Letter pubs.acs.org/JPCL
Potential-Dependent Structural Memory Effects in Au−Pd Nanoalloys Jakub S. Jirkovský,*,†,‡,⊥ Itai Panas,*,§ Simon Romani,∥ Elisabet Ahlberg,‡ and David J. Schiffrin† †
Chemistry Department, University of Liverpool, L69 7ZD, United Kingdom Department of Chemistry, University of Gothenburg, SE-412 96, Gothenburg, Sweden § Chalmers University of Technology, Department of Chemistry and Biotechnology, SE-412 96, Gothenburg, Sweden ∥ Centre for Materials and Structures, University of Liverpool, L69 3GH, United Kingdom ‡
S Supporting Information *
ABSTRACT: Alloying of metals offers great opportunities for directing reactivity of catalytic reactions. For nanoalloys, this is critically dependent on near-surface composition, which is determined by the segregation energies of alloy components. Here Au−Pd surface composition and distribution of Pd within a Au0.7Pd0.3 nanoalloy were investigated by monitoring the electrocatalytic behavior for the oxygen reduction reaction used as a sensitive surface ensemble probe. A time-dependent selectivity toward the formation of H2O2 as the main oxygen reduction product has been observed, demonstrating that the applied potential history determines surface composition. DFT modeling suggests that these changes can result both from Pd surface diffusion and from exchange of Pd between the shell and the core. Importantly, it is shown that these reorganizations are controlled by surface adsorbate population, which results in a potential-dependent Au−Pd surface composition and in remarkable structural memory effects. SECTION: Surfaces, Interfaces, Catalysis
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formation instead of H2O2.10 Hence, the ORR selectivity toward either H2O2 or H2O is very sensitive to the Au−Pd surface ensemble arrangement. Similar observations have been recently made in relation to the differential reactivity of monomers, dimers, and trimers for the hydrogen oxidation and evolution reactions.11 Importantly, both Au and Pd are relatively stable in the alloy regardless of the extent of components segregation.22,23 The use of gold ensures weak interactions with surface adsorbates, and these properties render Au−Pd nanoalloys an ideal model system to study adsorbate-induced structural reorganizations. In the present Letter, we utilize the H 2O 2 production selectivity as a sensitive analytical tool. Electrochemical techniques are ideally suited to probe the yield of peroxide from the ORR and hence provide a measure of the surface composition due to the known peroxide formation selectivity as a function of surface composition.10 Au0.7Pd0.3 nanoalloys of uniform size and composition dispersed on Vulcan XC72 carbon were employed.10,24 This alloy composition was chosen to allow a sensitive detection of surface Pd composition; previous work showed that peroxide selectivity passes through a maximum for a Pd composition of 8 atomic % and decreases markedly as the concentration of Pd
etal alloys are of great interest in heterogeneous catalysis and electrocatalysis.1 Nanoparticulate alloys provide opportunities to achieve enhanced catalytic performance2−4 from tuning electronic properties and from size-induced geometry effects.1,5 In addition, surface ensemble effects can also alter significantly their (electro)catalytic behavior6−11 and this, in turn, depends on the surface segregation energies of the alloy components. The segregation energy of a single atom (the “impurity”) of an element present in a matrix (the “host”) has been studied theoretically in attempts to predict likely surface enrichment by the components of bimetallic alloys. Among other approximations, these calculations ignore the influence of specific interactions of the medium on surface composition; that is, the alloy is considered to be present in vacuum.12,13 For example, the adsorption of hydrogen14 or CO8,15 can induce a displacement of Pd atoms in a Au−Pd alloy, from a subsurface