Process and catalyst-electrolyte combination for electrolysis

Abstract

The invention relates to a process for electrolysis comprising a cathode and an anode comprising a catalyst, both the cathode and anode at least partly immersed in an electrolyte, the process characterised in that the electrolyte at least partly inhibits further oxidation of a product formed at the anode. Typically the catalyst comprises one or more metal-(Group VIb) semiconductors, and one or more metal-(GroupVIb))-phosphorous species.

Claims

1. A process for water electrolysis comprising: providing a cathode and an anode, the anode containing a catalyst, providing an electrolyte, immersing both the cathode and anode at least partly in the electrolyte, and selectively transforming water at the anode to hydrogen peroxide or radicals capable of forming hydrogen peroxide, wherein the electrolyte at least partly inhibits further oxidation of the hydrogen peroxide formed at the anode, wherein the catalyst comprises one or more metal-(GroupVlb))-phosphorous species, wherein the one or more metal-(Group Vlb)-phosphorous species satisfies the formula M′.sub.m′A′.sub.a′P.sub.y wherein: M′ is a metal, A′ is a Group Vlb species, m′ and a′ have a value of between 1 and 5, P is a phosphorous species, and y has a value 0<y <5.

2. The process-according to claim 1, wherein the electrolyte comprises a proton-accepting species.

3. The process according to claim 1, wherein the electrolyte comprises a solvent and one or more dissolved species comprising a proton accepting species.

4. The process according to claim 3, wherein the electrolyte also comprises an ionic component.

5. The process according to claim 1, wherein the electrolyte is butyl ammonium sulphate dissolved in water.

6. The process according to claim 1, wherein the catalyst comprises: one or more metal-(Group Vlb) semiconductors.

7. The process according to claim 6, wherein the one or more metal-(Group Vlb) semiconductors correspond to the formula M.sub.mA.sub.a wherein: M is a metal, A is a Group Vlb species, and m and a have a value of between 1 and 5.

8. The process-according to claim 7, wherein the metal is selected from the group consisting of Ti, Mo, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ir, Cd, In, Sn, the rare-earth metals, and combinations thereof.

9. The process according to claim 7, wherein the Group Vlb species is selected from the group consisting of oxygen, sulphur, selenium and tellurium.

10. The process according to claim 1, wherein the electrolyte comprises an ionic liquid or a hydrated ionic liquid.

11. A process for water electrolysis comprising: providing a cathode and an anode, the anode containing a catalyst, providing an electrolyte, immersing both the cathode and anode at least partly in the electrolyte, and selectively transforming water at the anode to hydrogen peroxide or radicals capable of forming hydrogen peroxide, wherein the electrolyte at least partly inhibits further oxidation of the hydrogen peroxide formed at the anode, and wherein the electrolyte is butyl ammonium sulphate dissolved in water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further disclosure, objects, advantages and aspects of preferred and other embodiments of the present application may be better understood by those skilled in the relevant art by reference to the following description of embodiments taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the disclosure herein, and in which:

(2) FIG. 1 illustrates linear scan voltamograms of the MnOx electrodes in BAS electrolyte and aqueous NaOH electrolyte performed at scan rate of 1 mV s.sup.−1;

(3) FIG. 2A illustrates absorbance of MnO.sup.4− at 565 nm added to the BAS electrolyte after electrolysis (200 mC) to form hydrogen peroxide;

(4) FIG. 2B illustrates absorbance of MnO.sup.4− at 565 nm added to the BAS electrolyte after addition of a known amount of standard hydrogen peroxide;

(5) FIG. 3 illustrates the concentration of H.sub.2O.sub.2 detected over time in the BAS electrolyte with and without MnO.sub.2 disproportionation catalyst; and

(6) FIG. 4 illustrates diphenylamine redox indicator in as prepared BAS electrolyte, BAS with electro chemically produced hydrogen peroxide and BAS with standard hydrogen peroxide.

DETAILED DESCRIPTION

(7) Aspects of the invention will be further described with reference to the following non-limiting examples:

(8) Materials: Manganese (II) acetate tetrahydrate 99.99% (Mn(Ac).sub.2.3H2O), butylamine 99.5%, ethylamine 99.5% and sulphuric acid were purchased from Sigma-Aldrich Pty. Ltd. Nitric acid was purchased from Merck. Potassium permanganate, oxalic acid, diphenylamine. Hydrogen peroxide. Unless otherwise stated, reagent grade water (18 MΩ-cm resistivity) was used for all experiments. All chemicals were used as received.

