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Oxygen-Driven Surface Evolution of Nanoporous Gold: Insights from Ab Initio Molecular Dynamics and Auger Electron Spectroscopy Yong Li, Wilke Dononelli, Raphaell Moreira, Thomas Risse, Marcus Bäumer, Thorsten Kluener, and Lyudmila V. Moskaleva J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08873 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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The Journal of Physical Chemistry

Oxygen-Driven Surface Evolution of Nanoporous Gold: Insights from Ab Initio Molecular Dynamics and Auger Electron Spectroscopy

Yong Li,1 Wilke Dononelli,2 Raphaell Moreira,3 Thomas Risse,3 Marcus Bäumer,1 Thorsten Klüner,2 Lyudmila V. Moskaleva1,* 1 Institute of Applied and Physical Chemistry and Center for Environmental Research and Sustainable Technology, University of Bremen, 28359 Bremen, Germany 2 Institut für Chemie, Carl von Ossietzky Universität Oldenburg, 26129 Oldenburg, Germany 3 Institute of Chemistry and Biochemistry, Freie Universität Berlin, 14195 Berlin, Germany

Abstract Nanoporous gold (np-Au) has recently emerged as a highly selective environmentally friendly catalyst for low-temperature applications. Despite seeming simplicity of this material, which consists of almost pure gold, its surface chemistry turns out to be more complex than anticipated. Interactions between gold, chemisorbed O atoms generated and consumed during catalysis, and trace amounts of Ag impurities present in np-Au lead to complex surface dynamics. In this work theoretical modeling by means of ab initio molecular dynamics (AIMD) is combined with an Auger electron spectroscopic study to investigate oxygen-driven Ag surface diffusion on Au model surfaces exhibiting structural characteristics of np-Au. AIMD simulations reveal that surface O atoms dynamically form –(Au–O)– chain structures on the stepped Au(321) surface and lead to surface restructuring, but no chain formation is found on the flat Au(111). Ag impurities at low concentration lower the activation barrier for the –(Au–O)– chain formation, whereas formation of –O–Ag–O– links is energetically slightly unfavorable, especially at high Ag concentration. Further, our study reveals the migration of subsurface Ag atoms onto the surface toward O-rich areas. Using the stepped Au(332) surface with Ag impurities under UHV conditions as a model system, we show that atomic oxygen is able to induce surface segregation of Ag already at 200 K. Our results suggest that atomic surface oxygen should be one of the driving forces leading to the ligament coarsening in np-Au.

*

Corresponding author. E-mail address: [email protected], Tel.: +49-421-21863187, Fax. +49-421-21863188

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Introduction Free, ligand-stabilized, and oxide-supported gold nanoparticles have been the most active domains of research related to gold catalysis.1-5 Different from the well-studied gold-nanoparticle catalysts, the mechanistic understanding of catalytic processes on nanoporous gold (np-Au), a threedimensional nanoporous material, is far less developed. While bulk gold is notoriously inert, nanoporous gold foams demonstrated remarkable catalytic activity and high selectivity for a range of practically interesting reactions in gas and liquid phases, from CO oxidation to more complex organic transformations.6-8 Several explanations for the unexpected chemical activity of np-Au have been proposed, including the role of low-coordinated surface Au atoms, the involvement of Ag impurities (inherently present as residuals after fabrication from an Au-Ag alloy), and the possible effects on the structure and composition arising from not quite well characterized forms of chemisorbed oxygen on its surface.9 Oxygen atoms in the surface region of “as-prepared” and activated np-Au in at least three different chemical states have been identified by XPS and may have been incorporated during the fabrication through dealloying.10 Remarkably, some of the oxygen species appeared to be quite resistant to thermal treatment and could not be completely removed even upon heating to 900 K.11 Theoretical studies suggest that Ag residuals as well as onsurface/subsurface atomic oxygen species could exert notable influence on the surface structure and catalytic properties of np-Au.9, 12 Recent in situ electron microscopy studies by Fujita et al.13 and Friend et al.14 revealed dynamic restructuring of np-Au induced by chemical reactions. In particular, Fujita et al.

