A molecular view of heterogeneous catalysis

The establishment of a molecular view of heterogeneous catalysis has been hampered for a number of reasons. There are, however, recent developments, which show that we are now on the way towards reaching a molecular-scale picture of the way solids work as catalysts. By a combination of new theoretical methods, detailed experiments on model systems, and synthesis and in situ characterization of nano-structured catalysts, we are witnessing the first examples of complete atomic-scale insight into the structure and mechanism of surface-catalyzed reactions. This insight has already proven its value by enabling a rational design of new catalysts. We illustrate this important development in heterogeneous catalysis by highlighting recent examples of catalyst systems for which it has been possible to achieve such a detailed understanding. In particular, we emphasize examples where this progress has made it possible to propose entirely new catalysts, which have then been proven experimentally to exhibit improved performance in terms of catalytic activity or selectivity.     

A molecular view of bond rupture

Molecular mechanisms of rupture will be discussed in the light of recent computational studies. Bonds formed by materials as diverse as simple crystalline solids, glassy polymers, and biological ligands and receptors, reveal similar behavior. When these bonds are pulled apart at constant velocity, they rupture through a series of sudden yield events during which the material reorganizes. Yield events are separated by periods of elastic deformation where the stress builds until the system becomes unstable. The nature of the structural change at yield events varies from system to system. Small cavities form in the polymer film, an additional atomic layer is formed in the crystal, and hydrogen binding sites rearrange in the biological system. The work required to rupture these bonds is determined by the full sequence of yield events.     

Positive-tone nanopatterning of chemical vapor deposited polyacrylic thin films

Terpolymers of methyl alpha-chloroacrylate (MCA), methacrylic acid (MAA), and methacrylic anhydride (MAH) were synthesized by initiated chemical vapor deposition (iCVD) of MCA and MAA followed by low-temperature annealing that partially converts MAA into MAH. The MAA composition in the iCVD copolymer can be systematically varied between 37 and 85 mol % by adjusting the gas feed fractions of monomers. Study of the monomer reactivity ratios and the copolymer molecular weights supports the hypothesis of a surface propagation mechanism during the iCVD copolymerization. The carboxylic dehydration reaction at the annealing temperature of 160 degrees C is dominated by a mechanism of intramolecular cyclization, resulting in intramolecular MAH anhydride formation while preventing crosslink formation. The incorporation of highly electron-withdrawing anhydride functionality enhances chain scission susceptibility under electron-beam irradiation. P(MCA-MAA-MAH) terpolymer thin films can be completely developed at dosages as low as 20 microC/cm2 at 50 kV. High-quality positive-tone patterns were created with 60 nm feature size achieved in the vapor deposited films.     

J-aggregates in vapor deposited films of a bisazomethine dye

This is the first report of J-aggregate formation of a non-ionic bisazomethine dye in vapor deposited films; this dye allows us to prepare easily large homogeneous and very stable J-aggregate thin films and to investigate intrinsic properties of low-dimensional Frenkel excitons.     

Dynamics of glass-forming liquids. XV. Dynamical features of molecular liquids that form ultra-stable glasses by vapor deposition

The dielectric relaxation behavior of ethylbenzene (EBZ) in its viscous regime is measured, and the glass transition temperature (T(g) = 116 K) as well as fragility (m = 98) are determined. While the T(g) of EBZ from this work is consistent with earlier results, the fragility is found much higher than what has been assumed previously. Literature data is supplemented by the present results on EBZ to compile the dynamic behavior of those glass formers that are known to form ultra-stable glasses by vapor deposition. These dynamics are contrasted with those of ethylcyclohexane, a glass former for which a comparable vapor deposition failed to produce an equally stable glassy state. In a graph that linearizes Vogel-Fulcher-Tammann behavior, i.e., the derivative of -logτ with respect to T/T(g) raised to the power of -1/2 versus T/T(g), all ultra-stable glass formers fall onto one master curve in a wide temperature range, while ethylcyclohexane deviates for T ? T(g). This result suggests that ultra-stable glass formers share common behavior regarding the dynamics of their supercooled liquid state if scaled to their respective T(g) values, and that fragility and related features are linked to the ability to form ultra-stable materials.     

Electrogenerated chemiluminescence: A molecular electrochemistry point of view

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Efficient screening of materials and fast optimization of vapor deposited OLED characteristics

A combinatorial approach combining vapor deposition of organic molecules and a movable mask technique was used to screen and optimize materials and organic light emitting device configurations fast and efficiently. Some low molecular weight triphenyldiamine derivatives with different electronic and thermal properties were compared in two layer, ITO/TPD/Alq₃/Al device configurations. The optimum thickness for Alq₃ layer was obtained by evaporating a linear gradient of Alq₃ on top of various TPD layers. Further, a landscape library with two orthogonal linear gradients of TPD and Alq₃ was prepared to investigate the dependence of efficiency on thickness of both layers simultaneously. The necessity and the efficiency of an additional spiro‐qunioxaline compound as electron transporting/hole blocking layer was also investigated using a landscape library of Alq₃ versus spiro‐quinoxaline on top of TPD. The efficiency of the two layer device depends not only on the Alq₃ layer thickness, but also on the TPD layer thickness. The photometric efficiency of a TPD/Alq₃ device can be improved by replacing the optimum Alq₃ layer thickness by certain combinations of Alq₃/spiro‐quinoxaline layers.     

