Palladium

ARTICLE pubs.acs.org/JPCC

Surface Properties of Supported, Colloid-Derived Gold/Palladium Mono- and Bimetallic Nanoparticles Stefan Marx, Frank Krumeich, and Alfons Baiker* Department of Chemistry and Applied Biosciences, ETH Z€urich, H€onggerberg, HCI, CH-8093 Z€urich, Switzerland ABSTRACT: Mono- and bimetallic Au/Pd nanoparticles deposited on different supports (Al2O3, TiO2, SiO2) have been prepared using a colloidal route. Electron microscopy revealed a narrow particle size distribution of as-prepared nanoparticles with a mean diameter in the range of 34 nm. CO adsorption combined with DRIFTS measurements was employed to gain information about the available adsorption sites on these nanoparticles. On Pd nanoparticles, linear, μ2-briged, and μ3-bridged CO was observed. Upon reductive treatment of the particles, the signal for the μ2-bridged species on Pd(111) faces decreased significantly. The gold particles revealed a surprisingly strong red shift of the signal at around 2110 cm1 after reduction, indicating a morphological change of the particles upon treatment in hydrogen. In contrast, treatment in oxygen at the same temperature did not alter the IR spectrum notably. EXAFS and DRIFTS indicated a redistribution of the constituents of the bimetallic nanoparticles upon reduction: the coreshell structure of the as-prepared particles changed to a structure with more evenly distributed constituents.

1. INTRODUCTION Among the various techniques used to investigate the surface properties of unsupported and supported metal nanoparticles,18 CO chemisorption combined with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has been particularly powerful for the elucidation of surface properties relevant for catalysis. Compared to the widely used monometallic catalytic nanoparticles, bimetallic nanoparticles provide considerably higher tunability of their chemical and structural properties and therefore often show superior catalytic properties.3,914 In addition to changes of the electronic properties,15,16 structural modifications have also been observed.10 However, gaining knowledge about the surface properties of bimetallic nanoparticles is often demanding. DRIFTS offers the possibility of probing, in a direct way, the different adsorption sites existing on nanoparticle surfaces by exposing the sample to a suitable probe molecule in an IR cell.1,17,18 However, interpretation of the signals is often not straightforward because several signals due to different adsorptions sites can be observed. The position of a band due to a specific adsorption site can depend on coverage,19 particle morphology, and size.20 Moreover, the intensity of a signal can be strongly altered by dipole coupling, and an intensity transfer from the lowenergy band to the high-energy signal can occur.21 Despite these drawbacks, the application of CO as a probe molecule combined with DRIFTS measurements has great potential for the elucidation of the sites exposed on the surface of nanoparticles because it provides information not easily accessible with other techniques. Here, we targeted the surface properties of colloid-derived mono- and bimetallic nanoparticles consisting of palladium and gold deposited on different supports (Al2O3, TiO2, SiO2). For this purpose, we applied CO adsorption combined with r 2011 American Chemical Society

DRIFTS, electron microscopy, and X-ray absorption spectroscopy (XAS) after different pretreatments of the catalysts.

2. EXPERIMENTAL SECTION 2.1. Particle Preparation. Mono- and bimetallic nanoparticles were prepared using the method previously developed in our group.22,23 Metal nanoparticles were preformed in solution and deposited on different supports. In brief, to a basic solution of THPC [tetra(hydroxymethyl)phosphonium chloride] in water was added an aqueous solution of the metal precursor under vigorous stirring. HAuCl4 and Na2PdCl4 were used as precursor materials, and NaOH was utilized as a base. The ratio of base to THPC to total metal was kept constant at 6:1.4:1 in accordance with the original description. The pure gold colloid was brownish in color, whereas the Au0.5Pd0.5 and palladium colloids were black. After the colloidal solution had been stirred for 1 h at room temperature, the mixture was separated into several batches and added to different supports (400 mg each) that had been suspended in ca. 100 mL of water. The total metal loading was adjusted to a nominal value of 2 wt %. As supports, we used silica (Aerosil 380 V, Degussa, specific surface area = 380 m2/g), γ-alumina (Alfa Aesar, specific surface area = 80120 m2/g), titania (Aeroxide P25, Acros, specific surface area = 50 m2/g), and ceria (calcined at 500 °C, 5 h, Acros, specific surface area = 116 m2/g). To allow the particles to adsorb on the support, the pH value of the suspension was adjusted to 2 with Received: January 15, 2011 Revised: March 15, 2011 Published: April 01, 2011 8195

