Process and apparatus

Abstract

The present invention relates to a method of producing syngas, the method comprising: supplying a first gas comprising CO2 to a first unit for the capture and release of CO2, wherein said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; capturing CO2 from said first gas on said adsorber material at said first condition; releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2; supplying said second gas to a second unit, wherein said second unit comprises a catalyst system for the conversion of CO2 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support; and converting CO2 to syngas.

Description

[0001] Process and Apparatus

[0002] INTRODUCTION

[0003] The present invention relates to a method and an apparatus for producing syngas. The present invention also relates to a catalyst system for the conversion of C02to syngas, to a reactor unit, preferably a gas phase reactor unit, comprising the catalyst system, and to a method of converting C02 to syngas which incorporates the reactor unit. The present invention also relates to a reactor unit for the capture and release of CO2, and to a method for the capture and release of CO2 which incorporates the reactor unit.

[0004] BACKGROUND

[0005] The capture, including the direct air capture (DAC), of carbon dioxide (CO2) stands out as a promising CO2 removal technology for actively removing CO2 from the atmosphere and combating the climate crisis.

[0006] Despite the advancements and pilot-scale implementations, DAC remains an energy-intensive process, leading to high costs for CO2 removal while not producing any products of economic value. This limitation confines the technology to localized environments with cheaply available energy sources to ensure economical operation.

[0007] Current state-of-the-art DAC technologies mostly operate by carbon capture and storage (CCS), where the captured atmospheric CO2 is stored under geological rock formations due to the low intrinsic value of CO2, hindering its reintegration into the economy. However, the long-term effects of storing huge amounts of CO2 underground over decades remain uncertain.

[0008] A more sustainable approach to managing the captured CO2 would be its utilization to produce renewable fuels, thereby creating value while also closing the carbon loop. Nevertheless, the most prominent CO2 utilization technologies operate with pure CO2 as the carbon feed while requiring high energy input by themselves, discouraging their overall integration in the DAC framework.

[0009] Solar-powered technologies are promising for a sustainable future to harness the Sun’s energy directly to fuel our economy. Solar-driven CC>2-to-fuel technologies have been explored extensively in recent decades, primarily in solid-liquid interfaces. However, the solution-based systems often face limitations due to the low solubility of CO2 in the aqueous medium.

[0010] To circumvent this limitation, gas-phase CO2 photoreduction processes are emerging which offer enhanced localized CO2 concentration, better mass transport, and reduced light scattering among other advantages. Despite the benefits, gas-phase reports show limited activities mostly due to the challenging thermodynamics of gas-phase CO2 reduction coupled to water oxidation. Further, these processes are limited by the requirement of pure CO2, whose production from emission sources remains cost-intensive ($125-335 ton-1from the air), thereby hindering their scale-up applications. From a materials perspective, gas-phase CO2 photoreduction catalysts typically use metal / metal oxide composites that can require high overpotentials resulting in low efficiency and selectivity.

[0011] The present inventors therefore had the idea to develop an integrated process which: offers strong levels of CO2 capture, ideally directly from air; exhibits controllable release of CO2, to provide concentrated CO2 suitable for utilisation; and provides efficient conversion of CO2 to useful products, with resilience to the presence of other gases.

[0012] SUMMARY

[0013] Viewed from a first aspect, the present invention provides a method of producing syngas, the method comprising: supplying a first gas, preferably air, comprising CO2 to a first unit for the capture, preferably direct air capture, and release of CO2, wherein said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; capturing CC^from said first gas on said adsorber material at said first condition; releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2; supplying said second gas to a second unit, wherein said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support; and converting CO2 to syngas.

[0014] Viewed from a further aspect, the present invention provides an apparatus for producing syngas, the apparatus comprising: a first unit for the capture, preferably direct air capture, and release of CO2; and a second unit for the conversion of gaseous C02to syngas, wherein: said first unit has an inlet for a first gas, preferably air, comprising CO2 and an outlet for a second gas comprising CO2 in fluid connection with an inlet of said second unit, and said second unit has an outlet for syngas; said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; and said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support.

