Active Carbon Catalyst: Improved Spectroscopic Data Analysis by

Article pubs.acs.org/JPCC

Ru/Active Carbon Catalyst: Improved Spectroscopic Data Analysis by Density Functional Theory Izabela Czekaj,*,†,§ Sonia Pin,‡ and Jörg Wambach† †

General Energy Department, Paul Scherrer Institute, 5232 Villigen, Switzerland Swiss Light Source, Paul Scherrer Institute, 5232 Villigen, Switzerland

‡

ABSTRACT: We describe a DFT study on the self-organizing of ruthenium (Ru) nanoparticles deposited on a sp2 carbon layer (either of graphite type or of graphene type) using cluster models. The calculations provide insights into the nature and structure of the supported Ru clusters and the active sites of B5-type. Different sizes of Ru nanoparticles have been tested with up to 17 atoms. The carbon support has been modeled using a single sp2 carbon layer (graphene type) and double sp2 carbon layer (graphite type). Our DFT results suggest that for clusters with more than 4 Ru atoms, epitaxial growth of Ru on the carbon support is possible only in certain configurations. We were able to build and stabilize threedimensional Ru nanoparticles with several B5-type active sites on the carbon support. Such Csupported Ru-nanoparticles exhibiting B5-type sites give potential for further molecule adsorption and reaction mechanism studies. The all investigated structures and configurations for different Ru nanoparticles are presented. The supported Ru nanoparticles have been used for interpreting valence band and EXAFS spectra of a commercial Ru/ activated carbon catalyst. The Ru nanoparticles supported on graphene gives the best interpretation of the spectroscopic data.

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INTRODUCTION Carbon-supported Ru catalysts exhibit very good catalytic properties and are applied in the Haber−Bosch process,1 the Fischer−Tropsch synthesis,2 hydrogenation reactions,3−5 and the methanation of CO/CO26,7 as well as for conversion of biomass to methane in supercritical water.8,9 High surface area graphite (HSAG) is used for these applications to avoid selfgasification of the catalyst support via the water-gas-shift (WGS) reaction.10,11 It was recognized that the above-mentioned reactions are structure-sensitive, and in several reports,12−16 the observed reactivity is correlated with the existence of so-called active B5 sites. These were originally postulated by Van Hardeveld and Van Montfoort17 investigating the adsorption and dissociation of nitrogen on Ni, Pd, and Pd crystallites. The B5 sites are described as consisting of five metal atoms in a certain threedimensional arrangement exposing a 3-fold hollow site (i.e., a kind of terrace) and a bridge site (i.e., a step edge) close together. The majority of such active sites were identified to be present at edges related to low-coordinated surface atoms, e.g., edge atoms on (110) and (113) planes and formed only on medium-sized clusters (diameters within the range 1.5−7.0 nm). However, it must be stated that the shape of catalytically active clusters on a support might be different. Investigating the catalytic ammonia synthesis over several different types of supported Ru catalysts, Jacobsen et al.12 adopted the calculation of Van Hardeveld and Van Montfoort to the case of Ru. They calculated the optimum Ru particles sizes exhibiting the maximum number of B5 sites. It was found that these should be in the range between 1 and 3 nm in © 2013 American Chemical Society

diameter. These Ru particles are considered to possess an irregular crystalline geometry allowing the formation of B5 sites in the intersection between the crystallographic planes. Further, the authors state that on very small crystals the likeliness for the formation of B5 ensembles is very low, whereas with increasing crystal size, the fraction of B5 sites decreases due to the fast ́ decline in the fraction of edge atoms. Garcia-Garci á et al.13 investigated the ammonia decomposition over Ru supported on commercial activated carbon (AC). They report that the catalytic activity is influenced either by the degree of graphitization of the carbon support or by the use of a promoter. Sintering resulted in Ru particles with an average diameter in the order of 3−5 nm, which resulted in a remarkably increased catalytic activity than the one found on smaller sized crystallites. The activity of a catalyst is connected with the particle dispersion and their interactions with the support.12 Additionally, electron transfer from the support may occur, which depends on the status or pretreatment of the support (e.g., oxidation state, extend of graphitization). Therefore, knowledge about metal−support effects is important for understanding structure-sensitive catalytic reactions. The growth of Ru on highly oriented pyrolytic graphite (HOPG) as model catalysts was investigated by Nielsen et al.18 and Song et al.19 Both groups found that, depending on the applied preparation method, Received: June 28, 2013 Revised: November 20, 2013 Published: November 20, 2013 26588