13

have shown that catalytic reactions on np-Au promote faceting and

ligament coarsening. Thermodynamically driven ligament coarsening and a loss of active surface by npAu as a result of thermal treatment or during catalytic cycling is ultimately a consequence of the enhanced surface diffusion. A recent study by Balk et al.15 demonstrated that thermal coarsening of npAu occurs more rapidly in contact with nitrogen than in vacuum, suggesting that even adsorption-desorption of unreactive molecules, such as N2, promotes higher mobility of surface Au atoms and causes more rapid restructuring. Friend et al.14 emphasized the effect of ozone pretreatment on changing the catalytic properties of np-Au. Their work demonstrated that ozone activation and subsequent removal of most reactive O species lead to compositional and structural changes in np-Au and resulted in a catalyst with different chemical reactivity compared to as prepared np-Au material. In view of these findings, understanding the restructuring and dynamic evolution of gold surfaces driven by adsorbed O atoms and Ag impurities becomes very intriguing and important for elucidating the catalytic behavior of np-Au on the atomic scale. 2 ACS Paragon Plus Environment

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Oxygen-induced silver segregation in AuAg alloys has been anticipated but not systematically addressed until now. Recently, Schaefer et al.10 used high-resolution photoelectron spectroscopy along with other surface–science techniques to study oxidation and cleaning of nanoporous gold. They observed Ag enrichment in the surface region of np-Au after ozone treatment and a depletion of Ag surface content after a reduction using CO. Could these changes be caused by oxygen driven diffusion of silver due to Figure 1. Example of theoretically predicted stable gold oxide chain structure on the stepped Au(321) surface with impurities. Silver prefers positions close to the chain. Color coding: Au, yellow; Ag, blue; O, red.

preferential formation of surface Ag–O bonds over Au–O bonds on the surface? Some adsorbate-induced surface rearrangements are expected to occur on the time scale (seconds) of catalytic reactions, while those involving

massive restructuring of the surface may take longer time (hours).16 Thus, one of the objectives of the present work is to find out whether Ag diffusion from subsurface layers to the surface could be induced by oxygen adsorbates and to characterize the structural rearrangements in the surface region associated with this diffusion process. In addition to anticipated silver diffusion to the surface triggered by stronger silver bonding with adsorbed oxygen, we should also mention the tendency of the Au/O system to form one- or two-dimensional –(O–Au)– chain structures. According to our recent theoretical study on Au(321)9 as well as similar studies of O adsorption on Au(111)17 and Au(110),18 such –(O–Au)– chain structures represent the thermodynamically most stable form of adsorbed surface oxygen on gold even at low O coverage. Recent experiments finally confirmed this theoretical prediction.19 The chains are significantly stabilized compared to individually adsorbed O atoms, by ~0.15 eV per O atom, whereas in the case of analogous –(O–Ag)– chains, the energy gain is much smaller.9 Therefore, on bimetallic AuAg surfaces with adsorbed O, –(O–Au)– chains are preferred to –(O– Ag)– chains. Silver’s affinity to oxygen thus propells it to occupy positions close to –(O–Au)– chains, such as in the structure shown in Fig. 1.20 In this work, we carried out theoretical modeling and experimental Auger electron spectroscopic study of oxygen-driven silver surface diffusion on flat and stepped Au surfaces. Theoretical DFT-based simulations employed the Au(321) and Au(111) model surfaces and addressed the structural and dynamic properties of Ag impurities. These surfaces were chosen to represent rough and flat structural motifs present in nanostructured gold and gold-silver catalysts. 3 ACS Paragon Plus Environment


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The Au(321) surface possesses narrow terraces of (111) type as well as zigzag-shaped steps with 6-and 8-fold low-coordinated Au atoms at the step edge. Au(321) and Au(111) have been successfully employed to model reactions on gold catalysts in earlier theoretical studies.21-26 The terraces and kinked steps on the Au(321) surface make it a suitable model to represent the curved structure of the ligaments of np-Au. Fajín et al.21 previously studied oxygen behavior on Au(321) and identified most favorable adsorption sites. They also investigated surface restructuring at high O coverage by considering different arrangements of O.22 Although these authors did not specifically look for the formation of –(O–Au)– chains, the lowest-energy structures identified in their study22 contain structural motifs with linear O–Au–O units connected in infinite chains. Herein, ab initio molecular dynamics (AIMD) simulations have been carried out to provide deeper insight into the surface reconstruction processes leading to the formation of oxygen chain structures on gold and into oxygen-driven silver diffusion and segregation. In the experiments the Au(332) surface was used, which exhibits (111) oriented terraces separated by straight, close-packed steps. Due to imperfections, steps were shown to include kinks, like those present on the Au(321) surface, however, in a non-periodic fashion.27 Therefore, the computational and experimental models are rather closely related to each other.

