Title of Invention

PROCESS FOR THE ELECTROLYSIS OF TECHNICAL-GRADE HYDROCHLORIC ACID CONTAMINATED WITH ORGANIC SUBSTANCES USING OXYGEN-CONSUMING CATHODES.

Abstract TITLE: PROCESS FOR THE ELECTROLYSIS OF TECHNICAL-GRADE HYDROCHLORIC ACID CONTAMINATED WITH ORGANIC SUBSTANCES USING OXYGEN-CONSUMING CATHODES. A process for electrolysis an aqueous solution of hydrochloric acid to chlorine in an electrochemical cell provided with an anode compartment and a cathode compartment including at least one gas diffusion cathode comprising an electrically conductive web provided on at least one side thereof with a coating of a catalyst for the electroreduction of oxygen comprising rhodium sulphide and optionally containing at least one fluorinated binder incorporated therein, comprising introducing aqueous hydrochloric acid containing contaminant species into the anode compartment and oxygen intot he cathode compartment while impressing a direct electric current on the cell.
Full Text process For The Electrolysis Of Technical-Grade Hydrochloric Acid Contaminated
With Organic Substances Using Oxygen-Consuming Cathodes
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 HCI.
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 HCI. The third main source of HCI is the thermal
decomposition of chlorine-containing compounds, in which chlorine issues
completely in the form of HCI. If no direct use exists for the gaseous HCI, 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 HCI 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
HCI 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) Rh2O3?5H2O dissolves readily in HCI
and other acids. These problems have been partially mitigated with the disclosure of the
rhodium / rhodium oxide based catalyst described in concurrent U.S. Pat. Application
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. Patent
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 1o 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 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, rapidly exhaust the adsorptive capacity of the adsorber resin
at the high impurity contents, so that these organic substances break through the
adsorption column and have a negative effect on the operating voltage of the
electrolysis. The cell voltage increases accordingly. The regeneration of the
adsorber resin with methanol according to the manufacturers" specifications would
be relatively laborious and, given the above contents of impurities, would have to be
carried out every few days. Due to the risk of explosion which must be taken into
account the adsorber resin container would have to be removed and regenerated
externally.
If the hydrochloric acid does however stem from a direct connection to an
isocyanate unit the content of impurities is considerably lower and consists
essentially of the constituents mono- and dichlorobenzene, which can be removed
very successfully by means of activated carbon as well as adsorber resins to levels
below the detection limit, and the regeneration cycles of the adsorber resin packing
extend to several months up to about half a year, depending on the content of
impurities.
Tests with platinum catalysed oxygen-depolarised cathodes all showed a
similar high sensitivity towards organic impurities. In tests using rhodium oxide-
catalysed oxygen-depolarised cathodes the sensitivity towards organic substances
was found to be slightly less, although it was still quite considerable. The rhodium
oxide catalyst had been developed in order to be able to dispense with polarisation
upon switching the apparatus off. This catalyst did however reveal in tests that its
structural stability was not sufficient. Thus the activation of an electrode in which
this catalyst was used decreased by approx. 30% within only a few weeks.
OBJECTS OF THE INVENTION
It is an object of the invention to provide a novel catalyst for oxygen reduction
having desirable and unexpected chemical stability towards highly corrosive media.
It is another object of the invention to provide a novel catalyst for oxygen
reduction having desirable and unexpected electrocatalytic activity in presence of
organic contaminants.
It is another object of the invention to provide novel gas diffusion electrodes with
a novel catalyst therein having desirable and unexpected electrocatalytic properties.
It is another object of the invention to provide a novel electrolytic cell containing a
gas diffusion electrode of the invention and to provide an improved method of
electrolysing hydrochloric add to chlorine.
These and other objects and advantages of the invention will become obvious
from the following detailed description.
THE INVENTION
A more effective catalyst having the advantages of the chemical stability of
rhodium in the presence of hydrochloric acid is rhodium sulphide. Test electrodes in
which RhSx is used as a catalyst displayed the expected stability after switching off
the electrolysis, without polarisation, and the required resistance to catalyst losses
due to washing out.
It was however surprisingly found that electrodes in which RhSx is used as the
catalyst are almost completely non-sensitive to the broad spectrum of organic and
inorganic impurities. Whereas Pt-catalysed electrodes underwent an increase in
the operational voltage of up to 260 mV within ten days, even when purified
hydrochloric acid was used, and RhOx-catalysed electrodes also underwent an
increase of 100 mV under similar conditions, tests using RhSx-catalysed electrodes
and purified hydrochloric acid revealed only a slight increase of about 20 mV
compared with cells operated with chemically pure hydrochloric acid and only an
increase of about 40 mV compared with the value obtained using purified
hydrochloric acid even when completely non-purified hydrochloric acid was used.
This increase proved to be reversible when purified acid was once again
subsequently used. The difference in the operation of the cell when purified
technical-grade hydrochloric acid was used as opposed to chemically pure
hydrochloric acid has also been demonstrated in additional tests to be between a