dx.doi.org/10.1021/jp406402a | J. Phys. Chem. C 2013, 117, 26588−26597

The Journal of Physical Chemistry C

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and two sp2 carbon layers, C91, graphite-type, have been chosen, respectively. The Ru aggregates were cut from the ideal Ru bulk structure and consist of one to three layers. The electronic structure of all clusters was calculated by ab initio density functional theory (DFT) methods (program code StoBe22) using the nonlocal generalized gradient corrected functionals according to Perdew, Burke, and Ernzerhof (RPBE),23,24 to account for electron exchange and correlation. All Kohn−Sham orbitals are represented by linear combinations of atomic orbitals (LCAO’s) using extended basis sets of contracted Gaussians from atom optimizations.25,26 The following orbital basis sets of Ru(+14) (2211/3111/311) and C (621/41/1) has been considered in the calculations. Auxiliary basis sets, such as Ru(+14) (3,4;3,4) and C (4,3:4,3) were applied to fit the electron density and the exchange−correlation potential. Detailed analyses of the electronic structure in the clusters are carried out using Mulliken populations27 and Mayer bond order indices.28,29 During relaxation, the Ru atoms in the supported clusters had allowed to move in 3D space, whereas the support is fixed. In the calculation of the DOS, a fwhm of 0.6 eV was applied.

different nanoparticle morphologies can be obtained, which exhibits different catalytic behavior. There are several theoretical investigations of Ru catalysts, mainly based on periodic calculations.13−15 However, to our best knowledge, no investigation on the interaction between the Ru nanoparticles and the graphitic support has been done. The other interesting issues are metal−support influence on the location of the Ru clusters, their structure, and the possible number of active sites. In this paper, we present our DFT studies about the structure and the stabilization of Ru nanoparticles deposited on two different sp2 carbon surfaces, namely of graphene type and of graphite type, carried out in parallel with a detailed analysis of EXAFS and X-ray photoelectron valence band (XP-VBS) spectra. Additionally, we will present a new point of view on structure and localization of B5 active sites.

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CRYSTAL STRUCTURE OF GRAPHITE, RU, AND A RU MONOLAYER ON CARBON Hexagonal graphite20 is described by the space group P63/mmc (No. 194) with lattice constants a = b = 2.4640 Å and c = 6.7110 Å. The crystal unit cell contains four atoms, where carbon occupies two different types of positions, namely 2b and 2c sites. The nearest neighbor C−C distances are 1.42 and 2.46 Å for the second-order distance. Metallic Ru also is of hexagonal phase21 and described by the space group P63/mmc (No. 194) with lattice constants a = b = 2.7059 Å and c = 4.2815 Å. The crystal unit cell of Ru contains two atoms, where Ru occupies 2a positions. The nearestneighbor Ru−Ru distance is 2.71 Å, and each Ru atom is surrounded by six neighboring atoms on the surface. Figure 1 shows two possible epitaxial growth orientations of Ru(0001) suggested by Song et al.19 Marked in red is an

