Infrared Reflection Absorption Spectroscopy Measurements Research Articles

In Situ Tracking of Nonthermal Plasma Etching of ZIF-8 Films.

Surface characterization is critical for understanding the processes used for preparing catalysts, sorbents, and membranes. Nonthermal plasma (NTP) is a process that achieves high reactivity at low temperatures and is used to tailor the surface properties of materials. In this work, we combine the capabilities of infrared reflection absorption spectroscopy (IRRAS) with NTP for the in situ interrogation of zeolitic imidazolate framework-8 (ZIF-8) thin films to probe modifications in the material induced by oxygen and nitrogen plasmas. The IRRAS measurements in oxygen plasma reveal etching of organic ligands with sequential removal of the methyl group and imidazole ring and with the formation of carbonyl moieties (C═O). In contrast, nitrogen plasma induces mild etching and grafting of nitrile groups (-C≡N). Scanning electron microscopy imaging shows that oxygen plasma, at prolonged times, significantly degrades the ZIF-8 film at the grain boundaries. Treatment of ZIF-8 membranes using mild plasma conditions yields a fivefold enhancement for H2/N2 and CO2/CH4 ideal selectivities and an eightfold enhancement for CO2/N2 ideal selectivity. Additionally, the new tools described here can be used for spectroscopic in situ tracking of plasma-induced chemistry on thin films in general.

Monolayer behavior of pure F-DPPC and mixed films with DPPC studied by epifluorescence microscopy and infrared reflection absorption spectroscopy

The monolayer behavior of a l-DPPC derivative with a single fluorination in one of its terminal methyl groups (F-DPPC) at air-water interface was investigated by epifluorescence microscopy and infrared reflection absorption spectroscopy (IRRAS). Epifluorescence microscopy was utilized to study the shape and morphology of liquid-condensed (LC) domains observed upon compression of the film. IRRAS was employed for the determination of chain order and orientation. The shapes of LC-domains in a monolayer of F-DPPC are more dependent on the rate of compression than those of DPPC. The LC domains of F-DPPC display pronounced fractal growth patterns depending on the compression speed. The evolution of LC domain occurs under dominating electrostatic dipolar forces in F-DPPC. IRRAS measurements with the analysis of the frequency of the methylene stretching vibrations as a function of film compression show that the acyl chains in an F-DPPC monolayer in the LE-phase are more disordered than those in a DPPC film. The reason for the higher chain disorder in LE phase F-DPPC monolayers is a back folding of the fluorinated sn-2 chain terminus towards the air-water interface leading to larger molecular area requirement. Angular dependent IRRA spectra of monolayers at a surface pressure of 30 mN m−1 show that in the LC phase DPPC and F-DPPC exhibit a similar tilt of the acyl chains of ca. 28−30 ° relative to the surface normal. F-DPPC is ideally miscible with l-DPPC-d62 having the same chirality, as indicated by epifluorescence images and by IRRAS. However, the LC domains in an equimolar mixture of d-DPPC and F-DPPC having opposite chirality show multi-lobed complex domain patterns indicating chiral phase separation within LC domains.

Characterization of neurocalcin delta membrane binding by biophysical methods.

Neurocalcin delta (NCALD) is a member of the neuronal calcium sensors protein family. In the retina, NCALD is expressed by ganglion and amacrine cells. NCALD is composed of 4 EF-hand motifs but only 3 of them may bind calcium. The binding of calcium induces a conformational change of the protein which leads to the extrusion of its N-terminal myristoyl group as well as some hydrophilic residues. This mechanism, named calcium-myristoyl switch, is presumably involved in its membrane binding. The parameters responsible for the interaction of NCALD with membranes are only partially known. The purpose of this study was thus to gather more information on the membrane binding behavior of NCALD using lipid monolayers, including the influence of the lipid composition, the calcium and the myristoyl group. NCALD was injected underneath different lipid monolayers and this model membrane allowed the determination of the binding parameters as maximum insertion pressure (MIP) and synergy. The values of MIP are larger when monolayers were composed of a saturated phospholipid with phosphoethanolamine polar head. This trend is confirmed by polarization modulation infrared reflection absorption spectroscopy measurements. Moreover, the observations by fluorescence microscopy show that NCALD preferentially interacts with phospholipids which are in the liquid-condensed physical state, as found in membrane microdomains. This observation could explain the changes of NCALD expression level in the brains of patients suffering from Alzheimer's disease because of the alteration of lipid composition in microdomains structures.

