Peter C. Thüne

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.

Characterization of Ga/HZSM-5 and Ga/HMOR synthesized by chemical vapor deposition of trimethylgallium

Chemical vapor deposition (CVD) of trimethylgallium (TMG) has been studied as a method to disperse extraframework Ga in acidic ZSM-5 and mordenite zeolite. Various samples were extensively characterized by ICP, XPS, NMR, and FTIR. Silylation with tetramethyldisilazane is explored as a method for deactivating the external zeolite surface. The deposition of TMG in silylated ZSM-5 results in a gallium-to-aluminum ratio close to unity, which indicates a homogeneous metal distribution in the micropore space. However, pore blockage in the one-dimensional channels of mordenite results in a inhomogeneous distribution and a low Ga loading. Upon exposure to moistened air, the adsorbed methylgallium species decompose and alkoxy groups are formed. Subsequent oxidation or reduction leads to the complete removal of methyl groups. The reductive route is the preferred one resulting in a better dispersion of Ga, since oxidation of the methyl groups leads to water formation and hydrolysis of cationic Ga species.

Planar model system for olefin polymerization: the Phillips CrOx/SiO2 catalyst

A planar CrOx/SiO2/Si(100) model for the Phillips ethylene polymerization catalyst has been prepared by spincoat impregnation from an aqueous solution of CrO3. The model catalyst polymerizes ethylene from the gas phase at 160°C with a constant activity and forms a 400 nm thick layer of polyethylene in 1 h. The superior definition of the polymer films produced on this catalyst and the control over the distribution of active sites on its flat surface make this model catalyst an ideal substrate for kinetic studies on catalytic polymerization and for morphologic studies of the polymer product by scanning force microscopy. At extremely low catalyst loading we observe isolated polymer islands formed on single chromium sites. The work also opens attractive opportunities for future studies of nascent morphology of catalytically formed polymers.

In Situ ATR-FTIR Studies on MgCl2-Diisobutyl Phthalate Interactions in Thin Film Ziegler–Natta Catalysts

To study the surface structure of MgCl(2) support and its interaction with other active components in Ziegler-Natta catalyst, such as electron donors, we prepared a thin film analogue for Ziegler-Natta ethylene polymerization catalyst support by spin-coating a solution of MgCl(2) in ethanol, optionally containing a diester internal donor (diisobutyl-ortho-phthalate, DIBP) on a flat Si crystal surface. The donor content of these films was quantified by applying attenuated total internal reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). Changes in the interaction of DIBP with MgCl(2) at various temperatures were monitored by in situ ATR-FTIR. Upon increasing the temperature, a shift in the (C═O) band toward lower wavenumbers was observed together with the depletion of (O-H) stretching band due to the desorption of residual ethanol. We assign this shift to gradual redistribution of adsorbed DIBP from adsorption sites on the MgCl(2) (104) surface toward the more acidic MgCl(2) (110) surface. The morphologies of MgCl(2) and MgCl(2)/DIBP films were studied by transmission electron microscopy (TEM) revealing a preferential orientation of ClMgCl layers (001) parallel to the lateral film dimensions. This orientation becomes more pronounced upon annealing. In the absence of donor, the MgCl(2) grow in to large crystals aligned in large domains upon annealing. Both crystal growth and alignment is impeded by the presence of donor.

The CrOx/SiO2/Si(100) catalyst – a surface science approach to supported olefin polymerization catalysis

Depositing catalytically active particles onto flat, thin and oxidic support forms an attractive way to make supported catalyst suitable for surface science characterization. Here we show how this approach has been applied to the Phillips (CrO x / SiO 2 ) ethylene polymerization catalyst. The model catalyst shows a respectable polymerization activity after thermal activation in dry air (calcination). Combining the molecular information from X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) we can draw a molecular level of the activated catalyst that features exclusively monochromate species, which are anchored to the silica support via ester bonds with the surface silanol groups. These surface chromates form the active polymerization site upon contact with ethylene. Upon increasing calcination temperature we observe a decrease in chromium coverage as some of the surface chromate desorbs from the silica surface. Nevertheless, we also find an increasing polymerization activity of the model catalyst. We attribute this increase in catalytic activity to the isolation of the supported chromium, which prevents dimerization of the coordinatively unsaturated active site. Diluting the amount of chromium to 200 Cr-atoms/nm 2 of silica surface enables the visualisation of polyethylene produced by a single active site.

Nanoparticulate PdZn—pathways towards the synthetic control of nanosurface properties

This paper reports an in-depth structural investigation of PdZn nanoparticulates prepared over an entire compositional range. By using a combination of HRTEM, ICP-OES, EDX and XPS alongside PXRD, we are able to show how a liquid-type reduction process can be exploited to target different PdZn bimetallic structures while maintaining reproducibly narrow particle size distributions and average particle diameters of approximately 3 nm. Samples have been further analyzed by quantitative phase analysis of the Rietveld refined diffraction data, providing indications as to how variations in specific surface compositions are obtained when Zn is used as the alloying metal. The influence of nanolattice strain is investigated by geometric analysis of TEM data. Results suggest, in conjunction with previously published catalytic data, how different compositions of this specific bimetallic system may be exploited in catalytic processes to control substrate/product affinity. We thus demonstrate a new and simplified approach to PdZn bimetallics, which may offer novel perspectives for applications in industrial catalysis.

