Field of the Invention

The present invention relates to a method for the photonically-driven electrochemical reduction of carbon dioxide to carbon. The method can be used to permanently store captured carbon dioxide in a safe and efficient manner so is of huge potential in the fight against climate change. The present invention also relates to a photonically- driven electrochemical method for depositing carbon, wherein the carbon may be in a form such as graphene or fullerene or a novel allotrope with commercially important physical properties. In addition the present invention relates to an electrochemical cell which can be used in these methods.

Background to the Invention

The increasing concentration of the greenhouse gas carbon dioxide in the atmosphere and the ensuing environmental effects are widely seen as one of the most serious problems facing mankind. Many people have been working on different ways to try to tackle this problem. Carbon capture refers to the process of capturing carbon dioxide from its source, such as a fossil-fuel burning power plant, or from ambient air. The captured carbon dioxide can then be stored, for example in the ground in geological formations or deep under the sea, such as in disused oil wells. Carbon capture and storage has been proposed as a way to slow the increase in the atmospheric carbon dioxide concentration, or even to decrease it. This technique does not aim to actually reduce the levels of carbon dioxide in the world, but to store some of the carbon dioxide so that it does not enter the atmosphere.

However, the long term storage of carbon dioxide is very risky due to the possibility of the carbon dioxide leaking out of the storage facility, for example in the event of an earthquake. This could have disastrous effects, particularly for under-sea storage where any carbon dioxide released could dissolve in the sea and cause acidification. In addition, the number of geographic formations that are suitable for storage is limited and transporting the carbon dioxide from its place of capture to the storage facilities can be costly and difficult. Therefore, the storage of carbon dioxide as a gas is not a very attractive solution. The reduction of carbon dioxide (CO ₂ ) to carbon (C) and oxygen (O ₂ ) is an extremely compelling approach to carbon storage as it takes away the possibility of carbon dioxide escaping from storage to the atmosphere. Indeed, it would practically eliminate any storage needs because carbon in solid form is hugely more compact than gaseous carbon dioxide and could be stored anywhere. However, rather than simply being stored the carbon produced could be used for energy or in industrial processes. Furthermore, the reduction of carbon dioxide to carbon gives the possibility of depositing carbon in different forms including as graphene or fullerene or even in a novel form. Hence the carbon itself has much value and may have commercially important physical properties. The reduction reaction would also form oxygen, which is beneficial to the environment and has many industrial applications.

There has been some limited research into this possibility but the overriding sentiment is that the reduction of carbon dioxide back to carbon would require a similar amount of energy as is produced by burning carbon to form carbon dioxide so this area has not been given much serious attention.

In the 1960s speculation about the existence of naturally occurring graphite led to a proposal that graphite can be produced by the reduction of atmospheric carbon dioxide

(i.e. carbon dioxide in gaseous form) in an electrochemical process, see Marx, P. C. (1971) The American Mineralogist 56, 336-338 and Tee, P. A. H., and Tonge, B. L.

^• (1963) Journal of Chemical Education 40, 117-123.

In a recent study motivated by the problem of recovery of oxygen (O ₂ ) from carbon dioxide in space, NASA demonstrated the complete reduction reaction in the solid state to form carbon and oxygen, but at temperatures in excess of 400° C using very complex composite electrodes, see Duncan, K. L., Hagelin-Weaver, H.E., Bishop, S.R., Neal, L., Pedicone, R. Wachsman, E.D. (University of Florida) and Paul, HX. (NASA: Johnson Space Center) 2007-01-3247, SAE International, 37 ^th International Conference on Environmental Systems (ICES) Chicago, Illinois July 9-12, 2007.

Hence, despite the enormous potential for this area of technology, the complete reduction of carbon dioxide to carbon has not yet been achieved under conditions which would be suitable for employment on an industrial scale. In particular, the ability to operate the reduction in the solid or liquid state would be highly advantageous. One of the aims of the present invention is to provide an improved method for the complete reduction of carbon dioxide to carbon, especially a method that can operate under conditions that will be more favourable to application on an industrial scale than existing methods. Another aim of the present invention is to provide a new method of depositing carbon.

