METHOD OF REDUCING THE CATHODIC OVERVOLTAGE OF AN ELECTROLYTIC CELL

Abstract: In a method of reducing the cathodic overvoltage of an electrolytic cell, which contains a cathode compartment, a cathode and a catholyte solution situated within said cathode compartment, an anode compartment, an anode and an anolyte solution situated within said anode compartment and a permeable barrier between said cathode compartment and anode compartment, wherein a) a nobel metal, a heat-treated noble metal, a noble metal compound or a heat-treated noble metal compound; b) copper, heat-treated copper, a copper compound or a heat-treated copper compound; and c) at least one inorganic oxygen-containing compound, being different from components a) and b), or a heat-treated product thereof are introduced into the catholyte solution.

Description: METHOD OF REDUCING THE CATHODIC OVERVOLTAGE OF AN ELECTROLYTIC CELL Background of the Invention The present invention relates to a method of reducing the cathodic overvoltage of an electrolytic cell. Operation of electrolytic cells is carried out on a large industrial scale, for example for the production of caustic and chlorine by electrolysis of brine. To operate an electrolytic cell efficiently and economically, the current efficiency has to be increased and the operating voltage has to be as low as possible. During the operation of electrolytic cells overvoltage effects are observed which mainly occur at the surfaces of the electrodes. As it is well known, one of the overvoltage effects is the hydrogen overvoltage during the electrolysis of brine. Since very large quantities of caustic and chlorine are produced by electrolysis of brine daily, even a small reduction of the hydrogen overvoltage will lead to large savings in energy and money. Accordingly, much effort has been made by the skilled artisan to reduce the overvoltage of electrolytic cells. Various methods for reducing the overvoltage of an electrolytic cell are known. One method consists in roughening the electrode surface, however such surfaces are generally thermodynamically and electrocatalytically unstable. German Offenlegungsschrift DE-A-29 19 981 discloses a cathode for a halogen-alkali electrolysis which is made of copper or a copper alloy and has a roughened surface with a coating of rhodium or a rhodium alloy. The activation of electrodes by galvanic coating of the electrodes with other noble metals or noble metal alloys is also known, but this method is very expensive and, accordingly, not useful in large scale processes. European patent 0 133 468 B1 discloses a process wherein the overvoltage of an electrode is reduced by in-situ activation of the electrode. An electrophoretically depositable activator substance is added to the electrolyte. The activator compound is a chemical compound which consists of i) at least one of the elements B, C, O, S, Se, Te; ii) at least one transition metal; and iii) optionally at least one of the elements of the first and/or second group of the periodic table. The chemical compound is added to the electrolyte in the form of a colloidally disperse suspension. U.S. Patent No. 4,160,704 discloses a method for in situ reduction of cathode overvoltage wherein a low overvoltage ion, such as iron, cobalt, tungsten, nickel, chromium, molybdenum, vanadium, or a noble metal is introduced into the catholyte solution and the low overvoltage metal ions are plated in metallic form on the cathode. While many prior art processes achieve a reduction in cathodic overvoltage, only few prior art references address the problem of long-term stability. Often a reduction in cathodic overvoltage is achieved during a few days or weeks but the reduction gets less and less significant. This is evidently undesirable in industrial, large scale processes. In view of the very large quantities of chemicals, such as caustic and chlorine, that are produced by electrolysis, it would be highly desirable to find a new method of reducing the cathodic overvoltage of an electrolytic cell. It would be particularly desirable to find a method which is stable over an extended period of time. Summary of the Invention One aspect of the present invention is a method of reducing the cathodic overvoltage of an electrolytic cell, containing a cathode compartment, a cathode and a catholyte solution situated within said cathode compartment, an anode compartment, an anode and an anolyte solution situated within said anode compartment and a permeable barrier between said cathode compartment and anode compartment, wherein a) a noble metal, a heat-treated noble metal, a noble metal compound or a heat-treated noble metal compound; b) copper, heat-treated copper, a copper compound or a heat-treated copper compound; and c) at least one inorganic oxygen-containing compound, being different from components a) and b), or a heat-treated product thereof are introduced into the catholyte solution. Another aspect of the present invention is the use of a composition comprising a) a noble metal, a heat-treated noble metal, a noble metal compound or a heat-treated noble metal