Abstract
The normal incidence X-ray standing wave (NIXSW) technique has been used to follow the evolution of the adsorption geometry of Ni adatoms on the Fe3O4(001)-(√2 × √2)R45° surface as a function of temperature. Two primary surface region sites are identified: a bulk-continuation tetrahedral site and a sub-surface octahedral site, the latter site being preferred at higher annealing temperatures. The ease of incorporation is linked to the presence of subsurface cation vacancies in the (√2 × √2)R45° reconstruction and is consistent with the preference for octahedral coordination observed in the spinel compound NiFe2O4.
Highlights
Maximising the surface area of a catalyst is one of the most common methods of enhancing its catalytic activity
Soft X-ray photoelectron spectroscopy Normalised Ni 2p3/2 soft X-ray photoelectron (SXP) spectra from the first (0.2 ML, 300 K/room temperature), and third (0.5 ML, 875 K) samples are shown in Fig. 2
A satellite feature, at even higher binding energy, is observed. These two chemically distinct Ni species are assigned, as in the work of Bliem et al.,[15] to Ni adatoms, Niad, that are proposed to occupy tetrahedral sites above the surface and incorporated Ni atoms, Nisub.[15]. Note that these assignments were based on a comparison of STM images and XP spectra, but are confirmed by the normal incidence X-ray standing wave (NIXSW) data presented here
Summary
Maximising the surface area of a catalyst is one of the most common methods of enhancing its catalytic activity. The term ‘‘support material’’ becomes a misnomer for many reducible metal oxides, most famously in the case of supported Au nanoparticles,[1,2,3] for which it is the interface between the nano-particles and the oxide surface that is the active site for CO oxidation Such effects, termed ‘‘active supports’’ are widely reported in the literature,[4] and are very common with iron oxide supports, often via Mars–van Krevelen (MvK) type reactions.[5,6,7]. Wustite (FeO) almost never achieves stoichiometric FeO, with compositions of Fe(1Àx)O being common in natural crystals,[10] while the relative ease of transition between the two spinel iron oxides maghemite (g-Fe2O3) and magnetite (Fe3O4) is a direct consequence of the facile creation and mobility of cation defects within their bulks. The origin of the active support behaviour of the iron oxides is ascribed to the ease of cation diffusion, both towards and away from their surfaces, in response to surface redox reactions.[11]
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