GB2513103A

GB2513103A - Electrochemical flow reactors for hydrogen peroxide synthesis

Info

Publication number: GB2513103A
Application number: GB1303756.9A
Authority: GB
Inventors: Qian Li; Martin Henstridge; Christopher Batchelor-Mcauley; Richard Guy Compton; Nathan Lawrence; Robert Seth Hartshorne
Original assignee: Schlumberger Holdings Ltd
Current assignee: Schlumberger Holdings Ltd
Priority date: 2013-03-04
Filing date: 2013-03-04
Publication date: 2014-10-22
Anticipated expiration: 2033-03-04
Also published as: GB2513103B; GB201303756D0

Abstract

A method of electrochemical generation of hydrogen peroxide in aqueous solution comprises supplying a solution containing dissolved oxygen to an electrolytic cell having an anode 20, a carbon cathode 10 with a catalyst immobilised thereon and a continuous path through the solution between the anode and the cathode, and applying electrical potential to the cathode (10, 30) to cause catalysed reduction of dissolved oxygen to hydrogen peroxide. The catalyst may be covalently bound to the cathode and may be a quinone. The cell may comprise a reference electrode 22 and electrical potential may be supplied to the cathode to maintain it at a controlled potential relative to the reference electrode. The solution is preferably made to flow through the cell to contact both the anode and the cathode. A majority portion of the solution flowing into the cell may be directed into a stream passing over the cathode (30, figure 3) and a minority portion directed to a stream passing over the anode (50) and rejoining the majority portion.

Description

tM:;: INTELLECTUAL

PROPERTY OFFICE

Application No. 0B1303756.9 RTM Date:12 June 2013 The following terms are registered trademarks and should be read as such wherever they occur in this document: "Cypress Systems" and "Goodfcllow".

Intellectual Properly Office is an operaling name of Ihe Patent Office www.ipo.gov.uk Electrochemical Flow Reactors for Hydrogen Peroxide Synthesis

BACKGROUND

Large-scale manufacture of hydrogen peroxide as a commercial product is carried out using a thermochemical process. However, there have been number of proposals for making hydrogen peroxide on a smaller scale by electrochemical processes which may be carried out close to the point at which the hydrogen peroxide is used.

The desired reaction for the electrochemical process is a two Jectron reduction of oxygen at the cathode of an electrochemical cell 25+ 02+ H20 -*1-10/ + OH-The reaction at the anode may produce oxygen, thus 2011 -* 1⁄4O2+ 1420+25 These electrochemical reactions can provide net conversion of oxygen to hydrogen peroxide, the combination of the two eleetroehemical reactions resulting in, overall: 1/202 + OH-Undesirable interfering reactions which can occur are decomposition of peroxide at the anode and decomposition of peroxide ions by further reduction at the cathode.

A number of electrolytic processes have made use of an alkaline electrolyte at the cathode of an electrolytic cell in which the anode and cathode are in compartments which are separated by a porous material or a selectively permeable membrane. The anode and cathode chambers may have separate inlets and outlets and the cathode chamber may contain an alkaline aqueous solution while being supplied with oxygen gas, so that there are three phases in contact: the solid phase cathode, liquid phase electrolyte and the oxygen gas which undergoes reduction to form peroxide ions.

Oloman and co-workers have described processes in which a mixture of alkaline electrolyte and oxygen are supplied together to a so-called trickle bed reactor in which the cathode is a bed of graphite particles. See for example US Patent 4118305.

The cathode and anode have generally been separated by a porous material so as to guide solLition from the cathode away from direct contact with the anode. Such processes have frequently operated with an alkali concentration giving a pH of 14 or more and with substantial amounts of oxygen gas supplied at elevated pressure so that the flow of oxygen considerably exceeded the flow of alkaline liquid electrolyte.

There is also some literature concerned with electrochemical reduction of oxygen to hydrogen peroxide in neutral solution. Some papers have investigated chemistry at a carbon cathode with an immobilised quinone catalyst but have not disclosed any form of apparatus for carrying out a process: for example Golabi et al in Journal of Electroanalytical Chemistry 416 (1996) 75-82.

