采用CO吸附原位红外光谱研究Pt/TiO_2光催化体系中助催化剂作用

采用CO作为探针分子,将原位透射红外光谱应用于研究助催化剂Pt在Pt/TiO2光催化体系中的作用.实验发现,光照条件下,CO的覆盖度及样品温度没有明显变化,CO在Pt/TiO2上的吸附峰红移11 cm–1,在Pt/Al2O3上的吸附峰没有位移,表明CO在Pt/TiO2上的红移来源于TiO2上的光生电子转移到Pt上,这解释了为什么加入Pt助催化剂可提高光催化产氢活性.     

Dynamics of CO in mesoporous silica monitored by time-resolved step-scan and rapid-scan FT-IR spectroscopy

Carbon monoxide molecules generated in the channels of mesoporous MCM-41 silica sieve from a precursor (diphenyl cyclopropenone) by photodissociation with a nanosecond laser pulse were monitored by time-resolved Fourier transform infrared (FTIR) spectroscopy using the step-scan and rapid-scan methods. A very broad absorption of CO is observed in the region 2200-2080 cm(-1) at room temperature that decays in a biphasic mode. Two-thirds of the band intensity decays on the hundreds of microsecond scale (lifetime 344 +/- 70 micros). The process represents the escape of the molecules through the mesopores into the surrounding gas phase, and a diffusion constant of 1.5 x 10(-9) m(2)/s is derived (assuming control by intra-MCM-41 particle diffusion). The broad profile of the absorption is attributed to contact of the random hopping CO with siloxane and silanol groups of the pore surface. Measurements using MCM-41 with the silanols partially capped by trimethyl silyl groups gave further insight into the nature of the IR band profile. These are the first observations on the diffusion behavior of carbon monoxide in a mesoporous material at room temperature. The residual carbon monoxide remains much longer in the pores and features distinct peaks at 2167 and 2105 cm(-1) characteristic for CO adsorbed on SiOH groups C end on and O end on, respectively. The bands decrease with time constants of 113 +/- 3 ms (2167 cm(-1)) and 155 +/- 15 ms (2105 cm(-1)) suggesting that CO in these sites is additionally trapped by surrounding diphenyl acetylene coproduct and/or precursor molecules.     

A polymer double-layer studied by step-scan FTIR photoacoustic spectroscopy

Photoacoustic (PA) magnitude and phase spectra of a polyethylene/polyurethane double-layer are presented. The extension of the Rosencwaig-Gersho theory to a double-layered polymer film introduced by N.C. Fernelius in 1980 is applied to compare the experimental spectra with simulations. A reasonable agreement between experiment and simulation is observed. It is shown that FTIR-PAS in combination with simulation provides an important technique for depth profiling in polymeric laminates.     

Spectroelectrochemical studies of magnesium deposition by in situ FTIR spectroscopy

Magnesium can be reversibly deposited electrochemically from solutions of ethereal solvents, with Grignard reagents (RMgX) or complexes of Mg(AX_3−nR_n+1)₂ stoichiometry as the electrolytes (A=Al, B; X=Cl, Br; R=alkyl or aryl groups). These processes are far from being simple reactions of the Mg/Mg⁺⁺ couple, since the above electrolytes in solutions have complicated structures in which the ether molecules play an important stabilization role. In addition, Mg deposition processes in all of the above solutions are accompanied by adsorption phenomena. The surface chemistry of magnesium electrodes was studied in situ by FTIR spectroscopy, using an internal reflectance mode. The electrolyte solutions studied included tetrahydrofuran (THF) solutions of the RMgX electrolytes (R=butyl, ethyl, methyl benzyl, and X=Cl, Br); Mg(AlCl₂BuEt)₂; Mg(AlCl₃Bu)₂ and Mg(BPh₂Bu₂); Bu, Et, Ph=butyl, ethyl and phenyl groups, respectively. It was clear from these studies that Mg electrodes do not develop stable passivation in these solutions (i.e. formation of surface films). The nature of the adsorbed species in the above systems is discussed, based on the spectral results.     

