Adsorbed Pyridine Molecules Research Articles

Charge transfer effect: a new assignment of the abnormal optical absorption band of gold nanoparticles

As a significant accompanying phenomenon of surface-enhanced Raman scattering (SERS), the addition of foreign molecules to colloidal gold or silver nanoparticles results in a new abnormal optical absorption (AOA) band, which usually appears in the long-wavelength region. The assignment of this AOA band has long been debated as an important issue that is desired to be addressed in the SERS field, which is crucial for a clear understanding of the SERS enhancement mechanism and beneficial to surface plasmonics. In this study, both the calculated and measured optical absorptions of gold nanoparticle monomers and dimers as well as their interactions with adsorbed molecules, showed that the AOA band in the long-wavelength region which was assigned to the characteristic longitudinal localized surface plasmon resonance (LSPR) effect of gold nanoparticle chain aggregates in conventional SERS electromagnetic theory, should be attributed to the charge-transfer resonance absorption from gold nanoparticles to adsorbed molecules. This was further confirmed by the corresponding SERS effects. As the excitation wavelength at 785 nm was resonant with the broad AOA band centered at 750 nm, the SERS peaks of the adsorbed pyridine molecules could be dramatically enhanced due to the charge-transfer resonance effect. In contrast, under an excitation wavelength of 532 nm, the SERS peaks appeared very weak, although the excitation wavelength was resonant with the LSPR absorption band of the individual gold nanoparticles.

Detection of heavy metals, SERS and antibacterial activity of polyvinylpyrolidone modified plasmonic nanoparticles

Selective and sensitive optical sensor based on surface plasmon resonance for detection of various heavy metals in water using polyvinylpyrolidone modified silver nanoparticles was explained in this present study. The prepared nanoparticles were characterized by UV–visible spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FTIR) and Transmission electron microscopy (TEM). UV–visible spectra show the surface plasmon resonance (SPR) peak at 409 nm corresponding to silver nanoparticles. Crystalline nature of the nanoparticles was evident from TEM images and XRD analysis. TEM images showed average size of 10 nm for prepared silver nanoparticles. FTIR analysis provides the presence of various functional groups responsible for the reduction and stability of the prepared silver nanoparticles. SERS gives the molecular orientation of the adsorbed pyridine molecules via its nitrogen lone pair of electrons on the surface of silver. Prepared polyvinylpyrolidone modified silver nanoparticles (AgNPs) are demonstrated to detect the concentration of heavy metal contaminant Fe3+ ions in water based on linear change in surface plasmon resonance absorption strength. In addition, AgNPs showed promising activity towards E.coli. These observed optical properties suggest the possible utilization of prepared nanoparticles in the application of water purification.

Molecular insights into the hydrodenitrogenation mechanism of pyridine over Pt/γ-Al2O3 catalysts

The hydrodenitrogenation of nitrogen-containing heterocycles over noble metals has fundamental importance for energy and environmental science. To develop more efficient catalyst, mechanistic investigation has been conducted with a method combining in situ Fourier transformation infrared experiments and density functional theory calculations in this work. The in situ experiments indicate flatly adsorbed pyridine molecules convert to pyridinium and α-pyridyl species at higher temperature on metallic Pt of Pt/γ-Al2O3 catalysts. Pyridine hydrogenation distinctly takes place at 150 °C with the appearance of methylene stretching vibrations, and a stepwise mechanism is identified as the temperature further increases. The adsorption, hydrogenation and hydrogenolysis of pyridine on Pt are studied in detail by theoretical calculations. In line with these findings, the geometry optimization confirms pyridine preferentially adsorbs on Pt(111) and Pt(211) both in a parallel configuration. Based on the Langmuir–Hinshelwood mechanism, the results show successive hydrogenation markedly lowers the energy barrier for subsequent hydrogenolysis. The CN bond cleavage occurs via nucleophilic attack of pentahydropyridine, rather than piperidine, which determines the reaction products, including piperidine, n-pentylamine and n-pentane. The comparative study reveals both hydrogenation and hydrogenolysis are kinetically and thermodynamically more competitive on Pt(211) than Pt(111). Especially for hydrogenolysis, the coordinatively unsaturated Pt step atoms play an essential role in CN bond cleavage. Thus, hydrogenolysis is more geometric‒dependent than hydrogenation. This provides instructive information for the design of catalysts with adjustable product selectivity.

