Patent Grant
6855242
1. Technical Field
The present invention relates to the electrochemical production of peroxo-disulfuric acid with the use of diamond-coated electrodes.
2. Related Art
With a normal potential (Eo) of 2.01 V, peroxo-disulfuric acid (H2S2O8) is one of the strongest known oxidizing agents, which is used in a wide variety of fields. The most important areas of application of peroxo-disulfuric acid include etching processes in the electronics industry and the production of particular plastics, for example use in the polymerization of acrylonitrile.
Peroxo-disulfuric acid also has applications in waste treatment, the oxidation of dyestuffs and the bleaching of fibers. In addition to this, peroxo-disulfuric acid is an important intermediate product for the electrochemical production of hydrogen peroxide.
The mechanism of the formation of peroxo-disulfuric acid by the anodic oxidation of sulfuric acid is complex. It is assumed that it comprises the formation of hydroxyl radicals. According to the mechanism water is first of all discharged at the anode with the formation of adsorbed hydroxyl radicals (see Equation 1). The hydroxyl radicals, which are present adsorbed at the diamond surface, react with the hydrogen sulfate ions (see Equation 2) contained in the electrolyte, which form the actual peroxo-disulfuric acid in a subsequent dimerization step (see Equation 3).
H2O→OH.+H++ε− (1)
HΣO4−→HSO4.+e− (2)
(2 HSO4.→H2S2O8). (3).
High concentrations of sulfuric acid and high current densities are required in this case, because when dilute solutions and small current densities are present, the low concentration of discharged sulfate ions results in the latter not reacting with one another (see Equation 3), but with the water, with the formation of oxygen:
SO4+H2O→H2SO4+½O2 (4)
There may furthermore be formed as by-products: oxygen by the decomposition of water, ozone, peroxo-monosulfuric acid and hydrogen peroxide, according to the following (see Equations 5 and 6:
H2S2O8+H2O→H2SO5+H2SO4 (5)
H2SO5+H2O→H2SO4+H2O2 (6).
The effectiveness of the electrochemical peroxo-disulfuric acid production depends substantially on the electrode material used, of which high requirements are made because of the prevailing oxidative and corrosive conditions.
For example, the electrode material must be corrosion-resistant and stable against anodic dissolution.
Furthermore, the peroxo-disulfuric acid formation takes place in a potential range in which water is already decomposed with the production of oxygen. In order to suppress the competing oxygen production, therefore, the electrode material must exhibit a high overvoltage for the reaction.
Flat, large-area platinum electrodes are currently used for the large-scale electrochemical production of peroxo-disulfuric acid, with high sulfuric acid concentrations and high current densities. However, the platinum electrodes are gradually dissolved in the course of the reaction, so that the corrosion products obtained have to be removed by complicated cyclical means.
In order to obtain a satisfactory yield, a highly-concentrated sulfuric acid solution containing 7.5 moles must furthermore be used as electrolyte. Highly-concentrated sulfuric acid solutions of this kind may now, however, because of the oxidative and corrosive properties, be handled in special units and are therefore expensive in equipment terms.
Because of the expensive equipment required, peroxo-disulfuric acid is produced in plants specially equipped for it and has to be procured from there. It would be desirable, however, for peroxo-disulfuric acid to be able to be produced as required directly on site, that is to say at the place of use, since peroxo-disulfuric acid is because of its extremely reactive properties difficult to store and in addition free peroxo-disulfuric acid is subject to rapid hydrolysis in aqueous solution.
Just recently, diamond-coated electrodes have because of their high chemical stability been attracting increasing interest for applications in electrochemical processes.
Such electrodes, in which a boron- or nitrogen-doped diamond layer is applied to a suitable support material, may be obtained in the main by means of the known CVD (Chemical Vapor Deposition) technique.
For example, EP 0 714 997 B1 discloses the use of an electrode of a metal-containing substrate, in particular titanium, to which a boron-doped diamond layer has been applied, for the oxidation of spent photographic baths and in the electronics or optoelectronics sectors.
