Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/28—Per-compounds
  • C25B1/29—Persulfates

Definitions

• the present invention relates to the electrochemical production of peroxodisulfuric acid using diamond-coated electrodes.
• peroxodisulfuric acid H 2 S 2 O 8
• E 0 normal potential
• peroxodisulfuric acid H 2 S 2 O 8
• the most important areas of application for peroxodisulfuric acid include etching processes in the electronics industry and the production of certain plastics, such as use in the polymerization of acrylonitrile.
• Peroxodisulfuric acid is also used in wastewater treatment, oxidation of dyes and fiber bleaching.
• peroxodisulfuric acid is an important intermediate for the electrochemical production of hydrogen peroxide.
• Oxygen can also be formed as by-products by decomposing water, ozone, peroxomonosulfuric acid and hydrogen peroxide according to the following equations:
• the effectiveness of the electrochemical production of peroxodisulfuric acid essentially depends on the electrode material used, to which high demands are made due to the prevailing oxidative and corrosive framework conditions.
• the electrode material must be corrosion-resistant and stable against anodic dissolution.
• the formation of peroxodisulfuric acid takes place in a potential range in which water is already decomposed while generating oxygen.
• the electrode material In order to suppress the competing generation of oxygen, the electrode material must therefore have a high overvoltage for this reaction.
• peroxodisulfuric acid Due to the necessary outlay on equipment, peroxodisulfuric acid is produced in specially designed factories and must be obtained from there. However, it would be desirable to be able to produce peroxodisulfuric acid directly on site, that is to say at the place of use, since peroxodisulfuric acid is difficult to store due to its extremely reactive properties and free peroxodisulfuric acid in aqueous solution is also subject to rapid hydrolysis.
• Such electrodes in which a boron or nitrogen-doped diamond layer is applied to a suitable carrier material, can in principle be obtained using the known CVD (Chemical Vapor Deposition) technique.
• EP 0 714 997 B1 describes the use of an electrode made of a metal-containing substrate, in particular titanium, to which a boron-doped diamond layer has been applied, for the oxidation of used photographic baths and in electronics or optoelectronics.
• EP 0 730 043 A1 therefore provides an intermediate layer between the carrier material and the diamond layer, which consists of the decomposition products of a metallocene, preferably biscyclopentadienyltitanium dichloride.
• ozone can be obtained with silicon microelectrodes with silicon as a carrier (A. Perret, W. Haenni, P. Niedermann, N. Skinner, Ch. Comninellis, D. Gandini: Electrochemical Society Proceedings Volume 97-32 (1997 ) 275).
• the diamond-coated electrodes described above generally have the disadvantage that either the diamond layer can only be deposited on small areas (GM Swain loc. Cit.) Or, as described in EP 0 730 043 A1, electrochemically stable electrodes with a sufficiently firmly adhering diamond layer only using a specially applied intermediate layer can be obtained. It was an object of the present invention to provide a process for the electrochemical production of peroxodisulfuric acid and peroxodisulfates, with which peroxodisulfuric acid or the peroxodisulfate can be obtained on an industrial scale, even at low sulfuric acid concentrations, in economically sensible yields.
• the object of the invention is achieved by a method according to which peroxodisulfuric acid and peroxodisulfate are produced electrochemically using electrodes coated with doped diamond.
• electrodes coated with doped diamond are excellently suitable for the electrochemical production of peroxodisulfuric acid or peroxodisulfates.
• peroxodisulfuric acid is used in summary for the compounds peroxodisulfuric acid and peroxodisulfates produced.
• the electrodes coated with doped diamond are also abbreviated as "doped diamond electrodes”.
• doped diamond electrodes the electrodes coated with doped diamond are also abbreviated as "doped diamond electrodes”.
• FIG. 1 shows schematically the structure of an embodiment of a doped diamond electrode which is preferred according to the invention
• FIG. 2 shows a diagram in which the dependence of the effectiveness of the doped diamond electrodes used according to the invention on the sulfuric acid concentration and the current density is shown.
