PNNL

Abstract

Characterization of an a-(Fe0.75,Cr0.25)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 Fe2+/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 ?-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 a-(Fe0.75,Cr0.25)2O3(0001) surface, Cr3+ 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.