It has been found, however, that the adhesive strength of diamond layers on metal-containing support materials such as titanium is not satisfactory.
To improve the adhesiveness, therefore, in EP 0 730 043 A1 an intermediate layer is provided between the support material and the diamond layer, which consists of the decomposition products of a metallocene, preferably biscyclo-pentadienyltitanium chloride.
The production of diamond-coated electrodes with silicon as support material for small areas of not more than 1 cm2 is described for example by G. M. Swain in: Adv. Mater. 6 (1994), p.388.
In a number of papers, moreover, a very great potential range is reported for diamond electrodes, in which no water decomposition and hence oxygen production occurs (H. B. Martin, A. Argoitia, U. Landau, A. B. Anderson, J. C. Angus: J. Electrochem. Soc. 143 (1996) L 133; F. Beck, H. Krohn, W. Kaiser, M. Fryda, C. P. Klages, L. Schäfer: Electrochimica Acta 44 (1998) 525).
However, there occurs also there, in the potential ranges favorable for the electrochemical production of peroxo-disulfuric acid, a significant evolution of oxygen, so that a suitability in principle of the electrodes for a peroxo-sulfuric acid production in large, economically significant yields-specifically also with low sulfuric acid concentrations-was not able to be assumed.
It is also reported that ozone may be obtained with diamond electrodes having silicon as support, (A. Perret, W. Haenni, P. Niedermann, N. Skinner, Ch. Comninellis, D. Gandani: Electrochemical Society Proceedings, Volume 97-32 (1997) 275).
The diamond-coated electrodes described above exhibit in general the disadvantage that either the diamond layer may be deposited only on small surfaces (G. M. Swain op. cit.) or, as disclosed in EP 0 730 043 A1, electrochemically stable electrodes with sufficiently firmly adhering diamond layers may be obtained only with the use of a specially applied intermediate layer.
The object of the present invention is to provide a process for the electrochemical production of peroxo-disulfuric acid and peroxo-disulfates, with which peroxo-disulfuric acid or the peroxo-disulfate may be obtained in economically significant yields on a large scale, including with low sulfuric acid concentrations.
The object according to the invention is achieved by a process in which peroxo-disulfuric acid and peroxo-disulfates are produced electrochemically with the use of electrodes coated with doped diamond.
Surprisingly, it was found according to the invention that contrary to expectations electrodes coated with doped diamond are extremely well suited to the electrochemical production of peroxo-disulfuric acid or peroxo-disulfates.
Below the term “peroxo-disulfuric acid” will be used collectively for the compounds peroxo-disulfuric acid and peroxo-disulfates produced.
In particular, if such electrodes are used, sufficiently high yields of peroxo-disulfuric acid may still be obtained even with low sulfuric acid concentrations. The finding is completely contrary to the prevailing view, according to which a high sulfate ion concentration is essential for obtaining high yields and avoiding subsidiary reactions.
Below the electrodes coated with doped diamond will also be referred to by the shortened form “doped diamond electrodes”.
The invention will be explained in detail below with reference to the figures, where:
If doped diamond electrodes are used, satisfactory yields are obtained even with sulfuric acid concentrations as low as only 0.1 mole.
For the process according to the invention, the concentration of the sulfuric acid solution is preferably set in a range of 0.1 mole to 7.5 mole, in particular 1 mole. If the concentration is less than 0.1 mole, the yields become uneconomic. Although the doped diamond electrodes used according to the invention are because of their high stability and electrochemical properties suitable in principle for use in highly concentrated sulfuric acid solutions, a sulfuric acid solution containing more than 7.5 mole may nevertheless be handled industrially only with difficulty.
A current density suitable for the process according to the invention lies in a range of 10 mA/cm2 to 5000 mA/cm2, in particular 100 mA/cm2 to 1000 mA/cm2, preferably 100 mA/cm2 to 400 mA/cm2.
