RESUMEN
In low-temperature flash photolysis of NH3/O2/N2 mixtures, the NH2 consumption rate and the product distribution is controlled by the reactions NH2 + HO2 â products (R1), NH2 + H (+M) â NH3 (+M) (R2), and NH2 + NH2 (+M) â N2H4 (+M) (R3). In the present work, published flash photolysis experiments by, among others, Cheskis and co-workers, are re-interpreted using recent direct measurements of NH2 + H (+N2) and NH2 + NH2 (+N2) from Altinay and Macdonald. To facilitate analysis of the FP data, relative third-body collision efficiencies compared to N2 for R2 and R3 were calculated for O2 and NH3 as well as for other selected molecules. Results were in good agreement with the limited experimental data. Based on reported NH2 decay rates in flash photolysis of NH3/O2/N2, a rate constant for NH2 + HO2 â NH3 + O2 (R1a) of k1a = 1.5(±0.5) × 1014 cm3 mol-1 s-1 at 295 K was derived. This value is higher than earlier determinations based on the FP results but in good agreement with recent theoretical work. Kinetic modeling of reported N2O yields indicates that NH2 + HO2 â H2NO + O (R1c) is competing with R1a, but perturbation experiments with addition of CH4 indicate that it is not a dominating channel. Measured HNO profiles indicate that this component is formed directly by NH2 + HO2 â HNO + H2O (R1b), but theoretical work indicates that R1b is only a minor channel. Based on this analysis, we estimate k1c = 2.5 × 1013 cm3 mol-1 s-1 and k1b = 2.5 × 1012 cm3 mol-1 s-1 at 295 K, with significant uncertainty margins.
RESUMEN
In this work we have investigated the mechanism of nanoparticle synthesis in a low pressure, premixed, laminar flat flame of CH4-O2, doped with iron pentacarbonyl using a combined quartz-crystal-microbalance-particle-mass-spectrometry apparatus. We have unambiguously demonstrated that the formation of nanoparticles in iron pentacarbonyl-doped flames occurs very early, in close proximity to the burner surface, prior to the flame front. This early rise of nanoparticle mass concentration is followed by a sharp drop in nanoparticle concentration at the high temperature flame front. This "prompt" nanoparticle generation is consistent with kinetic models describing iron cluster formation. The observation of this phenomenon in a quasi-one-dimensional premixed flat flame strengthens our previous findings and points out that the "prompt" nanoparticle formation is a general phenomenon, not limited to diffusion flames. It presents a challenge and a trigger for further development of the existing mechanisms for gas phase synthesis of iron oxide particles in flames.