• FIG. 3 shows a cyclovoitagram of an electrode which is preferably used according to the invention and which has not been subjected to complete oxidative pretreatment
• FIG. 4 schematically shows the electrolytic used according to the example
• the concentration of the sulfuric acid solution is preferably set in a range from 0.1 mol, in particular 1 mol, to 7.5 mol. If the concentration is less than 0.1 mol, the yields become uneconomical. Because of their high stability and electrochemical properties, the doped diamond electrodes used according to the invention are suitable in principle for use in highly concentrated sulfuric acid solutions, but one can Technically difficult to handle sulfuric acid solution with more than 7.5 moles.
• a current density suitable for the method according to the invention is in a range from 10 mA / cm 2 to 5000 mA / cm 2 , in particular 100 mA / cm 2 to 1000 mA / cm 2 , preferably 100 mA / cm 2 to 400 mA / cm 2 .
• the diamond electrodes used can have any configuration.
• Plate, expanded metal, grid or mesh electrodes can be used.
• a so-called expanded metal form is particularly suitable for large-scale plants. This brings out advantageous properties such as good electrolyte exchange, cost-saving use of expensive base metals and largely homogeneous power delivery through homogeneously distributed preferred locations for the anode reaction such as tips and edges. In addition, this form can be coated particularly reliably.
• This form of electrode is also particularly suitable for electrolyte solutions with a low H 2 SO concentration.
• Sintered plate electrodes which can be porous or dense, can be used as plate electrodes.
• so-called three-dimensional electrodes such as spherical electrodes can also be used.
• Spherical electrodes can be formed from a multiplicity of coated spherical electrodes through which the electrolyte flows in the manner of a fluidized bed.
• the cell type is also not subject to any particular restrictions.
• Monopolar or bipolar cells with or without separation or subdivision of the electrode spaces by, for example, ion-selective membranes can be used.
• Doped diamond electrodes as described in the applicant's earlier, unpublished German patent application with official file number 198 42 396.9, which are expressly referred to here, are particularly suitable for the method according to the invention.
• base body With the measures described there for the known gas phase deposition process (CVD), sufficiently large, even complex shaped electrode carrier materials (hereinafter referred to as “base body”) can be coated with continuous, easily adherable diamond layers.
• base body complex shaped electrode carrier materials
• suitable metallic base bodies are niobium, tantalum, titanium and zirconium, with tantalum being particularly preferred.
• suitable ceramic base bodies are silicon, silicon carbides such as silicon-filtered SiSiC or SiC, and silicon nitride, which have sufficient conductivity.
• a self-passivating material in particular a self-passivating metal
• a self-passivating metal is used for the base body, as a result of which the electrode or the base body is impaired or damaged due to electrolyte solution, which may result from pores or cracks in the deposited layer in the gas phase deposition the inside of the electrode could penetrate.
• self-passivating metals are the above-mentioned elements titanium, niobium, tantalum or zircon as well as alloys made of these materials or also other self-passivating metals. For cost reasons, however, titanium is the first choice.
• the diamond layer can preferably be doped with boron, nitrogen, phosphorus or sulfur, boron and nitrogen being particularly preferred.
• the boron content can be between 0.05 ppm or 10 ppm and 10000 ppm, preferably between 0.05 ppm and 100 ppm, and that of nitrogen between 5 ppm and 100 ppm.
• the diamond electrodes described in the above-mentioned German patent application with file number 198 42 396.9 are distinguished by a particularly high adhesion of the diamond layer to the base body. It is assumed that the excellent adhesion through the process-controlled formation of a metal carbide layer at the so-called interface, the transition area between the base body and the layer of diamond, is effected, which results in a significantly improved mechanical stability.
• the electrode can be designed as a composite material electrode, the core 1 of the electrode being e.g. is formed from a copper or aluminum core, which is characterized by particularly high conductivity and relatively low costs.
• This core 1 is covered with a dense casing 2 made of a preferably self-passivating metal, in particular titanium.
