Effects of Cerium Oxide and Ferrocene Nanoparticles Addition As Fuel

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Effects of cerium oxide and ferrocene nanoparticles addition as fuel-borne catalysts on diesel engine particulate emissions: Environmental and health implications Zhi-Hui Zhang, and Rajasekhar Balasubramanian Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00920 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 29, 2017

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Environmental Science & Technology

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Effects of cerium oxide and ferrocene nanoparticles addition as fuel-

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borne catalysts on diesel engine particulate emissions: Environmental

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and health implications

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Zhi-Hui Zhang, Rajasekhar Balasubramanian*

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Department of Civil and Environmental Engineering, Faculty of Engineering, National University of Singapore, 1 Engineering Drive 2, E1A 02-19, Singapore 117576, Singapore

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* Corresponding address: Tel.: +65 65165135; fax: +65 67744202 E-mail address: [email protected] (R. Balasubramanian).

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1 ACS Paragon Plus Environment


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ABSTRACT

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This study systematically examined the potential impacts of doping CeO2 and Fe(C5H5)2

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nanoparticles as fuel-borne catalysts (FBCs) to ultralow sulfur diesel (ULSD) fuel on the

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physical, chemical and toxicological characteristics of diesel particulate matter (DPM). The

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FBCs-doped fuels are effective in promoting soot oxidation and reducing the DPM mass

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emissions, but lead to a significant increase in the total particle counts due to the formation of

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self-nucleated metallic nanoparticles. Compared to undoped ULSD, the FBCs-doped fuels result

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in higher concentrations of particle-phase polycyclic aromatic hydrocarbons (PAHs) and n-

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alkanes, higher fractions of organic carbon (OC) and water-soluble organic carbon (WSOC) in

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particles, show slight alterations in soot nanostructure, reduce soot ignition temperature and

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activation energy. Exposure of the human-type II cell alveolar epithelial cells (A549) to DPM

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derived from FBCs-doped fuels shows a decrease in cell viability and alterations in the global

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gene expression with a broad range of biochemical pathways. The overall variations in DPM

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characteristics are mainly caused by the catalytic combustion process, and are related to the type,

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properties and contents of FBCs used in diesel fuel as well as the engine operating conditions.

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Environmental and health implications of the study are highlighted.

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Keywords: Diesel particulate matter; Fuel-borne catalysts; Cerium oxide; Ferrocene; Toxicity


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INTRODUCTION

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High emissions of airborne particulate matter (PM) from diesel engines used in both on-

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road and off-road applications are of global concern as they have adverse impacts on urban air

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quality, human health, and also affect global climate change.1,

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technology and the application of post combustion-treatment systems in conjunction with diesel

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fuel reformulations are critically needed for abatement of diesel particulate matter (DPM)

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emissions.3 One of the fuel reformulation technologies under consideration is the addition of an

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ultra-low dose of fuel-borne catalysts (FBCs) in the form of organometallic nanoparticles to

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diesel fuels. This approach is explored for suppressing soot formation and/or promoting soot

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oxidation during the combustion process, and thus reducing DPM mass emissions.3-8

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additional consideration is that if FBCs are used in combination with diesel particulate filters

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(DPFs), the FBCs embedded in the emitted DPM could enhance soot oxidation in the DPFs and

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therefore promote DPFs regeneration.9 FBCs containing Ce, Fe and Pt are now commercially

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available, among which both Ce- and Fe-based FBCs have been increasingly used in Europe and

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elsewhere as diesel fuel additives to ensure fast and complete DPFs regeneration.7-11 These

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FBCs have also been used without DPFs in existing off-road diesel engines and commercial

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vehicles to reduce soot and DPM emissions.6-8 However, these FBCs seem to be restricted from

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using in several countries. For example, in the U.S., current regulations restrict their use to on-

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road diesel engines. In Switzerland, FBCs can only be used in combination with DPFs, but their

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use without particle traps is generally prohibited. These restrictions have been placed because of

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concerns over the potential environmental and public health impacts of FBCs.

2

Improvements in engine

An

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DPM emissions vary significantly in physical characteristics and chemical composition

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between different engine types, engine operating conditions, and fuel formulations. Some recent



65

studies have reported that the catalytic combustion process, caused by FBCs-doped fuels, not

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only reduce DPM mass concentrations efficiently, but also trend to alter the physical and

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chemical properties of DPM.7-8,

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composition of DPM into account are deemed to be more effective and efficient because the

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chemical characteristics of DPM rather than the DPM mass determine their potential influence on

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the environment and human health. In addition, the application of metal-based FBCs in diesel

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fuels inevitably causes the emissions of the metals themselves in DPM which, combined with

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changes in chemical composition and/or particle size distributions, might lead to changes in the

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overall toxicity of DPM.15-17 However, the potential adverse environmental effects and health

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risks resulting the use of Ce-based and Fe-based FBCs are poorly understood due the lack of

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systematic investigations.

12-14

Emission reduction strategies that take the chemical

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To address these knowledge gaps, we systematically investigated the changes in the

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physicochemical and toxicological characteristics of DPM emissions resulting from the diesel

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engine fuelled by CeO2- and Fe(C5H5)2- doped fuels by using a series of complementary

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analytical techniques. The central goal of this work is to assess their potential environmental and

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human health implications. Specifically, we examined the influence of doping different

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concentrations of these additives to ultralow sulfur diesel (ULSD) fuel on particulate mass

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concentration as well as particle number concentration with their corresponding size distributions.

