Novel method for preparing cellulose model surfaces by spin coating

A new, simplified method for preparing model surfaces of cellulose is introduced. Non-polar cellulose derivative trimethylsilyl cellulose (TMSC) was deposited onto untreated silicon substrate by spin coating, after which the coated TMSC was regenerated back to cellulose by vapour phase acid hydrolysis. By optimising the parameters of spin coating, a smooth cellulose film of ca 20 nm was obtained with roughness variation of max. 3 nm. With the well-defined morphology and chemical structure, combined with easy preparation, these model surfaces provide excellent means to explore the molecular level phenomena, taking place during various processes involving cellulose. Films were characterized using atomic force microscopy to illustrate the morphology and X-ray photoelectron spectroscopy to determine the chemical structure of the layers.     

Zeolite NaY-supported gold complexes prepared from Au(CH3)2(C5H7O2): reactivity with carbon monoxide

Mononuclear gold complexes in zeolite NaY were synthesized from initially physisorbed Au(CH₃)₂(C₅H₇O₂), and their reactions with CO in a flow system at 298 K and 760 Torr were investigated by infrared (IR) spectroscopy and mass spectral analysis of the effluent gases. CH₄ and CO₂ were formed as CO flowed through the sample either steadily or as successive pulses. The results are consistent with the inferences that (a) CO reacted with the supported gold to form gold carbonyls, (b) CH₄ formed by reaction of methyl groups on gold with traces of H₂O or hydroxyl groups on the zeolite and (c) CO on cationic gold reacted with traces of O₂ and/or H₂O to form CO₂. In samples treated in steadily flowing CO, cationic gold was reduced to zerovalent gold, but the cationic gold in samples exposed to CO pulses was not reduced to zerovalent gold, although CO₂ formed. Thus, CO adsorbed on cationic gold reacts to give CO₂ in the absence of zerovalent gold, consistent with the inference that gold catalysts for CO oxidation need not contain zerovalent gold.     

Introducing open films of nanosized cellulose—atomic force microscopy and quantification of morphology

A method for preparing open, sub-monolayer cellulose films on a silicon substrate is introduced, and the open films were quantified using the three-dimensional information from atomic force microscopy (AFM) height images. The preparation method is based on spin coating low concentrations of trimethylsilyl cellulose (TMSC) on silicon and hydrolysing the TMSC to cellulose using a vapour phase acid hydrolysis. AFM showed that the surfaces consist of nanosized cellulose patches which are roughly 50-100 nm long, 20 nm wide, and 1 nm high. The volume of the cellulose patches was quantified. Examination of the cross section of the cellulose patches revealed that the exaggeration of the lateral dimensions by the AFM tip is small enough to account for a mere ±2% error in the volume quantification. Pilot experiments showed that the volume of the cellulose was largely restored in a wetting/drying cycle but the morphology changed considerably. Because of their small size, the cellulose patches provide a novel approach for interpretation on the molecular architecture of cellulose.     

Low surface energy polymeric films from partially fluorinated photocurable solventless liquid oligoesters

Summary We have recently shown that low surface energy polymeric films can be readily obtained from partially fluorinated hydroxyl-end-capped solventless liquid oligoesters by thermal curing [1]. Photocurable liquid oligoesters can be prepared by further substituting the remaining hydroxyl end groups with acrylic double bonds. A conventional photoinitiator containing 2-hydroxy-2-methylpropiophenone was used to cure the liquid oligoesters. As the fluorine content increases from 0 to about 1.57 wt %, the surface energy decreases by about 14 mN/m. The fluorine level in the outer few nanometers is significantly greater than that in the bulk, as revealed by XPS studies. The driving force for the surface segregation of fluorinated species comes from the large difference in surface energy between the oligoesters and their fluorinated counterparts. Received: 14 August 2001/Revised version: 10 October 2001/Accepted: 10 October 2001     

Realistic Surface Science Models of Supported Catalysts: The Chromium Oxide Ethylene Polymerization Catalyst

Depositing catalytically active particles onto flat, thin oxide films which have electric conductivity forms an attractive way to make supported catalysts suitable for surface science investigations. Here we describe the development of a model system for the chromium oxide on silica catalyst used in industry for the polymerization of ethylene, the so-called Phillips catalyst. The model system, consisting of molecularly dispersed chromium ions, deposited by wet chemical impregnation on a thin silica film and anchored to it by treating it in oxygen, displays a representative activity for polyethylene formation.     

