Adsorption of Rh, Pd, Ir, and Pt on the Au(111) and Cu(111) Surfaces

Article pubs.acs.org/JPCC

Adsorption of Rh, Pd, Ir, and Pt on the Au(111) and Cu(111) Surfaces: A Density Functional Theory Investigation Rafael L. H. Freire,*,† Adam Kiejna,*,‡ and Juarez L. F. Da Silva*,§ †

São Carlos Institute of Physics, University of São Paulo, PO Box 369, 13560-970, São Carlos, SP, Brazil Institute of Experimental Physics, University of Wrocław, Plac M. Borna 9, 50-204 Wrocław, Poland § São Carlos Institute of Chemistry, University of São Paulo, PO Box 780, 13560-970, São Carlos, SP, Brazil ‡

S Supporting Information *

ABSTRACT: Several experimental and theoretical studies have suggested that the formation of surface alloys or the deposition of strained transition metal (TM) monolayers (ML) on TM supports can be considered as a route for the designing of new catalysts. In this work, we report an extensive first-principles investigation based on density functional theory of the adsorption of TM (Rh, Pd, Ir, Pt) on the Cu(111) and Au(111) surfaces considering TM coverages ranging from 1/9, 2/9, up to 1 ML. Although there are clear differences in the atomic radii of the Cu, Rh, Pd, Ir, Pt, and Au atoms, at low TM coverages, both systems exhibit similar behavior, namely, the lowest energy adsorption site for a single TM adatom is not in the hollow sites on the surface, but in the lattice sites located in the topmost layer. For TM/Au(111), this trend follows adatom by adatom up to the limit in which all the substrate Au atoms are exposed to the vacuum region, with the underlying TM adatoms, and it is also valid for Rh/Cu(111). For Pd, Ir, and Pt on Cu(111), the same trend is observed up to 4/9, 8/9, and 6/9 TM coverages, and the adatoms are exposed to the vacuum region for higher coverages. For TM/Au(111), our analyses indicate a tensile strain built-in due to the mixture of adatoms with smaller radii with Au with a larger radius in the same first (topmost) surface layer, while a compressive strain can be seen for TM/Cu(111), in particular for Pd, Ir, and Pt at high coverages, which favors the location of the TM adatoms on Cu(111). Judging by the Pauling electronegativity scale, we would expect a different behavior for the substrate work function change upon TM adsorption on Cu(111) and Au(111). However, a similar behavior was obtained for the lowest energy configurations on both substrates. This is rationalized in terms of the electronegativity differences, geometrical effect of atomic smoothing, the insertion of the adatoms in the first (topmost) surface layer, and the exposed layer to the vacuum region.

I. INTRODUCTION

Several experimental and theoretical studies have suggested that TM monolayer (ML) and multilayer structures supported on well-defined flat or stepped TM surfaces can be used to design new catalytic devices due to the possibility to tune the catalytic properties employing a substrate that induces a compressive or tensile strain on the TM layers.2,11−20 Furthermore, similar ideas have been explored in the formation of core−shell nanoparticles, in which the core sites are occupied by a chemical species and the surface sites are occupied by different chemical species, for example, a RuPt core−shell.21 Thus, several studies have been performed with the aim to understand the role of strain in the electronic properties of bimetallic systems and their effects in the reactivity properties. For example, Kitchin et al.12 found that strain effects can change the width and center of the surface d-band and, hence, affect the binding energy of adsorbate species and reactivity. This conclusion was also supported by subsequent studies.13,18 Thus, it has been suggested that the reactivity can be tuned by

Heterogeneous catalysis plays a crucial role in many areas of the chemical and energy industries, which have motivated intensive studies with the aim to understand, improve, and designing new catalyst devices. In general, the catalytic properties of materials depend on their atomic structure, composition, nature of the electronic states near the Fermi level, and so on.1−5 It has been well-known that changes in the atomic structure affect the relative position of the electronic states, and hence, they affect a large number of properties including reactivity. For example, stepped TM surfaces6,7 have been employed to understand the role of low-coordinated atoms in catalysis,1 while a large number of experimental results have demonstrated that the catalytic properties of supported nanoparticles are strongly dependent on the particle size, shape, and support surface termination.8,9 Furthermore, recent experimental results have indicated that subnano Pt particles supported on Al2O3 can enhance the reactivity for the oxidative dehydrogenation of propane by 40−100 times.10 Thus, specifically designed atomic structures can be used to tune the electronic properties and, hence, to affect the reactivity properties. © XXXX American Chemical Society

Received: April 3, 2014 Revised: July 24, 2014

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dx.doi.org/10.1021/jp5033228 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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geometry with a vacuum separation of 16 Å, and the PBE equilibrium lattice constants.42 That is, a0 = 3.63 Å and a0 = 4.16 Å for bulk Cu and Au in the face-centered cubic (fcc) structure, respectively, which is in good agreement with experimental results, namely, 3.61 and 4.08 Å, respectively.43 The TM atoms (Rh, Pd, Ir, Pt) were adsorbed on both sides of the slab to keep the inversion symmetry, and hence, dipole correction is not required. We considered all TM coverages varying from ΘTM = 1/9, 2/9 up to 1.0 ML. That is, we varied the number of adsorbate atoms, n, on X(111) from n = 1 up to n = 9. For all surface calculations, the atoms in the center of the slab, just one layer, were frozen in their bulklike positions, while all the remaining atoms and adatoms in the slab were allowed to relax; see Figure 1. All configurations were optimized using the conjugate gradient algorithm as implemented in VASP.

