Methyl radical chemistry on single crystal hematite surfaces and supported on single crystal hematite.

摘要

The conversion of methane to its partially oxidized products, such as methanol and formaldehyde, remains one of the most challenging processes in heterogeneous catalysis. Direct transformation of methane in these products is thermodynamically favored, but hindered by the selectivity. There has been an increase in evidence suggesting that the selectivity of this conversion is controlled by the surface reaction of methyl radicals. However, there has been no report of studying a methyl radical surface reaction on metal oxide surfaces using ultrahigh vacuum (UHV) techniques, even though the partial oxidation of methane conversion is carried out on metal oxide based catalysts. In this research, methyl radical chemistry was studied on two model samples in an UHV chamber. One was a single crystal hematite mineral sample oriented along the (0001) direction. The other was UO3 supported on a crystalline thin film of hematite. TPD spectra showed that methyl radicals adsorbed on the Fe3O4 (111)-terminated hematite (0001) surface and desorbed at higher temperatures. At the saturated methyl radical coverage, the XPS C(1s) line position indicated the formation of methoxide ions on the surface, and the carbon-surface bond strength calculated by the threshold TPD analysis agreed with the carbon-oxygen bond strength of surface methoxide ions. On the other hand, methyl radicals displayed tiny desorption features on the biphase-terminated hematite (0001) surface.; A TPD spectrum quantification technique was developed in this study. This quantification proved to be very important not only for probing surface adsorption, desorption, and reaction mechanisms, but also for relating the surface reactivity to the surface structure. The quantification results showed that methyl radicals adsorbed on the regular surface sites of the Fe 3O4 (111)-terminated surface, but on the defect sites of the biphase terminated surface. Based on the surface structure differences between these two surfaces, it is proposed that methyl radicals adsorbed on surface oxygen atoms with a dangling bond perpendicular to the surface plane.; The methyl radical sticking probability on the Fe3O4 (111)-terminated surface was measured at various coverages. The sticking probability-coverage profile follows a linear form, which suggests that methyl radical adsorption is governed by a site-blocking mechanism.; On the hematite-supported UO3 surface, partial oxidation products, such as methanol, formaldehyde, and CO were identified by TPD as the methyl radical surface reaction products. XPS quantification results indicated that UO3 formed a monolayer structure on the hematite support. A surface methoxide ion was the proposed reaction intermediate.