position to the surface. As observed for Pt−Co nanoalloys,16 Pt binds CO more strongly than Co, resulting in the formation of a Pt skin, but on CO removal, Co is able to diffuse back to the surface through the segregated Pt layer, as demonstrated from linear sweep voltammetry.17 Au−Pd alloys have recently attracted a great deal of attention for possible applications in the direct18−20 and the electrochemical synthesis of H2O2 in fuel cells.10 The enhancement of selectivity of the oxygen reduction reaction (ORR) toward H2O2 originates from the presence of individual surface Pd atoms surrounded by a Au matrix on the Au−Pd surface;10,21 in contrast, the presence of contiguous Pd atoms leads to H2O © 2012 American Chemical Society
Received: December 19, 2011 Accepted: January 6, 2012 Published: January 6, 2012 315
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in the nanoalloy is increased.10 Therefore, the effect of vicinal surface Pd atoms is very marked, and an increase in surface Pd concentration would result in a very marked decrease in peroxide selectivity, whereas the opposite effect would be observed for a decrease in surface concentration of Pd. HRTEM images shown in Figure 1A,B indicate good particle
Cyclic voltammetry was used to probe the surface composition of the particles by analyzing the oxidation/reduction of the corresponding surface oxides.6,10 Cyclic voltammograms (CVs) measured in Ar-saturated 0.1 M HClO4 for Au0.7Pd0.3/C electrodes are shown in Figure 1C, where clearly distinguishable features related to the two alloy components can be recognized. The formation of a Pd-rich oxide (PdOy) between 0.5 and 1.0 V and of a Au-rich oxide (AuOy) layers between 1.0 and 1.25 V can be observed. During the backward sweep to negative potentials, the counterparts to these processes appear; that is, the peaks at 0.4 and 0.8 V correspond to the reduction of PdOy and AuOy surface oxides, respectively,6,10 providing evidence of the presence of both alloy components on the particles surface.10 Note that all electrodes measured further down have been first characterized by cycling seven times according to Figure 1C to ensure reproducibility and the same starting electrode conditions. Figure 2A shows O2 reduction currents measured on a Au0.7Pd0.3/C rotating ring disk electrode (RRDE). The ORR is
Figure 2. Oxygen reduction on the Au0.7Pd0.3/C electrode. (A) Polarization curves measured in O2-saturated 0.1 M HClO4 at (1) 200, (2) 400, (3) 800, (4) 1500, and (5,6) 2500 rpm; 6 was measured in the first potential sweep, and 5−1 were recorded after 40 subsequent cyclic potential sweeps between 0.6 and −0.3 V. Sweep rate = 10 mV s−1. (B) Selectivity toward H2O2 production as a function of potential calculated from the Koutecky−Levich (K−L) plots and the rotating ring disk electrode measurement (RRDE).
clearly observed at potentials more negative than 0.5 V. The selectivity toward H2O2 production, SH2O2, was determined by two independent methods, from the slope of the Koutecky− Levich (K−L) plots, as described by equation (ES1)25,26 and by the simultaneous detection of H2O2 at a Pt ring surrounding the Au−Pd/C electrode surface. (See the Supporting Information for details.)10,25,26 Combining equations ES1 and ES2 of the Supporting Information, the dependence of SH2O2 on potential was calculated from the current density−potential (j−E) data. The results obtained for the first sweep in Figure 2A started at the rest potential, 0.5 V, that is, for a surface that had not been preoxidized, are shown in Figure 2B. In contrast with results for pure Au nanoparticles,25 sweeping the potential between −0.3 to 0.6 V lead to a gradual decrease in the
Figure 1. Characterization of Au0.7Pd0.3/C. (A) HAADF and (B) HRTEM images of Au0.7Pd0.3/C. Results of particle size distribution are shown in the inset to panel A. (C) Cyclic voltammetry on fresh Au0.7Pd0.3/ C electrode in Ar-saturated 0.1 M HClO4. Sweep rate = 200 mV s−1.
