Abstract:
Preparation and use of a new electrochemical active substance for the cover layer of metal anodes, which substance is thallium palladate and is obtained as a cardinal-red compound, which is cubic-face-centered, sufficiently characterized in pure crystalline form and, thus, constitutes a new electrochemical active substance for the cover layer of metal anodes.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the preparation and use of thallium palladate as a new electrochemical active substance for the cover layer of metal anodes. 
     Certain patents and patent applications, e.g. German Democratic Republic Pat. No. 55,323, German Democratic Republic Pat. No. 77,963, German Published Application No. 1,671,422, German Published Application No. 1,917,040, German Published Application No. 1,813,944, German Published Application No. 1,962,860, and German Published Application No. 2,200,500 disclose metal anodes containing cover layers of the most varying composition in order to eliminate the drawbacks of the anode carrier members of passivatable metal, such as titanium, tantalum, zirconium, niobium, etc. 
     Most of the above substances, however, suggested for application as cover layer or as essential component thereof have unfortunately turned out to be disadvantageous, because their electrical conductivity and/or their chemical and electrochemical stability is not satisfactory or, respectively, a number of the suggested substances cannot at all or not very easily be applied to the anode carrier member without adversely affecting the requirements made respecting a cover layer. 
     Whereas for example alkaline earth palladates such as CaPd 3  O 4 , SrPd 3  O 4  and NaPd 3  O 4  could already be synthesized in pure crystalline form, a preparation and adequate characterization of TlPd 3  O 4  was impossible till now. 
     DESCRIPTION OF THE INVENTION 
     It is therefore an object underlying the invention to develop a preparation and adequate characterization of a new electrochemical active substance for the cover layer of metal anodes. 
     It is a further object of the invention to make the new electrochemical active substance adaptable for the use as cover layer of metal anodes. It is moreover a specific object to be solved by the invention to prepare and use cardinal-red thallium palladate for the cover layer of metal anodes, which thallium palladate has favourable electrochemical and electrical properties as active material for coating metal anodes. 
     According to the invention it is possible to obtain TlPd 3  O 4  as cardinal-red compound in stoichiometric composition. The reaction takes place already at 500° C. A gas solid-reaction is concerned. As is known, at this temperature the equilbrium ##STR1## IS PRESENT. 
     The preparation is conducted according to the reaction: ##STR2## 
     The direct reaction of Tl 2  O 3  with PdO also leads to the same result. 
    
    
     Below an example for the preparation of the above new electrochemically active substance is given. 
     EXAMPLE 
     1 Mol PdO is intimately mixed with 4 to 5 Mol of TlNO 3  and is slowly heated to 500°-600° C in sintered corundum trays. In this connection, by decomposition of the thallium nitrates above 300° C the cubic Tl 2  O 3  results in jet-black crystals which react with PdO starting from 500° C. Excess Tl 2  O 3  is removed by sublimation at 650° C; palladium by boiling with aqua regia. The cardinal-red reaction product according to guinea photos contains no further impurities. 
     The analyses of this compound is complicated in so far as TlPd 3  O 4  is well soluble only in hydrobromic acid, however, the presence of bromide ions is disturbing in the course of the analyses by the formation of the TlBr that is difficult to dissolve and PdBr 2 . Thus, it was necessary to boil the hydrobromic acid solution of the TlPd 3  O 4  with a few drops of elementary bromine in order to bring the TlBr which always precipitates in smaller amounts in solution as TlBr 3  ; subsequently to quantitatively expel bromine by boiling with concentrated HNO 3  and, since the oxidizing properties of the nitric acid also adversely affect the applied precipitation with organic reagents, to fume them off with sulphuric acid. The gravimetric determination of the thallium was conducted by precipitation with thionalide. Palladium is precipitated with dimethylglyoxim. 
     
         ______________________________________TlPd.sub.3 O.sub.4      content of Tl  content of Pd______________________________________calculated 34,8 %         54,3 %test results      34,6 %         54,8 %      34,9 %         54,6 %      34,9 %         54,5 %______________________________________ 
    
     TlPd 3  O 4  under decomposition is readily soluble in constantly boiling hydrobromic acid, is moderately soluble in aqua regia and perchloric acid, is difficult to dissolve in sulphuric acid, nitric acid, formic acid and alkaline solvent. Above 750° C TlPd 3  O 4  is decomposed while forming elementary palladium and Tl 2  O 3 . The pyknometric density was found to be 8.99 g/cm 3  and is well concurring with the X-ray density of 8.83 g/cm 3 . The radiographic evaluation for TlPd 3  O 4  resulted in a cubic elementary cell: 
     
         ______________________________________lattice constant          a        =      9,596 ± 0,002 Avolume of the elemen-          V.sub.Ez =      883,68 A.sup.3tary cellnumber of formula          Z        =      8unitsX-ray density  S.sub.Ro =      5,11 oz/cu.in.______________________________________ 
    
     From the crystal lattice plane statistic the extinction conditions resulted 
     
         ______________________________________    hKl: h + k   = 2n-1         k + 1   = 2n-1         (h + 1) = 2n-1______________________________________ 
    
     On the basis of these extinction conditions, the space groups Fm3m--O h   5 , F432--O 3 , F43m -- T d   2 , Fm3 T h   3  and F23--T 2  come into consideration for TlPd 3  O 4 . 
     The raster-electromicroscopic examination resulted in the presence of partially twinned octahedrones. 
     Viewing the results in combination, they confirm the presence of a cubic face-centered structure. 
     If one now applies the TlPd 3  O 4  to alloyed or unalloyed metal anode base members together with co-adhesives the electrolyses of NACl--, KCl, chlorate- and HCl-solutions can be conducted, as well as of course other electrochemical processes. 
     A satisfactory electrochemical effectiveness is ensured already when 20% are present in the cover layer. In permanent operation (10.000 A/m 2 ) a thus coated metal-anode worked well over a period of time of 12 months without showing an alteration of the electrical properties as current yield, cell voltage, or electrochemical activity, such as chlorine deposition, resistance of the cover layer against wear. 
     It is, of course, to be understood that the present invention is, by no means, limited to the particular example, but also comprises any modifications within the scope of the appended claims.