In general it may be stated that the effectiveness rises with increasing concentration of the sulfuric acid solution and increasing current density. The dependence of the effectiveness on the sulfuric acid solution concentration and the current density is shown in FIG. 2. In
Thus there is achieved with the process according to the invention with the use of doped diamond electrodes having an H2SO4 concentration of 1 mole and a current density of only 30 mA/cm2 an effectiveness of 47%, which may be increased to up to 75% if the concentration is increased to 7.5 mole and the current density to 200 mA/cm2.
The diamond electrodes used for the process according to the invention may be of any form. Plate, expanded metal, lattice or mesh electrodes may be used. A so-called expanded metal form is particularly suitable for large-scale plants. Advantageous properties are thereby utilized, such as good electrolyte exchange, economical use of expensive base metals and a largely homogeneous current output due to homogeneously distributed preferred areas for the anode reaction such as tips and edges. In addition, the form may be coated particularly reliably. The electrode form is particularly suitable also for electrolyte solutions with low H2SO4 concentration.
Sintered plate electrodes, which may be porous or dense, may be used as plate electrodes.
According to a particular embodiment it is also possible to use so-called three-dimensional electrodes such as ball electrodes. Ball electrodes may be formed of a multiplicity of coated ball-shaped electrodes, which are swept by the electrolyte in the manner of a fluidized bed.
The cell type is also not subject to any particular limitations. Monopolar or bipolar cells with or without separation or subdivision of the electrode chambers by, for example, ion-selective membranes may be used.
A separation of the electrode chambers by, for example, ion-selective membranes is however to be recommended in order to prevent a cathodic reaction of the peroxo-disulfuric acid formed. The yields may be improved still further by such a measure.
Particularly suitable for the process according to the invention are doped diamond electrodes such as are described in the subsequent printed publications DE 198 42 396 A1 and EP 0 994 074 A2 of the applicant, to which reference is explicitly made here.
Using the measures described there for the known gas-phase deposition process (CVD), sufficiently large support materials and ones with complex shapes (hereinafter called “base bodies”) may be coated with continuous, easily adhering diamond layers.
Thus the homogeneous coating of sufficiently large areas up to a few thousand cm2 in size is possible with the process.
It has been found that with the process described there electrically conductive diamond layers with a specific resistance of an order of magnitude of between 0.005 and 10 Ωcm may be deposited directly on a metallic, graphite or ceramic base material in an electrochemically stable manner.
Examples of suitable metallic base materials are niobium, tantalum, titanium and zirconium, wherein tantalum is particularly preferred. Examples of suitable ceramic base materials are silicon, silicon carbides such as silicon-filtered SiSiC or SiC, and silicon nitride, which exhibit a sufficient conductivity.
Preferably use is made for the base material of a self-passivating material, in particular a self-passivating metal, whereby impairment or damage of the electrode or the base material due to electrolyte solution which could penetrate into the electrode interior through pores or cracks possibly appearing in the deposited layer during the gas-phase deposition is prevented. Examples of such self-passivating metals are the aforementioned elements titanium, niobium, tantalum or zirconium, as well as alloys of the materials or else other self-passivating metals. Titanium, however, is the first choice on cost grounds.
According to the invention the diamond layer may preferably be doped with boron, nitrogen, phosphorus or sulfur, wherein boron and nitrogen are particularly preferred. The content of boron may lie between 0.05 ppm or 10 ppm and 10,000 ppm, preferably between 0.05 ppm and 100 ppm. The content of nitrogen may lie between 5 ppm and 100 ppm.
The diamond electrodes described in the above-mentioned subsequent printed publications DE 198 42 396 A1 and EP 0 994 074 A2 are characterized by a particularly high adhesion of the diamond layer to the base material. It is assumed that the exceptional adhesion is caused by the process-controlled formation of a metal carbide layer at the so-called interface, the transition area between the base material and the layer of diamond, whereby a significantly improved mechanical stability is obtained.
