Volker Engels

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)).

Bimetallic PdZn Nanoparticles for the Partial Hydrogenation of Phenylacetylene

Novel catalytic systems based on PdZn alloys have been synthesized by polyol reduction over the entire compositional range [1, 2] and characterized by means of HRTEM, EDS and ICP. The expected bulk compositions were reproducible to within a 2 mol% margin and the purified particle suspensions stable for several months after preparation. The EDS results indicated the presence of considerable amounts of oxygen, especially in those samples containing high fractions of zinc. The particle size distributions were shown to be narrow and the mean sizes slightly decreased with higher molar fractions of palladium (diameter range 2.6 to 3.2 nm). In the catalytic hydrogenation of phenylacetylene, a strong dependence of the substrate conversion time on the Pd concentration was established, and selectivity towards the semihydrogenation product (styrene) was found to be close to 100 %. The selectivity dropped only shortly before the initial substrate was fully depleted.

Nanoparticulate PdZn as a Novel Catalyst for ZnO Nanowire Growth

ZnO nanowires have been grown by chemical vapour deposition (CVD) using PdZn bimetallic nanoparticles to catalyse the process. Nanocatalyst particles with mean particle diameters of 2.6 ± 0.3 nm were shown to catalyse the growth process, displaying activities that compare well with those reported for sputtered systems. Since nanowire diameters are linked to catalyst morphology, the size-control we are able to exhibit during particle preparation represents an advantage over existing approaches in terms of controlling nanowire dimensions, which is necessary in order to utilize the nanowires for catalytic or electrical applications.(See supplementary material 1)

Cobalt Catalyzed Carbon Nanotube Growth on Graphitic Paper Supports

The catalytic growth of multi-wall carbon nanotubes on carbon paper is reported. The study employed three cobalt carbonyl clusters as catalyst precursors. These were deposited on graphitic paper prior to chemical vapour deposition (CVD) of methane or ethyl- ene. The clusters show differentiated growth behaviour in accordance with precursor size, and with Co2(CO)8 displaying additional activ- ity in the growth of helical nanotube structures. We therefore report an approach for the decoration of graphitic papers with carbon nano- tubes with a view to the production of high area supports. Since their identification by Iijima in 1991 (1), carbon nano- tubes (CNTs) have been the subject of intensive research owing to their unique electronic and mechanical properties (2). While various synthetic strategies for their production have been devised, chemi- cal vapor deposition (CVD) has been established as one of the more commonly used techniques by virtue of its versatility in terms of the effect of experimental parameters (such as flow speed and composi- tion of the gaseous carbon precursors, deposition temperature and time) on the type of CNTs produced. Nevertheless, the development of procedures that are able to achieve sufficient morphological and structural product control remains an ongoing challenge. This is, on the one hand, due to there being an as yet incomplete understanding of the tube formation mechanism and, on the other hand, groups reporting a lack of control over catalyst morphology. The result of these issues is that irregular tube structures are generally obtained. In the case of morphological control, both experimental and compu- tational studies have demonstrated the importance of catalyst parti- cle size in influencing the resulting tube diameters (3). More fun- damentally, particle diameters in the nanometer regime are known to affect the metal melting point through the Gibbs-Thomson effect. In addition, the temperature at which liquefaction of nanoparticulate species occurs (and which can be assumed to influence the agglom- eration behavior), has been shown to depend on saturation of the metal species with carbon (4). These issues pose intrinsic problems in that it is not trivial to maintain narrow particle size distributions for commonly employed nanoparticulate catalysts derived from either physical vapour deposition or wet chemical methods (such as sputtering and metal salt reduction). In seeking to overcome these limitations, researchers have more recently focused on the employ- ment of molecular clusters as catalyst precursors, and on the deposi- tion of these on various supports. Most notably, use of the high pressure carbon monoxide (HiPco) process to produce single- walled carbon nanotubes (SWCNTs) using iron pentacarbonyl has offered control over nanotube yield and morphology by means of controlling the deposition parameters (5). In recent years, considerable effort has been directed towards addressing both the issue of seed size control and the development of a wider range of suitable supports with which to provide CNT- based high surface area systems. In this context, some of the present authors have previously reported CNT syntheses using both nickel formate precursors and colloidal cobalt nanoparticles on supports