Particle Size Effect of Hydride Formation and Surface Hydrogen

Aug 3, 2009 - Particle Size Effect of Hydride Formation and Surface Hydrogen Adsorption of Nanosized Palladium Catalysts: L3 Edge vs K Edge X-ray Abso...
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J. Phys. Chem. C 2009, 113, 15140–15147

Particle Size Effect of Hydride Formation and Surface Hydrogen Adsorption of Nanosized Palladium Catalysts: L3 Edge vs K Edge X-ray Absorption Spectroscopy Min Wei Tew,† Jeffrey T. Miller,‡ and Jeroen A. van Bokhoven*,† Institute for Chemical and Bioengineering, ETH Zurich, 8093 Zurich, Switzerland, and Chemical Technology DiVision, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439 ReceiVed: March 20, 2009; ReVised Manuscript ReceiVed: June 30, 2009

The particle size effect on the formation of palladium hydride and on surface hydrogen adsorption was studied at room temperature using in situ X-ray absorption spectroscopy at the Pd K and L3 edges. Hydride formation was indirectly observed by lattice expansion in Pd K edge XANES spectra and by EXAFS analysis. Hydride formation was directly detected in the L3 edge spectra. A characteristic spectral feature caused by the formation of a Pd-H antibonding state showed strong particle size dependence. The L3 edge spectra were reproduced using full multiple scattering analysis and density of state calculations, and the contributions of bulk absorbed and surface hydrogen to the XANES spectra could be distinguished. The ratio of hydrogen on the surface versus that in the bulk increased with decreasing particle size, and smaller particles dissolved less hydrogen. 1. Introduction Transition metal nanoparticles play an important role in many industrial catalytic reactions. They are supported to expose a large surface area and to maintain an optimum active surface area. Because the particle size and the kind of support affect the activity and selectivity of a reaction, obtaining the structure-performance relationship is essential to design and construct better catalysts. Numerous studies describe the effect of the particle size of palladium catalysts.1 Palladium is wellknown for being able to dissociate molecular hydrogen and absorb hydrogen into the crystal matrix to form hydrides at high hydrogen pressure and low temperature. The formation of the palladium hydride has been studied extensively because the hydrides affect the catalytic performance of the catalyst.2,3 The bulk dissolved hydrogen is much more energetic than adsorbed surface hydrogen and can hydrogenate surface adsorbates upon emerging to the surface.4,5 The subsurface hydrogen strongly enhanced the total hydrogenation of acetylene to ethane, whereas surface hydrogen alone without any subsurface population was much more selective toward ethylene.6 Distinction of bulk dissolved hydrogen from surface hydrogen in a palladiumcatalyzed process remains challenging. Nuclear reaction analysis distinguished surface adsorbed and bulk dissolved hydrogen on or in palladium nanocrystals on Al2O3/NiAl(110).7 Different phases of palladium hydrides (PdHx, x reflecting the stoichiometry) are formed upon absorption of hydrogen. An R-phase forms when 0 e x e 0.03. The structure is characterized by an increase of the lattice parameter of palladium metal from 3.891 to 3.894 Å, but it retains the FCC lattice of palladium atoms. A mixture of R- and β-phases is present in the range 0.03 e x e 0.58. For x g 0.58, hydrogen randomly occupies the octahedral interstices in the lattice to form the β-phase.8 At a stoichiometric concentration of x ) 1, all of the octahedral sites are fully occupied by hydrogen and the ideal sodium chloride structure is formed.9 The formation of bulk hydrides * To whom correspondence should be addressed. Address: ETH Zurich, HCI E 127, Wolfgang-Paulistr. 10, 8093 Zurich, Switzerland. Phone: +41 44 632 55 42. E-mail: [email protected]. † ETH Zurich. ‡ Argonne National Laboratory.

