Aging Oxidation Reactions on Atmospheric Black Carbon by OH

Article pubs.acs.org/JPCA

Aging Oxidation Reactions on Atmospheric Black Carbon by OH Radicals. A Theoretical Modeling Study Laura Rojas, Alexander Peraza, and Fernando Ruette* Laboratorio de Química Computacional, Centro de Química, IVIC, Apartado, 21827 Caracas, Venezuela ABSTRACT: Aging processes of black carbon (BC) particles require knowledge of their chemical reactivities, which have impact on cloud condensation nuclei (CCN) activities, radiant properties and health problems related to air pollutions. In the present work, interactions between several OH radicals with BC (modeled with a coronene molecule) were calculated by using DFT and PM6 codes as described by Mysak et al. Water interaction with BC was also included. Results show that OH radical adsorption is preferred on border sites, independent of the theoretical method employed. Potential energy curves using DFT(TPSS-D3) approach for OH chemisorption showed small-energy barriers, as reported in previous work with PM6. A dipole moment has been created, and the hydrophobic coronene surface is transformed to hydrophilic after the first OH chemisorption. Several stages were found in the BC aging by OH radicals, thus (a) Hydroxylation of coronene by several OH radical would lead to H abstractions directly from the substrate. (b) Abstraction of H from adsorbed OH (at the border sites) drives a C−C bond breaking and the formation of carboxyl groups. (c) Hydrogen abstraction from carboxyl group produces decarboxylation (CO2 plus water) as experimentally obtained. Potential energy curves of one of the reactive path were calculated with the PM6 method. The formation of products was confirmed using DFT. Coronene interaction with O2 was also considered to have a realistic atmospheric environment.

1. INTRODUCTION Black carbon (BC) is one of the products of incomplete fossil fuel and biomass combustion. BC is found as fine particles (0.5 to 1.0 μm) and ultrafine particles (0.05 to 0.12 μm).1−3 Cooke and Wilson3 estimated that annual emissions of BC are 7.96 and 5.98 Tg from fossil fuel consumption and from biomass burning, respectively. The authors have also found that BC has a lifetime between 6 and 10 days, depending on its transformation rate from a hydrophobic to hydrophilic states. BC may significantly contribute to the irradiative force of the Earth’s climate.4−6 For example, Ming et al.7 reported that increasing BC concentrations in the atmosphere have reached the Himalayas which have noticeable effects on recent glacier melting. Furthermore, a review by George and Abbatt8 concluded that atmospheric aerosol influences both climate and air quality. Polycyclic aromatic hydrocarbons (PAHs), present in BC, have been found to be one of the most prominent groups of toxic air pollutants. BC may operate as a universal carrier of a wide variety of semivolatile organic constituents of high toxicity to sensitive pulmonary and cardiovascular targets.9 Epidemiological studies provide evidence of an association between daily variations in BC concentrations and cardiovascular and cardiopulmonary diseases.10 It is generally known that hydroxyl (OH) radical and nitrates (NO3) initiated reactions which often lead to the formation of mutagenic and carcinogenic nitro-PAH; moreover, other nitropolycyclic aromatic compounds, nitrodibenzopyranones,11,12 are included. © 2015 American Chemical Society

The presence of BC in the atmosphere may also affect the formation of cloud condensation nuclei (CCN).13 In this sense, Merikanto et al.14 showed that nucleated particles are transported large distances in regions far from where combustion products were emitted. Consequently, there are important factors in shaping the global CCN distribution. After emission, the BC hydrophobic particles are poor CCN agents. Then, BC proceeds to aging reactions with increasing the size and the hygroscopicity of internally mixed PAHs by enhancing the CCN activity.15 Atmospheric heterogeneous oxidation of BCs occurs through reactions with OH, O2, O3, and NOx.16,17 The details of the BC aging process are poorly understood. However, it is known that in troposphere, the high photoreactivity rate induced by O2 photochemical oxidation from sunlight irradiation has an important influence in soot aging processes.18 Composition of aerosol particles from carbonaceous combustion is very complex.19−21 Soot and related substances are known to be quasi-solids and undergo chemical reactions at the surface rather than in the bulk. Coronene has been used as a representative molecule in BC17 because it has been found in emissions from gasoline engines and biomass combustion sources.22−24 This compound has been employed by Mysak et al.17 as an experimental model for anthropogenic pollution molecule emitted to the environment. They have studied Received: July 21, 2015 Revised: November 25, 2015 Published: November 25, 2015 13038

DOI: 10.1021/acs.jpca.5b07073 J. Phys. Chem. A 2015, 119, 13038−13047

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The Journal of Physical Chemistry A

chemisorbed OHs on coronene were calculated in the CCN process in section 3.3. Coronene aging reactions are presented in different parts of section 4. General conclusions and comments in future works are summarized in the last section.