Spectroscopic characterization–catalytic activity correlation of molybdena based catalysts

Titania-supported molybdena catalyst was prepared by calcination at 773K of ammonium heptamolybdate-impregnated TiO2 pellets with the equivalent of 5 monolayers of MoO3. Then, potassium-modified versions of the catalyst were prepared at 0.5–5wt%-K. The modified and unmodified catalysts were characterized, before and after H2-reduction at 673K, by means of ISS, XPS, UPS and FT-IR spectroscopy techniques. Results obtained enabled revealing the presence of density of states (DOS) at the Fermi level and, hence, metallic properties related to the deformed rutile structure of MoO2. On the other hand, Brønsted (MoOH) acidic function was found to form on the surface of the MoO2 structure, thereby producing a metal-acid (bifunctional) MoO2−x(OH)y catalyst. The presence of Brønsted acid MoOH groups was evidenced by in situ IR spectra of adsorbed pyridine molecules. On the other hand, the K-modification was found to enhance the molybdena dispersity and reducibility, but to suppress the surface acidity by transforming the bifunctional MoO2−x(OH)y species into the monofunctional MoO2−x(OK)y species. Subsequent catalytic measurements employing hydrotreating reactions of 1-hexene and n-hexane could confirm the exposure of bifunctional or monofunctional sites on the catalysts tested. The combination of research methods adopted in the present investigation has shown competency in unravelling the nature of active sites on heterogeneous catalysts.

Catalytic activity of cobalt on nanotextured polymer films for hydrogen production

We describe the mechanism of cobalt and ligand binding on nanotextured poly(chloro-p-xylylene) (PPX) films as supports for catalytic release of H2 from alkaline aqueous solutions of sodium borohydride. Cobalt catalysts are prepared on nanotextured PPX substrates via electroless plating using a Sn-free Pd(II) colloid with adsorbed pyridine ligand as an adhesion promoter. Gas physisorption studies on PPX, using N2 and CO2 as probe gases, indicate the presence of micropores (∼1 to 2nm width) responsible for the adsorption and non-covalent stabilization of pyridine molecules on the nanotextured surface. The strongly adsorbed pyridine molecules promote Co adhesion onto the PPX surface during subsequent electroless deposition, thereby retaining the metal's catalytic activity for H2 evolution even after multiple reaction cycles. In contrast, conventionally deposited PPX is devoid of any nanotexture and contains fewer micropores capable of stabilizing pyridine adsorption, resulting in poor metallization and catalytic activity for H2 evolution. We also demonstrate the effect of patterning the PPX substrate as a means to further improve the activity of the Co catalyst to achieve H2 evolution rates comparable to those obtained using precious metal catalysts.

ChemInform Abstract: Acid Strength of Silica-Supported Oxide Catalysts Studied by Microcalorimetric Measurements of Pyridine Adsorption.

Microcalorimetric measurements of the differential heat of pyridine adsorption were used to probe the distribution of acid strength on a series of silica-supported oxide catalysts. Depositing oxides of the following cations onto silica increased the acid strength of the catalyst: Ga{sup 3{plus}}, Zn{sup 2{plus}}, Al{sup 3{plus}}, Fe{sup 3{plus}}, Fe{sup 2{plus}}, Mg{sup 2{plus}}, and Sc{sup 3{plus}}. The acid strength distribution curves for the supported oxide samples showed either two or three regions of constant heat of adsorption while silica had an energetically homogeneous surface. The Ga, Al, and Sc samples were found to have both Bronsted and Lewis acidity while the remaining samples showed only Lewis acidity. Incremental adsorption of pyridine indicated that the initial region of highest heat corresponds to strong Lewis acidity while intermediate heats seemed to be due to weaker Lewis acid sites or a combination of Lewis and Bronsted acid sites. The final region of lowest heat was due to H-bonded pyridine on silica. Estimates of the entropies of adsorption were determined, providing information about the mobility of the adsorbed pyridine molecules. The initial differential heat of adsorption increases proportionally to the Sanderson electronegativity of the added oxide.

