J. Phys. Chem. C 2010, 114, 16677–16684
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Determination of the Size of Supported Pd Nanoparticles by X-ray Photoelectron Spectroscopy. Comparison with X-ray Diffraction, Transmission Electron Microscopy, and H2 Chemisorption Methods R. Wojcieszak,† M. J. Genet,‡ P. Eloy,† P. Ruiz,† and E. M. Gaigneaux*,† UniVersite´ Catholique de LouVain, Institute of Condensed Matter and Nanosciences (IMCN), Croix du Sud, 2/17, 1348 LouVain la NeuVe, Belgique ReceiVed: July 26, 2010; ReVised Manuscript ReceiVed: August 25, 2010
Supported palladium nanoparticles with different diameters were synthesized by the water-in-oil microemulsion method using TiO2 as support. The materials were characterized by different physicochemical methods such as X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and H2 chemisorption. The results confirmed that the microemulsion method permits well-dispersed palladium nanoparticles to be obtained. The size of the nanoparticles was estimated by XPS intensity ratios using models proposed by Davis and by Kerkhof and Moulijn and compared with XRD, TEM, and H2 chemisorption analysis. Good accordance of the two models was found for very small Pd particles (smaller than 3 nm). The Kerkhof-Moulijn model seemed to be very sensitive to the small variation in the particle size distribution. The Davis model seemed to be more adequate to determine the size of small and biggest particles as compared with the Kerkhof-Moulijn model. A good accordance between TEM results and the Davis model was found. The results obtained using the Davis model permitted also understanding of the differences observed between XRD and TEM studies. XPS analysis could be a good and probably more accessible alternative to determine rapidly and with high accuracy nanosize particles of materials, in particular when others physicochemical techniques are not accessible or have a limited resolution. 1. Introduction Presently, considerable attention is paid to the preparation of very small metallic particles because of their unique properties and potential application in many processes. The metal nanoparticles dispersed on an inert support play also a crucial role in catalytic processes. It is well-recognized that the size of metal nanoparticles determines the activity and selectivity of the catalyst.1 It is also considered that in some cases the drastic change of catalytic properties of nanoparticles is due to their new properties (physical and chemical) that are created when their size decreases. One of the main difficulties to study the small metallic nanoparticles (below 3 nm) is to determine the exact size of these nanoparticles.2 The complications arise because particles are usually not spherical and their distribution on the support is not homogeneous. Moreover, some techniques are not useful because of the size limitation. In the case of an X-ray diffraction (XRD) study, the applicability is restricted by several factors such as the weight fraction or the size of crystallites. Size limitation is also the problem with transmission electron microscopy (TEM) measurements; in fact the detection is limited by contrast effects which may result in the difficulty to detect the small clusters containing a few atoms. Moreover, several images of the same sample on different zones have to be analyzed and distribution can be only statistically established by measuring a great number (about 1000) of particles.2 Chemical methods such as chemisorption of an adequate gas * To whom correspondence should be addressed, eric.gaigneaux@ uclouvain.be. † Division of Molecules, Solids and Reactivity (MOST). ‡ Division of “Bio and Soft Matter (BSMA).
also have some limitations. The presence of metal-support interaction, formation of hydrides, spillover effect, or contaminants can indeed alter the gas uptake. X-ray photoelectron spectroscopy (XPS) is a powerful tool to study the external surface of catalysts.3,4 XPS study is able to determine the chemical composition and oxidation degrees of components present on the catalyst surface. In addition the sizes of the nanoparticles can be estimated from the XPS elemental intensity ratios using an adequate modeling of the signal. Different XPS models could be applied for estimation of average particle size. Some very good reviews and articles present in the literature explained the basic aspects and claimed the important role of XPS techniques in the determination of the size of particles.5–7 The main objective of this work was to carry out a study of the size of the particles modeling the XPS signal using two models proposed in the literature. In the first, the Davis model,3 the particle size could be obtained from the experimental intensity ratio between two core levels with different kinetic energy arising from the same dispersed phase. The second, the Kerkhof-Moulijn model,4 predicts the average particle size from intensity ratio of monolayer and crystallite samples. This model depends strongly on the physical properties of the support. One advantage of the first model is a certain independence from the physical properties of support and for the second model is the possibility to use XPS data already collected. Both models are described in detail in the literature. Their principal bases are reported below. In order to make an attempt to validate both models, the results of the particle sizes obtained from XPS are compared to the results obtained from XRD (Scherrer formula), TEM, and H2 chemisorption analysis.
