Evolution of Pt Nanoparticles Supported on Fishbone-Type Carbon

Article pubs.acs.org/JPCC

Evolution of Pt Nanoparticles Supported on Fishbone-Type Carbon Nanofibers with Cone−Helix Structures: A Molecular Dynamics Study Hong-Ye Cheng,† Yi-An Zhu,*,† Per-Olof Åstrand,‡ De Chen,§ Ping Li,† and Xing-Gui Zhou† †

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Department of Chemistry and §Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway

‡

ABSTRACT: Molecular dynamics simulations based on a reactive force field (ReaxFF) are performed to examine the effects of the variable morphologies of fishbone-type carbon nanofibers (f-CNFs) on the microstructures of supported Pt100 clusters. Four f-CNF cone−helix models with different basalto-edge surface area ratios and edge plane terminations are employed. Calculated results indicate upon adsorption of Pt100 clusters a fraction of Pt atoms migrates from the metal particles onto the f-CNFs either to accumulate at the metal−support interface or to attain a single atom adsorption on the supports. With decreasing apex angle or introduction of H termination, the Pt atoms are more likely to be coordinated to the basal planes and the binding energies of the Pt100 clusters to the f-CNFs are lowered, accompanied by a lower degree of the cluster reconstruction. On the contrary, if more f-CNF edge planes are exposed, a higher Pt dispersion, lower surface first-shell Pt−Pt coordination numbers, and longer Pt−Pt surface bonds are attained. Considering the interplay between the geometric and the electronic structures of transition metal surfaces, the relationship among the support morphologies, the metal−support interactions, and the catalytic properties of the active Pt clusters is eventually elucidated.

1. INTRODUCTION Carbon-supported Pt catalyst plays an increasing role in proton exchange membrane fuel cells (PEMFC). It exhibits high catalytic activities for both hydrogen/methanol oxidation1,2 and oxygen reduction3 at low temperatures. Carbon has many advantages as a support, including high surface area, inertness in basic and acidic solutions,4 and high conductivity.5,6 Among the new carbon materials, carbon nanofiber (CNF) that is made up of orientated graphene sheets has unique microstructures and superior physical and chemical properties.7−14 For instance, Pt supported on CNF was reported to exhibit better tolerance to CO poisoning in the electrochemical oxidation of methanol than Pt supported on Vulcan carbon (XC-72).15 According to the orientation of graphene sheets, CNF is classified into three categories, platelet type (p-CNF), tubular type (t-CNF), and fishbone type CNF (f-CNF),16 with their graphene sheets perpendicular, parallel, and tilted to the principal axis, respectively. The p-CNF is dominated by edge planes, while the t-CNF is surrounded by basal planes. As for fCNF, however, it has both of the two planes exposed and its variable apex angle makes the on-purpose control of the basalto-edge surface area ratio possible. It was reported that the basal and edge planes play different roles in the CNF chemical and physical behaviors.8−10,17,18 For instance, the basal planes were proposed to be capable of strengthening adsorption of ethylbenzene, while the edge planes that were modified by oxygen-containing species catalyzed the oxidative dehydrogenation reactions.9 As another example, when CNF acted as a © 2013 American Chemical Society

support the binding energy of its exposed surface sites to the doped Pt atoms was found to vary from −2.03 eV on the basal plane to −11.01 eV on the edge plane.19 It is therefore reasonable to expect that the basal-to-edge surface area ratio has a significant effect on the interaction between CNFs and deposited metal particles, which in turn determines how metal particles are dispersed15,20 and their catalytic activity and stability.15,21−25 A substantial amount of experimental and theoretical efforts have been devoted to a better understanding of the interaction between metal clusters and carbon supports and hence the relationship between the interaction and the catalytic performance. The CNF microstructure was found to have a considerable effect on the interaction between Pd and CNFs using temperature-programmed reduction (TPR).26 The temperature at the maximum reduction peak was used to quantify the interaction between the metal clusters and the supports, where a higher reduction temperature signified a stronger bonding. According to the TPR results, the binding strength, ranked in descending order, was as follows Pd/p-CNF > Pd/f-CNF > Pd/t-CNF, that is, a lower basal-to-edge surface area ratio yields a stronger interaction. Bessel et al. suggested that the interaction between Pt and CNF was stronger than that between Pt and Vulcan carbon and resulted in highly Received: February 5, 2013 Revised: June 13, 2013 Published: June 14, 2013 14261

