“Cerium(IV) oxide”的意思、由来-开放百科全书

Cerium occurs naturally as a mixture with other rare-earth elements in its principal ores bastnaesite and monazite. After extraction of the metal ions into aqueous base, Ce is separated from that mixture by addition of an oxidant followed by adjustment of the pH. This step exploits the low solubility of CeO₂ and the fact that other rare-earth elements resist oxidation.^[2]

Cerium(IV) oxide is formed by the calcination of cerium oxalate or cerium hydroxide.

Cerium also forms cerium(III) oxide, {{chem|Ce|2|O|3}}, which is unstable and will oxidize to cerium(IV) oxide.^[3]

Structure and defect behavior

Cerium oxide adopts the fluorite structure, space group Fm3m, #225 containing 8-coordinate Ce⁴⁺ and 4-coordinate O²⁻. At high temperatures it releases oxygen to give a non-stoichiometric, anion deficient form that retains the fluorite lattice. This material has the formula CeO_(2−x), where 0 < x < 0.28.^[4] The value of x depends on both the temperature, surface termination and the oxygen partial pressure. The equation

has been shown to predict the equilibrium non stoichiometry x over a wide range of oxygen partial pressures (10³–10⁻⁴ Pa) and temperatures (1000–1900 °C).^[5]

The non stoichiometric form has a blue to black color, and exhibits both ionic and electronic conduction with ionic being the most significant at temperatures > 500 °C.^[6]

The number of oxygen vacancies is frequently measured by using X-ray photoelectron spectroscopy to compare the ratios of {{chem|Ce|3+}}to {{chem|Ce|4+}}.

Defect chemistry

In the most stable fluorite phase of ceria, it exhibits several defects depending on partial pressure of oxygen or stress state of the material.^[7]^[8]

The primary defects of concern are oxygen vacancies and small polarons (electrons localized on cerium cations). Increasing the concentration of oxygen defects increases the diffusion rate of oxide anions in the lattice as reflected in an increase in ionic conductivity. These factors give ceria favourable performance in applications as a solid electrolyte in solid-oxide fuel cells. Undoped and doped ceria also exhibit high electronic conductivity at low partial pressures of oxygen due to reduction of the cerium ion leading to the formation of small polarons. Since the oxygen atoms in a ceria crystal occur in planes, diffusion of these anions is facile. The diffusion rate increases as the defect concentration increases.

The presence of oxygen vacancies at terminating ceria planes governs the energetics of ceria interactions with adsorbate molecules, and its wettability. Controlling such surface interactions is key to harnessing ceria in catalytic applications. ^[9]

Catalysis and surface activity

The primary emerging application of applied CeO₂ materials is in the field of catalysis. Surfaces of ceria, in its most stable fluorite phase, are dominated by the lower energy (111) planes, which tend to exhibit lower surface energy. The reaction most commonly catalysed by cerium(IV) is the water gas shift reaction, involving the oxidation of carbon monoxide. Ceria has been explored towards the catalysis of various hydrocarbon conversion reactions including CO₂ methanation and the catalytic oxidation of hydrocarbons such as toluene. ^[10]

The surface functionality of CeO₂ stems largely from its intrinsic hydrophobicity, a trait that is common among rare earth oxides. Hydrophobicity tends to impart resistance to water-deactivation at the surfaces of catalysts and thus enhances the adsorption of organic compounds. Hydrophobicity, which can be conversely seen as organophilicity, is generally associated with higher catalytic performance and is desired in applications involving organic compounds and selective synthesis. ^[11]

The interconvertibility of CeO_x materials is the basis of the use of ceria for an oxidation catalyst. One small but illustrative use is its use in the walls of self-cleaning ovens as a hydrocarbon oxidation catalyst during the high-temperature cleaning process. Another small scale but famous example is its role in oxidation of natural gas in gas mantles.^[12]

Building on its distinct surface interactions, ceria finds further use as a sensor in catalytic converters in automotive applications, controlling the air-exhaust ratio to reduce NO_x and carbon monoxide.

Further Applications

Polishing

The principal industrial application of ceria is for polishing, especially chemical-mechanical planarization (CMP).^[2] For this purpose, it has displaced many other oxides that were previously used, such as iron oxide and zirconia. For hobbyists, it is also known as "opticians' rouge".^[13]

Optics

CeO₂ is used to decolorize glass by converting green-tinted ferrous impurities to nearly colorless ferric oxides.^[2]

Cerium oxide has found use in infrared filters, as an oxidizing species in catalytic converters and as a replacement for thorium dioxide in incandescent mantles^[15]

Mixed conduction

Due to the significant ionic and electronic conduction of cerium oxide, it is well suited to be used as a mixed conductor ^[16], with significant value in fuel cell research and development.

