EP3793951A1

EP3793951A1 - Catalytic materials for wastewater treatment

Info

Publication number: EP3793951A1
Application number: EP19726085.4A
Authority: EP
Inventors: Daniel MALKO; Anthony R. J. KUCERNAK; Javier Rubio-Garcia
Original assignee: Imperial College of Science Technology and Medicine; Imperial College of Science and Medicine
Current assignee: Ip2ipo Innovations Ltd
Priority date: 2018-05-15
Filing date: 2019-05-15
Publication date: 2021-03-24
Also published as: WO2019220114A1; GB201807860D0

Abstract

The present invention relates to catalytic materials and methods for treating a wastewater stream using the catalytic materials. In particular, the present invention relates to materials and methods for reducing organic contaminants in wastewater streams. We describe a method for treating a waste stream comprising the steps of providing a catalytic material; supplying a waste stream comprising an organic compound which is a liquid or dissolved in a solvent and contacting the catalytic material with the waste stream; supplying oxygen to the waste stream; and oxidising the organic compound at the surface of the catalytic material with simultaneous reduction of oxygen. Novel catalytic materials and processes for preparing the catalytic materials and devices which comprise the catalytic materials are also disclosed.

Description

CATALYTIC MATERIALS FOR WASTEWATER TREATMENT FIELD OF THE INVENTION

The present invention relates to catalytic materials and methods for treating a wastewater stream using the catalytic materials. In particular, the present invention relates to materials and methods for reducing organic contaminants in wastewater streams.

BACKGROUND

Industrial wastewater produced by various industries such as chemical, food and beverage and agriculture is typically contaminated with organic compounds. Producers of the waste must pay a surcharge to the municipal treatment supplier, which is often defined by the level of contamination as determined per chemical oxygen demand (COD) level. Alternatively, waste producers must employ cost-intensive, on-site advanced oxidation processes. The overall treatment costs are estimated to be around US$100 billion globally (at 2015 values). Therefore, efficient pre-treatment options are highly desirable. Filtration and removal of solids can reduce COD levels by 10-30%, and is a multi-billion-dollar market. However, to date, for dissolved COD, only biological treatments, which may have many drawbacks (such as being time-consuming, difficult to control and not suitable for toxic waste streams) or chemical treatments, which are harsh and costly have been available. The present invention seeks to provide an alternative approach.

Dissolved COD is decreased by oxidation of the organic matter. The oxidation of organic species on metal catalysts such as platinum has shown to lead to the formation of adsorbed carbon monoxide on the metal surface which can lead to the poisoning of the catalyst (J. Hydrogen Energy 36, 15731-15738 (2011)). This has led to the introduction of metals such as lead or ruthenium which, when alloyed with platinum, enhance the formation of adsorbed species, such as hydroxyl groups, which help oxidize the adsorbed CO from the metal surface (J. Phys. Chem. 97, 12020-12029 (1993)). Besides the formation of CO adsorbates, the oxidation of organic species in industrial wastewater is particularly challenging when using metal and metal alloy catalysts. This is due to presence of cations and anions which have the ability to strongly attach to the catalyst surface further decreasing the number of sites available for the oxidation of the organic contaminants.

This drawback prevented the utilisation of electrochemical devices with precious metal anodes at moderate to low potentials. Industrially, only electrochemical devices utilising metal oxide catalysts, doped-diamond or other compositions are available. These require energy input and operate at relatively high oxidation potentials.

The present inventors have sought to develop catalytic materials which will oxidise the organics in the wastewater using oxygen from the air which, unlike traditional catalytic wet air oxidation, operates under mild conditions. The usual temperature range of 150-320°C requires high pressure to maintain a liquid phase. Typical conditions for wet air oxidation are temperatures of 150-320°C, pressures of 2-15 MPa and residence times of 15-120 min. The preferred chemical oxidation demand (COD) load ranges are from 10 to 80 kg/m³. By using a catalytic method with milder conditions, the operating plant is simplified and cost and energy requirements are reduced, while decreasing the contamination level (measured as COD) of the water by up to 50%, with very substantial savings in treatment costs.

The present inventors have unexpectedly determined that, in the presence of dissolved 0₂, the competitive reaction of oxygen reduction can simultaneously occur with oxidation of organic molecules. The reduction of O2 can occur through a 4-electron pathway leading to the production of water (or hydroxide at high pH) or, through a 2-electron transfer, which yields hydrogen peroxide. This second mechanism becomes more prominent as the oxidation potential falls below 0.5V vs RHE and it is enhanced by the presence of salts. While O2 reduction would decrease the efficiency of the oxidation catalyst with less surface available for the oxidation of organic molecules, the present inventors observed a surprising increase in the resistance of the catalyst to deactivation.

This enhanced durability is thought to be attributed to: i) the ability of hydrogen peroxide, which can be produced during the oxygen reduction process on the catalyst surface, to oxidize species which may have strongly attached to the surface of the catalyst, or ii) the formation of a mixed electrochemical potential which raises the electrochemical potential of the catalyst surface sufficiently to strip off attached contaminants.

This characteristic was further validated by putting in close vicinity, and in electrical contact, a catalyst with the ability to oxidise organic species which are normally present in industrial wastewater and materials with ability to enhance formation of either water or H2O2 upon reaction with molecular oxygen. This enabled better removal of contamination within industrial wastewater and enhanced material durability. As H2O2 is a potent oxidant which is occasionally used to decompose organic matter in wastewater, the enhanced production of H2O2 during the oxygen reduction process might not only support in the cleaning of the catalyst surface of the oxidation catalyst but also aid increasing of the cleaning efficacy of the overall catalytic system.

Additional benefits of using this type of catalytic material with in-situ promotion of H2O2 formation include:

- H2O2 is a disinfectant which kills bacteria present in the waste stream. This minimises possible deactivation of the catalyst due to formation of bacterial bio- films.

H2O2 is a strong oxidant which can directly attack organic species dissolved in the waste stream. This contributes to enhancing the water cleaning capabilities of the material.

- The activity of sites involved in the direct oxidation of organic species are preserved. Furthermore, a catalyst that can promote the efficient reduction of oxygen at high potentials (>0.7V vs RHE) might lead to the increase of the electrochemical potential on the surface of the oxidation catalyst. This might then lead to stripping off of contaminants (such as CO or ions) attached to that surface and/or drive the electrochemical oxidation of the organic contaminants on the catalyst surface, thereby enhancing the performance of the catalytic system. Such a catalyst would not necessarily produce hydrogen peroxide at this high potential, but water as a product of the oxygen reduction.

It is with these considerations in mind that the catalytic materials, methods and other aspects of the present invention have been devised.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a method for treating a waste stream to reduce organic compounds in the stream. The method comprises the steps of: providing a catalytic material, supplying a waste stream comprising at least one organic compound which is a liquid or dissolved in a solvent and contacting the catalytic material with the waste stream, supplying oxygen to the waste stream; and oxidising the organic compound at the surface of the catalytic material with simultaneous reduction of oxygen.

Advantageously, the method is carried out without any electrical energy input to the catalytic material.

Preferably, the method is carried out at a temperature of about 200°C or less, more preferably about 100°C or less, even more preferably about 50°C or less, most preferably at ambient temperature.

Preferably, the method is carried out at a pressure of about 3000kPa or less, more preferably about 2000kPa or less, even more preferably about 1000kPa or less, yet more preferably about 500kPa or less, most preferably about 100kPa or ambient pressure.

In one embodiment, the organic compound comprises formaldehyde and the method further comprises the step of removing the formaldehyde from the waste stream.

Preferably, the solvent is water.