[0015] Viewed from a further aspect, the present invention provides a reactor unit for the capture, preferably direct air capture, and release of CO2, the unit comprising: an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; and a means for achieving said second condition, preferably wherein said means for achieving said second condition is a reflector, preferably a parabolic trough reflector, and / or a thermal absorbent material.

[0016] Viewed from a further aspect, the present invention provides a method for the capture, preferably direct air capture, and release of CO2, the method comprising: supplying a first gas, preferably air, to a reactor unit as hereinbefore defined; capturing CC^from said first gas on said adsorber material at said first condition; releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2.

[0017] Viewed from a further aspect, the present invention provides a reactor unit, preferably a gas phase reactor unit, comprising a catalyst system for the conversion of CO2 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support, preferably a solid alumina or silica support, more preferably a solid alumina support.

[0018] Viewed from a further aspect, the present invention provides a method of converting CC>2to syngas, the method comprising: supplying a gas comprising C02to a reactor unit as hereinbefore defined; and converting said CO2 to syngas

[0019] Viewed from a further aspect, the present invention provides a catalyst system for the conversion of CC^to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support, preferably a solid alumina or a silica support, more preferably a solid alumina support.

[0020] Viewed from a further aspect, the present invention provides use of the methods, apparatuses, reactor units, and / or catalyst systems as hereinbefore described in the conversion to syngas of CO2 from air, from waste, and / or from exhaust gas mixtures, preferably from air.

[0021] DEFINITIONS

[0022] As used herein, the term “CO2 utilisation” or equivalently “CO2U” refers to the reaction of CO2 to form useful products. In the present invention, CO2U preferably refers to the conversion of CO2 to syngas.

[0023] As used herein, the term “syngas” refers to a mixture of H2 and CO, preferred molar ratios being as disclosed herein.

[0024] As used herein, the term “direct air capture” refers to the capturing of C02from air.

[0025] As used herein, the term “ambient temperature” preferably refers to a temperature of about 0 to 50 °C, more preferably about 10 to 35 °C, still more preferably about 15 to 35 °C, yet more preferably about 20 to 30 °C, e.g. about 25 °C.

[0026] As used herein the term “day” refers to a period of time wherein it is light, e.g. wherein light levels, e.g. environmental light levels, are greater than 1 lux.

[0027] As used herein the term “night” refers to a period of time wherein it is dark, e.g. wherein light levels, e.g. environmental light levels, are less than 1 lux.

[0028] As used herein, the ligand “tpyP” is a phosphonated terpyridine ligand, preferably having the following structure (which may be ionised):

[0029] As used herein, the term “CotpyP” refers to a transition metal complex comprising cobalt and two tpyP ligands. The term is thus used entirely interchangeably with the term “Co(tpyP)2” and the term “Co(tpyP)”. The same considerations apply to “NitpyP”, “Ni(tpyP)2” and “Ni(tpyP)”.

[0030] As used herein, the term “gas” encompasses mixtures of different gases, as well as pure gases. As used herein, and unless specified otherwise, discussion relating to the terms “first gas” and “second gas” excludes consideration of any carrier gas which may be present. As used herein, the term “photothermal heating” refers to heating achieved by the action of light, preferably sunlight. Photothermal heating effects can be amplified using light intensifying means and / or thermal absorption means, examples of which are discussed herein.

[0031] DETAILED DESCRIPTION

[0032] The present invention provides a method of producing syngas, the method comprising: supplying a first gas, preferably air, comprising CO2 to a first unit for the capture, preferably direct air capture, and release of CO2, wherein said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; capturing C02from said first gas on said adsorber material at said first condition; releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2; supplying said second gas to a second unit, wherein said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support; and converting CO2 to syngas.

