Papers

AVS1996 Session SS2-ThA: Atmospheric and Environmental Surface Chemistry

Thursday, October 17, 1996 1:30 PM in Room 203B

Thursday Afternoon

Start	Invited?	Item
1:30 PM		SS2-ThA-1 Adsorption and Reaction at Model Cloud Particle Surfaces J. Roberts (University of Minnesota) Chemical reactions that occur in the surface and near surface regions of ice and sulfuric acid particles have been implicated in the sequence of events that ultimately opens the Antarctic ozone hole. We seek to understand the underlying mechanisms of these surface-mediated chemical transformations. Our experimental method involves the study of ultrathin (<200 \Ao\ thick) films of ice and sulfuric acid that are deposited on single-crystal metal substrates. The films serve as convenient samples on which to study adsorption and desorption kinetics, and adsorbate structure. Importantly, the large surface area to volume ratios of the films makes it experimentally simple to distinguish between surface- and bulk-mediated processes. The following aspects of our recent work will be discussed: (i) the extent to which ice surface structure dictates reactivity, (ii) the surface chemical properties of solid and liquid sulfuric acid, (iii) the thermal and photochemistry of stratospherically abundant chlorine oxides on ice, and (iv) the mechanistic consequences of the fact that the surface of an atmospheric ice particle is highly dynamic, i.e. the evaporation and condensation rates of H\sub 2\O on ice are very large.
2:10 PM		SS2-ThA-3 Surface and Bulk Diffusion of HDO and H\sub 2\\super 18\O on Single-Crystal H\sub 2\\super 16\O Ice Multilayers F. Livingston, G. Whipple, D. Brown, S. George (University of Colorado, Boulder) Heterogeneous reactions on polar stratospheric clouds (PSCs) play a critical role in the depletion of Antarctic ozone. These PSCs are composed of ice (Type II PSC) and HNO\sub 3\ hydrates (Type I PSC). To understand the dynamical nature of the Type II PSCs, the surface and bulk diffusion of HDO and H\sub 2\\super 18\O on single-crystal H\sub 2\\super 16\O ice multilayers grown epitaxially on Ru(001) were examined using laser-induced thermal desorption (LITD) techniques. No H\sub 2\O surface diffusion was observed at 140 K on the time scale of the LITD experiments. This negligible diffusion set an upper limit for surface migration of D\sub s\\<=\5x10\super -9\ cm\super 2\s\super -1\ and suggested that the isotopically labeled H\sub 2\O may be diffusing into the ice multilayer. The bulk diffusion of HDO and H\sub 2\\super 18\O were measured utilizing a novel technique of isothermal desorption/depth profiling combined with LITD probing. The isotopically labeled H\sub 2\O readily diffused into the underlying multilayer. An Arrhenius analysis yielded bulk diffusion kinetics of E=16.4\+-\1.1 kcal/mol and D\sub o\=6.0(\+-\0.3)x10\super 7\ cm\super 2\s\super -1\ for HDO and E=16.7\+-\1.6 kcal/mol and D\sub o\=9.7(\+-\0.5)x10\super 7\ cm\super 2\s\super -1\ for H\sub 2\\super 18\O. The similar activation barriers for H\sub 2\\super 18\O and HDO diffusion suggest that HDO migration is controlled by molecular diffusion as opposed to isotopic H/D exchange processes. The bulk diffusion coefficients predicted at stratospheric temperatures indicate that the ice surfaces are dynamic with diffusion into the bulk competitive with thermal desorption.
2:30 PM		SS2-ThA-4 The Structure of Water Films formed by Condensation on Mica Surfaces M. Salmeron, L. Xu (Lawrence Berkeley Laboratory) The structure of water films condensed on mica substrates has been studied by Scanning Polarization Force Microscopy (SPFM), an electrostatic, non-contact, Atomic Force Microscopy operating mode. Freshly cleaved mica surfaces have a high energy, and a high positive potential. This induces the condensation of ice-like films (phase-II), with an apparent topographic contrast of up to 20 \Ao\ relative to phase I. This probably corresponds to multilayers of ice. The films can be locally "melted" by contact with the AFM tip to form liquid-like phase I water. The new areas convert back to phase II with time and exposure to water. The surface potential, high (> 10 V) and positive after initial cleavage decays with time to less than about \+-\ 0.5 V. The topographic contrast between the ice-like phase II, and the liquid- like phase I decreases to about 1.5 \Ao\ at the lower potencials. The frequency response of the polarization force in the two phases has been studied, and used to determine the various contributions to the local dielectric constant. This includes ionic motion, dipolar orientation and others
2:50 PM		SS2-ThA-5 Dehydroxylation and Rehydroxylation of \alpha\-Al\sub 2\O\sub 3\ Surfaces S. George, M. Cameron, C. Nelson, M. Tolbert (University of Colorado, Boulder) The rocket exhaust of the space shuttle deposits ~300,000 kg of \alpha\-Al\sub 2\O\sub 3\ and \gamma\-Al\sub 2\O\sub 3\ into the atmosphere during each launch. To determine the expected surface coverage on \alpha\-Al\sub 2\O\sub 3\, the dehydroxylation and rehydroxylation of \alpha\-Al\sub 2\O\sub 3\ were examined using laser induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The calibrated H\sub 2\O LITD measurements revealed that the fully hydroxylated \alpha\-Al\sub 2\O\sub 3\ surface at 300 K contains a hydroxyl coverage of 1.3 x 10\super 15\ cm\super -2\. These hydroxyl groups are lost by dehydroxylation during annealing at higher temperatures and the hydroxyl coverage is negligible at T> 600 K. The TPD measurements observed H\sub 2\O desorption