competitive reaction pathways for functionalization and volatilization of coronene thin films using hydroxyl radicals and ozone. For the above-mentioned reasons, coronene is used as a model of BC in the present work. Chemical aging of carbonaceous organic species from heterogeneous oxidation modifies the physical-chemistry properties of the particle such as size, morphology, composition, hygroscopicity, and the ability to act as cloud nuclei. Therefore, aging may significantly impact the role of organic aerosols on climate, atmospheric chemistry, and relevant environmental processes. Several theoretical works have been carried out to study heterogeneous interactions between solid phase soot and gas phase molecules. Kubicki studied modeling of hexane soot structure, interactions with pyrene25 and molecular simulations of benzene with soot.26 Several theoretical modeling of COOH, H2O, O2, and OH interaction on a soot modeled surface with small graphene clusters have been carried out by Picaud et al.27−34 They performed standard molecular dynamics simulation to study water adsorption on hydroxylated graphite surfaces.27,29 Quantum chemistry calculations using DFTB3LYP approach and ONION method were performed by interaction of H2O with oxidized and vacancy sites on small carbonaceous nanoparticles modeling soot.31,32 Recently, Rojas et al.35 carried out a preliminary modeling of BC aging using coronene with OH radicals by employing DFT exchangecorrelation functional (PBE99-PBE plus dispersion) with a TZVP basis set. They also included the semiempirical codes. This work showed a partial description of coronene functionalization due to OH radicals. The authors also discarded the analysis of multiple ways of OH chemisorption, potential energy curves for OH, the formation of volatile compounds, and O2 interaction. Other recent work by Edwards et al.36 calculated pathways for soot oxidation reactions with OH on phenanthrene, using DFT at a B3LYP/6-311G(d,p) level. They found that OH reacting with three types of PAH edge sites results in the CO formation at high temperatures. Additional calculations for OH interaction on carbonaceous PAH (coronene)37 have been carried out in order to analyze adsorption sites and the H2O formation in the interstellar medium using a semiempirical method. A composite of semiempirical and DFT were reported by Bentarcurt et al.38 of small molecules in hydrogenated charged model (coronene+), including H2O formation by the OH radical. Guennoun et al.39 joined experimental and theoretical works, at the B3LYP/6-311++G(d,p) level of theory, to explain the oxygen containing coronene formation after photochemical treatment with water at 10 K. In spite of several reported works, little is known about heterogeneous aging reaction mechanisms, due to reaction complexity that leads to volatilization and product derivatives in the atmosphere.40 In the present work, a BC aging model was carried out, considering OH radical interactions by hydroxyl radicals and ozone with a BC (coronene molecule) as reported.17 The interaction with OH radical is only considered since it is the most reactive species, and O3 is transformed in OH in the presence of water. This work is organized as follows: Section 2 delineates theoretical methods and computational details, as well as the BC model employed. Descriptions of quantum calculations of OH interactions with different coronene surface sites, and potential energy curves for chemisorption are described in sections 3.1 and 3.2, respectively. Water interactions with free OH and with multiple

2. MODELS AND THEORETICAL METHODS As mentioned before, PAHs are representative molecules for aerosol soot.21−24 Particularly, a single sheet of coronene (C24H12) has been used as a BC model17,27,35 because the interaction between coronene layers is small and does not affect chemisorption. A picture of the coronene molecule is depicted in Figure 1 with the different labeled adsorption sites for OH radicals.

Figure 1. Coronene molecule with adsorption sites labeled as border (a, a′, a″, and a‴), intermediate (b and b′), and center (c and c′) sites.

Chemisorption of OH radicals yields coronene radical species, which have been reported to be stable.40 Thus, the approach of UHF and UKS was employed for PM6 and DFT open shell calculations, respectively. In previous work,35 the approximated activation barriers for single OH chemisorptions were evaluated with the PM6 approach41 when considering multiple calculations to evaluate the potential energy curve (PEC). The OH radical was located around the reaction site, in a rectangular mesh, perpendicular to the coronene plane (xy plane), and the geometry optimizations were carried out at each fixed z coordinates of the O from OH. Similar procedure was performed with DFT calculations, using the same starting point of semiempirical coordinates that leads the OH to chemisorption and reaction. Noteworthy is the exploration of different pathways, considering several configurations for OHs, while impractical by using DFT. Nevertheless, the use of a simple method, such as PM6, may be quite useful to discard reaction paths and select the most reactive OH configurations. All calculations for possible reaction paths were corroborated with DFT geometry optimizations, starting at the top of the reaction barriers obtained by PM6. The construction of OH chemisorption configurations, reactions, and interaction with water molecules were performed with the IVIChem web interface.42,43 The DFT package used was ORCA44 with the hybrid TPSS meta-GGA exchangecorrelation functional45 by employing the Becke-Johnson dispersion correction46 (TPSS-D3) and a TZVP basis set.47 Counterpoise corrections (CP) for basis set superposition error (BSSE) were also considered. 13039


