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Catalysts 2017, 7(2), 45; doi:10.3390/catal7020045

Aqueous and Surface Chemistries of Photocatalytic Fe-Doped CeO2 Nanoparticles

1
Department of Chemistry, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand
2
Research Center for Academic Excellence in Petroleum, Petrochemicals and Advanced Materials, Naresuan University, Phitsanulok 65000, Thailand
3
Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
4
Department of Industrial Engineering, Faculty of Engineering, Naresuan University, Phitsanulok 65000, Thailand
5
Centre of Excellence for Innovation and Technology for Water Treatment, Naresuan University, Phitsanulok 65000, Thailand
6
School of Materials Science and Engineering, Faculty of Science, UNSW Australia, Sydney, NSW 2052, Australia
*
Authors to whom correspondence should be addressed.
Academic Editors: Enrique Rodríguez-Castellón, Agustín Bueno-López and Elisa Moretti
Received: 7 November 2016 / Revised: 23 January 2017 / Accepted: 23 January 2017 / Published: 29 January 2017
(This article belongs to the Special Issue Ceria-based Catalysts)
View Full-Text   |   Download PDF [4985 KB, uploaded 15 February 2017]   |  

Abstract

The present work describes the effects of water on Fe-doped nanoparticulate CeO2, produced by flame spray pyrolysis, which is a critical environmental issue because CeO2 is not stable in typical atmospheric conditions. It is hygroscopic and absorbs ~29 wt % water in the bulk when exposed to water vapor but, more importantly, it forms a hydrated and passivating surface layer when immersed in liquid water. In the latter case, CeO2 initially undergoes direct and/or reductive dissolution, followed by the establishment of a passivating layer calculated to consist of ~69 mol % solid CeO2·2H2O and ~30 mol % gelled Ce(OH)4. Under static flow conditions, a saturated boundary layer also forms but, under turbulent flow conditions, this is removed. While the passivating hydrated surface layer, which is coherent probably owing to the continuous Ce(OH)4 gel, would be expected to eliminate the photoactivity, this does not occur. This apparent anomaly is explained by the calculation of (a) the thermodynamic stability diagrams for Ce and Fe; (b) the speciation diagrams for the Ce4+-H2O, Ce3+-H2O, Fe3+-H2O, and Fe2+-H2O systems; and (c) the Pourbaix diagrams for the Ce-H2O and Fe-H2O systems. Furthermore, consideration of the probable effects of the localized chemical and redox equilibria owing to the establishment of a very low pH (<0) at the liquid-solid interface also is important to the interpretation of the phenomena. These factors highlight the critical importance of the establishment of the passivating surface layer and its role in photocatalysis. A model for the mechanism of photocatalysis by the CeO2 component of the hydrated phase CeO2·2H2O is proposed, explaining the observation of the retention of photocatalysis following the apparent alteration of the surface of CeO2 upon hydration. The model involves the generation of charge carriers at the outer surface of the hydrated surface layer, followed by the formation of radicals, which decompose organic species that have diffused through the boundary layer, if present. View Full-Text
Keywords: Cerium oxide; nanoparticles; photocatalysis; surface chemistry; aqueous chemistry; thermodynamic stability diagrams; speciation diagrams; Pourbaix diagrams Cerium oxide; nanoparticles; photocatalysis; surface chemistry; aqueous chemistry; thermodynamic stability diagrams; speciation diagrams; Pourbaix diagrams
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Channei, D.; Phanichphant, S.; Nakaruk, A.; Mofarah, S.S.; Koshy, P.; Sorrell, C.C. Aqueous and Surface Chemistries of Photocatalytic Fe-Doped CeO2 Nanoparticles. Catalysts 2017, 7, 45.

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