Thermal stability of cubic boron nitride films deposited by chemical vapor deposition

Thermal stability of well-crystallized cubic boron nitride (cBN) films grown by chemical vapor deposition has been investigated by cathodoluminescence (CL), Raman spectroscopy, and scanning electron microscopy (SEM) with the cBN films annealed at various temperatures up to 1,300 degrees C. The crystallinity of the cBN films further improves, as indicated by a reduction of the relevant Raman line width, when the annealing temperature exceeds 1,100 degrees C. Structural damage or amorphization was observed on the grain boundaries of the cBN crystals when annealing temperature reaches 1,300 degrees C. The CL spectra are found to be unchanged up to 1,100 degrees C after annealing at 500 degrees C, showing the stability of the cBN films in electronic properties up to this temperature. New features were observed in the CL spectra when annealing temperature reaches 1,200-1,300 degrees C.     

A molecular design view on the first hyperpolarizability of salicylideneaniline derivatives

Electron correlation effect was incorporated into the two-level framework through configuration interaction (CI) calculation including both occupied and unoccupied frontier orbitals, to evaluate the first hyperpolarizabilities of title compounds. Theoretical results are in excellent agreement with experimental data obtained by the solvatochromic method. Some detailed chemical and electronic information of the electron excitation process related to the nonlinear optical (NLO) properties were produced as well. Based on these data, effects of the character, number and position of donor and acceptor groups on the NLO properties are discussed from the viewpoint of molecular design. Not only the experimental data but also the theoretical analysis have suggested that a large number of intensive and properly located donor and acceptor groups may yield the optimal hyperpolarizabilities. Received: 22 May 1996 / Accepted: 9 December 1996     

A molecular spectroscopic view of surface plasmon enhanced resonance Raman scattering

The enhancement of resonance Raman scattering by coupling to the plasmon resonance of a metal nanoparticle is developed by treating the molecule-metal interaction as transition dipole coupling between the molecular electronic transition and the much stronger optical transition of the nanoparticle. A density matrix treatment accounts for coupling of both transitions to the electromagnetic field, near-resonant energy transfer between the molecule-excited and nanoparticle-excited states, and dephasing processes. This fully quantum mechanical approach reproduces the interference effects observed in extinction spectra of J-aggregated dyes adsorbed to metal nanoparticles and makes testable predictions for surface-enhanced resonance Raman excitation profiles.     

Stability of molecular form of HCl in water vapor: A computer simulation

The free energy and entropy of the dissociation of HCl molecule into ions in water vapor, HCl(H₂O) _n + mH₂O → H₃O + (H₂O) _n+m -1Cl^?, were calculated. The dependences of various parameters on the interionic distance at 273 K and various vapor pressures were obtained. A detailed model taking into account unpaired covalent-type interactions, polarization interactions, charge transfer effect, and hydrogen bonds was applied. The numerical values of the parameters were reconstructed from the experimental data on the free energy and enthalpy of the first reactions of addition of vapor molecules to ions, and also from the results of quantum-chemical calculations of the energy and geometry of locally stable configurations of clusters HCl(H₂O) _n . Despite lower internal energy of the dissociated state, the molecular form is absolutely stable in clusters of water molecules. The dissociated state is relatively stable. Accumulation of unrecombined ion pairs in clusters is possible with a decrease in the temperature to 200 K.     

Influence of substrate temperature on the stability of glasses prepared by vapor deposition

Physical vapor deposition of indomethacin (IMC) was used to prepare glasses with unusual thermodynamic and kinetic stability. By varying the substrate temperature during the deposition from 190 K to the glass transition temperature (Tg=315 K), it was determined that depositions near 0.85Tg (265 K) resulted in the most stable IMC glasses regardless of substrate. Differential scanning calorimetry of samples deposited at 265 K indicated that the enthalpy was 8 J/g less than the ordinary glass prepared by cooling the liquid, corresponding to a 20 K reduction in the fictive temperature. Deposition at 265 K also resulted in the greatest kinetic stability, as indicated by the highest onset temperature. The most stable vapor-deposited IMC glasses had thermodynamic stabilities equivalent to ordinary glasses aged at 295 K for 7 months. We attribute the creation of stable IMC glasses via vapor deposition to enhanced surface mobility. At substrate temperatures near 0.6Tg, this mobility is diminished or absent, resulting in low stability, vapor-deposited glasses.     

Effects of water vapor on the radiolysis of methane over molecular sieve 5A

Effects of the addition of H₂O on the radiation-induced chemical reaction of methane over molecular sieve 5A at 460°C have been studied by product analysis. Hydrogen, carbon monoxide, carbon dioxide and hydrocarbons consisting mainly of C₂ and C₃ alkanes and alkenes were produced from CH₄ + H₂O mixtures at high conversion levels. The yields of hydrocarbons from 3:1 and 3:2 CH₄ + H₂O mixtures decreased slightly with time but those from 3:4 mixture showed no decrease with time. When the molecular sieve 5A that had been irradiated in flowing methane was reirradiated in the presence of H₂O, carbonaceous solid produced from methane on molecular sieve 5A was readily decomposed to carbon dioxide, carbon monoxide, hydrogen and hydrocarbons, mainly alkanes. Therefore, it is concluded that the suppression of decrease of product yields with time by the addition of H₂O is mainly ascribed to decomposition of the carbonaceous solid by H₂O under electron beam irradiation. The role of added H₂O is also discussed in connection with the conventional methane-steam reforming reaction.     

The microdetermination of molecular weight by vapor pressure equilibration

An improved apparatus for the determination of molecular weight is described. It uses the isopiestic principle in which three solutions of the same solvent and different solutes equilibrate by distillation to equal vapor pressure in a common container. The main advantages claimed for this apparatus are ease of manipulation and more precise weighing of the solutions.     

Interaction of a solid supported liquid-crystalline phospholipid membrane with physical vapor deposited metal atoms

A metal (Ag, Au, Pd, Sn, Bi, In, Co, Al) layer directly deposited onto a liquid-crystalline lipid membrane resulted in different morphologies, ranging from a monolayer of discrete particles to a continuous film, depending on the metal's oxidation tendency.