Analysis of flame-formed organic nanoparticles by UV laser photoionization measurements (Abstract/Comunicazione in atti di convegno)

Type
Label
  • Analysis of flame-formed organic nanoparticles by UV laser photoionization measurements (Abstract/Comunicazione in atti di convegno) (literal)
Anno
  • 2013-01-01T00:00:00+01:00 (literal)
Alternative label
  • M. Commodo, P. Minutolo, A. D'Anna (2013)
    Analysis of flame-formed organic nanoparticles by UV laser photoionization measurements
    in COST action CM0901 Soot and PAHs Joint Working Group Meeting, Hilton Sorrento Palace Sorrento, Italy., April 10-12, 2013
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • M. Commodo, P. Minutolo, A. D'Anna (literal)
Note
  • Comunicazione (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#affiliazioni
  • Istituto di Ricerche sulla Combustione, CNR, P.le Tecchio 80, 80125, Napoli, Italy Dip. Ingegneria Chimica dei Materiali e della Produzione Industriale - Università Federico II, P.le Tecchio 80, 80125, Napoli, Italy (literal)
Titolo
  • Analysis of flame-formed organic nanoparticles by UV laser photoionization measurements (literal)
Abstract
  • Particle inception in flames is a very complex phenomenon involving: gas-phase free radical reactions, particle nucleation through polymerization and/or clustering pathways, particle growth by both heterogeneous gas-to-solid reactions and physical coagulation and/or coalescence processes, particles annealing and oxidation. To control particle formation, for both emission reduction and/or particle synthesis, the full understanding of the complex chemical-physical processes occurring in the flame reactor is required. In the recent years, the understanding of mechanisms leading to the formation of incipient organic nanoparticles in flames has attracted the interest of the combustion research community not only because of their role as soot precursors but also as possible constituent of the combustion aerosol emissions [1]. Photoelectric charging of particles is a powerful tool for the on-line characterization of submicron aerosol particles [2]. Photoionization based techniques have high sensitivity and chemical selectivity. Furthermore, photoemission studies yield information on the electronic properties of the investigated compounds, i.e. valence and conduction bands, and are very sensitive to the surface composition of aerosol particles [2]. In the present study an UV laser beam coupled to a differential mobility analyzer (DMA) has been used to measure the photoionization efficiency of carbon nanoparticles produced in flames. Ultrafine particles have been sampled on-line from well characterized laboratory laminar flames and size selected particles have been investigated in terms of their propensity to be ionized by interaction with UV photons with energy of 5.82 eV given by the fifth harmonic of a Nd:YAG laser at the wavelength of 213 nm. The size range of the investigated particles is 2-10 nm. Such particles are representative of the organic nanoparticles produced in slightly sooting flames across the soot inception threshold. The aim of this work is to gain further insights into the chemical/physical properties of the organic nanoparticles and early soot nuclei formed in combustion systems. This on-line aerosol based technique offers the advantage, over conventional off-line methods, to gain information on the pristine optical and electronic properties of the particles without interference of substrate interaction effects, condensation of molecules on the particle surface, and/or particles coagulation. Ethylene/air flames were stabilized on a McKenna burner, with a cold gas velocity of the unburned premixed gasses of 10 cm/s. Carbonaceous nanoparticles where collected by keeping constant the height above the burner (HAB), and changing the fuel/air equivalence ratio, from ?=1.73 (C/O=0.57) to ?=2.03 (C/O=0.67). Particles were sampled through a small orifice into a dilution tube probe operated with N2 as the diluent. Collected particles, were passed through an electrostatic precipitator to remove the ions in the gas stream before the interaction with the light source occurs. The remaining neutral particles entered the photoionization cell and were irradiated by the fifth harmonic of a Nd:YAG laser (?0=213 nm, h?=5.82 eV photon energy). The aerosol was then sent to the classifier of the DMA operated with a constant voltage in order to select single size particles and finally measured by electrometer. The number of neutral species was instead measured by substituting the ionization source with a commercial bipolar charger. Once both the number of neutral (N0) and photoionized particles (N+PI) were measured, the photoionization charging efficiency (CE) is obtained by dividing the number of the particle charged via photoionization by the number of neutral particles. Particle size distribution (PSDs) were measured by positioning the sampling probe at the HAB=15 mm. Depending on the flame equivalence ratio three different particle modes were measured, i.e. centered at 2.42 nm, 4.94 nm, and 10.58 nm. An example of PSD for the C/O=0.67 flame is reported in Fig.1a. The fraction of the particles with a fixed diameter, related to each of the modes, photoionized by the light beam were measured as function of the laser power density, I. A typical result, obtained by measuring selected particles with diameter of 2.42 nm, is shown in Fig. 1b. At low laser power density, usually below 0.5-0.6 MW/cm2, a linear trend is observed, indicating a proportionality between number of photo-ionized particles and number of incident photons. Such a result, therefore, is a clear evidence of a single photon photoionization process. Moreover, linearity of the photoionization CE as function of the laser power density was observed for all the investigated carbonaceous nanoparticles. Therefore, these experimental evidences allow to conclude that such particles have a photothreshold or ionization potential lower or at most equal to the energy of the used photon, thus IP<=5.8eV. Photofragmentation and/or saturation Single-photon ionization C/O=0.67 (a) (b) (c) (d) 0.57 0.61 0.63 0.65 0.67 0.57 0.61 0.63 0.65 0.67 Fig. 1: (a) Particle size distribution; (b) photoionization charging efficiency as function of laser power density; (c) and (d) particle photoelectric yields. The particle photoelectric yield, yparticle, i.e. the probability of electron emission per incident photon per particle, can be derived by the measure of particle photoionization efficiency CE by dividing by the photon flux, I. In Fig.1c and 1d, the yparticle measured for all the investigated particles is plotted for comparison. Increasing the C/O ratio a clear increase of yparticle for all the three classes of investigated particles, D=10.58 nm, 4.94 nm, and 2.42 nm is observed. Particularly evident is the case of particles in the first mode, d=2.42, as shown in Fig.1d. It is interesting to note, that for this latter class of particles, the yparticle is lower when they are produced and collected from flames whose PSD is uni-modal, e.g. C/O=0.57, 0.61, and 0.63 and increases when bi-modal PSD are generated, e.g. C/O=0.65 and 0.67. Variations in the yparticle is an indication of the particles chemical transformations. Due to the higher electron delocalization, the larger the aromatic structures are, the lower is the photoionization potential. Therefore, the data showed in the Fig.s 1c and 1d may be consistent with the assumption that in rich flame conditions, the high fuel concentration permits the building-up of macromolecular structures, particles in the nucleation mode, characterized by more extended aromatic functional groups. As a result, this technique results to be very sensitive to the changes in the chemical-physical properties of the flame-formed nanoparticles and demonstrate to have the potential to be used, for example, as fingerprint method for combustion aerosol. Future work by using tunable UV light sources may certainly lead to more useful information to link particle electronic properties to the possible different structures. [1] Bockhorn H., D'Anna A., Sarofim A.F., Wang H. (Eds.), \"Combustion Generated Fine Carbonaceus Particles\" KIT Scientific Publishing, Karlsruhe, 2009. [2] Burtscher, H., J. Aerosol Sci. 23(6): 549-595 (1992). (literal)
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