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3. RESULTS AND DISCUSSION A paramount oxidant in the troposphere is the OH radical which is mainly the product of O3 photolysis interacting with the H2O molecule to give OH radicals.48 These radicals react in the gas phase with BC surfaces to produce functionalized PAH radical species. In this respect, OH interactions with coronene were examined at different stages, considering single and multiple OH adsorptions, water adsorption effects, and surface reactions that lead to aging and destruction of BC. 3.1. OH Interaction with Different Adsorption Sites. Calculations of OH adsorption on different sites (Figure 1, panels a−c) were performed with DFT when considering dispersion and CP corrections. Results of adsorption energy [Eads(x), x = a, b, c; border, intermediate, and center sites), equilibrium bond distance (EBD) and dipole moment (DM) are given in Table 1. Comparison with previous work35 using

Figure 2. Potential energy curves (PECs) calculated with DFT approach for adsorptions on a (X), b (●), and c (⧫) sites.

Table 1. Values of Adsorption Energy (Eads), Dipole Moment (DM), and Equilibrium Bond Distances (EBD) for Chemisorption on a, b, and c Sites method

Eads (kcal/mol)

DM (Debye)

distortion. Moreover, at the border site a, only rotation out of the surface plane for an H atom is required. Similarly, chemisorption barrier heights follow the order center > intermediate > border. These results consistently suggest OH chemisorption is favored on a sites (border sites). 3.2. Multiple OH Adsorptions. The interaction of OH with BC, a competition between the addition of oxygenated functional groups (functionalization) and the loss of mass (volatilization) to the gas phase may occur. Mysak et al.17 found that the O/C ratio of 0.1 functionalization dominates in BC changes, while O/C > 0.3 volatilization is the most important reaction process. Additionally, once functionalization occurs, water condensation process takes place at the BC surface. Consequently, calculations for chemisorption of multiple OH were carried out considering different combinations for OH radical locations. Calculations with two and three adsorbed OH on coronene were performed with the DFT approach at different adsorption sites (Figure 1). The total energy (Etot), Eads, and DM values are displayed in Table 2, considering different configurations. All described calculations are in singlet or doublet states for two or three OH, respectively, because higher multiplicity states are less stable. Results for two OH show that chemisorption of the second OH is thermodynamically more favorable with respect to a single adsorption. This means, the first adsorption promotes the adsorption of a second OH. The most stable of the OH pair adsorptions occurs at contiguous adsorption sites (a, a′). The Eads for a second OH [Eads = E(coronene-(OH)2) − E(coronene−OH) − E(OH)] is about three times (−78.3 kcal/mol) that of one single OH adsorption on a site (−25.7 kcal/mol). This fact may be explained because the surface is already distorted for the first adsorption, and the created radical is removed after the second adsorption occurs. In the first OH adsorption, the coronene aromatic stabilization is lost because of π-bond is breaking passing the C(a) atom from sp2 to sp3 hybridization. The formation of a radical on coronene occurs; while in the second OH adsorption, the radical is eliminated and no hybridization changes take place. In this case, adsorption energy is coming from the C(a′)−OH bond formation. Additional adsorptions close to the former adsorption site [ortho positions, (c,c′), (b,c), and (a′,b)] are about 1.7 times stronger than one single OH adsorption. The exception for those pairs that are in meta positions [see (a,b), (a,c′), (a′,a″), (b,c′)]; adsorption values are indicated in Table 2). Meta

EBD (Å)

Chemisorption on site c −35.3 −7.5 −8.6 Chemisorption on site b PM6a −38.2 DFT (PBE99-PBE-D)a −8.6 DFT(TPSS-D3) −10.1 Chemisorption on site a PM6a −52.9 DFT (PBE99-PBE-D)a −24.6 DFT(TPSS-D3) −25.7

PM6a DFT (PBE99-PBE-D)a DFT(TPSS-D3)

a

1.70 1.42 1.28

1.46 1.53 1.52

1.95 1.84 1.78

1.46 1.52 1.51

2.08 1.81 1.81

1.44 1.48 1.47

From ref 35.

the PBE99-PBE-D functional with different functional (TPSSD3) and the inclusion of CP correction gives quite similar values. In all cases, the results indicate that OH chemisorption is exothermic (negative values of Eads). This Eads tends (|Eads(a)| > |Eads(b)| > |Eads(c)|) with those obtained with the PM6 code but with overestimation of adsorption energy of the latter. Values of EBD follow an inverse relationship EBD(a) < EBD(b) < EBD(c), and in all cases are about 1.5 Å. It has also been found that OH chemisorption produces a polarization in a direction perpendicular to the surface and follows the dipole moment DP(a) > DP(b) > DP(c) tendency. Information about the adsorption process, PECs for OH interaction on each site was calculated with the DFT approach, as shown in Figure 2. Calculations were performed using as starting points those employed in previous PM6 calculations. That is, by fixing O···C(adsorption site) distance and the z coordinates of two distant atoms from the adsorption site (the coronene is located in the xy plane). The rest of the system is allowed to move during optimization at each point of the PEC. As in the case of PM6 calculations,35 a physisorbed state is observed for adsorption on the c site with a corresponding small barrier for chemisorption. However, on border sites (a), no physisorbed state or chemisorption barrier are obtained. A possible explanation of this feature is based on the requirement on passing from sp2 to sp3 hybridization, which produces a barrier for chemisorption on the b and c sites due to surface 13040