Evidences for the relationship between surface structure and reactivity of goethite nanoparticles based on advanced molecular-probe methods

Surface properties of two goethites have been studied in order to compare the amount of acid surface sites and their distribution over the various surface domains. For this purpose, ammonia, pyridine and nitrogen were used as basic molecular probes. Calorimetry measurements of ammonia adsorption provided the image of the average surface acidity being moderate. This conclusion was supported by the moderate resistance of the adsorbed pyridine molecules to degassing conditions. Adsorption and desorption of pyridine prior to gaseous nitrogen adsorption resulting in masking/unmasking of acid surface sites on the goethite surface allowed confirmation of the acid character of the specific adsorption sites characterized by the high-energy adsorption of electron-donating molecular nitrogen. The amount of acid sites probed by nitrogen and ammonia were of the same order of magnitude but systematically higher for ammonia. The subsequent analysis of the argon and nitrogen derivatives of first-layer adsorption isotherm led to determine the distribution of {101} and {121} crystallographic faces and discuss the location of acid sites on these surface domains.

Molecular Dynamics of Pyridine Adsorbed on the Silica Surface

The molecular dynamics of pyridine adsorbed onto dry high-surface-area silica gel was studied using solid-state deuterium NMR spectroscopy. Selectively and totally deuterated pyridine samples were used to identify the spectral signatures associated with each ring location and to identify the dynamical modes present in the surface adsorbed species. Loading levels corresponding to 0.8 and 0.2 monolayer coverage were studied. At 174 K and 0.8 monolayer, the adsorbed pyridine molecules are found in three motional modes: a fraction is found to be static, a larger fraction is found executing fast discontinuous rotation (180° ring flips) about the pyridine Cγ−N axis, and a very small fraction is found in fast continuous rotation (diffusion) about the pyridine Cγ−N axis, all referred to a 5 × 10-6 s time scale. No evidence of O−H−N rotation about the Si−O bond (“swinging arm” motion) is observed. The pyridine flips and rotations permit measurement of the angle between the Cγ−N axis and each of the Cα−D and Cβ−D axes are found to be 55.9 ± 0.3° and 60.4 ± 0.5°, respectively. As the temperature is lowered, the fraction of static pyridine rings increases, while the fractions of flipping and freely rotating pyridine rings decrease. At 78 K and 0.8 monolayer, pyridine ring flip motion is still observed, implying a very low potential barrier to discontinuous rotation for some of the surface population. At 221 K, the flipping rings dominate, rotating rings are a minority, and no static rings remain. Above 221 K, the chemical exchange rate among pyridine molecules becomes significant and further motional averaging is observed. At 221 K and below, the spectra show no contributions corresponding to intermediate-rate motions; the notable absence of intermediate-rate patterns is explained in terms of a previously proposed model based on a broad distribution of motional correlation times. The surface structures and dynamics with 0.2 monolayer are found to be substantially similar to those found at 0.8 monolayer. These dynamical data are interpreted in terms of the structure of the silica surface as probed by pyridine molecules.

Surface characterization of zirconia, holmium oxide/zirconia and sulfated zirconia catalysts

Zirconia-supported holmium oxide, as well as their sulfated catalysts using (NH 4) 2SO 4 were prepared by wet impregnation and calcined at different temperatures. The samples thus obtained were subjected to bulk and surface texture analysis using X-ray powder diffractometry (XRD) and N 2-sorptiometry, respectively. The surface acidic properties were characterized by in situ FT-IR spectroscopy of adsorbed pyridine molecules. The decomposition of gas phase 2-propanol molecules on the catalysts was examined at different temperatures, also using in situ FT-IR spectroscopy. The results obtained revealed that the presence of Ho 2O 3 on ZrO 2 induces the formation of cubic and/or tetragonal ZrO 2 phase at low temperatures (≤800 °C) without significant effect on the surface area. Whereas, the presence of Ho 2O 3 and sulfate ions on ZrO 2 results in the highest formation of cubic and/or tetragonal ZrO 2 phase at low temperatures and induces stabilization of the surface area. The surface of Ho 2O 3/ZrO 2 samples exposes Lewis acid and reactive basic sites, whereas the sulfated samples show at least two types of Lewis and Brönsted acid sites with enhanced acid strength. The decomposition of 2-propanol results show that the sulfate-free samples are dual functioning catalysts at 300 °C; 2-propanol molecules decompose thereon via dehydration into propene and dehydrogenation into acetone. However, the sulfated samples are found to only catalyze the dehydration into propene at all reaction temperatures applied.