Computational and Experimental Details In the computational part of this work we applied a combination of ab initio molecular dynamics (AIMD) simulations and standard (static) DFT methods to study surface evolution processes. Whereas AIMD simulations allow us to explore time-evolution of complex condense-phase systems at a given temperature and find low-energy reaction pathways, static DFT was used to compute activation barriers, which are not directly available from AIMD. AIMD simulations. We employed a p(3×2) unit cell of the regular Au(321) surface with a slab thickness of ~12 Å and ~18 Å vacuum space separating the slab from its periodic image in Z direction. The bottom half of the slab was frozen while the top layers were allowed to relax without constraints. The ab initio molecular dynamic (AIMD) simulations were performed using the CP2K package.28 All initial geometries that served as input for AIMD simulations were fully optimized to a local minimum by means of electronic-structure computations performed with CP2K. The generalized-gradient approximation in the parameterization of Perdew, Burke, and Ernzerhof (PBE)29-30 was used to compute the exchange-correlation energy. The choice of the PBE functional 4 ACS Paragon Plus Environment

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is justified by its very good performance in describing bulk properties of transition metals. 31-32 The valence electrons (5d106s1 for Au, 4d105s1 for Ag and 2s22p4 for O) were described using hybrid Gaussian and plane-wave (GPW) basis sets,33 and the cutoff energy of 500 Rydberg of auxiliary plane wave basis sets was adopted. We employed special double-ζ valence plus polarization (DZVP) basis sets optimized to minimize basis set superposition errors.34 Core electrons were described with scalar relativistic norm-conserving pseudopotentials.35 Brillouin zone integration was performed with a reciprocal space mesh consisting of only the gamma point. In the simulations, we used Nose–Hoover thermostat (NVT) to sample from the canonical ensemble.36-37 The relatively short time scales of AIMD (time step of 1.0 fs and total time of a simulation run of up to 30 ps) limit the sampling to fast and low-energy-barrier events. To rapidly explore a large phase space volume of surface configurations, a statistical sampling was performed at an elevated temperature of 700 K. This strategy was previously successfully employed in a number of AIMD studies.38-39 Static calculations with the VASP code. To identify the structures and energies of the transition states and intermediates involved in the first steps of oxygen chain formation, we used the plane-wave based VASP code 40-41 and a projector augmented wave (PAW) method 42-43 with an energy cutoff of 415 eV. The exchange-correlation potential was described by the PBE parameterization of the generalized gradient approximation. For these calculations a p(2×2) unit cell was chosen. The slabs were ~7.2 Å thick, and the vacuum spacing between periodically repeated slabs was ~8.5 Å. Atoms in the bottom half of the sab were kept frozen at their bulkoptimized positions, whereas the upper half of the slab was allowed to relax without constraints. For the integrations over the Brillouin zone we used a 5×5×1 k point mesh generated by the Monkhorst-Pack procedure 44 and we invoked a generalized Gaussian smearing technique45 (with the smearing width of 0.05 eV). The structures were relaxed until the force acting on each atom was ≤ 2×10−2 eV Å-1. Transition states (TSs) were determined by applying the nudged elastic band (NEB) method46 and were subsequently refined using the dimer method. 47 Auger spectroscopy on Au(332). The Au(332) surface (Matek, Germany) was cleaned using several sputter (Ar-ions, 1000 V, 15 min) and anneal cycles (1000 K, 10 min). Long-range order and chemical composition were checked using LEED (Omicron, Germany) and Auger spectroscopy (Physical Electronics). Ag (99.99 %, Alfa Aesar) was evaporated at 200 K onto the cleaned surface using an electron beam evaporator (EFM3, Focus). The amount of Ag was calibrated using a quartz crystal microbalance. Prior to oxygen exposure the sample was heated to 650 K for 10 minutes, which leads to a strong decrease in the amount of silver on the surface, in 5 ACS Paragon Plus Environment


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line with earlier reports in the literature.48 The annealed surface was exposed to a beam of oxygen atoms (7 % oxygen atoms in O2) at 200 K as created by a thermal cracker (Dr. Ebel MBE Komponenten). Auger spectra were taken at 100 K if not stated otherwise.

