J. Phys. Chem. C 2010, 114, 9257–9263
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Generation of Pd Model Catalyst Nanoparticles by Spark Discharge Maria E. Messing,*,† Rasmus Westerstro¨m,‡ Bengt O. Meuller,† Sara Blomberg,‡ Johan Gustafson,‡ Jesper N. Andersen,‡ Edvin Lundgren,‡ Richard van Rijn,§,| Olivier Balmes,§ Hendrik Bluhm,⊥ and Knut Deppert† Solid State Physics, Lund UniVersity, Box 118, 221 00 Lund, Sweden, Synchrotron Radiation Research, Lund UniVersity, Box 118, 221 00 Lund, Sweden, ESRF, B. P. 220, F-38043 Grenoble, France, Kamerlingh Onnes Laboratory, Leiden UniVersity, P.O. Box 9504, 2300 RA Leiden, The Netherlands, and Chemical Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: February 15, 2010; ReVised Manuscript ReceiVed: April 17, 2010
We present a method to deposit Pd nanoparticles with a very small size distribution by an aerosol process onto oxide substrates for the creation of model systems in catalytic research. The Pd nanoparticles are characterized by transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. We confirm the small size dispersion from the desired particle size, and we show that the particle surface coverage can be highly controlled. Further, our measurements indicate that an amorphous shell surrounding a crystalline core of the Pd particles may form during the particle synthesis and that the shell contains carbon. 1. Introduction More than 80% of all chemical products are produced engaging catalysts, which are used to facilitate a specific reaction without being consumed. In most cases, the catalyst is in another phase than the reactants, for example, a solid catalyst and gaseous reactants, a situation referred to as heterogeneous catalysis. Because of its practical importance, heterogeneous catalysis has been studied for centuries. A catalyst used for industrial purposes or for cleaning of exhaust gases is a complicated system of materials usually consisting of an insulating oxide support with dispersed metal nanoparticles of the active catalyst, as well as a wide range of additives to promote or poison specific reactions. Because of the material complexity, atomic scale information on the inner workings of the catalyst is, at best, limited, and as a consequence, catalyst development is, for a large part, based on a trial-and-error approach. However, because of the need to obtain fundamental information on catalytic reaction pathways, model systems have been developed. Studies of processes related to heterogeneous catalysis under ultra-high-vacuum (UHV) conditions on welldefined single-crystal surfaces have been a major part of surface science for decades.1 In recent years, more complex material model systems have been developed by depositing pure metal or alloy nanoparticles by molecular beam epitaxy (MBE)2,3 or by wet chemical methods4 on a thin oxide film formed on a conducting material. In this way, the electric conductivity of the sample is maintained, allowing for the use of electron-based surface analysis techniques under UHV conditions or even in the millibar range.4 Further, recent in situ experiments on MBE grown epitaxial Rh * To whom correspondence should be addressed.
[email protected]. † Solid State Physics, Lund University. ‡ Synchrotron Radiation Research, Lund University. § ESRF. | Leiden University. ⊥ Lawrence Berkeley National Laboratory.
E-mail:
Figure 1. Schematic of the aerosol generator used to produce palladium particles by spark discharge between two electrodes.
particles on insulating MgO and Al2O3 substrates demonstrate the potential of surface X-ray diffraction (SXRD)5,6 for in situ studies of catalysts in a working environment. Here, we present a different route to a model system: sizeselected metal nanoparticles deposited onto any substrate by aerosol deposition. The system presented consists of Pd nanoparticles with a diameter of 15 or 35 nm deposited on HF-etched SiO2 or on Al2O3 substrates. The conducting SiOx is used for the sake of characterization using X-ray photoelectron spectroscopy (XPS), but the type of metal deposit or substrate can be chosen almost arbitrarily. Our studies show that the particles can be produced within a narrow size distribution and that the particle coverage can be well-controlled. Both of these properties are desirable for comparative studies of catalytic properties. For example, when using different substrates or when adding additives to control certain reaction pathways or hinder compound formation and sintering. Our studies also show that the main contaminant of the pristine particles is carbon, either in the form of hydrocarbons or, more likely, contained in an amorphous shell around the crystalline Pd core of the particles.