• the electrically conductive doped diamond layer 3 is then deposited on this sheath 2.
• the core 1 and the shell 2 together form the base body 1, 2, on which the electrically conductive diamond layer 3 is deposited.
• a carburized metal layer 4 which in the above example consists of titanium carbide.
• a preferred carbon source is methane and a preferred boron source is trimethyl borate, these compounds preferably being used in a ratio of 1: 1.
• Trimethylboron can also be used in an amount of 0.05 ppm to 100 ppm.
• the boron content of the diamond layer can be adjusted via the borate content in the gas phase.
• the information on the proportion of the individual components in the gas phase relates to the volume.
• the gas phase consists of 95% to 99.9%, in particular 99%, hydrogen (H 2 ) and 0.1% to 5%, in particular 0.5% to 1%, methane (CH 4 ) and from trimethyl borate in a proportion of about 1 ppm to 1%, the ratio of trimethyl borate: methane not exceeding 1: 1.
• the proportion of carbon source can be selected to be lower or higher depending on the type of carbon source used.
• a proportion of about 0.5% to 2% in the gas phase has proven to be particularly advantageous for methane. If the proportion is lower, the growth rate becomes uneconomical, if the proportion is too high, the quality of the layer obtained suffers.
• the trimethyl borate or trimethyl boron used as the boron source also represents a further carbon source.
• the process pressure is set to 5 to 50 hPa, but can also be up to 300 hPa if required.
• the temperature of the heating or glow wires used (also referred to as "filaments”) is generally 2000 ° C. to 2400 ° C., although it can also be up to 2800 ° C., in particular for electrodes with a ceramic base body. This ensures a high activation of the gas phase for the Coating process achieved. On the substrate side, however, it is ensured that, depending on the material, temperatures of 600 ° C to 950 ° C are not exceeded.
• the substrate temperature can be adjusted by adjusting the filament diameter, the filament spacing and / or the filament-substrate spacing. External heating or cooling can also be used.
• the boron content in the diamond layer is preferably between 10 ppm and 10000 ppm, ie it can be up to 1%, the boron content in the diamond layer generally being well below 1%.
• doped diamond layers with a thickness between 0.5 ⁇ m and 50 ⁇ m can be obtained.
• the base body is not ceramic, somewhat thicker layers are preferred, for example with a thickness of preferably 2 ⁇ m to 50 ⁇ m, although smaller thicknesses are also possible.
• the carburization at the interface (interface) between the base body 1, 2 and the diamond layer 3 deposited thereon can, for example, be carried out in advance before the actual deposition of the diamond layer or, alternatively, integrated into the gas phase deposition process.
• the surface carburization of the base metals takes place by heating them in discrete steps to the process temperature in the presence of hydrocarbon and hydrogen.
• metal carbide When the base metals are coated without prior carburization according to the second alternative, metal carbide also forms in the interface area due to the presence of methane and possibly trimethyl borate in the gas phase due to chemical reactions, until the simultaneous deposition of diamond and the resulting isolation of the existing metal surface compared to the methane and trimethyl borate, the metal carbide formation is terminated.
• a mixture of both alternatives is also possible for carburizing. However, separate carburization is particularly preferred because it allows more precise control of the process.
• a nitrogen source preferably nitrogen as such, is first added as the reactive gas, which reacts with the base body surface, in this case preferably a ceramic base body, to form nitride.
• the base body consisting of the core 1 with the copper or aluminum core and the casing 2 made of the preferably passivating metal, is roughened on the surface, for example by sand or shot peening.
• the roughening serves to support liability.
• Pre-germination then takes place in a suspension of nanodiamond and 0.25 ⁇ m diamond powder in ethanol.
• doped diamond electrodes are used which have not been subjected to a complete oxidative pretreatment before being put into operation for the first time.
• the expression “complete oxidation” means that the surface of the electrode, which is hydrogenated in the untreated state, is oxidized to the highest possible oxidation level, it being assumed that carbonyl groups form here.
