Palladium Film

Garcia, J.A. and Mandelis, A., Study of the thin-film palladium/hydrogen system by an optical transmittance method, Review Scientific Instruments, 67(11), 3981,1996. 

Additionally, the corrosive nature of the syngas stream can have significant impacts on the mechanical and chemical stability of the membrane. Researchers have shown that a pure palladium membrane can develop a corrosion scale as thick as 35 microns after only 5 days of operation in the presence of 0.1%H2S-H2, making thin film palladium membrane technologies impractical in such an environment [91]. 

Anisimkin V. 1., Kotelyanskii 1. M., Verardi P., and Verona E., Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors. Sens. Actuators B, B23, 203-208, November 1-4, 1994, Hotel Martinez Cannes, France. 

An essential element of the Hysep technology is the use of thin film palladium composite membranes to enable low cost and reliable hydrogen separation. The supported palladium layer in the Hysep module has a thickness as low as 3-9 pm, a substantial improvement over current commercial available palladium membranes, which are based on self supporting metal foils with a thickness of 20-100 pm. 

One disadvantage of microporous inorganic membranes is that they are porous. They can never produce 100% pure gas streams as can thin-film-palladium or ion-transport membranes. However, when microporous membranes are coupled with pressure swing adsorption (PSA), the combined system can produce 100% hydrogen. In this scenario, PSA would only be required to separate the final 1% of the impurities, and the coupling of the two technologies should result in a very compact and efficient separation system. 

Figure Al.7.10. STM image (1000 A x 1000 A) of the (111) surface of a tungsten single crystal, after it had been coated with a very thin film of palladium and heated to about 800 K (courtesy of Ted Madey).

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

Figure Bl.26.1. Sorption isothemi for chemisorption of hydrogen on palladium film at 273 K (Stephens S J 19597. Phys. Chem.. 63 188-94).

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. 

Electronic Applications. The PGMs have a number of important and diverse appHcations in the electronics industry (30). The most widely used are palladium and mthenium. Palladium or palladium—silver thick-film pastes are used in multilayer ceramic capacitors and conductor inks for hybrid integrated circuits (qv). In multilayer ceramic capacitors, the termination electrodes are silver or a silver-rich Pd—Ag alloy. The internal electrodes use a palladium-rich Pd—Ag alloy. Palladium salts are increasingly used to plate edge connectors and lead frames of semiconductors (qv), as a cost-effective alternative to gold. In 1994, 45% of total mthenium demand was for use in mthenium oxide resistor pastes (see Electrical connectors). 

C-21 dicarboxyhc acids are produced by Westvaco Corporation in Charleston, South Carolina in multimillion kg quantities. The process involves reaction of tall oil fatty acids (TOFA) (containing about 50% oleic acid and 50% hnoleic acid) with acryhc acid [79-10-7] and iodine at 220—250°C for about 2 hours (90). A yield of C-21 as high as 42% was reported. The function of the iodine is apparendy to conjugate the double bond in linoleic acid, after which the acryhc acid adds via a Diels-Alder type reaction to form the cycHc reaction product. Other catalysts have been described and include clay (91), palladium, and sulfur dioxide (92). After the reaction is complete, the unreacted oleic acid is removed by distillation, and the cmde C-21 diacid can be further purified by thin film distillation or molecular distillation. 

The ideal electroless solution deposits metal only on an immersed article, never as a film on the sides of the tank or as a fine powder. Room temperature electroless nickel baths closely approach this ideal electroless copper plating is beginning to approach this stabiHty when carefully controUed. Any metal that can be electroplated can theoretically also be deposited by electroless plating. Only a few metals, ie, nickel, copper, gold, palladium, and silver, are used on any significant commercial scale. 

Electroless reactions must be autocatalytic. Some metals are autocatalytic, such as iron, in electroless nickel. The initial deposition site on other surfaces serves as a catalyst, usually palladium on noncatalytic metals or a palladium—tin mixture on dielectrics, which is a good hydrogenation catalyst (20,21). The catalyst is quickly covered by a monolayer of electroless metal film which as a fresh, continuously renewed clean metal surface continues to function as a dehydrogenation catalyst. Silver is a borderline material, being so weakly catalytic that only very thin films form unless the surface is repeatedly cataly2ed newly developed baths are truly autocatalytic (22). In contrast, electroless copper is relatively easy to maintain in an active state commercial film thicknesses vary from <0.25 to 35 p.m or more. 

Metallic Palladium films pass H9 readily, especially above 300°C. Ot for this separation is extremely high, and H9 produced by purification through certain Pd alloy membranes is uniquely pure. Pd alloys are used to overcome the ciystalline instability of pure Pd during heat-ing-coohng cycles. Economics limit this membrane to high-purity apphcations. 

Figure 2 Surface EXAFS spectra above the Pd L edge for a 1.5 monolayer evaporated film of Pd on Sid 11) and for bulk palladium ailicide, Pd2Si and metallic Pd.

In the massive state none of these elements is particularly reactive and they are indeed very resistant to atmospheric corrosion at normal temperatures. However, nickel tarnishes when heated in air and is actually pyrophoric if very finely divided (finely divided Ni catalysts should therefore be handled with care). Palladium will also form a film of oxide if heated in air. 

The deliberate raising of the electrical potential of titanium, either by the attachment of discrete particles of a noble metal, such as platinum or palladium, at the surface, or by the application of positive direct current to force the formation of a protective film, is dealt with at a later point. The electrochemical aspect of the corrosion of titanium is comprehensively treated in a number of papers ... 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

In sliding electrical contact applications, palladium plating has been criticised on the basis of a tendency due to its catalytic activity to cause polymerisation of organic vapours from adjacent equipment with the formation of insulating films on the surface. This effect is important in certain circumstances, but is not serious in many practical applications... 

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