Article pubs.acs.org/JPCC
Oxidation and Surface Segregation Behavior of a Pt−Pd−Rh Alloy Catalyst Paul A. J. Bagot,*,† Karen Kruska,† Daniel Haley,† Xavier Carrier,‡,§ Eric Marceau,‡,§ Michael. P. Moody,† and George D. W. Smith† †
Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom Sorbonne Universités, UPMC Univ Paris 06, UMR 7197 CNRS, Laboratoire de Réactivité de Surface, F-75005 Paris, France § CNRS, UMR 7197 CNRS, Laboratoire de Réactivité de Surface, F-75005 Paris, France ‡
ABSTRACT: Platinum gauze catalysts are used extensively in the production of nitric acid from ammonia, where they are subject to harsh operating conditions combining elevated temperatures and oxidizing environments. These cause significant loss of metal species as volatile oxides. How different metallic species behave in these environments at a fundamental, atomic-scale level is not well understood. In this work, we study the early stages of oxidation of a Pt−Rh−Pd gauze at temperatures of 873−1273 K. Using a combination of advanced experimental methods, we explore how the oxidation behavior can strongly influence the surface and near-surface gauze microstructure. We show that Rh and Pd can segregate on different areas of the same surface and discuss how such atomic migration can be linked to mechanisms of metal loss from such alloys.
surface,10 the thickness of which is dependent on the oxygen exposure. However, in Pd−Rh, the same treatment cycle instead produces nanometer-sized Rh-rich clusters throughout the APT volume.11 In Pt−Pd, the oxidation response and species retained at the surface appear to be highly sensitive to the exposure temperature.12 At a deeper level of detail, the curved nature of APT specimens closely approximates the shape and size of catalytic nanoparticles, allowing simultaneous investigation of different crystallographic surfaces following a single gaseous exposure. Such studies have shown striking differences following NO exposure on different surfaces of Pt− Rh9 and Au−Pd.13 In addition to an improved basic understanding gained from studying these model alloys, these experiments also demonstrate a route for creating nanoengineered surfaces that are optimized to operate under specific conditions.12 We are now in a position to explore ternary and higher-order alloys. These are becoming increasingly of interest in a number of the catalytic systems mentioned above.14−16 With a move to more complex formulations of catalyst alloys, the traditional empirical approach of selecting suitable elements, compositions, and catalyst structures becomes increasingly arduous. Preliminary APT investigations using nitric oxide/oxygen exposures to Pt−Rh−Ir and Pt−Rh−Ru alloys have already demonstrated distinct differences in surface segregation and microstructure compared with the parent binary alloys.17,18
1. INTRODUCTION In heterogeneous catalysts, the active components are generally transition metals, each of which offers different catalytic properties of selectivity, yield, and stability under demanding, dynamic operating conditions. These elements are often combined as alloys to ensure acceptable performance. Binary alloy catalysts are the most common, including Pt−Rh for automobile exhaust catalysts and ammonia oxidation,1,2 Pd− Ni3,4 and Pt−Re5 for selective hydrotreating and reforming reactions, Fe−Co for Fischer−Tropsch liquid fuel production,6 and Pt−Ru in direct methanol fuel cells.7 In many of these systems there is a pressing need to reduce or replace expensive noble metals, particularly as there are concerns over the future availability of many of the rarer elements.8 However, this must be balanced with the need to adjust the formulation of existing catalysts in order to meet industrial demands and comply with tightening environmental legislation in the short term. To achieve these goals, there is a fundamental need to understand the responses of different metal combinations to realistic operating environments in terms of structural and/or surface composition changes at the atomic level. A growing catalog of fundamental segregation and restructuring information has been built up for Pt-group binary alloys under reactive exposures by exploiting the advantages of near-atomic-scale spatial resolution and chemical sensitivity offered by atom probe tomography (APT). These alloys exhibit remarkable diversity in their response to commonly encountered oxidizing and reducing environments.9−12 For example, in Pt−Rh an oxidation/reduction cycle drives a layer of Rh to the © XXXX American Chemical Society
Received: August 11, 2014 Revised: October 16, 2014
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Figure 1. SEM images of single wires in Pt−Pd−Rh gauze following oxidation in 1 bar O2 at (a) 873 K, (b) 1073 K, and (c) 1273 K. Representative areas for APT lift-out samples of grain boundary/facet surfaces are highlighted by red circles in (b).
