Time-resolved FTIR Spectra Research Articles

Efficient visible light photocatalytic antibiotic elimination performance induced by nanostructured Ag/AgCl@Ti3+-TiO2 mesocrystals

Efficient visible light photocatalytic elimination of pharmaceutical factory wastewater is one of widely concerned and greatly significant attempts in dealing with increasingly serious environmental pollution problems. In this work, hybrid Ag/AgCl nanoparticles were successfully introduced into Ti3+ self-doped TiO2 mesocrystals via precipitation and photo-reduction methods for efficient visible light photocatalytic antibiotic elimination. Some characterization measurements, such as XRD, SEM, TEM, XPS, UV–vis DRS spectra, fluorescence emission spectra, time-resolved fluorescence spectra, FT-IR spectra, and electrochemical measurements, were applied to investigate the physico-chemical properties of nanostructured Ag/AgCl@Ti3+-TiO2 mesocrystals. The experiment results showed that nanostructured Ag/AgCl@Ti3+-TiO2-60 composites presented the best visible light photocatalytic performance for tetracycline degradation, which was approximately 3.52 times of Ti3+-TiO2 mesocrystals and 22.43 times of commercial TiO2. Trapping experiments of active species and DMPO spin-trapping ESR experiments further confirmed that the visible light photocatalytic tetracycline degradation by Ag/AgCl@Ti3+-TiO2 mesocrystals were mainly due to the continuous attacks of photo-generated holes, OH and O2− radicals on tetracycline molecules. Besides, nanostructured Ag/AgCl@Ti3TiO composites were also employed in visible light photocatalytic industrial para-ester wastewater degradation. Finally, possible tetracycline transformation pathways and visible light photocatalytic tetracycline degradation mechanism over plasmonic Ag/AgCl@Ti3+-TiO2 composites were also proposed. Current work indicates that defective semiconductor mesocrystals and their composites have huge prospects in the refractory organic elimination.

Diffusion in polymers as investigated by two-dimensional correlation spectroscopy: The H2O/PCL system

Time-resolved FTIR spectra collected in situ during a sorption-desorption cycle constituted the experimental basis for a molecular level characterization of the H2O/PCL system. The data were analysed by a number of techniques (difference spectroscopy, least-squared curve-fitting (LSCF) and 2D-FTIR correlation spectroscopy (2D-COS) which provided relevant and complementary information on the diffusion mechanism and the molecular interactions formed (H-bonding). Particular attention was paid to the application of 2D-COS spectroscopy to diffusion studies and to the potential of this approach to disclose subtle details on the dynamic behaviour of the molecular specie involved. Its reliability was confirmed by comparison with simulation studies mimicking the system’s evolution during a sorption/desorption cycle. The conclusions drawn by 2D-COS were also supported by an analysis of the ν(OH) band of sorbed water with least-squared-curve-fitting techniques. The study has revealed two spectroscopically discernible water species – a first-shell, and a second-shell layer – which were quantified. It was concluded that on sorption, the concentration build-up of the first-shell layer is accelerated with respect to that of the second-shell, while the opposite occurs on desorption.

Gain and loss mechanisms for neutral species in low pressure fluorocarbon plasmas by infrared spectroscopy

This article examines the chemical reaction pathways of stable neutral species in fluorocarbon plasmas. Octafluorocyclobutane (c-C4F8) inductively coupled plasma discharges were found to primarily produce stable and metastable products downstream from the discharge, including c-C4F8, C2F4, C2F6, CF4, C3F8, C4F10, C3F6, and CF2. A novel analysis technique allows the estimation of gain and loss rates for neutral species in the steady state as functions of residence time, pressure, and discharge power. The gain and loss rates show that CF4, C2F6, C3F8, and C4F10 share related gain mechanisms, speculated to occur at the surface. Further analysis confirms that CF2 is predominantly produced at the chamber walls through electron impact dissociation of C2F4 and lost through gas-phase addition reactions to form C2F4. Additionally, time-resolved FTIR spectra provide a second-order rate coefficient of 1.8 × 10−14 cm3/s for the gas-phase addition of CF2 to form C2F4. Finally, C2F4, which is much more abundant than CF2 in the discharge, is shown to be dominantly produced through electron impact dissociation of c-C4F8 and lost through either surface or gas-phase addition reactions.

