bluuebunny/crossref_metadata_2025_split huggingface dataset:
Surface Properties and Catalytic Performance of Novel Mesostructured Oxides
Authors: Schüth F. 
Published: March 2014

AbstractMesoporous oxides of the M41S family, discovered by scientists of the Mobil Oil Corporation in 1991, are an exciting class of porous materials. Their unique pore system with the extremely sharp pore size distribution, which is adjustable in the range between about 2 and 10 nm at will, and the extremely high surface areas up to 1500 m2/g open very promising prospects with respect to the development of new catalysts. The oxides were initially prepared based on silicon and aluminum, but later other elements could be incorporated. Moreover, totally non siliceous oxides could be synthesized using surfactants, similar to the synthesis of the M41S materials. First examples for the use of these oxides as catalysts have been reported. In addition, also the properties of the materials as supports for noble metals have been investigated recently.
Production and Properties of Nanostructured Metal Oxide Particles
Authors: Becker A. , Benfer S. , Hofmann P. , Jacob K.‐H. , Knözinger E. 
Published: March 2014

AbstractTwo fundamentally different procedures of metal oxide production, which provide nanometer‐sized oxide particles, are described here. Earthalkaline oxides, e.g., are produced by chemical gas phase deposition with MeO molecules (Me = Ca, Mg) and clusters being directly deposited after gas phase reaction between Me atomes and O2 molecules. The procedure also allows the dotation of MgO with a second component such as CaO and vice versa. In order to obtain information on surface properties like reactivity and surface composition, OH groups as probe species are produced on the surface of the resulting particles. The respective chemisorption as well as thermal desorption of OH groups are monitored by FT‐IR spectroscopy. X‐ray powder diffraction provides information on the bulk composition and bulk structure. Cu2O, on the other hand, is produced by deposition of Cu metal from the vapour and subsequent oxidation of the resulting solid particles, which are then characterized by X‐ray powder diffraction and UV/VIS spectroscopy.
Reactivity and Dynamics of Bifunctional Phosphines Immobilized on Oxidic Supports – A Solid‐State NMR Study
Authors: Blümel Janet 
Published: March 2014

AbstractThe successful immobilization of transition metal catalysts on oxidic surfaces via bifunctional phosphines like PPh2(CH2)3Si(OEt)3 is crucially dependent upon the knowledge of the reactions of both their functionalities with the surface. In this study, solid‐state NMR spectroscopy served as a powerful analytical tool. The nature of a P(V) byproduct, generated by the combined action of ethoxysilane groups and silica surface, has been investigated and ways have been shown to avoid it. The study of the reactions of ethoxysilanes with silica surfaces led to the discovery, that addition reactions with surface siloxane groups play a dominant role. Therefore, the surface coverage can be optimized and later “leaching” of the catalyst prevented by using rigorously dried silica. The success of the developed concepts was demonstrated by the selective immobilization of di‐ and tricarbonylnickel complexes. Both types of complexes can be distinguished by their characteristic 31P solid‐state NMR spectra. The dynamics of immobilized phosphines and carbonylnickel complexes in the presence of different solvents has been studied by 31P suspension NMR spectroscopy. Polar, non‐viscous solvents gave the narrowest resonances, corresponding to the greatest mobility of the surface‐bound species.
Two‐Dimensional Imaging of Reactively Desorbing CuF: State‐Selective Angular and Velocity Distributions
Authors: Sugawara K. , Sun W. , Wach Th. , Wanner J. 
Published: March 2014

AbstractDuring the dry etching reaction of fluorine on copper surfaces, copper monofluoride is the main desorbing product at conditions of low fluorine dosing and surface temperatures above 900 K. Results from a detailed internal product‐state analysis of CuF molecules by laser‐induced fluorescence has been reported recently by Bracker et al., Can. J. Chem.; J.C. Polanyi, Special Issue 72, 643–651 (1994). The present contribution focuses on the state‐selective angular and velocity distributions of the desorbing CuF molecules. The product excitation is based on an actively stabilized ring dye laser in connection with spatially resolved fluorescence imaging by a CCD camera. For all investigated rovibrational product levels the desorbing molecules exhibit a cosn φ distribution with n values between 1.2 and 1.4. The state‐selective velocity distributions of the desorbing CuF molecules are Maxwellian with translational temperatures equal to the surface temperature within the error limits. These results imply that the reaction proceeds indirectly at submonolayer conditions with the following steps: 1/2F2(g)→F(ad) + Cu(s)→CuF(ad)→CuF(g). Furthermore, the associative desorption of CuF from copper surfaces is fully consistent with a non‐activated desorption mechanism.
A PIR, HAS and LEED Study of the Monolayer CH4‐NaCl(001)
Authors: Heidberg J. , Schönekäs O. , Weiss H. , Lange G. , Toennies J.P. 
Published: March 2014