dx.doi.org/10.1021/jp200431s | J. Phys. Chem. C 2011, 115, 8195–8205

The Journal of Physical Chemistry C some droplets of nitric acid; then, the samples were filtered and washed until no chlorine could be detected by adding AgNO3 solution. In the case of alumina-supported samples, the particles were too small to be filtered, and a repeated sedimentationdecantation procedure had to be applied instead. The samples were dried at 80 °C overnight and stored under ambient atmosphere. 2.2. TEM Investigations. The transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and energy dispersive X-ray (EDX) investigations were performed on a Tecnai F30 microscope (FEG, operated at 300 kV, resolution of ca. 2 Å). The samples were crushed, suspended in ethanol, and deposited on a holey carbon foil supported on a copper grid. TEM images were recorded with a slow-scan CCD camera; STEM images were recorded with a high-angle annular dark-field detector (HAADF-STEM) with the metal particles appearing bright (Z contrast). An energy-dispersive X-ray spectrometer attached to the Tecnai F30 instrument allowed the elemental analyses at spots and areas selected in the HAADFSTEM images as well. Elemental mapping was performed on an HD-2700CS microscope (Hitachi). A few drops of the suspension of the material in ethanol were deposited onto a perforated carbon foil supported on a copper grid. After drying, the grid was mounted on the single tilt holder of the microscope. The holder with the sample was treated for ca. 5 s in a plasma cleaner (75% Ar, 25% O2) before being inserted into the microscope vacuum. STEM investigations were performed on an aberration-corrected STEM microscope [HD-2700CS (Hitachi)], operated at an acceleration potential of 200 kV (electron gun: cold-field emitter). Analytical investigations of selected spots and areas were performed in the normal mode of the microscope with an energy-dispersive X-ray spectrometer [EDXS, Genesis Spectrum version 6.2 (EDAX)] attached to this microscope. 2.3. CO Adsorption Studies. DRIFTS measurements were performed on an Equinox 55 IR spectrometer (Bruker Optics) equipped with a praying mantis diffuse reflection accessory (Harrick) and a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector. IR spectra were recorded in the range between 400 and 4000 cm1 and with a spectral resolution of 2 cm1 to distinguish gaseous CO from adsorbed CO by resolving the rotation bands. Four hundred scans were averaged for one background spectrum, and 200 scans were combined into one spectrum. The DRIFTS cell was connected to a gas supply system so that different gases could be passed through the cell. In a typical experiment, the sample was introduced into the cell, and a constant He flow (5.0, Pangas) of 30 mL/min was passed through it. After 1 h at room temperature, a spectrum was taken and used as background for the following adsorption experiments. After stabilization of the sample, CO (10% in He; 19 mL/min) was applied, and several spectra were recorded until saturation of the particle surface was reached. In a typical experiment, saturation was reached after 10 min. For desorption of CO, the gas flow was switched back to He, and spectra were recorded until no change occurred. Then, the sample was heated to 250 °C. After the first CO adsorption, the sample was reduced in a stream of hydrogen (10 mL/min) at 250 °C. After reduction, the sample was cooled in He, and a new background spectrum was recorded; then, a second CO adsorption and desorption cycle was performed. In one case, the sample was calcined in oxygen (10 mL/min, 250 °C) to reveal the difference in the behavior induced by oxygen and hydrogen treatment. 2.4. X-ray Absorption Measurements. These experiments were performed at Hamburger Synchrotronstrahlungs-Labor

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(HASYLAB) at the Deutsches Elektronen-Synchrotron (DESY) at DORIS III (4.45 GeV, 120 mA current) at beamline X1 (energy range 7100 keV). The monochromator was a Si(111) crystal for the measurement at the Au LIII edge (11.918 keV) and a Si(311) crystal for the measurement at the Pd K edge (24.35 keV). Particles adsorbed on alumina were filled into a flow reactor and measured in transmission mode at both edges, whereas the spectra at the gold edge of particles adsorbed on titania had to be measured in fluorescence mode. A five-element Ge solid-state detector (Canberra) was applied to measure the Au LR fluorescence of Au (fluorescence window from 9.60 to 9.75 keV; excitation at Au LIII, 11.919 keV). For this purpose, the sample was pressed into a pellet and positioned at a 45° angle in the beam. To follow the influence of reduction with hydrogen, a flow of 20 mL/min H2 (5% in He) was passed through the flow reactor, and the temperature was raised to 250 °C. XANES spectra were recorded, and the temperature was held until no change in two subsequent spectra could be discerned. The raw data were energy-calibrated toward gold or palladium foil, background-corrected using a two-polynomial fit, and normalized. The χ(k) function was extracted from the EXAFS data in the range between 1.7 and 16.0 Å1, and Fourier transformation was performed on the k3-weighted data in the intervals k = 3.515.9 Å1 for gold and k = 3.314.9 Å1 for palladium spectra. Data analysis was performed using scattering paths calculated with FEFF 7.0.24 The feff input file was generated using crystal structures of gold and palladium metals. The calculated amplitudes and phase shifts were fitted to the experimentally obtained data. In a first step, the reference spectra of gold and palladium foils were fitted taking only the first shell into consideration. The values for S0 and the DebyeWaller factor obtained from these fits were then used as the basis for the fit of the mono- and bimetallic nanoparticles.