Analysis of the time-resolved FTIR spectra produced by the photolysis of alkyl phenylglyoxylates

The photochemistry of alkyl phenylglyoxylates (APG) was further investigated using time-resolved infrared spectroscopy. The primary focus was on the analysis of weak transient bands around 1828 and 1730 cm(-1) in the time-resolved FTIR spectra of glyoxylates. The observed transients were assigned to benzoyl and alkyl mandelate ester radicals, respectively. The formation of benzoyl radical was fast and attributed to the Norrish Type I process. In addition, the intensities of the strong FTIR bands around 1680 and 2100 cm(-1) were used to analyze the yields of the triplet state and ketene, respectively. These new and previous data on APG photochemistry are discussed in relation to the acrylate polymerization photoinitiation by alkyl phenylglyoxylates.

Multivariate curve resolution of rapid-scan FTIR difference spectra of quinone photoreduction in bacterial photosynthetic membranes

Photosynthetic reaction centres and membranes are systems of particular interest and are often taken as models to investigate the molecular mechanisms of selected bioenergetic reactions. In this work, a multivariate curve resolution by alternating least squares procedure is detailed for resolution of time-resolved difference FTIR spectra probing the evolution of quinone reduction in photosynthetic membranes from Rhodobacter sphaeroides under photoexcitation. For this purpose, different data sets were acquired in the same time range and spectroscopic domain under slightly different experimental conditions. To enable resolution and provide meaningful results the different data sets were arranged in an augmented matrix. This strategy enabled recovery of three different species despite rank-deficiency conditions. It also results in better definition (identity and evolution) of the contributions. From the resolved spectra, the species have been attributed to: 1. the formation of ubiquinol, more precisely the disappearance of Q/appearance of QH(2); 2. conformational change of the protein in the surrounding biological medium; 3. oxidation of diaminodurene, a redox mediator. Because, moreover, results obtained from augmented data sets strategies enable quantitative and qualitative interpretation of concentration profiles, other effects, for example the consequence of repeated light excitation of the same sample, choice of illumination power, or the number of spectra accumulated could be compared and discussed.

Direct observation of the kinetically relevant site of CO hydrogenation on supported Ru catalyst at 700 K by time-resolved FT-IR spectroscopy

Time-resolved FT-IR spectra of carbon monoxide hydrogenation over alumina-supported ruthenium particles were recorded on the millisecond time scale at 700 K using pulsed release of CO and a continuous flow of H(2)-N(2) (ratio 0.067 or 0.15, 1 atm total pressure). Adsorbed carbon monoxide was detected along with gas phase products methane (3016 and 1306 cm(-1)), water (1900-1300 cm(-1)), and carbon dioxide (2348 cm(-1)). Aside from adsorbed CO, no other surface species were observed. The rate of formation of methane is 2.5 +/- 0.4 s(-1) and coincides with the rate of carbon dioxide growth (3.4 +/- 0.6 s(-1)), thus indicating that CH(4) and CO(2) originate from a common intermediate. The broad band of adsorbed carbon monoxide has a maximum at 2010 cm(-1) at early times (36 ms) that shifts gradually to 1960 cm(-1) over a period of 3 s as a result of the decreasing surface concentration of CO. Kinetic analysis of the adsorbed carbon monoxide reveals that surface sites absorbing at the high frequency end of the infrared band are temporally linked to gas phase product growth. Specifically, a (linear) CO site at 2026 cm(-1) decays with a rate constant of 2.9 +/- 0.1 s(-1), which coincides with the rise constant of CH(4). This demonstrates that the linear CO site at 2026 cm(-1) is the kinetically most relevant one for the rate-determining CO dissociation step under reaction conditions at 700 K.