According to the invention it was furthermore established that an improvement in adhesion may also be improved by a carbonitride layer at the interface, wherein particularly good results are observed in this case with ceramic base materials.
In a particularly preferred embodiment, such as that shown in
The center 1 and the envelope 2 together form the base material 1, 2, on which the electrically conductive diamond layer 3 is deposited.
Between the diamond layer 2 and the surface area of the envelope 2, the interface area, is located a carburized metal layer 4 which in the aforementioned example consists of titanium carbide.
Below will be described in principle a gas-phase deposition process, taking as an example the so-called Hot Filament CVD technique, for producing the preferably used electrodes.
Use will generally be made, for the gas-phase deposition of a diamond layer on the base material, of a gas mixture which contains a carbon source, hydrogen and a source for the doping means, which according to the example described here is a boron source.
A preferred carbon source is methane and a preferred boron source tri-methylborate, wherein the compounds are preferably used in the ratio of 1:1. Trimethyl borane may also be used in an amount of 0.05 ppm to 100 ppm.
The boron content of the diamond layer may be adjusted by means of the boron content in the gas phase.
For the present invention the figures for the quantitative proportion of the individual components in the gas phase refer to the volume.
In the preferred embodiment the gas phase consists of 95% to 99.9% hydrogen (H2); 0.1% to 5% methane (CH4); and trimethylborate with a content of about 1 ppm to 1%, wherein the trimethylborate: methane ratio does not exceed 1:1. In the preferred embodiment, the hydrogen concentration preferably ranges from 95% to 99% and the methane concentration ranges preferably from 0.5% to 1%.
A smaller or higher proportion of carbon source may be selected depending on the nature of the carbon source used. For methane a proportion of about of 0.5% to 2% in the gas phase has proved to be particularly advantageous. If the proportion is smaller, the growth rate becomes uneconomic, and if the proportion is too high, the quality of the layer obtained suffers.
It should be borne in mind that the trimethylborate or trimethylborane used as boron source represents simultaneously a further carbon source.
The process pressure is set at 5 to 50 hPa, but may also come to up to 300 hPa if required.
The temperature of the heating or glow wires used (also termed “filaments”) comes as a rule to 2000° C. to 2400° C., where it may in particular for electrodes with ceramic base material also be up to 2800° C. A high activation of the gas phase for the coating process is thereby obtained. Care is however taken on the substrate side that, depending on the material, temperatures of 600° C. to 950° C. are not exceeded.
The setting of the substrate temperature may also be carried out by adapting the filament diameters, the filaments spacings and/or the filament-substrate distance. External heating or cooling may also be used.
The content of boron in the diamond layer comes preferably to between 10 ppm and 10 000 ppm, it may therefore come to up to 1%, wherein the boron content in the diamond layer as a rule lies far below 1%.
Doped diamond layers with a thickness of between 0.5 μm and 50 μm may be obtained with the process described. If the base material is not ceramic, somewhat thicker layers are preferred, for example with a thickness of preferably 2 μm to 50 μm, wherein however smaller thicknesses are also possible.
The carburizing at the boundary layer (interface) between base material 1, 2 and diamond layer 3 deposited thereon may for example be carried out in advance prior to the actual deposition of the diamond layer or alternatively be incorporated in the gas-phase deposition process.
In the first alternative the surface carburizing of the base material metals takes place by their being heated to the process temperature in discrete steps in the presence of hydrocarbon and hydrogen.
If the base material metals are coated without prior separate carburizing according to the second alternative, the presence of methane and optionally trimethylborate in the gas phase results in metal carbide also being obtained in the interface area by virtue of chemical reactions, until, as a consequence of the simultaneously occurring deposition of diamond and the thereby occasioned insulation of the existing metal surface from the methane and the trimethylborate, the metal carbide formation is completed.
A mixed form consisting of both alternatives is also possible for the carburizing. However, the separate carburizing is particularly preferred, since it permits a more targeted control of the process.
