Keywords

1 Introduction

Diesel engines are widely used all over the world as a prime mover for transportation (Wallington and Wiesen 2014; Dhanasekaran and Mohankumar 2014) and also for power generation (Breeze 2005). The popular usage of diesel engines is attributed to its high-performance characteristics and higher thermal efficiency as compared to spark ignition (SI) engines. Even though diesel engines are much more economical and efficient than the petrol engines, they are major contributors of harmful exhaust emissions such as volatile organic fraction (VOF) of burned and unburned hydrocarbons, carbon dioxide (CO2), carbon monoxide (CO), polynuclear aromatic hydrocarbons (PAHs), particulate matter (PM), carbon soot, oxides of nitrogen (NOx), aldehydes, sulfur oxides, which are mainly responsible for biological phenomena such as acid rain and photochemical contamination. Particulate matter or soot is a by-product of incomplete combustion of diesel, consisting of unburned long-chain hydrocarbons, chemicals like sulfates, ammonium, nitrates, elemental carbon, carcinogenic compounds, and heavy metals. Size of soot varies from small particles of nanometers to large particles of micrometers (Fino et al. 2007; Lloy and Cackette 2001). Smoke is a visible product of combustion, which occurs in the early stages of combustion cycle in the localized volume of rich air–fuel mixture. It is formed by the aggregation of chain line clumps of carbon in the exhaust. Exposure to fumes of diesel engines leads to irritation to eyes and respiratory tracts, and prolonged exposure to diesel fumes leads to cough and breathlessness. Diesel engine emissions cause cancer (Vermeulen et al. 2014), respiratory and cardiovascular health effects (Mauderly et al. 2014), air and water pollution, reduction in visibility, and global climate change (Lloy and Cackette 2001). The harmful components of diesel engine exhaust gas become a major threat for the environment. Despite these ill effects, the usage of diesel as a fuel in CI engines is increasing, as diesel engines are inevitable in all sectors of life, as a prime mover for transportation and stationary applications. Hence, stringent environmental legislation is essential to control these harmful emissions. Extensive research has been carried out worldwide for controlling the harmful emissions, thereby adhering to stringent regulations. Various exhaust gas after-treatment technologies being adopted for the control of harmful emissions include electronic fuel injection system (Parlak et al. 2012; Youn et al. 2011; Yao et al. 2010), diesel particulate filter (DPF) for PM reduction (Hsieh et al. 2011), NOx absorber catalyst, selective catalytic reduction (SCR), and exhaust gas recirculation (EGR) for the reduction of nitrous oxides (NOx) (Basha et al. 2014; Ibrahim and Ramesh 2013; Yamashita et al. 2014).

The toxic diesel engine emissions cannot be controlled solely by modifications in engine or exhaust after-treatment systems. An attractive method for the reduction of harmful emissions, especially particulate matters, is the use of metal oxides as catalysts in fuel, to catalyze the combustion reactions, resulting in the reduction of emissions (Brijesh and Sreedhara 2013). Catalysts can be used in the form of coating in catalytic convertors, diesel particulate filters, etc., or as fuel-borne additive, especially in nanosized form. Advantage of using catalytic nanoparticle in fuel is increase in the catalytic reaction time, as catalytic reactions occur in engine cylinder and exhaust pipe also. Due to the small size, nanoparticles have high surface-to-volume ratio which increases the surface energy than bulk material. Nanomaterial catalyst has high reactivity and selectivity as compared to non-nanocatalysts, and hence, the concentration required also will be less. Various metal oxide nanoparticles used as fuel-borne additives include aluminum oxide, copper oxide, and ceria (Mehta et al. 2014). Among these metal oxides, ceria is a potential catalyst for both oxidation and reduction of harmful diesel engine emissions.

