Spectra Of Pyridine Research Articles

Surface-enhanced Raman scattering measurements on silver nanoparticles covered with differently formed platinum films

Gold and silver electromagnetic nanoresonators covered by a thin layer of platinum are often used to study adsorption of various molecules on “model platinum surfaces” with surface-enhanced Raman scattering (SERS) spectroscopy. In this contribution spectra of pyridine adsorbed on films formed from core–shell Ag@Pt and Ag@Ag–Pt nanoparticles and pure Pt or Ag nanoparticles were measured using a confocal Raman microscope. The SERS spectra of pyridine adsorbed on alloy Ag@Ag–Pt nanoparticles could not be obtained as a linear combination of spectra measured on pure Ag and Pt surfaces. In other words, for silver electromagnetic nanoresonators covered by platinum there is no simple correlation between the “quality” of the deposited Pt layer and the relative intensity of SERS bands characteristic for adsorbate interacting with silver. The SERS spectra accumulated from various places of a film formed from Ag@Pt or Ag@Ag–Pt nanoclusters may differ significantly. Using Ag@Pt nanoparticles with practically negligible amount of Ag on the surface (as per the stripping measurement), it is possible to record SERS spectrum in which the contribution characteristic for pyridine adsorbed on the Ag surface is well visible. It means that, even for macroscopic samples of core–shell Ag–Pt nanoparticles, averaging of many spectra measured at various locations of the sample should be carried out to characterize reliably their properties.

Vibrational studies of benzene, pyridine, pyridine-N-oxide and their cations

IR and Raman spectra of pyridine and pyridine-N-oxide have been recorded and analyzed. The optimized molecular geometries, APT charges and vibrational characteristics for benzene, pyridine, pyridine-N-oxide and their cations have been computed using DFT method. Due to attachment of O atom at N site or removal of electron all the modes are affected in magnitudes. However, significant changes are noticed in their IR intensities, Raman activities and depolarization ratios of the Raman bands in going from pyridine to its N-oxide or in going from neutrals to their cations. It is interesting to note that in going from benzene to benzene cation charge redistribution takes place to reduce the symmetry from D6h to D2h. The calculated frequencies have been correlated with the experimental frequencies for the pyridine and pyridine-N-oxide molecules.

Nanostructure sulfated tin oxide as an efficient catalyst for the preparation of 7-hydroxy-4-methyl coumarin by Pechmann condensation reaction

Nanocrystalline sulfated tin oxide with different sulfate contents (5–30wt%) were prepared from hydroxylated tin oxide obtained by the precipitation method, followed by wet impregnation with SO42− species using a sulfuric acid solution. In samples calcined at 400, 500 and 600°C the characterization of the solids was made by DTA, XRD and nitrogen adsorption. The strength and number of acid sites were determined by nonaqueous titration of n-butylamine in acetonitrile. Both Bronsted and Lewis acid sites were determined by FTIR spectra of pyridine adsorbed. Sulfated tin oxides were tested in the synthesis of 7-hydroxy-4-methyl coumarin via solvent free Pechmann reaction of resorcinol:ethylacetoacetate (molar ratio 1:2), at 120°C. DTA measurements showed that SO42− addition tends to slow down the formation of SnO2 crystallites. XRD profiles showed that the sulfating inhibits the SnO2 crystal growth. The SO4 species remained strongly bonded at the SnO2 surface stabilizing its crystallite size against sintering and it acts as a structure porogen director mediating nanoparticle growth and assembly yielding a mesostructured form of SnO2 with wormhole morphology and high thermal stability. The surface areas of the investigated samples were influenced with the sulphate content and calcination temperature. The acidity measurements showed that the total acidity increases with the rise of sulfate content up to 25wt%. FTIR spectra of pyridine adsorbed on the catalysts showed the presence of both Bronsted and Lewis acid sites.The formation of 7-hydroxy-4-methyl coumarin increases with the increase of surface acidity showing a maximum when the sulfate content and calcination temperature were 25wt% and 400°C, respectively.

