Oxygen-Induced Changes of the Au30Pd70(110) Surface Structure

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 30

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Oxygen-Induced Changes of the Au pd (110) Surface Structure and Composition under Increasing O Pressure 2

Marie-Claire Saint-Lager, Marie-Angélique Languille, Francisco José Cadete Santos Aires, Aude Bailly, Stephanie Garaudee, Eric Ehret, and Odile Robach J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07068 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 16, 2018

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The Journal of Physical Chemistry

Oxygen-Induced Changes of the Au30Pd70(110) Surface Structure and Composition under Increasing O2 Pressure Marie-Claire SAINT-LAGER*1, Marie-Angélique LANGUILLE2,3, Francisco J. CADETE SANTOS AIRES2,4, Aude BAILLY1, Stéphanie GARAUDEE1, Eric EHRET2 and Odile ROBACH5

1

CNRS Institut Néel and Université Grenoble Alpes, 38000 Grenoble, France

2

Univ. Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON – UMR 5256, 69626 Villeurbanne, France.

3

Present address : Sorbonne Universités, Muséum National d’Histoire Naturelle, Ministère de la Culture et de la Communication, CNRS, 75005 Paris, France.

4

Laboratory for Catalytic Research, National Research Tomsk State University, 634050 Tomsk, Russia. 5

Univ. Grenoble Alpes, CEA, INAC-MEM, 38000 Grenoble, France. *e-mail : [email protected] phone : (+33) 4 76 88 74 15

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Abstract

The structure and composition of the Au30Pd70(110) surface were followed by grazing incidence x-ray diffraction (GIXRD) under increasing oxygen pressure from ultrahigh vacuum (UHV) to 500 mbar at moderate temperatures (300 to 420K). These measurements were complemented by Auger electron spectroscopy and environmental STM images with atomic resolution up to 500 mbar. After the cleaning and preparation procedures in UHV, the Au30Pd70(110) surface is (1x1) with a quasi-pure topmost layer of segregated gold. Under oxygen pressure, Pd segregation occurs and progressively enriches the near surface region. The surface evolves through different states which slightly differ from that of pure Pd(110). First, the (1x2) missing-row reconstruction induced by oxygen adsorption that is stable in a large range of pressures depending on the temperature. Actually, STM images show regular vacancies every two atoms along the dense [1-10] rows. Then, for higher oxygen pressures, a transition phase appears before the formation of an oxidized pure Pd film growing in the [100]PdO direction.


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Introduction Bimetallic alloys are a class of important heterogeneous catalysts as they frequently exhibit much enhanced catalytic stability, activity and selectivity, as compared with their singlemet,al constituents.1,2,3,4,5 Under reactive gas pressure, several effects due to the interactions between the two metals have to be taken into account to understand their catalytic properties, such as electronic effects and geometric ensemble effects (active sites formed by a specific association of the two metal atoms and/or site dilution).6 The fact that in UHV the topmost layer of Au-Pd alloys is heavily enriched in gold even for high palladium concentration was first evidenced by Jablonski7 and is now well documented. 8,9,10,11,12

Among the driving forces for segregation, the difference in surface energy of the

components plays a major role:13,14,15,16 calculated values being 1.626 and 2.043 J.m-2 respectively for Au and Pd at 298 K.17 The high strength of Pd-Au bonds, which are stronger than Au–Au and even Pd–Pd bonds, is another factor that promotes, in UHV, segregation of surface Pd atoms toward subsurface or deeper inside the bulk but it also prevents the formation of Pd first neighbors on bimetallic Au-Pd surface.18 Under oxygen pressure, palladium, that forms stronger bonds with oxygen compared to gold, segregates to the surface modifying the structure and the composition of catalyst surfaces.19 As early as 1981, Hilaire et al. mentioned a significant palladium surface enrichment starting around 570K and a complete PdO surface coverage on the Pd60Au40 alloy at 770-850K.20 Carbon Monoxide (CO) can also induce Pd segregation towards the AuPd surface as evidenced for Au30Pd70(110) under 500 mbar at room temperature.11,12 The Pd segregation in PdAu alloy has a great influence on the catalytic properties as shown by experimental investigations for model surfaces21,22,23,24 and supported bimetallic Nanoparticles (NPs)25 under reaction conditions of CO oxidation. The alloying of Pd with gold modifies the binding energy with oxygen and CO. For instance, high activity in CO oxidation was achieved on AuPd(100) at 400K thanks to the contiguous Pd sites existing at the surface (at least Pd dimers) created by Pd segregation under CO pressure.22 But the key for the improvement of the catalytic activity would be the reduction of the binding energy of CO with AuPd(100) compared with pure Pd. This would allow facile CO desorption even near room temperature and thus O2 adsorption and dissociation which is an exceedingly slow process on pure Pd at such temperatures.26 However a recent theoretical study on this 3 ACS Paragon Plus Environment