dispersion and uniform size. X-ray diffraction confirms a high degree of alloying according to Vegard’s law, and, importantly, no indication for the presence of a pure Au or Pd phase was observed; EDX analysis confirmed a uniform composition for individual particles.10 316
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measured O2 reduction current. After ∼40 potential cycles (∼2 h), no further changes in the j−E dependence were observed (Figure 2A). This Figure also shows the results at different rotating rates obtained for a cycled electrode. H2O2 selectivity results (Figure 2B) show very good agreement between the K−L and the RRDE analyses, demonstrating quantitative H2O2 detection. Figure 2A,B shows that the decrease in current on cycling is associated with switching of the reduction channel, from a 4 to a 2 e− mechanism. Because the Pd surface concentration determines H2O2 selectivity, it is proposed that the changes observed reflect rearrangement of the Au−Pd surface. This was further investigated for the same electrode in an O2 saturated solution. The potential was swept rapidly (200 mVs−1) from 0.5 V first to 1.25 V, a potential where complete surface oxidation takes place, and then cycled at 200 mV s−1 between −0.3 and 1.25 V, as indicated in Figure 3A (without rotation, 10 cycles ∼3 min). Changes in Au−Pd surface composition were investigated from oxide reduction waves in subsequent CV cycles, but only the first cycle differed from the rest. The onset of surface oxide formation appears at ∼0.7 V in the first sweep. In subsequent cycles, however, the onset of the current due to surface oxidation matches that of a fresh Au0.7Pd0,3 electrode (Figure 1C) and does not change upon further cycling (Figure 3A). The reduction scans (negative currents) qualitatively differ from those in Figure 1C only due to the presence of the ORR, and no new oxide(s) reduction features are in evidence. This experiment demonstrates that all changes observed in the product distribution of the ORR are related to the state of the surface. The voltammetric signature of the surface oxides appears to be uncoupled from the oxygen reduction process. (Compare Figures 1C, 2A, and 3A.) The shift in oxide layer formation potential by more than 100 mV in the first sweep (Figure 3A) compared with the fresh electrode in Figure 1C implies an increased resistance for surface oxidation (increased nobility),27 which, however, disappears after the first oxidation scan at potentials more positive than 0.7 V. (See inset to Figurer 3A). The increase in surface oxide stability between the first and the 10th scan in Figure 3A is indicative of another, very fast, Au−Pd surface rearrangement from a Au- to a Pd-rich surface.6,10 Considering the time taken for these changes to take place within the voltammetric scans, this process takes less than 3 min to complete. The changes in surface composition in Figure 3A are reflected in changes of H2O2 selectivity, as shown in Figure 3B−D. The higher reduction currents in the initial polarization curves in Figure 3C are due to the higher contribution of the ORR channel to water. The low H2O2 selectivity observed is comparable to that at the beginning of the experiment described in Figure 2, which suggests a similar Au−Pd ensemble composition of the surface. Indeed, during the CV treatment in Figure 3A, the surface is repeatedly oxidized and reduced. Studies on Pt single crystals28,29 have recently shown that this process induces severe surface reordering, and thus the fast rearrangements observed here will probably be facilitated, which was the reason for choosing this electrode pretreatment. A more detailed study is required to relate the dependence of surface reordering on potential cycling. In addition, the CV scans in Figures 1C and 3A and, in particular, their similarity after multiple cycles, demonstrate good stability of the investigated catalytic material and that both Au and Pd are present at the surface during these experiments. No Pd dissolution was noticeable, in agreement with the known stabilization effects by Au on Pd segregation.4,6,23
Figure 3. Oxygen reduction on the Au0.7Pd0.3/C electrode with different pretreatment history. (A) Cyclic voltammetry on the Au0.7Pd0.3/C electrode in O2-saturated 0.1 M HClO4. Sweep rate = 200 mV s−1. (B) Ring currents (ring potential = 1.0 V) recorded simultaneously with (C) polarization curves measured in O2 saturated 0.1 M HClO4 at 200 rpm. Sweep rate = 10 mV s−1. (D) ORR selectivity toward H2O2 production as a function of potential.
The above results indicate that the changes in surface composition during the oxidation cycle increase the yield of water by populating the nanoparticle surface with a high concentration of Pd atoms, and this effect does not appear to revert to a Au-rich surface in the short time of the cycling scans. To investigate if the surface rearrangements can be reversed, we reduced an electrode that had been oxidized as previously described at the start of the cycling treatment by applying a repeated sequence of constant potential steps at 0.1 V of 30 min each. The potential chosen 317
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Figure 4. Au−Pd (111) slabs with different Pd ensembles. Slabs obtained from Au(111) by replacing some surface Au (yellow) atoms by Pd (blue) to get (A) homogeneous distribution, (B) Pd trimers, (C) homogeneous distribution with oxygen atom (red) adsorbed, and (D) Pd trimers with oxygen atoms adsorbed. Slabs with oxygen atoms on the surface with (E) homogeneously distributed Pd within the solid and Pd segregation to the oxygen-covered surface from (F) the bottom layer (oxygen free surface) and (G) the middle layer; see also the Supporting Information for additional explanation. Energy differences obtained from comparing the total energy of actual slabs with and without adsorbed oxygen are indicated in the Figure.