Millisecond step-scan FT-IR transmission spectroscopy under transient reaction conditions: CO oxidation over Pt/Al2O3

Time-resolved FT-IR spectra of CO oxidation over Pt/Al2O3 catalysts were collected in situ in transmission mode under transient reaction conditions. Using the step-scan acquisition mode of a commercial FT-IR spectrometer, time-resolution of a few milliseconds, compared to several hundred milliseconds in the normal rapid-scan mode, could be achieved. The experiments were triggered by reproducible oxygen gas pulses at constant temperature. Infrared light transmission, uniform heating and fast gas exchange were realized by pressing the pure catalyst powder onto a thin stainless steel wire mesh using a low volume reaction cell. By comparing the results of both acquisition modes, rapid-scan and step-scan, we will demonstrate, that it is possible, in a rather simple way, to investigate heterogeneously catalysed reactions under reaction conditions by means of FT-IR spectroscopy with time-resolutions down to a few milliseconds, and, in principle, lower. CO oxidation between 443 and 573K over 1%Pt/γ-Al2O3 and 4%Pt/γ-Al2O3 catalysts has been investigated as a test reaction. By using the high time-resolution of the step-scan acquisition mode we could obtain new details of the dynamics of the CO oxidation reaction over a supported Pt catalyst at high temperatures and 1atm pressure.

Direct Observation of Surface Ethyl to Ethane Interconversion upon C2H4 Hydrogenation over Pt/Al2O3 Catalyst by Time-Resolved FT-IR Spectroscopy

Time-resolved FT-IR spectra of ethylene hydrogenation over alumina-supported Pt catalyst were recorded at 25 ms resolution in the temperature range of 323-473 K using various H2 concentrations (1 atm total gas pressure). Surface ethyl species (2870 and 1200 cm(-1)) were detected at all temperatures along with the gas-phase ethane product (2954 and 2893 cm(-1)). The CH3CH2Pt growth was instantaneous on the time scale of 25 ms under all experimental conditions. At 323 K, the decay time of surface ethyl (122 +/- 10 ms) coincides with the rise time of ethane (144 +/- 14 ms). This establishes direct kinetic evidence for surface ethyl as the relevant reaction intermediate. Such a direct link between the temporal behavior of an unstable surface intermediate and the final product in a heterogeneous catalytic system has not been demonstrated before. A fraction (25%) of the asymptotic ethane growth at 323 K is prompt, indicating that there are surface ethyl species that react much faster than the majority of the CH3CH2Pt intermediates. The dispersive kinetics is attributed to the varying strength of interaction of the ethyl species with the Pt surface caused by heterogeneity of the surface environment. At 473 K, the majority of ethyl intermediates are hydrogenated prior to the recording of the first time slice (24 ms), and a correspondingly large prompt growth of ethane is observed. The yield and kinetics of the surface ethylidyne are in agreement with the known spectator nature of this species.

In situ step-scan time-resolved microscope FTIR spectroscopy applied in irreversible electrochemical reactions

We demonstrated in the present paper, for the first time, the application of step-scan time-resolved FTIR external reflection spectroscopy in studies of irreversible electrochemical reactions. Through combination of a particularly designed flow thin layer IR cell and a nanostructured microelectrode, the solution in the thin layer between IR window and microelectrode as well as adsorbates on electrode surface can be rapidly renewed, which allows irreversible reactions to be repeated quickly. Electrooxidation of CO adsorbed on a nanostructured platinum microelectrode (nm-PtME) in H 2SO 4 solutions was employed as a test reaction to illustrate the performance of the newly developed technology. It has confirmed that to regenerate a CO adlayer on the nm-PtME after stripping completely CO adsorbates may take about 6 s in CO-saturated solution (CO concentration ∼1.0 mM) at a flow rate of 20 ml min −1, which satisfies the requirement of the repeatability in step-scan FTIR data collection. Time-resolved FTIR spectra for CO irreversible oxidation have been recorded at a time resolution of 250 μs. It has been revealed that the IR band center of CO slightly decreases with time within initial 4 ms and then maintains a constant value in the evolvement of the oxidation. Such variation demonstrates that CO oxidation on nm-PtME is mainly occurred in through a nucleation and growth mechanism. The present study has thrown light on investigation of fast dynamic processes and kinetics of irreversible electrochemical reactions by using in situ step-scan time-resolved FTIR spectroscopy.

