Experimental investigation of the effect of cerium oxide nanoparticles as a combustion-improving additive on biodiesel oxidative stability: mechanism

Abstract

Nano cerium oxide, a combustion-improving fuel additive, was investigated for its impact on biodiesel oxidative stability. The findings of the present study revealed for the first time that nano cerium oxide addition at the concentrations generally used to improve combustion (<100 ppm) severely reduced the oxidative stability of biodiesel.

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1. Introduction

Biodiesel, an alternative to petroleum-derived diesel fuel (petrodiesel) is produced from vegetable oils, animal fats or used frying oils. Biodiesel has become widely acceptable in the energy market owing to its unique features including reduction of most exhaust emissions in comparison with petrodiesel, higher cetane number, biodegradability, lack of sulphur, inherent lubricity, positive energy balance, higher flash point, compatibility with the existing fuel distribution infrastructure, renewability and domestic origin.^1,2 Despite these advantages however, some significant problems still exist from the technical and commercial point of view i.e. higher NOx emissions, higher pour point and cloud points, and limited storage life due to low oxidative stability.^1,3 Among those, oxidative degradation of biodiesel during storage could severely compromise its quality with respect to effects on kinematic viscosity, acid value, cetane number, total ester content, and formation of hydroperoxides, soluble polymers, and other secondary products.⁴ This in turn could lead to decreased biodiesel stability and could consequently jeopardize long-term and widespread application of this green fuel.

Nitrogen oxide emissions – another disadvantage of biodiesel – known as NOx are produced during the combustion of biodiesel and pose serious problems to both public health and the environment.⁵ In light of that, a number of investigations have aimed at addressing the mentioned drawbacks of biodiesel by addition of different additives e.g. metal based additives to biodiesel and its blends,^5–7 On the other hand, new approaches and advances achieved in nanotechnology have encouraged researchers to take advantages of such metal-based additives in their nano-size.⁸ For instance, Sajith et al. (2010) and Hong (2011) mentioned that nanosized metal oxides (e.g. nano cerium oxide and nano aluminium oxide) exhibited significantly higher activities and resulted in better impacts on combustion characteristics of fuels when compared to the conventional additives.^6,9 Moreover, another important advantage of nano-sized metal additives over their micron-sized counterparts would be the prevention of fuel injector and filter clogging.¹⁰ Such improvement could be ascribed to the fact that at these dimensions the surface-area-to-volume ratio of the particles increases considerably, leading to a larger contact surface area and consequently more efficient performance.^6,11

Among the additives studied nano cerium oxide has attracted a great deal of attention recently under various labels such as combustion improver, engine performance improver, combustion-chamber surface cleaner,¹² and NOx emission reducer. Arul Mozhi Selvan et al. and Sajith et al.^6,13 studied the effects of cerium oxide nanoparticles on the engine performance and emission characteristics and physicochemical properties of biodiesel and diesel biodiesel blends. They found a significant reduction of harmful emissions, and improved brake thermal efficiency owing to the enhanced catalytic activity. Oxonica, a UK-based company also manufactured a fuel additive, Envirox, consisting of cerium oxide nanoparticulate, and reported fuel savings of 10% when it was added to diesel fuel due to (a) better catalytic effect between diesel and air and (b) better oxygen absorption and consequently reduced NOx emissions.¹⁴ In a different study, Jung et al. studied the kinetics of oxidative combustion of biodiesel in the presence of cerium oxide nanoparticles and revealed significantly increased oxidation rate in the combustion chamber.¹⁵ Despite all these promising attributes associated with the application of nano cerium oxide as a combustion enhancer, however, little is known about its impact on the stability of biodiesel and its shelf life.

Therefore, the present study was set to achieve an insight into the potential positive or negative impacts of nano cerium oxide on oxidative stability of biodiesel. To accomplish that, nano cerium oxide suspensions of neat biodiesel produced from sunflower and waste cooking oils (SOB and WCOB respectively) were examined using the Rancimat method (EN14112).

2. Experimental

2.1. Materials

Pure sunflower oil was purchased from a vegetable oil factory and used frying oil was collected from a restaurant. Nano cerium oxide powder (99.97%) with 10–30 nm particle size was purchased from US Research Nanomaterials, Inc. (USA) (http://www.us-nano.com).

