Nieck E. Benes

Three-dimensional porous hollow fibre copper electrodes for efficient and high-rate electrochemical carbon dioxide reduction

Aqueous-phase electrochemical reduction of carbon dioxide requires an active, earth-abundant electrocatalyst, as well as highly efficient mass transport. Here we report the design of a porous hollow fibre copper electrode with a compact three-dimensional geometry, which provides a large area, three-phase boundary for gas–liquid reactions. The performance of the copper electrode is significantly enhanced; at overpotentials between 200 and 400 mV, faradaic efficiencies for carbon dioxide reduction up to 85% are obtained. Moreover, the carbon monoxide formation rate is at least one order of magnitude larger when compared with state-of-the-art nanocrystalline copper electrodes. Copper hollow fibre electrodes can be prepared via a facile method that is compatible with existing large-scale production processes. The results of this study may inspire the development of new types of microtubular electrodes for electrochemical processes in which at least one gas-phase reactant is involved, such as in fuel cell technology.

Microporous silica and doped silica membrane for alcohol dehydration by pervaporation

The aim of this work is the development of inorganic membranes that will enable broad application of pervaporation/vapour permeation technology in the chemical industry. This can be achieved by improvement of the existing microporous membranes and the development of new types with enhanced thermochemical stability and separation characteristics. The materials in the system, SiO2---Al2O3---TiO2---ZrO2---MgO, were investigated with respect to their chemical stability and pervaporation performance in alcohol dehydration processes. It was found that, depending on the nature and amount of dopant, composite membranes with improved pervaporation characteristics and chemical stability were obtained.

Spectroscopic Ellipsometry analysis of a thin film composite membrane consisting of polysulfone on a porous α-alumina support

Exposure of a thin polymer film to a fluid can affect properties of the film such as the density and thickness. In particular in membrane technology, these changes can have important implications for membrane performance. Spectroscopic ellipsometry is a convenient technique for in situ studies of thin films, because of its noninvasive character and very high precision. The applicability of spectroscopic ellipsometry is usually limited to samples with well-defined interfacial regions, whereas in typical composite membranes, often substantial and irregular intrusion of the thin film into the pores of a support exists. In this work, we provide a detailed characterization of a polished porous alumina membrane support, using variable-angle spectroscopic ellipsometry in combination with atomic force microscopy and mercury porosimetry. Two Spectroscopic ellipsometry optical models are presented that can adequately describe the surface roughness of the support. These models consider the surface roughness as a distinct layer in which the porosity gradually increases toward the outer ambient interface. The first model considers the porosity profile to be linear; the second model assumes an exponential profile. It is shown that the models can be extended to account for a composite membrane geometry, by deposition of a thin polysulfone film onto the support. The developed method facilitates practicability for in situ spectroscopic ellipsometry studies of nonequilibrium systems, i.e., membranes under actual permeation conditions.

Quantitative analysis of the microstructural homogeneity of zirconia-toughened alumina composites

The Voronoi diagram approach was applied to quantify the level of microstructural homogeneity of ceramic ZTA samples. From SEM pictures of polished cross-sections of ZTA samples a point pattern representing the distribution of the zirconia phase in the composite was generated. This point pattern was converted into a Voronoi diagram. The level of microstructural homogeneity was quantified by statistical analysis of the relevant properties (area, perimeter and number of faces) of the Voronoi polygons. A dimensionless parameter defining the level of microstructural homogeneity was calculated from the different sets of statistical data. The calculated parameters indicated significant differences in homogeneity between the ZTA samples. These differences were in qualitative agreement with previously published wear rates of the same ZTA composites. This illustrates the relevance of microstructural homogeneity for wear performance.

Sieving of Hot Gases by Hyper-Cross-Linked Nanoscale-Hybrid Membranes

Macromolecular networks consisting of homogeneously distributed covalently bonded inorganic and organic precursors are anticipated to show remarkable characteristics, distinct from those of the individual constituents. A novel hyper-cross-linked ultrathin membrane is presented, consisting of a giant molecular network of alternating polyhedral oligomeric silsesquioxanes and aromatic imide bridges. The hybrid characteristics of the membrane are manifested in excellent gas separation performance at elevated temperatures, providing a new and key enabling technology for many important industrial scale applications.

Development and comparative study of different nanofiltration membranes for recovery of highly charged large ions

The development of membranes for polyoxometalate (POM) recycling based on charge or size effects is described. POM recycling via nanofiltration with surface charge membranes is a new approach. From the properties of solute—membrane interaction three different kinds of nanofiltration membranes have been selected to find out most suitable one for recycling process. A new generation surfactant templated inorganic silica film has been prepared and characterised. This type of material could be promising for future NF membrane applications.

Quasi-elastic neutron scattering study of the mobility of methane in microporous silica

The dynamics of translation and rotation of methane in microporous bulk silica have been studied with quasi-elastic neutron scattering. At T=200 K the self-diffusion coefficient of translation is DS=1.1×10−8 m2 s−1 with an estimated activation energy of 4 kJ mol−1. Any variation of DS with occupancy is within the experimental error of 50%. The isotropic rotational diffusion constant DR is of the order of 1011 s−1, with an experimental error of a factor of 2. The pore-size distribution has been studied with small angle X-ray scattering. A broad pore-size distribution is observed, with pores smaller than 10 A. The largest fraction of pores is observed at the lower experimental limit, corresponding to a pore size of 2 A.

Formation and prevention of fractures in sol–gel-derived thin films

Sol-gel-derived thin films play an important role as the functional coatings for various applications that require crack-free films to fully function. However, the fast drying process of a standard sol-gel coating often induces mechanical stresses, which may fracture the thin films. An experimental study on the crack formation in sol-gel-derived silica and organosilica ultrathin (submicron) films is presented. The relationships among the crack density, inter-crack spacing, and film thickness were investigated by combining direct micrograph analysis with spectroscopic ellipsometry. It is found that silica thin films are more prone to fracturing than organosilica films and have a critical film thickness of 300 nm, above which the film fractures. In contrast, the organosilica films can be formed without cracks in the experimentally explored regime of film thickness up to at least 1250 nm. These results confirm that ultrathin organosilica coatings are a robust silica substitute for a wide range of applications.

Multi-component lattice gas diffusion

In this paper, diffusional transport of multi-component mixtures within the framework of a three-dimensional lattice gas is studied using dynamic Monte Carlo simulations. The mobile species, instantaneously hopping from one site to another, are assumed to have no mutual interactions, other than the usual ‘hard core’ interactions. Most strikingly, percolation phenomena occur for multi-component mixtures with significant differences in mobility. These greatly reduce the flux of the mobile component and cause failure of the standard macroscopic theories, including, e.g., the Maxwell–Stefan theory. Furthermore, we demonstrate that the well-known correlation effect disappears for systems in which gradients in the vacancy concentration are present. For systems in which co-operative displacements of two or more molecules are allowed to occur the effect of correlation between successive jumps vanishes, while the plot of the mobility versus occupancy shows a maximum. This intricate relation between mobility and occupancy again complicates the use of standard theories for describing mass transport.

Numerical scheme for simulating multicomponent mass transport accompanied by reversible chemical reactions in porous media

A numerical scheme is presented for computer simulation of multicomponent gas transport possibly accompanied by reversible chemical reactions in a macroporous medium, based on the dusty gas model. Using analytical solutions for simple systems it is shown that the derivation of the scheme is mathematically correct and the implementation into Borland Dephi code is performed without vital programming errors. Simulations showed remarkable accuracy, robustness and efficiency.