EMSL: Kinetics and Reactions Publications

Bell RC, K Wu, MJ Iedema, GK Schenter, and JP Cowin. 2009. "The Oil-Water Interface: Mapping the Solvation Potential." Journal of the American Chemical Society 131(3):1037-1042. doi:10.1021/ja805962x Abstract Ions moving across the oil water interface are strongly impacted by the continuous changes in solvation. The solvation potential for Cs+ is directly measured as they approach the oil-water interface (“oil” = 3-methylpentane), from 0.4 to 4 nm away. The oil-water interfaces are created at 40K using molecular beam epitaxy and a softlanding ion beam, with pre-placed ions. The solvation potential slope was determined at each distance by balancing it against an increasing electrostatic potential made by increasing the number of imbedded ions at that distance, and monitoring the resulting ion motion. The potential approaches the Born model for greater than z>0.4nm, and shows the predicted reduction of the polarizability at z<0.4nm.

Kwak JH, D Mei, CWW Yi, DH Kim, CHF Peden, L Allard, and J Szanyi. 2009. "Understanding the nature of surface nitrates in BaO/gamma-Al2O3 NOx storage materials: A combined experimental and theoretical study ." Journal of Catalysis 261(1):17-22. Abstract The special role of the interface between the active catalytic phase (metal or metal oxide) and the oxide support in determining the properties of practical catalysts has long been recognized; however, it is still very poorly understood in most systems

Smith RS, T Zubkov, Z Dohnalek, and BD Kay. 2009. "The Effect of the Incident Collision Energy on the Porosity of Vapor Deposited Amorphous Solid Water Films." Journal of Physical Chemistry B 113(13):4000-4007. Abstract Molecular beam techniques are used to grow water films on Pt(111) with various incident angles and collision energies from 5 to 205 kJ/mole. The effect of the incident angle and collision energy on the porosity and surface area of the vapor deposited water films was studied using nitrogen physisorption and infrared spectroscopy. At low incident energy (5 kJ/mole), the infrared spectra, which provide a direct measure of the surface area, show that the surface area increases with incident angle and levels-off at angles > 65°. This is in contrast to the nitrogen uptake data which display a maximum near 70° due to the decrease in nitrogen condensation in the larger pores that develop at high incident angles. Both techniques show that the morphology of vapor deposited water films depends strongly on the incident kinetic energy. These observations are consistent with a ballistic deposition-shadowing model used to describe the growth of highly porous materials at glancing angle. The dependence of film morphology on incident energy may have important implications for the growth of porous materials via glancing angle deposition and for the structure of interstellar ices.

Petrik NG, and GA Kimmel. 2009. "Nonthermal Water Splitting on Rutile TiO2: Electron-Stimulated Production of H-2 and O-2 in Amorphous Solid Water Films on TiO2(110)." Journal of Physical Chemistry C 113(11):4451-4460. Abstract Electron-stimulated desorption (ESD) of H2, O2 and H2O from 0 - 60 ML films of amorphous solid water (ASW) adsorbed on TiO2(110) are investigated as function of film thickness and isotopic composition. For 100 eV incident electrons, both the H2 and O2 ESD yields have maxima when the ASW coverage is ~ 20 monolayer (ML), while the H2O ESD yield increases monotonically with water coverage. All the products reach a coverage-independent yield above 40 - 50 ML. Experiments using isotopically layered films of H2O and D2O demonstrate that the molecular hydrogen is produced in reactions that occur preferentially at or near both the ASW/TiO2 interface and the ASW/vacuum interface. However, electronic excitations or ionic defects created within the interior of the ASW films by the energetic electrons can subsequently migrate to the interfaces where they initiate reactions. Electron irradiation of ASW films results in the formation of bridge-bonded hydroxyls on the TiO2(110). These hydroxyls do not contribute to the H2 produced near the ASW/TiO2 interface. Instead, the results suggest that this H2 is produced from a stable precursor, trapped near the substrate. The proposed mechanism for the H2 production near the ASW/TiO2(110) interface is supported by a kinetic model that semi-quantitatively reproduces the main features of the non-thermal reactions.

Du Y, NA Deskins, Z Zhang, Z Dohnalek, M Dupuis, and I Lyubinetsky. 2009. "Imaging Consecutive Steps of O2 Reaction with Hydroxylated TiO₂(110): Identification of HO₂ and Terminal OH Intermediates." Journal of Physical Chemistry C 113(2):666-671. Abstract We report results of the combined experimental and theoretical investigation of the molecular oxygen reaction with a partially hydroxylated TiO₂(110) surface. The consecutive steps of both primary and secondary site-specific reactions have been tracked with high-resolution scanning tunneling microscopy (STM). For the first time, we have directly imaged stable, adsorbed hydroperoxyl (HO₂) species, which is believed to be a key intermediate in many heterogeneous photochemical processes but generally metastable and “elusive” until now. We also found terminal hydroxyl groups, another critical but never directly observed intermediates. A conclusive evidence that O₂ reacts spontaneously with a single bridging OH group as an initial reaction step is provided. The experimental results are supported by density functional theory (DFT) calculations that have determined species energies and configurations. Reported observations provide a basis for a consistent description of the elementary reaction steps and offer molecular-level insight into the underlying reaction mechanisms. In a broader perspective, the results are expected to have far reaching implications for various catalytic systems involving the interconversion of O₂ and H₂O.

