Influence of Rare Earth Doping on the Structural and Catalytic Properties of Nanostructured Tin Oxide
© to the authors 2008
Received: 3 March 2008
Accepted: 7 May 2008
Published: 28 May 2008
Nanoparticles of tin oxide, doped with Ce and Y, were prepared using the polymeric precursor method. The structural variations of the tin oxide nanoparticles were characterized by means of nitrogen physisorption, carbon dioxide chemisorption, X-ray diffraction, and X-ray photoelectron spectroscopy. The synthesized samples, undoped and doped with the rare earths, were used to promote the ethanol steam reforming reaction. The SnO2-based nanoparticles were shown to be active catalysts for the ethanol steam reforming. The surface properties, such as surface area, basicity/base strength distribution, and catalytic activity/selectivity, were influenced by the rare earth doping of SnO2and also by the annealing temperatures. Doping led to chemical and micro-structural variations at the surface of the SnO2particles. Changes in the catalytic properties of the samples, such as selectivity toward ethylene, may be ascribed to different dopings and annealing temperatures.
The importance of the morphological properties of materials can be evidenced by the large number of publications on their synthesis. The development of new synthesis methods may lead to materials, such as catalysts, with superior performance. It is interesting to produce materials with nanometric-scale structures to obtain specific properties. Tin oxide nanoparticles have been investigated in our laboratory. This oxide has been used in a large range of technological applications, including sensors, catalysts, and electrocatalytic materials. It is well known that semiconductor oxides, such as SnO2, have an excellent potential for these applications due to their high capacity to adsorb gaseous molecules and promote their reactions [1–8]. We recently showed that the modification of the nanometric-scale structure and the composition of particles led to interesting selectivity changes for the methanol decomposition and aldolization reaction between acetone and methanol [2–4]. However, the influence of the nature of the active sites (the surface basicity of the oxide) on the performance of the catalysts was not totally investigated. The study of basicity, in more sensitive reactions, is very important as a source of information on the different kinds of active sites. In order to investigate the catalytic properties of the tin oxide samples prepared, we present the preliminary results in the catalytic steam reforming of ethanol. This reaction is promoted not only by basic sites but also by acidic sites of the oxide catalysts. Thus, it may be suggested that the control of surfaces and modifications of the nanostructures of the tin oxide particles, undoped and doped with rare earths used as catalysts in this reaction, can be used to obtain additional information on the catalytic properties and application of these nanostructured materials. Nowadays, this process has gained increasing attention due to the possibility of obtaining hydrogen for fuel cell applications, as well as ethylene which is considered a valuable raw material in the polymeric industry [9–11].
Doped and undoped SnO2samples were synthesized by the polymeric precursor method. This method is based on the chelation of cations (metals) by citric acid, in aqueous solution containing tin citrate, in the present case. Ethylene glycol was then added to polymerize the organic precursor. The aqueous tin citrate solution was prepared from SnCl2 · H2O (Mallinckrodt Baker, USA, purity >99.9%) and citric acid (Merck, Germany, purity >99.9%) with a citric acid:metal molar ratio of 3:1. For the synthesis of the rare earth-doped SnO2samples, an aqueous solution of a rare earth citrate was prepared from a rare earth nitrate (Y and Ce-nitrates, Alfa Aesar, USA, purity >99.9%) and citric acid with a citric acid:metal molar ratio of 3:1. The aqueous rare earth citrate solution was added to the aqueous tin citrate solution in the appropriate amount to obtain a doping level of 5 mol% in all cases. Ethylene glycol was then added to the citrate solutions, at a mass ratio of 40:60 in relation to citric acid, to promote the polymerization reaction. After several hours of polymerization at approximately 100 °C, the polymeric precursors were heat-treated in two steps, initially at 300 °C for 6 h in air to promote the pre-pyrolysis, and then at several temperatures (550–1,100 °C) for 2 h, also in air, to allow the organic material to be completely oxidized and to promote the crystallization of the SnO2phase.
The specific surface area of the samples was determined by N2adsorption/desorption isotherms (BET method) at liquid nitrogen temperature in an Quantachrome Autosorb-1C instrument. The CO2adsorption isotherms were determined with the same instrument. The amount of irreversible CO2uptake was obtained from the difference between the total adsorption of CO2on the catalyst and a second adsorption series of CO2determined after evacuation of the catalyst sample for 20 min. X-ray diffraction (XRD; Siemens, D5000, equipped with graphite monochromator and Cu Kα radiation) was used for the crystal phase determination. The X-ray photoelectron spectra were taken using a commercial VG ESCA 3000 system. The base pressure of the analysis chamber was in the low 10–10 mbar range. The spectra were collected using Mg Kα radiation and the overall energy resolution was around 0.8 eV. The concentration of the surface elements was calculated using the system database after subtracting the background counts.
Catalytic performance tests were conducted at atmospheric pressure with a quartz fixed-bed reactor (inner diameter 12 mm) fitted in a programmable oven, at a temperature of 500 °C. The catalysts (undoped SnO2sample calcined at 1,000 °C, Sn#1000, Y-doped SnO2samples calcined at 550 and 1,000 °C, SnY#550 and SnY#1000, respectively, and Ce-doped SnO2samples calcined at 550 and 1,000 °C, SnCe#550 and SnCe#1000, respectively) were previously treated in situ under nitrogen atmosphere at 500 °C for 2 h. The water:ethanol mixture (molar ratio 3:1) was pumped into a heated chamber and vaporized. The water–ethanol gas (N2) stream (30 mL/min) was then fed to the reactor containing 150 mg of the catalyst. The reactants and the composition of the reactor effluent were analyzed with a gas chromatograph (Shimadzu GC 8A), equipped with a thermal conductivity detector (TCD), Porapak-Q, and a 5A molecular sieve column with Ar as the carrier gas. Reaction data were recorded for 4 h.
Results and Discussion
Crystallite sizes measured by the Rietveld refinement and specific surface areas determined by N2adsorption (BET), as a function of the annealing temperature
Crystallite size (Å)
Specific surface area (BET) (m2 g−1)
The total and irreversible CO2adsorption capacity, uptake at 27 and 300 °C, of undoped and doped samples of tin oxide
Total CO2adsorption (μmol/m2)
Irreversible CO2adsorption (μmol/m2)
The SnO2-based nanoparticles were shown to be active catalysts for the ethanol steam reforming reaction. The surface properties, such as surface area, basicity/base strength distribution, and catalytic activity/selectivity, were influenced by the rare earth doping of SnO2and also by the annealing temperatures. Doping led to chemical and micro-structural variations at the surface of the SnO2particles. Also, changes in the catalytic properties of the samples, such as selectivity toward ethylene, may be ascribed to different dopings and annealing temperatures. This suggests a new pathway to produce catalysts by means of controlling their surface. A super-saturated solid solution yields a nanostructured metastable material that will undergo foreign cation segregation to the outer surface and then a de-mixing process. This process can effectively be used to control the surface chemistry.
In the present study, the effect of the different operational conditions, such as reaction temperature and water:ethanol molar ratio, on the catalytic behavior was not determined. However, this study is under way, and it will be the subject of future reports.
The authors gratefully acknowledge CNPq, FAPERGS, and FINEP for financial support.
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