non-detectable increase in voltage and a maximum increase of 30 mV in the
operating voltage of a cell operated under typical electrolysis conditions (current
density: 5 kA/m2, operating temperature: 70°C, 13 - 14% HCI).
It is thus by all means advantageous for the technical-grade hydrochloric acid
to be pre-purified via an activated carbon line and possibly in addition via an
adsorber resin bed, in order to avoid even small increases in the operating voltage.
Purification is at any case recommendable, in order to avoid the further reaction of
mono- and dichlorobenzene at the anode to form hexachlorobenzene, since the
latter is deposited|as a solid in the electrolysis unit and the hydrochloric acid loops
and can lead to problems especially in valves and pumps after long periods of
operation.
An additional finding is noteworthy: oxygen depolarised cathodes of the flow-
through type in which the carbon fabric was directly catalysed and which have an
open structure, were able to be operated continuously at up to 5 kA/m2 not only with
. pure oxygen but also with air or depleted oxygen and using organically contaminated
hydrochloric acid. The other type used, in which the catalyst is applied to the carbon
fabric in a form embedded in electrically conductive carbon dust (the single-sided
type) already reached its limits at a content of nitrogen in the oxygen of approx. 30%:
The operating voltage was 300 to 350 mV higher and thus already on the borderline
of effective operation.
The novel electrochemical catalyst of the invention is comprised of rhodium
sulphide, which may be either supported on a conductive inert carrier or unsupported.
This catalyst does not require any activation step prior to its use, and surprisingly retains
all of its electrocatalytic activity towards oxygen reduction in presence of chloride ions
and organic molecules. Moreover, the catalyst is surprisingly not dissolved by the
complexing action of aqueous hydrochloric acid / chlorine mixtures, thereby requiring no
particular precautions during shut-downs when used in hydrochloric acid electrolysers.
The catalyst is preferably coated on at least one side of a web, and may be used alone,
with a binder, blended with a conductive support and a binder, or supported on a
conductive support and combined with a binder. The binder may be hydrophobic or
hydrophilic, and the mixture can be coated on one or both sides of the web. The web
can be woven or non-woven or made of carbon cloth, carbon paper, or any conductive
metal mesh.
Examples of high surface area supports include graphite, various forms of carbon
and other finely divided supports but carbon black is preferred.
Such catalyst coated webs can be employed as gas diffusion cathodes exhibiting
cell voltages, current densities and a lifetime that could not be previously obtained under
normal operating conditions, especially when used in highly aggressive environments
and with low purity reactants, such as the case of electrolysis of by-product hydrochloric
acid.
The catalyst may be easily prepared upon sparging hydrogen sulphide gas in an
aqueous solution of a water soluble rhodium salt. Nitrogen gas may be used as a carrier
for hydrogen sulphide, and a pure nitrogen flow may advantageously be used to purge
excess hydrogen sulphide upon completion of the reaction. The resulting solids are
recovered by filtration, washing and drying to constant weight at 125°C, for example. The
rhodium sulphide obtained in this way is unsupported (unsupported catalyst). However,
when the aqueous solution of the water soluble rhodium salt further contains a
suspension of a suitable conductive support, then the rhodium sulphide is preferentially
deposited as tiny particles on the surface of the conductive particles (supported
catalyst). The resulting hydrated form of rhodium sulphide must be heated in an inert
atmosphere at 550 to 650°C, and preferably above 600°C to form an anhydrous form of
rhodium sulphide catalyst. The heating may be for several hours depending on the size
of the batch, and the choice of the temperature is crucial for the formation of a sufficiently
stable catalyst.
If the temperature is too low such as 300°C, the resulting crystallites are not well-
defined and the catalyst stability is not sufficient. If the temperature is too high, i.e.,
725°C, the unsupported catalyst has excellent acid stability but is not electrically
conductive enough.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic of reaction set-up for the generation of supported or unsupported
rhodium sulphide.
Figures 2 shows X-ray diffraction patterns for rhodium sulphide precursors as a function
of oven temperature. Trace 1: 30% RhSx on carbon, dried at 125°C. Trace 2: 30%
RhSx on carbon, 300°C in argon. Trace 3: 30% RhSx on carbon, 650°C in argon.
Figure 3 is a schematic of flow system for the generation of Cl2 from HCI using an
oxygen depolarised gas diffusion electrode.
Figure 4 shows typical platinum catalyst data, incorporated in a standard ELAT™
structure with 30% Pt/C, 1.1 mg/cm2, coated with 0.70 mg/cm2 Nafion, operating in
HCI/Cl2 solution at 3 kA/m2. ELAT is a trademark of E-Tek, Natick (MA), U.S.A. which
identifies gas diffusion electrodes comprising a carbon web and a mixture of catalyst and
fluorinated binder incorporated therein.
Figure 5 shows data obtained with rhodium-rhodium oxide , incorporated in a single-
sided ELAT™ structure with 30% Rh/C, 1.01 mg/cm2, coated with 0.70 mg/cm2 Nafion,
operating in HCl/CI2 solution at 3 kA/m2.
Figure 6 shows data obtained with 30% RhSx/C, incorporated in a single-sided EU\T™
structure with 30% Rh/C, 1 mg/cm2, coated with 0.70 mg/cm2 Nafion, operating in HCI/CI2
solution at 3 kA/m2.
Figure 7 shows potentiostated current - cathode potential curves for samples of single-
sided ELAT™ with 1 mg Pt/cm2, 30% Pt/C in 0.5 M H2SO4, at 70+/- 2°C, with and without
methanol. Methanol is added as 1, 5, or 10% by volume. Platinum foil 3 cm x 2 cm
serves as the counter electrode. A standard calomel electrode serves as the reference.
Reported potentials are corrected for IR using the current interrupt method.
