Development of a novel heterogeneous flow reactor: soot formation and nanoparticle catalysis

2013-11-20The development of novel experimental approaches to investigate fundamental surface kinetics is presented. Specifically, fundamental soot formation and surface catalysis processes are examined in isolation from other competing processes. In terms of soot formation, two experimental techniques are presented: the Burner Stabilized Stagnation (BSS) flame configuration is extended to isolate the effect of the parent fuel structure on soot formation and the fundamental rate of surface oxidation for nascent soot is measured in a novel aerosol flow reactor. In terms of nanoparticles, the physical and chemical properties of freely suspended nanoparticles are investigated in a novel aerosol flow reactor for methane oxidation catalyzed by palladium. ❧ The role of parent fuel structure within soot formation is examined by following the time resolved formation nascent soot from the onset of nucleation to later growth stages for premixed BSS flames. Specifically, the evolution of the detailed particle size distribution function (PSDF) is compared for butanol, butane and C₆ hydrocarbons in two separate studies where the C/O ratio and temperature are fixed. Under this constraint, the overall sooting process were comparable as evidenced by similar time resolved bimodal PSDF. However, the nucleation time and the persistence of nucleation with time is strongly dependent upon the structure of the parent fuel. For the C₆ hydrocarbon fuels, the fastest onset of soot nucleation is observed in cyclohexane and benzene flames and this may be due to significant aromatic formation that is predicted in the pre-flame region. In addition, the evolution of the PSDF shows that nucleation ends sooner in cylclohexane and benzene flames and this may be due to relatively quick depletion of soot precursors such as acetylene and benzene. Interestingly, within the butanol fuels studied the effect of the branched chain in i-butanol and i-butane was more significant than the presence of fuel bound oxygen. A numerical analysis of the gas-phase chemistry for butanol and butane indicates the fuel structure effect is largely exhibited in the relative importance of C₂ versus C₃ intermediate species formed during the initial stage of fuel breakdown. ❧ Oxidation kinetics of soot are typically measured with carbon black or well aged soot as substrates. The soot surface is also assumed to be graphitic in theoretical soot oxidation rate calculations. However, recent experimental and theoretical studies show that nascent soot can have structures and surface composition drastically different from mature, graphitized soot. In the current study, oxidation of nascent soot by O₂ was observed at T= 950 and 1000K for oxygen concentrations ranging from 1000 to 7800 ppm in a laminar aerosol flow reactor at ambient pressure. Oxidation behavior of primary particles (Dp < 20 nm) of nascent soot from a premixed BSS ethylene flame was observed by tracking the shift in the particle size distribution function (PSDF) under a given residence time. The measured rate of the surface reaction ranges from 1x10⁶-3x10⁶ g/cm²s for nascent soot. The rate of oxidation observed at the given conditions is an order of magnitude faster than predicted by the classical Nagle Strickland-Constable (NSC) correlations derived from graphite oxidation. Heterogeneous surface reaction rates are highly sensitive to the surface composition. Thus the faster rate of surface reaction by the nascent soot observed currently suggests that the surface composition of nascent soot is more reactive than the conventional graphite surface. ❧ Catalytic activity in reacting flow laden with suspended nanoparticle catalyst is measured in a novel aerosol flow reactor. Similar to conventional gas phase kinetics, heterogeneous reactions are the product of collisions between the particle surface and surrounding gas. However, particles below 10 nm in diameter are in a transition region where collisions do not always result in perfectly elastic scattering. The inelastic scattering provides more opportunities for reaction to occur than elastic scattering. It is this extra chemical behavior of nanoparticles which may serve as a novel parameter for tuning and optimizing catalytic activity. Specifically, the size dependence of catalytic activity is examined by observing methane oxidation catalyzed by freely suspended palladium nanoparticles. The role of the nanoparticle size is explored in flow reactor measurements and a novel in-situ process for rapid synthesis of aerosolized palladium nanoparticles is introduced. Under catalytic conditions, the temperature at which methane is oxidized is found to be 300K lower than conventional gas phase combustion. In addition, the preliminary measurements indicate that catalytic activity may be greater in nanoparticles relative to bulk surfaces