Palladium foils

As early as 1923 Hinshelwood and Topley (27) noted the exceptionally erratic behavior of palladium foil catalyst in the formic acid decomposition reaction within 140-200°C. The initially very high catalytic activity decreased 102 times during the exposure of palladium to hydrogen, which is a product of the reaction. Though the interpretation does not concern the /3-hydride formation, the authors observation deserves mentioning. 

Fig. 8. Arrhenius plots for the formic acid decomposition on palladium foil (1) and small pieces of this foil (2) at a higher temperature range, when hydrogen evolving as a product of the reaction was absorbed by Pd and transformed into the /3-Pd-H hydride phase. At the lower temperature range the reaction proceeds on the a-Pd-H phase, with a higher activation energy when the foil was hydrogen pretreated (2a), and a lower activation energy for a degassed Pd foil (3a). After Brill and Watson (57).

Fig. 22. Variation in the fractions of various forms of adsorbed acetylene on palladium foil with temperature of annealing [148].

Differentiation Between Palladium Foil and Platinum Foil. 

On placing 1 drop of an alcoholic solution of iodine on palladium foil, and allowing it to evaporate spontaneously in the air, a black spot will be formed on the palladium which will disappear on heating the foil to redness. On platinum foil similarly treated, no spot is formed. 

Figure 8. Experimental evidence of nuclear reactions produced by high-density charge clusters (HDCCs) (a) HDCC strike on a deuterium-loaded palladium foil (b) X-ray analysis of the crack illustrated above, showing new materials produced.

Preparation of Thin Tilms of TiOx on Palladium Foils. Following the procedure reported by Takahashi, an isopropanol based sol of titanium tetra-isopropoxide (TnP) was prepared (10 mL of TUP in 48.4 mL of 2-propanol), to which 6.7 mL of diethanolamine was added to inhibit the formation of oxides. This mixture was stirred for two hours, at which time gel formation was initiated by the addition of 28.1 mL of a dilute water solution (1.42 mL water in 30 mL of 2-propanol). The resulting molar ratio of water to TUP was 2 1, as was recommended by Takahashi and co-workers (22). The palladium substrates were dipped vertically into this solution and then clamped on edge while draining to allow the excess to drip off and to prepare as even a coating as possible. The membranes were then air-dried in a vertical position at 100°C for one hour, followed by a post-treatment with hydrogen (see Figure 2). 

Figure 4 Scanning electron micrographs of a) virgin palladium foil (2000x), b) after 40 hr of ethylene hydrogenation at 150 ° C (500x), c) after 40 hr of ethylene hydrogenation at 200 ° C (400x), d) close of etch-pit on 200 ° C sample (4000x).

Through 0.003 in palladium foil, the hydrogen permeation rate was taken as the sum of hydrogen and ethane in the product stream. 

Recently, attempts have been made to use the unique permselectivity of Pd for H2, and to overcome the problem of the low permeabilities of conventional palladium foils by depositing Pd on a porous inorganic support [16, 17], If the amount of deposited Pd is well controlled, one can expect a thin Pd layer with high selectivity and permeability towards H2. 

Ferric salts and potassium ferricyanide are completely reduced by charged palladium foil or wire, and the reduction may be carried out quantitatively if required for analytical purposes. 

Catalytic Activity of Compact Palladium.—Hydrogen combines with oxygen in the presence of palladium foil at 280° C., yielding water 2 on... 

Ammonia is oxidised to oxides of nitrogen by means of oxygen in the presence of palladium foil heated to redness1 and, as has already been pointed out, palladium foil saturated with hydrogen effects the reduction of ferric salts, chlorine water, iodine water, etc., to ferrous salts, hydrochloric acid, and hydriodic acid respectively. Hydrocarbons are oxidised to carbon dioxide and water when passed with air over palladium wire heated to redness. In the absence of air they are decomposed, yielding a deposit of carbon. After a time the palladium becomes brittle, and its surface, seen through a lens, resembles coke.2... 

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. 

Hydrogen permeation flux data of activated and non-activated pure palladium foil membranes (100 pm thick)... 

Examples of dense membranes are palladium foils (selective passage of hydrogen), and films of organic polymers such as polyvinyl alcohol (selective passage of water). 

Deactivation of the catalyst is always an industrially important problem. For fixed-bed reactors, to which class the cross-flow reactors also belong, catalyst poisoning is a particularly delicate matter, since the reactivation is often complicated and expensive. Some poisoning effects may be difficult to explain and understand, and this of course causes extra uncertainty. One example of such poisoning was the observation by Amor and Farris [33] that a special deactivation effect appeared in liquid-phase hydrogenation of toluene using a spiral tubular membrane reactor. Toluene was not hydrogenated at all over the palladium foil used. This phenomenon and reactivation of the catalyst have recently been studied by Ali et al. [56]. 

For the transport of hydrogen through a palladium membrane, please refer to Section 4.2.5.1. The membranes may be prepared as pure palladium membranes, but the trend has moved in the direction of preparing composites and using Pd alloys. There seems to be a number of advantages using composite palladium membranes supported on porous substrates over palladium foils... 

Fig. 1.14 Plot of Ratep/VP(H ) versus for ethylene hydrogenation on a palladium foil.

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