2.2. Biodiesel synthesis and characterization

Biodiesel was produced from sunflower oil and pre-treated waste cooking oil with methanol and an alkali catalyst (KOH) in a stirred tank reactor at 60 °C for 1 h. The pre-treatment process on waste cooking oil was carried out with the methanol-to-oil ratio of 0.3 (v/v) in the presence of 1% H₂SO₄ (v/v) as an acid catalyst for 1 hour at 60 °C. After the reaction, the mixture was allowed to settle for 1 h and the methanol–water fraction separated at the top was removed. The produced biodiesel samples were characterized based on the ASTM D6751 standard test methods (Table 1).

Table 1 Produced biodiesel specifications (limits are based on ASTM D6751 standard test method)

Property	Unit	SOB	WCOB	Limits
Density	g cm^−3	0.86	0.87	0.86–0.90
40 °C viscosity	mm^2 s^−1	3.9	5.2	1.9–6.0
Flash point	°C	178.3	170.2	>130
Acid value	mg KOH per g	0.15	0.6	<0.5
Moisture	% Volume	0.045	0.05	<0.05
Iodine value	g iodine per 100 g	58.5	109	120

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2.3. Oxidation stability analysis

Nano cerium oxide was added to biodiesel samples at inclusion rates ranging from 0 to 500 ppm. The dosing levels of the cerium oxide nanoparticles required for each sample was measured using a precision electronic balance apparatus and were mixed with the fuel samples by means of a probe ultrasonic instrument model MISONIX (40 kHz, 5 min). The oxidation stability of the samples were then investigated by determining the induction period as described by the EN14112 standard test method using a modified Metrohm 743 Rancimat instrument (Herisau, Switzerland). In this method, air (10 L h⁻¹) was sparged through the samples and then through a water trap. Volatile oxidation products (primarily formaldehyde and short-chain acids such as carboxylic acid) were absorbed by the water, causing an increase in conductivity. The water conductivity was monitored to determine the onset of oxidation and to define an induction time. Also, propyl gallate – a conventional antioxidant for biodiesel – was used as control (200–1000 ppm).

3. Results and discussion

Nano cerium oxide is being increasingly promoted as a biodiesel-combustion improver and in particular NOx emission reducer. However, as mentioned earlier its impact on one of the key quality parameters of biodiesel i.e. stability has not yet well investigated. The overall findings of the present study obtained through Rancimat analyses revealed that nano cerium oxide addition (0–500 ppm), led to a significant deterioration of oxidative stability of both SOB and WCOB samples (Table 2).

Table 2 Induction periods of biodiesel samples

Oxidation stability	Biodiesel feedstock	Nano cerium oxide dosing levels (ppm)
		0	50	100	200	500
Induction period (IP)	Sunflower oil	1.48	0.81	0.83	1.47	2.05
	Waste cooking oil	0.17	0.09	0.11	0.13	0.23

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As presented in Fig. 1, nano cerium oxide worsened the oxidative stability of both biodiesel samples at the lowest concentration investigated (i.e. 50 ppm) but caused slight improvements in comparison with neat biodiesel samples when concentrations over 200 ppm were used. Nonetheless, even at the highest concentration of 500 ppm, nano cerium oxide addition failed to meet the ASTM/EN requirement for biodiesel oxidative stability (the induction period of 6 h). On the other hand, application of nano-additives such as nano cerium oxide at concentrations over 100 ppm for improving fuel combustion characteristics is economically impractical.

Fig. 1 IP values determined using Rancimat test method for biodiesel samples dosed with 0–500 ppm nano cerium oxide.

Given the worsening effect of nano cerium oxide addition (within its concentration range as combustion improver, <100 ppm) on biodiesel oxidative features and in order to still be able to benefit its combustion improving effects i.e. NOx reduction, the application a conventional antioxidant – propyl gallate (PrG) (200–1000 ppm) – was also considered in the present study. The results obtained revealed that 500 and 1000 ppm PrG were required to reach an induction period of above 6 h, for sunflower biodiesel and WCO biodiesel, respectively, in the presence of 100 ppm nano cerium oxide. Whereas, significantly lower concentrations of 200 and 800 ppm for neat sunflower and WCO biodiesel samples, respectively, were required to meet the standard (Fig. 2).

Fig. 2 IP values determined using Rancimat test method for neat and 100 ppm nano-additivated biodiesel samples versus the concentration of propyl gallate.

3.1. Action mechanism of nano cerium oxide in biodiesel

As discussed earlier, nano cerium oxide is widely used as a fuel additive for the elimination of toxic exhaust emission gases. In fact, nano ceria can act as a chemically active component, working as an oxygen store by releasing oxygen in the presence of reductive gases, and removing oxygen by interaction with oxidizing species.¹⁶ More specifically, cerium oxide may absorb oxygen for the reduction of NOx or may provide oxygen for the oxidation of CO and soot through the combustion⁶ (Fig. 3). The key to the use of ceria for catalytic purposes is the low redox potential between the Ce³⁺ and Ce⁴⁺ ions (1.7 V) allowing the below reaction to easily occur in exhaust gases.¹⁶ Therefore, both oxidative and reductive properties for nano cerium oxide could be expected.