strain effects; however, it is a great challenge to identify the best TM combinations to enhance particular chemical reactions due to the complex nature of the effects. The adsorption of Pt and Pd atoms on Au(111) has been considered by several groups,15,16,22−29 while only a few studies have reported on Pd adatoms on Cu(111).23,27,28,30 Experimental studies of submonolayer amounts of Pd on Cu(111) have shown that islands of a disordered alloy are formed by exchange between Pd and Cu from the layer underneath.31 For one ML of Pd evaporated on Cu(111), Pd incorporates itself into the first three surface layers of the Cu crystal and forms a substitutional random alloy.32 Recently, epitaxial overlayers of Rh on Au(111) were also studied.33 Despite those studies, our understanding of these systems is far from being complete. For example, several studies have addressed ML and multilayer deposition on TM surfaces, while some others have addressed the adsorption of a few TM atoms on TM surfaces. Many of them, however, did not consider the mixing between different chemical species in the first (topmost) layers. Submonolayer coverages under the effect of compressive or tensile strain combined with electronic effects can lead to the formation of complex structures such as surface alloys, etc. Thus, our understanding of the role of strain and electronic effects from one atom to a full ML on TM surfaces can be improved. To obtain a better understanding of the role of tensile and compressive strain, as well as electronic effects on TM structures, in this work, we report an extensive density functional theory34,35 (DFT) investigation of TM (TM = Rh, Pd, Ir, Pt) atoms supported on the Au(111) and Cu(111) surfaces. The emphasis here is to explore geometric, energetic, and electronic properties of many different configurations formed by the TM/Au(111) and TM/Cu(111) systems for the TM coverages varying from a single atom (1/9 coverage) to a full ML (1 ML). For each coverage, a putative lowest energy configuration, formed on the surface or by alloying with the topmost surface layer atoms, is determined. This particular choice of the TMs and substrates is supposed to be well-suited to study the role of strain effects in two extreme cases: stretched TM structures on Au(111) and compressed TM structures formed at the Cu(111) surface.

Figure 1. Top and side views of the slab model employed to study the adsorption properties of Rh, Pd, Ir, and Pt on the Cu(111) and Au(111) surfaces using a 3 × 3 surface unit cell.

For all surface total energy calculations, we employed a 4 × 4 × 1 k-point mesh for the integration of the Brillouin zone (BZ), which implies from 4 to 10 k-points in the irreducible part of the BZ, whereas, for the density of states and work function calculations, a higher k-point mesh, 8 × 8 × 1, was used. For all calculations, we employed a total energy convergence of 10−4 eV, while the geometric optimization was stopped once all the atomic forces on all atoms were smaller than 0.05 eV/Å. To obtain the adsorption energies, we performed spin-polarized total energy calculations for the free TM (Rh, Pd, Ir, Pt) atoms using an orthorhombic box of dimensions 20.0 × 21.8 × 21.0 Å3 and the Γ-point for the BZ integration. B. Atomic Structure Configurations. To identify a reliable set of surface models for TMn/X(111), which includes the putative lowest configuration and high energy isomers, many possibilities for the initial configurations were considered, resulting from varying the composition and the adsorbed TMn atom positions (sites). The following steps were performed employing the slab shown in Figure 1: (i) In the first step, a single adatom, n = 1, was placed in one of the high-symmetry adsorption sites (on-top, bridge, hollow-like fcc or hcp) on the X(111) surfaces. (ii) Alternatively, the TM adatom was exchanged with a substrate atom (Cu or Au) located in the

II. THEORETICAL APPROACH AND COMPUTATIONAL DETAILS A. Total Energy Calculations. Our calculations are based on spin-polarized DFT within the semilocal exchangecorrelation (xc) energy formulation proposed by Perdew, Burke, and Ernzerhof (PBE).36 The electron−ion core interactions are described by the projector augmented wave (PAW) potentials37,38 as implemented in the Vienna Ab-initio Simulation Package (VASP),39,40 where the valence electrons are treated within the scalar-relativistic approximation. The following states, 3p64s13d10, 5s14d8, 5s14d9, 6s15d8, 6s15d9, and 6s15d10, were considered in the valence for the Cu, Rh, Pd, Ir, Pt, and Au elements, respectively.41 For all total energy calculations, we employed a plane-wave basis set with a cutoff energy of 230 eV for [Rh, Ir, Pt]/Au(111), 251 eV for Pd/ Au(111), and 369 eV for [Rh, Pd, Ir, Pt]/Cu(111), while, to obtain the equilibrium volumes of the respective TM bulk systems using stress tensor calculations, we employed higher cutoff energies (e.g., 460, 502, and 738 eV, respectively). The closed-packed X(111) surfaces were modeled using a 3 × 3 surface unit cell and the repeated slab (5 atomic layers) B



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first (topmost) surface layer, which is exposed to the vacuum region. (iii) All those configurations for n = 1 were optimized, and the findings were taken into account to construct new initial configurations for n = 2, and so forth, until n = 9. (iv) For all of those configurations, symmetry analyses were taken into account to avoid surface models with great similarity and to minimize the number of configurations. All the initial models were calculated for every TMn/X(111) system. Thus, although the number of configurations is finite (793 in total for TMn/ Cu(111) and TMn/Au(111)), the calculated configurations provide a representative set to describe the most important physical properties of the TMn/X(111) systems.

III. RESULTS AND DISCUSSION A. Relative Total Energies. Following the steps summarized above, we obtained a large number of surface models for each TMn/X(111) system, which includes the putative lowest energy configuration and high energy isomers. In the Supporting Information, we show the number of considered configurations and their relative total energies with respect to the lowest energy configurations. All the TMn/ X(111) configurations can be separated into two groups, namely, (i) configurations with all the TMn adatoms on X(111), namely, TMn/X9/X(111), and (ii) configurations with all the TMn adatoms exchanged with the surface atoms (Cu or Au), such as Xn/TMnX9−n/X(111). Thus, the relative total energy per adatom, ΔEtot, between the lowest energy configurations, namely, TMn/X9/X(111) and Xn/TMnX9−n/X(111) can be calculated as

Figure 2. Relative total energies per adatom, ΔEtot, between the lowest energy on-surface, TMn/X9/X(111), and the lowest energy subsurface, Xn/TMnX9−n/X(111), configurations as a function of the number of TMn adatoms; TM = Rh, Pd, Ir, Pt, and X = Cu, Au. The positive/ negative energy difference means that, respectively, the subsurface or on-surface configuration is energetically more favorable.

