UV Reflectance Spectroscopy Research Articles

Oxidation Activity of Ti-Beta Synthesized by a Dry-Gel Conversion Method

Ti-beta zeolite was synthesized in the presence of sodium cations by a dry-gel conversion method using tetraethylammonium hydroxide as a structure-directing agent. As evidenced by diffuse reflectance UV spectroscopy, incorporated titianium species were tetrahedrally coordinated in the zeolites and neither anatase-like phase nor hexacoordinated titianium species belonging to an amorphous titanosilicate phase was detected at the sodium content of lower than 1.0 mol%. It was found that the Ti-beta catalysts gave very high H2O2conversion and selectivity in the oxidation of cyclohexene in methanol used as solvent. The C6-C8cyclic alkenes and alkanes reacted faster than corresponding linear alkenes and alkanes, showing the lack of steric impediments in the pores of Ti-beta. Dimethylcyclohexanes were more reactive when tertiary H atoms predominantly occupy the equatorial position than when they occupy the axial position. Ti-beta samples exhibited high activity also for the oxidation of C6-C8cyclic alcohols.

Cure Characterization in Bis(maleimide)/Diallylbisphenol A Resin by Fluorescence, FT-IR, and UV-Reflection Spectroscopy

Curing bis(maleimide)/diallylbisphenol A (BMI/DABPA) results in the formation of a high-temperature thermoset resin. FT-IR, fluorescence, and UV-reflectance spectroscopy were used to investigate the cure behavior of this material under three different cure schedules. Fluorescence signals were quenched before curing due to the BMI component but increased and eventually leveled off as cure time increased. The largest fluorescence intensity increases occurred after 80% of the phenylmaleimide units were converted to phenylsuccinimide. Fluorescence signals were observed in both short-wavelength and long-wavelength regions. Model compound studies indicated that the phenolic portion of the BMI/DABPA resin has a higher quantum yield for fluorescence at a shorter wavelength than phenylsuccinimide derivatives. Therefore, fluorescence emission observed near 356 nm during curing is attributed to phenolic structures. FT-IR was used to quantify the extent of succinimide formation and to identify cross-linking processes...

Photo-bromination and photo-induced graft polymerization as a two-step approach for surface modification of polyacrylonitrile ultrafiltration membranes

A two-step heterogeneous polymer surface modification was developed, involving photo-bromination as activation and subsequent UV-induced graft polymerization with acrylic monomers. With polyacrylonitrile (PAN) ultrafiltration (UF) membranes, extent and effects of photo-bromination compared with the thermal reaction were studied with gravimetry, diffuse reflectance UV and ATR-IR spectroscopy, and scanning electron microscopy (SEM) as well as the UF properties. The yields of thermal and photo-bromination reached constant values of about 60 and 150 μmol g , after 24 and 3 hr, respectively. The former Br content was assigned to Br 2 addition to double bonds, the latter to Br-radical CH substitution plus addition products. Excess bromine was bound as a charge-transfer complex to PAN nitrile groups, causing partially irreversible changes of the membrane matrix indicated by UF permeability reductions. The formation of polymer radicals after UV irradiation of photo-brominated PAN was verified with ESR spectroscopy. UV irradiation-induced graft polymerization of acrylic acid and methyl acrylate from the gas phase on to PAN membranes was accomplished. It was found that activation by thermal bromination is sufficient to initiate heterogeneous graft polymerization. Sorbed bromine promoted homopolymerization, increasing concentrations of hydroquinone improved the grafting efficiency. The graft-polymer modified surfaces were characterized by ATR-IR, SEM and contact angles. A pronounced selectivity towards the UV exposed membrane surface was achieved. For relatively low degrees of modification (up to 150 μg/cm 2), thin and smooth graft polymer layers were created which specifically altered the membrane surface hydrophilicity. UF membrane permeability and selectivity were a direct function of the degree of modification, suggesting adjustability of average separation layer pore size.

