Frequency Generation Vibrational Spectroscopy Study Research Articles

The Influence of Sodium Iodide Salt on the Interfacial Properties of Aqueous Methanol Solution by a Combined Molecular Simulation and Sum Frequency Generation Vibrational Spectroscopy Study.

Understanding the influence of salt ions on the microscopic properties of liquid interfaces is of both fundamental and practical importance. A large number of previous experimental and theoretical investigations have explored the salt effects on the surfaces of either pure water or neat organic liquid. However, how the salt ions affect the interfacial structures of water/organic liquid mixtures has rarely been studied. Here, the molecular dynamics (MD) simulations and sum frequency generation vibrational spectroscopy (SFG-VS) were carried out to investigate the influence of sodium iodide (NaI) on the air/liquid interfaces of the methanol-water mixtures. The SFG-VS spectral intensities were discovered to increase with the addition of 3 M NaI, while the center frequencies of the C-H stretching vibrations at high methanol concentrations showed a ∼2 cm-1 blue shift compared with those obtained before adding NaI. The MD results indicated that Na+ and I- can only affect Part I (near the bulk phase) but not Part II (near the gas phase) of the interfacial region. It was also found that the average orientations of interfacial methyl groups were constant and not effectively disturbed by the changes of methanol concentrations or the addition of NaI. It is therefore concluded that the changes of the SFG-VS intensities upon the addition of NaI salts were mainly caused by the increasing number of interfacial methanol molecules. Further analysis showed that the existence of NaI affects the surface tensions more for the interfaces with higher bulk methanol concentrations, which is in agreement with the SFG-VS results. It is noteworthy that the maximum number density of methanol molecules with the net nonzero orientations is reached near the Gibbs dividing surface, the reasons of which are worth further investigating.

Identifying the Decomposition of Diethyl Carbonate in Binary Electrolyte Solutions in Contact with Silicon Anodes - A Sum Frequency Generation Vibrational Spectroscopy Study

The key factor in the use of lithium ion batteries is the formation of an electrically insulating solid layer that allows lithium ion transport but stops further electrolyte redox reactions on the electrode surface, therefore termed the solid electrolyte interphase (SEI). We have studied the preceding stages to the SEI formation, specifically the reduction of diethyl carbonate (DEC) mixed with ethylene carbonate (EC) with dissolved 1.0 M LiPF6, a common binary electrolyte. We used a p-doped crystalline silicon (100)-hydrogen terminated wafer as the anode in a lithium (Li) half-cell system. We employed in situ sum frequency generation (SFG) vibrational spectroscopy with interface sensitivity to probe the molecular composition of the SEI surface species under various applied potentials where only diethyl carbonate reduction to form a Si-ethoxy (Si-OCH2CH3) surface is expected. We found that even at open circuit potential (OCP) DEC decomposes on the silicon (100)-hydrogen surface to form Si-ethoxy bonds. These findings shed new light on the interfacial Si anode-binary electrolyte solutions chemistry at stages preceding the formation of the SEI on Si anodes.

Alcohol Oxidation at Platinum–Gas and Platinum–Liquid Interfaces: The Effect of Platinum Nanoparticle Size, Water Coadsorption, and Alcohol Concentration

Alcohol oxidation reaction over platinum nanoparticles with size ranging from 2 to 8 nm deposited on mesoporous silica MCF-17 was studied in the gas and liquid phases. Among methanol, ethanol, 2- propanol, and 2-butanol oxidations, the turnover frequency increased as the nanoparticle size became large in both reaction phases. The activation energy in the gas phase was higher than that in the liquid phase. Water coadsorption decreased the turnover rate of all the gas and liquid phase oxidations except for the gas-phase 2-butanol case, while a certain amount of water promoted 2-propanol oxidation in the liquid phase. Sum frequency generation vibrational spectroscopy study and DFT calculation revealed that the alcohol molecules pack horizontally on the metal surface in low concentration and stand up in high concentration, which affects the dissociation of β-hydrogen of the alcohol as the critical step in alcohol oxidation.

Compact ultrahigh vacuum/high-pressure system for broadband infrared sum frequency generation vibrational spectroscopy studies

We have designed a compact ultrahigh vacuum/high-pressure system for in situ broadband infrared (IR) sum frequency generation vibrational spectroscopy (SFG-VS) studies. In this system, we have achieved a significant reduction in the distance between the sample and the optical window (<5 mm), which in turn considerably reduces the IR absorption from the gas phase under high pressure conditions. Moreover, with this new system, the IR transmission under high pressure conditions can be measured in situ for calibrating the SFG spectra. Therefore, this modified technique can allow us to study the vibrational spectra of adsorbates on single crystals or polycrystalline foils under high pressure. The preliminary results from SFG measurements of a model CH3OH/TiO2(110) system under both ultrahigh vacuum and high pressure conditions are reported here. These results suggest that this newly developed system is potentially a powerful tool for investigating adsorbate structures and surface reactions under both ultrahigh vacuum and real conditions.

