Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing nonmetal element
Reexamination Certificate
2000-09-01
2002-06-11
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing nonmetal element
C205S618000, C205S620000, C205S622000
Reexamination Certificate
active
06402930
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a novel rhodium sulphide catalyst for reduction of oxygen in industrial electrolysers. The catalyst is highly resistant towards corrosion and poisoning by organic species, thus resulting particularly suitable for use in aqueous hydrochloric acid electrolysis, also when technical grade acid containing organic contaminants is employed.
The invention also relates to a process for the electrolysis of contaminated hydrochloric acid.
Hydrochloric acid is obtained as a waste product in a number of chemical processes. This applies in particular to addition reactions using phosgene, such as in isocyanate chemistry, where the chlorine used issues completely in the form of HCl. Hydrochloric acid is however also formed in substitution reactions, such as for example in the production of chlorobenzenes and chlorotoluenes, in which half of the chlorine used issues in the form of HCl. The third main source of HCl is the thermal decomposition of chlorine-containing compounds, in which chlorine issues completely in the form of HCl. If no direct use exists for the gaseous HCl, such as for example in oxychlorination processes, concentrated hydrochloric acid is formed by absorption in water or dilute hydrochloric acid. Chemically non-usable quantities can be very advantageously recycled to form chlorine by means of hydrochloric acid electrolysis, and in particular by means of hydrochloric acid electrolysis using oxygen-depolarised cathodes.
STATE OF THE ART
The electrolysis of aqueous HCl solutions is a well known method for the recovery of high-value chlorine gas. Aqueous hydrochloric acid is an abundant chemical by-product, especially in chemical plants making use of chlorine as a reactant: in this case, the chlorine evolved at the anodic compartment of the electrolyser can be recycled as a feedstock to the chemical plant. Electrolysis becomes extremely attractive when the standard hydrogen-evolving cathode is substituted with an oxygen-consuming gas diffusion electrode due to the significant drop in energy consumption. The ability of the gas diffusion electrode to operate successfully in this context is crucially dependent on the nature and performance of the catalyst, but also on the structure of the gas diffusion electrode.
Platinum is generally acknowledged as the most effective catalyst for the electroreduction of oxygen in a wide range of conditions; the activation of gas diffusion electrodes with platinum based catalysts is well known in the art, and finds widespread application in fuel cells and electrolysers of many kinds. However, the case of aqueous HCl electrolysis poses some serious drawbacks to the use of platinum as cathodic catalyst, as it is inevitable for the gas diffusion cathode to come at least partially in contact with the liquid electrolyte, which contains chloride ion and dissolved chlorine. First of all, platinum is susceptible to chloride ion poisoning which negatively affects its activity toward oxygen reduction; a second source of poisoning is constituted by contaminant species, especially organic species, which are in most of the cases dissolved in the by-product hydrochloric acid undergoing electrolysis. Even more importantly, the combined complexing action of hydrochloric acid and dissolved chlorine gas changes the platinum metal into a soluble salt which is dissolved away, making this material inappropriate for use in gas diffusion electrodes.
Other platinum group metals appear to follow a similar fate. For example, according to Pourbaix' Atlas of Electrochemical Equilibria in Aqueous Solutions, finely divided rhodium metal dissolves in hot concentrated sulphuric acid, aqua regia, and oxygenated hydrochloric acid. Similarly, (hydrated) Rh
2
O
3
.5H
2
O dissolves readily in HCl and other acids. These problems have been partially mitigated with the disclosure of the rhodium/rhodium oxide based catalyst described in concurrent U.S. patent application Ser. No. 09/013,080. In particular, the rhodium/rhodium oxide system, although slightly less active than platinum towards oxygen reduction, is not poisoned by chloride ions. Also the chemical resistance to aqueous hydrochloric acid with small amounts of dissolved chlorine is sensibly enhanced with respect to platinum. However, an activation step is needed to obtain a sufficiently active and stable form of this catalyst, and some limitations arise when such catalyst has to be included in a gas diffusion electrode; for instance, the chemical and electronic state of the catalyst is changed upon sintering in air, a very common step in gas diffusion electrode preparations known in the art. Cumbersome and/or costly operations have to be carried out to replace this step, or to restore the active and stable form of the catalyst afterwards, as disclosed in U.S. Pat. No. 5,598,197. Furthermore, the required chemical stability is displayed only in the potential range typical of the electrolysis operation; extremely careful precautions have to be taken during the periodical shut-downs of the electrolysers, otherwise the sudden shift in the cathodic potential, combined to the highly aggressive chemical environment, causes the dissolution of a significant amount of catalyst, and the partial deactivation of the remaining portion. While tailored procedures for planned shut-downs of the electrolysers can be set up, although resulting in additional costs, little or nothing can be done in case a sudden, uncontrolled shut-down due to unpredictable causes (for instance, power shortages in the electric network) should occur. There is also no evidence that rhodium/rhodium oxide based catalysts are more insensitive to contaminants with respect to platinum based catalysts.
Technical-grade hydrochloric acid of the kind obtained for example in the above mentioned processes, is usually contaminated with partially chlorinated organic substances, such as for example monochlorobenzene or ortho-dichlorobenzene from the processes themselves, as well as possibly with organic substances from vessel linings, packing materials or pipelines. Such organic substances are obtained for example in the form of surfactants or acrylic esters. The total concentration measured in the form of the TOC can in fact greatly exceed 20 ppm. In the electrolysis of hydrochloric acid using oxygen-depolarised cathodes in initial tests in which platinum was used as the catalyst, the operating voltages were found to be sensitive to the degree of contamination: over a period of several weeks, and in some cases only a few days, an increase in the cell voltage by 150 to 300 mV was observed, a phenomenon which was at least partially reversed during experimental operation using chemically pure hydrochloric acid. Similar results were obtained after switching off the apparatus although the reduction in voltage did however disappear again after a few days. The object was to find a process which avoids this disadvantage of increased operational voltage in the presence of contaminated hydrochloric acid.
The hydrochloric acid typically recycled in production processes usually emerges from several feed streams with corresponding fluctuations in the content of organic or inorganic impurities. Besides the mentioned organic impurities typical inorganic contaminants are in particular sulphates, phosphates and sulphides. One attempt to solve this problem was the purification of technical grade hydrochloric acid using activated carbon. The effect of the reduction in the highly fluctuating TOC from between 20 and 50 ppm to approx. 10 ppm, accompanied by the reduction in the content of chlorinated organic substances to <1 ppm, already produced a considerable improvement in the operation of the cell.
Subsequent purification of the concentrated, approx. 30% hydrochloric acid, with the aid of adsorber resins, allowed a reduction in the content of chlorinated organic substances to below the detection limit of 6 ppb. It was however also found that the non-chlorinated organic substances, which did after all make up the main proportion of impurities,
Allen Robert J.
Czerwiec Daniel
De Castro Emory S.
Gestermann Fritz
Giallombardo James R.
Bell Bruce F.
Bierman, Muserlian and Lucas
De Nora Elettrodi S.p.A.
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