system found that because of its strong ability to adsorb CO, the Pd chains configuration poorly oxidize CO.27 As a consequence second Pd neighboring configuration turns out to be more reactive in the CO2 formation step, since it is able, as the first neighboring configuration, to dissociate O2 molecules. Pd segregation at the surface of Au-Pd bimetallic NPs supported on TiO2 was also evidenced under O2 and O2+CO with the formation of Aucore-Pdshell structure but Au-Pd synergy effect towards CO oxidation was not observed.25 This was attributed to the possible replacement of Au by Pd atoms in low coordination sites, while such Au atoms would be at the origin of the high activity in CO oxidation of the supported gold NPs. The purpose of this article is to better understand the interaction of oxygen with an AuPd surface. We thus studied the latter by following the structure and composition of Au30Pd70(110) single crystal surface using Grazing Incidence X-Ray Diffraction (GIXRD), STM and Auger Electron Spectroscopy (AES).

Experiment The setup, used for the x-ray measurement, consists of an UHV preparation chamber coupled to a batch reactor to allow sample transfer in UHV conditions. The reactor runs from UHV to ambient pressure and makes possible both X-ray scattering and activity measurements in static conditions. It is described in detail elsewhere.28 It is usually settled in our laboratory for studies on the sample preparation and its low pressure characterization, using Low Energy Electron Diffraction (LEED) and AES in the UHV chamber, and reactivity measurements in the reactor chamber. Such investigations can be done prior to the allocated beam time at the European Synchrotron Radiation Facility (ESRF) and after those to obtain complementary measurements. The whole setup can be moved and installed at ESRF on beamline BM32 to perform X-ray measurements from UHV up to near ambient pressure of reactive gases. The surface preparation can be performed indifferently in the reactor and in the UHV chamber, both being equipped with an ion gun and a sample heating stage. The sample heating is supplied by electronic bombardment from a tungsten filament in the UHV chamber and, in the reactor chamber, by a high-power fiber-coupled laser diode to avoid hot filament which would be responsible for undesirable side reactions under reactive gases. For 4 ACS Paragon Plus Environment

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the same reason, the temperature was measured with an external infrared pyrometer (Impac IPE 140 MB 10). LEED and AES were conducted in the UHV chamber by using a SPECTALEED apparatus from Omicron. The AES detector is a retarded field analyzer in the 4grid LEED optics. GIXRD was performed in the reactor chamber connected to a gas manifold. N55 oxygen gas purity was used. The gas pressure in the reactor was monitored by a combination of a low pressure cold cathode gauge from UHV up to 10−4 mbar and, for higher pressures, two capacitive membrane gauges working from 10−3 to 1 mbar and from 1 mbar to 1 bar, respectively. GIXRD was performed with a photon energy of 18 keV, the incidence angle with the surface sample being 0.3°, i.e. about twice the critical angle for a better stability during sample heating. The detector was a standard NaI scintillator. The crystal basis (a1, a2, a3) used to describe the (HKL) directions is expressed in terms of the cubic Au30Pd70 lattice with a1=1/2[1-10]c and a2=[001]C lying in the surface plane and a3=1/2[110]C out of plane, the c subscript indicates that directions are indexed in the cubic frame (see figure 1c). In this basis α=β=γ =90°, a1 = a3 = a0/√2 and a2= a0, a0 being the cubic Au30Pd70 lattice constant. This unit cell is tetragonal centered and the Bragg peak appear for H+K+L even. The environmental STM setup is a custom-built system based on a modified Omicron MicroLH variable temperature STM. This system was used for the study of surface morphology and structure under gas pressures ranging from UHV (< 10-9 mbar) up to elevated pressures conditions;29,30,31 the limit being a slightly above 1 bar which is the highest pressure supported by the viewports necessary for safe manipulation the sample/instruments within the STM chamber. This environmental STM system is composed by two chambers. The UHV surface preparation chamber is equipped with an ion sputter gun operated at 2 kV and a heating stage consisting of a boron nitride ceramic plate heated by an internal (protected) graphite filament (Bora Electric); the setpoint temperature being controlled by a PID system. The sample temperature can be modulated from 120 to 500 K. After the cleaning procedure the sample is transferred under UHV to the STM chamber where the sample surface morphology and structure are controlled in UHV by STM, prior to its environmental STM study under oxygen pressure (in this study from 1 to 670 mbar) at room temperature. In order to limit side reactions during environmental gas experiments, most of the metallic (in particular copper) parts of the STM and the magnet locking the 5 ACS Paragon Plus Environment


