CO and soot oxidation activity of doped ceria: Influence of dopants
Graphical abstract
Introduction
Designing advanced doped ceria (CeO2) materials has drawn immense research interest due to their extensive use in several environmental, energy related, and other industrial catalytic applications [1]. The importance of CeO2 had primarily emerged from its oxygen storage capacity (OSC) and redox catalytic property [2]. Easy toggling between the oxidation states of cerium ion (Ce3+/Ce4+) accompanied with the formation of internal oxygen vacancies imparts excellent redox property in ceria [3]. Moreover, compared to bulk, nanoscale ceria shows outstanding catalytic activity in many applications, which is attributed to an increased specific surface area and relative ease of oxygen vacancy formation than that of the bulk material [4]. Despite its wide advantage, pure ceria lacks some important features necessary for commercial applications. Low OSC, significant loss of active surface area due to thermal sintering, deactivation of the redox couple, reduction of catalytic activity are few among them [5], [6], [7]. Even a small degree of sintering causes a huge effect on the crystallite size and the existence of oxygen vacancies, enough to carve down the catalytic activity significantly [8]. However, presence of a foreign metal ion in the ceria lattice is well proven to strengthen its hand against thermal sintering and loss of catalytic activity, along with significant increase of OSC [9]. In addition, incorporation of dopants results in enhanced BET surface area and oxygen vacancies; thereby, reasonably improve catalytic performance even at higher temperatures [10].
The choice of a suitable dopant however, still remains a major challenge to the scientific community in terms of oxygen storage capacity, thermal stability, and economical considerations. Numerous attempts have been performed to correlate the influence of dopant features, such as oxidation state, ionic radius, electronegativity, etc with the physicochemical properties and redox ability of ceria-based catalysts both from theoretical understanding and experimental observations. To mention a few, Nolan has correlated the ionic radii of trivalent dopants with the kind of defect formed on CeO2 (110) surface from DFT + U calculations [11]. Hu et al. have illustrated the variation in oxygen vacancy formation energy in terms of valance of dopant ion from theoretical calculations [12]. Anderson et al. have correlated redox catalytic efficiency of doped material with the dopant ionic radius applying DFT calculations [13]. On the other hand, Liu et al. have illustrated a linear correlation between CO oxidation activities of doped ceria with Pauling electronegativity of the dopants from experimental observation [14]. Further, Sun et al. have demonstrated size dependent OSC of ceria nanocrystals after carrying out an in-depth analysis [15]. But combining the outcomes of those studies towards better understanding of the influence of dopant features is not straight forward. Therefore, still there exists a gap at the bottom.
Considering this fact, we have focused particularly on the correlation of physicochemical properties of doped ceria catalysts with their catalytic activity in CO and soot oxidations, which are typically performed in ceria based automobile catalysts [16], [17], [18], [19], [20]. CO and soot are generated due to partial burn of fossil fuel in auto mobile engine and exhibit severe effects on environment and human health [21], [22], [23], [24], [25]. Therefore, abatement of CO and soot has been a big concern throughout. Ceria based catalysts have been highly explored for this application from 1980s. Number of doped ceria materials have been reported to exhibit considerable activity. Lack of appropriate substitute of fossil fuel and progressively tightened emission standards have provoked more interest in this area [26], [27], [28]. Though a plenty of doped ceria catalysts are already reported to exhibit considerable activity, but their comparative study is necessary towards basic understanding of influence of dopants, which have not been well explored yet. To achieve this perception we have undertaken this comparative study and have discussed the outcomes on the basis of present observations and previous literature reports. Catalytic materials have been prepared using different class of dopant metal ions with widely varied ionic radii and reducibility, such as, zirconium (Zr), hafnium (Hf), iron (Fe), manganese (Mn), lanthanum (La), and praseodymium (Pr). Transition metals Zr and Hf have comparatively smaller ionic radii than Ce and are hardly reducible under the redox condition of ceria [20]. They are known to exist in +4 oxidation state and are capable of introducing only intrinsic oxygen vacancies. On the other hand, Mn and Fe are even smaller in size and easily reducible [7], [10]. They are known to exist in multiple valance state and thereby impart extrinsic oxygen vacancy along with the intrinsic oxygen vacancies present in ceria. On the contrary, rare earth lanthanum (La) and praseodymium (Pr) exhibit higher ionic radii than Ce but Pr is easily reducible and exhibit in multiple valance state in contrary to La [19], [23]. However, both of these metal ions introduce extrinsic oxygen vacancies in addition to intrinsic oxygen vacancies. Moreover, according to literature reports, La modifies the surface property of ceria which leads to formation of comparatively stable surface carbonates [19], [20]. Hence, here it will be interesting to comparatively study those doped ceria materials together and analyze their physicochemical properties with their activity.
The catalysts have been prepared by choosing optimized Ce/M (M = Zr, Hf, La, Pr, Fe, Mn) mole ratio according to our previous investigations [20], [29], [30], [31]. All the catalysts have been prepared via simple facile coprecipitation method under similar laboratory conditions to exclude any difference in activity caused by morphology. All the prepared materials have been calcined at 773 K and characterized using state of art techniques like X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman, UV–vis diffuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectronspectroscopy (XPS). Reducibility of the prepared materials has been investigated using temperature programmed reduction (TPR). Activity has been studied by performing CO oxidation in presence of oxygen under atmospheric pressure, 300–850 K temperature range and soot oxidation in presence of air in thermogravimetric analyzer within 300–1273 K temperature range.
Section snippets
Catalyst preparation
The optimized solid solutions of CeO2-ZrO2 (CZ, 8:2), CeO2-La2O3 (CL, 8:2), CeO2-Pr2O3 (CP, 8:2), CeO2-HfO2 (CH, 8:2), CeO2-Fe2O3 (CF, 9:1) and CeO2-Mn2O3 (CM, 8:2) were synthesized by means of simple and economical coprecipitation method. The employed metal precursors were Ce(NO3)3·6H2O (Sigma Aldrich, AR grade), Zr(NO3)4·5H2O (Fluka, AR grade), La(NO3)3·6H2O (Sigma Aldrich, AR grade), Pr(NO3)3·6H2O (Sigma Aldrich, AR grade), HfCl4 (Sigma Aldrich, AR grade), Fe(NO3)3·9H2O (Sigma Aldrich, AR
Characterization studies
XRD is a powerful technique to determine the crystal structure and phase purity. The XRD patterns of the prepared catalysts CM, CF, CH, CP, CL, and CZ calcined at 773 Kare illustrated in Fig. 1. The diffraction pattern of pure ceria is also provided for the purpose of comparison. A careful observation of Fig. 1 reveals that, XRD patterns of all the prepared doped CeO2 samples are indexed to (111), (200), (220), (311), (420), and (422) planes, which are characteristic of fluorite like cubic
Conclusions
Nano-crystalline ceria solid solutions were successfully synthesized by incorporating Zr, Hf, La, Pr, Fe, and Mn metal ions via simple coprecipitation method. XRD, TEM and Raman spectroscopic investigation proved the formation of cubic fluorite phase ceria-based solid solution with perfect incorporation of the dopants. XPS examination was carried out to probe the surface properties, whereas, H2–TPR study provided the reducibility of the samples. The redox catalytic activities were investigated
Acknowledgement
DM and BG thank the Council of Scientific and Industrial Research (CSIR) and University Grant Commission (UGC) New Delhi for junior research fellowships. Financial support for this project was received from Department of Science and Technology, New Delhi, under SERB Scheme (SB/S1/PC-106/2012).
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