dx.doi.org/10.1021/jp401319n | J. Phys. Chem. C 2013, 117, 14261−14271

The Journal of Physical Chemistry C

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allowed us to tune the basal-to-edge surface area ratio smoothly by changing the apex angle.41 Compared with the conventional stacked-cup model,36 the cone−helix model succeeds in accounting for the broad distribution of the experimentally observed f-CNF apex angles and can rationalize the f-CNF superior electrical and mechanical properties. Therefore, application of the cone−helix structures is more rational to investigate the Pt-CNF interaction. In this contribution, four cone−helix f-CNF models with different basal-to-edge surface area ratios and edge plane terminations were adopted to investigate the effects of the variable morphologies of f-CNFs on the microstructures of supported Pt100 clusters through MD simulations based on a reactive force field. The binding energies of the Pt clusters to the f-CNFs were calculated and correlated to the number of the Pt atoms that were bonded to the C surfaces in order to quantify the Pt-CNF interaction. With the change in the f-CNF apex angle and considering the H termination, the variations in the bulk and surface first-shell Pt−Pt coordination number and Pt−Pt bond length distribution and that in the Pt dispersion were calculated to examine the evolution of the geometries of the Pt100 clusters upon adsorption. Finally, we conclude by discussing the relationship between the geometric structures of active metal surfaces and their catalytic properties on the basis of the d-band model.

crystalline facets on the metal clusters, which offered an explanation for the higher catalytic activity for oxidation of methanol.15 Chesnokov et al. investigated the effect of the CNF microstructure on the catalytic properties of Pd/CNF catalyst in the selective hydrogenation of 1,3-butadiene to butene.27 The p-CNF had a stronger interaction with Pd than f-CNF and t-CNF, and more electrons were transferred from Pd to CNF (i.e., a higher fraction of Pd2+). However, the increase in the amount of Pd2+ led to a decrease in the overall catalytic activity and selectivity. In the theoretical aspect, density functional theory (DFT) calculations have also been carried out to examine the metal− support interaction. Kong et al. investigated the binding geometries and energetics of a Pt atom on the p-CNF and tCNF surfaces and found the CNF with a lower basal-to-edge surface area ratio has a better electrode performance arising from a higher Pt dispersion.19 Okazaki-Maeda et al. studied the geometric and electronic structures of the Ptn (n = 1−13) clusters adsorbed on graphene and proposed that the interfacial interaction between a Pt cluster and graphene was determined by the shape and size of a cluster and the manner in which they were contacted.28,29 For large Pt clusters of a few hundreds of atoms, molecular dynamics (MD) simulations based on a relatively simple Lennard−Jones potential have been performed to study their interaction with carbon surfaces, e.g., Pt256 and Pt260 clusters adsorbed on graphite substrates30 and a Pt249 cluster supported on carbon nanotube (CNT) bundles.31 In these simulations, physisorption of metal clusters on the basal planes of graphite and CNT could be well described by the Lennard−Jones potential. However, calculations using the same method failed in accounting for chemisorption of the Pt clusters, merely because the Pt−C bond formation and breaking took place. To address this issue, the bond-order force fields, such as the Brenner32 (BrennerFF) and Reax33 force fields (ReaxFF), were developed, which could depict several different bonding states of an atom using the same parameters and could thus describe chemical reactions to some extent. More recently, Sanz-Navarro and co-workers deposited Pt and Ni particles of approximately 100 atoms on the edge and basal planes of both p-CNFs and fCNFs34−36 and examined their interactions by employing the ReaxFF. Since the adsorption energies of Pt on the CNF surfaces (−6.76 eV at the armchair edge and −11.01 eV at the zigzag edge)19 were much higher than the cohesive energy of bulk Pt (−5.84 eV),37 the Pt atoms could migrate readily from the Pt cluster to the f-CNF surface, leading to a reconstruction of the Pt100 cluster to form a flattened particle, in good agreement with experimental observations.7,38 As a consequence, the Pt−Pt bonds were elongated and lattice strain resulted. It is well accepted that the lattice strain in the metal clusters generally gives rise to a surface d-band shift which resulted in a significant change in the catalytic performance.39,40 In the work by Sanz-Navarro et al., an increased bond length was observed upon adsorption and signified a decrease in the bond order, thereby leading to more reactive metal atoms.36 However, as only an f-CNF stacked-cup model with a disclination angle of 60° and a p-CNF model were used to represent the support surfaces in their work,34,36 a more comprehensive investigation regarding the effects of the CNF basal-to-edge surface area ratio on the Pt-CNF interaction (and hence on the microstructures of the supported Pt clusters) is highly desired to enable rational catalyst design. We previously constructed several cone−helix models for f-CNF, which

