pubs.acs.org/JPCL
Exploring the Role of PAHs in the Formation of Soot: Pyrene Dimerization Hassan Sabbah,†,§ Ludovic Biennier,† Stephen J. Klippenstein,*,‡ Ian R. Sims,*,† and Bertrand R. Rowe† †
Institut de Physique de Rennes, Equipe “Astrochimie Exp erimentale”, UMR 6251 du CNRS, B^ at. 11c, Universit e de Rennes 1, Campus de Beaulieu, 35042 RENNES Cedex, France, and ‡Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.
ABSTRACT A critical step in currently accepted models for soot formation in combustion is the dimerization of polycyclic aromatic hydrocarbons as small as pyrene, which is necessary within these models to reproduce correctly the soot particle size distribution. We present experimental measurements on the kinetics of pyrene dimerization performed in low-temperature supersonic flows with photoionization mass spectrometric detection, coupled with theoretical results based on careful consideration of the intermolecular interaction energies, binding energy, equilibrium constant, and intermolecular dynamics. These results demonstrate that the equilibrium of the reaction strongly favors the dissociation of the pyrene dimer at high temperature and that physical dimerization (involving van der Waals forces) of pyrene cannot be a key step in carbon particle formation in hot environments such as flames and circumstellar shells. SECTION Kinetics, Spectroscopy
T
This hypothesis has subsequently been taken up within the wider combustion modeling community. There now exists a general consensus that the early chemistry of the soot formation process results in the formation of highly condensed aromatic structures and that at some point in their growth, nonbonding interactions will be strong enough so that chemical bonding is no longer a requirement for sticking. Currently, many numerical simulations of soot formation in combustion7-11 invoke irreversible binding of molecules as small as pyrene, despite the relatively low binding energy of the pyrene dimer (∼41 kJ mol-1 according to a recent calculation12) and despite the fact that there are no definitive experimental data available to support the claim.3 The current work aims, therefore, to rectify this deficiency. The exploration of the role of PAHs in the formation of carbonaceous particles is also of interest for astrophysical environments. Conditions in circumstellar envelopes of C-rich stars bear some resemblance to those in combustion. Progress in modeling the formation and growth of PAHs in combustion has motivated astrophysicists to propose essentially the same chemistry in space, with PAHs considered as the building blocks of stellar dust. Frenklach and Feigelson13 were the first to apply this chemistry of PAH formation to the circumstellar envelopes of carbon-rich AGB stars. Calculations performed by Miller et al.14 of the equilibrium concentrations of several PAH dimers under terrestrial flame
he formation of soot in combustion is of prime importance for fuel engine efficiency, the environment, and human health.1 Intense efforts have therefore been made in recent years to understand the associated physics and chemistry and, in particular, to develop detailed mechanistic models. At the microscopic level, such models consist of two principal components, elementary gas-phase chemistry and subsequent soot particle dynamics. The link between these two components, the formation of a critical nucleus from the gas phase enabling the transition to the solid phase, is crucial but poorly understood. The objective of this study is to explore this key step in relation to the widely used model first proposed by Frenklach and Wang.2 The central role of condensed polycyclic aromatic hydrocarbon (PAH) systems in soot formation has been recognized since the mid 1980s.3 The early modeling work of Frenklach et al.4 constituted a pioneering step toward the use of detailed kinetic modeling in order to understand PAH and soot formation in combustion.5 In this initial model, the transition from gaseous species to solid particles was assumed to result from pure chemical growth. The model was able to predict the correct order of magnitude of the incipient soot particle concentration, but it underestimated the mean soot particle size. In subsequent work,2,6 Frenklach and co-workers proposed that the formation of critical nuclei for subsequent condensation via physical association of PAHs as small as pyrene was necessary to reproduce correctly the soot particle size distribution. The nucleation model was modified, and PAH clusters bound by van der Waals forces were incorporated as critical nuclei.
