Effect of Organometallic Fuel Additives on Nanoparticle Emissions

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Environ. Sci. Technol. 2010, 44, 2562–2569

Effect of Organometallic Fuel Additives on Nanoparticle Emissions from a Gasoline Passenger Car JEREMY T. GIDNEY,† MARTYN V. TWIGG,‡ AND D A V I D B . K I T T E L S O N * ,§ Johnson Matthey, Emission Control Technologies, Orchard Road, Royston, Hertfordshire., SG8 5HE U.K., Johnson Matthey Plc, Orchard Laboratories, Orchard Road Royston, Hertfordshire., SG8 5HE, U.K., and Department of Mechanical Engineering, University of Minnesota 111 Church Street SE, Minneapolis, Minnesota 55455

Received July 7, 2009. Revised manuscript received February 9, 2010. Accepted February 14, 2010.

Particle size measurements were performed on the exhaust of a car operating on a chassis dynamometer fuelled with standard gasoline and gasoline containing low levels of Pb, Fe, and Mn organometallic additives. When additives were present there was a distinct nucleation mode consisting primarily of sub-10 nm nanoparticles. At equal molar dosing Mn and Fe gave similar nanoparticle concentrations at the tailpipe, whereas Pb gave a considerably lower concentration. A catalytic stripper was used to remove the organic component of these particles and revealed that they were mainly solid and, because of their association with inorganic additives, presumably inorganic. Solid nucleation mode nanoparticles of similar size and concentration to those observed here from a gasoline engine with Mn and Fe additives have also been observed from modern heavyduty diesel engines without aftertreatment at idle, but these solid particles are a small fraction of the primarily volatile nucleation mode particles emitted. The solid nucleation mode particles emitted by the diesel engines are likely derived from metal compounds in the lubrication oil, although carbonaceous particles cannot be ruled out. Significantly, most of these solid nanoparticles emitted by both engine types fall below the 23 nm cutoff of the PMP number regulation.

Introduction There has been a tremendous reduction in the levels of tailpipe pollutants from gasoline engines in cars. The demanding California Super Ultra Low Emission Vehicle (SULEV) standards have driven hydrocarbon (HC) and nitrogen oxide (NOx) emissions to levels that would have seemed impossible when catalytic emissions control systems were first fitted on cars (1, 2). In Europe high speed turbocharged direct injection diesel engines in passenger cars have become popular because of their desirable characteristics that include high bottom end torque, exceptionally good fuel economy and low carbon dioxide emissions (3). However, diesel engines produce more particulate matter (PM) than their gasoline * Corresponding author phone: (612) 625-1808; fax: (612) 6241578; email: [email protected]. † Emission Control Technologies. ‡ Orchard Laboratories. § University of Minnesota. 2562

