J. Phys. Chem. C 2009, 113, 1411–1417
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Molecular and Atomic Hydrogen Interactions with Au-Ir Near-Surface Alloys Peter A. Ferrin,†,# Shampa Kandoi,†,‡,# Junliang Zhang,§,| Radoslav Adzic,*,§ and Manos Mavrikakis*,† Department of Chemical and Biological Engineering, UniVersity of WisconsinsMadison, 1415 Engineering DriVe, Madison, Wisconsin 53706, and Materials Science Department, Building 555, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: May 27, 2008; ReVised Manuscript ReceiVed: October 11, 2008
Using a combination of density functional theory and experimental electrochemical methods, we have investigated Au-Ir near-surface alloys and their ability to take up hydrogen (H). Despite the relative instability of H in the subsurface of pure Au(111) and Ir(111), H in the subsurface of a near-surface alloy made of a monolayer of Au over Ir(111) (Au*/Ir) is stabilized compared to surface H. While dissociative adsorption of H2 is activated on this alloy surface, the presence of subsurface H stabilizes the transition state for further H2 dissociation. This is explained by the upshift in the d-band center of the surface Au atoms induced by the presence of subsurface H in Au*/Ir. Submonolayers of Au on Ir(111) stabilize H at the Au-Ir interface while allowing for nonactivated H2 dissociation at the exposed Ir atoms. Synthesis of submonolayer alloys of Au on Ir(111) shows that Au does not block hydrogen adsorption on the Ir surface, despite the inability of pure Au to take up hydrogen under these conditions. The possible existence of subsurface hydrogen in these alloys may explain this unexpected behavior. Introduction The adsorption and absorption of hydrogen on and in transition metals is of both theoretical and practical interest. The interaction between hydrogen and metals is of major significance for production of many chemicals and pharmaceuticals and hydrogen storage technologies.1 In addition, these interactions play a key role in hydrogen fuel cells electrocatalysis.2 Recent studies have shown that a new class of alloys, called near-surface alloys (NSAs), has promise for hydrogen-related applications.3-7 Some of these alloys bind atomic hydrogen (H) as weakly as the noble metals, while retaining the ability to dissociate molecular hydrogen (H2) much easier than the noble metals. The weak H binding can make subsequent steps in chemical reactions easier, allowing for lower reaction temperatures on NSAs. Lower reaction temperatures, in turn, provide opportunities for increased reaction selectivities and energy savings. NSAs involving Au are of special interest. Au-Ni surface alloys have been shown to possess increased resistance to coking in steam-reforming, which is attributed to the electronic structure changes of Ni in the presence of Au.8-11 Au-Pd surface alloys show good catalytic activity for many reactions, such as acetylene conversion to benzene,12 vinyl acetate synthesis,13-15 and ethylene hydrogenation.16 The alloying of Au on Pd has some other interesting propertiessit stabilizes adsorbed acetate while retarding its decomposition to carbon.17 One very interesting system investigated for its anomalous interactions with hydrogen is Au-Ir. Okada et al. recently * Corresponding authors. E-mail addresses:
[email protected]; manos@ engr.wisc.edu. † University of WisconsinsMadison. ‡ Present address: UTC Power Corporation, South Windsor, CT 06074. § Brookhaven National Laboratory. | Present address: General Motors Corporation, Fuel Cell Research Laboratory, 10 Carriage Street, Honeoye Falls, NY 14472. # These authors contributed equally to the work.