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We then analyzed the effects of these FBCs-doped fuels on the carbonaceous contents of the

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DPM including elemental carbon (EC), organic carbon (OC), and water-soluble organic carbon

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(WSOC). We selected PAHs and n-alkanes as appropriate chemical species of OC for further

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discussion as PAHs are suspected human carcinogens and n-alkanes represent an important class

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of the organic compounds originating from unburned fuel and/or lubricating oil. In addition, we


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studied the particle volatility and soot oxidation behavior to understand the effects of these

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FBCs-doped fuels on nanoparticle formation and soot oxidation reactivity. Determination of the

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influence of FBCs-doped fuels on the above-mentioned properties of DPM is essential for

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understanding how FBCs-doped fuels affect the DPM formation process and its chemical

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composition. The changes in the physico-chemical characteristics are likely to affect the toxicity

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of DPM. In order to examine this possibility, we exposed the particles derived from both FBCs-

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doped and undoped fuels to the human-type II cell alveolar epithelial cell line (A549) and

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assessed their toxicity by using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium

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bromide) assay. The cDNA microarray technique was then used to assess the changes in the

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expression of gens A549 in an attempt to gain an insight into the complex mechanisms that

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underlie the health effects induced by DPM derived from FBCs-doped fuels in comparison with

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undoped diesel fuel. We selected an off-road diesel engine for this study as such engines are

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widely used and emit a substantial fraction of DPM on a global level due to the lack of effective

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environmental regulations.18 A long-term exposure to high concentration of freshly emitted DPM

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has been reported to pose a significant carcinogenic risk to occupational workers in various

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working microenvironments where off-road engines are frequently used.19

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EXPERIMENTAL SECTION

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The essential details of the experimental work are explained briefly here, and more detailed information is given in Supporting Information (SI).

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Engine, Fuels and FBCs. The schematic of the experimental system is given in Figure

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S1. Experiments were carried out on a single-cylinder, naturally aspirated, four-stroke, direct-

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injection diesel engine (L70AE, Yanmar Corporation) connected to a 4.5 kW generator. The

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diesel engine has a capacity of 296 cm3 with bore and stroke of 78 mm and 62 mm, a fixed speed 5 ACS Paragon Plus Environment


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of 3000 rpm (revolutions per min). The more detailed specifications of the engine are provided in

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Table S1. Similar small off-road diesel engines have been used previously for investigating the

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influence of FBCs-doped fuels on engine exhaust emissions.4-8, 13, 14

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ULSD with less than 10-ppm (parts per million) by weight of sulfur was used as a base

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fuel, and the major properties of this fuel can be found elsewhere.20 The FBCs used in this study

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include cerium oxide (CeO2; Sigma-Aldrich, ≥ 99.9%) nanopowder with particle size of less than

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25 nm, and the analytical grade ferrocene (Fe(C5H5)2; Sigma-Aldrich, ≥ 98%) in the form of

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commercially available nanoparticles. CeO2 and Fe(C5H5)2 were added to ULSD in three

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different concentrations (25, 50, and 100 ppm Ce and Fe by weight, which are designated as

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Ce25, Ce50 and Ce100 for the CeO2-doped fuels, and Fe25, Fe50 and Fe100 for the Fe(C5H5)2-

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doped fuels, respectively.). Both CeO2 and Fe(C5H5)2 were added to the diesel fuel as per the

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established procedure, 8, 14, 21 which can be found in the SI.

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Particulate sampling and measurement. A two-stage Dekati mini-diluter (DI-2000,

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Dekati Ltd.) was used for diluting the engine exhausts for DPM sampling and online evaluation.

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The dilution ratio from each stage was determined, and all data presented in this article have

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been dilution-corrected to represent tailpipe conditions. DPM emissions from the first-stage

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diluter were collected onto 47 mm Teflon filters (Pall Life Sciences, Ann Arbor, MI, 2 µm pore

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size) and on 47 mm quartz fiber filters (Whatman, USA, 2.2 µm pore size) for the subsequent

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analyses, by concurrently using two Mini-Vol particulate samplers (Air metrics Ltd.; 5 L/min

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flow rate). The blank quartz fiber filters were pre-combusted in air (650 ℃ for 12 h) to remove

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any residual carbon contamination prior to being used for particles collection. Further details

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about determination of the dilution ratio and the handling of filters and DPM samples are

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provided in the SI. The number concentrations and size distributions of volatile and solid


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particles in the secondary dilution stage were measured by a Fast Mobility Particle Sizer (FMPS,

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Model 3091, TSI Incorporated, USA) for particles over the size range of 5.6 to 560 nm. In this

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setup, two diluters were used in series, with the first stage being heated to maintain a temperature

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of 190 ℃ to minimize thermophoretic deposition. During the solid particle number emission

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experiments, a thermodenuder (TD, Dekati Ltd) with a heated temperature of 265 ℃ was placed

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in-line between the secondary stage diluter and the FMPS for removing the volatile and semi-

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volatile compounds of particles. The TD diffusion losses were estimated using a widely used

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method,22 and the diffusion loss-corrected particle size distributions are presented in this article.

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Physical analyses. A thermogravimetric analyzer (Discovery TGA, TA instruments) was

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used to investigate the ignition temperature of soot followed by the method reported previously,

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22

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Arrhenius expression.23 Raman spectra of the DPM samples were recorded with a Renishaw

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microscope system (Renishaw, System 2000) using a 514 nm Ar ion laser as an excitation source.

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The spectra were then analyzed by using the Renishaw WIRE 2.0 software running under

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GRAMS/32 (Galactic, Levenberg–Marquardt nonlinear least-squares fitting algorithm) to obtain

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the D (Defect)/G (Graphite) band intensity ratios (ID/IG) for investigation of the graphite-like

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structure of soot particles. Further details of these analyses are provided in the SI.

and the activation energy for the soot was then estimated using a modified form of the

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Chemical analyses. A thermal/optical carbon aerosol analyzer (Sunset Labs, Forest

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Grove, Oregon, USA) was used to quantify EC and OC fractions of particles collected onto

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quartz filters according to the NIOSH 5040 reference method.24 Meanwhile, the particles

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collected on Teflon filters were extracted with organic-free Milli-Q water (>18 MΩ cm) using a

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water-bath sonicator (Elmasonic S 60H, ELMA, Germany) for 2 x 30 min. The extracts were

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then filtered through a 0.20 µm PTFE membrane filter (Satorius, Germany), acidified by 7 ACS Paragon Plus Environment


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ultrapure HCl (0.1 ml of 2M HCl), and purged for 2 min with ultrapure air. 100 µl of the

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acidified filtrate was injected into an organic carbon analyzer (TOC-VCSH, Shimadzu) for

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determination of WSOC.