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counterparts, and attention has been directed toward reducing these emissions that are associated with potential harmful health effects by engine measures, and more recently, by the fitment of diesel particulate filters (DPFs) (4-7). The most common DPFs are of the ceramic wall-flow type, and those that have been in service over prolonged periods contain “inorganic ash” mainly derived from components in lubrication oil that have passed through the combustion system (8). In diesel engine exhaust there are a large number of carbon particles and these provide scavenging sites for the inorganic species, so it might be expected there would be relatively more small inorganic particles if there was less carbonaceous PM present (9). Except at high load and cold start conditions conventional port-fuelled gasoline spark ignition engines emit much less PM than diesel engines so it is expected inorganic materials present will more likely form nucleation mode particles, typically smaller than 30 nm (10). The presence of organometallic fuel additives might increase the number of inorganic particles present in the exhaust gas. The main organometallic gasoline additive that has been used over many years as an octane booster is tetraethyl lead (PbEt4, Et ) CH3CH2). Its use has been progressively withdrawn because of an association with increased bloodstream lead concentrations in urban environments with detrimental health effects (11) and the intolerance of three-way catalysts to lead poisoning (12, 13) since it is possible to achieve appropriate octane ratings without the use of an organometallic additive by refining to produce higher octane hydrocarbon components and adding oxygenates. Nevertheless, ferrocene (FeCp2, Cp ) C5H5) and the related manganese tricarbonyl compound methylcyclopentadienyl manganese tricarbonyl (CH3C5H4Mn(CO)3) have been used in some countries as a replacement for PbEt4 as an gasoline octane booster and PbEt4 is still widely used in aviation gasoline. There are major concerns about the use of such fuel additives and an extensive recent review (14) highlighted problems associated with the adoption of CH3C5H4Mn(CO)3 including increased vehicle exhaust emissions and deposits within the engine and on catalyst systems that cause significant inefficiencies (15-17). As a result, the use of CH3C5H4Mn(CO)3 as a gasoline additive has been discouraged by car makers and it is rarely used in the West although up to 8.3 mg Mn/L is permissible in the U.S. (14), and it was used in Canada at up to 18 mg Mn/L until it was voluntarily phased out in 2005 (18). Nevertheless, levels of 30 mg and 41 mg Mn/L have been reported in China and Latvia respectively (19) and FeCp2 is widely used in Russia at levels up to 37 mg Fe/L (20). Recently it was demonstrated that manganese oxide particles of about 31 nm in size are able to translocate along the olfactory nervous system directly to the brain in rats so bypassing the blood-brain barrier (21). This is a concern since a form of neurodegeneration similar to Parkinson’s Disease called “manganism” has been linked to manganese exposure among miners and smelters, and excessive exposure to iron is also associated with neurodegeneration (22). There is debate about whether iron containing particles can translocate along the olfactory nervous system to the brain as manganese nanoparticles do, and it has been reported relatively very large (2.99 µm) radiolabeled iron sulfate aerosols do not translocate in the way similar size manganese particles do (23). However, this difference might be caused by the manganese particles having solubility in aqueous media whereas the iron particles may have been rendered insoluble because of formation of iron(III) hydroxide species. Additional experiments with nanosized iron particles 10.1021/es901868c

 2010 American Chemical Society

Published on Web 03/01/2010

TABLE 1. Vehicle, Catalyst and Other Details vehicle

subcompact car

model year

2000

engine type fuel system engine catalyst volume emissions conformance vehicle age catalyst substrate catalyst cell density catalyst age catalyst formulation lubrication oil

gasoline spark ignition multi point injection 4 cylinder, 1.6 L displacement 1.66 L European Stage 3 11 000 miles cordierite 400 cpsi, 6 mil wall thickness 4000 miles palladium/rhodium fully synthetic 5-30 SAE

derived from combustion processes are needed to clarify if they also translocate. The recent European particle measurement program (PMP) highlighted the importance of particles emitted from internal combustion engines because although huge numbers of nucleation mode particles have very little mass, they may contribute to adverse health effects (24). The PMP program led to the new EURO 6 regulations on solid particle number emissions from diesel passenger cars. These regulations were intended to promote the use of DPFs. They excluded particles smaller than 23 nm because of measurement difficulties in that size range despite concerns that had been raised about their potential health effects. This was consistent with the view that the standards would be met using particle filters which are least efficient in the 100-300 nm size range and become more efficient for both larger and smaller particles, thus a DPF that eliminated the larger particles would be even more effective in the sub 23 nm range. Thus the regulations do not imply that particles below 23 nm are benign and it is important to understand situations where such particles might be emitted. To clarify the fate of organometallic gasoline fuel additives from a gasoline engine we undertook the present study that involved measurement of particulate number and size distributions in the exhaust line of a typical European family passenger car fuelled with standard gasoline and gasoline containing an organometallic additive.