Visualization of local ethylene polymerization activity on a flat CrO x /SiO2/Si(100) model catalyst

Flat model catalysts give access to fundamental aspects of olefin polymerization over heterogeneous catalysts. They are especially suited for the investigation on the early stages of polymer film growth employing scanning probe and electron microscopy. The polymerization centers are confined to a well defined two dimensional plane that remains constant during the polymerization. Using the Phillips (CrO _x /SiO₂) catalyst as an example we demonstrate how this approach can be used do visualize the lateral distribution of polymerization activity in the initial stages of polymer growth.     

The observation of nanometer-sized entities in sulphided Mo-based catalysts on various supports

The progressive sulphidation of molybdenum-based hydrotreating model catalysts is studied using HRTEM and XPS. The catalysts are sulphided quasi in situ and prepared for TEM measurements in a glovebox. Subsequently, a specially developed vacuum-transfer specimen holder is used for transfer to the TEM. Using this procedure, a previously unknown feature is observed in the catalysts. Apart from the well-known slab-like structure of bulk molybdenum disulphide, nanometer-sized spherical entities of molybdenum sulphide or oxysulphide are observed. For a series of model CoMo/silica catalysts, these spots predominantly occur after mild sulphidation procedures. At higher sulphidation temperatures, these spots exist next to the slabs of MoS₂-like material. Comparison with practical molybdenum-based hydrotreating catalysts shows that these, too, can contain spots next to or instead of slabs, depending on precursor preparation and sulphidation procedures.     

Vibrational Stark tuning rates from periodic DFT calculations: CO/Pt(1 1 1)

DFT periodic calculations have been used to study the influence of an external electric field on the adsorption of CO on Pt(1 1 1). Particular attention has been focused on the determination of the CO and metal-CO vibrational Stark tuning rates. Stark tuning rates have been calculated at various CO coverages; a linear dependence between the CO Stark tuning rate and the CO surface coverage has been found. We have calculated a value of 68.94 cm⁻¹/(V/Å) for the zero-coverage limit CO Stark tuning rate, in good agreement with the experimental value of 75 ± 9 cm⁻¹/(V/Å). Like the CO Stark tuning rate, the metal-CO vibrational Stark tuning rate also increases as CO surface coverage decreases. In addition, we have found (at 0.25 ML) that the CO Stark tuning rate is similar at different adsorption sites, being only slightly larger at high-coordinated sites. CO vibrational Stark tuning rates of 45.58, 47.96, 47.61 and 48.49 cm⁻¹/(V/Å) have been calculated for ontop, bridge, hcp and fcc hollow sites, respectively. Calculations at high coverage using a (2 × 2)-3CO model yield a CO Stark tuning rate of 21.08 and 25.93 cm⁻¹/(V/Å) for ontop and three-fold hollow CO, respectively. These results show that the CO Stark tuning rate for CO adsorbed at high coordinated sites is only slightly larger than that at ontop sites. This result is in contradiction with experiments, which reported larger CO Stark tuning rates at high-coordinates sites than at ontop sites. Furthermore, the calculated metal-CO stretch is larger for ontop sites than for high-coordinated sites; this result is in disagreement with previous DFT cluster model calculations. Unfortunately, there is not experimental information available to support either result. Finally, we have also studied the CO adsorption site preference dependence on electric fields. We have found that CO adsorbs preferentially at high coordinated sites at more negative fields, and at ontop sites at more positive fields, in agreement with previous experiments and DFT cluster model calculations.