high-purity argon.The button was melted three times and subjected to a heat treatmentat 950 °C for 72 h to ensure chemical homogeneity. Unless otherwise

Fig. 1. Segregation energies, Eseg, of Cu50Pd43.75Y6.25 (011) layer stacks for clean surfaces andto the energy calculated for Y placed in the fifth layer below the surface, which represents th1 refers to surface layer, label 2 to subsurface layer, and so on.

noted, characterizations were performed on a polished, as-prepared(i.e., exposed only to ambient atmosphere) alloy sample. Microstruc-ture of the bulk alloy was characterized by scanning electron microsco-py (SEM) and X-ray diffraction (XRD). An FEI Inspect F50 SEM equippedwith a field emission gun was used to obtain both secondary electronand back-scattered electron images. EDS micro-chemical analysis wasalso performed on the SEM. A Rigaku Ultima III XRD system with CuKα characteristic radiation was used for phase determination.

The alloy surface composition and the chemical state of its Y com-ponent were determined by X-ray photoelectron spectroscopy (XPS).Measurements were performed with a PHI 5600ci spectrometer usinga monochromatic Al Kα X-ray source and an analyzer pass energy of58.7 eV. Photoelectrons were collected at an emission angle of 45° rela-tive to the surface normal. Alloy compositions were calculated from thepeak areas of the Pd3d, Cu2p and Y3d spectral features using sensitivityfactors provided by the instrument manufacturer. Oxygen was not in-cluded in composition calculations because its strongest photoemissionoverlaps with a peak from Pd. Elemental depth profiles were measuredby performing periodic XPS composition analyses during sputteringof the surface with an Ar+ion beam. A differentially pumped ion gunwas operated at 1.5×10−2 Pa and 25 mA to deliver a sputtering rateof approximately 12 nm/min.

4. Results and discussion

We begin our study by assuming that the Cu50Pd43.75Y6.25 alloy isentirely composed of a phase with the B2 structure (Prototype CsCl)[7]. We focus on a 1×1 surface cell model of Cu50Pd43.75Y6.25 corre-sponding to a slab of eight (011) atomic planes containing two elementseach.We consider twomodel-types: a Cu-richmodel containing 8 Cu, 7Pd, and 1 Y atoms, and a Pd-rich model containing 8 Pd, 7 Cu, and 1 Yatoms. For clean surfaces, the minimum energy i.e. the most stableconfiguration in both models is obtained when Y occupies a sub-surface lattice site (Fig. 1). Our results predict that, in vacuum, yttri-um does not segregate to the surface, but instead substitutes for Cu

in the presence of an oxygen ad-layer. For each system, the energy is reported relativee bulk configuration. The numbers on the x-axis refer to the layer numbers so that label

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in the subsurface, then for Pd in the subsurface, before finally movingto the surface where it substitutes for Cu.

To evaluate the effect of oxygen on the ternary alloy surface, wefirst calculate the adsorption of oxygen on CuPd (011) surface. Ourcalculations show that the most favorable site for oxygen adsorptionon CuPd (011) surface is the four-fold hollow site surrounded by 2Cu and 2 Pd atoms in the top layer and a Pd atom below the site inthe subsurface layer. The calculated adsorption energies are −1.42 and−1.40 eV for oxygen coverage of 1/2 and 1.0 monolayer (ML), respec-tively. The least favorable sites are O on top Cu and O on top Pd withadsorption energies in the order of −0.2 eV and −0.1 eV, respectively.

Next, we examine oxygen adsorption by placing oxygen atomsinto the on-surface four-fold hollow sites. Contrary to the trend ob-served for the clean surface models, yttrium surface segregation

Fig. 2. Views of the atomic geometry of sixteen (2×1) surface unit-cell arrangements of Cu50

est energy configurations. Also plotted are the segregation energies of these models in the vvalues are relative to bulk energy configuration (configuration 1).

becomes energetically favorable in the presence of oxygen ad-layer.The energy gain for going from bulk site to surface site is ~2.75 eVin the presence of 1.0 ML oxygen ad-layer, indicating a strong ther-modynamic force for oxidation-driven Y segregation (see Fig. 1).

To go beyond the limitations imposed by 1×1 surface models, weconsider larger alloy surface models of Cu50Pd43.75Y6.25 composed ofmixed atomic layers. To account for a possible segregation at the sur-face, we consider (2×1) surface unit-cell atomic configurations inwhich the composition in the topmost four layers varies systematical-ly. This leads to more than 16 possible configurations with differentpseudo-random arrangements of Y on the lattice site. In Fig. 2 wepresent the atomic geometry of the most stable arrangements alongwith their segregation energies. The first arrangement correspondsto bulk configuration. The last arrangement, which is the most stable

Pd43.75Y6.25 alloy considered in this study (top). The shown configurations are the low-acuum and in the presence of oxygen at coverage of ¼ and ¾ ML (bottom). The energy

Fig. 3. XRD pattern showing the phases in the CuPdY alloy. The inset shows the XRDpattern acquired using a quartz sample (reference material) prior to the acquisitionof the CuPdY XRD pattern.