compound; b) copper, heat-treated copper, a copper compound or a heat-treated copper compound; and c) at least one inorganic oxygen-containing compound, being different from components a) and b), or a heat-treated product thereof for reducing the cathodic overvoltage of an electrolytic cell. Yet another aspect of the present invention is an electrolytic cell, containing a cathode compartment, a cathode and a catholyte solution situated within said cathode compartment, an anode compartment, an anode and an anolyte solution situated within said anode compartment and a permeable barrier between said cathode compartment and anode compartment, wherein the catholyte solution comprises a) a noble metal, a heat-treated noble metal, a noble metal compound or a heat-treated noble metal compound; b) copper, heat-treated copper, a copper compound or a heat-treated copper compound; and c) at least one inorganic oxygen-containing compound, being different from components a) and b), or a heat-treated product thereof. Yet another aspect of the present invention is a catholyte solution comprising a) a noble metal, a heat-treated noble metal, a noble metal compound or a heat-treated noble metal compound; b) copper, heat-treated copper, a copper compound or a heat-treated copper compound; and c) at least one inorganic oxygen-containing compound, being different from components a) and b), or a heat-treated product thereof. Short Description of the Drawing Fig. 1 illustrates the reduction of the cathodic overvoltage over an extended time period according to the process of the present invention and according to a comparative process. Detailed Description of the Invention The present invention provides means for reducing the cathodic overvoltage of an electrolytic cell which is effective to a high degree and over a surprisingly long time period. It has been found that a much more effective reduction of cathodic overvoltage is achieved when the above-mentioned components a), b) and c) are fed to the catholyte solution than when only components a) and b) are fed to the catholyte solution. The weight ratio between component b) and component a) generally is from 10 to 400:1 , preferably from 20 to 300:1 , more preferably from 50 to 200:1 , and most preferably from 100 to 200:1. The weight ratio between component c) and component a) preferably is from 0.5 to 100:1 , more preferably from 2 to 50:1 , most preferably from 5 to 30:1. The weight ratio between component b) and component c) preferably is from 1 to 100:1 , more preferably from 2 to 50:1 , most preferably from 5 to 20:1. It is understood that more than one type of component a) and/or component b) and/or component c) can be fed into the catholyte solution. The total amounts of component a), b) and/or c) are preferably within the ranges indicated above. An optionally heat-treated noble metal or noble metal compound is used as component a). Useful noble metals or noble metal compounds are those of group 8 or 1b of the Periodic Table of Elements, preferably ruthenium, -rhodium, palladium, rhenium, osmium, iridium, platinum or gold. The noble metal is preferably in its elementary form. However, a noble metal compound can also be used, such as Rh(OH) 3 or RhCI 3 . The most preferred noble metal is ruthenium or, more preferably, rhodium. The most preferred noble metal is a rhodium sponge, that means rhodium metal in powder form. Optionally heat-treated copper or an optionally heat-treated copper compound is used as component b). Preferred copper compounds are copper oxides, such as Cu 2 O or CuO; CuCI 2 , CuCI, Cu(OH) 2 or Cu(OH). Cu 2 O is the most preferred copper compound. The third component c) is at least one inorganic oxygen-containing compound, which is different from components a) and b), or a heat-treated product thereof. Preferably, the third component c) is a blend of inorganic oxygen-containing compounds, which are different from components a) and b), or a heat-treated product thereof. Without wanting to be bound to the theory, it is believed that the third component acts as a promoter and stabilizer for reducing the cathodic overvoltage if it is used in combination with components a) and b). Useful inorganic oxygen-containing compounds are for example silicon oxides, preferably silicates like sodium silicate, silicon oxide dispersions or silicon oxide containing compounds and minerals; phosphates; carbonates, such as calcium carbonate; hydroxides or oxides, such as iron oxides, calcium oxide, calcium hydroxide, barium oxide, diatomaceous earth, barium ferrite or beryllium oxide or pumice. Preferably, the inorganic oxygen-containing compound contains an element of the groups 2a, 3a (such as Al), 4a (such as Si) or 5a (such as P) of the Periodic Table of Elements, or a transition metal of the groups 3b, 4b, 5b, 6b, 7b, 8 or 2b of the Periodic Table of Elements other than noble metals. Thereof the oxides and hydroxides are preferred, such as FeO, Fe 2 O 3 , CoO, ZrO 2 , Y 2 O 3 , TiO 2 or La 2 O 3 ; alkaline earth metal oxide or hydroxides, such as BaO, CaO, Mg(OH) 2 , Ca(OH) 2 , or Y 2 Ba 4 Cu 6 O 13 ; or