2. COMPUTATIONAL DETAILS 2.1. ReaxFF. The ReaxFF was employed to represent the interactions in the Pt/C/H system. This reactive force field is based on a bond-order approach and a charge equilibrium scheme42 parametrized from quantum chemical data, providing a good compromise between accuracy and computational efficiency. The ReaxFF was originally proposed for hydrocarbons33 and has been extended to model the systems such as Si/SiO2,43 Al/AlO,44 Ni/Cu/Co/C,45 Mg/MgH,46 Li/LiC,47 BiMoOx,48 and Au/S/C/H.49 More recently, it succeeded in accouting for the CNT growth mechanism on Ni.50−52 In the ReaxFF, the analytical expression of each energy contribution and a detailed explanation of their physical meanings can be found in ref 33 and the Supporting Information of ref 45. The Pt/C/H ReaxFF parameters were optimized using DFT calculations with the Perdew−Burke− Ernzerhof (PBE) exchange-correlation functional and normconserving pseudopotentials.34 In order to evaluate the reliability of the ReaxFF in depicting the Pt crystal structure, the surface energy of Pt(111) was calculated and the result (0.13 eV/A2) compared closely to the DFT prediction (0.13 eV/A2)53 and experimental measurement (0.16 eV/A2).54 SanzNavarro et al. made a comparison between the BrennerFF and ReaxFF and found that the latter was better to reproduce the DFT and experimental results regarding the single Pt atom adsorption on graphite edges,34 implying the ReaxFF was well suited to investigate the interaction between Pt clusters and fCNF surfaces. 2.2. MD Simulation. MD simulations have been performed using the LAMMPS code.55,56 A time step of 0.25 fs was adopted to ensure energy conservation through a few trial ReaxFF simulations in the microcanonical ensemble (NVE).34 The system energy was preliminarily minimized before MD simulations were conducted. Upon dynamics simulations, it took 50 ps to equilibrate the system at 600 K using the canonical ensemble (NVT) and Nosé−Hoover chain thermostat.57 Above 600 K, gasification of CNF will take place in the 14262



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presence of Pt and H2.58 An MD simulation was subsequently carried out in equilibrium for 250 ps. 2.3. Model Construction. Pt clusters of 100 atoms (denoted as Pt100) were constructed on the basis of the facecentered cubic (fcc) Pt crystal structure and had a diameter of about 15 Å. Before the Pt100 clusters were supported on fCNFs, simulated annealing was performed to get stable cluster structures. Minimization was combined with 20 cycles of MD simulations from 300 to 1500K and then back from 1500 to 300 K. The cone−helix model for f-CNF was obtained with a continuous graphite ribbon spiraling along the principal axis.41 Several notions have been brought in to depict the cone−helix structures. The disclination angle is the angle of the sector which is cut from the graphene sheet. The overlap angle is the rotation angle by which graphene sheets rotate clockwise or counterclockwise with respect to each other in their basal planes. With introduction of different disclination and overlap angles into graphene sheets, the resultant helical cones have variable apex angles and basal-to-edge surface area ratios: A greater apex angle gives rise to a lower basal-to-edge surface area ratio. The f-CNF cone−helix models employed in the present work included two standard models with disclination angles of 60° and 180° (denoted as f-CNF/60 and f-CNF/180, respectively) and two nonstandard models having a disclination angle of 60° and an overlap angle of 13.17° without and with H atoms terminated (denoted as f-CNF/60-13.17 and f-CNF/6013.17-H, respectively). Structural parameters of the resulting apex angles are summarized in Table 1.

Figure 1. Schematic representations of initial structures of Pt100 clusters supported on (a) f-CNF/60, (b) f-CNF/60-13.17, (c) fCNF/180, and (d) f-CNF/60-13.17-H.

the z direction have been determined in our previous work41 and are given in Table 1. In order to eliminate the lateral interactions, the cell lengths in the x and y directions were two times greater than the diameters of the f-CNFs (see Table 1). The initial structures of Pt100 clusters supported on different f-CNFs are illustrated in Figure 1. The Pt cluster was placed at a distance of about 2 Å away from the surface sites of f-CNFs. For f-CNF/60 and f-CNF/180, one Pt100 cluster was initially positioned at an armchair arrangement [denoted as Pt100(I)], a second Pt particle was located at a zigzag arrangement [denoted as Pt100(II)], and the last one was placed at the junction between an armchair and a zigzag arrangement [denoted as Pt100(III)], as shown in Figure 1a and 1c. As for the f-CNF/6013.17 and f-CNF/60-13.17-H, three Pt100 clusters were placed randomly at different surface sites of f-CNFs, as illustrated in Figure 1b and 1d. Because the metal clusters are not spherical and nonsymmetric, the orientation of the Pt clusters with respect to the f-CNFs has a considerable effect on the contact region between them, leading to a variation of the lattice strain in the Pt clusters.34,36 Therefore, in the present simulations the Pt clusters had the same initial adsorption orientation with respect to the f-CNFs.

