Surface stabilization of organics on hematite by conversion from terminal to bridging adsorption structures |
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| Authors: | Michael A Henderson |
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| Institution: | 1 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-93, Richland, WA 99352, USA |
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| Abstract: | Insight into the complexation of organic molecules on hematite surfaces was obtained from molecular-level studies of a simple probe molecule (methanol) with the R-cut surface of hematite. The R-cut crystal orientation of hematite, designated in this paper as α-Fe2O3(012), has two stable surface structures under ultrahigh vacuum (UHV) conditions based on low-energy electron diffraction (LEED) measurements. These are a (1×1) structure consisting of a bulk terminated arrangement of undercoordinated Fe3+ and O2− surface sites and a (2×1) reconstructed structure with unknown atomic structure. Whereas the (1×1) surface is essentially free of Fe2+, the (2×1) surface possesses a high surface concentration of Fe2+ sites based on electronic structure measurements using electron energy loss spectroscopy (EELS). Methanol adsorbs dissociatively on the (1×1) surface by coordination of the molecule’s oxygen atom at a Fe3+ site followed by transfer of the alcohol proton to a bridging O2− surface site, resulting in terminal OCH3 and bridging OH groups. Most of the dissociated methanol molecules recombine during heating and desorb in vacuum as methanol at 365 and 415 K for the (1×1) and (2×1) surfaces, respectively. However, a significant amount of the terminal OCH3 and bridging OH groups interchange as the surface is heated above room temperature (RT), resulting in bridging OCH3 and terminal OH groups. The bridging OCH3 groups are retained on the surface to higher temperature than the terminal OCH3 groups, but eventually decompose at about 550 K via a disproportionation reaction that forms gaseous CH3OH and H2CO. As a result of the disproportionation reaction, some surface Fe3+ sites are reduced to Fe2+ sites. The exchange process competes more successfully with recombinative desorption of methanol (from reaction of terminal OCH3 and bridging OH groups) on the (2×1) surface, despite the fact that this surface is already partially reduced, because terminal OCH3 groups are more stable on this surface than on the (1×1) surface. Based on these molecular-level findings, extensive exchange terminal organic ligands and bridging OH groups may play a significant role in stabilizing organics on hematite mineral surfaces. Such exchange processes may also play a role in destabilizing hematite surfaces toward reductive dissolution. |
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