Results and Discussion Formation of oxygen chain structures on a stepped gold surface. Atomic oxygen species are crucial players in aerobic oxidation reactions catalyzed by gold. Experimental characterization suggests that np-Au has a relatively high O content in the near-surface region, whereas at least three different types of O species can be distinguished.11 The chemical nature of these O phases has not been understood yet. Some of them are more reactive toward CO oxidation, whereas the others, are reacting only slowly and may serve as oxygen reservoirs. Furthermore, as alluded to above, supply and consumption of surface O may cause restructuring and redistribution of Ag impurities. Therefore, studying the surface evolution of gold in the presence of O atoms should help to better understand the interplay between gold, oxygen and silver impurities during the catalysis. Chain structures on Au surfaces were first predicted theoretically17-18 but recently also detected experimentally on Au(110)-(1×2).19 Figure 2. Snapshots of an AIMD simulation showing (Au-O)- chain formation from individually adsorbed O atoms on Au(321) without Ag impurities. (3×2) unit cell and O coverage of 0.17 ML. Color coding: Au, yellow; O, red.

The AIMD simulations described in the following reveal atomic-level insight into the dynamics of the chain formation process. The power of AIMD is that new reaction pathways,

not necessarily foreseen before starting the simulation, such as complex surface rearrangements involving multiple atoms, would be difficult to foresee and explore by conventional DFT. A simulation explores on the order of 104 structures and after a sufficient time it drives the system to a thermodynamically more stable configuration than the initial state. Approximate transition states and minima identified in a simulation can then be refined by conventional DFT. A simulation 6 ACS Paragon Plus Environment

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(Movie S1) started from a model of the Au(321) surface with five individually adsorbed O atoms per (3×2) unit cell corresponding to a coverage of 0.17 ML (Fig. 2, 0 ps). The coverage is defined as the ratio between the density of the adsorbate overlayer and the atom density of the topmost surface layer. An atom is concidered to belong to the top surface layer if its coordination number is 11 or less. Hence, there are 30 Au atoms in the top layer of the Au(321)-(3×2) surface unit cell. We have chosen the initial positions of O atoms at the energetically most favorable 3-fold fcc adsorption sites at the terrace edges of Au (321) (Figure 2, 0 ps).12 The dynamic evolution of this model was simulated at 700 K for 22 ps. Note that because of the elevated temperature used in our AIMD simulations for technical reasons (see Computational Methods), the simulation times at which processes occur are faster than they would be at room temperature. Assuming a simple Arrhenius rate law and a preexponential factor of 1013 s-1, a process with Ea = 0.2 eV would occur on a timescale of ~3 ps at 700 K, whereas the same process would require >80 ps at 300 K. The simulation (Fig. 2, Movie S1) revealed the formation of a –(Au–O)– chain structure mainly by diffusion of O atoms and accompanied by relatively large displacements of adjacent Au atoms. We labeled those O and Au atoms on the surface which primarily contributed to the chain formation with numbers from 1 to 11, Fig. 2. The first –O–Au–O– chain fragment is formed during the first 7 ps and involves the movement of O(3) toward Au(4) already connected to O(5). Meanwhile, Au(1), initially located at the edge of a lower terrace, lifts up and attaches to Au(2) of the upper terrace. Au(8) is also lifted up to form a link to Au(6). Concomitantly, Au(11) moves towards the chain formed between Au(8)-O(9)-Au(10) and binds to O(9). It is interesting to note that –O–Au–Au–O– links also form at this stage on both sides of the –O(3)–Au(4)–O(5)– link. These –O–Au–Au–O– structures seem to be slightly stabilized with respect to regularly adsorbed O atoms, see below. During the next 7 ps of the simulation O(7) migrates and attaches to Au(6) and Au(8) forming a long chain of –O(3)–Au(4)–O(5)–Au(6)–O(7)–Au(8)–O(9). The long chain formed keeps stable to the end of this simulation at 22 ps; therefore we can assume that the simulation has aproached a local minimum. This simulation clearly demonstrates the tendency of surface O to form chain structures on a stepped Au surface. To estimate the barrier heights associated with chain formation, we performed static DFT calculations and computed the energy profile for the formation of the first –O–Au–O– fragment, Fig. 3. For this calculation we chose a smaller sized (2×2) unit cell than in AIMD simulations (3×2); however, the oxygen adsorbates were arranged on the Au(321) substrate in such a way as to 7 ACS Paragon Plus Environment