sample were gold-plated and the piezoelectric tube was protected by a polymer coating to avoid possible corrosion and/or sparks between piezo-tube sectors (at pressures typically ranging from 10-3 to 1 mbar depending on the gas) under pure gas pressures.

Figure 1 : Experimental (circles) and calculated (full lines) structure factors of the clean (1x1) Au30Pd70(110) surface in UHV with, in (a) the (10) and (01) CTRs, and in (b) the (20) and (02) CTRs. CTRs with K=0 are in dark blue and with H=0 in light blue. (c) Schematic representation of the (1x1) Au30Pd70(110) surface with orange balls for gold atoms and blue balls for rich Pd layers with shade that depends on its concentration. The axes show how (a1, a2, a3), defining the (H,K,L) Miller indices, are related to the cubic lattice of the bulk alloy.

Results UHV characterization of the clean Au30Pd70(110) surface The composition of the Au30Pd70(110) sample was checked by Energy Dispersive X-ray (EDX) microanalysis within a SEM (Scanning Electron Microscope) and yields 29.5±0.5% as gold concentration. The Au30Pd70 lattice parameter a0 is obtained using x-ray diffraction by optimizing 12 Bragg peaks and is found to be 3.942±0.005 Å. This is consistent with the expected value for this composition (3.946 Å).32 A clean Au30Pd70(110) surface is prepared by repeated cycles of ion sputtering (argon pressure ≈ 10-6 mbar, V=2 keV) and annealing at 750 K in UHV leading to gold segregation. The clean surface exhibits a well-defined (1x1) LEED pattern. AES analysis shows that the 6 ACS Paragon Plus Environment

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ratio, RAu/Pd , of the peak to peak height of 69 eV Au (NVV) transition on that of the 327 eV (MNN) Pd is 4.3 ±0.2. We detected no trace of sulphur peak at 152 eV, the most frequent pollutant of the AuPd alloys. The (1x1) structure of the clean Au30Pd70 (110) surface in UHV (labeled stage I in the following) was determined by GIXRD. The diffracted intensities were collected by performing azimuthal rocking scans along a large set of Crystal Truncation Rods (CTRs) ((0 1), (1 0), (0 2), (2 0), (1 1), (1 2), (2 1) and (2 2)) and a second set for equivalent reflections. The structure factors were extracted according to standard correction procedures33 and the fit to the data was performed using the program package ROD.34 During the fit, the spacing and composition of the five topmost layers were allowed to relax. In addition the roughness was modeled by the β factor35 and the best fit (χ=0.9) was obtained for β=0.19. Moreover the static disorder was described with in-plane and out-ofplane Debye-Waller factors for each plane. The topmost plane is referenced with the “0” subscript and the following planes by negative figures, as going deeper inside the bulk. We found an almost pure gold plane at the surface with CAu(0)=95 ± 5 %. The gold concentration strongly decreases in the underneath planes with a small tendency to oscillations : CAu(1)=35 ± 1%, CAu(-2) = 20 ± 1 %, CAu(-3) = 36 ± 1%, CAu(-4) = 25 ± 1 %, to finally recover the bulk concentration CAu(≤-5)=30%. There are relaxations of the interplanar distances perpendicularly to the surface. The first inter-planar spacing is contracted relative to the bulk value by -2.5 ± 0.2 % and expanded for the second one by +2.4 ±0.2 %. No relaxation is found for the atomic planes underneath the second one. The experimental and calculated structure factors are reported in figure 1 for [H0 K0] rods with H0 or K0 equal to zero. It evidences different profiles of the CTR depending on the inplane component which was taken into account with anisotropic Debye-Waller factors; the bulk thermal value being 0.4 Å2. The best fit of (0 K0) CTRs (with K0 = 1, 2) was obtained with an out-of-plane Debye-Waller factor B⊥(0) = 2.8 Å2 for the surface plane. For (H0 0) CTRs (with H0 = 1, 2), it was with in-plane Debye-Waller factors B//(0) = 1.9 and B//(-1) = 1.1 Å2 for the first and second topmost planes, respectively. This corresponds to a static disorder with displacement of atoms from their mean positions along the dense rows at the surface (in the [1-10]c direction) and which extends in the two topmost planes. This behavior can be explained by the compressive strain in the gold rich surface plane induced by the 7 ACS Paragon Plus Environment


underneath alloy with a lattice parameter 3.5% smaller than that of bulk gold.