reported experiments where 1-10 ML of Au on an Ir single crystal surface is able to dissociate H2,18,19 despite the inability of bulk Au to do so. In addition, they suggest that hydrogen can be trapped in the interface between Au and Ir, if hydrogen is first dissociated on Ir and Au is overlaid on the Ir-H system.19-21 Contrary to this behavior, they reported that Au overlayers on Pt were not able to dissociate H2.22 In this work, we use density functional theory (DFT) to examine the relative stability of surface and subsurface hydrogen in Au-Ir model systems. To provide possible explanations for the anomalous results of Okada et al., we then study H2 dissociation on and atomic H diffusion in these model systems. We identify Au-Ir alloys that have the ability to (1) dissociate H2 easily and (2) store hydrogen in their subsurface. Furthermore, using experimental electrochemical techniques, we deposit Au on an Ir(111) surface. Confirming our first-principles-based predictions, Au deposited on Ir(111) is shown not to inhibit the uptake of hydrogen by iridium. Methods Theoretical. DACAPO, the total energy calculation code,23,24 is used throughout this study. We model the Au(111) and Ir(111) surfaces using a slab consisting of four layers (five for subsurface H calculations) of metal atoms, periodically repeated in a supercell geometry, with the equivalent of five layers of vacuum between any two successive metal slabs. The top two (three for five-layer slabs) layers in a metal slab are allowed to relax. The calculated minimum-energy lattice constants for Ir and Au are 3.86 Å25,26 and 4.18 Å,27 respectively, in good agreement with the corresponding experimental values (3.84 and 4.08 Å, respectively).28 Calculations are performed on a 2 × 2 surface unit cell. To create the near-surface alloy consisting of one monolayer of Au over Ir (hereafter referred to as Au*/Ir), the top layer of Ir atoms is replaced by Au atoms. Ionic cores are described by Vanderbilt ultrasoft pseudopotentials,29 and the Kohn-Sham one-electron valence states are expanded on the
10.1021/jp804758y CCC: $40.75 2009 American Chemical Society Published on Web 01/08/2009
1412 J. Phys. Chem. C, Vol. 113, No. 4, 2009 basis of plane waves with a kinetic energy cutoff of 25 Ry. The surface Brillioun zone is sampled at 18 special Chadi-Cohen k-points.30 The exchange-correlation energy and potential are described by the PW91 generalized gradient approximation.31 The electron density is determined self-consistently by iterative diagonalization of the Kohn-Sham Hamiltonian, Fermi population of the Kohn-Sham state (kBT ) 0.1 eV), and Pulay mixing of the resulting density. All total energies are extrapolated to absolute zero. To calculate minimum energy paths for H2 dissociation and atomic H diffusion, the climbing image nudged elastic band (NEB) method is used.32 Vibrational frequencies are calculated for each transition state to ensure the existence of a single negative mode. Vibrational frequencies are calculated by numerical differentiation of forces using a second-order finite difference approach with a step size of 0.015 Å for the hydrogen atoms. During these frequency calculations, all metal atoms are kept fixed at the respective transition state coordinates. The Hessian matrix is mass weighted and diagonalized to yield frequencies and normal modes of the corresponding systems.33 Experimental. Single crystals of Ir(111), 8 mm in diameter and 3 mm thick, used as working electrodes, were obtained from Metal Crystals and Oxides, Cambridge, England. The single crystal surfaces were oriented to better than 0.2°. Crystals were polished with diamond pastes and alumina powder, from 1 µm gradually down to 0.05 µm. After polishing, crystals were annealed using an induction heating system (HU 2000, Himmelwerk, Tubingen, Germany). The crystal temperature could reach 2000 °C in 60 s and was monitored using a pyrometer. Before annealing, ultrapure argon gas was purged for half an hour to ensure an oxygen-free atmosphere. After annealing, the crystal was cooled down and protected by a drop of ultrapure water before it was quickly transferred into the electrochemical cell. Au submonolayer quantities (or clusters) were deposited on the Ir(111) single crystal by using a novel method of galvanic displacement of a Cu adlayer by Au. The procedure involved two steps: (1) Cu was deposited on the substrate metal using underpotential deposition (UPD) from a solution containing 50 mM CuSO4 and 50 