segregation of Pd by adsorption of oxygen species on the nanoparticle surface is fast its reduction and further realloying is slow, taking over 2 hours to complete at a reduction potential of 0.1 V Ensembles of individual Pd atoms dispersed within a Au surface enhance the H2O2 selectivity to very high values, whereas the presence of Pd dimers or higher aggregates supports the formation of water.10 Therefore, the observed pronounced changes in H2O2 selectivity suggest that the Au− Pd surface composition varies depending on the pretreatment potential and hence on the adsorption of oxygen species. Adsorbate-induced segregation and consequent changes in surface reactivity for Pd−Au alloys, for instance, for adsorbed CO, are well known.8,15,30 Contiguous Pd atoms that form on the oxidized Au−Pd surface above ∼0.7 V disperse within the Au matrix when reductive conditions are re-established at potentials more negative than 0.6 V. This can happen either by diffusion of Pd inside the nanoparticles or by surface diffusion of Pd monomers to yield a lower surface coverage by dimers or trimers. The properties of Au−Pd(111) slabs were compared by DFT modeling for different Au−Pd ensembles. First, energies of slabs with Pd atoms homogeneously distributed at the surface (Figure 4A) and for the sake of simplicity, slabs with segregated Pd trimers only (Figure 4B) were calculated. The change in surface structure indicated by Figure 4A → B (ΔE = +0.17 eV per 3 × 3 unit cell surface) shows that as previously observed, individual Pd atoms dispersed within a Au surface are more stable than when forming aggregates (dimers, trimers).6,9,14,15 A detailed analysis of other ensembles in vacuum indicating this general trend can be found in ref 14. The same comparative calculation was performed for the two different ensembles of Pd at a Au@Pd(111) surface covered by oxygen. Figure 4C,D
ensures that hydrogen adsorption does not contribute to the phenomena observed. After each reduction step at 0.1 V, the H2O2 selectivity of the resulting surface was measured between −0.3 and 0.5 V, where no further alloy oxidation was observed, in the expectation that according to the results in Figure 2, the surface composition would not be significantly altered in between the reduction potentials steps at 0.1 V by a single limited potential sweep. This experiment can provide information about the time scale of surface composition rearrangements. The results of these experiments are summarized in Figure 3C,D. Again, a gradual decrease in the oxygen reduction current corresponding to an increase in H2O2 production (Figure 3D) was observed with increasing reduction times at 0.1 V until a steady j−E profile was reached after 2 h reduction. A similar time was required to reach the highest H2O2 production as during the cycling shown in Figure 2. It was possible to repeat the whole procedure several times, that is, to sweep the potential rapidly to 1.2 V as in Figure 3A and then obtain the same suppressed H2O2 selectivity in the potential region between −0.3 and 0.5 V. In addition, two peaks are clearly distinguishable for the ring currents in Figure 3B, where the yield of peroxide passes through two maxima at 0.3 and 0.0 V. This is a direct indication of surface segregation to at least two types of surfaces with different dependencies of SH2O2 on potential,25,26 probably corresponding to a Pd- and a Au-rich surface, respectively. The peak splitting diminishes upon cycling between −0.3 and 0.6 V or upon holding the potential at 0.1 V as shown in Figure 3B−D indicating realloying of the segregated metals. It should be noted that the H2O2 selectivity measured close to the onset of the ORR has low accuracy due to the low currents measured and that the apparent minor increases in H2O2 selectivity above 0.40 V are likely an artifact. Importantly, these experiments demonstrate that although 318
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Scheme 1. Schematic Model of Au−Pd Nanoparticle under Different Conditionsa
a
(A) Au−Pd alloyed nanoparticle in vacuum. (B) Same particle transferred into aqueous electrolyte saturated with oxygen and polarized below 0.50 V and (C) Au−Pd particle polarized above 0.70 V. For the given experimental conditions x, y, and z represent Pd molar fraction and x ≥ y > z; y* reflects shell Pd content for panel B.