with peak temperatures that varied from ~400-600 K versus progressively higher annealing temperature. The integrated H\sub 2\O TPD peak areas versus annealing temperatures were in excellent agreement with the H\sub 2\O LITD measurements. Rehydroxylation of the dehydroxylated \alpha\-Al\sub 2\O\sub 3\ was difficult and negligible hydroxyl coverages were observed after H\sub 2\O exposures as large as ~700 x 10\super 6\ Langmuir at 300 K. The \alpha\-Al\sub 2\O\sub 3\ surface could be easily rehydroxylated using a H\sub 2\O plasma at 200 mTorr. These results predict that the \alpha\-Al\sub 2\O\sub 3\ particles produced by the space shuttle will not be hydroxylated by the ambient H\sub 2\O pressures. Rather, the dehydroxylated \alpha\-Al\sub 2\O\sub 3\ surface will be available for reactions with HCl in the exhaust plume or chloroflurocarbons (CFCs) in the atmosphere.
3:10 PM		SS2-ThA-6 Adsorption and Photoreaction of Oxygenates on Tropospheric Metal Oxide Particulates F. Thomas, H. Idriss, A. Miller, E. Seebauer (University of Illinois, Urbana) # Tropospheric particulates comprising substantial amounts of transition metal oxides are found in high concentrations in urban areas and near deserts. This laboratory has previously shown that photocatalysis of certain oxygenates (such as MTBE) on combustion-generated fly ash can yield reaction products at rates comparable to those involving purely homogeneous pathways. The present work examines a larger class of oxygenates on a broader array of particulates. Both adsorption and photoreaction kinetics are examined as appropriate for several organic acids, esters, ethers, aldehydes, and related compounds. Substrates include fly ash and several soils with high Fe content. These solids adsorb organic acids at particularly high rates, probably on Lewis base sites that can be quantitatively titrated in the laboratory with CO\sub 2\. Photoreactions also occur at these sites, which appear to be associated largely with surface Fe and Ti. Implications of these chemistries for tropospheric reaction networks where particulates are present in large quantities will be discussed.
3:30 PM		SS2-ThA-7 XPS Studies of Chromium Adsorption on Iron Mineral Surfaces P. Watson, H. Abdel-Samad (Oregon State University) Iron oxide minerals, particularly goethite (FeOOH), are widely distributed in soils and probably play a more important role in the transmission of pollutants in groundwater than their bulk abundance would indicate. We have studied the adsorption of chromate (CrO\sub 4\\super 2-\) from aqueous solution on the surface of the mineral goethite (\alpha\-FeOOH) as a function of pH and adsorbate concentration in 0.05M NaNO\sub 3\ solution. Results obtained from the dried surface by X-ray photoelectron spectroscopy (XPS) are in good agreement with data from spectrophotometric analysis of chromate remaining in the supernatant liquid. Chromate adsorption increases with decreasing pH of the solution and eventually reaches a maximum at pH of 6.5. Adsorption isotherms at constant pH are a poor fit to the Langmuir isotherm. The chromium XPS signal indicates that initially a small amount of chromium adsorbs in the +3 oxidation state via a redox reaction, but that the large majority of chromium remains in the +6 oxidation state.
3:50 PM		SS2-ThA-8 Growth Kinetics of Phosphate Films on Metal Oxide Surfaces T. Murrell, M. Nooney, E. Rusert, D. Goodman (Texas A&M University) The kinetics of phosphate uptake by hematite, titania, and alumina were examined by exposing freshly prepared thin films to phosphate solutions at incremental times and subsequently analyzing the surfaces by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), scanning tunneling microscopy (STM), and atomic force microscopy (AFM). Thin film hematite exposed to a sodium phosphate solution demonstrated initially rapid phosphate uptake during the first 10 minutes of solution exposure, followed by a 10 min. induction period after which phosphate accumulated as a species different than that adsorbed initially. For titania and alumina exposed to a calcium phosphate solution, the initially rapid reaction was completed after 1 and 3 hours, respectively. Subsequent rapid accumulations occurred after 3-4 hours for titania and 25-30 hours for alumina. The data for these two oxides were comparable to in-situ studies using the same solutions. We postulate that phosphate chemisorbs initially as an inner-sphere complex, but with continued exposure, it forms large polymeric PO\sub 4\ structures. Three dimensional growth of phosphate was further confirmed by AFM images showing the formation of step structures and islands on surfaces exposed to phosphate solutions.
4:10 PM		SS2-ThA-9 Structure and Phase Transition in Thin Pyridine Films Adsorbed on Ag(111) and Ag(100): A Second Harmonic Generation Study T. Sjodin, K. Song, H. Dai (University of Pennsylvania) Pyridine multilayers (5 - 1000 ML) adsorbed on Ag(111) and Ag(100) have been studied by examining the second harmonic generation from the Ag/pyridine interface as well as the pyridine overlayers. An irreversible transition from a disordered, low-temperature adsorption structure to an ordered structure was observed at 135 K. The ordered sturcture is speculated to have ordered domains in the pyridine film on length scale of 1 micron, but not as the crystalline form. The orienation of the domains in layers within the first 50 ML appears to be in registary with the Ag crystal surface, and becomes increasingly isotropic for layers far from the interface.