• Diamond electrodes which have been subjected to such an anodic pretreatment or polarization are generally considered to be particularly stable and should remain electrochemically unchanged over a very long time.
• Theoretical considerations can be found in HB Martin, A. Arguitia, U. Landau, AB Anderson, JC Angus, in: J. Electrochem. Sok 143 (1996) L 133.
• experimental studies have shown that the desired stabilized state of the electrodes can be achieved by pre-polarization, but this is at the expense of intensity and effectiveness.
• the voltage must be kept in a range in which complete prepolarization does not take place.
• the method is therefore always operated in the potential range below the voltage at which oxygen can develop, that is, polarization occurs.
• the voltage should be kept as close as possible below this potential range.
• partial oxidation means that the oxidation is stopped at a lower oxidation state than that which occurs during the complete pre-polarization. It is assumed that hydroxyl groups are formed on the electrode surface.
• FIG. 3 This unexpected behavior of diamond electrodes is shown in FIG. 3 using a cyclovoltagram.
• SHE standard hydrogen electrode
• the potential versus a standard hydrogen electrode (SHE) in volts is plotted on the right, the one above Current density in A / cm 2 .
• the temperature prevailing when this cyclovoltagram was recorded was 25 ° C., the counter electrode consisted of platinum and 1n H 2 SO 4 was used as the electrolyte.
• the measuring speed was 200 mV / s.
• the solid line of the cyclovoltagram shows the behavior of a diamond electrode after it has been pre-polarized. So here there has been an oxidative pretreatment, for example by applying a very high voltage over a longer period of time.
• the other differently dotted lines show the behavior of the diamond electrode which is preferably used according to the invention without or with a partial prepolarization.
• the first cycle already shows a small maximum in the area around 2.2 V, or a turning point at around 2.35 V.
• the charge can be applied to other electrodes of the same type in a targeted manner, i.e. not in several cycles, but by correspondingly applying this charge to each electrode. It has been found experimentally that the optimal range is approximately 0.01 to 1, in particular approximately 0.1, coulombs per square centimeter (C / cm 2 ).
• the present method is illustrated below using an example.
• a boron-doped diamond layer was produced by means of HF-CVD (Hot Filament Chemical Vapor Deposition) technology on single-crystal p-Si (100) wafers (0.1 ⁇ cm, sold under the name Siltronix).
• HF-CVD Hot Filament Chemical Vapor Deposition
• the temperature of the filaments ranged from 2440 ° C to 2560 ° C, the substrate was kept at 830 ° C.
• Methane in excess of hydrogen 1% methane in H 2
• Trimethylborane in a concentration of 3 ppm was used for doping.
• the gas mixture was added to the reaction chamber at a flow rate of 5 dm 3 / min, resulting in a growth rate of 0.24 ⁇ m / h for the diamond layer.
• the diamond layer obtained had a thickness of approximately 1 ⁇ m. Column-shaped, statistically textured polycrystalline layers were obtained.
• Peroxodisulfuric acid was produced using electrodes obtained according to Example 1. The production took place in a single-cell electrolytic flow cell A (FIG. 4) with H 2 SO as electrolyte 7 with an electrolyte inlet 8 and an outlet 9 as well as electrical connections 10, 11.
• the diamond electrode was the anode 5 and zirconium the cathode 6. Both electrodes were round with a diameter of 80 mm and an area of 50 cm 2 each. The distance between the electrodes was 10 mm.
• a thermoregulated glass storage container of 500 cm 3 was used for the electrolyte 7 and circulated through the cell A with the aid of a pump.
• the electrolysis was carried out under galvanostatic conditions and an electrolyte temperature of 25 ° C.
• the concentration of peroxodisulfuric acid was determined using iodometric titration and recorded as a function of the specific electrical charge (Ah / dm 3 ) used (FIG. 5).
• the formation of peroxodisulfuric acid was confirmed by means of the specific Ni (OH) 2 test in the presence of silver nitrate to avoid interfering reactions with other oxidizing agents such as H 2 O 2 .

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