The effect of Rh particle size on the catalytic performance of porous silica supported rhodium catalysts for CO hydrogenation

Silica-supported Rh catalysts with different Rh particle dimensions were investigated for CO hydrogenation. The catalysts were characterized by various techniques such as TEM, H-2-TPR and N-2 adsorption to study the catalyst morphology, the size distributions of Rh particles and the silica pores. It was found that the distribution and the size of Rh particles were affected by the silica pores, and the metal grains were enclosed in the pores of the support, and thereby their growth was limited. The catalytic activity and selectivity to C-2-oxygenates for CO hydrogenation were found to be significantly controlled by the Rh particle sizes, and the higher activity and selectivity to C2-oxygenates were obtained over bigger Rh particles, within the range of the reported particle sizes.

Particle size distributions from Fraunhofer diffraction

info:eu-repo/semantics/published

Biological controls on resuspension and the relationship between particle size and the Kolmogorov length scale in a shallow coastal sea

Measurements of suspended particle matter (SPM) and turbulence have been obtained over five tidal surveys during spring and summer 2010 at station L4 (5025 degrees N 04.22 degrees W, depth 50 m), in the Western English Channel. The relationship between turbulence intensity and bed stress is explored, with an in-line holographic imaging system evaluating the extent to which material is resuspended. Image analysis allows for the identification of SPM above a size threshold of 200 pm, capturing particle variability across tidal cycles and the two seasons. Dissipation of turbulent kinetic energy, which exceeds 10(-5) W kg(-1), yields maximum values of bed stress of between 0.17 and 0.20 N m(-2), frequently resulting in the resuspension of material from the bed. Resuspension is shown to promote aggregation of SPM into flocs, where the size of such particles is theoretically determined by the Kolmogorov microscale, l(k). During the spring surveys, flocs of a size larger than lk were observed, though this was not repeated during summer. It is proposed that the presence of gelatinous, biological material in spring allows flocculated particles to exceed l(k). This suggests that under specific circumstances, the limiting factor on the growth of flocculated SPM is not only turbulence, as previously thought, but the presence or absence of certain types of biological particle.

Modulation of surface charge, particle size and morphological properties of chitosan-TPP nanoparticles intended for gene delivery

This work investigates the polyanion initiated gelation process in fabricating chitosan-TPP (tripolyphosphate) nanoparticles in the size range of 100-250 nm intended to be used as carriers for the delivery of gene or protein macromolecules. It demonstrates that ionic gelation of cationic chitosan molecules offers a flexible and easily controllable process for systematically and predictably manipulating particle size and surface charge which are important properties in determining gene transfection efficacy if the nanoparticles are used as non-viral vectors for gene delivery, or as delivery carriers for protein molecules. Variations in chitosan molecular weight, chitosan concentration, chitosan to TPP weight ratio and solution pH value were examined systematically for their effects on nanoparticle size, intensity of surface charge, and tendency of particle aggregation so as to enable speedy fabrication of chitosan nanoparticles with predetermined properties. The chitosan-TPP nanoparticles exhibited a high positive surface charge across a wide pH range, and the isoelectric point (IEP) of the nanoparticles was found to be at pH 9.0. Detailed imaging analysis of the particle morphology revealed that the nanoparticles possess typical shapes of polyhedrons (e.g., pentagon and hexagon), indicating a similar crystallisation mechanism during the particle formation and growth process. This study demonstrates that systematic design and modulation of the surface charge and particle size of chitosan-TPP nanoparticles can be readily achieved with the right control of critical processing parameters, especially the chitosan to TPP weight ratio. (c) 2005 Elsevier B.V. All rights reserved.

SERS enhancement by aggregated Au colloids: effect of particle size

Aggregated Au colloids have been widely used as SERS enhancing media for many years but to date there has been no systematic investigation of the effect of the particle size on the enhancements given by simple aggregated Au colloid solutions. Previous systematic studies on isolated particles in solution or multiple particles deposited onto surfaces reported widely different optimum particle sizes for the same excitation wavelength and also disagreed on the extent to which surface plasmon absorption spectra were a good predictor of enhancement factors. In this work the spectroscopic properties of a range of samples of monodisperse Au colloids with diameters ranging from 21 to 146 nm have been investigated in solution. The UV/visible absorption spectra of the colloids show complex changes as a function of aggregating salt (MgSO4) concentration which diminish when the colloid is fully aggregated. Under these conditions, the relative SERS enhancements provided by the variously sized colloids vary very significantly across the size range. The largest signals in the raw data are observed for 46 nm colloids but correction for the total surface area available to generate enhancement shows that particles with 74 nm diameter give the largest enhancement per unit surface area. The observed enhancements do not correlate with absorbance at the excitation wavelength but the large differences between differently sized colloids demonstrate that even in the randomly aggregated particle assemblies studied here, inhomogeneous broadening does not mask the underlying changes due to differences in particle diameter.