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be similar to the initial structure of the AIMD simulation, cf. Figs. 2 and 3. The O coverage in this model is 0.15 ML. The NEB search revealed that the migration of O(3) via TS1 is preceded by the formation of a very shallow minimum (MIN, Fig. 3), in which Au(1) atom from the lower terrace lifts up and forms a bonding interaction with Au(2) of the upper terrace. This small rearrangement leads to the formation of a metastable –O–Au(1)–Au(2)–O(3)– fragment, also observed in the simulation discussed above. In the intermediate MIN the –O–Au(1)–Au(2)–O(3)– fragment is still bent, whereas it becomes almost linear in the final state (FS). The main rearrangement from MIN to FS is, however, the shift of O(3) and the formation of a –O(3)–Au(4)–O(5)– link. The overall process is exothermic by 0.18 eV with respect to IS, indicating a stabilization of the chain structure with respect to individually adsorbed O atoms. Both Au–O and Au–Au bonds within the chain (for values see Fig. 3) shorten compared to their regular values, ~2.14-2.17 and 2.85-2.92 Å, respectively. This is consistent with a stronger bonding within the chain and also with more directional Au–O and Au–Au bonds in the chain than in the initial structure with individually adsorbed O atoms. As our previous study demonstrated, such chains may diffuse as an entity on the surface retaining the linking sequence.20 The activation barrier associated with the formation of the first link is only 0.35 eV with respect to IS and should be very easy to overcome at elevated temperature. We have not calculated the activation barriers for the subsequent steps revealed by AIMD simulation but they should be of comparable height to the first one or lower, since the next link is added to the chain on the same time scale as the first one is formed. We have also calculated two additional pathways for the formation of a short –O–Au–O– chain

starting

from

different

initial arrangement of O atoms on the surface, Fig. S1. Both of them show barrier heights below 0.5 eV Figure 3. First steps of the chain formation. (2×2) unit cell and O coverage of 0.15 ML. Energies are in eV, bond distances in Å. Color coding: Au, yellow; O, red.

and are slightly exothermic. Interestingly,

in

our

simulation the chains started to 8 ACS Paragon Plus Environment

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form along the step edges of the Au(321) surface. We expect that the enhanced flexibility of the Au atoms near or at the step edge contribute to the facile chain formation. To test this hypothesis, we also studied the formation of chains at the flat Au(111) surface starting with the comparable initial O coverage, Fig. S2. No chain formation was found after 24 ps, confirming the crucial role of steps for the kinetics of this process. We thus expect facile formation of –(Au– O)– chains on the surface of nanoporous gold, which shows a high density of steps and lowcoordinated Au atoms49 if O atoms were generated there during the catalytic cycle or deposited to the surface deliberately (e.g. by ozone treatment). Effect of silver impurities on the formation of metal-oxygen chain structures. Nanoporous gold contains

silver

impurities

(typical

bulk

concentration of Ag is ~1-5 atom%) originating from the incomplete leaching of Ag from a binary AuAg alloy. The concentration of silver in the surface region is even higher (6-20 atom%) and may increase if samples are stored in air,10,

11

which renders silver an important constituent to understand the surface chemistry of np-Au. Earlier studies already suggested that silver impurities

may

assist

in

adsorption

and

dissociation of O2, providing active O species to the Au surface.9,

12

On the other hand, it was

reported that high Ag concentrations did not seem to improve the activity and were rather detrimental for the selectivity of partial oxidation of alcohols.7, 50 Below we study the effect of Ag impurities

Figure 4. Effect of Ag impurities on the –O–M–O– chain formation (M = Au, Ag). (2×2) unit cell and O coverage of 0.15 ML. One (a, b), three (c), four (d), or twenty (e) Ag atoms per unit cell are substituted in the top layer. Energies are in eV and are referenced to the energy of IS, bond distances in Å. Color coding: Au, yellow; Ag, blue; O, red.