3. RESULTS AND DISCUSSION 3.1. Pd Nanoparticles. To produce homogeneous nanoparticles the colloidal route previously developed in our group was applied. This method uses tetra(hydroxymethyl)phosphonium chloride (THPC), which is capable of reducing gold and palladium according to the following reaction sequence

PðCH2 OHÞ4 þ þ OH f PðCH2 OHÞ3 þ H2 O þ HCHO Under basic conditions, the phosphonium salt releases formaldehyde, and the phosphor atom becomes reduced. The phosphane reacts further in a subsequent reaction, forming tris(hydroxymethyl)phosphonium oxide (THPO) and two reduction equivalents PðCH2 OHÞ3 þ H2 O f OPðCH2 OHÞ3 þ H2 In the last step of the sequence, THPO is converted to bis(hydroxymethyl)phosphinic acid, releasing two more reduction equivalents ðOH- Þ

OPðCH2 OHÞ3 þ H2 O sf ðCH2 OHÞ2 POðOHÞ þ HCHO þ H2 Electron microscope investigations revealed similar size distributions for Pd nanoparticles on TiO2 and Al2O3 (Figure 1ad). After the reduction procedure, a very slight increase in particle size could be observed for Pd on Al2O3 and TiO2 (Table 1). 8196

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The Journal of Physical Chemistry C

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Table 1. Mean Diameters of Supported Monometallic and Bimetallic Nanoparticles in Different States (As-Prepared, Reduced, and Calcined) support TiO2

metal Pd Au

treatment

mean (nm)

as-prepared

4.0

reduced

4.3

as-prepared reduced calcined as-prepared

3.4 2.7

reduced

3.0

as-prepared

3.5

reduced

3.6

as-prepared

4.4

reduced

5.8

AuPd

as-prepared

3.7

Au

reduced reduced

3.5 5.6

AuPd Al2O3

Pd Au

SiO2

Figure 1. HAADF-STEM micrographs and corresponding histograms of Pd nanoparticles on (a) TiO2 (as-prepared), (b) TiO2 (after reduction), (c) Al2O3 (as-prepared), and (d) Al2O3 (after reduction) and (e) HRTEM image of Al2O3 (after reduction, enhanced particle contrast).

To study the different adsorption sites on the nanoparticles, CO was adsorbed on the particles until no change in two subsequent DRIFTS spectra was discernible. After the gas flow had been switched to helium, no gas-phase CO disturbed the measurements, and desorption of CO was followed over time with DRIFT spectroscopy. Considering the results obtained from CO adsorption on single crystals, one can differentiate

3.9 10.2

between linear, μ2-bridged, and μ3-bridged species on (111) faces and μ2-bridged species on (100) faces.19,25,26 The situation is somewhat more complicated on nanoparticles, which contain, in addition to the two main faces, (100) and (111), also defect sites such as edges, corners, and steps, where linear and bridged adsorption modes are possible.17,20,2738 Figure 2a presents a series of spectra obtained for desorption of CO over time for Pd on Al2O3. Because complete removal of the adsorbed CO could not be achieved at room temperature (293 K), the sample was heated to 523 K (start of heating is indicated by the dotted line). The spectra show three different features: (i) a sharp line located at 2092 cm1, which exhibits a broad shoulder at around 2067 cm1, followed by (ii) an intense feature at 1987 cm1 and (iii) a smaller one located at 1932 cm1. These features resemble the spectra found in the literature for CO adsorbed on Pd nanoparticles.20,28,29 When the sample was flushed with He, the band at 2092 cm1 for the saturated surface (Θmax) was slightly shifted to 2088 cm1, and the intensity decreased. Upon heating to 523 K, this band was further shifted to 2076 cm1 (dotted line in Figure 2a, Pd/Al2O3) and disappeared completely. At the same time, a second band at 2065 cm1, that had been visible only as a shoulder, started to increase and was still visible at 523 K. These two bands could be clearly assigned to linear-bound species. Closer inspection of the position and behavior of the two bands led to the assumption that the high-frequency signal is due to CO adsorbed on top of (111) faces. Because the spectra were measured at high saturation of the surface, linear-bound species were formed, as indicated by several single-crystal studies.19,25,26 The position of the band at 2092 cm1 corresponds to CO on top of (111) faces.36 DFT calculations revealed a very weak adsorption energy of only 0.92 eV, which clearly explains the easy with which this species can be removed.36 Additionally, the position of this band shifted upon decrease of Θ to lower wavenumber by almost 16 cm1, indicating a close contact to other CO molecules by dipole coupling, which is most possible on flat planes. On the other hand, the low-frequency part at 2065 cm1 could not be removed by heating and was still visible at 523 K, indicating a stronger adsorption bond reflected by a higher heat of adsorption for this species. Again, DFT calculations 8197



Figure 2. DRIFT spectra of CO desorption from (a) as-prepared Pd nanoparticles and (b) reduced Pd particles on Al2O3 (dotted line indicates start of heating). Spectra were recorded over time (first spectrum on top, last spectrum on bottom).

confirmed that CO adsorbed on low-coordinated Pd atoms exhibits a higher heat of adsorption (1.43 eV) than do linearbound molecules on (111) faces. The calculated absorption frequency for this species falls into the range between 2076 and 2068 cm1, which corresponds to the frequency observed here.36 The second feature (denoted ii), located at 1987 cm1, can be assigned to 2-fold-bridged carbonyls on Pd(100). When the coverage was decreased, the signal was shifted to 1955 cm1. This is in good agreement with the data collected on single crystals, where a shift of μ2-bridged CO on (100) faces was reported to be between 1995 cm1 for Θmax and 1894 cm1 for Θmin.19,25,26 The proper identification of the third feature (denoted iii) is somewhat difficult because both 3-fold- and 2-fold-bridged carbonyls on Pd(111) fall into that range, depending on the coverage. At Θmax, the band at 1932 cm1 might be due to μ2-bridged CO molecules on (111) faces, because hollow sites are occupied only at low coverage. During desorption at room temperature, the signal shifted moderately to 1921 cm1. Upon heating, a huge jump of 20 cm1 was observed, and the subsequent spectra became broader than the earlier ones. At the same time as the jump was observed, the high-frequency component of the linear-bound CO disappeared, as discussed above. Both effects indicate a reordering phenomenon involving the complete desorption of linear-bound CO on (111) faces and the movement of μ 2 -bridged species to 3-fold-hollow sites. This reordering