mM H2SO4; (2) the Ir electrode surface covered by the Cu adlayer was emersed from the solution, rinsed with water to remove Cu2+ from the solution film, and placed into a (AuCl4)- solution. Since Au has more positive reduction potential than Cu, the desired quantity of Au was deposited on the substrate metal by displacement of Cu. Following 1 min of immersion, the electrode was rinsed again. After deposition, the crystal was transferred into an electrochemical cell containing 0.1 M HClO4 solution. All these procedures were carried out in a multicompartment electrochemical cell purged with Ar. We note here that due to the different valence state of Cu in Cu2+ and Au in (AuCl4)-, a 2/3 monolayer of Au can be deposited on an Ir(111) surface by this method. A full monolayer of Au was deposited on Ir(111) by direct electrochemical deposition under diffusion control conditions. The Ir(111) surface in contact with the electrolyte formed a meniscus, which facilitates the exposure of the desired plane and a clean and quick formation of the cell. The solution contained 5 µM of (AuCl4)- in 50 mM H2SO4. The potential of the Ir single crystal was held at 0.3 V until the charge corresponding to a full monolayer Au was collected. After deposition, the crystal was rinsed with water and transferred into an electrochemical cell containing 0.1 M HClO4 solution. The electrochemical measurements were taken at 25 °C. A leak-free Ag|AgCl, 3MCl- reference electrode was used with a
Ferrin et al. double junction reference chamber (Cypress, Lawrence, KS). All potentials are given with respect to a reversible hydrogen electrode (RHE) unless otherwise specified. A platinum flag was used as counter electrode. The hydrogen uptake was calculated from voltammetric curves in 0.1 M HClO4 solution, by integrating the charge associated with the hydrogen adsorption/ desorption process, corrected for charging of the double layer capacitance. Results and Discussion Complete Monolayer of Au on Ir (Au*/Ir). Hydrogen Stability. The binding energy (BE) and geometric information for atomic hydrogen (H) at 1/4 ML coverage on Au(111), Ir(111), and Au*/Ir, are given in Table 1. For the Au*/Ir system, the surface bridge site is the most stable (BE ) -2.00 eV); however, the surface fcc, hcp, and top sites (see Figure 1 for sites) are almost isoenergetic to that (BE of -1.98, -1.92, and -1.90 eV, respectively). On Ir(111), we find the top site to be the most stable (BE ) -2.73 eV), by more than 0.10 eV over other sites. On Au(111), the fcc site is the most stable (BE ) -2.22 eV) although the hcp site is nearly isoenergetic (BE ) -2.17 eV). Preferred adsorption sites and binding energies are in good agreement with both past theoretical4,26 and experimental34 work. Unlike on Au and Ir, the presence of surface hydrogen leads to a significant corrugation of the Au*/Ir surface. The binding energy of surface H on Au*/Ir is lower than that on either pure Au or pure Ir. This lower binding energy is anticipated by the downward d-band center shift of the Au overlayer, as calculated by Ruban et al.,35 as well as in this work. For all adsorption sites, the H-surface distance on Au*/ Ir is larger than that on pure Ir (see Table 1). On the pure Au(111) and Ir(111) surfaces, H in the first subsurface layer (BE of -1.47 and -1.15 eV, respectively) is much less stable than surface H (BE of -2.22 and -2.73 eV, respectively). However, on Au*/Ir(111), H at the Au-Ir interface is slightly more stable than surface H (BE of -2.05 eV versus -2.00 eV, respectively). As with surface H, the presence of subsurface hydrogen corrugates the Au overlayer significantly without much change in the Ir first subsurface layer in Au*/Ir. Subsurface H in pure Au and Ir also creates some surface corrugation, although the magnitude of the corrugation as measured by the displacement of atoms in the same layer in the direction perpendicular to the surface is smaller. The H-metal bond length at the Au*/Ir interface shows that the H-Au distance is much larger than the H-Ir distance. In fact, the H-Ir distance is very similar to that of subsurface H in pure Ir and toward the second layer of Ir atoms. While H at the Au-Ir interface is more stable than hydrogen in the first subsurface of either Au or Ir, this is not the case in the second subsurface layer. For Au*/Ir(111), H in the second subsurface (between two iridium layers) is ca. 0.7 eV less stable than H at the Au-Ir interface. That state is somewhat less stable than H in