Table 1. Structural Parameters of f-CNF Models cell parameters (Å)

f-CNF/60 f-CNF/ 60-13.17 f-CNF/180 f-CNF/ 60-13.17-H

apex angle (degrees)

no. of helical cones in one cell

a

b

c

112.89 105.69

9 9

150 150

150 150

36.48 38.16

60.00 105.69

5 9

150 150

150 150

33.83 38.16

In the standard f-CNF/60 and f-CNF/180, the adjacent helical cones have the same sequence of armchair and zigzag arrangements, while the nonstandard f-CNF/60-13.17(-H) has the edges of the adjacent layers interlaced, as illustrated in Figure 1. Among the four models, the f-CNF/60, which has the same basal-to-edge surface area ratio as the previously investigated stacked-cup model,36 was chosen to examine the effect of the spiral packing arrangement of hexagonal carbon rings on the Pt−CNF interaction. From the f-CNF/60 to the fCNF/60-13.17 and further to the f-CNF/180, the cone−helix models have more and more basal planes exposed, which makes it possible to reveal the roles of the basal and edge planes in bonding Pt clusters. On the other hand, as H is the most abundant species on catalyst surfaces in PEMFC, the influence of H termination on the binding strength of Pt clusters was taken into account in the f-CNF/60-13.17-H. The diameters of the f-CNFs were raised to 60 Å to eliminate the interactions among three Pt100 clusters. Periodic boundary conditions were imposed on the cone−helix models to include the interactions among adjacent helical cones and depict the f-CNF morphologies along the principal axis. The numbers of helical cones involved in one cell and the corresponding cell lengths in

3. RESULTS AND DISCUSSION 3.1. Interaction between Pt100 Clusters and f-CNFs. Schematic representations of the Pt100 clusters adsorbed on different cone−helix models upon 250 ps of equilibrium are shown in Figure 2. The comparison between Figures 1 and 2 indicates upon adsorption the Pt100 clusters are positioned much more closely to the f-CNF surfaces because covalent bonds are formed between them. Carefully analyzing the geometries we find one Pt atom at the Pt/CNF interface is probably bonded to several C atoms. The Pt−C bond length distribution was then investigated to describe the interaction between the Pt100 clusters and the f-CNFs, as given in Figure 3. The probability distributions of the bond lengths are normalized in the range [x1, x2] in such a way that ∫ xx21P(dx) = 1, where x1 = 1.8 Å and x2 = 2.8 Å for the Pt−C bonds and x1 = 2.4 Å and x2 = 3.2 Å for the Pt−Pt bonds, respectively. Atomic coordinates are recorded every 25 fs after the system is in equilibrium. Data are all grouped every 500 fs, and the 14263



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taken as small as 0.0125 Å to get a smooth curve, and the average error bars are given every five points to obtain a clear plot. From Figure 3, one can see for all the Pt−CNF systems two pronounced peaks are observed at Pt−C bond lengths of about 2.0 and 2.5 Å, which are close to the DFT-calculated Pt−C bond lengths for the single Pt atom adsorption on the surfaces of p-CNF19 and the data by previous ReaxFF simulations.34 Experimentally, early EXAFS investigations also indicated the Pt−C bond length primarily took two values of about 2.0 and 2.6 Å at the interface between Pt clusters and CNFs.59,60 On the other hand, the Pt−C bond length distribution varies with the basal-to-edge surface area ratio and depends on whether the f-CNF is terminated by H. The Pt100 clusters adsorbed on the f-CNF/60 and f-CNF/60-13.17 have more Pt−C bonds with a length of about 2.0 Å but lower bond length probabilities at 2.5 Å than those on the f-CNF/180 and f-CNF/ 60-13.17-H. As can be seen in Figure 2, the apparent reconstruction of the metal clusters reflects the fact that the interaction between the Pt100 clusters and f-CNFs is rather strong. In all four Pt/f-CNF models, a fraction of Pt atoms migrates from the metal particles onto the CNFs either to accumulate at the interface between the clusters and the fCNFs or to attain a single atom adsorption on the supports. Thus, variation in the Pt−C bond length distribution might arise from the difference in the bonding geometry of Pt atoms to f-CNFs. Three types of Pt adsorption configurations have been observed on the f-CNFs. First, a fraction of Pt atoms is located in the vicinity of the edge planes. Pt is bound to the open edge arrangements through two short Pt−C bonds (about 2.0 Å in