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A

Unity/Ultra

thermal

desorption

system

coupled

with

Agilent

6890N

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chromatography/5973N mass spectrometry (TD-GC/MS) was used for the determination of

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particle-phase PAHs and n-alkanes. The details of the method are provided by Ho and Yu.25 In

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this study, the 16 US priority PAHs were measured and then classified into three groups based

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on their molecular weights, namely low molecular weight (LMW, 2 and 3 rings) PAHs, medium

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molecular weight (MMW, 4 ring) PAHs and high molecular weight (HMW, 5 and 6 rings) PAHs,

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as suggested in our previous study.20 Therefore, the total-PAH data represents the sum of the 16

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individual PAHs. The overall toxicity is defined as the sum of Benzo[a]pyrene equivalent (total

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BaPeq), which was estimated based on the method of Nisbet and LaGoy.26 For particle-phase n-

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alkanes, concentrations of the homologous series from C14 to C26 were quantified. The

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concentrations of n-alkanes with molecular weight lower than C14 and higher than C26 were

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quite low and therefore are not reported in this article.

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Toxicological experiments. In this study, DPM emitted from pure ULSD and from 100

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ppm Ce- and 100 ppm Fe-doped fuels at medium engine load was chosen for a comparative

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evaluation of their toxicological characteristics. We have made this selection because particles

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emitted under these conditions contained the highest amount of Ce and Fe when the engine was

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operated with the most commonly used engine load. These particles were physically removed

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from the Teflon filters by sonication, and re-suspended in DMSO (Dimethyl sulfoxide,

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Bioreagents, Sigma-Aldrich) and subsequently in cell exposure medium using previously


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validated procedures,27 as summarized in the SI. The extraction efficiencies were evaluated, and

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the extracted particulate mass was efficiency-corrected.

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We exposed the particles to the human-type II cell alveolar epithelial cell line (A549).

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The cytotoxicity of these particles was measured by using the MTT assay and the resulting

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changes in the expression of global genes were carefully assessed by using the cDNA microarray

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technique. Method details are given in the SI. For the MTT assay, the cells were exposed to

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concentrations of particles in the cell culture wells that corresponded to 25, 50, 100, and 200

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µg/mL of particles, respectively, for 24 hours. Each sample was tested in six replicate wells, and

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the data on cell viability are reported as relative decrease compared to the control, considered as

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100% of viable cells. As for gene expression, the A549 cells were exposed to 50 µg/ml of

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particles for 6 hours. The gene expression level was considered significantly altered based on a

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1.5-fold cut-off for the fold-change. The shortlisting of genes obtained from microarray data was

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conducted using the Gene Ontology (GO) analysis. The identified gene list has undergone

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functional annotation clustering using the DAVID web resource (http://david.abcc.ncifcrf.gov/).

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The examined categories are Biological Processes and KEGG.

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QA/QC and statistical analysis. The analytical quality of the data obtained on the

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chemical composition of DPM was ascertained using the lower limit of detection (LOD),

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recovery, the linearity of calibration, and the relative standard deviation (RSD) of replicate

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analyses. The details of quality assurance and control (QA/QC) protocols for chemical analyses

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are provided in SI. The LODs for OC and EC were below 0.47 mg/m3. The RSD of duplicate

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analyses was lower than 5.8 % for TC (total carbon), and below 10% for OC and EC. The LOD

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for WSOC was below 0.6 µg/m3. After the correction for the blank contribution, the recoveries

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for WSOC were 82.9-94.8% by the TOC method, and the RSD of four replicate analyses was



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estimated to be within 6.5%. A five-point calibration for WSOC standard solution was

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performed, and the correlation coefficients (R2) obtained from the linear regression of the

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calibration curves was larger than 0.99. For PAHs and n-alkanes, the LODs were calculated to be

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in the range of 2.1–35.8 ng/m3 for PAHs and 2.9–29.2 ng/m3 for n-alkanes, respectively. The

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RSDs of four replicate analyses were 4.5 to 10.3% for the PAHs, and were 4.8 to 7.5% for n-

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alkanes, respectively. An eight-point calibration over a concentration range of 0.1– 5.0 ng for

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each PAH and n-alkane from the liquid standard mixtures (Sigma-Aldrich, Bellefonte, PA, USA)

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was established, and the correlation coefficients (R2) for linear regressions of the calibration

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curves was greater than 0.99. In this study, all analytical data were corrected for the average

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value of the blanks, and are reported as the mean values. The significance of the differences of

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all tested parameters between diesel fuel and FBCs-doped fuels, and between the two kinds of

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FBCs-doped fuels under the same FBC concentration was assessed by Student's t test at p < 0.05.

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The results of particle number size distributions are reported as the mean values, with the relative

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standard deviations ranging from 2.5 to 3.2%, respectively. With the exception of particle-phase

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n-alkanes and particle number size distributions, all other reported values are presented as mean

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± SD (standard deviations).

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RESULTS AND DISCUSSION

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Particulate mass and carbonaceous fractions. Figure 1(a) shows that both Ce- and Fe-

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doped fuels could effectively reduce the particulate mass concentrations, which is associated

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with the variation of the EC and OC emissions, as evident from Figures 1(b) and (c), respectively.

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Specifically, the EC emissions generally decreased with an increase in both Ce and Fe

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concentrations in ULSD. The degree of the EC reduction at lower engine load was greater than

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that at higher engine load. In addition, the Fe100 was generally more effective in inhibiting EC 10 ACS Paragon Plus Environment

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emissions than the Ce100 at a higher engine load. Similar results in the soot or smoke emission

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reduction were reported for Ce-doped fuels, 4, 5 and for Fe-doped fuels.7, 8 Interestingly, we found

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that with the exception of Ce100, all other FBCs-doped fuels did not make significant changes in

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OC emissions. Thus, our finding suggests that the decrease in DPM emissions, caused by FBCs-

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doped fuels, can be attributed mainly to the EC emission reduction. Moreover, the substantial

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reduction in EC emissions coupled with the relatively minor change in OC emissions for both

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Ce- and Fe-doped fuels contributes to the higher OC/EC ratios (Figure 1(d)) than that for

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undoped ULSD. Compared to Ce100, the higher EC emission reduction coupled with the lower

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OC emission reduction for Fe100 led to higher OC/EC ratios. These observations suggest that a

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high proportion of semi-volatile organics deposited on the surface of soot particles emitted from

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these FBCs-doped fuels, especially from Fe-doped fuels. Another interesting finding of our study

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is that the Ce50 and Ce100 led to a 3.7 to 8.1% higher particulate mass emission reduction than

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that of Fe50 and Fe100, indicating Ce-based FBC being more effective in inhibiting DPM

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emissions than that of Fe-based FBC under these conditions.