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for clean surfaces, corresponds to subsurface segregation with one Yin the bulk position. This result suggests that Y atoms that are nearone another exert mutual repulsion, and this repulsion (perhapsdue to strain from the large atomic radius of Y) limits the coveragethat Y can attain in any given metal layer. As Y moves to the outerlayers of the CuPd matrix, the segregation energy decreases andreaches its minimum value at −1.42 eV.

In oxygen environments with oxygen coverage varying from 1/2to 3/4 ML, the segregation energy of Y follows a trend that differsfrom that of clean surfaces (Fig. 2). Yttrium surface segregation be-comes energetically favorable. The lowest energy corresponds to con-figuration 15, where Y atoms substitute for Cu to form an yttriumoxide layer while the Cu atoms form a Cu-rich subsurface layer. Weobserve similar behavior for the other three lowest energy configura-tions with the exception of configuration 3 where the subsurface isCu–Pd rich. In all four configurations Y surface segregates by formingan oxide layer. These results are consistent with that of Fig. 1.

The XRD pattern acquired from the polished surface of the CuPdYalloy sample is presented in Fig. 3. The indexed pattern indicatesthat the primary phase in the alloy is CuPd with ordered bcc (B2)crystal structure (Prototype CsCl). The secondary phase is identified

Fig. 4. Back scattered electron micrograph showing the microstructure of t

as Pd3Y (Prototype Cu3Au). In addition to these two major phases,a small amount of CuPd phase with an fcc crystal structure is alsodetected. Back-scattered electron image in Fig. 4 and EDS data are con-sistent with the XRD findings, identifying the CuPd (B2) and Pd3Yphases. While B2 phase does not contain any Y, the other two phases(Pd3Y and CuPd-fcc) have high Y content.

Fig. 5 displays an electronmicrograph and XRD pattern of the CuPdYalloy after a 120 hour exposure to a flowing simulated post-WGSgas mixture of 50 H2, 30 CO2, 19 H2O, and 1 CO (in mol %), which, dueto the presence of water vapor, results in a constant oxygen partialpressure of 10−29 atm at 500 °C. Although the kinetics of Y oxidationat these conditions are much faster than for the as-prepared (exposedto air at ambient conditions) sample examined in the XPS experiments,the results are helpful for understanding the oxidation mechanism ofthe alloy. The Pd3Y phase on the surface and subsurface of the specimenoxidizes preferentially, producing Y2O3. The CuPd (B2) phase appearsstable. There is no evidence of formation of either Cu or Pd oxides.

Motivated by these experimental findings, we now turn our atten-tion to surface segregation of Pd3Y in oxygen rich environments tounderstand how the segregation of Y to the surface of CuPdY alloytakes place. In particular, we address the O-induced segregation pro-file of Pd3Y (111) surface. In the computational model, we account forpossible segregation at the surface by considering (2×2) surfaceunit-cell atomic configurations in which the composition in the top-most two layers varied systematically from pure Pd to pure Y. Oxygenadsorption is then examined by placing O atoms into the on-surfacefcc sites with O coverage varying from 0 to 1 monolayer (ML). Fig. 6displays the energies of different Pd3Y surface configurations consid-ered as a function of Y concentration in the topmost layer. The differ-ent data points at the same topmost layer Y concentration representthe variations due to differing Y concentration in the second layer. At0ML coverage, the lowest configuration energy (most stable configura-tion) has a bulk like first layer composition (3:1) ratio. This stability per-sists even at 1/4 ML coverage. As the O coverage increases from 1/4 MLto 1 ML the stability profile changes. Hence, at 1/2 ML and 1 ML thelowest energy configurations correspond to 50 and 75% of Y in the top-most layer, respectively.

Results of XPS depth profile characterization of the polished, asprepared (i.e., exposed to ambient atmosphere, but not oxidized atsevere conditions) alloy appears in Fig. 7. The probe diameter of theXPS is about 300 μm, which is much larger than the size of individualphase domains (~10 μm). As a result, the XPS data should be viewed

he CuPdY alloy. The chemical compositions were acquired using EDS.

Fig. 5. Back scattered electron micrograph showing the cross-section of the surface region of an oxidized CuPdY specimen. The XRD data was collected from the top surface after theoxidation of the alloy.

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as approximating overall, or average, composition of the alloy at eachdepth in the profile.