Synthesize 2-methylpyrazine using aqueous glycerol and ethylenediamine over zinc oxide–zinc chromite catalysts: Structure–activity relationship

ZnO–ZnCr2O4 mixed oxides (Zn–Cr–O) were obtained by decomposition of Zn–Cr hydrotalcite precursors synthesized by co-precipitation method at pH∼7 and 9, with a constant Zn/Cr ratio of 2. Based on the product distribution, mechanisms were proposed. The differences in dehydrocyclization activity of Zn–Cr–O mixed oxides were established based on adsorption and spectroscopic data that emphasized changes in the structural properties of Zn–Cr–O obtained at different pHs. X-ray photoelectron (XPS) lines indicated Cr6+ and these species were reduced to Cr3+ during H2-temperature programmed reduction (H2-TPR) analysis. The higher NH3 and O2 uptakes of Zn–Cr–O (pH∼7) than Zn–Cr–O (pH∼9) are attributed to the presence of Cr6+ ions on the catalyst surface. Disorder in the cubic ZnCr2O4 sub-lattice may result in multiple Raman bands appeared in the Zn–Cr–O samples. The characteristic bands of Lewis and Bronsted acid sites at 1450 and 1540cm−1 assigned by FT-IR spectra of pyridine adsorbed on Zn–Cr–O samples, and their relative peak intensities corresponding to Lewis and Bronsted suggested higher ratios for Zn–Cr–O (pH∼7) than for Zn–Cr–O (pH∼9). It is demonstrated that the dehydrocyclization reaction proceeds on ZnO and/or ZnCr2O4 resulting in 2MP synthesis, and catalysts that possess mainly acid sites show intermolecular cyclization of EDA leading to high selectivity of pyrazine.

A discrete interaction model/quantum mechanical method for simulating surface-enhanced Raman spectroscopy

We have derived and implemented analytical gradients for the discrete interaction model/quantum mechanics (DIM/QM) method. DIM/QM combines an atomistic electrodynamics model with time-dependent density functional theory and thus enables modeling of the optical properties for a molecule while taking into account the local environment of a nanoparticle's surface. The DIM/QM analytical gradients allow for geometry optimizations, vibrational frequencies, and Raman spectra to be simulated for molecules interacting with metal nanoparticles. We have simulated the surface-enhanced Raman scattering (SERS) spectra for pyridine adsorbed on different sites of icosahedral nanoparticles with diameters between 1 and 8 nm. To describe the adsorption of the pyridine molecule onto the metal surface, we have implemented a coordination-dependent force field to differentiate the various local surface environments. We find that the DIM/QM method predicts geometries and frequencies that are in good agreement with full QM simulations and experiments. For the simulated SERS spectra of pyridine, we find a significant dependence on the adsorption site and the size of the metal nanoparticle. This illustrates the importance of accounting for the local environment around the molecule. The Raman enhancement factors are shown to roughly mirror the magnitude of the nanoparticle's local field about the molecule. Because the simulated nanoparticles are small, the plasmon peaks are quite broad which results in weak local electric fields and thus modest Raman enhancement factors.

Surface‐enhanced Raman scattering investigations on silver nanoparticles deposited on alumina and titania nanotubes: influence of the substrate material on surface‐enhanced Raman scattering activity of Ag nanoparticles

Silver nanoparticles deposited on various ‘inert’ porous materials (mainly Al2O3 and TiO2) are often used as substrates for surface‐enhanced Raman scattering (SERS) measurements. In this study, we used the sputter deposition technique to cover tubular arrays of Al2O3 and TiO2 with Ag nanoparticles. Raman spectra of pyridine (as a probe molecule) and of two selected dyes (5‐(4‐dimethylaminobenzylidene)rhodanine and 5‐(4‐(dimethylamino)benzylidene)‐3‐(3‐methoxypropyl)rhodanine) adsorbed on fabricated Ag/TiO2‐n/Ti and Ag/Al2O3‐n/Al substrates were measured. We found that the SERS spectra of pyridine adsorbed on Ag nanoparticles deposited on an Al2O3‐n/Al substrate are distinctly different from those measured for an Ag/TiO2‐n/Ti composite. Similar effects were observed for dyes adsorbed on the surface of both composites. The spectral differences between two kinds of composites (Ag/TiO2‐n/Ti and Ag/Al2O3‐n/Al) are discussed in terms of (1) the modified electronic structure of the Ag nanoparticles due to their interaction with different substrate materials and (2) the different atomic topology of the metal particles thus deposited on the surfaces of the substrates. Composite samples were also studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to reveal their characteristic morphological and chemical features. Copyright © 2012 John Wiley & Sons, Ltd.