Yamanaka ct al, in Angewandtc Chemic mt. Ed. Vol 47 pp1900-1902 (2008) have described an electrolytic process carried out with deionised water as electrolyte in anode and cathode chambers which were separated by a cation permeable membrane (Nafion). Oxygen gas was supplied to the cathode chamber. The hydrogen peroxide yield was observed to be very low when the cathode was completely immersed in water and improved when the water level was lowered, allowing a substantial proportion of cathode to be in direct contact with oxygen gas.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below. This summary is not intended to limit the scope of the subject matter claimed.

Broadly, in a first aspect, the present invention provides a method of electrochemical synthesis of hydrogen peroxide in solution, the method comprising supplying a solution containing dissolved oxygen as electrolyte to at least one electrolytic cell having an anode and cathode where the cathode is carbon with a catalyst immobilised on the cathode, and applying electrical potential to the cathode to reduce the dissolved oxygen to hydrogen peroxide.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 diagrammatically illustrates laboratory-scale apparatus for the process; Figs 2 and 3 both show, diagrammatically, other forms of apparatus for carrying out the process.

U DETAILED DESCRIPTION

The method disclosed here utilises a carbon cathode with a catalyst for peroxide ion formation immobilised on the cathode. One possibility is that the cathode is formed of glassy carbon having a sLirface which is in contact with the aqueous solution ftrnctionalised with a catalyst. A glassy carbon electrode may take the form of a tube defining a flow path for the solution.

Another possibility is that the cathode comprises a carbon mesh or other porous carbon structure extending across the path of flow of the aqueous solution. Utilising a mesh structure for the cathode may increase contact with the flowing solution. It is also possible that contact between the cathode surface and the flowing solution may be increased by deliberately inducing turbulent flow. This could be done with static vanes or a moving part in the flow upstream of the cathode or with a step or other discontinuity in the wall of the flow path upstream of the cathode. The turbulence might be localiscd or the whole flow could become turbulent The catalyst on the cathode is a material which undergoes an electrochemical reaction form a first state to a second state. It then reacts with a precursor of hydrogen peroxide, and at the same time is returned to its first state. This catalyst may be an organic compound and it may be an organic quinone. An organic quinone catalyst may be covalently attached to the carbon cathode. One traditional approach has been to expose the carbon to a strong oxidising agent, leading to formation of carboxylic acid, ketone and other groups on its surface, and then immohilise a substance by bonding to these groups. A number of other processes for derivitisation of carbon by covalent attachment of molecules are known and may be used. The electro-reduction of diazonium salts has been described by P. Allongue eta! J. Am. diem. Soc., vol 119, page 201 (1997).

There have also been a number of disclosures of routes for derivatization of carbon, without electrochemistry. These include the homogeneous reduction of diazoniuni compounds in reducing media as described by Pandurangappa et al Analyst, vol 127, page 1568 (2002) and Leventis eta!, Talanta vol 63, page 1039 (2004). Also in this category is W02005/066618 (Schlumberger) which includes description of the diazocoupling of anthraquinonyl and nitrophenyl groups onto carbon nanotubes by means of the reduction of diazonium salts. Li eta!, in New J. Chem vo135, pp2462- 2470 (2011) describe a procedure for depositing anthraquinonyl groups on an edge plane pyrolytic graphite electrode by thermal decomposition of a diazonium salt ahsorbcd on the clectrode. Bonding of thc anthraquinonyl groups to carbon was attributed to esterification of carhoxylic groups already present at sites on the graphite.

The presence of a catalyst such as a quinone on the cathode allows reduction of oxygen to hydrogen peroxide to proceed in two stages. The quinone is electrochemically reduced to the corresponding hydroquinone and this in turn undergoes reaction with dissolved oxygen to form hydrogen peroxide while at the same time reverting to the quinone form. If the quinone is anthraquinone, the reaction scheme at the cathode is -0 2e /

OH A th

The steps of the cathode reaction can be written as AQ + 2e + 21120 -* AQH2 + 20ff AQII2 + 02 -* AQ + 11702 The electrochemical reaction at the anode may be the same as mentioned above V202+OH--÷02W Because the cathode reaction takes place in these two stages, it can take place at a cathode potential which is independent of the rate of flow of solution, whereas the potential required for direct reduction of oxygen at a cathode increases with rate of flow of the solution ( so-called over potential) because the reaction entails diffusion of the oxygen to the cathode.