Research

Fuel cells

Ceria is of interest as a material for solid oxide fuel cells (SOFCs) because of its relatively high oxygen ion conductivity (i.e. oxygen atoms readily move through it) at intermediate temperatures (500–650 °C) and lower association enthalpy compared to Zirconia system.^[19]

Water splitting

The cerium(IV) oxide–cerium(III) oxide cycle or CeO₂/Ce₂O₃ cycle is a two step thermochemical water splitting process based on cerium(IV) oxide and cerium(III) oxide for hydrogen production.^[20]

References

1. ^Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, {{ISBN|0-07-049439-8}}
2. ^¹²³Klaus Reinhardt and Herwig Winkler in "Cerium Mischmetal, Cerium Alloys, and Cerium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry 2000, Wiley-VCH, Weinheim. {{DOI|10.1002/14356007.a06_139}}.
3. ^{{cite web |url=http://courses.chem.indiana.edu/c360/documents/thermodynamicdata.pdf |dead-url=yes |archive-url=https://web.archive.org/web/20131029204441/http://courses.chem.indiana.edu/c360/documents/thermodynamicdata.pdf |archive-date=October 29, 2013 |title=Standard Thermodynamic Properties of Chemical Substances }}
4. ^Defects and Defect Processes in Nonmetallic Solids By William Hayes, A. M. Stoneham Courier Dover Publications, 2004.
5. ^{{Cite journal | doi = 10.1021/jp406578z| title = Analytical Model of CeO₂ Oxidation and Reduction| journal = The Journal of Physical Chemistry C| volume = 117| issue = 46| pages = 24129–24137| year = 2013| last1 = Bulfin | first1 = B.| last2 = Lowe | first2 = A. J.| last3 = Keogh | first3 = K. A.| last4 = Murphy | first4 = B. E.| last5 = Lübben | first5 = O.| last6 = Krasnikov | first6 = S. A.| last7 = Shvets | first7 = I. V.| hdl = 2262/76279}}
6. ^{{cite book| first1=K.| last1=Ghillanyova| first2=D.| last2=Galusek| editor1-first=Ralf |editor1-last=Riedel|editor2-first=I-Wie|editor2-last=Chen|title=Ceramics Science and Technology, Materials and Properties, vol 2|publisher=John Wiley & Sons |year=2011 |chapter=Chapter 1: Ceramic oxides|isbn=978-3-527-31156-9}}
7. ^{{cite journal |last=Munnings |first=C. |first2=S.P.S. |last2=Badwal |first3=D. |last3=Fini |journal=Ionics |year=2014 |doi=10.1007/s11581-014-1079-2 |title=Spontaneous stress-induced oxidation of Ce ions in Gd-doped ceria at room temperature|volume=20 |issue=8 |pages=1117–1126 }}
8. ^{{cite journal |last=Badwal |first=S.P.S. |author2=Daniel Fini |author3=Fabio Ciacchi |author4=Christopher Munnings |author5=Justin Kimpton |author6=John Drennan |title=Structural and microstructural stability of ceria – gadolinia electrolyte exposed to reducing environments of high temperature fuel cells |journal=J. Mater. Chem. A |volume=1 |issue=36 |doi=10.1039/C3TA11752A |date=2013 |pages=10768–10782}}
9. ^ [https://www.sciencedirect.com/science/article/pii/S0169433219302399 Hydrophobicity of low-index CeO₂ surfaces]
10. ^{{cite journal |last=Ruosi Peng |last2=et a.| title=Size effect of Pt nanoparticles on the catalytic oxidation of toluene over Pt/CeO2 catalysts | journal= Applied Catalysis B: Environmental | volume= 220 |year=2018}}
11. ^ [https://www.researchgate.net/publication/330633086_Theoretical_insights_into_the_hydrophobicity_of_low_index_CeO2_surfaces Theoretical insights into the hydrophobicity of low index CeO₂ surfaces] Applied Surface Science (2019), Volume 478,Pages 68-74
12. ^{{Greenwood&Earnshaw2nd}}
13. ^Properties of Common Abrasives (Boston Museum of Fine Arts)
14. ^MFA Materials database.
15. ^{{cite web |url=http://www.nanopartikel.info/cms/lang/en/Wissensbasis/Cerdioxid |title=Cerium dioxide |website=DaNa |archive-url=https://web.archive.org/web/20130302081012/http://www.nanopartikel.info/cms/lang/en/Wissensbasis/Cerdioxid |archive-date=2013-03-02 |dead-url=yes}}
16. ^{{cite web | url=http://www.fkf.mpg.de/2698712/MixedConductors | title=Mixed conductors | publisher=Max Planck institute for solid state research | accessdate=16 September 2016}}
17. ^{{cite journal|title=UV-shielding property, photocatalytic activity and photocytotoxicity of ceria colloid solutions|doi=10.1016/j.jphotobiol.2010.09.002|year=2011|last1=Zholobak|first1=N.M.|last2=Ivanov|first2=V.K.|last3=Shcherbakov|first3=A.B.|last4=Shaporev|first4=A.S.|last5=Polezhaeva|first5=O.S.|last6=Baranchikov|first6=A.Ye.|last7=Spivak|first7=N.Ya.|last8=Tretyakov|first8=Yu.D.|journal= Journal of Photochemistry and Photobiology B: Biology|volume=102|issue=1|pages=32–38}}
18. ^{{cite news|url=http://www.dailymail.co.uk/health/article-1208720/Suncream-linked-Alzheimers-disease-say-experts.html|title=Suncream may be linked to Alzheimer's disease, say experts|date=24 August 2009|accessdate=2009-08-25 | location=London | work=Daily Mail}}
19. ^{{cite journal|title=Electrical conductivity of the ZrO2–Ln2O3 (Ln=lanthanides) system|date=June 1999|doi=10.1016/S0167-2738(98)00540-2|volume=121|issue=1–4|journal=Solid State Ionics|pages=133–139|last1=Arachi|first1=Y.}}
20. ^{{cite web |url=http://www.solarpaces.org/Tasks/Task2/HPST.HTM |title=Hydrogen production from solar thermochemical water splitting cycles |archive-url=https://web.archive.org/web/20090830011704/http://www.solarpaces.org/Tasks/Task2/HPST.HTM |archive-date=August 30, 2009 |dead-url=yes |website=SolarPACES }}

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