lowest ΔEtot = [Etot (TM n/X 9/X(111)) lowest − Etot (X n/TM nX 9 − n/X(111))]/2n

(1)

Thus, ΔEtot yields an indication of the preference of the TMn adatoms for adsorption sites on X(111) or for lattice sites in the first (topmost) surface layer. The results are shown in Figure 2. For both systems, it can be seen that ΔEtot decreases by increasing the TMn coverage, which indicates a strong preference for the TMn adatoms for the lattice sites in the topmost surface layer instead of the high-symmetry adsorption sites on the surface; however, there is a clear dependence on the adatom coverages. For TMn/Au(111), we found that ΔEtot > 0 for every TM coverage, and hence, there is a strong preference for the first layer (topmost) surface sites. For ΘTM = 1.0 ML, the substrate Au atoms are exposed to the vacuum instead of the TM adatoms. Those results showed us the strong antisegregation for TMn/Au(111) systems. For TMn/Cu(111), the trends are slightly different, in particular for Pd and Pt (Figure 2). For example, at low TM coverages, ΔEtot > 0, which indicates an energetic preference for the first (topmost) surface sites instead of the adsorption sites on the surface. However, at high Pd and Pt coverages such as ΘPt = 1.0 ML, we found that ΔEtot > 0, and hence, the adatoms are exposed directly to the vacuum region, which can be explained by the large compressive strain induced by the Cu(111) support. In the case of TMn/Cu(111), we can say that the segregation is coverage-dependent, since, at low coverage, we observed an antisegregation behavior, whereas, at high coverage, we observed a strong segregation trend. B. Lowest Energy TMn/X(111) Configurations. The putative lowest energy configurations (top and side views) are shown in Figure 3 for TMn/X(111). For low TM coverages,

both systems exhibit similar behavior. The lowest energy adsorption site for a single TM adatom is in the topmost layer (first layer) site. For example, the TM (Rh, Pd, Ir, Pt) adatoms exchange position with the substrate atoms (Au, Cu), which occupy a hollow site on the surface nearest to the inserted TM adatom. The second TM adatom replaces a substrate atom on the topmost layer, and the substrate atoms occupy sites nearest to the adatoms. Thus, this result indicates an attractive interaction among the adatoms located in the first (topmost) layer, which gives rise to the formation of a substrate overlayer on the adatoms. For TMn/Au(111), this trend follows adatom by adatom up to the limit in which all the substrate Au atoms are exposed to the vacuum region, while the TM adatoms are beneath it. It should be noted that the lowest energy configurations for every TM coverage are composed of TM adatoms inserted in the topmost layer, while the substrate atoms are exposed to the vacuum region, and hence, there is no intermixing of TM and Au atoms in the same layer for the lowest energy configurations (i.e., no indication of alloying). The trends are not exactly the same for all TM adatoms on Cu(111). For example, Rhn/Cu(111) follow the same trends as described above for TMn/Au(111), and hence, the lowest energy Cu9/Rh9/Cu(111) configuration is formed at the limit of ΘRh = 1.0 ML. For Irn/Cu(111), the same trend is observed up to n = 8, while for n = 9, the Ir adatoms are exposed to the vacuum region. For Pdn/Cu(111), the differences start for n = 4, in which the lowest energy configuration is composed of Pd and Cu atoms intermixed in the overlayer and topmost layer, C



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Figure 3. Lowest energy PBE configurations for TMn/Au(111) and TMn/Cu(111), where the gold and blue balls indicate Au and Cu atoms, respectively, while the TM adatoms are indicated by different colored balls. As the slab is symmetric, here we show just half of it, that is, from the middle (frozen) to overlayer.

adatoms tend to occupy lattice sites. This trend occurs, principally, when there is a large difference in the atomic sizes of the adsorbate and host atoms. In this case, there is a relief strain lattice in the bulk or in the early surface layers that, consequently, leads to a decrease in the total energy of a system.46 As we said above, for the TM/Au(111), our results agree with that study, since the atomic radius of Au (1.47 Å) is larger than that of Rh (1.35 Å), Pd (1.39 Å), Ir (1.37 Å), and Pt (1.40 Å), while for Cu, it is 1.28 Å. Also, the interaction of the adatoms with Au is stronger than that between Au−Au atoms. Hence, by replacing the Au host

and at the limit of high coverage, all the Pd adatoms are exposed to the vacuum region. For Ptn/Cu(111), the Pt preference for overlayer sites starts for n = 6, at the limit of high coverage, all the Pt adatoms are also exposed to the vacuum region. Thus, the main differences occur only for high coverages, whereas, for low adatom coverages, both TMn/ X(111) systems show the same behavior. Our results for TMn/Au(111) agree with the studies on TM alloys based on broken bonds and electronic models.12,44−48 For example, according to Dowben et al.,46 a larger atom tends to occupy adsorption sites on the surface, while smaller D



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Figure 4. Effective coordination number, ECN, as a function of the number of TM adatoms on the Au(111) and Cu(111) surfaces.