In a second aspect, the present invention also provides a method for treating a waste stream comprising the steps of: providing a catalytic material; supplying a waste stream comprising an organic compound which is a liquid or dissolved in a solvent and contacting the catalytic material with the waste stream; and reducing the organic compound at the surface of the catalytic material with a reductant.

Preferably, the reductant is hydrogen. Suitably, the organic compound is a compound containing a nitro-group either in pure form or dissolved in a solvent; a chlorinated organic compound either in pure form or dissolved in a solvent; or a mixture thereof.

In a further aspect, the present invention provides a catalytic material comprising at least one active site capable of catalysing oxidation of the organic compounds and at least one active site capable of catalysing reduction of oxygen.

Preferably, the catalytic material comprises an oxidation catalyst and an oxygen reduction catalyst.

Preferably, the oxidation catalyst and oxygen reduction catalyst are in contact.

In one embodiment, the oxidation catalyst and oxygen reduction catalyst are in contact by being formed as adjacent contacting layers.

In an alternative embodiment, the oxidation catalyst and oxygen reduction catalyst are in contact by being prepared as a mixture comprising the oxidation catalyst and the oxygen reduction catalyst.

In one embodiment, the catalytic material further comprises a support. In an alternative embodiment, one of the oxidation catalyst and the oxygen reduction catalysts provides a support for the other catalyst.

Preferably, the oxidation catalyst:

i) is a metal comprising or consisting essentially of a precious metal or non- precious transition metal;

ii) is a metal alloy comprising or consisting essentially of precious metals and/or non-precious transition metals;

iii) comprises or consists essentially of one or more polyoxometalates such as H₄PW_HV0_4O or PW_{I I} MO0₄₀, and/or one or more metal oxides or carbides, such as PO2, WC, C02O3.

Suitably, the precious metal is selected from gold, silver, ruthenium, rhodium, osmium, platinum and palladium and mixtures thereof.

Suitably, the non-precious transition metal is selected from iron, cobalt, nickel, tin, copper, ruthenium, iridium, manganese, titanium, vanadium, tungsten, molybdenum and chromium and mixtures thereof.

Preferably, the metal alloy is a binary, ternary or quaternary metal alloy.

Suitably, the metal alloy is selected from PtNi, Ptlr, PtSn, PtCu, PtRu, PtPd, Pdlr, PdRh and PdNi. Preferably, the oxidation reduction catalyst is selected from:

(a) a carbonaceous material comprising;

(i) 80 to 95 wt% carbon;

(ii) 0 to 20 wt% of at least one transition metal;

(iii) 0 to 20 wt% nitrogen;

(iv) 0 to 20 wt% sulphur; and

(v) 0 to 20 wt% phosphorus.

(b) precious metals and/or metal alloys which specifically produce hydrogen peroxide as a reaction product when measured using the rotating ring disk technique;

(c) nitrogen doped carbon compounds which comprise from 50 - 98wt% carbon and 10-50wt% nitrogen;

(d) transition metal carbides;

(e) transition metal nitrides and carbonitrides;

(f) metal chalcogenides; and

(g) transition metal oxides.

Preferably, the carbonaceous material has a high surface area of about 200m²g ¹ or higher as determined by nitrogen adsorption analysis.

Advantageously, the carbonaceous material is obtainable by oxidative polymerisation of diaminonaphthalene. Preferably, the diaminonaphthalene is 1 ,5-diaminonaphthalene or 1 ,8-diaminonaphthalene, more preferably 1 ,5-diaminonaphthalene.

Optionally, the carbonaceous material is obtainable by oxidative polymerisation of a diaminonaphthalene in the presence of a metal salt.

Suitably, the metal salt is a ferrous or cobalt salt, preferably as a halide, more preferably as the chloride.

Advantageously, the catalytic material is formed with the oxidation catalyst in contact with the oxygen reduction catalyst such that, in use, a short-circuit condition is generated.

Suitably, the catalytic material is formed by over-layering layers of the oxygen reduction catalyst and oxidation catalyst on a substrate.

Alternatively, the catalytic material is formed by mixing the oxygen reduction catalyst and oxidation catalyst and applying the mixture to a substrate.

Preferably, the substrate is a porous substrate.

Preferably, the substrate is a conductive substrate. The catalytic material may be extruded, formed or moulded.

In some embodiments, the catalytic material is formed as pellets or granules.

In alternative embodiments, the catalytic material is formed as a porous plate.

Suitably, the catalytic material is deposited onto a plate or electrode substrate such as a gas diffusion layer.

Suitably, the catalytic material is: i) in form of pellets made from the oxidation catalyst finely dispersed and supported on carbon, silica, alumina or any other support commonly known in the art; or ii) formed by depositing the oxidation catalyst directly on carbon pellets; or iii) formed by depositing the oxidation catalyst onto a separate oxygen reduction catalyst in the form of powder or pellets. Preferably, in these embodiments, the oxidation catalyst is a metal or metal alloy.

Conveniently, the porous plate is a porous metal mesh, a carbon electrode of the type typically used in fuel cells, or a porous polymer sheet such as polyethylene or polypropylene.

The catalytic material may be coated onto a surface such as a waste pipe, catalyst support pellets (which can be selected from those well known in the art) or a reactor wall. A catalytic support is used when the catalyst is supplied in form of a coating. It merely serves as“support” to keep the coating in place. In the examples described below there is provided an alternative method to making the catalyst. Instead of making the pellets out of the catalytic material itself, and therefore creating a self-supporting structure, an existing support structure such as S1O₂ pellets, AI₂O₃ pellets, carbon pellets or any other commonly used supports may be used.

Preferably, the catalytic material further comprises at least one binder.

Suitably, the at least one binder is one or more binders selected from microcrystalline cellulose, carboxymethylcellulose, phenolic resin, ion exchange resins (Nafion, Aquivion, Tokoyama Cation exchange resin), sugars, humic acid-derived sodium salt (HAS), polyvinylalcohol, proprietary binderfrom Waterlink Sutcliffe Carbons (WSC), PTFE (Teflon etc), adhesive cellulose-based binder (ADH) (Saint Honore), wax, linseed oil, gum arabic, gum tragacanth, methyl cellulose, gums, protein, polyvinylpyrrolidone, polyisobutylene or styrene-butadiene rubber and other common binder polymers.

The catalytic material once mixed with the binder may be heat treated to remove or convert the binder to form a monolithic structure.

BRIEF DESCRIPTION OF THE FIGURES The above and other aspects of the present invention will now be described in further detail, by way of example only, with reference to the following examples and to the accompanying figures, in which:

Figure 1 is a schematic view of a reaction vessel for implementing the methods of the present invention;

Figure 2 is a schematic cross-sectional view of a first embodiment of a catalytic material in accordance with the present invention; and

Figure 3 is a schematic cross-sectional view of a second embodiment of a catalytic material in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 schematically illustrates the principal elements of an apparatus 10 suitable for treating wastewater. The apparatus 10 includes a column 1 1 and a catalyst material 12 supported within the column. Column 1 1 includes a wastewater inlet 13 arranged such that wastewater entering the column 1 1 flows under gravity through the catalyst material 12 and exits the column through an outlet 14. The column 1 1 also has an air inlet 15 below the supported catalyst material 12 for admission of air, suitably supplied from an air compressor (not shown).

Oxidation catalyst

For the purpose of describing the present invention, the term“oxidation catalyst” refers to a catalyst which is capable of catalysing the oxidation of compounds. The oxidation of organic compounds occurs using the oxygen as the oxidant, conveniently oxygen from ambient air.

