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Effects of combustion catalyst dispersed by a novel microemulsion method as fuel additive on diesel engine emissions, performance and characteristics Hossein Kazerooni, Amir Rouhi, Abbas Ali Khodadadi, and Yadollah Mortazavi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00004 • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 12, 2016

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Energy & Fuels

Effects of combustion catalyst dispersed by a novel microemulsion method as fuel additive on diesel engine emissions, performance and characteristics Hossein Kazeroonia, Amir Rouhia, Abbas Ali Khodadadia,b, Yadollah Mortazavia,b, ∗

a

Catalysis and Nanostructured Materials Research Laboratory, School of Chemical Engineering,

College of Engineering, University of Tehran, Tehran, P.O. Box 11155/4563, Iran. b

Oil and Gas Process Center of Excellence, College of Engineering, University of Tehran,

Tehran, P.O. Box 11155/4563, Iran.

KEYWORDS: Oxygen storage capacity; Microemulsion; Emissions; Engine performance; Fuel additive.

*Corresponding author: Catalysis and Nanostructured Materials Research Laboratory, School of Chemical Engineering, University of Tehran, P.O. Box 11155/4563, Tehran, Iran. Tel/Fax: +98 (21) 6696 7793. E-mail address: [email protected].

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ABSTRACT

The effects of oxygen storage properties of combustion catalysts on pollutant emissions and engine performance, using an automated diesel engine, were studied. A novel microemulsionbased protocol was utilized to disperse the catalysts in the diesel fuel. The catalystmicroemulsion (catalyst-µE) system remains stable upon its addition to the diesel fuel. Different concentrations of iron doped cerium oxide (0, 5, 10 and 20 mole%) were synthesized in microemulsion system and characterized by dynamic light scattering (DLS) and UV-visible spectrometry. Various characterization techniques were employed. X-ray diffraction (XRD) was used to show the effect of iron presence on crystallites structure of the catalysts. Hydrogen temperature programed reduction (H2-TPR) and oxygen temperature programed desorption (O2TPD) were utilized to quantify the total oxygen storage capacity (OSC) and types of oxygen species, respectively. The effects of oxygen storage capacity and oxygen types of the samples on emission of the pollutants and performance of engine were studied afterwards. Regarding the emissions, it is shown that Ce0.95Fe0.05O2-α catalyst decreased the soot, unburned-hydrocarbons (HC) and carbon monoxide (CO) by 20.0, 57.7 and 26.6% respectively. It is also shown that the catalyst with more OSC and more mobile oxygen (the so-called α-oxygen) was more influential in decreasing the mentioned pollutants. The results of in-cylinder pressure and heat release rate analyses indicate that the oxygen storage/release capacity of the catalysts can improve the burning process by decreasing the ignition delay with no significant change in the combustion duration. The results also reveal that addition of the catalysts decreases the brake specific fuel consumption (BSFC) by 3-4% in average and therefore, improving the fuel economy.


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Energy & Fuels

1. Introduction Diesel engines present many advantages over gasoline spark-ignited engines, e.g. 20 to 40% better fuel efficiency and lower CO2 emission. However, some disadvantages, including higher soot and NOx emissions, are related to the performance of the diesel engines. Considering the fuel price and the more restrict emission regulations, diesel engines are expected to be used more and more for future transportation systems with better performance and less toxic gas emissions including soot as the most challenging emission of diesel engines. Introducing additives into diesel without modification of engines is an interesting research area; believed to be beneficial with regard to both fuel consumption and pollutant emissions. Among all types of additives, combustion catalysts including both metal-containing and ash free catalysts have attracted much attention [1-7]. The metal containing catalysts are said to be much more effective as compared to the ash–free catalysts [1]. These additives should be dispersible or soluble in diesel fuel without settling or agglomeration during consumption or storage. They should neither change the fuel specification nor generate secondary pollutants. The metal-based additives are usually in the form of organometallic compounds to guarantee the mentioned specifications. These compounds are believed to change into metal oxide nanoparticles in the combustion cycle which in turn act as catalyst. However, by utilizing metal or metal oxide nanoparticles instead of organometallic compounds directly, i.e. during the synthesis, a more efficient control of the catalyst properties may be achieved. Some metal ions have been reported to be highly effective in improving the diesel fuel combustion such as iron [6, 8-12], cerium [12-18] and copper [19, 20]. Among these, cerium has attracted more attention due to the oxygen storage capacity (OSC) property of its oxide. It is


