PubMed Health⌕ Search

Biomedical subjects

Cynthia M Friend

Publications and source records attributed to Cynthia M Friend.

13 recordsLinked to original sources
PubMed2006

Surface-mediated NH and N addition to styrene on Ag(110).

In this paper, we report the investigation of the reaction of adsorbed NH and N with styrene on Ag(110) using temperature-programmed reaction spectroscopy. Using O2 and NH3 as the starting reagents, NH and N species were deposited on a Ag surface. The reaction of styrene on NH- and N-covered Ag surface appears to yield 2-phenylaziridine and benzonitrile, with additional products HCN and NH3. The formation of aziridine, the nitrogenous analogue of styrene epoxide, is proposed to be due to the cycloaddition of adsorbed NH to the carbon-carbon double bond. These results suggest that Ag-based heterogeneous catalysts may be useful for the aziridination of olefins.

Journal Article↗
PubMed2006

Partial oxidation of propene on oxygen-covered Au(111).

Partial oxidation of propene is promoted by Au following deposition of atomic oxygen (0.3 ML) via O3 decomposition on Au(111) at 200 K. Several partial oxidation products--acrolein, acrylic acid, and carbon suboxide (O=C=C=C=O)-are produced in competition with combustion to CO2 and H2O. Acrolein is the primary partial oxidation product, and it is further oxidized to the other products by excess oxygen. We propose that acrolein is derived from allyloxy intermediate that is formed via insertion of oxygen into the allylic C-H bond. While no propene epoxide formation is detected from oxidation of C3H6, a small amount of epoxidation is observed during reaction of C3D6 and CD3CH=CH2. These results are strong indications that small changes in the energy required for allylic C-H activation, in this case due to a kinetic isotope effect, may dramatically change the selectivity; thus, small modifications of the properties of oxygen on Au may lead to the more desirable epoxidation process. Our results are discussed in the context of the origin of activity of Au-based catalysts.

Journal Article↗
PubMed2006

Rich coordination chemistry of Au adatoms in gold sulfide monolayer on Au(111).

The rich chemistry of Au nanoparticles and ions has attracted tremendous interest, in part because the surfaces of bulk Au have traditionally been considered to be chemically inert. On the other hand, large-scale mass transport and the formation of vacancy islands have been observed on the Au(111) surface following the deposition of adsorbates, such as sulfur and thiols, that can interact strongly with the Au surface. In this work, we revisit the structure and chemistry of an ordered incommensurate AuS adlayer that forms on Au(111) following the deposition of sulfur and annealing to 450 K. A structural model of this AuS surface phase has not yet been determined experimentally due to the complexity of the system. Here, we use state-of-the-art density functional theory to develop an atomic-scale model that is consistent with the previously reported Au-S stoichiometry. In particular, we introduce theoretical techniques to take into account the charge transfer in an incommensurate system. Our model reproduces convincingly STM images and is remarkably robust. Bonding analysis based on Wannier functions shows that the model exhibits rich coordination chemistry corresponding to different Au oxidation states. The extraordinary stability and rich chemistry of this structure have implications for related S-Au interfaces and previously reported surface features of this system.

Letter↗
PubMed2005

Selective oxidation of styrene on an oxygen-covered Au(111).

For the first time, we demonstrate olefin epoxidation promoted by an extended Au surface. The oxidation of styrene to styrene epoxide, benzoic acid, and benzeneacetic acid is promoted on Au(111) covered with 0.2 ML of oxygen atoms. The estimated selectivity for styrene epoxide formation is approximately 53%. Total combustion to CO2 accounts for approximately 20% of the styrene reaction. We propose that styrene epoxide, benzoic acid, and benzeneacetic acid are produced via two possible oxametallacycle intermediates. Our work demonstrates that extended Au is an effective material for olefin oxidation, which has implications for understanding the activity of nanoscale Au catalysts.

Journal Article↗
PubMed2005

Synthesis of TiO2 nanoparticles on the Au(111) surface.