Potential application of UV reflection spectroscopy on solid pharmaceutical formulation analysis

An imaginary complex refraction index (extinction coefficient) has been deduced for a number of common pharmaceutical forms from their specular reflection spectra. It seems that reflectivity technique is preferred to the transmission one in those special case in which non-destructive analysis of samples is requested. Furthermore, the obtained results are independent of solvent effects and sample thickness.

Cure Characterization in Diamine-Cured Epoxy and Polyimide by UV Reflection Spectroscopy

UV specular reflection spectroscopy has been investigated for the characterization of cure reactions in an aromatic diamine cured epoxies and their composites, as well as in the imidization characterization of an aromatic polyamic acid. The UV reflection peak due to the reaction of the curing agent bis(p-aminophenyl) sulfone (DDS) with epoxide shows a red shift of about 24 nm in the spectral range of 300 nm to 350 nm. After the Kramers-Kronig conversion of the reflection spectra which is shifted to longer wavelength than the transmission spectra, the DDS peak occurs at a similar position as the transmission peak. The spectral shifts obtained by three different techniques, UV reflection, UV transmission, and fluorescence excitation were similar as a function of cure time at 160 °C. Therefore, the peak position in the reflection spectra may be used without the Kramers-Kronig conversion in order to correlate to the extent of cure, as established for the excitation spectral shift. The UV reflection spectral analysis is also obtained in DDS containing epoxy reinforced with glass or carbon fiber. Since the reflection probes only the first 0.5 μm layer of the resin, the presence of the fiber does not influence the spectral quality. In following the imidization of a polyamic acid made from 1,4-phenylenediamine (PDA) and an aromatic dianhydride, the spectral deconvolution of the UV reflection spectra provides a way to determine the relative composition of polyamic acid and polyimide. The resulting extent of imidization was compared with the result by IR analysis. Thus, UV reflection technique can be a useful cure characterization method with the samples which have smooth surfaces, strong UV absorption, but weak fluorescence.

Mars Ozone: Mariner 9 Revisited

The efficacy of the UV reflectance spectroscopy technique used by Mariner 9 to remotely measure ozone abundance at Mars is discussed. Due to temporal and spatial variability in cloud and dust amount, previously inferred ozone abundances could be underestimated by a factor of 3. Until the large uncertainty in cloud and dust scattering properties and opacities can be reduced, the ozone abundance inferred by the reflectance spectroscopy technique will always have significant uncertainty.

In situ measurements of water adsorption on a platinum electrode by an electrochemical quartz crystal microbalance

Adsorption of water at solid electrode-aqueous electrolyte solution interfaces is well known. According to the double-layer model proposed by Bockris and Reddy [l], the first layer closest to the electrode surface is composed only of water molecules when no specific adsorption takes place. In the case where a specifically adsorptive species is introduced into the solution, this species replaces the first-layer water molecules. Some of the adsorbed water molecules remain in the first layer and can interact laterally with specifically adsorbed species. In order to understand the adsorption and electron transfer processes at the molecular level, therefore, it is essential to examine the details of water adsorption, particularly to quantify the number of adsorbed water molecules and to determine the orientation and structure of the adsorbed water layer. Besides electrochemical measurements, adsorption of water at solid electrodeaqueous solution interfaces has been examined by in situ techniques such as the radiotracer method [2-61, surface-enhanced Raman scattering (SERSI [7-lo], UV reflectance spectroscopy [l l] and electrochemically modulated IR reflectance spectroscopy (EMIRS) [12,13]. The recently developed electrochemical quartz crystal microbalance (EQCM) [14] is a useful instrument for examining adsorption at the monolayer level because of its high sensitivity. However, EQCM studies of phenomena at the monolayer level are rather limited, probably owing to the difficulty of constructing a stable EQCM in solution. So far the EQCM has been used for studying the underpotential deposition of metals [15-181, the electrosorption of anions [19], adsorbed oxygen monolayers [20] and self-assembled monolayers of viologen derivatives [21] and ferrocene derivatives [22-251. Among these

