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Core–shell Pd–Pt nanocubes for the CO oxidation
Journal of Catalysis 336 (2016) 33–40
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Core–shell Pd–Pt nanocubes for the CO oxidation Astrid De Clercq a,b, Olivier Margeat a, Georges Sitja a, Claude R. Henry a, Suzanne Giorgio a,⇑ a b
Aix Marseille Université, CNRS, CINaM UMR 7325, 13288 Marseille, France Aix Marseille Université, CNRS, MADIREL UMR 7246, 13397 Marseille, France
a r t i c l e
i n f o
Article history: Received 26 August 2015 Revised 23 December 2015 Accepted 13 January 2016
Keywords: CO oxidation Core–shell Pd@Pt Nanocubes
a b s t r a c t Pd–Pt core–shell nanocubes with increasing outer Pt layers have been studied for the CO oxidation reaction by gas phase chromatography. The maximal reactivity of the core–shell nanocubes was found for an average value of 0.4 atomic layer of Pt, with a preferential growth at the corners. High resolution electron microscopy (HRTEM) observations show a rough Pt layer, epitaxially crystallized on the Pd surfaces. In situ observations of the Pd and Pt@Pd nanocubes by Environmental Transmission Electron Microscopy (ETEM) during oxidation–reduction cycles show the formation of high index facets during the adsorption of O2. Similar behavior of the reactivity was reported in the literature for electrocatalytic reactions and interpreted in terms of decrease in the adsorption energy of the rate limiting species on the metal surface. This is explained by the compressive strain and the proximity of Pd atoms in the epitaxial Pt layer on Pd cubes, resulting in a change in the electronic structure of the Pt. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction Bimetallic particles containing platinum become more and more important in catalysis [1,2]. They have already been used since 1950 in the oil refining industry, mainly for reforming reactions, where better stability and selectivity were obtained. The second application of these bimetallic catalysts is TWC (Three Way Catalysts) for car exhausts, with CO oxidation and NOx decomposition reactions. The third and more recent use of the combination between Pt and another metal (Pt–M) is in electrocatalysis, mainly for proton exchange membrane fuel cells (PEMFC). Traditionally pure Pt particles are used as catalyst in PEMFC, but they present two drawbacks: the poisoning of CO contained in the hydrogen fuel and the limited resources of Pt in the earth. Alloying Pt with another metal permits to avoid CO poisoning [3]. Moreover Pt–M nanoparticles have shown higher reactivity for the oxygen reduction reaction (ORR) [4,5]. In order to save Pt it could be interesting to prepare nanoalloys with a low concentration of Pt or ideally to make a core–shell nanostructure with a thin Pt shell and a cheap metal core. Bimetallic catalysts are classically prepared by impregnation, but with this method it is difficult to precisely control the size and the composition of bimetallic particles. In addition it is not possible to prepare controlled core–shell structures or particles ⇑ Corresponding author. E-mail address: [email protected] (S. Giorgio). http://dx.doi.org/10.1016/j.jcat.2016.01.005 0021-9517/Ó 2016 Elsevier Inc. All rights reserved.
with a unique shape. Great progress was achieved by colloidal preparation methods, for which the preparation of mono- or bimetallic nanoparticles with a sharp size distribution, a control of the composition and a single shape, is now possible [6–8]. As the colloidal nanoparticles self-organize on a flat surface they can form a regular array of particles that can be used as flat model catalysts [9]. However after synthesis the colloidal nanoparticles are capped by stabilizing molecules (ligands) that can strongly decrease the accessibility of surface atoms to reactants during the catalytic reaction. Nowadays, several efficient methods to remove the ligands are available [10–12]. Among Pt–M systems, the catalytic properties of Pd–Pt core– shell systems have been extensively studied in the recent years [13–20]. Most of them concern the oxygen reduction reaction in fuel cells and an increase in activity is always found relatively to commercial pure Pt catalysts. Some studies used Pd particles with a well-defined shape (cube [17] or octahedron [20]) on which successive Pt monolayers (ML) are deposited. In the case of a cubic shape, a maximal activity for the ORR was observed for 2–3 ML of Pt. Another study using a 2.5 nm Pt layer on Pd cubes also showed an increase in activity for methanol electro-oxidation [18]. The only study not concerned by electrocatalysis investigates the hydrogenation of p-chloronitrobenzene (p-CNB) on Pt layers (1–4 ML) on 6–8 nm Pd octahedra and shows the best activity for 1 ML of Pt [19]. In the present work we study the catalytic activity for the CO oxidation reaction of Pd–Pt core–shell structures, as a function of
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the thickness of the Pt shell. Pd cubes prepared by colloidal chemistry are used as seed to grow Pt layers with different thicknesses (0.2–2.2 equivalent ML). The core–shell structures are characterized by HRTEM. For the CO oxidation measurements the particles are supported on silica and the ligand is removed by UV–ozone treatment. The core–shell structures are found to be more active than pure Pd particles and the highest activity is obtained for 0.4 equivalent ML of Pt. The effect of oxygen and hydrogen adsorption on the morphology of pure Pd and Pd–Pt core–shell particles is studied in situ at RT by ETEM. 2. Experimental 2.1. Synthesis of Pd cores and deposition of Pt amounts The synthesis of Pd cores was inspired by a method of Yang et al. [21]. 25 mg of Pd(acac)2 (99%, Sigma–Aldrich) was mixed in 2 ml Dimethylsulfoxide (DMSO) and 77.5 mg Polyvinylpyrrolidone (PVP) (Mw 1.3 M Alfa Aesar) and 69 mg KI were dissolved in 2 ml DMSO. Then both solutions were mixed together in a glass reactor resistant up to 3 bar. The reactor was closed and heated up to 140 °C for 2 h in an oil bath, before cooling down to room temperature. A sample containing 0.02 mmol Pd was extracted and a first amount of Pt(acac)2 (for example 2.4 mg in 2 ml DMSO) was added. This mixture was again heated up to 120 °C for 1 h 30. This procedure of cooling to room temperature and adding a certain amount of Pt(acac)2 was repeated to prepare the samples containing different amounts of Pt. From EDX analysis we could evaluate the molar ratio of Pt/Pd for an average of 10 measurements on collections of particles. With the estimation that about 8 mol% is at the surface of the Pd cubes, we calculate the equivalent Pt layers deposited on the Pd cubes (even if the layers were not completely smooth). As such, samples with 0.2, 0.4, 0.5, 1.6 and 2.2 equivalent monolayers could be prepared. In further discussion, the sample names are based on this calculation and written as Pd@Pt(x-equivalent-layers). All particles were washed by a standard centrifuge technique and re-dispersed at concentrations of 14 mM in an ethanol–water (4:1) mixture. 2.2. Preparation of the supported NPs and ligand removal The NPs were loaded on amorphous SiO2 powder by ultrasonically mixing the solution containing 0.010 mmol Pd with 45 mg SiO2 (US Research Nanomaterials, 160–600 m2/g). This gives an approximate loading for pure Pd of 2.3 wt%. Total Pt/Pd metal loadings for the other samples was estimated based on the EDX analysis, which gives a maximal loading of 3.1 wt% for the highest amount of Pt. The powder was then dried in vacuum and the PVP ligands were removed at room temperature by UV–ozone exposure for 2 h [12]. 2.3. Characterization methods TEM and HRTEM observations were acquired with a Jeol 3010 operated at 300 kV. For the in situ TEM observations, an environmental sample holder (E-cell) was used in the standard JEOL 3010 microscope [22], so that the low pressure in the column of the microscope was not affected. In these conditions, the characteristics of the microscopes correspond to a resolution of 0.21 nm, with a Cs = 1.4 mm. The morphological changes of the nanocubes were in situ observed during oxido reduction cycles in pure H2 and pure O2 at pressures below 10 mbars. The cubes were deposited and encapsulated between gas proof carbon layers separated by 0.1 mm for the gas circulation.