The growth of titanium oxide nanoparticles on reconstructed Au(111) was investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. Ti was deposited by physical-vapor deposition at 300 K. Regular arrays of titanium nanoparticles form by preferential nucleation of Ti at the elbow sites of the herringbone reconstruction. The titanium oxide nanoclusters were synthesized by subsequent exposure to O(2) at 300 K. Two-and three-dimensional titanium oxide nanocrystallites form during annealing in the temperature range from 600 to 900 K. At the same time, the Au(111) surface assumes a serrated 110-oriented step-edge morphology suggesting step-edge pinning by titanium oxide nanoparticles. The oxidation state of the titanium oxide nanoparticles varies with annealing temperature. Specifically, annealing to 900 K results in the formation of stoichiometric TiO(2) nanocrystals as judged by the Ti(2p) binding energies measured in the x-ray photoelectron data. The nanodispersed TiO(2) on Au(111) is an ideal system to test the various models proposed for the enhanced catalytic reactivity of supported Au nanoparticles.

Journal Article↗
PubMed2005

Enhancement of O2 dissociation on Au111 by adsorbed oxygen: implications for oxidation catalysis.

We show that the dissociation probability of O2 on the reconstructed, Au111-herringbone surface is dramatically increased by the presence of some atomic oxygen on the surface. Specifically, at 400 K the dissociation probability of O2 on oxygen precovered Au111 is on the order of 10(-3), whereas there is no measurable dissociation on clean Au111, establishing an upper bound for the dissociation probability of 10(-6). Atomic oxygen was deposited on the clean reconstructed Au111-herringbone surface using electron bombardment of condensed NO2 at 100 K. The dissociation probability for dioxygen was measured by exposing the surface to 18O2. Temperature programmed desorption (TPD) was used to quantify the amount of oxygen dissociation and to study the stability of the oxygen in all cases. Oxygen desorbs as O2 in a peak centered at 550 K with pseudo-first-order kinetics; i.e., the desorption peak does not shift with coverage. Our interpretation is that the coverage dependence of the activation energy for dissociation (deltaE(dis)) and/or preexponential factor (nu(d)) may be responsible for the unusual desorption kinetics, implying a possible energy barrier for O2 dissociation on Au111. These results are discussed in the context of Au oxidation chemistry and the relationship to supported Au nanoparticles.

Adsorption↗
PubMed2005

Heteroepitaxial nucleation and oriented growth of manganese oxide islands on carbonate minerals under aqueous conditions.

Manganese redox cycling and the accompanying dissolution and precipitation reactions are important processes in natural waters. In the present study, Mn2+ (aq) is reacted with O2(aq) at circumneutral pH to form manganese oxide islands on the (1014) surface of MnCO3. The islands grow heteroepitaxially. The effects of the substrate surface morphology, the substrate atomic structure, and the aqueous concentration of Mn2+ are investigated. On terraces, rhombohedral oxide islands form with 90 degrees rotation relative to the crystallographic axis of the underlying carbonate substrate. Although the island heights self-limit between 2 and 3 nm depending on reaction conditions, the islands grow laterally to several square microns before separate islands collide and coalesce. The islands do not grow over substrate steps or down dissolution-pit edges. Comparison studies done with MgCO3 and CaCO3 show that the former also promotes heteroepitaxial growth whereas the latter does not. This difference is explained by the relative bond length mismatch between the structures of the carbonate substrates and the atomic structures of manganese oxides. A free energy model is also presented to explain why the heights of the manganese oxide islands self-limit. Our results provide an improved basis both for the development of predictive models of contaminant fate and transport and for the modeling of hydraulic flow through carbonate aquifers.

Carbonates↗
PubMed2005

Revisiting the S-Au(111) interaction: static or dynamic?

