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, the residue of a heat-treated noble metal catalyst is 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 of 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. In all practical examples disclosed in the patent, the activator compound contains the noble metal ruthenium or rhenium, which renders the overvoltage reduction expensive. 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. Again, the use of noble metals renders the overvoltage reduction expensive. 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 relatively inexpensive and which can be used on a large scale. 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 residue of an at least partially exhausted, heat-treated noble metal catalyst is introduced into the catholyte solution. 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 contains a residue of an at least partially exhausted, heat-treated noble metal catalyst. Yet another aspect of the present invention is the use of a residue of an at least partially exhausted, heat-treated noble metal catalyst for reducing the cathodic overvoltage of an electrolytic cell. Yet another aspect of the present invention is a catholyte solution containing a residue of an at least partially exhausted, heat-treated noble metal catalyst. Short Description of the Drawings Fig. 1 illustrates the reduction of the cathodic overvoltage over an extended time period according to the process of the present invention. Fig. 2 represents a picture of the surface of a steel cathode in a comparative run. Fig. 3 represents a picture of the surface of a steel cathode in the process of the present invention. Detailed Description of the Invention It has been surprisingly found that the residue of an at least partially exhausted, heat-treated noble metal catalyst can be used for reducing the cathodic overvoltage of an electrolytic cell. The method of reducing the cathodic overvoltage according to the process of the present invention is significantly less expensive than known methods which make use of noble metals. The present invention provides a new use for noble metal catalysts which are at least partially exhausted due to their previous use in other processes. Processes wherein noble metal catalysts typically are used are, for example, the production of trichloroethylene from perchloroethylene, hydrogenation and dehydrogenation processes. By the term "at least partially exhausted" is meant that the catalyst only contains a part of its original activity and selectivity, generally only about 90 percent or less, typically only about 50 percent or less, in most cases even only about 20 percent or less. Noble metal catalysts are generally known in the art. They contain noble metals, preferably a noble metal of Group VIII or IB of the Periodic Table of Elements, more preferably ruthenium, rhodium, palladium, silver, rhenium, osmium, iridium, platinum or gold. The noble metal catalyst can comprise the noble metal in its elementary form or as a noble metal compound. Additionally, the noble metal catalyst may contain a co- catalyst, for example a transition metal, such as Cu, Fe, Co, Zr, Y, Ti, V or La; a transition metal oxide, such as CuO, Cu 2 O, FeO, Fe 2 O 3 , CoO, ZrO 2 , Y 2 O 3 , TiO 2 or La 2 O 3 ; an alkaline earth metal, such as Ba, Ca or Na; an alkaline earth metal oxide or hydroxide, such as BaO, CaO or NaOH; or an aluminum, silicon or phosphorus compound, such as AI(OH) 3 , AI 2 O 3 , SiO 2 or P 2 O 5 . The catalyst can be in the form of a powder or in granular or pelletized form. The noble metal catalyst typically contains a carrier, however its presence is not mandatory. The carrier can be non-combustible, such as aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide, however the carrier is preferably combustible, such as activated carbon. The amount of the noble metal (compound) is generally at least about 0.01 percent, based on the total weight of the noble metal catalyst including the carrier. The preferred amount of the noble metal (compound) depends on the type of carrier. If the noble metal catalyst contains a non-combustible carrier, the amount of the noble metal (compound) is preferably at least about 0.3 percent, more preferably at least about 1 percent, based on the total weight of the noble metal catalyst including the carrier. If the noble metal catalyst contains a combustible carrier, the amount of the noble metal (compound) is preferably at least about 0.01 percent, more preferably at least about 0.05 percent, based on the total weight of the noble metal catalyst including the carrier. The noble metal catalyst which is at least partially exhausted is subjected to heat treatment before it is used in the process of the present invention, generally at a temperature of at least about 350°C. If the catalyst carrier is combustible, such as activated carbon, the noble metal catalyst is preferably treated at a temperature of from 350°C to 1200°C, more preferably from 500°C to 1000°C, most