Suitably, the oxidation catalyst:

iii) comprises or consists essentially of one or more polyoxometalates such as H₄PW_{I I}V0_4O or PW_H MO0_4O, and/or one or more metal oxides or carbides, such as Ti0₂, WC, C02O3.

The term“non-precious metal catalyst” describes a catalyst which is substantially free of precious metals. The term“substantially free” means that the sum of all precious metals in the catalyst does not exceed about 0.01 wt%, preferably does not exceed about 0.005 wt%, more preferably does not exceed about 0.001 wt%.

The precious metal may be gold, silver, ruthenium, rhodium, osmium, platinum or palladium. The non-precious transition metal may be iron, cobalt, nickel, tin, copper, ruthenium, iridium, manganese, titanium, vanadium, tungsten, molybdenum or chromium.

The metal alloy may be a binary, ternary or quaternary metal alloy. Examples of metal alloys include, but are not limited to, PtNi, Ptlr, PtSn, PtCu, PtRu, PtPd, Pdlr, PdRh and PdNi.

The catalyst is suitably formed as a catalytic material in which:

i) the catalytic material is in form of pellets made from the catalyst finely dispersed and supported on carbon, silica, alumina or any other support commonly known in the art;

ii) the catalyst is deposited directly on carbon pellets; or

iii) the catalyst is deposited onto a separate oxygen reduction catalyst in the form of powder or pellets.

Suitably, in these embodiments, the catalyst is a metal or metal alloy.

Oxygen reduction catalyst

The term “oxygen reduction catalyst” refers to a catalyst that can catalyse an electrochemical oxygen reduction reaction to electrochemically reduce oxygen (as measured by the rotating disk electrode technique or other suitable electrochemical characterisation technique).

The oxidation reduction catalyst is generally selected from:

(a) carbonaceous materials which have a high surface area of >200m²g ¹ as determined by nitrogen adsorption analysis;

(b) precious metal and metal alloys which specifically produce hydrogen peroxide as a reaction product when measured using the rotating ring disk technique;

(d) transition metal carbides;

(e) transition metal nitrides and carbonitrides;

(f) metal chalcogenides; and

(g) transition metal oxides.

Advantageously, wherein the carbonaceous material comprises: (i) 80 to 95 wt% carbon; (ii) 0 to 20 wt% of at least one transition metal; (iii) 0 to 20 wt% nitrogen; (iv) 0 to 20 wt% sulphur; and (v) 0 to 20 wt% phosphorus.

Catalysts suitable for use as the oxygen reduction catalyst are also described in WO 2015/049318A1 which is incorporated herein by reference and to which further reference should be made. The terms“hydrogen peroxide” and Ή2O2” are used interchangeably throughout this document.

As used herein, the term“poison-resistant oxygen reduction catalyst” refers to a catalyst which is resistant to poisons, such as organic molecules that typically deactivate platinum- based catalysts, for example nitrogen or sulfur containing compounds or salts (e.g., amines, sulphides, thiols, benzene and benzene derivatives). Therefore, the poison resistant catalyst will continue to function when contacted with a waste stream (e.g., it is capable of functioning in the presence of a variety of compounds, including different organic compounds, and poisons).

The elemental composition characterisation of the catalysts may be determined as is standard in the art and as set out, for example, in Malko, D., Kucernak, A. & Lopes, T., Nature Communications 1, 13285 (2016) and Malko, D., Lopes, T., Symianakis, E. & Kucernak, A. R J. Mater. Chem. A 4, 142-152 (2015), the entire contents of which are incorporated herein by reference and to which further reference should be made. For example, the elemental composition may be determined by X-ray photoelectron spectroscopy and/or total reflection X-ray fluorescence.

Total reflection x-ray fluorescence may be carried out, for example, using a Bruker S2 Picofox. For example, samples may be prepared from a suspension of 10 mg of the poison resistant cathode catalyst in 1 ml H₂0 (MiliQ 18.2 MW-cm), which may contain 1 wt% Triton X-100 (Sigma Aldrich) as surfactant, 0.2 wt% polyvinylalcohol (Mowiol® 4-88, Sigma-Aldrich) as binder and 100pg Ga, as internal standard (from 1 g/l Standard Solution, TraceCert®, Sigma-Aldrich). 10 may be deposited onto a quartz glass sample carrier and dried at room temperature in a laminar flow hood to give a homogenous thin film.

X-ray Photoelectron Spectroscopy (XPS) analyses may be performed, for example, using a Kratos Analytical AXIS UltraDLD spectrometer. For example, a monochromatic aluminium source (Al Ka = 1486.6 eV) may be used for excitation. The analyser may be operated in constant pass energy of 40 eV using an analysis area of approximately 700mhi x 300mhi. Charge compensation may be applied to minimise charging effects occurring during the analysis. The adventitious C1 s (285.0 eV) binding energy (BE) may be used as internal reference. The pressure may be about 10KPa during the experiments. Quantification and simulation of the experimental photopeaks may be carried out using CasaXPS and XPSPEAK41 software. Quantification may be performed using non-linear Shirley background subtraction. As used herein, wt% means, unless the context indicates otherwise, dry weight percentage of said elemental component of the total of weight of the catalyst.

The term“waste stream” encompasses any discharge of liquid waste comprising at least one organic compound. The at least one organic compound may be liquid (e.g. an alcohol such as methanol, ethanol, or glycerol) or the organic compound may be dissolved in a solvent. Thus, the term waste stream encompasses waste water (also written as wastewater), e.g. where the solvent is water. It will be appreciated that the waste stream may comprise one type of organic compound, or a mixture of organic compounds. A waste stream encompasses the effluent from domestic, industrial, commercial or agricultural activities. Thus, the waste stream may be effluent from a petroleum refinery, chemical or petrochemical plant, paper of pulp production, food or beverage production processes (including those from breweries, wineries, distilleries, abattoirs, creameries, sugar manufacturers and refineries, confectionery (such as chocolate and candy) production, and pharmaceutical and pesticide manufacturing processes). The materials and process of the present invention are particularly useful and suitable for waste streams from food or beverage production processes.

The waste stream may additionally comprise solids which are suspended or dispersed in the stream. It may also comprise further compounds which are dissolved in the liquid of the stream, such as nitrogen-containing compounds (e.g. ammonia, nitrogen heterocycles, amino acids, urea, etc), sulphur-containing compounds (e.g. thiocyanates, sulphides, sulphur-containing heterocycles, sulfoxides, and thiosulphates), and salts which may comprise a metal cation (such as an alkali metal cation or alkaline earth metal cation) or a halide anion (e.g. chloride, bromide or iodide).

Oxidation process

The present invention relates to a method for treating a waste stream comprising the steps of:

(a) providing a catalytic material;

(b) supplying a waste stream comprising at least one organic compound which is a liquid or dissolved in a solvent and contacting the catalytic material with the waste stream;

(c) supplying oxygen to the waste stream; and

(d) oxidising the at least one organic compound at the surface of the catalytic material with simultaneous reduction of oxygen.

The catalytic material may comprise at least one active site capable of oxidising the organic compound and at least one active site capable of reducing oxygen.

The catalytic material may perform the oxidation and the oxygen reduction at the same time. The catalytic material suitably has an extended surface which has specific sites such that, if the catalyst is exposed to oxygen and the at least one organic compound at the same time, the surface of the catalyst might be covered partially by the at least one organic compound and partially by oxygen. Consequently, both oxidation of organic compounds and reduction of oxygen can occur concurrently.

