AVS2016 Session SS-TuP: Surface Science Poster Session

It is known that dimethyl trisulfide (DMTS) is mainly responsible for an off-flavor that develops during the storage of Japanese sake. Recently, we found that the supported gold catalysts are effective for the adsorption and removal of DMTS. In this study, in order to clarify the reaction properties of DMTS over gold, we investigated the adsorption and decomposition of DMTS using the Au(111) single-crystal surface.

First, we examined the influence of the exposure temperature on the adsorption properties of DMTS. X-ray photoelectron spectroscopy (XPS) results indicated that DMTS is dissociatively adsorbed as CH₃S and CH₃SS species at 100–300 K. Furthermore, both the dissociative adsorption rate and the saturation coverage were the same regardless of the exposure temperature.

Metal oxide substrates are often riddled with defect sites, imperfections in metal-oxide atomic arrangements. One such defect is an oxygen vacancy at the surface. Commonly, the substrate is exposed to O₂ to reestablish the proper metal-oxygen coordination. Much like O₂, hydrogen peroxide may be used to oxidize the surface of metal oxide nanopowders, such as titania (TiO₂), as well as drive off impurities remaining and or derived from the synthesis of these materials, to establish a more pristine surface. Various commercially available nanosized rutile and anatase structured titania nanopowders are treated with hydrogen peroxide and any changes in crystallinity are monitored using a confocal Raman microscope as well as powder X-ray Diffraction. In situ DRIFTS coupled with a high temperature reaction chamber is used to assess any changes in the substrate upon treatment, including evolving water and hydroxyl features on the surface and the disappearance of impurities, both as the pretreatment conditions change and as a function of substrate temperature.

The surface chemistry of late transition-metal (TM) oxides has drawn significant attention due to the observation and prediction of facile C-H bond cleavage of molecularly adsorbed n-alkanes at low temperatures. Previous studies have shown that PdO(101) readily promotes the dissociation of alkanes by a mechanism in which adsorbed σ-complexes serve as precursors to initial C-H bond cleavage. Density functional theory (DFT) calculations further predict that the formation and facile C-H bond activation of alkane σ-complexes should also occur on RuO₂ and IrO₂ surfaces, suggesting that the σ-complex mechanism is a common pathway for alkane activation on late TM oxides.

In this study, we investigated the adsorption and oxidation of n-butane on the stoichiometric RuO₂(110) surface using temperature-programmed reaction spectroscopy (TPRS) and DFT calculations. At low coverage, molecularly adsorbed n-butane achieves a binding energy of ∼100 kJ/mol on RuO₂(110), consistent with a strongly bound σ-complex that forms through dative bonding interactions between the n-butane molecule and coordinatively unsaturated (cus) Ru atoms. We find that a fraction of the n-butane reacts with the RuO₂ surface during TPRS to produce CO, CO₂, and H₂O that desorb above ∼400 K and present evidence that adsorbed σ-complexes serve as precursors to the initial C−H bond cleavage and ultimately the oxidation of n-butane on RuO₂(110). From measurements of the product yields as a function of surface temperature we estimate that the initial reaction probability of n-butane on RuO₂(110) decreases from 9% to ∼4% with increasing surface temperature from 280 to 300 K and show that this temperature dependence is accurately described by a precursor-mediated mechanism. From kinetic analysis of the data we estimate a negative, apparent activation energy of −35.1 kJ/mol for n-butane dissociation on RuO₂(110) and an apparent reaction prefactor of 6 × 10⁻⁸. The low value of the apparent reaction prefactor suggests that motions of the adsorbed n-butane precursor are highly restricted on the RuO₂(110) surface. DFT calculations confirm that n-butane forms strongly bound σ- complexes on RuO₂(110) and predict that C−H bond cleavage is strongly favored energetically. The n-butane binding energies and energy barrier for C−H bond cleavage predicted by DFT agree quantitatively with our experimental estimates. Our results support the idea that the σ-complex mechanism is a common pathway for alkane activation on late TM oxide surfaces that expose pairs of cus metal and oxygen atoms.

Many heterogeneously catalyzed reactions have been shown to be strongly structure dependent.^[1] Catalytically active materials can feature a wide spectrum of defect densities on the same sample and may include various step types.^[2] Thus, curved crystals with continuously changing average step densities provide a good alternative to flat single crystals for the investigation of surface structure dependencies.^[3] In this study we use two curved Ag single crystals to exemplify the strength of this approach to studying structure dependencies. The crystals have two different apex orientations. One Ag crystal with a [111] apex contains two different step sites on either side of the center, generally referred to as the (100) and (111) or A and B step types. The step density gradually increases until at the edges of the crystal we reach 5-atom wide (111) terraces. The crystal with the [100] apex has only one step type, that resembles the B step type from the first crystal, but has adjacent (100) terraces. We study the surface structure of the clean crystals with LEED and STM and show that the surface behaves as may be expected with single-atom high steps.