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phenomenon takes places between Θ = 0.5 and Θ = 0.33 ML.19,25,39 Therefore, the signals observed for Θmin are attributed to a mixture of μ2- and μ3-bridged species under the condition that, as soon as 3-fold-bridged species occur, the linear (111) species is diminished, because linear, 2-fold, and 3-fold CO on Pd(111) have not been found to coexist.28 After reduction, CO was adsorbed until no further uptake was visible. Desorption spectra of CO from Pd/Al2O3 are presented in Figure 2b, and the start of heating is again indicated by a dotted line. The features are, in principle, the same as observed for the as-prepared Pd particles, but closer examination reveals some interesting changes. The two signals for linear-bound CO (i) are visible again. The high-frequency part [linear on (111) faces] is slightly more intense than in the as-prepared case, whereas the low-frequency counterpart (CO linear on defects) is less prominent. The large feature at 1984 cm1 [bridged on (100) faces] (ii) is also slightly more intense than in the as-prepared case, whereas the signal corresponding to μ2-bridged CO on (111) faces (iii) is strongly reduced. The increase of the signal for CO bonded linearly to (111) faces might be due to the fact that, upon reduction, the number of defect sites can be reduced compared to the as-prepared particles. In Figure 1e, a typical representative of a Pd particle on Al2O3 after reduction with well-ordered faces is presented. Thus, the slight differences of the as-prepared and reduced Pd particles might stem from ordering processes rather than from sintering because only negligible growth of the particles was observed. The increase of the intensity of feature ii could also be related to ordering processes. By the reduction of defect sites and formation of smoother faces, the signals of CO bonded to the (111) and (100) faces should increase. When comparing the spectra of Pd on Al2O3 before and after reduction, it is obvious that only the signal corresponding to CO on (100) faces (feature ii) is dominant. The decrease of the intensity of signal iii could be explained by an intensity transfer from the lower band toward the higher-frequency band through dipole coupling.21 Interestingly, the above-mentioned difficulties in differentiating between 2-fold- and 3-fold-bridged CO on (111) faces were not present for the reduced sample because the signal for the μ3-bridged species was well resolved at around 1800 cm1. The onset of growth of this band coincides again with the disappearance of the high-frequency signal for linear CO on (111) faces, as observed for the unreduced sample. Parts a and b of Figure 3 show that the same features with the same absorption frequencies as described above were observed also for Pd on TiO2. Again, two signals for CO adsorbed on top (i) are discernible (one is dominant at high Θ, whereas the second becomes more prominent at low Θ), the feature for Pd bridged on (100) faces (ii) and the low-frequency signal for μ2bridged and μ3-bridged are discernible. The spectra taken after reduction resemble that of Pd on Al2O3, except that the 3-foldhollow side is not as prominent as observed for the aluminasupported Pd particles. The spectra recorded for Pd on CeO2 (not shown) were similar, indicating that the support had only a minor influence on the Pd particles. Because support effects are reported in the literature,28 we reasoned that the preparation method plays a crucial role. However, it seems that the colloidal preparation method used in this work leads to relatively weak support interactions. Note that the particles had been preformed in solution and stabilized by a protective agent. The preformed colloid was deposited onto the support, and the stabilizer could be washed away as confirmed by XPS investigations.23 Thus, the 8198



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Figure 3. DRIFT spectra of CO desorption from (a) as-prepared Pd nanoparticles and (b) reduced Pd particles on TiO2 (dotted line indicates start of heating). Spectra were recorded over time (first spectrum on top, last spectrum on bottom).

contact with the support is not as strong as in the case of a conventional impregnationcalcination method, where the support is present during the whole period of particle formation and growth. The weak interaction of the support was even more evident when SiO2 was used. Stabilization of the Pd nanoparticles on this support was very weak so that after reduction, only sintered particles could be found on which no significant CO adsorption was possible. 3.2. Gold Nanoparticles. For gold particles deposited on TiO2, an average size of 3.9 nm was determined. The diameter for gold nanoparticles on alumina falls in the same range (4.4 nm). After reduction, an increase of the particle size was observed. Interestingly, on titania, a much broader size distribution was observed after reduction, with the main fraction of the particles being between 1 and 8 nm and some particles having agglomerated into significantly larger particles (Figure 4). The same holds true for particles after reduction on Al2O3 and SiO2. Therefore, treatment of pure gold particles with hydrogen at 523 K leads to an agglomeration, which is not observed for gold nanoparticles if they are treated at that temperature in a stream of oxygen. Upon calcination in oxygen the size of gold particles on titania was almost unchanged as indicated by an average diameter of 3.4 nm (Figure 4c). Desorption spectra for the as-prepared gold nanoparticles on TiO2 are shown in Figure 5a. Only one feature at 2111 cm1 could be observed. The position is assigned to CO adsorbed linearly on metallic gold atoms in accordance with numerous

