In situ study of ozone and hybrid plasma Ag-Al catalysts for the oxidation of toluene: Evidence of the nature of the active sites

Silver colloids have been prepared by reducing AgNO3 in aqueous solution and embeded in alumina following a sol-gel procedure in the presence of Pluronic 84 ((EO)(19)(PO)(39)(EO)(19)), as surfactant. Plasma-catalytic experiments aimed at the mineralization of toluene showed that the selectivity to CO2 was significantly increased in the presence of Ag catalysts compared with results obtained using the plasma alone. In-situ studies of the ozone interaction with catalysts provide an insight into the nature of the active sites of supported silver colloids for mineralization reactions. It is noticeable that when ozone is chemisorbed on embedded Ag colloidal catalysts no change in the silver oxidation state or size is found. The population of the chemisorbed species is higher at lower temperatures, where the non-selective decomposition of ozone is smaller. The catalysts exhibit high stability, preserving the structural and textural properties after the catalytic tests, that is indeed very important in the presence of ozone. (C) 2011 Elsevier B.V. All rights reserved.

Nano-structural investigation of Ag/Al2O3 catalyst for selective removal of O-2 with excess H-2 in the presence of C2H4

Ag/gamma-Al2O3 catalysts have been characterized in-depth during different thermo-chemical treatments by in situ diffuse reflectance UV-visible spectroscopy and quasi in situ Transmission Electron Microscopy. The combination of these techniques indicates that sintering and redispersion of silver is clearly observed from the increases and decreases in the absorption band intensity over the range of 250-600 nm due to the presence of silver clusters and silver nanoparticles. These results allow us to study the effect of the reaction feed on the metal dispersion at different operation conditions and discuss the formation of active sites during the selective catalytic reduction of O-2 with excess H-2 in the presence of unsaturated hydrocarbons. In this case high catalytic activity and selectivity toward the oxygen removal was achieved for this catalyst. (C) 2010 Elsevier B.V. All rights reserved.

Surface-Enhanced Raman Evidence of Protonation, Reorientation, and Ag+ Complexation of Deoxyadenosine and Deoxyadenosine-5 '-Monophosphate (dAMP) on Ag and Au Surfaces

Surface-enhanced Raman (SERS) spectra of deoxyadenosine and 5'-dAMP on Ag and Au surfaces showed the protonation of both compounds in the N1 position, their orientation geometry on metal surfaces, and the formation of Ag+ complexes at alkaline pH on hydroxylamine-reduced Ag colloids. Interestingly, substitution at the N9 position caused dramatic changes in the relative band intensities within the spectra of both deoxyadenosine and 5'-dAMP compared to that of simple adenine, although they continued to be dominated by adenine vibrations. Concentration-dependent spectra of 5'-dAMP were observed, which matched that of adenine at high concentrations and that of deoxyadenosine at lower concentration (

Modification of Ag nanoparticles with mixed thiols for improved SERS detection of poorly adsorbing target molecules: detection of MDMA

Here we report an example of a mixed thiol monolayer on the surface of Ag nanoparticles which promotes adsorption and quantitative SERS detection of 3,4-methylenedioxymethamphetamine (MDMA, “Ecstasy”); the thiols in the mixed monolayers act synergistically since MDMA does not adsorb onto colloids modified with either of the thiols separately.

Structure of Adenine on Metal Nanoparticles: pH Equilibria and Formation of Ag+ Complexes Detected by Surface-Enhanced Raman Spectroscopy

The SERS spectra of adenine recorded under a broad range of pH values and concentrations using both silver and gold colloids provided evidence for the existence of several distinct species. At high concentration (0.5-10 ppm), the spectra recorded between pH 1 and 11 showed only two distinct spectra, rather than the three forms that would be expected for a compound with two pK(a) values of 4.2 and 9.8. The spectra at neutral and alkaline pH were identical and assigned to the deprotonated form of adenine on the basis of DFT calculations, isotope shifts, and comparison with the normal Raman spectra of neutral and deprotonated adenine. The spectra at acidic pH were different, consistent with adenine protonation. Neutral adenine was not detected at any pH studied. At low adenine concentration (

Triply-bridged diphos disilver helical complexes [Ag-2(mu(2)-dppa-P,P')(3)(anion)(2)] [dppa = bis(diphenylphosphino)acetylene]

Triply-bridged disilver complexes [Ag-2(mu(2)-dppa-P,P')(3)(anion)(2)] form selectively and are stabilised by many aromatic interactions.

Anion-templated formation of a unique inorganic 'super-adamantoid' cage [Ag-6(triphos)(4)(O3SCF3)(4)](2+) [triphos = (PPh2CH2)(3)CMe]

In the presence of templating anions, 2:3 molar mixtures of triphos and silver(I) cations unexpectedly give novel hexanuclear cages, which result from an unusual 'endo-methyl' geometry of the triphos ligands.