The STM images of the clean Au30Pd70(110) surface are displayed in figure 2. The large scale image (figure 2a) evidences numerous terraces, [1 -1 0]C preferentially elongated and separated by monoatomic-high steps (∼1.4-1.5 Å).The surface terraces exhibit a (1x1) structure (figure 2b). This structure seen locally by STM is confirmed for the entire surface by complementary LEED characterization (figure 2c). Further details on the STM study can be found elsewhere.12 We note that the (1 0 0.05) surface peak in x-ray diffraction has a lorentzian profile in the K direction with a width ∆K = 0.027, as well as the (0 1 0.05) in the H direction with ∆H = 0.0066. This corresponds to a domain size of about 420 nm along the dense rows and 120 nm perpendicularly, consistently with STM images.

Figure 2 : STM images of the clean Au30Pd70(110) surface in UHV at room temperature : (a) large scale image (400 × 400 nm2), Ut= 80 mV, It= 0.3 nA; (b) atomic-resolution image (3 × 2 nm2) showing periodicities of ≈0.03 nm along [1 -1 0]C and ≈0.04 nm along [0 0 1]C, Ut= 80 mV, It= 2 nA; (c) corresponding LEED pattern (incident electron energy : 130 eV).

Evolution under exposure to pure oxygen from UHV up to 500 mbar The Au30Pd70(110) surface was exposed to increasing oxygen pressure from UHV up to 500 mbar at room temperature (RT) and at 420K. As shown in Figure 3, the evolution of the diffracted intensity along the K radial scan, at H=0 and L=0.05, is quite similar at these two temperatures. We can define different stages. First (stage I), the clean (1x1) UHV Au30Pd70(1 8 ACS Paragon Plus Environment

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1 0) surface exhibits a well-defined surface peak at (0 1 0.05). This peak slowly decreases when increasing oxygen pressure while a broad peak grows around K=1.50±0.01. It is the signature of the (1x2) reconstruction induced by O2 adsorption into the Pd70Au30(110) surface and will be discussed later (stage II). It was observed in a wide pressure range depending on the temperature: from 2 to 500 mbar at RT and from 5.10-6 to 2 mbar at 420K. However there is a long delay to grow it at the low pressure limit (for instance 30 min under 2 mbar at 300K); on the other hand the lifetime is reduced at the high pressure limit. After this stage, both diffraction peaks decrease to completely disappear (stage III). Then a small surface peak reemerges at K=1. It was observed under 500 mbar at RT or after long exposures to 2 mbar of O2 at 420K (stage IV). At 420K, a higher oxygen pressure makes the surface peak vanish and a new sharp diffraction signal rises at K= 1.47 assuming a complete surface transformation to a new crystalline phase (stage V) not observed at RT. This new peak can be related to the PdO-like phase.

Figure 3 : Diffracted intensity along K at H= 0 and L=0.05 under increasing oxygen pressure at room temperature (RT) (left plot) and at 420 K (right plot); the double bars on the vertical scale indicate a new sample preparation. The arrows point to the small peaks; the reality of which being checked by azimuthal scan, in black the surface peak at K = 1 and in grey at K=1.5 of the (1x2) reconstruction; the large peak of the upper curve at 420K is at K = 1.47. The curve with a star (*2 mbar + 1h20) was recorded after cooling at RT and pumping down to UHV then the pressure and the temperature were



rose again to 2 mbar and 420K, respectively. The AES stamps correspond to the Auger spectra of figure 4 performed on the same sample as X-ray diffraction after transfer in the preparation chamber; stamps are grey-shaded for spectra recorded on a different sample but after a similar treatment. The central column indicates the five stages defined in the text.

The chemical composition and the structure of the surface were probed by AES and LEED respectively. Up to few 10-6 mbar O2 pressure, AES spectra and LEED patterns were collected in-situ in the preparation chamber. At higher O2 pressure, the sample was exposed to O2 in the reactor and directly transferred in the UHV preparation chamber for ex situ AES and LEED investigations. Figure 4 shows AES spectra of the Pd M5N4,5N4,5 and Au N7V4,5V4,5 signal at 327 and 69 eV, respectively. The Auger electron energy of 69 eV is at the minimum of the electron inelastic mean free path that corresponds to about 0.4-0.5 nm36 ensuring a good surface sensitivity for gold detection.