[0033] The method benefits from an integration of CO2 capture, release, and utilisation. This enables CO2 to be captured from a source, such as air, and subsequently to be released in a more concentrated form at an appropriate time for conversion to useful products. The method therefore not only allows a gas to be decarbonised, but for the captured CO2 then to be upgraded to useful syngas products. This contrasts favourably with traditional carbon capture processes, where the captured carbon is often confined to longterm storage, without being exploited in any useful way. The method also makes use of a photocatalytic process for the utilisation of CO2, which means that economically valuable syngas can be obtained using a renewable and zero-emission energy source.

[0034] Advantageously, the second unit allows for the gas-phase conversion of CO2 to syngas with conversion rates comparable with the reported activities of the catalytic system in solution, without the need for any organic solvent or buffer solution, and without the solubility issues associated with solution processes. The use of a second unit suitable for gas-phase conversion presents the possibility for direct integration with the carbon capture and release of the first unit, the benefits of which include those discussed above. The present invention also analogously provides an apparatus for producing syngas, the apparatus comprising: a first unit for the capture, preferably direct air capture, and release of CO2; and a second unit for the conversion of gaseous C02to syngas, wherein: said first unit has an inlet for a first gas, preferably air, comprising CC^and an outlet for a second gas comprising CC^ in fluid connection with an inlet of said second unit, and said second unit has an outlet for syngas; said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; and said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support.

[0035] It will be understood that the apparatus can be an apparatus for performing the method of producing syngas as hereinbefore described. It will equally be understood that the method of producing syngas can be performed using the apparatus for producing syngas as hereinbefore described.

[0036] Preferred embodiments for the method and apparatus for producing syngas will be discussed together below. Preferred embodiments are to be understood as applying equally preferably to the method and / or apparatus.

[0037] First unit

[0038] Both the method and the apparatus of the present invention make use of a first unit for the capture and release of CO2. This unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition. A purpose of this unit is to allow for the first gas to be decarbonised, followed by the controlled release of the captured CO2. This is reflected in the method steps of: capturing CC^from said first gas on said adsorber material at said first condition; releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2.

[0039] A purpose of the capturing step is to remove CO2 from the first gas, or, said another way, to decarbonise the CC>2-containing first gas. The removal of CO2 from gas mixtures is important in directly or indirectly reducing the levels of CO2 in the atmosphere, in response to the climate crisis. A purpose of the subsequent CO2 release is to provide a second gas, the CO2 comprised therein being exploited for the production of syngas. Temporally separating capture and release of CO2 means that control can be exerted over the concentration and purity of CO2 supplied to the second unit, which may make for more efficient conversion to syngas. The timing of release can also be controlled according to when the conversion is desired to be performed. The integration of CO2 capture, release, and utilisation means that, in addition to decarbonising the first gas, the captured CO2 product can be converted to useful products, rather than merely being placed in long-term storage.

[0040] While in principle the first gas can consist of CO2, it will be understood that the capture and release of CO2 is less beneficial if no other gaseous species are present. The first gas is thus preferably a gas mixture comprising CO2. Preferred examples of gas mixtures include biogas, exhaust gas, and other waste gas mixtures, such as flue gas. The methods of the present invention allow CO2 to be removed from such gas streams, minimising the environmental impact associated with their release.

[0041] It is preferred that the first gas is air. Said another way, it is preferred that the first gas has a composition substantially corresponding to air. In such a case, the first unit can be considered a first unit for the direct air capture and release of CO2. The corresponding method steps can therefore comprise: supplying air to a first unit for the direct air capture and release of CO2, wherein said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; capturing CC^from said air on said adsorber material at said first condition.

[0042] Direct air capture is particularly advantageous as it facilitates the possibility of removing CO2 directly from the atmosphere, in response to the climate crisis.