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on the potential energy surface of –O–Au–O– chain formation along the same path and with the same initial arrangement of O adsorbates as disscussed above for pure Au(321). Fig. 4 compares several possible arrangments of Ag atoms with respect to oxygen atoms involved in the chain formation, and different concentrations of Ag in the top layer. These structures were chosen as representative examples to illustrate the dependence of the activation barrier of a chain formation (i) on the size of Ag ensembles and (ii) on the question of whether the forming chain contains a Ag atom or not. Calculations reveal that Ag impurities generally lower the activation barrier for the migration of O(3) atom. In the case, where the migrating O atom is completely surrounded by Ag, the barrier reduces by a factor of 2, to only 0.16 eV. Fig. 4 also shows that the formation of a -O– Ag–O– chain (Fig 4b, d and e) is in all cases a slightly endothermic process at variance to –O–Au– O– chain formation, which is always found to be exothermic. The most favorable reaction energy, -0.20 eV, was found for the case, where an individual Ag atom is located near the migrating O atom but is not a part of the chain (Fig. 4a). Whereas silver can also form –O–Ag–O– chains, with Ag–O distances, 2.09-2.10 Å, that are shorter than typical Ag–O distances for adsorbed single O atoms, 2.13-2.15 Å, calculations show that these chains are not stabilized with respect to individually adsorbed O bound to Ag. Here, two competing effects come into play: the ionic component of the bond between Ag and O favors high coordination (adsorption at 3fold sites) and the repulsion between charged O atoms favors structures with Figure 5. An AIMD simulation showing vertical Ag diffusion from a subsurface layer to the surface. See also Movie S2. Ag1-Rsurf represents the distance of Ag(1) atom initially located near the step edge to a reference surface and is plotted along the whole simulation trajectory. The reference surface is chosen as a Z coordinate slightly above the Au atom of the O-Au-O fragment at 0 ps and is kept fixed during the simulation.

maximum separation between adsorbates, whereas the covalent component favors a linear O–Ag–O arrangement with the most efficient orbital overlap. In case of Au, the covalent component is stronger, as seen by

electronic structure analysis;20 hence, the formation of linear O–Au–O structures is energetically favorable. Interestingly, if all top-layer Au atoms are replaced by Ag (Fig 4e), the formation of the 10 ACS Paragon Plus Environment

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–O–Ag–O– chain is most unfavorable with the reaction energy of +0.18 eV and a barrier with respect to IS of 0.25 eV, which suggests that silver alone should not form chains, at least not at the relatively low coverage of O assumed in this work. The trends discussed above already shed some light on the complex interplay of different energetic effects in Au/Ag/O ternary systems, which should become even more intricate with respect to the silver and oxygen speciation at the highly stepped surfaces of np-Au. Dynamic evolution of nanoporous gold with Ag impurities: oxygen-driven segregation of silver. The diffusion and segregation of Ag atoms in nanoporous gold may have important implications for catalysis, especially for O2 activation.9 Under real catalytic conditions, both Ag impurities and surface O atoms are present in the nanoporous gold catalyst.11 Even though the transient concentration of oxygen is considered to stay low, these species may influence the chemical and physical processes taking place on np-Au. Two types of Ag diffusion in the surface region shall be distinguished here: the lateral migration of Ag exposed to the surface and the Ag migration from subsurface sites onto the surface of the system. The first possibility has already been elucidated in our recent work,20 where we have shown how a weakly bonded silver atom at a surface step migrates toward a short O–Au–O chain and forms a bond with oxygen. It would be even Figure 6. An AIMD simulation showing vertical Ag diffusion from a subsurface layer to the surface. See also Movie S3. Ag atom is initially located below the surface gold layer. The distance of Ag to the surface, Ag2-Rsurf, and the coordination number of migrating Ag were monitored along the whole simulation trajectory. The reference surface is chosen same as in Fig. 5.

more interesting to know whether silver atoms located in deeper surface layers can also

“feel”

the

presence

of

oxygen

adsorbates on the surface and diffuse from the bulk to the surface.

