AVS 52nd International Symposium
    Surface Science Monday Sessions
       Session SS2-MoA

Paper SS2-MoA7
Methyl Radical Chemistry on Clean and UO@sub3@-Covered Single Crystal Hematite Surfaces

Monday, October 31, 2005, 4:00 pm, Room 203

Session: Oxide Surfaces Structure and Reactivity
Presenter: L. Liu, Columbia University
Authors: L. Liu, Columbia University
P.C. Stair, Northwestern University
Correspondent: Click to Email
The partial oxidation of methane over oxide catalysts is controlled by the surface reaction of methyl radicals. Methyl radical chemistry was studied on two model surfaces in UHV: Clean, (0001) orientation single crystal hematite and UO@sub 3@ supported on a crystalline hematite thin film. Temperature Programmed Desorption (TPD) showed that methyl radicals adsorb on the Fe@sub 3@O@sub 4@ (111)-terminated hematite (0001) surface at 300 K and desorb intact at higher temperatures. At saturation coverage, the XPS C(1s) line position is consistent with methoxide ions on the surface, and the carbon-surface bond energy determined by threshold TPD analysis is similar to the C-O bond energy of surface methoxide ions. In contrast, methyl radicals produce only very small desorption features on the biphase-terminated hematite (0001) surface. The coverage of adsorbed methyl was obtained by quantification of the TPD data. Saturation coverage was 1x10@super 14@/cm@super 2@ on the Fe@sub 3@O@sub 4@ (111)-terminated surface but only 3.7x10@super 12@/cm@super 2@ on the biphase-terminated surface, consistent with adsorption on regular surface sites and defect sites, respectively. Since the two surfaces both contain Fe@super 2+@, Fe@super 3+@, and O the differences in methyl radical adsorption must be due to differences in surface structure. Based on an analysis of the structures reported in the literature it is proposed that methyl radicals adsorb on surface oxygen atoms with a dangling bond perpendicular to the surface plane on the Fe@sub 3@O@sub 4@ (111)-terminated surface. On the hematite-supported UO@sub 3@ surface, partial oxidation products, such as methanol, formaldehyde, and CO were identified by TPD. XPS quantification indicates that UO@sub 3@ forms a monolayer structure on the hematite support. The increased reducibility of UO@sub 3@ compared to hematite is responsible for the change in surface chemistry. A surface methoxide ion is the proposed reaction intermediate.