Figure 4 : Evolution of the Auger spectra as a function of the exposure to gas (with the same color as stamps of figure 3) : (Black) Clean (1x1) Au30Pd70(110) surface in UHV, RAu/Pd = 4.3±0.2 (stage I); (blue) (1x2) surface induced by 2.10-6 mbar oxygen at 500 K, RAu/Pd = 3.7±0.2 (stage II); (Green) post mortem in UHV , after exposure to 2 mbar oxygen at 420 K (small peak at K=1), RAu/Pd = 2.4±0.2 (stage IV); (Red) after 1 mbar CO + 500 mbar O2 at 470K on oxidized surface (sharp peak at K=1.47), also recorded post-mortem, RAu/Pd ∼ 0. For each curve, the intensity is normalized to the peak to peak height of the Pd(MNN) transition.

The Au/Pd peak to peak ratios RAu/Pd were estimated as a function of the O2 exposure. This 10 ACS Paragon Plus Environment

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ratio was found to be 2.3±0.2 (not shown here) for the surface after argon ion sputtering and before annealing. After annealing it increased to 4.3 indicating a surface enrichment in Au on the UHV clean surface (stage I). Under increasing O2 pressure, RAu/Pd decreases. RAu/Pd is equal to 3.7 for 2.10-6 mbar O2 pressure at 500K, associated to the (1x2) formation (stage II). It decreases to 2.4±0.2 after a long exposure under 2 mbar O2 pressure at 420K (stage IV). When the pressure is increased up to 100 mbar (at 420 K), RAu/Pd is almost zero. Gold disappears from the spectra and only the Pd signature is observed (stage V). These results show that the surface is progressively enriched with palladium when oxygen pressure is increased.

The (1x2) oxygen induced reconstruction The (1x2) reconstruction induced by oxygen pressure (stage II) was studied by combining STM and GIXRD measurements. STM images were recorded during increasing oxygen exposure at room temperature. At the beginning it was not possible to obtain a clear image due to atom mobility yielding poor surface organization. Well defined images with atomic resolution were obtained when higher pressures were reached. Figure 5a shows a large scale STM image (500x500 nm2) recorded under 670 mbar of O2. Steps are visible and allow to define several levels of terraces that are significantly less numerous than what was observed in UHV at the same scale (figure 2a).12 The terraces still have a preferential orientation but they are not regular with respect to their width and their contours. The surface of the terraces is clearly rough (0.05-0.06 nm corrugation). The smaller scale images provide further details: Figure 5b shows alternation of bright islands and dark islets, forming a pattern close to that called "chessboard" by Niehus and Achete on pure Pd(110).37 The level difference between islands is of the order of a monoatomic height (roughly 1.4-1.5 Å). The terraces have an average size of 15 nm and 8 nm in the directions [1-10]C and [001]C respectively. The surface consists of a large number of parallel rows along the [1-10]C direction (see Figure 5c). Two bright rows are spaced with a depression corresponding to a dark row, which is characteristic of a (1x2) missing row reconstruction, the variation of intensity perpendicularly to the rows, along the [001]C direction, being of the order of 0.80.9 nm. Similar image contrasts were observed by Han et al. on Pd(110).38 Figure 5c also reveals a regular alternation of dark and bright spots along the dense [1-10]C direction with a distance (between minima or between maxima) corresponding to 2a1 or 3a1 (a1 being the 11 ACS Paragon Plus Environment


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lattice unit along [1 -1 0]c). It clearly shows that the dark spots correspond to depressions

separated by about twice or thrice the interatomic distance indicating the occurrence of one vacancy every two or three atoms. The STM images do not reveal a relationship between the distribution of these “holes” from one row to another. This can explain why no (Nx2) reconstruction was observed, nor by LEED neither by GIXRD.

a

b

c

2

500 x 500 nm

2

50 x 50 nm

2

10 x 10 nm

Figure 5 : STM images of the Au30Pd70(1 1 0) under O2 pressure (500 Torr) at room temperature : (a) large scale 500 × 500 nm2 image, Ut= 600 mV, It= 0.4 nA; (b) more detailed 50 x 50 nm2 image, Ut= 800 mV, It= 0.3 nA, showing structure within the corrugated terraces; (c) atomic-resolution 10 × 10 nm2image, Ut= 800 mV, It= 0.3 nA showing Nx2 (N=2,3) arrangements on the surface (the blue arrows point to vacancies along [1 -1 0]c).