A numhcr of quinoncs may bc employcd. Thc quinone may bc a compound with ftiscd rings, such as naphthoquinone. antroquinone or phenanthrene quinone. The quinone may bear substituents which do not impede the reaction or induce decomposition of hydrogen peroxide.

The anodc of thc clcctrochcmical cell may bc in communication with thc aqucous solution flowing ovcr thc cathode. There arc a numbcr of possibilities for this. In a simple arrangemcnt, the anodc may bc placed in the flow of the aqucous solution which passes over the cathode, possibly at a position downstream from cathode.

Another possibility is that a path of communication through the aqueous solution from cathodc to anode is shaped or restricted so that flow from thc cathode is largely directcd away from contact with thc anodc. A number of expedients for this are alrcady known: for instance, the anode may be in a branch from the main flow of the aqueous solution so that although there is still a continuous path between the cathode and anode through the aqueous solution, the main flow of aqueous solution containing peroxide items passes the branch without contact with the anode surface. A similar effect could be achieved by a liquid porous material placed between cathode and anode. A further possibility is that aqueous solution is supplied separately to the vicinity of the anode and flows over the anode before merging with the main flow which has passed over the cathode.

The anode may be made from a material which does not catalyse the decomposition of peroxide ions. Thus it may be formed of carbon without quinone or other catalyst on its surface. A graphite rod or a carbon mesh may be used.

Another possibility is that the anode is a sacrificial metal anode, such as zinc or magnesium. The metal would be stripped electrochemically to form the corresponding ion (eg Zn2 or M?) therefore inhibiting any chemical reaction at the anode.

Placing the anode downstream of the cathode helps to ensure that no products generated at the anode affcct thc cathode reaction. However, thc anode might bc placed upstream in the case where the water being pumped is clean' so that efkctively no reactions can occur at the anode other than possible solvent breakdown.

The electrochemical cell may also be provided with a reference electrode in communication with the flow and the electrical potential applied to the cathode may then be held at a constant potential relative to this reference electrode. Electronic devices able to supply a constant potential relative to a reference electrode are widely available as laboratory potentiostats. Such devices can also be scaled up to have a larger current-carrying capacity if required.

The reference electrode may be located in the flow from the cathode, such as between cathode and anode if the anode is downstream of the cathode.

The solution supplied to the electrochemical cell provides its electrolyte. The solution may contain water as the only solvent or it may contain other solvent which is miscible with water. Tt is also envisaged that a conductive non-aqueous solvent could be used in some embodiments. Ionic solutes in the solution enable it to pass electric current.

The solution supplied to the electrochemical cell may come from a single source even if an incoming flow is separated into a part which flows over the cathode and another part which flows over the anode. All flow from the electrochemical cell may go into a single outflow.

The solutions supplied to an electrochernical cell may have p1-i in a range from p1-I 4 or pH 5 up to pH 9 or pH 10, thus being neutral or only mildly acidic or alkaline. The solution which is employed may be water from a natural source such as a river or lake, possibly with some added solute, and generation of hydrogen peroxide in this water may serve to sanitise the water by killing or inhibiting the growth of unwanted microorganisms.

Fig 1 of the drawings shows a laboratory scale electrochernical cell for carrying out the method. As shown, a glassy carbon tube 10 was fitted between sections of electrically insulating plastic tubing 12 and provided the cathode of the electrochemical cell. The glassy carbon tube 10 was surrounded by a copper ring 14 to conduct electricity to it.