If a nitride layer is to be formed as intermediate layer, first and foremost a nitrogen source, preferably nitrogen as such, is added as reactive gas, which reacts with the base material surface, in this case preferably a ceramic base material, with the formation of nitrides.
For the gas-phase deposition the base material, consisting of the center 1 with the copper or aluminum core and the envelope 2 consisting of the preferably passivating metal, is roughened at the surface, for example by sand or ball blasting. The roughening serves to support the adhesion. Thereafter a pre-seeding in a suspension of nanodiamond and 0.5 μm diamond powder in ethanol takes place.
According to a particularly preferred embodiment, doped diamond electrodes are used which have not been subjected to a complete oxidative pre-treatment prior to initial start-up.
For the purpose of the invention the term “complete oxidation” means that the surface of the electrode, which is hydrogenated in the untreated state, is oxidized up to the highest possible oxidation state, it being assumed that carbonyl groups are thereby formed.
Diamond electrodes which have been subjected to such an anodic pre-treatment or polarization are generally considered to be particularly stable and are said to behave unchanged electrochemically over a very long period. Theoretical discussions of this are found in H. B. Martin, A. Arguitia, U. Landau, A. B. Anderson, J. C. Angus, in: J. Electrochem. S.o.k. 143 (1996) L. 133.
Experimental tests have shown, however, that although the desired stabilized state of the electrodes may be achieved by pre-polarization, this is nevertheless at the cost of the intensity and the effectiveness.
Surprisingly, it was found that even non-pre-polarized diamond electrodes, or ones only partially pre-polarized, exhibit a sufficient stability for the production of peroxo-disulfuric acid.
If non-pre-polarized diamond electrodes or partially pre-polarized ones are used, the voltage must be held in a range in which complete pre-polarization does not take place. The process is therefore always operated in the potential range below that voltage at which oxygen may evolve, i.e. polarization occurs. In order nevertheless to obtain the highest possible levels of effectiveness, the voltage should however be kept as close as possible below the potential range.
A closer investigation of the effect showed that the effectiveness of the electrode may be increased if prior to actual start-up it is charged deliberately with a load of approximately 0.1 C/cm2 electrode surface for partial oxidation.
“Partial oxidation” means for the purpose of the invention that the oxidation is stopped at a lower oxidation state than that which is set for complete pre-polarization. It is assumed that in so doing hydroxyl groups form at the electrode surface.
The effectiveness increases up to approximately the load, until with further loading it again decreases significantly and finally drops below the original value as previously described.
In
The cyclogram shows in the solid line the behavior of a diamond electrode after it has been pre-polarized. An oxidative pre-treatment has occurred here, therefore, for example by the application of a very high voltage over a prolonged period.
The other dashed lines of various kinds show the behavior of the diamond electrode preferably used according to the invention without or with a partial pre-polarization. Already during the first cycle a small maximum appears in the range around about 2.2 V, or a turning point at about 2.35 V.
The maximum points to an electrochemical reaction. Only at a still higher voltage does the expected maximum in current density appear due to the increasing evolution of oxygen.
If the same electrode is now subjected to a further cycle, the maximum soon becomes very clear at about 2.2 V and the subsequent minimum at 2.2 V is clearly discernible. The trend continues with further cycles. It becomes strongest in the 6th cycle, in which the maximum has here shifted to about 2.4 V and the subsequent minimum now also occurs at a higher voltage value of about 2.65 V.
In still further cycles the value of the maximum then again decreases significantly. The curve for the 10th cycle is still represented. The maximum shifts further to higher voltage values, but declines in its level. The subsequent minimum also shifts to higher voltage values. The trend continues in further cycles not shown here in the figure.
The non-reversible, electrochemical reaction therefore initially increases in intensity with each cycle, and then decreases again. In the last analysis the solid black line is at the same a trend-like development which is also aimed for as a maximum value in further cycles.