This reconstruction can be correlated to the (1x2) evidenced by GIXRD. The width of the (1x2) reconstruction peaks was broad denoting a poor organization. The coherence domain of the reconstruction was deduced from (0 ½ 0.05) and (0 3/2 0.05) reconstruction peaks, each having a width of ∆H=0.03 and ∆K=0.145. It gives coherence lengths of 9 nm along the [1 -1 0]C rows and of 3 nm perpendicularly, which are slightly smaller than the average size of the terraces measured by STM. Moreover, as already mentioned, we did not detect any signal of a N reconstruction along the rows. Anyway, the reconstruction was stable enough under 2 mbar at room temperature to allow recording of several CTRs ((1 0), (0 1) and (1 1)) so that the surface structure determination could be achieved. The set of CTRs is limited but the STM images help to converge towards a realistic solution. The best fit (χ=1.3) was obtain for a roughness parameter β=0.1 and is reproduced in figure 6a-d. Figure 6e compares the Pd and Au concentrations per atomic plane of the (1x2) and of the (1x1) clean surface in 12 ACS Paragon Plus Environment

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UHV. For the (1x2) reconstructed surface, significant changes from the bulk alloy values spread only on the three topmost planes. In the surface plane one over two rows is empty and the other is occupied at 64± 5 % by palladium. This low occupancy is compatible with regular vacancies every two atoms along the [1-10]C rows, as seen on the atomic resolution STM image.

Figure 6 : The (1x2) missing row reconstruction under 2 mbar of oxygen at RT . (a), (b), (c) and (d): Structure factor as a function of L along the (10), (01), (11) and (0 3/2) CTRs, respectively; the calculated (0 ½) is also reported in (d) with dashed line. Points represent the experimental data and lines the fits. (e) Schematic picture of a (1x2) Au30Pd70(110) surface. The small horizontal arrows indicate the row pairing; blue balls correspond to pure palladium composition, light and dark yellow balls to Au0.47Pd0.53 composition in plane -1 and Au0.65Pd0.35 composition in plane -2, respectively. The small red balls figure the oxygen position according to the c(2x4)-O reconstruction of Pd(110) represented in the bottom right insert.

The gold concentration of the plane -1 is slightly higher than the nominal composition, but it remains less than fifty percent (47± 5 %), whereas the plane -2 is richer in gold (65± 5 %) (figure 6e). The distance of the Pd row at the surface with respect to the underneath plane is contracted by -5.5 ± 0.5% relatively to the bulk inter-planar distance (1.393Å). This is significantly more than on the clean surface (-2.5%) but in the latter case the surface was almost pure gold. Moreover, in the missing row reconstruction, the contraction of the topmost layer is usually accompanied by a lateral pairing of the atomic position in the plane below.39 The shift of two rows under the missing row was found to be closer from 3±1% and those under the filled one farther so that the average distance remains the same as in the 13 ACS Paragon Plus Environment


alloy (3.94 Å). The inter-planar dilatations induced by gold enrichment of planes -1 and -2 are negligible in the frame of the accuracy of these fits.

Formation of a thin strained oxide As already mentioned an increase of the oxidizing conditions makes the surface peaks vanish as well as the (1x2) reconstruction ones. There is a stage where no diffraction peaks were observed while there is a palladium enrichment of the surface (stage III). Then a small peak at K=1 makes a quick reappearance (stage IV) before the oxide formation (stage V). Stage V exhibits a well-defined diffraction peak at (0 1.47 0.05) (figure 3). Auger spectra show that the surface planes become purely composed of palladium (figure 4). The diffracted sharp peaks were measured at (0 -1.47 0), (0.93 0 0), (-0.93 0 0), (0.93 0.73 0), (0.93 -0.73 0), (0.93 1.46 0), (1.85 1.47 0) (0 0.73 0.95) and (0.93 1.45 0.95) and are illustrated in figure 7. These peaks detected at fractional values of H, K, and L, can be related to PdO tetragonal bulk oxide grown in the [100]PdO direction, with the long axis perpendicular to the dense [1 -1 0]C rows of the Au30Pd70(110) surface as shown in the right insert of figure 7.

