Abstract
Cobalt oxide nanomaterials show high activity in several catalytic reactions thereby offering the potential to replace noble metals in some applications. We have developed a well-defined model system for partially reduced cobalt oxide materials aiming at a molecular level understanding of cobalt-oxide-based catalysis. Starting from a well-ordered Co3O4(111) film on Ir(100), we modified the surface by deposition of metallic cobalt. Growth, structure, and adsorption properties of the cobalt-modified surface were investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and infrared reflection absorption spectroscopy (IRAS) using CO as a probe molecule. The deposition of a submonolayer of cobalt at 300 K leads to the formation of atomically dispersed cobalt ions distorting the surface layer of the Co3O4 film. Upon annealing to 500 K the Co ions are incorporated into the surface layer forming ordered two-dimensional CoO islands on the Co3O4 grains. At 700 K, Co ions diffuse from the CoO islands into the bulk and the ordered Co3O4(111) surface is restored. Deposition of larger amounts of Co at 300 K leads to formation of metallic Co aggregates on the dispersed cobalt phase. The metallic particles sinter at 500 K and diffuse into the bulk at 700 K. Depending on the degree of bulk reduction, extended Co3O4 grains switch to the CoO(111) structure. All above structures show characteristic CO adsorption behavior and can therefore be identified by IR spectroscopy of adsorbed CO.
Highlights
Egerlandstrasse 3, 91058 Erlangen, Germany or the Fischer–Tropsch reaction.[30,31] Noteworthy, in several of these applications cobalt oxide may replace noble metals such as platinum or rhodium, thereby providing a low-cost alternative to noble metal catalysts.In various respects the adsorption behavior and reactivity of nanostructured cobalt and cobalt oxide catalysts is quite different from its noble metal counterparts
The infrared reflection absorption spectroscopy (IRAS) experiment were performed in a ultrahigh vacuum (UHV) system with a base pressure of p = 2 Â 10À10 mbar, containing all necessary preparation characterization methods (LEED, Auger Electron spectroscopy, thermal desorption spectroscopy)
The CoO(111) texture appears in scanning tunneling microscopy (STM) as crystallites protruding from the average Co3O4 surface level as indicated
Summary
In various respects the adsorption behavior and reactivity of nanostructured cobalt and cobalt oxide catalysts is quite different from its noble metal counterparts. A feature that is rather unique for cobalt oxide is its outstanding structure dependency.[4] For the case of CO oxidation, it has early been recognized that the Co3O4 shows very high activity even at low temperature.[12] Haruta and coworkers demonstrated outstanding activity of Co3O4 nanorods for CO oxidation at temperatures as low as À77 1C.1 These nanorods expose a large fraction of (110) facets, and on the basis of theoretical calculations, different mechanisms have been proposed for the activation of lattice oxygen.[1,16,32] These pathways involve either threefold or two-fold coordinated oxygen, a species that may be present at the bulkterminated (110) surface. These model systems will be used to study the interaction and the catalytic conversion of hydrocarbons and hydrocarbon oxygenates
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