Silver–platinum core–shell nanoparticles for surface-enhanced Raman spectroscopy

Core–shell Ag@Pt nanoparticles have been synthesised by the means of seed-growth reaction including reduction of PtCl 4 2− with silver and replacing Ag atoms with Pt. Surface-enhanced Raman scattering (SERS) spectra of pyridine (which gives slightly different spectra when interacting with various metals) adsorbed on synthesised Ag@Pt clusters were measured. SERS measurements have revealed that deposition of the platinum layer causes near elimination of the spectral interferences from pyridine directly interacting with the silver core. The average SERS enhancement factor for pyridine adsorbed on the Ag@Pt clusters was estimated as equal to about 10 3–10 4, significantly higher than the SERS enhancement factor achievable on the pure platinum nanostructures. Using the silver core (instead of the previously used gold cores) allows for measurement of strong SERS spectra on the Pt covered nanostructures for the wider range of the excitation radiation. This procedure of platinum deposition was tested with various silver nanoparticles – produced with borohydride, citrate and citrate/borohydride methods – which substantially differ in size distribution. The application of formed Ag@Pt structures for obtaining intense Raman spectra for molecules adsorbed on only slightly modified platinum surfaces is discussed.

Synthesis and characterization of new solid acid catalysts, H 3PW 12O 40 supported on nanoparticle tin oxide: An efficient catalyst for the preparation of 7-hydroxy-4-methylcoumarin

A series of 12-tungstophosphoric acid (TPA) modified SnO 2 samples (TPA–SnO 2) with varying TPA content were prepared by wet impregnation technique. The TPA–SnO 2 catalysts were characterized by thermal analysis, powder X-ray diffraction (XRD) patterns and N 2 adsorption at −196 °C. The surface acidity was measured by potentiometric titration and FTIR spectra of chemically adsorbed pyridine. The DTA shows a peak maximum at 600 °C which was ascribed to the crystallization of tin oxyhydroxide into SnO 2. The thermograms of TPA show no exothermic effect due to the decomposition of TPA up to 25 wt%. When TPA content reached 30 wt% TPA/SnO 2 an exothermic effect at 618 °C indicated that the thermal stability of TPA on SnO 2 seems to be comparable to that of pure TPA. XRD measurements showed that there are no diffraction patterns of TPA for the catalyst calcined at 500 °C. The presence of TPA on tin oxide inhibits sintering of the particles during calcination by reducing the crystallite size of SnO 2 from 27.7 to 19.8 nm. The surface area values increase gradually with increasing TPA loading to reach a maximum limit of 25 wt% TPA. When calcination temperature is increased to 500 °C the specific surface is decreased. The acidity measurements showed that the total acidity increases with the rise of TPA content up to 30 wt%. FTIR spectra of pyridine adsorbed on the catalysts showed the presence of both Bronsted and Lewis acid sites. An optimum reaction performance (78.0% of 7-hydroxy-4-methyl coumarin formed) was achieved at 30 wt% loading of TPA calcined at 400 °C and reaction conditions of 120 °C, 2 h reaction and 1:2 (resorcinol:ethyl acetoacetate) molar ratio.