the second subsurface layer of Au(111), although somewhat more stable than H in the second subsurface of Ir(111). The stability of subsurface H in the Au*/Ir alloy helps to explain the results of Okada et al.21 Using nuclear reaction analysis, they probe the location of hydrogen in a system of Au deposited on an Ir single crystal; they find that H can be trapped at the subsurface. This is energetically extremely unfavorable in pure Au or Ir; it is much more stable at the interface of this Au*/Ir model. Hydrogen Diffusion. The potential energy surface for the diffusion of hydrogen from the surface into the first and second subsurface layers of Au(111), Ir(111), and Au*/Ir(111) is shown
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TABLE 1: Binding Energies and Structural Data for Surface and Subsurface Hydrogen on and in Au(111), Au*/Ir(111), and Ir(111)a H site Au*/Ir (all sites) hcp fcc top bridge 1st subsocta 1st substetrahcp 1st substetratop 2nd subsocta 2nd substetrahcp 2nd substetratop pure Ir (best site only) top first substetratop 2nd subsocta pure Au (best site only) fcc first substetratop second substetratop
binding energy (eV)
dH-TransitionMetal (Å)
δzTM (Å)
-1.92 -1.98 -1.90 -2.00 -1.68 unstable -2.05 -1.32 -1.21 -0.99
1.89 1.89 1.61 1.77
0.14 0.20 0.17 0.74
2.46
1.83
0.36
0.01
2.40 1.78 1.67 1.67
1.87 2.22 1.84 1.85
0.90 0.04 0.07 0.32
0.01 0.00 0.10 0.06
1.89 1.92
0.34 0.03
0.09 0.07 0.02
1.94 1.91
0.23 0.14
0.09 0.01 0.03
-2.73 -1.15 -1.18
1.62 2.03
-2.22 -1.47 -1.47
1.89 1.89
1.59
1.88
a H-coverage is always 1/4 ML (θH ) 0.25 ML). octa ) octahedral, tetrahcp ) tetrahedral site (under hcp); tetratop ) tetrahedral under top. The reference state is the clean slab and a gas phase hydrogen (H) atom. The calculated H2(g) bond energy is 4.57 eV. dH-TransitionMetal is the distance between the H atom and the closest transition metal atom. The first number gives the distance to the nearest metal atom in the plane above the hydrogen; the second number gives the distance to the nearest metal atom in the plane below the hydrogen. δzTM is the distance perpendicular to the surface between the highest and lowest metal atoms in the same layer, a measure of the corrugation of the surface (a value of 0 for δzTM indicates no corrugation). The first number is the plane above the hydrogen atom; the second number is the plane below the hydrogen atom.
Figure 1. Schematics of surface and subsurface chemisorption sites. Absorption sites are directly below the surface sites indicated in the tables. Further explanation of subsurface absorption sites can be found in ref 33.
in Figure 2. Diffusion of H into the first subsurface layer of Ir(111) has an activation energy barrier of 1.68 eV, whereas in Au(111) that barrier is only 0.83 eV. Diffusion of H into the Au-Ir interface of Au*/Ir(111) has a much smaller barrier (0.39 eV), which is similar to that of Pd(111).36 Further diffusion into the second subsurface layer of Au*/Ir has a barrier of 0.98 eV. While that barrier is smaller than that of diffusion into the first subsurface of pure Ir(111), the energy of the respective transition states relative to gas phase hydrogen is similar; see Figure 2. The geometric characteristics of the H diffusion path for all the systems studied are similar. Hydrogen migrates from a surface fcc site to an octahedral site directly underneath and then to the tetrahedral under-top site in the first subsurface layer. Further diffusion into the bulk occurs by movement to an octahedral site in the second subsurface layer. In the case of Au(111) and Au*/Ir(111), there is further diffusion into the second subsurface tetrahedral under-top site, which is a more stable site. Preceding this diffusion into Ir(111) and Au*/Ir(111), there is surface diffusion from the top and bridge site, respectively, to the fcc site. Reverse diffusion from the first subsurface to the surface for Au*/Ir(111) is much more difficult than diffusion from the
Figure 2. Potential energy surface for H diffusion into Au(111), Au*/ Ir(111), and Ir(111). 1/2 H2(g) with the corresponding clean slab are the reference states. If a sequence of transition states exist within the diffusion path of H between two layers, only the highest transition state is reported. TS1 is the transition state between surface and first subsurface layer; TS2 is the transition state between the first and second subsurface layers. θH ) 0.25 ML. Cartoons show the relative positions of hydrogen along the diffusion pathway.