FTIR spectroscopy of alcohol and formate interactions with mesoporous TiO2 surfaces

The effects of pH and ultraviolet (UV) light with ligated formic acid on mesoporous TiO2 were characterized by transmission Fourier transform infrared (FTIR) spectroscopy and compared with adsorbed formate complexes. Surface-modified anatase thin films were prepared from acidic aqueous nanoparticulate anatase suspensions diluted with methanol and ethanol. Bands assigned to carboxylic acid groups displayed unique bonding character in the ligated formic acid on the anatase surface. For increased proton concentrations in the films, separation in -COO stretching bands (delta nu) for formic acid increased (increase in frequency for nuC=O and decrease in frequency for nuC-O). With UV exposure, surface-bound organics were rapidly removed by photocatalytic oxidation at 40 degrees C and 40% relative humidity (RH). In addition, the delta nu of the formic acid bands decreased as organics were mineralized to carbonates and CO2 with UV light. Aqueous formic acid adsorption experiments showed a distinctly different bonding environment lacking carbonate, and the delta nu for the carboxylic groups indicated a bridging bidentate coordination. The delta nu of the bands increased with increasing proton concentration, with both bands shifting to higher wavenumbers. The shifts may be ascribed to the influence of protonation on surface charge and the effect of that charge on the electronegativity of carboxylate groups bound to the surface. As alcohols are used in the mesoporous TiO2 solar cell preparation, implications of these surface modifications to dye-sensitized photovoltaics are discussed.     

CO inner-shell excitation studied by electron impact spectroscopy

The inner-shell excitation and decay of the CO molecule have been studied in electron impact experiments. The dipole-forbidden transition (1sσ_c)⁻¹(2pπ) ³Π has been characterized by angular resolved electron energy loss spectroscopy and its decay via the measurement of resonant Auger spectra. The contribution of the (1sσ_c)⁻¹(2pπ) ³Π state to the CO resonant Auger spectrum in the region of the “spectator transitions” has been isolated and the population of CO⁺ quartet final states has been observed.     

Hydration of aprotic donor solvents studied by means of FTIR spectroscopy

The paper attempts to explain the mutual influence of nonpolar and electron-donor groups on solute hydration, the problem of big importance for biological aqueous systems. Aprotic organic solvents have been used as model solutes, differing in electron-donating power. Hydration of acetonitrile, acetone, 2-butanone, and triethylamine has been studied by HDO and (partially) H2O spectra. The quantitative version of difference spectra method has been applied to determine solute-affected water spectra. Analysis of the data suggests that solvent-water interaction via the donor center of the solute is averaged between water-water interactions around the solute. Such behavior can be simply explained by the model of solute rotating in a cavity of water structure, which is formed by clathratelike hydrogen-bonded water network. On the basis of the band shape of solute-affected HDO spectra and the corresponding distribution of intermolecular distances, the criterion for hydrophobic type hydration has been proposed. From that point of view, all the studied solutes could be treated as hydrophobic ones. The limiting band position and the corresponding intermolecular distance of affected water, gained with increasing electron-donating power of solutes, has been inferred from the data obtained. These observations are important for interpretation of vibrational spectra of water as well as for volumetric measurements of solutions. The simple model of hydration, proposed to better justify the results, connects the values obtained from the methods providing microscopic and macroscopic characteristics of the system studied.     

Charge transport in luminescent polymers studied by in situ fluorescence spectroscopy

Modulation of the photoluminescence of poly-[2,7-(fluorene)-1,4-(phenylene)] can be attained by reversible electrochemical modification of the conjugated chain (p- or n-doping). Controlled injection of charge quenches the fluorescent emission of the conjugated polymer. The injection of holes completely eliminates the emission, while the electrons only quench up to one-third of the initial fluorescence of the polymer. Analogous quenching effects have been previously reported for solid-state organoelectronic devices. Electrochemical Stern-Volmer plots permit the estimation of the relative mobility of charge carriers in the polymer layer. The mobility of holes is 1 order of magnitude higher that the mobility of electrons, as determined by this method.     

Copper-on-copper homoepitaxy studied by infrared spectroscopy of adsorbed CO

Infrared spectroscopy of adsorbed CO was used to characterize the dependence of surface structure on deposition temperature during homoepitaxial growth on Cu(100). Intensity borrowing due to dipole coupling greatly enhances the absorption signal due to defect-bonded CO, making it possible to detect and quantify defect concentrations at the level of a few percent. For deposition temperatures between 300 and 400 K, the defect density increases slightly with decreasing deposition temperature but remains below 2%. There is a sharp increase in defect density, up to 5%-6%, as the deposition temperature is decreased from 300 to 250 K. At lower deposition temperatures, there is some sign of a leveling off in defect density, but the IR absorption spectrum becomes so broad that meaningful analysis becomes impractical, while visible degradation of the low-energy electron diffraction pattern indicates worsening surface order. No indication of "re-entrant" ordering at low temperatures was observed for deposition temperatures down to 150 K.     

