Palladium, deposition

From acid chlorides by selective hydrogenation in the presence of a catalyst (palladium deposited upon a carrier, which is usually barium sulphate but is... 

A typical catalyst for the final stage would be 10% palladium deposited on finely divided asbestos. 

Palladium Advantages have been claimed for new baths (e.g. using chelated complexes ). Antler summarised the use of palladium as coatings, inlays and weldments in electronic connectors . Crosby noted that palladium deposits are of two kinds (1) soft but continuous or (2) hard but porous or cracked. To resist wear and substrate corrosion on contacts, he proposed the application of type 1 (from a bath with tetranitropalladium(ii) anion) over type 2 (from solution containing tetramminepalladium(ii) cation) . 

Lindlar s catalyst metallic palladium deposited on calcium carbonate and is poisoned with lead acetate and quinoline. 

There are three major classes of palladium-based hydrogen sensors [4], The most popular class of palladium-based sensors is based on palladium resistors. A thin film of palladium deposited between two metal contacts shows a change in conductivity on exposure to hydrogen due to the phase transition in palladium. The palladium field-effect transistors (FETs) or capacitors constitute the second class, wherein the sensor architecture is in a transistor mode or capacitor configuration. The third class of palladium sensors includes optical sensors consisting of a layer of palladium coated on an optically active material that transforms the hydrogen concentration to an optical signal. 

The dependence of adsorption properties and texture on electrolytic palladium deposits on the deposition potential was investigated by Tsirlina et al. [116]. It was found that, in sulfuric acid solutions, the palladium dissolution, which accompanies the oxygen adsorption, obscures the adsorption behavior of the deposits with respect to oxygen. Assumptions were made about the predominant crystallographic orientation (100) of the surface of some palladium deposits being dependent on the deposition potential and about the presence of regions whose adsorption properties are anomalous with respect to copper. The assumptions are based on a comparative analysis of the copper adsorption data and x-ray diffraction patterns. In addition to sites of crystalline palladium, the deposits were found to have disordered areas as well. 

In a series of papers, Kolb and coworkers have presented CV and in situ STM studies on palladium deposition on various gold single-crystal electrodes. They have found [430] that PdCU is adsorbed on Au(lll), forming a distorted hexagonal structure, which plays a crucial role in Pd deposition and dissolution. It has also been found that Pd deposition starts from the formation of a pseudomorphic layer in the underpotential region, followed by the formation of the second Pd monolayer at overpotentials. Pd nucleated... 

Some properties of palladium deposited on different amorphous or zeolitic supports were determined, including catalytic activity per surface metal atom (N) for benzene hydrogenation, number of electron-acceptor sites, and infrared spectra of chemisorbed CO. An increase of the value of N and a shift of CO vibration toward higher frequencies were observed on the supports which possessed electron-acceptor sites. The results are interpreted in terms of the existence of an interaction between the metal and oxidizing sites modifying the electronic state of palladium. 

Determination of the Number of Electron-Acceptor Sites. This determination was done in a few cases using perylene and phenothiazine as reactants. The catalysts were reduced for 2 hours at 400°C under H2. Since we presume that silica-alumina may be modified by the reaction medium during palladium exchange, we used as our blank a support treated under conditions similar to that of the catalysts. The results (Table V) show a clear decrease of the number of oxidizing sites after palladium deposition. 

The obvious decrease in the number of electron-acceptor sites with palladium deposition on silica-alumina strongly suggests an interaction between the metal and these sites. Turkevich (28) first demonstrated that palladium behaves like an electron-donor toward tetracyanoethylene we suppose that it can be the same toward an electron-acceptor site of a solid support. In that hypothesis, palladium should have a partial positive charge on the second class of supports. This is actually observed by the adsorption of CO. This adsorbate can be considered as a detector of the electronic state of palladium. The shift toward higher frequencies of the CO band reflects a decrease in the back donation of electrons from palladium to CO. Thus, palladium on silica-alumina or HY is electron-deficient compared with the silica- or magnesia-supported metal. Moreover, the shift of CO vibration frequency is roughly parallel to the increase of activity thus, these two phenomena are connected. We propose that the high activity of palladium on acidic oxides is related to its partial electron deficiency. 

