Article pubs.acs.org/JPCC
Ceria and Its Defect Structure: New Insights from a Combined Spectroscopic Approach Anastasia Filtschew, Kathrin Hofmann, and Christian Hess* Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Darmstadt, Germany
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S Supporting Information *
ABSTRACT: Ceria is an interesting component for a variety of catalytic and fuel cell applications. In the study described here, ten different commercial ceria samples as well as synthesized ceria samples were investigated in detail regarding their (defect) structure and characteristic properties using XRD, N2 adsorption−desorption, and optical spectroscopy (Raman, DRIFTS, UV−vis). The investigations revealed correlations of surface defect features (Raman, DRIFTS) as well as those of bulk defects (Raman, UV−vis). The Raman feature at around 250 cm−1 was demonstrated to be related to surface defects rather than a 2TA vibration as described in the literature. A correlation between UV−vis band gap values and the presence of Raman bulk defects was established based on the observed decrease of the band gap energy with increasing number of defects. Detailed Raman analysis revealed that the frequently mentioned linear equation for the determination of the crystal size from the half-width of the F2g Raman feature is erroneous, since the F2g half-width depends on ceria bulk defects. Apart from these universal observations, differences in the properties depending on synthesis conditions were observed. In particular, it is shown that the type and quantity of ceria defects are influenced not only by crystal size but also by the preparation method.
1. INTRODUCTION
properties, in particular the combination of methods was used to extract further information.
Ceria is used for a variety of applications, such as exhaust emission catalysis,1,2 as a catalyst for hydrogen production and purification,3 as an electrolyte in solid oxide fuel cells,4,5 and as a photocatalyst.6 Special characteristics of ceria are its facile oxygen release and therefore facile defect creation.7,8 Defects in ceria are of particular interest, since they can participate in a reaction or influence it.2,5,6,9,10 The creation of intrinsic oxygen vacancies in ceria is responsible for oxygen-ion conductivity in solid oxide fuel cells.5 Recently, the importance of oxygen vacancies for catalytic applications, e.g. CO oxidation and NOx storage reduction, has been demonstrated.6,7,9,11−13 Defects in ceria have been studied extensively in the past, focusing on their formation by doping,14−17 their treatment in reductive or oxidative atmosphere,18−21 their dependence on crystal size,22−26 and their atomic surface structures.27−29 Investigations on ceria are mostly based on just one type of ceria sample, which is doped or treated. However, such an approach does not account for the influence of synthesis conditions on ceria properties. In this context, the questions arise, which (defect) properties define a specific ceria, and how are they influenced by synthesis? In this work, different commercial ceria samples as well as synthesized ceria samples were investigated in detail to identify their characteristic properties. For that purpose, ceria samples covering a broad range of crystal sizes and defect concentrations were chosen. Besides the characterization of ceria © 2016 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Material Synthesis. The commercial and prepared ceria samples are listed in Table 1. The commercial samples were used as received. The synthesized samples were prepared by 2-fold calcination of cerium(III) nitrate hexahydrate (SigmaAldrich, 99.99% trace metal basis) in air for 12 h at the indicated temperatures using a heating rate of 6 °C/min. After calcination the samples were sieved (mesh size: 200 μm). Table 1. Investigated Ceria Samples commercial
synthesized
Fluka SA 99.995% SA