[0043] Preferably, the first gas comprises greater than 0.01 vol%, preferably greater than 0.025 %, more preferably greater than 0.03 vol%, still more preferably greater than or equal to about 0.04 vol% CO2 based on the total volume of the first gas. Preferably, the first gas comprises less than 1 vol%, more preferably less than 0.5 vol%, still more preferably less than 0.1 vol%, yet more preferably less than 0.075 vol%, e.g. less than 0.05 vol% CO2 based on the total volume of the first gas. Preferably, the first gas comprises between 0.01 to 0.1 vol%, more preferably between 0.02 to 0.075 vol%, still more preferably between 0.03 to 0.05 vol% CO2 based on the total volume of the first gas. Such concentrations reflect the utility of the method in direct air capture.

[0044] Preferably, the first gas comprises less than 100 vol%, preferably less than 75 vol%, more preferably less than 50 vol%, still more preferably less than 25 vol%, 10 vol%, 5 vol%, 2.5 vol%, 2 vol%, 1 vol%, 0.5 vol%, or 0.1 vol% CO2 based on the total volume of the first gas. Such concentrations reflect the ability of the method to capture CO2 even from gases with low CO2 concentrations, such as air.

[0045] The first unit comprises an adsorber material for the capture and release of CO2. It is preferred that the adsorber material comprises: a solid support; and a molecular CO2 capturing agent.

[0046] It will be understood that the role of the molecular CO2 capturing agent is to provide the chemical functionality necessary to effect the capture of CO2 from the first gas. The molecular CO2 capturing agent is preferably a nucleophilic agent. Preferably, the molecular CO2 capturing agent comprises an amine, an alcohol, or an amine alcohol. Amines are particularly preferred, more preferably polyamines, e.g. polyethylenimine (PEI).

[0047] The solid support is preferably an inorganic solid support. Preferably, the solid support is alumina or silica, preferably silica. It is preferred that the molecular CO2 capturing agent is impregnated or loaded onto the solid support, to afford a composite material. Means of achieving this will be known to the skilled person, for example wet impregnation. The degree of loading can be tailored by the skilled person to particular circumstances. However, some preferred example degrees of loading may be between 20-80 %, more preferably 30-70 %, still more preferably 40-60 or 45-55 %, e.g. about 50 %.

[0048] As noted, the adsorber material captures CO2 under a first condition and releases CO2 under a second condition. This differential release and capture advantageously enables control over the process. For example, release need not occur until certain criteria are met, e.g. until sufficient CO2 has been captured to offer optimised levels of CO2 conversion in the second unit and / or until light conditions are suitable for the photocatalysed steps in the second unit. Such criteria having been met, a change to a second condition then allows the CO2 to be released and optimally exploited in the second unit.

[0049] It will be understood that capture and release of CO2 will be occurring simultaneously as a dynamic equilibrium. The important consideration is which of the two reactions dominates under a given condition. It will be understood that the rate of capture dominates over the rate of release at the first condition, while the rate of release dominates over the rate of capture at the second condition. In other words, there is a net capturing of CC^ at the first condition and a net release of CO2 at the second condition.

[0050] Preferably, the method comprises changing from the first condition to the second condition. In other words, the method preferably comprises: capturing CC^from said first gas on said adsorber material at said first condition; changing from said first condition to second condition; releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2.

[0051] Changing from the first to the second condition may be an active process or a passive process. The changing may also comprise both active and passive contributions. For example, a user may be able to apply artificial means to actively effect a change in conditions. Alternatively, passive or natural changes in environmental conditions may allow for the change to be achieved. Equally, a user may be able to exploit or enhance a change in environmental condition using artificial means to enable the change to be made. More specific examples of each of these possibilities are discussed below.

[0052] Preferably, the first and second conditions are different temperatures. It can be rationalised from principles of thermodynamics and equilibria that the release of CO2 may in some cases be preferred at higher temperatures, and capture preferred at lower temperatures. It is thus preferred that the second condition is a higher temperature than the first condition.

[0053] The exact temperature at which capture begins to dominate over release, or vice versa, will depend on the exact details of the system, and will be determinable by the person skilled in the art. For example, when using a more nucleophilic molecular CO2 capturing agent, the favouring of release over capture may occur at a higher temperature as compared with the use of a less nucleophilic molecular CO2 capturing agent.