Cholette F, T Zubkov, RS Smith, Z Dohnalek, BD Kay, and P Ayotte. 2009. "Infrared Spectroscopy and Optical Constants of Porous Amorphous Solid Water." Journal of Physical Chemistry B 113(13):4131-4140. Abstract Reflection-absorption infrared spectra (RAIRS) of amorphous solid water (ASW) films grown at 20K on a Pt(111) substrate at various incidence angle (θBeam = 0-85o) using a molecular beam are reported. They display complex features arising from the interplay between refraction, absorption within the sample, and interference effects between the multiple reflections at the film-substrate and film-vacuum interfaces. Using a simple classical optics model based on Fresnel equations, we obtain optical constants [i.e., n(ω) and k(ω)] for porous ASW in the 1000-4000cm-1 (10-2.5 μm) range. The behaviour of the optical properties of ASW in the intramolecular OH stretching region with increasing θBeam is shown to be strongly correlated with its decreasing density and increasing surface area. A direct comparison between the RAIRS and calculated vibrational spectra shows a large difference (~200cm-1) in the position of the coupled H-bonded intramolecular OH stretching vibrations spectral feature. Moreover, this band shifts in opposite directions with increasing θBeam in RAIRS and vibrational spectra demonstrating RAIRS spectra cannot be interpreted straightforwardly as vibrational spectra due to severe optical distortions from refraction and interference effects.

Lower BH, R Yongsunthon, L Shi, L Wildling, HJ Gruber, NS Wigginton, CL Reardon, GE Pinchuk, T Droubay, JF Boily, and SK Lower. 2009. "Antibody recognition force microscopy shows that outer membrane cytochromes OmcA and MtrC are expressed on the exterior surface of Shewanella oneidensis MR-1." Applied and Environmental Microbiology 75(9):2931-2935. Abstract Antibody-recognition force microscopy showed that OmcA and MtrC are expressed on the exterior surface of living Shewanella oneidensis MR-1 cells during anaerobic growth, when Fe(III) served as the terminal electron acceptor. OmcA was localized to the interface with hematite, while MtrC was more uniformly displayed on the bacterium’s exterior cell surface. Both cytochromes were also found associated with extracellular material.

Parkinson GS, Z Dohnalek, RS Smith, and BD Kay. 2009. "Reactivity of Fe-0 Atoms, Clusters, and Nanoparticles with CC14 Multilayers on FeO(111)." Journal of Physical Chemistry C 113(5):1818-1829. Abstract The interaction of Fe0 atoms and clusters with CCl4 multilayers was investigated using a novel "atom dropping" method at 30 K over a FeO(111) thin film. Temperature programmed desorption experiments over a range of Fe0 and CCl4 coverages demonstrate a rich surface chemistry with several reaction products (C2Cl4, C2Cl6, OCCl2, CO, FeCl2, FeCl3) observed. X-ray photoelectron spectroscopy data show that the initial reactive interaction occurs spontaneously at 30 K, with the experimentally observed reaction products formed at higher temperature, in agreement with the results of theoretical calculations. The formation of OCCl2 and CO is concluded to occur through abstraction of O atoms from the generally inert FeO(111) substrate. The buffer layer assisted growth technique is used to show that the reactivity, and interestingly the reaction products, is determined by the size of Fe0 nanoparticles which interact with CCl4.

Yi C, and J Szanyi. 2009. "Reaction of NO2 with a pure, thick BaO film: the effect of temperature on the nature of NOx species formed." Journal of Physical Chemistry C 113(6):2134-2140. Abstract The adsorption and reaction of NO2 on a thick (>30 ML), pure BaO film deposited onto an Al2O3/NiAl(110) substrate were investigated in the temperature range of 300 – 660 K using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and x-ray photoelectron spectroscopy (XPS) techniques. The adsorption of NO2 on BaO at room temperature results in the formation of nitrite-nitrate ion pairs. During thermal desorption the nitrite species decompose first, releasing an NO molecule and leaving an O on the surface, while nitrate species decompose in two steps at higher temperatures: at lower temperature as NO2 only, then, at higher temperature, as NO + O2. In cyclic experiments when the BaO film was exposed to NO2 at 300 K, followed by annealing to 575 K, a large amount of NOx was stored as nitrates, and no saturation was achieved even after the 10th adsorption/anneal cycle. This suggests the gradual conversion of the BaO film into barium nitrate clusters at elevated temperatures. The rate of nitrate formation increases as the sample temperature during NO2 exposure increases up to 610 K, while at even higher temperatures the amount of nitrates formed decreases. NO2 adsorption on the thick BaO film at 610 K results in the formation of strongly bound nitrates as the major NOx species.

Trevisanutto PE, PV Sushko, KM Beck, AG Joly, WP Hess, and AL Shluger. 2009. "Excitation, Ionization, and Desorption: How Sub-band gap Photons Modify the Structure of Oxide Nanoparticles." Journal of Physical Chemistry C 113(4):1274-1279. Abstract Nanoparticles of wide-band-gap materials MgO and CaO, subjected to low-intensity ultraviolet irradiation with 266 nm (4.66 eV) photons, emit hyperthermal oxygen atoms with kinetic energies up to ~ 0.4 eV. We use ab initio embedded cluster methods to study theoretically a variety of elementary photoinduced processes at both ideal and defect-containing surfaces of these nanoparticles and develop a mechanism for the desorption process. The proposed mechanism includes multiple local photoexcitations resulting in sequential formation of localized excitons, their ionization, and further excitations. It is suggested that judicious choice of sub-band-gap photon energies can be used to selectively modify surfaces of nanomaterials.