Experimental Section Measurements were made on a passenger car with a 1.6 L four cylinder spark ignition engine (Table 1) operating under emission testing conditions using standard gasoline and gasoline containing an additive. The car conformed to European Stage 3 emissions standards that were in force from 2000 to 2005 in the EU and currently in China. The car had been driven for 17 703 km (11 000 mi) and was fitted a new OEM three-way catalyst, then driven for 6437 km (4000 mi) using mixed driving patterns on public roads before the present tests. The vehicle, engine, and emissions control system were not modified apart from having fitted gas sample points before and after the catalyst. The car was serviced by an authorized dealership at the manufacturers recommended intervals; it was not serviced specifically before the present tests. The regular unleaded gasoline used contained 50 ppm sulfur, and 20 L batches of fuel containing the organometallic additive were prepared by adding appropriate amounts of pure organometallic compound in a small volume of gasoline to the 20 L batch followed by mixing. Three additives were investigated: CH3C5H4Mn(CO)3, Fe(Cp)2 and PbEt4. They were laboratory grade chemicals from commercial sources and used as received. Two concentrations of CH3C5H4Mn(CO)3 were chosen, 8.3 mg/L (Mn-8) and 18 mg/L (Mn-18) of

TABLE 2. Vehicle Tailpipe Regulated Emissions THC g/km CO g/km NOx g/km CO2 g/km stage 3 emission limits average emissions

0.2 0.09

2.3 0.63

0.15 0.06

164.54

manganese, representing levels that were historically permitted in the U.S. and Canada, respectively (20). Only one concentration was used for the FeCp2 and PbEt4 tests (Fe8-8.4 mg/L and Pb-30-31.3 mg/L), and this was the same molar concentration of metal in the fuel as the Mn-8 case. The car was tested on a twin 50.8 cm (20 in) rolls MRW DC-60 chassis dynamometer using the appropriate road load model at 23 °C. At the beginning and end of each series of PM measurements, European Stage 3 drive tests were performed to confirm the regulated emissions: carbon monoxide (CO), hydrocarbons (HCs), and nitrogen oxides (NOx) remained unchanged and very well within the permitted limits, Table 2. Emission certification of light duty vehicles in Europe is performed on a chassis dynamometer using the ECE + EUDC test cycle. The ECE cycle also know as the urban driving cycle simulates congested city driving while the EUDC simulates higher speed suburban driving. Most of the work reported here was done using the EUDC portion of the European Stage 3 test cycle. This avoided cold soak, cold start and frequent idling associated with the ECE portion of the test. Prior to the each set of EUDC tests, a steady state warm-up was performed at a constant 80 km/h for 10 min, to enable the lubrication oil and water coolant to attain steady temperatures. This approach was used to minimize variability associated with storage and release and other cold start effects. The EUDC tests were performed in sets of three. A small number of steady state tests were also performed at idle and 50 km/h to obtain PM emissions data over a wider range of operating conditions. A Cambustion DMS500 fast particle spectrometer capable of aerosol particle determinations over the range 5-2000 nm was used to measure the particle emissions. A short (0.95 mm diameter, 150 mm long) stainless steel sampling line was used to convey the sample from the tailpipe exit to the DMS500 dilution sampler. The line was as short as possible to minimize loss of particles and their precursors (25, 26). The Cambustion dilution sampler (27) consists of heated concentric tubes with a mixing zone at the tip where the sample is diluted 4:1 with HEPA filtered air. The diluted sample flows through a removable inner tube (190 cm) composed of electrically conductive silicone that is maintained at 65 °C. The conductive line minimizes electrostatic losses and the combination of the initial dilution with the heated wall prevents condensation and stabilizes formation of volatile particles. Although silicone boots have been found to produce sampling artifacts, those problems are associated with much higher temperatures than those used here. To clarify the composition of the particle emissions in terms of the solid and volatile content, measurements for Mn-8 and Fe-8 were repeated using a catalytic stripper (28, 29). The catalytic stripper has a small 2581 cells/cm2 (400 cells/in2) monolith coated with an oxidation catalyst formulation, about 50 mm diameter and 150 mm in length that was electrically heated to 400 °C by heating tape wrapped around the outside of its stainless steel container. The sample supplied to the catalytic stripper was diluted using a Dekati ejector diluter which was integrated with the catalytic stripper. The dilution ratio was determined by comparing CO2 concentrations pre-ejector and post-ejector, and the actual ratio used was 26:1. The outlet of the ejector was connected directly to the sampling line of the DMS 500 which was VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Average tailpipe size distributions measured over the EUDC cycle for the five fuels tested. Results are corrected for dilution ratio to tailpipe concentrations. operated at 30 C with no additional dilution because the sample had already been diluted and cooled in the ejector. Measurements of solid particles in heavy-duty diesel engine exhaust made at the University of Minnesota are reported here for comparison. Those particle measurements were made with a scanning mobility particle sizer (SMPS) built at the University Minnesota and similar in design to the TSI model 3934. It consisted of an electrostatic classifier and a TSI 3010 condensation particle counter (CPC). It was used to classify particles by an electrical mobility diameter. The SMPS was configured to cover the range of 7.5-317 nm. Diesel engine exhaust was diluted using a two-stage microdilution tunnel with filtered, temperature controlled and dried air using air ejectors. First stage dilution was approximately 10:1 and second stage dilution was approximately 20:1. The NOx concentration changes were used to measure the primary and secondary dilution ratios. Measurements were taken after the second stage of dilution and a catalytic stripper. The engine used was a Cummins 2002 ISM running on 50 ppm sulfur fuel. It was operated at two steady state test modes from the AVL eight-mode approximation of the FTP. The test modes were mode 1: 700 rpm, 0 ft-lbs (idle) and mode 5: 1800 rpm, 264 Nm. Additional diesel results for a Caterpillar C12 engine obtained as described elsewhere (29) are also shown.

