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Langmuir 1987, 3, 244-254
electrodeposit. These findings and ref 1-9 are consistent with the idea that an electrodepositing metal competes for adsorption sites with supporting electrolyte and other species present in the solution. For instance, according to this line of reasoning, the observed greater stability of P b deposits in chloride media than in bromide or iodide, items i and ii, is due to the lesser activity of chloride for attachment to the platinum surface 2nd thus stronger attachment of lead. Halogen atoms evidently prefer lower coordination numbers than P b atoms and thus adopt positions on or in the outermost layer of metal atoms, item iii. As a result of the differing adsorbabilities of the halides on Pt, namely, I- > Br- > C1> F-,l7-I9as well as the differing atomic/ionic radii, dif-
ferent electrodeposited monolayer structures are formed, item iv. Electrodeposited P b atoms, halogen atoms, and the substrate interact at both short and long range to form ordered layers, adlattices, the structure of which is a sensitive function of the packing densities of P b and halogen atoms constituting the layer, item v. The free energies of these adlattices differ sufficiently that the formation of the adlattices is resolved voltammetrically, as in Figure 4. That is, mixed adlattices are formed, the structures and compositions of which are specific to a given set of conditions. Primary variables include the identity of the electrodeposited material, substrate, electrolytic anion, and solvent; the crystallographic orientation, structural perfection, and degree of cleanliness of the substrate surface; the pH; and the amount of material electrodeposited.
(18) Stern, D. A.; Baltruschat, H.; Martinez, M.; Stickney, J. L.; Song, D.; Lewis, S.K.; Frank, D. G.; Hubbard, A. T. J. EEectroanaL. Chem., in
Acknowledgment. This work was supported by the National Science Foundation. Registry No. HBr, 10035-10-6; CaBrP, 7789-41-5; Br-, 24959-67-9; Pb, 7439-92-1; Pt, 7440-06-4.
press. (19) Salaita, C. N.; Lu, F.; Baltruschat, H.; Hubbard, A. T. J. Electroanal. Chem., in press.
Soot Particle Formation in Laminar Diffusion Flamed Robert J. Santoro** and J. Houston Millers National Bureau of Standards, Gaithersburg, Maryland 20899 Received October 7, 1986. In Final Form: January 5, 1987 Soot formation processes in laminar diffusion flames are described. Experimental results obtained at the National Bureau of Standards are reviewed and show that fuel molecules are rapidly converted to acetylene and other key precursors (such as the vinyl radical) in flame positions adjacent to and on the fuel-rich side of the high-temperature reaction zone. These molecules react rapidly to form small aromatic species, such as benzene, and eventually small soot particles. This soot particle inception process occurs in a time period on the order of about 1 ms. Once formed, particles may grow rapidly by surface chemistry if they travel through flame regions with high concentrations of growth species such as acetylene. A systematic variation of the fuel flow rate shows that the final particle size, as well as the volume fraction occupied by soot particles, is determined by the particle residence time in this growth region. During the time that surface growth is occurring, particle agglomeration leads to a decrease in total soot particle number concentration. As particles continue along streamlines, they eventually cross the high-temperature reaction zone where they may be oxidized. The oxidative process is also affected by fuel flow rate in the sense that the larger amount of soot formed in flames with relatively high fuel flow rates results in an increase in the radiative transfer from the flame. This reduces the flame temperature and, hence, the rate of oxidation.
Introduction The formation of soot particles in combustion systems has been an active area of research for the past 25 years. Interest in this topic spans a wide range of problems and expertise and can involve experimental situations as simple as a laboratory-scale premixed burner or as complex as a jet aircraft combustor. However, in each of these systems the fundamental processes which govern the formation of the soot particles share a common sequence of events. These include (1)a chemical kinetically controlled reaction sequence which results in the formation of precursor species needed to form the first particles, (2) a particle 'Presented a t the symposium on "Fine Particles: High Temperature Synthesis", 60th Colloid and Surface Science Symposium, June 15-18, 1986, Atlanta, GA; G. Mulholland, Chairman. Center for Chemical Engineering. Permanent address: Department of Mechanical Engineering, T h e Pennsylvania State University, University Park, PA 16802. 5 Center for Fire Research. Permanent address: Department of Chemistry. T h e George Washington University, Washington, DC 20052.
*
inception stage which results in the formation of large numbers of small primary particles, (3) a particle growth period in which surface growth and particle coagulation processes contribute to the increase in particle size, and (4) a stage in which material is no longer added to the soot particles and size is controlled by agglomeration or even may be reduced by oxidative attack. Within this general framework a variety of studies have been carried out to examine specific steps and mechanisms which contribute to the formation and evolution of soot particles in flames. As mentioned above, these may involve quite complicated experimental environments in which only a limited number of measurements are possible. Many of these studies have aimed a t providing a general engineering understanding of the parameters important in the soot formation process. Such studies can provide useful information for aiding in the design of hardware or in establishing proper operating procedures. However, it is difficult to gain direct information on the fundamental processes which govern the formation of soot particles from these studies. To address this area, research on more fundamental combustion systems has been undertaken. In particular, laboratory-
This article not subject to U S . Copyright. Published 1987 by the American Chemical Society.
