(20f) Catalysis of the Oxidative Coupling of Methane By La2O3-CeO2 Nanofibers: Effects of Fiber Diameter and La/Ce Ratio

Direct conversion of methane into useful products, such as olefins (e.g. ethylene), has been a highly desirable goal in industrial catalysis for decades. However, in spite of significant global efforts, no major commercial catalyst has been developed. An important common feature of all earlier catalysts researched for the oxidative coupling of methane (OCM) is that they were based on quasi-spherical nanoparticles (powders), and thus were prone to metal dispersion, agglomeration, sintering and pore diffusion problems [1,2]. Recently, we reported the development of a novel fabric catalyst, comprising ~50 nm diameter La₂O₃-CeO₂ nanofibers, that is capable of ameliorating these problems. Initial OCM results with these fabric catalysts exhibited a CH₄ conversion of about 25% with a 70% C₂₊ product selectivity for a La/Ce ratio of 15/1 [3]. In this presentation, we will report on the effects of nanofiber diameters and composition (i.e. La/Ce ratio) on C₂₊ product selectivity and methane conversion. Nanofibers of bimetallic La₂O₃-CeO₂ are prepared by electrospinning homogeneous solutions of the nitrate salts of La and Ce in polyvinyl pyrollidone (PVP) and H₂O and EtOH as solvents (EtOH/H₂O ratios ranging between about 2:1 and 1:1). Nanofiber diameters in the 20-150 nm range are obtained by varying the solution PVP concentration (3-12 wt%), applied voltage (30-50 kV) and the distance between the solution source and the collector plate (30-60 cm). Both C₂₊ selectivity and CH₄ conversion are found to increase as the fiber diameters decreased. The La/Ce ratios of the fabrics are varied by simply changing only the metal salt ratio in the electrospinning solution. Addition of CeO₂ offers a small advantage over a pure La₂O₃fabric in OCM performance, peaking at a La/Ce ratio of about 3/1, while performance rapidly deteriorates when the ratio exceeds 1/1.

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