2 Cerium Oxide—An Excellent Catalyst

Cerium oxide is an abundant element in rare earth family group with good thermal stability. Cerium oxide is a highly stable, refractory ceramic material with a melting point of 2600 °C and a density of 7.13 g/cm3. In the face-centered cubic (FCC) structure of ceria as shown in Fig. 8.1, Ce4+ ions form a cubic close-packing arrangement and oxide ions occupy all the tetrahedral sites, whereas the octahedral sites remain vacant. The crystal structure is fluorite face-centered cubic with a lattice constant of 5.11 Å and has exceptional magnetic and electronic properties due to their unfilled 4f electronic structure. In the case of cerium oxide nanoparticles, most of the Ce ions are located near the surface, which enhances the stability of oxygen vacancies, as compared to the bulk. Cerium oxide has an electronic configuration of 4f25d 6s2, and it can exist in cerium oxide (CeO2) or cerous oxide (Ce2O3) form, i.e., with oxidation states of +3 and +4, respectively. Being thermodynamically unstable, the conversion of cerium oxide (CeO2) to cerous oxide (Ce2O3) takes place easily and the conversion mainly depends on the partial pressure of oxygen and temperature. The +3 state closely resembles the other trivalent rare earths, the +4 state is stable in an aqueous environment and it is, therefore, a strong oxidizing agent. At elevated temperature and low oxygen pressure, the CeO2 reduces to oxygen-deficient non-stoichiometric oxides and will reorganize on cooling.

Fig. 8.1
figure 1

Structure of ceria (Eyring 1991)

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Cerium oxide is a good antioxidant owing to the presence of oxygen vacancies on its surface and the auto-regenerative cycle of its oxidation states, Ce3+ and Ce4+. Being an efficient oxygen buffer, cerium oxide enhances the reduction and oxidation reactions under fuel-rich and lean conditions, respectively, and hence can be used as an excellent catalyst for the reduction of harmful emissions from diesel engines (Mullins et al. 1998; Das et al. 2007; Qiu et al. 2006; Zhang et al. 2004, 2009; Huang et al. 2009; Vidmar et al. 1997). In a diesel engine, a significant amount of soot is formed during its operation, which may adhere to the surface of combustion chamber as deposits, along with the lubricating oil mist. These carbon deposits lead to the friction losses and variations in the internal surface heat transfer behavior. The addition of ceria nanoparticles in diesel leads to a reduction in the activation temperature of carbon, and the carbon deposits get oxidized, leading to cleaner and efficient engine. Cerium oxide provides the oxygen for the reduction of the soot as well as the hydrocarbon and gets converted to cerous oxide (Ce2O3), which in turn is reoxidized to CeO2 through the reduction of nitrogen oxide, as per the following reactions.

Hydrocarbon combustion:

$$ \left( {2x + y} \right){\text{CeO}}_{2} + {\text{C}}_{x} {\text{H}}_{y} \to \left( {\frac{2x + y}{2}} \right){\text{Ce}}_{2} {\text{O}}_{3} + \left( {\frac{x}{2}} \right){\text{CO}}_{2} + \left( {\frac{y}{2}} \right){\text{H}}_{2} {\text{O}} $$
(8.1)

Soot burning:

$$ 4{\text{CeO}}_{2} + {\text{C}}_{\text{soot}} \to 2{\text{Ce}}_{2} {\text{O}}_{3} + {\text{CO}}_{2} $$
(8.2)

NOx reduction:

$$ {\text{Ce}}_{2} {\text{O}}_{3} + {\text{NO}} \to 2{\text{CeO}}_{2} + \frac{1}{2}{\text{N}}_{2} $$
(8.3)

Various commercial applications of cerium include metallurgy, glass and glass polishing, ceramics, phosphors, and catalysts. Ceria (CeO2) is commonly used as an oxygen ion conductor in solid oxide fuel cells and oxygen pumps due to its high oxygen ion conductivity. Cerium has high refractive index and is used as a pacifying agent in glass polishing. Being an antioxidant agent, cerium oxide nanoparticle is considered to be one of the most interesting nanomaterials for application in therapy, as many disorders are due to the oxidative stress and inflammation (Xia et al. 2008; Horie et al. 2011). The toxicity studies of CeO2 nanoparticle on human health, based on local site of contact (dermal) irritation, general cytotoxicity, mutagenicity, and environmental effects, revealed that there is no difference in biological effects between non-nano- and nanocerium oxide (Park et al. 2007).