depends strongly on the palladium particle size. The amount of hydride that forms decreases with increasing dispersion of palladium in supported catalysts.10 In contrast to the bulk metal, palladium particles smaller than 2.6 nm do not form the β-hydride phase even at high hydrogen pressures (PH2 ) 1 atm and T ) 298 K).3 The detailed lattice structure of palladium nanoparticles and the formation of palladium hydride have been studied in detail using various techniques, including X-ray absorption spectroscopy (XAS).11-13 Geometrical and electronic information of palladium nanoparticles and of palladium hydride can be obtained by performing XAS at the K and L edges. XAS at the K edge (∼24 350 eV) probes the transition of an electron from a 1s to a 5p orbital. The X-ray absorption near-edge structure (XANES) is sensitive to the geometric environment. The edge position is indicative of oxidation state and geometry. The extended X-ray absorption fine structure (EXAFS) gives detailed structural information, such as coordination number, interatomic distance, and Debye-Waller factor. On substrate-free palladium nanoparticles, Pd K edge EXAFS showed a small contraction of the nearest neighbor distance, a reduction of the average coordination number, and an increase in Debye-Waller factor as the particle size decreased from 23 to 7 nm.14 No contraction of particles was observed when palladium was supported on silica and carbon.13 The formation of palladium hydrides can be determined from the lattice expansion, which is indicated by an increased interatomic distance. Expansion of the palladium lattice by 2.2 to 7% after hydride formation was observed on supported palladium particles.11,12 The empty d density of states (DOS) can be obtained from L3 edge XAS (∼3175 eV) which probes the 2p to 4d transition and is characterized by an intense whiteline which reflects the number of holes in the d band.15 Changes in the L3 edge XANES of transition metals after adsorption of an adsorbate on the metal originatefromorbitalhybridization,chargetransfer,metal-adsorbate scattering, and differences in metal-metal scattering16 and can be theoretically simulated.17 Palladium hydride gives rise to a signature peak at ∼6 eV above the Fermi level in Pd L3 edge

10.1021/jp902542f CCC: $40.75  2009 American Chemical Society Published on Web 08/03/2009

Hydrogen Sorption on and in Palladium Catalysts XAS.18-20 Detection of palladium hydride by Pd K edge XANES is less successful because of the low scattering power of hydrogen. Here, we performed XAS at the Pd K and L3 edges to determine the geometric and electronic structural changes that occur in supported nanosized palladium particles after exposure to hydrogen as a function of their size. XANES analysis of the L3 edge data enabled us to distinguish hydrogen that was adsorbed on the surface from that absorbed into the particle. 2. Experimental Section 2.1. Synthesis of Silica and Alumina Supported Palladium Nanoparticles. All samples were prepared by incipient-wetness impregnation using Pd(NH3)4(NO3)2 as a precursor. For silica supported palladium nanoparticles, 10.0 g of silica (Davisil 644 from Aldrich, 300 m2/g and 1.1 mL/g) was impregnated with 6.2 g of Pd(NH3)4(NO3)2 in 12 mL of H2O. The sample was dried overnight at 373 K and then reduced at 523 K in 1 atm of flowing hydrogen (200 mL/min) to obtain Pd/SiO2-A(10.5). Similarly, having the impregnated sample calcined at 523 K for 5 h and reduced at 523 K in 1 atm flowing hydrogen gave Pd/SiO2-B(2.8). The number in parentheses represents the particle size in nm (vide infra). Pd/Al2O3-A(3.6) and Pd/Al2O3B(1.3) were prepared using the same procedures as that of Pd/ SiO2-A(10.5) and Pd/SiO2-B(2.8) but on high-purity alumina (sulfate and sodium free; 180 m2/g and 0.7 mL /g). The weight loading for palladium was 1.93% for Pd/SiO2-A(10.5) and Pd/ SiO2-B(2.8) and 1.87% for Pd/Al2O3-A(3.6) and Pd/Al2O3B(1.3). 2.2. Carbon Monoxide Chemisorption. Carbon monoxide chemisorption measurements were performed in a conventional static volume apparatus (Micromeritics ASAP 2010). The samples were reduced in pure hydrogen at 423 K for 90 min using a ramp of 2 K/min and then evacuated at 423 K for 30 min. Carbon monoxide adsorption isotherms were taken at 308 K. A fixed volume of carbon monoxide was added to a known volume containing a known amount of catalyst. From the decrease in pressure, the absolute amount of adsorbed carbon monoxide was calculated. After the first carbon monoxide adsorption isotherm was taken, the sample was evacuated for 45 min at 308 K to remove weakly adsorbed carbon monoxide and a second carbon monoxide adsorption isotherm was taken. The difference between both isotherms gave the amount of strongly adsorbed carbon monoxide. A CO:Pd ratio of 1:1 was used to determined the palladium dispersion. 2.3. Scanning Transmission Electron Microscopy. Scanning transmission electron microscopy (STEM) measurements were performed on prereduced samples. Several milligrams of catalyst was added to isopropanol and sonicated for about 10 min. A drop of the slurry was then placed onto a carbon-coated copper grid (200 meshes, CuPK/100) from SPI supplies, and dried thoroughly. High-angle annular dark field (HAADF) imaging, or Z contrast imaging, was done using a JEM-2010F FasTEMm FEI electron microscope (manufactured by JEOL) operated at 200 kV and an extracting voltage of 4500 V. Typically, STEM images from 10 different regions of a catalyst were imaged for particle size analysis. Around 500 to 2000 palladium particles were measured for the particle size distribution. 2.4. X-ray Absorption Measurements. The X-ray absorption measurements were performed in transmission mode (K edge) at the insertion-device beamline of the Materials Research Collaborative Access Team (MRCAT) at the Advanced Photon Source of Argonne National Laboratory and in fluorescence mode (L3 edge) at the Line for the Ultimate Characterisations