The lower end of the insulating tubing 12 was connected by flexible tubing 16 to a supply reservoir (not shown) of aqueous electrolyte positioned at a height above the electrochemical cell so as to give some hydrostatic pressure enabling solution from this reservoir to flow upwardly through the electrochemical cell as indicated by vertical arrows. The upper end the tubing 12 was connected by flexible tubing 18 to a container (not shown) receiving discharged solution. The rate of flow of solution through the cell was controlled by incorporating a restriction such as a capillary in the tubing 16 connecting the supply reservoir to the lower end of the tubing 12.

The anode 20 of the cell was platinum mesh positioned above, and hence downstream of, the glassy carbon cathode 10. A leakless Ag/AgCI reference electrode 22 supplied by Cypress Systems, Lawrence, ICansas, USA was positioned to project into the path of flow intermediately between the cathode 10 and the anode 20.

A potentiostat 24 connected to the three electrodes 10, 20, 22 was used to apply electrical potential to the cathode 10 and anode 20 relative to the reference electrode 22.

The effectiveness of anthraquinone catalyst on the glassy carbon electrode was shown experimentally. Tn a preliminary experiment no catalyst was applied to the glassy carbon tube 10. Phosphate-buffered saline solution at pH 7 was made to flow through the tubing 12 at various flow rates. At each flow rate cyclic voltammetry was carried out, app'ying varying potential to the cathode 10 and recording both the potential applied and the current flowing.

It was observed that the steady state currents increased with flow rate, showing that the electrochemical reaction was dependant on convection-diffusion to bring oxygen to the electrode. The half-wave potential was also determined and it was observed that half wave potential increased with the flow rate of solution through the tubing, rising from -o.54 V at a very low flow rate of3.5 x I o-cm3/sec to -o.78 volt at 0.85 cm3/sec. This demonstrated that the voltage required to drive the reduction of oxygen increased with increasing flow rate.

For a subsequent experiment anthraquinone was covalently attached to the interior surfacc of thc tubing 10. Thc covalcnt grafting of 2-anthraquinonyl functional groups onto the interior surface of tubing 10 was achieved by electrochemical reduction of anlhraquinone-2-diazonium telrafluorohorate sail al open circuil polential This is Ihe potential obtained at zero current and is therefore the potential of the solution or components within it. The open circuit potential will be at a potential in which nitrogen can be lost and the radical formed. Aqueous solution containing 1 mM diazonium salt was injected into the glassy carbon tube 10 while it was sealed at both ends, and reaction was allowed to take place at room temperature for 5 minutes. The carbon tube was then sonicated in pure water for at least 1 minute to remove any physisorbed material.

Voltammetry was used to confirm that anthraquinone had become attached to the glassy carbon. Thc trcatcd glassy carbon tubc was fittcd as thc cathodc in apparatus as shown in Fig 1. Phosphate buffered saline at p117 which had been stripped of dissolved oxygen by bubbling nitrogen through it was used as electrolyte within that apparatus. Voltanirnetry was carried out at arrange of scan rates and the redox reaction of anthraquinone was observed to occur at -0.52 volt relative to the same Ag/AgCI reference electrode 22. The maximum current was found to be proportional to scan rate, which is consistent with the reactant being located at an electrode. Calculation based on the voltammetry results indicated that a near monolayer of anthraquinone had been deposited on the interior surface of the glassy carbon tube.

The apparatus was then used to reduce oxygen to hydrogen peroxide. Air-saturated phosphate-buffered saline at pH 7 was again made to flow through the tubing 12 at various flow rates and cyclic voltammetry was carried out as before. it was now observed that the half wave potential was almost constant at -0.5 volt over the whole range of flow rates and it was also observed that the current at -0.5 volt corresponding to the reduction of anthraquinone was the same at each flow rate but consideraHy greater than the current observed at this cathode potential in the preliminary experiment without any quinone on the glassy carbon tube 10.

Fig 2 shows an arrangement in which thc cathodc is providcd by a scquence of panch of glassy carbon foam cxtcnding across thc cross-scction of a rnctal pipc 32 which connccts thcsc foam panels electrically and is surroundcd by an insulating pipc 34.

