J. Phys. Chem. C 2009, 113, 5629–5635
5629
Synthesis and Characterization of Ce1-xEuxO2-x/2 Mixed Oxides and Their Catalytic Activities for CO Oxidation Willinton Y. Herna´ndez,* Miguel A. Centeno, Francisca Romero-Sarria, and Jose A. Odriozola Instituto de Ciencia de Materiales de SeVilla, Centro Mixto UniVersidad de SeVilla-CSIC, AVenida Ame´rico Vespucio 49, 41092 SeVille, Spain ReceiVed: October 21, 2008; ReVised Manuscript ReceiVed: January 26, 2009
A series of Ce1-xEuxO2-x/2 mixed oxides was synthesized by coprecipitation. The solids were characterized by means of XRF, SBET, XRD, UV-vis, and Raman techniques, and their catalytic activities toward CO oxidation were tested. A solid solution, with CeO2 F-type structure, is formed for europium contents (measured as Eu2O3 by XRF) e20% wt. For higher contents, the solid solution is not formed, but a physical mixture is detected. The existence of oxygen vacancies in the solids with Eu2O3 contents between 3 and 17% wt was demonstrated by the presence of bands at 532 and 1275 cm-1 in their Raman spectra. The catalytic performances of the solids correlate with the amount of these punctual defects in the solid solution. 1. Introduction The oxidation of CO is one of the most studied heterogeneous chemical reactions. The reported active catalysts include supported transition metals; free transition metal clusters; and more recently, transition and noble metal clusters/nanoparticles supported on Al2O3, SiO2, TiO2, CeO2 and other metallic oxides.1-3 In a general way, the key steps of the CO oxidation reaction are (i) the weakening or breaking of the O-O bond in the adsorbed O2 molecules and (ii) the formation of the CO2 molecule. Various models have been postulated to account for the role of oxygen as a function of the catalyst’s nature. It has been reported that oxygen adsorption proceeds directly on the noble metal cluster4 or on the support, in particular at oxygen vacancies, where the O2 molecule is activated to react with an adsorbed CO molecule.5 In the case of gold-based catalysts, it has been probed by scanning tunneling microscope measurements and density functional theory calculations that in Au/ TiO2 samples, gold atoms prefer binding to oxygen vacancies before any other site on the titania surface.6 Therefore, the presence of vacancies of oxygen affects the gold dispersion on the supports and modifies the electronic properties of the gold particles, which is extremely important in catalysis by gold. CeO2 has been established as a principal oxide support used in catalytic applications in which oxidation-reduction processes are involved.1,7 The importance of ceria in catalysis originates from its remarkable oxygen storage capability and its ability to undergo rapid and repeatable redox cycles. This feature is strongly related to the easy creation and diffusion of oxygen vacancies, especially at the ceria surface. In addition, ceria possess a high potential in applications in fuel cell technologies.8,9 To increase the number of oxygen vacancies in the CeO2, the elimination of oxygen, either from the surface or the bulk oxide, can be promoted through thermal treatment,10 irradiation with electrons,11 exposure to X-rays,12 or chemical reduction.13 However, the doping of ceria with aliovalent cations, introduced into the network of the fluorite type oxide, has been an efficient way to generate oxygen vacancies, permitting a high mobility of lattice oxygen. In addition, the doping of the ceria positively * Corresponding author. Phone: +34 954489501, ext. 9221. Fax: +34 954460665. E-mail:
[email protected].