Summary of the Invention

According to a first aspect, the present invention relates to a method of reducing carbon dioxide to carbon, the method comprising the steps of:

(i) providing an electrochemical cell comprising an anode, a cathode and a reference electrode, wherein the cathode comprises diamond which has been doped;

(ii) contacting the anode, cathode and reference electrode with carbon dioxide, wherein the carbon dioxide is either in the solid or liquid form;

(iii) applying a potential difference across the anode and cathode; and

(iv) irradiating the surface of the cathode.

According to a second aspect, the present invention relates to a method of depositing carbon on a surface, the method comprising the steps of:

(i) providing an electrochemical cell comprising an anode, a cathode and a reference electrode, wherein the cathode comprises diamond which has been doped;

(ii) contacting the anode, cathode and reference electrode with a carbon- containing substance;

(iii) applying a potential difference across the anode and cathode; and

(iv) irradiating the surface of the cathode,

so that carbon is deposited on the surface of the cathode. According to a third aspect, the present invention relates to an electrochemical cell which comprises:

an anode, a cathode and a reference electrode, wherein the cathode comprises diamond which has been doped;

means of applying a potential difference across the anode and the cathode; and a laser positioned so as to be capable of irradiating the surface of the cathode.

The electrochemical reduction of carbon dioxide to carbon is thought to proceed via a two-stage process involving a carbon monoxide intermediate. It has, to date, been extremely difficult to effect the second part of the reaction, reduction of carbon monoxide to carbon. The inventors have surprisingly found that the combination of using a diamond which has been doped as the cathode, irradiating the surface of the cathode, and applying a potential difference across the electrodes, provides an energetically favourable pathway for the reaction to occur. This allows the reaction, particularly the difficult second stage, to proceed at a more favourable rate and under more favourable conditions for industrial application than has previously been achieved.

In more detail, and without wishing to be bound by theory, it is thought that for solid or liquid carbon dioxide, the electrochemical mechanism for the reduction of carbon dioxide to carbon proceeds via one of the following reaction schemes.

Case 1 : O ^2" Oxide Anion is Intermediate Carrier

Cathode:

CO ₂ + 2e ^' — > CO + O ^2" 1 ^st reduction (1)

CO + 2e ^" — ^■ — -I K C + O ² - 2 ⁿ reduction (2)

Net cathode: CO ₂ + 4e ^" → C + 2O ^2' (3)

Anode:

2O ^2" ► O ₂ + 4e ^" (4) NET Cell Reaction: CO ₂ ► C + O ₂ (5) Case 2: CO ₃ ^2" Carbonate Anion is Intermediate Carrier

Cathode:

2CO ₂ + 2e ^" ► CO + CO ₃ ^2' 1 ^st reduction (6) CO + 2e ^" ► C + O ^2" 2 ^nd reduction (7)

Net cathode: 2CO ₂ + 4e ^' ► C + CO ₃ ^2' + O ^2" (8)

Anode:

2CO ₃ ^2" ► 2CO ₂ + O ₂ + 4e ^" (9) 2O ^2" ► O ₂ + 4e- (10)

Net anode: CO ₃ ^2" + O ^2" ► CO ₂ + O ₂ + 4e ^" (11)

NET Cell Reaction: CO ₂ ► C + O ₂ (12) In Case 1 and Case 2 the overall net cell reaction is identical. The difference is in the nature of the carrier anion. The second electrochemical reduction step of CO to C and O ^2" (equations 2 and 7) apparently does not readily occur. This may be due to energetic considerations since CO is the most stable of diatomic molecules and has a bond energy of about 1076.5 kJ/mol. The inventors believe that the second reduction process at the cathode will necessarily involve the adsorbtion/chemisorption of CO to the cathode surface forming the intermediate species (CO)^ ₅ which will be in dynamic equilibrium with CO. The inventors have discovered that it is this adsorption step which is critical to the efficiency of the final reduction process and that the properties and preparation of the cathode surface are key to this reaction.