preferably from 500°C to 900°C. If the catalyst carrier is non-combustible, the noble metal catalyst is preferably treated at a temperature of from 500°C to 1200 2 C, more preferably from 600°C to 1000 a C, most preferably from 800°C to 900°C. The heat-treatment is generally carried out in the presence of oxygen or an oxygen-containing gas, such as air. The heat-treatment is preferably carried out during a period of from 1 to 50 hours, more preferably from 2 to 50 hours, most preferably from 4 to 20 hours. It can be carried out in one or more, preferably in one or two steps. If the heat-treatment is carried out in a single step, it is preferably carried out at a temperature of from 500°C to 1000°C, more preferably from 800°C to 900°C. The heat-treatment is preferably carried out in two steps wherein a) the noble metal catalyst is incinerated at a temperature of from 350 2 C to 1000 2 C, preferably from 500°C to 800°C, more preferably from 500°C to 700°C, to obtain an incineration residue, and b) the incineration residue obtained in step a) is calcinated at a temperature of from 600°C to 1200°C, preferably from 700°C to 1000°C, more preferably from 800°C to 900°C. The incineration step a) generally takes 1 to 24 hours, typically 3 to 12 hours. The calcination step b) generally takes 1 to 24 hours, typically 2 to 5 hours. The remaining residue of the at least partially exhausted noble metal catalyst, after the above-described heat-treatment, is preferably ground and suspended in a caustic solution before it is introduced into the cathode compartment of an electrolytic cell. 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. 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 caustic solution preferably contains from 0.1 to 50 percent, more preferably from 1 to 20 percent, most preferably from 4 to 10 percent suspended residue of the at least partially exhausted, heat-treated noble metal catalyst, based on the total weight of the caustic solution. The caustic solution preferably is an aqueous solution containing 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 containing the residue of the at least partially exhausted noble metal catalyst 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 aqueous alkali metal hydroxide. More preferably, the anolyte solution is an aqueous solution containing from 15 to 36 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 containing 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 caustic solution containing the residue of an at least partially exhausted, heat-treated noble metal catalyst 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, the residue of an at least partially exhausted, heat-treated noble metal catalyst 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 concentration of the residue of the at least partially exhausted, heat-treated noble metal catalyst 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 2 percent, based on the weight of the catholyte solution. 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 about 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 residue of an at least partially exhausted, heat-treated noble metal catalyst. Preferably the coating load is from 20 to 300 g/m 2 , more preferably from 50 to 250 g/m 2 , most preferably from 60 to 210 g/m 2 cathode surface. 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. The residue of an at least partially exhausted, heat-treated noble metal catalyst 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 stable over an extended period of time, usually at least about 50 days after the electrophoretic coating of the electrode with the above- described residue of an at least partially exhausted, heat-treated noble metal catalyst. 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. Example 1 An exhausted catalyst was used which contained about 10 percent of copper, about 1 percent of phosphorus and about 0.045 percent of rhodium, with the residual amount being an activated carbon carrier which contained minor amounts of impurities, such as silicon, potassium, calcium, iron and aluminum. The exhausted catalyst was incinerated at 850 e C for three hours. Based on the original amount of the exhausted catalyst, 7.4 percent ash remained. The analyzed ash contained 0.5 percent Rh, 52 percent Cu, 6.3 percent P, 1.4 percent Si, 0.2 percent K, 0.6 percent Ca, 0.2 percent Fe, and 0.2 percent Al, the remaining amount being 0. The ash was ground in a hand mortar to an average particle size of about 2.5 μm. The ground ash was suspended in a 34 weight percent aqueous solution of sodium hydroxide at a ratio of 1 g of ash per 15 g of the sodium hydroxide solution in an ultrasonic bath for 15 minutes. 