New Cu-Based Catalysts Supported on TiO2 Films for Ullmann SNAr-Type C-O Coupling Reactions

New routes for the preparation of highly active TiO(2)-supported Cu and CuZn catalysts have been developed for C-O coupling reactions. Slurries of a titania precursor were dip-coated onto glass beads to obtain either structured mesoporous or non-porous titania thin films. The Cu and CuZn nanoparticles, synthesized using a reduction by solvent method, were deposited onto calcined films to obtain a Cu loading of 2 wt%. The catalysts were characterized by inductively coupled plasma (ICP) spectroscopy, temperature-programmed oxidation/reduction (TPO/TPR) techniques, (63)Cu nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), scanning and transmission electron microscopy (S/TEM-EDX) and X-ray photo-electron spectroscopy (XPS). The activity and stability of the catalysts obtained have been studied in the C-O Ullmann coupling of 4-chloropyridine and potassium phenolate. The titania-supported nanoparticles retained catalyst activity for up to 12 h. However, catalyst deactivation was observed for longer operation times due to oxidation of the Cu nanoparticles. The oxidation rate could be significantly reduced over the CuZn/TiO(2) catalytic films due to the presence of Zn. The 4-phenoxypyridine yield was 64% on the Cu/nonporous TiO(2) at 120 °C. The highest product yield of 84% was obtained on the Cu/mesoporous TiO(2) at 140 °C, corresponding to an initial reaction rate of 104 mmol g(cat) (-1) s(-1). The activation energy on the Cu/mesoporous TiO(2) catalyst was found to be (144±5) kJ mol(-1), which is close to the value obtained for the reaction over unsupported CuZn nanoparticles (123±3 kJ mol(-1)) and almost twice the value observed over the catalysts deposited onto the non-porous TiO(2) support (75±2 kJ mol(-1)).

Controlled amino-functionalization by electrochemical reduction of bromo and nitro azobenzene layers bound to Si(111) surfaces

4-Nitrobenzenediazonium (4-NBD) and 4-bromobenzenediazonium (4-BBD) salts were grafted electrochemically onto H-terminated, p-doped silicon (Si) surfaces. Atomic force microscopy (AFM) and ellipsometry experiments clearly showed layer thicknesses of 2-7 nm, which indicate multilayer formation. Decreasing the diazonium salt concentration and the reaction time resulted in a smaller layer thickness, but did not prevent the formation of multilayers. It was demonstrated, mainly by X-ray photoelectron spectroscopy (XPS), that the diazonium salts not only react with the H-terminated Si surface, but also with electrografted phenyl groups via azo-bond formation. These azo bonds can be electrochemically reduced at Ered = -1.5 V, leading to the corresponding amino groups. This reduction resulted in a modest decrease in layer thickness, and did not yield monolayers. This indicates that other coupling reactions, notably a biphenyl coupling, induced by electrochemically produced phenyl radicals, take place as well. In addition to the azo functionalities, the nitro functionalities in electrografted layers of 4-NBD were independently reduced to amino functionalities at a lower potential (Ered = -2.1 V). The presence of amino functionalities on fully reduced layers, both from 4-NBD- and 4-BBD-modified Si, was shown by the presence of fluorine after reaction with trifluoroacetic anhydride (TFAA). This study shows that the electrochemical reduction of azo bonds generates amino functionalities on layers produced by electrografting of aryldiazonium derivatives. In this way multifunctional layers can be formed by employing functional aryldiazonium salts, which is believed to be very practical in the fabrication of sensor platforms, including those made of multi-array silicon nanowires.

Superhydrophobic Polyethylene Films By Catalytic Ethylene Polymerization

Polyethylene films were grown on a flat silica surface modified by the bis(imino)pyridyl iron(II) catalyst during ethylene polymerization in an organic solvent. The resulting films show under certain polymerization conditions superhydrophobic properties. The development of the nascent polymer morphology which leads to the superhydrophobic behavior depends strongly on the polymerization conditions, especially the solvent and the polymerization temperature. Advancing water contact angle as high as 169° and sliding angles as low as 2° for a 10 μl droplet are obtained on these films. SEM images reveal special surface structures of these films containing micrometer-sized islands, submicrometer particles on the islands, and stress nanofibers between the islands, which — in combination — render superhydrophobicity to the polyethylene surfaces.

Surface model for gas-phase polymerizations of ethylene and propylene using supported metallocene/methylalumoxane catalysts

In order to obtain more detailed information about supported metallocene/methylaluminoxane catalysts, two catalysts are supported onto a flat silicon wafer by spincoating impregnation. These model catalysts are characterized by SEM and EDX showing a film of metallocene catalyst dispersed inside the methylaluminoxane matrix, as well as regions of a localized increased concentration of the catalyst. Applying these model catalysts in a gas-phase polymerization reactor results in rather homogeneous films of polyethylene and polypropylene, respectively. In addition, crater-like and spherical polymer structures can be observed, probably formed by inhomogeneous catalyst distribution.