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Figure 1(e) shows that WSOC mass concentrations increased significantly for both Ce-

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and Fe-doped fuels at the high engine load, leading to the higher WSOC/OC (Figure 1(f)) ratios

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for these conditions. However, at low and medium engine loads, the higher WSOC/OC ratios

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only occurred for the addition of 100 ppm Ce to ULSD. In other cases, the WSOC/OC ratios for

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FBCs-doped fuels showed no significant difference compared to that for undoped ULSD. In

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addition, for each test condition, the WSOC/CO ratios for Ce-doped fuels showed slightly higher

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than that of Fe-doped fuel, but were not statistically significant. Our results indicate that the

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FBCs-doped fuels tend to increase the water soluble fraction of OC in these cases, suggesting an

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increased fraction of polar organic compounds in OC in these cases. These observations imply



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that the FBCs-doped fuels have potential to change in the particle toxicity due to the increased

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emission of both OC and WSOC fractions in DPM although they showed an overall decrease in

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total particle mass concentrations.

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A fundamental understanding of the formation of new particles and their growth in diesel

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engines may explain how FBCs-doped fuels affect the carbonaceous particulate emissions. The

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details can be found in the SI. Briefly, with the addition of FBCs to diesel fuel, the organic

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component of ferrocene decomposes in the early stage of the combustion process before the

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onset of soot particle inception due to its lower melting point, 28 and is most dominantly present

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in the form of amorphous Fe (III) oxide crystallizing to hematite α-Fe2O3.29 Then, the iron oxide

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nuclei and nanoparticles are formed for the subsequent deposition of the carbonaceous phase. As

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for CeO2, its nanoparticulate structure would remain unchanged due to its very high melting

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point (2200–2400℃). During the combustion process, these metal oxide catalysts have a high

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potential to promote carbon oxidation due to the redox mechanisms, where metal oxide catalysts

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act as oxygen suppliers for the gasification of carbon and are then re-oxidized by external

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oxygen.30-32 Therefore, the high external surface area of the catalyst is crucial for the oxidation of

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soot. In this study, we found that the doping of both CeO2 and Fe(C5H5)2 can lead to higher

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counts of metallic nanoparticles as explained in following section. As the iron metal oxide

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particulates tend to affect the soot formation, they are likely to serve as condensation nuclei for

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early than the soot inception and therefore enhance soot formation. Conversely, our observations

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show that the higher concentrations of Fe in diesel fuel led to the lower EC emissions. We

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therefore believe that the surface enrichment of the catalysts with the soot formed from FBCs-

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doped fuels can facilitate easier oxygen transfer from the surrounding air to the metal oxide,

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accelerating the redox process and consequently enhancing the oxidation behavior. With an


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increase in the engine load, the counts of metallic nanoparticles decreased as found in this study,

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which suggests a reduced amount of contact points between soot and catalysts, and therefore led

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to lower percentage reduction of soot emission. Moreover, when compared to CeO2, the

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decomposition of the same dose of Fe(C5H5)2 produced higher counts of smaller metal oxide

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nuclei as evident from the higher counts of nanoparticle emissions (see the following section).

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These Fe-containing nanoparticles, combined with their higher mobility due to the lower melting

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point, create more contact points for enhancing the soot/catalyst contact under diesel combustion

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conditions, and therefore appear to be more effective than the cerium oxide catalyst as observed

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in this study.

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The addition of FBCs to diesel fuel could also considerably reduce the ignition 12, 13, 33

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temperature (light-off temperature) of diesel soot as proposed by some previous studies,

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which was further confirmed by our current study (see a later section).

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temperature, the oxidation rate of diesel and soot in the last stage of engine combustion will

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decrease leaving more unburned fuels left to condense on the surface of the soot. Therefore, the

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reduction of ignition temperature may be one of the factors that led to higher OC emissions.

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Finally, the lower EC emissions may provide less surface area for OC adsorption, which may in

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turn lead to lower OC emission. The two factors compete with each other, and therefore lead to a

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minor variation of OC caused by FBCs-doped fuels.

With a drop in

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Particle-phase PAHs and n-alkanes emissions. Compared to the undoped ULSD, the

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total particle-phase PAHs concentrations generally increased with an increase in the Ce and Fe

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contents in the fuels (Figure 2(a)). The percentage increase in both MMW-PAHs and HMW-

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PAHs was higher than that of LMW-PAHs. Another important finding is that with the same dose

293

of FBCs in the fuel, the Fe-doped fuel led to higher total particle-phase PAHs emissions than that



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of the Ce-doped fuel. A similar effect of both additives on the total BaPeq was also observed in

295

this study, suggesting that both Ce- and Fe-doped fuels tend to increase the toxicity of the DPM

296

due to an increase in the particle-phase PAHs emissions. Our observation is consistent with a

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previous report that the total particle-phase BaPeq was increased by CeO2-doped diesel fuels.8

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Figure 2(b) shows that for each fuel, the distribution of particle-phase n-alkanes represents a

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similar bell-shaped curve with C21 as the most abundant. As in the case of FBCs, both Ce- and

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Fe-doped fuels led to higher n-alkanes emissions compared to the undoped ULSD, with Fe-

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doped fuels leading to higher emissions of n-alkanes than that of Ce-doped fuels. It is known that

302

particle-phase PAHs and n-alkanes mainly originate from the unburned diesel fuel or/and

303

lubricating oil. Diesel fuels have a complex mixture of hydrocarbons with chain lengths mostly

304

between C12 and C26 while crankcase oils tend to have a longer chain of carbon atoms. Given

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the nature of diesel fuel and lubricating oil combined with the distribution of particle-phase n-

306

alkanes observed in this study, it is reasonable to assume that most of the n-alkanes observed in

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this study are a result of incomplete combustion of diesel fuel and not lubricating oil. The same

308

reason may also be responsible for higher PAHs emissions. This finding further confirms our

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previously proposed theory that a reduction in the engine ignition temperature due to FBCs can

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cause an increase in unburned hydrocarbon emissions as discussed in the previous section.