10.1021/jp106956w 2010 American Chemical Society Published on Web 09/13/2010
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One difficulty is to synthesize nanoparticles homogeneously distributed on the surface of a support. Among the several preparation methods proposed in the literature to obtain nanoparticles, the chemical reduction of metal salts from the solution is of particular interest since it allows a better control of the structure of supported active phase. The chemical reduction in the solution is probably the most versatile, economical, and easy to perform method.8 Generally, it is performed using several chemical reducing agents such as hydrazine, NaBH4, NaH, polyols, citrates, or aldehydes.9–11 Colloidal Pt nanoparticles with a narrow size distribution via a polyol process was reported by Bonet et al.12 However, the obtained colloid of Pt particles with a mean particle size of 7 nm was not stable. The use of stabilization agents is necessary to prepare stable metal particles with small particle size.13 The citrate reduction introduced by Turkevich14 showed a double role of citrate salts, namely as a reducing agent of metal ions and then as the stabilizing agents for metallic particles formed. Boutonnet et al.15 showed with success that metallic nanoparticles could be obtained homogeneously by the simple mixing of two water-in-oil microemulsions, one containing a salt or a complex of the metal and the other containing a reducing agent. Metal cations constrained in the water droplets can react with reducing agent to form the metallic nanoparticles. It was shown that the use of appropriate surfactant is the key factor for the control of the metal particle size. In this work the supported nanoparticles of palladium were prepared using a water-in-oil microemulsion method. Different organic solvents such as n-alcohols, n-octane, and cyclohexane were used to form the oil phase. The anionic AOT (bis(2ethylhexyl)sulfosuccinate) and nonionic Brij30 (ethylene glycol monolauryl ether) surfactants and organic polymer PVP (poly(vinylpyrrolidone)) have been chosen as stabilizing agents. More details of the preparation of the supported palladium particles are given in ref 16. 2. Experimental Section 2.1. Catalyst Preparation. Palladium(II) chloride (from Aldrich, 99.9%) was used as the Pd precursor and TiO2 oxide (Merck, specific surface area of 8 m2 g-1. TiO2 was used as received without any additional treatment. The anatase structure was confirmed by XRD study.) as the support. The organic surfactants were purchased from Sigma (sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and ethylene glycol monolauryl ether (Brij30)). PVP (poly(vinylpyrrolidone)) surfactant was purchased from Fluka (purity 99%). Organic solvents (cyclohexane, 1-butanol, 1-octanol, 1-decanol, and n-octane) and hydrazine hydrate solution (80% in water) were purchased from Fluka. All reagents were used without further purification. All materials were prepared as follows: 0.066 g of PdCl2 (to obtain 2% wt of palladium) was dissolved in 5 mL of distilled water in the presence of NaCl (0.05 g, Merck, 99.5%). Then the solution was evaporated at 60 °C to a volume of 1 mL. The microemulsion was then formed using 50 mL of organic solvent, 1 mL of precursor solution, and an appropriate quantity of organic surfactant. The water to surfactant molar ratio was 6. This microemulsion was then heated to 50 °C and then 2 g of TiO2 was incorporated to the reactant flask under magnetic stirring. After 30 min, 3 mL of hydrazine was injected. The reactant solution color changed from light red to black indicating the palladium reduction and nanoparticle formation. This solution was maintained at 50 °C for another 30 min. The solution was then filtered, washed with acetone and water for 1 h, and dried at 100 °C for 30 min. The reduction was carried