Figure 2. Schematic representations of Pt100 clusters upon 250 ps of equilibrium on (a) f-CNF/60, (b) f-CNF/60-13.17, (c) f-CNF/180, and (d) f-CNF/60-13.17-H.

statistical error is calculated as the root-mean-square deviation in the resulting samples. The size of the distribution bins is

Figure 3. Probability densities of Pt−C bond length distributions at the interface between Pt100 clusters and (a) f-CNF/60, (b) f-CNF/60-13.17, (c) f-CNF/180, and (d) f-CNF/60-13.17-H. 14264



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E b = E Pt100 /f ‐ CNF − E Pt100 − Ef ‐ CNF

bond length) or two short and two long Pt−C bonds (about 2.5 Å in bond length), and simultaneously, it is coordinated to the basal plane through six long Pt−C bonds (see Figure 4a).

(1)

where EPt100/f‑CNF is the total energy of the Pt100 cluster adsorbed on the f-CNF upon 250 ps of equilibrium, EPt100 is the total energy of an isolated Pt100 cluster with fixed geometry upon adsorption, and Ef‑CNF is the total energy of the f-CNF. Under this definition, the binding energy offers a direct measure of how strongly the Pt clusters and the f-CNFs interact, where a more negative binding energy indicates a stronger binding strength. The relationship between the binding energies of the Pt100 clusters and the numbers of the Pt atoms bonded to fCNFs is shown in Figure 5. It is apparent that with the increase

Figure 5. Relationship between the binding energies of Pt100 clusters and the numbers of the Pt atoms bonded to f-CNFs. Figure 4. Schematic representations of Pt bonded to f-CNF: (a) a Pt atom bonded to both the f-CNF basal and the edge planes; (b) a Pt atom bonded to the f-CNF edge plane; (c) a Pt atom bonded to the fCNF basal plane. Surrounding Pt atoms in the clusters are not included for clarity.

in the number of the Pt atoms which are transferred from the Pt100 clusters to the nearby f-CNF surfaces a stronger cluster reconstruction and a greater binding strength result. 3.3. Degree of Metal Cluster Reconstruction. 3.3.1. Number of Pt Atoms Bonded to f-CNFs. As aforementioned, the reconstruction degree of the supported Pt100 clusters can be quantified by the number of the Pt atoms bonded to the f-CNFs. From Table 2, one can see the

Second, a few Pt atoms interact only with the edge carbon atoms, leading to two short Pt−C σ bonds or two short and two long Pt−C bonds, as shown in Figure 4b. Third, Pt atoms can be positioned at a distance far away from the edge planes and interact with carbon atoms in the basal plane through six long Pt−C covalent bonds, as shown in Figure 4c. Consequently, the Pt−C bond length distribution is determined by how frequently Pt atoms are in contact with the edge (or basal) planes, that is, the f-CNF basal-to-edge surface area ratio plays a significant role. For the Pt100 clusters on the f-CNF/60, the number of long Pt−C bonds is about three times greater than that of the shorter ones. With increasing basal-to-edge surface area ratio, the f-CNF/180 has more Pt atoms bonded to the basal planes and simultaneously fewer coordinated to the edge planes. As a result, the probability of the short bonds decreases while the number of the long Pt−C bonds increases. On the Pt/f-CNF/ 60-13.17-H, the peak at about 2.0 Å is dramatically diminished because the edge planes around the metal clusters are dominated by H atoms, and therefore, σ bond formation between single Pt atoms and edge surface C atoms is significantly suppressed, in accordance with experimental findings.59 3.2. Binding Strength of Pt100 Clusters to f-CNFs. In order to quantitatively represent the Pt−CNF interaction, the binding energy of a Pt100 cluster on an f-CNF is defined as

Table 2. Structural Properties of Supported and Isolated Pt100 Clusters mean number of Pt atoms bonded to f-CNFs f-CNF/60 f-CNF/ 60-13.17 f-CNF/180 f-CNF/ 60-13.17 -H isolated Pt100

mean first-shell Pt−Pt coordination number

Pt100(I)

Pt100(II)

Pt100(III)

Pt100(I)

Pt100(II)

Pt100(III)

26.36 31.36

34.04 28.69

28.40 22.45

7.50 7.23

6.90 7.49

7.28 7.93

20.78 12.99

22.29 17.51

25.33 15.93

7.85 8.29

7.55 8.21

7.52 8.29

8.73

reconstruction degree, ranked in descending order, is as follows: Pt100/f-CNF/60 ≈ Pt100/f-CNF/60−13.17 > Pt100/fCNF/180 > Pt100/f-CNF/60-13.17-H. Since it was reported that the Pt adsorption energies on the edge planes of graphite were much more negative than those on the basal planes,19 the f-CNF with a smaller apex angle and more basal planes exposed would give rise to a smaller number of the Pt atoms that migrate from the Pt clusters onto the f-CNF supports and 14265



