A DFT study of the interaction between large PAHs and atomic chlorine or hydrogen chloride molecule: Toward a modelling of the influence of chlorinated species on the trapping of water by soot

Abstract

First-principle calculations have been performed to characterize the interaction of chlorinated species (HCl and Cl) with large polycyclic aromatic hydrocarbon (PAH) molecules and radicals. Whereas the characterization of the interaction process on the face of the PAH molecules requires taking into account long-range dispersion interactions in the calculations, trapping at the edge of PAH radicals involves stronger interactions that lead to the dissociation of the HCl molecule. Then, the first steps of water adsorption on the corresponding chlorinated species has been characterized, showing that chlorine may act as an efficient nucleation center for water molecules on such aromatic systems mimicking part of the carbonaceous surfaces that are likely present in soot. These results represent a first but necessary step for a better understanding of soot behavior in industrial or domestic fire situations.

Introduction

The role of atmospheric aerosols in the transport of irritant gases into the lungs is now recognized as a major public health problem [1], [2]. However, despite a growing number of related research works, this role is still far from being understood and quantified [3]. Similarly, aerosols are suspected to be implicated in inhalation of smokes generated in fires, although parameters like gas and aerosol concentrations, aerosol particle sizes and compositions, temperature and exposure duration are certainly different [4]. However, common to both situations is the fact that irritant gases have to be sufficiently bound to the aerosol to be transported, but also bound loosely enough to be easily released.

Smoke can be viewed as a mixture of gases, vapors and particulates among which carbonaceous agglomerated structures, commonly known as soot, are widely present [5]. Soot formation and growth involve a number of processes from precursor formation to agglomeration of primary particles and subsequent growth promoted by adsorption of surrounding gases like water, acid gases and hydrocarbons including polycyclic aromatic hydrocarbons (PAH) [5], [6].

Widely accepted structural models for carbonaceous primary particles issued from combustion assume the presence of graphene-like sheets, stacked on concentric spheres of increasing radii. This arrangement results in typical onion-like structures containing also small fractions of other elements (mainly oxygen) besides carbon due to partial oxidation [7], [8] and structural defects like edges or atom vacancies formed during the recombination of soot precursors [9]. The surface of soot is thus expected to be highly reactive and thus, prone to structural modifications and ageing [10]. Chemistry at the surface of these primary particles is therefore of critical importance to changes in hygroscopic properties of soot and to its impact on both the transport and the release of irritant gases.

Among these gases, hydrogen chloride (HCl) is one of the main chloride species produced in combustion or thermal decomposition of, for instance, polyvinyl chloride (PVC), electrical elements or electrical cables. This acid gas may cause severe irritant effects at low concentrations (around 100 ppm) and even results in death at higher concentrations and long time exposures (at least for small animals [11]). Since the 70’s, it has been inferred that smoke aerosol could be particularly effective in transporting hydrogen chloride (HCl) past the respiratory defence during fire situations in confined spaces where such chlorinated materials abound [4], [12]. More recently, Ouf et al. [13] reported significant contribution of Cl atoms in the elemental composition of soot particles emitted during thermal degradation of electrical cables. In addition to toxicological effects, the question of the influence of chloride species, bounded at soot surfaces, on the affinity between soot and water molecules has been poorly investigated. The modelling of water trapping (adsorption/absorption) on such soot surfaces is however of main importance to develop a predictive model of evolution of the air resistance of filters used as protective equipment for fire workers and also for maintaining the containment of highly toxics substances (radioactive, biological, nanoparticles…) of industrial facilities. As recently reported [14], [15], increasing the amount of Cl within the fuel composition involved in fires representatives of the nuclear industry increases the amount of condensable species (water, acids, PAHs…) on soot surfaces. This feature has been related to the increase of the amount of water content at the surface of high efficiency particulate air filters when the amount of Cl increases in fuel composition. Thus, the amount of Cl on soot is suspected to strongly impact filter clogging [14], [15].

As a consequence, characterizing adsorption of chlorinated species, especially HCl, on soot surface appears as an important task and, surprisingly, as far as we know, there are no detailed investigations in the literature related to this yet interesting topic. Indeed, the vast majority of works devoted to the interaction between chlorinated species and carbonaceous surfaces have been related to graphene technology and, thus, mainly focused on electronic properties of this material [16], [17], [18], [19], [20], [21], [22]. A few other studies have been devoted to the effect of chlorine addition on fuel combustion, showing that chlorine atoms can inhibit hydrocarbon oxidation and, as a consequence, activate the transformation of polycyclic aromatic hydrocarbons to soot [23].

To overcome this lack of information, experimental as well as theoretical studies are therefore required, aiming at a detailed characterization of the interaction processes between chlorinated species and soot, and of their influence on, for instance, subsequent water adsorption.

In a series of recent papers, we used first-principle calculations to characterize the interaction of various oxidants (atomic and molecular oxygen, ozone, nitric oxide) and also of water molecules with perfect and defective carbonaceous surfaces, aiming at modelling the first stages of atmospheric species adsorption on soot [24], [25], [26], [27], [28], [29], [30]. The results of these studies have shown that soot may indeed chemically interact with the surrounding molecules, leading to the formation of oxygenated groups at its surface (mainly phenol, carbonyl, carboxyl, ether- and epoxy-like groups) that can participate in the trapping of water molecules by soot. Besides, these studies showed that atomistic approaches based on quantum chemistry were able to usefully complement information on soot reactivity, which is of great interest to a better understanding of the fundamental processes involved in, e.g., atmospheric chemistry [31].

Here, we thus choose to use similar approaches based on the DFT method to characterize the details of the adsorption processes of chlorinated species (namely HCl molecule and, for comparison, Cl atom) on large PAHs mimicking the surface of soot. Indeed, these molecules have been previously shown to describe accurately enough the local surrounding viewed by an adsorbate during its interaction process with the soot surface [24], [32], [33]. As in our previous studies, both PAH molecules and radicals have been considered in the calculations as proxies for the perfect and defective carbonaceous surfaces existing in combustion soot, although we are aware that soot is made up by finite graphene-like layers certainly larger than the PAH molecular models considered here [8], [9].

The computational methods used in this work and the details of the calculations are briefly given in Section 2. The various results are presented in Section 3 and they are discussed in Section 4. Finally, the main conclusions are summarized in Section 5.