It is clearly discemible in turn that the electrochemical reaction continues to occur only with low intensity after the pre-polarization. The formation of highly reactive oxygen compounds is involved in the reaction.
The reaction also takes place on other electrode materials, but only with the simultaneous formation of oxygen and hence with far lower efficiency.
In practice the potential will not be applied to the electrodes in the form of one or more cycles, as this is very expensive.
It is nevertheless possible by means of the cycles to determine very precisely for a particular electrode type the applied load at which it operates most effectively, that is to say the load from which the effectiveness decreases again.
Considered in detail, the load per unit of area of the electrode surface is involved here. The optimum load amount per unit of area varies, however, with different electrode types. This is because the surface structure, i.e. the crystal orientation or else the form of the electrode, for example, influences the maximum range.
If however the load content is determined experimentally for a particular electrode type by subjection to a plurality of cycles, the load may be applied specifically for further electrodes of the same type, i.e. not in a plurality of cycles, but by corresponding charging over time to each individual electrode of precisely the load.
It has been found experimentally that the optimum range lies at approximately 0.01 to 1, in particular at approximately 0.1, Coulomb per square centimeter (C/cm2).
The background to this unexpected effect, which has not been reported to date in publications, probably lies in the fact that the load application promotes the formation on the crystal surface of OH groups, which then increase the effectiveness of the electrode. If a greater load than for maximum effectiveness is applied, the OH groups possibly start to react with one another and thereby reduce the effectiveness again after reaching the maximum.
It becomes clear from this investigation that the production of reactive oxygen components such as peroxo-disulfuric acid on such diamond electrodes may be influenced advantageously by dispensing with the conventional, that is to say complete pre-polarization. The present process will be illustrated below by means of an example.
Production of an Electrode with a Boron-doped Diamond Layer
A boron-doped diamond layer was produced by means of HF-CVD (Hot Filament Chemical Vapor Deposition) technique on monocrystalline p-Si (100) wafers (0.1 Ωcm, sold under the name Siltronix).
The temperature of the filaments lay in a range of 2440° C. to 2560° C., and the substrate was held at 830° C. Methane was used as a reactive gas in an excess of hydrogen (1% methane in H2). Trimethylborane in a concentration of 3 ppm was used for the doping. The gas mixture was added to the reaction chamber at a flow rate of 5 dm3/min, wherein a growth rate of 0.24 μm/h was obtained for the diamond layer. The diamond layer obtained had a thickness of about 1 μm. Columnar, randomly textured polycrystalline layers were obtained.
Production of Peroxo-disulfuric Acid
Peroxo-disulfuric acid was produced using electrodes obtained according to Example 1. The production took place in a single-cell electrolytic flow cell A (
The electrolysis was carried out under galvanostatic conditions and an electrolyte temperature of 25° C. During the electrolysis the concentration of the peroxo-disulfuric acid was determined by means of iodometric titration and plotted as a function of the specific electric charge (Ah/dm3) used (FIG. 5). The formation of peroxo-disulfuric acid was confirmed by means of the specific Ni(OH)2 test in the presence of silver nitrate in order to avoid disturbing reactions with other oxidizing agents such as H2O2.
In order to prevent the electrochemical reduction of peroxo-disulfuric acid at the cathode and its hydrolysis to peroxo-monosulfuric acid, the electrolysis took place with a low sulfuric acid conversion rate (<5%) and with short electrolysis times (<1 h).
The result is shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 199 48 184 | Oct 1999 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCTEP00/09712 | 10/4/2000 | WO | 00 | 9/25/2002 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO0125508 | 4/12/2001 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4802959 | Lipsztajn | Feb 1989 | A |
| 6375827 | Kurosu et al. | Apr 2002 | B1 |
| 6503386 | Lehmann et al. | Jan 2003 | B2 |
| Number | Date | Country |
|---|---|---|
| 0 714 997 | Jun 1996 | EP |
| 0 730 043 | Sep 1996 | EP |