Results and Discussion Size Distribution Measurements. Figure 1 shows the exhaust particle size distributions with the car fuelled with gasoline, and gasoline containing Mn-8, Mn-18, Fe-8, and Pb-30. Particle size distributions resulting from all of the fuels containing additives showed a bimodal distribution with distinct nucleation and accumulation modes. Both diesel and spark ignition engine exhaust aerosol typically exhibit bimodal particle size distributions in the submicrometer region with an accumulation mode usually containing most of the particle mass in range from about 30 nm to about 500 nm, and a nucleation mode usually containing most of the particle number in the 3-30 nm range. The boundary between these modes can shift with engine type, fuel, and operating conditions (9, 10). 2564

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The size distribution with standard gasoline fuel was unimodal showing only an accumulation mode. Concentrations in this mode are similar to those reported elsewhere (10, 30, 31). The absence of a nucleation mode with standard gasoline is likely due to a combination of removal of HCs by the three-way catalyst and suppression of volatile particle formation by the heated dilution air used in the DMS 500 dilution system (32). In the accumulation mode region where most of the particle mass is typically found, all the fuels show similar distributions, suggesting similar mass emissions. However, the presence of metal containing species clearly increased the particle concentrations in the nucleation mode where most of the number emissions are found. In this region the Mn-18 gave the highest concentration followed by Fe-8, Mn-8, and Pb-30. Tailpipe emissions, especially in the nucleation mode region, showed considerable test-to-test variability as indicated by the error bars on the plots which are based on the standard error of the mean plus an additional 10% to account for uncertainties in dilution ratio. PM emissions from spark ignition engines are very dependent on operating conditions so that small variations in the test conditions can produce large changes in emissions (10, 33-36). Storage and release of particles and their precursors in the muffler and tailpipe during operating over the highly transient EUDC will likely further increase variability (36-38). Despite these uncertainties the differences between the fuels in the nucleation mode region are well outside the estimated errors. Solid Particle Size Distributions. Solid particle size distributions were measured with Fe-8 and Mn-8 fuels using a Dekati ejector dilutor and a catalytic stripper that removes organic material by catalytic oxidation in the presence of excess oxygen, and the results are shown in Figure 2. Particle losses in the catalytic stripper are mainly due to thermophoresis and diffusion effects. Thermophoretic effects are almost independent of particle size, but diffusion losses increase with decreasing particle size and they are roughly 50% for 8 nm particles where the nucleation mode peak number occurs. The penetration through the catalytic stripper estimated using a simple fully developed flow model (39) is also shown in Figure 2. The peak nucleation mode concen-

FIGURE 2. Comparison of average total and solid particle size distributions measured over the EUDC cycle with Mn-8 and Fe-8 fuels. Results are corrected for dilution ratio to tailpipe concentrations but not for losses in the catalytic stripper used to remove volatiles. Estimated penetration through the stripper is also shown.