Adsorption behavior of human serum albumin on ATR crystal studied by in situ ATR/FTIR spectroscopy and two-dimensional correlation analysis

The time-dependent adsorption behavior of human serum albumin (HSA) onto an ATR (ZnSe) crystal was investigated by two-dimensional (2D) correlation analysis and in situ ATR-FTIR spectroscopy following the secondary structural changes in the amide I region. The two major advantages of the generalized 2D correlation spectroscopy were first tested. New extra bands have been resolved by 2D correlation analysis, but they are either artifacts or a result of uncertainty on band position in generalized 2D correlation spectroscopy. The sequence of the intensity variations of the three sub-bands under the amide I band profile deduced from the 'sequential order' rules is contradictory to the experimental observation, which supports our argument on the 'sequential order' rules in generalized 2D correlation spectroscopy (H. Huang, Anal. Chem., 2007, 79, 8281-8292). Subsequent detailed analysis on the in situ ATR-IR spectra shows that the adsorption process of HSA on the ATR (ZnSe) crystal in aqueous solutions can be divided into three stages: no obvious conformational transitions in the first 25 min of adsorption of HSA molecules; large structural rearrangement from α-helix to random coil and short extended chain structures in a fully cooperative way from 25 to 50 min of adsorption; and further slight conformational transformation of short extended chain and turn structures into random coil with no sequential order after 50 min of adsorption.     

In situ high pressure study of ZIF-8 by FTIR spectroscopy

ZIF-8 as a promising storage material was investigated at high pressures up to ~39 GPa by in situ FTIR spectroscopy for the first time. Structural modifications are found to be reversible in a low-pressure region but irreversible in a high-pressure region. Overall, the ZIF-8 framework exhibits an unusual chemical stability even under extreme compression.     

Degradation behaviour of polyurethane studied by linear temperature programmed pyrolysis FTIR spectroscopy

FTIR spectrometers with two measurement beams offer the possibility to record and process spectra from 4000 to 400 cm⁻¹ taken every 30 seconds in situ from a polyurethane film which is subjected to a programmed linear rise of temperature from ambient to 400°C and to take quasi simultaneously the spectra of the gaseous pyrolysis products. As an example, polyurethane (PUR) prepared from diphenylmethane diisocyanate (MDI) with polyesteralcohol from adipic acid was thermally degraded to isocyanate and polyester alcohol. Carbodiimid was found in the pyrolysis products and carbon dioxide was detected in the pyrolysis gas. The isocyanate evaporation and the thermal degradation of polyester alcohol were studied.     

Kinetics of the CO oxidation reaction on Pt(111) studied by in situ high-resolution x-ray photoelectron spectroscopy

High-resolution x-ray photoelectron spectroscopy has been used to study the kinetics of the CO oxidation reaction on a Pt(111) surface in situ. The study focuses on the interaction of a preadsorbed p(2x2) layer of atomic oxygen with CO dosed using a supersonic molecular beam. Measurements of O 1s and C 1s spectra at 120 K show that CO adsorbs on the oxygen precovered substrate, but no reaction occurs. A maximum CO coverage of 0.23 ML (monolayer) is observed, with CO exclusively bound on on-top sites. In accordance with the literature, bridge sites are blocked by the presence of atomic oxygen. The reaction of CO with preadsorbed O to CO(2) is studied isothermally in a temperature range between 275 and 305 K. The reaction rate initially increases with CO pressure, but saturates at 9x10(-7) mbar. The data indicate that a certain amount of disordered oxygen within the p(2x2) layer acts as a starting point of the reaction and for a given temperature reacts with a higher rate than O in the well-ordered oxygen p(2x2) phase. For the reaction of CO with this ordered phase, the results confirm the assumption of a reaction mechanism, which is restricted to the edges of compact oxygen islands. The activation energy of the reaction is determined to (0.53+/-0.04) eV, with a prefactor of 4.7x10(6+/-0.7) s(-1).     

FTIR spectroscopy study of CO adsorption on Pt-Na-mordenite

Different carbonyls are formed after CO adsorption at ambient temperature on a Pt-Na-mordenite (Pt-Na-MOR) sample. Pt(3+)(CO)(2) dicarbonyls (nu(s) at 2205 cm(-1) and nu(as) at 2167 cm(-1)) are decomposed without formation of monocarbonyls. The respective mixed-ligand species, Pt(3+)((12)CO)((13)CO), formed after (12)CO-(13)CO coadsorption, display bands at 2192 and 2131 cm(-1), in excellent agreement with the theoretically calculated values. Pt(2+)-CO species absorb at 2145 cm(-1) and are not able to accept a second CO molecule. Pt(+)-CO carbonyls are characterized by a band at 2111 cm(-1). Under CO equilibrium pressure, these species are converted into dicarbonyls (nu(s) at 2135 cm(-1) and nu(as) at 2101 cm(-1)). The respective mixed-ligand species, Pt(+)((12)CO)((13)CO), manifest bands at 2123 and 2069 cm(-1), in good agreement again with the theory. Different carbonyls of metallic platinum are observed below 2100 cm(-)(1). In addition, weakly adsorbed CO was registered as Na(+)-CO complexes (2177 and 2165 cm(-1)) and Na(+)-OC-Na(+) species (2138 cm(-1)). It was found that during desorption of CO platinum was reduced, ultimately to metal. However, heating in a NO + O(2) mixture leads to reoxidation of the metal particles and restoration of the initial state of the sample.     

