Ordered mesoporous Co 3 O 4 as highly active catalyst for low temperature CO-oxidation w

Since the introduction of the nanocasting (hard templating) pathway, increasing efforts have been devoted to the preparation of ordered mesoporous metal oxides such as Co3O4, 2–4 Cr2O3, 5,6 CeO2, 7 MgO, Fe3O4 9 and ferrihydrite. These materials have high surface area compared to bulk materials, and therefore can be used as efficient catalyst supports and as catalysts themselves. Low temperature CO oxidation is very important in many applications including air purification and pollution control devices, automotive emission control, gas purification of the hydrogen to feed PEM fuel cells, closedcycle CO2 lasers and CO gas sensors. 11 Although the most often studied materials for low temperature CO oxidation are gold based catalysts several studies have also used Co3O4 (bulk or supported) which shows a very high activity for this reaction as well. Depending on the preparation method for the materials and the reaction conditions, Co3O4 shows different activity in CO oxidation. Recently Wang et al. reported catalytic activity of a series of Co3O4 samples, 20 and they showed that CO conversion of the samples can reach 100% at ambient temperature and even below, using a gas mixture consisting of 0.5 vol% CO, 14.4 vol% O2, and 85.1 vol% N2 with a total flow rate of 20 mL min , corresponding to a space velocity of 4000 mL gcat 1 h . Deactivation was observed to be severe, and reliable activities of the catalysts could not be extracted from the data, since the experiments were carried out at full conversion, and the deactivation patterns did not show consistent trends. Haruta’s group has reported light-off temperatures (temperature of 50% conversion, T50) as low as 54 1C under conditions close to the ones used in our study, but only if the reaction gas was meticulously dried. At normal operation, the T50 was around 40 1C. Since the nanocasting pathway allows the synthesis of highly defined pore systems and the generation of high surface areas, we studied the catalytic activity of nanocast Co3O4 in CO-oxidation and the dependence of the catalytic activity on the porosity of the samples. Ordered mesoporous Co3O4 with different textural parameters was prepared via the nanocasting pathway. The catalytic performance of these materials was in the same range as that of the best reported materials with respect to conversion at room temperature, but a more precise comparison is difficult due to the different conditions used in previous publications. Cubic ordered mesoporous silica (KIT-6) was synthesized according to the literature. The pore size of the KIT-6 was varied by changing the aging temperature (40, 100 and 135 1C). With increasing aging temperature during the hydrothermal process, pore size and pore volume of the synthesized KIT-6 increase while silica wall thickness decreases. KIT-6 was used as a hard template to fabricate ordered mesoporous Co3O4. Briefly, 0.5 g of KIT-6 was dispersed in 5 ml of 0.8 M Co(NO3)2 6H2O in ethanol and stirred for 1 h at room temperature, followed by evaporation of the ethanol at 50 1C. The composite was calcined at 200 1C for 6 h. The material was re-impregnated again, followed by calcination at 450 1C for 6 h. The silica template was then removed by leaching with 2 M NaOH aqueous solution. Finally, the resulting Co3O4 was washed several times with water and then dried at 50 1C. All samples were characterized by nitrogensorption, X-ray diffraction, (XRD), transmission electron microscopy (TEM) and high resolution scanning electron microscopy (HR-SEM). The activities of the catalysts for CO oxidation were measured in a plug flow reactor using 200 mg of catalyst (250–500 mm size fraction) in a gas mixture of 1 vol% CO in air (Air Liquide, 99.997% purity) at a flow rate of 60 mL min , corresponding to a space velocity of 18 000 mL gcat 1 h . Temperatures during these tests were ramped at 2 1C min 1 while CO conversion was recorded. Control experiments proved that this transient operation gives the same activity as a steady state measurement of activity. For clarification, the samples were labelled as Co3O4-T, with T representing the aging temperature of the hard template. The structure and porosity of nanocast Co3O4 strongly depend on the parameters of the nanocasting process, and the results of a detailed study will be reported elsewhere, since this would exceed the scope of this contribution. In summary, Co3O4-40 which had been fabricated from KIT-6 aged at low temperature has uncoupled sub-frameworks while Co3O4-100 and Co3O4-135 have a coupled framework. The parent material aged at higher temperature contains a high fraction of micropores connecting the two mesopore systems. Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr, 45470, Germany. E-mail: schueth@mpimuelheim.mpg.de; Fax: +49 208 306 2995; Tel: +49 208 306 2373 w Electronic supplementary information (ESI) available: Fig. S1. Deactivation plot for Co3O4-40 catalyst. Fig. S2, XPS spectrum for Co3O4-100 before and after catalytic test. See DOI: 10.1039/b808815b