Triangular {Ni3} coordination cluster with a ferromagnetically coupled metal-ligand core

The coordination characteristics of a tridentate, π-conjugated Schiff base (HL·SMe) combined with 4-(methylthio)benzoic acid (Hbza·SMe) were explored in methanol solution. A two-step aerobic process involving the precursor NiCl2·6H2O afforded the trinuclear coordination compound [NiII3(L·SMe)4(bza·SMe)(MeOH)2]Cl0.5(CH3O)0.5·5.5MeOH·H2O (1), while the precursor Ni(ClO4)2·6H2O resulted in [NiII3(L·SMe)4(bza·SMe)(MeOH)2](ClO4)0.75(CH3O)0.25·1.5MeOH·0.75H2O (2); in both cases the thioether-containing ligands are key to isolation of crystalline products. Their solvent-free cationic motif [Ni3(L·SMe)4(bza·SMe)]+, supported by two different types of monodeprotonated chelate ligands, has been studied by electrospray ionization mass spectrometry. 1 and 2 crystallize in the triclinic space group P1¯ and monoclinic space group Cc, respectively, and feature a hitherto not observed triangular metal-ligand skeleton that is characterized by predominantly ferromagnetic exchange coupling between Ni(II) ions. Proof-of-concept infrared reflection–absorption spectroscopy measurements of 1 deposited on a gold substrate indicate that this moisture-stable compound retains its main structural features upon adsorption on the metal surface, which enables subsequent studies of catalytic properties of 1 on a solid support.

IR spectroscopic study of the displacement of an SF6 monolayer on graphite by Xe.

We report a study of displacement by xenon of a monolayer of sulphur hexafluoride initially condensed on a graphite surface. Earlier work showed that, below 112 K, Xe displaces SF6 almost completely in a first-order transition. Working at higher temperatures, we show that this system has a simple eutectic-like phase diagram, at least for SF6 not too dilute. In our experiment, both adsorbates are in equilibrium with their respective vapors in a cold cell. In our infrared reflection-absorption spectroscopy measurements, the SF6 coverage on the surface is monitored by the frequency shift due to dynamic dipole coupling of the collective mode of the strong SF6 ν3 vibrational resonance. Simulations relate this frequency shift to the SF6 areal density. Below T ≈ 134 K, with increasing Xe pressure, a small amount Xe dissolves in the solid SF6 monolayer preceding its displacement by a solid predominantly Xe monolayer in a first-order transition. Above 134 K, there is a weaker first-order transition to a mixed liquid monolayer, followed by continuous increase in Xe concentration. If the initial SF6 monolayer is near its melting line, the melting transition on adding Xe appears to become continuous.

Co-adsorption of oxygen and formic acid on rutile TiO2 (110) studied by infrared reflection-absorption spectroscopy

