The fate of metal (Fe) during diesel combustion: 

Morphology, chemistry, and formation pathways of 

nanoparticles 

Art Miller a,∗, 

a NIOSH/Spokane Research Lab, Spokane, WA 99208, USA 

Gib Ahlstrand b,

b College of Biological Sciences, University of Minnesota, St. Paul, MN 55108, USA 

 David Kittelson c, 

c Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA 

Michael Zachariah d 

d Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA 

Abstract

This report describes an investigation in which we used iron-doped diesel fuel to generate metal-bearing diesel 

particles and a subsequent analysis of the particles using transmission electron microscopy (TEM) and energydispersive spectroscopy (EDS). For this study, DPM was generated by a 1.5-L diesel engine and the fuel was doped 

with ferrocene to enhance the level of iron in the system. The exhaust particles were collected on TEM grids and 

analyzed using the Philips CM12 TEM/EDS instrument. Results show that when the iron-to-carbon (soot) ratio 

(Fe/C) in the engine is low, the exhaust particles have morphologies similar to those for the undoped case, but 

at a threshold Fe/C value of 0.013 (for this engine), homogeneously nucleated metallic nanoparticles are formed 

and begin agglomerating. The number and size of these nanoparticles increase with level of doping. Metal-bearing 

particles that span a wider size range are also formed. Agglomeration of metallic and carbon particles is observed 

in two distinct modes: attachment of iron primary particles (5–10 nm in diameter) to carbon agglomerates, and 

coagulation of iron agglomerates (20–200 nm in diameter) with carbon agglomerates. Results of this work imply 

that as new engine technologies reduce soot levels in the engine and/or levels of trace metals in the fuel are 

increased, the generation of metallic nanoparticles may ensue, creating a potential health concern. 

 

1. Introduction

Diesel engines produce a trimodal size distribution of diesel particulate matter (DPM) [1]. The 

newer “cleaner” engines, although they produce 

a smaller mass of particles, can sometimes produce 

a greater number of nanosized particles [2]. This 

“nuclei mode” of DPM is quite variable and can be 

attributed in many cases to the homogeneous or selfnucleation of volatile species during the dilution and 

simultaneous cooling of the exhaust aerosol as it exits the tailpipe [3]. It has also been shown that some 

nanoparticles may form prior to dilution and that this 
phenomenon (early formation) can be enhanced by introducing trace amounts of metals into the fuel [4–6]. 

The dynamics of particle formation during and after combustion is a topic of much interest and investigation. A report summarizing the diesel combustion 

process [7] describes how the fuel jet quickly disintegrates as it exits the nozzle and vaporizes as it entrains 

hot air, subsequently forming a mixture plume of fuel 

vapor and air. It has been shown that soot precursors 

originate from partial oxidation of fuel in the fuel-rich 

region of this plume. It is postulated that within the 

plume, the high concentration of precursors interact 

to form soot primary particles that are quite spherical. 

Such particles begin to agglomerate and are mostly 

oxidized in the diffusion flame at temperatures around 

2500 K, but some survive, presumably due to localized quenching and enter the combustion chamber. 

As combustion by-products exit the diffusion flame 

and as the piston subsequently moves downward, they 

undergo rapid cooling. During this cooling process, 

other nonvolatile gaseous species (such as metals) 

may begin to deposit onto the existing carbon particles. If the vapor concentration of such species is high 

enough, the rapid cooling may also drive their saturation ratios so high that they begin to self-nucleate, 

forming a new population of nanoparticles. The newly 

formed particles may then coagulate with existing 

particles as the aerosol travels through the exhaust 

system. Further cooling of the system en route to the 

tailpipe causes semivolatile species such as lube oil 

vapors and polycyclic aromatic hydrocarbons (PAHs) 

to condense onto the agglomerate particles. When the 

aerosol exits the tailpipe, it again cools rapidly and 

other more volatile species condense suddenly and/or 

self-nucleate into nanoparticles. The resulting complex mixture of particles is what we call DPM. 

Both chronic and acute health effects have been 

shown to be associated with DPM, but the causal 

mechanisms are unclear. One theory suggests that the 

ultrafine and nanosized particles, i.e., particles less 

than about 100 nm in diameter, may play a key role 

in this association [8,9]. Although new techniques 

have been developed to characterize the size and morphology of such small particles, the issue of chemical 

composition and its links to particle formation scenarios is complex and remains elusive. The role of trace 

metals is of particular interest due to their possible 

toxicity [10,11] as well as their potential roles in atmospheric chemistry, and the research presented here 

thus focuses on the fate of metals in DPM, and their 

effects on particle morphology and chemistry. 

