C. Laffon

Structure of the water ice surface studied by x-ray absorption spectroscopy at the O K-edge

Vapor-deposited H2O ice films grown between 38 and 150 K under ultrahigh vacuum conditions have been investigated using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy at the oxygen K-edge, in conventional mode—which is bulk sensitive-, and using the photon-stimulated desorption mode (PSD-NEXAFS), which is surface sensitive. By recording simultaneously those two signals, we have evidenced the differences between the surface and bulk electronic and atomic structures, for both amorphous porous ice condensed at 40 K and crystalline ice condensed at 150 K. We have also followed the bulk and surface evolutions of an amorphous ice film annealed from 38 to 147 K. A steep change in the local atomic structure of the bulk is observed, likely related to the high-density amorphous ice→low-density amorphous ice phase transition between 38 and 55 K. We have shown that the surface of crystalline ice is well ordered, but this order is different from that of the bulk. We have evidenced that the H2O–H2O int...

Adsorption of some substituted ethylene molecules on Pt(111) at 95 K Part 1: NEXAFS, XPS and UPS studies

Abstract The present work examines the interactions of propanoic acid, acrylic acid, acrolein and methylmethacrylate (MMA) with Pt(111) at 95 K to identify the nature of the interactions on this surface. The investigations are carried out by XPS, UPS and NEXAFS on monolayer and multilayer. Theoretical molecular orbital calculations are firstly performed to determine the nature of the bonding and antibonding orbitals of these molecules. The NEXAFS results show that the condensed multilayers of acrylic acid and acrolein are almost oriented parallel to the surface when propanoic acid and MMA are randomly oriented. The monolayer formed at 95 K for all these molecules are also oriented flat on Pt(111). However two different interaction processes are observed depending on the chemical structure of the compound: acrolein and propanoic acid are physisorbed when MMA and acrylic acid are in strong interaction with the metal but with an uncertainty on the chemisorption mode between a π-bonded state or a “di-σ like” state.

NO ICE HYDROGENATION: A SOLID PATHWAY TO NH2OH FORMATION IN SPACE

Icy dust grains in space act as catalytic surfaces onto which complex molecules form. These molecules are synthesized through exothermic reactions from precursor radicals and, mostly, hydrogen atom additions. Among the resulting products are species of biological relevance, such as hydroxylamine—NH2OH—a precursor molecule in the formation of amino acids. In this Letter, laboratory experiments are described that demonstrate NH2OH formation in interstellar ice analogs for astronomically relevant temperatures via successive hydrogenation reactions of solid nitric oxide (NO). Inclusion of the experimental results in an astrochemical gas–grain model proves the importance of a solid-state NO + H reaction channel as a starting point for prebiotic species in dark interstellar clouds and adds a new perspective to the way molecules of biological importance may form in space.

Efficient surface formation route of interstellar hydroxylamine through NO hydrogenation. I. The submonolayer regime on interstellar relevant substrates

Dust grains in the interstellar medium are known to serve as the first chemical laboratory where the rich inventory of interstellar molecules are synthesized. Here we present a study of the formation of hydroxylamine--NH(2)OH--via the non-energetic route NO + H (D) on crystalline H(2)O and amorphous silicate under conditions relevant to interstellar dense clouds. Formation of nitrous oxide (N(2)O) and water (H(2)O, D(2)O) is also observed and the reaction network is discussed. Hydroxylamine and water results are detected in temperature-programmed desorption (TPD) experiments, while N(2)O is detected by both reflection-absorption IR spectroscopy and TPD techniques. The solid state NO + H reaction channel proves to be a very efficient pathway to NH(2)OH formation in space and may be a potential starting point for prebiotic species in dark interstellar clouds. The present findings are an important step forward in understanding the inclusion of interstellar nitrogen into a non-volatile aminated species since NH(2)OH provides a solid state nitrogen reservoir along the whole evolutionary process of interstellar ices from dark clouds to planetary systems.

How the Intricate Interactions between Carbon Nanotubes and Two Bilirubin Oxidases Control Direct and Mediated O2 Reduction.

Due to the lack of a valid approach in the design of electrochemical interfaces modified with enzymes for efficient catalysis, many oxidoreductases are still not addressed by electrochemistry. We report in this work an in-depth study of the interactions between two different bilirubin oxidases, (from the fungus Myrothecium verrucaria and from the bacterium Bacillus pumilus), catalysts of oxygen reduction, and carbon nanotubes bearing various surface charges (pristine, carboxylic-, and pyrene-methylamine-functionalized). The surface charges and dipole moment of the enzymes as well as the surface state of the nanomaterials are characterized as a function of pH. An original electrochemical approach allows determination of the best interface for direct or mediated electron transfer processes as a function of enzyme, nanomaterial type, and adsorption conditions. We correlate these experimental results to theoric voltammetric curves. Such an integrative study suggests strategies for designing efficient bioelectrochemical interfaces toward the elaboration of biodevices such as enzymatic fuel cells for sustainable electricity production.

