Abstract
Manganese catalysts containing templated mesostructured porous silica were prepared using different methods of preparation, namely, the direct hydrothermal (DHT), solid-state ion exchange (SSI), template ion exchange (TIE), and impregnation (Imp) methods. The physical-chemical properties of materials were characterized by X-ray diffraction (XRD), N2 adsorption-desorption, FT-IR, TEM, EDX, UV-Vis, EPR, and H2 TPR techniques. The results of this study indicate that the obtained catalysts retained their hexagonal mesopore structure after introducing Mn into MCM-41. On the contrary, the crystalline phase of manganese oxide was stabilized on the external surface and inside the mesoporosity of the MCM-41 and seems to be dependent on the synthesis method used. Catalytic performances of synthesized materials were then investigated in methane oxidation at atmospheric pressure. The results showed that the metal loading and catalysts synthesis procedure influence the catalytic performance of the obtained materials. Moreover, the activity of the catalyst depends on the crystalline phase and particularly on the environment of the active phase.
Highlights
Given the global energy consumption and the decline of fossil fuels such as petroleum and charcoal, interest has been focused on methane, the major component of natural gas because of large world reserves and low level of impurities [1]
The conventional flame combustion process of methane produces emissions that are harmful to the environment, namely, nitrogen oxides (NOx) and unburned hydrocarbons (UHC) which is a powerful greenhouse gas that contributes to global warming, it has a more powerful impact of about 25 times on the greenhouse effect than CO2 [2]. erefore, the use of a catalytic route has proved to be an alternative way to overcome this inconveniency and in order to increase the performance of the methane combustion, highly efficient and inexpensive catalysts are needed
E X-ray diffraction patterns at high angles exhibit a broad diffraction peak, centered approximately at 2θ 23°, which is due to amorphous silica. e XRD pattern of different catalysts showed no peak characteristic of manganese oxide which could be explained by a very high metal dispersion, except MnLUSSSI (Figure 1(b) (E)) which reveal three main diffraction peaks at 28°, 32°, and 36° that can be related to the formation of manganese oxides (MnOx), probably MnO2 and/or Mn2O3 (Bixbite, JCPDF 24-508). e observation of these phases agrees with other literature reports [31]
Summary
Given the global energy consumption and the decline of fossil fuels such as petroleum and charcoal, interest has been focused on methane, the major component of natural gas because of large world reserves and low level of impurities [1]. Hu et al [9] compared the performances of Mn-based catalysts on different supports (Al2O3, SiO2, and TiO2) for methane combustion, and manganese oxide supported on Al2O3 was found to be more active owing to the high rate of mobile oxygen in the support and the increase of acidity which enhances the catalytic activity. The metal content can influence the catalytic activity of materials in methane oxidation as was investigated by Laugel et al [15] who showed that Mn/SBA-15 catalyst with a high loading of manganese oxide (30 wt.%) leads to a decrease in activity compared with the catalysts at lower Mn loading (7 wt.%). Special attention will be paid to determine how the method of preparation of catalysts and the metal loading can affect the textural, structural, and catalytic properties of manganese containing mesoporous LUS silicatype MCM-41 for methane combustion
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