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Table 2. Values of Total Energy (Etot), Adsorption Energy (Eads), and Dipole Moment (DM) for Multiple OH Radicals Adsorptions [(OH)n (n = 1,2,3)] Using DFT Approach. X corresponds to C24H12 and Y to OH radicala surface process X + Y(a) → X−Y(a) X + Y(b) → X−Y(b) X + Y(c) → X − Y(c)

a

X X X X X X X X X X X X X X

− Y(a) + Y(a′) → X − (Y)2(a,a′) − Y(a′) + Y(b) → X − (Y)2(a′,b) − Y(a) + Y(b) → X − (Y)2(a,b) − Y(b) + Y(a) → X − (Y)2(b,a) − Y(b) + Y(b′) → X− (Y)2(b,b′) − Y(a) + Y(a″) → X − (Y)2(a,a″) −Y(a) + Y(c′) → X − (Y)2(a,c′) − Y(c) + Y(a′) → X − (Y)2(c,a′) − Y(a′) + Y(a″) → X−(Y)2(a′,a″) − Y(b) + Y(c′) → X −(Y)2(b,c′) − Y(a) + Y(c) → X − (Y)2(a,c) − Y(a) + Y(a‴) → X − (Y)2(a,a‴) − Y(b) + Y(c) → X−(Y)2(b,c) − Y(c) + Y(c′) → X − (Y)2(c,c′)

X X X X X X X X X X X X X X

− − − − − − − − − − − − − −

(Y)2(a,a′) + Y(a″) → X − (Y)3(a,a′,a″) (Y)2(a,a′) + Y(b) → X − (Y)3(a,a′,b) (Y)2(a,a′) + Y(c) → X − (Y)3(a,a′,c) (Y)2(a,a′) + Y(a‴) → X − (Y)3(a,a′,a‴) (Y)2(a,a″) + Y(b) → X − (Y)3(a,a″,b) (Y)2(c,c′) + Y(b) → X − (Y)3(c,c′,b) (Y)2(a,b) + Y(c) → X − (Y)3(a,b,c) (Y)2(a,c) + Y(b) → X − (Y)3(a,c,b) (Y)2(a′,c′) + Y(c) → X − (Y)3(a′,c′,c) (Y)2(a′,b) + Y(c) → X − (Y)3(a′,b,c) (Y)2(a′,b) + Y(c′) → X − (Y)3(a′,b,c′) (Y)2(b,c) + Y(b′) → X − (Y)3(b,c,b′) (Y)2(c′,c) + Y(a″) → X − (Y)3(c′,c,a″) (Y)2(a′,c) + Y(b′) → X − (Y)3(a′,c,a)

Etot (hartree) Adsorption of One Y −998.2310 −998.2062 −998.2039 Adsorption of Two Ys −1074.1265 −1074.0745 −1074.0270 −1074.0270 −1074.0490 −1074.0729 −1074.0250 −1074.0249 −1074.0356 −1074.0018 −1074.0730 −1074.0420 −1074.0504 −1074.0351 Adsorption of Three Ys −1149.9253 −1149.9520 −1149.9288 −1149.9467 −1149.8867 −1149.8708 −1149.8805 −1149.8805 −1149.8720 −1149.9014 −1149.8911 −1149.8459 −1149.8650 −1149.9288

Eads kcal/mol

DM (debye)

−25.7 −10.1 −8.6

1.47 1.78 1.28

−78.3 −45.6 −15.8a −31.3 −45.2 −44.6 −17.1a −31.6 −21.2a −15.5a −44.7 −25.3 −46.0 −37.9

2.69 2.44 3.33 3.33 2.78 2.72 2.75 3.15 4.10 2.80 2.57 2.19 1.30 2.25

−17.6a −34.3 −19.8a −31.0 −27.0a −40.7 −51.9 −23.0a −17.7a −35.2 −28.8a −15.5a −37.1 −83.5

3.23 3.82 3.48 3.90 3.03 3.24 3.95 3.95 2.63 3.39 3.95 3.27 3.91 3.48

Values correspond to less stable adsorptions for two and three OH radicals.