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Figure 6. Comparison of first-shell Pt−Pt coordination number distributions between isolated and supported Pt100 clusters on (a) f-CNF/60, (b) fCNF/60-13.17, (c) f-CNF/180, and (d) f-CNF/60-13.17-H.

isolated Pt100 cluster is measured to be 8.73, which is in good agreement with another theoretical prediction (8.561 for a Pt100 cluster). In the supported Pt100 clusters, the number of bulk atoms that have a first-shell Pt−Pt coordination number of 12 is significantly reduced, and simultaneously some Pt atoms are atomically adsorbed on the supports with coordination numbers being 0 or 1, as shown in Figure 6. Consequently, the mean first-shell Pt−Pt coordination number is decreased considerably from 8.73 to ∼7 (see Table 2). Some EXAFS investigations indicated that the mean first-shell Pt−Pt coordination numbers of the Pt clusters supported on CNFs and CNTs were about 5.5 with cluster diameters of 1.1 nm59 and 7.5 ± 0.7 with cluster diameters of 1.1 ± 0.3 nm, respectively.62 Considering the particle sizes (∼1.5 nm) of the Pt clusters employed in the present calculations, our estimation on the first-shell Pt−Pt coordination number agrees well with experimental measurements. 3.3.3. Pt−Pt Bond Length. Upon adsorption some Pt atoms migrate to the interfaces between Pt and f-CNF or onto the support surfaces, and therefore, lattice strain is inherently introduced with cluster reconstruction. The degree of lattice strain in the supported clusters can be described by comparison of the radial distribution function and the Pt−Pt bond length distribution between the supported and isolated Pt100 clusters, as given in Figures 7 and 8, respectively. For the sake of comparison, we assumed that the Pt100 clusters have the same average density. From Figure 7, it is apparent that all supported Pt100 clusters undergo a significant reconstruction upon adsorption, though the reconstruction degree varies with the f-CNFs. In Figure 8a−d, a fraction of Pt−Pt bonds becomes longer and simultaneously the probability of short Pt−Pt bonds

hence a lower reconstruction degree of Pt clusters. The C atoms on the edge planes of the f-CNF/60-13.17-H are initially coordinated to H atoms. Upon Pt100 adsorption, some of the surface C atoms are bonded to the Pt atoms instead and the detached H atoms migrate to the surfaces of the Pt100 clusters. Nevertheless, much fewer Pt atoms can find a C atom to form a covalent bond than those on the f-CNF/60-13.17, and consequently, a lower degree of metal cluster reconstruction results. 3.3.2. First-Shell Pt−Pt Coordination Number. In general, it is technically feasible to get some experimental structural information of supported metal particles, such as the mean firstshell Pt−Pt coordination number and the Pt−Pt bond length distribution, which makes it possible to compare the reconstruction degree between theoretical and experimental data. Here the distributions of the first-shell Pt−Pt coordination number in the Pt100 clusters are calculated as ∑i 12 = 0Ni = 100, where Ni is the number of the Pt atoms that have i Pt neighbors and are counted on the basis of the bond orders. Data are all grouped every 500 fs, and the size of the distribution bins is 1. The statistical error is calculated as the root-mean-square deviation in the resulting samples. In the isolated Pt100 cluster, the first-shell Pt−Pt coordination number falls within two ranges. It might take the values more than 10, signifying the bulk nature of the Pt atoms. Alternatively, the Pt atoms could act as surface atoms that are exposed to vacuum and have a first-shell Pt−Pt coordination number of no more than 10. As one can see in Figure 6, the maximum probability appears at 12, and therefore, most of the bulk Pt atoms are coordinatively saturated. On the other hand, the surface atoms mostly have 6−9 nearest neighbors. With consideration of the contributions from both bulk and surface atoms, the mean first-shell Pt−Pt coordination number in the 14266



