Preparation, Characterization and Catalytic Activity for CO Oxidation of SiO2 Hollow Spheres Supporting CuO Catalysts
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- Song, C., Wang, C., Zhu, H. et al. Catal Lett (2008) 120: 215. doi:10.1007/s10562-007-9272-9
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Silica hollow spheres were synthesized by sol–gel process using carbon microspheres as templates, and used as supports for CuO/SiO2 catalysts. The samples were characterized by TEM, nitrogen adsorption–desorption, XRD and TPR, and furthermore, the catalytic performance for CO oxidation was approached. The results indicated that the catalytic activity of CuO supported on SiO2 hollow spheres exhibited much higher as compared to that supported on commercial SiO2. Enhancement of the catalytic activity may be attributed to the fact that the unique hollow spherical texture should facilitate the formation of main active species and gas diffusion in catalysts.
KeywordsHollow spheresCuO/SiO2CO oxidationCatalytic activity
Hollow spheres have increasingly attracted interest due to their unique properties and potential applications in chemistry, biotechnology, and materials science [1–3]. Inorganic materials with hollow spherical structure have exhibited excellent properties in many aspects such as magnetic, optical [4–6] and electric properties [7, 8]. However, little attentions have been paid to the research on catalytic properties of hollow spheres in the past years. Recently, the hollow spheres as catalysts have been applied to several liquid phase reactions [9–11]. For example, the PdCo bimetallic hollow spheres were successfully applied to catalysis of the Sonogashira Reaction .
In recent years, the CO catalytic oxidation has become an important research topic because of its various applications in pollution control devices for vehicle exhaust, CO gas sensors, and catalytic combustion, and so on [13–15]. Copper-contained catalysts show a potential activity for the CO oxidation and have been extensively investigated during the past decades [16–18]. It is well known that SiO2 is a good catalyst support and the performance of catalysts has a close relationship with the properties of supports [19–23]. Therefore, silica with hollow spherical structure was selected as supports of copper oxide (CuO) in present work, in order to study the influence of morphology of supports on catalytic activity.
Herein we used carbon spheres as templates  to fabricate silica hollow spheres by a succinct sol–gel path and researched the catalytic performance for CO + O2 reaction. The results indicated that catalytic activity of CuO supported on SiO2 hollow spheres exhibited much higher as compared to that supported on commercial SiO2. The possible reason of this phenomenon was also discussed.
2.1 Preparation of CuO/SiO2 Hollow Spheres Catalysts
For the sake of simplicity, the hollow spheres catalysts were signed as H-xCuSi (x = 2, 10, 20), which corresponded to 2 g CuO/100 g SiO2, 10 g CuO/100 g SiO2, and 20 g CuO/100 g SiO2, respectively. Similarly, the commercial SiO2 with different CuO loading amounts catalysts were signed as C-xCuSi.
The size and morphology of the products were observed by using a Hitachi Model H-800 transmission electron microscope (TEM), with a tungsten filament at an accelerating voltage of 200 kV. Nitrogen adsorption–desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2020 apparatus, and the pore size distribution was calculated from the nitrogen desorption isotherm by the Barrett–Joyner–Halenda (BJH) method. The phase analysis of the products was examined by X-ray diffraction (XRD) using a Philips X’pert Pro diffractometer with Ni-filtered CuKa radiation (0.15418 nm). The X-ray tube was operated at 40 kV and 40 mA.
Temperature-programmed reduction (TPR) was carried out in a quartz U-tube reactor, and a 100 mg sample was used for each measurement. Prior to the reduction, the sample was pretreated in a N2 stream at 100 °C for 1 h and then cooled to room temperature. After that, a H2–Ar mixture (7% H2 by volume) was switched on, and the temperature was increased linearly at a rate of 10 °C/min. A thermal conductivity cell detected the consumption of H2 in the reactant stream.
The activities of the catalysts for CO + O2 reaction were carried out under steady state, involving a feed stream with a fixed composition 1.6% CO, 20.8% O2 and 77.6% N2 by volume. A quartz tube was employed as the reactor and the requisite quantity of catalysts (25 mg for each test) were used. The catalysts were pretreated in N2 stream at 100 °C for 1 h and then heated to reaction temperature, after that, the mixed gases were switched on. The reactions were carried out at different temperatures with the same space velocity of 30,000 mL g−1 h−1. Two chromatogram columns and thermal conduction detection (TCDs) were used for the purpose of analyzing the production. Column A was packed with 13X molecular sieve (30–60 M) for separating O2, N2 and CO while column B packed with Porapak Q for monitoring CO2.
3 Results and Discussion
The turn-over numbers of CO on every copper ions per hour in different catalysts
CuO loading (g/100 g SiO2)
The hollow spheres SiO2 supports have been prepared by using carbonaceous spheres as templates in a simple sol–gel procedure.
The catalytic activity of CuO supported on hollow spherical SiO2 exhibited much higher than that supported on commercial SiO2 for CO + O2 reaction. The results indicate the effect of the copper species on the CO oxidation activity of the CuO/SiO2 catalysts has the following order: small CuO crystalline particles > larger CuO crystalline particles > highly dispersion copper species. The unique hollow spherical texture of supports should be in favor of the formation of the main active species and gas diffusion in catalysts at current condition.
The financial supports of the National Natural Science Foundation of China (No. 20573053), and the National Basic Research Program of China (Grant No. 2003CB615804) are gratefully acknowledged.