Adsorption of formic acid and co-adsorption with oxygen have been investigated on the rutile TiO2(110) surface using p- and s-polarized infrared reflection-absorption spectroscopy (IRRAS) at O2 exposures between 45 L to 8100 L and at temperatures between 273K and 343K. On the clean surface formic acid dissociates into a formate ion (formate) and a proton. Formate binds to two five-fold coordinated Ti atoms in the troughs along the [001] direction, and the proton binds to neighboring bridging O atoms. Exposure of adsorbed formate to O2 leads to a decrease in the asymmetric νas(OCO) band at 1532cm−1 and to the concomitant formation of a new vibration band at 1516cm−1. From the s-and p-polarized IRRAS measurements performed at different O2 exposures, surface pre-treatments and substrate temperatures, and by comparisons with previous reports, we conclude that the new species is a bidentate surface hydrogen carbonate, which is formed by reaction between formate and oxygen adatoms on the surface. The σv reflection plane of the surface hydrogen carbonate molecule is oriented along the [001] direction, i.e. the same direction as the adsorbed formate molecule. On the clean TiO2(110) surface exposed to O2 prior to formic acid adsorption, similar results are obtained. The reaction rate to form surface hydrogen carbonate from formate is found to follow first-order kinetics, with an apparent activation energy of Er=0.25eV.

Silanization of Sapphire Surfaces for Optical Sensing Applications.

Well-characterized silane layers are essential for optimized attachment of (bio)molecules enabling reliable chem/biosensor performance. Herein, binding properties and orientation of 3-mercaptopropyltrimethoxysilane layers at crystalline sapphire (0001) surfaces were determined by water contact angle measurements, infrared reflection absorption spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Infrared reflection absorption spectroscopy measurements suggest an almost perpendicular arrangement of the MPTMS molecules to the substrate surface. Adhesion force studies between a silicon nitride AFM tip and modified sapphire, gold, and silicon dioxide substrates were investigated by peak force tapping atomic force microscopy and used to define the silane binding properties on these surfaces. As expected, the Al-O-Si bond was determined to be responsible for the layer formation at the sapphire substrate surface.

Sucrose esters as biocompatible surfactants for penetration enhancement: An insight into the mechanism of penetration enhancement studied using stratumcorneum model lipids and Langmuir monolayers

Up to now, the molecular mechanism of the penetration enhancing effect of sucrose esters (SEs) on stratumcorneum (SC) has not been explained in details. In this study, variety of surface sensitive techniques, including surface pressure-area (π-A) isotherms, infrared reflection-absorption spectroscopy (IRRAS), and Brewster angle microscopy (BAM), have been used to investigate interactions between SEs and SC intercellular lipids. A monolayer of the mixture of ceramide AS C18:18, stearic acid and cholesterol in the molar ratio of 1:1:0.7 on an aqueous subphase is a good model to mimic a single layer of intercellular SC lipids. The π-A isotherms of mixed monolayers and parameters derived from the curves demonstrated the interaction between nonionic surfactants such as SEs and SC lipids. With increasing SE concentration, the resultant monolayer films became more fluid and better compressible. IRRAS measurements showed that SEs disordered the acyl chains of SC lipids, and the BAM images demonstrated the modification of the domain structures in SC monolayers. Longer chain-SE has a stronger disordering effect and is better miscible with ceramides in comparison to SE with a shorter hydrophobic part. In conclusion, this study demonstrates the disordering effect of SEs on the biomimetic SC model, pointing out that small changes in the structure of surfactant may have a strong influence on a penetration enhancement of lipophilic drugs through intercellular lipids of skin.

ChemInform Abstract: Surface Science Studies of Selective Deoxygenation on Bulk Molybdenum Carbide

AbstractReview: 57 refs.

Surface Science Studies of Selective Deoxygenation on Bulk Molybdenum Carbide

The carbonyl bond selective dissociative chemisorption of several ketones and acetaldehyde on bulk β-Mo2C was investigated using a combination of reflection absorption infrared spectroscopy (RAIRS), temperature-programmed desorption and X-ray photoelectron spectroscopy measurements. The planar, continuous β-Mo2C sample was prepared by temperature-programmed carburization of pure molybdenum foil. We review our results showing selective deoxygenation of cyclobutanone, cyclopentanone, cyclohexanone, acetone and acetaldehyde to form coadsorbed alkylidene and atomic oxygen species on the metal carbide. RAIRS measurements were used to detect surface oxo species. Olefin metathesis reaction steps were used to detect surface alkylidene species. The selective deoxygenation activity of the β-Mo2C foil is related to the C/Mo ratio at the surface.