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Compared to Ce-doped fuels, Fe-doped fuels resulted in a lower ignition temperature and

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therefore higher particle-phase PAHs and n-alkanes emissions. Moreover, we found that the

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relative increase in C16-C25, caused by FBCs-fuels, was much higher than that of both C14-C15

314

and C26. This can be explained on the rationale that the heavier n-alkanes such as C26 probably

315

originated from the lubricating oil, and therefore the emissions of these species are expected to

316

show little variations. As for the shorter-chain alkanes such as C14-C16, on the one hand, they


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are more likely to undergo nearly complete combustion than the heavier ones. On the other hand,

318

they are not easily attached to the soot particles due to their higher volatility than heavier n-

319

alkanes. As a result, they were only slightly affected by the addition of FBCs. The same reason

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can also be used to explain the observation that the percentage increase in both MMW-PAHs and

321

HMW-PAHs was higher than that of LMW-PAHs.

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Particle number concentrations and size distributions. As shown in Figures 3(a)-(f),

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the variation of both volatile and solid particle number size distributions, caused by the FBCs-

324

doped fuels, seems to be associated with their proportion used in ULSD and the engine load. At

325

the low engine load (Figure 3(a)), both Ce- and Fe-doped fuels resulted in the formation of new

326

particles with the nucleation mode within the peak diameters of 10-20 nm. Progressive increases

327

in the concentrations of Ce and Fe in fuels resulted in the size distribution of these nucleation

328

mode particles being shifted upward and toward a larger size, leading to an increase in the

329

emission of total particle counts as shown in Table S2. Moreover, for each dose of FBCs in

330

ULSD, the peak of nucleation mode particles for the Fe-doped fuels was higher and shifted

331

toward a larger size than that of the peak associated with the Ce-doped fuels, resulting in the Fe-

332

doped fuels having higher total particle counts than those of Ce-doped fuels, as evident from

333

Table S2. At the medium engine load (Figure 3(b)), with the exception of 25 ppm Ce, all other

334

FBCs-doped fuels showed obvious peaks in the nucleation mode. However, at the high engine

335

load (Figure 3(c)), only the 100 ppm Fe-doped fuel showed a peak in the nucleation mode. In

336

addition, with an increase in the engine load, the peak in the distribution of nucleation mode

337

particles for FBCs-doped fuels became flatter, indicating that the counts of nucleation mode

338

particles decreased with an increase in the engine load. Table S2 reveals that for each engine

339

load, with an increase of Ce and Fe concentrations in ULSD, the concentrations of particles with



Page 16 of 35

340

diameter less than 20 nm increased significantly, while the concentrations of particles larger than

341

50 nm decreased generally. Table S2 also shows that the FBCs-doped fuels significantly

342

increased particles of less than 50 nm in some cases, including at the low engine load with all

343

tested additions of FBCs, with the exception of 25 ppm Ce at the medium engine load, and also

344

at the high engine load with 100 ppm Fe. Our findings indicate that the increase in the total

345

particle counts as well as the decrease in the geometric mean diameter (GMD) caused by FBCs-

346

doped fuels was a result of a significant increase in the counts of smaller particles, while the

347

reductions in large diameter particles were mainly responsible for the decrease in particulate

348

mass, as evident from Figure 1(a). Additionally, our observations also reveal that with the same

349

amount of FBCs doped in the diesel fuel, the increase in the counts of nanoparticles is higher at

350

the lower engine load, and also generally higher for the Fe-doped fuel than compared to the Ce-

351

doped fuel, suggesting the variations in particle counts are associated with the FBC types used

352

and the engine operating conditions. Another interesting finding from Figures 3(d)-(f) is that

353

after TD, the counts of nanoparticles emitted from both Ce- and Fe-doped fuels were

354

significantly reduced due to the suppression of the formation of new particles from volatile

355

species after the dilution and cooling processes, but did not disappear altogether, which were still

356

much higher than those from pure ULSD. Our findings, therefore, confirm that more and larger

357

self-nucleated metallic particles and their agglomerates are generated in the cylinder and emitted

358

to the engine exhaust with an increase in Ce and Fe concentrations in ULSD. The explanations

359

regarding the formation and the evolutionary processes of metallic nanoparticles, and their

360

influence of particle size distributions in response to changes in the proportion of FBCs used in

361

ULSD and the engine load are provided in the SI. The presence of nanoparticles containing Ce


Page 17 of 35


362

and Fe may have profound implications on human exposure and subsequent health effects, and

363

this topic is discussed in a later section.

364

Soot oxidation behavior and soot nanostructure. Compared to undoped ULSD, a

365

lower soot ignition temperature was obtained from all Ce- and Fe-doped fuels (Figure 4(a)). The

366

extent of the reduction in the ignition temperature seems to have increased with an increase in

367

the Ce concentrations in fuels. However, the reduction in the ignition temperature showed no

368

clear trends with an increase in Fe concentrations. For each engine load, the activation energy of

369

soot generally decreased with an increase in the concentrations of both Ce and Fe in the fuels.

370

Our results suggest that the use of Ce- and Fe-doped fuels offers co-benefits in that these fuels

371

not only produce lower engine output i.e. soot, but also make it easier to oxidize soot by after-

372

treatment systems. In addition, at the high engine load, the soot emitted from Fe100 showed a

373

lower activation energy than that from Ce100, indicating that the soot emitted from Fe100 can be

374

more easily oxidized than that from Ce100 under high engine load. We believe that the Ce and

375

Fe embedded in the diesel soot act as catalysts that reduce the activation energy needed to start

376

soot oxidation and lower the temperature at which soot is oxidized. It should be noted that both

377

Ce- and Fe-based FBCs could effectively promote soot oxidation during the engine combustion

378

process as discussed previously. The reasons responsible for lower soot (EC) emissions can also

379

be used to explain the results in this section. Apart from that, previous observations also

380

suggested that the change in the volatile organic fraction (VOF) in particles,23 the nanostructure

381

of soot

382

oxidation of soot, which might vary with the Ce and Fe concentrations in the fuels as well as the

383

engine operation conditions. In this study, the higher OC fraction in particles and the higher

384

counts of total particles, caused by FBCs-doped fuels, suggest that the soot from these FBCs-

34

as well as the spreading of the metal oxide on the soot surface 33 affects the catalytic



385

doped fuels could have higher surface areas than that of undoped ULSD, and thus should also be

386

considered as one of factors that led to higher soot oxidation reactivity.