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believed that the OSC property of ceria, gives it the ability to store oxygen under oxidizing conditions and release oxygen under reducing conditions, and thereby, reducing the local inhomogeneity in the fuel oxygen mixture. Such a property would result into faster and more efficient burning inside the combustion chamber which in turn decreases the toxic emissions and improve the engine performance [21]. However, the improvements attributed to the OSC property of the catalysts have been just elaborated in a few articles [22, 23] and need a more thorough investigation. Nowadays there are some cerium based diesel fuel additives in the market (e.g. OXONICA and EOLYS [24]). They are claimed to improve efficiency and reduce harmful emissions [25]. These kind of additives are dosed directly into the diesel fuel without the need to modify the engine at ratios ranging from 1:16000 to 1:200, depending on the concentration of the catalytic ingredients in the additive [8]. These additives usually include pure cerium oxide and are made in more than two steps to become compatible with the diesel fuel. Moreover, to the best of our knowledge, no detailed scientific study is available in the literature regarding the catalytic impact of the cerium oxide based additives and their OSC property on the combustion process. Among the challenges of using metal oxide based materials as combustion catalysts, dispersion of these hydrophilic nanoparticles without agglomeration or precipitation in the diesel fuel is the most critical step. Water in oil microemulsions or reverse micellar systems are considered as common methods for synthesizing metal oxide nanoparticles in hydrophobic solutions. However, the main problem associated with these systems is that by addition of microemulsions, if considered as additives to diesel fuel, the whole system would collapse and nanoparticles would settle down. This is attributed to the concentration of surfactant which


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would decrease below the critical micelle concentration (above which reverse micelle forms) in the diesel fuel. The additives used in the present study are specific microemulsion based formulas which remain stable even after it is added to the diesel fuel. These additives contain some new developed cerium oxide based materials as combustion catalysts which were promoted by iron ions as dopant in different concentrations. The interactions between the specific anionic surfactants and the nanoparticles which take place, due to strong electrostatic attraction forces in these systems, avoid the fading of reverse micelles even after its addition to the diesel fuel. The synthesis method used in this study is proposed to be substituted for the costly multistep methods that are employed by the researchers. 2. Experimental 2.1. Experimental apparatus The engine used for the present study is a commercial direct injection, water cooled, four cylinders, in line, turbocharged diesel engine which its major specifications are presented in Table 1. Table 1. Engine specifications. Engine type

MT1440C-105AD Motorsazan Co.

Stroke

127 mm

Bore

100 mm


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Displacement

3.99 lit

Combustion chamber

Reentrant

Compression ratio

17.5:1

Maximum power output

78 kW @ 2000 rpm

Maximum torque output

375 N.m @ 1500 rpm

Aspiration

2-Valve/cylinder, turbocharged

Fuel injection pump

DPA rotary

Injection type

Direct injection

Injection pressure (typical)

275 bar @ 2000 rpm

Fuel injection nozzle

5 holes

Max pressure of the injection pump

400-450 bar

Nozzle diameter

0.276 mm

Needle lift

0.68 mm

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The engine was coupled to an eddy current dynamometer (400 hp) manufactured by Pmid company, model E400, to exert various loads on the engine. All the engine experiments were performed with three replicates. Speed, load, fuel and air consumed as well as lambda (the airfuel ratio) were measured in each test. Engine emissions were measured using an AVL Dicom-


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class1 exhaust gas analyzer. CO and HC were measured using a non-dispersive infrared detector (NDIR). Chemical luminescence detector (CLD) and paramagnetic detector were also used for NOx and O2 measurements respectively. Soot was determined using an AVL 415S smoke meter. To ensure the measurement accuracy, the emission analyzers were calibrated before and after each run. Measuring instruments and accuracy of the measurements is presented in Table 2. Table 2. Specification of the measurement devices and measurement accuracy. Measuring instrument

Dynamometer

Pmid company (model E400)

Accuracy

Measurement range

Power: 1 hp

------------

Torque: 1 N.m

------------

Speed: 1 rpm

------------

Fuel flow meter

Pmid company

0.01 kg/hr

------------

Air flow meter

ABB Sensyflow P (Germany)

0.3 kg/hr

------------

CO

AVL Digas 4000

0.01%

0-10%

NOx

AVL Dicome 4000

1 ppm

0-5000 ppm

HC

AVL Dicome 4000

1 ppm

0-20000 ppm

Soot

0.01 mg/m3

------------

0.001 FSN

0-10 FSN

AVL 415S smoke meter


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Pollutant emission and performance measurements were performed according to the ECER96, 8 mode procedure as shown in Table 3. Table 3. ECE-R96 8 mode procedure. Mode