The catalytic activity of ceria is mainly influenced by its oxygen storage capacity (OSC). A major drawback of ceria is that significant deactivation occurs due to particle sintering, especially at high temperatures, which will diminish the OSC of ceria particle. Doping of ceria is one of the methods to improve its thermal stability. Ceria-based binary and ternary mixed oxides have been developed for achieving thermal stability, even at elevated temperatures. The modification of ceria by doping with transition metals, such as La3+, Zr4+, Y3+, is likely to improve the surface area, enhance the redox properties of ceria, and also prevent the decline of oxygen storage capacity due to thermal deactivation (Shehata et al. 2012). Studies show that, out of these lanthanides, zirconium- and yttrium-based mixed oxides showed most promising results (Scheffe et al. 2013). Incorporation of zirconium into ceria structure results in modifications involving non-equivalent oxygen atoms. The oxygen anions near to the doping center have considerably lower reduction energies and larger displacements, resulting in higher mobility of ions. An oxygen vacancy is most easily created close to zirconium centers, and hence, Zr doping centers might serve as nucleation centers for vacancy clustering (Yang et al. 2006) causing the distortion of the O2− sub-lattices. The distortion of the O2− sub-lattices in the mixed oxides permits a higher mobility of lattice oxygen. Hence, the reduction not only occurs on the surface but also extends deep into the bulk (Damyanova et al. 2008; Yeste et al. 2013). In the case of yttrium, the Y3+ surface enrichment hinders the crystallite growth (Atribak et al. 2009). Yttrium doping increases the oxygen ion conductivity of ceria, and the surface segregation of Y3+ results in oxygen vacancies (Dudek and Molenda 2006; Shih et al. 2011). When both zirconium and yttrium are used as dopants, the deformation on the lattice due to zirconium doping favors yttrium incorporation, while zirconium promotes the formation of cerium-rich surfaces and yttrium hinders the accumulation of cerium on the surface (Atribak et al. 2009). The addition of catalytic nanoparticles in diesel increases the time of catalytic reaction leading to better reduction of harmful emissions. Though the use of catalysts in nanoscale form as fuel-borne additive has got much more attention recently, one of the main challenges is the lack of stability of the catalytic nanoparticles in fuel. Most of the earlier works reported the enhancement in the efficiency and reduction of emissions, with the use of catalytic nanoparticle in diesel, but much emphasis has not been given on stability aspect.

This chapter reports a comparative study on the catalytic activity and various physicochemical properties of CeyZr1−yO2, CeyY1−yO2, and CexZryY1−x−yO2 mixed oxide nanoparticles. The nanoparticles were synthesized using co-precipitation method and were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–Vis spectroscopy, Raman spectroscopy, temperature-programmed reduction with H2 (H2-TPR), thermogravimetric analysis (TGA), and Brunauer–Emmett–Teller (BET) analysis. Mixed oxides of ceria were dispersed in diesel, and their stability in diesel was improved by the use of surfactant. Oleic acid was used as the surfactant. Stability of catalytic nanoparticle-added diesel was studied systematically by means of zeta potential measurements. Effect of these catalytic nanoparticles on the performance and emissions of a water-cooled four-stroke single-cylinder diesel engine was also investigated.

3 Synthesis of Ceria-Based Mixed Oxide Nanoparticles

3.1 Precipitation Method

Precipitation method is one of the popular methods for the synthesis of cerium oxide nanoparticles. Chemicals required are reagent-grade cerium nitrate (Ce(NO3)3·6H2O) (Sigma-Aldrich, purity 99%), analytical-grade isopropanol, and 3 M aqueous ammonia. Cerium (III) nitrate solution (0.08 M) in water–isopropanol mixture (1:6) was prepared and vigorously stirred by means of a magnetic stirrer. Fivefold excess of 3 M aqueous ammonia solution is added to the stirring solution, while noting down the pH of the solution. The pH was adjusted to 10 by adding ammonium hydroxide to the reacting solution, as alkaline medium contributes smaller particles than acidic one. The reaction was left stirring at room temperature for 2 h. After one hour of the reaction, the red color of reactants turned yellow, showing the formation of cerium oxide nanoparticles. The particles precipitate at the bottom of the round bottom flask after completion of reaction. The resultant precipitate was washed three times with isopropanol to wash out contaminants and unreacted reagents. Samples were purified by centrifuging the solution. The resultant precipitate was then dried at 60 °C in an oven for 2 h, and nanocrystalline cerium oxide was obtained. The reaction is as follows.