Accordingly, the present invention is concerned with providing conditions at the cathode surface that will stimulate this reaction to occur. In particular, the use of a diamond which has been doped for the cathode, in combination with irradiation at the surface of the cathode and a potential difference across the electrodes can provide conditions under which dissociation of (CO) _ads is stimulated. The first step of the first aspect of the invention involves providing an electrochemical cell comprising an anode, a cathode and a reference electrode, wherein the cathode comprises diamond which has been doped. The key feature in promoting the reduction reaction of carbon dioxide to carbon, particularly the difficult second electrochemical reduction step of CO to C and O ^2" that occurs at the cathode, is the use of diamond that has been doped as the cathode. By "diamond which has been doped" we mean a material that comprises diamond (i.e. the allotrope of carbon where the carbon atoms are in a diamond lattice crystal structure) which contains some impurities. The type and level of impurities preferably alters the properties of the diamond. The cathode may also comprise other materials, particularly on its surface, as in one of the embodiments of the invention described below.

In a preferred embodiment, the cathode comprises boron-doped diamond. The presence of boron improves the electrical conductivity of the electrode. Typically the boron will be present at 10 ²⁰ cm ^'3 . The boron-doped diamond electrodes are provided by Diamond Detectors Ltd. Other dopants used include nitrogen which also typically forms part of the growth process.

Diamond electrodes have been used in the past for various purposes, such as in sensors and in electrochemical reactions. They have been used in the partial reduction of carbon dioxide in aqueous and non-aqueous solutions to species other than carbon, such as carbon monoxide, formic acid, methanol, oxalic acid and hydrocarbons (see Sanchez-Sanchez, C. M., Monteil, V. Tryk, D. A. and Aldaz, A. (2001) Pure Applied Chemistry, Vol. 73, No. 12, pp.1917-1927 and Spataru, N., Tokuhiro, K., Terashima, C, Rao, T. N., and Fujishima, A. (2003) Journal of Applied Electrochemistry, Volume -33, Number 12 / December, pp. 1205-1210). - However, the complete reduction to carbon, which involves the difficult second stage of the reaction, is not considered in these studies. In the present invention a diamond doped material is used for the cathode because of its special combination of properties. In particular, diamond has catalytic properties which are thought to promote the surface reactions involved in the complete reduction of carbon dioxide to carbon and has optical properties which are exploited in the present invention to stimulate the dissociation of adsorbed CO by photonic excitation. In particular, the optical transparent nature of diamond facilitates laser irradiation of the cathode surface at the required wavelengths, which is essential in the present invention.

The electrochemical cell used in the invention can be of any design which is suitable for electrochemicaUy reacting carbon dioxide in solid or liquid form. Where solid or liquid carbon dioxide is to be reacted, the cell will normally provide insulation or a cooling system in order to keep the carbon dioxide at the temperatures needed. One example of an electrochemical cell on a small scale is shown below in Fig 3. However, the invention can be scaled up for use on an industrial scale to convert captured carbon dioxide into solid carbon. For example, an electrochemical cell according to the invention can be incorporated into any industrial situation where carbon dioxide is produced, for example in energy generation plants such as coal fired power stations.

In addition to the cathode which is discussed above, the electrochemical cell includes an anode and reference electrode. The anode can be of any suitable material such as silver or nickel but silver is preferred. The reference electrode material may be gold or platinum and is used to monitor the electrical potential of the cathode.