Figure 7 : PdO peaks measured along several directions of the reciprocal space in the frame of the Au30Pd70(110) surface defined by (a1, a2, a3). Black and red curves along K in the plane L=0 with H=0 (k=0) and H=0.92 (k=1), respectively; blue along the L direction at H=0.92 and K=0. The oxide peaks are labelled with the Miller indices (h,k,l) in the PdO frame(aoxi, boxi, coxi).


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We deduced the corresponding oxide Miller indices h,k,l with h=0.95L, k=0.93H and l=0.73K leading to the following parameters for the PdO film unit cell : aoxi=2.93± 0.03 Å, boxi=3.00 ± 0.05 Å and coxi=5.39± 0.01 Å. The lattice parameter is contracted perpendicularly to the surface and the structure is no longer exactly tetragonal as for the usual bulk PdO oxide. We notice that peaks at (0 ±0.73 0) were also measured even if they are not expected in the usual bulk-like PdO. This indicates that this phase is not exactly the same. Another particularity of this oxide film is its narrow thickness (≤ 1 nm) as shown by the broad (110)PdO oxide peak in the L direction plotted in the right insert of figure 7.

Figure 8 : K-scan at H=0 and L=0.05 in various conditions when a small diffraction signal is detected at K=1, with in insert the corresponding omega scan at K=1; for the clean surface UHV-RT curves the intensity scale has to be multiplied by 40. We notice the increase of the level of the noise at low K when pressure increases which is due to the x-ray diffusion by gas in the chamber.

Discussion : In UHV, the clean surface is (1x1) with the last plane almost pure gold as expected from Pd and Au surface energies.13-17 Contrary to Au3Pd(110), which has two quasi-pure gold planes at the surface,9 the Au30Pd70(110) does not have the (1x2) missing row reconstruction typical of the Au(110) surface.39,40,41 The present (1x1) gold surface alloy is quite similar to the 15 ACS Paragon Plus Environment


growth of gold on Pd(110) that presents a (1x1) Au surface up to one monolayer (ML) gold deposit; the (1x2) missing row reconstruction being established above a deposit of 1.5 Au ML.42 Gold atoms on the (1x1) Au30Pd70(110) surface are strongly strained, the gold bulk lattice being 3.5% larger than the alloy one. This induces a large disorder described by an anisotropic Debye-Waller factor in the CTR fits and the large number of terraces yielding a stepped surface as observed in the STM images. When increasing oxygen pressure, the surface is Pd-enriched and a (1x2) reconstruction first appears. This oxygen-induced reconstruction (stage II), as characterized by our STM and Xray diffraction measurements, is of missing row type. The remaining row at surface is filled by palladium and the pure gold surface plane of the (1x1) clean surface is dissolved over the two underneath planes. This allows to partially relax the strain induced by the pure gold surface plane of the (1x1) clean surface. This results in steps significantly less numerous on the STM images of the (1x2) surface than in the (1x1) one and also in a lower roughness parameter β of the CTR fits. However the coherence domains of the reconstruction are smaller. An oxygen-induced missing row reconstruction is observed on pure Pd(110) and was also studied by X-ray diffraction;43 but it is clear from the K-scan of this study that it was more organized on the pure Pd(110) surface than on the present Au30Pd70(110) alloy. However, the main difference is the appearance of regular vacancies every two or three atoms along the filled rows shown by the STM images, consistently with the low occupancy of about 60%, deduced from X-ray diffraction. On pure Pd(110), the oxygen-induced missing-row reconstruction exhibits a strong outwards relaxation43 whereas on Au30Pd70(110) alloy, there is an important contraction of the topmost Pd plane spacing (-5.5 %). In this case, the gold composition of the second plane is about 50% which modifies the equilibrium state with, in one hand the Au-Pd binding stronger than Pd-Pd one,18 and, in the other hand, the weakening of the Pd-O binding due to the neighboring gold atoms.44 The formation of regular vacancies along the topmost [1 – 1 0] Pd row could then be the way to relax the strain induced by oxygen adsorption on the Au30Pd70(110) surface. On pure Pd(110), this (1x2) missing row is associated to the oxygen atoms adsorption forming the well-known c(2x4)-O reconstruction.45 The latter was modeled with the remaining close packed rows decorated by adsorbed oxygen atoms located in tetrahedral 16 ACS Paragon Plus Environment

