CeO2 Nanoparticles –

Overview and Scope of Application

Submitted by

Sikder Ashikuzzaman Ayon | ID: 201111 022

Ayeman Mazdi Nahin | ID: 201111 029

Date of Submission: 09 November, 2016

MME 457 | Powder Metallurgy

Structure of CeO2 Cerium is a rare earth metal belonging to the lanthanide series

of the periodic table. When combined with oxygen in a

nanoparticle formulation, cerium oxide adopts a fluorite

crystalline structure that has unique antioxidant properties.

For particles in the size range of 3−10 nm, the particle shape is

dominated by truncated octahedral. CeO2 particles of large size

are dominated by the octahedral shape with flat surfaces. [1]

Synthesis There are many available methods for synthesis of CeO2

nanoparticles or nanoceria. Some of them are precipitation method, electrostatic method, sol-gel method,

hydrothermal method etc.

Precipitation Method In this method, required reagents are cerium nitrate Ce(NO3)3·6H2O, iso-propanol and aqueous ammonia.

The reaction is carried out at room temperature. 0.06 M cerium (III) nitrate solution in water–iso-propanol

mixture in the volume ratio of 1:8 is prepared and stirred afterwards. A five-fold excess of 2 M aqueous

ammonia solution is then added to the solution so that the pH of the solution can be adjusted to 10, as an

alkaline medium gives smaller particles than the acidic medium. After 1 hour, the pale red colored reactants

turn yellow, indicating the formation of amorphous cerium hydroxide nanoparticles. These synthesized

nanoparticles are calcined at 450°C for 3 hours to obtain crystalline nano-sized particles.

Conversion of amorphous cerium hydroxide to cerium oxide: 𝐶𝑒(𝑂𝐻)4 + 𝐻𝑒𝑎𝑡 → 𝐶𝑒𝑂2 + 2𝐻2𝑂

Many-a-times suitable surfactant is used to obtain a better usefulness, depending on the application.

Subsequent methodology may also be required. For example, in order to obtain a uniform suspension of

nanoparticles in diesel, a standard ultrasonic shaker has been used for mixing the nanoparticle corresponding

to the required dosing level. [2]

Role of Nanoceria in Diesel Engine Diesel engines are one of the major contributors to the emissions such as hydrocarbons, particulates and

nitrogen oxides. These emissions are very harmful to human beings and subject to strict environmental

legislation. Improvement in the performance of diesel engines is an important challenge to be addressed,

especially in the current era.

Effect on Fuel Properties

Flash Point

Flash point is the temperature at which a flash is

momentarily produced by a spark on the surface of a

heated fuel. Higher the volatility of a fuel, lower will

be the flash point and vice versa. Tests were carried

out to observe the variation of flash point of diesel

after adding nanoparticle up to a dosing level of

80 ppm. The obtained result is shown in Figure 2,

which indicates a decrease in the volatility of the fuel

with the nanoparticle addition. The addition of

catalytic nanoparticles like CeO2 in the fuel, increases

its flash point. Therefore, nanoparticle added fuel is

inherently safer to handle as compared to its base fuel.

Figure 1: Unit cell of fluorite crystal structure

Figure 2: Variation of flash point of diesel as a function of concentration of nanoceria

Kinematic Viscosity

Kinematic viscosity of the diesel is found to increase with nanoparticle addition as shown in Figure 3. The

addition of nanoparticles in the fuel increases the resistance between the fluid layers and hence increases

the viscosity. Higher fuel viscosities provide sufficient lubrication for fuel injection pumps or injector plungers

resulting in less wear.