Fig. 3 (a) Cerium oxide shift reaction between the two states of cerium by oxygen releasing-absorbing mechanism. (b) Cerium oxide role in combustion process: it absorbs oxygen from NO mediates produced due to the high temperature of combustion chamber, then donates this oxygen to the soot (C) particles produced by incomplete combustion of hydrocarbons and converts them to CO₂ molecules.

In case of the effect of nano cerium oxide on biodiesel stability, based on the Rancimat results obtained, nano cerium oxide's effect on biodiesel molecules did not follow a certain improving or deteriorating trend. More precisely, despite the sharp reduction occurred in oxidative stability of the neat biodiesel samples by the addition of 50–100 ppm of nano cerium oxide, the stability increased slightly as the additive concentration increased to 500 ppm. In fact, nano cerium oxide seems to have mainly acted as an oxidizing agent for biodiesel molecules and facilitated peroxide radicals production by donating oxygen species to the biodiesel radicals (Fig. 4), while exerted some antioxidant capabilities when it was present at high concentrations. It is worth quoting that anti-free radical and antioxidant properties of this material have been previously observed in biological systems such as DNA molecules, brain cells, neurons, visual cells and lipid cells of living organisms.^17,18 For instance, Estevez and Erlichman argued that nano ceria acts as a protective agent against oxidative cell damages by moderately reducing the potent oxidizing agents in cells i.e. reactive oxygen species (ROS) accumulation.¹⁸

Fig. 4 Nano cerium oxide oxidative-reductive acting mechanism on biodiesel molecules.

Cerium in nano cerium oxide lattice can reversibly bind oxygen and shift between Ce⁴⁺ and Ce³⁺ states under oxidizing and reducing conditions.¹⁹ The loss of oxygen and the reduction of Ce⁴⁺ to Ce³⁺ are accompanied by the creation of oxygen vacancies in the nanoparticle lattice. Ce³⁺ lattice defects as well as the resultant oxygen vacancies are abundant at the surface of nano ceria.^18,20,21 These oxygen vacancies can absorb oxygen free radicals, neutralize them and then release oxygen molecules to the media around them.²⁰ It has been reported that the concentration of Ce³⁺ ions on the surface of the particle increases by reducing nanoparticle size. Consequently, these would lead to increased anti-free radical ability of ceria nanoparticles.²² Because of all these properties, several studies have proposed that nano ceria could act as free-radical scavenger.²¹ These free radicals including peroxide and hydroxyl radicals could be produced during the degradation of biodiesel as well. However, based on the findings of the present study, nano cerium oxide (in practical concentrations) did not seem capable of scavenging the large biodiesel peroxide radicals.

To describe the different effects of nano cerium oxide in low and high dosing levels a presumption could be postulated: at low concentrations, cerium oxide lattice structure seems incapable of getting sufficiently close to the peroxide radicals produced during the first step of biodiesel degradation to scavenge them. This is due to the high steric hindrance of large biodiesel peroxide radicals. Under such conditions, only the oxygen donating property of cerium oxide could be observed whereas at high concentrations of nano cerium oxide, the accumulation of nanoparticle species around the peroxide radicals seems to overcome the steric hindrance. Then, the nanoparticles surround the peroxides produced in the propagation step of biodiesel oxidation chain reactions and prevent their contact with the other biodiesel molecules. Thus, further degradation of biodiesel molecules is inhibited by converting the free radicals to the former biodiesel molecules. This assumption is schematically shown in Fig. 4.

4. Conclusions

Nano cerium oxide is increasingly gaining attention in the biodiesel industry owing to its combustion-improving features. The findings of the present study revealed for the first time that nano cerium oxide (10–30 nm particle size) addition within the concentration range ppm generally practiced to improve combustion (<100), severely worsened the oxidative stability of biodiesel. An average 45% reduction in the IP values for both biodiesel samples was observed using 50 and 100 ppm nano additive. As a result, elevated concentrations of conventional antioxidants such as PrG were required to make up for this adverse effect while still enjoying nano cerium oxide beneficial impacts on biodiesel combustion. Considering the effects of decreasing nano cerium oxide particle size on its properties, such as increasing its free oxygen radical absorbance potential, using smaller particle sizes of this material may show better impacts on the stability of biodiesel, however, more research is required.

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