The first structural characterization of a [2.2]PHANEPHOS - Transition-metal complex: Structure of rac-[Pd(4,12-bis(diphenylphosphino)[2.2]paracyclophane)Cl-2]

The solid-state structure of the [2.2]PHANEPHOS-transition-metal complex rac-[Pd(4,12-bis(diphenylphosphino)[2.2]paracyclophane)Cl-2] has been established by single-crystal X-ray diffraction. The P-Pd-P bite angle is ideally suited to catalytic processes such as carbon-carbon cross-coupling reactions, which involve reductive elimination as the rate-determining step.

Diphosphine analogues of proton sponge: X-ray crystal structure of [Pd(eta(3)-allyl)(dppn)]BF4 center dot CH2Cl2 (dppn equals 1,8-bis(diphenylphosphino)naphthalene)

The X-ray crystal structure of [Pd(eta(3)-allyl)(dppn)]BF4 . CH2Cl2 (1) where dppn = 1,8-bis(diphenylphosphino)naphthalene is reported. Comparison of the conformation of the ligand in 1 with that in the free state shows that there is a relief of strain on complexation analogous to the relief of strain observed upon protonation of proton sponge.

Catalytic properties of beta zeolite exchanged with Pd and Fe for toluene total oxidation

Dealuminated beta zeolites exchanged with Pd and Fe were prepared to investigate the influence of iron and dealumination on the activity and selectivity of Pd/BEA zeolite for toluene total oxidation. The specific areas determined by BET method and EPR studies allowed to know that the palladium would be more easily agglomerated on the BEA than on the DBEA. Moreover, a quantification of the palladium saturation on the BEA zeolite was deduced by EPR. Effects of dynamic and static oxidation and weak and strong reduction treatments were studied by EPR. Several isolated and interacted Pd+ species and hole centers were detected. The Pd was much reduced after the catalytic test in dealuminated and Fe doped samples. This result could be directly correlated to the catalytic deactivation. The deactivation could be also explain by the type of coke deposed on the catalyst and by the hydroscopic behavior of the samples. Addition of Fe or dealumination could prevent the deactivation and then lead to better catalysts for VOCs oxidation.

Characterization of novel Ag on TiO2 films for surface-enhanced Raman scattering

Novel Ag on TiO₂ films are generated by semiconductor photocatalysis and characterized by ultraviolet-visible (UV/Vis) spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM), as well as assessed for surface-enhanced Raman scattering (SERS) activity. The nature and thickness of the photodeposited Ag, and thus the degree of SERS activity, is controlled by the time of exposure of the TiO₂ film to UV light. All such films exhibit the optical characteristics (λ_max ≅ 390 nm) of small (<20 nm) Ag particles, although this feature becomes less prominent as the film becomes thicker. The films comprise quite large (>40 nm) Ag islands that grow and merge with increasing levels of Ag photodeposition. Tested with a benzotriazole dye probe, the films are SERS active, exhibiting activity similar to that of 6-nm-thick vapordeposited films. The Ag/TiO₂ films exhibit a lower residual standard deviation (∼25%) compared with Ag vapor-deposited films (∼45%), which is, however, still unacceptable for quantitative work. The sample-to-sample variance could be reduced significantly (<7%) by spinning the film during the SERS measurement. The Ag/TiO₂ films are mechanically robust and resistant to removal and damage by scratching, unlike the Ag vapor-deposited films. The Ag/TiO₂ films also exhibit no obvious loss of SERS activity when stored in the dark under otherwise ambient conditions. The possible extension of this simple, effective method of producing Ag films for SERS, to metals other than Ag and to semiconductors other than TiO₂, is briefly discussed. 

Anodic stripping voltammetry with photochemical preconcentration at nanocrystalline TiO2 films: Detection of Ag+ and Hg2+

Nanocrystalline TiO2 deposited on conducting glass plates is shown to be an excellent material for preconcentration of silver and mercury, via photochemical reaction, prior to their detection by anodic stripping voltammetry (ASV). During the first stage of growth in the photoreduction of silver or mercury, 3D nuclei are formed on the TiO2 film. As the deposition proceeds micrometer size agglomerates grow on the surface. The conical morphology of the silver nuclei grown on a (110) rutile single crystal in the initial stages of growth suggests that there is a preferential deposition of silver at the centre of the growing nuclei. When the nuclei size reach a critical value (ca. 400 nm diameter, 40 nm height) the morphology changes to a globular shape without any preferential site for deposition on the surface of the silver nucleus. It was observed that micromolar concentrations of silver or mercury can be detected by anodic stripping voltammetry and relatively large amounts of these metals (micrometer scale nuclei) can be loaded on the nanocrystalline TiO2 film surface. The latter opens the possibility of analytical applications of nanocrystalline TiO2 electrodes for the selective detection of silver or mercury via photochemical anodic stripping voltammetry.