The second step of the first aspect of the invention involves contacting the anode, the cathode and reference electrode with carbon dioxide, wherein the carbon dioxide is either in the solid or liquid form. The present invention aims to address the problem of excess carbon dioxide in the environment which is a large scale problem. Using carbon dioxide in the solid form, or in the liquid form allows- large amounts of carbon - dioxide to be treated at one time which can provide a valuable approach to this problem. Conventional methods can be used to capture and purify environmental carbon dioxide or carbon dioxide from a power generating plant and cool it to low enough temperature or high enough pressure to liquefy or solidify it. In a preferred embodiment, the carbon dioxide is in solid form. This allows for large scale reduction of carbon dioxide. The solid state has advantages for readily controlling and analysing the mechanisms of the reduction process and determining the structure of different types of carbon that may be deposited on the cathode. It is also easier to work in the solid state, since liquid CO ₂ requires operating at higher pressures. Once the optimum efficiency can be established for the overall process in the solid state and this mechanism is shown to operate to the liquid state, then scaling up to the industrial conversion of liquid CO ₂ to C and Oj gas may be preferable depending on the nature of the initial CO ₂ capture process and the relative costs involved.

The third step of the first aspect of the invention involves applying a potential difference across the anode and cathode. The potential difference is applied to effect the electrochemical reactions at the anode and the cathode which take place in the reduction reaction. Any conventional means can be used to provide the potential difference. In the small scale electrochemical cell described below a CV37 potentiostat can be used to provide the facilities for voltarnmetric and amperometric measurements. In the present invention the potential difference applied across the electrodes is typically between O and 10 volts. Preliminary results in a standard electrochemical cell (as in Figure 4) not subject to photonic excitation indicated that for a silver cathode potentiostatically held constant at about -3.2 volts relative to the anode the first electrochemical reduction step of CO ₂ to CO goes to completion (see equations 1 and 6 given above). The observed currents vary depending on the thickness of the solid CO ₂ separating the electrodes and may vary over a wide range from nA to μA.

In an electrochemical cell with optically transparent cathode electrode (Fig 3) held at constant applied potential and then subjected to photonic excitation, changes in the current flowing will reflect changes in the surface reactions induced by the laser radiation. In this way a method for elucidating the mechanisms of the reduction process as function of the applied potential and laser excitation wavelength is established. The fourth step of the first aspect of the invention involves irradiating the surface of the cathode. The purpose of this is to photonically promote the surface reactions involved in the reduction of carbon dioxide to carbon, in particular to photonically promote the dissociation of adsorbed CO. This step is key to the invention as, without it, the difficulty of overcoming the bond energy of the very stable CO means that conditions are needed that are not suitable for industrial application. Accordingly, it is this irradiation step in combination with the used of a doped diamond electrode which has optical properties allowing this irradiation to take place (as well as advantageous electrochemical properties) that enables the advantages of the invention to be achieved.

Near UV and near IR lasers will be used depending on which photonic induced dissociation mechanism is being probed: UV induced plasmon field generation to destabilise the adsorbed CO bond and IR photonic induced stretching & bending of the adsorbed CO bond respectively. The optical properties of diamond mean that it is transparent to the laser light and hence the laser is normally shone through the cathode to irradiate the surface. However, the laser can be positioned in any location, provided that it irradiates the surface of the cathode. Irradiation has been used in the past to assist in dissociation of bonds between atoms in fields other than electrochemical reactions. One recent study has focussed on the dissociation of near critical ncCO ₂ (carbon dioxide clusters formed at the gas-liquid critical points 31 °C and pressure 7.38MPa) and the creation of carbon particles when the carbon dioxide clusters are irradiated with laser beams at 213 and 266 run. See Fukuda, T., Maekawa, T., Hasumura, T., Rantonen, N., Ishii, K., Nakajima, Y., Hanajiri, T., Yoshida, Y., Whitby, R. and Mikhalovsky, S. (2007) New Journal of Physics 9, 321. Irradiation has not been used before to drive electrochemical reactions, such as the electrochemical reduction of carbon dioxide to carbon. There are three main mechanisms for photonically promoting the dissociation of absorbed CO using laser irradiation:

(1) The first mechanism relies on the laser excitation of silver nano -particles on the cathode surface to create a plasmon field with electrons of matching or greater energy than the LUMO (lowest unpopulated molecular orbital) 2π* anti-bonding molecular orbital of the adsorbed CO bond. These plasmon generated electrons tunnel into the anti-bonding orbital thus resulting in the destabilisation of the CO bond.