Metals in diesel exhaust may originate from 

sources such as trace metals in the fuel [12] or from 

the use of metallic fuel additives for enhancing regeneration of diesel particulate filters [13], but under normal operating conditions most are believed to 

originate from lube oil additives and by-products of 

engine wear that enter the combustion chamber via 

reverse blow-by of the piston rings [14]. The research 

reported here provides insight into the role of such 

metals in the chemistry of DPM formation by investigating the formation of metal-rich nanoparticles via 

homogeneous nucleation and the overall distribution 

of metals across a wide range of particle sizes due to 

vapor deposition and coagulation. 

The main goals of this work are to quantify the fate 

of a fuel-borne metal (Fe) during diesel combustion 

and to investigate how metal levels during combustion may affect the particle formation process. The 

research approach entailed using a diesel engine to 

generate exhaust particles and doping the fuel supply 

with ferrocene to introduce additional metal (Fe) into 

the combustion process, in amounts above the normal 

trace levels. For collecting data in the lab the engine 

was run and a sample of the exhaust was extracted 

and cooled/diluted. The aerosol was then routed to 

a suite of instruments (Fig. 1). The size distribution 

of the diesel aerosol was measured using a scanning 

mobility particle sizer (SMPS) and carbon levels in 

the exhaust were determined from filter samples using 

NIOSH method 5040 [15]. Particle samples were collected with a low-pressure impactor (LPI) or an electrostatic precipitator (ESP) and were analyzed using 

transmission electron microscopy (TEM) and energy 

dispersive spectroscopy (EDS). 

2. Experimental setup and procedures

2.1. Experimental setup

The source of diesel particulate for this work was 

an Onan–Cummins “QuietDiesel” genset powered by 

a three-cylinder, 1.5-L Isuzu engine. This unit is capable of providing 10 kW of continuous ac power at a 

fuel flow rate of about 3 kg/h. In order to maintain a 

steady 60 Hz of ac current from the generator, the fuel 

flow to the engine is controlled by an electronic governor actuator that maintains a constant engine speed 

of 1800 rpm. The load on the engine is provided by 

a resistive load bank made by the Simplex Company. 

For the purposes of our experiments, the engine was 

run at conditions between 0 and 8 kW load. The corresponding fuel flow rates varied linearly from 1.1 to 

2.6 kg/h. The fuel used for all tests was #2 diesel fuel 

with a nominal sulfur level of 350 ppm, doped with 

varying amounts of ferrocene to yield a fuel mixture 

containing from 20 to 600 ppm of iron. 

During each test, the particulate matter for sampling/analysis was drawn isokinetically from the 

tailpipe, as shown in Fig. 1. The 3.8-cm-diameter exhaust line was connected to a fume hood system via 
a 10-cm-diameter pipe, and a 3-mm-ID stainless steel 

probe drew a small sample about 15 cm from the muffler exit. The probe was inserted through the tailpipe 

wall and directed into the exhaust stream. The opposite end of the probe was attached to the inlet of an 

AirVac TD260H vacuum pump that acted as an ejector dilutor [3]. The dilutor was fed a supply of 30 psi 

(200 kPa) dry air. Cold-flow calibration using a bubble flow meter at the dilutor inlet and a rotameter at 

the outlet yielded a dilution ratio of approximately 

10:1 for this setup. For measurements of particle size 

distribution with the SMPS, the flow was further diluted as shown in Fig. 1. 

Fig. 1. Experimental setup. 

It should also be noted that 

the size distribution of particles generated by this engine varies somewhat from that of typical heavy-duty 

engines. Under normal operating conditions, which 

include a constant rpm of 1800, it produces no discernable nuclei mode and the accumulation mode 

decreases with engine load rather than increasing as 

it typically does for larger engines. 

2.2. Particle sampling

Two methods were used to collect particles on 

TEM grids. The first method employed a low-pressure 

cascade impactor (LPI) to collect size-selected samples of particles larger than 50 nm (aerodynamic diameter). The second collection method employed an 

electrostatic precipitator (ESP), which does not yield 

a representative sample of particles due to charge biasing, but has the advantage of collecting a wide variety of particle sizes on one grid, including particles 

less than 50 nm (aerodynamic diameter). The samples were collected on 200 mesh copper TEM grids 

coated with either carbon (carbon-coated Formvar) or 

SiO films. The carbon films are more robust (therefore less damaged by the impactor jets) and, since 

they are thinner, yield better TEM images. The SiO 

films, since they contain no carbon, allow better EDS 

analysis of the carbonaceous particles. 