Characterization of surface acidity of an acid montmorillonite activated with hydrothermal, ultrasonic and microwave techniques

The surface acidity of montmorillonite treated by conventional hydrothermal reaction with acids as well as ultrasonic- and microwave-assisted acid digestion was studied using diffuse reflectance FT-IR (DRIFT) spectroscopy of adsorbed pyridine molecules. An attempt for semi-quantitative measurement of Brønsted acidity generated during the acid digestion was also carried out. The conventional hydrothermal, ultrasonic- and microwave-assisted acid digestion yielded strong Brønsted acid sites of similar nature and strength, which retained pyridine up to 400–450 °C. Strong Lewis acid sites were also developed at higher temperature (>300 °C), which were able to retain pyridine at 450 °C. The results show that surface acidity of similar strength can be generated with conventional hydrothermal treatment at 80 °C for 4 h or ultrasonic-treatment at room temperature for 1 h or microwave-assisted acid digestion for 10 min using 3 N concentration of H2SO4. Therefore, microwave- and ultrasonic-assisted acid digestions are potential and rapid alternative methods to conventional hydrothermal acid digestion for preparing acid-activated clays.

Atomic and Electronic Structure of Pyridine on Ge(100)

We have performed ab initio pseudopotential calculations in order to investigate the atomic and electronic structure of pyridine adsorbed on the Ge(100) surface. A large number of pyridine/Ge(100) adsorption configurations possibly resulting from cycloadditions and Lewis acid−base reactions are presented. The configuration having the Ge−N linkage formed by dative bonding with adsorbed pyridine molecules tilted is the most stable, which explains the experimental STM images well. The dative bonding character is investigated by comparing the charge densities for the clean and pyridine-adsorbed Ge(100) surfaces. Finally the difference between the Ge(100) and Si(100) surfaces is discussed.

The hydroxyl species and acid sites on diatomite surface: a combined IR and Raman study

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), Raman spectroscopy of adsorbed pyridine molecules (Py-Raman) and in situ Py-IR have been used to investigate the hydroxyl species and acid sites on diatomite surfaces. The Lewis (L) and Brønsted (B) acid sites, and various hydroxyl species, including isolated hydroxyl groups, H-bonded hydroxyl groups and physically adsorbed water, are identified. The L acid sites in diatomite samples are resulted from the clay impurities, and the B acid sites are resulted from some moderate strength H-bonded hydroxyl groups. At room temperature, both of the isolated and H-bonded silanols associate with the physically adsorbed water by hydrogen bond. After calcination treatment, physically adsorbed water will be desorbed from the silanols, and the silanols will condense with the increase of temperature. Generally, the H-bonded silanols condense more easily than the isolated ones. The properties of surface hydroxyl species of diatomaceous silica are more similar to precipitated silica rather than fumed silica.

Formation of highly ordered organic monolayers by dative bonding: pyridine on ge(100).

The adsorption of pyridine onto the Ge(100) surface has been studied using both real-time scanning tunneling microscopy (STM) and ab initio pseudopotential density functional calculations. The results show that pyridine molecules adsorb on the electron-deficient down-Ge atoms of the Ge=Ge dimers via Ge-N dative bonding, with the pyridine ring tilted to the surface. The electron-rich up-Ge atoms remaining after adsorption of pyridine induce an asymmetric dimer row, which is mainly reconstructed to the c(4 x 2) structure. At pyridine coverage of 0.25 ML, the adsorbed pyridine molecules form a perfectly ordered monolayer. The entire Ge substrate underlying this organic monolayer rearranges into the c(4 x 2) structure.