Wojcieszak et al. out in a thermostated water bath at 50 °C. A reduction time of 30 min was counted from the hydrazine injection to the microemulsion. The synthesis was carried out under nitrogen atmosphere. This procedure was also described in ref 16. Samples were denoted as AOT/n-octane, AOT/cyclohexane, AOT/1-octanol, Brij30/1-decanol, AOT/1-butanol, and PVP/ cyclohexane. A sample denoted as Pd/TiO2(H2O) was prepared in water and without any organic surfactants and reduced by hydrazine. The aim of this preparation was to obtain the sample with the same Pd loading but lower dispersion of active phase. The absence of protective agents should indeed permit the agglomeration and growth of palladium nanoparticles. 2.2. Characterization of Physical and Chemical Properties. 2.2.1. X-ray Photoelectron Spectroscopy (XPS) Analysis. (a) Surface Science Instruments (SSI). XPS analysis was performed on an SSX-100/206 spectrometer (Surface Science Instruments, USA). The analysis chamber was operated under ultrahigh vacuum with a pressure close to 5 × 10-7 Pa, and the sample was irradiated with a monochromatic Al KR (1486.6 eV) radiation (10 kV; 22 mA). Charge stabilization was achieved by using an electron flood gun adjusted at 8 eV and placing a nickel grid 3 mm above the sample. Pass energy for the analyzer was set at 150 eV for both, wide and narrow scans, and the analyzed area was approximately 1.4 mm2. In these conditions the full width at half-maximum (fwhm) of the Ag 3d5/2 peak of a silver standard sample was about 1.6 eV. The surface atomic concentrations were calculated by correcting the intensities with theoretical sensitivity factors based on Scofield cross sections17 and the mean free path varying according to 0.7 power of the photoelectron kinetic energy. Peak decomposition was performed using curves with an 85% Gaussian type and a 15% Lorentzian type, and a Shirley nonlinear sigmoid-type baseline. The following peaks were used for the quantitative analysis: O 1s, C 1s, Ti 2p, and Pd 3d. Moreover, the Cl 2p, S 2p and N 1s peaks were also monitored and C 1s again to check for charge stability as a function of time. (b) Kratos Axis Ultra (Kratos Analytical). The PdM45N45N45 Auger peak needed to be recorded, but the corresponding kinetic energy (about 330 eV) was lower than the minimum kinetic energy (386.6 eV) measurable with the SSI spectrometer. Therefore XPS peaks were also recorded on a Kratos Axis Ultra spectrometer (Kratos Analytical, U.K.). The spectrometer was equipped with a monochromatized aluminum X-ray source (powered at 10 mA and 15 kV). The pressure in the analysis chamber was about 10-6 Pa. The angle between the normal to the sample surface and the direction of photoelectrons collection was about 0°. Analyses were performed in the hybrid lens mode corresponding to a combination of magnetic and electrostatic lenses. The analyzed area was about 700 µm × 300 µm. The pass energy of the hemispherical analyzer was set at 160 eV for the wide scan and 40 eV for narrow scans. In the latter conditions, the full width at halfmaximum (fwhm) of the Ag 3d5/2 peak of a standard silver sample was about 0.9 eV. Charge stabilization was achieved by using the Kratos Axis device. Peak decomposition was performed using curves with an 70% Gaussian type and a 30% Lorentzian type and a Shirley nonlinear sigmoid-type baseline. The following peaks were used for the quantitative analysis: O 1s, C 1s, Ti 2p, and Pd 3d. Molar fractions were calculated using peak areas normalized on the basis of acquisition parameters after a Shirley background subtraction and corrected with experimental sensitivity factors and transmission factors provided by the manufacturer. The same sequence of spectra
Determination of the Size of Supported Pd Nanoparticles
Figure 1. XRD analysis. Patterns of the Pd/TiO2 materials prepared by the microemulsion method.