Speed (rpm)

Load (%)

Load (N.m)

Weighting factor

1

2000

100

380

0.15

2

2000

75

285

0.15

3

2000

50

190

0.15

4

2000

25

95

0.1

5

1500

100

390

0.1

6

1500

50

292

0.1

7

1500

25

195

0.1

8

1500

-

97

0.15

For heat release analysis, a piezoelectric type pressure transducer (Indi Module 621) was ﬂush-mounted with the combustion chamber for routine sampling of the cylinder pressure traces. Cylinder pressure was measured at every 0.1 crank angles (oCA). Engine crank shaft position was determined by a crank angle encoder. The fuel injector was instrumented with a hall-effect needle lift sensor which provided the indications of start and end of fuel injection events. These


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measurements are used in the heat release analysis. A schematic diagram of the experimental setup is depicted in Fig. 1.

Figure 1. Schematic of the diesel engine test bench. 2.2.Microemulsion preparation CexFe1-xO2-α (x = 1.00, 0.95, 0.90, 0.80) microemulsion solution was synthesized by addition of 0.55 M aqueous metals salt solution to anionic surfactant-heptane solution. To make the metal salt solution, Ce(NO3)3.6H2O and Fe(NO3)3.9H2O were added in an appropriate portion, by considering the molar ratio of iron and cerium, to 15 mL of deionized water. The solution was stirred for 10 minutes. The solution was then added drop wise in 10 minutes to 200 mL of surfactant-heptane solution. Sodium Dodecyl Sulfate (SDS) was used as an anionic surfactant. The anionic surfactant concentration was 0.1 M, and the amount of the aqueous solution was chosen so that the system was just below its emulsification failure boundary [26].


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The mixing was carried out by drop wise addition of the 35% hydrogen peroxide solution over 1 hr with continuous stirring. After that, stoichiometric amount of 25% ammonia solution was added over 30 minutes. The stirring continued for 1 more hr for the reaction to be completed. The final solution was used as the concentrated additive. It was added into the diesel fuel in an appropriate proportion in such a way that the final concentration of CexM1-xO2-α remained 10 molecules per million molecules of the diesel fuel. The fuel properties used in the present study are shown in Table 4. Table 4. Properties of diesel fuel. Properties

Unit

Diesel Fuel

Standard

Flash Point

o

56

ASTM D93

Kinematic viscosity @ 40oC

mm2/oC

2.92

ASTM D445

Cetane number

-

45

ASTM D613

Cloud point

o

-6

ASTM D2500

Gross heating value

MJ/kg

137.6

ASTM D240

Low heating value

MJ/kg

42.4

ASTM D240

Specific gravity

-

0.825

ASTM D240

Initial boiling point

o

174

ASTM D86

Final boiling point

o

378

ASTM D86

C

C

C C


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2.3.Characterization methods 2.3.1. Characterization of the microemulsion 2.3.1.1 Dynamic light scattering (DLS) DLS was performed on a Micromeritics (Nanoplus1 autocorrelator, λ = 656 nm) for measurements of the scattered intensity s and the collective diffusion constant o. In dynamic light scattering, the collective diffusion coefficient o was determined from the second order autocorrelation function of the scattered light. From the value of the coefficient, the hydrodynamic diameter (DH) of the nanoparticles was calculated according to the Stokes– Einstein relation, DH = β /3so; where β is the Boltzmann constant, is the temperature (T = 298 K) and s is the solvent viscosity. This way we could measure the hydrodynamic diameter of the combustion catalyst dispersed in the additive. 2.3.1.2 UV-visible transmittance spectrometry (UV-vis) UV-vis transmittance spectra of the catalyst-containing additive were recorded using a T90+ UV-visible spectrometer (PG instrument LTD) with vision pro software. Transmittance spectra were obtained in the range of 200–700 nm at 1 nm scan interval with a scan speed of 600 nm/min. The background employed for the catalyst-µE additives were the pure microemulsion (water/heptane/surfactant). 2.3.2. Characterization of the combustion catalyst


