Chemistry of nitrogen-containing polycyclic aromatic formation under combustion conditions
Introduction
In recent years, ammonia (NH) as a potential carbon-neutral alternative fuel has drawn interest in the combustion community. Many studies have investigated its combustion chemistry, properties, and performance [1], [2], [3], [4], [5], [6]. From these studies, ammonia is considered as a promising fuel additive rather than a single fuel because of its higher NO emissions [2] and much narrower range of equivalence ratios for stable flames compared with hydrocarbon fuels [1]. A new subset of chemistry rises from these considerations, i.e., the formation of nitrogen-containing polycyclic aromatic compounds (NPACs). In fact, NPACs have been identified to be important for applications other than nitrogen-containing fuels. For example, typical products formed during the pyrolysis of polyurethane, a widely used class of polymers, contain a lot of NPACs, including nitrogen-embedded pentagonal and hexagonal rings (e.g., pyrrole, pyridine, quinoline) [7]. Heterocyclic amines, a category of carcinogenic compounds, are the NPACs detected in cooked meats [8]. Another example is the combustion of biomass, which produces a lot of aromatics and nitrogen species [9], [10]. This is likely to be an ideal environment for growing NPACs as well.
Polycyclic aromatic hydrocarbons (PAHs) are well-known soot precursors during combustion process [11], [12], [13]. For pure hydrocarbon flames, the PAHs formed contain mostly only carbon and hydrogen, with some oxygenated polycyclic aromatic compounds (e.g., furan-embedded PACs) forming in the maximum temperature region of the flame [14], [15], [16]. Nitrogen participates in combustion chemistry when the oxidizer contains nitrogen molecules (N) or when N is added as dilution gas, by forming small species such as NO emissions. These small gas-phase molecules are rarely found to directly interact with PAHs, as observed in soot particle samples identified by experiments [4], [17], [18], [19]. However, the picture completely changes when the flames are doped with nitrogen-containing fuels, such as ammonia, pyrrole, and pyridine. Nitrogen-containing species such as hydrogen cyanide (HCN) would have a considerably higher concentration [20], [21], thereby potentially influencing PAH chemistry by changing the pool of reacting species, and by directly forming NPACs.
Multiple studies have shown that adding ammonia into pure hydrocarbon flames reduces both soot volume fraction and average soot particles diameter by inhibiting the precursors [4], [5], [22]. The chemistry remains somewhat a mystery and that only a few recent studies have started to investigate. A recent computational work by Deng et al. [6] simulated a set of ethylene (CH) counterflow diffusion flames with different percentage of ammonia as fuel additive using the KM2-Glarborg mechanism [23], [24]. The reaction pathway analysis shows that the influence of adding ammonia on large PAHs such as phenanthrene (CH) and pyrene (CH) is mostly through dilution and thermal effects, while the chemical effects are rather small. However, recent experimental studies by controlling either thermal or dilution effects observed that ammonia could still effectively inhibit soot formation. Bennett et al. [22] reveals that the PAH planar laser induced fluorescence (PLIF) signals are noticeably reduced by increasing the doping percentages of ammonia in a set of laminar counterflow ethylene flames, even when the temperature profiles of the flames are similar. Zhou et al. [5] compared the sooting characteristics between N and NH flames with the same mixing ratio to minimize dilution effect, and concluded that ammonia could inhibit soot formation much more effectively than nitrogen. These experiments indicate that, apart from the thermal and dilution effects, direct chemical effects by forming NPACs may also play an important role in these ammonia flames, and these pathways are yet to be described in current kinetic models.
Despite these needs, studies on the formation pathways of NPACs in combustion are limited in the literature. Early in 1981, Kausch et al. [25] studied the NPACs in sooting flames. By examining the effect of doping pyrrole or pyridine in a mixture of benzene vapor and methane, neither exerted any discernible effect on the quantities of PAH or soot formed in the benzene/methane flames. Seven NPACs were identified from these flames, all of which contain cyano groups. The formation mechanism of these NPACs was unknown at the time. Bouwman et al. [26] recently explored the possibility of forming quinoline from phenyl radical, similar to hydrogen-abstraction-acetylene(CH)-addition (HACA) mechanism [27], [28], [29], [30] from phenyl radical to naphthalene. The results show that while naphthalene formation is de facto barrierless, quinoline formation is kinetically hindered by a barrier at 65.1 kJ mol. This result can explain the experimental findings by Bennett et al. that the PLIF signal of two-ring PAHs was not noticeably affected by ammonia addition in ethylene counterflow flames after minimizing the thermal effects, since HACA mechanism and recombination of cyclopentadienyl radical [31], [32] are still the dominating pathways in such conditions. In addition, the formation of quinoline and isoquinoline via HACA and hydrogen-abstraction-vinylacetylene(CH)-addition (HAVA) mechanisms was recently explored in two combined theoretical and experimental studies [33], [34]. Liu et al. [35] studied the reaction pathways for 1-naphthyl + HCN, in comparison with HACA mechanism (1-naphthyl + CH), and concluded that the formation of nitrogen-embedded PAHs on the zig-zag site is not kinetically favoured. In general, there is a lack of systematic studies on the formation of NPACs under combustion conditions.
Inspired by these experimental and computational discoveries, this work aims to systematically study the formation pathways of NPACs, and discuss their importance of explaining experimental observations and impact on current kinetic mechanisms. The formation of nitrogen-embedded pentagonal and hexagonal rings is considered on the zig-zag and armchair sites correspondingly, as well as the formation of cyano groups on the aromatic structure. For the armchair site aromatics, both biphenyl-type and phenanthrene-type are considered as they represent distinct electronic structures and kinetics, as shown in Figs. 1 and 2. Starting from 1-naphthyl radical, o-biphenynyl radical, and 4-phenanthryl radical, the addition of HCN could form different NPACs, similar to the role of CH in HACA mechanisms. HCN was chosen because it is one of the most abundant nitrogen-containing species in ammonia flames other than ammonia itself [4], as well as the pathway’s similarity for HCN to react like CH in HACA mechanism. These new pathways could offer insights on the direct chemical effects of ammonia doping on the formation of PAHs, and provide new routes for NPACs formation in other fields as well.
Section snippets
Electronic structure calculations
Electronic structure calculations were carried out using Gaussian 16 software suite [36]. Geometry optimizations of local minima and transition states (TSs) were conducted with unrestricted hybrid density function theory (DFT) at B3LYP/6-311G(d,p) level. For each geometry optimization, an optimized wave function [37] was obtained from the initial geometry by the keywords “guess=mix stable=opt nosymm”, and then provided to the geometry optimization by the “guess=read” keyword. The wave function
Potential energy surfaces
In the discussion below, the pathways starting from naphthalene, biphenyl, and phenanthrene are named as Pathway A, B, and P correspondingly.
Conclusions
In this work, we systematically examined the reaction pathways leading to the formation of nitrogen-containing polycyclic aromatic compounds (NPACs) on zig-zag and armchair sites, in comparison with the hydrogen-abstraction-acetylene(CH)-addition mechanism. Using ab initio G3//B3LYP/6-311G(d,p) electronic structure calculations and RRKM-one dimensional master equation approach, we explored the potential energy surfaces of 6 pathways, calculated the high-pressure limit rate constants of 30
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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