Soot Particle Formation scale flames have proven to be attractive systems in which t o examine the fundamental physical and chemical processes involved in the soot particle production problem. Initially these studies involved premixed flames a t stoichiometric or slightly rich conditions often operating a t low pressure (20-50 torr).’g2 Under these conditions molecular beam/mass spectrometric sampling could be carried out providing a wealth of information on the chemical evolution of the flame. Particle measurements were also attempted in some of these studies by applying electron microscopy techniques to particles collected through the sampling ~ r i f i c e . As ~ diagnostic tools have been developed, particularly laser-based technique^,^ the number of studies examining soot particle formation has increased substantially. Recently emphasis has been given to investigations which examine soot particle formation in diffusion flame^.^-^ One motivation for increased interest in diffusion flame studies lies in the fact that, in most practical systems, fuel and oxidizer species are introduced separately with mixing processes strongly influencing the combustion events. In terms of studying a system in which mixing processes have an effect, laminar diffusion flames represent one of the simpler systems available for study. The steady-state nature of these flames allows measurement of a wide variety of variables and provides a suitable environment for following the combustion processes in both the spatial and temporal coordinates. Thus, simple laminar diffusion flames represent a tractable system in which to examine the formation and evolution of the soot particle field. In the study of soot formation in diffusion flames it is important to keep in mind that there is a close coupling between the particle, gas-phase species, and temperature and velocity fields. The specific nature of these fields has significant effects on the spatial distribution of soot particles in the flame, the rates of growth, and the extent to which oxidation reduces the soot concentration in the latter stages of the combustion process. In order to address this problem in a comprehensive manner, an intensive study of soot formation in diffusion flames has been ongoing a t the National Bureau of Standards (NBS) over the last several years. As part of this effort, a variety of measurement techniques have been applied to laminar diffusion flames in order t o gain a detailed understanding of the evolution of the soot formation process in these flames. A goal of these studies has been to examine the complete series of processes which determine the formation, growth, and burnout of soot particles. Specific attention has been given to resolving the important chemistry leading to the formation of the key precursor molecules to soot particle formation, to following the evolution of the particle field throughout the flame, and t o obtaining quantitative measurements of the rates of these processes. To achieve this goal, measurements of the important intermediate (1) Homann, K. H.; Wagner, H. Gg. Symp. (Int.) Combust., [Proc.] 1966,I l t h , 371. (2) Bittner, J. D.; Howard, J. B. Symp. (Int.) Combust., [Proc.]1981, 18th, 1105. (3) Wersborg, B. L.; Howard, J. B.; Williams, G. C. Symp. (Int.) Combust., [Proc.] 1972,I l t h , 929. (4) D’Alessio, A.; DiLorenzo, A,; Sarofim, A. F.; Berretta, F.; Masi, S.; Venitozzi, C. Symp. (Int.) Combust., [Proc.] 1975,15th, 1427. (5) Smyth, K. C.; Miller, J. H.; Dorfman, R. C.; Mallard, W. G.; Santoro, R. J. Combust. Flame 1985,62, 157. (6) Kent, J. H.; Jander, H.; Wagner, H. Gg. Symp. (Int.) Combust., [Proc.]1981,18th, 1117. (7) Santoro, R. J.: Semeriian, H. G.: Dobbins. R. A. Combust. Flame 1983,51, 203. (8) Vansburger, U.; Kennedy, I. M.; Glassman, I. Symp. (Int.) Combust., [Proc.] 1984,20th,1105.
Langmuir, Vol. 3, No. 2, 1987 245 gas-phase species, soot particle size and concentration, temperature, and velocity fields have been required. The results of these studies provide one of the most comprehensive data bases from which to examine the phenomena of soot particle formation in flames. In the present paper, a review of some of the results of these studies will be given along with some discussion of areas of future interest. Because our goal is to concentrate on the studies performed at NBS, our treatment of efforts by other workers will not be as comprehensive as it should be. Several excellent reviews of studies of soot formation exist and the interested reader is urged to refer to those sources for the details of other work in this area.*14 Throughout the paper appropriate literature references will be made, but it will not be possible to discuss that work in this paper.