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In order to separate the combustion catalysts, the catalyst-µE solutions were evaporated at 80°C, followed by several times washing with acetone and distilled water to remove the attached surfactant. The remained powder was collected and treated at 550°C as the highest reported temperature prior to the combustion reaction in diesel engines. It is supposed that any obtaining and release of oxygen by the catalyst should happen before this stage [27, 28]. 2.3.2.1.Hydrogen-temperature programmed reduction (H2-TPR) H2-TPR was conducted following degassing of the treated catalyst at 300°C for 1hr under Ar purging by Autosorb-1 Quanta chrome using a sample of 10 mg from the combustion catalyst and a gas (5% H2 in Ar) flow rate of 10 mL/min. The heating rate was set at 10°C/min. 2.3.2.2.Oxygen-temperature programmed desorption (O2-TPD) O2-TPD was performed in an experimental set-up similar to that used for TPR studies. For this experiment 20 mg of each sample was used. Before each TPD experiment, the treated catalyst was heated up to 300oC under 10 mL/min flow of oxygen. After maintaining the sample at 300oC for 1 hr, it was then cooled down to ambient temperature under the same flow of oxygen. This way, the catalyst is not only oxidized but also absorbs oxygen on its surface. In order to remove the weakly absorbed surface oxygen, the catalyst was exposed to 10 mL/min helium flow at the room temperature for a period of 30 minutes [29]. The sample was then heated up to 1000oC, by a heating rate of 10oC/min, under helium flow and the desorbed oxygen was detected by a thermal conductivity detector (TCD). 2.3.2.3.X-Ray Diffraction (XRD)


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XRD was carried out in a diffractometer using Cu Kα radiations on D8 advance X-ray diffracto meter for the treated catalyst. 3. Results and discussion 3.1.UV-visible spectrometry and Dynamic light scattering study of the microemulsion additives To confirm the formation of nanoparticle inside the microemuslion system, optical experiments including UV-vis spectrometry and dynamic light scattering were carried out on the microemulsion containing pure cerium oxide.

Figure

2.

(a)

UV-visible

transmittance

spectrometry

of

the

micoemulsion

(heptane/surfactant/CeO2) (b) DLS investigation of the microemulsion containing pure cerium oxide (heptane/surfactant/CeO2). Figure 2(a) illustrates the transmittance spectra of cerium oxide. It shows a strong adsorption band in the UV region due to charge transfer transitions from O 2p to Ce 4f bonds,


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which overrun the well-known f to f spin-orbit splitting of the Ce 4f state [30, 31] and confirms the formation of cerium oxide. Figure 2(b) represents DLS of the microemulsion system including cerium oxide which is widely used to measure hydrodynamic diameter of the nanoparticle. D10, D50 and D90 are the diameters that split the distribution with 90, 50 and 10 percent of the particles number above the diameter and 10, 50 and 90 percent of the particles number below the diameter respectively. D50 is usually employed as the median in particle size distribution. The results are summarized in Table 5. Table 5. DLS results for (heptane/surfactant/nanoparticle) system. D10 (Number

D50 (Number

D90 (Number

Polydispersity

distribution)

distribution)

distribution)

Index (PI)

32.5 nm

32.9 nm

41.0 nm

0.75

Average diameter of the nanoparticles can be estimated by subtracting the thickness of surfactant layer (which is about 1.2 nm for the employed surfactant [32]) from D50. Therefore 30.5 nm can be considered as the hydrodynamic diameter of cerium oxide. The results were similar for all the iron doped cerium oxides which show the importance of reverse micelle diameter in determining size of the nanoparticles in µE-systems. For investigating the stability of the additive, DLS measurement for the microemulsion solution was repeated after 6 months. The results indicated that D50 of the nanoparticles was 32.5 nm which doesn’t show a meaningful


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change in the size of the nanoparticles measured earlier, indicating that the solution is quite stable. Each catalyst-µE additive was added to the diesel fuel at ratio of 1:500 which led to 10 nanoparticles per one million diesel molecules in final fuel. 3.2.Effects of catalyst-µE additives on pollutant emissions The emissions for each of the additive after dosing to diesel fuel are measured at 8 combinations of engine speeds and loading torques and the results are weighted together to obtain a single value for comparison. Figure 3 shows variations in the soot, CO, HC and NOx emissions according to ECE R96 standard for addition of different percent of iron to cerium oxide catalysts.


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Figure 3. Variation in pollutant emissions, according to ECE-R96 weighting factors, versus iron content of cerium oxide.

