CO Adsorption on Pd-Au Alloy Surface: Reversible Adsorption Site. Switching Induced by High-Pressure CO

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1 Supporting Information for CO Adsorption on Pd-Au Alloy Surface: Reversible Adsorption Site Switching Induced by High-Pressure CO Ryo Toyoshima, Nana Hiramatsu, Masaaki Yoshida, Kenta Amemiya, Kazuhiko Mase, Bongjin Simon Mun and Hiroshi Kondoh,* Department of Chemistry, Keio University, Hiyoshi, Kohoku-Ku, Yokohama, Kanagawa , Japan Institute of Materials Structure Science, High Energy Accelerator Research Organization, and The Graduate University for Advanced Studies, 1-1 Oho, Tsukuba, Ibaraki , Japan Department of Physics and Photon Science, and Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology, Gwangju , Republic of Korea *Tel.: Index Page 1. Atomic fraction of Pd at Pd 70 Au 30 alloy surface S2 2. CO adsorption on Pd 70 Au 30 (111) surface S4 3. CO adsorption on Pd(111) surface S6 4. Optimized geometric parameters and corresponding adsorption energies S7 5. Calculated core-level shifts and comparison with the observed ones S9 6. Surface free energy of clean and CO adsorbed Pd-Au alloy S10 7. References S13 S1

2 1. Atomic fraction of Pd at Pd 70 Au 30 alloy surface In x-ray photoelectron spectroscopy (XPS) measurements, collection of XP spectra at low take-off angles relatively enhances signals from (near) surface species. Angular dependence of Pd 3d 5/2 XP spectra is shown in Figure S1. The normal emission (90º) spectrum exhibits an asymmetric shape centered at around ev. The grazing emission (50º) spectrum shows two remarkable changes; a shift of peak top toward the lower-energy side, indicating of a surface component at the lower-energy side and appearance of a shoulder structure at the higher-energy side. Therefore the Pd 3d XP spectra consist of three components including the bulk component and curve-fitted as shown in Figure 1(e) in the text. The lower-energy component is assigned to surface Pd atoms located in the 1st layer. While the higher-energy component can be attributed to Pd atoms situated in the 2nd layer. 1 These assignments are well supported by core-level shift (CLS) calculations as described later. Figure S1. Take-angle dependence of Pd 3d 5/2 level taken for a clean Pd 70 Au 30 (111) surface with emission angles of 90º and 50º from the surface parallel. From the angular dependence of XP spectra, the fraction of Pd atoms in each layer in near-surface region is quantitatively estimated using the following equations; A S A B = A S A B = n=3 χ n n=3 χ n χ 1 exp (, (n 1)d λ cos θ ) χ 2 ( d λ cos θ ), (n 1)d exp ( λ cos θ ) S2

3 where A S, A S and A B are the XP component intensities for the 1st, 2nd and bulk layers, respectively, and χ n is the atomic fraction of Pd in the n-th layer, of which signal is attenuated depending on the depth. d, λ and θ indicate the interlayer distance (2.32 Å), inelastic mean free path (4.75 Å), and take-off angle (50º and 90º), respectively. The fraction of Pd in the bulk χ n (n 3) is fixed to 0.70, being the same as that of the Pd 70 Au 30 alloy. With numerical curve fitting analysis, the Pd fractions in the 1st (χ 1 ) and 2nd (χ 2 ) layers were determined as 32±3% and 33±1%, respectively. Since the obtained atomic fraction values are independent of take-off angle, the photoelectron diffraction effects on the values seem rather small. This is partly because the photoelectron kinetic energy used here (95 ev) is such low that it induces no significant effect via forward scattering. The formation of Au-rich surface layers is consistent with the previous results for Pd 70 Au 30 (111). 2 The Au segregation is due to the lower surface free energy of Au. S3