2.4. CO oxidation reaction The activity for the CO oxidation reaction was evaluated in a fixed-bed quartz tubular reactor (inner diameter 3 mm, length 20 cm) at atmospheric pressure for a total flow rate of 22.3 ml/ min (8% CO, 12% O2 in N2). About 20 mg of catalyst powder was loaded into the quartz reactor. A thermocouple placed near the reactor bed gave the estimation of the catalyst temperature. A gas chromatograph equipped with a thermal conductivity detector (TCD) and two columns (Porapak Q 1/800 2 m 80/100 and Molecular sieve 1/800 2 m 13 60/80) was used to analyze the reaction products. Due to the exothermic nature of the reaction, rapid temperature rises in the catalyst bed can occur. Therefore it was opted to compare the samples in the low conversion regime (below 10% conversion), in the kinetically-controlled regime, with a space velocity of 600–800 ml s1 g (metal)1. After loading, samples were exposed to N2 flow for 1 h at 160 °C. Then, temperature increments of 10–20 °C at 0.5 °C/min were done and the total measurement time for each curve was around 12 h. No decrease in activity is observed after 20 h of continuous operation. The TOF is referred to the number of surface sites obtained by taking cubic particles with the mean size determined by TEM and exact amount of metal loading. 3. Results 3.1. Standard Transmission Electron Microscopy (TEM) observations Fig. 1a is a standard TEM overview of pure Pd seeds spontaneously (0 0 1) oriented, dropped on a carbon film. Most of them are cubes with an average edge length of 14.6 ± 2.3 nm, or elongated along a crystallographic [0 0 1] direction, with (0 0 1) external faces. The enlarged image of the cubes (Fig. 1b) clearly shows very weak truncations by (1 1 0) faces at the edges and truncations by (1 1 1) faces at the corners. The extension of these truncations approximately corresponds to anisotropy ratio between the surface free energies of r(0 0 1)/r(1 1 0) ffi r(0 0 1)/r(1 1 1) ffi 0.7, which is lower than the values obtained for clean fcc metal crystals of the same size, at the equilibrium. Indeed, the equilibrium shape is only obtained after high temperature annealing in ultra-high vacuum conditions for nanocrystals in this size range. So the cubic shape obtained during the growth of the particles is not the equilibrium shape of clean Pd particles. Pd cubes covered with Pt atomic layers (from 0.2 to 2.2 equivalent layers) were observed in the same conditions. An intermediate composition is shown in Fig. 1c and d. Compared to pure Pd nanocubes, no noticeable size increase could be observed, because of the very low Pt quantities. However the crystal shape evolved from cube to ‘‘frame Pd–Pt concave nanocrystal” as previously described by Lu et al. [23] and was interpreted as the preferential growth of Pt at the corners of Pd cubes. The high resolution image of the edge and the corner of such a frame concave nanocrystal, seen in Fig. 2, corresponds to the epitaxial growth of Pt on the (0 0 1) Pd faces. Despite of a mismatch of 1.4% between the Pt and Pd atomic radius, the (0 0 2) lattice planes are continuous at the interface, due to the accommodation of Pt on the Pd substrate, with some distortions. It confirms that Pt is mainly deposited near the corners, creating some higher index facets. 3.2. CO oxidation reaction The CO oxidation TOF of the different samples can be compared in Fig. 3. The pure Pd cubes/SiO2 sample was found to be very little active, only starting to convert at high temperature, in the range where the SiO2 support starts to show some activity. This low
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Fig. 1. TEM images of (a and b) the pure Pd seeds and (c and d) the Pd seeds covered with 0.4 equivalent layers of Pt, with sharpening of the corners.
Fig. 2. HRTEM images of the corner of a Pd seeds covered with 2.2 equivalent layers of Pt.
activity for the cubic shaped Pd colloidal NPs was observed by other groups (for similar size, support and gas composition), while Pd particles with an octahedron shape started to convert around 250 °C [24], for the same metal loading and nearly the same size. In the case of Pd nanocubes covered with low quantities of Pt, with a coverage equivalent to 0.2 Pt layers, little improvement
was noticed. However, a significant superior performance was obtained for a coverage of 0.4 equivalent Pt layers. The turnover frequency (TOF) at 220 °C is given in Table 1. From this, it can be seen that the TOF increased by a factor 33. As seen in Fig. 3 and Table 1, a further increase in the Pt coverage does not improve any more the activity (TOF). The TOF for Pd@Pt 0.5, 1.6 and 2.2 equivalent layers, even decreased with a factor 3 compared to the TOF for [email protected]. TOF values can be compared with values reported for Pt nanocubes prepared by a colloidal method [25] or small Pt nanoparticles prepared by wet impregnation of SiO2 [26] (Table 1). It should be noticed that the given values were obtained for slightly different gas compositions. For the CO oxidation reaction with a first order in O2 and negative first order in CO, this would give an adjustment of the listed TOF values with a factor 1.5 and 3, corresponding to 0.03 and 0.3 s1 respectively. From this table, it can be seen that the coverage of 0.4 equivalent Pt layers gives a higher TOF than pure Pt nanocubes [25]. The samples with higher coverage (0.5–2.2 equivalent layers) give a similar TOF as Pt nanocubes [25]. The most active sample, with 0.4 equivalent layers of Pt, has a TOF in the same range as very small Pt/SiO2 nanoparticles prepared by wet impregnation, although in this case, no residual ligands are present, which can strongly reduce the number of active sites and then the activity [27]. In comparison with previous publications on the catalytic activity of Pd–Pt core–shell nanoparticles, we found similar trends of the reactivity as for the ORR and hydrogenation reactions as a function of the thickness [17,19,20]. The Arrhenius plots with associated apparent activation energies can be seen in Fig. 4.