(2) The second mechanism is the laser-excitation of the stretching and bending modes of the CO. In this embodiment, charge transfer tunnelling is facilitated, thus lowering the Gibbs free energy of activation for the dissociation process.

(3) The third mechanism is the laser excitation of the adsorbed CO to a higher electronic state. Electrons in upper excited electronic states rapidly decay and can populate the LUMO anti-bonding 2π* orbital and induce dissociation.

The first mechanism is based on the indication that the dissociation can be triggered by populating the LUMO (lowest unpopulated molecular orbital) 2π* anti-bonding molecular orbital of the adsorbed CO bond which according to quantum mechanics destabilises the CO bond. Figures 1 and 2 and the discussion of these figures provides further details on this theory. The generation of surface plasmons of the appropriate energy requires structured metallic surfaces on cathodes. Suitable surface structures can be applied to the doped diamond using focussed ion beam etching. This mechanism is thought to work because electrons tunnel from the metal into the 2π* anti-bonding molecular orbital of the CO molecule whilst still in its electronic ground state. Resonance electron tunnelling results between Fermi electrons in the metal and the CO anti-bonding orbital which will destabilise the CO molecule and result in dissociation to C and O.

The metallic structures that are applied to the cathode surface in this- embodiment can include any suitable metal, such as gold, nickel, palladium, molybdenum or silver and preferably include silver particles. In particular, it is most preferably that the silver particles are nanoparticles, preferably nearly spherical in shape.

The size of the particles and the frequency of the irradiation is selected to create plasmon energies bracketing the energy range of the LUMO 2π* anti-bonding orbital (E _2π * -3.2 eV) of the CO molecule. A distribution of particle sizes providing a range of plasmon energies about E _2π * is appropriate to compensate for shifts in plasmon wavelength due to temperature effects, index of refraction changes and effects of the applied electric field. Estimates indicate that nearly spherical silver particles with peak particle size distribution preferably around 18 (±15) nm will generate the required plasmon energies.

In one embodiment of the invention, it is possible to use two different silver particle sizes to generate two plasmon fields, one to provide the energy required to destabilise the CO molecule and one plasmon field to track the charge transfer electrochemical reduction process.

The surface of the cathode, including the metallic structures, is then irradiated by laser to create plasmon energies bracketing the energy range of the LUMO 2π* anti- bonding orbital (-3.2 eV) of the CO molecule. The energy of the plasmon field Ep generated by laser excitation of silver nanoparticles equals the energy E2 _π * of the LUMO 2π* anti-bonding orbital of the adsorbed CO molecule thus promoting the dissociation of the adsorbed CO molecule to solid C and O ^2" anion (E _p = E ₂₇₁ *)

Hence, electrons from the plasmon field populate the antibonding orbital, destabilise the adsorbed CO bond and promote dissociation of the molecule. The added presence of an applied electric field then helps to drive the reaction to completion producing pure C at the cathode and O ₂ gas at the anode.

In this embodiment the structured surfaces will enable the use of surface enhanced Raman microspectrometry to characterise the nature of the layers deposited on the electrode surfaces, particularly of the carbon deposited on the cathode. The creation of plasmons is discussed in Weimer, W. A. and Dyer, M. J. (2001) Applied Physics Letters VoI 79 Number 19 3164, Emory, S. R., Haskins, W. E. and S. Me (1998) J. Am. Chem. Soc, 120, 8009-8010 and Mock, J. M., Smith, D. R. and Schulz, S. (2003) Nano Lett, Vol3, No. 4, 485-491.