Accordingly, the catalytic material comprises an oxidation catalyst and an oxygen reduction catalyst. The catalytic materials are in contact with each other. The functional catalytic material described herein selectively oxidises organic molecules present in a waste stream, using molecular oxygen, conveniently provided by ambient air. The catalytic material may comprise a catalyst which specifically activates oxygen (such as those described in WO2015/049318A1 to which further reference should be made), combined with a catalyst that catalyses electrochemical oxidation of organic compounds. The combination of catalysts allows an electrochemical reaction to take place, without the requirement for a power source, such as an electrolysis cell, or for a supporting structure to extract energy, such as a fuel cell. In a conventional system two catalysts would exist separately on electrodes and would be connected through an electrochemical device. This is only necessary if it is required to produce electricity. In the case of the present invention, neither electricity nor a fuel source is required. Instead, the catalysts are integrated into the material rather than added by means of a device, electrodes or other means. As the oxygen reduction catalyst is highly specific to oxygen it produces the driving force for the oxidation catalyst to drive the oxidation reaction. The catalytic material, in effect, internally short-circuits in use. This forms a further aspect of the present invention.

As discussed above already, the oxidation of 0₂ can occur through a 4-electron pathway which would lead to H₂0 or through a 2-electron pathway which yields H2O2. Whilst O2 reduction would decrease the efficiency of the oxidation catalyst, with less surface being available for the oxidation of organic molecules, a surprising increase in the poisoning resistance of the catalyst was observed.

Preferably, the catalytic material has greater than about 1 % selectivity toward hydrogen peroxide production, such as from 5 to 20%, or from 1 to 5% selectivity toward hydrogen peroxide production.

The oxidation catalyst is selected from pure metal or metal alloys comprising precious metal and/or non-precious transition metals, polyoxometalates, metal oxides and carbides.

The oxidation catalyst may consist of a pure metal or a metal alloy. The metal may be a precious metal, such as gold, silver, ruthenium, rhodium, osmium, platinum or palladium. The metal may be a transition metal which is not a precious metal, such as iron, cobalt, nickel, tin, copper, ruthenium, iridium, manganese, titanium, vanadium, tungsten, molybdenum or chromium. Examples of metal catalysts include, but are not limited to, PtNi, Ptlr, PtSn, PtCu, PtRu, PtPd, Pd, Pdlr, PdRh and PdNi.

When the catalyst is a metal or metal alloy, it can be deposited on carbon pellets directly or on the oxygen reduction catalyst or pellets made thereof.

The oxygen reduction catalyst is preferably a carbonaceous material which has a high surface area of >200m²g ¹ as determined by nitrogen adsorption analysis. In preferred embodiments, the carbonaceous material comprises: (i) 80 to 95 wt% carbon;

(ii) 0 to 20 wt% of at least one transition metal;

(iii) 0 to 20 wt% nitrogen;

(iv) 0 to 20 wt% sulphur; and

(v) 0 to 20 wt% phosphorus.

An example of carbonaceous material can be produced by oxidatively polymerising 1 ,5- diaminonaphthalene in an ethanolic solution, optionally whilst adding 1wt% iron ions in the form of a salt such as FeCl_2*4H₂0, and pyrolyzing the dry precursor at 900°C under inert atmosphere in a tube furnace for 2h. The result is a catalyst that can electrochemically reduce oxygen.

The oxygen reduction catalyst may be a precious metal or metal alloy which specifically produces an enhanced amount of hydrogen peroxide as a reaction product when measured using the rotating ring disk electrode technique. Particularly good oxygen reduction catalysts are those which allow large outputs of H2O2, for example cobalt oxides, cobalt phosphides, PtHg, PtAu, Au and nitrogen-doped carbons. This contrasts with precious metal alloys which produce water rather than H2O2.

The above precious metal and metal alloys can be deposited on a conductive substrate, which can be a high surface area carbon, such as VulcanXC 72, KetjenBlack, Black Pearls, carbon pellets, pellets made from the oxidation catalyst or pellets made from metal e.g., porous silver.

The oxygen reduction catalyst may also be selected from: nitrogen-doped carbon compounds which comprise from 50-98wt% carbon and 10-50wt% nitrogen; transition metal carbides such as; FeC and WC; transition metal nitrides and carbonitrides, such as TiN and TaC_xN_y; metal chalcogenides such as transition metal compounds with S, Se or Te, for example, Ru2Mo₄SE₈MoS; and transition metal oxides, for example, Zr0₂-x, Co₃0₄-x and TaO.

The oxygen reduction catalyst can electrochemically reduce oxygen (as measured by the rotating disk electrode technique or another suitable electrochemical characterisation technique).

Oxidation of organic matter occurs in combination with oxygen, hydrogen peroxide or another suitable oxidising agent. T ypical organic substances for oxidation in a wastewater stream are outlined in WO2017/0493754A1 , which is incorporated herein by reference and to which further reference should be made.

The catalytic material may include one or more binders. Suitable binders include binders selected from microcrystalline cellulose, carboxymethylcellulose, phenolic resin, ion exchange resins (Nafion, Aquivion, Tokoyama Cation exchange resin), sugars, humic acid-derived sodium salt (HAS), polyvinylalcohol, proprietary binder from Waterlink Sutcliffe Carbons (WSC), PTFE (Teflon etc), adhesive cellulose-based binder (ADH) (Saint Honore), wax, linseed oil, gum arabic, gum tragacanth, methyl cellulose, gums, protein, polyvinylpyrrolidone, polyisobutylene or styrene-butadiene rubber and other common binder polymers.

The catalytic material mixed with the binder may be heat treated to remove the binder and form a monolithic structure.

Structure

Both the oxidation catalyst and the oxygen reduction catalyst may be deposited on carbon pellets or particles as a support or substrate.

Pellets or particles may also be made from the oxidation catalyst or the oxygen reduction catalyst. Alternatively, both catalysts may be combined and pelletized. Additionally, either catalyst may be manufactured as microparticles or nanoparticles deposited onto a support.

The catalytic mixture may be deposited onto catalyst beads which may then be filled into a column as shown in Figure 1 that acts as reaction vessel.

In an alternative embodiment, as shown schematically in Figure 2, the oxygen reduction catalyst 21 and the oxidation catalyst are supported on a supporting substrate 20 with the oxidation catalyst over-layering the oxygen reduction catalyst, or vice versa. Suitably, the substrate is a porous substrate, to allow the wastewater and/or oxygen to access the layer mounted immediately to the substrate. Furthermore, the substrate is advantageously electrically conductive. In preferred embodiments, the substrate 20 is formed of carbon fibres.

In a yet further embodiment, shown schematically in Figure 3, the oxygen reduction catalyst and oxidation catalysts are mixed to form a mixed catalyst 23 which is supported by a porous, and preferably electrically conducting, substrate 20 as described above.

Although Figures 2 and 3 illustrate the catalytic material as being planar, this is for illustrative purposes only. The materials may be formed into any suitable shape.

The catalytic material may be extruded, formed or moulded and is preferably in the form of a pellet or a porous plate. The catalytic material may be deposited onto a plate or electrode substrate such as a gas diffusion layer. The porous plate can be either a porous metal mesh, carbon electrode that is typically used in fuel cells or a porous polymer sheet (i.e. polyethylene, polypropylene etc).

If the catalytic material only comprises the oxidation catalyst, then the pellet is preferably made from carbon. The catalytic material may be coated onto a surface such as a waste pipe, catalyst support pellets (which can be selected from those well known in the art) or a reactor wall. A catalytic support is used when the catalyst is supplied in form of a coating. It merely serves as“support” to keep the coating in place. In the examples described below there is provided an alternative method to making the catalyst. Instead of making the pellets out of the catalytic material itself, and therefore creating a self-supporting structure, an existing support structure such as S1O₂ pellets, AI₂O₃ pellets, carbon pellets or any other commonly used supports may be used.