10.1021/jp101390a 2010 American Chemical Society Published on Web 05/03/2010
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Figure 2. Schematic of the aerosol nanoparticle system setup.
2. Experimental Methods To produce palladium model catalyst particles, a commercially available aerosol generator, Palas, model GFG 1000, primarily constructed for carbon soot particle production was used. The mechanism behind particle formation is based on spark discharge between two conducting electrodes positioned in the middle of a polymer chamber with their flat ends separated by a distance of 2 mm (Figure 1). A 20 nF capacitor is connected to one of the electrodes and charged by a high-voltage supply with an adjustable output current.7 When the breakdown voltage of 2 kV is reached, the capacitor discharges instantaneously in a spark across the electrode gap. The local temperature of the spark reaches approximately 20 000-30 000 K,8 leading to evaporation of electrode material. Primary particles are formed by homogeneous nucleation of the supersaturated vapor and are subsequently transported toward the aerosol outlet by a stream of carrier gas focused between the electrodes. Primary particles are small particles, typically with a diameter between 1 nm and a few nm, with a homogeneous atomic structure. Further growth of primary particles by condensation and coagulation results in the production of highly charged, nanometer-sized agglomerate particles.9 To ensure a constant particle production, that is, a constant breakdown voltage, the distance between the electrodes is maintained by an electrical motor. Two parameters can be varied in order to affect particle production of this apparatus. The spark discharge frequency can be set between 0 and 300 Hz, and the carrier flow rate can be set between 2 and 8 L/min. In this investigation, particles were generated at spark discharge frequencies of 30, 60, 120, 180, 240, and 300 Hz and flow rates of 3.4, 3.9, 4.4, 4.9, 5.4, and 5.9 L/min. Furthermore, the carrier gas type is known to affect particle production.10 In the original setup, an argon-air mixture is used as the carrier gas, but to comply with cleanliness requirements, the carrier gas was replaced with ultrapure nitrogen. In addition, the cylindrical carbon electrodes used in the original setup were replaced by high-purity palladium rods (99.99%) with diameters of 3 mm, mounted to cylindrical stainless steel holders with diameters of 6 mm in order to fit the apparatus. The spark generator was connected to an aerosol nanoparticle system setup (Figure 2) in order to enable size distribution measurements, reshaping of the agglomerate particles into compact particles, and controlling the deposition of particles. A β-emitting 63Ni source11 was used as a neutralizer in order to achieve a reproducible and known charge distribution on the agglomerate particles before size selection in a differential mobility analyzer (DMA), labeled DMA 1 in Figure 2. The DMA, a standard instrument in aerosol science, classifies charged particles according to their mobility inside an electric field.12 This mobility is roughly inversely proportional to the
particle diameter. Following size selection, the agglomerate particles could be reshaped into more compact particles inside a compaction tube furnace (route 2 in Figure 2). Alternatively, knowing that a majority of the particles that pass the DMA carry one single charge, each in the size range applied here,11 particle concentration measurements could be directly performed using an electrometer (route 1 in Figure 2). By stepwise scanning the voltage of DMA 1 and measuring the resulting particle concentration, size distribution measurements of the agglomerate particles were obtained. To size select and measure particle concentrations of the reshaped particles, a second DMA, labeled DMA 2 in Figure 2, was scanned in a similar fashion. The compaction behavior of the particles was examined by scanning the reshaping temperature and measuring the peak value of the size distributions for each temperature.13 Depositions of particles for further characterization were done directly onto Si substrates as well as lacey carbon film Cu TEM grids, positioned inside an electrostatic precipitator (ESP) (Figure 2). The ESP focuses charged particles onto a collector electrode14 and allows for a high-efficiency deposition of particles. The setup used allows for deposition of particles with diameters of up to 100 nm onto a spot of about 1-3 cm in diameter. Transmission electron microscopy (TEM) (JEOL, model 3000F) operated at 300 kV and equipped with a field emission gun and an X-ray energy-dispersive spectrometer (XEDS) together with scanning electron microscopy (SEM) (FEI, model Nova Nanolab 600) was used for morphological, structural, and chemical investigations. X-ray diffraction (XRD) measurements of the samples were carried out in a combined UHV/high-pressure