The second mechanism is the laser-excitation of the stretching and bending modes of the CO. In this embodiment, charge transfer tunnelling is facilitated, thus lowering the Gibbs free energy of activation for the dissociation process. For pure CO, stretching and bending modes range from 1700 cm ^'1 to 2200 cm ^"1 . Therefore, by photonic excitation of CO vibrational stretching in the energy range with wavenumbers 1700 cm ^"1 to 2200 cm ^"1 , dissociation of absorbed CO is promoted. This requires the frequency of irradiation to be in the Near IR corresponding to energy levels of -0.2 IeV to ~0.27eV for pure CO.

The theoretical analysis of electron transfer at electrified interfaces has previously been proposed in a different context, see Kuznetsov, A. M., Ulstrup, J. (2000) Electrochimica Acta 45 2339-2361, which provides background. The third mechanism is the laser excitation of the CO to a higher electronic state. Electrons in upper excited electronic states (such as the 1 ^st excited electronic state) rapidly decay and can populate the LUMO anti-bonding 2π* orbital and induce dissociation. This involves irradiating the cathode surface at a laser wavelength in the Near UV with wavelengths around 209 nm. Excitation to the 1 ^st electronic excited state occurs with near UV laser irradiation at ~209nm which corresponds to ~5.93eV.

The optimum induced dissociation of CO may require combination of the three mechanisms proposed above. The present invention covers the use of different frequencies of irradiation to optimise the dissociation of CO using any combination of the three methods above.

What is clear is that any or all three photonic mechanisms taken together require less energy than the thermodynamic enthalpy ~11.15eV required to break the CO bond. The mechanisms proposed rely on the subtleties of quantum mechanics to destabilise the CO molecule by using energy level specific photonic excitation in combination with the high electric field generated at the electrode surface by electrochemical means. It is this combination that promotes the dissociation of the CO molecule in an energy favourable way and drives the overall electrochemical reduction of solid CO ₂ to completion yielding solid C and O ₂ gas.

As well as promoting dissociation of the CO bond on the surface of the cathode, the implementation of the three mechanisms outlined above promotes the formation of carbon at the cathode as may be summarised in the following equations:

CO + 2e ^" ► C + O ^2" (II) 2 ^nd reduction (2a) hυ ₂

C + C ►£> (2b) where CO and C are in solid state and adsorbed at the cathode surface. t>i is frequency to facilitate the breaking of CO bond.

As noted above, according to the preferred embodiment, the CO dissociation mechanism may be promoted by generating surface plasmon resonance at an estimated wavelength of -387 nm by irradiating silver nanoparticles with peak particle size distribution around 18 (±15) nm and/or by photonic excitation of CO vibrational stretching in the energy range with wavenumbers 1700 cm ^'1 to 2200 cm ⁴ . υ ₂ is the frequency to stimulate formation of adsorbed C ₂ and then possible further frequency changes can be made to promote further polymerisation: hi) ₃ hv ₄

C ₂ + C ^C ₃ + C ^► C ₄ etc. (2c)

The exact electrode composition, application of electric field and particular light frequencies enable the reduction of targeted activation energies in the reaction process and the determination of which molecular orbital energies participate in and regulate the reaction rates.

According to the second aspect of the invention, the method of the first aspect of the invention can be used to deposit carbon on a surface but with other carbon-containing substances. Suitable carbon containing substances include carbon dioxide or other carbon-containing substances such as hydrocarbons like methane. Using methane would be particularly advantageous as the reduction reaction products would yield deposited carbon and H ₂ gas. The H ₂ gas could provide fuel and, moreover, the overall effect of the reduction process remediates the impact on global warming of methane. The carbon-containing substance can be in any form, including in the solid or liquid form, as in the first aspect of the invention.

Depending on conditions of the reaction, such as the electric field and the irradiation, the carbon may be deposited in different forms. Hence, these conditions may be altered to produce the type of carbon desired. It is possible that amorphous carbon can be produced, or allotropes of carbon with special properties such as graphene, fullerene, Buckminster fullerene, graphite, or carbon nanotubes. It is also possible that novel forms of carbon are produced under different conditions. This has a huge significance for the development of carbon-based technologies. It may be possible to produce carbon particles of the correct size and structure as the substrate for microorganisms to create 'Carbon Preta', a supersoil for providing food worldwide.