The LPI is a simple method for classification of 

particles by aerodynamic size [16]. It consists of several sizing stages stacked on top of each other. The 

aerosol is drawn through the stack using a relatively 

low-pressure vacuum pump, which is where the LPI 

gets its name. The cascading stages are designed so 

that the aerosol sample is divided into predetermined 

size classes and impacted onto substrates, convenient 

for offline analyses of size-dependent particle characteristics. The LPI used in this work was designed to 

operate at −90 kPa outlet pressure and 1.0 lpm flow 

rate. For sample collection, cellophane tape was used 

to attach the TEM grids onto the impactor plates of 

the final three stages, i.e., stages 6, 7, and 8, which 

have design cut points of (Daero) 120, 75, and 50 nm, 

respectively. 

The ESP used for these experiments was built inhouse and is a simplified version of the Rochester 

design [17]. It consists of a brass tee plumbed so that 

the aerosol flows through the straight section. A small 

probe with a molybdenum platform is inserted into 

the tee connection with a TEM grid attached to the 

platform. The base end of the probe consists of a highvoltage coaxial fitting, wired so that the probe is at 

high voltage while the tee fitting is grounded. This 

design, unlike the point-to-plane (Rochester) design, 

does not generate a corona discharge and so depends 

on the intrinsic charge of particles to attract them 

onto the grid. For this reason its collection efficiency 

is much less, and the particles are collected with a 

charge bias. Despite these limitations, it provided a 

simple and effective way to collect a variety of particles from the aerosols sampled. 
 

2.3. Particle imaging and analysis

Particles were imaged and photographed using 

a Philips CM12 TEM (FEI Company, Hillsboro, 

OR, USA) operating at 60 kV accelerating voltage 

and using TEM spot size settings of 2–5. Images 

were recorded on Kodak electron microscopy film 

4489 and developed in Kodak developer D-19 at full 

strength for 3.0 min. Particles and nearby support 

substrate backgrounds (C and SiO films) were analyzed for elemental composition using a Power MX 

EDS system (EDAX, Inc., Mahwah, NJ, USA) fitted with a Super Ultra Thin Window X-ray detector 

(resolution 144.5 eV at MnKα line, 40 μs time constant, 10 eV/channel, 45◦ takeoff angle). For EDS 

analysis, the TEM was operated at 60 kV, spot sizes 

10–11, with objective aperture removed. Elemental 

ratios were calculated from spectra using the EDAX 

MX-TEM program “Materials Thin,” which employs 

a thin-section approximation analysis strategy. When 

appropriate, substrate support film background spectra were subtracted from particle spectra. 

3. Results and discussion

In this work we focus on the characterization of 

the nuclei and accumulation modes of DPM as defined previously [1]. Under normal operating conditions for this engine, the nuclei mode is absent and the 

accumulation mode consists of agglomerates of elemental carbon spheres, with hydrocarbons and other 

trace species condensed/adsorbed onto the surfaces of 

the agglomerates. When iron is added to the system 

in small quantities, it mainly attaches to particles via 

surface deposition [18]. At a certain threshold level 

of iron doping, vapor deposition cannot keep up with 

the locally high cooling rate and the supersaturation 

ratio of iron rises quickly. Homogeneously nucleated 

metallic nanoparticles are subsequently created and 

begin coagulating with each other and with other particles. These processes result in the evolution of distinct new particle morphologies, which are outlined 

and discussed below. 

3.1. Identifying common particle morphologies

Five particle types were identified, representing 

the most common particle morphologies observed in 

samples. Each of the particle types is described below, 

including comments on the processes of condensation, nucleation, and coagulation as they relate to the 

fate of iron in the system. These processes may occur for other species as well, most notably for carbon, 

and the formation of carbon agglomerates is therefore 

covered first to provide a baseline for subsequent discussions of the interaction between metals and carbon 

in such a system. 

3.1.1. “Type 1” particles: carbon agglomerates

(accumulation mode of soot)

Under normal operating conditions, most particles 

are carbon agglomerates formed by coagulation of 

“spherules” of elemental carbon. The spherules are 

formed from soot precursors near the flame front, 

where temperatures are high and combustion chemistry dominates [19]. The extremely high concentration of free carbon and numerous collisions result in 

the formation of these 20–80 nm clusters of atoms 

and molecules consisting mainly of elemental carbon [20]. Due to the enhanced molecular mobility at 

high temperatures [20], these clusters arrange themselves into spherical shape and have been shown to 

have a layered structure [21]. These spherical particles are sometimes referred to as primary particles. 