The preparation of silica supported Pd catalysts: the effect of pretreatment variables on particle size

The effect of pretreatment on the dispersion of Pd catalysts supported on silica has been studied. The catalysts were prepared from [Pd(NH3)4] (NO3)2 as the metal precursor at a pH = 9. The resulting catalysts were characterized using both CO and H2 chemisorption, transmission electron microscopy, and in-situ UV reflectance spectroscopy. Pretreatment in H2 resulted in poor Pd dispersions while pretreatment in He or Ar resulted in very high dispersions. Pretreatment in O2 resulted in moderate dispersions. The results are explained by considering the chemical structure of the adsorbed surface complex under different pretreatment conditions. The chemistry of the decomposition process is considered in detail.

Ultraviolet Spectroscopy of the Outer Solar System

This talk will focus on the information to be gained through UV reflectance spectroscopy of atmospheres in the outer solar system. Here, we are concerned with the spectral region below 3000 Å, where many interesting molecular species have significant absorption cross sections, and observations must be made from sounding rockets or space-based observatories. With the exception of a few important rocket measurements the bulk of the observations in this region to dat have been made with the Voyager spacecraft and IUE observatories. The Voyager Ultraviolet Spectrometer (UVS) measurements offer the advantage of relatively high spatial resolution while IUE permits repeated measurements over a long time base. HST will combine both features and should led to a significant advancement in our understanding of outer solar system atmospheres.

XPS Studies of characterized Cu/Al2O3, Zn/Al2O3 and CuZn/Al2O3, catalysts

AbstractBy XPS it is confirmed that copper‐containing catalysts supported on alumina and calcined at high temperature (673–873 k for 24 h.), with or without zinc addition, have a ‘surface spinel’ structure.Combining the XPS results with previous results obtained by vis–UV reflectance spectroscopy and XRD we show that the limit of solubility of Zn and Cu ions in γ‐alumina, at a calcinations temperature of 873 k, about 20 and 10 wt.% (by weight), respectively. In the CuZn/Al2O3, catalysts prepared from Zn/Al2O3, a surface enrichment of copper occurs if the Zzinc ions are already presents in an amount larger than the surface solubility. The presence of zinc on the surface increases the reducibility under H2 of the Cu(II) ions compared to the same ions in the pure copper surface spinel. This may be due to the role of Zn(II)O(II) aggregates in the activation of the hydrogen molecule and/or to the segregation of CuO species on the Zinc surface spinel.

Assessment of the Surface Quality of SIMOX Wafers by UV Reflectance

The surface quality of SIMOX wafers implanted under various conditions of implant temperature, voltage, and dose has been characterized in terms of surface contamination, roughness, and amorphization by UV reflectance spectroscopy.

Surface modification of inorganic pigments with organic UV absorbers

Surfaces of tale and zinc oxide are coated with titanium oxide gel, containing UV-absorbing carboxylates of p-methoxycinnamate and 3-(2-benzotriazoyl)-4-hydroxyphenylacetate. Diffuse reflectance and diffuse transmittance UV spectroscopy disclose the newly attained UV-absorbing properties of talc. Zinc oxide, which absorbs light below 390 nm, however, shows increased surface reflection after the coating modification. Diffuse transmitted UV light, in contrast, was increasingly absorbed by the modified zinc oxide. Results of scanning electron photomicrograph and diffuse reflectance IR spectroscopy are also presented to characterize the modified surfaces.

Structure Determination of the Anorexic Agent Mazindol

The anorexic agent mazindol was shown to exist as the carbinolamine tautomer 5-p-chlorophenyl-2,3-dihydro-5H-imidazo[2,1-a]isoindol-5-ol in solution and in the solid state. The later was established by diffuse UV reflectance spectroscopy.