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Gasphase tailoring of nanoparticle properties by surface reactions
Abstracts of the European Aerosol Conference 2004
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GASPHASE TAILORING OF NANOPARTICLE PROPERTIES BY SURFACE REACTIONS A.P. WEBER Institut fuer Mechanische Verfahrenstechnik und Mechanik, Strasse am Forum 8, Universitaet Karlsruhe (TH), D-76131 Karlsruhe, Germany Keywords: catalytic activity, sintering kinetics, mechanical strength INTRODUCTION In many processes nanoparticles are produced in the gasphase, for instance when high purities are required. Further conditioning of these particles may be necessary and is most effectively done when the particles are still gasborne. In this contribution, some ways to control sintering kinetics, catalytic activity and mechanical strength of nanoparticles and their agglomerates in the aerosol state will be shown. METHODS AND RESULTS An important step in understanding and controlling nanoparticle properties is the ability to determine in situ the surface state of the gasborne particles. A powerful tool for the surface analysis of particles is Aerosol Photoemission Spectroscopy (APES) where particle work function and electron emission activty are measured on gasborne aerosol particles (Weber et al., 2001). Although the exact nature of adsorbates has to be specified with the support of additional off-line techniques, APES allows t o quantify the amount of adsorption and to differentiate between adsorption of molecules on the particles and surface reactions. The catalytic activity of aerosol particles was determined for a few rather well known reactions such as the hydrogen oxidation on Pt nanoparticles and the methanation on Ni nanoparticles. For these experiments the term ’aerosol catalysis’, which was originally introduced by Glikin (1996), applies quite well. Since the catalytic behavior of the particles was of main concern only low yields of gas phase products were investigated so that the educt concentration did not change and reaction specific parameters such as the order of reaction were irrelevant. The product concentrations were measured on-line using Fourier transformed infrared spectroscopy (FTIR) with a special aerosol flow cell. With this setup it was possible to study the reactions for contact times as low as 70 ms and to investigate poisoning kinetics on this time scale (Weber et al., 1999, Seipenbusch, 2003, Weber et al., 2003). During the methanation, which is an activated process requiring temperatures above 300 °C, partial sintering of the nanoparticles was observed. However, the sintering kinetics was found to depend critically on the carrier gas composition (Seipenbusch et al., 2003). Therefore, the sintering kinetics was studied in well defined gas atmospheres by varying kind and amount of trace gases. The influence of the gas composition on the nanoparticle size distribution and surface state was investigated by employing transmission electron microscopy (TEM), nitrogen adsorption (BET), thermogravimetric analysis (TGA) coupled to FTIR and APES. Finally, the mechanical stability of the nanoparticle agglomerates against restructuring and fragmentation was studied. While for the fragmentation a novel method was employed (see below) restructuring of nanoparticle agglomerates was investigated using a technique originally introduced by Schmidt-Ott (1988). At higher temperatures, agglomerates tend to become more compact when enough energy is provided (Weber and Friedlander, 1997). However, similar to the results for sintering, minute amounts of surface contamination may greatly enhance the activation energy for restructuring (Seipenbusch et al., 2003). The structural changes were measured with a tandem differential mobility analyzer (TDMA) setup.
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Abstracts of the European Aerosol Conference 2004
For the fragmentation studies a low pressure impactor was used. By controlling the pressure in the impaction chamber the particle impact velocity on a TEM grid was varied. Subsequently, the fraction of broken bonds was determined as a function of the impact energy by TEM image analysis (Froeschke et al., 2003). Once this technique was established, the surface state of the nanoparticles was modified by thermal treatment and photochemistry and the effects of these surface changes on the interparticle bond strength were measured. In fact, it was found that by tempering of SiO2 particles and by partial oxidation of the surface of polystyrene particles a substantial enhancement of the mechanical stability of the agglomerates was achieved. On one hand, the kinetic energy needed for 50% fragmentation increased by these treatments. On the other hand, the fraction of bonds, which were not breakable up to the maximum impact velocity attainable in the current setup, increased also. Models to understand the phenomenon of increased bonds strengths on a molecular level are in preparation. CONCLUSIONS It was found that smallest amounts of adsorbates on the surface of gasborne nanoparticles can significantly alter their behavior with respect to catalytic activity, sintering, restructuring and fragmentation. In the formation of methane on Ni nanoparticles, carbon deposits form on the particle surface and eventually graphitize at temperatures above 385°C. Submonolayers of carbon are sufficient to reduce the activity by an order of magnitude indicating that an active surface site is composed of more than one Ni atom. Also in grain boundary growth (sintering of nanoparticles) or sliding (restructuring of nanoparticle agglomerates) oxygen or carbon atoms can act as a barrier. This so called pinning effect can in turn be used to tailor the thermal and mechanical stability of agglomerated structures by the controlled addition of trace gases during particle formation. ACKNOWLEDGEMENTS The contributions of M. Seipenbusch, S. Froeschke, M. Kirchhof, H.