The carbonylation of allylic methoxyoctadienes has proved to be very efficient in the presence of soluble palladium(O) and -(II) complexes to give selectively the linear methyl nona-3,8-dienoate, provided that hydrochloric acid is added to the reaction medium. Under the same conditions, 10% palladium deposited on charcoal exhibits similar activity and selectivity to soluble complexes. In both cases, careful examination of the results seems to agree with homogeneous catalytic pathways. 

Acid chlorides can be selectively hydrogenated in the presence of a catalyst (palladium deposited on a carrier, which is usually barium sulphate but is occasionally charcoal). The reaction, which involves the hydrogenolysis of the carbon-halogen bond, is known as the Rosenmund reduction and has been widely used for the synthesis of aromatic and heterocyclic aldehydes. 

Polymerization of triethoxysilane in water catalyzed by Pd(OAc)2 results in a palladium-deposited siloxane that can be used as a heterogeneous catalyst for reductions with hydrogen. 

The main reaction product of tetrachloroethylene hydrogenation catalyzed by Pd particles of average size 20-40 nm immobilized on the oxide substrate is thichloroethylene. At the same time, catalytic reaction with the use of black palladium deposited on the same substrate gives mainly the saturated compounds [113]. It has also been established that Pd particles of sizes 2.5 nm deposited on A1203 catalyze the dissociation of CO whereas the larger particles in the size 27 nm as well as a continuous film of metal Pd are inactive in this process [115]. 

Heterogeneous catalysts modified by the addition of chiral substances have been used to hydrogenate olefins asymmetrically, but only a few effective chiral heterogeneous catalyst systems have been found. Palladium deposited on silk fibroin was used to hydrogenate 4-benzylidene-2-methyl-5-oxazolone asymmetrically to give, after hydrolysis, optically active phenylalanine (Fig. lc). The optical purity1 of the product was found to be dependent on the origin of the fibroin and its chemical pretreatment (4-6). 

Alumina support modified with Ni is ready for palladium deposition which can be carried out in the same device. Pd (Pd alloy) deposition should be followed by additional thermal treatment for the complete inter phase diffusion of Ni and Pd. 

Except at very low palladium concentrations on the gel surfaces, the amounts of hydrogen sorbed at 30-mm. pressure by the palladium carrying gels appear to be constant at H/Pd ratio of from 0.70 to 0.75. At the low concentrations of palladium on the gel, the nature of the palladium, deposited on the surface or... 

The measured data give evidence that the susceptibility of palladium deposited on silica gel surfaces is dependent on both the surface to bulk ratio and the lattice constant. They also show definite surface states on the palladium crystallites with more holes in the d-band of the surface than in the rest of the metal. 

The palladium deposit is found, however, to slowly lose its catalytic activity in a solution of the hypophosphite. Neither palladium foil nor palladium wire will exhibit catalytic activity in this reaction. 

The finely divided metal is obtained for this purpose by reduction of palladous chloride with strips of metallic zinc, the palladium deposit being carefully washed and dried. After prolonged exposure to air it appears to lose its affinity for mercury, possibly on account of slow oxidation. 

Palladium deposited onto the mesoporous material MCM-41 by vacuum sublimation of CpPd(allyl) gives a heterogeneous catalyst for Heck arylations.87... 

Fig. 9.7. The Raman spectra of the palladium/a-Si H interface region (a) as deposited a-Si H (b) palladium on freshly deposited a-Si H (c) sample annealed to 300 C (d) annealed to 550 °C (e) palladium deposited on oxidized a-Si H and annealed to 400 °C (Nemanich et at. 1981).

Fig. 18. FT-RAIRS spectra of CO adsorbed at 300K on IML and 20ML films of palladium deposited by metal vapour deposition on TiO2(110) [56]. The switch from a local titania dielectric (transmission band) to that of the metal (absorption band) takes place at about lOML of palladium. The singleton frequency and the coverage dependent dipole shift are similar for both palladium layers indicating little perturbation of the CO adsorption behaviour on the palladium by the Ti02(l 10) substrate.

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