Figure 2. XRD analysis. Patterns of the Pd/TiO2 (H2O) sample.
as with the SSI spectrometer was recorded, but also the PdM45N45N45 Auger peak. Pd foil was used as the reference material for study of prepared catalysts. A Pd foil was analyzed before and after Ar+ etching during 50 min with the Kratos Minibeam I ion gun (4 kV, 15 mA) to remove Pd oxide from the foil surface. For both SSI and Kratos measurements (i) sample powders were pressed into small stainless steel troughs mounted on a multispecimen holder, (ii) the C-(C,H) component of the C 1s peak of adventitious carbon was fixed to 284.8 eV to set the binding energy scale, and (iii) the data treatment was performed using CasaXPS software (Casa Software Ltd., U.K.). 2.2.2. InductiWely Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The chemical composition of catalysts was determined by inductively coupled plasma atomic emission spectroscopy, using a Thermo Jarrel Ash Iris Advantage equipment. The samples were first brought into solution by alkali oxidative fusing NaOH/Na2O2 and subsequent dissolution with diluted HCl. 2.2.3. XRD. X-ray diffraction study was carried out on a Siemens D5000 diffractometer using the KR radiation of Cu (1.5418 Å). The 2Θ range was scanned between 2 and 70° at a rate of 0.01 deg s-1. The results for the 26-70° range are shown in Figures 1 and 2. Any additional phase was observed in the 2-26° range. The identification of the phases was
J. Phys. Chem. C, Vol. 114, No. 39, 2010 16679 achieved by using the ICDD-JCPDS database. X-ray diffraction line broadening analysis was used for characterizing supported palladium nanoparticles. The Scherrer formula2 was applied to estimate the average metal particle size. The X-ray pattern was fitted by least-squares fit assuming a Gaussian peak shape. In this form, the Scherrer equation has been applied to calculate the diameter assuming the spherical shape of particles and K constant of 0.9. 2.2.4. TEM. The transmission electron microscopy images were obtained with a TEM LEO 922 Omega microscope after placing a drop of the sample suspension on a carbon-coated copper grid. Energy dispersive X-ray spectroscopy (EDS) provides identification of elements with Z g 6 and was used for Pd identification on the catalysts surface. 2.2.5. H2 Chemisorption. Hydrogen chemisorption at 35 °C method was used for determination of dispersion and average size of palladium particles supported on TiO2 oxide. The amounts of chemisorbed hydrogen were determined at 35 °C on a Micrometrics device. The following procedure was applied in the measurements: preliminary evacuation and heating at 300 °C for 1 h in a He flow, then flow of hydrogen (Praxair, 99.%) at 300 °C for 1 h, evacuation at 300 °C for 2 h, cooling down to 35 °C, evacuation under He at 35 °C for 1 h, leak test, and finally measurement of hydrogen uptake at 35 °C. A double adsorption method was used: (i) first the adsorption isotherm was measured, which includes both physisorption and chemisorption; (ii) after a 2 h outgassing under He atmosphere at 35 °C, a second isotherm was measured which includes physisorption only. Both isotherms are determined in the pressure range from 0.5 to 700 mmHg. The difference between first and second isotherms gave the H2 chemisorption isotherm.2 Assuming a spherical shape, an average crystallite size d (given in nanometers) was calculated based on irreversible adsorption isotherm of hydrogen, according to the following: d ) 6000/SF, where S is the surface area (of the fraction of reduced palladium, given in m2 g-1) and F is the palladium density (given in cm3 g-1).2 S was calculated assuming H/Pd ) 1 and a surface area of 7.93 Å2 per palladium atom. 3. Results and Discussion 3.1. Preparation of the Supported Pd Nanoparticles. The water-in-oil microemulsion method permits very small metal particles to be obtained with a very good dispersion of the active phase. In this method, the metal precursor is reduced in solution in the presence of protective agents such as polymers or surfactants. In some cases the interaction between metal particles formed and surfactant could be very strong and the surfactant molecules can stay adsorbed on the metal surface after calcination of the sample or a cleaning step. The reduction of palladium in microemulsion is very fast. The transfer of electrons from reductant to the metal takes place according to the equation9
mMen+ + nRed f mMe0 + nOx
(1)
The number of nuclei formed at the very beginning of the reduction determined the number and size of the resultant particles.18 The reduced species that formed later were involved in the collision with the nuclei already formed instead of the new nuclei formation. This provides the creation of larger particles. In our case the hydrazine to palladium molar ratio was 100, which placed this preparation in the desired conditions (particle size not dependent on hydrazine concentration). These conditions ensured that the particle size would be mainly governed by the nature of solvent and surfactant.