The transformation of the waste water is driven by contacting the desired reactant, i.e. the organic compound, which is either dissolved in the waste stream (present as a gas or liquid in pure form or in a mixture with the catalyst) and supplying the respective reductant or oxidant (either as a gas or liquid or dissolved in a liquid or a gas). The reaction can be carried out at any temperature and pressure that is deemed favourable above an uncatalyzed reaction but preferable in the temperature range of 0 to 100°C, and preferably at ambient pressure and temperature. Advantageously, the method of the invention can operate under mild conditions, preferably temperatures less than 200°C and pressures of <3000 KPa.

Waste streams

The waste stream for use with the catalyst and methods of the present invention typically comprise one or more organic compounds. The organic compound may be a liquid (e.g. it may be liquid at the temperature at which the reaction is operated) or it may be dissolved in the waste stream, in other words the waste stream may comprise a solvent and an organic solute (e.g. the organic compound which is dissolved). It will be appreciated that if an organic compound is solid at room temperature (i.e. about 20°C), and has partial solubility in the solvent, the part which is dissolved in the solvent is referred to as the "solute".

Examples of organic compounds which are liquid at room temperature include ethanol, methanol and glycerol. Thus, the waste stream may comprise one or more liquid organic compounds. If the organic compound is dissolved in the waste stream, the solvent may be water, acetonitrile, an ether, ethyl acetate, a halogenated hydrocarbon (e.g. dichloromethane or dichloroethane), or N-methylpyrrolidone, or it may be a further organic compound, which is liquid (e.g. methanol, ethanol, glycerol, etc.), or a mixture thereof (e.g. glycerol and water, ethanol and water, methanol and waster etc.). Preferably the solvent comprises water, or in other words, the waste stream is preferably a wastewater stream.

It will be appreciated that the term "dissolved" in the context of the invention means that the organic compound is capable of dissolving, at least in part, in a solvent. The organic compound may be selected from a carbohydrate, an alcohol, an aldehyde, an ester, a ketone, a hydrocarbon, an acid, and amino acid, a protein and combinations thereof. When the organic compound is a carbohydrate, it may be a monosaccharide (such as glucose, galactose, fructose, mannose and ribose), a disaccharide (such as sucrose, lactose, maltose, isomaltose, isomaltulose, trehalose and trehalulose), an oligosaccharide (such as FOS, MOS or GOS), a polysaccharide (such as inulin), or mixtures thereof.

When the organic compound is an alcohol, it may be selected from an alcohol, such as methanol, ethanol, glycerol, isomalt, lactitol, maltitol, mannitol, sorbitol, xylitol, and erythritol.

When the organic compound is an acid, it may be a carboxylic acid or dicarboxylic acid, for example the acid may be selected from citric acid, tartaric acid, malic acid, lactic acid, acetic acid, or propionic acid. When the organic compound is an amino acid or protein, it may be a selected from bovine serum albumin (BSA), cysteine, lysine, alanine, arginine, asparagine, aspartic acid, glutamine, glutamic acid, glycine, histidine, isoleucine, leucine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine and valine.

When the organic compound is an ester, it may be selected from ethyl acetate, n-butyl acetate, n-propyl acetate, isopropyl acetate, ethyl formate, and methyl formate.

When the organic compound is an aldehyde, it may be selected from formaldehyde (methanal), acetaldehyde (ethanal), propionaldehyde (propanal), butyraldehyde (butanal), pentanal, benzaldehyde, cinnamaldehyde, vanillin, tolualdehyde, furfural, retinaldehyde, glyoxal, malondialdehyde, succindialdehyde, glutaraldehyde, and phthalaldehyde.

When the organic compound is a ketone, it may be selected from acetone, propanone, butanone, 3-pentanone, cyclohexanone, dimethyl ketone, methyl ethyl ketone, methyl isobutyl ketone, and isophorone.

When the organic compound is a hydrocarbon, it may be a branched or unbranched, saturated, partially saturated or unsaturated, cyclic or acyclic compound consisting of hydrogen and carbon atoms. The hydrocarbon preferably contains from about 1 to about 20 carbon atoms. The hydrocarbon may be an aromatic hydrocarbon which comprises one or more five or six membered rings. If more than one ring is present, the rings may be linked by a single bond, or may be fused to give larger polycyclic compounds. Exemplary hydrocarbons include methane, ethane, propane, butane (n or iso), pentane (n, iso or cyclo), hexane (n, iso or cyclo), benzene, naphthalene, anthracene, phenanthracene, pyrene, chrysene etc.

The nature of the organic compound in the waste stream will depend on the source of the waste stream. For example, if the waste stream is from a winery or a brewery, the waste stream may comprise water, and the organic compound may include carbohydrates, alcohols, organic acids, esters and combinations thereof. Organic compounds of a winery or brewery waste stream typically include ethanol, glycerol, phenolic compounds (e.g. tannins), acids (e.g. citric acid, tartaric acid, malic acid, lactic acid and acetic acid), monosaccharides and disaccharides (e.g. glucose and sucrose), and starches.

During treatment of the wastewater, the waste stream comprising the organic compound is contacted with the catalytic material. Upon exposure to the oxidation active site of the catalytic material, or upon contact with the oxidation catalyst, at least a part of the organic compound may be oxidised. The skilled person will appreciate that the organic compound may be completely oxidised (e.g. broken down to carbon dioxide and water) or may be partially oxidised (e.g. increasing the oxidation state of the organic compound). The method of the invention lowers the COD of the waste stream which has contacted the catalytic material, by at least partially oxidising the one or more organic compound(s) present in the waste stream.

It will be appreciated that exposing the waste stream to the catalytic material referred to herein will reduce the concentration of the organic compound(s) in the waste stream. For example the concentration of the organic compound(s) in the waste stream which have contacted and reacted with the catalytic material may be at least about 5% less, preferably at least about 10% less, more preferably at least about 20% less, even more preferably at least about 30% less, even more preferably at least about 50% less, even more preferably about 70% less, even more preferably at least about 90% less, even more preferably at least about 95% less than the concentration of the organic compound(s) in the waste stream prior to contacting the catalytic material.

It will be appreciated that if the waste stream comprises more than one type of organic compound, then each of these organic compounds may be oxidised.

The organic compound in the waste stream may be directly electrochemically oxidised at the surface of the catalytic material. By "direct electrochemical oxidation", it is meant that the organic compound is oxidised on the surface of the catalytic material, without the involvement of other chemical reagents.

Reduction process

In a further aspect of the present invention, there is a method for treating a waste stream comprising the steps of:

(a) providing a catalytic material;

(b) supplying a waste stream comprising an organic compound which is a liquid or dissolved in a solvent and contacting the catalytic material with the waste stream; and

(c) reducing the organic compound at the surface of the catalytic material with a reductant.

This is the counter process of the above-described oxidation method. In accordance with this embodiment, the reductant may be hydrogen. The organic compound may be a compound containing a nitro-group either in pure form or dissolved in a solvent. Or, the organic compound may be a chlorinated organic compound either in pure form or dissolved in a solvent.

The catalytic material may also comprise silver, which is known to have antimicrobial properties, and thus using a silver may assist in the ability of the catalytic material to treat the waste stream. Thus, the present disclosure also provides a process for disinfecting a waste stream using a catalytic material which comprises silver as described herein. The term "disinfecting a waste stream" refers to a process which eliminates or reduces the amount of microbes in a waste stream.