Figure 2. EDX maps of Pt−Pd−Rh gauze oxidized at 1073 K for 5 h in 1 bar oxygen. (a) SEM secondary electron image. The light-blue box indicates the region for EDX mapping. (b) Integrated EDX spectrum. (c) Pt Mα1 map. (d) Rh Lα1 map. (e) O Kα map.
Appendix). Together, these build a comprehensive, multiscale understanding of changes to the surface structure/chemistry.
In the current work, our goal is to investigate the early stages of oxidation in a ternary Pt−Pd−Rh alloy gauze by a multitechnique approach. Such gauze catalysts are of great industrial importance because of their role in the ammonia oxidation reaction, a key stage in the production of nitric acid for fertilizers and plastics.19 Pt is the active element for breaking bonds in ammonia, but additions of Rh and Pd have been shown to greatly improve the yield20 and stability21 of the catalyst. However, little work has been undertaken either to quantify the optimum compositions or to examine how the nominal surface composition may be altered under the initial oxidation conditions as the catalyst is ramped up to temperature. For example, clear evidence is presented in the literature showing how detrimental loss of metal species at high temperatures can be constrained by adding Pd to the alloy.21 A detailed picture of how Pd prevents such losses is missing, although an earlier scanning electron microscopy (SEM)/ electron probe microanalysis (EPMA) study by Rubel and Pszonicka22 showed possible indications of Pd surface enrichment within the center of grains on a similar ternary alloy, along with Rh enrichment at the grain boundaries. Here we explore the origins of this action as part of a broader study of alloy element segregation behavior and partitioning within the early stages of oxidation behavior in a gauze. This is performed utilizing a diverse set of experimental methods including APT, SEM, 3D focused ion beam (FIB) slicing, energy-dispersive Xray analysis (EDX), and thermogravimetric analysis (TGA). Molecular dynamics (MD) simulations are also included (see
2. EXPERIMENTAL SECTION A Pt−Pd−Rh gauze (83−8−9 atom %, 90−5−5 wt %) woven from 0.076 mm diameter wire was obtained from Alfa Aesar. For control APT experiments, small lengths of wire were separated from the gauze, electropolished in a molten salt mixture (4:1 v/v NaNO3/NaCl), and then cleaned with ethanol. All of the APT samples were analyzed using a Cameca LEAP 3000X-HR instrument at a specimen stage temperature of 50 K with a 532 nm pulsed laser running at 200 kHz and producing pulses at 0.5 nJ. Oxidation treatments were carried out using small gauze sections placed inside a quartz crucible held within a 27 mm ID tube furnace. Oxygen was then passed through at 35 cm3/min, before the furnace was quickly ramped (over a period of 40−45 min) to the desired temperature. It was held at the set-point temperature for 5 h before being allowed to cool to room temperature with oxygen still flowing. Following exposure, small sections (approximately 5 mm2) were cut and mounted for SEM/FIB analysis using a Zeiss NVision dual-column FIB instrument, along with a similar sample from the as-received material. The Zeiss NVision instrument was also used for FIB 3D sequential sectioning following a previously established methodology23 and to prepare APT samples from specific areas using the lift-out method.24 EDX mapping was performed using an Oxford Instruments INCA system in a JEOL JSM 6300 scanning electron microscope. For TGA studies at UPMC, a B
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Figure 3. (a) FIB secondary electron image of gauze oxidized at 1073 K. The area selected for FIB slicing shown in the blue rectangle. (b) Final 3D reconstructed volume of the oxide network. (c) Depth distribution profile of oxide penetration.
Figure 4. Thermogravimetric analysis of Pt−Pd−Rh gauze oxidized for 5 h at 1073 K (black lines) and 1273 K (red). The temperature ramp profiles (thin dashed lines) and total sample weights as percentages of the initial weights (thick lines) are plotted.