aluminum, silicon, germanium or phosphorus oxides or hydroxides, such as AI(OH) 3 , AI 2 O 3 , SiO 2 or P 2 O 5 . More preferably, an oxide or hydroxide of an element of group 4a is used, such as SiO 2 . Often it is recommended to use more than one inorganic oxygen-containing compound or a heat- treated product thereof in the method of the present invention. Most preferably, an oxide or hydroxide of an element of group 4a, such as SiO 2 , is used in combination with an oxide or hydroxide containing an above-mentioned transition metal other than noble metals, such as Fe 2 O 3 , and/or with an oxide or hydroxide of an element of the groups 2a and/or 3a, such as Ca(OH) 2 and/or AI(OH) 3 . Preferably components a) and b), more preferably components a), b) and c) are used as a physical combination. Preferably at least components a) and b), more preferably components a), b) and c), are subjected to heat treatment at a temperature of from 350°C to 1200°C and to a grinding step before they are introduced into the catholyte solution. The heat-treatment is preferably carried out in the presence of oxygen or an oxygen-containing gas, such as air. A preferred heat-treatment is calcination at a temperature of from 600°C to 1200 °C, more preferably from 700°C to 1000°C. The calcination step b) generally takes 1 to 24 hours, typically 2 to 10 hours. Known grinding devices are suitable, such as hand mortars, ball mills or planetary ball mills. The average particle size of the ground material is preferably from 0.1 to 100 μm, more preferably from 0.5 to 20 μm, most preferably from 1 to 5 μm. For obtaining very good reduction of cathodic overvoltage and stability, it is advisable and highly preferred to carry out the above- mentioned heat-treatment and grinding steps. The heat-treatment and grinding steps can be repeated several times, preferably up to 5 times. When heat-treating and grinding components a), b) and optionally c) several times, the cathodic overvoltage can be further reduced and the reduction is stable over a longer time period in the method of the present invention. The ground material can be suspended in any liquid which is compatible with the catholyte solution, however it is preferably suspended in a caustic solution. The liquid preferably comprises from 0.1 to 50 percent, more preferably from 1 to 20 percent, most preferably from 4 to 10 percent of suspended components a), b) and c), based on the total weight of the liquid and components a), b) and c). The caustic solution preferably is an aqueous solution comprising from 1 to 40 percent, more preferably from 10 to 40 percent, most preferably from 20 to 35 percent, of an alkali metal hydroxide, based on the total weight of the caustic solution. The preferred alkali metal hydroxide is potassium hydroxide or, more preferably, sodium hydroxide. Preferably, the caustic solution comprising components a), b) and c) is agitated in an ultrasonic bath, with a Turrax mixer or with a propeller mixer. Electrolytic cells containing a cathode compartment, a cathode and a catholyte solution situated within said cathode compartment, an anode compartment, an anode and an anolyte solution situated within said anode compartment and a permeable barrier, such as a diaphragm or a membrane, between said cathode compartment and anode compartment are generally known in the art and are not described in more details herein. The present invention is particularly suitable for electrolytic cells wherein the anolyte solution is an aqueous alkali metal chloride, preferably potassium chloride or, more preferably, sodium chloride, and the catholyte solution is an aqueous alkali metal hydroxide. More preferably, the anolyte solution is an aqueous solution comprising from 15 to 26 percent, more preferably from 22 to 26 percent of sodium chloride, based on the total weight of the anolyte solution. The catholyte solution preferably is an aqueous solution comprising from 6 to 40 percent, more preferably from 8 to 35 percent of sodium hydroxide, based on the total weight of the catholyte solution. The cathode is preferably made of steel, nickel, activated steel or nickel with ruthenium, rhenium or other noble metals or their compounds. The liquid, preferably the caustic solution, comprising components a), b) and c) can be directly fed into the cathode compartment of an electrolytic cell or can be fed into a separate container from where it is fed in batches or continuously into the cathode compartment. Alternatively, components a), b) and c) can be fed as a solid into the catholyte solution in the cathode compartment or into a separate container. The feeding can be carried out before or while the electrolytic cell is in operation. Whatever manner of feeding is selected, it is carried out such that the total weight of components a), b) and c) in the catholyte solution in the cathode compartment is generally from 0.01 to 5 percent, preferably from 0.05 to 3 percent, more preferably from 0.1 to 1 percent, based on the total weight of catholyte solution. The total preferred weight of components a), b) and c) per square meter of cathode