An infrared and inelastic neutron scattering spectroscopic investigation on the interaction of eta-alumina and methanol

The industrially important interaction of methanol with an eta-alumina catalyst has been investigated by a combination of infrared spectroscopy (diffuse reflectance and transmission) and inelastic neutron scattering (INS) spectroscopy. The infrared and INS spectra together show that chemisorbed methoxy is the only surface species present. Confirmation of the assignments was provided by a periodic DFT calculation of methoxy on eta-alumina (110). The thermal conversion of adsorbed methoxy groups to form dimethylether was also followed by INS, with DFT calculations assisting assignments. An intense feature about 2600 cm(-1) was observed in the diffuse reflectance spectrum. This band is poorly described in the extensive literature on the alumina/methanol adsorption system and its observation raised the possibility of a new surface species existing on this particular catalyst surface. INS measurements established that the 2600 cm(-1) feature could be assigned to a combination band of the methyl rock with the methyl deformation modes. This assignment was reinforced by an analysis of the neutron scattering intensity at a particular energy as a function of momentum transfer, which confirmed this particular adsorbed methoxy feature to arise from a second order transition. Similar behaviour was observed in the model compound Al(OCH3)3. The anomalous infrared intensity of the 2600 cm(-1) peak in the diffuse reflectance spectrum is a consequence of the different absorption coefficients of the C-H stretch and the combination mode. The implications for catalyst studies are discussed.     

Optimization of sample packing in diffuse reflectance infrared spectroscopy

Parameters of sample packing in diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) have been investigated and optimized. One MPa pressure applied for 1 min was found to provide simultaneously at least ±3% reproducibility and appropriate band intensity. It has been also demonstrated that this precision can be obtained if the sample mass is controlled to ±2% and the sample is dry. A simple sample packing accessory was designed and constructed for reproducible sample preparation. Quartz concentration of dust samples has been determined and good agreement has been found with the results of the conventional pellet technique.     

Comparison of calibration methods in quantitative diffuse reflectance infrared spectroscopy

Four different calibration methods were used for quantitative analysis of quartz and calcite in atmospheric aerosols by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS): (A) conventional calibration with one measurement on each standard (single calibration); (B) calibration with an internal standard; (C) calibration with parallel (n = 4) measurements on each standard (multiple calibration); (D) multiple calibration followed by reference reflectance correction. The accuracy and the precision of the methods were compared and it was found that by using method D the reliability of the conventional pellet preparation transmission technique can be achieved.     

Characterization of estuarine sediments by near infrared diffuse reflectance spectroscopy

It has been developed a partial least squares near infrared (PLS-NIR) method for the determination of estuarine sediment physicochemical parameters. The method was based on the chemometric treatment of first order derivative reflectance spectra obtained from samples previously lyophilized and sieved through a lower than 63 μm grid. Spectra were scanned from 833 to 2976 nm, averaging 36 scans per spectrum at a resolution of 8 cm⁻¹, using chromatographic glass vials of 9.5 mm internal diameter as measurement cells. Models were built using reference data of 31 samples selected through the use of a hierarchical cluster analysis of NIR spectra of sediments obtained from the Ria de Arousa estuary and prediction parameters were established from a validation set of 50 samples of the same area. pH, redox potential (Eh), carbon (C), nitrogen (N) and hydrogen (H) content together with Sn, Pb, Cd, As, Sb and total Cr and also acid soluble, reducible and oxidable Cr fractions were employed as characteristic parameters of the studied sediments. Standard error of prediction values for C and N content were of the order of 4 and 1.3 mg g⁻¹ for H. Prediction errors for pH and Eh were 0.15 units and 37 mV, respectively, thus indicating the good prediction capabilities of the method. Regarding trace metal concentrations PLS-NIR provided prediction error levels for unknown samples around 20% for Sn, Pb, As and Sb and root mean square errors of prediction around 40% for concentration levels of 400 ng g⁻¹ Cd and 100 μg g⁻¹ Cr. For the different extractable fractions of Cr the residual prediction deviation varied from 1.3 to 1.7 but relative errors found for samples of the validation set were only useful for screening purposes.     