adsorption order, the process may be more favorable with respect to the previous two OH adsorptions [i.e., (a, c, and b) is −23.0 while (a, b, and c) is −51.9 kcal/mol. A particular case in which the three OH are in meta positions (a′, c, and b′) is unstable and the OH(b′) migrate to an ortho location (a) with respect to a′ giving (a′, c, and a) (last row of Table 2). It is possible to conclude from this section that OH chemisorption energies depend on previous adsorptions; consequently, the kinetics and dynamics of these interactions are very complicated. In general, a clear trend in the DM is observed, that is more OH adsorption higher DM values. Therefore, adsorption of an OH radical makes the coronene surface more hygroscopic because of the increase of DM. For example, DM for coronene augments (0.0 D) when the OH are adsorbed in the coronene(OH)n to values as high as 1.78, 3.33, and 4.07 D units, for n = 1, 2, and 3, respectively. 3.3. Water Interaction with OH Radicals. As previously described, OH adsorption polarizes the carbonaceous surface and creates a net DM, when OH is absorbed on border sites. It is well-known that intermolecular forces are mainly due to hydrogen bonds [H(δ+)····O(δ−)H], dipole−dipole interactions, permanent dipole with induced dipole, and London

adsorptions create a biradical, because two double bonds are broken in the aromatic configurations with alternated double bonds. On the other hand, on para adsorptions, this fact does not occur [see (a,c) and (b,b′)] due to the fact that the two formed radicals recombine to form a double bond. Sites that are far away, at the meta location show low Eads [example, pair (a,a‴)]. It is important to mention that Eads of a second OH depends on the adsorption of the first one. For example, adsorption on a and then on b (a,b) gives an Eads of −15.8 kcal/ mol (see adsorption of two OH in Table 2), while in the reverse way (b,a), a higher Eads of −31.3 kcal/mol is obtained. Generally, values of DM also increase with the adsorption of a second OH because they are in the same surface face. This means, the surface polarity increases with the OH adsorption coverage. Adsorptions of three OH in several sites are shown in Table 2. All calculations are displayed in doublet state because they are more stable than quadruplet. Results seem to indicate that chemisorption, generally, is more energetically stable than for a single OH. The less stable chemisorptions occur when meta adsorptions take place, as in (a, a′, and a″), (a, a′, and c), (a, c, and b), (c′, a′, and b), (a′, c′, and c), and (b, c, and b′) shown by Eads values (indicated in the footnote). Depending of the 13041


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The Journal of Physical Chemistry A dispersion. This means, these forces are involved in water condensation that may occur on BC particles, as abovementioned for CCN formation.13,14 Prior to the study of water interaction with adsorbed OH on coronene, a simple water cluster (trimer) was calculated to evaluate the water−water interaction. The selection of a water trimer derives from a prototype of three-body forces in liquid and solid water. The expected results show a tendency to form molecular water aggregates. Values from literature (−16.6 using MP4, −16.4 employing CCSD(T),49 agree reasonably well with −18.2 kcal/mol obtained in the present work) (Figure 3a).

Figure 3. DFT calculations for (a) total water trimer interaction among themselves and (b) water trimer interaction with OH radical.

Calculations were evaluated with dispersion and counterpoise corrections for BSSE. Previous DFT calculations35 with PBE99PBE functional plus dispersion and a TZVP basis set and PM6 yielded values of −15.4 and −14.1 kcal/mol, respectively. Water clusters may also interact with OH free radicals. Calculations of the water trimer with a single OH radical were also performed to get the most stable configuration (Figure 3b). The interaction is mainly through hydrogen bridge bonds (−12.6 kcal/mol) between water hydrogen atoms and the oxygen of OH. This result suggests that water clusters may also trap OH radicals. It is important to note that physisorption of water molecules could maintain an important OH concentration around the coronene surface. However, radicals have to migrate from the particle surface coated with water to its core in order to continue the functionalization of the BC species. This diffusion is possible because BC has a high capability of light absorption, and the barrier for H−O···H−O−H → H− O−H···OH reaction is low (∼4 kcal/mol).50 In this sense, BC aging may strongly be influenced by the atmospheric humidity and radiation. In latitudes where relative humidity is low (i.e., midlatitudes), this H2O effect might be reduced. This influence is reported by Wei et al.51 where an enhanced light absorption on atmospheric BC at relatively high humidity was obtained. To evaluate the net contribution of OH adsorptions in the water interaction, a calculation of clean coronene with the water trimer was carried out (Figure 4a). Results show that the Eads is small (−4.5 kcal/mol), including BSSE correction. On the other hand, calculations for water cluster with the hydrolyzed coronene (C24H12(OH)3), using the most stable configurations of Table 2, exhibits a stronger interaction (−32.8 to −43.1 kcal/mol) with respect to the separated water molecules (Table 3 and Figure 4b). The water cluster has slightly better interaction than the separated water molecules on the corresponding OH sites. Yet, the most stable system (a, a′, and a″) with −1379.4026 au (Table 3) corresponds to the strongest adsorption energy (−43.1 kcal/mol). Therefore, the H2O trimer prefers to be adsorbed on edge sites.

Figure 4. Geometrical structures and adsorption energies (Eads) calculated with DFT: (a) for a water trimer on clean coronene (b) with three OH radicals located on the most stable configuration (a, a′, and a‴).

