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18: Iron oxide model catalysts: preparation & characterization
要旨:
The surface structures formed by annealing of the potassium overlayer on the Fe3O4(111) surface were
investigated by low electron energy diffraction (LEED), Auger electron spectroscopy (AES) and scanning
tunneling microscopy (STM). Annealing at 600-700°C in vacuum or 10-6 mbar of oxygen resulted in
well-ordered surfaces depending on the amount of potassium pre-deposited. As the K coverage
increased, it transformed gradually from a (4x4) to a (2x2) and then to a (1x1) structure relative to the
original Fe3O4(111)-(1x1) surface. At low coverage, the (4x4) structure was formed by a long-range
modulation of the surface with an ~24 Å periodicity which exhibited an internal 6 Å periodicity
characteristic for the Fe3O4(111)-(1x1) surface. At mid-coverage, two sorts of (2x2) domains were
observed, which were distinguished by the different diameter of protrusions forming a STM image. They
were attributed to the different states of potassium in top layer. These domains coexisted on the surface
and were found at both oxidative and vacuum preparations. At high K coverage, the surface exhibited a
(1x1) structure with a high density of vacancy defects. Auger depth-profile measurements confirmed the
diffusion of potassium into iron oxide bulk at elevated temperatures. The formation of a
non-stoichiometric K2O/K2Fe22O34/Fe3O4(111) interface with a gradual decreasing of potassium
concentration from the surface has been suggeste