In a preferred embodiment, the method of the present invention is used to remove formaldehyde from a waste stream.

According to the present invention there is also provided a process for preparing a catalytic material, wherein the process comprises the steps of;

(a) providing a reaction mixture for the oxygen reduction catalyst; and/or

(b) providing a reaction mixture for the oxidation catalyst; and

(c) providing at least one heating step.

The catalytic material may be further subjected to pyrolysis to form a monolithic structure.

Preferably, the heating step/pyrolysis is carried out at a temperature of from about 300 to about 1 100 C, more preferably at a heating rate of about 0.1 to about 20°C per minute.

The wastewater treatments of the present invention can advantageously be carried out at low temperatures, that is, the invention does not require additional external heat to be supplied. However, it will be appreciated that the waste stream may comprise residual heat, e.g. if it is provided from a reactor which has been subjected to heating. Preferably, and advantageously, the reaction conditions are mild (ambient pressure and room temperature).

The processes of the various aspects of the present invention may be carried out at temperatures from about 0°C to about 100°C, preferably from about 15°C to about 70°C, more preferably from about 18°C to about 60°C. The pH of the waste water can vary widely. For example, the pH of the waste water directly contacting the catalytic material may be in the range of between about 1 to about 14. However, it will be appreciated that one or more additives may be added to the waste stream prior to contacting the waste stream with the catalytic material to adjust the pH. Examples of such additives include a base, such as a group 1 or group 2 metal (e.g. Na, K, Mg or Ca) hydroxide or carbonate, including NaOH, KOH etc, or an acid, such as sulphuric acid, acetic acid, or hydrochloric acid. For example, the acid or base (including NaOH and KOH) may be added, e.g. in a concentration of between 0.1 M and 2M, such as about 0.5M or about 1 M. EXAMPLES

Example 1

The following method describes an example of making the catalyst materials of according to the present invention and using them to treat wastewater and simulated wastewater: The method follows the following general steps:

Preparation of the oxygen reduction catalyst.

Pelletizing the oxygen reduction catalyst and making carbon monoliths.

Deposition of the oxidation catalyst or Coating catalyst support with a mixture of the oxidation catalyst and the oxygen reduction catalyst, or only the oxidation catalyst alone.

Preparation of water treatment reactor and treatment of wastewater 1) Preparation of the oxygen reduction catalyst.

The oxygen reduction catalyst ODAN was synthesized by dispersing 500 mg (3.16 mmol) of 1 ,5-diaminonaphthalene (97% from Alfa Aesar) and 500 mg (2.19 mmol) of (NH₄)₂S₂0s (98% from Sigma-Aldrich) in 50 ml of ethanol (absolute from VWR). The dispersion was stirred for 24 h at room temperature to obtain oligomers of 1 ,5-diaminonaphthalene. The resulting mixture was heated to 80 °C in order to evaporate the solvent. When dry, the resulting residue was transferred to an alumina boat (11 cm long by 2 cm wide by 1 cm deep, approximately 10 ml of volume capacity) and heat treated at 1000 °C for 2 h in a tube (quartz) furnace (Carbolite) at a heating rate of 20 °C min^-1. This heat treatment was performed in an inert atmosphere, under a continuous flow of nitrogen (50 seem). After cooling down under this nitrogen atmosphere, the resulting material was removed from the quartz boat and ready for use.

The catalyst Fe-N/C was prepared by dissolving 1 .0 g (6.4 mmol) of 1 ,5- diaminonaphthalene (97%, Alfa Aesar), 1 .0 g (4.4 mmol) of (NH₄)₂S₂0s (98%, Sigma- Aldrich) and 35.6 mg of FeCI₂-4H₂0 (99%, Sigma-Aldrich) in 250 ml of ethanol (absolute, VWR). The solution was stirred for 24 h at room temperature. The solvent was then removed with a rotary evaporator. When dry, the resulting residue was transferred to an alumina boat (1 1 -cm long by 2 cm wide by 1 cm deep, ~10 ml of volume capacity) and heat treated at 950 °C for 2 h, after reaching the end temperature, in a tube (quartz) furnace (Carbolite) at a heating rate of 10 °C min^-1. This heat treatment was performed in an inert atmosphere, under a continuous flow of nitrogen (50 com). After cooling down under nitrogen, the resulting material was removed from the quartz boat and refluxed in 0.5 M H₂S0₄ for 8 h, to remove any soluble metal phases. The material was then filtered and dried. The dried powder was then subjected to a second heat treatment at 900 °C for 2 h after reaching the target temperature at a heating rate of 20 °C min^-1 under nitrogen and allowed to cool as above. The resulting powder was then ready to use as the oxygen reduction catalyst.

1 .707 g 2-Methylimidazole (2Mlm), 0.638 g ZnO and 28.8 mg FeC₂0₄-2H₂0 was uniformly mixed and sealed in an autoclave under Ar atmosphere and followed by heating at 220°C for 18 hours. The mole ratio between N-ligands (2Mlm) and metal ions was slightly higher than the stoichiometric ratio to ensure the conversion of metal ions during the solid-state reaction. The obtained crystals were subsequently ball-milled with 6 mm stainless steel ball for 2 hours at 400 rpm under Ar protection. The obtained powder (approximately 1 g each time) was dispersed in a PVP solution in DMF (2.5 mg ml-1 , 200 ml) with a sonication. The precipitates were collected by centrifugation and dried at 80°C in vacuum. The final catalysts were obtained by calcination of the crystal precursors at 1000°C for 2 hours under Ar atmosphere at a ramping rate of 10°C min ¹

The catalyst Co-N/C was synthesized by dissolving 1.0 g (6.4 mmol) of 1 ,5- diaminonaphthalene (97%, Alfa Aesar), 1.0 g (4.4 mmol) of (NH₄)2S₂0s (98%, Sigma- Aldrich) and 100 mg of C0CI₂ 6H₂O (99%, Sigma-Aldrich) in 250 ml of ethanol (absolute, VWR). The solution was stirred for 24 h at room temperature. The solvent was then removed with a rotary evaporator. When dry, the resulting residue was transferred to an alumina boat (1 1 -cm long by 2 cm wide by 1 cm deep, ~10 ml of volume capacity) and heat treated at 950 °C for 2 h, after reaching the end temperature, in a tube (quartz) furnace (Carbolite) at a heating rate of 10 °C min-¹. This heat treatment was performed in an inert atmosphere, under a continuous flow of nitrogen (50 com). After cooling down under nitrogen, the resulting material was removed from the quartz boat and refluxed in 0.5 M H₂S0₄ for 8 h, to remove any soluble metal phases. The material was then filtered and dried. The dried powder was then subjected to a second heat treatment at 900 °C for 2 h after reaching the target temperature at a heating rate of 20 °C min^-1 under nitrogen and allowed to cool as above. The resulting powder was then ready to use as an oxygen reduction catalyst.

Metal precursors, Co(N0₃)₂-6H₂0 was dissolved in 2 ml of ultrapure water (Millipore MilliQ, 18.2 MW cm). Carbon black powder (Vulcan XC72R) was added into the solution to achieve a weight percentage of 60% Co and 40% Carbon. This was followed by 2.5 hours of sonication. Phosphorus precursor, (NH₄)2HP0₄ of stoichiometric ratio to the Co was then dissolved in 4 ml of water and added to the metal precursor/carbon solution to form Cobalt phosphate on carbon. This was followed by another sonication for 1.5 hours to yield a homogenous dispersion. The sample was subsequently calcinated in a drying oven and left overnight at 120°C. The powder formed after the calcination process was ground and pyrolyzed in a tube furnace at 800°C with a constant flow rate of 5 seem under H₂/N₂ (< 5 vol. % of H₂) environment.