surface depends on various factors, such as the type of noble metal used and the weight ratio between components b) and a). When using rhodium as component a) and a weight ratio between component b) and component a) of 100 to 200:1 , the total weight of components a), b) and c) preferably is from 10 to 300 g, more preferably from 20 to 250 g, and most more preferably from 30 to 210 g per square meter of cathode surface. When using another noble metal as component a) or another weight ratio between components b) and a), the preferred total weight of components a), b) and c) can be found out by a series of trials based on the information herein. When the electrolytic cell is in operation, it generally takes only a few minutes to achieve a remarkable reduction in cathodic overvoltage. The current density is generally more than 200 A/m 2 , preferably from 400 to 8000 A/m 2 , more preferably from 500 to 6000 A/m 2 , most preferably from 700 to 5000 A/m 2 cathode surface. During the operation of the electrolytic cell, the cathode is coated in situ with components a), b) and c). According to the process of the present invention the cathodic overvoltage can generally be reduced by at least about 50 mV, in most cases by at least about 80 mV. Components a), b) and c) can be added any time prior to or during the operation of the electrolytic cell without interruption of the cell operation. The cell voltage remains substantially stable over an extended period of time, usually at least about 50 days, in most cases at least about 100 days, after the electrophoretic coating of the electrode with the above-described components a), b) and c). The cell voltage can generally be reduced by at least about 50 mV, in most cases by at least about 60 mV. The invention is illustrated by the following examples which should not be construed to limit the scope of the present invention. Unless stated otherwise all parts and percentages are given by weight. Comparative Example A Cu 2 O and a rhodium sponge are mixed in water in a weight ratio of Cu 2 O:Rh:H 2 O of 125:1 :131. The mixture is then dried at 90°C for 6 hours and at 130°C for 6 hours, then calcinated at 850°C in air for 3 - 5 hours. The calcinated powder is ground in a planetary mill at 340 RPM for 4 hours and then suspended in a 32 percent NaOH solution by means of an ultrasonic bath. The solid concentration in the suspension is 7 percent. 10 mL of suspension (containing 0.68 g of suspended solid) is fed into the cathode compartment of a laboratory chlorine diaphragm cell which contains 154 cm 2 cathode surface. The cathode compartment contains about 450 mL of 10 weight percent aqueous sodium hydroxide as a catholyte solution. The cathode has a steel surface. The anolyte is a 22 weight percent aqueous sodium chloride solution. The cell temperature is 75°C. Current of various intensities is fed to the cell, the lowest being 10 A, the highest being 30A. At a higher current intensity a greater overvoltage reduction is achieved than at a lower current intensity. Example 1 Ca(OH) 2 , Fe 2 O 3 , AI(OH) 3 , SiO 2 , Cu 2 O and a rhodium sponge are mixed in water in a weight ratio of Ca(OH) 2 Fe 2 O 3 :AI(OH) 3 :SiO 2 :Cu 2 O:Rh:H 2 O of 2.3:0.6:6.2:6.2:125:1 :131. The mixture was treated according to the procedure described in Comparative Example A. The solid received after the thermal treatment and grinding contained 74.1 percent Cu, 0.674 percent Rh, 1.4 percent Al, 0.82 percent Ca, 0.29 percent Fe, 2 percent Si and the rest is oxygen. The ground solid was suspended in a 32 percent NaOH solution by means of an ultrasonic bath. The solid concentration in the suspension was 7 percent. 10 mL of suspension (containing 0.68 g of suspended solid) is fed into the cathode compartment of a laboratory chlorine diaphragm cell as described in Comparative Example A above. Current of various intensities is fed to the cell, the lowest being 10 A, the highest being 30A. At a higher current intensity a greater overvoltage reduction is achieved than at a lower current intensity. The y-axis in the Fig. illustrates the relative cathode potential in mV before and after feeding the calcinated, ground solid suspended in the aqueous sodium hydroxide solution into the cathode compartment when the cell was operated at 10 A current. The x- axis illustrates the time in days, designated as d. The feeding point 1 of the suspended solid is at the time d = 0. The curve 2 represents the reduction in cathode potential according to Comparative Example A. A decrease in cathode potential of 70 mV was achieved shortly after feeding, however after about 20 days of operation, the remaining decrease in cathode potential was less than 40 mV and the test was stopped. The curve 3 represents the reduction in cathode potential according to Example 1. A decrease in cathode potential of over 100 mV was achieved. After over 90 days of operation, the decrease in cathode potential was still between 80 and 100 mV. The test was continued for 380 days, although not shown in the Fig. After 250 days of operation, the decrease in cathode potential was still 60 mV.