Molecular adsorption of NO on PdO(101)

We investigated the adsorption of NO on the PdO(101) surface using temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations. From TPD, we find that NO reaches a saturation coverage of ~0.54 monolayer (ML) on PdO(101) at 90K, and that desorption from the saturated layer produces a complex TPD trace with five pronounced maxima between 100 and 550K. We observe negligible quantities of desorbing products other than NO during TPD, indicating that NO does not react on PdO(101) under the conditions studied. For low coverages, RAIR spectra reveal that NO adsorption at 90K produces both flat-lying and atop configurations of NO on the coordinatively unsaturated (cus) Pd atoms of PdO(101), but that these configurations are meta-stable and transform to bridge-NO species at higher temperature. RAIRS measurements further show that NO binding on atop-Pdcus sites becomes preferred with increasing coverage above ~25% of the Pdcus density, and provide evidence of a weakly-bound state at coverages above 100% (0.35 ML). Consistent with our experimental findings, DFT predicts that NO preferentially adsorbs on bridge-Pdcus sites at low coverage and that the site preference changes to atop-Pdcus sites with increasing NO coverage, resulting in mixtures of atop-bridge NO configurations as well as an all-atop configuration. DFT also predicts that NO can adopt a flat-lying configuration on the Pdcus row at low coverage, and can also achieve stable bonding on the fourfold Pd sites of PdO(101) at high NO coverages. Both the computational and experimental results show that NO binds on PdO(101) in multiple, local configurations even at low coverage.

The quantum nature of C-H···metal interaction: vibrational spectra and kinetic and geometric isotope effects of adsorbed cyclohexane.

The nature of C-H···M (M = metal surface) interactions is reviewed based mainly on our recent investigations of cyclohexane on Rh(111). Infrared reflection-absorption spectroscopy measurements at low temperature (∼20 K) have shown that the softened CH stretching band consists of several sharp peaks. At temperatures above 80 K, each peak is broadened, most probably by anharmonic coupling with thermally excited low-energy frustrated translational modes. The origin of fine structure in this band and its similarity to that in hydrogen bond systems are discussed. In addition, novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) using temperature programmed desorption, ultraviolet photoelectron spectroscopy, and spot profile analysis low-energy electron diffraction. The desorption energy of deuterated cyclohexane (C6 D12 ) is lower than that of C6 H12 (inverse kinetic isotope effect). In addition, the work function change by adsorbed C6 D12 is smaller than that by adsorbed C6 H12 . These results indicate that C6 D12 molecules are slightly more distant from the surface than C6 H12 molecules (vertical geometric isotope effect). A lateral geometric isotope effect was also observed for the two-dimensional cyclohexane superstructures as a result of the repulsive interaction between interfacial dipoles (= work function change). These isotope effects are ascribed to the quantum nature of hydrogen involved in the C-H···M interaction.

Structure and electrochemical activity of WOx-supported PtRu catalyst using three-dimensionally ordered macroporous WO3 as the template

PtRu/WOx catalyst is prepared in a polyol process with three-dimensionally ordered macroporous (3DOM) WO3 as the template in combination with an ammonia-leaching treatment. The morphology, composition and structure of the prepared catalysts are characterized by scanning transmission electron microscopy, energy dispersion X-ray spectroscopy, N2 adsorption, X-ray photoelectron spectroscopy and X-ray diffraction. The electrochemical activities are evaluated by linear sweep voltammetry, cyclic voltammetry, and chronoamperometry measurements in combination with in situ IR reflection absorption spectroscopy (IRRAS). The composition and particle size of the PtRu/WOx catalyst are similar to those of the PtRu catalyst prepared without the 3DOM WOx template, and both of the catalysts have a uniform element distribution, however, the PtRu/WOx catalyst has a more porous structure and greater metallic Pt proportion due to the support role of the residual WOx template and the interaction between WOx and PtRu nanoparticles. Electrochemical and in situ IRRAS measurements indicate that the PtRu/WOx catalyst has a greater electro-catalytic activity for methanol oxidation than the PtRu catalyst due to the assisting catalytic role of WOx and mass transport benefit of the porous structure.