Precipitation of cerium hydroxide

$$ {\text{Ce}}\left( {{\text{NO}}_{3} } \right)_{3} \cdot 6{\text{H}}_{2} {\text{O}} + {\text{NH}}_{4} {\text{OH}} \to {\text{Ce}}\left( {\text{OH}} \right)_{4} + {\text{NH}}_{4} {\text{NO}}_{3} $$
(8.4)

Conversion of amorphous cerium hydroxide to cerium oxide on heating

$$ {\text{Ce(OH)}}_{4} \xrightarrow{\Delta }{\text{CeO}}_{2} \cdot 2{\text{H}}_{2} {\text{O}} $$
(8.5)

3.2 Co-precipitation Method

Cerium zirconium mixed oxide nanoparticles were synthesized by co-precipitation method using Ce (IV) precursor (Rossignol et al. 1999). The chemicals used in this method are ammonium cerium (IV) nitrate, zirconium oxychloride (ZrOCl2·8H2O), yttrium nitrate (Y(NO3)3·6H2O), and aqueous ammonia. The precursors were dissolved in distilled water to obtain desired molarity as shown in Table 8.1 and stirred for 30 min at 60 °C by means of a magnetic hot plate stirrer. To this solution, aqueous ammonia (NH3·H2O) was added drop-wise, and a pale purple precipitate started to form, which turned to light yellow on continuous stirring. Ammonia addition was continued till the pH of the solution reached more than 10. The solution was further stirred for two hours to get monodispersed particles. The precipitate was filtered and washed repeatedly with water to remove excess ammonia and unreacted precursors. The sample was dried for 8 h in an oven at 60 °C, and the yellow powder obtained was grounded in a mortar to get fine powder. The powder was then calcined at 500 °C for 4 h.

Table 8.1 Precursors for mixed oxide preparation
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3.3 Flame Spray Pyrolysis

Flame spray pyrolysis (FSP) is a popular technique for the synthesis of high-purity nanosized materials with controlled size and crystallinity in a single step. A wide array of high-purity nanopowders ranging from single to complex mixed oxides, metals, and catalysts can be synthesized by means of FSP. In this method, flame is used to force chemical reactions of precursors resulting in the formation of clusters, which increase their size to a range of nanometers by coagulation and sintering.

Flame spray pyrolysis is a single-step process in which a metal precursor(s) dissolved in a solvent is sprayed with an oxidizing gas into a flame zone. The spray is combusted, and the precursors are converted to nanosized metal or metal oxide particles, depending on the operating conditions. FSP allows the use of a wide range of precursors, solvents, and process conditions, with proper control over particle size and composition. In flame spray pyrolysis (FSP) process, the chemical and physical properties of nanoparticles depend on various parameters such as the burner design, gas-to-liquid mass ratio, oxygen content of the dispersion gas, atomization, fuel and precursor properties, concentration of the precursor in the solution. Basically, it is necessary to optimize all of the relevant experimental parameters in order to control the phase, particle size, and morphology.

Figure 8.2 shows the experimental setup of FSP process. FSP setup consists of a burning chamber made of quartz tube with a filter at the top and a spray nozzle at the center surrounded by six nozzles between two brass plates. A filter is connected to a vacuum pump to draw the combustion products from the quartz tube. The spray nozzle made of stainless steel is used to pump the precursor to the burning chamber. The spray nozzle consists of a capillary tube inside another tube. Precursor and fuel flow through the capillary tube, while the dispersion gas passes through the annular gap. The annular gap is controlled to vary the atomization of the precursor and hence the flame height. Cooling water circulation was provided around the nozzle to maintain the nozzle temperature.

Fig. 8.2
figure 2

Flame spray pyrolysis setup

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Cerium(III) acetate hydrate and zirconium acetylacetonate were used as precursors for the synthesis of cerium zirconium mixed oxide nanoparticles. The precursors were mixed and dissolved in acetic acid mixture, pumped to center spray nozzle, with the aid of a syringe pump, and further atomized by means of oxygen gas. A mixture of methane and oxygen was fed to the six nozzles around the precursor nozzle to obtain premixed supporting flames. The precursor burns in the flame at the center nozzle forming nanoparticles, which are collected in a filter paper with the aid of a vacuum pump. The filter paper was washed in toluene to recover the nanoparticles adhered on it. The collected samples were again washed and centrifuged with toluene to remove excess amount of impurities.