Experimental Section A. Burners. A series of studies of laminar diffusion flames have been undertaken in which spatially detailed measurements of the gas-phase species, particle, and velocity and temperature fields have been obtained. Two burner configurations have been used for these studies: a Wolfhard-Parker slot burner and a coannular diffusion flame burner. The majority of the studies have been carried out for flames burning methane or ethene in air. All of the gas-phase species measurements reported here have been done on the Wolfhard-Parker slot burner in which methane/air diffusion flames were studied at atmospheric pressure conditions. The design of this burner has been described previously in detail and will only briefly be summarized here.6 The fuel flowed through an 8 mm wide by 41 mm long slot sandwiched between two 16 mm by 41 mm air slots (see Figure 1). The resulting flame sheets were symmetric about the plane through the center of the burner. The flame was stabilized by a wire screen chimney with “gulls” similar to the design of Kent et aL6 The burner assembly was mounted on a two-dimensional, computer-controlled, micrometer stage so that movement laterally, through the flame sheets, and vertically was possible. Lateral profiles of temperature, velocity, and species concentration were collected at relatively high spatial resolution (0.2 mm) at a series of heights (consecutive profiles were taken every 2 mm). Particle size measurements have been obtained in a coannular diffusion flame burner also operated at atmospheric pressure condition^.^ The coannular burner consists of an 11.1-mm4.d. fuel passage surrounded by 101.6-mm outer air passage. The fuel tube extends 4 mm beyond the exit plane of the air passage. The flame is enclosed in a 405 mm long brass cylinder to shield the flame from laboratory air currents. Slots were machined in the chimney to provide optical access to the flame. Screens and a flow restrictor were placed at the exhaust of the chimney to achieve a stable flame by reducing any air recirculation down the side walls and minimizing air entrainment through the slots. The burner/chimney assembly was mounted on a milling machine base which provided for three-dimensional positioning capability. Radial traversing of the burner was accomplished by using a motorized translation stage which was mounted on the milling base. A positioning accuracy of 0.025 mm was possible with this system. B. Species Concentration Measurements. Species concentrations were determined by a direct-samplingquartz microprobe system with mass spectrometric analysis. A probe after the design of Fristrom and We~tenberg’~ was inserted into the ~~~
(9) Haynes, B. S.; Wagner, H. Gg. Prog. Energy Combust. Sci. 1981, 7,229. (10) Wagner, H. Gg. In Particulate Carbon Formation During Combustion; Siegla, D. C., Smith, G. W., Eds.; Plenum: New York, 1981; p 1. (11)Wagner, H. Gg. Symp. (Int.) Combust., [Proc.]1979, 17th, 3. (12) Glassman, I. Princeton University Mechanical and Aeronautical Engineering Report No. 1450, 1979. (13) Palmer, H. B.; Cullis, C. F. In The Chemistry and Physics of Carbon;Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1965; Vol. 1, p 265. (14) Calcote, H. F. Combust. Flame 1981,42,215.
246 Langmuir, Vol. 3, No. 2, I987
Santoro and Miller
Flame 'oz-nse
Figure 1. Schematic diagram of the Wolfhard-Parker slot burner and the mass spectrometric sampling system. flame parallel to the fuelfair flow separators (see Figure 1). The probe orifice was estimated to be =I40 pm with an effective sampling volume diameter of 700 pm. The pressure was maintained at =0.3 torr downstream of the first probe by a rotary pump. A second, identical microprobe sampled this low-pressure gas flow directly into the ionizer of a quadrupole mass spectrometer. Signals were calibrated against room temperature mixtures and the resulting calibration factors were corrected for the temperature dependence of the molecular flow through the flame probe orifice. The resultant mole fractions are expected to be accurate to *lo% for most flame species. In a related series of experiments, molecular iodine which was stared in a side arm on the probe was mixed with the flame gas sample just inside the orifice?6 Iodine reacts quantitatively with hydrocarbon radicals to form the alkyl iodides, which could survive the remainder of the sampling train and be detected mas8 spectrometrically. In some experiments, mass spectrometric probe detection was replaced hy laser-based spectroscopictechniques. In all of these experiments,the incoming laser beam was aligned parallel to the flow separators. Both a pulsed Nd-YAG dye laser and a CW argon ion laser were used in these experiments. In some cases, fluorescence emission from excited molecules was used to determine relative concentrations of flame species; in other experiments, ions formed in the excitation process were detected. C. Particle Measurements. The particle size measurements were obtained by using a laser scattering/extinction technique.' Laser extinction and scattering measurements were carried out hy using a 4-W argon ion h e r which was operated at the 514.5-nm laser line. The incident laser power Io was 0.5 Wand was modulated by using a mechanical chopper. The laser beam was foeused to a diameter of 0.2 mm in the flame; the transmitted power I was measured hy using a photodiode and the scattered light was detected at 90" with respect to the incident beam by using a photomultiplier tube. A pinhole aperture restricted the length of the sample volume to 1mm. Signals from each detector were input to a lock-in amplifier and subsequently digitized signals were stored on a minicomputer. The differential scattering cms section per unit volume Q(W"), which is the power scattered in the 90° direction, per unit solid angle, per unit volume, and per unit incident flux (cm-' sr-') is (15) Fristom, R. M.: Westenberg, A. F.F k " S t m t m : Mffirsw-Hill New York, 1965. (16) Miller, J. H.; Taylor, P. H. Cambwt. Sei. Teehnal., in press.
obtained by using a calihration procedure based on the known scattering crass section of ethene." In all the experiments, the incident laser beam was vertically polarized and Q, is the scattering cross section applicable for the vertically polarized scattered light. The transmittance measurements represent the integrated value of the local extinction coefficient along the optical path through the flame. These measurements were used to reconstruct the local values of the extinction coefficient, k,,, by applying an appropriate inversion technique?6 The ratio of scattering crass section, Qw(90"), to the extinction coefficient,k, was used to determine the partide size. For these calculations, the particle size analysis was carried out by using Mie theory. In general, the soot particle sizes allow a Rayleigh limit analysis (D