AbstractThe adsorbate CH4‐NaCl(001) at monolayer coverage has been studied by means of helium atom scattering (HAS), electron diffraction (SPA‐LEED) and polarization Fourier‐transform infrared spectroscopy (PIR) in the temperature range between 45 K and 10 K. Above 40 K a splitting of the v4 mode of CH4 into a polarization‐dependent doublet absorption was observed in the FTIR spectra. HAS and SPA‐LEED measurements reveal a (1×1) superstructure with one CH4 molecule per NaCl ion pair. Calculations of the potential energy support these experimental results, yielding a perfect dipod arrangement of the CH4 molecules in the (1×1) structure. Isosteric cooling of the adsorbate results in a two‐dimensional phase transition of order > 1. By means of HAS a p(2×2) superstructure was found at T < 30 K, whereas SPA‐LEED suggests a (2×1) symmetry. Beside of a remarkable sharpening of all IR absorptions two additional features are observed. The phase transition is reversible. Two to three hours after preparation both diffraction methods yield the (1×1) structure even at lowest temperatures. This time behaviour may be indicative of a quantum effect, namely nuclear spin conversion, in the adsorbed methane layer.Knowledge of the interaction between CH4 and NaCl is necessary to predict the gas migration and therewith the safety aspects for a final repository of nuclear waste in permian salt domes.
Nucleation and Growth of Plasma‐Polymerised Hexamethyldisilazane on Iron‐Substrates
Authors: Grundmeier G. , Stratmann M. 
Published: March 2014

AbstractPlasma polymer films of hexamethyldisilazane (HMDSN) were deposited on iron substrates by an audio frequency discharge. The investigations focus on the activation of the iron surface by an oxygen plasma, the adsorption of the monomer and the initial states of polymer formation. Surface compositions were examined by means of in‐situ Infrared‐Reflection‐Absorption‐Spectroscopy (IRRAS) and Quartz‐Crystal‐Nanoweighting (QCN). It turned out that during the oxygen plasma treatment an additional layer of iron oxihydroxide is formed. The resulting high density of surface hydroxyls leads to the hydrolysis of the adsorbed silazane. Plasma‐polymerised HMDSN is characterised by a reduced carbon and nitrogen content and the incorporation of siloxane units. In the range of about 4 nanometer the polymer structure changes significantly with the film thickness. With increasing film thickness the concentrations of siloxane and N‐H units decrease. The interface between the oxide and the polymer is characterised by strong polar interactions between surface hydroxyls and the siloxane group and additionally by directed bonds via silanolate units.
Time Resolved Charge Transfer with Hot Electrons at the Silver/Electrolyte Interface
Authors: Diesing D. , Rüße S. , Otto A. , Lohrengel M.M. 
Published: March 2014

AbstractTunnel junctions of the type metal/oxide/noble metal are contacted with an electrolyte where the noble metal is the working electrode. Hot electrons passing through the oxide film reach the metal/electrolyte interface and occupy protonic states in the electrolyte. Pulse experiments indicate an adsorbed hydrogen phase at the silver/electrolyte interface.
In‐Situ Photoelectron‐Spectroscopy of Oxygen Electrodes on Stabilised Zirconia
Authors: Zipprich W. , Wiemhöfer H.‐D. , Vohrer U. , Göpel W. 
Published: March 2014