The carbon agglomerates are subsequently formed 

when these primary particles begin to coagulate into 

larger chainlike agglomerates and eventually, as the 

system cools, take on other more volatile species by 

surface interactions. 

For the undoped case, most observed particles 

are these “typical” carbon agglomerates (Fig. 2) with 

mean mobility diameter of around 80 nm. 

Fig. 2. Typical agglomerate of diesel soot (no metal doping). 

The mean 

diameter of the primary particles is approximately 

20–30 nm, similar to that reported earlier [22]. Since 

the fuel is not doped, metal content is relatively low 

and virtually no “pure” metallic particles are observed. The trace metals in the system, derived from 
fuel contaminants, lube oil additives, and engine wear, 

likely react in the gas phase to form metallic compounds [23], and as the system subsequently cools, 

they adsorb or condense relatively evenly onto carbon particles. For this undoped case the amount of 

metals residing on carbon particles is generally too 

low for detection using EDS. However, using particleinduced X-ray analysis (PIXE) on bulk samples of 

soot collected for the 20 ppm doping case (Fig. 3), 

we can detect that in addition to the iron, there are 

also other metals (fuel, oil, and engine-wear-related) 

present in smaller quantities. 

Fig. 3. PIXE data for Fe-doped case. Replicate samples A and B show presence of lube-oil additive metals (P, S, Ca, Zn) and 

engine wear metal (Cr). 

Others have done bulk 

analysis of diesel soot samples using inductively coupled plasma mass spectrometry (ICP-MS) with similar results [23]. 

3.1.2. “Type 2” particles: primary particles of metal

(self-nucleated)

When the fuel is doped with just enough iron to 

trigger homogeneous nucleation, we see the appearance of a nuclei mode of small (<10 nm mobility diameter) metallic “primary particles” as evidenced by 

the beginnings of a nuclei mode for the 20-ppm doping case in Fig. 4. At and above this threshold condition, the amount of metal in the system is much higher 

than that for the undoped case, and the EDS technique 

can now be used to measure the metal on individual 

particles. For the lowest doping condition (20 ppm), 

EDS spectra from large particles show small amounts 

of Fe, but TEM images show no visibly dense regions 

indicating concentrated iron content. It is concluded 

that for this case the Fe on the particles is mainly 

from vapor deposition, with perhaps some coagulation of very fine nanoparticles or molecular clusters, 

and is thus not visible on the images. This was verified by our parallel study using single-particle mass 

spectrometry [18]. 

As the metal content in the system increases, more 

and larger self-nucleated metallic particles are generated. The primary particle size is initially less than 

10 nm, as evidenced by both SMPS data (20-ppm case 

in Fig. 4), and TEM images such as in Fig. 5. Analysis 

of small particles such as those in Fig. 5 using EDS 

yields background levels of all species except Fe, suggesting they are composed mainly of iron. Note that 

the size of both primary particles and agglomerates 

appears to increase with increase in doping levels, especially notable for the 600-ppm doping case. 

At lowest doping level (20 ppm), it is difficult to 

observe the primary iron particles using TEM. The 

SMPS data confirm their existence, and the shapes of 

particles such as those in Fig. 5a suggest that they 

coagulate into larger particles. However, they are so 

small that even when they coagulate into particles 

such as those in Fig. 5a, it is not possible to resolve 

the primary particles with our TEM. At higher doping 

rates, the primary iron particles are larger and we are 

then able to resolve the individual primary particles. 

The potential role of iron oxide primary particles that may form early in the combustion process, 

perhaps even in the core of the diffusion flame as 

described in an earlier work [24], is unclear. For 

premixed flames [25,26], addition of ferrocene was 

shown to trigger the homogeneous nucleation of iron 

oxide nanoparticles, which formed prior to the soot, 

and in fact reportedly enhanced soot production, since 

their high surface reactivity encouraged soot deposition. These soot particles were described as then 

being immune from surface oxidation as they passed 

through the remainder of the flame, since the metal 

could not diffuse through the carbon and reach the 
surface fast enough. On the other hand, laminar diffusion flames [24] have been shown to display the 

same behavior, i.e., early generation of metal oxide 

nanoparticles, but (perhaps due to reduced oxygen 

availability) this process appeared to be limited, leaving sufficient metal in the system to form metal oxide 

coatings on the soot particles, evidenced by their high 

rate of oxidation later in the flame. Neither of these 

studies indicated that metal played a role in suppression of soot during the combustion process. However, 

a later study [27], in which soot concentration was 

measured in the cylinder as a function of crank angle, 

suggested not only that during diesel combustion of 

metal-doped fuel, soot oxidation is enhanced due to 

its metallic surface film, but also that its formation is 

suppressed. The validity of that approach may be in 

question, however, since the soot measurements were 

not taken inside the diffusion flame, but rather in the 

prechamber, i.e., where the flame initiated. 