Figure 4. HAADF-STEM micrographs and corresponding histograms of Au nanoparticles on (a) TiO2 (as-prepared), (b) TiO2 (after reduction), (c) TiO2 (after calcination), (d) Al2O3 (as-prepared), (e) Al2O3 (after reduction), and (f) SiO2 (after reduction).

studies for a wide variety of systems including single-crystal surfaces,4042 planar model catalysts,4348 and gold/powder systems.18,4958 No signals appearing at wavenumbers higher than 2120 cm1 were found for any of the samples, indicating the absence of adsorption on the support material (normally found at 8199



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Figure 5. DRIFT spectra of CO desorption from (A) as-prepared Au nanoparticles, (B) reduced Au particles, (C) calcined Au particles, and (D) calcined and reduced Au particles on TiO2 (dotted line indicates start of heating). Spectra were recorded over time (first spectrum on top, last spectrum on bottom).

2180 cm1 for CO on TiO2).56 Additionally, no positively charged gold could be identified by CO adsorption because the signals for CO on Au3þ are expected between 2148 and 2207 cm1; those for Auþ, between 2120 and 2191 cm1; and those for Auδþ, between 2127 and 2152 cm1.50,52 When the coverage was decreased, a blue shift was observed from 2111 to 2114 cm1. Two possible explanations have been given for this unusual shift: Boccuzzi et al. proposed that CO molecules adsorbed on Ti4þ ions of the support induce a ligand effect that is responsible for the observed shift.59 Another possibility includes the broadening of the 2π* orbital of CO with increasing coverage, which results in an enhanced backdonation into the 2π* orbital and explains the observed red shift (with increasing Θ).42 Because no signals for the adsorption of CO on TiO2 were observed in our work and additionally the same red shift was observed on Al2O3, we prefer the latter explanation, leaving the support out of the model. After reduction, the CO spectrum revealed a completely new picture (Figure 5b, Au/TiO2). Again, only one main feature was discernible, but it was red-shifted by about 35 cm1 compared to the as-prepared sample and exhibited broadening to lower wavenumbers. Additionally, a shoulder at about 2020 cm1 appeared. During the desorption process, a new set of weak bands increased at about 1900 cm1. These bands disappeared together with the main feature. As indicated by electron microscopy, the reduction procedure led to a significant increase of the particle size. To differentiate whether the temperature or the

hydrogen exposure was responsible for the sintering, the sample was treated at the same temperature in a stream of oxygen. The electron micrograph (Figure 4c, Au/TiO2) indicates no sintering upon treatment in oxygen. DRIFT spectra obtained after calcination are presented in Figure 5c (Au/TiO2). The position of the CO signal is almost unchanged compared to the signal for the asprepared material. A slightly higher wavenumber was found (2115 cm1 for Θmax and 2117 cm1 for Θmin), as was previously reported for gold particles that had been exposed to oxygen due to coadsorption of oxygen60 and confirmed by DFT calculations.61 The calcined sample was then reduced with hydrogen, and again, the signal was red-shifted to 2074 cm1, and a shoulder at 2020 cm1 was observed (Figure 5d, Au/ TiO2), as described before. For gold nanoparticles adsorbed on Al2O3, the same behavior was observed (Figure 6a,b). The as-prepared particles exhibited a signal at 2114 cm1 that shifted toward 2119 cm1 with decreasing coverage. After reduction the signal, was significantly red-shifted to 2076 cm1. Again, a shoulder at around 2020 cm1 appeared, and a signal at 2040 cm1 was also observed, indicating a high heterogeneity of adsorption sites. After the gas flow had been switched from CO to pure helium, a broad band at around 1900 cm1 developed, growing in intensity. The same behavior was observed for gold on ceria (not shown here). On silica, the band at around 2040 cm1, which was discernible only as shoulder on the samples discussed above, became now most prominent (Figure 6c, Au/SiO2). 8200



Figure 6. DRIFT spectra of CO desorption from (a) as-prepared Au nanoparticles, (b) reduced Au particles on Al2O3, and (c) reduced Au particles on SiO2. Spectra were recorded over time (first spectrum on top, last spectrum on bottom).

Signals below 2090 cm1 are discussed as CO bound to negatively charged gold atoms.52,57 In the literature, it is described that reducible supports such as titania, ceria, Fe2O3, and MgO with a sufficient high number of defect sites (like Ti3þ, or F centers in MgO) are capable of transferring charge density to the metal cluster, which leads to the observed red shift upon reduction.7,6264 Note that, in this work, gold nanoparticles were deposited on supports that are not reducible such as Al2O3 and SiO2; still, a red shift was observed after reduction. Because these supports do not exhibit any reduced defect sites, they are not capable of donating electron density to the gold particles.