J. Phys. Chem. C, Vol. 113, No. 34, 2009 15141 by Imaging and Absorption (LUCIA) beamline of the Swiss Light Source (Villigen, Switzerland). At the MRCAT beamline, a cryogenically cooled double-crystal Si(111) monochromator was used in conjunction with an uncoated glass mirror to minimize the presence of harmonics. The Pd K edge spectra were collected using ionization chambers optimized for the maximum current with linear response using a mixture of nitrogen and helium in the incident X-ray detector and a mixture of ∼20% argon in nitrogen in the transmission X-ray detector. Catalyst samples were pressed into a cylindrical holder in a continuous-flow EXAFS reactor cell of 18 in. long and with a 0.75 in. diameter, fitted at both ends with polyimide windows and valves to isolate the reactor from the atmosphere. A palladium foil spectrum was acquired simultaneously with each measurement for energy calibration. At the LUCIA beamline, a water-cooled double crystal Si(111) monochromator was used. Prior to the monochromator, high-order harmonics and thermal load received on the monochromator crystals were reduced with a set of two flat, nickel-coated silicon mirrors with a variable incidence angle of 0.4-1.3°. The Pd L3 (3175 eV) edge XANES spectra were obtained using a silicon drift detector with an energy resolution of ∼130 eV.21 The detector was mounted with a special thin window which allows the fluorescence of elements down to the carbon K edge to be detected. A sieve fraction of the catalyst was packed in a flow-through cell and covered by a kapton film. The cell was then put on a heating block which can be heated to 800 K. The design of the flow through cell and the heating block is described elsewhere.22 In K edge experiments, the palladium catalysts were first reduced at a ramp of 2 K/min to 423 K in 4% H2/He. After hydrogen removal and cooling in helium to room temperature, the catalysts were in a reduced form and a first spectrum of bare palladium particles was collected. Subsequently, the catalysts were exposed to hydrogen at room temperature to enable hydride formation. At this point, a spectrum was collected. Finally, the system was purged with helium at room temperature and a third spectrum was recorded. In L3 edge experiments, the palladium catalysts were first reduced at a ramp of 2 K/min to 423 K and then cooled down to room temperature in 4% H2/He. The catalysts were then exposed to hydrogen at room temperature to enable hydride formation and a spectrum was collected. Finally, the system was purged with helium and heated to 573 K and a second spectrum was recorded. The gases were passed through an oxygen trap (CNYS model 1000) and carrier gas dry tube (Supelco) prior to the cell to remove trace oxidants. Standard procedures based on the WINXAS9723 and XDAP software packages (version 2.3)24 were used to process the Pd K and L3 edges data, respectively. Phase shifts and backscattering amplitudes were obtained from reference compounds: Pd(NH3)4(NO3)2 (Aldrich) for Pd-O and palladium foil for Pd-Pd. The coordination parameters were obtained by a leastsquares fit in R-space using k2 weighting. 2.5. L3 Edge XANES Calculations. Self-consistent field full multiple-scattering calculations were performed using the JFEFF gui software code.25-27 Pd L3 XANES of the center and surface atoms of a 13-atom cluster, Pd13, and of β-palladium hydrides of varying cluster sizes were calculated. The Pd13 had a facecentered cubic crystal structure with a central atom surrounded by 12 surface atoms. All of the surface atoms were equivalent so that they all had the same potential. The β-palladium hydride has an FM-3M crystal structure. β-palladium hydride clusters of different sizes were used to simulate the effect of particle size on hydrogen dissolution in the palladium bulk structure and to distinguish between hydrogen adsorption on the surface

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Figure 1. Schematic picture which illustrates the metallic palladium particles before and after hydrogen adsorption and absorption.

