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

Abstract

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.

Heterogeneous reactions of mineral dust aerosol: implications for tropospheric oxidation capacity

“…In addition, photolysis of materials (such as nitrate) formed on mineral surface can also be sources for some trace gases (Nanayakkara et al, 2013(Nanayakkara et al, , 2014Gankanda and Grassian, 2014). Although the effects of photoradiation in heterogeneous reactions with mineral dust have been recognized for more than 1 decade, it largely remains unclear to which extent these reactions are photoenhanced under ambient solar radiation, and thus quantitative evaluation of impacts of heterogeneous photochemistry on tropospheric oxidation capacity is lacking.…”

308nm photochemical reaction of gaseous HNO3 and benzene on α-Fe2O3 surfaces

“…The gaseous HNO 3 irreversibly adsorbed onto the a-Fe 2 O 3 surfaces to form adsorbed NO 3 À and H + [4] and restricted to the reaction site on particle surfaces. a-Fe 2 O 3 was a semiconductor material with a lower band gap of 2.2 eV and it can absorb UV light of l < 530 nm [33].…”

“…When 308 nm UV irradiation, NO 2 and OH were formed from photolysis of adsorbed HNO 3 and then p-NP was also produced. The following reactions likely occur on a-Fe 2 O 3 surface during adsorption, where (g) and (a) denoted gas phase and adsorbed phase, respectively [4,34,19].…”

“…Hematite (a-Fe 2 O 3 ) is the most thermodynamically stable iron oxide phase at ambient temperature and a semiconductor material with a lower band gap of 2.2 eV that can absorb solar radiation [3]. It can provide reaction beds for atmospheric chemical reactions of gaseous materials or can actively precipitate atmospheric chemical reactions, thereby directly influencing the transmission and transformation of gaseous pollutants [4].…”

“…In recent years, the heterogeneous reactions and photochemistry of NO X in the gas phase and on various surfaces has been extensively studied [1,[3][4][5]. Miet et al [10] reported the reactivity of pyrene and 1-nitropyrene with NO 2 adsorbed on the silica particles using a fast-flow-tube in the absence of light and observed a linear dependence of pseudofirst order rate constant of polycyclic aromatic hydrocarbon (PAH) consumption on the concentration of NO 2 in the gas phase.…”