subsurface to the surface in Au(111) and Ir(111). The activation energy barrier for Au*/Ir is 0.44 eV; for Au(111), 0.08 eV; for Ir(111), 0.13 eV. While at high temperatures, H resurfacing will be relatively easy on all of these surfaces, at lower temperatures hydrogen that is in the subsurface of Au*/Ir(111) is much more likely to be kinetically trapped than in the subsurface of either of the pure metals, in agreement with the findings of Okada et al.20
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TABLE 2: Barriers to H2 Dissociation on Selected Au-Ir(111) Systemsa
system
Ea (H2 dissociation) (eV)
d-f of surface atom (eV)
Ir Au Au (1/4 ML Hsub) Au*/Ir Au*/Ir (1/4 ML Hsub) Au*/Ir (1/2 ML Hsub) Au0.75*/Ir Au0.75Ir0.25*/Ir
0b 0.96 0.94 1.05 0.85 0.57 0.65 0b
-2.85 (Ir) -3.33 (Au) -3.62 (Au) -4.10 (Au) -3.02 (Au) -3.05 (Au) -3.28 (Au) -4.07 (Au); -2.49 (Ir)
Ea is the activation barrier for H2 dissociation. d-f is the weighted center of the d-band with respect to the Fermi level. The label in parentheses refers to which type of atom’s d-band center is reported. b Dissociation takes place on an Ir surface atom. a
Figure 4. Activation energy (Ea) for H2 dissociation on Au(111) and Au*/Ir(111) as a function of subsurface hydrogen coverage (θHsub).
Figure 5. Side and top views of submonolayer models. On the far left is the “pinhole” model, with a Au coverage of 0.75. To the right are submonolayer alloys with Au:Ir ratios of 0.75:0.25, 0.5:0.5, and 0.25:0.75, respectively. Gold spheres indicate Au atoms; green spheres indicate Ir atoms.
Figure 3. D-bands of surface Au in Au(111) (in black) and Au*/Ir(111) (in red), both clean (solid lines) and with 1/4 ML of subsurface hydrogen (dashed line, respective colors). The vertical line indicates the position of the d-band center for each system. The number immediately to the right of the system name is the d-band center in eV. The d-band of surface Ir in pure Ir(111) (in blue) and 1/4 ML subsurface-hydrogen -modified Ir(111) surfaces (dashed line, blue) are also included. All values are referenced to the Fermi level.
H2 Dissociation. H2 dissociation on Au(111) is an activated process, with an activation energy barrier of 0.96 eV (see Table 2). On Au*/Ir(111), the activation energy barrier is somewhat larger, 1.05 eV, whereas on pure Ir(111), H2 dissociation is a spontaneous process. This increase in the activation energy for dissociation can be predicted by the d-band center of the Au surface atoms on the two surfaces: in the case of Au*/Ir, the d-band center of surface Au is lower than that of pure Au, because of geometric and electronic effects. As the Au layer is pseudomorphic to the Ir surface, Au atoms are compressed, leading to a downshift of the Au d-band.37,38 In addition, bonding of the Au surface atoms to the Ir atoms right below them leads to an additional downshift of the Au d-band.23 Our calculations support this picture (see Figure 3 and Table 2 for values of the d-band center). According to the theory proposed by Nørskov et al., the downshift in the d-band center would increase the barrier for H2 dissociation. Our results on the electronic structure of these surfaces are in agreement with those of Ruban et al.35 in their study of Au overlayers on transition metals. If there was exactly 1 ML of Au on top of Ir(111) in the work by Okada et al., then our results would indicate that in the absence of any defects and/or Ir atoms exposed to the surface no H2 dissociation
on Au*/Ir would occur. However, those experiments suggest that H2 dissociation occurs at relatively low temperatures on the Au*/Ir surface.18 Effect of Subsurface Hydrogen on Electronic Structure and Surface ReactiWity. Because of its increased stability in Au*/ Ir(111), subsurface H can have an effect on Au*/Ir(111) surface chemistry. Therefore, we investigate these effects here. In addition, this is of more general interest, as several other studies have pointed to the possible role of subsurface hydrogen as a reactive intermediate.39-42 First, we studied the electronic structure of Au*/Ir(111) with and without 1/4 ML of subsurface H, and compared this with the electronic structure of pure Au(111) with and without subsurface hydrogen (see Figure 3). As mentioned earlier, without subsurface hydrogen, the d-band center of the surface Au on Au*/Ir(111) is lower than that of pure Au (d-f is -4.10 and -3.33 eV, respectively). However, in the presence of 1/4 ML subsurface H, Au in the Au*/Ir(111) surface is strongly activated, with a d-band center upshift of ca. 1.1 eV. Pure Au(111), in contrast, shows a d-band center downshift of ca. 0.3 eV in the presence of 1/4 ML subsurface H. The large upshift in d-band center in the presence of subsurface H is partly due to the rearrangement of the surface Au atoms, resulting in a relaxation of the compressive strain induced on Au by Ir. While there is a general upshift in the d-band center of all peaks in the density of states of Au*/Ir with subsurface H versus that without subsurface H, the most pronounced difference in the electronic structure in the two systems is a large peak at ca. -2.4 eV, which develops in the presence of subsurface hydrogen. This peak is mainly due to a significant increase in the density of states of the dz2 component of the d-band caused by the presence of subsurface H; the
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Figure 6. Binding energy of surface and subsurface hydrogen versus Au coverage for Au-Ir surface alloys. The bars indicate the binding energy of surface and subsurface H (θH ) 0.25 ML) from a reference of gas phase atomic hydrogen. The black dots indicate the d-band center of the surface atoms with respect to the Fermi level. For the left four systems, the d-band centers are for Ir surface atoms. For Au0.75*/Ir(111), Au*/ Ir(111), and pure Au(111), the d-band centers indicated are for Au surface atoms.