4 2. CO adsorption on Pd 70 Au 30 (111) surface From comparison of C 1s peak intensities of CO between Pd(111) and Pd 70 Au 30 (111), the CO saturated coverage under ultrahigh vacuum (UHV) conditions is estimated to be monolayer (ML) on the Pd 70 Au 30 (111) surface, where the CO saturated coverage on Pd(111) is assumed as 0.50 ML 3. Figure S2. C 1s XP spectra taken from CO-saturated Pd(111) and Pd 70 Au 30 (111) surfaces under UHV conditions at 298 K. Under 10 mtorr CO, the Au 4f 7/2 peak shifts to the higher binding energy side. It is noted that CO exposure to a Au(111) surface up to 100 mtorr causes no change in the Au 4f level, suggesting that the CO molecules are not stably adsorbed on the Au(111) surface. Thus this shift is caused by the presence of Pd. At the moment it is unclear whether this shift is due to direct adsorption of CO on surface Au atoms in the vicinity of Pd or due to electronic effects of CO-binding Pd on surrounding Au atoms. 4 Figure S3. Au 4f 7/2 XP spectra taken from a Pd 70 Au 30 (111) surface before and under 10 mtorr CO exposure. S4

5 The absence of irreversible pressure-induced segregation of Pd atoms to the surface layer was checked by the CO exposure up to 1 Torr. Figure S4 shows the XP spectra of a CO-saturated Pd 70 Au 30 (111) surface under UHV taken before and after CO gas exposure at 1 Torr and subsequent evacuation. Here the background pressure was reduced below 10 8 Torr. The spectra were normalized by peak area of Pd 3d 5/2 to cancel out the attenuation by residual CO. The spectral shapes of Pd 3d 5/2 and Au 4f 7/2 levels are essentially unchanged. After the 1 Torr gas exposure, the Au 4f 7/2 peak area slightly decreases (~2.5%), suggesting that irreversible segregation of a small amount of Pd to the surface. The enhancement of shoulder peak of Pd 3d 5/2 also suggests a small increase of surface Pd atoms. However, since these changes are rather small, it can be considered that the surface structure primarily remains unchanged after the high-pressure CO exposure. The slight increase in the surface Pd may occur exclusively at irregular sites such as steps and defects. Figure S4. Pd 3d 5/2 (a) and Au 4f 7/2 (b) XP spectra of a CO-saturated Pd 70 Au 30 (111) surface under UHV taken before and after CO gas exposure at 1 Torr and subsequent evacuation. S5

6 3. CO adsorption on Pd(111) surface The CO adsorption on Pd(111) surfaces is one of typical adsorption model systems to understand adsorbate-substrate interactions. Therefore it has been extensively investigated over wide ranges of pressure and temperature using various surface science techniques. Figure S5 shows the CO adsorption behavior under different pressure conditions at 298 K. The clean (CO-free) surface exhibits two components in Pd 3d 5/2 level (a), which reflect surface (S) and bulk (B) Pd atoms. CO exposure causes a shift of the surface component to the higher-energy side (c). Curve fitting analyses lead us to conclude that CO molecules are adsorbed at idge and hollow sites, which is consistent with C 1s level (d). The CO coverage is estimated to be 0.50 ML. 3 Under 10 mtorr CO, another component appears at the higher-energy side, which is associated with CO adsorption on top sites (e and f). This is in good agreement with a previous result performed under a similar pressure condition. 5 A hump structure at around 284 ev is due to a carbon contamination. These peak assignments are supported by CLS calculations mentioned below. Figure S5. Pd 3d 5/2 and C 1s XP spectra taken from Pd(111) under different CO pressures at 298 K. All the XP spectra are curve fitted, and each component is labelled by B, S and Pd x in Pd 3d 5/2 and h, and t in C 1s, of which peak position and assignment are given in Table S4. S6

7 4. Optimized geometric parameters and corresponding adsorption energies Optimized geometric structures and averaged adsorption energies (E ad ) are obtained for CO/Pd(111), CO/Au(111) and CO/Pd 75 Au 25 (111) systems using density functional theory (DFT) calculations. The averaged adsorption energy is estimated via the following equation, E ad = E CO/metal E metal N CO E CO N CO, where E CO/metal, E metal and E CO are the total energies of CO-adsorbed metal, bare metal and isolated CO molecule in vacuum, respectively. N CO is the number of CO molecule included in the system. The CO molecules could be found stable at typical high-symmetry sites. The results are summarized in Table S1. Table S1. Optimized geometric structures and corresponding adsorption energies of CO adsorbed on Pd(111), Au(111) and Pd 75 Au 25 (111). The CO molecules are located at high symmetry sites; hollow (h), idge () and top (t) sites. d M C (M= Pd, Au) and d C O are the bond lengths, whereas, h M C is the difference in vertical height. The CO molecules are perpendicularly aligned from the surface parallel. The CO coverage is 1/4 ML for CO/Pd(111) and CO/Au(111), and 1/16 ML for CO/Pd/Pd 75 Au 25 (111) and CO/Au/Pd 75 Au 25 (111). For the alloy systems, the fractions of Au atom in the 2nd and bulk layers are modelled by 75 and 25%, respectively. system Site d M C (Å) d C O (Å) h M C (Å) E ad (ev) CO/Pd(111) h t CO/Au(111) h t CO/Pd/Pd 75 Au 25 (111) CO/Au/Pd 75 Au 25 (111) h t h t S7