FIGURE 3. Comparison of solid particle size distributions at idle of two modern heavy-duty diesel engines without aftertreatment and those of the passenger car fuelled with Mn-8 and Fe-8 gasoline. All measurements were made downstream of a catalytic stripper so that volatile particles are removed and are corrected for dilution ratio to tailpipe concentrations. tration was reduced by about 50-70% by the catalytic stripper which is only slightly more than the expected penetration loss. This suggests that these particles consist mainly of nonvolatile residual inorganic particles. Figures 3 and 4 compare solid particle measurements made with a catalytic stripper using modern heavy-duty diesel engines with no exhaust aftertreatment with those made with the car running on Fe-8 and Mn-8 fuels. Solid inorganic particles in diesel exhaust are generally associated with ash from inorganic fuel and oil additives (41, 42). Figure 3 compares emissions for idling engines. Most nucleation mode

particles emitted by diesel engines are volatile but a solid mode may appear when volatiles are removed, especially when not much soot is present. The size distribution for the car with Mn-8 falls nearly on top of that of 11 L heavy-duty diesel. The particles emitted with Fe-8 are somewhat larger and the concentration is lower but the solid nucleation mode concentration reported in ref 29 for a 12 L heavy-duty diesel is the lowest. Figure 4 shows solid particle number concentrations for 50 kph cruise for the car running on Mn-8 and Fe-8 fuels and for the two diesel engines running under light load cruise conditions, roughly 20-25% load. For these VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Comparison of solid particle size distributions under cruise conditions of two modern heavy-duty diesel engines without aftertreatment and those of the passenger car fuelled with Mn-8 and Fe-8 gasoline. All measurements were made downstream of a catalytic stripper so that volatile particles are removed and are corrected for dilution ratio to tailpipe concentrations. conditions the diesel engines show no evidence of solid and Mn compounds leading to different partitioning between nucleation mode formation while both Mn-8 and Fe-8 gave particles in the nucleation and accumulation modes and a large nucleation mode. On the other hand, the diesel engines in-cylinder and exhaust surfaces. Figure 5a and b show the produce a large solid mode in the accumulation mode region, results of chemical equilibrium calculations for Fe and Pb presumably carbonaceous soot, and thus much higher mass compounds in a spark ignition engine exhaust environment. emissions than the gasoline engine cases. The absence of For comparison, Figure 5c shows predicted equilibrium solid nucleation mode particles for the diesel cruise condiconcentrations of Ca compounds expected to be present in tions is likely due to scavenging of nuclei and precursors by diesel exhaust as a result of lubrication oil consumption. the accumulation mode particles. DPFs like those being used Calculations for gasoline combustion were performed using on post-2007 heavy-duty diesel engines in the U.S. and diesel the NASA CEAgui chemical equilibrium program for gasoline passenger cars in Europe essentially eliminate solid particles containing 50 ppm S, with 8.4 and 31.3 mg/L Fe or Pb, in both modes. On the other hand, gasoline engines are not respectively, in air at 100 kPa with an excess air factor of 0.98 required to use filters so solid nucleation mode particles which is representative of how the car used in these associated with the inorganic additives used in some parts experiments operated. Calculations for a diesel engine were of the world could be an issue. It should be noted that port done for fuel containing 50 ppm S and lubrication oil fuel injection (PFI) gasoline engines running on standard containing 5000 ppm Ca with oil consumption of 0.1% of unleaded gasoline like the one tested here emit very few that for fuel burning in air at 100 kPa and an excess air factor solid nucleation mode particles and modern PFI engines of 1.7, typical of diesel operation. The calculations assume easily meet the PMP number emission that have been set for chemical equilibrium and are limited to the species present diesel passenger cars in Europe. However our work shows in the NASA thermodynamic data files. that the use of inorganic fuel additives may lead to the high At flame temperatures and down to about 1300 °C the solid particle number emissions and most of these are too principal compounds of all three metals are in the gas phase. small to even be counted by the current PMP method. However, as the gases cool, Fe is predicted to start to form Nucleation Mode Formation. Nucleation mode particles solid oxides at about 1250 °C, but Pb is predicted to stay in are formed by nucleation of hot vapors (43, 9). Metals gas phase compounds until much lower temperatures, introduced into the combustion chamber either with the around 650 °C, are reached, and then liquid Pb starts to fuel or lubrication oil evaporate at flame temperature and condense, followed by solid lead sulphide at lower tempernucleate to form particles as the exhaust cools during atures. Manganese oxides have similar melting and boiling expansion. Lubrication oil metals have been found enriched points to iron oxides, so they ought to behave in a similar in the nucleation mode while wear metals like Fe are enriched manner as the Fe species. Temperatures in the exhaust of a in the accumulation mode (44). Thus, it would appear that spark ignition engine upstream of the catalyst in the exhaust additive metals rather than wear metals are the main sources manifold often exceed 600 °C when driving the EUDC and of metal compounds in the nucleation mode. Particle size temperatures at the exhaust valves will be even higher, and distributions, averaged over the EUDC drive cycle, obtained under these conditions Pb compounds will be mostly in the with fuel containing CH3C5H4Mn(CO)3 and Fe(Cp)2 produced gas phase. Such molecular gas phase species diffuse much similar nanoparticle emissions at the same dosing. Increasmore rapidly to walls and deposit there than particle species. ing the Mn concentration in the fuel resulted in an increase In addition, if liquid particles are present they are more likely in nanoparticles. The molar equivalent dosing of gasoline to stick to walls than solids. Furthermore, there may be other with PbEt4 however resulted in relatively low tailpipe factors that change the partitioning of Pb between nucleation nanoparticle emissions. This may be due in part to the greater and accumulation mode particles. It is important to recognize volatility of the Pb compounds compared to that of the Fe that even for Fe and Mn a mass balance on the solid 2566