Page 18 of 35

387

The D (Defect)/G (Graphite) band intensity ratios (ID/IG) were used to further explore if

388

and how FBCs-doped fuels affect the oxidation of soot through the change in the graphite-like

389

structure of soot particles. With the decrease in this ratio, the graphite-like structure becomes

390

dominant, implying the soot is not easily oxidized.33 Figure 4(b) shows that the ID/IG ratios of the

391

soot particles emitted from 25 and 50 ppm Ce-doped fuels were lower than that from undoped

392

ULSD, indicating that the soot had more ordered graphite-like structures and lower amorphous

393

carbon concentrations. However, the soot emitted from all Fe-doped fuels and from the 100 ppm

394

Ce-doped fuel showed a marginal change in ID/IG ratios, indicating the nanostructure of the soot

395

from these fuels was not significantly changed. The variation in soot nanostructures from the

396

combustion of FBCs-doped fuels in diesel engines could be associated with the changes in the

397

combustion chemistry and in-cylinder combustion and pyrolysis conditions, such as temperature

398

and residence time, etc. The results from this study indicate that the factors that lead to the

399

changes in soot nanostructure are related to the type and amounts of FBCs doped in fuels. Our

400

results further reveal that the changed soot nanostructure, caused by FBCs-doped fuels, did not

401

significantly affect the soot oxidation based on our observations that the soot from Ce25 and

402

Ce50 had more ordered graphite-like structures but still had higher oxidation activity than that

403

from undoped ULSD.

404

Toxicological Studies. Exposure of the cells to a dose of 25, 50 and 100 µg/ml Ce100-

405

derived particles and a dose of 25 and 50 µg/ml Fe100-derived particles led to lower cell

406

viability than those to ULSD-derived particles (Figure 5). Meanwhile, with the exception of the

407

highest exposure does of 200 µg/ml, the cell viability for particles derived from Ce100 was 18 ACS Paragon Plus Environment

Page 19 of 35


408

slightly lower than that from Fe100, indicating that the Ce-based additive leads to the particles

409

having a higher cytotoxicity than that of the Fe-based fuel additive.

410

As shown in Table 1, a short time exposure of A549 cells to the ULSD-derived particles

411

significantly modulated 115 genes, with 70 of them being up-regulated and the 45 down-

412

regulated. While the Ce100 and Fe100-derived particles triggered the modulation of 148 genes

413

(102 up- and 46 down-regulated) and 106 genes (79 up- and 27 down-regulated), respectively.

414

The strikingly altered genes in this assay were widely involved in transcription, cell

415

proliferation/cell cycle/cell aging, metabolism, response to stimulus, cell adhesion, cell signal

416

transduction, response to organic substances/drugs and transport (Table S3). Specifically, we

417

observed that the particles emitted from Ce100 caused the down-regulation of genes of CTGF,

418

CDKN2D, OSGIN1, CYR61, DDIT3, UHMK1 and FOS while the particles emitted from Fe100

419

caused the down-regulation of genes of CTGF, OSGIN1, CYR61. Additionally, when compared

420

to undoped ULSD, the particles emitted from Fe100 significantly up-regulated gene DDIT3. The

421

regulation of these genes is involved in the function of cell proliferation inhibition and/or

422

apoptosis promotion.35

423

Interestingly, compared to the control, the particles emitted from both Ce100 and Fe100

424

up-regulated the cytochrome P450 family genes (CYP1A1 and CYP3A5), which can be altered

425

by PAHs through the aryl hydrocarbon receptor.36-39 Additionally, the increased emissions of

426

PAHs were also reported to be related to the modulation of the genes involved in signal

427

transduction, apoptosis, metabolic activation process and DNA damage.38 Apart from PAHs,

428

alkanes were shown to up-regulate several genes related to cell cycle.37 Therefore, it is possible

429

that the higher emissions of particle-phase PAHs and n-alkanes, caused by FBCs-doped fuels,

430

may contribute to the variation of gene expressions as observed in this study. Notably, when 19 ACS Paragon Plus Environment


Page 20 of 35

431

compared to the control, the particles emitted from Fe100 specifically induced an up-regulation

432

of genes related to the response to reactive oxygen species (ROS)/oxidative stress (ACOX2,

433

ALDH1A3, CYP1A1, DHRS9, HMOX2, FA2H and PPARGC1A). Moreover, among these up-

434

regulated genes, ACOX2 is reported as a shared genetic risk factor for preeclampsia and

435

cardiovascular diseases,

436

addition, the cytochrome P450 family genes together with ALDH1A3 are more potent in the

437

induction of many pro-carcinogenic genes (i.e. JUN, FOS).38 Here, we noticed a significant up-

438

regulation of P450 family genes, ALDH1A3 and their corresponding pro-carcinogenic gene FOS

439

was stimulated by the particles emitted from Fe100. These results suggest that Fe100 may

440

potentially increase the pathogenicity of the diesel exhaust.

40

and CYP1A1 is associated with the increased lung cancer risk. 39 In

441

Interestingly, when we compared to undoped ULSD, we found that the particles emitted

442

from Fe100 induced a significant up-regulation of DDIT3, EGR1, EGR2, FOS, which were

443

associated with multiple biological functions including transcription, cell proliferation, metabolic,

444

and response to ROS. The particles emitted from Ce100 only significantly up-regulated CFB and

445

SERPINA3, which were involved in the regulation of immune and inflammatory responses.41

446

These results are consistent with previous reports in the literature that the exposure of cells to

447

Fe2O3 nanoparticles through the inhalation route could affect biological targets through

448

generation of ROS by oxygenation of reduced Fe species, 16, 42 and a short time exposure of cells

449

to CeO2 nanoparticles may lead to an acute inflammatory response.17 Therefore, we believe that

450

emission of the metallic nanoparticles from FBC-doped diesel as found in this study may affect

451

the gene expression and cytotoxicity.

452

Apart from the genes with specific functions described above, our data also reveals a

453

broad range of biochemical pathways that are illustrated by genes regulating transcription, 20 ACS Paragon Plus Environment

Page 21 of 35


454

transport and cell signal transduction. This observation implies that apart from PAHs and metal

455

oxides, the changes in polar and nonpolar organic compounds in the OC fraction, the physical

456

properties of metallic nanoparticles, caused by FBCs-doped fuels, could also affect the biological

457

reactions occurring at the particles/cell interface and therefore may lead to synergistic or

458

antagonistic biological effects. Notably, the results from this preliminary study suggest that a

459

panel of cytochrome P450 family genes, and the genes that are related to oxidative stress and

460

inflammatory response proposed here deserve considerable attention when conducting further

461

studies on this topic.