Before sputtering (time=0 in Fig. 7; two replicate measurementsshown), the relative concentration of Y is significantly higher thanit is in the bulk. Cu concentration before sputtering is lower than inthe bulk, while Pd concentration does not change significantly.These results reveal an apparent overall segregation pattern inwhich Y atoms replace Cu atoms at the top-surface of the alloy. How-ever, this is the net effect of concurrent segregation/oxidation pro-cesses that take place on the surface as observed experimentally atthe microstructural level and computationally at the atomistic level.In the presence of oxygen, Y atoms replace Pd atoms in Pd3Y (3:1)on the surface at a ratio of 2:2 or 1:3 as shown by the DFT calculations.As the Y-rich surface of the Pd3Y phase oxidizes and produces Y2O3,the remaining Pd atoms on the top surface layer either diffuse tothe CuPd phase or segregate as a Pd-rich phase on the surface. Sincewe have not observed any new phases on the surface, a plausible ex-planation is that the remaining Pd atoms are incorporated in the CuPd(B2) phase. In this case there would be a decrease in the overall Cuconcentration on the surface as observed with the XPS analysis.

After less than 1 min of sputtering (~12 nm into the sample), thecomposition of the sample is essentially ~Cu44Pd50Y6. The differencebetween this composition and the bulk alloy composition determinedby XRF (Cu49.8Pd44.8Y5.4) is within the error typically associated withthe application of XPS manufacturer-determined sensitivity factors; itmay also reflect more efficient removal of Cu (relative to Pd) duringthe sputtering process [16]. At long sputter times (>435 s), the Cucontent of the surface region is slightly smaller than at intermediatesputter times (b165 s); such a gradient is likely the result of segrega-tion of low surface energy Cu from the bulk of the alloy to its surfaceregion [16].

Fig. 8 shows how the Y3d region of the XPS spectrum changes withsputtering time at the beginning of the depth profile experiment.Before sputtering (time=0; two replicate spectra shown), the Y3dpeak is characteristic of oxidized Y, with 3d5/2 and 3d3/2 componentsat approximately 158 and 160 eV, respectively [17]. Immediatelyafter sputtering is started, a new feature appears at 156 eV and the160 eV feature disappears. The peaks at approximately 156 eV and158 eV are assigned to the 3d5/2 and 3d3/2 components of metallicY, respectively [17]. These results indicate that the oxidized surface

is immediately removed upon sputtering and further suggest thatthe surface segregation of Y observed in the depth profile experimentis induced by the formation of Y-oxides at the top-surface of the alloy.They are consistent with the results of the calculations for the case of1/2 ML adsorbed O.

5. Conclusions

In summary, we have shown that yttrium in a ternary CuPdY alloydoes not surface segregate in vacuum, but instead substitutes first forCu and then Pd at the sub-surface lattice site. XRD and back-scatteredelectron image revealed the presence of two major phases, B2 CuPdand Pd3Y phases. In the presence of oxygen, however, Y preferentiallyoccupies surface sites due to the oxidation of Pd3Y phase on thesurface and subsurface, leading to the formation of Y-oxides at thetop-surface alloy. Characterization of a Cu50Pd43.75Y6.25 alloy by XPSdepth profiling confirms oxidation-induced segregation of Y to thetop-surface of the alloy.

Acknowledgments

This technical effort was performed in support of the NationalEnergy Technology Laboratory's (NETL) ongoing research on Hydro-gen and Clean Fuels under the RES contract DE-FE-0004000. The com-putational effort in this research used the resources of the NationalEnergy Research Scientific Computing Center, which is supported bythe Office of Science of the U.S. Department of Energy under ContractNo. DE-AC02-05CH11231.

This report was prepared as an account of work sponsored by anagency of theUnited States Government. Neither the United States Gov-ernment nor any agency thereof, nor any of their employees, makes anywarranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information,apparatus, product, or process disclosed, or represents that its usewould not infringe privately owned rights. Reference herein to anyspecific commercial product, process, or service by trade name, trade-mark, manufacturer, or otherwise does not necessarily constitute orimply its endorsement, recommendation, or favoring by the UnitedStates Government or any agency thereof. The views and opinions of

Fig. 7.XPSdepth profile of (nominal) Pd43.75Cu50.00Y6.25 alloy. Sputtering rate ~12 nm/min.

Fig. 6. Atomic geometry of surface unit-cell arrangements of Pd3Y considered in thisstudy (top). Energies of different Pd3Y surface configurations considered as a functionof Y concentration in the topmost layer (bottom). The different data points at the sametopmost layer Y concentration represent the variations due to the differing Y concen-tration in the second layer.

Fig. 8. Y3d region of the XPS depth profile. The feature at ~160 eV is characteristic ofY-oxides (3d3/2); the feature at 156 is characteristic of reduced Y (3d5/2); the featureat ~158 eV is likely a combination of Y-oxide (3d5/2) and reduced Y (3d3/2). Y existsas oxide only at top-surface, before sputtering begins.

66 D.N. Tafen et al. / Surface Science 608 (2013) 61–66

authors expressed herein do not necessarily state or reflect those ofthe United States Government or any agency thereof.

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