A preference to form stable on-surface or subsurface TM structures on the Au(111) and Cu(111) surfaces is consistent with the trends for segregation energy of TM alloy components presented by Ruban et al.48 They found that, based on the difference in the calculated surface energies of the alloy constituents, and accounting for crystal structure differences between the host and the impurity that alter the local d-state character around the impurities, one can predict whether a given component of an alloy will segregate (i.e., is more stable on the surface) or not. According to their results for all the TM/Au(111) systems (TM = Rh, Pd, Ir, Pt) considered by us, strong antisegregation (0.7−0.3 eV) should be expected. This indicates that, consistent with Figure 3, the bulk or subsurface positions of TMs are more favorable. For the TM/Cu(111) systems, the agreement is less clear. For Rh atoms in the Cu host, strong antisegregation (i.e., subsurface positions of Rh atoms) is expected, which agrees with our findings (Figure 3). For Pd and Pt in the Cu host, respectively, a moderate segregation or no segregation is predicted. In a latter case, one can see in Figure 3 that, in our calculations, the preference for on-surface or subsurface position is ΘTM-dependent. Such coverage dependence seems to hold also for the Ir/Cu(111) system where, for the lower coverages, the subsurface positions are favored (no segregation), in agreement with Ruban et al.,48 whereas, for a complete monolayer, the on-surface configuration is most stable. C. Local Environment Parameters: Coordination and Bond Lengths. To obtain a better understanding of the structural properties of the lowest energy TMn/X(111) configurations, we calculated two local environment parameters employing the effective coordination concept51,52 as a function of the number of TM adatoms. We calculated the effective coordination number (ECN), which yields the effective number of NN atoms surrounding a particular atom, and the weighted average bond lengths, dav, which is an average weighted distance to the NNs atoms. The most important results are shown in Figure 4, while extra results are shown in the Supporting Information. For clean X(111), the substrate atoms in the first (topmost) surface layer have ECN = 9 NNs, while the substrate atoms below the first layer have bulklike coordination (i.e., ECN = 12 NNs). For a single TM adatom on X(111) in the hollow (fcc, hcp), bridge, and on-top sites, ECN = 3, 2, and 1 NNs, respectively.

atom with Rh, Pd, Ir, or Pt adsorbates, we will have a relief strain lattice, as well as an energy decrease related to broken bonds on the surface. The outcome is a decrease in the total energy of a system.46 Surface segregation studies using electronic models12,44,47,48 have also pointed to such a behavior for TMn/Au(111). A difference in the lattice constants of a TM adsorbate and the host crystals (and, consequently, in the nearest-neighbor (NN) distances at the surface) will give rise to different types of an elastic lattice strain. The NN distance at (111) surfaces of Au (2.94 Å) and Cu (2.57 Å) is, respectively, larger/smaller than that of any TM considered (Rh, 2.69 Å; Pd, 2.78 Å; Ir, 2.72 Å; Pt, 2.81 Å; all values for TM(111) surface) and will introduce a stretching/compressive strain to adsorbate structures at the Au(111)/Cu(111) surfaces. The resulting strain will affect the overlap of metal d-states at neighboring sites and thus modify the d-band structure and the d-band center position. Thus, according to the Hammer−Nørskov model,49 it will influence a catalytic activity of the involved metals. Ruban et al.50 have analyzed the d-band center shifts in an extensive database of surface TM impurities and overlayers and concluded that an overlayer or a surface alloy of metals with small lattice constants that formed on metals that are characterized by larger lattice constants shifts the d-band center up in energy. Metals with larger lattice constants that alloy on a metal surface with smaller lattice constants shifts the d-band center down in energy. These trends result from a combination of d-bandwidth changes upon alloying, and the d-band shifts to maintain a constant local d-band filling. For TMn/Cu(111), however, many points still remain unclear. The atomic size for Cu (1.28 Å) is smaller than that of Rh (1.35 Å), Pd (1.39 Å), Ir (1.37 Å), and Pt (1.40 Å); thus, according to the broken bond model of Dowben et al.,46 subsurface configurations become unfavorable. Nevertheless, some studies that involve electronic models indicate subsurface configurations as the most favorable ones.45,47,48 A comparison of results for the TMn/Cu(111) systems with these models shows that, in the limit of lower coverages, there is an agreement with the electronic models; that is, the subsurface configurations are the most stable. For the highest coverage, however, the results agree with the broken bond models indicating the on-surface configurations as the most stable. E



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We found that the ECN for TM adatoms in the TMn/ Au(111) and Rhn/Cu(111) systems increases from 9 to 12 NNs. This happens because the adatoms occupy surface sites in the first (topmost) surface layer and are covered with the substrate atoms expelled from the first (topmost) surface layer. Thus, at the limit of ΘTM = 1.0 ML, the adatoms form a bulklike environment with ECN = 12 NNs, while the substrate atoms are exposed to the vacuum region with ECN = 9 NNs. For [Pd, Ir, and Pt]/Cu(111), the TM adatoms behave in a distinctly different way; see Figure 4. After an initial increase from 9 NNs to 12 NNs, the ECN curve breaks down and shows a sudden decrease to ECN = 9 NNs for 1.0 ML of Ir, about ECN = 6 NNs for ΘPd = 4/9 ML, and about ECN = 7 NNs for ΘPt = 6/9 ML. Then, in the latter two cases, the ECN increases again to 9 NNs at 1.0 ML. This sudden change reflects the adatom preference for adsorption sites on the surface for those systems for higher adatom coverages. For the overlayer of the host Au or Cu atoms, in general, the ECN increases from 3 to 9 NNs. This is expected, since the ECN for the first atomic layer of a clean surface is 9 NNs and a single atom placed in a hollow site on the surface has 3 NNs. Thus, an analysis of the results for the ECN allows us to state if the Au(111) or Cu(111) surface is being covered with the TM adatoms or with the Au or Cu host atoms, respectively. However, as can be seen in Figure 4, there are some points missing in the plot, because of the exchange between subsurface and on-surface-like structures that occurs for these configurations. Thus, the atoms that appear on the surface, after the break seen in the ECN curves, are the adatoms; see Figure 4. On the other hand, in the case of TMn/Au(111), the adatoms do not appear in the overlayer. In this case, the adatoms exchange with the host Au atoms to form a mixed first surface layer (Figure 3). For TMn/Au(111), the Au atoms located in the first layer keep their average bond lengths (dav) almost the same as the parent metal as a function of the number of adatoms, changing less than 0.05 Å in this interval. The decrease observed in dav for Au atoms can be explained, in principle, by the difference between Au−Au and Au−TM bonds. As the Au−Au bond is higher than Au−TM, the bond length for Au atoms decreases because of the contributions to NNs distances that come from Au and TM atoms, and on average, the NNs distances decrease a bit. We can use the same idea to TMn/Cu(111). However, in this case, the Cu−Cu bond is less than Cu−TM bond, so for those systems, we observed a little increasing of the dav for Cu atoms, less than 0.05 Å, too. For host atoms in the first layer, we have a distinct behavior when we compare TMn/Au(111) and TMn/Cu(111) systems. For TMn/Au(111), the dav values for TM atoms are a bit larger than to the parent metal, which indicates that the first layer is under a tensile strain; that is, there is an expansion of surface. In case of TMn/Cu(111), the dav values for TM atoms are less than to the parent metal. Therefore, in this case, the first layer looks under a compressive strain. For the Au atoms on the surface (overlayer), their bond length dav increases almost linearly from about 2.70 to 2.90 Å with the number of adatoms increasing from n = 1 to 9. This behavior can be explained by the coordination of the Au atoms in the overlayer, which increases from about 3 to 9 NNs for n = 1−9. D. Adsorption Binding Energies. To characterize the TMn-X(111) interactions, we have analyzed the adsorption energy per TM adatom. It was calculated using the equation