Figure. 8 shows potentiostated current - cathode potential curves for of single sided flow-
through electrode as in Example 4 with 1.05 mg/cm2 Rh as RhSx, 30% RhSx/C in 0.5 M
H2SO4, at 70+/- 2°C, with and without methanol. Methanol is added as 1, 5, or 10% by
volume. Platinum foil 3 cm x 2 cm serves as the counter electrode. A standard calomel
electrode serves as the reference. Reported potentials are corrected for IR using the
current interrupt method.
Figure 9 shows the experimental set-up for the high current density and upset
temperature comparative testing of the various oxygen depolarised cathode (ODC) types
with continuous temperature and concentration control. The effective cell area was 100
cm2.
Figure 10 shows the behaviour of the cell voltage of a Platinum catalysed ODC
during an electrolysis of chemical pure and technical grade hydrochloric acid of
different provenience, of an isolated isocyanate plant and a multi plant input site
system with different sources of hydrochloric acid which was purified respectively
with activated carbon and subsequently with an adsorption resin of type OC 1066
manufactured by Bayer AG, Germany.
Figure 11 shows the results of the electrolysis of hydrochloric acid with an ODC with
rhodium/rhodium oxide catalyst. Chemical pure and technical grade hydrochloric
acid, purified with activated carbon and subsequently with a resin of type EP63
manufactured by Bayer AG, was electrolysed.
Figure 12 shows the results of the electrolysis of different kinds of hydrochloric acid
with the new catalyst on rhodium sulphide basis in comparison to a platinum
catalysed ELAT.
Figure 13 shows the results of rhodium sulphide catalysed ODC in alternating
operation with chemical pure and technical grade hydrochloric acid, purified with
activated carbon
Figure 14 shows the long time behaviour of the catalyst in a 4 element pilot
electrolyser with an active area of 0.85 m2.
Figure 15 shows the comparative electrolysis of technical grade hydrochloric acid,
pre-purified with activated carbon, in a laboratory cell with pure oxygen and with air.
The ODC was of the flow through type.
Figure 16 shows the sensitivity of different type morphology ODC against operation
with depleted oxygen: a carbon powder carried catalyst (single sided type) and a
directly catalysed electrically conductive web (flow through type)
In the following examples, there are described several preferred embodiments to
illustrate the invention. However, it is to be understood that the invention is not intended
to be limited to the specific embodiments.
EXAMPLE 1
100 grams of supported rhodium sulphide were prepared by the following procedure:
57.3 grams of RhCI3xH2O (39.88% given as rhodium metal) were dissolved in 2 litres of
de-ionised (D.I.) water, without any pH adjustment. 53.4 grams of Vulcan XC-72 active
carbon were added, and the mixture was slurried with a magnetic stinrer.
Hydrogen sulphide gas was then sparged through the slurry at ambient temperature
using nitrogen as a carrier gas, according to the scheme of Figure 1. The mixture has
been allowed to react as described for 7 hours. Upon completion of the reaction,
nitrogen was purged through the system to remove residual H2S. The remaining
solution was vacuum filtered to isolate the solids, which were then washed with de-
ionised water and dried at 125°C to a constant weight.
The resulting catalyst cake was finally ground to a fine powder and subjected to 650°C
under flolving argon for two hours. A load of catalyst on carbon of 30%, given as rhodium
metal, was obtained.
As already stated before, this final thermal treatment is a crucial step in the preparation
of the desired stable and active metal sulphide. Figure 2 shows the development of a
preparation of rhodium sulphide as outlined above as a function of treatment
temperature. In particular, figure 2 shows the results of a powder sample XRD scan on
a.) the supported catalyst after filtration and drying, b.) the supported catalyst of a.) after
heating to 300°C in argon, and c.) the supported catalyst of b.) after heating to 650°C.
The increase in number and clarity of peaks in these scans indicates the formation of
well-defined crystallites containing rhodium and sulphur. These changes induced in the
XRD spectrograph by the temperature treatment also reflect corresponding substantial
gains in catalyst stability.
EXAMPLE 2
8 grams of unsupported rhodium sulphide were prepared by the following procedure:
12.1 grams of RhCI3 xH2O (39.88% given as rhodium metal) were dissolved in 700 ml of
de-ionised water, without any pH adjustment. Hydrogen sulphide gas was then sparged
through the slurry at ambient temperature using nitrogen as a carrier gas, according to
the scheme of Figure 1. The mixture has been allowed to react as described for 4 hours.
Upon completion of the reaction, nitrogen was purged through the system to remove
residual H2S. The remaining solution was vacuum filtered to isolate the solids, which
were then washed with de-ionised water and dried at 125°C to a constant weight. The
resulting catalyst cake was finally ground to a fine powder and subjected to 650°C under
flowing argon for two hours.
COMPARATIVE EXAMPLE 1
A rhodium oxide/rhodium catalyst on Vulcan XC-72 was prepared following the
method disclosed in co-pending U.S. Patent Serial No. 09/013,080 (26 Feb. 98) and
herebelow repeated. 9.43 g of RhCI3?xH2O (39.88% given as rhodium metal) were
dissolved in 2 litres of de-ionised water at room temperature, and the resulting solution
was added to a dispersion of 8.75 g of Vulcan XC-72 in 500 ml of D.I. water. The
mixture was stirred to maintain a uniform carbon slurry while slowly adding (2-3 ml/min) a
0.5 molar solution of ammonium hydroxide. Besides the 220 ml of ammonium hydroxide
theoretically required to form Rh(OH)3, a 20% excess of ammonium hydroxide was
added to set a basic environment. The basic slurry was then stirred at 60-70°C for 30-
60 minutes and filtered hot. The filter cake was washed with about 200 ml D.I. water at
60-70oC and dried in air at 125°C for 15 hours.
The resulting cake was then ground to a fine powder and heated at 650°C under flowing