Engine Performance The brake thermal efficiency was increased by about 6% on the addition of cerium oxide nanoparticle in

diesel. The cerium oxide nanoparticles present in the fuel promote longer and more complete combustion

as compared to the base fuel, as cerium oxide acts as an oxygen buffer, releasing and storing oxygen

depending upon the partial pressure of oxygen. The efficiency of cerium oxide as a catalyst is related to its

ability to undergo a transformation from the stoichiometric CeO2 (+4) state to the Ce2O3 (+3) valence state

via relatively low energy reactions. The efficiency increases with the increase of dosing level of cerium oxide

up to 35 ppm. Beyond that, a slight decrease in efficiency was observed, especially at higher loads. (Shown

in Figure 4). This is because, chance of agglomeration is more when dosing level of nanoparticles is increased.

As a result, size of nanoparticles increases which affects the catalytic action.

Reduction of Exhaust Emissions

Hydrocarbon Emissions

The hydrocarbon emissions were decreased on addition of catalytic nanoparticles by about 40 to 45%,

especially at higher load as shown in Figure 5. It was also observed that HC emissions reduces with the

increase in the concentration of cerium oxide from 5 ppm to 35 ppm. Previously stated, Cerium oxide has

the ability to undergo a transformation from (+4) valance state to (+3) state, releasing the oxygen. Thus it

supplies oxygen for the oxidation of the hydrocarbon as well as the soot and gets converted to cerous oxide

(Ce2O3) as follows, [3]

(2𝑥 + 𝑦)𝐶𝑒𝑂2 + 𝐶𝑥𝐻𝑦 → [(2𝑥+𝑦)

2] 𝐶𝑒2𝑂3 + 𝑥𝐶𝑂 +

𝑦

2𝐻2𝑂

Carbon soot burning: 4𝐶𝑒𝑂2 + 𝐶 𝑠𝑜𝑜𝑡 → 2𝐶𝑒2𝑂3 + 𝐶𝑂2

NOx Emissions

The NOx emissions were found to be decreased by a maximum of 30%, on the addition of nanoceria in diesel,

especially at higher load, as shown in Figure 6. It was observed that NOx emissions reduce with the increase

in the concentration of ceria from 5 ppm to 35 ppm. Cerous oxide (Ce2O3) formed after the oxidation of

hydrocarbon and soot gets re-oxidized to CeO2 through the reduction of nitrogen oxide as per the following

reaction: [3]

𝐶𝑒2𝑂3 + 𝑁𝑂 → 2𝐶𝑒𝑂2 + 12⁄ 𝑁2

Figure 3: Variation of kinematic viscosity of diesel as a function of concentration of cerium oxide nanoparticles

Figure 4: Variation of the brake thermal efficiency with load.

Treatment of Neurological Oxidative Stress Diseases Due to their potent free radical scavenging properties, CeO2 nanoparticles can also be used in the treatment

of diseases resulting from neurological oxidative stress like Ischemia, Alzheimer’s disease, Parkinson’s

disease etc.

Biology of Oxidative Stress Reactive oxygen and nitrogen species (ROS and RNS, respectively) are potent oxidizing and nitrating agents

that include superoxide (O2•_), nitric oxide (NO) etc. Most ROS & RNS are also considered to be free radicals

(i.e. molecules with unpaired electrons in the outer orbital shell). They are generated from several biological

processes. For example, mitochondria produce several ROS as a byproduct of cellular respiration and

immune cells generate O2•_ and NO as defense mechanisms against infectious organisms. In addition to

biological sources, we are exposed to free radicals from environmental sources including cigarette smoke,

car exhaust and ultraviolet radiation from the sun. To combat the high reactivity, cells and organisms possess

endogenous systems to neutralize free radicals and reactive oxygen species. In addition to these, exogenous

antioxidants are even obtained from dietary sources. When the level of free radicals exceeds the ability of

antioxidant systems to inactivate or neutralize them, the result is a state of oxidative stress. Oxidative stress

plays a role in aging as well as a variety of human disease states. [4]

Parkinson’s Disease Parkinson’s Disease (PD), the second most common

neurodegenerative disorder, is caused by the death of

dopaminergic nigra neurons of the midbrain. Symptoms

of the disease which do not become apparent up until

about 50-80% of the cells have died, include tremor at

rest, difficulty initiating movements and slow movements.