20 mL of suspension (containing 1.6 g of suspended ash) was fed into the cathode compartment of a laboratory chlorine diaphragm cell which contained 154 cm 2 cathode surface. The cathode compartment contained about 450 mL of 10 weight percent aqueous sodium hydroxide as a catholyte solution. The cathode had a steel surface. The anolyte was a 22 weight percent aqueous sodium chloride solution. The cell temperature was 75°C. Current of various intensities was fed to the cell, the lowest being 10 A, the highest being 30 A. At a higher current intensity a greater overvoltage reduction was achieved than at a lower current intensity. Fig. 1 illustrates the relative cathodic overvoltage and relative cell voltage before and after feeding the ground ash suspended in the aqueous sodium hydroxide solution into the cathode compartment when the cell was operated at 10 A current density. Fig. 1 illustrates that the cathodic overvoltage immediately decreased and that a reduction of the cathodic overvoltage of 80 to 110 mV and a reduction of cell voltage of 60 to 70 mV were achieved over an extended period of time. Fig. 2 is a picture of the surface of a steel cathode after having operated an electrolytic cell for 40 minutes as described in Example 1 at 10 A current intensity, but without the addition of ash to the catholyte solution (Comparative run). Fig. 3 is a picture of the surface of a steel cathode after having operated an electrolytic cell for 40 minutes according to Example 1 at 10 A current intensity, wherein ash had been suspended in the catholyte solution as described in Example 1. The pictures of Fig. 2 and 3 were obtained by light microscopy with 40 times magnification. Example 2 Example 1 was repeated, except that the ground ash was suspended in a 34 weight percent aqueous solution of sodium hydroxide at a ratio of 2 g of ash per 15 g of the sodium hydroxide solution. 20 mL of suspension (containing 3.2 g of suspended ash) was fed into the cathode compartment of a laboratory chlorine diaphragm cell described in Example 1. Current of various intensities was fed to the cell, the lowest being 10 A, the highest being 30 A. At a higher current intensity a greater overvoltage reduction was achieved than at a lower current intensity. A reduction of the cathodic overvoltage of 120 to 150 mV was achieved. Examples 3 to 5 Example 1 was repeated except that the exhausted catalyst was first incinerated at 500°C for three hours. The ash obtained from incineration is then calcinated in a lab Muffle furnace at 550°C (Ex. 2), 700°C (Ex. 3) and 850 2 C (Ex. 4) respectively. The remaining ash was ground in a ball mill to an average particle size of about 2.5 μm, suspended in aqueous sodium hydroxide and fed into the cathode compartment of a laboratory chlorine diaphragm cell as in Example 1. The highest cathodic overvoltage reduction and the best long-term stability was achieved in Example 5 wherein the calcination was carried out at 850°C.

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 residue of an at least partially exhausted, heat-treated noble metal catalyst is introduced into the catholyte solution. 2. The process of Claim 1 wherein the concentration of the residue of the heat-treated noble metal catalyst in the catholyte solution is from 0.01 to 5 percent, based on the weight of catholyte solution. 3. The process of Claim 1 or Claim 2 wherein the anolyte solution is an aqueous alkali metal chloride and the catholyte solution is an aqueous alkali metal hydroxide. 4. The process of any one of Claims 1 to 3 wherein, prior to heat-treatment, the noble metal catalyst contains a noble metal on a combustible carrier. 5. The process of claim 4 wherein the noble metal catalyst additionally contains a transition metal, a transition metal oxide, an alkaline earth metal, an alkaline earth metal oxide, an aluminum compound, a silicon compound or a phosphorus compound. 6. The process of any one of Claims 1 to 5 wherein the noble metal catalyst is heated to a temperature of from 350 to 1200 a C before it is introduced into the catholyte solution. 7. The process of Claim 6 wherein the heat treatment is carried out by a) incinerating the noble metal catalyst at a temperature of from 350°C to 1000 2 C to obtain an incineration residue, and b) calcinating the incineration residue obtained in step a) at a temperature of from 600°C to 1200 2 C. 8. The process of any one of Claims 1 to 7 wherein the residue of the at least partially exhausted, heat-treated noble metal catalyst is ground and suspended in a caustic solution before it is introduced into the catholyte solution. 9. The process of any one of Claims 1 to 8 wherein the electrolytic cell is operated at a current density of more than 200 A/m 2 . 10. 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 contains a residue of an at least partially exhausted, heat-treated noble metal catalyst set forth in any one of Claims 1 to 5. 11. Use of a residue of an at least partially exhausted, heat-treated noble metal catalyst set forth in any one of Claims 1 , 4 and 5 for reducing the cathodic overvoltage of an electrolytic cell. 12. A catholyte solution containing a residue of an at least partially exhausted, heat-treated noble metal catalyst set forth in any one of Claims 1 to 5.

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