shows the calculated energy difference between geometry optimized surface structures obtained by adding same amount of oxygen atoms on the surfaces from Figure 4A,B. Interestingly, these results show that the formation of surface oxide reverses the trend observed for the surface in vacuum and Pd segregation becomes thermodynamically more favorable by ∼1 eV per periodically repeating 3 × 3 unit cell surface box. This is a consequence of the much stronger bonding of adsorbed oxygen to contiguous Pd atoms compared with individual Pd atoms embedded in Au (∼1 eV for three-fold hollow O adsorption on a (111) surface).10 In addition to segregation trends resulting from surface diffusion, Pd(O)ads, co-aggregation of Pd from both non-oxidized surface and “bulk” was also investigated. For an oxygen-free Au−Pd surface, when considering the energetics of segregation of components from the bulk alloy, Au should segregate preferentially to the surface; that is, the predicted most stable composition in vacuum is a Au−Pd alloy with homogeneously distributed Pd terminated by a Au skin surface,12−14 as observed experimentally for the Au−Pt system.31 A different situation prevails when strongly interacting adsorbates such as oxides, are present. Results from a comparative study with varying oxide coverage shown in the Supporting Information (Figure S2) indicate that the Pd segregation driving force, the segregation energy, depends on the degree of oxidation of the alloy surface and that an increase of oxygen coverage enhances Pd segregation. This is also illustrated in Figure 4E−G for a 0.3 ML of oxygen. (See the Supporting Information for additional details.) These results show that the experimentally observed changes in the H2O2 selectivity and the proposed surface Au− Pd rearrangements reflect the stronger affinity of Pd toward surface oxide formation with respect to gold.27 The reorganization according to Figure 4 and Figure S2 of the Supporting Information described above indicates that surface oxidation not only pulls Pd to the surface but also can induce PdO co-nucleation and island formation, which in turn acts as seeds for Pd surface segregation. The above discussion and the behavior observed suggest that the surface composition is strongly dependent on adsorbate population and differs significantly from the nanoparticle “bulk”. Hence, the formation of structures analogous to those of core−shell materials is likely. A simplified model shown in Scheme 1 describes the results presented. This is based on the similarity to a three-phase system where the solution represents one phase, the nanoparticle shell represents the second phase, and the core represents the third phase. In this model, the composition of the nanoparticle shell is directly affected by the population of
surface adsorbates resulting from either the applied potential or the chemical environment at the surface. The distribution of Pd between the “core” and the “shell”, a key parameter determining the O2 reduction pathway, reflects the partition equilibrium resulting from the opposing trends of interaction of Pd with the surface and the thermodynamically preferred Au1−xPdx alloy formation terminated by a Au skin in vacuum (Scheme 1A).12−14 It is proposed that shell enrichment by Pd atoms becomes energetically favorable when oxygencontaining species (oxide, OH) are adsorbed, with the core becoming Au-rich. Hence, the core composition will correspond to Au1−zPdz, where z < x in Scheme 1C. When the surface is reduced, for example, at 0.1 V in the experiment described in Figure 3, realloying takes place and Pd dissolves back into the Au-rich core according to Scheme 1C → B to form Au1−yPdy in the core and Au1−y*Pdy* in the shell. The Pd content in the shell, y*, is determined by the strength of surface interaction with adsorbates under the given experimental conditions. The maximum in SH2O2 observed at 0.2 V is, therefore, related to the maximum concentration of Pd monomers, and the decrease in the SH2O2 observed after the oxidation-induced segregation at the more positive potentials indicates that the surface must be enriched by contiguous Pd atoms. This implies that Pd surface coverage is higher than 0.3 ML, at which a Pd monomer-rich surface is still sustainable; at higher Pd coverage the formation of dimers and higher ensembles of neighboring Pd atoms is greatly favored. Considering the number of Pd atoms in a ∼8 nm Au0.7Pd0.3 nanoparticle (∼3400 atoms in the surface monolayer, ∼15 000 atoms in total, considering a spherical nanoparticle), the extent of Pd exchange between the core and the shell according to Scheme 1B,C is likely significant to explain the pronounced decrease in H2O2 selectivity upon oxidation from ∼55% below 10% (at 0.2 V). Note that SH2O2 of ∼15% was, for example, obtained for Au−Pd containing 50% of Pd, whereas pure gold shows SH2O2 of ∼75%.10 Surprisingly, the changes in Pd surface composition observed occur on two different time scales. Pd surface segregation facilitated by surface oxidation is very fast (