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Fig. 3. CO oxidation activities over the different samples of cubic Pd–Pt core–shell NPs supported on SiO2 with increasing equivalent atomic layers of Pt (for PO2/PCO = 1.5).
Table 1 Turnover frequencies for Pd cubes covered with increasing equivalent Pt atomic layers at 220 °C. Our measurements (PO2/PCO = 1.5)
TOF @ 220 °C (s1)
Pd nanocubes [email protected] nanocubes [email protected] nanocubes [email protected] nanocubes [email protected] nanocubes [email protected] nanocubes
<4 103 <4 103 0.13 0.02 0.04 0.02
Literature results Pt nanocubes/Al2O3 (compensated PO2/PCO = 1.0) [25] Standard Pt/SiO2 wet impregnation (compensated PO2/PCO = 0.5) [26]
0.02 (0.03) 0.1 (0.3)
3.3. ETEM observations By environmental TEM (ETEM), we have studied the shape evolution of the pure Pd cubes and Pd–Pt core–shell cubes under oxygen and hydrogen gas. As discussed in previous works, even RT observations are representative of the shape evolution during real catalytic reactions as the electron beam increases the surface diffusion on the NPs. Before the measurements of the CO oxidation, the samples are exposed to O2. Then, the morphological and chemical structure changes of the NPs under oxidizing and reducing gas environments have been studied by ETEM. Similar experiments were performed in previous works with pure Pt nanocubes, prepared by wet chemistry with different organic ligands [28]. A clear shape evolution was observed, with
Fig. 4. Arrhenius plots of different samples of cubic Pd–Pt core–shell NPs supported on SiO2 with increasing equivalent atomic layers of Pt (for PO2/PCO = 1.5).
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a change of r(0 0 1)/r(1 1 1) from 0.8 (cubic shape) in oxygen to r (0 0 1)/r(1 1 1) of 1.12 in hydrogen. First, the pure Pd cubes were observed during oxidation–reduction cycles (Fig. 5) in pressures of around 1 mbar. In hydrogen gas (Fig. 5a), not much shape changes were detected compared to vacuum observation. The absorption of hydrogen to form the hydride phase was previously studied for similar Pd cubes, and an expansion of the lattice parameter was observed, but with no influence on the cubic shape [29]. On the other hand, Fig. 5b clearly shows the extension of (1 1 0) faces during O2 exposure and the rounding of the corners. This transformation corresponds to the increase in the anisotropy ratio r(0 0 1)/r(1 1 0) from 0.7 to about 0.8. This change is coherent with previous ETEM studies of fcc crystals [22,28,30] showing the formation of higher index facets in pure O2. This extension of the (1 1 0) faces is also visible in weak beam dark field imaging in Fig. 5c and d and the associated 3D shape is illustrated in the drawing of Fig. 5e and f. Next, the most active [email protected] cubes for the CO oxidation were observed in similar conditions, in pure H2 and pure O2 by ETEM (Fig. 6). They keep the original concave shape in pure H2, as seen in Fig. 6a, with no noticeable difference with the assynthesized shape. Then, the same [email protected] cubes in pure O2 are rounded at the corners (Fig. 6b). Such rounding at the corners in pure O2 was observed in ETEM on Pt cubes [28]. By environmental SXRD (surface X-ray diffraction) it was shown for Pd and Pt nanoparticles that the rounding under oxygen is due to the formation of high index facets under oxygen which disappear reversibly under CO [31,32]. As in the case of pure Pd nanocubes in O2, for [email protected] nanocubes, the (1 1 0) facets are extended compared to the same samples in H2. The increase in stability of high index facets on Pt and Pd under high pressure of oxygen is also justified by ab initio calculations [33]. In the studied large nanocubes, no bulk oxidation was observed under oxygen. In a recent ETEM study, in pure O2, at higher magnification with an aberration corrector and a higher electron beam current density, the oxidation of the top layer could be observed for smaller supported NPs (4–5 nm) which was reversible under CO [34]. It has been also observed by a combination of in situ STM and ambient pressure XPS at RT in 1.33 mbar of oxygen, that stepped Pt surfaces become oxidized and that nucleation of oxide islands occurred only on steps [35]. The surface oxidation is reversible and on Pt (1 1 1) in the same conditions the surface is not oxidized but some oxide nuclei appear on atomic steps. Thus for our rather large particles with flat (1 0 0) facets the oxidation of Pt is not expected.