Highly Selective, Facile NO2 Reduction to NO at Cryogenic Temperatures on Hydrogen Precovered Gold

We have discovered that NO(2) is reduced to NO at 77 K by hydrogen precovered gold in vacuum. Here, we investigate the partial reduction of NO(2) to NO on an atomic-hydrogen populated model gold catalyst for a more fundamental understanding of the surface chemistry of hydrogenation. Gold-based catalysts have been found to be active for many hydrogenation reactions, but few related fundamental studies have been conducted. Our experimental results reveal a high catalytic activity for gold: indeed, NO(2) is reduced to NO with 100% conversion and 100% selectivity at temperatures lower than 120 K. Density functional theory calculations and reflection-absorption infrared spectroscopy measurements indicate that HNO(2) and N(2)O(3) are intermediates which are highly dependent on surface hydrogen concentrations; subsequent hydrogenation of HNO(2) and dissociation of N(2)O(3) upon annealing induces the production of NO and H(2)O.

Leaching studies of potash–lime–silica glass with medieval composition by IRRAS

Potash–lime–silica glasses were extensively used during medieval times for several art objects and stained glass windows which are considered as a fundamental part of the decorative plan of churches and cathedrals in Northern Europe. The high potash content of this particular type of glass is the reason for its low chemical durability and hence accelerated deterioration, which entails the formation of leached layers and corrosion products, especially when exposed to atmospheric agents and to air pollutants.In order to preserve the value and to avoid the degradation of these artistic glasses, it is important to understand the chemical processes occurring on the glass surfaces by studying the role of glass composition and corrosion agents. For this purpose the leaching progress of two types of potash–lime–silica glasses (glass M1 with a silica content lower than 50% and high amounts of K2O and CaO, and glass M3 with 60% SiO2, 25% CaO and 15% K2O) was studied by using Infrared Reflection Absorption Spectroscopy (IRRAS) and the dependency of the corrosion rate on the nature of the acids (sulphuric, nitric and hydrochloric acids) was tested. The corrosion rate during the early stages of corrosion is different, depending on the type of acid used during the experiments: The results showed that glass M1 presents a high corrosion rate when leached in sulphuric acid in comparison to nitric and hydrochloric acids; in contrast to this result glass M3 is more resistant to the attack of sulphuric acid compared to the other acidic solutions and has a much higher chemical stability in comparison to glass M1.Furthermore the applicability of a self-built unit for IRRAS measurements was tested monitoring the progression of the leaching processes, in order to continue with further in-situ aging studies on the weathering mechanisms occurring on the glass surfaces.

Dissociative adsorption of hydrogen on the ZrB2(0001) surface

We have studied the dissociation of H2 on the ZrB2(0001) surface using density functional theory and reflection absorption infrared spectroscopy (RAIRS). Our results show that H2 readily dissociates on the Zr-terminated (0001) surface up to a H coverage of 1/2ML. Furthermore, we show that H is very mobile on the surface and that it desorbs between 545 and 625K. The calculated vibrational frequencies for the adsorbed H are in excellent agreement with our RAIRS measurements and with previously reported high resolution electron energy loss spectra.