Figure 7. Linear sweep voltammetry curves of Ir(111) (dark line) and of Ir(111) with 0.67 ML Au overlaid on top (red line) in Ar-saturated 0.1 M HClO4. The scan rate was 50 mV/s. The 0.67 ML Au was deposited by galvanic displacement of a Cu monolayer deposited at underpotential on Ir(111). The calculated H desorption charges on Ir(111) and Au (0.67 ML)/Ir(111) were 254 and 247 µC/cm2, respectively.
remaining four components of the d-band are less affected. This upshift found in the density of states in Au*/Ir(111) due to subsurface H is in contrast to the general downshift of all peaks in the d-band of pure Au(111) when subsurface H is present. Upon subsurface H absorption, pure Ir(111) shows a small upshift in its d-band center (ca. 0.3 eV), clearly not as dramatic as in Au*/Ir(111), and with the upshift distributed over all five d-band components. This change in the d-band center38 predicts that bond-breaking events on Au*/Ir surfaces could be easier in the presence than in the absence of subsurface hydrogen. Indeed, as shown in Figure 4, our DFT calculations suggest that the addition of 1/4 ML of subsurface H lowers the barrier to H2 dissociation by
0.20 eV on Au*/Ir(111). We have also studied H2 dissociation in the presence of more than 1/4 ML of subsurface H in that model system. With 1/2 ML of subsurface H, H2 dissociation is even more facile, with a barrier of only 0.57 eV, nearly 0.5 eV lower than that of clean Au*/Ir(111). Conversely, on pure Au, the presence of 1/4 ML of subsurface hydrogen has little effect on the barrier to H2 dissociation. Submonolayer Quantities of Au on Ir. While the existence of subsurface hydrogen can be explained by the stability of hydrogen at the Au-Ir interface in Au*/Ir(111), the high H2 dissociation barrier on the pure Au*/Ir(111) surface makes it an inadequate model to explain all the results of Okada et al. In fact, the change in electronic structure of the surface Au atoms due to the underlying Ir substrate makes H2 dissociation more difficult on Au*/Ir(111) than on pure Au(111). Even in the presence of subsurface hydrogen (Hsub), which stabilizes the H2 dissociation transition state somewhat on the Au*/Ir(111) nearsurface alloy, the barrier to H2 dissociation remains significant (0.85 and 0.57 eV for 1/4 and 1/2 ML Hsub, respectively). To explain the experimentally observed H2 dissociation better, we examine H2 dissociation on model systems characterized by local regions of submonolayer Au-Ir alloys, where some Ir atoms are directly exposed to the gas phase. In particular, we studied two models: (1) the “pinhole” model (Au0.75*/Ir), where one Au surface atom is removed from the 2 × 2 unit cell of the Au*/Ir(111) surface, and (2) the “submonolayer Au surface alloy” model (AuxIr1-x*/Ir), where between one and three of the Au surface atoms in the 2 × 2 unit cell of the Au*/ Ir(111) surface are replaced by Ir atoms; schematics of these models are shown in Figure 5. While in the bulk Au and Ir are immiscible, submonolayer surface alloys of other immiscible elements have been created, as shown in the work of Nørskov et al.,8,43,44 and, therefore, we anticipate that preparing surface alloys between Au and Ir would be possible. Even if these systems are not thermodynamically favorable, they can be used to describe possible local “defects” in the idealized NSA structure.