TABLE 1: Characteristics of Samples CO chemisorption (%) sample Pd/Al2O3-B(1.3) Pd/SiO2-B(2.8) Pd/Al2O3-A(3.6) Pd/SiO2-A(10.5) d

support % Pda d (nm)b dispersionc dispersiond alumina silica alumina silica

1.9 1.9 1.9 1.9

1.3 2.8 3.6 10.5

74 55 37 13

54 41 28 9

a Determined by ICP. b Determined by STEM. c Total adsorption. Strong adsorption.

and hydrogen absorption into the particle. The size effect was simulated by increasing the number of atoms forming the cluster. All clusters were generated by TKATOMS based on crystallographic data from the literature.28 A third cluster of six palladium atoms, Pd6,29 was used to study the signature of hydrogen adsorbed on different surface sites, namely, 3-fold, bridged, and on-top sites. In all calculations, a Hedin-Lundquist exchange correlation potential and the L3 XANES, LDOS (angular projected density of states), NO HOLE (complete corehole screening), SCF (self-consistent field), and FMS (full multiple scattering) cards were used. A schematic picture, which illustrates the metallic palladium particles before and after hydrogen adsorption and absorption is shown in Figure 1. 3. Results 3.1. Characterization of Samples. Characteristic features of the samples, such as type of support, metal loading, palladium particle size, and the corresponding metal dispersion are summarized in Table 1. The palladium particle sizes of Pd/SiO2A(10.5), Pd/Al2O3-A(3.6), Pd/SiO2-B(2.8), and Pd/Al2O3-B(1.3) as determined with STEM were 10.5, 3.6, 2.8, and 1.3 nm, respectively (Figure 2). Pd/SiO2-A(10.5) and Pd/Al2O3-B(1.3) had uniform particle size distributions, which enables distinguishing very large from very small particles. Pd/Al2O3-A(3.6) and Pd/SiO2-B(2.8) had broader particle size distributions. The dispersion obtained using carbon monoxide chemisorption paralleled the palladium particle size determined with STEM, i.e., lower dispersion corresponded to larger particle size and vice versa. However, because Pd/Al2O3-A(3.6) and Pd/SiO2B(2.8) showed a relatively wide size distribution, some caution in interpretation of their properties related to particle size must be taken. 3.2. Experimental K Edge XANES and EXAFS. Table 2 summarizes the structural parameters from the EXAFS fitting of Pd/SiO2-A(10.5), Pd/SiO2-B(2.8), Pd/Al2O3-A(3.6), and Pd/ Al2O3-B(1.3) under different conditions. Figure S1 shows the EXAFS and corresponding Fourier-transformed spectra of supported bare palladium nano-particles of different size at room temperature. After reduction, hydrogen removal, and being cooled down to room temperature in helium, metallic palladium particles of Pd/SiO2-A(10.5), Pd/SiO2-B(2.8), Pd/Al2O3-A(3.6), and Pd/Al2O3-B(1.3) were obtained. Pd/SiO2-A(10.5) showed