Table 3. DFT Calculations for Interaction of a Water Trimer and Three Separated Water DFT Calculations for Interaction of a Water Trimer and Three Separated Water Molecules with OHs Located on (a,a′,b), (a,c,a′), and (a,a′,a‴) Surface Sitesa water adsorption process

Etot (hartree)

Eads (kcal/mol)

DM (Debye)

water trimer above OH sites −1379.3915 −32.8 1.92 C24H12(OH)3(a,a′,b) + (H2O)3 −1379.3743 −36.5 1.00 C24H12-(OH)3(a,c,a′) + (H2O)3 −1379.4026 −43.1 4.59 C24H12-(OH)3(a,a′,a‴) + (H2O)3 three separated water molecules above the corresponding OH sites C24H12-(OH)3(a,a′,b) + 3H2O −1379.3899 −31.8 4.03 C24H12-(OH)3(a,c,a′) + 3H2O −1379.3747 −36.8 1.76 C24H12-(OH)3(a,a′,a‴) + −1379.3910 −35.8 1.92 3H2O a

Etot is the total energy of the C24H12(OH)3-(H2O)3 system. Eads is adsorption energy [Eads = Etot(C24H12(OH)3-(H2O)3) − Etot((H2O)3) − Etot(C24H12(OH)3)] molecules with OHs located on (a,a′,b), (a,c,a′), and (a,a′,a‴) surface sites. Etot is the total energy of the C24H12(OH)3-(H2O)3 system. Eads is adsorption energy [Eads = Etot(C24H12(OH)3-(H2O)3) − Etot((H2O)3) − Etot(C24H12(OH)3)].

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Results of the reaction path also indicate that the activation barrier is negligible. A geometry optimization calculation, where the reaction starts (Figure 5, vertical arrow) was also performed with DFT using coordinates from PM6. Results also showed the H abstraction from coronene after optimization with an Ereact value about −61.1 kcal/mol and a final distance C(a′)··· OH2 ≈ 4 Å (Figure 5 in □). This H abstraction has been reported for aromatic systems, such as benzene,53 because the hydroxyl radical is generally considered to be an electrophilic species. 4.2. C−C Bond Scission and Carboxyl Group Formation. In the previous work,35 the adsorption of other OHs on the a′ and a sites may occur. Moreover, the present investigation has examined several channels for the OH attack to the four OH groups at the coronene border sites with PM6 (see Figure 6). One of the channels gives a negligible barrier,

Generally, the water interaction decreases the DM on the coronene surface; however, the polarity of the surface is not null in the range of 1.0−4.59 D units. This means, other water layers may be formed on BC surface model. In this sense, Martin et al.52 found that biomass burning is the most important source of CCN in the Amazonian region, due to BC products interacting strongly with clouds to yield rainfalls.

4. AGING REACTIONS ON ATMOSPHERIC CARBON GRAINS Atmospheric degradation reactions are very relevant in studies related to environments and behavior of chemical pollutants. As described before, BC has a lifetime between 6−10 days, depending on the transformation rate between hydrophobic and hydrophilic property. BC is a strong light absorber and has a proportionately greater effect on radiation absorption than the volatile organic compounds (short-lived organic carbon). Little research has been done to understand details of the BC aging processes occurring in the atmosphere. In this section, we are aimed at studying several reactions with multiple OH interactions to transform BC to volatile products, as proposed by Mysak.17 4.1. Hydrogen Abstraction from PAH. A previous work, OH interaction on coronene in two adjacent sites produces weakening of C···C bond due to bond distance elongation.35 In addition, the adsorption of an OH on border sites will also lead to a decrease of the C···H bond strength. Calculations with PM6 indicated that configuration with two OH, on a′ and b sites, leads to an H abstraction from the border site (a′) by an incoming third OH, with water formation. In this work, calculations of PEC with PM6 fixing C(a′) and O coordinates of OH were performed. The rest of the atoms were fully optimized at each point of the PEC. The starting point of reactants and the final products are given in an inset of Figure 5. At the distance C(a′)···OH ≈ 2.6 Å, the energy of the system dramatically drops and the reaction C24H12(OH)2 + OH → C24H11(OH)2 + H2O occurs. This process is exothermic and starts at H···OH distances ≈1.7 Å. A full optimization was performed without restrictions, and as expected, the H2O leaves from the surface at 3.5 Å, and the system stabilizes with reaction energy (Ereact) of −65.5 kcal/mol (Figure 5 in ■).