[0054] Accordingly, the exact temperatures of the first and second conditions are not particularly limited. However, in some methods and apparatuses, the first condition is preferably a temperature of less than 60 °C, more preferably less than 50 or 40 °C, still more preferably less than or equal to 35 °C. In some methods and apparatuses, the first condition is preferably ambient temperature. In other words, the adsorber material preferably captures (i.e. is capable of capturing) CO2 under ambient temperature, e.g. ambient room temperature and / or ambient outdoor temperature. Preferably, the first condition is a nighttime temperature, e.g. an ambient night-time temperature and / or an outdoor night-time temperature. In such cases, the CO2 capture may occur under ambient conditions, preferably under night-time conditions.

[0055] In some methods and apparatuses, the second condition is preferably a temperature of greater than 50 °C, more preferably greater than 70 °C, still more preferably greater than 90 °C, yet more preferably at least 100 °C. Preferred ranges include 50-150 °C, preferably 70-130 °C, more preferably 80-120 °C, and still more preferably 90-1 10 °C. Such temperatures may be consistent with the use of certain nucleophilic molecular CO2 capturing agents, such as amines. A change from a first lower temperature condition to a second higher temperature may preferably be associated with a passive change in environmental temperatures, e.g. a change from night to day.

[0056] Such a change may also be associated with active or artificial heating means, including (but not limited to) ovens, hotplates, radiators, and / or heated water baths or water jackets.

[0057] Preferred artificial means include the use of thermal absorption means and / or light intensifying means. Preferred thermal absorption means include the provision of thermally absorbing material, e.g. tape, around the first unit. Such a material will absorb ambient heat, e.g. from sunlight, to contribute to an increase in temperature of the first reactor unit. Preferred light intensifying means include mirrors, lenses, and reflectors, preferably parabolic reflectors, more preferably parabolic trough reflectors. Other preferred examples of reflectors include linear Fresnel reflectors and parabolic dish reflectors. Such means can harness light energy, e.g. from sunlight, to facilitate an increase in temperature via a photothermal effect. Such artificial means can work in combination with a passive change in environmental conditions, e.g. a change from night to day. For example, as described in the examples, CO2 capturing can occur at a first night-time condition (which may be characterised by a night-time temperature), while CO2 release can occur at a second daytime condition achieved by the heating effect of the sun in combination with optional thermal absorption and / or light intensifying means. Advantageously, the steps occurring in the first unit can operate on a diurnal cycle, thereby maximising use of the sun’s energy.

[0058] In some methods and apparatuses, it is thus preferred that: said second condition is a higher temperature than said first condition; and / or said second condition is achieved by photothermal heating.

[0059] The first and second conditions may preferably be different light intensities. Preferably said first condition is a light intensity of less than 3 suns, more preferably less than two suns, still more preferably less than or equal to 1 sun, yet more preferably less than 1 sun. The first condition can also preferably be darkness, which may be complete darkness. Preferably, the first condition is an ambient light intensity, e.g. an ambient daytime light intensity or, more-preferably, an ambient night-time light intensity. In other words, the adsorber material preferably captures (i.e. is capable of capturing) CO2 under ambient light conditions, e.g. ambient room light conditions and / or ambient outdoor light conditions, preferably under ambient night-time light conditions (e.g. ambient outdoor light conditions), more preferably in darkness. The second condition is preferably a light intensity of greater than 1 sun, preferably greater than or equal to 2 suns, more preferably greater than or equal to 2.5 suns, still more preferably greater than or equal to 3 suns. Preferably, the light intensity is less than or equal to about 10 suns, more preferably less than or equal to about 6 suns.

[0060] Preferably, increases in light intensity are associated with higher temperatures, as demonstrated in the Examples.

