print pageprinter friendly
Reports: DNI653105-DNI6: Experimental Investigations of Radical-Particle Reactions Relevant to Hydrocarbon Pyrolysis

Fabien Goulay, West Virginia University

Overview Carbon particles have been demonstrated to significantly lower the temperature required for pyrolysis and reforming of hydrocarbons. This catalytic effect may be explained by the enhancement of the unimolecular dissociation at the gas-particle interface in addition to the pure gas-phase reactions. During the past funding period, an innovative experimental set up has been designed and assembled in order to investigate the unimolecular decomposition of simple test molecules at the surface of hot carbon nanoparticles. The technique is based on pump-probe laser spectroscopy to initiate the reaction and detect the products. Before injection of the particles, the pump probe laser system has been validated by investigating the OH + phenylacetylene gas phase reaction over a wide range of pressures and temperatures. Experimental The experimental cell is made of a custom-made 7-way stainless steel cross with 5 optical access ports (Figure 1). The particles are injected using a pulsed aerodynamic chopper generating a 2 to 3 ms burst of buffer gas containing a high volume fraction of suspended carbon nanoparticles. The aerosol is then carried to the center of the flow cell by a laminar flow. A flow of pure helium concentric to the particle flow minimizes the diffusion of the particles to the optics. The particle flow is crossed by the fundamental output of a Nd:YAG laser with a 8-ns pulse width. The absorption of the laser light by the particles leads to an increase of the particle surface temperature within the time scale of the laser pulse. The gas phase products formed by reaction of gas molecules a the surface of the hot particles are detected by Laser Induced Fluorescence (LIF) using the fundamental or second harmonic of a tunable Nd:YAG-pumped dye laser. Figure 1 Experimental setup for the investigation of heterogeneous pyrolysis using IR laser-heating coupled to laser-induced fluorescence. Combustion particles are directly extracted from a diffusion ethylene flame and carried to the reaction cell. This requires removing any gas phase molecules by flowing the particles through an activated charcoal denuder and a diffusion dryer. The particle number density and diameter distribution at the cell entrance as well as after the reaction are monitored using a scanning mobility particle sizer and laser induced incandescence. Any optics inside the main vacuum chamber is protected from particle deposition using counter-flow aerodynamic windows. The pulse injection minimizes the amount of particles used in the experiment as well as the contamination of the optics and vacuum lines. The reaction flow-cell displayed in Figure 1 has been designed and is being assembled in the PI's laboratory. The particle generation system is being coupled to the reaction cell using a pulsed aerodynamic chopper. The homogeneity of the aerosol flow will be verified by measuring the temporal profile of the laser-induced incandescence signal following 1064-nm laser heating of the carbon particles. Initial experiments will look at OH radicals formed by decomposition of gaseous hydrogen peroxide using laser photolysis at 266 nm or laser induced particle heating. The detection of radicals after laser heating will demonstrate the catalytic effect of hot carbon nanoparticles. Further experiments will look at the effect of particle temperature and gas composition on the temporal behavior of the gaseous products. Gas phase kinetics using pump-probe spectroscopy The pump-probe laser system for measuring OH radical temporal behavior has been validated by measuring the bimolecular rate coefficient for the OH + phenylacetylene reaction. In this case the OH the radicals are formed by 266-nm photodissociation of H2O2 or 355-nm photodissociation of HONO in the presence of gaseous phenylacetylene with a known partial pressure. The concentration of the OH radicals is followed in time by exciting the OH radical on the A2S-X2P (1,0) band at 281.954 nm. The following fluorescence from the A2S-X2P (1,1) band is collected through a 310 nm (FWHM ±10nm) band pass filter by a photomultiplier tube. Radical number density temporal profiles are obtained by averaging 20 laser shots per point while changing the delay time between the lasers and integrating the signal over a 1.1 µs gate. The OH temporal decay is measured over a wide range of phenylacetylene partial pressure in order to infer the absolute reaction rate coefficient. Figure 2 Reaction rate for the OH + phenylacetylene as a function of cell temperature

Figure 2 displays the measured rate as a function of cell temperature. The average value is greater than expected based solely on the electrophilic attack of the OH radical onto the aromatic ring. Together with high-level calculations of the entrance pathways exothermicities, these results are consistent with the formation of an association pre-reactive complex involving the ethynyl group. The Van der Waals intermediate is then likely to evolve without or with a small activation energy to a resonantly stabilize addition intermediate (see Figure 3). Figure 3 Mostly likely reaction products formed by addition of the OH radical onto the acetylenic group of phenylacetylene. The OH + phenylacetylene reaction rate is measured to be several order of magnitude greater than the values currently used in combustion models and may therefore play a significant role in phenylacetylene removal and formation of oxidation products.