2a) Pelletizing the oxygen reduction catalyst and making carbon monoliths

The oxygen reduction catalyst was mixed well with a binder (Balocel [50% Microcrystalline cellulose, 15% CMC, 35% Lactose]) in a mass ratio of 3:2 and 1.3 ml water per gram of mixture was added. The mass was then extruded with a screw extruder into 3mm diameter extrudates. The extrudate was then subjected to a spheroniser for 2 minutes to obtain uniform pellets. The pellets were then pyrolized under a nitrogen atmosphere at 800°C in a tube furnace at a heating rate of 1 °C to obtain catalyst monolithic pellets. 50mg of catalyst was added to 3 ml of DVB (divinylbenzene) and sonicated in an ice bath for 2 h, azobisisobutyronitrile AIBN (1 mol% with respect to DVB,) was added and gently stirred. Gradually aqueous CaCI₂ solution (9 ml, 10 g/l) was added and it was stirred vigorously for 5 minutes. The mixture was manufactured into pellets and dried at 70°C. The pellets were then washed 3 times in an ethanol bath for 2h at a time, with slight agitation and then left to dry in an oven at 1 10°C overnight (water removal). Then the samples are heated at a ramp rate of 2°C/min to 800°C under a N₂ atmosphere.

Pellets were produced from the catalyst instead of activated carbon according to the preparation method described in: US 5,389,325.

3a) Deposition of the oxidation catalyst

The required amounts of H₂PtCl₆-6H₂0, SnCI₂-2H₂0 and pellets were added to the mixture of EG and deionized water with stirring. The mixture was heated to 130°C and kept at this temperature for 3 h. Then the black solid sample was filtered, washed and dried at 80°C for 10 h in an oven. The nominal loading of Pt in the catalysts was 5 wt% with a PtSn ratio of 3:1.

The required amounts of H₂PtCl₆-6H₂0, NiCI₂-2H₂0 and pellets were added to the mixture of EG and deionized water with stirring. The mixture was heated to 130°C and kept at this temperature for 3 h. Then the black solid sample was filtered, washed and dried at 80°C for 10 h in an oven. The nominal loading of Pt in the catalysts was 5 wt% with a PtNi ratio of 3:1.

The required amounts of H₂PtCl₆-6H₂0, RuCh and pellets were added to the mixture of EG and deionized water with stirring. The mixture was heated to 130°C and kept at this temperature for 3 h. Then the black solid sample was filtered, washed and dried at 80°C for 10 h in an oven. The nominal loading of Pt in the catalysts was 5 wt% with a PtSn ratio of 3:1.

2b/3b) Coating catalyst support with a mixture of the oxidation catalyst and the oxygen reduction catalyst, or only the oxidation catalyst alone

PtNi/C (60wt% on Carbon, Premetek) was mixed in a 1 :1 ratio with Fe-N/C catalyst in isopropyl alcohol. Nafion (5wt% perfluorinated resin in lower aliphatic alcohols) was added to achieve a 1 :1 weight ratio of Nafion to Carbon. Solvent was removed until a ~1wt% suspension was reached. Catalyst pellets Si02 (Si02 Pellets 1 -5mm, Pi-Kem) were either dip or spray coated with the suspension. The coated pellets were dried at 60°C and then heated to 130°C for 5 minutes in order to“set” the Nafion binder.

4) Preparation of water treatment reactor and Treatment of wastewater

The pellets of catalytic material obtained from 3a or2b/3b above were loaded into a tubular vessel, such as a polypropylene tube. The wastewater was then trickled through the reactor and contacted with oxygen through admission of air from below the catalytic material. Example 2

In this example, the Fe-N/C and ODAN carbonaceous catalyst materials prepared in Example

1 were chosen as the catalysts for the oxygen reduction reaction and combined with various binary metal alloys as the oxidation catalyst for analysis as catalytic materials. The materials were compared as a layered catalytic material, in which the oxygen reduction catalyst and oxidation catalyst are overlayered on a carbon support, and as mixed catalytic material, in which the oxygen reduction catalyst and oxidation catalyst are mixed and applied as a layer to a carbon support. The efficacy of the catalytic materials were then compared against two solutions, prepared to have the same COD value of 55,000 mg(0₂) I ¹ as typical American Sugar Refining Company (ASR) wastewater. The first solution was a glucose solution and the second solution was a simulated wastewater.

For the glucose solution, 12.899 g of D(+)-glucoseanhydrite (99.5% VWR) and 10 g of sodium hydroxide (98.5% VWR) were dissolved to make up a 250 ml solution by water (MilliQ 18.2 MW cm). For the simulated wastewater solution, 5 ml of NaCI solution (0.999M), Na₂S0₄ solution (0.094M), Na₃P0₄ solution (3.4mM), and Na₂S solution (0.36mM) were added to the glucose solution.

The catalytic materials were formed on carbon supports formed of carbon fibre formed to allow passage of oxygen through the carbon support. Carbon fibre substrates commonly used for carbon electrodes are suitable for this purpose. The catalysts were formulated as paintable inks for application to the carbon substrate.

Layered catalytic material

The target Pt loading for each oxidation catalyst is 0.5 mg. cm ². Accordingly, 30 mg of carbon-supported Pt-Ru powder (60wt%; Pt:Ru 1 : 1 , Fuel Cell Store) was dispersed in a solution containing 0.5 ml of isopropanol (VWR) and 0.5 ml of water (MilliQ 18.2 MW cm) with 5 min of sonication. Nafion^® 60mg (20 wt% solution, Sigma-Aldrich) was added to increase ink stability, decrease the particle size of carbon aggregates, and facilitate proton transfer between oxidation catalyst and reduction catalyst. The mixture was sonicated for

2 h to prepare a uniform ink.

The same procedure was used for preparation of a Pt-Ni oxidation catalyst ink, using 0.122 g of carbon black supported Pt-Ni powder (7.4wt% Pt; 2.1wt% Ni, Johnson Matthey Technology Centre), 1 ml of isopropanol (VWR), 1 ml of water (MilliQ 18.2MW cm), and 0.550 g of Nafion^® (20 wt% solution, Sigma-Aldrich).

The prepared inks of Pt-Ru and Pt-Ni were respectively brushed onto ODAN cathodes (loading:3.7 mg/cm²) evenly and heated at 45°C in order to evaporate the isopropanol solvent.

Mixed catalytic material For preparation of the mixed catalyst, 72 mg of ODAN and 36 mg of Nafion^® was added to each of the ink compositions described above brushed on the carbon fibre support. At an isopropanol (VWR) to water (MilliQ 18.2 MW cm) ratio of 1 :1 , the mixed catalyst ink tended to seep through the spaces between carbon fibre in the carbon fibre substrate. Accordingly, further inks were prepared with an in used in ink were prepared isopropanol (VWR) to water (MilliQ 18.2 MQ cm) ratio of 4:1. The appearance of electrodes was tested by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX). (Hitachi TM3030 Tabletop Microscope)

The catalytic materials were tested against the glucose solution and simulated wastewater solutions described above. It was found that layered catalytic materials are generally more efficient at COD removal than mixed catalytic materials against glucose solution. However, a mixed catalytic material of Pt-Ru and ODAN prepared at an isopropanol water ratio of 4:1 provided very good efficiency against the glucose solution with COD removal rates in the region of 900-1 110g(O₂)r¹cm ²h ¹.

In contrast, against the simulated wastewater solution, the mixed catalyst catalytic materials generally performed better than the layered catalytic materials, with COD removal rates in the region of 550-930g(O₂)r¹cm ²h ¹.