Claims: Claims: 1. A method of reducing the cathodic overvoltage of an electrolytic cell, containing a cathode compartment, a cathode and a catholyte solution situated within said cathode compartment, an anode compartment, an anode and an anolyte solution situated within said anode compartment and a permeable barrier between said cathode compartment and anode compartment, wherein a) a noble metal, a heat-treated noble metal, a noble metal compound or a heat-treated noble metal compound; b) copper, heat-treated copper, a copper compound or a heat-treated copper compound; and c) at least one inorganic oxygen-containing compound, being different from components a) and b), or a heat-treated product thereof are introduced into the catholyte solution. 2. The process of Claim 1 wherein component a) is rhodium, ruthenium, heat-treated rhodium or heat-treated ruthenium. 3. The method of Claim 1 or Claim 2 wherein component b) is a copper oxide or a heat-treated copper oxide. 4. The method of any one of Claims 1 to 3 wherein component c) is a silicon oxide, phosphate, carbonate, hydroxide or oxide or a heat-treated product thereof. 5. The method of any one of Claims 1 to 4 wherein component c) is an oxide or hydroxide of the groups 2a, 3a, 4a or 5a of the Periodic Table of Elements, an oxide or hydroxide of a transition metal of the groups 3b, 4b, 5b, 6b, 7b, 8 or 2b of the Periodic Table of Elements other than noble metals, or a heat-treated product thereof. 6. The method of any one of Claims 1 to 5 wherein the weight ratio between component b) and component a) is from 10 to 400:1 . 7. The method of any one of Claims 1 to 6 wherein the weight ratio between component c) and component a) is from 0.5 to 100:1 8. The process of any one of claims 1 to 7 wherein the total weight of components a), b) and c) in the catholyte solution in the cathode compartment is from 0.01 to 5 percent, based on the total weight of catholyte solution. 9. The process of Claim 8 wherein components a) and b) or components a), b) and c) are subjected to heat treatment at a temperature of from 350°C to 1200 °C and to a grinding step before they are introduced into the catholyte solution. 10. The process of any one of Claims 1 to 9 wherein the anolyte solution is an aqueous alkali metal chloride and the catholyte solution is an aqueous alkali metal hydroxide. 1 1 . Use of a composition comprising a) a noble metal, a heat-treated noble metal, a noble metal compound or a heat-treated noble metal compound; b) copper, heat-treated copper, a copper compound or a heat-treated copper compound; and c) at least one inorganic oxygen-containing compound, being different from components a) and b), or a heat-treated product thereof for reducing the cathodic overvoltage of an electrolytic cell. 12. An electrolytic cell, containing a cathode compartment, a cathode and a catholyte solution situated within said cathode compartment, an anode compartment, an anode and an anolyte solution situated within said anode compartment and a permeable barrier between said cathode compartment and anode compartment, wherein the catholyte solution comprises a) a noble metal, a heat-treated nobie metal, a noble metal compound or a heat-treated noble metal compound; b) copper, heat-treated copper, a copper compound or a heat-treated copper compound; and c) at least one inorganic oxygen-containing compound, being different from components a) and b), or a heat-treated product thereof. 13. A catholyte solution comprising a)a noble metal, a heat-treated noble metal, a noble metal compound or a heat-treated noble metal compound; b) copper, heat-treated copper, a copper compound or a heat-treated copper compound; and c) at least one inorganic oxygen-containing compound, being different from components a) and b), or a heat-treated product thereof for reducing the cathodic overvoltage of an electrolytic cell.

Inventors: MENSCHIG KLAUS-RUEDIGER, ZHOU JIANQING, MENSCHIG, KLAUS-RUEDIGER, ZHOU, JIANQING
Applicants: DOW CHEMICAL CO, MENSCHIG KLAUS RUEDIGER, ZHOU JIANQING, MENSCHIG, KLAUS-RUEDIGER, THE DOW CHEMICAL COMPANY, ZHOU, JIANQING
Application year: 1999
Main classification: C25B15
IPC codes: C25B11, C25B15