PNNL

Abstract

Nitric oxide chemistry and photochemistry on the Cr-terminated surface of a-Cr2O3(0001) was examined using temperature programmed desorption (TPD), sticking coefficient measurements and photodesorption. NO adsorbed at 100 K binds at surface Cr cation sites forming a strongly bound surface species that thermally desorbs at 320-340 K, depending on coverage. No thermal decomposition was detected in TPD in agreement with previous results in the literature. Sticking probability measurements of NO at 100 K indicated near unity sticking for NO coverages up to ~1.3 ML, with additional adsorption with higher exposures at decreased sticking probability. These results suggest that each surface Cr cation site on the a-Cr2O3(0001) surface was capable of binding more than one NO molecule, although it is unclear whether this was as separate NO molecules or as dimers. Photodesorption of adsorbed NO was examined for surface coverages below the 1 ML point. Both visible and UV light were shown to photodesorb NO without detectable NO photodecomposition. Visible light photodesorption of NO occurred with a cross section of ~2x10-19 cm2, which was estimated to greater than that obtained with UV light. The visible light photodesorption event was not associated with bandgap excitation in a-Cr2O3(0001), but instead was linked to excitation of a surface Cr3+-NO- charge transfer complex. These results illustrate that localized photoabsorption events at surface sites with unique optical properties (relative to the bulk) can result in unexpected surface photochemistry. 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 is operated by Battelle for the US Department of Energy.