Quasi-homogeneous catalytic activities of hydrocarbon dispersible HZSM-5 nanocrystals grafted with different alkyl groups

HZSM-5 nanocrystals grafted with different alkyl groups (ethyl, butyl, hexyl, dodecyl, and hexadecyl groups) were prepared to study their stable dispersion in the hydrocarbons as well as the quasi-homogeneous catalytic activities. The physical properties of the prepared zeolites were characterized by solid-state nuclear magnetic resonance, X-ray diffraction, N 2 adsorption–desorption, fourier transform infrared spectra of pyridine adsorbed, and dynamic light scattering. The results showed that the ZSM-5 crystal structure were kept without obvious change after grafting alkyl groups, but that their stable dispersions in the hydrocarbon significantly improved with increasing chain length of grafted alkyl groups by reducing aggregate size of HZSM-5 nanocrystals. Quasi-homogeneous catalytic cracking of n-dodecane at 427 °C showed that the cracking rates with hydrocarbon dispersible HZSM-5 nanocrystals improved by more than 2 times compared with thermal cracking, wherein the zeolites grafted hexyl group had the best catalytic activity in good agreement with its larger acid amount.

Theoretical study on contribution of charge transfer effect to surface-enhanced Raman scattering spectra of pyridine adsorbed on Ag n ( n = 2–8) clusters

We investigate surface-enhanced Raman scattering (SERS) spectra of pyridine–Ag n ( n = 2–8) complexes by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In simulated normal Raman scattering (NRS) spectra, profiles of pyridine–Ag n ( n = 2–8) complexes are analogical with that of isolated pyridine. Nevertheless, calculated pre-SERS spectra are strongly dependent on electronic transition states of new complexes. Wavelengths at 335 nm, 394.8 nm, 316.9 nm and 342.6 nm, which are nearly resonant with pure charge transfer excitation states, are adopted as incident light when simulating pre-SERS spectra for pyridine–Ag n ( n = 2–8) complexes, respectively. We obtain enhancement factors from 10 3 to 10 5 in pre-SERS spectra compared with corresponding NRS spectra. The obvious increase in Raman intensities mainly result from charge transfer resonance Raman enhancement. A charge difference densities (CDDs) methodology is adopted in describing chemical enhancement mechanism. This methodology aims at visualizing charge transfer from Ag n ( n = 2–8) clusters to pyridine on resonant electronic transition, which is one of the most direct evidences for chemical enhancement mechanism.

Synthesis, characterization and catalytic properties of Y-β zeolite composites

Y-β zeolite composites were hydrothermally synthesized by using high silica Y zeolite as the precursor and characterized by XRD, N2 adsorption, SEM and IR spectra of pyridine. The result showed that the N2 adsorption-desorption isotherm of the zeolite composites had a distinct hysteresis loop, and the SEM result showed that the zeolite composites had a different morphology from Y, β and the corresponding physical zeolite mixture. The acid catalytic performance of the zeolite composite catalysts was investigated in the hydrocracking and hydroisomerization of n-octane, and the results showed that the composite materials exhibited an excellent hydrocracking activity and good hydroisomerization performance. The yield of i-C4 over the zeolite composite catalyst was 4.45% higher than that on the corresponding zeolite mixture in the n-octane hydrocracking process at 553 K. The isomerization ability of n-octane over the composite catalyst was 3.6 fold that of the corresponding mixture at 503 K.

SERS spectra of pyridine adsorbed on nickel film prepared by magnetron sputtering

As a repeating well and cheaper enhancement substrate, the nickel film was fabricated with magnetron sputtering coating instrument. Surface enhanced Raman spectra (SERS) of pyridine adsorbed on this nickel film are compared with the experimental values of gaseous pyridine, the theoretical value of pyridine solution listed in other literatures and our method is better than electro-chemical etching electrode method for large scale preparation. The enhancement factor of the nickel film is calculated and the result indicates that magnetron sputtering coating technology is feasible for obtaining good SERS active surface.