Reflection absorption infrared spectroscopy and temperature-programmed desorption studies of the adsorption and desorption of amorphous and crystalline water on a graphite surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) have been used to perform a detailed investigation of the adsorption of water on highly oriented pyrolytic graphite (HOPG) at 90 K. RAIRS shows that water is physisorbed on HOPG at all coverages, as expected. Experiments at higher surface temperatures show marked changes in the O-H stretching region of the spectrum which can be assigned to the observation of the amorphous to crystalline ice phase transition. The infrared signature of both phases of solid water has been determined on HOPG and can be used to identify the phase of the ice. TPD spectra show the desorption of multilayers of crystalline ice. At high exposures a small bump appears in the TPD spectrum, on the low temperature side of the main peak, which is attributed to the amorphous to crystalline phase transition. At very low exposures of water, it is possible to distinguish the desorption of water from two- and three-dimensional islands and hence to determine the growth mode of water on the HOPG surface. Isothermal TPD studies have also been performed and show that the desorption of water does not obey perfect zero-order kinetics. Desorption orders, derived directly from the TPD spectra, confirm this observation. Desorption energies and preexponential factors have also been determined for this adsorption system.     

Reflection absorption infrared spectroscopy and temperature programmed desorption investigations of the interaction of methanol with a graphite surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to investigate the adsorption of methanol (CH(3)OH) on the highly oriented pyrolytic graphite (HOPG) surface. RAIRS shows that CH(3)OH is physisorbed at all exposures and that crystalline CH(3)OH can be formed, provided that the surface temperature and coverage are high enough. It is not possible to distinguish CH(3)OH that is closely associated with the HOPG surface from CH(3)OH adsorbed in multilayers using RAIRS. In contrast, TPD data show three peaks for the desorption of CH(3)OH. Initial adsorption leads to the observation of a peak assigned to the desorption of a monolayer. Subsequent adsorption leads to the formation of multilayers on the surface and two TPD peaks are observed which can be assigned to the desorption of multilayer CH(3)OH. The first of these shows a fractional order desorption, assigned to the presence of hydrogen bonding in the overlayer. The higher temperature multilayer desorption peak is only observed following very high exposures of CH(3)OH to the surface and can be assigned to the desorption of crystalline CH(3)OH.     

Optimization of some instrumental factors in diffuse reflectance infrared Fourier transform spectroscopy

The effects of some instrumental factors on infrared spectroscopy analysis were investigated in the case of diffuse reflectance infrared Fourier transform (DRIFT) mode. It is usually said that quantitative analysis is possible only if both particle size distribution and sample density are perfectly controlled. However, even if these conditions are checked, instrumental factors are of great interest for the goodness of a curve fitting procedure, which is often necessary in solid sample studies. A factorial design achieved on anthron made it possible to obtain major trends concerning the required values for one instrumental parameter (resolution) and two mathematical treatments (zero filling and Savitsky-Golay (S-G) smoothing). Resolution was found to have the greatest effect on measured responses. A value of 2 cm(-1) according to the corresponding aperture is sufficient to approach the real width of bands for powdered samples. The use of a zero filling factor (ZFF) improves the apparent resolution by data interpolation. The best values found for instrumental parameters were applied to an anthron-anthracen mixture. The optimization of spectral features made it possible to obtain semi-quantitative results quite easily, in good agreement with the data corresponding to each pure compound.     

Detection of volatile organic compounds by temperature-programmed desorption combined with mass spectrometry and Fourier transform infrared spectroscopy

A temperature-programmed desorption (TPD) device connected to a mass spectrometer was used to detect volatile organic compounds from air samples. The main aim was to develop an analytical method, by which both non-polar and polar organic components can be detected in the same run. In TPD, the adsorbed compounds are desorbed from the resin more slowly than in the conventional trapping techniques, such as purge-and-trap technique, in which the resin is flash-heated and the compounds are desorbed at the same time to a cryogenic trap or an analytical column. In TPD, the adsorbent resin acts also as an analytical column. In this way it is possible to obtain more rapid analysis, and also a more simple instrumentation, which can be used on-line and on-site. In this work, a new version of TPD device, which uses a resistor for heating and a Peltier element for rapid cooling, was designed and constructed. Various adsorbent resins were tested for their adsorption and desorption properties of both polar and non-polar compounds. When using a mixture of adsorbent resins, Tenax TA and HayeSep D, it was possible to analyze both polar, low-molecular weight compounds, such as methanol and ethanol, and non-polar volatile organic compounds, such as benzene and toluene, in the same run within 15 min including sampling. The same TPD principle was also tested using a Fourier transform infrared spectrometer as an analytical instrument, and the results showed that it was possible to obtain a separation of similar compounds, such as hexane and heptane, and still retaining the same sensitivity as the original on-line FTIR instrument.     

