Influence of Adsorption Site and Wavelength on the Photodesorption of NO from the (Fe,Cr)3O4(111) Mixed Oxide Surface

The chemical and photochemical properties of a mixed oxide single crystal surface were examined in ultrahigh vacuum (UHV) with temperature-programmed desorption (TPD), photon stimulated desorption (PSD), and low energy electron diffraction (LEED) using nitric oxide as a probe molecule. A mixed Fe and Cr corundum oxide film with a (0001) orientation prepared on an alpha-Al2O3(0001) crystal was employed. After sputter/anneal cleaning, the films surface was transformed into a magnetite-like (111) termination based on LEED images which revealed a (2 x 2) surface periodicity. The sputter/anneal surface, designated as (Fe,Cr)(3)O-4(111), also showed faint (root 3 x root 3)R30 degrees LEED spots suggestive of the presence of a minority termination or of a poorly order surface structure within the (2 x 2) surface phase. TPD of NO from the (Fe,Cr)(3)O-4(111) surface revealed three chemisorbed states at 220, similar to 315, and 370 K tentatively assigned to NO binding at Fe3+, Cr3+, and Fe2+ sites, respectively. The relative populations of these sites followed the trend: Fe2+ > Fe3+ >> Cr3+. No significant thermal chemistry of NO was detected. Photodesorption was the primary photochemical pathway for NO on the (Fe,Cr)(3)O-4(111) surface in UHV. The NO photodesorption rate was sensitive to the adsorption site, following the trend: Fe3+ > Fe2+ > Cr3+. Multiexponential rate behavior seen in the overall NO PSD spectra was shown to be due to site heterogeneity (i.e., a convolution of the individual rates at the three types of surface sites) and not due to carrier-dependent or charge trapping effects. The photodesorption rate with UV light (365 nm) was similar to 10x greater than that in the visible, and the per photon rates in the visible (between 460 and 630 nm) were essentially independent of the wavelength. Results in this study illustrate the importance of identifying adsorption sites in interpreting photochemical events on the surfaces of mixed oxide photocatalysts.