-J. Schmid, J. Salas Vicente, S.H. Bossmann, A.M. Braun, G. Kasper and S.K. Friedlander and the financial support by the Deutsche Forschungsgemeinschaft (DFG) for parts of this work are highly appreciated. REFERENCES Froeschke, S., Kohler, S., Weber, A.P., Kasper, G. (2003). Impact fragmentation of nanoparticle agglomerates, J. Aerosol Sci., 34, 275-287. Glikin, M.A. (1996). Aerosol catalysis. Theoretical Foundations of Chemical Engineering, 30, 390-394. Schmidt-Ott, A. (1988) New approaches to in situ characterization of ultrafine agglomerates, J. Aerosol Sci., 19, 553-563. Seipenbusch, M. (2003). Katalytische und photoelektrische Aktivität gasgetragener Nanopartikel-Agglomerate, Ph.D. thesis, Institut für Mechanische Verfahrenstechnik und Mechanik, Universität Karlsruhe (TH), Karlsruhe, Germany. Seipenbusch, M., Weber, A.P., Schiel, A., Kasper, G. (2003). Influence of the gas atmosphere on restructuring and sintering kinetics of nickel and platinum aerosol nanoparticle agglomerates, J. Aerosol Sci., 34, 1699-1709. Weber, A.P., Friedlander, S.K. (1997). In situ determination of the activation energy for restructuring of nanometer aerosol agglomerates, J. Aerosol Sci., 28, 179-192. Weber, A.P., Seipenbusch, M., Thanner, Ch., Kasper, G. (1999). Aerosol catalysis on nickel nanoparticles, J. Nanoparticle Res., 1, 253-265. Weber, A.P., Seipenbusch, M., Kasper, G. (2001). Application of Aerosol Techniques to Study the Catalytic Formation of Methane on Gasborne Nickel Nanoparticles, J. Phys. Chemistry A, 105, 8958-8963. Weber, A., Seipenbusch, M., Kasper, G. (2003). Size effects in the catalytic activity of unsupported metallic nanoparticles, J. Nanoparticle Res., 5, 293-298.
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GASPHASE TAILORING OF NANOPARTICLE PROPERTIES BY SURFACE REACTIONS A.P. WEBER Institut fuer Mechanische Verfahrenstechnik und Mechanik, Strasse am Forum 8, Universitaet Karlsruhe (TH), D-76131 Karlsruhe, Germany Keywords: catalytic activity, sintering kinetics, mechanical strength INTRODUCTION In many processes nanoparticles are produced in the gasphase, for instance when high purities are required. Further conditioning of these particles may be necessary and is most effectively done when the particles are still gasborne. In this contribution, some ways to control sintering kinetics, catalytic activity and mechanical strength of nanoparticles and their agglomerates in the aerosol state will be shown. METHODS AND RESULTS An important step in understanding and controlling nanoparticle properties is the ability to determine in situ the surface state of the gasborne particles. A powerful tool for the surface analysis of particles is Aerosol Photoemission Spectroscopy (APES) where particle work function and electron emission activty are measured on gasborne aerosol particles (Weber et al., 2001). Although the exact nature of adsorbates has to be specified with the support of additional off-line techniques, APES allows t o quantify the amount of adsorption and to differentiate between adsorption of molecules on the particles and surface reactions. The catalytic activity of aerosol particles was determined for a few rather well known reactions such as the hydrogen oxidation on Pt nanoparticles and the methanation on Ni nanoparticles. For these experiments the term ’aerosol catalysis’, which was originally introduced by Glikin (1996), applies quite well. Since the catalytic behavior of the particles was of main concern only low yields of gas phase products were investigated so that the educt concentration did not change and reaction specific parameters such as the order of reaction were irrelevant. The product concentrations were measured on-line using Fourier transformed infrared spectroscopy (FTIR) with a special aerosol flow cell. With this setup it was possible to study the reactions for contact times as low as 70 ms and to investigate poisoning kinetics on this time scale (Weber et al., 1999, Seipenbusch, 2003, Weber et al., 2003). During the methanation, which is an activated process requiring temperatures above 300 °C, partial sintering of the nanoparticles was observed. However, the sintering kinetics was found to depend critically on the carrier gas composition (Seipenbusch et al., 2003). Therefore, the sintering kinetics was studied in well defined gas atmospheres by varying kind and amount of trace gases. The influence of the gas composition on the nanoparticle size distribution and surface state was investigated by employing transmission electron microscopy (TEM), nitrogen adsorption (BET), thermogravimetric analysis (TGA) coupled to FTIR and APES. Finally, the mechanical stability of the nanoparticle agglomerates against restructuring and fragmentation was studied. While for the fragmentation a novel method was employed (see below) restructuring of nanoparticle agglomerates was investigated using a technique originally introduced by Schmidt-Ott (1988). At higher temperatures, agglomerates tend to become more compact when enough energy is provided (Weber and Friedlander, 1997). However, similar to the results for sintering, minute amounts of surface contamination may greatly enhance the activation energy for restructuring (Seipenbusch et al., 2003). The structural changes were measured with a tandem differential mobility analyzer (TDMA) setup.