Observation of the two triplet state conformations of alkyl phenylglyoxylates.

Step-scan time-resolved FT-IR spectra of alkyl phenylglyoxylates in hexane with a 4 cm(-1) spectral resolution reveal splitting of the transient absorption signal near 1650 cm(-1) into two closely located peaks with different lifetimes on the nanosecond time scale. This signal had been previously assigned to the triplet state of the starting material that gives rise to the alpha-hydroxyphenylketene. In the current article, evidence is presented to assign these two peaks to different triplet state conformers only one of which undergoes fast Norrish Type II photoelimination. This assignment was made on the basis of chemical reactivity, steric effects, and kinetic data.

Structure and Properties of the Mesophase of Syndiotactic Polystyrene IV. Release of Guest Molecules from δ Form of Syndiotactic Polystyrene by Time Resolved FT-IR and WAXD Measurement

Syndiotactic polystyrene(sPS) forms clathrates with organic solvent molecules between helical chains of trans–trans–gauche–gauche(TTGG) conformation, so called δ form complex. By annealing or extraction, the δ form transforms to the mesophase which has highly regulated molecular cavities between helical chains. Time resolved FT-IR spectra of the sPS films containing δ form complex with toluene were obtained to clarify the relationship between the release of toluene from the δ form and conformational changes of sPS backbone chain. From the results of FT-IR, content of TTGG conformation decreased with annealing time, and increased with extraction time by acetone, indicative of the formation of γ form from amorphous phase. The mesophase extracted holds high content of TTGG conformation and is stable, compared to annealed sample. The solvent extraction method is thus a better technique to make mesophase membranes containing molecular cavities.

Application of Time-Resolved Step-Scan Fourier Transform Infrared Spectroscopy to Photochemical Mechanistic Investigations of Alkyl Phenylglyoxylates,

The photochemical reactivity of alkyl phenylglyoxylates has been investigated using time-resolved step-scan Fourier transform infrared spectroscopy. The time-resolved FTIR spectra of hexane solutions of methyl (MPG), ethyl (EPG), and isopropyl (iPPG) phenylglyoxylates and benzene solutions of MPG display two major transient absorption peaks in the carbonyl fundamental region around 1661 and 2105 cm-1. On the basis of the spectroscopic and kinetic evidence, the 1661 cm-1 transitions are assigned to the carbonyl fundamental of the triplet state of the corresponding alkyl phenylglyoxylate. The 2105 cm-1 band is attributed to the carbonyl stretch of α-hydroxyphenyl ketene, which is the postulated intermediate of the intramolecular hydrogen abstraction in alkyl phenylglyoxylates. These assignments are supported by the results of DFT calculations. From kinetic time profiles for the two observed transient infrared bands, the rate constants for the intra- and intermolecular γ-hydrogen abstraction were obtained an...

Photolysis of gamma-(alpha-carboxy-2-nitrobenzyl)-L-glutamic acid investigated in the mcrosecond time scale by time-resolved FTIR.

The photolytic release of substrates from caged substrates has proven to be an excellent method to generate concentration jumps for kinetic measurements in the microsecond time scale. In this report we use time-resolved FTIR in the step-scan mode to probe the photolysis mechanism of one such caged compound, namely gamma-(alpha-carboxy-2-nitrobenzyl)glutamate, and to obtain a direct measure of the rate of photorelease of the substrate glutamate. The time-resolved difference FTIR spectra exhibit specific signals that can be assigned to the reactant caged glutamate, photolytically released product glutamate, as well as to the aci-nitro intermediate, the key intermediate of the photolysis reaction. Therefore these signals allow the characterization of the kinetics of formation and decay of the intermediate and products. This is the first such report that provides a direct determination of the rate of formation of the photolysis product from a caged compound in the microsecond time scale. Furthermore, the results presented provide a good basis for further time-resolved FTIR studies of molecular reaction mechanisms, such as ligand protein interactions, in the microsecond time scale through the photolytic release of substrates from caged compounds.

