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4Hydrogen in metals and alloys
USE OF A CHEMICAL METHOD IN THE PROCESS OF CATALYTIC PALLADIUM SEDIMENTATION AT THE HYDROGEN SENSOR DEVELOPMENT
B. A. Spiridonov, V. N. Ermilin
Voronezh State Technical University 14 Moskovskiy pr., Voronezh, 394026, Russia
The problem of the development of effective catalysts for hydrogenation, reduction by hydrogen is relevant. It is well-known that palladium is one of the few metals with the required properties for this application. For example, one volume of Pd at 80 0C can absorb up to 900 volumes of hydrogen. In the presence of Pd, hydrogen, even at low temperatures in darkness, can easily reduce halogens, transform SO2 into H2S, ClO3 into Cl2, etc. [1]. Palladium has found wide application as a catalyst for hydro- and dehydrogena-tion, isomerisation of organic compounds, the removal of O2 traces in hydrogen, the manufacture of low-frequency electrical contacts, for the manufacture of jewellery products, thermocouple resistors, for the application of protective and decorative coatings, etc. [2].
In preliminary investigations, it has been established that palladium can be used in sensors for monitoring the hydrogen content in gaseous media. In order to develop such sensors, a glass-ceramic material, produced by volumetric glass crystallization, used as the carrier material. The monoc-rystal size is less than 1 mm and the crystalline phase content is within the limits from 25% to 95%. Glass-ceramic materials are characterised by a complex of valuable properties depending upon the nature of oxides generating the crystalline and glass-like phases and by the quantitative ratio of these phases. The majority of glass-ceramic materials have a high mechanical strength, are thermally resistant, have a low thermal expansion coefficient, a high dielectric permeability, water stability and gas permeability. The glass-ceramic materials based on Li2O-Al2O3-SiO2 are optically transparent, those based on MgO-Al2O3-SiO2 are radiotransparent, and those based on Cs2O-Al2O3-SiO2 are fireproof, inert in respect to melted metals [2].
In the application of oxides of certain metals on the glass-ceramic material surface, the composition being generated is characterised by the ability to respond to the hydrogen content in the gaseous phase. One can assume that the introduction of Pd into this composition should enhance the effect obtained and so improve the sensor sensitivity to hydrogen.
As the composition of glass-ceramic material-MeO is a dielectric, it does not seem possible to perform introduction of palladium by the electrochemical method. The most practically suitable method is the chemical method of palladium introduction.
It is well-known from the metallisation of dielectrics that the widest use has been taken by the chemical-electrochemical method [3, 4], which includes three basic processes: surface preparation, electroconductive layer production, application of galvanic coatings. Of these three processes, only the first one, i.e. surface preparation, can be used for palladation. This process includes the following operations:
degreasing, etching, sensitizing and activation. The last operation includes the application of palladium onto the prepared dielectric surface. Activation means that extremely small amount of palladium is applied onto the dielectric surface by the chemical method, which then catalyses the chemical reduction reaction. The generation of the catalytic layer of palladium in the colloidal state is a highly complicated process. It is carried out in two stages: the first stage, called sensitizing involves the application of a solution onto the dielectric surface, which reduces palladium. The second stage involves the immersion into a palladium chloride solution and the reduction of palladium to the metallic state in the solution film adjoining the dielectric surface. The acidic solutions of tin chloride (II) are subjected to hydrolysis with the generation of low-solubility products of Sn(OH)15Cl05 and more complicated chemical compounds. They are fixed on the surface being treated in a layer with the thickness from 10-4 to 10-1 mm. Most often, a solution containing SnCl2 and HCl is used for sensitizing.
For these investigations, model test plates were manufactured from a glass-ceramic material with the dimensions of 30x10x1 mm. The oxides of certain metals (MnO, SnO, etc.) were applied to the surface of these materials by the electronic beam spraying method.
The test plate degreasing was performed in a solution containing trisodiumphosphate, potassium carbonate, sodium hydroxide and the OS-20 surface-active additive at the temperature of 60-70 0C for 10-15 minutes. After washing in hot and running water, the test plates were subjected to etching in a mixture of concentrated H2SO4 and potassium bichromate (20-30 g/m) for 20 minutes. After washing in water, the test pieces were placed into the sensitizing solution containing tin chloride (II) and HCl a for 1-10 minutes depending upon thickness of the oxide metal film applied on the glass-ceramic material surface. After washing in running cold water, the test pieces were placed into the activation solution containing palladium chloride (II) and HCl and were held at room temperature for 1-3 minutes.
It has been established by these investigations that the most positive results have been obtained for the composition of glass-ceramic material-SnO-Pd.
References
1. Akhmetov N. S. Inorganic chemistry.- Textbook for Universities. 2nd edition. M.: Vysshaya shkola, 1975.672 pp.
2. Chemical encyclopedic dictionary/Edited by I. L. Knounyants. - M.: Sov. encyclopedia, 1983. - 792 pp.
3. Il'yn V. A. Metallisation of dielectrics. L.: Mashi-nostroyenie, 1977. - 80 pp.
4. Shalkauskis M. I., Vashkyans A. Chemical metallisation of plastics. L.: Khimiya, 1977. - 135 pp.
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