4. Discussion Previous studies of the catalytic activity of Pd@Pt core–shell NPs for the oxygen reduction reaction (ORR) have shown an increased activity compared to pure Pt nanoparticles [13–17,20]. The increase in activity has been explained by a decrease in stability of OH species, which has been shown from DFT calculation to be the rate limiting step in the ORR reaction [36]. The decrease in the adsorption energy of OH would be due to a modification of the electronic structure of the surface Pt atoms due to the presence of the Pd substrate (core), through both strain and ligand effects. The strain effect includes the compressive strain of the Pt shell due to the misfit between Pt and Pd lattices. This misfit is moderate (1.4%) but the strain effect can be increased by the small size and the roughness of the particles [37]. The ligand effect is due to the modification of the electronic structure of the Pt atoms by the presence of Pd atoms as nearest neighbors. This
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ligand effect was first described by Nørskov and coworkers [38] for metal alloys. Moreover the morphology of the particles may influence the ORR activity by the presence of facets with different coordinations of the Pt shell atoms [39]. Indeed a larger activity has been obtained for octahedra Pd@Pt exposing (1 1 1) facets [20] compared to cubes with (1 0 0) facets [17]. But a more striking result in these studies is the observation of a maximum activity in ORR which corresponds to 2–3 ML of Pt on Pd cubes [17]. The atomic structure and defects of such Pd@Pt nanocubes and octahedra were also studied in detail by aberration corrected STEM [17,20,23]. The Pt layers were uniformly deposited on the Pd seeds except on the cube corners where the thickness was larger. HAADF-STEM profiles showed a partial mixing between Pt and Pd atoms which is more important close to the interface. For higher amount of Pt ML on Pd cubes, the activity again decreased. The presence of this maximum was explained by a decrease in the ligand effect when the number of Pt layers increases [17]. In the case of methanol electro-oxidation of Pd–Pt core–shell cubes an increase in activity relatively to pure Pt was observed for a mean thickness of the Pt shell of 2.5 nm, which corresponds to about 12–13 atomic layers [18]. For this large number of Pt layers the ligand and strain must be very weak and thus the increase in reactivity small. However, in this study, it could be seen from the TEM pictures that the Pt film is very rough [18] compared to the study by Xie et al. [17,23] which showed smooth Pt deposits. The observed increase in activity can be explained by the roughness of the Pt layer which can shorten the Pt–Pt distance as seen by EXAFS [37]. In the case of hydrogenation of p-CNB on Pd–Pt core–shell octahedral [19] with Pt shell thickness between 1 and 4 atomic layers, a maximum of activity was observed for about 1 ML of Pt. Again the increase in activity could also be explained by a combination of ligand and strain effect. The CO oxidation has been studied in the recent years on colloidal nanoparticles with well-defined unique shapes (cubes, octahedra, cubo-octahedra). On monometallic Pt [25] and Pd [24] the catalytic activity showed important dependence on the shape explained by the presence of different facets, mainly (1 1 1) and (1 0 0). It was not possible before to evidence such dependence on catalysts prepared by wet impregnation or physical deposition because it was not possible by this method to obtain a single shape and the size distribution was large. Moreover, by selecting a single shape and different size it was possible to evidence a clear size effects on Pd cubes, namely the TOF increased by decreasing particle size [40]. The dependence on the shape and on the size of particles clearly proves that the CO oxidation is structure sensitive on Pt group metals, at least in the O2-rich regime [9,25]. In the present work we have studied the CO oxidation on Pd–Pt core–shell cubes as a function of the thickness of the Pt. The curves of the activity as a function of temperature (Fig. 3) show that the activity of Pd–Pt core–shell particles is larger than the activity of pure Pd cubes which is very low (as previously observed by others [24]). We have tried to synthesize pure Pt cubes in the same size range as the Pd cubes but we did not succeed, since we obtained small more or less round Pt particles. Murray and coworkers succeeded to prepare Pt cubes with a comparable size of 9.3 nm [25]. After removing the ligands they have studied the activity in CO oxidation at different partial pressures of CO and O2 with the closest conditions from the present study at PO2/PCO = 1. In our study the maximum of reactivity is observed for 0.4 eq. atomic layer of Pt and it is higher than for the pure Pt cubes of Murray’s group [25] (Table 1). By increasing further the thickness of the Pt shell up to 2.2 equivalent atomic layers the activity decreases, but stays around the value of pure Pt nanocubes and it is still much higher than for pure Pd cubes.
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Fig. 5. ETEM observations of different pure Pd cubes under (a) 3 mbar H2 with limited effect on the shape and (b) under 1 mbar O2 with extension of the (1 1 0) facets. Dark field images under H2 (c) and O2 (d) also clearly evidence the (1 1 0) facets, as presented in the model (e and f).
In the operation conditions we have used for the CO oxidation experiments, we are certainly in the regime of high CO coverage, which limits the dissociation of oxygen. Then the reaction is negative first order in CO partial pressure [41]. Experimentally we find a 0.8 order in CO. In this case, the only way to increase the reaction rate is to decrease the adsorption energy of CO. As for previous electrocatalysis experiments the deposition of Pt layers on top of the Pd cubes induced a down-shift of the d-band center due to combination of ligand and compressive strain effects, which result in a decrease in the CO bond strength on the surface of the particles [42]. Thus we can explain the increase in reactivity by increasing the thickness of the Pt layer. A maximum of reactivity is observed for 0.4 equivalent atomic layer. However, this is only an average thickness because of the preferentially deposition at the corners
and the presence of some larger bars. For larger Pt thickness the activity is lower and close to that of Pt cubes previously measured in similar conditions [25]. Probably for these larger thicknesses the Pt top layer is relaxed to the bulk value and the ligand effect is also decreased by the screening of the Pd atoms by the Pt layer. In the ETEM experiment under 1 mbar of oxygen we have seen the development of (1 1 0) facets on the cube edges of the Pd–Pt core–shell particles. This evolution is explained by a high oxygen coverage. However in our condition of CO oxidation the coverage of oxygen is very low and the development of (1 1 0) facets is unlikely, but it is not excluded that the particle shape also changes with high coverage of CO. Unfortunately it was not possible to observe by ETEM the shape of the particles in the conditions of CO oxidation.
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Fig. 6. ETEM observations of a series of Pt0.4eqL@Pd under (a) 1 mbar H2 with limited effect on the shape and (b) under 1 mbar O2 with rounding of the sharp corners and extension of the (1 1 0) facets.
5. Conclusions
References
Pd–Pt core–shell structures have been obtained by growing successive amounts of Pt on Pd cubes. The Pt layer grows epitaxially on the Pd cubes, but is more concentrated at the corners, which induces sharp corners. The reactivity of the Pd–Pt core–shell cubes has been studied in the CO oxidation reaction as a function of the thickness of the Pt ad-layer. A maximum of reactivity has been found for a thickness of 0.4 equivalent atomic layers. For this thickness the core–shell particles are more active than pure Pt cubes or pure Pd cubes. In the case of Pt NPs prepared by wet impregnation, for which no ligand molecules are used to stabilize the NPs, the reactivity is generally higher than for colloidal Pt NPs. However, the primary aim of this work was not to prepare better catalysts, but to understand the effect of a thin layer of Pt on a Pd NP, which can be better controlled in the colloidal process. This better activity in [email protected] is explained by a decrease in the adsorption energy of CO due to a compressive strain, induced by the misfit between the two metal bulk lattices and by a ligand effect due to the modification of the electronic structure of Pt atoms in contact with Pd atoms. A similar qualitative evolution as a function of thickness of the Pt layer was already observed in electrocatalysis and also explained by the decrease in the strength of adsorbed species. By ETEM it has been observed that under 1 mbar of hydrogen, the particles keep the cubic shape and under 1 mbar of oxygen, (1 1 0) facets and rounded corners appear at the edges of the cube.