LDL particle size in habitual exercisers, lean sedentary men and abdominally obese sedentary men

Habitual exercisers enjoy considerable protection from coronary heart disease (CHD). Often, however, only modest differences in traditional CHD risk factors are apparent between habitual exercisers and their sedentary counterparts. For this reason, there is increasing interest in novel predictors of CHD, such as a preponderance of small, dense low-density lipoprotein (LDL) particles. Polyacrylamide gel electrophoresis was used to separate lipoprotein subfractions in 32 lean exercisers, 36 lean sedentary men and 21 obese sedentary men aged 30-45 years. Well-validated equations were used to determine LDL concentration and peak particle diameter. Waist girth was used to identify lean (<100 cm) and obese ( >= 100cm) individuals. LDL concentration was lower in lean exercisers than in lean sedentary men (2.64 +/- 0.44 vs. 3.76 +/- 0.79 mmol.l(-1), p <0.001), suggesting that habitual exercise influences this risk factor. In contrast, there were no significant differences in LDL peak particle diameter between lean exercisers, lean sedentary men and obese sedentary men (27.92 +/- 0.67, 28.09 +/- 0.62 and 27.77 +/- 0.77 nm, respectively). In multiple linear regression analysis, triglyceride concentration was the only significant predictor of LDL PPD. These data suggest that habitual exercise influences LDL concentration but does not influence LDL particle size in men aged 30-45 years.

Problems with determining the particle size distribution of chalk soil and some of their implications

Particle size distribution (psd) is one of the most important features of the soil because it affects many of its other properties, and it determines how soil should be managed. To understand the properties of chalk soil, psd analyses should be based on the original material (including carbonates), and not just the acid-resistant fraction. Laser-based methods rather than traditional sedimentation methods are being used increasingly to determine particle size to reduce the cost of analysis. We give an overview of both approaches and the problems associated with them for analyzing the psd of chalk soil. In particular, we show that it is not appropriate to use the widely adopted 8 pm boundary between the clay and silt size fractions for samples determined by laser to estimate proportions of these size fractions that are equivalent to those based on sedimentation. We present data from field and national-scale surveys of soil derived from chalk in England. Results from both types of survey showed that laser methods tend to over-estimate the clay-size fraction compared to sedimentation for the 8 mu m clay/silt boundary, and we suggest reasons for this. For soil derived from chalk, either the sedimentation methods need to be modified or it would be more appropriate to use a 4 pm threshold as an interim solution for laser methods. Correlations between the proportions of sand- and clay-sized fractions, and other properties such as organic matter and volumetric water content, were the opposite of what one would expect for soil dominated by silicate minerals. For water content, this appeared to be due to the predominance of porous, chalk fragments in the sand-sized fraction rather than quartz grains, and the abundance of fine (<2 mu m) calcite crystals rather than phyllosilicates in the clay-sized fraction. This was confirmed by scanning electron microscope (SEM) analyses. "Of all the rocks with which 1 am acquainted, there is none whose formation seems to tax the ingenuity of theorists so severely, as the chalk, in whatever respect we may think fit to consider it". Thomas Allan, FRS Edinburgh 1823, Transactions of the Royal Society of Edinburgh. (C) 2009 Natural Environment Research Council (NERC) Published by Elsevier B.V. All rights reserved.

Analysis of ochres from Clearwell Caves: the role of particle size in determining colour

Three ochre samples (A (orange-red in colour), B (red) and C (purple)) from Clearwell Caves, (Gloucestershire, UK) have been examined using an integrated analytical methodology based on the techniques of IR and diffuse reflectance UV-visible-NIR spectroscopy, X-ray diffraction, elemental analysis by ICP-AES and particle size analysis. It is shown that the chromophore in each case is haematite. The differences in colour may be accounted for by (i) different mineralogical and chemical composition in the case of the orange ochre, where hi,,her levels of dolomite and copper are seen and (ii) an unusual particle size distribution in the case of the purple ochre. When the purple ochre was ground to give the same particle size distribution as the red ochre then the colours of the two samples became indistinguishable. An analysis has now been completed of a range of ochre samples with colours from yellow to purple from the important site of Clearwell Caves. (C) 2004 Elsevier B.V. All rights reserved.