Although the cause of dopaminergic neurodegeneration

is unknown, age, environmental factors and genetics have

all been shown increase the risk of developing PD. Many

hypotheses regarding the etiology of sporadic PD revolve

around mitochondrial dysfunction and oxidative stress. [4,

pp. 258-259]

Ways for Treatment Currently approved pharmacological treatments for

Parkinson’s Disease are oriented to reduce the symptoms of the disease. For example, the dopamine

Figure 7: Position of Substantia nigra in human brain and its damage due to Parkinson's disease. [11]Manipulated

Figure 5: Variation of HC emissions with load for diesel and modified diesel.

Figure 6: Variation of NO𝑥 emissions with load for diesel and modified diesel.

precursor levodopa (L-dopa) or dopamine receptor agonists are given with the aim of enhancing

dopaminergic transmission in the remaining substantia nigra neurons. Although L-dopa is considered the

‘gold standard’ for the treatment of PD, effectiveness typically wanes after 5 years and the side effects, such

as dyskinesia (jerky, involuntary movements) and psychosis can be debilitating. Currently, development of

therapies aimed at halting dopamine neuron degeneration is necessary.

Therapy by Nanoparticle In the last years, increasing biological interest is emerging for nanotechnology that can improve

pharmacological treatments, by using nanomaterials. In particular, cerium oxide nanoparticles, considered

one of the most interesting nanomaterials for their catalytic properties, show a promise for application in

therapy. Due to the presence of oxygen vacancies on its surface and autoregenerative cycle of its two

oxidation states, Ce3+ and Ce4+, nanoceria can be used as an antioxidant agent.

Cerium 4+ oxidation state is the most stable in its oxide form. But in the oxide form, cerium can switch from

one oxidation state to another depending on external stimuli, and this feature is very important because it

allows cerium oxide to participate in many chemical reactions. The surface of cerium oxide contains more

defects, both Ce3+ ions and oxygen vacancies. Therefore, when in the form of nanoparticles, given the

increase in the surface to bulk ratio, the Ce3+ ions concentration increases, leading to improved reactivity.

[5]

Figure 8: Model of the reaction mechanism for the oxidation of hydrogen peroxide by nanoceria and the regeneration via reduction by superoxide. An oxygen vacancy site on the nanoceria surface (1) presents a 2Ce4+ binding site for H2O2 (2), after the release of protons and two-electron transfer to the two cerium ions (3) oxygen is released from the now fully reduced oxygen vacancy site (4). Subsequently superoxide can bind to this site (5), and after the transfer of a single electron from one Ce3+, and uptake of two protons from the solution, H2O2 is formed (6) and can be released. After repeating this reaction with a second superoxide molecule (7) the oxygen vacancy site returns to the initial 2Ce4+ state (1). It is also possible that the third Ce3+ indicated, which gives rise to the oxygen vacancy, could participate directly in the reaction mechanism. The square Ce–O matrix is shown here only to illustrate the model and does not correspond to the actual spatial arrangement of the atoms in the crystal structure. [6]

Recent research suggests the possibility of an autoregenerative mechanism for nanoceria treated with 10

mM of hydrogen peroxide, with a continuous oscillation of cerium between the Ce4+ to the Ce3+ states

(Reaction model shown in Figure 8). Interestingly, it has been reported that the anti-radical ability of ceria

nanoparticles increases with a reduction in nanoparticle size, resulting from increased concentration of Ce3+

ions.

Biocompatibility It is demonstrated that nanoceria can penetrate in the cells, but often tends to form agglomerates, which

may cause loss of activity. For its real use in therapy, it will be necessary to improve the reactivity of

nanoceria with the cells, by modifying chemical and physical properties, such as its size and tendency to

aggregate. Furthermore, unlike free molecules, nanoceria can be localized in the right place at the right time,

being a nanoparticle. An in vivo (on laboratory animals) experiment demonstrated that nanoceria can act in

a similar manner, injecting the nanoparticles in the retinas of albino rats, which are extremely sensitive to

photon induced damages. [7]

Some Other Outstanding Applications Cerium oxide nanoparticles have been shown to have significant interactions in plants. Results showed

that lettuce (Lactuca sativa) treated in 100 mg per kg nanoceria grew significantly faster than others.