AbstractEvaporated silver films as an oxygen electrode material were studied in‐situ and ex‐situ by XPS and UPS under UHV conditions. The experimental set‐up consisted of a miniaturised solid state galvanic cell with yttria stabilised zirconia as solid electrolyte and a solid oxide reference electrode. The oxygen activity at the working electrode was changed by controlling the electrode potential versus the reference electrode consisting of a metal/metal oxide system Me/MeO (with Me = Pd or Fe). The electrode surface was investigated by XPS and UPS under cathodic polarisation with low oxygen activity as well as under anodic polarisation with steady‐state oxygen evolution. The measurements have been performed between 720 K and 1100 K.With XPS, several oxygen species have been detected at the silver surface. Their concentration depends on the applied electrode potential. A species with low XPS binding energy was found whose intensity increased with anodic polarisation. It is attributed to oxygen species at the surface and at subsurface sites. The latter corresponds to oxygen atoms dissolved in the bulk. Under cathodic polarisation, the oxygen species at low binding energy disappeared, but a second oxygen species at higher XPS binding energy grew in intensity with increasing cathodic polarisation. In view of the reducing conditions and the presence of water, this latter oxygen signal is attributed to a steady state concentration of hydroxyl species at the surface. With UPS, the work function of electrons at the silver surface was determined as a function of the applied overpotential. The work function of silver increased by about 1.6 eV in changing from a cathodic polarisation of −1.5 V to an anodic polarisation of +1.5 V. This pronounced change is caused by a concentration decrease of adsorbed OH and a corresponding increase of adsorbed O atoms leading to a higher surface electron affinity. Applying high cathodic as well as anodic polarisation led to the formation of insulated silver particles within open pores of the silver layer on the surface of the zirconia. These particles which are electronically coupled to the Fermi level of the underlying zirconia cause a superposed but slightly shifted second UPS spectrum with the shift depending on the effective overvoltage. The as determined overvoltage at the silver electrode depends linearly on the cell voltage.
Nanostructuring of Electrode Surfaces by Tip‐Induced Metal Deposition
Authors: Ullmann R. , Will T. , Kolb D.M. 
Published: March 2014

AbstractIt is demonstrated how the tip of an STM can be used as a tool for the deposition of small Cu clusters onto flat surfaces at preselected positions. This is done by depositing Cu onto the tip which then acts as a reservoir from which Cu is handed over to the surface during an appropriate tip approach. It is shown how the instrumental parameters can be used to vary the cluster size or, by inducing tip oscillations, to generate cluster trains instead of a single cluster. Cu clusters can also be selectively erased by applying very positive tip potentials. The influence of the substrate on the shape of the tip‐induced Cu clusters is briefly discussed and preliminary results on the electrochemical stability of single clusters of various sizes are presented.
Chemical Sensors for Acid‐Base‐Active Gases: Applications to CO2 and NH3
Authors: Holzinger Michael , Fleig Jürgen , Maier Joachim , Sitte Werner 
Published: March 2014

AbstractIt is shown how acid‐base interactions of complex gases such as CO2, NH3 or H2O with solids can be used to measure the partial pressures of these gases. Two examples developed in our laboratory are presented:
The interactions of acid‐base‐active gases such as NH3 (Lewis base) or BF3 (Lewis acid) with the ionic conductors AgCl or CaF2 cause ionic space charge regions giving rise to a change of the ionic surface conductivity. The response time can be significantly improved by applying a microelectrode impedance method.
The use of a potentiometric arrangement the cell reaction of which is a pure acid‐base reaction, leads to a fast, precise and drift‐free CO2‐sensor. Since the reference electrode is not affected by CO2 itself, the entire cell can be exposed to the CO2 and O2 containing atmosphere without the necessity of sealing the reference electrode. The chemical potential of oxygen even though important for defining the component activities at each electrode cancels in the overall reaction of our cells. The performance of two cells is presented. While the measuring electrode consists of Na2CO3, the reference electrodes discussed are made of Na2Ti6O13/TiO2 or Na2Ti6O13/Na2Ti3O7 two‐phase mixtures, both being stable with respect to a reaction with CO2. The sensors are very fast, response times are of the order of seconds.
Thermodynamics of Fluid Sulfur Hexafluoride from Molecular Dynamics Simulations
Authors: Lustig Rolf 
Published: March 2014

AbstractAn octahedral 6‐center Lennard‐Jones model for sulfur hexafluoride is used in classical molecular dynamics simulations. A variety of thermodynamic properties are computed over a wide range of the fluid state and compared to data obtained from a preliminary fundamental equation developed by de Reuck et al. [IUPAC Thermodynamic Tables Project Centre, London 1991]. Properties which are correlated well by the empirical fundamental equation are also reproduced accurately by the model.
Pulsed Ultraviolet Laser Photolysis of Substituted Phenyl Diazosulphonates at 248 nm
Authors: Nomayo M. , Wokaun A. 
Published: March 2014