462

Environmental and health implications. The study reveals that the use of FBCs-doped

463

fuels in diesel engines effectively reduces the DPM mass and EC emissions due to the enhanced

464

soot oxidation, caused by FBCs during engine combustion. The reduction in DPM mass tends to

465

improve urban air quality while that in EC is beneficial from the climate change mitigation

466

viewpoint as EC is a second major contributor to global warming after CO2. However, the FBCs-

467

doped fuels facilitate the formation of self-nucleated metallic nanoparticles, resulting in the

468

increased emissions of nanosize particles. In addition, the use of FBCs-doped fuels increases the

469

organic carbon (OC) fraction of particles and the water-soluble fraction of OC, suggesting an

470

increased fraction of both polar and non-polar organic compounds in particles, including the

471

specific toxic organic compounds such as PAHs. Another important observation is a decrease in

472

cell viability, as revealed by the in vitro toxicity assessment. Exposure of human epithelial cells

473

to DPM exhibited alterations in the global gene expression with a broad range of biochemical

474

pathways, which could be attributed to the changes in the physico-chemical characteristics of

475

DPM caused by the FBCs-doped fuels. Emission of ultrafine particles (diameter < 100 nm) with

476

high toxicity is of major health concern as such particles penetrate deeper in the lungs and can



477

cross cellular membranes, extending their toxicological impacts on other organs.43 From the

478

findings of the study, it becomes clear that the DPM mitigation strategies involving the use of

479

chemical additives in diesel engines must take into account the different characteristics of DPM

480

holistically in order to provide both environmental and public health benefits.

Page 22 of 35

481

Limitations. The knowledge gained from this preliminary investigation with a small

482

diesel engine (gen-set) can be valuable for selecting appropriate fuel additives or designing after-

483

treatment devices and alternative combustion systems for reduction of DPM emission from

484

diesel engines. However, further studies will still be needed to investigate detailed new particle

485

formation mechanisms in large commercial diesel engines as well as in on-road engines and to

486

examine the influence of fuel properties and operating conditions on the detailed characteristics

487

of DPM while using toxic metal-containing chemical additives. Apart from PAHs and n-alkanes,

488

other toxic organic compounds such as nitrated- and oxygenated-PAHs, hopanes, steranes, and

489

quinones, should also be determined while using FBCs and their role in the toxicity of DPM

490

should be assessed. Expression profiles of genes induced by DPM in human epithelial cells

491

provided a fundamental understanding of possible biological mechanisms involved in pulmonary

492

diseases, caused by FBCs-doped fuels. The finding from the cDNA microarray technique needs

493

to be confirmed with additional analyses of gene and protein expressions in other conditions such

494

as varied exposure time and dose, and different human pulmonary cell lines using a

495

complementary technique such as quantitative real-time PCR (qPCR). Additionally, particles

496

emitted from different engine types and operating conditions will allow us to gain a better

497

understanding of how the changes in the physico-chemical characteristics of DPM, induced by

498

FBCs, trigger the multiple biological effects. It should be acknowledged that the protocols used

499

in the current toxicity study could significantly change the nature of particles emitted from the


Page 23 of 35


500

engine, which might not be representative of in vivo exposure. Therefore, in vivo inhalation

501

exposure studies using animal models over a long time of exposure are needed. Despite these

502

limitations, the findings from this study emphasize the importance of a balance that needs to be

503

maintained between the environmental regulation-driven strategies involving the use of FBCs to

504

reduce the emission of the DPM mass and the key environmental and public health benefits that

505

can be derived, leading to a win-win situation.

506

Supporting Information available: The schematic of the experimental system employed in this

507

study, specifications of the engine, the procedures of doping CeO2 and Fe(C5H5)2 to diesel fuel,

508

the methods of determination of the dilution ratio and the handling of filters and DPM samples,

509

the procedures of physical analyses and toxicological experiments, and the details of quality

510

assurance and control (QA/QC) protocols for chemical analyses. The discussion regarding the

511

DPM formation processes, the explanations regarding the formation and the evolutionary

512

processes of metallic nanoparticles and their influence of particle size distributions in response to

513

changes in the proportion of FBCs used in ULSD and the engine load.

514

ACKNOWLEDGMENTS

515

Z.-H. Zhang thanks the Singapore − Peking − Oxford Research Enterprise (COY-15-EWI-

516

RCFSA/N197-1) for providing scholarship in support of his doctoral study.



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(43) Oberdörster, G.; Maynard, A.; Donaldson, K.; Castranova, V.; Fitzpatrick, J.; Ausman, K.; Carter, J.; Karn, B.; Kreyling, W.; Lai, D.; et al. Principles for characterizing the potential human health effects from exposure to nanomaterials: Elements of a screening strategy. Part. Fibre Toxicol. 2005, 2, 8.

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Page 28 of 35

Table 1 Functional classification of genes up- and down-regulated following exposure of A549 cells after 6 h to particles derived from both FBCs-doped and undoped ULSD fuels

Category Transcription Cell proliferation /growth/cycle/aging Metabolism Stimulus Cell adhesion Signal transduction Response to organic substance/inorganic substance/drug Response to reactive oxygen species/oxidative stress Regulation of inflammatory response Transport Total

ULSD vs Control

Ce100 vs Control

Fe100 vs Control Downregulated 0 3

Ce100 vs ULSD Upregulated 0 0

Fe100 vs ULSD Upregulated 5 2

Upregulated 8 10

Downregulated 13 0

Upregulated 0 15

Downregulated 11 10

Upregulated 3 13

13 8 4 9 10

15 11 2 0 11

15 10 0 5 15

5 8 2 0 0

17 8 0 19 5

6 4 2 0 5

0 0 0 0 0

4 0 0 0 0

0

6

0

0

7

0

0

2

3

0

5

0

3

0

2

0

9 70

2 45

9 102

0 46

5 79

2 27

0 17

0 7


Page 29 of 35


Figure Captions Figure 1. Effect of FBCs-doped fuels on (a) the concentrations of diesel particulate matter (DPM) mass and carbonaceous species, (b) EC (elemental carbon), (c) OC (organic carbon), and (e) WSOC (water-soluble organic carbon)), and the ratios of (d) OC/EC and (f) WSOC/OC. Significantly different from the ULSD (p < 0.05).