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Figure 8. STM images (200 × 200 nm) of (a) 1 ML of Au on Ir(111) deposited by direct electrochemical methods under diffusion control conditions and (b) 2/3 ML of Au on Ir(111) deposited by galvanic displacement of a Cu monolayer deposited at underpotential. The tunneling current was 1.24 nA.
Hydrogen Adsorption, Absorption, and Dissociation on the Pinhole Model (Au0.75*/Ir). The adsorption of H on and at the interface of the pinhole Au0.75*/Ir model system is somewhat different than its adsorption on Au*/Ir(111) (see Figure 6). H on the Au part of the surface of the pinhole model is somewhat stabilized (BE ) -2.19 eV) as compared to surface H on Au*/ Ir(111) (BE ) -2.00 eV), although it is less stable than H on pure Au(111) or Ir(111) (BE ) -2.22 and -2.73 eV, respectively). However, the most stable site for H in the pinhole model is in the “pinhole” created by the removal of the Au atom, with a BE of -2.29 eV. H binds with -1.71 eV at the Au-Ir (subsurface) interface of Au0.75*/Ir and is significantly destabilized with respect to the corresponding state at the Au*/Ir(111) system (BE ) -2.05 eV). H2 dissociation on the pinhole model Au0.75*/Ir is preferred at the vicinity of the missing surface Au atom and has a lower barrier (0.65 eV) than that on the clean Au*/Ir(111) (1.05 eV). However, dissociation is still activated. Thus, the pinhole model may not entirely explain the experimental results of Okada et al. Hydrogen Adsorption, Absorption, and Dissociation on Submonolayer Au Surface Alloys. Replacing some of the Au atoms of the Au*/Ir(111) surface layer with Ir atoms significantly changes the binding energy of H on that system. The binding energy of surface and subsurface H on several of these alloys (Au0.25Ir0.75*/Ir(111), Au0.50Ir0.50*/Ir(111), Au0.75Ir0.25*/Ir(111)), and for comparison on Au*/Ir(111), Au0.75*/Ir(111), and Au(111), is shown in Figure 6. Surface H is always most stable in the vicinity of surface Ir atoms in these model surfaces. Counterintuitively, the binding energy of surface and subsurface H increases somewhat with increasing Au coverage until there is no Ir on the surface. While pure Ir(111) has a surface H binding energy of -2.74 eV, a surface that is 25%, 50%, or 75% Au has a H binding energy of -2.77, -2.83, and -2.88 eV, respectively. The stabilization of subsurface H is even larger: pure Ir(111) binds subsurface H with -1.06 eV; this increases to -1.95 eV for Au0.75Ir0.25*/Ir(111). Binding at the Au-Ir interface of submonolayer alloys is also significantly stronger than binding in the subsurface of pure Au(111), which shows a binding energy of -1.47 eV. The stabilization of H on the surface of submonolayer Au-Ir alloys can be explained by changes in the surface electronic structure induced by the Au-Ir surface alloy formation. In particular, we find that there is an upshift in the d-band center of the remaining surface Ir atoms as Au is progressively alloyed into the surface (see Figure 6). In other words, Ir embedded in
a surface dominated by Au atoms is more reactive that Ir in an all-Ir surface. H2 dissociation on submonolayer Au-Ir surface alloys is not activated, as the Ir on the surface is adequate to allow for spontaneous dissociation, even in the limit of high Au coverages. This combination of non-activated dissociation with stabilized subsurface H better explains the experimental results of Okada et al.; according to our findings on these Au-Ir surface alloy model systems, there can be significant trapping of H at the Au-Ir (subsurface) interface as well as H2 dissociation taking place spontaneously on the non-noble Ir surface atoms. Experimental Hydrogen Adsorption on Au-Ir Alloys. Using the electrochemical methods explained above, 2/3 ML of Au was deposited on an Ir(111) surface. The peaks associated with hydrogen adsorption in voltammetry current-potential curves of the clean and Au-modified Ir(111) were integrated to obtain a measure of the hydrogen uptake by each one of the surfaces (see Figure 7). Assuming an atomically flat surface, the charge for 1 ML of H atoms on an Ir(111) surface is 264 µC/cm2; thus both the clean and Au-modified surface measured here contain ca. 0.95 ML of H atoms. While STM images show that a significant amount of Au has been deposited on the surface (see Figure 8b), there is only a very small change in hydrogen uptake between clean and Au-modified Ir surfaces, suggesting that Au does not block hydrogen uptake on Ir. We note that while the total uptake of H on the Ir(111) and Au-modified surface is nearly constant, there is a significant difference in the shape and features of the cyclic voltammagram, indicating that there is some effect of the Au on Ir, rather than a H-induced phase separation of the two components of the alloy. Depositing 1 ML of Au on the surface using the direct electrochemical methods under diffusion control conditions, we find little change in the hydrogen uptake. The STM images of 1 ML Au on Ir also show the existence of mostly 1 ML of Au with a small coverage of 2 ML Au islands, decorated by a large amount of pockets of Ir (see Figure 8a). The size of the Ir pockets ranges from less than 1 nm to around 10 nm. Therefore, our experimental results are in line with the theory-based arguments proposed above: the existence of even small amounts of Ir on the surface allows for the spontaneous dissociation of H2, followed by atomic H diffusion to the Au-Ir interface. This could explain the nearly invariant experimentally measured H-uptake of the Ir single crystal, even after the adsorption of Au on its surface.