Therefore, we established a computational model on the basis of the (3×2) supercell of the 11 ACS Paragon Plus Environment


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Au(321) surface to compute various diffusion processes of Ag atoms in nanoporous gold. In the following AIMD simulation, we started from a structure shown in Fig. 5 (0 ps) and in more detail in Fig. S3, where 16 Ag atoms occupy subsurface positions under the two adjacent O–Au–O fragments. Ag atoms were placed in a somewhat random fashion but not too far from the surface, so that the diffusion to the top layer would have a chance to happen on the timescale of the simulation. To elucidate the effect of surface O on Ag diffusion we compared below the evolution of starting structures with and without surface oxygen. Short O–Au–O chains are the simplest chains one could imagine and may serve as prototype chain structures. Such fragments could form even at relatively low O coverage. Calculations show that already these short chains are energetically more favorable than the individually adsorbed O atoms at the same O coverage. For instance, the work of Fajín et al.21 reported a short O–Au–O chain at the step edge of Au(321), similar to the structure considered here, to be the lowest-energy final state of O2 dissociation. Assuming short chains in the starting structure of our simulation brings an advantage that the rest of the surface is not significantly perturbed with respect to the oxygen-free surface. Therefore, we can separate the Au-O chain formation from Ag diffusion assuming that the chain formation takes place first and the diffusion of silver follows as the next step. The structural evolution of the surface was studied using AIMD simulations, Movies S2, S3. The snapshots of the top view of the surface are given in Fig. 5, see also Movie S2. Fig. 5 shows that already after 8 ps a silver atom labeled Ag(1) initially located in the subsurface layer directly under the step lifts up and links the two short chains into a longer one. By linking two O–Au–O fragments a migrating Ag atom creates two new Ag–O bonds resulting in the overall energy gain. Thereafter, the extracted Ag atom retains its bonds with oxygen; however, the linear O–Ag–O fragment becomes bent after 32 ps of the simulation. This is in line with the observation that for O–Ag–O the directional bonding is not so pronounced, in contrast to O–Au–O fragments, which remain linear once formed. Meanwhile, another subsurface Ag atom labeled as Ag(2) in Figs. 5,6, which is initially ~4 Å away from the nearest adsorbed oxygen atom, also diffuses toward the surface and finally finds itself in the top surface layer, Figs. 5,6. This transition occurs at ~14 ps of the simulation. By the end of the simulation the distance of Ag(2) to the surface, Ag-Rsurf, decreases from initially ~4 to ~0 Å, indicating that Ag(2) has migrated onto the surface. Fig. 6 shows snapshots from the simulation as cuts through the slab illustrating the movement of Ag(2). We also monitored the coordination number of that Ag atom, which changes from 12 to 6-8 during our simulation, manifesting the transition from a bulk to a surface position. 12 ACS Paragon Plus Environment