argon gas to dehydrate and stabilise the catalyst. The load of catalyst on carbon was
30%, given as rhodium metal. The catalyst powder was further subjected to an
activation step by heating at 500°C for 30 minutes under flowing hydrogen gas to further
reduce some of the rhodium oxide to rhodium metal. As emphasised in co-pending U.S.
Patent Serial No. 09/013,080 (26 Feb. 98), activation of rhodium-rhodium oxide catalyst
Is essential to obtain the most active form of this catalyst.
COMPARATIVE EXAMPLE 2
100 grams of supported platinum sulphide were prepared according to the
procedure of the above Example 1, whereby a solution of chloroplatinic acid was
employed instead of the rhodium chloride salt.
EXAMPLE 3
The catalysts of all the above reported examples, along with commercially available
platinum on Vulcan XC-72 (for example from E-TEK, Inc.), can be utilised in several
different configurations. The catalyst of this invention is not limited by the structure of
the gas diffusion electrode: for instance, in the present case, each catalyst of the above
examples and comparative examples was incorporated in four different types of
electrode structure, thereby obtaining sixteen different samples, according to the
following procedures:
a) ELAT: A web of carbon cloth with a warp-to-fill ratio of unity and about 25 to 50 yarns
per inch, and a 97-99% of carbon content was selected from a commercially available
product with a thickness of 10 to 15 mils. Carbon cloth with a thickness of 5 to 50 mils
could have advantageously been used for this purpose. A mixture of fluorinated polymer
(polytetrafiuoroethylene, P.T.F.E., commercialised by DuPont under the trademark
Teflon®) and Shawinigan Acetylene Black (SAB) carbon, commercialised by Cabot
Corp., was coated on each side of the carbon cloth, air drying at room temperature after
each coat, until reaching at a total loading of 8 to 10 mg/cm2. A mixture of the powdered
catalyst and Teflon® was then applied on one side of the carbon web in multiple coats
until obtaining a layer of 0.5 to 2 mg of catalyst per square cm. After the final coat, the
carbon cloth was heated to 340°C for 20 minutes.
b). Single-sided ELAT: The above procedure for preparation of the ELAT was repeated
except the SAB/Teflon® mixture was applied to only one side of the carbon cloth, with a
loading of 4 to 5 mg/cm2. The catalyst coat was applied on the same side, on top of the
SAB/Teflon® layer.
c). Flow-through Electrode: A carbon cloth with the same specifications for the ELAT
electrode was selected and 2 to 5 coats of a mixture of catalyst powder and Teflon® were
applied to one side thereof. The coated fabric was then heated at 340°C for about 20
minutes to obtain 1.03 mg/cm2 of rhodium metal. The final heating step or sintering step
is believed to melt the Teflon® and distribute it across the carbon catalyst. However, the
sintering step may be successfully omitted for this electrode.
d). Membrane Electrode Assembly: An ink was formulated consisting of approximately
3 parts catalyst and 1 part (as dry weight) Nafion® ionomer, such as that sold by
Solutions Technology, (Mendenhall, Penn.) as a suspension in a mixture of water and
lower aliphatic alcohols such as methanol, propanol, and/or butanol. The ink was applied
to a Nafion® 324 ion exchange membrane, commercialised by DuPont, held in place with
a heated vacuum table, via spraying or painting. Other ion exchange membranes known
in the art may have alternatively been utilised. Subsequent layers of the ink were applied
until depositing 0.05 to 1 mg metal/cm2 of catalyst. The assembly was further heated to
remove solvents, and assembled with an appropriate electrode backing such as those
disclosed in co-pending Patent Serial Number 09/184,089 (30 October 98). The catalyst
ink as described could alternatively have been applied to an electrode backing,
subsequently heated to remove solvents and assembled with an ion exchange
membrane to form an equivalent membrane electrode assembly.
EXAMPLE 4
Prior to incorporation in gas diffusion electrodes, the resistance of this invention"s
catalyst to corrosive media such as boiling solutions of HCI/CI2 can be simply determined
and compared to prior art catalysts as well as rhodium sulphide prepared at various
temperatures. One to five grams of the catalysts of Table 1 were placed in a 250 ml
beaker containing 130g/l chlorine-saturated HCI and heated to boiling. The formation of
a deep colour indicates the dissolution of the metal from the catalyst, thus providing
evidence for whether the catalyst would be appropriate for use in systems for the
recovery of chlorine from aqueous HCI solutions.
Table 1 Summary of stability experiments for supported platinum and rhodium
compounds, in boiling chlorine-saturated HCI
From this Table it is evident that in order to produce a stable form of rhodium
sulphide, some heat treatment step is mandatory. It is also possible to conclude that not
all sulphides of precious metals are stable in these conditions, and furthermore, in view
of the instability of supported platinum sulphide, it is surprising to find supported rhodium
sulphide relatively inert in these conditions.
EXAMPLE 5
The electrodes of Example 3 were subjected to an electrolysis laboratory test according
to the scheme of Fig. 3. This configuration had a 3 mm gap between the cathode and
the anode. However, equivalent results were obtained with a "zero-gap" adjustment,
where the cathode and the anode were both pressed against the membrane. The
exposed electrode surface area was 6.45 cm2 and the membrane was Nafion 324. The
anode was titanium mesh activated with ruthenium oxide catalyst. Oxygen was fed to
the cathode at a rate of up to five-fold stoichiometric excess at 45-50 mbar pressure and
17% aqueous hydrogen chloride electrolyte (184+10 g/l) was fed to the anode. The
said electrolyte was recirculated until 50% of the hydrogen chloride was depleted and
then fresh electrolyte was added. The 50% depletion leads to a temporary increase in
cell voltage, and is exhibited as "spikes" on a graph of voltage versus time. The