Surface-enhanced Raman scattering (SERS) activity of Ag, Au and Cu nanoclusters on TiO 2-nanotubes/Ti substrate

Tubular arrays of TiO 2 nanotubes (ranging in diameter from 40 to 110 nm) on a Ti substrate were used as a support for Ag, Au or Cu deposits obtained by the sputter deposition technique, where the amount of metal varied from 0.01 to 0.2 mg/cm 2. Those composite supports were intended for surface-enhanced Raman scattering (SERS) investigations. Composite samples were studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to reveal their characteristic morphological and chemical features. Raman spectra of pyridine (as a probe molecule) were measured at different cathodic potentials ranging from −0.2 down to −1.2 V after the pyridine had been adsorbed on the metal-covered TiO 2 nanotube/Ti substrates. In addition, SERS spectra on a bulk standard activated Ag, Au and Cu substrates were also measured. The SERS activity of the composite samples was strongly dependent on the amount of metal deposit, e.g. at and above 0.06 mg Ag/cm 2, the intensity of SERS signal was even higher than that for the Ag reference substrate. The high activity of these composites is mainly a result of their specific morphology. The high SERS sensitivity on the surface morphology of the substrate made it possible to monitor very small temporal changes in the Ag metal clusters. This rearrangement was not detectable with microscopic (SEM) or microanalytical (AES) methods. The SERS activity of Au or Cu clusters was distinctly lower than those of Ag. The spectral differences exhibited by the three kinds of composites as compared to the reference metal samples are discussed.

A Novel Method for Fabricating the Surface-Enhanced Raman Scattering Substrates and Its Enhanced Properties

We have got large area surface-enhanced Raman scattering (SERS) substrates with uniform high enhancement factors by the so-called moulage method for the first time. A silver film (99.99%) with several millimeters thickness was thermally evaporated on the porous anodic alumina templates and the SERS substrate was got after moving off the templates. Surface-enhanced Raman scattering spectra of pyridine (0.01 Mol/L) were measured under 632.8 nm excitation. The experimental enhancement factors were more than 10(5) and S/N(p-p) around 100 was obtained. We have compared the SERS spectra of pyridine collected from different locations on the same SERS substrate and different substrates, which illustrate the well uniform enhance properties and the reproducibility of this method, respectively. The comparison of the SERS spectra, obtained from the SERS substrates and Ag film evaporated directly on glass slide, have proved that the electromagnetic coupling between two adjacent nanoparticles was important to the SERS effect. We also used rhodamine 6G as the probe molecules and found that the different molecules were very sensitive to the morphology of the SERS substrates.

Theoretical studies of surface enhanced hyper-Raman spectroscopy: The chemical enhancement mechanism

Hyper-Raman spectra for pyridine and pyridine on the surface of a tetrahedral 20 silver atom cluster are calculated using static hyperpolarizability derivatives obtained from time dependent density functional theory. The stability of the results with respect to choice of exchange-correlation functional and basis set is verified by comparison with experiment and with Raman spectra calculated for the same systems using the same methods. Calculated Raman spectra were found to match well with experiment and previous theoretical calculations. The calculated normal and surface enhanced hyper-Raman spectra closely match experimental results. The chemical enhancement factors for hyper-Raman are generally larger than for Raman (10(2)-10(4) versus 10(1)-10(2)). Integrated hyper-Raman chemical enhancement factors are presented for a set of substituted pyridines. A two-state model is developed to predict these chemical enhancement factors and this was found to work well for the majority of the molecules considered, providing a rationalization for the difference between hyper-Raman and Raman enhancement factors.

Raman investigations of SERS activity of Ag nanoclusters on a TiO 2-nanotubes/Ti substrate

Tubular arrays of TiO 2 nanotubes (ranging in diameter from 40 to 110 nm) on a Ti substrate were used as a support for Ag deposits obtained by the sputter deposition technique where the amount of Ag varied from 0.01 to 0.2 mg Ag/cm 2. Those composite supports were intended for surface-enhanced Raman scattering (SERS) investigations. Composite samples of Ag/TiO 2 nanotube/Ti were studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to reveal their characteristic morphological and chemical features. Raman spectra of pyridine (as a probe molecule) were measured at different cathodic potentials ranging from −0.2 down to −1.2 V after the pyridine had been adsorbed on the Ag-covered TiO 2 nanotube/Ti substrates. In addition, SERS spectra on a bulk standard activated Ag substrate were also measured. The SERS activity of the composite samples was strongly dependent on the amount of Ag deposit. At and above 0.06 mg Ag/cm 2, the SERS signal was even higher than that for the Ag reference substrate. The high activity of the composites is mainly a result of their specific morphology. The high SERS sensitivity on the surface morphology made it possible to monitor very small temporal changes in the Ag clusters. This rearrangement was not detectable with microscopic (SEM) or microanalytical (AES) methods.

