**1. Introduction**

ing coating applications, which leads to encapsulated morphology of CeO2 nanoparticles in‐ to the polymer particles resulting in lack of agglomeration during film formation. The

The fifth chapter is "Extraction and recovery of cerium from rare earth ore by solvent extrac‐ tion" by Kai Li et al. The authors describe the solvent extraction and recovery of cerium found in a variety of minerals and present in the highest concentration in light rate earth ores. The main approach used is based on the acid-leaching process of rare earth minerals,

The sixth chapter is "Pd-supported catalysts over mixed oxides based on cerium for envi‐ ronmental catalysis purposes" by Victor Ferrer. This chapter presents studies related to the use of CeO2 as a redox promoter in the oxidation reactions of hydrocarbon and the reduc‐ tion of nitrogen oxides, which are carried out in catalytic converters installed in vehicles. The study also discusses the improvement of CeO2 redox properties by the incorporation of elements such as terbium or zirconium in the cerium network, creating a mixed oxide with better performance. Catalytic activity resulting from the CH4 and CO oxidation reactions

and reduction of NO by CO in catalysts using Pd as an active phase are also studied.

**Prof. Dr. Sher Bahadar Khan and Prof. Dr. Kalsoom Akhtar**

Chemistry Department Faculty of Science

King Abdulaziz University Jeddah, Saudi Arabia

photoactivity behavior of CeO2 nanoparticles is also discussed.

VIII Preface

which produces the leaching solution as well as high purity CeO2products.

Cerium belongs to lanthanide series and available most abundantly in the crust of the earth with an average concentration of 50 ppm as a rare earth element. Elemental cerium is a flexible and malleable lustrous metal. Cerium metal is iron-gray in color and is highly reactive. It is also known as a strong oxidizing agent and exists as cerium oxide in association with oxygen atoms. It exists as either cerous (Ce3+, trivalent state) or ceric (Ce4+, tetravalent state) in the form of compounds [1].

It is clear from the title that this book is related to cerium oxide (CeO<sup>2</sup> ) which is one of the important transition metal oxides acting as n-type semiconductor materials. It possesses several features resulted from the combination of high amount of oxygen in its structure and the facile change between the reduced and oxidized states (Ce3+ and Ce4+) [2]. The CeO<sup>2</sup> has cubic fluorite structure, in which each cerium atom is surrounded by eight equivalent oxygen atoms and each oxygen atom is surrounded by a tetrahedron of four cerium atoms. Ideally, CeO<sup>2</sup> should have a formal charge of −2 and distance between oxygen–oxygen atoms should be 2.705 Å, in which the formal charge of cerium ions is +4 [3].

The main unique characteristics of cerium oxide involve a band gap of 3–3.6 eV, high value of dielectric constant up to κ = 23–26, high refractive index of n: 2.2–2.8, and high dielectric strength reached to 2.6 MV cm−<sup>1</sup> [4]. Such properties qualify cerium oxide-based materials to be employed in various applications, especially when they are in nanosized particles. The cerium oxide is a famous member of nanostructured materials having a wide range of applications. Cerium oxide materials/nanomaterials have been utilized in numerous fields including adsorption, catalysis, photocatalysis, sensing, fuel cells, hydrogen production, semiconductor devices as well as biomedical uses [5–10].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Commercial uses of CeO<sup>2</sup> could be utilized in the pure form or in a concentrated dose as a polishing powder for glasses as well as ophthalmic lenses or precision optics. Cerium oxide is also employed as a glass constituent for preventing solarization and discoloration, particularly in television screens. The CeO<sup>2</sup> contributes in heat-resistant alloy and ceramic coatings. Cerium oxide is also used in petroleum refining and emission controlling system in gasoline engines as well as a diesel fuel-borne catalyst to reduce particulate matter emissions. In recent years, CeO<sup>2</sup> nanoparticles have gained more consideration in biomedical research community since they could be used as inhibiting cellular agent along with their antimicrobial and antioxidant activity [1, 5].

[5] Rajeshkumar S, Naik P. Synthesis and biomedical applications of cerium oxide nanopar-

[6] Dhall A, Self W. Cerium oxide nanoparticles: A brief review of their synthesis methods

chemi-sensor and photo-catalyst for environmental applications. Science of the Total

[8] Faisal M, Khan SB, Rahman MM, Jamal A, Akhtar K, Abdullah MM. Role of ZnO-CeO<sup>2</sup> nanostructures as a photo-catalyst and chemi-sensor. Journal of Materials Science &

[9] Khan SB, Faisal M, Rahman MM, Akhtar K, Asiri AM, Khan A, et al. Effect of surfactant

[10] Khan SB, Karimov KS, Chani MTS, Asiri AM, Akhtar K, Fatima N. Impedimetric sens-

–Co<sup>3</sup> O4

on the particle size, photocatalytic activity and sensing properties of CeO<sup>2</sup>

International Journal of Electrochemical Sciences. 2013;**8**:7284-7297

ing of humidity and temperature using CeO<sup>2</sup>

Microchimica Acta. 2015;**182**:2019-2026

nanoparticles as a

3

Introductory Chapter: Cerium Oxide - Applications and Attributes

http://dx.doi.org/10.5772/intechopen.82757

nanoparticles.

nanoparticles in polymer hosts.

ticles–A review. Biotechnology Reports. 2018;**17**:1-5

Environment. 2011;**409**:2987-2992

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and biomedical applications. Antioxidants. 2018;**7**:97-109

[7] Khan SB, Faisal M, Rahman MM, Jamal A. Exploration of CeO<sup>2</sup>

Owing to the dramatical and widespread industrial uses of cerium oxide materials, the National Institute of Environmental Health Sciences is suggested and nominated CeO<sup>2</sup> for toxicological characterization because of its limited toxicity data, and a lack of toxicological studies for nanoscale CeO<sup>2</sup> . CeO<sup>2</sup> , which is one of important transition metal oxides, acts as n-type semiconductor materials that have diverse applications such as adsorption, catalysis, photocatalysis, sensing, fuel cells, hydrogen production, semiconductor devices as well as biomedical uses.