Fig. 4. Effect of iron doping level on the size distribution of diesel particles. Engine load of 0 kW and dilution of approximately 

700:1 for all cases. 

(a) (b) (c) 

Fig. 5. Iron agglomerate nanoparticles showing primary particle size and typical agglomerate diameters for three doping conditions: (a) 60 ppm, (b) 200 ppm, (c) 600 ppm. 

The creation of iron oxide particles during diesel 

combustion with 200 ppm Fe doping is suggested by 

EDS data such as in Fig. 6, which shows a distinct 

oxygen peak for a 78-nm metallic agglomerate. However, analysis of the relative mole fractions of iron 

and oxygen on a number of such particles did not 

yield consistent stoichiometric ratios commensurate 

with any particular FeOx compounds. It is also possible that the self-nucleated metallic particles are pure 

iron and that they oxidize en route to the TEM. This 

is supported by the lack of O peaks in the singleparticle mass spectra in our previous work [18]. Since 

the boiling point of iron is high, it is possible that 

the Fe primary particles form only after Fe passes 

through the diffusion flame. In that case, the local 
reducing environment of CO would limit the formation of oxides, which would explain our observation 

of “pure” iron nanoparticles. We therefore conclude 

that most observed nanoparticles are pure iron particles that self-nucleate after iron is vaporized in the 

diffusion flame and subsequently cool in the reducing environment outside the flame. It is hypothesized 

that such primary particles subsequently coagulate to 

a self-preserving size distribution, which is in fact the 

lognormal distribution making up the nuclei mode for 

the doped cases. 

Fig. 6. EDS spectrum of metallic nanoparticle showing iron 

and oxygen. 

3.1.3. “Type 3” particles: carbon agglomerates

decorated with iron nanoparticles

Before or during the mutual coagulation of the 

primary iron particles, they also individually collide 

with carbon agglomerates and “decorate” the agglomerate surfaces with “nanonuggets” of relatively 

pure iron. This is confirmed by observation of very 

small nuggets of dense material visible on TEM images of carbon agglomerates (Figs. 7a–7b). The dense 

nuggets are assumed to be self-nucleated primary particles of iron attached directly to carbon surfaces. 

This process is also documented for the case of Cedoped diesel combustion [13]. This decorating of carbon agglomerates may also explain the observation of 

minute dark specks on some carbon agglomerates at 

lower doping levels. For such cases, the size of the 

primary iron particles is potentially so small that they 

are not easily resolved in the TEM images, but rather 

appear as tiny specks (Fig. 7c). Note that for Figs. 7 

and 8, since the number and size of iron nuggets correlate with iron availability, the mass flow rate of Fe 

is used instead of simply the doping level. 

3.1.4. “Type 4” particles: metal agglomerates

After homogeneous nucleation of iron is established, further increase in metal above the threshold 

level simply creates more (and slightly larger) primary particles. Large numbers of such monodisperse 

particles drive coagulation and this creates the growing “nuclei mode” of metallic (agglomerate) particles 

we see in Fig. 4. The mean diameter of this nuclei 

mode of metal agglomerates depends on the quantity 

of metal available. For example, at 600 ppm doping 

(Fig. 4), the mean mobility diameter of such agglomerates increases to approximately 30 nm (see Fig. 5c, 

which is typical of such particles). 

