Surface stabilization of organics on hematite by conversion from terminal to bridging adsorption structures

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-Fe 2O 3(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 Fe 3+ and O 2− surface sites and a (2×1) reconstructed structure with unknown atomic structure. Whereas the (1×1) surface is essentially free of Fe 2+, the (2×1) surface possesses a high surface concentration of Fe 2+ 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 Fe 3+ site followed by transfer of the alcohol proton to a bridging O 2− surface site, resulting in terminal OCH 3 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 OCH 3 and bridging OH groups interchange as the surface is heated above room temperature (RT), resulting in bridging OCH 3 and terminal OH groups. The bridging OCH 3 groups are retained on the surface to higher temperature than the terminal OCH 3 groups, but eventually decompose at about 550 K via a disproportionation reaction that forms gaseous CH 3OH and H 2CO. As a result of the disproportionation reaction, some surface Fe 3+ sites are reduced to Fe 2+ sites. The exchange process competes more successfully with recombinative desorption of methanol (from reaction of terminal OCH 3 and bridging OH groups) on the (2×1) surface, despite the fact that this surface is already partially reduced, because terminal OCH 3 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.