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supported on the f-CNF/180. As for the Pt100 clusters adsorbed on the f-CNF/60-13.17-H, the changes in the mean first-shell coordination number and in the Pt−Pt bond length distribution are much less significant than that on the f-CNF/60-13.17 because the fewer exposed edge planes interact more weakly with the Pt100 clusters. These findings provide direct evidence in support of the conclusion that a stronger binding strength between Pt and f-CNF is achieved once more f-CNF edge planes are exposed, which in turn gives rise to a higher degree of metal cluster reconstruction. 3.4. Pt Dispersion. Pt dispersion is defined as the molar ratio of surface to overall Pt atoms in clusters, and a higher dispersion implies more exposed atoms and a higher catalytic efficiency for structure-insensitive reactions. The locations of Pt atoms can be identified by bringing in the notion of the firstshell coordination number. Considering the fcc crystal structure, a Pt atom with a first-shell coordination number of 12 indicates it is located in bulk metal and a lower coordination number signifies its surface nature.63 In the isolated nanoparticles Pt atoms are coordinated solely to the other Pt atoms, while in the supported clusters bond formation between Pt and C atoms should be taken into account as well. In order to identify the surface atoms, the Pt100 cluster is divided into two parts, namely, Pt atoms that are bonded to the f-CNFs and Pt atoms that are coordinated only to the other Pt atoms. For the Pt atoms in the isolated Pt cluster and the majority of the Pt atoms in the supported Pt clusters, Pt atoms are not bonded to the f-CNFs. In this case, the surface atom is identified by the first-shell Pt−Pt coordination number being no more than 10, which has been proposed by Lee and co-workers.64 For the Pt atoms bonded to the f-CNFs, the surface atom is determined by the first-shell Pt−Pt coordination number being no more than 4. Upon reconstruction the dispersion of the Pt100 clusters adsorbed on different f-CNFs is given in Table 3. The corresponding data of an isolated Pt100 cluster in vacuum is about 69.02%, which is close to a previously reported theoretical result (62%).61 Compared with the isolated Pt100 cluster, most of the supported metal particles have lower Pt dispersion, merely because a variety of surface sites is inherently eliminated upon adsorption. Nevertheless, adsorption-induced reconstruction tends to raise the dispersion of the supported clusters to be comparable to that of the cluster in vacuum. Moreover, one can see from Table 3 that a lower basal-to-edge surface area ratio of f-CNF leads to a higher degree of metal cluster reconstruction, which in turn gives rise to a higher Pt dispersion. While the isolated Pt100 cluster has a higher dispersion, it suffers from a higher surface energy and is readily sintered to its counterparts under experimental conditions. In contrast, the f-CNFs have the capability of stabilizing the Pt clusters by forming Pt−C bonds, and therefore a relatively high Pt dispersion prevails on the supported catalysts. 3.5. Pt−Pt Surface Bond Length. As the electronic structures of active metal surfaces are of vital importance to heterogeneous catalysis, we move on to the surface properties of the Pt100 clusters to examine the effects of the metal−support interactions on the catalytic performance. The mean first-shell Pt−Pt coordination number of the surface Pt atoms is given in Table 3. With the comparison between Tables 2 and 3, one can see that the surface Pt atoms have lower mean first-shell Pt−Pt coordination numbers than the overall Pt atoms in both the isolated and the supported Pt100 clusters, and the trend in the mean first-shell Pt−Pt coordination numbers of the surface Pt

Figure 7. Radial distribution functions of supported and isolated Pt clusters.

(below ∼2.7 Å in bond length) also increases upon adsorption, indicating significant lattice strain is present. As the metal− support interaction results in flattened metal clusters, elongation of Pt−Pt bonds is reasonable. However, the presence of the shortened Pt−Pt bonds is well beyond what is expected. Here the Pt−Pt bond length distributions on the surface and in the bulk are separated for both isolated and supported Pt100 clusters and illustrated in Figure 8e and 8f. Upon adsorption a considerable increase in the probability of long Pt−Pt bonds is observed on the cluster surface, while in the bulk of the supported Pt100 cluster both bond elongation and contraction are present, that is, the shortened Pt−Pt bond length arises from bulk reconstruction. On the other hand, both the first-shell Pt−Pt coordination number and the Pt−Pt bond length distribution vary with the basal-to-edge surface area ratio and depend on whether the edge planes are terminated by H. Pt100 clusters adsorbed on the f-CNF/60 and f-CNF/60-13.17 have a lower mean first-shell Pt−Pt coordination number, and the Pt−Pt bond length distributions are shifted up more significantly than those 14267



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Figure 8. Comparison of bond length distributions between isolated and supported Pt100 clusters on (a) f-CNF/60, (b) f-CNF/60-13.17, (c) fCNF/180, and (d) f-CNF/60-13.17-H. (e) Comparison of surface bond length between isolated Pt100 and supported Pt100(II) on f-CNF/60. (f) Comparison of bulk bond length between isolated Pt100 and supported Pt100(II) on f-CNF/60.