The fact that the growing nuclei mode is a direct result of metal nucleation and agglomeration is 

shown in Fig. 8. The linear growth of particulate volume (mass) in the nuclei mode corresponds directly 

to the increase in mass flow rate of Fe through the 

system. For this figure the nuclei mode volumes were 

calculated from SMPS size distributions and the Fe 

flow rates were calculated as the product of fuel flow 

rate and doping level. For the conditions of our tests, 

the “threshold” Fe flow rate for the onset of homogeneous nucleation is at about 0.025 g/h, i.e., where 

the trend line crosses the x-axis in Fig. 8. This value 

corresponds to the product of fuel flow rate at 0-kW 

load (1.1 kg/h) and approximately 23 ppm Fe doping of the fuel. Since the threshold is also affected by 

the availability of carbon surfaces for metal deposition, we use this threshold Fe flow rate along with the 

(baseline/undoped) carbon mass measurements presented later in section C to calculate a threshold ironto-carbon ratio of 0.013 for onset of homogeneous 

nucleation for this case. In Fig. 9 we show that the 

calculated Fe/C ratios for all loads are well above this 

threshold for the 60-ppm doping case, which is supported by experimental results, while the Fe/C ratio at 

0 kW and 20 ppm doping is near the threshold of nucleation. The latter was observed experimentally and 

is reflected in Fig. 4. Note that due to the wide range 

of metal-to-carbon ratios used to derive the trend line, 

the Fe/C ratio at 0 kW actually appears slightly below 

the threshold in Fig. 9, while the existence of a nuclei 

mode for the 20-ppm case in Fig. 4 confirms that it is 

in fact above the threshold. 

As metal levels increase, further coagulation of 

metallic particles leads to the formation of larger 

metal agglomerates. This is evidenced by observation of ∼100-nm “pure” iron particles in the singleparticle mass spectra for the case of 60 ppm doping 

[18]. For the lowest doping rate (20 ppm), such particles are virtually nonexistent, but for the highest doping rate (600 ppm), they are fairly common. 
(a) (b) (c) 

Fig. 7. TEM images showing metallic primary particles, the darker spots/specks that decorate lighter-colored carbon agglomerates at Fe flow rates of (a) 0.158 g/h, (b) 0.115 g/h, and (c) 0.063 g/h. 

Fig. 8. Volume of nuclei mode particles (<20 nm) as a function of Fe throughput. 

Fig. 9. Data showing variation in iron-to-carbon ratio with doping level and engine load. 
3.1.5. “Type 5” particles: combined metal/carbon

agglomerates

For iron doping levels at and above 60 ppm, we 

observe large agglomerate particles that contain significant amounts of both iron and carbon. An example 

of such a particle is shown in Fig. 10a. Three consecutive localized EDS scans of this particle (Figs. 10b– 

10d) show clearly the association between morphology and chemistry. The upper end of this particle consists of a carbon agglomerate structure with a few very 

small metallic primary particles visible, and the spectrum of Fig. 10b shows a relatively small iron peak 

for this case. The spectrum of Fig. 10c shows an even 

smaller iron peak for the low-density (carbonaceous) 

central portion of that particle. Finally, Fig. 10d shows 

a very large iron peak, suggesting that this end of 

the particle was formed by coagulation with a type4 metallic agglomerate. 

(a) (b) 

(c) (d) 

Fig. 10. TEM image and EDS spectra show how iron content is related to morphology. (a) Image of combination iron–carbon 

particle. (b) EDS spectrum for upper end of particle. (c) Spectrum for midsection. (d) Spectrum for lower section. 

Fig. 10a thus demonstrates the two different 

modes of coagulation for metallic and carbon species 

that may take place during the complex particle formation process. First, the very small iron primary 

particles visible on the upper left end of the chain 

show how self-nucleated particles attach directly to 

carbon surfaces. Second, the structure on the lower 

right indicates coagulation of larger metallic agglomerates with carbon particles, an observation supported 

by single-particle mass spectrometry [18]. Both these 

processes are collision-driven and so will increase in 
frequency as the metal in the system and thus the concentration of metallic particles increases. 

Fig. 11 further demonstrates the variation in particle size, morphology, and elemental composition for 

the metal-bearing exhaust aerosol. 

Fig. 11. Sample taken with ESP shows the great variety in particle morphologies for the case of 60 ppm Fe doping of fuel. 

This sample was 

collected from raw exhaust (using 60 ppm Fe doping) with an electrostatic precipitator. This collection 

method favors highly charged particles, so the sample 

is not necessarily representative, but it does clearly 

show the variation in particle morphologies that occur at these doping levels. Note that the agglomerates with larger primary particles were shown to be 

carbonaceous, while those with smaller (and more 

dark/dense) primary particles were rich in iron. 

3.2. Relative frequency of particle types as a

function of metal throughput

The five particle types described above appear 

with varying frequencies depending on doping level. 

This is best visualized by comparing TEM images of 

particles collected on the eighth stage of the LPI at 

different doping levels (Figs. 12–15). Note that the 

particle number density in the photos is not necessarily representative, due to variable loading of the TEM 

grids. In the following descriptions, references to iron 

content were verified by EDS analysis. 