Subsequently, water adsorption to the steps and their effect on the water-metal interface are investigated using spatially resolved Temperature Programmed Desorption. As Ag binds water only weakly, effects resulting from the available steps are expected to be rather small . We show how the different step types affect the desorption of water and how it would be nearly impossible to measure the effects using multiple flat Ag samples.

1. Somorjai, G.A. The structure sensitivity and insensitivity of catalytic reactions in light of the adsorbate induced dynamic restructuring of surfaces. Catal. Lett.1990, 7, 169–182.

2. Walter, A.L.; Schiller, F.; Corso, M., et al. X-ray photoemission analysis of clean and carbon monoxide-chemisorbed platinum(111) stepped surfaces using a curved crystal. Nat. Commun. 2015, 6, 8903.

3. (a) Besocke, K.; Krahl-Urban, B; Wagner, H. Dipole moments associated with edge atoms; A comparative study on stepped Pt, Au and W surfaces, Surf. Sci.1977, 68, 39–46.

(b) Hopster, H.; Ibach, H.; Comsa, G. Catalytic oxidation of carbon monoxide on stepped platinum(111) surfaces, J. Catal.1977, 46, 37–48.

(c) Pluis, B.; van der Gon, A.W.D.; Frenken, J.W.M.; van der Veen, J.F. Crystal-Face Dependence of Surface Melting, Phys. Rev. Lett.1987, 59, 2678–2681.

Gallium-indium metal alloys are remarkable materials that, at the eutectic composition, are liquid at room temperature and form a very thin (0.7 nm) passivating oxide film on the surface. This makes them valuable in the field of molecular electronics as soft conformal electrical contacts and as, potentially, self-repairing wires. For this project, EGaIn is put through a fluidic shearing process that produces 3-layered core-shell nano/micro-spherical particles composed of a chemisorbed organic outer layer on an oxide film around the liquid metal core that prevents their coalescence. We used Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) to monitor topographical changes in these particles as a function of temperature. The liquid metal has a different rate of expansion from the oxide shell, and AFM coupled with SEM are especially well-suited to monitor these changes both as a function of the rate of change of the temperature and the thickness of the oxide film. The nature of the external coating can also be tuned through exposure of the particles to strong oxidants or shearing the metal in the presence of different surfactants. The effect of the different film coatings and the expansion of the particles upon application of heat will be discussed.

Partial replacement of surface oxygen atoms with chlorine atoms may provide a means for modifying the activity and selectivity of oxide surfaces toward hydrocarbon oxidation. In this study, we investigated the adsorption and oxidation of ethylene on partially chlorinated RuO₂(110) surfaces using temperature programed reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS). Chlorination of the RuO₂(110) surface occurs when exposing the stoichiometric surface to gaseous HCl at 700 K, where the bridging oxygen atoms are selectively replaced by chlorine atoms. The degree of chlorination is controlled by the amount of HCl gas introduced, and characterized by XPS. Compared with stoichiometric RuO₂(110), we find that bridging Cl atoms weaken the binding and suppress the oxidation of ethylene, without shifting the selectivity toward partially oxidized products. We also find that on-top oxygen atoms significantly enhance the activity of both s-RuO₂(110) and chlorinated RuO₂(110) surfaces toward the complete oxidation of ethylene. The enhanced reactivity arises from an increase in the ethylene coverage achieved on the O-rich surfaces as well as more facile C‒H bond cleavage of ethylene via H-transfer to on-top vs. bridging oxygen atoms. Our results provide evidence that ethylene molecules achieve high coverages on the O-rich surfaces by preferentially binding to stranded Ru sites located between on-top oxygen atoms, and that such configurations are responsible for the high activity of the O-rich RuO₂ and RuO_xCl_y surfaces. These findings demonstrate that the relative reactivity of on-top vs. bridging oxygen atoms plays a decisive role in determining the chemical activity of partially-chlorinated RuO₂ surfaces, and that high reactivity can be achieved on O-rich RuO_xCl_y surfaces.