4 Characterization of Catalytic Nanoparticles

The particle size and morphology of the nanoparticles were characterized by means of scanning electron microscope (make: Hitachi SU6600 Field Emission Scanning Electron Microscopy (FESEM)). BET surface area of the catalysts was estimated by physical adsorption of N2 at −196 °C in an automatic volumetric system (make: Autosorb-6, Quantachrome). X-ray diffraction studies were done using Bruker AXS D8 Advance diffractometer (Cu Kα radiation λ = 1. 5406 Å). Thermogravimetric analysis was performed to study thermal stability of particles (make: NETZSCH STA 449 F3 thermal analyzer). Raman spectra were acquired with a Bruker RFS 100/S Fourier Transform Spectrometer (Nd:YAG laser source 1064 nm, 85 mW laser power). Experiments of temperature-programmed reduction with H2 (H2-TPR) were performed in a tubular quartz reactor coupled to a TCD analyzer for monitoring H2 consumption. The experiments were conducted with 50 mg of catalyst, while maintaining the mass flow rate as 30 ml/min flow of 7.7 vol.% H2 in argon atmosphere.

4.1 Textural and Structural Properties

The SEM (Fig. 8.3) images show that the particles are spherical in shape with a mean diameter of 60 nm. SEM images also confirmed that the size or morphology of nanoparticles is not affected by the formation of mixed oxide with zirconium and yttrium.

Fig. 8.3
figure 3

SEM images of a CeyZr1−yO2, b CeyY1−yO2, and c CexZryY1−x−yO2 mixed oxide nanoparticles

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BET surface area of the mixed oxides is estimated and presented in Table 8.2. Highest surface area of 116.3 m2/g was observed for the sample CexZryY1−x−yO2 nanoparticle as compared to others, due to a larger number of defects on the surface, and CeyY1−yO2 nanoparticles have the lowest surface area of 44 m2/g. From the point of view of surface area or the oxygen release from the surface, it can be seen that sample CexZryY1−x−yO2 nanoparticles exhibit more catalytic activity, as compared to other samples. Mixed oxides of cerium, zirconium, and yttrium can be synthesized by means of two methods. In the first method, oxides of cerium, zirconium, and yttrium are synthesized separately and mixed physically to obtain the mixture of oxides such as CeO2–ZrO2, CeO2–Y2O3, and CeO2–ZrO2–Y2O3. In the second method, mixed oxides of cerium, zirconium, and yttrium are synthesized by means of co-precipitation method.

Table 8.2 Surface area of various mixed oxide nanoparticles
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XRD patterns for the mixed oxides synthesized by means of co-precipitation method and by physical mixing of oxide nanoparticles are compared and are shown in Fig. 8.4. X-ray diffraction pattern shows the peaks’ characteristic for fluorite-type structures, with planes corresponding to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) (Atribak et al. 2009). Due to the incorporation of zirconium and yttrium, a shift of 2θ to higher angles was observed, which confirms the formation of solid solution (Yu et al. 2003). Small peaks were also observed in the XRD patterns, due to ordered arrangement of cations (Nagai et al. 2008). For physically mixed samples, two small peaks are observed, corresponding to (4 0 0) and (6 6 2) planes which are the characteristic planes of yttria and other two peak indexes on planes (1 1 2) and (1 2 1) being the characteristic planes of zirconia (Nagai et al. 2008). Mixed nanoparticles have broader peaks which indicates semicrystalline structure (Yu et al. 2003). The absence of characteristic planes of ZrO2 and Y2O3 in XRD patterns of in situ synthesized mixed nanoparticles also confirms the formation of a solid solution.

Fig. 8.4
figure 4

X-ray diffraction patterns of various mixed oxide nanoparticles

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Raman spectra of CeO2, CeyZr1−yO2, CeyY1−yO2, and CexZryY1−x−yO2 are shown in Fig. 8.5, where the Raman active F2g mode which can be regarded as a symmetric O–Ce–O stretching for the cubic fluorite structure is compared (Ma et al. 2009). In the Raman spectrum of pure ceria, which has a fluorite structure, the F2g mode is centered at 465 cm−1, as seen in Fig. 8.5. A shift in the peak is observed, mainly due to the coexistence of cubic (CeO2) and tetragonal (ZrO2) solid solution phase, as the band near to 465 cm−1 is observed together with the components typical of the tetragonal phase, though the cubic phase is still largely predominant. Comparison of Raman spectra of CeO2 and CeyZr1−yO2 samples shows that the formation of mixed oxides of ceria with zirconium improves the oxygen vacancies. Since Y2O3 has cubic structure and there is no formation of bonds with yttrium incorporation, CeyY1−yO2 exhibits the same Raman band as CeO2.