[0061] Changing from a first to a second light intensity can be associated with a change from night to day, and the increments therebetween. Active means for adjusting light intensity include the use of lamps or other sources of artificial light, as well as exposing the first unit to a light source, e.g. into sunlight. As described above, a change in light intensity can be associated with active or artificial means in combination with passive or environmental means. For example, as described in the examples, CO2 capturing can occur at a first night-time condition, while CO2 release can occur at a second day-time condition achieved by the light intensity of the sun in combination with light intensifying means, such as a reflector. Advantageously, the steps occurring in the first unit can operate on a diurnal cycle, thereby maximising use of the sun’s energy.

[0062] Alternatively, the first and second conditions may be different pressures. It can be rationalised using principles of equilibria that release of CO2 may in some cases be preferred at lower pressures. It is thus preferred that said second condition is a lower pressure than said first condition. Means for adjusting pressure will be known to the skilled person, such as pressure pumps.

[0063] Generally, it is preferred that the first condition is an ambient or resting condition, while the second condition is achieved by active means. For example, it is preferred that the first condition is an ambient temperature (e.g. an ambient night-time temperature), while the second condition is a higher temperature achieved at least partially by active means (e.g. through the use of light-intensifying means), preferably by photothermal heating. It is preferred that the first unit comprises means for achieving the second condition. Example means have been variously discussed above. Preferred means include light intensifying means, preferably a reflector, more preferably a parabolic trough reflector, and / or a thermal absorbent material. In all embodiments of the present invention, it is preferred that a reflector achieves a photo intensity of greater than 1 sun, preferably greater than 2 suns, more preferably at least 3 suns.

[0064] As suggested above, it is preferred that said first condition corresponds to a nighttime condition and said second condition corresponds to a day-time condition or vice versa, though it is more preferred that said first condition corresponds to a night-time condition and said second condition corresponds to a day-time condition. It will be understood that the steps in the second unit are best performed in the presence of light, given the use of a photocatalyst. The association of the CO2 release with a day-time condition therefore provides temporal synchronisation between the release of CO2 and the preferred operating conditions of the second unit, in which the released CO2 is exploited. In night-time conditions, when the second unit is less effective, the overall apparatus can still be performing useful work, as the first night-time condition allows CO2 capture to be performed during this time.

[0065] The step of capturing CO2 is therefore preferably conducted at night-time and the step of releasing CO2 is preferably conducted at day-time.

[0066] The first unit preferably comprises means for moving, e.g. flowing, the first gas through the first unit, e.g. over and / or through the adsorber material. The method preferably comprises moving, e.g. flowing, the first gas through the first unit, e.g . over the adsorber material. The flow rate can be controlled by the skilled person to optimise the rate of CO2 capture. The mass of the adsorber material can also be controlled by the skilled person to optimise the rate of CO2 capture. The flow rate and / or the mass of adsorber material are preferably selected so as to align the saturation time of the adsorber material with local night-time. The saturation time of the adsorber material is preferably between 5-18 h, more preferably 6-16 h, still more preferably 7 to 14 h, yet more preferably 8 to 12 h, still more preferably 9 to 10 h. Saturation is preferably 90 % saturation, more preferably 95 % saturation, still more preferably 99 % saturation, e.g. about 100 % saturation.

[0067] The steps in the first unit provide a second gas comprising CO2. Preferably, the second gas is more concentrated in CO2 than the first gas. Said another way, the second gas preferably exhibits a peak concentration of CO2 which is greater than the concentration of CO2 in the first gas. It will be understood that the concentration of CO2 in the second gas will decrease with time as the CO2 is increasingly released from the absorber material. The concentration of CO2 in the second gas is preferably judged at the point of exit of the second gas from the first unit.