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sites in a zigzag pattern37,46 (see insert of figure 6e). We checked by LEED and by GIXRD that such a reconstruction does not accompany the (1x2) Au30Pd70 structure. Anyway the same kind of sites for oxygen adsorption could exist, as illustrated on the schematic representation in figure 6e. Such (1x2) Au30Pd70(110) surface exhibits the two contiguous Pd atoms required for oxygen dissociation.47 Moreover in this geometry, taking into account the palladium composition in the two sub-surface planes, Pd surface atoms have statistically at least two more Pd atoms as first neighbors. The role of Pd first neighbors in the sub-surface was pointed out and makes the oxygen dissociation energetically more favorable for Pd/Au(100) than for Pd/Au(111).48 However X-ray diffraction is not sensitive to oxygen atoms, so we cannot deduce from our data their positions that could differ from oxygen adsorption on pure Pd(110) surface. When the oxidizing conditions are increased all diffraction peaks disappear (figure 3 – stage III). Then, in stage IV, a small peak reappears at K=1. This is illustrated in figure 8 which compares it on K-scan at H=0 and L=0.05 in several conditions, showing that this peak is, in all cases, very similar in intensity and in width along radial and azimuthal scans (∆K = 0.035±0.005, ∆ω=0.7±0.1°). Moreover, these values are close to those of the surface peak of the (1x1) clean surface even if the latter is much larger (x40). It is not the same for the (1x2) reconstruction peak at K=1.5 which is broad in both directions. It can be thus concluded that in any case the small (010) peak stays a surface peak, the intensity of which depends on the surface state. The reappearance of the (010) peak can be compared to what was observed by GIXRD on pure Pd(110) surface.43 It was attributed to a so called “complex” structure.49 This later is formed by (7x√3) and (9x√3) domains in which the [1 -1 0]C Pd rows are decorated by O atoms in a zig-zag pattern, as for the c(2x4)-O but with Pd filling of the missing row.50 In stage V a complete surface transformation to a new crystalline phase occurs corresponding to a strained PdO phase. A study of the growth of oxide on Pd(110) under oxygen pressure was performed by GIXRD.43 It showed that PdO has relaxed and the [100]PdO axis is ∼±7° tilted from the perpendicular to the Pd(110) surface. This result is significantly different from the strained film of the present work. But it was produced in more oxidizing conditions, above 0.5 mbar O2 at 620K, namely 150 to 200K higher than in our work. This



yielded diffraction peaks with a much thinner extension in the L direction meaning a much thicker PdO film.

Conclusion This study on Au30Pd70(110) under increasing oxygen pressure from UHV up to 500 mbar shows that its surface strongly evolves with the oxygen pressure. Five stages can be identified and some of them can be compared to the structures observed for pure Pd(110), but with noticeable differences: -

Stage (I) : the clean UHV (1x1)Au30Pd70(110) with an almost pure gold plane at surface which is rather stepped as seen in the large scale STM images;

-

Stage (II) : a (1x2) missing row reconstruction with Pd at surface, The formation of this missing row reconstruction allows to relax the large strain of the pure gold first atomic plane of the clean surface. Actually, the remaining row presents regular vacancies every two atoms. Unlike pure Pd(110), the (1x2) Au30Pd70(110) does not exhibit the well-known c(2x4)-O oxygen-induced reconstruction on Pd(110), but may have similar sites for oxygen atoms adsorption;

-

Stage (III) : a transition state with no long range order measured by x-ray diffraction;

-

Stage (IV): reappearance of a small diffraction peak similarly to what was observed for “complex” structure observed on Pd(110) surface;

-

Stage (V): a strained phase of PdO growing in the [100]PdO direction.

For this alloy, the strong tendency of Pd segregation towards the surface under oxygen pressure, even at room temperature, was also observed under CO pressure.12 In contrast to what was observed on the AuPd(100) surface, this is not favorable to the existence of a mixed AuPd surface highly reactive for CO oxidation.21-24 This can be due to a Pd richer alloy and also to the more open (110) surface. Anyway the formation of a strained epitaxial PdO thin film during the last step is very promising. Indeed, it was already evidenced by GIXRD for Pd(100) in similar conditions of temperature.51 It seems that it plays a major role in the high reactive regime for CO oxidation during the oscillatory regime. The study of this strained PdO phase, its structure and its reactivity for CO oxidation is under progress.


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Aknowledgements : The authors thank the technical staff (SERAS) of Institut Néel CNRS and of the BM32 CRG beamline at ESRF, in particular O. Ulrich, for their valuable help.52

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Oxygen-Induced Changes of the Au30Pd70(110) Surface Structure

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