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Therefore, an alternative explanation focusing more on the intrinsic properties of the gold particles, not taking support interaction into account, seems more feasible. Current DFT calculations on Au13/TiO2 revealed that the frequency of adsorbed CO depends not only on the overall charge of the particle but also on the morphology of the particle, the coordination number and distance of the coordinating gold atom from the support.61 It could be shown that, even on a slightly positively charged Au13 cluster, the absorption frequency was strongly redshifted for low-coordinated gold atoms that were not part of the first atomic layer and were not in direct contact with the support. The calculated wavenumber for this species was 2077 cm1, which is exactly the value we observed in our investigation. We assume that the reduction procedure causes high mobility of the particles, not only resulting in a slight increase in size but also leading to a morphological change and sintering of the particles resulting in more low-coordinated gold atoms, which are not in direct contact with the support.65 Another possible explanation is the coadsorption of hydrogen on the gold particles, which leads to the observed red shift after reduction. Additionally, the very low frequency band at around 1900 cm1 was assigned to bridged CO molecules. This assumption was driven by the fact that these species were observed during the desorption process only at low coverage. Moreover, theoretical calculations predicted such a band at 1927 cm1, which is in good agreement with the value observed here.61 3.3. Au/Pd Bimetallic Nanoparticles. Finally, we examined Au/Pd bimetallic nanoparticles by CO adsorption after different treatments. Several different approaches have been used to unravel properties such as bulk composition, surface composition, and electronic and geometric changes induced by alloying gold and palladium,2,10,15,16,6679 among which the adsorption of carbon monoxide has been one of the most frequently applied.8087 This method provides information not only about the different metals exposed on the surface but also about the populations of different adsorption sites (linear, 2-fold-, and 3-fold-bridged), which depends on the elemental composition of the surface. The investigations described above about the behavior of pure gold and palladium particles served as a reference aiding in the interpretation of the complex behavior of bimetallic particles. In Figure 7a, the electron micrographs and size distributions for as-prepared Au0.5Pd0.5 nanoparticles on Al2O3 are presented. Elemental mapping confirmed that they were made up of both metal constituents. Nevertheless, because the particles were in the lower nanometer range, the whole particle was probed by X-ray beam during the EDX experiment, and the bulk composition, rather than the surface composition, is visualized. Bimetallic nanoparticles deposited on TiO2 exhibited the same behavior; the corresponding micrographs are given in Figure 7c,d. Reduction of bimetallic particles on Al2O3 and TiO2 induced only slight changes in the size distribution observed for pure palladium particles, whereas pure gold particles showed some sintering, resulting in an increase in size. Alloying of bimetallic particles was also confirmed by EXAFS investigations. The Fourier transformed EXAFS spectra of bimetallic particles indicate mixing of the metals (Figure 8). Whereas the spectrum of pure gold nanoparticles showed only one main peak, the two spectra for the bimetallic particles were split because of the existence of two scattering paths for gold, namely, AuAu and AuPd. To obtain quantitative data about the local structure, the first coordination sphere was fitted in R space. The most informative 8201



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Figure 8. k3-weighted magnitude of Fourier transform of Au and Au/Pd nanoparticles at the Au LIII edge: (a) as-prepared Au/TiO2 (black, dashed), as-prepared AuPd/TiO2 (black, solid), as-prepared AuPd/ Al2O3 (gray, solid); (b) as-prepared AuPd/Al2O3 (black, solid), reduced AuPd/Al2O3 (gray, solid).

Table 2. Results of the EXAFS Fit of Al2O3- and TiO2Supported Metal Nanoparticles (As-Prepared and Reduced) for the Au LIII and Pd K Edges Rb sample

Figure 7. HAADF-STEM micrographs and corresponding histograms of Au/Pd nanoparticles on (a) Al2O3 (as-prepared) with elemental mapping at the Au M edge (red) and Pd K edge (green), (b) Al2O3 (after reduction), (c) TiO2 (as-prepared), and (d) TiO2 (reduced).

data are the coordination numbers of the single scattering paths, as well as the total coordination number, Ntotal (Table 2). The total coordination number, Autotal, of the titania-supported sample was smaller than Autotal of the alumina-supported sample. The lower total coordination number confirms the smaller average diameter of the titania-supported bimetallic particles observed by electron microscopy. Closer inspection of the differences between NAuAu and NAuPd indicates an enrichment of gold inside the cluster and the accommodation of Pd on the surface because NAuAu is greater than NAuPd for the as-prepared samples.15 The surface enrichment of Pd is also evident from the very small NPdPd value due to the fact that surface atoms have fewer next neighbors compared to bulk atoms (Table 2). Nevertheless, a certain amount of gold atoms might be exposed on the surface. The spectra for CO adsorbed on as-prepared Au0.5Pd0.5/Al2O3 particles (Figure 9a) strongly resemble the spectra observed for pure Pd on alumina. Again, three regions

edge

pair

N

a

(Å)

Δσ2 c 2

(Å )

Au/TiO2 Au LIII AuAu 9.7 2.83 0.0087 Au/Pd

Au LIII AuAu 4.2 2.79 0.005

TiO2 Pd K

ΔE0d (eV) 0.2

6.2

0.8

9.8

6.0

10.2

6.8

3.9

9.3

2.8

9.2

AuPd 1.9 2.79 0.0044

3.4

PdPd 2.3 2.78 0.0080 PdAu 3.5 2.78 0.0074

3.7 5.9

PdO

1.0 1.97 0.0027

2.0

Au/Pd

Au LIII AuAu 5.6 2.81 0.0077

4.4

Al2O3

AuPd 3.7 2.80 0.0077

4.0

3.3 2.81 0.0069

4.2

PdAu 5.9 2.81 0.0093

0.2

Au LIII AuAu 4.3 2.79 0.0061

3.2

3.2

8.8

AuPd 4.5 2.79 0.0074 PdPd 2.7 2.77 0.0066

3.9 4.9

11.5

9.5

PdAu 6.8 2.79 0.008

2.2

Pd K Au/Pd Al2O3 reduced

residual Ntotal

Pd K

PdPd

N = coordination number. b R = distance (Å). c Δσ2 = Debey-Waller factor (Å2). d ΔE0 = inner core correction (eV). a

could be distinguished: a high-frequency part at around 2080 cm1 assigned to linear-bound species on Pd (i) with a shoulder at around 2050 cm1 that is assigned to linear CO adsorbed on gold 8202