An electronically enhanced chiral sum frequency generation vibrational spectroscopy study of lipid-bound cytochrome c.

Electronically enhanced chiral SFG spectroscopy was employed to study the lipid bound cyt c in situ. It was directly observed that upon interacting with anionic phospholipids, the amino acid residues around the heme adopted the β-sheet conformation. In addition, the orientation of this newly formed β-sheet structure was found to be sensitive to the bulk pH.

In situ sum frequency generation vibrational spectroscopy study of CO adsorption on Au surfaces promoted by Ar + sputtering and FeO x additives

Identification of the active sites is a key factor in understanding the mechanism of gold-catalyzed reactions. In this work we investigated the CO adsorption properties of gold-based model catalysts ranging from native and surface-modified single crystal surfaces to polycrystalline thin films promoted by iron oxide layers. The results, completed with those obtained on a gold nanoparticle-containing system, clearly demonstrate the role of the defect sites in the CO adsorption. In addition, a promoter such as iron oxide can further enhance CO adsorption, probably through a mechanism connected to the interface formation between gold and iron oxide.

Amino acid adsorption on hydrophilic TiO 2: A sum frequency generation vibrational spectroscopy study

Sum frequency generation vibrational spectroscopy (SFG) is a powerful tool for in situ investigation of adsorption processes at biologically important solid–liquid interfaces. In this work adsorption of different amino acids was investigated by SFG at the titanium-dioxide–amino acid solution interface. While both naturally occurring acidic amino acids, aspartic acid and glutamic acid, were found to form an ordered adsorbate layer, amino acids with non-acidic side chains exhibited little affinity towards TiO 2. A detailed study of aspartic acid on TiO 2 pointed out that both carboxylic groups of the molecule are coordinated to the surface; data taken in the carboxyl stretching frequency range revealed that one of them is bonded monodentally to a surface Ti ion, while the other is most probably stabilized by a hydrogen bond. The results indicate that the acidic side chain is the prerequisite of the formation of a stable, ordered amino acid adsorbate layer on TiO 2.

Pyrrole Hydrogenation over Rh(111) and Pt(111) Single-Crystal Surfaces and Hydrogenation Promotion Mediated by 1-Methylpyrrole: A Kinetic and Sum-Frequency Generation Vibrational Spectroscopy Study

Pyrrole Hydrogenation over Rh(111) and Pt(111) Single-Crystal Surfaces and Hydrogenation Promotion Mediated by 1-Methylpyrrole: A Kinetic and Sum- Frequency Generation Vibrational Spectroscopy Study Christopher J. Kliewer, Marco Bieri and Gabor A. Somorjai* Department of Chemistry, University of California, Berkeley, California 94720, and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 E-mail: somorjai@socrates.berkeley.edu TITLE RUNNING HEAD: Pyrrole hydrogenation over Rh(111) and Pt(111) RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) CORRESPONDING AUTHOR FOOTNOTE: Gabor A. Somorjai, Tel: 510-642-4053. Fax: 510-643- 9668. E-mail: somorjai@socrates.berkeley.edu. ABSTRACT. Sum frequency generation surface vibrational spectroscopy and kinetic measurements using gas chromatography have been used to study the adsorption and hydrogenation of pyrrole over both Pt(111) and Rh(111) single-crystal surfaces at Torr pressures (3 Torr pyrrole, 30 Torr H 2 ) to form pyrrolidine and the minor product butylamine. Over Pt(111) at 298K it was found that pyrrole adsorbs in an upright geometry cleaving the N-H bond to bind through the nitrogen evidenced by SFG data.

Sum Frequency Generation Studies of Ammonium and Pyrrolidinium Ionic Liquids Based on the Bis-trifluoromethanesulfonimide Anion

A systematic sum frequency generation vibrational spectroscopy study is conducted on the gas-liquid interface of room-temperature ionic liquids. The compounds contain ammonium and pyrrolidinium based cations, to which alkyl substituents of different length and/or functional groups are attached, and they are all combined with the bis-trifluoromethanesulfonimide anion ([imide]-). The alkyl chain length shows a strong effect on the ordering of the chains at the topmost layer, reaching the maximum order for (C4H9)N+(CH3)2(C3H7) and (C6H13)N+C4H8(CH3). There is also evidence of gauche defects for longer alkyl chains, and changes in the spectral features for the shortest ones. The substitution of a carbon atom in the chain by oxygen, and its per-deuteration, provides the means to acquire a more complete description of the surface structure, and an unambiguous assignment of the vibrations detected in the SFG spectra. Finally, a brief comparison between alkylpyrrolidinium, and previously studied alkylimidazolium, cations is also established.