Active site for syngas production by direct partial oxidation of CH4 over ZrO2

Abstract

The production of high-value-added chemicals and their raw materials by partial oxidation of methane (POM) is advantageous. The screening of 31 simple oxide catalysts for direct POM showed that ZrO₂ had the highest syngas yield (CO and H₂) and was thus a promising catalyst. Kinetic analysis indicated that POM over the ZrO₂ catalyst proceeded in a Langmuir–Hinshelwood mechanism and that CH₄ activation was the rate-limiting step. Density functional theory calculations showed that CH₄ was activated on coordinatively unsaturated Zr⁴⁺ cations formed by the dehydration of the hydroxyl groups on the ZrO₂ surface. In situ diffuse reflectance infrared Fourier transform spectroscopy revealed that CH₄ was converted into CO and H₂ through CH₄-oxygenated intermediates, such as methoxy and formate species. The CH₄-oxygenated intermediates on the ZrO₂ catalyst were closely related to the catalytic performance of the oxide catalysts in POM. A comprehensive investigation of the POM reaction over ZrO₂-based catalysts was then conducted. ZrO₂ modification with tungsten oxide (WO_x) or lanthanum oxide (LaO_x) was examined to determine their ability to improve the catalytic properties of ZrO₂ for POM. ZrO₂ modification with WO_x and LaO_x enhanced its acidity and basicity, respectively. CO selectivity was increased by modifying ZrO₂ with a small amount of WO_x. Moreover, modification with LaO_x increased CH₄ conversion and H₂ yield at low temperatures.