Jennifer D. Schuttlefield

FTIR spectroscopy combined with quantum chemical calculations to investigate adsorbed nitrate on aluminium oxide surfaces in the presence and absence of co-adsorbed water

Surface reactions of nitrogen oxides with aluminium oxide particles result in the formation of adsorbed nitrate. Specifically, when alpha-Al(2)O(3) and gamma-Al(2)O(3) particles are exposed to gas-phase NO(2) and HNO(3) adsorbed nitrate forms on the surface. In this study, Fourier transform infrared (FTIR) spectroscopy is combined with quantum chemical calculations to further our understanding of the adsorbed nitrate product on aluminium oxide particle surfaces in the presence and absence of co-adsorbed water at 296 K. FTIR spectra of adsorbed nitrate on alpha-Al(2)O(3) and gamma-Al(2)O(3) particles are interpreted using calculated vibrational frequencies of nitrate coordinated to binuclear Al oxide cluster models. Comparison of the calculated and experimental vibrational frequencies of adsorbed nitrate establishes different modes of coordination (monodentate, bidentate and bridging) of the nitrate ion to the surface in the absence of adsorbed water. In the presence of co-adsorbed water, the nitrate ion becomes fully solvated, as shown by a comparison of the experimental nitrate infrared spectra as a function of relative humidity with the calculated nitrate vibrational frequencies for binuclear Al cluster compounds which contain both coordinated nitrate ions and water molecules. These calculations also suggest that adsorbed water can displace nitrate from direct coordination to the surface, leading to an outer-sphere nitrate adsorption complex as well as an inner-sphere complex. Furthermore, the relative humidity dependence of the spectra suggest that water does not evenly wet the surface even at high relative humidity, as there are open or bare surface sites where nitrate ions are not solvated. Besides adsorbed mondendate, bidendate, bridging and solvated nitrate, the presence of ion bound nitrate ion, partially solvated nitrate, molecular nitric acid, hydronium ion and H(3)O(+):NO(3)(-) ion pairs on the oxide surface are also discussed.

Surface reactions of carbon dioxide at the adsorbed water-oxide interface

In this study, FTIR spectroscopy is used to investigate surface reactions of carbon dioxide at the adsorbed water-oxide interface. In particular, FTIR spectra following CO 2 adsorption in the presence and absence of coadsorbed water on hydroxylated nanoparticulate Fe 2 O 3 and γ-Al 2 O 3 at 296 K are reported. In the absence of coadsorbed water, CO 2 reacts with surface O-H groups to form adsorbed bicarbonate on the surface. In the presence of coadsorbed water, this reaction is blocked as water hydrogen bonds to the reactive M-OH sites. Instead, CO 2 reacts with adsorbed water to yield adsorbed carbonate and protonated surface hydroxyl groups, M-OH 2 + , through a proposed carbonic acid intermediate. The carbonate spectra recorded between 10 and 90% RH are nearly identical to that of carbonate adsorbed on these surfaces in the presence of the liquid water. FTIR isotope studies show that there is extensive exchange between oxygen in adsorbed water and oxygen atoms in both adsorbed carbonate and gas-phase carbon dioxide. On the basis of these experimental results along with quantum chemical calculations, a mechanism is proposed for surface reactions of carbon dioxide at the adsorbed water-oxide interface.

Photochemistry of Adsorbed Nitrate

In the atmosphere, gas-phase nitrogen oxides including nitric acid react with particle surfaces (e.g., mineral dust and sea salt aerosol) to yield adsorbed nitrate, yet little is known about the photochemistry of nitrate on the surface of these particles. In this study, nitrate adsorbed on alumina surfaces, a surrogate for mineral dust aerosol, is irradiated with broadband light (lambda > 300 nm) in the absence and presence of coadsorbed water, at <1% and 45 +/- 2% relative humidity (%RH), respectively, and molecular oxygen. Upon irradiation, the nitrate ion readily undergoes photolysis to yield nitrogen-containing gas-phase products, NO2, NO, and N2O. Although NO2, NO, and N2O form under the different conditions investigated, both coadsorbed water and molecular oxygen change the gas-phase product distribution, with NO being the major product under dry and humid conditions in the absence of molecular oxygen and NO2 the major product in the presence of molecular oxygen. To the best of our knowledge, this is the first study to investigate the role of solvation by coadsorbed water in the photochemistry of adsorbates at solid interfaces and the roles that molecular oxygen, adsorbed water, and relative humidity may have in photochemical processes on aerosol surfaces that have the potential to alter the chemical balance of the atmosphere.

Attenuated Total Reflection Fourier Transform Infrared Spectroscopy to Investigate Water Uptake and Phase Transitions in Atmospherically Relevant Particles

In this study, attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy is used to investigate water uptake and phase transitions for atmospherically relevant particles. Changes in the ATR-FT-IR spectra of NaCl, NH4NO3, (NH4)2SO4, Ca(NO3)2, and SiO2 as a function of relative humidity (RH) are presented and discussed. For these various particles, water can (1) become adsorbed on the particle surface; and/or (2) become absorbed in the particle structure to form a hydrate salt; and/or (3) become absorbed by the particle to form a liquid solution. Spectral features and analyses that distinguish these various processes are discussed. For the salts that do undergo a solid to liquid phase transition (deliquescence), excellent agreement is found between the measurements made here with ATR-FT-IR spectroscopy, a relatively simple, inexpensive, and readily available analytical tool, compared to more expensive, elaborate aerosol flow reactor systems using tandem differential mobility analyzers. In addition, for particles that adsorb water, we show here the utility of coupling ATR-FT-IR measurements with simultaneous quartz crystal microbalance (QCM) measurements. This coupling allows for the quantification of the amount of water associated with the particle as a function of relative humidity (f(RH)) along with the spectroscopic data.

Photooxidation of Chloride By Oxide Minerals: Implications for Perchlorate On Mars

We show that highly oxidizing valence band holes, produced by ultraviolet (UV) illumination of naturally occurring semiconducting minerals, are capable of oxidizing chloride ion to perchlorate in aqueous solutions at higher rates than other known natural perchlorate production processes. Our results support an alternative to atmospheric reactions leading to the formation of high concentrations of perchlorate on Mars.

Interfacial Morphology and Photoelectrochemistry of Conjugated Polyelectrolytes Adsorbed on Single Crystal TiO2

The nanoscale morphology and photoactivity of conjugated polyelectrolytes (CPEs) deposited from different solvents onto single crystal TiO(2) were investigated with atomic force microscopy (AFM) and photocurrent spectroscopy. CPE surface coverages on TiO(2) could be incremenentally increased by adsorbing the CPEs from static solutions. The solvents used for polymer adsorption influenced the surface morpohology of the CPEs on the TiO(2) surface. Photocurrent spectroscopy measurements in aqueous electrolytes, using iodide as a hole scavenger, revealed that the magnitude of the sensitized photocurrents was related to the surface coverages and the degree of aggregation of the CPEs as determined by AFM imaging. Absorbed photon-to-current efficiencies approaching 50% were measured for CPE layers as thick as 4 nm on TiO(2). These results suggest that precise control of CPE morphology at the TiO(2) interface can be achieved through optimization of the deposition conditions to improve the power conversion efficiencies of polymer-sensitized solar cells.