The results show a decrease in the soot by up to 14.8% when pure cerium oxide was used in the additive. However, the declining trend in the soot emission was more pronounced in presence of the iron-doped cerium oxide catalyst, i.e. 20.0, 17.5 and 15.6 wt% for 5, 10 and 20 mol% iron as dopant, respectively. Soot is produced in the combustion chamber under oxygen deficient condition and its trend is similar to that of CO [33]. The results suggest that decrease in


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the soot and CO may be attributed to the oxygen provided by the catalyst for combustion reaction. Figure 3 also indicates that, similar to the soot and CO, the HC emission decreased upon addition of cerium oxide catalyst in the fuel. However, the extent of HC decrease is larger compared to those of soot and CO. For instance, when Ce0.95Fe0.05O2-α catalyst was added to the diesel fuel CO emission decreased by 26.6% whereas, the HC emission was suppressed by about 57.7%. The decrease in HC emission might also be due to a decrease in the ignition delay. Reduction of ignition delay in turn results in a decrease in the maximum heat release rate in the combustion chamber [34]. Thus, the decrease in HC emission is more influenced by the reduction of ignition delay. This will be discussed more thoroughly in the subsequent sections. The results of emissions also show the significant role of nanomaterials presence in the diesel fuel. It is observed that by introducing ultra-low amount of nano catalyst (10 ppm) in the diesel fuel, the amount of soot, CO and HC suppression would be comparable to those of the oxygenated fuel additives [33, 35-38]. It should be noticed that the recommended concentration of popular oxygenated fuel additive is up to 20 weight% or even more in some cases. Considering the ppm level of nano catalysts concentration as the diesel additive, makes them much more preferable. Moreover, comparing our results to those reported for other metal-based additives, including organometallic compounds [11, 39], indicates that the use of metal oxide nanoparticles is more effective in suppressing the pollutants, especially soot. As was discussed, decrease in the soot, CO and HC in presence of the catalyst in the fuel occur simultaneously. Suppressing these pollutants call for their oxidation upon formation in the


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combustion chamber. This suggests that improving the oxidation activity of the catalyst in the fuel lead to even larger reduction of these emissions. However, by addition of the catalysts to the diesel fuel, NOx emissions would increase. This may be as a result of an increase in the temperature of the combustion chamber and/or oxidative activity of the catalyst for oxidation of N2 present in the combustion chamber [40-43]. In regular diesel engines, there would be a trade-off between soot and NOx emissions. This is shown in Fig. 3. According to these results the catalyst that suppresses the soot more effectively increases NOx emission to some extent. This trend has been reported for several other oxygenated additives [44]. For instance, Wang et al. reported that by increasing the concentration of ethanol or biodiesel, soot formation is suppressed while NOx emission is enhanced [45]. Figure 4 shows the effects of different combustion catalysts on pollutant emissions at maximum power output speed (2000 rpm) and different loads. Any increase in the load at constant speed is accompanied by an increase in soot and NOx emission while no meaningful trend can be distinguished for HC and CO. However, in none of the cases the catalyst performance is suppressed by the load increase.


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Figure 4. Variation of pollutant emissions at 2000 rpm and different loads. (a) Soot (b) unburned hydrocarbons (c) nitrogen oxides (d) carbon monoxide. Addition of combustion catalysts also has some other effects e.g., soot particle size. For this purpose, soot emissions were also measured using smoke opacity meter. This instrument has been designed to quantify the visible black soot emission utilizing such physical phenomena as the extinction of a light beam by scattering and absorption of the soot collected on the filter in some specified time interval. A typical pattern of soot is shown in Fig.5. The quantified values are between 0 and 10 and are reported as filter smoke number (FSN) or Bosch number.


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Fig. 5. Images of the soot collected on filter (a) collected at 2000 rpm and full load without catalyst (b) collected at 2000 rpm and full load by the addition of Ce0.95Fe0.05O2-α catalyst.


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Fig. 6. Comparison between the values of soot emissions in mg/m3 and FSN. The large difference between the values of soot emissions in FSN and mg/m3, calculated based on ECE-R96 standard (as shown in Fig. 6), suggest that substantial portion of soot have smaller size upon addition of the combustion catalysts. This may be explained as follows; particles of 200 nm diameter or larger block green light proportional to their cross-sectional area. However, particles with sizes in the range of 50 nm block only about 15% of their surface area [34]. This means that the soot opacity readings depend on the particle size and will be underestimated in FSN if smaller particles are measured which would be the case when the