Fig. 8.5
figure 5

Raman spectra of various mixed oxide nanoparticles

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4.2 Thermal and Catalytic Properties

Thermal stability of mixed oxide is of utmost important, as it has to withstand high in-cylinder temperature, when mixed with diesel as fuel additive. TGA of calcined and uncalcined samples for a temperature range of 25–950 °C is shown in Fig. 8.6. For calcined sample, there is only a weight loss of 6.4%, whereas in the case of uncalcined sample a weight loss of 65% was observed. The weight loss below 100 °C is mainly due to the desorption of water. The weight loss above 100 °C is due to the removal of methoxide (Gnanam and Rajendran 2011) in the mixed oxide nanoparticles, and as temperature goes up, the complete removal of organic residues takes place.

Fig. 8.6
figure 6

Thermogravimetric analysis of calcined and uncalcined mixed oxide nanoparticles

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Catalytic activity of mixed oxides of ceria mainly depends on its reducibility. Temperature-programmed reduction (TPR) with hydrogen is a standard technique to characterize the reducibility of ceria-based materials. Samples selected for H2-TPR analysis include Ce0.90Zr0.10O2, Ce0.80Zr0.20O2, Ce0.85Y0.15O2, Ce0.90Y0.10O2, Ce0.70Zr0.20Y0.10O2, and Ce0.60Zr0.30Y0.10O2, and Fig. 8.5 shows the results obtained. The literature (Mehta et al. 2014; Yao and Yao 1984; Kennedy 1975) shows that the TPR profile of ceria has a two-peak pattern, one corresponding to surface reduction (around 500 °C) and the other for the bulk reduction (around 900 °C) peak, showing that the reduction of ceria may follow a two-step process. On doping cerium with zirconium and yttrium, TPR profiles of the mixed oxides essentially show a broad reduction signal in agreement with the expectation of an enhanced reduction of the bulk mixed oxide (Atribak et al. 2009; Meng et al. 2010). TPR results show a shift in peaks toward lower temperatures for the CeZrO2 mixed oxide nanoparticles. There exists a small peak around 450 °C which is the surface reduction peak and a broad peak around 600 °C which is the bulk reduction region (Meng et al. 2010).

It was observed that the catalytic activity is increased with increase in the doping level of ZrO2 and Ce0.80Zr0.20O2 gives better result. Shift in peaks for Y2O3-doped CeO2 nanoparticle is less compared to CeyZr1−yO2 nanoparticle. Among the yttrium-doped CeO2 nanoparticle, the sample Ce0.85Y0.15O2 nanoparticle shows the best catalytic property. In this case, a broad bulk reduction region was observed at a temperature near to 750 °C (Wang et al. 2003). On comparison with TPR results of various mixed oxides of ceria (Fig. 8.7), the mixed oxides containing both zirconium and yttrium were found to possess the best catalytic activity. Ce0.60Zr0.30Y0.10O2 mixed oxide nanoparticles show best catalytic activity even though it has only a single sharp peak; if the H2 consumption is compared, it has a sharp peak near 550 °C which is the surface reduction peak. It was also observed that as the proportion of zirconium increases while keeping the concentration of yttrium constant, catalytic activity also increases. On the basis of comparison of TPR results of various mixed oxides of cerium, Ce0.80Zr0.20O2, Ce0.85Y0.15O2, and Ce0.60Zr0.30Y0.10O2 mixed oxide nanoparticle samples were selected as additives in diesel for the stability studies and engine performance and emission studies.

Fig. 8.7
figure 7

H2-TPR of various mixed oxide nanoparticles

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5 Synthesis of Nanofluid

Nanofluids can be prepared by a single-step or two-step method. A two-step approach of the synthesis of nanofluid is more commonly used, especially for the bulk synthesis of nanofluids. In a two-step method, the catalytic nanoparticle is mixed with oleic acid, by means of ultrasonic shaker for the duration of 90 min. This highly concentrated additive was then dispersed in diesel and is sonicated by means of ultrasonic shaker for the duration of 30 min.