[0068] Preferably the peak concentration of the second gas is at least 1 .5 times more concentrated in CO2, e.g. by vol%, than said first gas (preferred concentrations for which are as discussed above). Preferably the peak concentration of the second gas is between 1.5-4 times more concentrated in CO2, e.g. by vol%, than said first gas (preferred concentrations for which are as discussed above). Such a range may be reflective of CO2 derived from a first gas which is biogas, where CO2 concentrations may be about 30 vol%. Preferably the peak concentration of the second gas is between 2.5-10 times more concentrated in CO2, e.g. by vol%, than said first gas (preferred concentrations for which are as discussed above). Such a range may be reflective of CO2 derived from a first gas which is exhaust gas, where CO2 concentrations may be about 15 vol%. Preferably the peak concentration of the second gas is between 1000-2750 times more concentrated in CO2, e.g. by vol%, than said first gas (preferred concentrations for which are as discussed above). Such a range may be reflective of CO2 derived from a first gas which is air, where CO2 concentrations may be about 0.04 vol%.

[0069] The second gas preferably consists, or substantially consists, of CO2. This may reflect the selectivity of the absorber material for CO2.

[0070] The present invention also provides a reactor unit for the capture, preferably direct air capture, and release of CO2, the unit comprising: an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; and a means for achieving said second condition, preferably wherein said means for achieving said second condition is a reflector, preferably a parabolic trough reflector, and / or a thermal absorbent material.

[0071] The present invention also provides a method for the capture, preferably direct air capture, and release of CO2, the method comprising: supplying a first gas, preferably air, to the reactor unit hereinbefore defined; capturing CO2 from said first gas on said adsorber material at said first condition; releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2.

[0072] Preferred embodiments for each of the features, as well as the nature of any additional features, of such a reactor unit and method are as discussed in relation to the method and apparatus for producing syngas. It will be understood that the first condition is preferably an ambient or resting condition, while the second condition is preferably achieved by active means.

[0073] As mentioned above, it is preferred that the first gas is gas mixture, preferred examples of which include biogas, exhaust gas, and other waste gas mixtures, such as flue gas. It is particularly preferred that the first gas is air.

[0074] As mentioned above, it is preferred that the absorber material comprises: a solid support; and a molecular CO2 capturing agent. Preferred features of the solid support and molecular CO2 capturing agent are discussed above.

[0075] As mentioned above, it is preferred that the first and second conditions are different temperatures, preferred details of which are discussed above. For example, the first condition is preferably a temperature of less than 60 °C, more preferably less than 50 or 40 °C, still more preferably less than or equal to 35 °C; and / or the second condition is preferably a temperature of greater than 50 °C, more preferably greater than 70 °C, still more preferably greater than 90 °C, yet more preferably at least 100 °C. As mentioned above, it is preferred that said first condition corresponds to a night-time condition and said second condition corresponds to a day-time condition.

[0076] Preferred means for achieving said second condition are as discussed above. Particularly preferred means include light intensifying means, preferably a reflector, more preferably a parabolic trough reflector, and / or a thermal absorbent material. Preferably, said second condition is achieved by photothermal heating, and suitable means are thus preferably chosen accordingly.

[0077] It will be understood that the steps associated with the first unit are preferably conducted in the gas phase. Preferably, the first unit is a gas phase unit. Said another way, the capture and release of CO2 is preferably a capture and release of gas phase CO2.

[0078] Said another way, the steps associated with the first unit are preferably not conducted in the liquid or solution phase. Preferably, the first unit is not a liquid or solution phase unit. Said another way, the capture and release of CO2 is preferably not a liquid or solution phase capture and release of CO2. For example, CO2 is preferably not provided to the first unit in solution. Preferably, the reactor unit does not comprise, or substantially does not comprise, liquid or solution. The adsorber material is preferably not provided in solution.

[0079] Second Unit

[0080] The method of the present invention of producing syngas comprises steps of: supplying said second gas to a second unit, wherein said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support; and converting CO2 to syngas.

[0081] A purpose of these steps, and therefore the second unit, is to utilise the CO2 captured and released in the first unit. This integrated approach advantageously makes useful exploitation of the captured and released CO2, rather than merely confining it to longterm storage. This is also reflected in the apparatus of the present invention for producing syngas, which comprises a second unit for the conversion of gaseous C02to syngas, wherein said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support.