8 Because the alloy surface consists of Pd and Au atoms, Pd-Au bi-elemental sites are available for CO adsorption. In order to check the possibility that such adsorption contributes significantly to the CO adsorption behaviour on Pd 70 Au 30 (111), the adsorption energies on the bi-elemental sites are calculated as shown in Table S2. CO adsorption energies on hollow sites of Pd 2 Au 1 and Pd 1 Au 2 units are estimated to be 1.37 and 0.84 ev, respectively. Considering the adsorption energies on the idge site of Pd 2 ( 1.50 ev) and the top site of Pd 1 ( 1.15 ev), the mono-elemental sites are energetically preferred compared to the bi-elemental sites. Similarly CO adsorption at idge site of Pd 1 Au 1 unit is avoided, since the top site of Pd 1 is more favourable. Table S2. CO adsorption energies at high symmetry sites of bi-elemental units, Pd 2 Au 1, Pd 1 Au 2 and Pd 1 Au 1. system site E ad (ev) CO/Pd 2 Au 1 /Pd 75 Au 25 (111) CO/Pd 1 Au 2 /Pd 75 Au 25 (111) CO/Pd 1 Au 1 / Pd 75 Au 25 (111) h h Site switching from the multiple-coordination (idge and hollow) sites to the single-coordination (top) site of Pd ensemble is experimentally observed under high-pressure conditions. The adsorption energies of single-coordination sites are summarized in Table S3. If two or three CO molecules are adsorbed at adjacent top sites of contiguous Pd, the steric repulsion should give rise to tilting away from each other. In fact the absolute value of adsorption energy of the tilted configuration (7 from the surface normal) of the 2CO(t)/dimer-Pd is increased by 0.08 ev from that of the perpendicular geometry. A similar stabilization via tilting is confirmed also for the 3CO(t)/trimer-Pd system. Table S3. Adsorption energies of the single-coordination (top) sites of contiguous Pd when two and three CO molecules occupy the top sites of dimer-pd and trimer-pd, respectively, with and without tilting of the CO molecular axes from the surface normal. system α ( ) E ad (ev) 2CO(t)/dimer-Pd 3CO(t)/trimer-Pd (fixed) (fixed) S8

9 5. Calculated core-level shifts and comparison with the observed ones The XP spectra taken from the Pd-Au alloy surfaces are deconvoluted into several components as shown in Figure 1 and 2 in the text. Each component was assigned based on its core-level shift (CLS), which is influenced depending on the chemical environment induced by CO adsorption. The CLSs were estimated by DFT calculations using the Slater-Janak transition state approximation 6 described below; [E(n i 1) E(n i )] = n i n i 1 E dn ε n i (n i 1 2 ), = [E(n i 1) E(n i )] [E ref. (n i 1) E ref. (n i )]. The binding energy of particular core-level i is defined by the difference of total energies of the ground (E(n i )) and photo-excited (E(n i 1)) systems, where n i is the number of electrons occupying the core-level i. In the Slater-Janak transition state approximation, the binding energy is approximated by the Kohn-Sham eigenvalue (ε i ) with a half occupation. The CLS ( ) is defined as a shift of binding energy with respect to that of the reference species; for instance, the bulk Pd atom is the reference for Pd 3d level. The calculated and experimental CLSs for Pd 75 Au 25 (111) are listed in Table S4 together with those for Pd(111). The experimental CLS values are deduced from the XP spectra shown in Figures 1, 2 and S5. Table S4. Comparison between calculated and experimental CLSs. For the clean surfaces, the CLSs of the 1st layer (S) and the 2nd layer (S ) atoms are calculated. For CO adsorbed surfaces, CO molecules are assumed to locate at hollow (h), idge () and top (t) sites. The CO-binding Pd atoms are labelled as Pd x, where x is the coordination number of CO to the Pd atom. The reference species of Pd 3d and C 1s XPS are the bulk Pd and CO adsorbed at the hollow site, respectively. system level label calcd CLS (ev) exp. CLS (ev) Pd(111) CO/Pd(111) Pd 75 Au 25 (111) CO/Pd 75 Au 25 (111) Pd 3d Pd 3d C 1s Pd 3d Pd 3d C 1s S S Pd 1/3 Pd 1/2 Pd 1 t S S Pd 1/3 Pd 1/2 Pd 1 t 0.26 ~ a b a the Pd atom in the1st layer is modelled by an isolated (monomer) Pd bonding with two internal Pd atoms. b the Pd atom in the 2nd layer is modelled by an isolated Pd atom underneath the Au surface layer. S9