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Acknowledgments This work was supported in part by a grant from the region PACA and by the ANR projects ‘DINAMIC’ and ‘SANAM’.
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Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Core–shell Pd–Pt nanocubes for the CO oxidation Astrid De Clercq a,b, Olivier Margeat a, Georges Sitja a, Claude R. Henry a, Suzanne Giorgio a,⇑ a b
Aix Marseille Université, CNRS, CINaM UMR 7325, 13288 Marseille, France Aix Marseille Université, CNRS, MADIREL UMR 7246, 13397 Marseille, France
a r t i c l e
i n f o
Article history: Received 26 August 2015 Revised 23 December 2015 Accepted 13 January 2016
Keywords: CO oxidation Core–shell Pd@Pt Nanocubes
a b s t r a c t Pd–Pt core–shell nanocubes with increasing outer Pt layers have been studied for the CO oxidation reaction by gas phase chromatography. The maximal reactivity of the core–shell nanocubes was found for an average value of 0.4 atomic layer of Pt, with a preferential growth at the corners. High resolution electron microscopy (HRTEM) observations show a rough Pt layer, epitaxially crystallized on the Pd surfaces. In situ observations of the Pd and Pt@Pd nanocubes by Environmental Transmission Electron Microscopy (ETEM) during oxidation–reduction cycles show the formation of high index facets during the adsorption of O2. Similar behavior of the reactivity was reported in the literature for electrocatalytic reactions and interpreted in terms of decrease in the adsorption energy of the rate limiting species on the metal surface. This is explained by the compressive strain and the proximity of Pd atoms in the epitaxial Pt layer on Pd cubes, resulting in a change in the electronic structure of the Pt. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction Bimetallic particles containing platinum become more and more important in catalysis [1,2]. They have already been used since 1950 in the oil refining industry, mainly for reforming reactions, where better stability and selectivity were obtained. The second application of these bimetallic catalysts is TWC (Three Way Catalysts) for car exhausts, with CO oxidation and NOx decomposition reactions. The third and more recent use of the combination between Pt and another metal (Pt–M) is in electrocatalysis, mainly for proton exchange membrane fuel cells (PEMFC). Traditionally pure Pt particles are used as catalyst in PEMFC, but they present two drawbacks: the poisoning of CO contained in the hydrogen fuel and the limited resources of Pt in the earth. Alloying Pt with another metal permits to avoid CO poisoning [3]. Moreover Pt–M nanoparticles have shown higher reactivity for the oxygen reduction reaction (ORR) [4,5]. In order to save Pt it could be interesting to prepare nanoalloys with a low concentration of Pt or ideally to make a core–shell nanostructure with a thin Pt shell and a cheap metal core. Bimetallic catalysts are classically prepared by impregnation, but with this method it is difficult to precisely control the size and the composition of bimetallic particles. In addition it is not possible to prepare controlled core–shell structures or particles ⇑ Corresponding author. E-mail address: [email protected] (S. Giorgio). http://dx.doi.org/10.1016/j.jcat.2016.01.005 0021-9517/Ó 2016 Elsevier Inc. All rights reserved.
with a unique shape. Great progress was achieved by colloidal preparation methods, for which the preparation of mono- or bimetallic nanoparticles with a sharp size distribution, a control of the composition and a single shape, is now possible [6–8]. As the colloidal nanoparticles self-organize on a flat surface they can form a regular array of particles that can be used as flat model catalysts [9]. However after synthesis the colloidal nanoparticles are capped by stabilizing molecules (ligands) that can strongly decrease the accessibility of surface atoms to reactants during the catalytic reaction. Nowadays, several efficient methods to remove the ligands are available [10–12]. Among Pt–M systems, the catalytic properties of Pd–Pt core– shell systems have been extensively studied in the recent years [13–20]. Most of them concern the oxygen reduction reaction in fuel cells and an increase in activity is always found relatively to commercial pure Pt catalysts. Some studies used Pd particles with a well-defined shape (cube [17] or octahedron [20]) on which successive Pt monolayers (ML) are deposited. In the case of a cubic shape, a maximal activity for the ORR was observed for 2–3 ML of Pt. Another study using a 2.5 nm Pt layer on Pd cubes also showed an increase in activity for methanol electro-oxidation [18]. The only study not concerned by electrocatalysis investigates the hydrogenation of p-chloronitrobenzene (p-CNB) on Pt layers (1–4 ML) on 6–8 nm Pd octahedra and shows the best activity for 1 ML of Pt [19]. In the present work we study the catalytic activity for the CO oxidation reaction of Pd–Pt core–shell structures, as a function of
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the thickness of the Pt shell. Pd cubes prepared by colloidal chemistry are used as seed to grow Pt layers with different thicknesses (0.2–2.2 equivalent ML). The core–shell structures are characterized by HRTEM. For the CO oxidation measurements the particles are supported on silica and the ligand is removed by UV–ozone treatment. The core–shell structures are found to be more active than pure Pd particles and the highest activity is obtained for 0.4 equivalent ML of Pt. The effect of oxygen and hydrogen adsorption on the morphology of pure Pd and Pd–Pt core–shell particles is studied in situ at RT by ETEM. 2. Experimental 2.1. Synthesis of Pd cores and deposition of Pt amounts The synthesis of Pd cores was inspired by a method of Yang et al. [21]. 25 mg of Pd(acac)2 (99%, Sigma–Aldrich) was mixed in 2 ml Dimethylsulfoxide (DMSO) and 77.5 mg Polyvinylpyrrolidone (PVP) (Mw 1.3 M Alfa Aesar) and 69 mg KI were dissolved in 2 ml DMSO. Then both solutions were mixed together in a glass reactor resistant up to 3 bar. The reactor was closed and heated up to 140 °C for 2 h in an oil bath, before cooling down to room temperature. A sample containing 0.02 mmol Pd was extracted and a first amount of Pt(acac)2 (for example 2.4 mg in 2 ml DMSO) was added. This mixture was again heated up to 120 °C for 1 h 30. This procedure of cooling to room temperature and adding a certain amount of Pt(acac)2 was repeated to prepare the samples containing different amounts of Pt. From EDX analysis we could evaluate the molar ratio of Pt/Pd for an average of 10 measurements on collections of particles. With the estimation that about 8 mol% is at the surface of the Pd cubes, we calculate the equivalent Pt layers deposited on the Pd cubes (even if the layers were not completely smooth). As such, samples with 0.2, 0.4, 0.5, 1.6 and 2.2 equivalent monolayers could be prepared. In further discussion, the sample names are based on this calculation and written as Pd@Pt(x-equivalent-layers). All particles were washed by a standard centrifuge technique and re-dispersed at concentrations of 14 mM in an ethanol–water (4:1) mixture. 2.2. Preparation of the supported NPs and ligand removal The NPs were loaded on amorphous SiO2 powder by ultrasonically mixing the solution containing 0.010 mmol Pd with 45 mg SiO2 (US Research Nanomaterials, 160–600 m2/g). This gives an approximate loading for pure Pd of 2.3 wt%. Total Pt/Pd metal loadings for the other samples was estimated based on the EDX analysis, which gives a maximal loading of 3.1 wt% for the highest amount of Pt. The powder was then dried in vacuum and the PVP ligands were removed at room temperature by UV–ozone exposure for 2 h [12]. 2.3. Characterization methods TEM and HRTEM observations were acquired with a Jeol 3010 operated at 300 kV. For the in situ TEM observations, an environmental sample holder (E-cell) was used in the standard JEOL 3010 microscope [22], so that the low pressure in the column of the microscope was not affected. In these conditions, the characteristics of the microscopes correspond to a resolution of 0.21 nm, with a Cs = 1.4 mm. The morphological changes of the nanocubes were in situ observed during oxido reduction cycles in pure H2 and pure O2 at pressures below 10 mbars. The cubes were deposited and encapsulated between gas proof carbon layers separated by 0.1 mm for the gas circulation.
