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ABSTRACT: A new type of detector which we call the Catalytic Activity Aerosol Monitor (CAAM) was investigated towards its capability to detect traces of commonly used industrial catalysts in ambient air in quasi real time. Its metric is defined as the catalytic activity concentration (CAC) expressed per volume of sampled workplace air. We thus propose a new metric which expresses the presence of nanoparticles in terms of their functionality - in this case a functionality of potential relevance for damaging effects - rather than their number, surface, or mass concentration in workplace air. The CAAM samples a few micrograms of known or anticipated airborne catalyst material onto a filter first and then initiates a chemical reaction which is specific to that catalyst. The concentration of specific gases is recorded using an IR sensor, thereby giving the desired catalytic activity. Due to a miniaturization effort, the laboratory prototype is compact and portable. Sensitivity and linearity of the CAAM response were investigated with catalytically active palladium and nickel nano-aerosols of known mass concentration and precisely adjustable primary particle size in the range of 3-30nm. With the miniature IR sensor, the smallest detectable particle mass was found to be in the range of a few micrograms, giving estimated sampling times on the order of minutes for workplace aerosol concentrations typically reported in the literature. Tests were also performed in the presence of inert background aerosols of SiO2, TiO2, and Al2O3. It was found that the active material is detectable via its catalytic activity even when the particles are attached to a non-active background aerosol.
Annals of Occupational Hygiene 03/2013; · 1.95 Impact Factor
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ABSTRACT: To study the pure catalytic activity of metallic nanoparticles, the formation of methane on gasborne Ni nanoparticles, so called aerosol catalysis experiments, were performed. Beside effects typical for the methanation such as poisoning of the particle surface at temperatures above 385°C, the maximum of the catalytic activity was observed for Ni particles of about 14nm, i.e. in a size range, which is quite uncommon for typical nanoeffects of metallic particles. To clarify, which catalytic phenomena are related to the aerosol state, the same reaction was performed on supported Ni nanoparticles, which were also generated and conditioned in the gas phase and deposited on a SiO2 surface by thermophoresis. For these supported particles, the same reaction conditions were established as before for the gasborne Ni nanoparticles. However, differences in the mass transport characteristics of educt and product molecules to the particles were encountered and led to lower overall reaction rates. While qualitatively poisoning kinetics and activation energies agreed for both cases, significant differences were observed for the size dependence of the catalytic activity and for the sintering kinetics. The observed shift of the optimum size for the methanation from 14nm (aerosol) to 25nm (on support) can be explained by different adsorption enthalpies of the educt gases on aerosol and supported Ni nanoparticles, respectively.
Journal of Nanoparticle Research 04/2012; 8(3):445-453. · 3.29 Impact Factor
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ABSTRACT: Starting point for this study was the experimentally found unusual high charge state of free submicron particles at elevated
temperatures. By employing an aerosol setup with single electron counting efficiency it is shown for Pt and TiO2 particles that the work function decreases with increasing temperature. Sign and amount of the work function decrease agree
with the results of other experiments on extended surfaces, single crystals and clusters. On the basis of the reduced work
function the observed high particle charge states are quantitatively understood.
Applied Physics A 04/2012; 95(3):629-634. · 1.63 Impact Factor
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ABSTRACT: We report the synthesis of composite nanoparticles by an integrated CVS/CVD process at atmospheric pressure. Iron oxide and silica support particles were generated by chemical vapour synthesis (CVS), using Fe(CO)5 and Si(OC2H5)4 and were directly coated in the aerosol state with molybdenum oxide by chemical vapour deposition of Mo(CO)6. Depending on the CVS temperature hematite (600 degrees C) or maghemite (1500 degrees C) iron oxide phases were determined by XRD and FTIR. Core-shell structures with a coating thickness in the lower nm range were obtained for CVD temperatures below 150 degrees C. Complete encapsulation of the core particles and uniform elemental distribution is shown by TEM and EELS measurements. Higher CVD temperatures lead to unwanted homogenous decomposition of the molybdenum precursor. Additional aerosol temperature treatment was used to reach further oxidation and the formation of a mixed oxide shell, indicated by FTIR measurements. The results show the potential of the process for the synthesis of structured core-shell nanoparticles.