b

a

Significantly different from between Ce-

doped fuel and Fe-doped fuel under the same doping dose (p < 0.05). Figure 2. Effect of FBCs-doped fuels on (a) the concentrations of particle-phase PAHs and their total BaPeq (benzo[a]pyrene equivalent), and (b) the concentration distributions of n-alkanes. LMW (low molecular weight, 2 and 3 rings)-PAHs, MMW (medium molecular weight, 4 rings)PAHs, HMW (high molecular weight, 5 and 6 rings)-PAHs. a Significantly different from the ULSD (p < 0.05). b Significantly different from between Ce-doped fuel and Fe-doped fuel under the same doping dose (p < 0.05). Figure 3. Effect of FBCs-doped fuels on (a-c) volatile (without TD) and (d-f) non-volatile (with TD) particle number size distributions. Figure 4. Effect of FBCs-doped fuels on (a) soot oxidation: ignition temperature and activation energy, and (b) the D (defect)/G (graphite) band intensity ratios (ID/IG). a Significantly different from the ULSD (p < 0.05).

b

Significantly different from between Ce-doped fuel and Fe-doped

fuel under the same doping dose (p < 0.05). Figure 5. Effect of particles derived from both ULSD and FBCs-doped fuels on cell viability. Significantly different from the ULSD (p < 0.05).

b

a

Significantly different from between Ce-

doped fuel and Fe-doped fuel under the same doping dose (p < 0.05).



(b) 50

(a) 120 Particulate mass

EC

75% Load

75% Load

3

DPM mass concentration (mg/m )

Page 30 of 35

25% Load

50% Load a a

3

80

40 a,b a,b a,b a,b

EC (mg/m )

100

60

a a

a,b a,b a,b a,b

40 a a a,b a,b a,b a,b

50% Load a

30

a a

50% Load

75% Load

75% Load

a,b

a b

OC/EC

3

OC (mg/m )

50% Load

25% Load

a,b

30 a,b

b

30 a,b a,b a,b

20

a a

a,b a,b a,b a,b a a

10

10

Fe100 Ce100 Fe50 Ce50 Fe25 Ce25 ULSD

(f) 0.30

WSOC 50% Load

a,b a,b a,b a,b a,b a,b

Fuel type

(e) 10 25% Load






0

Fuel type

75% Load

0.25

WSOC/OC

25% Load

a

4

WSOC/OC

6 a

a a

50% Load

a

0.20

3

WSOC (mg/m )


OC/EC

25% Load

40

8


(d) 50

OC

50

0

a a a,b a,b

Fuel type

40

20

75% Load

a,b a b

0.15 a,b a,b

0.10

a,b a,b

a a

2

a a

0.05

Fuel Type

Fuel type

Figure 1 30 ACS Paragon Plus Environment




0.00




0

a,b a,b

a a a a a,b a,b


0




60

a

20

Fuel type

(c) 70

a a

10

20 0

25% Load

Page 31 of 35


1.5 50% Load

Total BaPeq HMW-PAHs MMW-PAHs LMW-PAHs

50

a,b a,b

a,b

40

a,b

a,b

a,b a,b a,b

a,b

a,b

0.6

a,b

a,b

20

0.9

a,b a,b

b

30

a,b

10

a,b

a,b

a

a

a,b

Fe100

Ce100

Fe50

Ce50

Fe25

Ce25

ULSD

0

1.2

a,b

0.3

0.0

Fuel type

50% Load

ULSD Ce25 Ce50 Ce100 Fe25 Fe50 Fe100

3

Particle-phase n-alkanes (µg/m )

(b) 500 400

300

200

100

0 14

16

18

20

22

Number of carbon atoms Figure 2 31 ACS Paragon Plus Environment

24

26

Total BaPeq

3

Particle-phase PAHs (µg/m )

(a) 60


(a) 4 x 1014


14

2 x 10

50% Load, without TD

14

3

3

3 x 10

dN/dlog(Dp) (#/m )

14

dN/dlog(Dp) (#/m )

(b) 4 x 1014


14

1 x 10

3 x 10

14

2 x 10

14

10

0

100

10

Diameter Dp (nm)


14

2 X 10

25% Load, with TD


14

3

3 X 10

0

(f) 4 X 1014

50% Load, with TD


14

14

3

3

10

100 Diameter Dp (nm)

dN/dlog(Dp) (#/m )

10

dN/dlog(Dp) (#/m )

14

2 x 10

1 x 10

0

1 X 10

14

3 x 10


14

14

1 X 10

(e) 4 X 1014


(d) 4 x 1014

dN/dlog(Dp) (#/m )

3

dN/dlog(Dp) (#/m )

(c) 4 X 1014

2 X 10


1 x 10

0

3 X 10

Page 32 of 35

14

3 X 10


75% Load, with TD

14

14

2 X 10

14

1 X 10

0

0

10


10

Figure 3





25% Load

640

250 a,b

a,b

480

a,b a,b a,ba,b a,b

a,b a,b a a

a a

a a

a,b

a,b

a,b

a,b

a,b

a,b a,b a a

a a

320

300

75% Load

50% Load

Ignition temperature Activation energy

o

Ignition temperature ( C)

(a) 800

a a a a

a a

a a a b a b

160

150 100 50




0

200

0

Fuel type

(b) 2.0 ID/IG

50% Load b a,b

1.5

ID/IG

b a,b

1.0

0.5

Figure 4


Fe100

Fuel type

Ce100

Fe50

Ce50

Fe25

Ce25

ULSD

0.0

Activation energy (kJ/mol)

Page 33 of 35


670 671 ULSD 672 Ce100673 Fe100674

100

90 Cell viability (% control)

Page 34 of 35

a,b a,b

80

a,b a,b

b a,b

70

60

50 25

50 100 Particulate dose (µg/ml)

Figure 5


200

Page 35 of 35


229x94mm (150 x 150 DPI)

ACS Paragon Plus Environment

Effects of Cerium Oxide and Ferrocene Nanoparticles Addition As Fuel

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