According to the third aspect the present invention relates to an electrochemical cell which comprises an anode, cathode and reference electrode, wherein the cathode comprises diamond which has been doped; means of applying a potential difference across the anode and the cathode; and a laser positioned so as to be capable of irradiating the surface of the cathode. This electrochemical cell should be suitable for use in the methods of the first and second aspects of the invention. The specific components of the electrochemical cell are as described above in relation to the methods.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, of which:

Figure 1 shows a CO molecular orbital CAChe calculation showing effect of adding a silver atom to HOMO and LUMO orbitals. (See Vibrational and Electronic Spectra 2008

http://www2.okbu.edu/acadermcs/natsci/chemistry/people/rn rjordan/inorganicl/electro nic/ELECTRON.HTML (pgs 4 and 5 of 9) 10/03/2008 11:23:49.); Figure 2 shows a CO frontier molecular orbital diagram constructed from C and O atomic orbitals (See Moore, E. (2002) Molecular Modeling and Bonding, p.64 Published by The Royal Society of Chemistry, Cambridge CB4 OWF, UK. ISBN: 085404 675 5.); Figure 3 shows an electrochemical cell for use in the present invention; and

Figure 4 shows the electrochemical cell design used in preliminary experiments without photon stimulation. Detailed Description of the Figures

In more detail, Figure 1 provides a brief description of CO ligands and backbonding using CAChe software in the case for HOMO (highest occupied molecular orbital (sigma) and LUMO (lowest unoccupied molecular orbital (anti-bonding pi orbital)) for the carbonyl ligand shown both as pure CO and CO attached to a silver atom. For the pure CO, stretching modes range from 1700 cm ^"1 to 2200 cm ^"1 . What is clearly suggested from the Ag-CO LUMO diagram Figure 1 is that the anti-bonding 2π* orbital has a deficit of electron density between C and O atoms and an increased CO bond length. In this configuration, any of the three mechanisms outlined above can result in dissociation.

Figure 2 shows the frontier molecular orbital diagram for the CO molecule as constructed from C and O atomic orbitals, as discussed above.

Figure 3 shows a schematic of a cylindrically shaped electrochemical cell design with photonic stimulation capability consisting of three main sections: (1) anode assembly, (2) solid cylindrical CO ₂ section and (3) cathode and reference electrode assembly. Each section is described as follows:

Section 1 - Anode Assembly

Consists of Ag disc (13) and Ag contact wire (12) housed in PTFE rod (7) counter sunk at working end to fit Ag disc (13) surface flush or just beneath the PTFE rod surface - distance adjusted by PTFE micro-screw (7A) fitting in anode bulkhead. The PTFE rod (7) fits into the PTFE anode bulkhead (4) which itself is counter sunk at the working end to hold the cylindrical Neodymium cylindrically shaped gold plated magnet with central hole (5). The PTFE rod (7) threads the through both the anode bulkhead (4) and the hole of the magnet (5). The magnet is held in position flush to the working surface by PTFE set screws (6) positioned 120 degrees apart around the anode bulkhead (4) (1 of 3 set screws shown as 6). The working surface is defined by the plane of contact between the Ag anode disc (13) surface and the solid CO ₂ (2) interface. Section 2 - Solid Cylindrical CO ₂ Block (2).