TM n /X(111) X(111) TM atom Ead = [Etot − (Etot + nEtot )]/2n TM /X(111) Etot n ,

EX(111) , tot

(2)

atom ETM tot

where and are the total energies of the TMn/X(111), clean X(111) surface, and free TM atoms, respectively. n indicates the number of adatoms, and the factor 1 /2 is due to the presence of adatoms on both sides of the slab. The results are shown in Figure 5. In simpler terms, the more

Figure 5. Adsorption energy, Ead, versus the number of TM adatoms on the Au(111) and Cu(111) surfaces. Negative energy means a stable system.

negative is the adsorption energy, the stronger will be the adatom-substrate bond. That is, the adatoms will be more strongly bonded to the substrate. For all TMn/X(111) systems, except for Pt/Cu(111), we found that the adsorption energy increases almost linearly in absolute value with the number of TM adatoms on X(111). The TM adatoms bind stronger to the Cu(111) surface than to Au(111), except for Pd/Au(111) at ΘPd > 6/9 ML. It can be seen that the smaller differences happen for 4d adatoms on X(111), while the larger ones for 5d adatoms on X(111). Thus, the adsorption energy is higher by almost 0.7−1.0 eV for Irn/Cu(111) than for Irn/Au(111). Recently, Santana and Rösch27 employing DFT calculations reported that the binding energy of Pt3 on X(111) is higher than for Pd3/X(111). Furthermore, both clusters bind stronger on Cu(111) than on Au(111). Both trends are consistent with our results (Figure 5); however, our binding energies are distinctly higher, which can be explained as follows. Santana and Rösch considered the adsorption of triangular TM clusters only on the surface, while in the lowest energy configurations, the TM3 adatoms occupy lattice sites in the topmost surface layer and the substrate atoms (Au or Cu) are exposed to the vacuum region. Furthermore, our results indicate that the stronger binding of TM adatoms on Cu(111) compared with Au(111) can be explained by the large charge transfer among the adatoms and Cu atoms. This is supported by our Bader analysis results presented below. E. Density of States. One of the key parameters that is helpful in understanding reactivity is the position of the d-band center of the occupied d-states with respect to the Fermi level, which can be correlated with the magnitude of adsorbate binding energies with the substrate.49,53 We calculated the total and local density of states (LDOS) for every lowest energy TMn/X(111) configuration shown in Figure 3. The LDOS plots for all particular systems are shown in the Supporting Information. For all of these configurations, in general, two chemical species (the adatoms and substrate atoms) can be seen in the overlayer and first (topmost) surface layer, whereas F



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Figure 6. d-band center of the occupied d-states with respect to the Fermi level for the TMn adatoms in the overlayer and topmost surface layer (first layer), and substrate atoms located in the overlayer, first layer, and second layer for the lowest energy configurations of the (a) TMn/Au(111) and (b) TMn/Cu(111) systems.

results for each adsorbate (Rh, Pd, Ir, and Pt, respectively) on the specific surface. For TM1/Au(111), the d-band center of the Au atoms located in the overlayer are in the range from −2.60 to −2.70 eV, and with an increase in the number of Au adatoms in the overlayer, the d-band center is shifted far away from the Fermi level. At the limit of a full Au ML exposed to the vacuum region, the overlayer d-band center nearly reaches the d-band of the first layer of the clean Au(111) surface, −3.39 eV. For the Au atoms located in the first layer, the d-band center differs by about 0.20 eV from the clean Au(111) surface results, and

there is only one chemical species in the second layer (substrate atoms). Thus, we have separated the LDOS in five distinct groups, namely, the adatoms and substrate atoms at the overlayer and first layer, and the substrate atoms at the second layer. The d-band center was calculated for the average LDOS for each group, and the results are shown in Figure 6 along with the d-band center of the first and second layers for the clean Cu(111), Rh(111), Pd(111), Ir(111), Pt(111), and Au(111) surfaces. In Figure 6, on the right, we can observe a distinction for over-, first, and second layers. Also, the columns represent G



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since most of the reactions occur on the surface, and in this way, we are able to tune the reactivity of surfaces. When for the TM/Au(111) and Rh/Cu(111) surfaces, the coverage reaches 1 ML, the d-band center is shifted up by ≃0.2 eV relative to substrate metals; that is, it is almost the same. This small difference can be attributed to strain and ligand effects, which both occur simultaneously, because, in all of these cases, we have one layer of TM atoms below the Au or Cu overlayer. A slightly different behavior can be observed for the Ir/Au(111) at 1 ML. However, a closer look at the dav results allows us to realize that this system exhibits great deviations. This implies great structural distortions that could explain that equality. F. Work Function and Charge Transfer. To obtain a better understanding of the TMn/X(111) systems, we considered the work function, Φ, changes upon TM adsorption. The work function is calculated as the energy difference between the electrostatic potential at a point far from the surface (middle of the vacuum region), Ves(rvac), and the Fermi energy, EF, (Φ = Ves(rvac) − EF).55 The results are shown in Figure 7 along with the work function of the clean surfaces.