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Figure 9. DRIFT spectra of CO desorption from (a) as-prepared AuPd nanoparticles and (b) reduced AuPd particles on Al2O3. Spectra were recorded over time (first spectrum on top, last spectrum on bottom).

(vide infra), a region located around 1980 cm1 which is attributed to bridged CO species on Pd (100) faces (ii), and a third, low frequency section associated with CO 2-fold and 3-fold bridged on Pd (111) faces (iii). The most obvious difference between the pure Pd and AuPd samples is the increased intensity of the signal located at 2080 cm1 for the bimetallic sample. Furthermore, the intensity of the signal associated with bridged CO on Pd (111) faces in the low frequency region is smaller. These results indicate a higher fraction of Pd atoms where CO is able to bind in only a linear fashion. Thus, a significant amount of gold atoms must be exposed on the surface, which increases the number of isolated palladium atoms, leading to more linear-bound species on Pd faces and less molecules adsorbed in a bridged fashion. Reduction of the sample changed the alloy composition in a manner that the former coreshell structure became less pronounced, resulting in a more homogeneous alloy. The results of EXAFS fitting (Table 2) show a significant decrease of the homometallic coordination numbers NAuAu and NPdPd, whereas the heterometallic coordination numbers NAuPd and NPdAu increase. Thus each metal atom gets more neighbors of the other kind of metal constituent during reduction, and the degree of alloying becomes higher. This behavior could directly be followed by IR spectroscopy. The spectra for CO adsorption on the reduced bimetallic sample (Figure 9b) shows the same feature as the asprepared sample, but the trends described above are more pronounced. The high-frequency signal at around 2080 cm1 was more intense than for the as-prepared sample (and, thus, even higher than for pure Pd particles), whereas the signal in

Figure 10. DRIFT spectra of CO desorption from (a) as-prepared AuPd nanoparticles, (b) reduced AuPd particles, and (c) reduced AuPd nanoparticles with a Au/Pd ratio of 1:3 on TiO2 (dotted line indicates start of heating). Spectra were recorded over time (first spectrum on top, last spectrum on bottom).

region iii was very weak. Additionally, a new signal at 2050 cm1 appeared, which is assigned to a linear species adsorbed on either gold atoms or new adsorption sites with Pd being involved. Here, we assume the adsorption on gold sites. As shown in section 3.2 describing CO adsorption on Au particles, low-coordinated gold atoms are able to coordinate CO. Nevertheless, the observed wavenumber of only 2050 cm1 is rather low for CO adsorbed on Au atoms. However, as demonstrated in a previous work dealing with AuPd nanoparticles, Pd is able to donate electron density to Au atoms. Thus, the gold atoms in AuPd alloys are electronrich or negatively charged, which might explain the unusually strong red shift for CO on gold in bimetallic particles.10,15,16 8203


The Journal of Physical Chemistry C To gain some information about the possible influence of the reducibility of the support on the CO adsorption behavior, we investigated bimetallic particles on TiO2. Interestingly, as observed before, the application of titania as a support did not influence the behavior of CO adsorption significantly, as emerges from Figure 10. In the high-frequency region (i), all three signals observed for as-prepared particles on Al2O3 are discernible (Figure 10a). The signal assigned to linear-bound CO on Pd(111) faces is very intense but decreases very rapidly upon desorption. A second band at 2060 cm1 could be observed upon desorption and is assigned to CO on top of Pd atoms at defect sites, such as corners and edges. The intensity of this band did not increase because the number of corner atoms on a particle does not change dramatically upon alloying with gold, whereas the number of isolated Pd atoms on a facet is dramatically altered. A shoulder at around 2050 cm1 was also observed and assigned to CO bound to gold atoms. The most stable species was again CO on Pd (100) faces (ii), which exhibits a shoulder at 1940 cm1 that arises during desorption. This shoulder was not observed for pure Pd particles and is therefore assigned to bridged CO molecules on Pd/Au bimetallic faces. The signal of the μ2- and μ3-bridged species shifted from 1930 to 1852 cm1 (iii) during the desorption process. Upon reduction, more gold atoms were exposed to the surface. Therefore, Pd atoms segregated from each other, low-coordinated Pd sites became dominant, and the signal for linear-bound CO on Pd(111) increased (2080 cm1). At the same time, the signal at 2050 cm1 for CO bonded linearly on gold also became visible due to the higher number of exposed gold atoms. In Figure 10c, spectra measured on a reduced Au0.25Pd0.75/TiO2 sample are depicted. The spectra for this sample show features in between those obtained for Pd and Au0.5Pd0.5. Thus the signal for linear-bound species on Pd was smaller than for Au0.5Pd0.5, but more intense compared to the monometallic Pd sample. The signal at 2050 cm1 assigned to CO on gold was less intense compared to that of the Au0.5Pd0.5 sample, but it was still observed. The low-energy part of the spectrum (iii) also showed features between those observed for Pd and Au0.5Pd0.5. The shoulder at 1940 cm1 assigned to bridged species on bimetallic faces was still discernible but smaller than that assigned to Au0.5Pd0.5, whereas the feature at 1900 cm1 was more intense than the one observed for reduced Pd samples. Thus, we identified two new signals (2050 and 1940 cm1) that we assume to be indicative of gold atoms exposed on the surface of bimetallic nanoparticles.

