Enhancing Selectivity of La─Ce─O Catalyst in Oxidative Coupling of Methane by Na2WO4 Doping

Modification of the La─Ce─O by doping with sodium tungstate resulted in a decrease in its specific surface area and an increase in particle size, which in turn led to a decrease in catalytic activity below 650 °C and a significant increase in selectivity to C₂. It seems that temperature and crystallite diameter play a key role in this process.

Abstract

La─Ce─O catalyst doped with 5, 10, and 15 wt% of Na₂WO₄ prepared by wet impregnation was characterised by XRD, N₂-sorption, Raman spectroscopy, TPD-CO₂, and SEM-EDX and tested in the oxidative coupling of methane (OCM) reaction between 400 and 800 °C. In the presence of La─Ce─O and doped with 5 wt% Na₂WO₄ catalysts, the reaction starts at 450 °C while the catalysts with 10 and 15 wt% Na₂WO₄ are inactive below 600 °C. Moreover, doping with Na₂WO₄ improves the selectivity to C₂ hydrocarbons (ethane and ethylene). Over the catalysts with 15 wt% Na₂WO₄ content 65% selectivity to C₂ hydrocarbons with a methane conversion of 25% were obtained at 650 °C (CH₄/O₂ = 3.8). Changes in the catalytic properties of the Na₂WO₄ doped La─Ce─O catalysts are accompanied by significant changes in their physicochemical properties. It has been proposed that the modification of the chemical and phase composition and blocking the strong basic sites are primarily responsible for improving the selectivity, while the activity is also influenced by the reduction of the surface area and increase in the particle size caused by the Na₂WO₄ doping.

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