similar behavior is also observed for the Au atoms located in the second layer, except for small deviations found for ΘTM ≥ 5/9. For TMn/Au(111), the d-band center of the Rh, Ir, and Pt is shifted toward the Fermi level by 0.20−1.0 eV compared with the clean TM(111) surfaces, whereas, for Pd, it is nearly the same as that for clean Pd(111), −1.96 eV. Thus, the d-band center of the Pd adatoms is not affected by the presence of substrate Au atoms located in the overlayer. Our results for TMn/Au(111) differ from those of previous DFT calculations,27 by about 0.7−0.8 eV, which can be explained by the differences in the lowest energy configurations, as discussed above. In contrast to the results obtained for TMn/Au(111), which show smooth changes as a function of the TM adatoms coverage, the d-band center for TMn/Cu(111) shows oscillations. For a single Cu adatom located in the overlayer, the d-band center is nearly the same for all TM adatoms (i.e., −1.70 eV), which is about 0.80 eV above the d-band center of the first layer in the clean Cu(111) surface, −2.52 eV. For an increased number of substrate Cu atoms located in the overlayer, the d-band center moves toward that for the clean Cu(111) surface; however, it does not reach the clean Cu(111) surface result (e.g., it is 0.39 eV higher in energy for a full ML of Rh adatoms). For Pd, Ir, and Pt on Cu(111), the d-band center of the Cu atoms in the overlayer is higher by about 0.80, 0.35, and 0.50 eV, respectively, compared with the clean Cu(111) surface. For the Cu atoms in the first layer, except for a few cases, the d-band center position is similar to that of the first layer of the clean Cu(111) surface, which was also observed for TMn/Au(111), whereas there are relatively large changes in the d-band center for the Cu atoms in the second layer. For Rhn/Cu(111), we observe that the d-band center of the Rh atoms moves from −2.0 eV (n = 1) to about −2.60 eV (n = 9). A similar trend can be seen for Ir on Cu(111); however, for Pd and Pt, there are substantial differences due to the location of the TM adatoms in the overlayer for high coverages. In particular, we observe large changes in the d-band center position for a few TM adatoms located in the overlayer, while for an increased coverage, the d-band center is moved far away from the Fermi level. Compared with previous results,27 our dband centers are shifted down by about 1.0 eV. This can be explained by the fact that the Pd3 and Pt3 adatoms are located in the overlayer, which position the d-band center closer to the Fermi level due to the reduction in the coordination. The changes in the d-band center of the atoms in the overlayer (i.e., adatoms and substrate) can be explained by the changes in the coordination of the atoms. For example, it is known that, when more atoms bind to the substrate, their coordination increases and the position of the d-band center is shifted downward12 because of changes in the local electron density due to the redistribution of electrons or d-states overlapping. For example, if the number of NNs increases, the number of shared electrons increases too, so the density of states broadens and is shifted downward to keep the d-band filling. The d-band shift behavior observed for the Au and Cu atoms in the overlayer of the TM/Au(111) and TM/Cu(111) systems against the number of TM atoms in the systems confirms this idea. That is, when the number of TM atoms increases, then the ECN of the host atoms increases too (Figure 4). The d-band center of the host atoms in the overlayer roughly shifts up, which indicates a more reactive environment than the clean surface of the host metals. This is an important result

Figure 7. Work function of the TMn/Au(111) and TMn/Cu(111) systems as a function of the number of TM adatoms. The theoretical (horizontal continuous lines) and experimental54 (horizontal dashed lines) clean surface work functions are indicated for reference.

The clean surface results are close to the experimental values.54 A discrepancy between PBE and measured values is smaller than 0.40 eV for the worst case (i.e., Rh(111)), while it is only in the range of 0.10−0.15 eV for Cu(111), Pd(111), Ir(111), Pt(111), and Au(111). H



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e (Au). An opposite effect can be observed for TM adatoms on Cu(111); namely, the work function of the system increases for all TM coverages. Hence, it follows the expected behavior based on the electronegativity differences; however, several of those atomic structures are high energy configurations. As the number of adsorbate atoms increases, the surface becomes more smooth and the Φ starts to increase. Finally, when a complete monolayer of adsorbate is formed on Au(111), the layer exposed to the vacuum region is composed only of Au atoms, and hence, the Φ values are close to the clean Au(111) surface values. For TM9/Cu(111), the Pd, Ir, and Pt monolayers are exposed to the vacuum region, and hence, Φ increases and nearly reaches the work function of the clean TM(111) surfaces. The same trend can be observed for Rh9/ Cu(111); however, in this case, the Cu atoms are exposed to the vacuum region and Rh forms a subsurface monolayer.