The present invention is most advantageous in situations where biological techniques cannot be used, such as in treating chemical wastewaters, or where there are space constraints. Existing water treatment techniques often employ biological methods. Bacteria are very sensitive to reaction conditions, such as high salt content (phosphate or sulphate), pH and to bacteria-toxic substances such as aldehydes, phenols. The techniques currently used are very slow and result in a large system footprint.

For the removal of dissolved organics that are not accessible to biological methods there are only very harsh options, such as direct electrochemical oxidation, wet air oxidation, combustion, treatment with strong oxidising agents. All these options have significant drawbacks in terms of energy expenditure, which results in high costs. The technology presented by the present invention can be more than 100 times more energy efficient, as it operates with negligible energy input, namely that required for pumping the wastewater and for aeration, optionally supplemented by gentle heating and/or pressurised air or oxygen. This contrasts with catalytic wet air oxidation which operates under considerably less mild conditions.

Claims

1. A method for treating a waste stream comprising the steps of:

(a) providing a catalytic material;

(c) supplying oxygen to the waste stream; and

(d) oxidising the organic compound at the surface of the catalytic material with simultaneous reduction of oxygen

2. A catalytic material comprising at least one active site capable of oxidising the organic compound, and at least one active site capable of reducing oxygen.

3. A catalytic material as claimed in claim 2, wherein the catalytic material comprises:

(a) an oxidation catalyst; and

(b) an oxygen reduction catalyst.

4 A catalytic material as claimed in claim 2 or claim 3 wherein the oxygen reduction catalyst is selected from:

(a) carbonaceous materials comprising:

(i) 80 to 95 wt% carbon;

(ii) 0 to 20 wt% of at least one transition metal;

(iii) 0 to 20 wt% nitrogen;

(iv) 0 to 20 wt% sulphur; and

(v) 0 to 20 wt% phosphorus,

(c) nitrogen doped carbon compounds which comprise from 50 - 98wt% carbon and 10-50wt% of nitrogen;

(d) transition metal carbides;

(e) transition metal nitrides and carbonitrided;

(f) metal chalcogenides; and

(g) transition metal oxides.

5 A catalytic material as claimed in claim 4 wherein the oxygen reduction catalyst is a carbonaceous material which having a high surface area of about 200m²g ¹ or higher as determined by nitrogen adsorption analysis.

6. A catalytic material as claimed in claim 4 or claim 5 wherein the carbonaceous material is obtainable by oxidative polymerisation of diaminonaphthalene, 1 ,5- diaminonaphthalene or 1 ,8-diaminonaphthalene, more preferably 1 ,5- diaminonaphthalene.

7. A catalytic material as claimed in claim 6 wherein the carbonaceous material is obtainable by oxidative polymerisation of a diaminonaphthalene in the presence of at least one metal salt.

8. A catalytic material as claimed in claim 7 wherein the at least one metal salt is a ferrous or cobalt salt, preferably a halide, more preferably a chloride.

9. A catalytic material as claimed in any one of claims 2 to 8 wherein the oxidation catalyst is selected from pure metal or metal alloys comprising precious metal and/or non- precious transition metals, polyoxometalates, metal oxides and carbides.

10. A catalytic material according to any one of claims 2 to 9 having a selectivity toward hydrogen peroxide production, preferably a selectivity toward hydrogen peroxide production of about 1 % or higher.

1 1. A catalytic material according to any one of claims 2 to 10, wherein the oxidation catalyst is pelletized, deposited on carbon pellets or deposited on pellets made from the oxygen reduction catalyst.

12. A catalytic material according to any one of claims 2 to 10 wherein the oxygen reduction catalyst is pelletized, deposited on carbon pellets or deposited on pellets made from the oxidation catalyst.

13. A catalytic material according to any one of claims 2 to 12 wherein the catalytic material is a mixture of an oxidation catalyst and oxygen reduction catalyst.

14. A catalytic material according to any one of claims 3 to 13 wherein the catalytic material is formed with the oxidation catalyst in contact with the oxygen reduction catalyst such that, in use, a short-circuit condition is generated.

15. A catalytic material according to any one of claims 2 to 14 further comprising a binder bonding the catalysts, preferably wherein the binder is one or more of microcrystalline cellulose, carboxymethylcellulose, phenolic resin, ion exchange resin, sugars, humic acid-derived sodium salt, polyvinylalcohol, proprietary binder, polytetrafluorethylene, adhesive cellulose-based binder, wax, linseed oil, gum arabic, gum tragacanth, methylcellulose, proteins, polyvinypyrrolidone, polyisobutylene and styrene- butadiene rubber.

16. A catalytic material as claimed in any one of claims 2 to 15 wherein the catalytic material is formed by over-layering layers of the oxygen reduction catalyst and oxidation catalyst on a substrate.

17. A catalytic material as claimed in any one of claims 2 to 15 wherein the catalytic material is formed by mixing the oxygen reduction catalyst and oxidation catalyst and applying the mixture to a substrate.

18. A catalytic material as claimed in claim 16 or claim 17 wherein the substrate is a porous substrate and/or a conductive substrate.

19. A method as claimed in claim 1 wherein the catalytic material is a catalytic material as claimed in any one of claims 2 to 18.

20. A method as claimed in claim 1 or claim 19, wherein an energy input source such an electrolysis cell or energy extracting structure, such as a fuel cell, is not required.

21. A method as claimed in claim 1 , claim 19 or claim 20 wherein the method is carried out at a temperature of about 200°C or less, preferably about 100°C or less, more preferably about 50°C or less, even more preferably at ambient temperature.

22. A method as claimed in claim 1 or any one of claims 19 to 21 wherein the method is carried out at a pressure of about 3000kPa or less, preferably about 2000kPa or less, more preferably about 1000kPa or less, even more preferably at about 500kPa or less, most preferably at a pressure of about 100kPa or ambient pressure.

23. A method as claimed in claim 1 or any one of claims 19 to 22, wherein the organic compound includes formaldehyde and the method further comprises the step of removing formaldehyde from the waste stream.

24. A method as claimed in claim 1 or any one of claims 19 to 23 wherein the solvent is water.

25. A method for treating a waste stream comprising the steps of:

(a) providing a catalytic material;

(b) supplying a waste stream comprising an organic compound which is a liquid or dissolved in a solvent and contacting the catalytic material with the waste stream;

26. A method according to claim 25, wherein the reductant is hydrogen.

27. A method according to claim 25 or 26 wherein the organic compound is a compound containing a nitro-group either in pure form or dissolved in a solvent; a chlorinated organic compound either in pure form or dissolved in a solvent; or a mixture thereof.

28. A process for preparing a catalytic material, wherein the process comprises the steps of;

(a) providing a reaction mixture for the oxygen reduction catalyst; and/or

(b) providing a reaction mixture for the oxidation catalyst; and

(c) providing at least one heating step.

29. A process for preparing a catalytic material according to claim 28, wherein the catalytic material is further subjected to pyrolysis to form a monolithic structure.

30. A process according to claim 28 or claim 29 wherein the heating step is performed at a temperature of from 300 to 1 100°C at a heating rate of 0.1 to 20°C per minute.

31. A wastewater treatment device comprising a catalytic material as claimed in any one of claims 2 to 18.

32. A waste pipe or reactor wall which has been coated with a catalytic material as claimed in any one of claims 2 to 18.

33. Use of a catalytic material as claimed in any one of claims 2 to 18 in the oxidation of at least one organic compound in waste stream comprising at least one organic compound which is a liquid or dissolved in a solvent.