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Title: Impact of a Mixed Oxide’s Surface Composition and Structure on Its Adsorptive Properties: Case of the (Fe,Cr) 3O 4(111) Termination of the α-(Fe,Cr) 2O 3(0001) Surface

Characterization of an α-(Fe 0.75,Cr 0.25) 2O 3(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 Fe 2+ in XPS. The composition of the surface was close to that of M 3O 4 based on XPS, with the metal (M) content of Fe 2+/3+ and Cr 3+ 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, O 2, CO 2 and H 2O) was used to identify the active cation sites present in the (Fe,Cr) 3O 4(111) terminated surface. Although XPS and SIMS both indicated that the near-surface region was enriched in Cr 3+, no adsorption states typically associated with Cr 3+ sites on α-Cr 2Omore » 3 single crystal surfaces were detected. Instead, the TPD behaviors of O 2 and CO 2 pointed toward the main active sites being Fe 2+ and Fe 3+, with O 2 preferentially adsorbing at the former and CO 2 at the latter. NO was observed to bind at both Fe 2+ and Fe 3+ sites, and H 2O TPD looked nearly identical to that for H 2O on the Fe 3O 4(111) surface. Competition for adsorption sites between coadsorbed combinations of CO 2, O 2, H 2O 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) 3O 4(111) termination of the α-(Fe 0.75,Cr 0.25) 2O 3(0001) surface, Cr 3+ cations in the near-surface region appear to be fully coordinated and unavailable for adsorbing molecules. The authors thank Drs. Sara Chamberlin and Scott Chambers for supplying the film used in this work. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.« less
Authors:
;
Publication Date:
OSTI Identifier:
1173036
Report Number(s):
PNNL-SA-102190
48144; KC0304030
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry C, 118(50):29058-29067
Research Org:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
X-RAY PHOTOELECTRON SPECTROSCOPY; ELECTRON DIFFRACTION; ION MICROPROBE ANALYSIS; MASS SPECTROSCOPY; ADSORPTION; Oxide surface; XPS; TPD; LEED; SIMS; adsorption; Environmental Molecular Sciences Laboratory