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EXPERIMENTAL SECTION For comparison with the DFT results, a commercial Ru/AC catalyst (BASF-Engelhard) was studied. The support is AC, derived from coconut shell, and consists of small, irregular grains (roughly 1 × 1 × 2 mm3). Ru (2 wt %, 0.24 at. %) is deposited on the support, forming an egg-shell-type catalyst. For XPS, the wet, as-received Ru/AC catalyst grains were dried, exposed to pure hydrogen for 2 h at 350 °C for reduction, and mounted on conductive carbon tapes for the measurement without further treatment. The valence band spectra were measured by X-ray photoelectron spectroscopy (XPS) applying a pass energy Epass = 20 eV. Monochromatic Al Kα (hν = 1486.6 eV) radiation was used as the X-ray source in a VG Escalab 220i XL apparatus. The X-rays illuminate a small area of about 0.5 × 0.8 mm2. For details see ref 30. Due to the relatively low kinetic energy of the photoelectrons, the probing depth is low and the analysis covers the surface and the topmost layers of the sample, only.31−33 X-ray absorption spectroscopy (XAS) and in particular its region extended X-ray absorption fine structure (EXAFS) spectroscopy25 is one of the most widely used techniques to study nanoparticles (materials whose finite size ensures that not “true” long-ranged order can exists34−37) because of its capability to probe the local coordination environment of an atom. The inherent length scale of the measurement (within a radius of ca. 0.8−1.0 nm from the central atom) corresponds well to the extent of metal−metal interactions and the number of coordination shells that might be formed in a cluster in the 1−2 nm size range. EXAFS studies of supported nanoparticles typically assume, either implicitly or explicitly, that the particles adopt quasi-hemispherical shapes. Programming codes that calculate the theoretical EXAFS signal and data analysis package38 allow EXAFS to evolve into a quantitative spectroscopic tool for structural analysis of nanoparticles. A key advantage of these ab initio multiple scattering EXAFS theories38 is their ability to analyze distant coordination shells by explicitly including many-body contributions to the total absorption. The combination of theoretical calculations (DFT) with EXAFS allows the calculated structures to be verified by

Figure 1. Ru7 cluster aligned on sp2 carbon, following the suggestions by Song et al.19 in the two possible epitaxial geometries ((a) 0° and (b) 30°). The red and blue lines are guidelines for the eye only.

arrangement of the Ru lattice being in-line with the carbon layer and described by vectors with k1 = k2 = k3 = 2.84 Å, resulting in a lattice mismatch of about −4.8%, further called the “0° geometry”. The blue hexagon represents an arrangement of the Ru lattice rotated by about 30° with respect to the carbon layer and described by vectors with kr1 = kr2 = kr3 = 2.46 Å, resulting in a lattice mismatch of about +9.2%, further called the “30° geometry”.

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COMPUTATIONAL DETAILS In our studies, both the Ru and sp2 carbon support surfaces are modeled by clusters, which reflect local sections of the ideal surface. sp2 carbon clusters with one layer, C54, graphene-type, 26589



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Figure 2. Different Ru cluster topologies on graphene: (a) Ru4 (stable adsorption); (b) and (c) Ru10; (d) Ru13; (e) Ru16. (b)−(d) Unstable adsorption.

spectrum). The EXAFS signal, χ(k), which is the sum of all contributions, χi(k), from a group of atoms approximately at equal distances from the absorbing atom (i.e., the ith shell), was adjusted by applying the EXAFS equation written in the following extended form:

comparing the obtained radial distances and coordination number with those measured by EXAFS. All X-ray absorption data were measured at the SuperXAS beamline at the Swiss Light Synchrotron (SLS), located at the Paul Scherrer Institut in Villigen PSI (CH). The intensity of the incident beam (I0) was measured with an argon-filled ion chamber. The as-received Ru/C catalyst was crushed in a mortar and sieved to a grain size of 125−800 μm. A fixed catalyst bed (typically 200 mg of Ru/C) with a length of about 25 mm was used in a high-temperature, high-pressure reactor, described elsewhere.39 X-ray absorption data from the sample were measured in transmission mode by scanning from 110 eV below to 1100 eV above the Ru K edge. A second argon-filled ion chamber was used to measure I1. A thin sample of Ru metal foil was used to calibrate the X-ray energy during each scan. The calibration measurement was made with a third argonfilled ion chamber (I2) placed after I1. The position of the metal absorption edge (Ru, 22 117 eV) could then be determined by placing the Ru metal standard between ion chambers I1 and I2 and measuring the absorption coefficient in the metal standard. To collect in situ X-ray absorption data from the carbonsupported Ru particles,40 the XAS data were analyzed using the UWXAFS analysis package,41 using the following general procedure: (i) the smooth, isolated-atom background function was removed from the experimental data; (ii) theoretical photoelectron scattering amplitudes and phase shifts were calculated for a model structure; (iii) the theoretical EXAFS signal was fit to the experimental EXAFS data in k-space (k interval: 1−16 Å−1) and R-space (R interval: 1−6 Å). For the measured X-ray absorption spectrum, the AUTOBK code42 code was used to normalize the absorption coefficient, μ(k), and separate the EXAFS, χ(k), from the isolated atom absorption background, μ0(k), fitted with a straight line. This process is shown in the following relation χ(k) = [(μ(k) − μ0(k))/Δμ0(k)], where k is the photoelectron wavenumber, k = √2m(E − E0)/ℏ2, E is the photoelectron energy, and E0 is the photoelectron energy origin (chosen with correspondence to the maximum of the first derivative of the absorption