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catalysts have been added. It seems that the catalysts act as soot nuclei and hence soot with smaller particle sizes are formed [46]. 3.3.Effects of catalyst-µE additives on the combustion characteristics Single-zone model was used to calculate the heat release rate. According to this model, composition and temperature of the mixture inside the cylinder are assumed to be uniform. Internal energy of the mixture is also calculated based on the first law of thermodynamics. The detailed description of the calculations can be found in the literature [47, 48]. If the cylinder pressure and the instantaneous cylinder volume can be measured (by relating the crank angle to cylinder volume), the heat release rate may be calculated according to Eq. 1 [47, 48]. dq/dθ = (γ/(γ -1)).p.(dV/dθ) + (1/(γ -1)).V.(dp/dθ)

Eq. 1

Where p is the pressure and V is the cylinder volume, θ is the crank angle, γ is the ratio of the specific heat capacity of the mixture at constant pressure to specific heat capacity of the mixture at constant volume, i.e. Cp/Cv.γ is calculated from the slope of lnp vs lnV diagram and was estimated to be 1.3. Therefore, by calculating heat release rate, the typical combustion characteristics including the maximum heat release rate, total combustion duration and ignition delay can be determined. It should be noted that ignition delay is defined as the time interval between the start of injection (SOI) timing and start of combustion (SOC) timing of the fuel and usually reported as degrees of crankshaft rotation. The SOC is defined as the point at which the sudden slope change occurs in the heat release rate diagram. The end of the combustion process in a cycle is taken as the point where 90% of the cumulative heat release has occurred [47]. The difference between the end of the combustion and the ignition timing is taken as the total combustion duration. The duration of


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ignition delay is one of the most important criteria, having a great impact on the combustion process, mechanical stresses, engine noise and exhaust emission. For a complete combustion in the combustion chamber of a diesel engine a smaller ignition delay period is desired. In cylinder pressure measured versus crank angle is presented in Fig. 7(a). Based on these measurements and using Eq. 1, heat release rate and cumulative heat release rate were calculated which are presented in Fig. 7(b) and 7(c), respectively.

Figure 7. (a) Pressure (bar) versus crank angle (oCA) for base diesel and diesel with different catalyst at 2000rpm and full load, (b) Heat Release Rate (J/oCA) versus crank angle (oCA) for base diesel and diesel with different catalyst at 2000rpm and full load, (c) Cumulative Heat


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Release Rate (J/oCA) versus crank angle (oCA) for base diesel and diesel with different catalyst at 2000rpm and full load. The effects of combustion catalysts on different combustion parameters including maximum heat release rate and ignition delay are summarized in Table 6. Table 6. Ignition delay and maximum heat release rate of base diesel and diesel with the addition of different catalyst at 2000 rpm and full load. Ignition delay (oCA)

Maximum HRR (J/oCA)

Base

7.36

246.9

CeO2

6.21

230.9

Ce0.95Fe0.05O2-α

6.71

147.8

Ce0.9Fe0.1O2-α

5.85

220.0

Ce0.8Fe0.2O2-α

6.97

151.9

Nano catalyst

As is evident from Table 6 presence of the combustion catalysts led to a decrease in the ignition delay for all the samples while the combustion duration remained approximately unchanged at 44 degree of crank angle. This would also give a reasonable explanation for the effect of catalysts on the pollutant emissions. By a shorter ignition delay, less fuel is accumulated in the premixed combustion phase for diesel fuel with combustion catalyst. In other word, the premixed combustion phase would be less intense. This results in to smaller maximum heat release rate and the decrease of soot, CO and HC emissions [49]. Considering the fact that


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ignition delay is partly due to the fuel quality and the reduction of ignition delay indicate an increase in the cetane number [50], it may be concluded that the cetane number of fuel increases upon addition of the combustion catalysts. Moreover, by monitoring the maximum heat release rate, it may also be concluded that the increase in the NOx emissions is the result of the oxidative catalytic activity of the combustion catalyst and not the increase in temperature of the combustion chamber. 3.4.Effects of catalyst-µE additives on the fuel efficiency The effect of combustion catalyst dosing on the performance including torque and brake specific fuel consumption (BSFC) under the full loaded engine working condition at the speed of 1500 to 2000 rpm was investigated for Ce0.95Fe0.05O2-α sample, as the most effective catalyst in reducing pollutant emissions. The results are presented in Fig. 8.

Figure 8. (a) Brake specific fuel consumption vs speed at full load condition, (b) Torque vs speed at full load condition.