4. CONCLUSIONS Colloid-derived mono- and bimetallic nanoparticles of palladium and gold were deposited on different supports and characterized with regard to their surface properties by means of CO adsorption combined with DRIFTS, electron microscopy, and EXAFS. DRIFTS showed three spectral regions for Pd nanoparticles assigned to linear-bound CO on Pd(111) faces and on defect sites, 2-fold bridged on Pd(100) faces, and 2-fold- and 3-fold-bridged on Pd(111) faces. Different supports (alumina, titania, silica, ceria) had no significant influence on the position or on the intensity of the signals, indicating a rather weak metalsupport interaction for the nanoparticles prepared by the colloidal route. Gold nanoparticles showed a distinct difference between asprepared and reduced samples. Upon treatment in hydrogen, a strong red shift was observed. IR studies and electron microscope

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investigations indicated a morphological change of the particles: highly uncoordinated gold atoms were formed, and CO adsorption occurred. Additionally, a low frequency band was observed during desorption, which was assigned to μ2-bridge CO on gold nanoparticles. Bimetallic nanoparticles of gold and palladium showed a goldenriched core and a palladium-enriched surface. DRIFTS combined with CO adsorption revealed new specific bands for these particles. Upon reduction of the bimetallic nanoparticles, the coreshell character of the particles became less prominent, indicating a more homogeneous distribution of the metals. Changes of the bulk composition were followed with XAS, and the surface composition was examined with IR spectroscopy. The combination of these two complementary techniques allowed the final assignment of different CO absorption bands in the IR spectrum.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: þ41 44 632 3153. Fax: þ41 44 632 1163.

’ ACKNOWLEDGMENT Financial support by the “Fondation Claude et Giuliana” is kindly acknowledged. The authors thank Hasylab at DESY and Prof. Jan-Dierk Grunwaldt (KIT) for providing beam time at beam line X1 and European Union for financial support (contract number I-2009 0002 EC). ’ REFERENCES (1) Han, Y. F.; Zhong, Z. Y.; Ramesh, K.; Chen, F. X.; Chen, L. W.; White, T.; Tay, Q. L.; Yaakub, S. N.; Wang, Z. J. Phys. Chem. C 2007, 111, 8410. (2) Reifsnyder, S. N.; Lamb, H. H. J. Phys. Chem. B 1999, 103, 321. (3) Edwards, J. K.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Faraday Discuss. 2008, 138, 225. (4) Besenbacher, F.; Chorkendorff, I.; Clausen, B. S.; Hammer, B.; Molenbroek, A. M.; Norskov, J. K.; Stensgaard, I. Science 1998, 279, 1913. (5) Ponec, V. Surf. Sci. 1979, 80, 352. (6) Tenney, S. A.; Ratliff, J. S.; Roberts, C. C.; He, W.; Ammal, S. C.; Heyden, A.; Chen, D. A. J. Phys. Chem. C 2010, 114, 21652. (7) Manzoli, M.; Boccuzzi, F.; Chiorino, A.; Vindigni, F.; Deng, W. L.; Flytzani-Stephanopoulos, M. J. Catal. 2007, 245, 308. (8) Kolbeck, C.; Cremer, T.; Lovelock, K. R. J.; Paape, N.; Schulz, P. S.; Wasserscheid, P.; Maier, F.; Steinruck, H. P. J. Phys. Chem. B 2009, 113, 8682. (9) Chen, M. S.; Kumar, D.; Yi, C. W.; Goodman, D. W. Science 2005, 310, 291. (10) Marx, S.; Baiker, A. J. Phys. Chem. C 2009, 113, 6191. (11) Conte, M.; Carley, A. F.; Attard, G.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. J. Catal. 2008, 257, 190. (12) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362. (13) Ketchie, W. C.; Murayama, M.; Davis, R. J. J. Catal. 2007, 250, 264. (14) Landon, P.; Collier, P. J.; Carley, A. F.; Chadwick, D.; Papworth, A. J.; Burrows, A.; Kiely, C. J.; Hutchings, G. J. Phys. Chem. Chem. Phys. 2003, 5, 1917. (15) Liu, F.; Wechsler, D.; Zhang, P. Chem. Phys. Lett. 2008, 461, 254. (16) Baber, A. E.; Tierney, H. L.; Sykes, E. C. H. ACS Nano 2010, 4, 1637. (17) Borchert, H.; Jurgens, B.; Zielasek, V.; Rupprechter, G.; Giorgio, S.; Henry, C. R.; Baumer, M. J. Catal. 2007, 247, 145. 8204


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