χ (k ) =

∑ i

e

S0 2Ni kR i 2

⎡ ⎤ 4 |f ieff (k)|sin⎢2kR i − σ (3)k3 + δi(k)⎥ ⎣ ⎦ 3

−2σi 2k 2 −2R / λi(k)

e

where k is the photoelectron wavenumber, f eff i (k) and δi(k) are the photoelectron scattering-path amplitude and phase, respectively, S02 is the passive electron reduction factor, Ri, is the effective path length (which is equal to the interatomic distance for single-scattering paths), σi2 is the mean-square deviation in Ri, σi3 is the third cumulant that depends on the asymmetry of the bonds distribution, and λi(k) is the photoelectron mean free path. The computer code FEFFIT38 was used to fit theoretical EXAFS calculated with the FEFF8 code to experimental χ(k) data.

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RESULTS Stabilization of Ru Structures on Graphite and Graphene. Ru clusters with different sizes (Ru3 to Ru16) and two-dimensional (2D)- or three-dimensional (3D) shapes have been considered and tested for adsorption on the carbon support in different adsorption topologies (Figure 2); however, we only found the Ru3 (2D), Ru4 (3D), Ru7 (2D), and Ru11 (3D) clusters to result in a stable symmetric adsorbed configuration (e.g., Figure 2a). By unstable configurations we mean those Ru clusters not stabilizing on the surface, and thus desorbing from the support (e.g., Figure 2b or Figure 2c). This result proves the structural sensitivity of the adsorption of Ru nanoparticles on the carbon support. In a second step and applying the above-described adsorption geometries, we deposited a 2D Ru7 cluster on the carbon support. The two geometries are displayed in Figure 1a (left = “0°” (red), right = “30°” (blue)) showing the starting 26590



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Figure 3. Ru7 cluster aligned on sp2 carbon in the nonstable 30° epitaxial geometries after optimization. The blue lines are guidelines for the eye only.

Figure 5. Comparison of a (113)-B5 site arrangement (a) on Ru(0001) single crystal (left) and (b) on our Ru11 cluster (right).

geometry before geometrical optimization. The situation after the optimization is given in Figure 3. For the “30° geometry” (kri = 2.46 Å) geometry optimization resulted in a quite distorted, nonstable Ru ensemble on the carbon surface, and the Ru atoms shift to kinds of bridged positions (Figure 3). That can be related very well with the described differences in the lattice mismatch: the Ru arrangement in the “0° geometry”

has a smaller lattice mismatch with graphite surface than in the “30° geometry” case. The “0° geometry”, corresponding to a hexagon with Ru−Ru distances of k1 = 2.84 Å,13 resulted in a stable, symmetric, and epitaxial adsorption (Figure 3b_left) with an almost undistorted, but slightly elongated Ru hexagon (2.84 Å vs 2.71 Å for metallic Ru). Most of the Ru atoms are positioned in an almost on top position to the underlying carbon atom, and the center

Figure 4. Stable Ru11/C cluster on compatible epitaxial positions: (a) Ru11C54 graphene-case; (b) Ru11C91 graphite-case. Side and top view each. The pentagons mark the position of B5 sites. 26591



