Analysis of Flame-Formed Organic. Photoionization Measurements

Transcription

1 Analysis of Flame-Formed Organic Nanoparticles by UV Laser Photoionization Measurements Mario Commodo Istituto di Ricerche sulla Combustione, CNR, P.le Tecchio, 80, 80126, Napoli, Italy Patrizia Minutolo Istituto di Ricerche sulla Combustione, CNR, P.le Tecchio, 80, 80126, Napoli, Italy Andrea D Anna Andrea D Anna Dipartimento di Ingegneria Chimica dei Materiali e della Produzione Industriale - Università di Napoli Federico II, P.le Tecchio 80, 80125, Napoli, Italy

2 Soot formation in combustion Since 1994 about Particle Inception Num mber nucleation NOCs, PNPs Non-intrusive methods: UV/Visible extinction and fluorescence, Small angle X-ray scattering, Small angle neutron scattering, Fluorescence polarization anisotropy, Primary Dynamic soot particles light scattering; Soot formation in homogeneous mixtures (premixed flames) Bockhorn H. Soot formation in combustion: mechanisms and models (1994) Intrusive, online methods: Nano-scanning mobility particle sizing, Particle mass spectrometry; Intrusive, offline techniques: Atomic force microscopy, Electron microscopy, Size exclusion chromatography, Laser desorption mass spectrometry, Fourier transform infrared spectroscopy, Surface enhanced Raman spectroscopy D, nm

3 Soot formation in combustion NOCs: 2 3 nm molecular-like character high optical band gap: 2-3 ev UV light absorption fluorescence emission i high H/C ratio low coagulation efficiency at flame temperature hydrophilic Soot: nm NOCs Soot solid-like like character low optical band gap: ev UV-vis light absorption incandescence emission low H/C ratio higher coagulation efficiency at flame temperature hydrophobic

4 NOCs: chains vs. stacks model Temperature Concentration and characteristic of gaseous precursors Fuel Flame: Premixed vs. diffusive Flame stoichiometry: early inception vs. persistent

5 Photoemission processes Photoemission Photo-detachment Conduction band (LUMO) Photo-ionization Valence band (HOMO) Photoelectron spectroscopy (UPS) Photoionization mass spectrometry (PIMS) Aerosol Photoionization (PI-DMA)

6 Photoionization Vacuum h ionization potential (Φ) E h E E K electron kinetic i energy Conduction band (LUMO) UV radiation hν Eg e - Ionization potential Valence band (HOMO) Characteristic of the chemical/physical properties of the compounds Benzene Naftalene Pyrene Ovalene Graphite 9.24 ev ev ev ev 4.4 ev

7 Photoionization OH H3 C OH O OH OH O H3 C O OH OH CH3? 5.82 ev 213 nm NOCs ~ 4.4 ev Soot P. Desgroux et al. / Proceedings of the Combustion Institute 34 (2013)

8 Aerosol Photoionization 1980 ETH Zürich Schmidt-Ott A.; Schurtenberger, P.; Siegmann, H. C. Enormous Yield of Photoelectrons from Small Particles. Physical Review Letters H. Burtsher and co-authors ( ) R. Niessner and co-authors ( )

9 Experimental set-up FCE Flames F1 F2 F3 F4 F5 (E) N + PI(D) C/O ratio Equivalence ratio (Φ) DMA Cold gas velocity (cm/s) Probe position (mm) Quartz windows (D) N + PI PSD M: a M: a M: a B: a, b B: a, b Nd:YAG 213 nm (5.8 ev) Photo-ionization cell Joule meter Charging efficiency (CE): (C) N 0 Fuchs CE N N 0 I yparticle Particle photoelectric yield (y particles ): number of emitted photonelectrons per particle per unit laser power density N 2 HAB Inlet hole (diluition ratio 1/1000) ESP Stabilizing plate (B) (A) Am-241 diffusioni charger N 0 flame N + flame N - flame N 0 Fuchs N + Fuchs N - Fuchs

10 Results (a) (F1) Single photon ionization (F2) y particella Photo-fragmentation and/or ions/electrons recombination (F3) (b) (F4) (b ) (F5) Commodo et al./j. Phys. Chem. A (2013) 213 nm (5.8 ev) Single photon ionization Slope Photoelectric yield (y paticle )

11 Results Flames Particle modes F1 F2 F3 F4 F5 a 0.91± ± ± ± ±0.1 b 1.1± ±0.05 b 0.68±0.08 P ( I ) PI n 1 IP 5. 82eV n

12 Results Vacuum level Ec Ev Eg ~ ev Eg ~ ev E, ev 2-4 nm Cold gas flow (C 2 H 4 /Air) stacks OH H3C HO O HO O HO H3C O OH CH3 HO chains 2-4 nm Flame front Molecules Nucleating particles Soot Flame residence time

13 Results (a) (b) Three particle sizes are investigated Particles formed in richer flames have higher photoionization efficiency Commodo et al./j. Phys. Chem. A (2013) This results may be consistent with a larger extension of the aromatic islands in the formed nanoparticles as a result of the increased equivalence ratio

14 Conclusions An aerosol based experiment has been designed to measure photoionization properties of size-selected inception and grown particles formed in ethylene/air premixed flames; Particles have been selected by their electrical mobility in order to investigate species in the various modes of the size distribution. ib i Specifically, the inception i mode was centered in the nm size range, and the grown ones in the size range 3-6 nm, and 6-12 nm; Laser light at 213 nm has been used to photoionize the flame formed nanoparticles.

15 Conclusions A single photon ionization process was observed for all of the investigated conditions. This is an indication of the fact that these organic nanoparticles posses an ionization threshold below or at least equal to the energy of the employed photons, i.e ev; Since the ionization threshold corresponds to the energy of the HOMO level of the compound, this information, if coupled to the measure of the particle optical gap, allows the determination of the electronic properties, valence and conduction levels; The particle photoelectric yield, or CE/I, for size selected nanoparticles, increases by increasing C/O ratio. This may indicate a change in the chemical composition of the organic nanoparticles il and possibly a larger extension of the aromatic il island in the particles formed by richer flame conditions; Photoionization yield results to increase by increasing particle size. Thanks for your attention!