Letter pubs.acs.org/journal/estlcu
Fuel Sulfur and Iron Additives Contribute to the Formation of Carbon Nanotube-like Structures in an Internal Combustion Engine Jacob J. Swanson,*,†,‡ Ryder Febo,†,‡ Adam M. Boies,§,∥ and David B. Kittelson‡ †
Department of Integrated Engineering, Minnesota State University, Mankato, Mankato, Minnesota 55423, United States Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States § Department of Civil, Environmental, and Geo- Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States ∥ Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, United Kingdom ‡
S Supporting Information *
ABSTRACT: Recent work has indicated the presence of carbon nanotubes (CNTs) in laboratory diesel and gasoline exhaust, in ambient air, and in lung samples of children exposed to traffic exhaust. While it is already known that certain processes will produce some carbonaceous particles of fullerene-like crystallinity, the conditions responsible for their formation remain unknown. On the basis of a standard process for the gas-phase synthesis of CNTs, we hypothesized that the presence of a metal catalyst precursor and high levels of fuel sulfur would impact CNT formation in a diesel engine. A diesel engine was doped with varying concentrations of fuel-borne sulfur and ferrocene to produce conditioned iron (Fe) particles that acted as seed catalysts. Results showed that in the presence of Fe nuclei resulting from 36 ppm ferrocene doping, 4500 ppm of fuel sulfur produced CNT-like structures in 31% of images analyzed by transmission electron microscopy. The precursor concentrations required for high rates of CNT growth are comparable to those found in transportation fuels used in many regions of the world. These findings substantiate studies that indicate a global presence of CNT-like particles in ambient air. Formation of these structures is less likely with low-sulfur fuels, and the structures are effectively removed by particulate filters.
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INTRODUCTION Recent work indicated the presence of carbon nanotubes (CNTs) in the lungs of children exposed to ambient air in Paris.1 Fullerene particles have been measured in ambient air in China,2 in wood smoke,3 and in natural gas/methane combustion processes.4 Internal combustion processes are suspect in these examples, but all prevailing conditions responsible for the formation of these particles are unknown. There are concerns that CNTs, because of their high aspect ratio and fibrous morphology, will cause asbestos-like pathogenicity when inhaled by humans.5−7 Recent work using mice as surrogates has shown CNTs cause lung granulomas, interstitial fibrosis, and pleural inflammation,5,8−10 all of which are also associated with asbestos fibers. In the work presented here, we test a hypothesis for the production of CNTs in an internal combustion engine. It is already well-known that most combustion-generated particulate matter (PM) is composed of semivolatile material found in the nucleation mode and adsorbed to solid, carbonaceous aggregates that are found mostly in the accumulation mode.11 Nucleation mode particles are formed as exhaust is diluted and cools, allowing semivolatile material to nucleate and form new particles and adsorb or condense on existing particles. Accumulation mode (roughly 30−500 nm) particles are solid, carbonaceous aggregates with adsorbed hydrocarbon and sulfuric acid vapors. The combustion of metal © XXXX American Chemical Society
additives in lubrication oil or fuel leads to formation of solid metal oxide nanoparticles. These are found in the nucleation mode when the rate of oil consumption is high and soot concentrations are low, typically under idle or low-load conditions, or decorated on carbonaceous aggregates.12−16 Recently, other particle morphologies and chemistries have been reported. Lagally et al.17 measured emissions from a rickshaw spark ignition engine and found approximately 10% of the solid particles were CNTs, other fullerenes, and forms of crystalline carbon morphologically different from typical soot aggregates. Jung et al.18 analyzed transmission electron microscopy (TEM) grids from experiments conducted with three different diesel engines operated both with and without the addition of an iron (Fe) catalyst, and found CNTs on all three samples. The authors concluded these samples were “real” rather than contamination. Despite these recent findings, no study has yet to determine the necessary components required for CNT formation within gasoline and diesel engines. Standard processes for the gas-phase synthesis of CNTs provide more insight into the formation mechanisms of CNTs. A typical floating catalyst process includes the continuous Received: August 17, 2016 Revised: September 11, 2016 Accepted: September 12, 2016
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DOI: 10.1021/acs.estlett.6b00313 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
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diesel fuel has contained up to 50000 ppm of S, but S levels in fuel burned near coasts must be lower (∼1000 ppm). Experiments were conducted at one load, two speeds, and five doping levels, in the order shown in Table 1. Crosscontamination of the doped fuel and lubrication oil was avoided by conducting tests in order of increasing doping level.
