Palladium composite membranes temperature

Because Pd-based metal membranes, commonly used for hydrogen separation [11] are not resistant towards sulphur, not much research has been performed on the use of such membranes in H2S dehydrogenation reactors. Some success has, however, been reported by Edlund and Pledger [12], They developed a platinum-based layered metal membrane that could resist irreversible attack by H2S at 700°C. At this temperature a conversion of 99.4% was achieved in the membrane reactor. Without hydrogen removal the conversion was only 13%. No permeance data is provided, but platinum-based metal membranes are known for their low hydrogen permeance [14], Johnson-Matthey developed palladium composite membranes with a hydrogen permeance of about 1 10 mol/m sPa [14], but these are most probably not resis-... 

Electroplating. Basically in electroplating, a substrate is coated with a metal or its alloy in a plating bath where the substrate is the cathode and the temperature is maintained constant Membranes from a few microns to a few millimeters thick can be deposited by carefully controlling the plating time, temperature, current density and the bath composition. Dense membranes made of palladium and its various alloys such as Pd-Cu have been prepared. Porous palladium-based membranes have also been made by deposition on porous support materials such as glass, ceramics, etc. 

Besides the compact membrane catalysts described in Section II, there are two types of composite membrane catalyst porous and nonporous. Composite catalyst consists of at least two layers. The first bilayered catalyst was prepared by N. Zelinsky [112], who covered zinc granules with a porous layer of palladium sponge. The sponge became saturated with the hydrogen evolved during hydrochloric acid reaction with zinc and at room temperature actively converted hydrocarbon iodates into corresponding hydrocarbons. 

Composite membrane catalysts can also be assembled with polymeric supports or intermediate layers [117-119]. These membranes were tested as membrane catalysts for selective hydrogenation of some dienic hydrocarbons and proved to be as selective as monolithic palladium alloy membranes [117]. The use of polyarilyde has been proposed in order to widen the temperature range of polymer-supported membrane application... 

The composite membrane Pt/Si02/V proved to be resistant to irreversible poisoning by hydrogen sulfide after 8 hours exposure at temperatures between 700 and 800 C. Platinum coating layers were 25 xm thick. Similar palladium layers were damaged within 15 sec [124]. 

For ill-designed composite membranes, for example, formed by depositing palladium onto substrates which it does not wet, surface tension will force the thin film to contract and ball up if the palladium atoms acquire sufficient surface mobility. Pinholes may form as a prelude to complete de-wetting, or pinholes may remain from the initial fabrication if the palladium did not fully wet its substrate. Kinetics of de-wetting is accelerated at elevated temperature and in the presence of adsorbates such as CO, which increase surface mobility of Pd. If molten metals do not wet ceramics, they will be expelled from ceramic pores. During sintering of cermets, Pd and other metals will not adhere to the ceramic phase, if the metal and ceramic do not wet. 

Ma YH, Engwall EE, Mardilovich IP. Composite palladium and palladium-alloy membranes for high temperature hydrogen separations. ACS Fuel Chem Div Prepr. 2003 48(1) 333. 

Thorough cleaning and the avoidance of any extraneous debris are essential in any membrane fabrication scheme. After EP, a composite membrane can be soaked in hot water or dilute ammonia to help remove any impurities trapped in the porous support that may be detrimental to the palladium film during high temperature operation [72, 73]. However, traces of impurities from the EP bath such as chlorine, sodium, and carbon, inevitably become incorporated into the metal film. Membrane defects can be a consequence of preparation conditions. Fabrication in a dean-room environment has resulted in increased permselectivity [140]. Porous stainless steel (PSS) must be cleaned and pickled before electrodeposition or else activated for electroless plating. 

It follows from the work of Darling and others that formation of the beta phase palladium hydride must be avoided to prevent embrittlement, cracking and delamination of thin catalytic films of unalloyed palladium used on composite membranes of Nb, Ta, Ti, V, and Zr. This also holds true for alloys of palladium susceptible to hydrogen embrittlement. Likewise, transformation of Nb, Ta, Ti, V and Zr substrates into brittle hydrides at low temperatures or at high hydrogen partial pressures must also be avoided. 

In conclusion. Table 2.3 reports the permeation data of different palladium-based composite membranes, produced, principally, by ELP or CVD techniques. Many parameters are reported in the table membrane type and thickness, temperature and pressure ranges of the permeation experiments, hydrogen flux. 

J. Okazaki, T. Ikeda, D. A. Pacheco Tanaka, T. M. Suzuki and F. Mizukami, In situ high-temperature X-ray diffraction study of thin palladium/alpha-alumina composite membranes and their hydrogen permeation properties, J. Membr. Sci., 2009, 335, 126-132. 

Moreover, since also the mismatch of palladium and alumina thermal expansion coefficients is reduced, high temperature delamination and defects formation in the membrane are also reduced. It is also possible to control the palladium thickness (to about 5 pm) by increasing the hydrogen flux and so minimizing the cost of the composite membrane. 

The design of the Pd-membrane reactor was based on the chip design of reactor [R 10]. The membrane is a composite of three layers, silicon nitride, silicon oxide and palladium. The first two layers are perforated and function as structural support for the latter. They serve also for electrical insulation of the Pd film from the integrated temperature-sensing and heater element. The latter is needed to set the temperature as one parameter that determines the hydrogen flow. 

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