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As it is evident, by addition of the catalyst the average torque increased by approximately 4% and the average BSFC decreased by nearly 3% at different speeds. These are indicative of the improving effect of the catalyst in improving the engine performance by reducing the ignition delay, explained in Section 3.3. 3.5.Investigation on oxygen storage capacity and oxygen type of combustion catalysts via H2-TPR and O2-TPD H2-TPR along with O2-TPD experiments were carried out to analyze oxidative role of the combustion catalysts. H2-TPR technique is usually employed to measure the total OSC of the cerium oxide based materials [51, 52]. The TPR spectrum of pure ceria sample, reported in the literature [53], has usually two peaks which appear at different temperature ranges. The lower temperature peak attributed to reduction of the surface, and the higher temperature peak is due to bulk reduction. This behavior is explained according to the idea that the mobility of the bulk oxygen is relatively slow as compared to the surface oxygen [53]. The TPR spectra were obtained for the pure ceria and iron doped cerium oxide are presented in Fig.9. By addition of iron to cerium oxide, the peak related to reduction of the bulk has increased drastically while no considerable change appeared in surface reduction peak. This could be due to formation of other oxides, i.e. the mixed oxides of cerium and iron, as revealed by XRD pattern presented in Fig. 12. The generation of mixed oxide by addition of iron create a synergic effect between the cations (Fe3+, Ce4+). It means that when some quantity of iron is reduced first, it could catalyze the reduction reaction of the rest of the catalysts and have a significant influence on OSC [54].


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Figure 9. H2-TPR profile of cerium oxide and iron doped cerium oxide samples. In order to calculate total OSC, the TCD signal was calibrated by the reduction of Ag2O under similar conditions of the TPR experiments. The values of OSCs obtained according to calibration curve for different catalysts are summarized in Table 7. It should be mentioned that OSC is calculated based on Eq. 2. H2 + OS

H2O

Eq. 2


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Table 7. Oxygen Storage Capacity of different combustion catalysts. Combustion catalyst

OSC (mmol/gr cat)

CeO2

0.48

Ce0.95Fe0.05O2-α

6.18

Ce0.9Fe0.1O2-α

6.11

Ce0.8Fe0.2O2-α

5.66

The O2-TPD results are presented in Fig. 10. The profiles are characterized by more than one peak. The first peak is usually related to superficial oxygen, the so-called α-oxygen molecules [29]. It has been reported that α-oxygen molecules have better mobility as compared to the oxygen molecules appear at higher temperatures which is believed to be the bulk oxygen molecules. As is evident from Fig. 11, the first oxygen desorption peak for the entire catalyst samples lay below 580oC. As was explained, this type of oxygen molecules are more mobile and play a significant role as active catalytic oxygen. The percentage of α-oxygen molecules compared to the total oxygen molecules in each samples were calculated by dividing the integration of the TPD profile of the sample between 30 to 580oC to the corresponding integration between 30 to 1000oC.


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Figure 10. (a) O2-TPD of the catalysts, (b) percentage of α-oxygen molecules vs iron percentage of the catalysts. The activation energy of oxygen desorption (Ed) for each type of oxygen molecules, assuming a homogeneous surface, may be estimated from the temperature at which the rate of the oxygen desorption is maximum (TM) and the heating rate, i.e. b, 10oC/min for our case [55]. The activation energy of oxygen desorption can be estimated by Eq. 3 [55]. Ed = RTM ln((RTM2ʋ)/(Ed b))

Eq. 3

Where R is the gas constant, b is heating rate, and ʋ is obtained from Eq. 4. ʋ = kT/h

Eq. 4

Where k is the Boltzmann’s constant and h is the Planck’s constant. TM may also be found from the zeros of second order differential of O2-TPD profile. This is due to Eq. 5. T = To + bt

dT = b.dt

Eq. 5

Where To is the initial temperature and t is the time. Thus, for any unknown variable like x when ∂2x/∂t2 = 0 then ∂2x/∂T2 would be also zero.


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As an example the related diagrams, TPD profile, first order and second order derivatives of O2-TPD for Ce0.9Fe0.1O2-α sample are shown in Fig.11. The results of the calculations are summarized in Table 8.

Figure 11. (a) O2-TPD, first order and second order derivatives of O2-TPD for Ce0.9Fe0.1O2-α.