The Au-S interaction is probably the most intensively studied interaction of Au surfaces with nonmetals, as, for example, it plays an important role in Au ore formation(1) and controls the structure and dynamics of thiol-based self-assembled monolayers (SAMs). Various S-induced surface structures on Au(111) were recently reported for different conditions and predominantly interpreted in terms of a static Au surface. Here, we demonstrate that the Au(111) surface exhibits a very dynamic character upon interaction with adsorbed sulfur: large-scale surface restructuring and incorporation of Au atoms into a growing 2D AuS phase were observed in situ. These results provide new insight into the Au-S surface chemistry.

Journal Article↗
PubMed2005

Novel synthesis of two-dimensional TiS2 nanocrystallites on Au111.

We describe a novel approach to synthesize two-dimensional nanocrystalline TiS2 islands on Au111. Ti is deposited by physical vapor deposition at room temperature on AuS-covered Au111 surfaces. Subsequent annealing to temperatures between 670 K and 800 K leads to the formation of single-layer, triangular TiS2 islands. These TiS2 nanocrystallites reflect the structure of bulk TiS2, and are composed of S-Ti-S stacking units with hexagonally close-packed layers of sulfur atoms and titanium occupying the octahedral sites in between. The lattice constant of the hexagonal unit cell is 3.45 A. A superlattice with a repeat distance of 17.3 A results from the coincidence of five TiS2 units with six Au atoms and is observed in scanning tunneling microscopy and low energy electron diffraction. The triangular shape of the islands indicates a preference for one of the two possible edge terminations. The observation of two island orientations rotated by 60 degrees with respect to each other can be attributed to the formation of twin-related TiS2 domains. The population of the two different island orientations changes during annealing at 800 K indicating a thermodynamic preference for one of the possible stacking sequences.

Journal Article↗
PubMed2004

Growth of nanocrystalline MoO3 on Au(111) studied by in situ scanning tunneling microscopy.

The growth of nanocrystalline MoO3 islands on Au(111) using physical vapor deposition of Mo has been studied by scanning tunneling microscopy and low energy electron diffraction. The growth conditions affect the shape and distribution of the MoO3 nanostructures, providing a means of preparing materials with different percentages of edge sites that may have different chemical and physical properties than atoms in the interior of the nanostructures. MoO3 islands were prepared by physical vapor deposition of Mo and subsequent oxidation by NO2 exposure at temperatures between 450 K and 600 K. They exhibit a crystalline structure with a c(4 x 2) periodicity relative to unreconstructed Au(111). While the atomic-scale structure is identical to that of MoO3 islands prepared by chemical vapor deposition, we demonstrate that the distribution of MoO3 islands on the Au(111) surface reflects the distribution of Mo clusters prior to oxidation although the growth of MoO3 involves long-range mass transport via volatile MoO3 precursor species. The island morphology is kinetically controlled at 450 K, whereas an equilibrium shape is approached at higher preparation temperatures or after prolonged annealing at the elevated temperature. Mo deposition at or above 525 K leads to the formation of a Mo-Au surface alloy as indicated by the observation of embedded MoO3 islands after oxidation by NO2. Au vacancy islands, formed when Mo and Au dealloy to produce vacancies, are observed for these growth conditions.

Journal Article↗
PubMed2004

Mineral-assisted pathways in prebiotic synthesis: photoelectrochemical reduction of carbon(+IV) by manganese sulfide.

Photoelectrochemistry on mineral surfaces has the potential to play a central role in the prebiotic syntheses of building blocks for biomolecules. In this study, photoreduction of C(+IV) as bicarbonate is used as a probe to investigate the photoelectrochemical properties of alabandite (MnS) colloidal particles. Our experimental results show that photoreduction occurs and that formate is the initial photoproduct. A quantum efficiency of 4.2% is obtained (pH = 7.5). The quantum efficiency is temperature-independent from 298 to 328 K. In addition to formate, longer chain carbon products are also produced. Ion chromatography shows the presence of acetate and propionate. Infrared spectroscopy and mass spectrometry indicate the formation of longer chain organic molecules that contain oxygenated functional groups. Our results suggest that some prebiotic syntheses could have occurred via photoelectrochemical reactions on semiconducting minerals.

Acetic Acid↗