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To exclude other driving forces responsible for Ag diffusion, we repeated the simulation for a similar initial structure but without adsorbed O atoms, Fig. S4. The arrangement of Ag atoms is very similar to the initial structure of Fig. S3, except the two Ag atoms at the step edge, which were additionally replaced by Ag. These two Ag substituents were added to simultaneously check for Ag diffusion in the top surface layer. The structures of the system without oxygen adsorbates sampled from the trajectories at 0 ps, 8 ps, 16 ps and 24 ps are shown in Fig. S4. On this time scale no diffusion or surface reconstruction was observed, despite the strong concentration gradient of Ag between the top surface layer and the subsurface layers. Therefore, the diffusion driven by entropy to distribute the Ag evenly throughout the Au material can be excluded on the time scale of our simulation. The diffusion coefficient was calculated from Einstein relations, see the SI, Eq. (S1), that give a value of 0.028 Å2 ps-1 implying no diffusion. Hence, these results provide evidence for oxygen induced diffusion of Ag onto the surface of np-Au. This is in line with experiments reported in the literature.10 In the latter study photoelectron spectroscopy was used to show that oxidation of np-Au by ozone leads to an increase of the surface concentration of silver already at 300 K. To enable a more direct comparison to the calculations presented here, we also performed experiments on a Au(332) surface. To elucidate the effect of oxidation of the system we deposited about 1 ML Ag onto the surface at 200 K and annealed the system subsequently to 650 K. The annealing step reduces the amount of silver on the surface drastically as shown previously.48 This is directly reflected in the Ag/Au intensity ratio, which drops severely upon annealing (see Table 1). The reduced Ag intensity is in line with our expectations based on the very small mixing enthalpies of Ag and Au,20 which renders the entropically driven dissolution of Ag in Au an important process. Exposure of such a surface to an oxygen atom beam results in the oxidation of the system, which is evident from the appearance of an oxygen signal in the Auger spectra. The analysis of the Auger spectra, Fig. S5, reveals a drop of the Au intensity, while the intensity of the Ag line increases slightly, however, the change is close to the error bar of the experiment. It is known from high resolution XPS that both silver and gold are oxidized by such a treatment,10 which should cause an intensity drop of both metal signals. While this is clearly observed for the Au signal, the corresponding signal of Ag lacks this behavior. The resulting increase of the Ag/Au ratio (Table 1) is a clear indication for an increased concentration of Ag on the Au surface after exposure to oxygen. It shall be noted in passing that the effect is even more pronounced if the annealed system is subject to CO oxidation. These results indicate that the oxygen induced segregation of 13 ACS Paragon Plus Environment


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Ag onto the surface of Au is a process, which has to be considered at all temperatures in which npAu is used as an oxidation catalyst.

Table 1. Intensity of Ag and Au Auger lines (arbitrary units) before and after annealing and after oxygen exposure. Ag

Au

r = Ag/Au

Au(332) + 1 ML Ag

0.43

0.11

3.9

+ annealing 650 K

0.02

0.35

0.08

+ oxygen exposure at 200K

0.025

0.22

0.13

Conclusions In this study, we theoretically investigated the dynamic evolution of a model Au(321) surface containing adsorbed atomic oxygen, Ag impurities, or a combination of both. Our model has been chosen to mimic the surface morphology and composition of nanoporous gold, but our results can certainly be applied to other gold-based catalysts, such as bimetallic Au-Ag nanoparticles.51 The AIMD simulations of this work reveal that surface O atoms dynamically form –(Au–O)– chain structures and lead to surface reconstruction. Ag impurities at low concentration lower the activation barrier for the chain formation, whereas formation of –O–Ag–O– links is energetically slightly unfavorable, especially at high Ag concentration. Our results suggest that atomic surface oxygen is one of the driving forces leading to the ligament coarsening in nanoporous gold because oxygen chain formation and Ag diffusion result in surface reconstructions and generate Au atoms inside chain structures with low coordination numbers. The mobility of these low-coordinated Au atoms released through the consumption of O atoms during the reaction should prompt further reconstructions. In addition, our study reveals oxygen-induced segregation of subsurface Ag atoms onto the surface, which tend to migrate to the O-rich areas. Using the stepped Au(332) surface with Ag impurities under UHV conditions as a model system, we were able to show that atomic oxygen is able to induce surface segregation already at 200 K. These results tie in nicely with oxygen induced Ag surface segregation in nanoporous gold.10 The observed temperature at which this process is operative indicates that such processes have to be considered in case of catalytic reactions on np14 ACS Paragon Plus Environment

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Au, which are typically performed above 200 K.

Supporting Information Additional pathways of O-Au-O chain formation. AIMD simulations of Au(111) surface with adsorbed O. Details of the model used in AIMD simulations of Ag diffusion. Auger spectra of Au(332) surface. Movies illustrating selected AIMD simulations.

Acknowledgements We acknowledge the financial support from the German Research Foundation (DFG) within framework of research unit 2231 “NAGOCAT” Projects No. BA 1710/29-1, KL 1175/14-1, MO 1863/4-1, and RI 1025/3-1. We thank the North-German Supercomputing Alliance (HLRN) for providing computational resources. R.M. thanks DAAD and CNPq for support (Project No.: 290149/2014-2 DAAD/CNPq).

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