AbstractThe photochemical decomposition of aqueous phenyl diazosulphonate solutions by KrF excimer laser pulses at 248 nm is investigated. The present study complements earlier investigations (Franzke et al. 1991, 1992) of the photochemistry of this class of compounds induced by irradiation at 308 nm (XeCl excimer laser), and in the 370–390 nm wavelength range (dye laser pulses). From the comparison, conclusions with regard to the wavelength dependent photolysis pathway are drawn.Phenyl diazosulphonates carrying methoxy‐ or ester‐type substituents at the aromatic ring are observed to decompose in a two‐step‐reaction in aqueous solution: Photolysis proceeds via the phenyl diazonium ion as an intermediate, to yield the corresponding phenol as a product.In contrast, the photolysis of 3‐vinyl‐phenyl diazosulphonate in aqueous solution at 248 nm results in a complex reaction involving via more than one intermediate. UV spectra recorded during photolysis show that products characterized by an absorption maximum at 272 nm are formed; these spectra are identical with those of final products from 308 nm photolysis, which are identified as 3‐hydroxyethyl‐phenols. – The photolysis pathway of 4‐hydroxy‐phenyl‐diazosulphonate is found to proceed via the diazoquinone, instead of a diazonium ion.As a consequence of the high energy of 248 nm photons, the mentioned substituted phenols are not the final products of the photochemical reaction. These molecules are further decomposed by absorbing 248 nm photons; the quantum efficiency of these secondary photochemical reactions is lower by one to two orders of magnitude, as compared to the primary photolysis of the parent compounds.
Femtosecond Time‐Resolved Measurement of Electron Relaxation at Metal Surfaces
Authors: Aeschlimann M. , Pawlik S. , Bauer M. 
Published: March 2014

AbstractTime resolved two‐photon photoemission, based on the equal pulse correlation technique, is used to measure the energy relaxation of the photoexcited electrons in various polycrystalline metal samples. While for Ag the inelastic lifetime of excited electrons is, within our time resolution, in agreement with the Fermi‐liquid theory, the result obtained for Au is very different. Scanning tunneling microscopy studies reveal that the enhanced inelastic lifetime of gold is not caused by surface effects. The measured inelastic relaxation time for transition metals with unoccupied d‐orbitals is shorter as compared to the noble metals, indicating that the inelastic decay depends on the number of unoccupied d‐orbitals below the probed energy state.
On the Kinetics of the Anodic Oxidation of GaP
Authors: Heckner K.‐H. , Majoros G. , Kraft A. , Landsberg R. 
Published: March 2014

AbstractThe formation of anodic oxides on n‐GaP were studied by means of neutron activation analysis measuring the 32P activity in the film and in the solution. The oxide film formation under potentiostatic conditions and the competing oxide dissolution during film formation and under currentless conditions were investigated. The best description of the oxide growth is a combination of the parabolic growth law for film formation and of a zeroth order process for the simultaneous film dissolution. The rate constant for film formation linearily depends on the square of the applied voltage. The dissolution rate also increases with increasing the applied voltage. The dissolution rate of oxide films decreases under currentless conditions compared with dissolution during growth.
Temperature Programmed Surface Reaction Studies of the Methanol to Gasoline (MTG) Conversion over ZSM‐5
Authors: Jayamurthy M. , Vasudevan S. 
Published: March 2014

AbstractThe methanol to gasoline conversion over ZSM‐5 has been studied by the Temperature Programmed Surface Reaction (TPSR) technique. The technique is able to delineate the two steps in the process: the dehydration of methanol to dimethyl ether and the subsequent conversion of dimethyl ether to hydrocarbons. The activation barriers associated with each step was evaluated from the TPSR profiles and are 108 kJ/mol and 195 kJ/mol respectively. The methanol desorption profile shows considerable change with the amount of methanol molecules adsorbed per Brönsted site of the zeolite. The energy associated with the desorption process, (CH3OH)nH+ –ZSM‐5→(CH3OH)n–1H+ –ZSM‐5 + CH3OH, shows a spectrum of values depending on n. The spectrum is not continuous and has a gap for n = 1. The trend is similar to that for the corresponding gas‐phase reaction, (CH3OH)nH+ →(CH3OH)n–1H+ +CH3OH. The present investigation highlights the importance of the protonated methanol cluster, (CH3OH)nH+ –ZSM‐5, in the primary step of the MTG process, the formation of dimethyl ether. The reaction is compared with the corresponding gas phase ion‐molecule reaction of CH3OH2+.
Size Distribution and Characterization of Si Nanometer‐Size Particles Obtained by CO2 and Nd: YAG Laser Irradiation of SiH4
Authors: Cáceres Jorge O. , Nájera Juan J. , Lane Silvia , Ferrero Juan C. 
Published: March 2014