atoms with respect to the basal-to-edge surface area ratio is rather similar to that of the overall Pt atoms. On the other hand, because the surface Pt atoms have less nearest neighbors, they could move more freely than bulk Pt atoms and have longer Pt−Pt bond length distributions, as illustrated in Figure 9. According to the d-band model proposed by Hammer and Nørskov,40 the change in the adsorption energies of a small species on transition metals scales linearly with the shift in the surface d-band centers. In the linear muffin-tin orbital (LMTO) theory, the d-band center is sensitive to the Wigner−Seitz (WS) radius. With increasing the Pt WS radius, the embedding electron density from neighboring atoms decreases and consequently the d-band is narrowed and the d-band center is shifted up toward the Femi level to preserve the degree of dband filling. With the upshift in the d-band center, the antibonding states that arise from the hybridization between

Table 3. Pt Dispersions and Mean First-Shell Pt−Pt Coordination Numbers of Surface Pt Atoms in Isolated and Supported Pt100 Clusters Pt dispersion (%) f-CNF/60 f-CNF/ 60-13.17 f-CNF/180 f-CNF/ 60-13.17 -H isolated Pt100

mean first-shell Pt−Pt coordination number of surface Pt atoms

Pt100(I)

Pt100(II)

Pt100(III)

Pt100(I)

Pt100(II)

Pt100(III)

66.13 68.52

67.75 65.09

63.86 61.84

6.21 5.97

5.72 6.05

5.69 6.62

63.24 67.51

60.38 63.64

66.58 62.87

6.61 6.95

6.75 6.99

6.50 7.04

69.02

7.31

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Figure 9. Pt−Pt bond length distributions of surface and overall Pt atoms of Pt100(I) adsorbed on (a) f-CNF/60, (b) f-CNF/60-13.17, (c) f-CNF/ 180, and (d) f-CNF/60-13.17-H.

the adsorbate states and the metal d states are, at least in part, shifted up through the Fermi lever and become empty, which eventually gives rise to a stronger binding strength of the adsorbate to the metal surface. Thus, the variable f-CNF morphology has the capability of tuning the surface bond length distribution of supported Pt clusters, which in turn makes it possible to tailor the catalytic activity and selectivity of Pt for a particular chemical reaction.

clusters adsorbed on the f-CNF/60 and f-CNF/60-13.17 have more Pt−C bonds with a length of about 2.0 Å but lower bond length probabilities at 2.5 Å than those on the f-CNF/180. The variation in the Pt−C bond length distribution arises from the difference in the binding geometry of Pt atoms to f-CNFs. With increasing the basal-to-edge surface area ratio, the mean first-shell Pt−Pt coordination number in the Pt100 clusters is increased considerably from 6.90 to 7.85 and the Pt−Pt bond length distributions are shifted up less significantly. On the Pt100/ f-CNF/60-13.17-H, introduction of H termination has the same effect as the increased basal-to-edge surface area ratio. These findings provide direct evidence in support of the conclusion that a stronger binding strength between Pt and fCNF is achieved once more f-CNF edge planes are exposed, which in turn gives rise to a higher degree of metal cluster reconstruction. As a consequence, a higher Pt dispersion, lower surface first-shell Pt−Pt coordination numbers, and longer Pt− Pt surface bonds are attained. According to the d-band model, the increased Pt−Pt interatomic distance results in an upshift in the surface d-band center, which would eventually strengthen the binding of small species to the supported Pt clusters.

4. CONCLUSION MD simulations have been performed to examine the effects of the variable morphologies of f-CNFs on the bulk and surface structures of supported Pt100 clusters. Four f-CNF models with cone−helix structures are used to represent the support surfaces: f-CNF/60, f-CNF/60-13.17, f-CNF/180, and fCNF/60-13.17-H. Upon adsorption of Pt100 clusters, a fraction of Pt atoms migrates from the metal particles onto the f-CNFs either to accumulate at the interface between the clusters and the f-CNFs or to attain a single atom adsorption on the supports, merely because the adsorption energies of Pt on the fCNF surfaces are much higher than the cohesive energy of bulk Pt. Consequently, an apparent reconstruction of the metal clusters is observed, implying a strong binding strength of the Pt100 clusters to the f-CNFs. From the f-CNF/60 to the f-CNF/60-13.17, and further to the f-CNF/180, the basal-to-edge surface area ratio becomes progressively higher. As the Pt adsorption energies on the edge planes are much more negative than those on the basal planes, the f-CNF with a smaller apex angle and more basal planes exposed gives rise to a smaller number of migrating Pt atoms and hence a weaker Pt−CNF interaction. Meanwhile, the Pt100

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (No. 21003046) and 973 Project of Ministry of Science 14269



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Evolution of Pt Nanoparticles Supported on Fishbone-Type Carbon

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