Palladium adherence

Catalysts for dielectric surfaces are more complex than the simple salts used on metals. The original catalysts were separate solutions of acidic staimous chloride [7772-99-8J, used to wet the surface and deposit an adherent reducing agent, and acidic palladium chloride [7647-10-17, which was reduced to metallic palladium by the tin. This two-step catalyst system is now essentially obsolete. One-step catalysts consist of a stabilized, pre-reacted solution of the palladium and staimous chlorides. The one-step catalyst is more stable, more active, and more economical than the two-step catalyst (21,23). A separate acceleration or activation solution removes loose palladium and excess tin before the catalyzed part is placed in the electroless bath, prolonging bath life and stability. 

Biosensors fabricated on the Nafion and polyion-modified palladium strips are reported by C.-J. Yuan [193], They found that Nafion membrane is capable of eliminating the electrochemical interferences of oxidative species (ascorbic acid and uric acid) on the enzyme electrode. Furthermore, it can restricting the oxidized anionic interferent to adhere on its surface, thereby the fouling of the electrode was avoided. Notably, the stability of the proposed PVA-SbQ/GOD planar electrode is superior to the most commercially available membrane-covered electrodes which have a use life of about ten days only. Compared to the conventional three-dimensional electrodes the proposed planar electrode exhibits a similar... 

Abstract The dimerization of 1,3-dienes (e.g. butadiene) with the addition of a protic nucleophile (e.g. methanol) yields 2,7-octadienyl ethers in the so-called telomerization reaction. This reaction is most efficiently catalyzed by homogeneous palladium complexes. The field has experienced a renaissance in recent years as many of the platform molecules that can be renewably obtained from biomass are well-suited to act as multifunctional nucleophiles in this reaction. In addition, the process adheres to many of the principles of green chemistry, given that the reaction is 100% atom efficient and produces little waste. The telomerization reaction thus provides a versatile route for the production of valuable bulk and specialty chemicals that are (at least partly) green and renewable. The use of various multifunctional substrates that can be obtained from biomass is covered in this review, as well as mechanistic aspects of the telomerization reaction. 

Previous studies have shown that a trend exists in the behavior of some evaporated metals on polyimide surfaces x-ray and ultraviolet photoelectron (XPS, UPS) as well as high resolution electron energy loss (HREELS) measurements have indicated that while for some metals such as aluminum, titanium and chromium there is bond formation with the PMDA carbonyl oxygen of the polyimide (2, 10-13). other metals such as copper, palladium and gold undergo little reaction or interaction (10,12,14,15). It has, however, since been postulated that metals, in order to adhere well at all to a polymer under a wide variety of conditions, must form metal- polymer bonds (10). 

Catalyzation with a tin/palladium colloid, acceleration by removal of the tin coating and electroless copper deposition with commercially available products results in an adherent copper layer to the plastic. The plated sample is then heat treated, electroplated with copper to a thickness of 37.5/x and then heat treated again. 

Chlorophenyl)propionic acid A solution of 4-chlorocinnamic acid (5 g) in tetralin (180 ml) containing palladium black (1 g) is heated at the boiling point for 1.5 h in a Kjeldahl flask under an air condenser. The side chain is then completely reduced. The catalyst is filtered off, the acid components are removed from the filtrate in sodium carbonate solution, adhering tetralin is removed from the alkaline solution by ether, the alkaline solution is acidified by hydrochloric acid, and the 3-(p-chlorophenyl)propionic acid that separates is taken up in ether. On evaporation, the pure 3-(p-chlorophenyl)propionic acid of m.p. 123° is obtained. 

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. 

In the case of all-metal composite membranes formed by depositing palladium and its alloys onto metal substrates based on Nb, Ta, and V, strong coherent interfaces are highly desired not only for wetting and adherence, but also to ensure, at the atomic level, that the dissociated hydrogen is rapidly transferred between the catalyst and substrate layers. The rules of epitaxial growth and lattice matching are also used to optimize these solid-solid interfaces. 

Already by 1963, for a patent granted in 1966, Straschil and Lopez realized that the match of coefficient of thermal expansion between palladium membranes and (porous) substrates was critical, and stated that it would be virtually impossible to compensate for differences in dilation due to absorption of hydrogen [38]. They patented the use of dimpled or corrugated foils to accommodate differential thermal and chemical expansion [38]. Buxbaum and Hsu, in a 1992 patent, maintained that a rough substrate surface produced by abrasion with steel wool was critical for adherence of palladium on surfaces of Nb, Ta, V and Zr [39]. Other patents recommend corrugated or undulating configurations to allow for both thermal and chemical expansion [24, 26, 27, 29]. 

In a similar way, gold pattern layers have been put on PI substrates by micro contact printing. " In micro contact printing, a polymeric stamp that is wetted with a potassium hydroxide solution is pressed onto the PI substrate. The alkaline treated regions of the substrate become hydrophilic and are prone to hold a palladium(II) solution. In the same way as described above, the adhered palladium ions are reduced by NaBILj and further electroplated. 

Other types of porous glass materials are represented by Si02, AI2O3 and B2O3, giving excellent anchor effect and adherence [53]. Also porous stainless steel (PSS) can be considered as a valid support due to its mechanical durability, its thermal expansion coefificient close to that of palladium and the ease of gas sealing [53]. 

The selection of porous substrate. Ceramic supports are chemically stable, have small pore sizes and a more uniform pore size distribution, essential for the formation of thinner and uniform membrane layer. But they are brittle, and the large difference in thermal expansion coefficients between palladium layer and ceramic supports leads to membrane cracking and loss of adherence. On the other hand, metallic porous supports made of stainless steel (PSS) have a more similar thermal expansion coefficient, reducing thermal stresses, but the large pore size and wide pore size distribution make the formation of a uniform selective layer problematic. 

The protection of amorphous R-TM films is very difficult to carry out. Forester et al. (1983) have taken out a patent which demonstrates that a thin palladium layer (2-3 nm), overlaying a GdFe film, increases resistance to poisoning by atmospheric gases. However the sample rapidly absorbed hydrogen. In this case the films did not fracture or disintegrate and remained firmly adherent to the substrate. 

Deposition should be performed in controlled conditions to obtain the formation of pinhole-free, adherent palladium film a typical laboratory apparatus is shown in Fig. 3.3. The membrane support is inserted in a reactor and maintained in rotation at constant velocity by a variable-speed motor to allow the removal from the reaction zone of nitrogen produced by hydrazine reduction (Equation [3.5]).The reactor is immersed in a thermostatic bath to keep the temperature at a constant value. Hydrazine can be periodically added from the tube inserted in the middle of the reactor, while nitrogen can be evacuated from the reactor through the tube on the right side. 

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