AbstractSilicon ultrafine particles have been obtained from SiH4 by laser irradiation under different experimental conditions. The powders produced are amorphous except for those obtained at 350°C which are a mixture of amorphous and crystalline particles. Sizes are in the range 2–54 nm and mean diameters between 5.3 and 8.1 nm. The particle sizes fit a narrow, log‐normal distribution function (LNDF). No significant variation in size is observed with increase of pressure or temperature within the studied interval.
An Approach to the Intrinsic Kinetics of the Zirconium Oxide Carbochlorination
Authors: Bohé Ana E. , Pasquevich Daniel M. 
Published: March 2014

AbstractA procedure to overcome the experimental difficulties found in the study of the intrinsic kinetics of zirconia carbochlorination was analyzed. The effect of experimental variables on the carbochlorination kinetics was studied by means of thermogravimetry. The effect of temperature on the extent of two reaction stages was observed: the first reaction stage dominates the reaction at high temperatures and low reaction‐degrees. The second one was of first order with respect to the chlorine partial pressure at 1223 K.
Decrease in the Oxidation Potential of the Tris‐2,2′‐bipyridyl Ruthenium(II) Complex Upon Electrostatic Binding to Colloidal Silica as Evidenced on the Basis of the Reaction with the Azidyl Radical
Authors: Naik Devidas B. , Schnabel Wolfram 
Published: March 2014

AbstractThe azidyl radical which is otherwise unreactive towards the Ru(bpy)32+ complex reacts with this complex in aqueous solution provided negatively charged colloidal silica particles are present. The increase in reactivity is explained in terms of ion binding, i.e. Ru(bpy)32+ complexes becoming rather firmly attached to the negatively charged particles. The attachment causes a decrease in the redox potential of the system Ru(bpy)33+/Ru(bpy)32+ from E0 = 1.27 V vs. NHE (in the absence of colloidal silica) to E0 = 1.17 V vs. NHE (in the presence of colloidal silica). This value was obtained on the basis of E0 = 1.35 V vs. NHE for the system N3/N3−.
Kinetic and Mechanistic Studies of the Reactions of CF3O Radicals with NO and NO2
Authors: Fockenberg Chr. , Somnitz H. , Bednarek G. , Zellner R. 
Published: March 2014

AbstractThe reactions of CF3O radicals with (1) NO and (2) NO2 were studied using two different experimental techniques. A laser photolysis/LIF detection method was applied for measuring the rate constants as a function of temperature (T = 222–302 K) and total pressure (ptot = 7–107 mbar). Whereas the reaction with (1) NO was found to be independent of temperature and pressure with k1 = (4.5±1.2)×10−11 cm3 s−1, the reaction with (2) NO2 was found to be dependent on both of these variables. The temperature dependence of k2 in the high pressure limit can be given by the expression k2,∝ (T) = (8±5)×10−13 exp ((863±194) K/T) cm3 s−1. The product distributions of the two reactions were determined in separate experiments using steady‐state photolysis combined with FTIR spectroscopy. For reaction (1) only CF2O was found as a reaction product with a yield of 0.93±0.10, independent of temperature. For reaction (2) several products (CF3ONO2, CF2O, FNO2) were identified, the overall yield, however, is dominated (≥90%) by the recombination product CF3ONO2. A theoretical analysis of the detailed mechanisms of both reactions was made by performing ab initio energy and geometry predictions in combination with RRKM calculations. Both reactions were found to proceed via an initial addition mechanism involving the CF3ONOx (x = 1, 2) intermediate and a four‐center transition state. A direct abstraction of an F atom by NO or NO2 can be excluded.
Far Infrared Laser Magnetic Resonance Detection of CF3, CHF2, and CH2F
Authors: Nolte J. , Wagner H. Gg. , Temps F. , Sears T.J. 
Published: March 2014

AbstractFar infrared laser magnetic resonance (FIR‐LMR) spectra of CF3, CHF2, and CH2F radicals were observed in the 300–800 μm wavelength region in the reactions of F atoms with a series of different hydrocarbon and partially fluorinated hydrocarbon molecules. To identify the carriers of the observed resonance patterns and to exclude or identify contamination of the spectra by additional resonances from other paramagnetic species, numerous chemical tests were developed and performed. With optimized sources and the chemical tests in hand, many spectra could be unambiguously attributed to CF3, CHF2, and CH2F, respectively. The present study reports the first gas phase detection of CHF2 radicals in their ground electronic state.