23 research outputs found

    Surface Photochemistry of Adsorbed Nitrate: The Role of Adsorbed Water in the Formation of Reduced Nitrogen Species on α-Fe2O3 Particle Surfaces

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    The surface photochemistry of nitrate, formed from nitric acid adsorption, on hematite (α-Fe2O3) particle surfaces under different environmental conditions is investigated using X-ray photoelectron spectroscopy (XPS). Following exposure of α-Fe2O3 particle surfaces to gas-phase nitric acid, a peak in the N1s region is seen at 407.4 eV; this binding energy is indicative of adsorbed nitrate. Upon broadband irradiation with light (λ > 300 nm), the nitrate peak decreases in intensity as a result of a decrease in adsorbed nitrate on the surface. Concomitant with this decrease in the nitrate coverage, there is the appearance of two lower binding energy peaks in the N1s region at 401.7 and 400.3 eV, due to reduced nitrogen species. The formation as well as the stability of these reduced nitrogen species, identified as NO– and N–, are further investigated as a function of water vapor pressure. Additionally, irradiation of adsorbed nitrate on α-Fe2O3 generates three nitrogen gas-phase products including NO2, NO, and N2O. As shown here, different environmental conditions of water vapor pressure and the presence of molecular oxygen greatly influence the relative photoproduct distribution from nitrate surface photochemistry. The atmospheric implications of these results are discussed

    A Photoelectron Spectroscopy Study of Stoichiometric and Reduced Anatase TiO2 (101) Surfaces: The Effect of Subsurface Defects on Water Adsorption at Near-Ambient Pressures

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    X-ray photoelectron (XPS) experiments at normal and grazing emission are performed, demonstrating the labile nature of the anatase TiO<sub>2</sub>(101) surface after argon cluster ion sputtering and the propensity of oxygen vacancies to migrate subsurface at room temperature. Near-ambient XPS (NAP-XPS) shows that molecular water adsorbs on the anatase TiO<sub>2</sub>(101) surface at pressures of 0.6 mbar and above, at room temperature, in a mixed molecular and dissociated state. Water adsorbs in a similar fashion on both sputtered and stoichiometric surfaces and reaches a saturation point between 0.6 and 1.8 mbar at room temperature. This means there is little difference in reactivity with regards to water adsorption on both sputtered and stoichiometric surfaces, giving credence to the theory that anatase has superior photocatalytic activity over rutile due to the tendency of oxygen vacancies to lie subsurface, therefore being able to contribute to photocatalysis without being quenched by adsorbates
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