Dense metal membrane palladium

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. 

Palladium-based dense metallic membranes have been known to be completely selective for hydrogen permeation and are used in commercially available small-scale hydrogen purification units (e.g., Johnson Matthey, 2007 REB Research, 2007 Power + Energy, 2007 ATI Wah Chang, 2007). These hydrogen purification units typically use palladium-alloy... 

The manufacture of dense metal membranes or thin films can be effected by a number of processes casting/rolling, vapor deposition by physical and chemical means, electroplating (or electroforming) and electroless plating. By far, casting in combination with rolling is the predominant preparation and fabrication technique. It is noted that many of these processes have been demonstrated with palladium and its alloys because of their low oxidation propensity. Preparation of dense metal membranes is summarized in some detail as follows. 

Dense metallic membranes, in particular those based on palladium alloys, have been extensively studied for the selective transport of In the case of O2,... 

Thicker, self-supporting, dense metal membranes are known. These are tubular and are usually commercially successful palladium-silver hydrogen separation membranes were of this type.21 Currently, Power and Energy, Inc. also fabricates this type of membrane, although planar membranes are more common due to easier fabrication and a greater variety of fabrication methods. 

Researchers have reported the influence of various chemical species on the performance of palladium-based membranes. Observed reductions in hydrogen flux through dense metal membranes are typically attributed to the blocking of the adsorption of molecular hydrogen, which is a necessary step in the atomic transport mechanism [13, 80,112,114,120-123],... 

A large number of hydrogenation and dehydrogenation reactions were tested in the early studies of dense-metal membrane reactors (see listing in Shu et al. [34], Hsieh [35], and Gryaznov and Orekhova [36]). Many works tested the dehydrogenation of cyclohexane to benzene as a model reaction since it can be carried out at low temperature with no side reactions and no deactivation a conversion of 99.5% was achieved with a palladium membrane, compared with 18.7% at equilibrium, at 200°C [31]. 

Dense metal membranes exhibit an absolute permeability to specific species. Clear examples are given by palladium and palladium-alloy membranes, which are exclusively permeable to hydrogen, and by silver membranes. 

The discovery of hydrogen selective diffusion through a palladium membrane by Sir Thomas Graham in 1866 and its subsequent development, a palladium-hydrogen system, have been extensively investigated (Lewis, 1967). A self-sustained palladium dense metal membrane tube with thickness... 

Dense metal membranes have obtained huge successes in implementing reactors in this sense, the case of palladium membrane (for hydrogen separation) is very representative (Adhikari Fernando, 2006). 

The resulting metal membrane has a structure similar to the bulk pores of the anodic alumina but without the dense "skin layer and has straight through-pores. Nickel and platinum membranes having pore diameters in the 15 to 200 nm range can be produced this way. One of the critical steps of this method is depositing palladium catalyst on the surface, but not inside the pores, of the anodic alumina. The catalyst is used to facilitate the deposition of metal from the bottom to the surface of the cylindrical pores. 

The application of polymer membranes is generally limited to temperatures below 475 K and to the separation of mixtures that are chemically inert. Otherwise, membranes made of inorganic materials can be used. These include mainly microp-orous ceramics, metals, and carbon, and dense metals, such as palladium, that allow the selective diffusion of very small molecules such as hydrogen and helium. 

The materials of membrane construction can be classified as either dense or porous. Dense metal materials include palladium membranes that are semiperme-able to hydrogen, and silver membranes that are semipermeable to oxygen. The low permeation rates for silver membranes have led to the more recent use of solid oxide electrolyte dense membranes such as modified zirconias and perovs-kites, which have higher O2 permeation rates at high temperatures. ... 

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