2.4. CO oxidation reaction The activity for the CO oxidation reaction was evaluated in a fixed-bed quartz tubular reactor (inner diameter 3 mm, length 20 cm) at atmospheric pressure for a total flow rate of 22.3 ml/ min (8% CO, 12% O2 in N2). About 20 mg of catalyst powder was loaded into the quartz reactor. A thermocouple placed near the reactor bed gave the estimation of the catalyst temperature. A gas chromatograph equipped with a thermal conductivity detector (TCD) and two columns (Porapak Q 1/800 2 m 80/100 and Molecular sieve 1/800 2 m 13 60/80) was used to analyze the reaction products. Due to the exothermic nature of the reaction, rapid temperature rises in the catalyst bed can occur. Therefore it was opted to compare the samples in the low conversion regime (below 10% conversion), in the kinetically-controlled regime, with a space velocity of 600–800 ml s1 g (metal)1. After loading, samples were exposed to N2 flow for 1 h at 160 °C. Then, temperature increments of 10–20 °C at 0.5 °C/min were done and the total measurement time for each curve was around 12 h. No decrease in activity is observed after 20 h of continuous operation. The TOF is referred to the number of surface sites obtained by taking cubic particles with the mean size determined by TEM and exact amount of metal loading. 3. Results 3.1. Standard Transmission Electron Microscopy (TEM) observations Fig. 1a is a standard TEM overview of pure Pd seeds spontaneously (0 0 1) oriented, dropped on a carbon film. Most of them are cubes with an average edge length of 14.6 ± 2.3 nm, or elongated along a crystallographic [0 0 1] direction, with (0 0 1) external faces. The enlarged image of the cubes (Fig. 1b) clearly shows very weak truncations by (1 1 0) faces at the edges and truncations by (1 1 1) faces at the corners. The extension of these truncations approximately corresponds to anisotropy ratio between the surface free energies of r(0 0 1)/r(1 1 0) ffi r(0 0 1)/r(1 1 1) ffi 0.7, which is lower than the values obtained for clean fcc metal crystals of the same size, at the equilibrium. Indeed, the equilibrium shape is only obtained after high temperature annealing in ultra-high vacuum conditions for nanocrystals in this size range. So the cubic shape obtained during the growth of the particles is not the equilibrium shape of clean Pd particles. Pd cubes covered with Pt atomic layers (from 0.2 to 2.2 equivalent layers) were observed in the same conditions. An intermediate composition is shown in Fig. 1c and d. Compared to pure Pd nanocubes, no noticeable size increase could be observed, because of the very low Pt quantities. However the crystal shape evolved from cube to ‘‘frame Pd–Pt concave nanocrystal” as previously described by Lu et al. [23] and was interpreted as the preferential growth of Pt at the corners of Pd cubes. The high resolution image of the edge and the corner of such a frame concave nanocrystal, seen in Fig. 2, corresponds to the epitaxial growth of Pt on the (0 0 1) Pd faces. Despite of a mismatch of 1.4% between the Pt and Pd atomic radius, the (0 0 2) lattice planes are continuous at the interface, due to the accommodation of Pt on the Pd substrate, with some distortions. It confirms that Pt is mainly deposited near the corners, creating some higher index facets. 3.2. CO oxidation reaction The CO oxidation TOF of the different samples can be compared in Fig. 3. The pure Pd cubes/SiO2 sample was found to be very little active, only starting to convert at high temperature, in the range where the SiO2 support starts to show some activity. This low
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Fig. 1. TEM images of (a and b) the pure Pd seeds and (c and d) the Pd seeds covered with 0.4 equivalent layers of Pt, with sharpening of the corners.
Fig. 2. HRTEM images of the corner of a Pd seeds covered with 2.2 equivalent layers of Pt.
activity for the cubic shaped Pd colloidal NPs was observed by other groups (for similar size, support and gas composition), while Pd particles with an octahedron shape started to convert around 250 °C [24], for the same metal loading and nearly the same size. In the case of Pd nanocubes covered with low quantities of Pt, with a coverage equivalent to 0.2 Pt layers, little improvement
was noticed. However, a significant superior performance was obtained for a coverage of 0.4 equivalent Pt layers. The turnover frequency (TOF) at 220 °C is given in Table 1. From this, it can be seen that the TOF increased by a factor 33. As seen in Fig. 3 and Table 1, a further increase in the Pt coverage does not improve any more the activity (TOF). The TOF for Pd@Pt 0.5, 1.6 and 2.2 equivalent layers, even decreased with a factor 3 compared to the TOF for [email protected]. TOF values can be compared with values reported for Pt nanocubes prepared by a colloidal method [25] or small Pt nanoparticles prepared by wet impregnation of SiO2 [26] (Table 1). It should be noticed that the given values were obtained for slightly different gas compositions. For the CO oxidation reaction with a first order in O2 and negative first order in CO, this would give an adjustment of the listed TOF values with a factor 1.5 and 3, corresponding to 0.03 and 0.3 s1 respectively. From this table, it can be seen that the coverage of 0.4 equivalent Pt layers gives a higher TOF than pure Pt nanocubes [25]. The samples with higher coverage (0.5–2.2 equivalent layers) give a similar TOF as Pt nanocubes [25]. The most active sample, with 0.4 equivalent layers of Pt, has a TOF in the same range as very small Pt/SiO2 nanoparticles prepared by wet impregnation, although in this case, no residual ligands are present, which can strongly reduce the number of active sites and then the activity [27]. In comparison with previous publications on the catalytic activity of Pd–Pt core–shell nanoparticles, we found similar trends of the reactivity as for the ORR and hydrogenation reactions as a function of the thickness [17,19,20]. The Arrhenius plots with associated apparent activation energies can be seen in Fig. 4.