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where the above-described 2D layer represents the Ru−C interface. Ru clusters with different sizes on the carbon surface were tested and proved to be stable. That procedure is advantageous, because thus it is guaranteed that the interfacial Ru layer is in line with both the carbon support and the added Ru atoms of the second or higher layers. For our studies, we performed calculations with two support models: (a) carbon support represented by a single sheet of sp2 carbon (C54), referred to as “graphene”, and (b) carbon support consisting of two sp2 carbon layers (C91), called “graphite”. By doing so, we wanted to test the influence of the second carbon layer on the adsorption geometry of Ru. Such considerations are interesting with respect of the applicability on, e.g., various carbon fibres, such as single- or multiple-walled carbon nanotubes. For the formation of a 3D Ru nanoparticle, a cluster containing eleven Ru atoms (Ru11) was built (Figure 4) by adding three Ru atoms in the second, and one atom in the third layer. Analyzing that cluster, one can find an arrangement of five Ru atoms, with three atoms (in our case the atoms of the second and third layer) defining a plane, and two atoms (in the first, interface layer) being slightly shifted out of that plane, forming a step (Figure 5b). In other words, this arrangement consists of five Ru atoms exposing a 3-fold hollow hcp site and close together a bridge-site on an edge position. Following the terminology of Van Hardeveld and Van Montfoort,17 this is a (113)-B5-type site (Figure 5a), which was also found on the supported Ru cluster and described as the catalytically active B5 sites for ammonia synthesis.12−15 Both types of supports give almost identical Ru−C bond distances (graphene, 2.17 Å; graphite, 2.23 Å); however, the shape of the B5 sites are influenced by the support. In the case of the graphene-type support (Figure 6a), the shape of the obtained B5 sites corresponds well with the reported regular shape. As a consequence of electronic interactions to the support, the horizontal Ru−Ru bond distances of the interface atoms are 2.76 Å, almost the value of the pure metal (2.71 Å), whereas in the second layer 2.83 Å is found. On singlecrystalline Ru(0001) (Figure 5a), a monatomic step length of 2.14 Å is found, and for our models a slightly lower step length of about 2.09 Å was determined. Vertically, the Ru−Ru distances (the step height) are around 1.6 Å. For the graphite-type support (Figure 6b), the vertical Ru− Ru bond distances of the second and third layer are compatible with the graphene-type model, but significant changes of the horizontal Ru−Ru distances are observed. The additional interaction with the second layer carbon atom leads to a further compression of the interface Ru−Ru distance to 2.50 Å (Table 1), and in parallel, the second layer Ru−Ru distance is further elongated to 2.94 Å, leading to a nonregular, pentagon-like B5 site. Figure 6 additionally shows the obtained electronic parameters of the two B5 sites, i.e., Mulliken charges (white) and Mayer bond orders (red). The B5 sites of the Ru11C91/ graphite model exhibit a higher total charge (Σq = −0.84) than those of the Ru11C54/graphene model (Σq = −0.58). It is near at hand that, as a matter of the different electronegativity of the B5 sites, both types of sites could show a different reactivity, possibly additionally influenced by the different shapes. Concluding, these results suggest that the second graphite layer has important influence on the Ru nanoparticles’ shape and reactivity, and thus the influence of the support must be carefully considered and cannot be neglected easily.

Figure 6. Geometric and electronic parameters obtained for our B5 site: (a) Ru11C54, graphene type; (b) Ru11C91, graphite type. Key: distances (Å) (black); bond order (red); Mulliken charges (white).

Table 1. Internal Distances (Å) between Ru Centers and Support centers

Ru11/C54

Ru11/C91

Ru1−Ru1 Ru2−Ru2 Ru1c−C Ru1−C Ru1c−Ru1

2.76−2.77 2.83 2.37 2.55−2.64 2.77

2.50; 2.92 2.92−2.94 2.67−2.69 2.57−2.75 2.68−2.71

Figure 7. Partial density-of-states for Ru nanoparticles (including occupied states; theoretical spectra shifted about E − EF) compared with XP-VB spectra of the reduced Ru/C catalyst and a metallic Ru film, as well as with UP spectra (He I, 21.2 eV) of clean Ru(0001) single crystal.39 Note: the XP spectra are charge-shifted to match C1s = 284.45 eV.