At the highest doping condition of 600 ppm 

(Fig. 12), “pure” type-1 carbon particles do not exist, 

while there are numerous type-4 metal agglomerate particles and type-5 combined particles. This is 

the only condition at which the large type-4 metal 

agglomerates are readily found, and we also see considerable numbers of type-3 Fe-decorated carbon particles. For the latter, the Fe appears as tiny dots on the 

larger carbon particles, as seen in Fig. 12b. 

Fig. 13 shows that at the next lower doping level 

(200 ppm), the type-4 metal agglomerate particles are 

still prevalent. The type-5 combined particles are now 

less prevalent, and the number of primary metal particles attached directly to carbon agglomerates is also 

reduced (Fig. 13b). 

At the 60-ppm doping level (Fig. 14), the type-4 

metal agglomerate particles are still fairly prevalent. 

The type-1 carbon agglomerates are plentiful, while 

type-5 particles are now infrequent. Direct attachment 

of primary metal particles to carbon agglomerates is 

still evident but infrequent, and since the primary particles are very small, they are sometimes hard to resolve on TEM images such as in Fig. 14b. 

At the lowest doping level (20 ppm), the type-5 

particles are gone and the type-4 particles are rare 

(Fig. 15). Since few if any type-2 particles exist, the 

nanoparticles observed in these images are carbonaceous (this was confirmed using EDS). Under this 

condition, most of the particles are type-1 particles. 

These particles contain small amounts of iron but 

their appearance is similar to the undoped particles of 

Fig. 2. A potential exception to that similarity is the 

possibility of greater variation in primary particle size 

for the 20-ppm doped particles. 

This disparity in the size of primary carbon particles could be due to enhanced postcombustion oxidation of the doped particles as they make their way 

through the exhaust system. For the case of no doping the carbon primary particles have a fairly narrow 

lognormal distribution in size [22]. Since the relative 

amount of vapor deposition of both metallic and hydrocarbon species would favor smaller particles due 

to their high surface-to-volume ratio, these particles 

are likely to oxidize faster than their larger neighbors, leading to the skewed size distribution (greater 

number of smaller particles) observed in Fig. 15, as 

compared to Fig. 2. Further work should be done to 

confirm this statistically. 
(a) (b) 

Fig. 12. (a) LPI stage 8 sample, 600-ppm doping rate, 0 kW engine load, showing prevalence of iron agglomerates (type 3) and 

combination iron/carbon particles. (b) Zoom of same shows primary iron nanoparticles (type 2) decorating a carbon agglomerate. 

(a) (b) 

Fig. 13. (a) LPI stage 8 sample, 200 ppm doping, 6 kW load. (b) Zoom of same showing small iron agglomerates and iron 

nanoparticles decorating a carbon agglomerate. 

(a) (b) 

Fig. 14. (a) LPI stage 8 sample, 60 ppm doping, 6 kW load. (b) Zoom of same showing small metal agglomerates and very fine 

iron nanoparticles decorating a carbon agglomerate (upper right corner). 
(a) (b) 

Fig. 15. (a) LPI stage 8 sample, 20 ppm doping, 6 kW load. (b) Zoom of same showing carbonaceous agglomerates and nanoparticles, as well as variation in size of carbon primary particles. 

3.3. Metal enhanced decrease in soot production

The addition of ppm levels of metals to diesel 

fuel has been shown to decrease soot emissions [4]. 

We confirmed this result for the case of 60 ppm iron 

doping by using the NIOSH Standard Method 5040 

[15] to measure elemental carbon (EC) and organic 

carbon (OC) in the raw exhaust. In Fig. 16 we compare the baseline (undoped) values of total carbon 

(TC = EC + OC) with the case for the doped condition. 

Fig. 16. Comparison of total carbon (TC) measurements 

with and without 60 ppm Fe doping. 

Decreases in total carbon are on the order of 

20–40%, which is commensurate with metal-induced 

decreases in soot production reported earlier [13]. 

Using the NIOSH method 5040 allows us to look 

separately at metal-induced decreases in EC and OC. 

Notably, OC is decreased significantly and in fact accounts for most of the decrease in total carbon. Fig. 17 

shows OC decreases on the order of 65–85% for the 

three engine loads tested. 

The trend of decreasing total carbon with engine 

load is also reflected in Fig. 4, which indicate a decrease in accumulation mode particles for higher doping. Similarly, the data of Fig. 17 suggest greater relative reductions in total carbon at higher engine load. 

