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 (WOx) or lanthanum oxide (LaOx) was examined to determine their ability to improve the catalytic properties of ZrO2 for POM. ZrO2 modification with WOx and LaOx enhanced its acidity and basicity, respectively. CO selectivity was increased by modifying ZrO2 with a small amount of WOx. Moreover, modification with LaOx increased CH4 conversion and H2 yield at low temperatures.