Ru atom in a 6-fold hollow site above the center of a carbon hexagon. For our calculation the on top atoms are slightly distorted from the exact center by about 0.5 Å due to an electronic interaction with the other Ru atoms, resulting in a kind of coalescence effect. However, when a larger ensemble containing more Ru atoms is considered, the exact on top position is obtained by electronic interactions. Stabilization of 3D Cluster on Graphite and Graphene. The described stable 2D Ru7/carbon arrangement acted as the basis for the formation of 3D Ru nanoparticles, 26592



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Table 2. Comparison of the Peak Positions from Ru(0001) Gained by UPS45−47 as Well as by XPS from a Reduced Ru Crystal48 or Pressed Ru Powder49,50 with Our Data from a Polycrystalline Ru Sample, the Reduced Ru/C Catalyst, Both Obtained with m-XPS, and Our DFT Calculationsa peaks (eV) met Ru(0001) Blume43 Pelzer et al.44 Fuggle et al.46

method UPS UPS XPS

0.3 ∼0.4 ∼0.5

∼2.0

∼2.6 ∼2.3 ∼3.0

∼5.0 ∼4.9 ∼5.0

∼3.0

∼5.0 ∼5.0

∼2.5 ∼2.6 ∼2.4 ∼2.8

∼5.0 ∼5.0

7.3

Ru0 Powder Larichev et al. Shen et al.48

47

Ru0(poly) Ru0/AC catalyst Ru11/C54 model Ru11/C91 model a

XPS XPS

∼0.5 ∼0.5

m-XPS m-XPS DFT DFT

∼0.3 ∼0.3 ∼0.4 ∼0.5

∼2.0 ∼2.0

∼0.9 ∼0.8

Own Data 1.6 ∼1.6 1.5

∼7.0 ∼7.5

∼3.8 ∼4.2

Bold font: major peak. Small font: weak shoulder.

Figure 8. Estimation of the number of the B5 sites amount as function of laterally growing Ru cluster: (a) Ru11C54 with three B5 sites; (b) Ru38C96 with six B5 sites; (c) Ru50C96 with twelve B5 sites.

Figure 7 compares the calculated partial density-of-states (pDOS) for both types of supported Ru nanoparticles with experimental XP-VB spectra from a commercial Ru/AC catalyst from a metallic Ru0 sample, and with literature ultraviolet photoelectron spectroscopy (UPS) data from a clean Ru(0001) single crystal.43 Clean Ru(0001), top of Figure 7, exhibits UPS peaks centered at about 0.3, 2.6, and 5.0 eV and are mainly due to direct bulk transitions from the Ru 4d bands.44 The peak positions are comparable with data from Pelzer et al. recorded at hν = 21.2 eV (He I).44

The XP-VB spectra from a polycrystalline Ru sample and a reduced Ru/carbon catalyst are shown in the bottom of Figure 7. The carbon-derived background signal from the AC support was subtracted, leaving only the Ru−related signals. The poly crystalline, metallic Ru sample (120 nm Ru on a glass support) exhibits a shoulder at about ∼0.3 eV, a major signal at 1.6 eV, and additional shoulders at about 2.5, 5.0, and 7.0 eV. As can be seen from Table 2, the obtained spectrum and peak positions are comparable with published data having applied nonmonochromatic X-ray radiation.45−48 Reduction transfers the 26593



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Table 3. EXAFS Fitting Results and List of All the Paths Used in the Fit of the Carbon-Supported Nanoparticle

a

path

degeneracy, N

distance, R (Å)

σ2 (Å2)

SS1 SS2 SS3 SS4 TR1 TR2 TR3 DS TR

7.9(3) 2.8(8) 4(1) 5.9(9) 2.2(9) 2.2(9) 4.4(9) 11.8(9) 5.9(9)

2.68(2) 3.8(1) 4.68(7) 5.519(6) 4.11(1) 4.36(2) 5.19(3) 5.510(5) 5.52(6)

0.0057(2) 0.008(1) 0.005(1) 0.0023(4) 0.010(1) 0.003(2) 0.003(2) 0.0023(4) 0.0023(4)

The paths are defines as follows. SS1:

Ru 0 ⎯⎯⎯⎯→ Ru1(180°) ⎯⎯⎯⎯→ Ru 0 1NN

1NN

SS2: Ru 0 ⎯⎯⎯⎯→ Ru 2(180°) ⎯⎯⎯⎯→ Ru 0 2NN

2NN

SS3:

Ru 0 ⎯⎯⎯⎯→ Ru 3(180°) ⎯⎯⎯⎯→ Ru 0 3NN

3NN

SS4:

Ru 0 ⎯⎯⎯⎯→ Ru 4(180°) ⎯⎯⎯⎯→ Ru 0 4NN

4NN

TR1:

Ru 0 ⎯⎯⎯⎯→ Ru1(120°) ⎯⎯⎯⎯→ Ru1(120°) ⎯⎯⎯⎯→ Ru 0 1NN

1NN

1NN

TR2:

Ru 0 ⎯⎯⎯⎯→ Ru1(150°) ⎯⎯⎯⎯→ Ru1(150°) ⎯⎯⎯⎯→ Ru 0 1NN

Figure 9. Experimental χ(k) data (dotted line) and theoretical calculated EXAFS for the Ru bulk (red line, first panel from the top), the optimized Ru11C91 (blue line, second panel from the top) and Ru11C94 (magenta line, third panel from the top) clusters. For better understanding spectra are shifted along the y-axis.

3NN

1NN

TR3:

Ru 0 ⎯⎯⎯⎯→ Ru 3(150°) ⎯⎯⎯⎯→ Ru1(60°) ⎯⎯⎯⎯→ Ru 0 3NN

1NN

1NN

DS:

Ru 0 ⎯⎯⎯⎯→ Ru1(0°) ⎯⎯⎯⎯→ Ru 4(180°) ⎯⎯⎯⎯→ Ru 0

initial RuO2 to metallic Ru0, shifting the spectrum closer to the Fermi level. A major peak is centered at around 0.9 eV with a weak shoulder at about 0.3 eV. The peak shape indicates other signals at around 1.6 and 2.6 eV, as well as broad features centered near 5.0 and 7.5 eV. The fact that the major signals are shifted closer toward EF, is presumably connected with the small cluster size of the Ru deposits (≤1.5 nm30) and might indicate the observed excellent reactivity of the catalyst.49−51 The calculated VB spectra of the Ru nanoparticle on graphite (Ru11/C91) and graphene (Ru11/C54) obtained by DFT exhibit up to about 4.0 eV comparable peaks as the experimentally obtained spectra, although no signals are obtained at higher binding energies as in the measured spectra. For both model systems, the upper trace (thick line) is the sum of the pDOS from the Ru atoms located in the different layers of the nanoparticles. Ru 1l refers to the contribution from the first layer, i.e., the atoms forming the Ru−C interface layer. Ru 2l describes the middle layer and Ru 3l shows the pDOS of the top Ru atom (see also Figure 3, where the layers are shown). In the case of Ru11/C54, three major features are visible at 0.8, 2.4, and 3.8 eV together with a weak shoulder at about 0.3 eV,

1NN

1NN

4NN

TR:

Ru 0 ⎯⎯⎯⎯→ Ru1(0°) ⎯⎯⎯⎯→ Ru 4(180°) ⎯⎯⎯⎯→ Ru1(0°) ⎯⎯⎯⎯→ Ru 0 1NN

1NN

1NN

1NN

whereas in the case of Ru11/C91, four signals can be distinguished: 0.5, 1.5, 2.8, and about 4.2 eV (shoulder). The shape of the calculated VB spectrum is compatible with the experimental spectra from the Ru catalyst at activated carbon, and quite different than those from Ru(0001), which proves that the obtained supported cluster model is describing the commercial catalytic system quite well. Ru on graphene (Ru11/C54) represents possibly the reduced catalyst sample a bit more accurately, which could be explained by the existence of more graphene-like structures in the commercial Ru/C catalyst. Simulation of 2D Ru Nanoparticle Growth and B5 Sites Amount. As known from literature,52 the catalytic activity is directly related to the structure of the active sites. 26594



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Figure 10. Experimental Ru−K edge k3*χ(k) (left panel) and corresponding k3 Fourier transform (right panel) for the carbon-supported Ru nanoparticles: dotted line, experimental data; solid red line, full multiple scattering fit; solid green line, residuals.

paths) should be a fitting parameter, in addition to the bond length disorders σ 2 . All the parameters were varied independently for most paths; however, due to similarity of the geometries of the TR2 and TR3 paths, the σ2 was constrained to be the same as that of the TR2 path. The sample has a Ru−Ru distance of 2.68(2), 3.8(1), 4.68(4), and 5.52(0) Å for the first, second, third, and fourth shells, respectively. Those distances are consistent with what was obtained from the DFT calculations. Using the hemispherical cuboctahedron as model for the Ru nanoparticles structure, the average coordination numbers, obtained from shells 1 through 4, where all found to be in consistent with literature,55 corresponding to a particle diameter of ∼1.5 nm.