Lack of stability of catalytic nanoparticles in diesel is one of the main challenges to be addressed for their practical applications in diesel engines. The stability of catalytic nanoparticle dispersed in diesel was improved with the addition of oleic acid as surfactant. Optimized concentration of oleic acid in diesel for maximum stability of catalytic nanoparticles was determined based on the estimation of the formation of reverse micelle. The addition of surfactant in diesel leads to a decrease in surface tension and eventually the aggregation of surfactant molecules, as the concentration of surfactant increases. The concentration of surfactant at which sudden variation of surface tension occurs, corresponding to the formation of reverse micelle (critical micelle concentration), was measured by maximum bubble pressure method (Fainermanl et al. 1994). In this method, the pressure required to force a gas bubble out of a capillary tube which is vertically immersed in the liquid to be investigated changes with the surface tension of the liquid. An air pump was used for applying pressure and a valve to control the air flow. The variation in pressure was noted using a differential manometer with the aid of a CCD camera. The pressure required to force a gas bubble through diesel out of a capillary was determined, while varying the concentration of oleic acid in the range 0.01–0.1 vol.% and the surfactant concentration corresponding to the point of minimum pressure; i.e., the point of reverse micelle formation was determined.

Figure 8.8 shows the variation of bubble pressure with surfactant concentration in volume percentage. A sudden drop in pressure was observed at a surfactant concentration of 0.05% by volume, which corresponds to optimum surfactant concentration. The minimum concentration value corresponds to the formation of micelle of surfactants in the base fluids. Stability of surfactant-coated mixed oxide of cerium was compared with uncoated mixed oxide of cerium.

Fig. 8.8
figure 8

Variation of pressure with concentration of oleic acid in diesel

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5.1 Stability Study

Stability of nanofluid, i.e., catalytic nanoparticle-added diesel, was estimated from the measurement of zeta potential by using dynamic light scattering technique (Malvern Zetasizer Nano ZS). Particle with zeta potentials more positive than +30 mv and more negative than −30 mv is considered to be stable. The concentration of catalytic nanoparticle in diesel was varied from 2.5 to 15 ppm, with a 2.5 ppm interval. Surfactant was added in diesel, by employing two methods. In the first method, catalytic nanoparticle was coated with surfactant by mixing it with oleic acid and toluene and stirred for 24 h. The mixture was then centrifuged and dried to obtain the surfactant-coated catalytic nanoparticles, which is then added to diesel. In the second method, oleic acid was added along with the catalytic nanoparticles and mixed thoroughly by means of ultrasonic shaker. Stability of surfactant-coated and surfactant-uncoated nanoparticles was also compared, in order to determine the best method of stabilization of catalytic nanoparticle in diesel.

Figure 8.9 shows the variation of zeta potential with respect to concentrations for (a) CeyZr1−yO2, (b) CeyY1−yO2, and (c) CexZryY1−x−yO2 mixed oxide nanoparticles. It was found that both coated and uncoated nanoparticles show good stability (more than ±30 mV). Out of three mixed oxide nanoparticles, the CeyZr1−yO2 mixed oxide nanoparticles show the highest stability and CeyY1−yO2 shows the least. Based on the stability studies, it was concluded that sample with 10 ppm concentration catalytic nanoparticles shows the maximum stability.

Fig. 8.9
figure 9

Stability analysis at various concentrations for a CeyZr1−yO2, b CeyY1−yO2, and c CexZryY1−x−yO2 mixed oxide nanoparticles

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6 Engine Performance and Emission Study

Load test was carried out in a single-cylinder four-stroke diesel engine (Table 8.3) for investigating the effect of these nanoparticles on performance and emission characteristics of diesel engine. The experimental test rig consists of a single-cylinder four-stroke compression ignition engine, eddy current dynamometer, fuel supply system, and various sensors and instruments coupled with data acquisition system for online measurement of load, exhaust emissions, and smoke.

Table 8.3 Specifications of engine
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The performance parameters were determined by conducting a constant speed load test. Commercially available laboratory view-based engine performance analysis software package—engine soft LV—was used for online performance evaluation. The exhaust emissions of the engine were measured using a AVL exhaust gas analyzer (AVL DIGAS 444), and smoke intensity was measured in terms of filter smoke number (FSN) by means of AVL smoke meter (AVL Type 415SG002). Ce0.80Zr0.2O2, Ce0.85Y0.15O2, and Ce0.60Zr0.30Y0.10O2 mixed oxide nanoparticles were selected for performance and emission studies, based on the oxygen storage vacancies of catalytic nanoparticles, as mentioned earlier. Various emissions analyzed in the present work include smoke and NOx.