and Pd Lα1 lines at ∼2.84 keV is expected. However, the presence of a considerable Rh Lα1 peak (2.7 keV) and the lack of a significant Pd Lβ1 peak (usually at 3 keV) suggests that Rh is present at the surface to a greater extent than Pd. The relative spatial distributions of selected elements are shown in Figure 2c−e (insufficient signals were obtained to produce a Pd map). While only semiquantitative, these indicate the considerable nonuniform chemistry across the surface. Figure 2c clearly indicates that all of the grain boundary regions are Pt-depleted with respect to the matrix. Consideration of Figure 2d,e suggests that the grain boundaries principally contain Rh-rich oxide. The distributions of the key elements obtained here are furthermore in very close agreement with those seen by lowerresolution SEM/EPMA on a similar Pt−4 wt % Pd−3.5 wt % Rh alloy,22 with Rh and O concentrated at the grain boundaries and Pt within the grain facets. 3.3. FIB Depth Examination. Following the initial examination of the surface using SEM/EDX, FIB 3D sequential sectioning, an approach explained in detail previously,23 was carried out to explore the subsurface penetration of the oxide formed at 1073 K. The procedure and resulting map of the oxide are presented in Figure 3. A randomly selected area of interest in the gauze surface is indicated in the FIB image in Figure 3a. This area was sequentially milled to remove a series of thin slices, taking cross-sectional SEM images throughout. Image postprocessing enabled the oxide to be isolated and reconstructed in the 3D map presented in Figure 3b, which
TA Instruments SDT Q600 thermal analyzer was used. Separate gauze samples (about 70 mg each) were heated in the same flow rate of pure oxygen at a heating rate of 20 K/min and then held at the desired set points for 5 h.
3. RESULTS 3.1. SEM Examination. The SEM images in Figure 1 show the results of the oxidation treatment across the three temperatures studied. Figure 1a shows that the microstructure is little affected at 873 K, with only small patches of oxide decorating the grain boundaries. By comparison, at 1073 K a greater level of oxidation is obvious (Figure 1b), again localized along boundaries but now forming a continuous network extending up to 1 μm perpendicular from the boundaries. Following oxidation at the highest treatment temperature of 1273 K (Figure 1c), the surface appears almost oxide-free, presumably as a result of volatilization of the major oxidation products. The majority of the subsequent analyses were therefore carried out on samples oxidized at the intermediate temperature of 1073 K, where the extent of surface oxidation behavior is most evident. This temperature is also within the operating temperature of gauzes in service. 3.2. EDX Maps. EDX mapping provided an overview of the surface composition after oxidation at 1073 K. Figure 2a is an SEM image in which the mapped region is indicated by the light-blue box. Figure 2b shows the resulting integrated spectrum. Peak overlap due to contributions from the Rh Lβ1 C
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Figure 5. (a) Atom map of Pt−Pd−Rh gauze oxidized at 1073 K for 5 h in 1 bar oxygen. The sample was extracted from within 1 μm of a grain boundary. The upper volume shows a heavily oxidized surface region, demarcated from the metallic matrix below by the 40 atom % oxygen isoconcentration surface. (b) Proxigram analysis indicating the chemical distribution across the matrix−oxide interface. (c) 1D concentration profile through the full length of the atom map (the analysis cylinder is shown in the inset).
shows that the network of oxide penetrates into the bulk. Extraction of the quantitative depth distribution was also possible, and this is plotted in Figure 3c. From this we note that the peak in the oxide depth penetration profile is around 0.2 μm, with the oxide concentration tailing away to zero at a maximum distance of 1.2 μm from the surface. Figure 3b also reveals that the deepest penetration of the oxide correlates with the very center of the grain boundaries. Building up an overall view of the subsurface structures by this method is useful for identifying appropriate areas of interest for APT analysis, as described in section 3.5. 3.4. Thermogravimetric Analysis. Sections of gauze were oxidized in situ within the TGA apparatus at 1073 or 1273 K to investigate any volatilization of species. The TGA results are shown in Figure 4. The apparent drop in mass at the outset at both temperatures (up to around 40 min; point A in the figure) is unavoidable drift during the heating ramp, as confirmed in control studies. This is noticeable in the current study because the weight loss is particularly small. However, within the region enclosed by the dashed gray box, a clear difference in the responses of the gauze at the two temperatures is apparent. At 1073 K, there is no detectable mass loss within the time period. Therefore, the gauze surface at the end of this oxidation treatment has not lost any significant metal species, a fact which aids the interpretation of the prior SEM/FIB data and the
subsequent APT analysis. At 1273 K, however, a steeper drop occurs (point A to B), corresponding to a mass loss in the gauze of approximately 0.11%. Throughout the remainder of the holding time at that temperature, no further losses/gains (distinguishable from apparatus drift) are noted. The approximate mass loss of 0.11% at 1273 K translates to a weight loss per unit area (ΔW/s) of 0.023 mg/cm2 (using approximate gauze dimensions). This is in close agreement with the value of 0.03 mg/cm2 determined from a Pt−7Pd− 6Rh alloy for a similar exposure period in a prior TGA-based investigation.21 3.5. Atom Probe Tomography. APT was used to investigate atomic-scale changes to the elemental distribution within the gauze. First, control runs were carried out to verify the bulk composition of the as-received gauze, which was found to be 82.5 atom % Pt, 8.5 atom % Pd, and 9.0 atom % Rh (statistical uncertainties all less than ±0.04 atom %) with a homogeneous distribution throughout. This composition is in close agreement with the nominal bulk content (82.8% Pt, 8.4% Pd, 8.7% Rh). The FIB lift-out method24 was employed to extract APT specimens from the oxidized gauzes, first from a grain boundary. A protective carbon strip (∼20 μm × 3 μm) was deposited over the region of interest. The extracted samples were analyzed by the atom probe under conditions identical to D
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Figure 6. Proxigrams from interior regions of two separate grains of Pt−Pd−Rh gauze oxidized at 1073 K for 5 h in 1 bar oxygen.