flow reactor15 using a photon energy of 18 keV at beamline ID0316 at the European Synchrotron Radiation Facility (ESRF). The X-ray photoelectron spectroscopy measurements where performed using the ambient pressure photoelectron spectrometer at the Molecular Science beamline 11.0.217 at the Advanced Light Source (ALS) using a photon energy of 525 eV. A single-crystal Pd(100) was used as a reference for the bulk Pd 3d5/2 binding energy. The single-crystal Pd(100) surface was cleaned by cycles of Ar+ sputtering and subsequent annealing between 100 and 700 °C, keeping the crystal in 10-7 mbar of O2. After the cleaning procedure, no contaminants, such as carbon, could be detected. The binding energy was calibrated to the Fermi level, which, in the case of the pristine Pd particle samples, could easily be detected. 3. Results and Discussion 3.1. On-Line and Microscopy Characterization of the Pd Particles. Particle production by the spark discharge method was found to be a robust and simple way of producing palladium particles that can be used as a realistic model for catalysts. By
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Figure 3. Size distributions of the agglomerate particles generated at carrier flow rates of (a) 3.4 and (b) 5.9 L/min, for spark discharge frequencies between 30 and 300 Hz, measured by DMA 1.
varying the spark discharge frequency and carrier flow rate, the particle number concentration and the diameter were dramatically affected. An increase of spark discharge frequency results in an increase of particle number concentration and a shift to larger particle diameters. This is in good agreement with particles of other materials generated by spark discharge.7,10,18 Because a higher spark discharge frequency corresponds to an increase in the number of sparks per second that evaporates electrode material, a higher number concentration of particles can be reached. Furthermore, an increase in evaporated particle material leads to a higher coagulation rate and hence faster growth of particles, resulting in particles with a larger diameter. On the other hand, an increase of carrier flow rate leads to a decrease of particle number concentration and a slight shift to smaller particle diameters, which is in agreement with observations by Tabrizi et al.10 This is explained by the much shorter time available for coagulation with increased carrier flow rate. In Figure 3a,b, the particle concentration versus mobility diameter is displayed for the different spark discharge frequencies at a carrier flow rate of 3.4 and 5.9 L/min, respectively. The mobility diameter is the diameter given by the DMA measurements and does not necessarily correspond to the geometric diameter of a particle; this would only be true for spherical particles. For nonspherical particles, as measured in the scans presented here, it will allow, however, a comparison of the different production conditions. For the carrier flow rate of 3.4 L/min, the peak value of the number concentration increased from 4.8 × 105 to 1.8 × 106 cm-3 when increasing the spark discharge frequency from 30 to 300 Hz. At the carrier flow rate of 5.9 L/min, the same increase in spark discharge frequency resulted in an increased peak value of the particle number concentration from 2.3 × 105 to 8.7 × 105 cm-3. Because the spark discharge frequency of 300 Hz combined with the carrier flow rate of 3.4 L/min gave the highest particle yield, those parameters were used for all particles produced for further investigations. The increase of spark discharge frequency from 30 to 300 Hz also leads to a shift of peak particle diameter from 17 to 33 nm and from 10 to 20 nm for the carrier flow rates of 3.4 and 5.9 L/min, respectively. In agreement with measurements of gold particles produced by the same particle generation setup with nitrogen as the carrier gas,19 an increased carrier flow rate was observed to result in a decreased number concentration of particles for all spark discharge frequencies used. However, contradictory to the measurements of gold particle diameter that was almost unaffected by the same change of carrier flow rate,
a shift to smaller peak particle diameters was observed with increasing carrier flow rate for the palladium particles. This observation would need further investigations before the reason could be explained. From morphological investigations by high-resolution TEM (HRTEM), the as-produced agglomerate particles were found to consist of primary particles connected into a chainlike structure (Figure 4a). No significant difference of morphology or primary particle size was observed between particles produced at different carrier flow rates and/or spark discharge frequencies. The primary particles had diameters ranging from approximately 2 to 5 nm. In addition, XEDS measurements confirmed that the particles were actually palladium particles, within the general detection limit for XEDS of