STM images reveal that TIPT monomers are quite mobile on the Cu(100) surface at room temperature. At low coverage, molecules readily migrate and accumulate at step edges. We observe very few supramolecular features at the surface, and these structures often decompose after repeated STM scanning. In contrast to the as deposited samples, after annealing to 420°K more robust open pore structures are observed. The structure and size of these molecular frameworks are consistent with covalent linking. We have also studied TIPT adsorption on the √3-Ag surface. The structure of these films as a function of coverage and annealing temperature will be discussed.

1. D.F.Perepichka, and F. Rosei, Science 323, 216–217 (2009).

2. R. Liu et al., Surf. Sci. 647, 51–54 (2016).

Graphite oxide was successfully synthesized from graphite powder using the modified Hummers method*. The graphite oxide was then exfoliated to yield graphene oxide which was subsequently reduced to give reduced graphene oxide. This employed two different chemical reduction methods, and one effective combination of the two. The two methods being a weaker sodium borohydride/calcium chloride catalyst and a hydrogenation through hydrogen produced from the reaction of hydrochloric acid and aluminum. This can be seen through the removal of various functional groups from our graphene oxide sample after each reduction method, as shown in FTIR spectra of each sample. While the reduction methods employed did remove a number of oxygenated functional groups on the graphene oxide sheet, we still observe the presence of hydroxyl and some carboxylic acids that persist through. We also notice the appearance of a well-defined peak at ~1600 cm^-1 representing the conjugated system in the combined reduction method.

* W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339

Driven by growing concern of the effect of greenhouse gases on the environment, CO₂ chemistry has become an increasingly active area of research. The interaction of CO₂ with metal-organic complexes offers opportunities for CO₂ recycling, but those chemistries have not been developed in surface catalysts, which could offer much higher efficiency. We have developed a prototypical metal-organic network that shows chemical activity toward CO₂ by co-depositing bis-pyridinyltetrazine (DPTZ) and metallic vanadium on a Au(100) surface. These organize at room temperature into highly-ordered one-dimensional metal-organic chains. We characterized the assembly by high-resolution scanning tunneling microscopy. The chains align in specific orientations relative to the underlying gold surface due to their interaction with the gold. The assembly occurs by a redox-active self-assembly process, in which the vanadium oxidizes to the +2 state and there is corresponding reduction of the ligand, as observed by X-ray photoelectron spectroscopy. Exposure to CO₂ gas leads to a shift in the vanadium oxidation state to +4; the shift is gradual with increasing CO₂ exposure. The 1D chains generally remain intact during the CO₂ exposure, but become somewhat less ordered with increasing exposure time. Following gas exposure, the surface was annealed at various temperatures. At annealing temperatures of 250 °C and greater we observe desorption of the ligand and the shift of vanadium back to the +2 state, indicating a residual vanadium-oxo species on the surface. Developing single-site metal center surfaces systems with chemical activity toward CO₂ may lead to the development of new methods for CO₂ capture and recycling, as well as providing more general insight into the development of next-generation catalysts.

CO₂ is an important ice species in interstellar environments, often the second most abundant ice after H₂O. Astronomical infrared spectra of interstellar objects have revealed abundant CO₂ in a variety of protostellar environments as well as in cold dark clouds. The CO₂ ν₂ band has been used as a tracer of thermal processing due to its dependence on the ice temperature and local environment; however, there are still uncertainties involved in fitting the laboratory v₂ band to astronomical spectra. We report laboratory spectra of the CO₂ longitudinal optical (LO) phonon mode for a series of CO₂ ices at low temperature and for ice mixtures with polar (H₂O) and non-polar (CO, O₂) components. We show that the LO phonon mode is particularly sensitive to the mixing ratio of various ice components of astronomical interest. These spectra may be useful in constraining the bulk environment of CO₂ in astronomical ices as well as for tracing ice mixing in laboratory experiments.

Methylammonium Lead Iodide Perovskite (MAPbI₃) is a promising photoelectronic material for photovoltaics and LEDs. However, the stability of MAPbI₃ under the external application environments is a big concern. The underlying mechanism of decomposition of MAPbI₃ is not well understood yet. In this poster, we will use in-situ Diffuse reflectance infrared fourier transform spectroscopy for the first time to understand the surface reaction mechanism in the decomposition of MAPbI₃ in various related applications environments. With the unique setup and high-surface-area configuration, our results showed that the degradation rate is strongly affected by the temperature and chemical composition of the application environments. The degradation mechanism of MAPbI₃ changes with the application environments. Our results provide a fresh view of the degradation pathways of MAPbI₃ and will help optimize the synthesis of MAPbI₃ and provide potential solutions for stabilizing MAPbI₃.