large identified grain. The red circles in Figure 1b indicate typical regions targeted for APT sample extraction. APT reconstructions were generated from surfaces of two different grains. Proxigram analysis was used to study the nearsurface composition in each case (Figure 6). The overall trends for the two grains are similar, with Pd enrichment (20−35% over the first 0.5 nm) and slight Rh enrichment (12−18%) at the surface. A thin (1−2 nm) layer of oxygen is present, but far less oxygen is seen here than the amount penetrating down the grain boundaries. There are some slight differences between the two data sets, indicating small variations in the interaction of oxygen with different grains. Figure 6a shows greater Pd surface enrichment, while in Figure 6b oxygen is more prevalent, which may also explain the greater surface movement of Rh to form rhodium oxide. However, from inspection of the EDX maps in Figure 2, we expect such differences to be minor compared with those apparent between the grain boundaries and grain interiors. To verify that the Pd enrichment observed was genuine and not an experimental artifact arising as a result of slight differences in field evaporation between Rh and Pt,27 control APT experiments from electropolished samples were examined at the initial stages of the runs. In no case was Pd surface enrichment apparent. Although it is beyond the main scope of this study, the surface segregation behavior of high-order alloys under purely thermal conditions is also of interest. This knowledge allows separation of individual effects to enable better understanding of how even small changes in operating environments may alter surface chemistries. As an example, a 1D atom probe study on Pt−Rh demonstrated Pt surface segregation when the alloy was vacuum-annealed above 1000 K, but the presence of even trace (ppm) sulfur switched the surface to Rh-rich.28 In support of the current work, in the Appendix we present details of a molecular dynamics study on a Pt−Pd−Rh alloy revealing that a purely thermally driven system causes Pd to diffuse to the surface while Rh moves into the bulk.
those used for control runs. The exact distance of the analyzed volume from the grain boundary was difficult to control precisely, but all were extracted within 1 μm of the interface. Figure 5a shows the resulting 3D atom map, where the tip axis direction is perpendicular to the oxidized surface. This map is clearly separated into two distinct sections: the uppermost volume, with a depth of 10−25 nm, is an oxide cap under which lies the unoxidized alloy matrix. Only oxygen ions and various metal species (RhO3+, RhO2+, RhO+, RhO22+, RhO2+, and trace RhOxH, PtO+, and PtO2+) are observed in the oxide. In the matrix below the interface, oxygen and traces of RhO are seen along with all three metals. No molecular Pd oxide ions were detected in either volume, while contaminant levels (C, Ga from FIB milling) were negligible. To analyze the compositions of both regions and examine the interface structure in detail, an oxygen isoconcentration surface (40 atom %) was used to define an interface for the proximity histogram (proxigram)25 (Figure 5b), which maps the concentrations of species on either side of the interface. This emphasizes the marked changes in the gauze local chemistry in both the oxidized region and the adjacent region of the alloy. The sharp interface between the two regions is approximately 3 nm wide. The oxide is dominated by rhodium, with negligible levels of Pt and Pd. The proxigram also highlights the significant segregation of Rh in the oxidized material; the bulk Rh content is only 8 atom %. The composition of the oxide calculated from Figure 5b is 39.8 atom % Rh and 59.7 atom % O, which is very close to an oxide stoichiometry of Rh2O3the most stable form of rhodium oxide but catalytically inert for ammonia oxidation.26 Below the oxide, rhodium has been heavily depleted from the matrix (to a mean value of 0.4 atom %) within the analysis volume of the proxigram. To examine the alloy composition further away from the interface, a 4 nm-wide analysis cylinder was placed along the right-hand side of the atom map (Figure 5c inset) to sample the chemistry in the region furthest from the oxide along with the resulting 1D concentration profile. The profile confirms the depletion of Rh near the interface but also reveals that up to 15 nm away the Rh content is still less than 3 atom %. The surface composition at the center of the surface-exposed grains was also examined in order to understand how the overall gauze chemistry may be affected by the oxidation. Samples were again extracted by FIB from the central area of a
4. DISCUSSION A key goal is to link atomic-scale behavior to macroscale performance. For this we start at the atomic scale, beginning with the process of oxide formation on the surfaces. From thermodynamic data we can compare the (exothermic) enthalpies of formation for the relevant stable oxides, E