Further Reading: [8]

Very efficient glass polishing agents that can be used for precision optical polishing. Also it is applied to

decolor glass. Further Reading: [9]

Used in sunscreens and cosmetic creams. Nanoparticles of CeO2 is known to adsorb and neutralize

organophosphates and is an interesting topical skin protectant. Further Reading: [10]

Conclusion To implement the experimental achievements in practical field, more scientific studies and critical analyses

are still required. As a concluding remark on the topics discussed, efforts should be taken to obtain the

optimum combinations of the parameters for the best performance of the fuel by introducing nanoceria. In

the case of medical therapy, even though the scientific opinion is not still unanimous, we can conclude that

nanoceria is a potent and regenerative antioxidant agent.

References

[1] Z. L. Wang and X. Feng, "Polyhedral Shapes of CeO2 Nanoparticles," The Journal of Physical

Chemistry, vol. 107, no. 49, p. 13563, 15 November 2003.

[2] B. J. THIRUMAL, J. G. E, LOGANATHAN and C. G. SARAVANAN, "EMISSION REDUCTION FROM A DIESEL

ENGINE FUELED BY CERIUM OXIDE NANO-ADDITIVES USING SCR WITH DIFFERENT METAL OXIDES

COATED CATALYTIC CONVERTER," Journal of Engineering Science and Technology, vol. 10, no. 11

(2015), pp. 1407-1408, November 2015.

[3] A. C. Sajeevan and V. Sajith, "Diesel Engine Emission Reduction Using Catalytic Nanoparticles: An

Experimental Investigation," Journal of Engineering, vol. 2013, pp. 5-7, 29 January 2013.

[4] A. Y. Estevez and J. S. Erlichman, "Cerium Oxide Nanoparticles for the Treatment," ACS Symposium

Series, pp. 255-256, 17 November 2011.

[5] I. Celardo, E. Traversa and L. Ghibelli, "Cerium oxide nanoparticles: a promise for applications in

therapy," Journal of Experimental Therapeutics and Oncology, vol. 0, pp. 1-2, January 2011.

[6] I. Celardo, J. Z. Pedersen, E. Traversab and L. Ghibelli, "Pharmacological potential of cerium oxide

nanoparticles," Nanoscale, p. 1414, 02 March 2011.

[7] J. CHEN1, S. PATIL, S. SEAL and J. F. McGINNIS, "Rare earth nanoparticles prevent retinal

degeneration induced by intracellular peroxides," Nature Nanotechnology, 29 October 2006.

[8] X. Gui, Z. Zhuang, S. Liu, L. Liu and W. Cao, "Fate and Phytotoxicity of CeO2 Nanoparticles on Lettuce

Cultured in the Potting Soil Environment," 28 August 2015.

[9] AZoNano, "Cerium (Ce) Nanoparticles - Properties, Applications," AZoNano, 20 June 2013. [Online].

Available: http://www.azonano.com/article.aspx?ArticleID=3275. [Accessed 01 November 2016].

[10] A. Zenerinoa, T. Boutardb, C. Bignona, S. Amigonia, D. Jossed, T. Deversc and F. Guittarda, "New CeO2

nanoparticles-based topical formulations for the skin protection against organophosphates,"

Toxicology Reports, vol. 2, pp. 1007-1013, 2015.

[11] A.D.A.M., Inc., "Substantia nigra and Parkinson's disease: Mediline Plus Medical Encyclopedia Image,"

A.D.A.M., Inc., 13 August 2015. [Online]. Available:

https://medlineplus.gov/ency/imagepages/19515.htm. [Accessed 30 October 2016].