Effect of CO2 exposure on free volumes in polystyrene studied by positron annihilation spectroscopy

Free‐volume properties, size and distribution, in amorphous polystyrene exposed to CO₂ gases have been measured as a function of pressure to 800 psi (5.5 MPa), of time, and of temperature using positron annihilation lifetime spectroscopy. The free volume increases significantly and its distribution broadens as a function of pressure. The free volume relaxes as a function of time with a characteristic time of 15 h, and 5.7 h for 400, and 800 psi, respectively, after depressurizing under vacuum. A portion of free volume created by CO₂ exposure remains permanently in the polymer after CO₂ exposure. The glass transition temperature decreases significantly as a function of CO₂ pressure from the free‐volume data and is compared with the differential scanning calorimeter results. The observed free‐volume variations as a function of pressure, time, and temperature are discussed in terms of hole expansion, creation, free‐volume relaxation, plasticization, and hole filling in amorphous polymers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 388–405, 2008     

Sunlight-induced effective heterogeneous photocatalytic decomposition of aqueous organic pollutants to CO2 assisted by a CO2 sorbent, amine-containing mesoporous silica

Photocatalytic mineralization of aqueous formic acid and phenol on pure TiO(2) under sunlight irradiation was substantially accelerated to give a reliable photocatalytic efficiency by conducting the reactions in the presence of a CO(2) sorbent, amine-containing SBA-15, placed in the gas phase of the reactor.     

Subnanometer hole properties of CO2-exposed polysulfone studied by positron annihilation lifetime spectroscopy

Positron annihilation lifetime (PAL) spectroscopy has been employed to study subnanometer hole properties in polysulfone (PSF). In this study, hole properties of size, fraction, and distribution of PSF exposed to CO₂ are reported. In the PSF/CO₂ system, the hole size and fraction significantly increase and the free-volume distribution broadens as a function of CO₂ pressure in the range of 0–1000 psi. Hysteresis in hole properties is observed during CO₂ sorption/desorption cycle. The high sensitivity of PAL results due to CO₂ exposure in PSF is explained in terms of the microstructural changes in the polymer matrix, i.e., filling penetrant and plasticization, gas hydrostatic pressure effect, and creation of free volumes and holes. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3049–3056, 1998     

In situ SAXS investigation of dibromomethane adsorption in ordered mesoporous silica

A SAXS/WAXS apparatus with the aid of a specially designed sample cell capable for performing both SAXS and WAXS experiments was used for adsorption studies in nanoporous materials. The applicability of the instrument for structural investigations and its ability for adsorption experiments because of the advanced sample environment were demonstrated by carrying out in situ SAXS measurements during gas physisorption. SAXS profiles of ordered mesoporous silica were measured at selected equilibrium points alongside a dibromomethane (CH₂Br₂) adsorption isotherm at 293 K. SBA-15 was the adsorbent of choice because it consists of a regular 2D hexagonal array of cylindrical mesopores that gives rise to Bragg reflections in the small-angle regime. CH₂Br₂ was selected as a contrast-matching fluid because it has almost the same electron density as silica. We obtained high-quality data comparable to those resulting from experiments performed in synchrotron light sources which produce intense beams of x-rays and support advanced instrumentation for high-resolution diffraction and SAXS studies. The Bragg peaks of the pore lattice are clearly visible for the evacuated sample and at the early stages of the adsorption process. The intensity decrease and the elimination of the Bragg peaks for the saturated sample suggest that an almost perfect contrast matching was achieved. A model has been used for monitoring the fluid condensation and evaporation regime in SBA-15 by taking into account both the Bragg scattering and the diffuse scattering for spatially random pore filling. The results show the absence of spatial correlations between filled pores suggesting random pore filling.     

Basic sites on periodic mesoporous organosilicas investigated by XPS and in situ FTIR of adsorbed pyrrole

Oxygen sites in ethane-bridged and phenylene-bridged PMOs prepared using neutral templates in acidic conditions are characterized by means of XPS and FTIR spectroscopy of adsorbed pyrrole. Their electron donor ability is observed to be stronger than that of oxygens in pure amorphous silica of MCM-41 type and comparable to that reported for oxygen atoms in some alkali metal exchange zeolites. For pyrrole adsorbed on PMOs double interactions most probably occur, involving both silanols and electron donor sites at the surface. In the case of phenylene-bridged PMO preferential electron donor sites for interaction with pyrrole N-H group are aromatic rings rather than oxygens, as previously observed for adsorbed iodine.