(9) Ethyl ammonium nitrate (EAN) was prepared by mixing ethylammonium hydroxide with nitric acid. A typical procedure for EAN synthesis is as follows: 50 ml of ethylamine (0.76 mole) was added to 200 ml of water and kept agitated using magnetic stirring. Then a small amount 2 M nitric acid solution was added drop wise until pH 7 was recorded using a Mettler Toledo InLab® micro pH electrode. Finally water was removed from the mixture over 2 hours in a rotary evaporator at 50° C. and 20 kPa.

(10) The electrolyte for hydrogen peroxide synthesis—butyl ammonium sulphate (BAS) was prepared by mixing 0.08 moles of butyl amine with 0.04 moles of sulphuric acid in a 100 ml standard volumetric flask. High ionic strength butyl ammonium sulphate electrolyte (BAS-IL) was prepared by mixing 0.4 moles of butyl amine with 0.2 moles of sulphuric acid in a 100 ml standard volumetric; this yields salt water mixture. The pH of the electrolytes was adjusted to 10 through addition of small amount of butylamine with pH recorded using a Mettler Toledo InLab® micro pH electrode.

(11) Electrochemical Experiments: Electrochemical experiments were performed on a PAR VMP2Z potentiostat with a standard three-electrode configuration. The MnO.sub.x films deposited on Au electrodes were used as an anode with a Pt counter electrode used as cathode. The working area of the electrode was masked using Kapton tape, leaving a 0.5 cm×0.5 cm electrode area. A 66-EE009 (“No-Leak”) Ag/AgCl (Cypress Systems) and standard calomel electrodes were used as a reference. All electrochemical measurements unless otherwise stated were performed at room temperature (RT) of about 22° C.

(12) MnOx Electrodeposition: Thin films of MnOx were deposited on the gold electrodes from ethylammonium nitrate ionic liquid with 10 vol. % of water and 0.01 M manganese acetate at 120° C. using constant current density of 200 μA cm.sup.−2 for 10 minutes, as described in more detail elsewhere.

(13) Characterisation Techniques: The amount of hydrogen peroxide was determined using a solution of potassium permanganate. A stock solution of 0.119 M potassium permanganate was prepared and standardised. A diluted solution of 5.78.Math.10.sup.−3 M potassium permanganate was used for titration. In a typical procedure 0.25 ml of 0.8 M BAS electrolyte at pH 10 was added to the 0.25 ml of 1M H.sub.2SO.sub.4 and titrated using 5.78 10.sup.−3 M potassium permanganate solution.

(14) A useful second indicator for the presence of hydrogen peroxide involves its action as an oxidant, reflecting its possible use for in-situ oxidation reactions. Thus 1% of diphenylamine in concentrated sulphuric acid was oxidised in the H.sub.2O.sub.2 solution. In a typical procedure 0.1 ml of 0.8 M BAS electrolyte at pH 10 was added to the 0.2 ml of the diphenylamine solution.

(15) UV-Vis transmission spectra were recorded at room temperature using a Cary 1E UV-visible spectrophotometer.

(16) Results: Linear scan voltammograms of the manganese catalyst were measured in a range of electrolytes and are shown in FIG. 1. High oxidation current densities were observed in BAS and BAS-IL electrolytes at oxidation potential above 0.5 V vs. SCE. It can be seen, however, that there were only negligible current densities at potentials below 0.6 V vs. SCE when aqueous sodium hydroxide was used as an electrolyte. Similarly low currents were observed in the neutral BAS electrolyte, in accord with the previously reported low catalytic activity of manganese based catalysts in neutral electrolytes.

(17) The equilibrium potential for water oxidation at various pH's can be calculated from E.sub.anodic=1.23−0.059 (pH)−0.244, V vs. SCE. It can be seen that at pH 10, the equilibrium potential for water oxidation is around 0.4 V vs. SCE and high overpotential is required to oxidise water in the electrolyte containing sodium hydroxide. The neutral BAS electrolyte shows very little activity, indicating that this currents involved at pH 10 are the result of a strongly pH sensitive process.

(18) Attempts to determine oxygen production from the electrolysis at pH 10 produced only very small rates of oxygen evolution. Oxidation of the electrolyte was also investigated as oxidation of amines is a well known process, though typically at higher potentials than those in use here. Despite passage of substantial quantities of charge no evidence of decomposition was seen in Nuclear Magnetic Resonance or mass spectroscopy (ESI) results. This led us to suspect the formation of some product of water oxidation intermediate between water and oxygen. Classic colourmetric tests for hydrogen peroxide including potassium permanganate reduction and diphenylamine oxidation both proved strongly positive for the presence of H.sub.2O.sub.2.

