Abstract
<p>Reflectance spectroscopic methods the electronic, redox and structural properties of Mn-Na-W/SiO<sub>2</sub> catalysts prepared by the incipient wetness impregnation method and mixture slurry method were studied in detail. Since POSS nanotechnology (POSS = polyhedral oligomeric silsesquioxanes) has attracted attention as tooling for synthesis of catalysts with novel properties and functionalities, we expanded this method for the preparation of Mn-Na-W/SiO<sub>2</sub> catalyst. The physicochemical and catalytic properties of Mn-Na-W/SiO<sub>2</sub> catalysts prepared by conventional methods and POSS nanotechnology were examined comparatively. In all studied Mn-Na-W/SiO<sub>2</sub> catalysts both individual oxides (MnO<sub>x</sub>, WO<sub>3</sub>) and bimetal oxide phases (Na<sub>2</sub>WO<sub>4</sub>, MnWO<sub>4</sub>) are found in addition to oxide particles of high dispersion. The UV-Vis Diffuse Reflectance indicates that Na<sup>+</sup> cations facilitates stabilization of octahedrally coordinated Mn<sup>3+</sup><sub>Oh</sub> cations in the isolated state, while Mn<sup>3+</sup><sub>Oh</sub> promote the disordering of W<sup>6+</sup> cations in the supported system. The Mn-Na-W/SiO<sub>2</sub> prepared using metal-POSS precursors marks out presence of unglobular SiO<sub>2</sub> particles, higher dispersion of MnO<sub>x</sub> and MnWO<sub>4</sub> particles and more easily reducible metal-oxide species. The catalysts prepared by incipient impregnation method and mixture slurry method have practically similar catalytic performance while the catalyst prepared by POSS nanotechnology method shows lower activity and selectivity. At 800‒850 °C the increase of C<sub>2</sub> hydrocarbons yield from 4 to 15% and the rise of molar ratio С<sub>2</sub>Н<sub>4</sub>/C<sub>2</sub>H<sub>6</sub> from 0.2 to 1 are observed when impregnation or mixture slurry method are used for catalyst preparation instead of POSS nanotechnology method.</p>
Highlights
Development of effective catalysts for conversion of methane to valuable products with high selectivity will solve such problems as rational utilization of natural gas and environmental protection [1,2,3,4,5,6,7,8,9,10]
Using XPS, BET, X-ray diffraction (XRD), TG-DTA, HRTEM-energy dispersive X-ray analysis (EDX), Temperature-programmed reduction (TPR) and UV-Vis Diffuse Reflectance spectroscopic methods we thoroughly studied electronic, redox and structural properties of Mn-Na-W/ SiO2 catalysts prepared by the incipient wetness impregnation method and the mixture slurry method
The chemical composition of the prepared catalysts was determined by X-ray fluorescence spectroscopy (XFS) using an ARL analyzer with a Rh anode of an X-ray tube and inductively coupled plasma-atomic emission spectrometry (ICP-AES) method
Summary
Development of effective catalysts for conversion of methane to valuable products with high selectivity will solve such problems as rational utilization of natural gas and environmental protection [1,2,3,4,5,6,7,8,9,10]. The exploitation of the so-called “POSS nanotechnology” (POSS = polyhedral oligomeric silsesquioxanes) in which metal-containing silsesquioxanes are used as precursors for the preparation of catalytic materials provides a peculiar way to obtain nanosized catalysts [29, 30]. This method of preparing heterogeneous metal-containing materials has distinct advantage over traditional methods of preparing metal impregnated siliceous materials in that the metal atoms remain highly dispersed throughout support [29,30,31,32,33,34,35]. The peculiarity of the genesis of Mn-Na-W/SiO2 materials prepared by different methods was revealed
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