electrolyte flow rate was 4 ml per minute or 0.372 m3hour/m2 at a back-pressure of 120
mbar. Unless stated otherwise, the cells were run at 3 kA/m2 and all voltages were
uncorrected for current collector resistance. The temperature of the cell and electrolyte
was held at 55°C ± 5°C with heating tape applied to the cell metal end plates and an air
conditioning unit.
In commercial electrochemical plants, two common temporary operation modes
are encountered which reflect the situations of either scheduled repair or replacement of
worn-out components, or the unscheduled failure of these components. For the
scheduled shut-downs, one can induce a "controlled" procedure, whereby elements of
the plant are systematically turned off or attenuated to a lower operational level. In
particular, chlorine can be degassed on the anode side and oxygen can be substituted
with nitrogen on the cathode side. Conversely, during the unscheduled failures
("uncontrolled" shut-downs), components of the plant are typically subjected to the most
rigorous of operating conditions. In particular, chlorine and oxygen are left in the cell and
as a consequence severe corrosion conditions arise. Since it is an object of this
invention to provide a catalyst and gas diffusion electrode capable of operation in an
electrochemical plant, the catalyst-electrode assemblies were tested in simulated
controlled and uncontrolled shutdowns.
These two interventions differ in the manner of turning off various components.
For the controlled shutdown, an inert gas was fed to the cathode, and the rectifier current
was slowly decreased, followed by turning the rectifier off. Once the rectifier was off, the
pumps were halted. For the uncontrolled shut-down, oxygen flow was halted to the
cathode while the rectifier and pump circuits were suddenly shut off. without the gradual
decrease in current or flow rate.
The catalyst of this invention was subjected to testing under the uncontrolled shut-down,
and compared to current state-of-the art catalysts. Figure 4 shows the typical platinum
catalyst in an ELAT™ electrode. While the operating voltage is 1.15 volts, the
uncontrolled shut-down causes the catalyst to experience the full corrosive force of the
electrolyte, and the cell potential increases by over 500 mV. Figure 5 shows the case of
the rhodium/rhodium oxide of Comparative Example 1, incorporated in a single-sided
ELAT, as described in Example 3, paragraph b). Here the initial steady-state voltage is
just over 1.2 V, and only after activation does the voltage decrease below 1.2 V to
approximately 1.18 V. Figure 6 is the case of a single-sided ELAT made with the
rhodium sulphide catalyst of Example 1, as described in Example 3, paragraph b). The
steady-state voltage of 1.15 V was obtained without any form of activation of the catalyst,
either prior to assembly in the electrode or during operation in the laboratory test system.
Figure 6 demonstrates that this new catalyst obtains desirable performance without an
additional activation step, and that the catalyst activity is preserved after being exposed
to the full corrosive force of solutions of HCI/CI2.
EXAMPLE 6
Since much of the waste aqueous HCI is generated after chlorinating an organic
feedstock, there is often a significant level of organic contaminants in the recycled acid
solution. Although one object in the design of oxygen reduction catalysts is to provide a
catalyst that yields appreciable activity in the presence of high chloride ion
concentrations, it is another goal to provide an oxygen reduction catalyst that yields
appreciable activity in the presence of organic contaminants, as already mentioned.
Such a catalyst may find utility in other applications as well, such as a cathode in Direct
Methanol Fuel Cells (DMFC), whereby methanol crossing over from the anode to the
cathode acts as a poison toward the latter when a platinum based state of the art
catalyst, such as the commercial product cited in the Example 4, is used. In any case, it
is well known that methanol ranks among the organic molecules with the highest activity
towards adsorption on transition metals, therefore the behaviour in the presence of
methanol of a transition metal-based catalyst is fairly representative of the general
attitude of such catalyst to poisoning by organic contaminants.
The efficacy of the rhodium sulphide catalyst to reduce oxygen in the presence of
organic molecules has been assessed in a potentiostated three-electrode system. The
three-electrode or "half cell" method fits 1 cm2 sample of gas diffusion electrode into an
inert holder. The gas-fed side of the gas diffusion electrode is positioned into a plenum
whereby an excess of air or oxygen is passed at low pressures (on the order of 10 mm of
water or less). The face containing the catalyst (that would normally be against the
membrane of an electrolyser or DMFC) is held in a 0.5M H2SO4 solution at a fixed
temperature. The counter electrode is placed directly across the gas diffusion electrode,
and a reference electrode is held in-between the two. The fixed geometry is maintained
between the three electrodes through a specially constructed cap. A potentiostat is
employed to control the potential and measure the current. A current interrupt device is
placed in series with the electrodes and the internal resistance (IR) is subtracted from
the readings. The direct addition of organic molecules such as methanol to the sulphuric
acid solution allows the ready evaluation of catalyst performance in the presence of
contaminants.
Figure 7 shows the case of an ELAT activated with the commercial Pt on Vulcan
XC-72 catalyst of Example 4 operated as the cathode under a potential control in the
half-cell, at 70°C and in 0.5M H2SO4. For each addition of methanol since the very first
one, an instant and substantial reduction in oxygen reduction current due to the
methanol poisoning can be noticed. Figure 8 shows the ELAT™ of Example 3 paragraph
a) activated with the rhodium sulphide catalyst of Example 1, operating under the same
regimen. In, this case, a shift in cathodic potential was observed only at the highest