a bulk-like structure with a Pd-Pd first-shell coordination number (NPd-Pd) of 12, in agreement with the STEM particle size of about 11 nm. Pd/Al2O3-A(3.6), Pd/SiO2-B(2.8), and Pd/ Al2O3-B(1.3) had lower NPd-Pd, which indicates smaller particles.30 They had also higher DWF due to a higher degree of disorder. The NPd-Pd of each sample was insensitive to the measurement conditions and corresponded well with the particle sizes determined from the STEM images. All three samples showed an average Pd-Pd bond distance (RPd-Pd) of 2.75 Å, which is the same in bulk palladium.12 The corresponding K edge XANES spectra are shown in Figure S2 of the Supporting Information. As the particle size decreased, the maxima at ∼24 395 eV slightly shifted to lower energy and the overall amplitude of the oscillations decreased. Unlike for example platinum and gold,31 no contraction of the metal-metal bond was observed with decreasing size. Under hydrogen, lattice expansion due to hydride formation was observed for all four samples, as shown by the increased RPd-Pd (Figure S3, Supporting Information). The EXAFS and corresponding Fouriertransformed spectra are shown in Figure S4 of the Supporting Information. Higher disorder due to the interstitial hydrogen also led to higher DWF. Having the catalysts heated to 573 K and then cooled down to room temperature in helium reformed the metallic state of each sample, as shown by the decreasing RPd-Pd and DWF back to values similar to those before hydride formation. Pd/Al2O3-B(1.3) was not fully reduced during the measurement because of leakage of the cell, and hence, the structural parameters at the metallic palladium state after hightemperature reduction could not be fitted accurately. 3.3. Experimental L3 Edge XANES. Figure 3a shows the Pd L3 edge XANES spectra of the bare catalysts measured at 573 K, after reduction and hydrogen removal, and that of palladium foil. The near-edge region of the catalysts had less intense features than those of the foil, since the coordination number in the nanoparticles was lower. Absorption edges of Pd/Al2O3-B(1.3) and Pd/SiO2-B(2.8) were located at slightly higher energy with respect to that of the foil, and their whitelines were broader and less intense. The spectrum of Pd/SiO2-A(10.5) looked similar to that of palladium foil. Exposure to hydrogen shifted the edges of all catalysts to higher energy, as shown in Figure 3b. An extensive whiteline broadening and intensity loss together with the appearance of a new peak located at ∼6 eV above the whiteline (∼3181 eV) were observed. The peak at ∼6 eV decreased with particle size. 3.4. XANES Simulations. The simulated L3 edge XANES spectra of palladium hydride clusters of increasing size are shown in Figure 4a. As the particle size increased, the absorption edge shifted to higher energy position, the whiteline intensity decreased significantly, and a new peak appeared at ∼6 eV above the whiteline energy position. These changes were similar to the ones observed experimentally (Figure 3b). The intensity of the new peak increased with particle size. To calculate the XANES of a surface palladium atom with adsorbed hydrogen, a cluster of 27 atoms (PdH6Pd12H8) was constructed and after removing the absorbed hydrogen atoms only surface hydrogen remained (Pd_Pd12H8). The calculated XANES were compared to that of cluster model PdPd12 and of the palladium hydride with 27 atoms, as shown in Figure 4b. The simulated XANES spectrum of a palladium atom with only hydrogen adsorbed on the surface (Pd_Pd12H8) showed whiteline broadening and an intensity increase at the region after the whiteline. No peak was observed at ∼6 eV above the whiteline energy position. This suggests that the ∼6 eV peak originates from the bulk absorbed hydrogen and not from surface adsorbed hydrogen. To separate

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Figure 2. STEM results for (a) Pd/Al2O3-B(1.3), (b) Pd/SiO2-B(2.8), (c) Pd/Al2O3-B(3.6), and (d) Pd/SiO2-A(10.5).

TABLE 2: Structural Parameters of Different Samples under Different Conditions as Determined in the First-Shell EXAFS Analyses sample

treatment

da (nm)

shell

N

R (Å)

DWF (10-4 Å2)

Eo (eV)

Pd/SiO2-A (10.5)

red-He red-H2 He red-He red-H2 He red-He red-H2 He red-He red-H2 He

10.5

Pd-Pd Pd-Pd Pd-Pd Pd-Pd Pd-Pd Pd-Pd Pd-Pd Pd-Pd Pd-Pd n.d. Pd-Pd Pd-Pd

12.0 12.0 12.0 10.6 10.6 10.6 10.0 10.0 10.0 n.d. 6.1 5.8

2.75 2.80 2.75 2.75 2.80 2.74 2.75 2.80 2.75 n.d. 2.77 2.75

5.1 33 4.5 10 27 11 13 25 13 n.d. 50 40

5.2 3.5 3.9 5.1 4.5 3.8 5.6 4.6 5.5 n.d. 2.7 4.4

Pd/Al2O3-A (3.6) Pd/SiO2-B (2.8) Pd/Al2O3-B (1.3) a

3.6 2.8 1.3

Determined by STEM.