Synthesis and Catalytic Performance of Mesoporous Material by Two-step Crystallization

A mesoporous material was synthesized by using the precursor of MCM-22 zeolite through a two-step synthesis route,and characterized by XRD,N2 physical adsorption-desorption,TEM,27 Al MAS NMR,IR spectra of pyridine adsorbed methods and so on.It was found that the synthesized sample was not a mixture of microporous zeolite and mesoporous material but a new mesoporous molecular sieve with strong acid sites and good thermal stability.The acid catalytic activity for macromolecules was evaluated and compared with conventional mesoporous material MCM-41 through the cracking reaction of cumene and alkylation reaction of benzene with 1-dodecen.The results show that the conversions of cumene cracking at 350℃ over the new mesoporous material and MCM-41 are 68.98% and 48.80%,respectively.For the alkylation reaction of benzene with 1-dodecen at 210℃,the conversions of 1-dodecen over the new mesoporous materials and MCM-41 are around 95.20% and 86.89%,respectively.The selectivities of linear alkylbenzene over the new mesoporous materials and MCM-41 are around 88.11% and 90.06%,respectively.The results show that the catalytic property of the new mesoporous materials for macromolecules is superior to that of MCM-41.

Pyridine on Colloidal Silver. Polarization of Surface Studied by Surface-Enhanced Raman Scattering and Density Functional Theory Methods

Surface-enhanced Raman scattering (SERS) spectra of pyridine in various Ag colloids with different excitation wavelengths (514.5 and 1064 nm) were recorded and compared with the theoretical models computed with application of the static density functional theory method in order to simulate the chemical enhancement. Pyridine was chosen as a model compound to look at the surface enhancement since it is a well-known and extensively studied molecule by using SERS spectroscopy, and hence of great significance to the subject. Moreover, it was the first species for which a SERS spectrum obtained on electrochemically roughened silver surface was reported. The arrangement of pyridine on metal particles in the solution allows neglecting the solvent effects and placing the focus only on the interaction between adsorbent and metal surface to study the SERS mechanism. The intensity of bands in the fingerprint region of pyridine SERS spectra depends not only on the excitation wavelength but also on the applied colloid ...

Characterizing the surface acidity of bentonite by various methods

AbstractSurface acidity of bentonite was investigated by using three methods. The first method was inverse gas chromatography; the acid‐base character of bentonite surface was estimated by the retention time with different nonpolar and polar probes at infinite dilution region. The adsorption specific free energy (ΔGSP), specific enthalpy (ΔHSP), and specific entropy (ΔSSP) of polar probes on bentonite were determined. By correlating ΔHSP with donor and acceptor numbers of the probes, the acidic (KA) and the basic (KD) parameters of the sample were calculated. The second method was recording the spectrums of pyridine by Fourier transform infrared spectroscopy (FTIR); the spectrum of pyridine bonded by Bronsted and Lewis acid sites was obtained. The last method was modified amine titration by using Hammett indicators. Copyright © 2010 John Wiley & Sons, Ltd.

Structural features of binary microporous zeolite composite Y-Beta and its hydrocracking performance

Y-Beta zeolite composites denoted as HS-FBZ (High-Silica-Faujasite-Beta-Zeolite) were synthesized hydrothermally using high silica Y as precursor and characterized by XRD, N 2 adsorption, SEM, HRTEM, NH 3-TPD and FT-IR spectra of pyridine. The results reveal that the different ratios of Y and Beta zeolites in the composites induced the change of surface areas, pore diameters and acid properties of HS-FBZ, and the extended Beta layers on the zeolite Y surface were observed. The composite exhibited an excellent hydrocracking activity for heavy oil. Compared with the corresponding physical mixture of Y and Beta zeolites, the zeolite composite as a hydrocracking catalyst for vacuum gas oil increased the heavy naphtha yield by 5.61 wt% and the jet fuel yield by 11.52 wt%.