In general, it has been assumed that the substrate work function changes upon adsorption of chemical species (ΔΦ = ΦTMn/X(111) − ΦX(111)), which can be explained by the relative difference of the electronegativities of the adsorbate and substrate atoms.55,56 For example, for adatoms with larger electronegativity than the substrate, we would expect an increase in the work function due to the expected charge transfer from the substrate to the adatom, and vice versa.56−58 However, sometimes the ΔΦ behavior can be counterintuitive, as ΔΦ can depend on factors such as surface relaxations and reconstructions,59,60 charge redistribution (polarization),61−63 the nature of the adatoms−substrates chemical bonding,56,62 and coverage.56,64 Using the Pauling electronegativity scale, all the TM adatoms (Rh 2.28; Pd 2.20; Ir 2.20; and Pt 2.20) have larger electronegativity than Cu (1.90) and smaller than Au (2.40),65,66 and hence, we would expect a different behavior for TMn on Au(111) and Cu(111), for example, an increase for TMn/Cu(111) and a decrease for TMn/Au(111). However, a similar behavior is obtained for the lowest energy configurations of both substrates (Figure 7). We found that, upon TM adsorption, at low coverages, the work function of both substrates decreases, which is a surprising result at first sight. However, a closer look at the adatom configurations in the considered systems and the analysis of the electron charge transfer to/from surface layer atoms shows that the Φ lowering observed for small coverages can be rationalized in terms of the electronegativities difference (charge transfer), geometrical effect of atomic smoothing, and the insertion of the adatoms in the first layer for TMn/Cu(111). After an initial decrease of Φ from the clean surface values, it reaches a minimum at 2−4 adatoms and then increases to the values close to the ΦAu(111) values, in the case of adsorption on Au(111), or to the values corresponding to the work function of adsorbate metals, for the adsorption on Cu(111). Thus, ΔΦ can be understood as follows. Initial adsorption of one to four TM atoms makes the close-packed (111) surface more rough on the atomic scale, both in the case when the adatom stays on the surface or goes in the first layer. This geometric effect is strengthened, or weakened, by the electron density redistribution on the adsorbate and substrate atoms. The magnitude of the electron charge redistribution can be estimated from the Bader analysis67,68 of charges on atoms in the TM/Au(111) and TM/Cu(111) systems. In the Supporting Information, we present the structures and the Bader charges calculated on the most stable Pt1, Pt4, and Pt9 conformations on Au(111) and Cu(111). The Pt1 and Pt4 atoms are incorporated into the first surface layer, by moving, respectively, one or four Au or Cu atoms to on-surface positions. The Pt atoms that are built into the first substrate layer gain electron charge. However, this electron charge gain on Pt is much larger when adsorbed on Cu than Au(111). The Pt1 atom at the Cu(111) gains about 0.68 e, whereas that on the Au(111) gains 0.09 e, only. The Cu or Au atom expelled from the substrate surface layer and that moved to on-surface positions, respectively, loses 0.11 e (Cu) or gains 0.05 e (Au). This, respectively, strengthens/weakens the geometric effect of the Φ lowering in the Pt/Cu(111) and the Pt/Au(111) systems. Similar, but larger, changes in the electron charge distribution are observed for adsorbed Pt4 conformations. The Pt4 at the Cu(111) surface gains about 2.09 e, whereas that on the Au(111) loses 0.08 e. The Cu or Au atoms the moved to on-surface positions, respectively, lose 0.54 e (Cu) or gain 0.14

IV. SUMMARY In this work, we have performed extensive DFT-PBE calculations of the geometric and energetic properties of TM (Rh, Pd, Ir, Pt) adsorbed on the Au(111) and Cu(111) surfaces, for coverages ranging from ΘTM = 1/9 to 1.0 ML. Although there are clear differences in the atomic radii of the Cu, Rh, Pd, Ir, Pt, and Au atoms, at low TM coverages, both systems exhibit similar behavior; namely, the lowest energy adsorption site for a single TM adatom is not the hollow site on the surface, but one of the lattice sites located in the first (topmost) layer. For example, the TM adatom (Rh, Pd, Ir, Pt) exchanges position with the substrate atoms (Cu, Au), which occupy a hollow site on the surface nearest to the inserted TM adatom in the first (topmost) surface layer. The second TM adatom replaces a substrate atom in the first (topmost) layer, and the substrate atoms occupy sites nearest to the adatoms, which indicates an attractive interaction among the adatom and substrate atoms. For TMn/Au(111), this pattern follows adatom by adatom up to the limit in which all the substrate Au atoms are exposed to the vacuum region, while the TM adatoms are beneath it. Thus, the intermixing of TM and substrate atoms in the same layer occurs only for low coverages, whereas there is no intermixing at full monolayer. We obtained the same trend for Rhn/Cu(111); however, we found differences for Ir, Pd, and Pt on Cu(111) at the limit of high TM coverages. For Pd, Ir, and Pt on Cu(111), the same trend is observed up to n = 4, 8, and 6, and the adatoms are exposed to the vacuum region for high coverages. All of those results are supported by the analyses of the atomic structures and ECN results. For TMn/Au(111), the Au adatoms located in the first layer keep their average bond lengths (dav) almost the same as for the parent metal as a function of the number of TM adatoms coverage. For the TM atoms in the same layer, the dav is a little larger than for the parent metal atoms, which indicates a tensile strain built-in due to the mixture of adatoms with smaller atomic radii with the Au atoms with a larger atomic radius in the same layer; however, an indication of a compressive strain could be seen for TMn/Cu(111). That is, the average bond lengths decrease for the TM atoms on the first surface layer of the Cu(111) substrate. Thus, at high coverage, the on-surface sites are preferable due to the strong compressive strain over the system. We found that the adsorption energy increases almost linearly in absolute value as a function of the number of adatoms, except for Pt/Cu(111). The TM adatoms bind I