the effect of lattice expansion on the L3 edge spectra, we calculated the XANES of the central palladium atom in the PdPd12 cluster with expanded and contracted Pd-Pd interatomic distance (Figure S5, Supporting Information). This lead to a change in position of the peak at ∼3190 eV; the whiteline remained unchanged. The Pd L3 edge XANES of the Pd6 cluster with and without hydrogen adsorbed at FCC 3-fold, bridged, and on-top sites were calculated (Figure 4c). The spectra of 3-fold and on-top hydrogen showed slight whiteline broadening and a significant intensity increase between 3175 and 3180 eV. The spectrum of hydrogen adsorbed at a HCP 3-fold site was very similar to that of hydrogen at a FCC 3-fold site and is hence not shown

in the figure. Larger spectral differences were observed for bridged hydrogen. Besides a more prominent whiteline broadening, which shifted to higher energy, the intensity after 3185 eV differed from that of Pd6. Nevertheless, none of the spectra showed the ∼6 eV peak. This supported the earlier suggestion that the ∼6 eV peak originates from dissolved hydrogen. Adsorbed hydrogen is recognized by broadening of the whiteline (Figure 4b). Figure 5a, b, and c overplot the calculated DOS with the corresponding L3 XANES of a palladium cluster consisting of 13 atoms (PdPd12), a palladium hydride of 27 atoms (PdH6Pd12H8), and a Pd_Pd12H8 model where a surface palladium atom was set as the absorbing atom. Since for noble

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Figure 3. (a) L3 edge XANES spectra of supported bare palladium nanoparticles of different size at 573 K and of palladium foil (solid): Pd/Al2O3-B(1.3) (dash), Pd/SiO2-B(2.8) (dash-dot), and Pd/SiO2A(10.5) (dot). (b) L3 edge XANES spectra of supported palladium nanoparticles of different size under pure hydrogen at room temperature and of palladium foil (solid): Pd/Al2O3-B(1.3) (dash), Pd/SiO2-B(2.8) (dash-dot), and Pd/SiO2-A(10.5) (dot).

metals the L3 edge XANES is related to the projected DOS, such plots illustrate the origin of the ∼6 eV peak.32 The x-axis was normalized to the Fermi level, and the calculated unoccupied (antibonding) and occupied (bonding) DOS are distributed above and below the Fermi level. The unoccupied d-DOS correlate to the L3 edge XANES. Without bulk hydrogen (Figure 5a), the bonding states formed from the hybridization of s, p,

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Figure 4. (a) Calculated L3 XANES spectra of palladium hydride with a different number of shells. 19 atoms, PdH6Pd12 (black); 27 atoms, PdH6Pd12H8 (red); 33 atoms, PdH6Pd12H8Pd6 (blue); 57 atoms, PdH6Pd12H8Pd6H24 (green); 93 atoms, PdH6Pd12H8Pd6H24Pd36 (cyan). (b) Calculated L3 XANES spectra of model Pd_Pd12H8 with surface palladium atom set as absorbing atom (dash), palladium hydride PdH6Pd12H8 (dash-dot), and PdPd12 (solid). (c) Calculated L3 XANES spectra of model Pd6 with hydrogen adsorbed at FCC 3-fold (dash), bridged (dot), and on-top (dash-dot) modes. The spectra of bare Pd6 (solid) are included for comparison.

and d bands of palladium are located up to 5 eV below the Fermi level. No clear hybridization peaks were observed above the Fermi level. After hydrogen dissolution into the palladium lattice (Figure 5b), the palladium d band center shifted downward in energy by about ∼0.5 eV relative to the Fermi level. Meanwhile, new bonding and antibonding states appeared. The new bonding state of palladium hydride is positioned at ∼7 eV below the Fermi level. The presence of Pd-H bonding states below the Fermi level enables the identification of the new states at ∼6 eV above the Fermi level as the corresponding antibonding states.34 Note that the new antibonding state which formed from the overlapping of the palladium d band with the

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Figure 6. Calculated d density of states for palladium with a coordination number of 2 (dot), 7 (dash), and 12 (solid).

could be observed between 2.5 and 5 eV, which led to an overall intensity increase of the corresponding XANES right after the whiteline. A broadening of the d band at ∼3.5 eV caused a shoulder at the whiteline (Figures 3b and 4b).