S722
Abstracts of the European Aerosol Conference 2004
For the fragmentation studies a low pressure impactor was used. By controlling the pressure in the impaction chamber the particle impact velocity on a TEM grid was varied. Subsequently, the fraction of broken bonds was determined as a function of the impact energy by TEM image analysis (Froeschke et al., 2003). Once this technique was established, the surface state of the nanoparticles was modified by thermal treatment and photochemistry and the effects of these surface changes on the interparticle bond strength were measured. In fact, it was found that by tempering of SiO2 particles and by partial oxidation of the surface of polystyrene particles a substantial enhancement of the mechanical stability of the agglomerates was achieved. On one hand, the kinetic energy needed for 50% fragmentation increased by these treatments. On the other hand, the fraction of bonds, which were not breakable up to the maximum impact velocity attainable in the current setup, increased also. Models to understand the phenomenon of increased bonds strengths on a molecular level are in preparation. CONCLUSIONS It was found that smallest amounts of adsorbates on the surface of gasborne nanoparticles can significantly alter their behavior with respect to catalytic activity, sintering, restructuring and fragmentation. In the formation of methane on Ni nanoparticles, carbon deposits form on the particle surface and eventually graphitize at temperatures above 385°C. Submonolayers of carbon are sufficient to reduce the activity by an order of magnitude indicating that an active surface site is composed of more than one Ni atom. Also in grain boundary growth (sintering of nanoparticles) or sliding (restructuring of nanoparticle agglomerates) oxygen or carbon atoms can act as a barrier. This so called pinning effect can in turn be used to tailor the thermal and mechanical stability of agglomerated structures by the controlled addition of trace gases during particle formation. ACKNOWLEDGEMENTS The contributions of M. Seipenbusch, S. Froeschke, M. Kirchhof, H.-J. Schmid, J. Salas Vicente, S.H. Bossmann, A.M. Braun, G. Kasper and S.K. Friedlander and the financial support by the Deutsche Forschungsgemeinschaft (DFG) for parts of this work are highly appreciated. REFERENCES Froeschke, S., Kohler, S., Weber, A.P., Kasper, G. (2003). Impact fragmentation of nanoparticle agglomerates, J. Aerosol Sci., 34, 275-287. Glikin, M.A. (1996). Aerosol catalysis. Theoretical Foundations of Chemical Engineering, 30, 390-394. Schmidt-Ott, A. (1988) New approaches to in situ characterization of ultrafine agglomerates, J. Aerosol Sci., 19, 553-563. Seipenbusch, M. (2003). Katalytische und photoelektrische Aktivität gasgetragener Nanopartikel-Agglomerate, Ph.D. thesis, Institut für Mechanische Verfahrenstechnik und Mechanik, Universität Karlsruhe (TH), Karlsruhe, Germany. Seipenbusch, M., Weber, A.P., Schiel, A., Kasper, G. (2003). Influence of the gas atmosphere on restructuring and sintering kinetics of nickel and platinum aerosol nanoparticle agglomerates, J. Aerosol Sci., 34, 1699-1709. Weber, A.P., Friedlander, S.K. (1997). In situ determination of the activation energy for restructuring of nanometer aerosol agglomerates, J. Aerosol Sci., 28, 179-192. Weber, A.P., Seipenbusch, M., Thanner, Ch., Kasper, G. (1999). Aerosol catalysis on nickel nanoparticles, J. Nanoparticle Res., 1, 253-265. Weber, A.P., Seipenbusch, M., Kasper, G. (2001). Application of Aerosol Techniques to Study the Catalytic Formation of Methane on Gasborne Nickel Nanoparticles, J. Phys. Chemistry A, 105, 8958-8963. Weber, A., Seipenbusch, M., Kasper, G. (2003). Size effects in the catalytic activity of unsupported metallic nanoparticles, J. Nanoparticle Res., 5, 293-298.