Journal of Nanoscience and Nanotechnology 09/2011; 11(9):8313-7. · 1.56 Impact Factor
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ABSTRACT: The decomposition kinetics of the organometallic precursor cyclopentadienyl-allyl-Pd [Cp(allyl)Pd] on silica and titania substrates (as well as previously deposited Pd) are investigated by continuous metal-organic (MO)CVD under atmospheric pressure. The substrate particles are freshly generated in an aerosol process with well-defined and known densities of surface OH groups, and then coated immediately in a second reactor. Time-resolved data on size and number density of deposited Pd nanodots on the carrier particles are obtained by electron microscopy and used as a means to infer the decomposition kinetics. It is found that decomposition proceeds very rapidly for a few seconds, both by proton-initiated decomposition on surface OH groups and by autocatalysis, until the Pd nanodots have grown to a size of about 2 nm, where autocatalysis comes to a standstill, regardless of precursor concentration, residence time in the CVD reactor, or decomposition temperature. Proton-initiated decomposition on OH groups continues to depend measurably on these process parameters in terms of the number density of Pd dots, even after autocatalysis has stopped. Our observations indicate that autocatalysis is structure-sensitive.
Chemical Vapor Deposition 02/2011; 17(1‐3):54 - 57. · 1.80 Impact Factor
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ABSTRACT: Initial sintering rates of nearly monodisperse Pd model catalyst particles on silica, titania, and mixtures thereof were measured as a function of temperature on the time scale of a few seconds in the gas phase. Support particles and active phase were synthesized by aerosol based techniques permitting a high degree of experimental control. It was found that Pd particles sintered more slowly on silica than on titania. This is attributed to the higher surface roughness of the SiO2. Adding a small amount of the second oxide phase to either silica or titania increased the surface roughness and thus also the thermal stability of the Pd. This effect was more pronounced for titania. High amounts of the second oxide phase did not further enhance the stability, because phase separation occurred and core−shell structures with similar surface morphologies were formed in the carriers.
04/2010;
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ABSTRACT: With the increasing utilization of engineered nanomaterials (ENM), the potential exposure of workers to ENM is likely to increase significantly. Very little is known though, of the risks posed by ENM to human health, in particular concerning those characteristics that are technologically attractive: small size, high surface to mass ratio, and surface reactivity. ENM risk assessment is hampered by a lack of exposure as well as toxicity data specific to the multitude of ENM being developed. An economical approach to this problem urgently calls for intelligent testing strategies to capture essential features of ENM, thereby allowing over-arching ENM risk assessment. The data gaps of ENM risk assessment include (1) ENM aerosol standards and agreement on ENM key metrics; (2) dependable exposure scenarios, affordable monitoring technologies, exposure data and models; and (3) biomedical data on ENM translocation and toxicity, and associated testing strategies (which must be linked to the exposure scenarios). The special features of ENM do not, however, create a need to amend the current overall approach to the risk assessment of chemicals.
Toxicology 03/2010; 269(2-3):92-104. · 3.68 Impact Factor
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ABSTRACT: Aerosols of submicron polystyrene particles were oxidized by either vacuum-ultraviolet (VUV) irradiation in the presence of molecular oxygen (O(2)) and/or by ozone (O(3)). Different degrees of oxidation and oxidative degradation were reached by VUV-photolysis depending on radiant energy, O(2) and H(2)O concentrations in the bulk gas mixture as well as on particle diameter. The same functionalization was obtained by exposing the aerosol to O(3), however, oxidation, in particular oxidative degradation, was less efficient. The evolution of hydroxyl and carbonyl functions introduced was quantified by ATR-FTIR spectroscopy of filtered particles, and oxidative degradation of the polymer particles was confirmed by determining size and number of aerosol particles before and after oxidation. Efficiency analyses are based on the results of an O(3) actinometry and on an evaluation of the rate of absorbed photons by the aerosol particles in function of their size.
Photochemical and Photobiological Sciences 08/2009; 8(7):944-52. · 2.58 Impact Factor
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ABSTRACT: Due to improved quantification capabilities and enhanced signal-to-noise ratio (SNR), phase-corrected real reconstruction in magnetic resonance imaging is superior to the common magnitude reconstruction, especially at low SNR. This requires the development of an automated phase-correction algorithm. Existing methods are not well suited for multiple unconnected regions of very low SNR. For this situation, a method based on the real-signal maximization is implemented, in which the experimental image phase is approximated by a three-dimensional polynomial of up to third order. The presented implementation was successfully applied to data originating from different samples and pulse sequences.