Figure 5. Calculated density of states of palladium s (black), p (red), and d (blue) (a) overplotted with calculated L3 XANES for Pd cluster of 13 atoms (PdPd12) (bold black), (b) hydrogen s (green) and palladium d (blue) overplotted with calculated L3 XANES for palladium hydride of 27 atoms (PdH6Pd12H8) (bold black), and (c) hydrogen s (green) and palladium d (blue) overplotted with calculated L3 XANES for Pd_Pd12H8 with the surface palladium atom set as absorbing atom (bold black).

s band of hydrogen gave rise to the extra ∼6 eV peak in the L3 XANES of palladium hydride. In the presence of only surface hydrogen (Figure 5c), the palladium d band center remained virtually unchanged. Pd d-H s hybridization was weaker than that of palladium hydride. Intensity increase of the Pd d band

4. Discussion 4.1. Pd K Edge XANES and EXAFS. After exposing the nanoparticles to hydrogen, a palladium hydride phase formed in all catalysts. The EXAFS analysis showed that the NPd-Pd of all samples roughly remained the same after exposure to hydrogen, suggesting that changes in the K edge XANES (Figure S3, Supporting Information) spectra were due mostly to the presence of hydrogen. A completely reversible expansion of the palladium-palladium distance of 1.8% was observed for all samples, as a result of the incorporation of hydrogen in the lattice of palladium. The lattice structure of palladium hydride represents an isotropically expanded form of the FCC host lattice with the hydrogen atoms occupying part of the octahedral sites. The information from Pd K edge XAS was limited to structural changes, such as lattice expansion upon hydrogen adsorption and hydride formation. We were unable to determine the electronic structure and observe hydride formation and hydrogen adsorption solely based on the absorption K edge. 4.2. Pd L3 Edge XANES. The L3 edge spectra of bare palladium (Figure 3a) showed that absorption edges slightly shifted to higher energy while the whitelines were broadened and decreased in intensity for decreasing particle size. The L3 edge XANES whiteline intensity reflects the number of holes in the 4d band and thus the local d density of states. According to the band theory, the projected d band of lower coordinated atoms becomes narrower, since fewer atoms contribute to the bandwidth. This is shown in the calculated d density of states for palladium clusters with a coordination number of 2, 7, and 12 (Figure 6). The energy of the d band of the larger particles is positioned further from the Fermi level, which decreases its reactivity.35-38 Compared to the Pd K edge XANES, the Pd L3 edge XANES shows more prominent changes after formation of palladium hydrides, as it is sensitive to changes in electronic structure. The corresponding signatures (Figure 3b) were all very sensitive to the particle size. The newly formed antibonding states, which is characterized by a signature peak at ∼6 eV above the

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Tew et al. 5. Conclusion Pd K and L3 edge XANES provide complementary information on the formation of hydrides. Unlike the Pd K edge, the L3 edge XANES enables the direct observation of the formation of palladium hydrides, based on the new antibonding states form. The surface adsorbed hydrogen can be distinguished from the bulk dissolved hydrogen in the Pd L3 edge XANES. Hydride formation can be indirectly observed in K edge XANES and EXAFS via the lattice expansion. The formation of palladium hydrides is strongly particle size dependent, though all sizes showed the formation of interstitial hydrogen. Large particles provide more interstitial places for the formation of hydrides and lead subsequently to a more intense new antibonding state as compared to smaller particles, indicating that less hydrogen per palladium atom can be absorbed in the smaller particles. Due to the higher surface to bulk ratio, the smaller particles showed more surface adsorbed hydrogen.

Figure 7. Quantification of the amount of absorbed hydrogen from the areas of the new antibonding states of different samples: (a) Pd/ SiO2-A(10.5); (b) Pd/SiO2-B(2.8); (c) Pd/Al2O3-B(1.3).

TABLE 3: Area of the New Antibonding States of Different Samples sample

dTEM (nm)

Apeaka

∆Apeakb ((10-15%)

Pd/SiO2-A(10.5) Pd/SiO2-B(2.8) Pd/Al2O3-B(1.3)

10.5 2.8 1.3

0.65 0.30 0.25

53% 61%

a

Rounded to 0.05. SiO2-A(10.5).