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simulations and in DFT-based Monte Carlo simulations of various Pt−Pd−Rh surfaces.38 When Pd is present at the surface, it can form a barrier layer of PdO that acts to inhibit diffusion of Rh,36 preventing the formation of the more stable Rh oxide. Surface/subsurface migration of Rh toward the grain boundaries will also reinforce this effect. It is therefore also highly likely that the formation of this PdO capping layer is responsible for the reduction in Pt losses, as noted in prior TGA studies.21 In that work, increasing the Pd content (up to 20 atom %) in the three alloys tested resulted in substantial reductions in weight loss following oxidation for over 5 h; the rate constant was nearly 3 times smaller in the Pt−20Pd−6Rh alloy in than in the Pd-free Pt− 17Rh binary alloy. Greater Pd levels in the alloy would likely enable a protective PdO layer to form more rapidly. However, it must be remembered that the surface is heterogeneous following oxidation; methods with higher spatial resolution need to be combined with TGA data to fully understand any emerging trends. Overall, these results shed light on the mechanism by which Pd can considerably suppress metal loss under oxidizing conditions. APT analysis shows that even a thin Pd-rich layer is sufficient to enable this striking effect. Furthermore, it demonstrates that atomic-scale analyses are necessary to reach a proper mechanistic understanding of overall behavior. Additional tools such as analytical TEM and nanoSIMS may offer yet further insights into these types of problems, which have not traditionally exploited information from near-atomicscale methods. Gauze alloys are defined by global compositions, but even at this level there have been suggestions that the empirically determined upper limits of useful alloy element concentrations (e.g., ∼10 atom % Rh) have not yet been properly rationalized.2 However, we have further shown here that such a design approach is insufficient to fully understand the gauze behavior and consequent performance. The reason behind this is that there are important microchemical and microstructural factors to consider. The surface composition deviates significantly from the nominal value, and the deviations are nonuniform over the surface grain structure. In summary, we have demonstrated that multitechnique approaches spanning a wide range of length scales are vital in order to make progress in understanding mechanistic processes occurring under reactive environments and that such methodologies are essential in order to unlock further improvements in catalyst performance, stability, and cost reduction in increasingly complex alloy catalyst systems.
normalized to 1 mole of oxygen: 255 kJ/mol for Rh2O3, 170 kJ/mol for PdO, and 135 kJ/mol for PtO2.29−31 These show that Rh oxide is by some way the most likely oxide to form. It is also most probable to form at grain boundaries, as these are the locations most favorable for nucleation, an observation noted on a range of both catalytic17,32 and engineering alloys.33,34 A number of factors are likely to be at play here, including differences in local chemical potential and surface energies/ stresses between the bulk and grain boundary surfaces. Additionally, mass transport by short-circuit diffusion along grain boundaries may also relieve stresses arising from the volume change when oxide forms. Although efforts to isolate the dominant factors for oxide formation are impracticable without substantial in-depth dedicated studies, broadly they can all be linked to surface defects, which likely act as the initiation points for oxygen attachment. Once the oxide is nucleated at the grain boundaries, further growth of the oxide develops an interconnected boundary network. Analysis of the full set of images used to produce the FIB map in Figure 3b yields a total surface coverage of 40% catalytically inactive Rh2O3 following 1073 K oxidation. The growth of this network induces considerable Rh diffusion from the bulk, as shown in Figure 5. In fact, the Rh diffusion profiles in a prior APT Pt−Rh study10 and the current work are remarkably similar; there is a heavily Rh-depleted layer (≥15 nm) below the oxide−metal interface, after which the Rh level recovers to around only 2 atom %. The nature of the nascent oxide formed and the temperature trends across 873−1073 K are comparable with results from a number of prior APT studies on the binary Pt−Rh system,9,10,35 all of which indicate that Pd plays little role in the oxide formation kinetics in this ternary alloy. While Rh dominates the behavior at the grain boundaries, away from these there is evidence for movement of Pd to the gauze surface. Averaging the APT data in the near-surface region of both plots in Figure 6 gives a Pd content of