Size-Selected Nanoparticle Chemistry:  Kinetics of Soot Oxidation

Abstract

A new experimental method has been developed to conduct surface chemistry and extract surface kinetic rates on size-selected nanoparticles. The method utilizes a tandem differential mobility analyzer (TDMA) technique in which monodisperse particles are selected from a polydisperse aerosol input stream and then subjected to chemical processing. The change in particle size is measured and used to determine kinetic information for the relevant surface reaction. The method has been applied to measure the oxidation rate of soot in air over the temperature range 800-1120°C. Soot was generated in situ using an ethylene diffusion flame and sent to a differential mobility analyzer (DMA) to extract monodisperse particles. Three initial particle sizes of 40, 93, and 130 nm mobility diameter were subjected to oxidation in a high-temperature flow reactor, and the resulting change in particle size was measured with a second DMA. The measured size decreases were fit to a model utilizing a modified Arrhenius expression for the rate of decrease: Ḋ p ) -A nm T 1/2 exp(-E a /(RT)). The fit yielded an activation energy of E a ) 164 kJ mol -1 with a different preexponential factor, A nm , for each initial particle size. The size-decrease rates, and therefore the preexponential factors, differed by a factor of 1.7 between the 40 and 130 nm particles, with the 130 nm particles decreasing faster than the 40 or 93 nm particles. This may be the result of several factors including different effective densities or different soot particle compositions. The current experiments are the first measurements of the soot oxidation rate to be performed on size-selected, freshly generated soot particles using online aerosol techniques. Our results agree well with previous work over the temperature range covered, which is somewhat surprising given the wide range of techniques and materials previously studied in the effort to understand soot oxidation.

Investigation on Particulate Oxidation from a DI Diesel Engine Fueled with Three Fuels

“…Figure 8 shows an Arrhenius plot of oxidation rates for this study in comparison with results obtained in other studies, which have been shown in Higgins et al (2002Higgins et al ( , 2003. In order to keep the results comparable, a hypothesis of 120 m 2 /g surface area of particle samples was used to calculate the surface-specific oxidation from the mass decrease rate, following the same approach in Higgins et al (2002Higgins et al ( , 2003. For pure biodiesel soot, Jung et al (2006) reported an activation energy of 89 kJ/mol (line c), which is lower than results obtained in this study.…”

“…For diesel soot, the following reported lower activation energy than those obtained in this study : Jung et al (2005) with 25 ppm cerium-dosed case (line a, 107 kJ/mol); Jung et al (2003) with lube-oil-dosed case (line b, 101 kJ/mol); Higgins et al (2003) with regular diesel fuel (line e, 108 kJ/mol); Ahlström and Odenbrand (1989) with flow reactor study of diesel particles (line h, 106 kJ/mol). On the other hand, Miyamoto et al (1988) obtained higher results using TGA for uncatalyzed diesel soot (line f, 191 kJ/mol) and the following two obtained similar results: Higgins et al (2002) using diffusing flame-generated soot (line d, 164 kJ/mol) and Otto et al (1980) using TGA for diesel particles (142 kJ/mol). Clearly, the activation energy depends on the type of fuel, the treatment of the fuel or the soot, and the methods of measurement.…”

“…From top to bottom: (A) biodiesel particle collected at 0.08 MPa; (B) LSD particle collected at 0.08 MPa; (C) ULSD particle collected at 0.08 MPa; (D) biodiesel particle collected at 0.7 MPa; (E) LSD particle collected at 0.7 MPa; (F) ULSD particle collected at 0.7 MPa. Thin lines show previous works: (a) Jung et al (2005), 25 ppm cerium-dosed case (107 kJ/mol); (b) Jung et al (2003), lube-oil-dosed case (101 kJ/mol); (c) Jung et al (2006), pure biodiesel (89 kJ/mol); (d) Higgins et al (2002) using diffusion flame-generated soot (164 kJ/mol); (e) Higgins et al (2003), diesel particles with regular diesel fuel (108 kJ/mol); (f) Miyamoto et al (1988) TGA of uncatalyzed diesel soot (191 kJ/mol); (g) Otto et al (1980) TGA of diesel particles (142 ± 21 kJ/mol); (h) Ahlström and Odenbrand (1989) particulate agglomerates (Strzelec et al 2011). The larger specific surface area improves the exposure of the particles to oxygen in the oxidation process, leading to the higher oxidative reactivity of biodiesel particles, compared with the ULSD and LSD particles.…”

“…The ignition temperature increases with engine load for each fuel and at each engine load, it increases in the order of biodiesel, LSD, and ULSD. Figure 8 shows an Arrhenius plot of oxidation rates for this study in comparison with results obtained in other studies, which have been shown in Higgins et al (2002Higgins et al ( , 2003. In order to keep the results comparable, a hypothesis of 120 m 2 /g surface area of particle samples was used to calculate the surface-specific oxidation from the mass decrease rate, following the same approach in Higgins et al (2002Higgins et al ( , 2003.…”

Current Understanding of Ultrafine Particulate Matter Emitted from Mobile Sources

“…Several studies have been conducted to understand the kinetics of soot oxidation. Higgins et al 53 derived an expression for the kinetics of the oxidation of diesel soot particles as a function of size. Particle size was measured for selected monodispersed particle populations with a TDMA before and after oxidation at high temperature.…”