Monitoring the Allosteric Transition and CO Rebinding in Hemoglobin with Time-Resolved FTIR Spectroscopy

Time-resolved FTIR spectra are reported for the photocycle of carbonmonoxy hemoglobin, under saturating photolysis conditions, which are sufficient to drive the R−T allosteric transition. Direct evidence for this transition was provided by the microsecond time scale evolution of an 1857 cm-1 bisignate difference band (cysteine S−H stretching), which is a marker of the T state. The time course of the strong 1951 cm-1 band of bound CO showed the expected fast geminate and slower second-order rebinding phases. Two slow phases were observed, having time constants consistent with reported binding rates for R and T state molecules. The geminate yield was 50%, the majority (37%) rebinding with a 70 ns time constant, consistent with previous studies, but an additional low-amplitude (13%) phase was resolved, with an 890 ns time-constant. Difference FTIR bands are also observed in the 1300−1700 cm-1 region, where protein vibrations are expected. In the nanosecond regime these bands varied irregularly, due to instrument limitations, but in the microsecond regime they evolved (30 μs time constant) toward the static difference spectrum of HbCO minus deoxyHb, reflecting the expected evolution from R to T state photoproduct molecules. The difference spectra of R and T photoproduct molecules extracted from the data via kinetic analysis contain not only common bands but also bands that are distinctive. The R photoproduct difference spectrum contains a positive/negative band pair at 1649 and 1683 cm-1, which is interpreted as resulting from the breaking of one or more α-helical carbonyl H-bonds. Candidate H-bonds are those that connect the H-helix residues Tyr α140 and β145 with the F-helix residues Val α93 and β95, in both HbCO and deoxyHb. These H-bonds are believed to break and reform at intermediate stages of the allosteric pathway, on the basis of UV Raman evidence.

Two-Dimensional Correlation Analysis of Time-Resolved Step-Scan FT-IR Spectra of a Liquid Crystalline Guest—Host System in an Electric Field

The time-resolved step-scan FT-IR spectra of a solution of 2-naphthaldehyde in a nematic solvent were recorded during switching of an electric field. The mesogen of the nematic solvent consisted of a nitrile-substituted bicyclohexyl unit and had a flexible aliphatic chain attached to it. Two-dimensional (2D) correlation analysis of the dynamic spectra was undertaken in order to investigate the details of the molecular orientation and relaxation of the sample under the influence of this electric field. In addition, from the time-resolved spectra the relaxation times of selected functionalities were estimated. The results yield strong evidence that both the nematic solvent and the solute reorient in phase at similar rates. However, the movement of the hydrocarbon chain of the nematic solvent seems to be slightly delayed with respect to the mesogen and the solute. It has been shown that the rigidity of the mesogen plays an important role in the mechanism of the electric-field-induced reorientation of the aliphatic chain.

FT-IR Technique for Monitoring the Interfacial Interactions of a Surfactant Solution and a Solid Hydrocarbon

A cylindrical internal reflection element (IRE) has been used as a support for a solid hydrocarbon-aqueous nonionic surfactant interface. Time-resolved FT-IR spectra of the interface during the removal of the hydrocarbon from the IRE surface by the detergent action of flowing surfactant solutions have been recorded. Qualitative comparisons of the detergency of different surfactants can be made. With the use of subtraction techniques, the spectra of interfacially adsorbed and aqueous surfactant can be obtained for comparison. Significant adsorption of a relatively hydrophobic ethoxylated alcohol (Neodol 25–3) at the interface was detected. The ether group bands of the adsorbed surfactant were different from those in the spectrum of an aqueous dispersion of the surfactant. The removal of hydrocarbon from the IRE by the Neodol 25–3 was much slower than that caused by a more hydrophilic material, Neodol 23–6.5.