influences the thermal stability and the surface area compared to the pure oxide. The cations frequently employed for this purpose are transition metals such as yttrium14 and zirconium,15 and elements in the series of rare earth metals, such as Gd, Pr, Sm, Nd, and Eu,16-21 among others. Generally, these materials are used as electrolytes in solid oxide fuel cells due to their ionic conductivity characteristics. Among the solid solutions created from CeO2, special attention lies on the Ce-Eu-O system. Europium is an element appropriate to form solid solutions with the ceria due to the similarity of ionic radii (0.97 and 1.07 for the Ce4+ and Eu3+, respectively), and both oxides crystallize in similar crystalline structures (F-type cubic for CeO2 and C-type cubic for Eu2O322). Additionally, the Eu3+/Eu2+ redox pair whose potential is close to 0 (E° ) -0.36)23 is expected to promote the formation and stabilization of oxygen vacancies for low Eu/Ce ratios. Moreover, for the same materials synthesized by a hydrothermal method, Shuk et al. reported a beneficial effect on the ionic conductivity by increasing the europium content (maximum for x ) 15%), determined by a rise in the concentration of Vo¨ (vacancies of oxygen).19 This system has not yet been explored extensively, and more information about their physical and chemical properties, such as crystalline structure, surface area, vacancies generation and catalytic properties, is needed. This work investigates the potential of Ce1-xEuxO2-x/2 mixed oxides as active supports in the reaction of CO oxidation, which reflects the importance of oxygen vacancies on the promotion of redox processes. 2. Experimental 2.1. Preparation of the Samples. A series of oxides with general formula Ce1-xEuxO2-x/2 was prepared by coprecipitation. The appropriate amounts of Ce(NO3)3 · 6H2O (Alfa Aesar 99.5%) and Eu(NO3)3 · 6H2O (Alfa Aesar 99.9%) to achieve the desired Eu2O3-to-CeO2 weight percentage ratios (w/w) (0, 3, 6.5, 10, 13.5, 17, 20, 40, 70, and 100%), were dissolved in distilled water to obtain a 0.1 M solution. Then the cationic solutions were precipitated by addition of ammonium hydroxide (30% w/w) at room temperature under continuous stirring. The precipitated gels were washed by filtration with distilled water and dried overnight at 100 °C. Finally, the samples were calcined at
10.1021/jp8092989 CCC: $40.75 2009 American Chemical Society Published on Web 03/12/2009
5630 J. Phys. Chem. C, Vol. 113, No. 14, 2009 300 °C for 2 h (10 °C/min). The obtained solids were denoted as Ce_Eu(X), being X the nominal weight percentage (w/w) of Eu2O3. 2.2. Characterization. BET specific surface areas were measured by nitrogen adsorption at liquid nitrogen temperature in a Micromeritics ASAP 2000 apparatus. Before analysis, the samples were degassed 2 h at 150 °C in vacuum. The CeO2 and Eu2O3 contents of the samples were determined by X-ray fluorescence spectrometry (XRF) in a Panalytical AXIOS PW4400 sequential spectrophotometer with a rhodium tube as the source of radiation. X-ray diffraction (XRD) analysis was performed on a Siemens D 500 diffractometer. Diffraction patterns were recorded with Cu KR radiation (40 mA, 40 kV) over a 2θ range of 10-80° and a position-sensitive detector using a step size of 0.01° and a step time of 7 s. The lattice parameters of the samples where solid solution exist was calculated adjusting the refinement Rietveld (using the X’Pert Plus program to perform a refinement in “automatic” mode) to an FM3-M space group. For those materials with Eu2O3 contents >20%, the lattice parameters were calculated adjusting the refinement Rietveld to the Ia3j space group. The reflection from the (111) plane was used for the determination of the average crystallite size, D, calculated from the Scherrer equation. Ultraviolet-visible (UV-vis) diffuse reflectance spectrum was recorded on a Varian Cary 100 spectrophotometer equipped with an integrating sphere and using BaSO4 as reference. The Raman spectra were recorded in a dispersive Horiva Jobin Yvon LabRam HR800 microscope, with a 20 mW He-Ne green laser (532.14 nm), without filter, and with a 600 g mm-1 grating. The microscope used a 50× objective and a confocal pinhole of 100 µm. The Raman spectrometer is calibrated using a silicon wafer. 2.3. Catalytic Activity. The CO oxidation tests were carried out in a conventional continuous flow U-shaped glass reactor (7 mm inner diameter) under atmospheric pressure. The sample (80 mg, 100 µm < φ < 200 µm) was placed between glass wools. A thermocouple in contact with the sample assures the right measure of the temperature. The feed mixtures were prepared using mass flow controllers (Bronkhorst). The reaction was followed by mass spectrometry using a Balzers Thermostar benchtop mass spectrometer controlled by the software Balzers Quadstar 422 with capabilities for quantitative analysis. The light-off curves of CO oxidation (400 °C, 5 °C/min) were obtained with a mixture of 3.4% CO (Air Liquide, 99.997% pure,