The catalytic nanoparticle-added diesel was prepared by means of ultrasonic shaker and filled in the fuel tank. The concentration of catalytic nanoparticles in diesel was fixed as 10 ppm in the present study, based on the stability studies. For each sample, two load tests were conducted and average results are presented here. It was observed that there is a considerable reduction in the emission of smoke and NOx emissions with the addition of mixed oxides of ceria nanoparticles in diesel. Figure 8.10 shows the variation of smoke with respect to the BMEP. A considerable reduction in smoke was observed up to half load for all samples. Among the samples, Ce0.80Zr0.20O2 mixed oxide nanoparticles show a maximum smoke reduction of 54% and an average reduction of 33%. In a diesel engine, smoke is mainly formed due to the lack of oxygen (Kennedy 1975) and pyrolysis and nucleation are the first two steps in the soot formation.

Fig. 8.10
figure 10

Variation of smoke emission with BMEP for various nanoparticles

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The presence of oxygen in ceria-based nanoparticles may reduce the chance of pyrolysis and nucleation (Lenin et al. 2013; Keskin et al. 2008), thus reducing the chance of soot formation as per Eq. (8.2).

The doping of ceria with zirconium and yttrium increases the number of defects in the crystal lattice and hence the oxygen storage capacity. The high temperature in engine cylinder leads to the release of the oxygen from the crystal lattice of ceria-based mixed oxide nanoparticles, thus reducing the smoke. Figure 8.11 shows the variation of NOx emissions with respect to BMEP. Even though a considerable reduction in NOx emissions was observed at lower loads, an increase in NOx emissions was observed at higher loads for all samples. This may be due to the fact that at the higher loads the combustion chamber temperature is high, which will enhance NOx emissions as per Zeldovich thermal mechanism. Ce0.80Zr0.20O2 nanoparticles show a maximum reduction of 9% and an average reduction of 3%, while Ce0.85Y0.15O2 nanoparticles show a maximum reduction of 17% and an average reduction of 5%, in NOx emissions. For Ce0.60Zr0.30Y0.10O2 nanoparticles, a maximum reduction of 23% and an average reduction of 6% were observed (Sprague et al. 2006).

Fig. 8.11
figure 11

Variation of NOx emission with BMEP for various nanoparticles

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Figure 8.12 shows the variation of brake thermal efficiency with respect to BMEP and the entire samples exhibit an improvement in efficiency throughout the load range, while maximum enhancement was observed for Ce0.85Y0.15O2 sample. Addition of catalytic nanoparticles leads to the enhancement in combustion efficiency, through the enhancement of oxidation/reduction reactions in the cylinder, leading to the improvement of brake thermal efficiency. Ce0.85Y0.15O2 shows an average improvement of 4.5%, while Ce0.60Zr0.30Y0.10O2 nanoparticles show an improvement of 3% in the brake thermal efficiency. Possible explanations for the reduction of smoke and improvement of brake thermal efficiency are elucidated here. The catalytic nanoparticles in fuel result in the micro-explosion of fuel droplets leading to better atomization and hence shorter ignition delay (Mehta et al. 2014). In addition, the presence of catalytic nanoparticles in diesel increases the time available for the catalysis reaction and hence the emission reduction occurs up to the end of the exhaust tailpipe (Lenin et al. 2013). Even though higher reduction in the emissions and enhancement in brake thermal efficiency can be obtained by using higher concentration of mixed oxide of ceria nanoparticles, the maximum concentration is limited by the stability of the catalytic nanoparticles in diesel.

Fig. 8.12
figure 12

Variation of BTE with BMEP for various nanoparticles

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7 Conclusion

Various ceria-based mixed oxide nanoparticles were synthesized by co-precipitation method. It was observed that the synthesized particles have an average size of 60 nm and have a spherical morphology. Characterization of synthesized particles using XRD and Raman spectroscopy confirms the presence and the formation of mixed oxide nanoparticles. The thermal stability of nanoparticles was found to be increased by calcination. BET analysis shows that CexZryY1−x−yO2 nanoparticles have highest surface area. Temperature program reduction of samples with hydrogen shows that among all samples, and Ce0.60Zr0.30Y0.10O2 shows the best catalytic activity. The optimum amount of surfactant in diesel was found to be 0.05% by volume, and the mixed oxide nanoparticles in diesel show better stability at 10 ppm concentration. Load test on a single-cylinder diesel engine for performance and emission studies shows a considerable reduction in smoke as well as improvement in efficiency, with the addition of mixed oxide of ceria nanoparticles in diesel.