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FIGURE 5. (a) Predicted equilibrium composition of exhaust gases for gasoline containing 8.4 mg/L Fe. (b) Predicted equilibrium composition of exhaust gases for gasoline containing 31.3 mg/L Pb. (c) Predicted equilibrium composition of exhaust gases for diesel engine with 50 ppm sulfur fuel and lubrication oil containing 5000 ppm Ca. Oil consumption assumed to be 0.1% of fuel consumption. VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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nucleation mode reveals that it accounts for no more than about 10% of the metal, the rest must be lost to surfaces or emitted as larger particles. Although pure PbEt4 was used here, commercial leaded gasoline contains ethylene dibromide “lead scavenging agent” that is likely to enhance formation of volatile lead particles. The ethylene dibromide reacts with Pb or its oxide to form a mixture of relativity volatile lead bromide and lead oxybromides (40). Regardless of whether ethylene dibromide is present or not, it is appears that the partitioning of Pb between nucleation mode particles, accumulation mode particles and in-cylinder and exhaust surfaces is different than for Fe and Mn. The deposition of Pb on exhaust valves is known to improve seating and reduce wear because a layer of soft Pb is deposited on them. Thus a combination of factors that are likely related to the much lower temperature of gas to particle conversion and the formation of liquid particles results in a smaller fraction of Pb in the nucleation mode. Ca behaves in a similar manner to iron forming solid phase materials at temperatures below about 1300 °C. Typical diesel exhaust temperatures rarely exceed 650 °C where the main Ca compound is predicted to be CaSO4. The equilibrium calculations are consistent with the assumption that the solid nucleation mode observed with idling heavy-duty diesel engines (Figure 3) results from metal containing species originally present in the lubrication oil, mainly Ca, Zn, Mg, and P, although solid carbonaceous particles cannot be ruled out. The diesel engine results are a further example of how inorganic materials entering the engine can lead to the formation of a solid nucleation mode PM in the exhaust. It might be expected that the passenger car ULG case would show some evidence of a solid nucleation mode but neither the EUDC tests nor idle test (not presented) showed significant numbers of particles or particles in the nucleation mode range. This may be at least partially due to the much lower oil consumption of light-duty passenger car engines than heavy-duty diesel engines. Metallic lubrication oil additives are essential in reducing friction and wear, and at present, unlike organometallic fuel additives, there are few good alternatives.

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