Step or edge sites on single crystalline samples or nanoparticles were found to play an important role in catalysis. Therefore, for illustrative purposes, we tried to make a prediction of the number of active B5 sites by multiplying laterally our abovedescribed nanoparticles (Figure 8a), which contain eleven Ru atoms at a diameter of 0.55 nm and possess three B5-type sites. In the case of lateral growth, a nanoparticle with a diameter of 1.1 nm (Figure 8b) could contain six B5 sites, and at a diameter of 1.5 nm (Figure 8c), twelve B5 sites could be possible. The latter type of nanoparticle (with 1.5 nm diameter) is equivalent to the cluster size determined by analyzing in situ EXAFS data on the commercial catalyst. The analysis indicates a particle size of around 1.0 nm (38 Ru atoms) to about 1.4 nm (50 Ru atoms) and consisting of at least three atomic layers. These results are in good agreement with data obtained from TEM suggesting a Ru particle size between 1.2 and 1.4 nm.53 EXAFS Results. Theoretical EXAFS data have been calculated using data of the bulk metallic Ru structure21 as well as using the DFT optimized structure data from the Ru11C54 and Ru11C91 clusters. The calculation was performed by summing and normalizing all the individual Ru contributions. The EXAFS simulations were carried out using the FEFF8 code within the multiplescattering approximation. Figure 9 shows the experimental χ(k) data together with the EXAFS theoretical calculation for the Ru bulk, and for the optimized Ru11C91 and Ru11C54 clusters. As can be observed, the spectra using the data from the cluster models reflect much better the experimental spectra of the Ru/AC catalyst. The Ru11C54 cluster model proved to mirror the experimental EXAFS data best, similar to the corresponding pDOS data represented the VB spectra best. This last EXAFS model was then refined, to obtain the average structural parameters reported in Table 3, and the k3weighted Ru-K edge EXAFS (left) and corresponding Fouriertransformed (right) spectra are reported in Figure 10. The high-symmetry paths considered in the fit are the following: four single-scattering (SS1−SS4) paths from the central (absorbing) Ru0 atom to its neighbors in the first through fourth shells, TR1−TR3 triangular paths, and two collinear focusing double (DS) and triple-scattering (TS) paths. In this fit, the photoelectron energy origin correction, ΔE0, was set to be the same for all paths and S02 was calculated from the bulk to be 0.81 and fixed for the subsequent fit. Because of the size effect described in ref 54, the path degeneracies N (which are equal to the coordination numbers for the single scattering

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CONCLUSIONS The results show that DFT simulations contribute to a better understanding the catalyst by providing insight into the structure and electronic situation of the Ru nanoparticles on a carbon support. It has been proven that Ru nanoparticles are very structure sensitive and stabilized on the sp2 carbon support only in certain geometries thus resulting in specific shapes. Additionally, it was found that the type of the support has influence on the shape and the electronic properties of the cluster and the active B5 sites. The Ru-nanoparticle on the graphene support fits best to the experimental EXAFS and XPVB spectra. Exemplarily, this shows that it is advantageous to perform DFT modeling of supported nanoparticles for analyzing experimental data of catalytic systems. We have been able to show that lateral growth of Ru nanoparticles can result in obtaining materials with a large amount of active B5 sites.

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AUTHOR INFORMATION

Corresponding Author

*I. Czekaj: e-mail, [email protected]; phone, +41 (0) 44 632 7836; fax, +41 (0) 44 633 1405. Present Address §

Institute for Chemical and Bioengineering, ETH, HCI D-125, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The calculations were done partially using the Unix farm as well as the linux farm at the Paul Scherrer Institut. We are grateful to 26595



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Marian Dreher for supplying the used XANES and EXAFS spectra.

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