10 6. Surface free energy of clean and CO adsorbed Pd-Au alloy The surface free energies (γ) of CO/Pd-Au alloy under the presence of CO gas are evaluated using the atomistic thermodynamics framework based on DFT calculation. 7 A surface structure with the lowest surface free energy is preferentially formed under a certain condition. The surface free energy is defined as follows; γ = 1 A [G slab(t, p CO, N CO, N slab, χ Au, χ Pd ) N slab χ Au μ Au N slab χ Pd μ Pd N CO μ CO (T, p CO )]. The surface free energy is defined as normalized Gibbs free energy by surface area A. Then G slab is the Gibbs free energy of a slab with the surface area A at a given temperature (T) and a pressure (p). N slab is the number of atoms included in the slab and N CO is the number of CO adsorbed on the slab surface. χ Au and χ Pd are the mole fractions of Au and Pd, respectively, of the alloy (i.e. χ Au + χ Pd = 1). μ Au, μ Pd and μ CO are the chemical potentials of Au, Pd and CO, respectively. The chemical potential of bulk reservoir of the alloy (μ bulk ) can be expressed by the chemical potentials of metal species included in the alloy (μ Au and μ Pd ) as shown below. χ Au(bulk) and χ Pd(bulk) are the mole fraction of the bulk alloy. However, surface segregation causes a difference in the surface and bulk stoichiometries, which alters μ Au, μ Pd and μ bulk, and hence the difference of the chemical potentials μ Au Pd (= μ Au μ Pd ). Destruction of the bulk alloy proceeds when the chemical potentials of each component exceeds the potential of pure metal, μ Au > μ Au,pure or μ Pd > μ Pd,pure. The range of μ Au Pd where the alloy can be stably maintained is described as follows; μ bulk = χ Au(bulk) μ Au + χ Pd(bulk) μ Pd = χ Au(bulk) (μ Au μ Pd ) + μ Pd = χ Au(bulk) μ Au Pd + μ Pd, μ Au Pd(Au rich) μ Au Pd μ Au Pd(Pd rich), μ Au Pd(Au rich) = ( μ Au,pure μ bulk 1 χ Au(bulk) ), μ Au Pd(Pd rich) = ( μ bulk μ Pd,pure 1 χ Pd(bulk) ). Whereas, the chemical potential of CO on the surface in equiliium with the background CO gas with a CO pressure of p CO can be expressed by the chemical potential of gas-phase CO as follows; μ CO (T, p CO ) = E total CO + μ CO (T, p CO ) = E total CO + μ CO (T, p 0 ) + k B T ln ( p CO p 0 ) total where k B is the Boltzmann constant, and E CO is the total electron energy of isolated CO molecule. μ CO (T, p 0 ) is obtained from tabulated enthalpy and entropy values at the standard pressure p 0 (= 1 atm). 8 This equation means that the surface free energy of the system changes as a function of S10

