Surface activation of ordered mesoporous silica materials by cobalt and rhodium

Abstract

Three different types of ordered mesoporous silica materials, MCM-41, aluminum incorporated into the silica framework MCM-41 (Al-MCM-41) and SBA-15, were prepared. Apart from elemental composition, they differed in textural and structural properties. Furthermore, aluminum was grafted on the silica surfaces by Molecular Designed Dispersion (MDD) method. Finally, the transition metal (cobalt or rhodium) was applied for the surface activation of the silica supports by MDD technique. Much higher initial concentration of cobalt acetylacetonate complex (3.2 mmolCo(acac)2.gsupport-1) was applied than the concentrations of rhodium acetylacetonate complex (0.2–0.01 mmolRh(acac)3. gsupport-1). The prepared catalysts were characterized by different techniques such as AAS, EPMA, EDX, TGA, N2 physisorption, SEM, XRD, IR and Raman spectroscopy, DR UV-Vis spectroscopy, XPS, TPD-NH3, TPR-H2 and pulse chemisorption of H2 and their catalytic activities were tested for two reactions of environmental application: N2O decomposition (in inert and real conditions and in the presence of the reducing agent) and CO oxidation at low temperatures (below 450 °C). The Co-grafted catalysts showed poor activity (14 %) in catalytic N2O decomposition in the inert condition at 450°C and at GHSV 15648 h-1. This was in agreement with TPR-H2 study as Co-grafted catalysts showed very low hydrogen consumption within the temperature range of catalytic tests of N2O decomposition. The carbon monoxide as the reducing agent can facilitate the reduction of cobalt which was confirmed as the N2O conversion increased up to 26 % at 450°C (at GHSV 15648 h-1). The non-specified oxide CoxOy was detected by DR UV-vis and Raman spectroscopy as well as cobalt spinel phase by XRD analysis in the sample Al-MCM+Co. This catalyst contained both cobalt ions (Co2+ as well as Co3+) as determined by TPR-H2 experiments which seems to be beneficial for the catalytic activity of CO oxidation. On the other hand, a specific ration between Co2+ and Co3+ ions is not definitely required as the most active catalyst in the reaction of CO oxidation was SBA+Co containing tetrahedral Co2+ ions interacting with the support as determined by DR UV-vis analysis. More active sites were distributed on SBA+Co per unit surface area than on other Co-grafted catalysts by expressing the catalytic activity as converted CO molecules per unit BET surface area. Therefore, it is expected that the most important for the catalytic activity is better and higher distribution of cobalt species on the silica support. In the case of Rh-grafted catalysts, the activity of the catalysts with the same initial rhodium loading (0.2 mmolRh(acac)3.gsupport-1) in the reaction of N2O decomposition under the inert and real conditions (the presence of O2, water vapor and NO) was found in the order: Al-MCM+Rh < MCM+Al+Rh < SBA+Rh ≈ MCM+Rh. It was proven experimentally (by TPR-H2 and pulse chemisorption of H2) that aluminum incorporated into the structure of MCM-41 enhanced rhodium dispersion on the support which was beneficial for its catalytic activity in the reaction of N2O decomposition. Nearly the same catalytic activities of MCM+Rh(2.7 %) and SBA+Rh(2.7 %) supports the fact that differences in mesoporous structures of MCM-41 and SBA-15 did not play the key role in the reaction of N2O decomposition within the applied amount of rhodium and experimental conditions of N2O decomposition.