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TABLE 1: ICP, BET, and XPS Molar Concentrations of TiO2, Pd/TiO2(H2O) Sample, and Materials Prepared by Microemulsion Method
sample AOT/n-octane AOT/cyclohexane AOT/1-octanol Brij30/1-decanol AOT/1-butanol PVP/cyclohexane Pd/TiO2(H2O) TiO2-N2H4
BET % wt % Pd % C % N surface area Pd (ICP) (XPS) (XPS) (XPS) (m2 g-1) 2.2 1.7 1.6 1.6 1.8 1.7 1.6
2.8 2.3 1.1 1.6 1.4 1.5 1.5
23.2 21.4 18.8 17.3 21.7 37.4 18.3 25.8
0.45 0.32 0.47 0.39 0.64 4.20 0.36 2.76
8.4 8.3 7.9 8.0 8.2 8.2 8.3 7.6
The results of ICP analysis, given in Table 1, showed, that almost all palladium atoms were deposited on the support in the case of AOT/n-octane catalyst (about 2 wt %). Contrary to that the amounts of palladium in other samples were smaller (1.6-1.7 wt %) than expected (2 wt %). This could be explained by a loss of palladium during filtration due to the nanometer size of the particles. Some of them can pass through the filter. This is also true for supports because a weight loss of about 8% was observed for all catalysts after filtration and drying. However, the reasonable results obtained are comparable with other methods such as precipitation and ion exchange. 3.2. XRD, TEM, and Hydrogen Chemisorption Studies. XRD patterns of TiO2 support and hydrazine-treated TiO2 support (not reported here) showed well crystallized anatase structure. This confirmed that the agent used to perform the reduction of the metal precursor had no effect on the structure of the support. As expected, the XRD patterns of the obtained samples confirmed the metal reduction (Figure 1). However, the diffraction line at 40.15°, which corresponds to the metallic Pd(111) with face-centered cubic structure, was too weak to be exploited to determine particle size in the case of catalysts prepared by the microemulsion method. Indeed, it is well recognized that the XRD technique is limited by the size of particles. In the case that the size of the particle is lower than 5 nm, no measurable signal is obtained. Contrary to that, the average Pd particle size in the case of Pd/TiO2 (H2O) sample (Figure 2) was calculated. The obtained value (19 nm) was then compared to the TEM study. TEM micrographs were recorded on all samples and allowed the observation of very small palladium particles. Metal nanoparticles were evidence for the Pd/TiO2(H2O) catalyst. In this case the palladium was found homogeneously dispersed on the support surface (Figure 3A) with particle mean size of about 4-7 nm. However, some bigger particles (about 18 nm) were also visible. If we considered the spherical structure of these particles, the average number of atoms contained in these aggregates will be about 210000 atoms. These isolated big aggregates would be thus responsible for the XRD signal (Figure 2). On catalysts prepared by the microemulsion method, the TEM images differ from that of the surfactant-free sample (Figure 3B,C). The palladium nanoparticles were not visible on TEM images for AOT/1-octanol, AOT/cyclohexane (Figure 3B), and AOT/n-octane catalysts although the EDS surface analysis confirmed the presence of palladium on the surface. It could be supposed that, after the reduction with the hydrazine, the palladium particles formed are very small (