injection of a hydrocarbon source, a catalyst source (e.g., molybdenum, nickel, or iron), and a group 16 element promoter (e.g., selenium, tellurium, or sulfur) source in a reactor at >1000°C in a reducing atmosphere.19−21 Thermal decomposition of the iron source (typically ferrocene) leads to the formation of catalyst nanoparticles, which act as catalytic surfaces for CNT growth once sufficient carbon is available from the decomposition of the hydrocarbons. Elemental group 16 enhancers, such as sulfur (S), are critically important reactants, but there exact role is still under investigation.22,23 Molecular dynamics simulations suggest that S migrates to the surface of Fe particles, “conditioning” the surface.24−27 As the catalyst nanoparticles incorporate carbon into the particle, forming iron carbides, the S prevents formation of a continuous graphene shell upon saturation and promotes oriented growth of the graphene, forming CNTs from the floating catalyst. While group 16 elements are not common in substrate-grown CNTs, their use in floating catalyst reactors aids both catalyst efficiency and growth rate. Sulfur is the most typical catalyst enhancer, as oxygen competes with graphene formation to form CO2 and selenium and tellurium are rarer elements that have been used for only proof of concept.21 On the basis of the chemical constituents found in transportation fuels and known chemical constituents required for gas-phase synthesis of CNTs in a tube furnace reactor, we hypothesized that fuel S and Fe content would significantly affect the generation of CNTs in an internal combustion engine. The prevailing conditions are favorable: hydrocarbons, high temperatures, and locally rich (reducing) fuel zones. Experiments were conducted on a four-stroke, single-cylinder diesel engine. The engine was fueled with diesel fuel containing varying concentrations of S (from 10 to 4500 ppm of S) and ferrocene (from 0 to 36 ppm of Fe). Particle size distributions were measured with and without a catalytic stripper28 to determine the fraction of solid particles. Samples for TEM were collected and used to determine particle morphology and to detect the presence of CNTs.
Table 1. Engine Operating Conditions and Fuel Doping (i.e., the amount added to the base fuel)a test point
S (ppm)
Fe (ppm)
1 2 3 4 5
0 500 4500 0 4500
0 0 0 36 36
a
For all conditions, the engine load was 75% of the maximum and two speeds were evaluated, 1800 and 2400 rpm. The exhaust temperature was ∼350°C, and the brake mean effective pressure was ∼4.2 bar.
Exhaust Dilution and Sampling. Exhaust was sampled using a two-stage dilution system to simulate exhaust dilution and mixing in ambient air using known best practices.50 The primary dilution ratio was 21:1. The aerosol was aged in a residence chamber for ∼1 s before being diluted again in an ∼12:1 ratio. TEM samples were collected after primary dilution and the aging chamber. More experimental details are given in the Supporting Information. TEM Sampling, Imaging, and Quantification. TSI’s 3089 Nanoparticle aerosol sampler was used to collect particles on Ted Pella Formvar/Carbon (01800-F) TEM grids. Two grids were collected for each of 10 conditions. A FEI Tecnai T12 TEM instrument was used to image samples, typically at 47000× magnification. Each grid was physically divided into four sections. The microscope was randomly moved to 25 different locations in each section, focused, and an image was taken for a total of 100 images for each grid. All images were analyzed manually by two different researchers. For each image, CNT-like structures were identified as particles that have a projected diameter of 3:1. Image quality was not sufficient to determine whether these particles are carbon “rods” or carbon “tubes”.