Morphology-dependent activity of Pt nanocatalysts for ethanol oxidation in acidic media: Nanowires versus nanoparticles

The morphology of nanostructured Pt catalysts is known to affect significantly the kinetics of various reactions. Herein, we report on a pronounced morphology effect in the electrooxidation of ethanol and carbon monoxide (CO) on Pt nanowires and nanoparticles in an acidic solution. The high resolution transmission electron microscopy analysis showed the inherent morphology difference between these two nanostructured catalysts. Voltammetric and chronoamperometric studies of the ethanol electrooxidation revealed that these nanowires had a higher catalytic activity by a factor of two relative to these nanoparticles. The rate for CO monolayer oxidation exhibits similar morphology-dependent behavior with a markedly enhanced rate on the Pt nanowires. In situ infrared reflection–absorption spectroscopy measurements revealed a different trend for chemisorbed CO formation and CO2-to-acetic acid reaction product ratios on these two nanostructures. The morphology-induced change in catalytic activity and selectivity in ethanol electrocatalysis is discussed in detail.

Effect of Addition of SnOx to the Pt2Ru3/C Catalyst on CO Tolerance for the Polymer Electrolyte Fuel Cell

SnOx-modified Pt2Ru3/C catalysts were prepared by depositing SnOx nanoparticles onto a commercial Pt2Ru3/C catalyst at different pH values together with a post-treatment in reduction atmosphere to improve the catalytic activity for H2/CO electro-oxidation. The structure and morphology of Pt2Ru3/C and SnOx-modified Pt2Ru3/C catalysts were characterized by x-ray diffraction and scanning transmission electron microscopy. Electrochemical activities were evaluated by CO stripping voltammetry and a single cell test in combination with on line microgas chromatograph analysis and in situ infrared reflection–absorption spectroscopy (IRRAS). The Pt2Ru3/C and SnOx-modified Pt2Ru3/C catalysts had similar morphology and particle size. Electrochemical, on line microgas chromatograph analysis of anode outlet gas and in situ IRRAS measurements indicated that the SnOx-modified Pt2Ru3/C catalyst prepared at the pH value of 5 during the SnOx deposition process exhibited the greatest CO tolerance by improving the CO electro-oxidation efficiency.

12-Aminododecanoic acid as a corrosion inhibitor for carbon steel

The inhibiting effect of 12-aminododecanoic acid (AA) on corrosion of carbon steel (CS) was investigated in hydrochloric acid of different pH, temperatures and over a prolonged period of time, and also in some other selected corrosive solutions. It was found that AA inhibits both partial corrosion reactions, with a slightly stronger inhibition of the anodic corrosion reaction. The corrosion protection mechanism is by formation of a surface-adsorbed AA monolayer that offers a hydrophobic barrier to transport of solvated corrosive ions to the surface. A maximum inhibition efficiency of 98.8 ± 0.5% was achieved in 0.5 M HCl. The adsorption of AA onto the CS surface was described by the Langmuir adsorption isotherm. The corresponding standard Gibbs energy of adsorption was calculated to be −26 kJ mol −1. Polarization modulation infrared reflection absorption spectroscopy measurements revealed that the adsorbed AA monolayer is amorphous, which is due to the high heterogeneity of the CS surface.

Preferential CO oxidation in a large excess of hydrogen on Pt 3Sn surfaces

An experimental evaluation, under realistic conditions, of the catalytic properties of a Pt 3Sn(1 1 1) model catalyst in CO oxidation under excess of hydrogen is reported in this study. From a structural point of view, two different surface terminations are available in this alloy, viz. (2 × 2) and ( 3 × 3 ) R 30 ° , differing by their tin concentration and Pt–Sn local structural arrangements in the uppermost layers, thus offering specific active sites for the reaction to proceed. In this study, mass spectrometry and polarization modulation infrared reflection absorption spectroscopy measurements (PM-IRRAS) have been performed at near ambient pressure on both surfaces of the alloy and compared to those obtained on Pt(1 1 1) used as a reference. The presence of hydrogen promotes the rate of CO oxidation on the three surfaces. The influence of tin is remarkable, increasing the catalytic activity of Pt atoms by one order of magnitude. With the exception of CO, no other species such as formyls or carbonates are observed on the surface by IR spectroscopy in the range of temperature (293–425 K) investigated.