Testing of the Region of Murcia soils by near infrared diffuse reflectance spectroscopy and chemometrics

A partial least squares near infrared (PLS-NIR) method has been developed for the determination of several physicochemical parameters in soils from different locations of the Region of Murcia. The method was based on the proper chemometric treatment of diffuse reflectance spectra of soil samples. Reflectance spectra were scanned from samples stored in glass vials in the NIR region between 800 and 2600 nm, averaging 36 scans per spectrum at a resolution of 8 cm⁻¹. Models were built using reference data of 39 samples selected from a dendrogram obtained after hierarchical cluster analysis of NIR spectra of soils and prediction parameters were established from a validation set of 109 additional samples of the same area not considered to build the model. Organic matter, CaCO₃, pH, electrical conductivity (EC), together with several trace metals as Cr, Co, Ni, Cu, Zn, As, Se, Cd and Tl, were employed as characteristic parameters of the soils under study, and found results evidenced that PLS-NIR provides a valuable tool for screening purposes providing residual predictive deviations which ranged from 0.9 to 1.5 as a function of the considered parameter.     

On the structure of vanadium oxide supported on aluminas: UV and visible raman spectroscopy, UV-visible diffuse reflectance spectroscopy, and temperature-programmed reduction studies

Vanadia species on aluminas (delta- and gamma-Al2O3) with surface VOx density in the range 0.01-14.2 V/nm2 have been characterized by UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), and temperature-programmed reduction in hydrogen. It is shown that the alumina phase has little influence on the structure and reducibility of surface VOx species under either dehydrated or hydrated conditions. Three similar types of dispersed VOx species, i.e., monovanadates, polyvanadates, and V2O5, are identified on both aluminas under dehydrated conditions. Upon hydration, polymerized VOx species dominate on the surfaces of the two aluminas. The broad Raman band at around 910 cm(-1), observed on dehydrated V/delta-, gamma-Al2O3 at all V loadings (0.01-14.2 V/nm2), is assigned to the interface mode (V-O-Al) instead of the conventionally assigned V-O-V bond. The direct observation of the interface bond is of significance for the understanding of redox catalysis because this bond has been considered to be the key site in oxidation reactions catalyzed by supported vanadia. Two types of frequency shifts of the V=O stretching band (1013-1035 cm(-1)) have been observed in the Raman spectra of V/Al2O3: a shift as a function of surface VOx density and a shift as a function of excitation wavelength. The shift of the V=O band to higher wavenumbers with increasing surface VOx density is due to the change of VOx structure. The V=O stretching band in dispersed vanadia always appears at lower wavenumber in UV Raman spectra than in visible Raman spectra for the same V/Al2O3 sample. This shift is explained by selective resonance enhancement according to the UV-Vis DRS results. It implies that UV Raman has higher sensitivity to isolated and less polymerized VOx species while visible Raman is more sensitive to highly polymerized VOx species and crystalline V2O5. These results show that a multiwavelength excitation approach provides a more complete structural characterization of supported VOx catalysts.     

Investigation of a Co-MgO-ZSM catalyst by temperature-programmed reduction,temperature-programmed desorption,and IR spectroscopy

The 32% Co-3% MgO-ZSM (SiO₂/Al₂O₃=38) system has been studied by means of temperature-programmed reduction, temperature-programmed desorption, and IR spectroscopy. The data from temperature-programmed reduction show that cobalt exists on the surface of the catalyst in the form of Co²⁺, CoO, Co₃O₄, and CoO·MgO solid solutions. Reduction of the sample results in the formation of a very inhomogeneous surface with four groups of sites.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Prague. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 55–59, January, 1992.     

Surface study of hexagonal boron nitride powder by diffuse reflectance Fourier transform infrared spectroscopy

The surface chemistry of hexagonal boron nitride powder is examined. Surface functional groups are found to consist of a mixture of primary and secondary amine groups as well as hydroxyl groups. The surface has a net basic character indicating that the amine contribution to the overall surface chemistry is greater than that of the acidic hydroxyl contribution. A chemical surface treatment also is performed to modify the surface chemistry of hexagonal boron nitride powder. Copyright © 2005 John Wiley & Sons, Ltd.     

Discrimination of Ganoderma lucidum according to geographical origin with near infrared diffuse reflectance spectroscopy and pattern recognition techniques

A rapid and nondestructive near infrared (NIR) method combined with chemometrics was used to discriminate Ganoderma lucidum according to cultivation area. Raw, first, and second derivative NIR spectra were compared to develop a robust classification rule. The chemical properties of G. lucidum samples were also investigated to find out the difference between samples from six varied origins. It could be found that the amount of polysaccharides and triterpenoid saponins in G. lucidum samples was considerably different based on cultivation area. These differences make NIR spectroscopic method viable. Principal component analysis (PCA), discriminant partial least-squares (DPLS) and discriminant analysis (DA) were applied to classify the geographical origins of those samples. The results showed that excellent classification could be obtained after optimizing spectral pre-treatment. For the discriminating of samples from three different provinces, DPLS provided 100% correct classifications. Moreover, for samples from six different locations, the correct classifications of the calibration as well as the validation data set were 96.6% using the DA method after the SNV first derivative spectral pre-treatment. Overall, NIR diffuse reflectance spectroscopy using pattern recognition was shown to have significant potential as a rapid and accurate method for the identification of herbal medicines.     