Although those data are for 60 ppm doping only, the 

absolute throughput of iron increases about 2.5 times 

between the lower and higher engine load conditions 

due to increased fuel flow, and the added iron may 

account for the enhanced decrease in carbon (due to 

metal-enhanced oxidation) indicated by the data. 

3.4. Formation pathways for metal-bearing particles

Based on the results of our study, the following is 

a description of plausible formation pathways for the 

resulting particles. 

Using the assumption of a quasi-steady-state combustion process [7], the ferrocene-doped fuel exits the 

injection nozzle at high velocity and forms a plume 

of fuel vapors with a diffusion flame at its periphery 

(Fig. 18). The plume entrains hot air and temperatures 

inside reach about 1600 K. Since the ferrocene decomposes at around 750 K [4], it is expected that as 

the fuel droplets vaporize, the ferrocene also vaporizes. Subsequent thermal decomposition of ferrocene 

vapor would then lead to the existence of iron vapor 

at or near the hotter (∼2500 K) flame front, creating the conditions for vapor deposition and/or homogeneous nucleation of iron particles just outside the 

flame front. 

Iron vapor derived from the ferrocene experiences 

sudden cooling as it leaves the flame front and again 

as the piston retracts (Fig. 19). The cooling raises the 

saturation ratio of iron, and vapor begins adsorbing 

and condensing onto existing particle surfaces. If the 

amount of iron vapor is high compared to available 

surface area (mainly carbon agglomerates), condensation cannot convert the vapors fast enough (since it 

is rate-limited) and the saturation ratio rises quickly, 

triggering homogeneous nucleation of primary particles. The absolute amount of a given species, in this 

case iron, along with local temperature and cooling 

rate, would determine the extent of these processes 

and thus affect the size of the homogeneously nucle-
ated particles. During and after formation of these primary particles of iron, they coagulate with each other 

and with larger carbon agglomerates, leading to the 

five particle types described above. 

The presence of iron in the system leads to measurable decreases in soot production. There are two 

probable causes for this: 

(1) The first is the suppression of soot inside the diffusion flame. This was suggested in earlier work 

[27], but a definite mechanism was not identified. It could possibly result from iron vapors, 

thermally decomposed from ferrocene, interacting with free carbon and/or local soot precursors 

(carbon fragments) and catalyzing their destruction. Our data were inconclusive in this regard. 

(2) The second cause is the iron-accelerated oxidation of carbon in the cooler region outside the 

flame (and after the flame goes out). This would 

include both gas-phase reactions and the (later) 

oxidation of organic compounds attached to the 

surfaces of carbon agglomerates. The plausibility of the latter has been demonstrated in earlier 

work [13] and is supported by the data of Fig. 17. 

Fig. 17. Reductions of total carbon (TC) and organic carbon (OC) with engine load for the case of 60 ppm Fe doping of fuel. 

Fig. 18. Depiction of the diesel combustion process after Flynn et al. [7]. 
Fig. 19. Temperature history during typical diesel combustion. 

4. Conclusions

Using ferrocene doping of diesel fuel, we have 

shown that for this diesel engine, when the iron-tocarbon (soot) ratio is above a threshold of approximately 0.013, iron nanoparticles form during combustion and subsequently coagulate with each other 

as well as with larger carbon agglomerates. 

At conditions below this threshold, emitted particles will have relatively uniform morphology and elemental chemistry, consistent with agglomeration of 

carbon spheres followed by adsorption and condensation of other species onto the agglomerate surfaces. 

Above the threshold, five distinct morphologies are 

identifiable: (1) carbon agglomerates, (2) homogeneously nucleated primary particles of iron, (3) carbon agglomerates decorated with these iron primary 

particles, (4) agglomerates of the iron primary particles, and (5) combined agglomerates of iron and 

carbon. Plausible formation pathways were presented, 

based on the (combustion) temperature history in a 

diesel engine, and supported by data showing change 

in size, morphology, and iron content as doping levels 

were increased. 

The demonstrated fact that metallic nanoparticles 

will form at a threshold metal-to-carbon ratio has important implications. It suggests that as engine combustion technologies improve, the “cleaner” engines, 

i.e., those that generate less soot, will be more likely 

to generate metallic nanoparticles. The source of metals for such particle generation could be from fuel 

impurities, lube oil consumption (lube oil often contains some metal additives), or metals derived from 

engine wear. Due to their small size and thus efficient lung penetration, the generation of such metallic 

nanoparticles could introduce a new health concern. 

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