Section 3 - Cathode & Reference Electrode Assembly

Consists of the components: Boron-Doped Diamond Electrode (BDDE) (8) mounted on an optically transparent window (9), a Neodymium gold plated disc magnet with central hole (10) all housed in a PTFE plinth (11). The Neodymium gold plated disc magnet (10) serves as the reference electrode. Electrical contacts to the BDDE and reference electrodes are not shown. The presence of the optically transparent window (9) provides physical support and allows the BDDE (8) to be irradiated by laser light from below as shown in figure 3 (by the hυ arrows). The transparent optical properties of the BDDE (8) ensure the effective transmission of photons to the electrochemically active BDDE/CO ₂ interface. The additional function of the Neodymium gold plated disc magnets (5) and (10) is to use their magnetic attraction properties to ensure continuity of electrical contact between Ag anode (13), solid CO ₂ (2) and BDDE cathode (8) surfaces.

Figure 4 illustrates the electrochemical cell design used in preliminary experiments without photon stimulation. In the numbering of Figure 4, a cathode electrode assembly 1 consists of contact wire sealed in a glass tube terminating in an exposed silver plate electrode and an anode electrode assembly 2 also consists of contact wire sealed in a glass tube terminating in an exposed silver plate electrode. A reference electrode assembly 3 consists of contact wire sealed in a glass tube terminating in an exposed platinum bead electrode. An output gas assembly 4 consists of a glass tube with an in-line oxygen sensor 4A and a flow meter (not shown), to measure gases produced in the electrochemical reduction process. An input gas assembly 5 consists of a glass tube for nitrogen dry gas; a solid cylindrical carbon dioxide block 6 is centrally disposed within solid spherical carbon dioxide pellets 7 and a thermistor temperature monitor 8 is provided centrally of the carbon dioxide block 6. The electrode assemblies 1 to 3, output gas assembly 4 and input gas assembly 5 are introduced through glands 9 consisting of BoIa PTFE screw connectors with "O ring" seals, enabling the glass tubes to be positioned within the PTFE cell cylindrical body 10. An oxygen calibration port 12 is provided and an "O" ring 13 seals the cell cap 11 to the PTFE cylindrical body 10.

Initial Experimental Protocol Referring to Figure 3

The aim of the research is to elucidate the mechanisms of the electrochemical reduction of solid CO ₂ to solid carbon and O ₂ gas as a function of the applied electric field and applied photonic stimulation on Diamond Doped (DD) electrically conductive electrodes. The type and structure of any deposited carbon reaction products formed on the cathode surface under different photonic excitation methods is analysed by spectroscopic and X-Ray techniques. The determination of which analysed by spectroscopic and X-Ray techniques. The determination of which combination of techniques provide the least expenditure of energy for the complete dissociation Of CO ₂ to C and O ₂ is a clear objective. The photonic stimulation is explored for two methods of destabilising the CO bond adsorbed at the electrode surface thus promoting dissociation. In each case, the photonic stimulation is applied at various stages in the electrochemical reduction process and at different overpotentials: 1) Irradiating the diamond/solid CO ₂ interface from behind using near UV laser light at (375 - 413 nm) to create plasmons on pre-deposited layer of silver nano-particles on the diamond electrode surface. This process exploits the fact that electrons of matching energy will populate the relatively low energy levels of the adsorbed CO LUMO 2π* anti-bonding orbital.

2) Irradiating the diamond/solid CO ₂ interface from behind using near IR laser light at (1700 - 2200 cm ^"1 ) to excite bending and stretching modes of adsorbed CO.

3) By varying electrochemical parameters such as the overpotential and the frequency level of photonic stimulation it is possible to tune the reduction dissociation process to produce specific carbon structures. In each experimental case, electrochemicar parameters such as overpotential and current as function of time and photonic stimulation will be monitored. Potentiostatic measurements will be used in the first instance. Preparation of custom made electronically conductive Boron DD electrodes (BDDE) of the correct refractive index to enable transparent illumination of the electrode/CO ₂ interface from behind at the required wavelength range will be supplied by Diamond Detectors. Other aspects of BDDE electrode surface preparation (smoothness, etc) are provided by Diamond Detectors Ltd.

Finally, suitable catalytic materials such as molybdenum may be added to the surface preparation to further enhance the electrochemical reduction process and reduce the overall energy requirement.