Magnetic Resonance Imaging 09/2008; 27(3):393-400. · 1.99 Impact Factor
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ABSTRACT: The potential of diesel soot aerosol particles to break up into smaller units under mechanical stress was investigated by a direct impaction technique which measures the degree of fragmentation of individual agglomerates vs. impact energy. Diesel aerosol was generated by an idling diesel engine used for passenger vehicles. Both the aerosol emitted directly and aerosol that had undergone additional growth by Brownian coagulation ("aging") was investigated. Optionally a thermo-desoption technique at 280 degrees C was used to remove all high-volatility and the majority of low-volatility HC adsorbates from the aerosol before aging.
It was found that the primary soot agglomerates emitted directly from the engine could not be fragmented at all. Soot agglomerates permitted to grow additionally by Brownian coagulation of the primary emitted particles could be fragmented to a maximum of 75% and 60% respectively, depending on whether adsorbates were removed from their surface prior to aging or not. At most, these aged agglomerates could be broken down to roughly the size of the agglomerates from the primary emission. The energy required for a 50% fragmentation probability of all bonds within an agglomerate was reduced by roughly a factor of 2 when aging "dry" agglomerates. Average bond energies derived from the data were 0.52*10-16 and 1.2*10-16 J, respectively. This is about 2 orders of magnitude higher than estimates for pure van-der-Waals agglomerates, but agrees quite well with other observations.
Although direct conclusions regarding the behavior of inhaled diesel aerosol in contact with body fluids cannot be drawn from such measurements, the results imply that highly agglomerated soot aerosol particles are unlikely to break up into units smaller than roughly the size distribution emitted as tail pipe soot.
Particle and Fibre Toxicology 02/2008; 5:9. · 7.25 Impact Factor
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ABSTRACT: The detachment of barrel-shaped oil droplets from metal, glass and polymer fibres was examined using an atomic force microscope (AFM). The AFM was used to detach the droplets from the fibres while measuring the force-distance relationship. A novel fibre-droplet interfacial tension model was applied to predict the force required to draw the droplet away from its preferential axisymmetric position on the fibre, and also to predict the maximal force required to detach the droplet. The model assumes that the droplet retains a spherical shape during detachment, i.e., that droplet distortion is negligible. This assumption was found to be reasonably accurate for small radius oil droplets (<10 microm), however less accurate for larger droplets (>25 microm). However, it was found that the model produced a good agreement with the maximal detachment force measured experimentally--regardless of droplet size and degree of deformation--even though the model could not predict droplet extension beyond a length of one droplet radius.
Journal of Colloid and Interface Science 08/2007; 312(2):333-40. · 3.07 Impact Factor
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Aerosol Science and Technology 03/2007; 41(3):277-283. · 2.67 Impact Factor
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ABSTRACT: Applying MRI techniques to low-density fibrous filter media provides us with unique information about the initial structure and deposited mass within the same filter sample. This now enables us to obtain the necessary link between structure and deposition for validation and further enhancement of modeling filtration kinetics. However, additional work is needed before achieving a realistic understanding of filtration kinetics.
Magnetic Resonance Imaging 03/2005; 23(2):341-2. · 1.99 Impact Factor
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Chemie Ingenieur Technik 02/2004; 70(1‐2):115 - 119. · 0.59 Impact Factor
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ABSTRACT: The influence of the size of nanoparticles on their catalytic activity was investigated for two systems on unsupported, i.e. gasborne nanoparticles. For the oxidation of hydrogen on Pt nanoparticle agglomerates, transport processes had to be taken into account to extract the real nanoparticle size effects. The results indicate an optimum particle size for the catalytic activity below 5nm which points clearly toward a real volume effect. In the case of the methanation reaction on gasborne Ni nanoparticles, no transport limitations were observed and the product concentration was directly proportional to the activity of the primary particles. We found an activity maximum for particles of about 19nm in diameter. This size is too large to be attributed to a real nanoparticle size effect induced by the electronic band structure. Therefore, we concluded that the particle size influences the adsorption behavior of the carbon monoxide molecules. In fact, it is known that intermediate adsorption enthalpies may favor dissociation processes, which is an essential step for the reaction, as manifested in the so called volcano-shaped curve. Then, in addition to the material dependence of the adsorption, we would also encounter a direct size dependence in the case of methanation on gasborne Ni nanoparticles.