On Determination of Acid Site Densities on Sulfated Oxides

Combined infrared and gravimetric studies have been performed of pyridine adsorption on sulfated and non-sulfated silica-zirconia mixed oxides in order to quantify the site densities of both Bronsted and Lewis acid sites. The values determined for the integrated molar absorption coefficients for the non-sulfated silica-zirconia mixed oxides were 1.24 cm μmol-1 at 1540 cm-1 and 1.56 cm μmol-1 at 1450 cm-1 and were independent of the Si:Zr ratio and other pre-treatment and preparation variables. On the other hand, values ranging from 0.64 to 1.66 cm μmol-1 at 1540 cm-1 for the pyridinium ion and from 1.00 to 2.08 cm μmol-1 for the Lewis-bound pyridine were determined for the sulfated samples. It is proposed that the reduction in absorption coefficient for the pyridinium ion with increasing sulfate density corresponds with the transformation of monomeric to dimeric or polymeric sulfate species which bring two adjacent adsorbed pyridine molecules into close proximity. The findings of this study raise considerable concern over the validity of acid site densities determined for sulfated materials where calculations have been performed based on literature values for the absorption coefficients.

Infrared Study of Acetophenone Adsorption on Mordenite and Dealuminated Mordenite

Infrared spectra of acetophenone adsorbed on mordenite and acid-leached mordenite are reported. The effects of dealumination were characterized by pyridine adsorption and29Si and27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). Adsorption of pyridine gave bands at 1634 and 1545 cm−1due to vibrations of pyridinium ions and a band at 1621 cm−1characteristic of pyridine ligated to Lewis acidic Al3+sites.27Al NMR gave peaks at 55 ppm due to tetrahedral framework Al and at 0 ppm due to octahedral extraframework Al.29Si NMR gave peaks at −106 and −112 ppm due to Si(1Al) and Si(OAl) species, respectively. For acetophenone on mordenite, spectra contained ν(CO) bands at 1685, 1671, 1650, and 1630 cm−1due to physisorbed acetophenone containing unperturbed CO groups, acetophenone hydrogen bonded with external and internal silanol groups, acetophenone hydrogen bonded to Brønsted acidic Si(OH)Al groups, and acetophenone ligated to Lewis acidic Al3+ions, respectively. Dealumination converted 56% of the Si(1Al) species to Si(OAl), reduced the concentrations of both framework and extraframework Al, reduced the concentration of Si(OH)Al groups, and greatly reduced the population of Lewis acidic sites, but generated internal silanol groups which formed hydrogen bonds with acetophenone molecules. Residual Al3+sites and Si(OH)Al groups after dealumination could interact with adsorbed pyridine molecules but for steric reasons were unable to form adsorption complexes with acetophenone. Evidence for the protonation of acetophenone was inconclusive.

ピリジンの水銀電極への吸着‐脱着挙動

Adsorption-desorption phenomena of pyridine at a mercury electrode were studied by a flow injection method with mesuring differential capacity-time curves.At more positive potentials than -0.9 V the adso rbed pyridine molecules are never desorbed, which means that the adsorption is irreversible. At -1.35 V a very sharp desorption peak is observed, which means that the adsorption is reversible. Irreversibly adsorped pyridine molecules replace water molecules at the surface of the electrode. Reversibly adsorbed pyridine molecules, which form groups of molecules, are placed on the water molecules at the surface of the electrode. When the groups of pyridine moleculs are desorbed, very large peaks are observed.

Atomic Force Microscopy Determination of the Molecular Orientation and Array Structure of a Pyridine Adlayer on Heulandite(010) Adsorbed from Its Aqueous Solution

For the first time the array structure of liquid-phase-adsorbed aromatics on a zeolite surface was determined, and the molecular orientation within the adsorbed phase was estimated. Pyridine molecules adsorbed on a cleaved heulandite(010) surface from an aqueous solution were imaged with a contact-mode atomic force microscope. The adsorbed pyridine molecules formed a well-ordered two-dimensional (2D) hexagonal array on the surface, with the rhombic unit cell dimension being 0.55 nm. The molecular plane is found to be tilted, with the estimated tilt angle being ca. 43° to the surface. The ring plane also makes an estimated angle of ca. 20° with respect to one of the 2D lattice vectors of the adsorbed pyridine unit cell.