Hydrogen Interactions with Au-Ir Alloys Conclusions The thermodynamics and kinetics of the interaction of molecular and atomic hydrogen on several Au-Ir model systems has been studied using periodic, self-consistent density functional theory and electrochemical experimental methods. The addition of 1 ML of Au on an Ir(111) surface stabilizes hydrogen at the Au-Ir interface as compared with subsurface hydrogen in both Au(111) and Ir(111). The diffusion of atomic H through 1 ML Au overlaid on Ir(111) and into the Au-Ir interface is comparable to H diffusion from surface to subsurface in Pd(111). Due to the stabilization of H at the Au-Ir interface, however, it can be kinetically trapped at low temperatures. At submonolayer coverages of Au in an Au-Ir surface alloy overlaid on top of Ir(111), subsurface H is stabilized as compared with H in the subsurface of pure Au(111) or Ir(111). On these submonolayer Au-Ir surface alloys, gas phase H2 is still able to spontaneously dissociate on the Ir surface atoms. Even at very low Ir coverages on the Au-Ir(111) surface, therefore, one could expect H2 dissociation on the surface followed by trapping of some hydrogen at the Au-Ir interface. Acknowledgment. This work has been supported by DOEBES, Chemical Sciences Division (Hydrogen Fuel Initiative Grant Number DE-FG02-05ER15731). Supercomputing time at NERSC, PNNL, and ORNL was used. References and Notes (1) Chorkendorff, I.; Niemantsverdriet, H. Concepts of Modern Catalysis and Kinetics; Wiley-VCH: Weinheim, 2003. (2) Skulason, E.; Karlberg, G. S.; Rossmeisl, J.; Bligaard, T.; Greeley, J.; Jonsson, H.; Nørskov, J. K. Phys. Chem. Chem. Phys. 2007, 9, 3241. (3) Greeley, J.; Mavrikakis, M. Nat. Mater. 2004, 3, 810. (4) Greeley, J.; Mavrikakis, M. J. Phys. Chem. B 2005, 109, 3460. (5) Greeley, J.; Mavrikakis, M. Catal. Today 2006, 111, 52. (6) Alayoglu, S.; Nilekar, A. U.; Mavrikakis, M.; Eichhorn, B. Nat. Mater. 2008, 7, 333. (7) Chen, J. G.; Menning, C. A.; Zellner, M. B. Surf. Sci. Rep. 2008, 63, 201. (8) Besenbacher, F.; Chorkendorff, I.; Clausen, B. S.; Hammer, B.; Molenbroek, A. M.; Nørskov, J. K.; Stensgaard, I. Science 1998, 279, 1913. (9) Kratzer, P.; Hammer, B.; Nørskov, J. K. J. Chem. Phys. 1996, 105, 5595. (10) Chin, Y. H.; King, D. L.; Roh, H. S.; Wang, Y.; Heald, S. M. J. Catal. 2006, 244, 153. (11) Lahr, D. L.; Ceyer, S. T. J. Am. Chem. Soc. 2006, 128, 1800. (12) Baddeley, C. J.; Ormerod, R. M.; Stephenson, A. W.; Lambert, R. M. J. Phys. Chem. 1995, 99, 5146. (13) Han, Y. F.; Wang, J. H.; Kumar, D.; Yan, Z.; Goodman, D. W. J. Catal. 2005, 232, 467.
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