A modified diffuse reflectance infrared Fourier transform spectroscopy cell for depth profiling of ceramic fibers

A modified diffused reflectance infrared Fourier transformed spectroscopy (DRIFTS) accessory was used to analyze the surface properties of alumino-silicate fibers. The modifications are simple and involve a different way of performing depth-profiling from traditional DRIFTS by removing approximately 2 mm of salt from a full cup prior to placing the sample in for depth profiling. This method proved successful in elucidating the effects of quenching alumino-silicate fibers in mineral oil versus quenching in an air stream.     

A variable-temperature diffuse reflectance infrared fourier transform spectroscopy study of the binding of water and pyridine to the surface of acid-activated metakaolin

Four metakaolins were prepared by heating a Spanish kaolin at 600, 700, 800, and 900 degrees C for 10 h. Following preliminary optimization, these metakaolins were acid activated in 6 M hydrochloric acid at 90 degrees C for 6 h; the samples calcined at 600, 700, and 800 degrees C produced the highest surface area solids and were selected for further study. Variable-temperature diffuse reflectance infrared Fourier transform spectroscopy analysis of the resulting acid-activated metakaolins (AAMKs) identified a wide range of hydrogen bond strengths in adsorbed water at room temperature. Above 300 degrees C it was possible to fit the broad hydroxyl stretching band to seven contributing components at 3730, 3700, 3655, 3615, 3583, 3424, and 3325 cm(-1). As the sample temperature was increased, the 3730 cm(-1) band increased in intensity as the water hydrogen bonded to AlOHAl was thermally desorbed. The other six bands decreased in intensity. The spectra of adsorbed pyridine indicated the presence of both Br?nsted and Lewis acid sites on the surface of the air-dried AAMKs. Preheating the AAMK at 200 degrees C prior to pyridine sorption reduced the number of Br?nsted acid sites and increased the number of thermally stable Lewis acid sites. A reduction in the amount of adsorbed pyridine after pretreating the AAMK at 400 degrees C was tentatively attributed to a reduction in surface area. This was reflected in fewer thermally stable Lewis acid sites in the AAMK pretreated at 400 degrees C compared to the number present in the sample pretreated at 200 degrees C.     

A study on the protection to relics and the related problems with diffuse reflectance spectroscopy

The application of diffuse reflectance spectroscopy to relic protection is studied by using a self-made fiber optics reflectance spectrophotometer. The major work done includes: (1) the composition of pigment on colored relics and its changes are identified; (2) the change on metal surface is monitored; (3) the reflectance spectrum characteristics of relic protection materials are studied. The results tell that diffuse reflectance spectroscopy is a new protection technique, characterized by its quickness and non-destructiveness to the relic.     

The application of diffuse and specular reflectance to TLC-FTIR and other microsampling applications

Diffuse reflectance (DR) FTIR has been used to monitor components separated by thin layer chromatography (TLC). Analyses were performed both directly on the TLC plate [1] and after removal of the silica background by extraction. Better limits of detection (high nanogram versus low microgram) and a larger spectral range were observed for the extracted materials. Microspectrometry was used to advantage in analyzing small amounts of contamination on metal and coated concrete surfaces.     

Rapid determination of aluminum by UV-vis diffuse reflectance spectroscopy with application of suitable adsorbents

Diffuse reflectance spectroscopy (DRS) can be used as a rapid and sensitive method for the quantitative determination of low amounts of aluminum. In this analytical technique, the analyte in samples are extracted onto a solid sorbent matrix loaded with a colorimetric reagent and then quantified directly on the adsorbent surface. Alternatively, colored aluminum complexes formed in solution can also be immobilized onto adsorbent surface and be measured by DRS technique. Octadecyl silica disk, methyltrioctylammonium chloride–naphthalene and MCM-41 were examined as adsorbents. Eriochrome cyanine R and quinalizarin were used as coloring reagents. Optimal sorption conditions were found for each system of analyte–reagent–adsorbent. The concentration of analyte is determined using the appropriate form of the Kubelka–Munk function. We obtained for each of the aluminium–reagent–adsorbent system a calibration curve by plotting the absorbance versus the log 10²[Al³⁺] μg ml⁻¹. The linear dynamic range extends over two orders of magnitude within 0.01–15 μg ml⁻¹ with little differences in the range and in the correlation coefficients among the adsorbents. We consider that for a rapid determination of aluminum a spot-test-DRS combination with a detection limit of 1.0 × 10⁻² μg ml⁻¹ is the more facile and preferred technique.