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Fig. 3. CO oxidation activities over the different samples of cubic Pd–Pt core–shell NPs supported on SiO2 with increasing equivalent atomic layers of Pt (for PO2/PCO = 1.5).
Table 1 Turnover frequencies for Pd cubes covered with increasing equivalent Pt atomic layers at 220 °C. Our measurements (PO2/PCO = 1.5)
TOF @ 220 °C (s1)
Pd nanocubes [email protected] nanocubes [email protected] nanocubes [email protected] nanocubes [email protected] nanocubes [email protected] nanocubes
<4 103 <4 103 0.13 0.02 0.04 0.02
Literature results Pt nanocubes/Al2O3 (compensated PO2/PCO = 1.0) [25] Standard Pt/SiO2 wet impregnation (compensated PO2/PCO = 0.5) [26]
0.02 (0.03) 0.1 (0.3)
3.3. ETEM observations By environmental TEM (ETEM), we have studied the shape evolution of the pure Pd cubes and Pd–Pt core–shell cubes under oxygen and hydrogen gas. As discussed in previous works, even RT observations are representative of the shape evolution during real catalytic reactions as the electron beam increases the surface diffusion on the NPs. Before the measurements of the CO oxidation, the samples are exposed to O2. Then, the morphological and chemical structure changes of the NPs under oxidizing and reducing gas environments have been studied by ETEM. Similar experiments were performed in previous works with pure Pt nanocubes, prepared by wet chemistry with different organic ligands [28]. A clear shape evolution was observed, with
Fig. 4. Arrhenius plots of different samples of cubic Pd–Pt core–shell NPs supported on SiO2 with increasing equivalent atomic layers of Pt (for PO2/PCO = 1.5).
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a change of r(0 0 1)/r(1 1 1) from 0.8 (cubic shape) in oxygen to r (0 0 1)/r(1 1 1) of 1.12 in hydrogen. First, the pure Pd cubes were observed during oxidation–reduction cycles (Fig. 5) in pressures of around 1 mbar. In hydrogen gas (Fig. 5a), not much shape changes were detected compared to vacuum observation. The absorption of hydrogen to form the hydride phase was previously studied for similar Pd cubes, and an expansion of the lattice parameter was observed, but with no influence on the cubic shape [29]. On the other hand, Fig. 5b clearly shows the extension of (1 1 0) faces during O2 exposure and the rounding of the corners. This transformation corresponds to the increase in the anisotropy ratio r(0 0 1)/r(1 1 0) from 0.7 to about 0.8. This change is coherent with previous ETEM studies of fcc crystals [22,28,30] showing the formation of higher index facets in pure O2. This extension of the (1 1 0) faces is also visible in weak beam dark field imaging in Fig. 5c and d and the associated 3D shape is illustrated in the drawing of Fig. 5e and f. Next, the most active [email protected] cubes for the CO oxidation were observed in similar conditions, in pure H2 and pure O2 by ETEM (Fig. 6). They keep the original concave shape in pure H2, as seen in Fig. 6a, with no noticeable difference with the assynthesized shape. Then, the same [email protected] cubes in pure O2 are rounded at the corners (Fig. 6b). Such rounding at the corners in pure O2 was observed in ETEM on Pt cubes [28]. By environmental SXRD (surface X-ray diffraction) it was shown for Pd and Pt nanoparticles that the rounding under oxygen is due to the formation of high index facets under oxygen which disappear reversibly under CO [31,32]. As in the case of pure Pd nanocubes in O2, for [email protected] nanocubes, the (1 1 0) facets are extended compared to the same samples in H2. The increase in stability of high index facets on Pt and Pd under high pressure of oxygen is also justified by ab initio calculations [33]. In the studied large nanocubes, no bulk oxidation was observed under oxygen. In a recent ETEM study, in pure O2, at higher magnification with an aberration corrector and a higher electron beam current density, the oxidation of the top layer could be observed for smaller supported NPs (4–5 nm) which was reversible under CO [34]. It has been also observed by a combination of in situ STM and ambient pressure XPS at RT in 1.33 mbar of oxygen, that stepped Pt surfaces become oxidized and that nucleation of oxide islands occurred only on steps [35]. The surface oxidation is reversible and on Pt (1 1 1) in the same conditions the surface is not oxidized but some oxide nuclei appear on atomic steps. Thus for our rather large particles with flat (1 0 0) facets the oxidation of Pt is not expected.