(19) In order to determine efficiency of the process for hydrogen peroxide production a series of electrolytes was used during oxidation at constant potential of 0.59 V vs. Ag/AgCl. The amount of charge passed was kept at 200 mC and equivalent to 1.04 μmoles of H.sub.2O.sub.2 based on Faraday's law assuming a 2 electron oxidation of water. Standard titration of hydrogen peroxide with potassium permanganate solution assumes reduction to Mn(II) as per equation 7.
2MnO.sup.4−+5H.sub.2O.sub.2+6H.sup.+.fwdarw.2Mn.sup.2++5O.sub.2+8H.sub.2O  equation 7

(20) In order to improve sensitivity, electrolytes were titrated in a standard UV-Vis cell, with the amount of potassium permanganate determined by measuring absorbance value at 565 nm. The value of absorbance at 565 nm as a function of amount of potassium permanganate added to a pH 10 BAS electrolyte after 200 mC oxidation is shown in FIG. 2A. The end of titration was determined as a point where a linear fit function of the absorbance at high amount of titrant added extrapolates to zero. In the example of FIG. 1 the end point is taken as 55.4 μL, which is equivalent to 0.08 μmoles of H.sub.2O.sub.2 or a production yield of about 77% from Faraday's Law for a 2 electron process. In a control experiment where 1.04 μmoles H.sub.2O.sub.2 was directly added to the electrolyte medium, as can be seen in FIG. 2B, the detected amount of H.sub.2O.sub.2 was around 86%.

(21) The hydrated ionic liquid form of this electrolyte (BAS-IL) produces the highest currents at any given overpotential (FIG. 1). To the extent that the formation of a solvated hydrogen peroxide species involving the electrolyte is the rate determining step in this process, it is to be expected that the high salt concentration will enhance the rates of reaction. Lower production yields of about 64% were obtained from the titration of the BAS-IL electrolyte after 200 mC of electrolysis.

(22) The lower than 100% efficiency detected in these experiments may be due to the competing process of water oxidation to oxygen during the electrolysis experiment and/or continuous loss of hydrogen peroxide from the electrolyte through the standard disproportionation to water and oxygen (described by the equation 2H.sub.2O.sub.2═H.sub.2O+O.sub.2) which is known to be catalysed by alkaline solutions and the presence of ammonia. The lower yield in the BAS-IL electrolyte is also concordant with this due to the much higher free amine content in this electrolyte.

(23) In order to investigate the longer term stability of the hydrogen peroxide in the BAS electrolyte, the concentration of H.sub.2O.sub.2 was determined at various time intervals after electrochemical oxidation was complete. It can be seen from FIG. 3 that the concentration of H.sub.2O.sub.2 in the BAS electrolyte drops steadily over the course of 24 hours with some amount present even after 40 hours. When a small amount of manganese dioxide (which is a known catalyst for the disproportionation reaction) was added to the mixture, the amount of H.sub.2O.sub.2 drops significantly within first few hours and is negligible after 18 hours.

(24) The use of electrochemically prepared hydrogen peroxide as an oxidant was confirmed using standard test with diphenylamine as an indicator. The UV-Visible spectra of the electrolytes are shown in FIG. 4. It can be seen that presence of hydrogen peroxide leads to oxidation of diphenylamine and formation of violet diphenylbenzedine (III).

(25) Thermodynamics of Hydrogen Peroxide Production in Amine Electrolytes.

(26) Importantly the potentials involved in the processes in FIG. 1 are lower than the normally expected equilibrium potential for the H.sub.2O/H.sub.2O.sub.2 reaction at pH 10. However, the results obtained clearly support the hypothesis that the electrolyte solvation is a critical aspect of the reaction in this case. It appears that the solvation is sufficient to lower the E0 to the range studied in FIG. 1. This is not an unusual event—electrolyte solvation is capable of shifting redox potentials by as much as 1 V for example in the case of Au/(Au(III).

(27) The over potential for this process on the catalyst can be expected to be less than the full 4e H.sub.2O/O.sub.2 process. In fact the high overpotentials required for water oxidation over many catalysts are a result of mechanisms that proceed via peroxy intermediates. In the present case a solvated form of hydrogen peroxide becomes the main product because the solvation process carries it away from the electrode into the bulk.

(28) While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification(s). This application is intended to cover any variations uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.

(29) As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope of the invention as defined in the appended claims. The described embodiments are to be considered in all respects as illustrative only and not restrictive.

(30) Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention and appended claims. Therefore, the specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as performing the defined function and not only structural equivalents, but also equivalent structures.

(31) “Comprises/comprising” and “includes/including” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. Thus, unless the context clearly requires otherwise, throughout the description and the claims, the words ‘comprise’, ‘comprising’, ‘includes’, ‘including’ and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.