Figure 6. Approximated PEC for the formation of COOH group and C−C bond breaking by H abstraction from a chemisorbed OH using PM6. Calculated values of Ereact and O···C distance with DFT and PM6 are given with the symbols (□) and (■), respectively.

and the OH will abstract an H atom from a border OH group to generate the C(a)-C(a′) bond breaking and the formation of carboxyl and hydroxyl groups and water. The reaction C24H10(OH)6 + OH → C24H10(OH)5O + H2O was found to be exothermic with an Ereact of −54.1 kcal/mol (Figure 6 in ■). Similarly, H abstraction from chemisorbed OH was also confirmed by a DFT starting with coordinates previous from PM6 (Figure 6, vertical arrow). An Ereact value of −41.7 kcal/ mol was obtained and a C(a′)···OH2 distance ≈ 3.4 Å (Figure 6, □ symbol). The formation of an O radical on an adsorbed OH by H abstraction will boost the formation of the CO bond, and this will raise the C−C bond scission to obviate the formation of pentacoordinate carbon moiety. Noteworthy, bond breaking (C−H, O−H, and C−C) depends upon several OH chemisorptions to be reached. The present results are in agreement with Mysak et al.,17 employing X-ray photoelectron spectroscopy (XPS) to monitor the OH reaction with coronene. These authors reported the formation of CO and COOH groups by proposing that BC aging takes place when the O/C ratio exceeds 0.5 (adsorption of many OH). 4.3. Volatilization and CO2 Formation. Previous work,35 the BC destruction with formation of volatile products was not

Figure 5. Approximated PEC for the hydrogen abstraction from coronene calculated with PM6 to produce H2O. Calculated value of reaction energy (Ereact) from DFT is presented with the symbol (■) after optimization with the final O···C distance. 13043


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The Journal of Physical Chemistry A reported. Conversely, the present study has considered that once the carboxyl groups are formed, interactions with other incoming OH may lead the H abstraction from COOH groups. This fact creates a OCO· radical that prompts CO 2 formation by breaking the C(a′)···C(b) bond. Consequently, the calculated PEC for one reaction channel with PM6 method for a COOH···OH approach confirms this hypothesis (Figure 7). Results also revealed physisorbed states of 10 and 5 kcal/

Figure 7. Approximated PEC for the formation of CO2 and H2O by abstraction from COOH using PM6. Values of Ereact and O···C distance from DFT and PM6 after optimization are displayed with the symbols (□) and (■), respectively. Figure 8. PECs and Eint calculated with DFT for (a) O2 and (b) OH interactions on site a with respect to an OH in a′ (ortho location). Values of Eads after optimization are displayed with the symbol (○).

mol at C(a′)···OH distances of 3.1 and 2.2 Å, respectively. The PEC exhibits a reaction barrier for chemisorption about 15 kcal/mol. The products are 65.4 kcal/mol more stable than the reactants (Figure 7 in ■). Similarly, calculation with DFT starting about 2 Å arises to the formation of CO2 plus H2O. The Ereact of −71.0 kcal/mol and a final distance C(a′)···OH2 of about 4.9 Å is shown in Figure 7 with □ symbol. Formation of CO2 by OH has been proposed from the experimental work of Molina et al.16 on solid organic compounds. Moreover, Boehm54 studied that activated carbons are slowly oxidized at room temperature in the presence of water vapor. Surface oxides are formed, and some CO2 is gradually released in this aging reaction. The aging process may be forwarded to be channeled right by OH chemisorptions and H abstractions. The combination of these two reactions may lead to C−C bond breaking that will drive to BC functionalization and finally to its destruction. Note, decarboxylation may also occur in ionic mechanisms that may be catalyzed by acid or base,55 yet it is not considered in the present work. 4.4. Interaction with Molecular Oxygen. Interaction of coronene with multiple OH appears to explain the BC aging with Mysak’s model;17 however, the OH concentration in the atmosphere is very low as compared with O2. The effect of an O2 molecule is here considered after chemisorption of OH at the border site. As mentioned above (section 3.2), an OH radical chemisorption on a site creates a radical on the coronene surface which is very reactive to the adsorption of a second OH (ortho location a′, Table 2). For this reason, DFT calculations of PEC for O2 and OH interactions on site a′, when an OH is adsorbed on site a (Figure 8). These results indicate a relatively weak Eads for O2 (−15.7 kcal/mol, ○ of Figure 8a) compared with OH (−78.3 kcal/mol, ○ of Figure 8b). No chemisorption barriers have been observed in both cases. This low Eads for O2 forming peroxide radical, and the

strong light absorbance property of BC may lead to desorption by breaking the weak C···O−O bond. Chemisorptions of O2 on meta and para locations do not occur. Since OH radical production is maintained in the presence of water and sunlight, a new OH may be adsorbed on the C24H12−OH−O2 system. For example, chemisorption in para location (c site) with respect to the first OH (a site) gives an Eads value of −13.1 kcal/mol. In this sense, O2 desorption may take place because the process C24H12-(OH)2-O2 → C24H12(OH)2 + O2 is favored by 15.9 kcal/mol. This means O2 desorption is facilitated by other OH adsorption because the existing radical is eliminated. It is important to point out that O2 chemisorption may not be probable without the formation of an unpaired electron on the surface. Furthermore, migration of OH on the surface may be possible because diffusion barriers are very small (2.3 kcal/mol).56 Several authors40,57,58 have proposed that systems like the C24H12−OH-O2 radical are able to react with atmospheric gases (NO, NO2, H2O, and OH) or intermediates in OH formation (OH−, O2−, and O2H).54 Consequently, calculations performed by exploration of OH around the chemisorbed O2 reveal that the formation of C24H12-OH-(OOOH) is feasible (−34.7 kcal/mol). This stable intermediate is favored due to the elimination of the unpaired electrons located on O2. As described above, the OH radicals may diffuse on the surface. Therefore, modeling all species is a very complex process. This is because interactions with different species would lead to a great amount of consecutive reactions associated with many other reaction paths. Several considerations have been taken into account for OH concentration. Local concentration of OH around the BC grain 13044