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on transition metal surfaces: Theory and experimental evidence. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 2207−2211. (2) Wandelt, K. Properties and Influence of Surface Defects. Surf. Sci. 1991, 251, 387−395. (3) Bell, A. T. The Impact of Nanoscience on Heterogeneous Catalysis. Science 2003, 299, 1688−1691. (4) Nørskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Towards the Computational Design of Solid Catalysts. Nat. Chem. 2009, 1, 37−46. (5) Freund, H.-J.; Meijer, G.; Scheffler, M.; Schlögl, R.; Wolf, M. CO Oxidation as a Prototypical Reaction for Heterogeneous Processes. Angew. Chem., Int. Ed. 2011, 50, 10064−10094. (6) Da Silva, J. L. F.; Schroeder, K.; Blügel, S. First-Principles Investigations of the Multilayer Relaxation of Stepped Cu Surfaces. Phys. Rev. B 2004, 69, 245411. (7) Da Silva, J. L. F.; Barreteau, C.; Schroeder, K.; Blügel, S. AllElectron First-Principles Investigations of the Energetics of Vicinal Cu Surfaces. Phys. Rev. B 2006, 73, 125402. (8) Hvolbæk, B.; Janssens, T. V. W.; Clausen, B. S.; Falsig, H.; Chistensen, C. H.; Nørskov, J. K. Catalytic Activity of Au Nanoparticles. Nanotoday 2007, 2, 14. (9) Cuenya, B. R. Synthesis and Catalytic Properties of Metal Nanoparticles: Size, Shape, Support, Composition, and Oxidation State Effects. Thin Solid Films 2010, 518, 3127−3150. (10) Vajda, S.; Pellin, M. J.; Greeley, J. P.; Marshall, C. L.; Curtiss, L. A.; Ballentine, G. A.; Elam, J. W.; Catillon-Mucherie, S.; Redfern, P. C.; Mehmood, F.; et al. Subnanometre Platinum Clusters as Highly Active and Selective Catalysts for the Oxidative Dehydrogenation of Propane. Nat. Mater. 2009, 8, 213−216. (11) Larsen, J. H.; Chorkendorff, I. Increased Dissociation Probability of CH4 on Co/Cu(111). Surf. Sci. 1998, 405, 62−73. (12) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. Role of Strain and Ligand Effects in the Modification of the Electronic and Chemical Properties of Bimetallic Surfaces. Phys. Rev. Lett. 2004, 93, 156801. (13) Calleja, F.; Garcı ́a-Suárez, V. M.; Hinarejos, J. J.; Ferrer, J.; de Parga, A. L. V.; Miranda, R. Relationship Between Strain and the Surface Electronic Structure of Cu(111) Films on Ru(0001): Theory and Experiment. Phys. Rev. B 2005, 71, 125412. (14) Greeley, J.; Nørskov, J. K.; Kibler, L. A.; El-Aziz, A. M.; Kolb, D. M. Hydrogen Evolution over Bimetallic Systems: Understanding the Trends. ChemPhysChem 2006, 7, 1032−1035. (15) Groß, A. Reactivity of Bimetallic Systems Studied from First Principles. Top. Catal. 2006, 37, 29−39. (16) Pandelov, S.; Stimming, U. Reactivity of Monolayers and NanoIslands of Palladium on Au(1 1 1) with Respect to Proton Reduction. Electrochim. Acta 2007, 52, 5548−5555. (17) Kibler, L. A. Dependence of Electrocatalytic Activity on Film Thickness for the Hydrogen Evolution Reaction of Pd Overlayers on Au(111). Electrochim. Acta 2008, 53, 6824−6828. (18) Laurent, G.; Busnengo, H. F.; Rivière, P.; Martı ́n, F. H2 Reactivity on Strained Pseudomorphic Monolayers of Cu and Pd on Ru(0001). Phys. Rev. B 2008, 77, 193408. (19) Bae, S.-E.; Gokcen, D.; Liu, P.; Mohammadi, P.; Brankovic, S. R. Size Effects in Monolayer Catalysis-Model Study: Pt Submonolayers on Au(111). Electrocatalysis 2012, 3, 203−210. (20) Prieto, M. J.; Tremiliosi-Filho, G. Surface Restructuring of Pt Films on Au Stepped Surfaces: Effects on Catalytic Behaviour. Phys. Chem. Chem. Phys. 2013, 15, 13184−13189. (21) Alayoglu, S.; Nilekar, A. U.; Mavrikakis, M.; Eichhorn, B. Ru−Pt Core-Shell Nanoparticles for Preferential Oxidation of Carbon Monoxide in Hydrogen. Nat. Mater. 2008, 7, 333−338. (22) Behm, R. J. Interaction of Hydrogen with Bimetallic Surfaces. Z. Phys. Chem. 2009, 223, 9−36. (23) Santos, E.; Quaino, P.; Schmickler, W. On the Electrocatalysis of Nanostructures: Monolayers of a Foreign Atom (Pd) on Different Substrates M(1 1 1). Electrochim. Acta 2010, 55, 4346−4352.

stronger to the Cu(111) surface than for Au(111), except for Pd/Au(111) with ΘPd > 6/9 ML. It can be seen that the smaller differences happen for 4d adatoms on X(111), while the larger ones for 5d adatoms on X(111). For TM1/Au(111), the d-band centers of the Au atoms located in the overlayer are in the range from −2.60 to −2.70 eV, and an increase in the number of Au adatoms in the overlayer shifts the d-band center down from the Fermi level. Thus, at the limit of full coverage, the d-band center nearly reaches the d-band center of the first layer of the clean Au(111) surface (i.e., −3.39 eV). The changes in the d-band center are due to the reduction in the coordination of the Au adatoms exposed to the vacuum region. In contrast to the TMn/Au(111) results, the d-band center for the TMn/Cu(111) systems shows strong oscillations due to the change in the lowest energy configurations from subsurface sites for on-surface sites. Using the Pauling electronegativity scale, we would expect a different behavior for TMn on Au(111) and Cu(111); however, a similar behavior was obtained for the lowest energy configurations of both substrates. Namely, the work function of both substrates decreases upon TM adsorption at low coverages, it reaches a minimum at 2−4 adatoms, and then it increases to the values close to the ΦAu(111) values, in the case of adsorption on Au(111), or to the values corresponding to the work function of adsorbate metals, for the adsorption on Cu(111). That is, at first sight, a surprising result. However, a closer look at the adatom configurations in the considered systems and the analysis of the electron charge transfer to/from surface layer atoms (Bader analysis) shows that the observed Φ lowering for small coverages can be rationalized in terms of the geometrical effect of atomic smoothing, and the insertion of the adatoms in the first layer for TMn/Cu(111). This geometric effect is strengthened, or weakened, by the electron density redistribution on the adsorbate and substrate atoms.

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ASSOCIATED CONTENT

S Supporting Information *

Extra data and analyses are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.L.H.F.). *E-mail: [email protected] (A.K.). *E-mail: [email protected]. Phone: +55 16 3373 6641. Fax: +55 16 3373 9952 (J.L.F.D.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Council of Technological and Scientific Development, CNPq, the Coordination for the Improvement of Higher Level Education, CAPES, and the São Paulo Research Foundation, FAPESP, and the infrastructure provided to our computer cluster by the São Carlos Center of Informatics, University of São Paulo, is gratefully acknowledged.

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REFERENCES

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Adsorption of Rh, Pd, Ir, and Pt on the Au(111) and Cu(111) Surfaces

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