Glassy carbon foam is supplied by Goodfellow Cambridge Ltd, Huntingdon UK as Carbon-Vitreous-Foam, it has high surface area and also has high porosity, thus allowing solution to flow through it while providing contact with the flowing solution.

A reference electrode 36 is provided and an anode is provided in the flow path downstream of thc rcfcrcncc electrode 36. This anodc also has carbon foam panels 40 extending across the cross-section of a metal pipe 42 within the surrounding insulating pipe 34. The carbon foam panels 30 of the cathode have anthraquinone immobilised upon them, for instance by electrochemical reduction whereas the carbon foam panels4o of the anode do not have any catalyst on them. Connections 44 lead to a potcntiostat.

Fig 3 shows a further arrangement in which thc cathodc is constructcd as in Fig 2 but thc anodc 50 is a graphitc rod located in a cavity 52 positioncd as a branch off thc main pipe 34. It can bc secn that although thc anodc is positioned out of the main part of the flow there is a continuous path through the solution from cathode foam panels 30 to the anode 50.

A small amount of the incoming solution does not pass through the carbon foam panels of the cathode but instead travels along a bypass pipe 54 of smaller internal diameter than the main pipe 34. This bypass pipe 54 leads into the cavity 52 containing the anode 50 so that solution in this cavity 52 is continually replaced by incoming solution. Flow out of the anode cavity 52 merges with the main flow along the pipe 34.

It will be appreciated that the example embodiments described in detail above can be modified and varied within the scope of the concepts which they exemplify. Features referred to above or shown in individual embodiments above may be Lised together in any combination as well as those which have been shown and described specifically.

Accordingly, all such modifications arc intended to be includcd within the scope of this

disclosure as defined in the following claims.

Claims

CLANS1. A method of electrochemical generation of hydrogen peroxide in aqueous solution, the method comprising supplying a solution containing dissolved oxygen to an electrolytic cell having an anode and a cathode and a continuous path through the solution between the anode and the cathode, where the cathode is carbon with a catalyst immobilised on the cathode, and applying electrical potential to the cathode to cause catalyscd reduction of dissolved oxygen to hydrogen peroxide.
2. A method according to claim 1 wherein the catalyst is covalently bound to the cathode.
3. A method according to claim I or claim 2 wherein the catalyst is a quinone.
4. A method according to claim 1 or claim 2 wherein the quinone comprises a molecule with two or three thsed benzene rings.
5. A method according to claim 1 or any other preceding claim wherein the cathode is glassy carbon.
6. A method according to claim 1 or any other preceding claim wherein the cathode comprises a porous carbon foam and the solution passes through the foam.
7. A method according to claim 1 or any other preceding claim wherein the anode is also carbon.
8. A method according to claim 1 or any other preceding claim wherein the electrochemical cell comprises a reference electrode and electrical potential is supplied to the cathode by circuitry which maintains the cathode at a controlled potential relative to the reference electrode.
9. A method according to claim 1 or any other preceding claim wherein the solution is made to flow into and through the cell to contact both the anode and the cathode and then to flow out of the cell while electrical potential is supplied to the anode and cathode generate hydrogen peroxide in the flowing solution.
10. A method according to claim 1 or any other preceding claim wherein the whole of the flowing solution passes over both the anode and the cathode.
11. A mcthod according to claim 1 or any othcr prcccding claim whcrcin a majority portion of the solution flowing into the cell is directed into a stream which passes over the cathode while a minority portion of the solution flowing into the cell is directed into a stream which passes over the anode but not over the cathode and thereafter rejoins the majority portion.
12. A method according to claim 1 or any othcr prcccding claim whcrcin thc anodc and cathodc arc fully immcrscd in solution supplicd to the clectrochcmical cell.
13. A method according to claim 1 or any other preceding claim wherein the solution supplied to the electrochemical cell is aqueous.
14. A method according to claim 13 wherein the aqueous solution supplied to the clectrochernical cell has a pH in a range from 4 to 10.
15. A method according to claim 14 wherein the aqueous solution is supplied to the electrochemical cell from a natural water source without chemical purification.