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continue to finally produce volatile oxygenate organic compounds. (5) A realistic model of the atmosphere would contain O2. Calculations with O2 showed that the formation of peroxide radical on coronene surface may occur without an energy barrier. However, the Eads of O2 on the coronene surface is about five times weaker than the corresponding to OH. (6) Further research of BC decomposition or aging due to multiple atmospheric oxidants may also be considered in computational modeling. It is known that interactions with primary oxidants (OH, O3, and NO3) is followed by reaction with other species (N2O5, O2, NO2, NO, and halogen radicals). The multiadsorption of these small molecules on activated BC may generate the formation of several volatile organic compounds.40 (7) It is worth noting that the general trends for OH reactions on coronene with the semiempirical method PM6 were qualitatively reproduced by more accurate DFT calculations that include dispersion and counterpoise corrections. (8) The mechanisms calculated correspond only to radical reactions. The two major pathways for aromatic decarboxylation reactions are ionic and free radical. In aqueous solution, ionic decarboxylations may also be catalyzed by acid as proposed by Manion et al.55 (9) Several possible chemical and physical processes must be considered for the conversion of soot from hydrophobic to hydrophilic and aging reactions. This includes coagulation with other particles and condensation of gas-phase organic and inorganic species.

could be higher than in the whole atmosphere. As mentioned above (section 3.3), the OH radical may be trapped on the water shell that covers the BC grain. In this sense, experimental results of UV photochemistry of coronene39 by the interaction with water, in argon cryogenic matrices, yield oxygenated products (1,10-dihydroxycoronene and 1,10-coroquinone). Similarly, Yamakoshi et al.,59 using fullerenes as photosensitizers in biological systems, observed the formation of active oxygen species (O2− and OH radical), through energy transfer (singlet oxygen 1O2) and electron transfer. In this sense, Wu et al.60 found that photo-oxidation of hydrogenated fullerene in water leads to a functionalized surface with several O and OH groups. These OH produced in situ may partially explain the formation of oxygenated species proposed by Mysak et al.17 for the case of coronene. The BC aging mechanisms in the atmosphere are complex because of the amount of intermediates formed due to a variety of components in the atmosphere. Additionally, different levels of light radiation and air humidity influence the OH and O3 radical concentrations and the lifetime of the BC grain.

5. CONCLUSIONS AND COMMENTS Several important issues due to the interaction of an OH radical atmosphere with a BC model (coronene) were considered, including its transformations by chemisorption, edge surface reactions, and water interaction. The most important findings and comments of further works are given as follows. (1) There is an energetic preference for OH adsorption on border sites a of PAH components of BC. Chemisorption barriers are small in all adsorption sites, therefore, from a kinetics point of view, PAH hydroxylation must readily occur. Moreover, adsorption of the first OH leads to stronger chemisorption for subsequent OH adsorptions. Generally, adsorptions on position meta are less favorable than ortho and para. (2) Due to the high reactivity and electronegativity of OH radical, chemisorptions originate a net dipole moment due to electronic transfer from coronene to OH groups. This dipole with the presence of hydrogen bonds enhances the water physisorption because water−dipole interactions will transform hydroxylated coronene to a hygroscopic entity. (3) The OH radical is stabilized by interaction with water clusters. The OH displacement through the adsorbed water cluster may occur by diffusion or by the following reaction: OH* + HOH ↔ HOH* + OH. It is important to notice that the aging reaction rate may be strongly influenced by changes in relative humidity. This fact leads to variations in the BC particle growing and diffusivity of the radicals due to hygroscopic water uptake. Notice that the probability of extracting an H atom would be proportional to the OH coverage at the coronene border sites. The latter depends on the atmospheric OH concentration. (4) There are five possible steps involved in coronene aging by OH radical reactions: (i) Multiple OH chemisorption inducing surface polarization and makes the carbonaceous material hydroscopic. (ii) Physisorption of water molecules on hydroxylated coronene sites. (iii) Abstraction of the H atom from coronene at the border sites in which an OH is previously chemisorbed, thus, leading to a hydroxyl group at the border site. (iv) New adsorption on border sites forms C-(OH)2. Then, if there are two adjacent C(OH)2 groups, abstraction of an H from OH will occur with a C−C bond breaking and formation of a COOH group. (v) Abstraction of H from the COOH group to form CO2 plus H2O may occur. This way, the destruction of back carbon components is materialized. This process may

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Drs. Loreto Donoso and Tibisay Pérez for a fruitful discussion of experimental data.

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REFERENCES

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