FTIR Spectroscopic Investigation of the Mechanism and Kinetics of the Heterogeneous Reactions of NO₂ and HNO₃ with Soot

Abstract

Soot samples from a spark generator, a flame, and a diesel passenger car were either collected on a Teflon filter and transferred to an IR-transparent window or deposited directly from a flame onto the window and investigated by Fourier transform infrared (FTIR) spectroscopy. The soot-covered windows were mounted in a 10 cm vacuum cell connected to a standard flow system with He as carrier gas. Reactive gases, such as NO2 and HNO3, were added to the carrier gas flow at a concentration of (0.016 to 2.5) × 1014 molecule cm-3. FTIR spectra of soot samples before and after exposure to HNO3, NO2, and O3 are presented. Formation of IR absorption bands was analyzed as a function of exposure time. IR bands attributable to soot surface oxidation products and nitrogen containing species, e.g. −CO, R−NO2, R−ONO2, and R−ONO were observed. The observed time dependence of the absorption bands of the spark generator soot can be fitted by two parallel reactions, a slow and a fast process. Both processes have a reaction order of n ≈ 0.2 (±0.3) for the NO2 + soot reaction and n ≈ 0.5 (±0.6) for the HNO3 + soot reaction. The number of active sites, N max = 2.2 × 1014 molecules cm-2 soot surface, has been estimated from saturation experiments. Surface reaction probabilities depend on reactant concentration and reaction time and were in the range of γ ≈ 10-6 to 10-8 for the slow, and γ ≈ 10-3 to 10-6 for the fast processes. The reaction probability on diesel engine soot was nearly 1 order of magnitude slower. It is concluded that the reaction of NO2 with soot cannot account for the HONO levels observed in urban air.

Pre-DPF water injection technique for pressure drop control in loaded wall-flow diesel particulate filters

“…As discussed by Jeguirim et al [39] it is attributed to the indicated potential formation of nitric and nitrous acids in presence of water and O 2 . This was also pointed out by Kirchner et al [40], who observed an increase in C-ONO2 and C-ONO groups in the reaction of soot when nitric acid is present in the exhaust flow. In the same way, Uchisawa et al [41] found the cause of the soot oxidation improvement due to water presence in the strong acidity produced by hydrolysis and decarboxylation, as also indicated by Ciambelli et al [42].…”

Influence of Combustion Conditions on Hydrophilic Properties and Microstructure of Flame Soot

“…13 carbonyl group, assigned to ketone species (1710 cm −1 ), was observed for decane soot. 13,20,21,40,41 As seen in Figure 7, compared to the functional groups on the rich toluene flame soot, no significant changes were observed on the lean toluene flame soot. For n-hexane and decane soot, the lean flame soot exhibited lower intensities in the peaks of alkynesC−H, aromatic C−H, unsaturated CH 2 , and highly substituted aromatic compounds, while the carbonyl (CO) group bound to an aromatic ring showed a larger intensity than that of the rich flame soot.…”

“…18 Functional groups, including aliphatic or aromatic C−H, carbonyl CO, and ethers C−O, have been confirmed through infrared spectroscopy. 13,20,21 Microstructures including graphitic carbon, disordered carbon, and amorphous carbon have also been detected using Raman spectroscopy. [15][16][17]19,22,23 Recent studies have shown that residence time in the flame and flame temperature can induce modifications in the microstructures and functional groups of soot.…”

CHARACTERIZATION OF THE CHEMICAL BEHAVIOR OF A DBD WIRE-CYLINDER REACTOR FOR NOx REMOVAL. DETERMINATION OF REACTIONAL PATHWAYS BY ISOTOPIC LABELING

“…Those species are present in excess in the mixture (about 100 to 1 NO molecule). But a significant part of NO "disappears", being trapped by C 3 H 6 and its fragments giving R-NO x [18] , adsorbed on the outlet pipes or reduced to N 2 +O 2 . R-NO x formation will be presented in the following section.…”

“…After it, we will determine the reactional pathways by substituting introduced O 2 by its 18 O 2 isotope. The mass spectrum of the different species created by oxidation from O 2 will indeed be modified.…”