11 pressure-dependent CO chemical potential. Taking into accounts of above equations, the surface free energy is rewritten as follows; γ = 1 A [G slab N slab μ bulk N slab μ Au Pd (χ Au χ Au(bulk) ) N CO (E total CO + μ CO (T, p 0 ) + k B T ln ( p CO p 0 ))] G slab and N slab μ bulk can be approximated as the total electron energy of the slab with and without CO adsorption, respectively, because in the case of solid phase the entropic contribution (viational free energy and configuration entropy) to the Gibbs free energy is negligibly small. 7 The third term in the above equation accounts for a possible difference in the surface and bulk stoichiometries. Figure S6 shows the surface free energy for different surface stoichiometries of clean Pd-Au alloy calculated with μ Au Pd values obtained under the Au- and Pd-rich conditions. The bulk alloy is modelled by L1 2 crystal structure with an atomic ratio of Au : Pd = 25 : 75. To check surface segregation, the fraction of Au atom in the 1st and the 2nd layers are varied from 0 to 100%. The most stable structure is covered by Au in the 1st layer, irrespective of μ Au Pd. This result clearly supports preferential Au segregation to the surface from the viewpoint of surface free energy. Figure S6. The surface free energy (γ) of Pd 75 Au 25 (111) as a function of Au fraction in the 1st and 2nd layers under the Au-rich (a) and Pd-rich (b) conditions. The black dots indicate the sampling points for γ value. The most stable surface composition is marked by white arrow. S11

12 As shown in Figure 4 in the text, the CO molecules at idge sites of the dimer Pd ensemble switch to top sites of the ensemble with accommodating more top CO from the gas phase under high pressure conditions. Figure S7 shows surface free energy changes of CO-adsorbed trimer Pd as a function of CO pressure. At low pressures, the CO preferentially adsorbs at the hollow site of the trimer Pd ensemble, while the presence of high-pressure CO induces a similar site switching to the top site occupation. As discussed above, the steric repulsion between the top CO molecules causes tilting of the molecular axes away from each other. Figure S7. Simulated relative surface free energies γ for two CO adsorbed systems, CO(h)/trimer-Pd and 3CO(t)/trimer-Pd, with respect to that for the clean surface as a function of CO pressure at 300 K. Structure models for the two adsorption systems are shown in figures. The yellow, blue, gray and red balls are Au, Pd, C and O atoms, respectively. The fractions of Au atom in the 1st, 2nd and bulk layers are modeled by 75, 75 and 25%, respectively. S12

13 7. References [1] Walle, L. E.; Grönbeck, H.; Fernandes, V. R.; Blomberg, S.; Farstad, M. H.; Schulte, K.; Gustafson, J.; Andersen, J. N.; Lundgren, E.; Borg, A. Surface Composition of Clean and Oxidized Pd 75 Ag 25 (100) from Photoelectron Spectroscopy and Density Functional Theory Calculations. Surf. Sci. 2012, 606, [2] Piccolo, L.; Piednoir, A.; Bertolini, J. C. Pd Au Single-Crystal Surfaces: Segregation Properties and Catalytic Activity in the Selective Hydrogenation of 1,3-Butadiene. Surf. Sci. 2005, 592, [3] Surnev, S.; Sock, M.; Ramsey, M. G.; Netzer, F. P.; Wiklund, M.; Borg, M.; Andersen, J. N. CO Adsorption on Pd(111): A High-Resolution Core Level Photoemission and Electron Energy Loss Spectroscopy Study. Surf. Sci. 2000, 470, [4] Gauthier, Y.; Schmid, M.; Padovani, S.; Lundgren, E.; Buš, V.; Kresse, G.; Redinger, J.; Varga, P. Adsorption Sites and Ligand Effect for CO on an Alloy Surface: A Direct View. Phys. Rev. Lett. 2001, 87, (1) (4). [5] Kaichev, V. V.; Prosvirin, I. P.; Bukhtiyarov, V. I.; Unterhalt, H.; Rupprechter, G.; Freund, H.-J. High-Pressure Studies of CO Adsorption on Pd(111) by X-ray Photoelectron Spectroscopy and Sum-Frequency Generation. J. Phys. Chem. B 2003, 107, [6] Zeng, Z.-H.; Ma, X.-F.; Ding, W.-C.; Li, W.-X. First-Principles Calculation of Core-Level Binding Energy Shift in Surface Chemical Processes. Sci. China Chem. 2010, 53, [7] Kitchin, J.; Reuter, K.; Scheffler, M. Alloy Surface Segregation in Reactive Environments: First-Principles Atomistic Thermodynamics Study of Ag 3 Pd(111) in Oxygen Atmospheres. Phys. Rev. B 2008, 77, (1) (12). [8] Chase, M. W., Jr. NIST-JANAF Thermochemical Tables, 4th ed; American Chemical Society: New York and American Institute of Physics for the National Institute of Standards and Technology: Woodbury, N. Y., 1998; 641. S13