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Table 8. Temperature of peaks in O2-TPD profile and corresponding activation energy of oxygen desorption process for different catalysts. Activation energy of Combustion

Temperatures of the ʋ, ×1013 s-1

catalyst

oxygen desorption at O2-

O2-TPD peaks TPD peaks (Ed kJ/mol)

CeO2

515oC, 725oC

1.64, 2.08

231.5, 293.9

Ce0.95Fe0.05O2-α

523oC, 620oC, 877oC

1.65, 1.86, 2.39

233.9, 264.1, 344.8

Ce0.9Fe0.1O2-α

510oC, 620oC, 877oC

1.63, 1.86, 2.39

229.9, 264.1, 344.8

Ce0.8Fe0.2O2-α

530oC, 620oC, 865oC

1.67, 1.86, 2.37

236.1, 264.1, 341.0

The activation energies somehow reflect the importance of oxygen type in the effectiveness of oxygen storage capacity of the catalysts. As was discussed, the presence of iron in the ceria structure improves not only the total oxygen storage capacity of the catalysts but also leads to formation of more mobile oxygen molecules (see Fig. 10(b)). In most of the cases the improvement of OSC along with larger contribution of α-oxygen results in the enhancement of oxidative catalytic activity which in turn suppresses the emission of pollutants that require to be oxidized. Figure 12 presents the change in pollutants emission versus OSC of the catalysts. The OSC of CeO2 is much lower than the iron doped cerium oxide catalysts. As it can be seen, a more reasonable trend between the OSC of the catalysts and the variations of soot and NOx emissions is observed. For the case of HC and CO however, some catalysts with lower OSC are more effective in suppressing the emissions. It should be noted that soot and NOx are much more


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challenging in diesel emissions compared to CO and HC. This would indicate the importance of OSC for the combustion catalysts.

Figure 12. Variation in pollutant emissions, according to ECE-R96 weighting factors, versus OSC. (a) soot (b) nitrogen oxides (c) unburned hydrocarbons (d) carbon monoxide. 3.6.X-Ray Diffraction study of the catalysts Figure 13 presents the XRD patterns of as synthesized CeO2, Ce0.95Fe0.05O2-α, Ce0.9Fe0.1O2-α and Ce0.8Fe0.2O2-α, calcined at 550oC. All the samples present highly resolved diffraction peaks indicating their crystalline structures are well-developed upon calcination at high-temperature. Searching for the XRD patterns in JCPDS powder diffraction ﬁles, it is found that the


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Energy & Fuels

synthesized CeO2, Ce0.95Fe0.05O2-α, Ce0.9Fe0.1O2-α and Ce0.8Fe0.2O2-α exhibit diffraction peaks corresponding to cubic structure CeO2 (JCPDS 00-001-0800) and related to (h k l) values of (3 1 1), (1 1 1) and (2 2 0). By addition of iron to cerium oxide several characteristic peaks including those of (1 1 2), (0 2 1) and (2 1 1) similar to those of CeFeO3 (reference code: 00-022-0166) with orthorhombic structure have been also detected.


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Figure 13. X-Ray diffraction pattern of combustion catalysts. 4. Conclusion


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The main goal of the present study was to investigate the influence of oxygen storage type catalysts on direct injection diesel engine performance, emissions and combustion characteristics based on ECE-R96 standard. The results indicated that presence of the catalyst suppressed HC, CO and soot emissions to a large extent. However, regarding suppression of the pollutants, Ce0.95Fe0.05O2-α was found to be the most effective catalyst; it could reduce CO, HC and soot emission by 26.6, 57.7 and 20.0% respectively. The maximum heat release rate of the engine and ignition delay at maximum power output was found to decrease for all the catalysts prepared in this study. In the meantime the combustion duration remained unchanged by addition of the catalysts. The results also revealed that the brake specific fuel consumption, i.e. BSFC, decreased in the presence Ce0.95Fe0.05O2-α as the best catalyst in the fuel. H2-TPR and O2-TPD experiments indicated that the total oxygen storage capacity and types of oxygen are the main features of the catalysts in lowering the emissions and improving the engine performance.

ACKNOWLEDGMENT The authors would like to thank Motorsazan Company, Tabriz, Iran, and its experts for their technical assistance.

ABBREVIATIONS µE, microemulsion; DLS, dynamic light scattering; XRD, X-ray diffraction; H2-TPR, hydrogentemperature programmed reduction; O2-TPD, oxygen temperature programed desorption; OSC, oxygen storage capacity; BSFC, brake specific fuel consumption; NDIR, non-dispersive infrared detector; CLD, chemical luminescence detector; CA, crank angle; SDS, sodium dodecyl sulfate;


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TCD, thermal conductivity detector; FSN, filter smoke number; SOI, start of injection; SOC, start of combustion.

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