r 2010 American Chemical Society
Received Date: July 27, 2010 Accepted Date: September 10, 2010 Published on Web Date: September 20, 2010
2962
DOI: 10.1021/jz101033t |J. Phys. Chem. Lett. 2010, 1, 2962–2967
pubs.acs.org/JPCL
conditions gave values significantly below the number densities observed for small soot particles. On the basis of this analysis, they questioned the assumption that soot nucleation begins with PAH dimerization. In subsequent work,15 Miller calculated size-dependent lifetimes of PAH dimers by a trajectory method on a Lennard-Jones potential and found that they approach chemical reaction times for aromatic growth only at a PAH monomer mass 4 times that of pyrene, calling into question the role of PAH dimers with a mass below 800 amu in soot nucleation. In response to this work, Frenklach and co-workers16 undertook a theoretical investigation of binary collisions between pyrene molecules, taking into account molecular structure and internal molecular motions, and obtained a pyrene dimer lifetime of 12 ps, similar to the result found by Miller. However, they continued to argue for pyrene dimers as soot precursors on the basis that Miller in his calculation had not treated internal free rotors, likely to increase substantially dimer lifetimes, and also that the dimer lifetime of 12 ps is actually greater than the mean collision time with the bath gas of ∼10 ps at 10 bar, a midrange pressure for diesel combustion. We present here an experimental and theoretical study of the dimerization and nucleation of pyrene at high and low temperatures. Our aim was to identify the range of temperatures in which the dimerization of pyrene (C16H10) monomers occurs, then to measure, and model, rate coefficients for dimerization over this range, and finally to use this validated model to predict the likely behavior of pyrene under combustion conditions. The experiments were performed using a continuous-flow CRESU17,18 apparatus, modified to use condensable species such as PAHs.19,20 This chemical reactor is designed to generate dense uniform flows using a Laval nozzle over a wide range of temperatures (60-470 K) and containing high concentrations of PAH vapors. Previous work on the dimerization of benzene21 had shown that while the CRESU technique was suitable for studying dimerization, a technique other than laser-induced fluorescence was necessary to follow evolving monomer and dimer concentrations. Accordingly, a time-of-flight mass spectrometer equipped with a vacuum ultraviolet excimer laser (λ = 157 nm) for photoionization was included to probe the reaction kinetics of the chemical species present in the reactor. Details of the experimental apparatus are provided below and in the Supporting Information. The first step was to determine the onset of nucleation. For this purpose, values of the pyrene monomer signal σpy were plotted as a function of pyrene concentration at each flow temperature employed. Four different temperatures were provided by the available Laval nozzles in the range of 60-470 K. Figure 1 shows two such plots, one recorded at 235 K and one at 120 K. Experiments performed at 470 and 235 K (Figures S3a (Supporting Information) and 1a) showed excellent linearity between σpy and the pyrene monomer concentration as calculated from the relative flow rates of pyrene from the oven and buffer gas and the total gas density in the flow. This is not the case, however, for flows at lower temperatures. At 120 K (Figure 1b), the signal first increases linearly with the increase of pyrene concentration, but above a certain value,
r 2010 American Chemical Society
Figure 1. Plots of the pyrene monomer signal as a function of the nominal density (as calculated from the pyrene flow rate, mixing ratio, and total flow density). The upper panel (a) demonstrates the absence of significant nucleation at 235 K at pyrene densities up to ∼4 1014 molecules cm-3 for a reaction time of 143 μs at a total density of 2.06 1016 molecules cm-3. In the lower panel (b), nucleation at a temperature of 120 K is evidenced by the collapse of the pyrene monomer signal above a nominal pyrene density of 1.7 1014 molecules cm-3 for a reaction time of 118 μs and a total density of 1.96 1016 molecules cm-3.
a sudden decrease of σpy is observed. This loss of monomer signal is explained by the onset of nucleation. We propose that, at these high degrees of supersaturation, pyrene dimers may be considered as critical nuclei within the framework of classical nucleation theory.22 Once they are formed, they go on to react rapidly with more monomer to form trimers and higher oligomers (as demonstrated in the mass spectrum shown in Figure S2, Supporting Information). The same phenomenon was observed at 60 and 80 K (Figure S3, Supporting Information). After the determination of the temperature range over which this nucleation was occurring, kinetics measurements on pyrene dimerization were performed within this range but at lower pyrene monomer concentrations, where such nucleation did not occur. In each set of experiments, values of the pyrene monomer signal were measured at a series of different distances d from the exit of the Laval nozzle, corresponding to a range of reaction times which could be calculated from the supersonic flow speed, determined beforehand via impact pressure measurements.23 Care was taken to ensure that the fraction of monomer consumed remained relatively small (