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EXPERIMENTAL PROCEDURES Engine Test Apparatus. Experiments were conducted on a four-stroke, 435 cm3 single-cylinder, overhead valve, All Power America LLC (http://allpoweramerica.com/) “Yanmar L100V clone” diesel engine that is air-cooled and naturally aspirated. The All Power and Yanmar L100V engines are nearly identical in geometry and performance, and their parts are interchangeable. The engine was connected to a dynamometer to control speed and load. Both the Yanmar L100V engine and clones have been used in studies of the performance and combustion of alternative liquid fuels29−35 and gaseous fuels,36−38 to study the potential for emissions reduction using alternative fuels,39−41 and for evaluation of diesel particulate filters.42,43 Standard ULSD fuel, containing nominally 10 ppm of S, was doped with ferrocene [Fe(C5H5)2], an iron-carrying organometallic compound that is sometimes used as a catalyst additive in diesel fuels44,45 and antiknock agent in gasoline fuels.46 Additional S was added in the form of benzothiophene (C8H6S) and dibenzothiophene (C12H8S) at a mass ratio of 67% to 33%. These are two of the most common forms of S in diesel fuel.47 The S level chosen represents very “high S”; however, many countries still have diesel fuel standards or observed levels that far exceed 1000 ppm of S, including Saudi Arabia, Indonesia, Oman, Argentina, and Kuwait,48 and the level of S is as high as 7000 ppm in Iran.49 Historically, marine
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RESULTS AND DISCUSSION Results of identifying CNT-like structures are shown in Figure 1. For conditions 1−4, at both speeds, only a few, small CNTlike structures were identified (these small CNT-like structures, which look less CNT-like, are shown in the Supporting Information). However, at 1800 rpm, the high-S and high-Fe condition, CNT-like structures were clearly identified in 31 of 100 images. Similarly, 22 of 100 images for the 2400 rpm conditions clearly contained CNT-like structures. The Supporting Information contains graphs of the number of particles and the number of CNT-like structures that were counted for all conditions. Figure 2 shows eight images of CNT-like structures from both these conditions. These images readily show highaspect ratio (∼10:1) structures that are morphologically quite different from carbonaceous aggregates. Typical structure lengths were 50−100 nm, which are similar but a bit smaller than the average length of 168 nm as described by Lagally et al.17 They are shorter and less tangled than commercially purchased CNTs or CNTs found in vehicle tailpipe scrapings and have lower aspect ratios.1 They are also dissimilar to the B
DOI: 10.1021/acs.estlett.6b00313 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
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Figure 1. Number of images out of 100 containing CNT-like structures for 1800 and 2400 rpm.
Figure 3. Images that suggest the growth of the CNT-like structure started with a small (∼5−10 nm) catalyst particle.
These results demonstrate that high levels of fuel S combined with the presence of metallic nanoparticles originating from fuel-borne catalysts such as ferrocene promote the formation of CNT-like structures in diesel engine exhaust. The high-S, zeroFe conditions and zero-S, high-Fe conditions produced very few, poorly defined CNT-like structures in comparison to many clear, CNT-like structures produced by the high-S, high-Fe condition. This highlights the importance of both S and Fe in this process. The consumption of lubricating oil also produces metallic seed particles, but in this case, the lubricating oil metals were not conducive to CNT formation as evidenced by a null CNT response for the 4500 ppm S, 0 ppm Fe condition. Overall, it is clear that with fuel S and appropriate seed catalysts present, gasoline and diesel combustion in many ways mirrors standard gas-phase CNT production processes. While the level of S that produced a response is not present in on-road diesel or gasoline fuels in the United States or many other developed countries, it is a relevant concentration for onroad diesel fuel used in many other countries and marine fuels used internationally and near the ASTM D1655 aviation fuel limit of 3000 ppm of S. More research is needed to understand if other combustion conditions such as lower speeds (marine engines), different temperature/time histories, different fuel/oil chemistries, etc., may more readily produce CNTs at different Fe and S levels. To reduce the risk of possible human exposure to combustion-generated CNTs, our results suggest that countries continue to reduce the use of S and organometallic additives in fuels or to use particulate filters when this is not possible.
Figure 2. TEM images showing CNT-like structures for the 4500 ppm S, 36 ppm Fe, and 1800 and 2400 rpm conditions.
long, “bundled” CNTs that are often administered by intratracheal instillation in health effects studies.5−10 Because the sampling system consisted of two ejector dilutors that are likely to remove most >500 nm particles and all >∼1 μm particles due to impaction on surfaces,51 it is unknown whether longer, more “fibrous” CNTs were produced in this process. Gas-phase CNT synthesis processes have demonstrated the importance of the catalytic seed particle.19,20 Fe is a particularly effective catalyst. Figure 3 visually highlights the fact that CNTlike structures appear to “grow” out of Fe catalyst particles, as evidenced by 5−10 nm particles present at the beginning of most particles, in a fashion similar to those reported for purposeful synthesis of CNTs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.estlett.6b00313. Schematic of the experimental apparatus, images of CNT-like structures for the zero-doping case, images of aggregates decorated by metal particles, particle size distributions, and additional images and analysis statistics (PDF) C
DOI: 10.1021/acs.estlett.6b00313 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 952-358-9194. Fax: 507-389-1095. E-mail: jacob.
[email protected]. Notes
The authors declare no competing financial interest.
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