Electron-induced chemistry of pyridine on Si(111)7 × 7: a LEED and TDS study

The adsorption of pyridine on Si(111)7 × 7 at room temperature (RT) is investigated by thermal desorption spectrometry (TDS). The molecular (mass 79) desorption profile, with desorption maxima at ∼ 380 and ∼ 470 K, is found to resemble that of five-membered heterocyclic aromatic compounds including thiophene and furan. The corresponding molecular TDS profile for pyridine exposed to a sputtered Si surface consists of a single broad peak at ∼ 420 K with similar intensity as that found for the 7 × 7 surface, thus suggesting that a specific type of active sites is not essential for the pyridine adsorption. On the other hand, the large reduction in mass 79 desorption observed for pyridine exposure to an oxidized Si surface shows that Si dangling bonds are required for the initial adsorption of pyridine. In addition, a 50 L RT exposure of pyridine on Si(111)7 × 7 results in a “modified” 7 × 7 LEED pattern in which the fractional-order spots become less intense except for the off-axis (± 37, ± 37), fractional-order spots that remain relatively intense. Furthermore, the rapid conversion of the modified 7 × 7 pattern to a 1 × 1 pattern during the LEED experiment suggests that the adsorbed pyridine molecules undergo an electron-induced dissociation reaction on the Si surface. The corresponding mass 79 TDS profile is found to become broaden and less intense with increasing electron irradiation current and incident electron energy. The electron irradiation also appears to significantly increase the mass 4 (D2) desorption for RT exposure of d5-pyridine to Si(111)7 × 7, which further indicates that electron-induced dissociation of pyridine results in the deposition of C, N, and D atoms on the Si surface. It is, however, unclear if the electron-induced dissociation is a direct process or a stepwise process involving smaller intermediate fragments that further decompose during thermal annealing. Comparison of the respective TDS profiles of pyridine and benzene shows that the heteroatom appears to have the effect of reducing the adsorption of pyridine on the Si surface (in comparison to that of benzene) while inducing a greater surface reactivity. The present work further illustrates that the similarities observed in the TDS profiles for a series of homologous five-membered and six-membered aromatic compounds may not necessarily entail similar adsorption configurations and that the complex surface chemistry involved can indeed be quite different for these molecules on the Si(111) surfaces.

FTIR and Raman spectroscopic investigation of 2, 2′-bipyridine adsorption on silica, alumina, zirconia and titania

The adsorption of 2,2′-bipyridine on silica, γ-alumina, monoclinic zirconia and mixed anatase/rutile titania have been studied by infrared (IR) and Raman spectroscopic techniques. These oxides were chosen because they constitute pillars in various pillar interlayered clay (PILC) catalysts which have been the subject of a separate 2,2′-bipyridine study. Four different types of adsorbed 2,2′-bipyridine molecule have been identified depending on the specific oxide. Physisorbed neutral molecules were common to all four oxides. Hydrogen-bonded neutral species were observed on silica, γ-alumina and titania, while neutral 2,2′-bipyridine, coordinatively bound in a bidentate manner to single Lewis sites was also detected on γ-alumina, zirconia and titania. An additional Lewis-bound species was observed on γ-alumina where the adsorbate was bound to two adjacent Al3+ Lewis sites. At high temperature these adsorbed molecules were observed to dissociate into individual adsorbed pyridine molecules. Physisorbed bipyridine molecules exhibited trans symmetry, while H-bonded and chemisorbed bipyridine molecules possessed cis symmetry.

Electroreflectance spectroscopy of Au(100) covered by adsorbed pyridine molecules

Abstract Electroreflectance spectroscopy in the visible and near-UV has been employed to study the influence of adsorbed pyridine on the surface reconstruction and the optical properties of Au(100). The pyridine-induced structural transition of the Au(100) electrode from a reconstructed (5 × 20) to an unreconstructed (1 × 1) surface has been monitored optically for various pyridine concentrations. It has been found that this surface structural transition coincides with a phase transition in the pyridine adlayer, which corresponds to a reorientation of the molecule from a flat π-bonded to a vertical N-bonded surface coordination. Pyridine adsorbs flat lying on the reconstructed surface, which is stable at negative surface charges, while on the unreconstructed, more open surface at positive charges the pyridine molecules assume a vertical, N-bonded position. Optical transitions involving surface states which display a strong dependence on the electrode potential (charge density) due to a Stark shift, were studied in the absence and presence of pyridine. Adsorption of pyridine causes a broadening and a gradual decrease of the surface state spectral features which suggests that the surface states undergo an adsorbate-induced level broadening, but are not quenched completely.