concentration levels of methanol. These last two figures illustrate the highly selective
nature of the rhodium sulphide catalyst inasmuch as the catalyst is able to readily reduce
oxygen in the presence of methanol.
COMPARATIVE EXAMPLE 3
In an arrangement as shown in fig. 9 with an electrochemical cell of 100 cm2
active area, a gap of 2 mm between anode and membrane of type Nafion 324 and an
ELAT cathode of the single side type catalysed with platinum supported on carbon
powder, technical grade hydrochloric acid was electrolysed. For this purpose the
anolyte cycle was under hydrostatic pressure of 400 mbar to press the membrane
against the ODC which itself was pressed against the cathodic current distributor
mesh to be electrically contacted. The concentration of the anolyte cycle was kept at
ca. 14 % as fed into the cell and the ca. 13% as leaving the cell. For this purpose the
anolyte was circulated with a pump and the concentration loss in the electrolysis was
compensated by continuously feeding fresh concentrated acid into the circuit. The
temperature of the anolyte leaving the cell was controlled to about 70°C via a heat
exchanger between pump and cell. The current density throughout the experiment
was 5 kA/m2.
As can be seen in fig. 10, during the electrolysis with chemical pure hydrochloric
acid the cell voltage was between 1.06 and 1.08 V. With feeding the acid coming
from the isocyanate plant which was purified with activated carbon and subsequently
with adsorption resin of type OC 1066 from Bayer AG (Germany) the cell voltage
gradually increased for 50 to 60 mV and stabilised at this level. In the same
arrangement in a next step with the same purification hydrochloric acid of a multi
plant site was supplied to the experiment. The effect was a dramatic increase of cell
voltage for about 260 mV, which was only to a minor part recovered after a shut down,
snowing the high sensitivity of the platinum catalyst with respect to organic contaminants
especially of the second type, partly identified as tensides from polymerised ethylene
and propylene oxides and acrylic acid esters. This effect is rather surprising in view of
the fact that the ODC is hydraulically separated from the anode compartment by the
Nafion membrane.
COMPARATIVE EXAMPLE 4
In another experiment with the same arrangement of fig. 9 but with a single
sided ELAT catalysed with carbon powder carried rhodium / rhodium oxide the cell
behaviour was found to be the following: the start-up voltage under chemical pure
acid proved to be ca 130 mV higher trfan comparative example 3, as can be seen in
figure 11. It should be noticed, that this test was run with a current density of 4 kA/m2
and a temperature of 60°C. After feeding technical grade hydrochloric acid of the
multi plant site mentioned in example 7 and purified with activated carbon and
subsequently with another resin of type EP63 from Bayer AG (Germany) the cell
voltage increased and stabilised about 100 mV higher. This behaviour was
promising with respect to the reduced sensitivity against organic contaminants still
passing the purification line. However, during this experiment the catalyst loss was
nearly 30 %, as measured in catholyte drain. This gave a hint, that the stability of
this catalyst against being leached out was not sufficient, as well as the cell voltage.
EXAMPLE 7
In this experiment with the same arrangement as for comparative examples 3 and 4
the new carbon powder carried catalyst on rhodium sulphide basis in a single side
ELAT was tested in comparison to a platinum catalyst in the same type ELAT. With a
current density of 5 kA/m2 and an operating temperature of 70 °C the starting voltage
proved to be merely 40 mV higher for the rhodium sulphide catalyst as for the
platinum catalyst during the first days of operation with chemical pure hydrochloric
acid. With acid coming from an isocyanate plant which was purified with activated
carbon and subsequently with adsorption resin of type OC 1066 from Bayer the cell
voltage stabilised only 20 mV higher, as can be seen in figure 12. The voltage
increase for the platinum catalyst was 40 mV. Changing to the technical grade
hydrochloric acid of the multi plant site, purified the same way, the voltage increased
further 10 mV only for the rhodium sulphide catalyst but about 260 mV for platinum
catalyst. Omitting the second step of purification with resin of type OC 1066 the
increase in cell voltage was 10 mV only for the rhodium sulphide. Electrolysis
without any purification resulted in an increase of cell voltage for 20 mV for the
ihodium sulphide catalyst only. These last two steps had not been performed with
the platinum catalyst. Going back to the full purification proved the effect of
increasing cell voltage to be reversible for the rhodium sulphide catalyst.
EXAMPLE 8
In a long time test run over 90 days with the same arrangement and using a rhodium
sulphide catalyst as in example 7 technical grade hydrochloric acid of a multi plant
site only purified with activated carbon or chemical pure hydrochloric acid were
supplied alternately. The surprising result was that nearly no effect due to the
technical grade acid was found, which proved that the purification with activated
carbon is sufficient, as can be seen in figure 13. The organic impurities,
predominantly mono and di-chlorobenzenes, can be reduced to a level of with the activated carbon.
EXAMPLE 9
In a pilot plant with a four element electr6lyser of an element size of 0.85 m2 a long time
test under industrial conditions was performed. In an analogous arrangement as in fig. 9
with 400 mbar anolyte pressure, the operating temperature was controlled to the hydrochloric acid inlet concentration to ca. 14% by weight. Except for the start-up
period with 3 kA/m2 throughout the operation a current density of 4 kA/m2 was kept. Over
a period of more than 280 days, predominantly technical grade hydrochloric acid of the
multi plant site type was electrolysed. The acid was purified with activated carbon only.
As can be seen from figure 14, the element voltage was surprisingly stable, showing
