Metallic membranes materials

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

Dense Metal Membrane Materials, Configurations, Mechanisms of Transport, and Permeability... 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

Porous metals have long been commercially available for particulate filtration. They have been used in some cases as microfiltration membranes that can withstand harsh environments, or as porous supports for dynamic membranes. Stainless steel is by far the most widely used porous metal membrane. Other materials include silver, nickel. Monel, Hastelloy and Inconel. Their recommended maximum operating temperatures range from 200 to 650°C. Elepending on the pore diameter which varies from 0.2 to 5 microns, the water permeability of these symmetric membranes can exceed 3000 L/h-m -bar and is similar to that obtained with asymmetric ceramic microfiltration membranes. Due to the relatively high costs of these membranes, their use for microfiltration has not been widespread. 

Linear polycarbosilanes and polycarbosiloxanes-especially those containing arylene units in the chain-have specific physico-chemical properties which can be applicable in heat-resistant materials [29-31]. Phenylene-silylene-ethylene-polymers, which may serve as potential substrates for applications as membrane materials are usually obtained in the presence of platinum catalysts [32], although other transihon-metal complexes have also been tested in this process. 

Other Important Considerations with Membranes Oxidizers such as sodium hypochlorite (i.e., CIO2), bromine, iodine, and ozone, which are typically used in the disinfection of wastewater, are not well tolerated by thin-hlm membranes. Such disinfectants can thus influence the efficacy of membranes in removing contaminants such as PPCPs. Furthermore, membranes can become fouled by microorganisms that can metabolize the membrane material. Thus, microbial counts of >100 cells/mL can be problematic. Likewise, dead-cell debris can also cause fouling. Membranes can also be fouled by heavy metals such as chromium. Thus, if heavy metals are deemed a problem, they should be precipitated from the wastewater prior to the filtration with membranes. 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

The objective of this protocol is the fabrication of a light-addressable potentiometric sensor (LAPS) for the detection of the pH value and the cadmium-ion concentration in aqueous solutions. For the pH-sensitive LAPS, use, e.g., Ta205 as a sensor membrane, and for the cadmium-selective LAPS, use a Cd2+-selective chalcogenide glass thin film as a heavy metal-sensitive material. The electrochemical sensor characterisation of the LAPS structure perform current vs. voltage (I/V) and constant current (CC) measurements. 

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