Journal of Nanoparticle Research 07/2003; 5(3):293-298. · 3.29 Impact Factor
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ABSTRACT: Thermal charging of submicron and nanometer particles has been studied for model aerosols of TiO2 and SiO2 as well as Al-Si (aluminosilicate) at 1 000 °C with a new quasi in-situ technique. The size dependence of the particle separation efficiency for electrostatic precipitation was determined. The charging state of the particles was obtained from evaluating the global Deutsch number for precipitation in an electric field applied to a laminar flow based on particle trajectory considerations.
Particle and Particle Systems Characterization 12/2002; 19(6):410 - 418. · 0.49 Impact Factor
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ABSTRACT: “Aerosol catalysis” is shown to be a powerful tool for investigating the catalytic properties of freshly formed nanoparticles in situ and without substrate interference. The method is first outlined conceptually, followed by an illustrative application to the catalytic formation of methane on a nickel nanoaerosol. Reaction order and activation energy were found conform with generally accepted values from supported Ni catalysts. The TOR decreases strongly during the first 10 s as the reaction proceeeds toward a steady value. The decrease correlates with a buildup of about 0.3 monolayer equivalents of carbon on the particle surface measured by TGA and a decline in particle photoelectric activity observed via measurement by aerosol photoemission spectroscopy (APES). APES is shown to be capable of detecting the progressive degradation of the freshly formed particle surface due to a heterogeneous surface reaction on a millisecond time scale. Furthermore, it was possible to induce order-of-magnitude changes in TOR via defined changes in particle morphology, induced by aerosol restructuring techniques preceding exposure to the catalytic reaction.
09/2001;
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ABSTRACT: Collision and electrostatic dispersion rates of airborne submicrometer TiO(2) agglomerates were measured and compared with the classical collision theory for spheres as well as with models accounting for the agglomerate structure in terms of the fractal dimension and electrostatic effects such as Coulomb and van der Waals interactions. According to the authors' knowledge, this is the first time that the agglomerate fractal dimension and electrostatic effects have been considered simultaneously in determining the collision frequency function of agglomerates. The observed enhancement in the collision frequency of agglomerates was found mainly to be a result of electrostatic particle interactions. Nonspherical particle shape has only a comparatively small influence on the collision probability, on the order of 10-20%. Electrostatic dispersion coefficients of agglomerates were found to be similar to those of spheres. Copyright 2001 Academic Press.
Journal of Colloid and Interface Science 09/2001; 240(1):67-77. · 3.07 Impact Factor
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ABSTRACT: Nickel nanoparticles produced by spark discharges were used as aerosol catalyst for the formation of methane. The available surface area of the particles was determined using different methods. It was found that the surface area available for nitrogen adsorption and, therefore, for the methanation reaction remained virtually constant during restructuring of the agglomerates while the surface area based on the mobility was significantly reduced. In general, the reaction parameters such as activation energy and reaction rates agree well with the values for single nickel crystals and foils. At temperatures above 350C the activation energy and the photoelectric activity of the particles decrease indicating the formation of graphite on the particle surface. Also the change of the work function points to the build up of multiple layers of graphite on the particle surface. The surprisingly low temperature for the surface deactivation may indicate an enhanced formation of carbon atoms at the surface.
Journal of Nanoparticle Research 05/1999; 1(2):253-265. · 3.29 Impact Factor
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ABSTRACT: This work examines the wetting of a range of low packing density (high porosity), porous, fibrous media by mineral oils. The fibrous media were suspended above a vessel of oil, and the rate of imbibation of oil was measured by means of a balance under the oil and a load cell above the media. It was found that the height of the oil column within the media over time generally resembled the classical capillary rise curve. Capillary rise models were fitted to the data to predict an equivalent capillary diameter, dynamic contact angle, and height of the liquid column as time approaches infinity, using the known filter and oil properties. Different capillary models were examined, and it was found that the modified Washburn equation was the simplest model to use and produced an acceptable agreement between theory and experiment. It was found that the adjustment phase of the curve from the fibrous media was generally more rapid than for a typical capillary, presumably since the fibrous media can be wetted not only from directly below (as with a capillary), but also through the meniscus at the sides of the media. It was found that a linear correlation existed between the effective capillary diameter of the media, and the packing density of the media divided by the fibre diameter. The results appear to be relatively independent of the material type. These results allow capillary diameters to be determined for fibrous media from easily measured parameters, without the requirement of conducting wetting experiments or ‘bubble-point’ tests.
Chemical Engineering Science.