again the high tolerance of inserted rhodium sulphide catalyst in ELAT type
electrodes. The other very positive result was the indifference of the electrodes
under shut down conditions. The plant was shut down without polarisation for 16
times and no influence on the voltage could be observed. The amount of catalyst
Joss as measured via rhodium content in the catholyte drain was in total about 6.5%
by weight with respect to the total amount of catalyst. The main loss occurred during
the first start up and the first shut downs (ca. 3%). During normal operation the loss
of catalyst was found to be 1.57% by weight only, promising together with the
decreasing losses during shut downs an electrode lifetime of several years.
EXAMPLE 10
In a laboratory cell in the experimental arrangement of examples 7 and 8 an oxygen
depolarised cathode of the flow through type catalysed with rhodium sulphide was
tested with technical grade hydrochloric acid of the multi plant site purified with
activated carbon only as in examples 7 through 9. The electrode was intermittently
supplied with pure oxygen and air. It could be proven that even with air as cathodic
feed-gas the cell could be operated up to 5 kA/m2. Despite the fact, that technical
grade acid was utilised, there was the surprising result of a good performance of the
cell: applying 4 kA/m2 the voltage increase was found to be 160 mV after three days
of conditioning, see fig 15. Raising the current density to 5 kA/m2 the voltage
increased for another 160 mV. The air flow was 1 mVh and 1.7 nrrVh respectively.
After reducing the depth of the cathodic gas room from 20 mm to 5 mm the air flow
rate could be reduced to 0.4 m3/h for 5 kA/m2 and the voltage was even lower with
the reduced air flow rate, showing the possibility for further optimisation for this
mode of operation in increasing the off-gas exchange rate with reducing the flow
channel dimension. The important result is that using a flow through type oxygen
depolarised cathode catalysed with rhodium sulphide an operation with air and with
technical grade hydrochloric acid is possible.
EXAMPLE 11
A comparative test with technical grade hydrochloric acid under the same conditions
as in example 10 with a single sided ODC versus a flow through type ODC, both
catalysed with RhSx was carried out. An increasing amount of nitrogen mixed into the
pure oxygen results in a voltage increase applying the single sided ODC. With only
30% of nitrogen the voltage reaches a level of >1.6 V with a strong exponential
behaviour towards higher percentages of nitrogen. The behaviour of the flow through
type ODC in the contrary showed a much smaller effect, as can be seen in figure 16.
In addition it could be shown that an increase of the gas flow by a factor of 5 through
the same non flow optimised cathode chamber reduces the increase of cell voltage
with the flow through type ODC from 70 mV to 30 mV only. This indicates that only
the flow through type ODC was capable to be operated with depleted oxygen or
even with air. At the same time the RhSx catalysed ODC was found to be tolerant
against organic impurities in the hydrochloric acid.
We claim:
1. A process for electrolysing an aqueous solution of hydrochloric acid to
chlorine in an electrochemical cell provided with an anode compartment and a
cathode compartment including at least one gas diffusion cathode comprising an
electrically conductive web provided on at least one side thereof with a coating of a
catalyst for the electroreduction of oxygen comprising rhodium sulphide and
optionally containing at least one fluorinated binder incorporated therein, comprising
introducing aqueous hydrochloric acid containing contaminant species into the
anode compartment and oxygen into the cathode compartment while impressing a
direct electric current on the cell.
as claimed in
2. The process of claim 1 wherein said species are organic contaminants
resulting from the production of the aqueous hydrochloric acid solution as the by-
product of the chlorination of an organic feedstock.
as- claimed in
3. The process of claim 1 wherein said species are organic contaminants
resulting from the production of the aqueous hydrochloric acid solution as the by-
product of the thermal decomposition of chlorinated organic compounds.
, as claimed in
4. The process of any preceding claims wherein said species are contaminants
resulting from the interaction of the aqueous hydrochloric acid with rubber or plastic
liner systems of the plant or other organic compounds leaching parts of the system.
as claimed in
5. The process of any preceding claims wherein said hydrochloric acid
containing contaminant species is pre-purified by means of activated carbon.
as claimed in
6. The process any claims 1 to 4 wherein said hydrochloric acid containing
contaminant species is pre-purified by means of activated carbon and at least one
adsorption resin.
as claimed in
7. The proeess of any claims 1 to 4 wherein said hydrochloric acid containing
contaminant species is pre-purified by means of at least one adsorption resin.
.as claimed in
8. The process of any claims 1 to 4 wherein the cathode compartment of the
electrochemical cell is fed with air or depleted oxygen.
as claimed in
9. The process of claim 8 wherein the gas diffusion cathode is a flow-through
type cathode.
as claimed in
10. The process of any preceding claims wherein the anode and cathode
compartments of the electrochemical cell are separated by an ion exchange
membrane.
1. A process for electrolysing an aqueous solution of hydrochloric acid to
chlorine in an electrochemical cell provided with an anode compartment and a
cathode compartment including at least one gas diffusion cathode comprising an
electrically conductive web provided on at least one side thereof with a coating of a
catalyst for the electroreduction of oxygen comprising rhodium sulphide and
optionally containing at least one fluorinated binder incorporated therein, comprising
introducing aqueous hydrochloric acid containing contaminant species into the
anode compartment and oxygen into the cathode compartment while impressing a
direct electric current on the cell.