Adsorption of Acetaldehyde on Ice As Seen from Computer Simulation and Infrared Spectroscopy Measurements

Detailed investigation of the adsorption of acetaldehyde on I(h) ice is performed under tropospheric conditions by means of grand canonical Monte Carlo computer simulations and compared to infrared spectroscopy measurements. The experimental and simulation results are in a clear accordance with each other. The simulations indicate that the adsorption process follows Langmuir behavior in the entire pressure range of the vapor phase of acetaldehyde. Further, it was found that the adsorption layer is strictly monomolecular, and the adsorbed acetaldehyde molecules are bound to the ice surface by only one hydrogen bond, typically formed with the dangling H atoms at the ice surface, in agreement with the experimental results. Besides this hydrogen bonding, at high surface coverages dipolar attraction between neighboring acetaldehyde molecules also contributes considerably to the energy gain of the adsorption. The acetaldehyde molecules adopt strongly tilted orientations relative to the ice surface, the tilt angle being scattered between 50° and 90° (i.e., perpendicular orientation). The range of the preferred tilt angles narrows, and the preference for perpendicular orientation becomes stronger upon saturation of the adsorption layer. The CH(3) group of the acetaldehyde molecules points as straight away from the ice surface within the constraint imposed by the tilt angle adopted by the molecule as possible. The heat of adsorption at infinitely low coverage is found to be -36 ± 2 kJ/mol from the infrared spectroscopy measurement, which is in excellent agreement with the computer simulation value of -34.1 kJ/mol.

Adsorption of some substituted ethylene molecules on Pt(111) at 95 K II. A FT-RAIRS study

Abstract This study was carried out in order to gain insight into the interactions processes between small organic molecules and the Pt(111) surface at 95 K. In the first part of the work, we have shown that acrolein and propanoic acid are weakly linked to the metallic surface, when acrylic acid and MMA are strongly interacting with Pt(111). In these last cases, we were not able to clearly differentiate between a “di-σ” or a “π-bonded” state. The FT-RAIRS experiments presented in this paper, complementary of the others data, allowed us to specify the chemisorbed state. Thus, we confirmed the physisorption for acrolein and propanoic acid, and we showed that acrylic acid and MMA are in interactions with the Pt(111) keeping their π-character.

Influence of Water in the UV-Induced Chemistry of Methanol in the Solid Phase

UV-irradiated methanol (CH3OH) in water ice at 3 K has been investigated with infrared spectroscopy and compared with pure methanol. The main byproducts detected are formaldehyde (H2CO), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and ethylene glycol (C2H4(OH)2). The production of H2CO, CO2, and CO is enhanced in water ice, resulting from cross reactions between the byproducts of methanol with those of water (OH and H2O2).

Photochemistry of carbon monoxide and methanol in water and nitric acid hydrate ices: A NEXAFS study

Soft X-ray induced chemistry of H(2)O, CO and CH(3)OH and the effects of the water and nitric acid hydrate (HNO(3).1.65H(2)O) matrix on the photochemistry of CO and CH(3)OH have been investigated using NEXAFS spectroscopy. For pure H(2)O, CO and CH(3)OH ices, we show that the destruction rates are strongly limited by back reactions, leading to strikingly high survival rates of these molecules upon the harsh irradiation conditions to which they are submitted. We also evidence the interplay between the photochemical reactions of CO and CH(3)OH and those of the matrix. The OH and O radicals released by the photolysis of H(2)O and HNO(3) react with the CO and CH(3)OH and their fragments, considerably reducing the survival rates compared to pure CO and pure CH(3)OH ices, especially in presence of nitric acid, and dramatically enhancing the formation of CO(2) at the expense of CO. Because NEXAFS spectroscopy allows identifying which reactions are important among those possible, it emerges a simple picture of the photochemical routes of CO and CH(3)OH in the H(2)O and HNO(3)/H(2)O environments.

A perfectly stoichiometric and flat CeO2(111) surface on a bulk-like ceria film.

In surface science and model catalysis, cerium oxide (ceria) is mostly grown as an ultra-thin film on a metal substrate in the ultra-high vacuum to understand fundamental mechanisms involved in diverse surface chemistry processes. However, such ultra-thin films do not have the contribution of a bulk ceria underneath, which is currently discussed to have a high impact on in particular surface redox processes. Here, we present a fully oxidized ceria thick film (180 nm) with a perfectly stoichiometric CeO2(111) surface exhibiting exceptionally large, atomically flat terraces. The film is well-suited for ceria model studies as well as a perfect substitute for CeO2 bulk material.