around 29 atom %, which is significantly higher than that in the bulk (8.4 atom %). The trends at these locations are less clear-cut than those at the grain boundaries, but it is reasonable to expect that a number of factors contribute to the presence of Pd at the surface. First, after Rh2O3 the next most thermodynamically stable oxide is PdO. There is also evidence that the formation of this Pd oxide is relatively rapid, as high-Pd-content binary alloys are particularly prone to oxidation at temperatures as low as 873 K. In APT studies of Pd−6.4 atom % Rh11 and Pt−31 atom % Pd,12 oxide materialized in the former after only 10 min of exposure. On Pd−Rh foils containing 85−95 atom % Pd, annealing in air at 873−1073 K also produces a Pd-enriched surface of PdO.36 Although the extent of oxidation seen in the current work is lower at 873 K, it has been proposed that a minimum Pd threshold in the alloy is required before significant Pd oxidation occurs, which a photoemission study on Pt−Pd foils37 suggested to be between 5 and 17 atom % Pd. Evidence for a second mechanism driving Pd to the surface comes from a study showing the limited miscibility of Rh and Pd oxides formed on Pd−Rh,11 attributed to the two different oxide crystal structures formed. In the context of the current study, the preference for Rh2O3 to rapidly nucleate at grain boundaries means that PdO is more likely to form elsewhere, away from the boundaries. Finally, even if there are areas of the surface where little oxygen interaction occurs (because of low sticking probabilities or surface diffusion), purely thermal effects will cause surface segregation of Pd, as shown in the MD
5. CONCLUSIONS The initial stages of oxidation in a ternary Pt−Pd−Rh alloy gauze have been studied at 873−1273 K using an advanced multicharacterization technique approach. The overall oxidation behavior is strongly dependent on temperature and microstructure, as SEM/FIB and EDX images of the gauze oxidized at 1073 K revealed a network of oxide along all of the grain boundaries. Subsequent APT studies on this sample have shown that (i) Rh2O3 dominates the grain boundary oxide, (i) below this oxide the alloy matrix is heavily depleted in Rh to at least 15 nm below the interface, and (iii) Pd does not play a significant role in the oxidation kinetics at the grain boundaries but instead forms a thin (1−2 nm) Pd-rich layer at the surface of the alloy grain interiors. The gauze surface is thus partitioned into two chemically distinct regions, both markedly different from the bulk alloy composition, even after only 5 h of F
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oxidation. On the basis of the correlation of the high-level characterization trends with TGA analyses, the noted improvement in metal species volatility at the highest temperatures (≥1273 K) in such gauzes with Pd content is attributed to the formation of this nanoscale protective Pd oxide layer.
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accounts for the local electronic density. The remaining parameters are element-specific, and the values used were computed by Ç ağin et al.41 and are reproduced in Table 1. The parameter ε has dimensions of energy, a is the face-centered cubic (fcc) lattice constant, and m and n are positive integers. For pairwise interactions of unlike atoms, standard mixing rules proposed for alloys were employed.41 Following equilibration at 2000 K (which is well above the melting point) to remove any memory of the initial configuration, the nanoparticle was then cooled to 1073 K in stages before running for a sufficiently long period to ensure equilibration (∼1 ns). This approach was used not in an attempt to achieve truly energy-minimized structures but rather to prepare physically realistic systems and to establish suitable models for predicting how elevated temperatures may change surface compositions. Figure 7a shows a snapshot from a typical simulation equilibrated at 1073 K. (Higher temperatures yielded similar results, although under such conditions sublimation effects can render the resulting behavior unphysical). In Figure 7b the size of the Pt atoms has been reduced to reveal the internal Pd and Rh distributions. It is evident that following thermal annealing the distributions of the minor elements are no longer random. Pd (blue atoms) is surface-segregated, while Rh (red atoms) diffuses into the bulk. This trend agrees with Monte Carlo simulations showing Pd enrichment in annealed 75 atom % Pt− 10 atom % Pd−15 atom % Rh (111) surfaces42 and also with simple inspection of the surface energies of the metals, which vary in the order Rh > Pt > Pd.43 Drawing quantitative conclusions from the simulations must be done with caution, but we examined in the composition− depth profile for the nanoparticle, plotting atom types versus radial distance from the core (Figure 7c). The analysis was truncated