Documents:

159-KOLNP-2003-(26-11-2012)-CORRESPONDENCE.pdf

159-KOLNP-2003-(26-11-2012)-FORM-16.pdf

159-KOLNP-2003-(26-11-2012)-OTHERS.pdf

159-KOLNP-2003-(26-11-2012)-PA.pdf

159-KOLNP-2003-FORM 27.pdf

159-KOLNP-2003-FORM-27.pdf

159-kolnp-2003-granted-abstract.pdf

159-kolnp-2003-granted-assignment.pdf

159-kolnp-2003-granted-claims.pdf

159-kolnp-2003-granted-correspondence.pdf

159-kolnp-2003-granted-description (complete).pdf

159-kolnp-2003-granted-drawings.pdf

159-kolnp-2003-granted-examination report.pdf

159-kolnp-2003-granted-form 1.pdf

159-kolnp-2003-granted-form 18.pdf

159-kolnp-2003-granted-form 26.pdf

159-kolnp-2003-granted-form 3.pdf

159-kolnp-2003-granted-form 5.pdf

159-kolnp-2003-granted-letter patent.pdf

159-kolnp-2003-granted-reply to examination report.pdf

159-kolnp-2003-granted-specification.pdf

159-kolnp-2003-granted-translated copy of priority document.pdf


Patent Number 215540
Indian Patent Application Number 159/KOLNP/2003
PG Journal Number 09/2008
Publication Date 29-Feb-2008
Grant Date 27-Feb-2008
Date of Filing 10-Feb-2003
Name of Patentee DENORA ELETTRODI S.P.A.
Applicant Address VIA DEI CANZI 1, I 20134 MILAN, ITALY, AN ITALIAN COMPANY.
Inventors:
# Inventor's Name Inventor's Address
1 ALLEN ROBERT J 130 ADAMS AVENUE SAUGUST, MA 01906, USA.
2 GIALLOMBARDO JAMES R 25 HULL STREET, BEVERLY, MA 01915, USA.
3 CZERWIEC DANIEL CEDER STREET WELLESLEY, MA 02481 USA.
4 DE CASTRO EMORY S 60 LITTLE NAHANT ROAD NAHANT, MA 01908-1028.
5 SHAIKH KHALEDA 17 FULLER LANE CONCORD, MA 01742 USA.
6 GESTERMANN FRITZ BERLINER STRASSE 83 51377 LEVERKUSEN GERMANY.
7 PINTER HANS-DIETER FORSTRING 20-42929 WERMELSKIRCHEN GERMANY.
8 SPEER GERD AM WEITHER 5 51399 BURSCHEID GERMANY.
PCT International Classification Number C25B 11/04
PCT International Application Number PCT/EP01/10068
PCT International Filing date 2001-08-31
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 09/654,553 2000-09-01 U.S.A.