Paper AS-ThP8
Impact of a Mixed Oxide’s Surface Composition and Structure on Its Adsorptive Properties: The Case of the α-(Fe,Cr)₂O₃(0001) Surface

Thursday, November 13, 2014, 6:00 pm, Room Hall D

Characterization of an α-(Fe_0.75,Cr_0.25)₂O₃(0001) mixed oxide single crystal surface was conducted using x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), low energy electron diffraction (LEED) and temperature programmed desorption (TPD). After sputter/anneal cleaning in ultra-high vacuum (UHV), the mixed oxide surface became terminated with a magnetite-(111) structure based on the presence of (2x2) spots in LEED and Fe²⁺ in XPS. The composition of the surface was close to that of M₃O₄ based on XPS, with the metal (M) content of Fe^2+/3+ and Cr³⁺ being close to 1.4:1, despite the fact that the film’s bulk was 3:1 with respect to the metal cations. The enrichment of the surface with Cr was not altered by high temperature oxidation in UHV, but could be returned to that of the bulk film composition by exposure to the ambient. Adsorption of various probe molecules (NO, O₂, CO₂ and H₂O) was used to identify the active cation sites present in the (Fe,Cr)₃O₄(111) terminated surface. Although XPS and SIMS both indicated that the near-surface region was enriched in Cr³⁺, no adsorption states typically associated with Cr³⁺ sites on a -Cr₂O₃ single crystal surfaces were detected. Instead, the TPD behaviors of O₂ and CO₂ pointed toward the main active sites being Fe²⁺ and Fe³⁺, with O₂ preferentially adsorbing at the former and CO₂ at the latter. NO was observed to bind at both Fe²⁺ and Fe³⁺ sites, and H₂O TPD looked nearly identical to that for H₂O on the Fe₃O₄(111) surface. Competition for adsorption sites between coadsorbed combinations of CO₂, O₂, H₂O and NO corroborated these assignments. These results indicate that the surface composition of a mixed oxide can vary significantly from its bulk composition depending on the treatment conditions. Even then, the surface composition does not necessarily provide direct insight into the active adsorption sites. In the case of the (Fe,Cr)₃O₄(111) termination of the α-(Fe_0.75,Cr_0.25)₂O₃(0001) surface, Cr³⁺ cations in the near-surface region appear to be fully coordinated and unavailable for adsorbing molecules.