b

Differences in area relative to that of Pd/

absorption edge, was largest for the largest particles and decreased with decreasing size. 4.3. Surface Adsorption versus Hydride Formation. Pd L3 XANES calculations indicated that signatures of surface hydrogen and bulk dissolved hydrogen differ. Bulk dissolved hydrogen leads to a large edge shift and the formation of a new antibonding state, while surface hydrogen caused a smaller edge shift and whiteline broadening (Figure 4b). The latter changes were observed in the experimental whitelines of Pd/SiO2-B(2.8) and Pd/Al2O3-B(1.3), along with the formation of the new antibonding state. As the particle size decreases, the surface to bulk ratio increases while the relative number of interstitial sites decreases, causing less hydrogen to dissolve into the palladium lattice. We quantified the amount of absorbed hydrogen from the area of the new antibonding states of different samples (Figure 7). Such an attempt was reasonable because the new antibonding states originate solely from the absorbed hydrogen. The results are summarized in Table 3. The intensity increased with increasing particles sizes, Pd/SiO2-A(10.5) > Pd/SiO2B(2.8) > Pd/Al2O3-B(1.3). The loss of intensity in the catalysts with the smaller particles suggests that about 40-50% ((10-15%) less hydrogen dissolves in the smaller particles. Relatively more hydrogen adsorbs on the surface for small particles and this does not contribute to the formation of hydride, which results in a smaller intensity of the antibonding state. The surface contribution of these smaller sized particles is observable, and hence, their signatures originate not only from the hydride but also from the surface hydrogen. In contrast, the experimental spectrum of Pd/SiO2-A(10.5) under hydrogen showed only the antibonding states of the hydride. Because of its particle size of 10.5 nm and low surface contribution, the signatures originated purely from the hydride and no surface contributions were observed.

Acknowledgment. The authors thank the Swiss National Science Foundation (SNF) for financial support, the MRCAT beamline at the Advanced Photon Source of Argonne National Laboratory, and the LUCIA beamline of the Swiss Light Source for the beam time and support. Support for this research was provided by the U.S. Department of Energy, Office of Basic Energy Sciences, through the Catalysis Science Grant No. DEFG02-03ER15466. The use of the Advanced Photon Source (APS) was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Materials Research Collaborative Access Team (MRCAT, Sector 10 ID) operations are supported by the Department of Energy and the MRCAT member institutions. Supporting Information Available: Pd K edge EXAFS and corresponding Fourier-transformed spectra of supported bare palladium nanoparticles of different size at room temperature, K edge XANES spectra of supported bare palladium nanoparticles of different size at room temperature and of palladium foil, K edge XANES spectra of supported palladium nanoparticles of different size under pure hydrogen at room temperature and of palladium foil, EXAFS and corresponding Fourier-transformed spectra of supported palladium nanoparticles of different size under pure hydrogen at room temperature, and Calculated Pd L3 edge XANES of model Pd_Pd12 with contracted or expanded Pd-Pd distances. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Takasu, Y.; Akimaru, T.; Kasahara, K.; Matsuda, Y. J. Am. Chem. Soc. 1982, 104, 5249–5250. Ichikawa, S.; Poppa, H.; Boudart, M. J. Catal. 1985, 91, 1–10. Takasu, Y.; Matsuda, Y.; Toyoshima, I. Chem. Phys. Lett. 1984, 108, 384–387. Gigola, C. E.; Aduriz, H. R.; Bodnariuk, P. Appl. Catal. 1986, 27, 133–144. Matolin, V.; Gillet, E. Surf. Sci. 1990, 238, 75–82. Chang, J.; Chou, T. Appl. Catal., A 1997, 156, 193–205. Borodzinski, A. Catal. Lett. 2001, 71, 169–175. Semagina, N.; Renken, A.; Kiwi-Minsker, L. J. Phys. Chem. C 2007, 111, 13933–13937. Schalow, T.; Brandt, B.; Starr, D. E.; Laurin, M.; Shaikhutdinov, S. K.; Schauermann, S.; Libuda, J.; Freund, H.-J. Phys. Chem. Chem. Phys. 2007, 9, 1347–1361. (2) Haug, K. L.; Buergi, T.; Trautman, T. R.; Ceyer, S. T. J. Am. Chem. Soc. 1998, 120, 8885–8886. Nag, N. K. J. Phys. Chem. B 2001, 105, 5945– 5949. (3) Palczewska, W. In Hydrogen Effects in Catalysis; Paal, Z., Menon, P. G., Eds.; Marcel Dekker: New York, 1988; p 373. (4) Johnson, A. D.; Daley, S. P.; Utz, A. L.; Ceyer, S. T. Science 1992, 257, 223–225. (5) Morkel, M.; Rupprechter, M.; Freund, H.-J. Surf. Sci. 2005, 588, L209–219.

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