4. Discussion Previous studies of the catalytic activity of Pd@Pt core–shell NPs for the oxygen reduction reaction (ORR) have shown an increased activity compared to pure Pt nanoparticles [13–17,20]. The increase in activity has been explained by a decrease in stability of OH species, which has been shown from DFT calculation to be the rate limiting step in the ORR reaction [36]. The decrease in the adsorption energy of OH would be due to a modification of the electronic structure of the surface Pt atoms due to the presence of the Pd substrate (core), through both strain and ligand effects. The strain effect includes the compressive strain of the Pt shell due to the misfit between Pt and Pd lattices. This misfit is moderate (1.4%) but the strain effect can be increased by the small size and the roughness of the particles [37]. The ligand effect is due to the modification of the electronic structure of the Pt atoms by the presence of Pd atoms as nearest neighbors. This
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ligand effect was first described by Nørskov and coworkers [38] for metal alloys. Moreover the morphology of the particles may influence the ORR activity by the presence of facets with different coordinations of the Pt shell atoms [39]. Indeed a larger activity has been obtained for octahedra Pd@Pt exposing (1 1 1) facets [20] compared to cubes with (1 0 0) facets [17]. But a more striking result in these studies is the observation of a maximum activity in ORR which corresponds to 2–3 ML of Pt on Pd cubes [17]. The atomic structure and defects of such Pd@Pt nanocubes and octahedra were also studied in detail by aberration corrected STEM [17,20,23]. The Pt layers were uniformly deposited on the Pd seeds except on the cube corners where the thickness was larger. HAADF-STEM profiles showed a partial mixing between Pt and Pd atoms which is more important close to the interface. For higher amount of Pt ML on Pd cubes, the activity again decreased. The presence of this maximum was explained by a decrease in the ligand effect when the number of Pt layers increases [17]. In the case of methanol electro-oxidation of Pd–Pt core–shell cubes an increase in activity relatively to pure Pt was observed for a mean thickness of the Pt shell of 2.5 nm, which corresponds to about 12–13 atomic layers [18]. For this large number of Pt layers the ligand and strain must be very weak and thus the increase in reactivity small. However, in this study, it could be seen from the TEM pictures that the Pt film is very rough [18] compared to the study by Xie et al. [17,23] which showed smooth Pt deposits. The observed increase in activity can be explained by the roughness of the Pt layer which can shorten the Pt–Pt distance as seen by EXAFS [37]. In the case of hydrogenation of p-CNB on Pd–Pt core–shell octahedral [19] with Pt shell thickness between 1 and 4 atomic layers, a maximum of activity was observed for about 1 ML of Pt. Again the increase in activity could also be explained by a combination of ligand and strain effect. The CO oxidation has been studied in the recent years on colloidal nanoparticles with well-defined unique shapes (cubes, octahedra, cubo-octahedra). On monometallic Pt [25] and Pd [24] the catalytic activity showed important dependence on the shape explained by the presence of different facets, mainly (1 1 1) and (1 0 0). It was not possible before to evidence such dependence on catalysts prepared by wet impregnation or physical deposition because it was not possible by this method to obtain a single shape and the size distribution was large. Moreover, by selecting a single shape and different size it was possible to evidence a clear size effects on Pd cubes, namely the TOF increased by decreasing particle size [40]. The dependence on the shape and on the size of particles clearly proves that the CO oxidation is structure sensitive on Pt group metals, at least in the O2-rich regime [9,25]. In the present work we have studied the CO oxidation on Pd–Pt core–shell cubes as a function of the thickness of the Pt. The curves of the activity as a function of temperature (Fig. 3) show that the activity of Pd–Pt core–shell particles is larger than the activity of pure Pd cubes which is very low (as previously observed by others [24]). We have tried to synthesize pure Pt cubes in the same size range as the Pd cubes but we did not succeed, since we obtained small more or less round Pt particles. Murray and coworkers succeeded to prepare Pt cubes with a comparable size of 9.3 nm [25]. After removing the ligands they have studied the activity in CO oxidation at different partial pressures of CO and O2 with the closest conditions from the present study at PO2/PCO = 1. In our study the maximum of reactivity is observed for 0.4 eq. atomic layer of Pt and it is higher than for the pure Pt cubes of Murray’s group [25] (Table 1). By increasing further the thickness of the Pt shell up to 2.2 equivalent atomic layers the activity decreases, but stays around the value of pure Pt nanocubes and it is still much higher than for pure Pd cubes.
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Fig. 5. ETEM observations of different pure Pd cubes under (a) 3 mbar H2 with limited effect on the shape and (b) under 1 mbar O2 with extension of the (1 1 0) facets. Dark field images under H2 (c) and O2 (d) also clearly evidence the (1 1 0) facets, as presented in the model (e and f).
In the operation conditions we have used for the CO oxidation experiments, we are certainly in the regime of high CO coverage, which limits the dissociation of oxygen. Then the reaction is negative first order in CO partial pressure [41]. Experimentally we find a 0.8 order in CO. In this case, the only way to increase the reaction rate is to decrease the adsorption energy of CO. As for previous electrocatalysis experiments the deposition of Pt layers on top of the Pd cubes induced a down-shift of the d-band center due to combination of ligand and compressive strain effects, which result in a decrease in the CO bond strength on the surface of the particles [42]. Thus we can explain the increase in reactivity by increasing the thickness of the Pt layer. A maximum of reactivity is observed for 0.4 equivalent atomic layer. However, this is only an average thickness because of the preferentially deposition at the corners
and the presence of some larger bars. For larger Pt thickness the activity is lower and close to that of Pt cubes previously measured in similar conditions [25]. Probably for these larger thicknesses the Pt top layer is relaxed to the bulk value and the ligand effect is also decreased by the screening of the Pd atoms by the Pt layer. In the ETEM experiment under 1 mbar of oxygen we have seen the development of (1 1 0) facets on the cube edges of the Pd–Pt core–shell particles. This evolution is explained by a high oxygen coverage. However in our condition of CO oxidation the coverage of oxygen is very low and the development of (1 1 0) facets is unlikely, but it is not excluded that the particle shape also changes with high coverage of CO. Unfortunately it was not possible to observe by ETEM the shape of the particles in the conditions of CO oxidation.
A. De Clercq et al. / Journal of Catalysis 336 (2016) 33–40
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Fig. 6. ETEM observations of a series of Pt0.4eqL@Pd under (a) 1 mbar H2 with limited effect on the shape and (b) under 1 mbar O2 with rounding of the sharp corners and extension of the (1 1 0) facets.
5. Conclusions
References
Pd–Pt core–shell structures have been obtained by growing successive amounts of Pt on Pd cubes. The Pt layer grows epitaxially on the Pd cubes, but is more concentrated at the corners, which induces sharp corners. The reactivity of the Pd–Pt core–shell cubes has been studied in the CO oxidation reaction as a function of the thickness of the Pt ad-layer. A maximum of reactivity has been found for a thickness of 0.4 equivalent atomic layers. For this thickness the core–shell particles are more active than pure Pt cubes or pure Pd cubes. In the case of Pt NPs prepared by wet impregnation, for which no ligand molecules are used to stabilize the NPs, the reactivity is generally higher than for colloidal Pt NPs. However, the primary aim of this work was not to prepare better catalysts, but to understand the effect of a thin layer of Pt on a Pd NP, which can be better controlled in the colloidal process. This better activity in [email protected] is explained by a decrease in the adsorption energy of CO due to a compressive strain, induced by the misfit between the two metal bulk lattices and by a ligand effect due to the modification of the electronic structure of Pt atoms in contact with Pd atoms. A similar qualitative evolution as a function of thickness of the Pt layer was already observed in electrocatalysis and also explained by the decrease in the strength of adsorbed species. By ETEM it has been observed that under 1 mbar of hydrogen, the particles keep the cubic shape and under 1 mbar of oxygen, (1 1 0) facets and rounded corners appear at the edges of the cube.
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