Oxidative methane coupling over Mg, Al, Ca, Ba, Pb-promoted SrTiO3 and Sr2TiO4: Influence of surface composition and microstructure
Graphical abstract
Introduction
For about 30 years the scientists are looking for a way of how to commercialize of oxidative methane coupling reaction (OCM) and to produce ethane and ethylene directly from natural gas. Among the difficulties encountered are both to overcome high heat emissions at 750–900 °C and to find a highly selective catalyst [1]. In spite of the numerous studies, the known OCM catalysts do not reach the required 80% C2 selectivity and 30% methane conversion for a single pass and there still exists a crucial need for a selective catalyst.
It was shown that OCM was the heterogeneous–homogeneous reaction and the catalytic effect of OCM catalysts was to generate methyl radicals to the gas phase [2], [3], [4] and not to promote their heterogeneous oxidation on the surface [5], [6]. Thus, among the properties of the catalysts the most important are both low oxygen mobility and presence of surface oxygen ion-radicals [7], [8], [9], being the active sites for methyl radical generation. Therefore, simple, complex or mixed oxides of alkaline, alkaline-earth and rare-earth elements traditionally showed high activity in OCM [1], [10], [11], [12], [13], [14], [15]. Another active system, which was found by Fang et al. [16], was a combination of alkaline element with transition metal oxide, for example, W-Na-Mn/SiO2 [17], [18], [19], [20], [21], [22]. Some authors showed that mixture of the rear earth oxides or their modification with the noble metals resulted in high activity in OCM because of the synergetic effects [23], [24]. However, homogeneous reactions give rise to the limiting upper bound on the yield of C2 hydrocarbons corresponding approximately to 28% [25]. To reach this level active catalyst should not only effectively generate methyl radicals, but also promote their efficient recombination in the gas phase. Thus, the active catalysts should be multicomponent and include in their composition both an active phase (for methyl radical generation) and an inert phase (for methyl radical recombination).
The active component of the composite catalyst can be considered to be perovskite-like oxide systems (ABO3) because of appropriate structure, which allows us to create electronic defects and oxygen vacancies by doping in A and B sublattices and thus to influence the concentration of active surface sites. The second reason is that in perovskites individual oxides of Sr, Ba and La can easily segregate to the surface, thus increasing the concentration of oxygen ion-radicals and modifying the surface properties of perovskites. Moreover, perovskites possess both high thermal and chemical stability, which make them promising materials for design of composite catalysts. Recently Fakhroueian et al. have studied Ba0.5Sr0.5TiO3 substituted with Li, Na, Mg and have shown that doping increases the catalytic activity of titanates [26]. However, the reasons of this still remained unclear. In this work we attempted to determine what governs high activity of substituted titanates in OCM. The aim of this work was to synthesize Mg, Al, Ca, Ba, Pb substituted SrTiO3 and Sr2TiO4 samples and to identify how the phase composition and the microstructure of the samples influence catalytic activity.
Section snippets
Catalyst preparation
SrTi0.9A0.1O3 (A = Mg. Al, Ca, Ba, Pb) and Sr2Ti0.9B0.1O4 (B = Mg, Al) samples were prepared by mechanochemical method [27] using SrCO3, TiO2, MgO, BaCO3, CaO, Al2O3 (obtained by decomposition of Al(NO3)3·9H2O at 600 °C), PbO2 (obtained from decomposition of Pb(NO3)2 at 600 °C), all being of chemical pure grade or pure for analysis grade. Stoichiometric amounts of these compounds were mixed and intensively milled for 3 min in a planetary mill APF-5 under the following conditions: air atmosphere, steel
Phase composition
XRD analysis showed that Ca, Ba, Mg substituted SrTiO3 were two-phases samples that consisted of cubic perovskite Sr1−xAxTiO3 (x < 0.1) and tetragonal “layered” perovskite – (Sr1−xAx)3Ti2O7 (A = Ca, Ba) or (Sr1−xMgx)2TiO4 (x < 0.1) (Table 1). The quantity of the “layered” perovskite was varied in a range from 11 wt.% (Mg) to 42 wt.% (Ba). The lattice parameters and the unit cell volumes of the both phases were a little increased as compared to SrTiO3 indicating that substituting cations slightly
Conclusions
In this work we have demonstrated that Mg- and Al-doped SrTiO3 and Sr2TiO4 are active catalysts for OCM reaction providing up to 25% of C2 yield and C2 selectivity 66% in the mixture with the inert quartz particles. The optimal reaction processing conditions correspond to the CH4/O2 = 2, the temperature 850–900 °C and the ratio catalyst: quartz = 1:4. It is found that prepared using mechanochemical method (sintering at 1100 °C in air for 4 h) Mg, Ca, Ba doped SrTiO3 besides the perovskite phase Sr1−xAx
Acknowledgement
The reported study was supported by RFBR, research project No. 12-03-31608 mol_a.
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