[0082] It will therefore be understood that the second unit is configured to receive the second gas comprising CO2. It is preferred that the second gas is directly supplied from the first unit to the second unit. In other words, the second gas is preferably not passed to any intermediate units and / or is not subject to any intermediate treatment. Viewed another way, the outlet of the first unit is preferably in direct fluid communication with the inlet of the second unit. This reflects the suitability of the second unit for gas-phase conversion, which means that the first unit can advantageously be directly integrated with the second unit. There is no need, for example, to bring the released CO2 into the solution phase, where poor levels of solubility are often observed.

[0083] A carrier gas is preferably used to convey the released CO2 to the second unit. Another role of the carrier gas may be to displace the released CO2 from the proximity of the adsorber material, which may encourage the further release of CO2. The apparatus preferably therefore comprises means for providing a carrier gas for carrying the second gas to the second unit. The method preferably comprises supplying said second gas to said second unit using a carrier gas.

[0084] The carrier gas is preferably an inert gas, preferred examples of which are N2and noble gases, more preferably N2. In some methods and apparatuses, the carrier gas may be air. As described in the examples, the method and apparatus of the present invention are resilient to gas compositions comprising species other than CO2. However, it is preferred that the carrier gas is anaerobic, or substantially anaerobic. The carrier gas preferably comprises less than 21 vol%, more preferably less than 10 vol%, still more preferably less than 5 vol%, yet more preferably less than 1 vol% O2, based on the total volume of the carrier gas.

[0085] When a carrier gas is used, it is preferred that greater than 0.04 vol%, more preferably greater than 1 vol%, still more preferably greater than 5 vol%, still more preferably greater than 10 vol%, 20 vol%, or 35 vol%, yet more preferably greater than or equal to about 50 vol% CO2 is present based on the combined volume of the second gas and the carrier gas. Such concentrations are preferably judged at the point of exit of the combined second gas and carrier gas from the first unit. Such concentrations are preferably peak concentrations. The concentration, e.g. average concentration, of CO2 exposed to the catalyst system can be controlled by the skilled person using the flow rate of the carrier gas. Preferred flow rates for the carrier gas are between 0.05 to 500 mL / min, more preferably 0.1 to 200 mL / min.

[0086] The second unit comprises a catalyst system for the conversion of C02to syngas. It will be understood that the catalyst system enables, or contributes to the ability of, the second unit to perform the conversion of CO2 to syngas. The catalyst system comprises: a photocatalyst; a molecular catalyst; and a solid support.

[0087] It has been found that such a catalyst system can provide for efficient CO2 conversion to syngas.

[0088] A further aspect of the present invention therefore relates to a catalyst system for the conversion of CC^to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support, preferably a solid alumina or a silica support, more preferably a solid alumina support.

[0089] Preferred features of the catalyst system are discussed below. Such embodiments are to be understood to apply equally preferably to the methods and / or apparatus of the present invention for producing syngas, and / or to the catalyst system of the present invention for the conversion of CCLto syngas.

[0090] The catalyst system comprises a photocatalyst. A purpose of the photocatalyst is to provide a means of driving the conversion reaction using energy derivable from sunlight. It will be understood that the photocatalyst, via the interaction of its energy band levels with radiation derivable from sunlight, contributes the electronic driving force for conversion of the CO2 by the catalyst system. The use of the photocatalyst thus advantageously enables the use of a renewable and zero-emission source of energy, in the form of sunlight. When the first and second conditions in the first unit represent day and night conditions respectively, as discussed above, both the release of CO2 from the first unit and the conversion of CC^ in the second unit can thus advantageously be temporally synchronised.

[0091] In the present invention, the nature of the photocatalyst need not be particularly limited, and certain examples will be known to those skilled in the art. It will be understood that the energy band levels of the photocatalyst must be suitable to catalyse the reduction of CO2. Preferred photocatalysts are selected from metal oxides, preferably transition metal oxides, examples being SrTiOs and TiC>2. Other types of photocatalyst may also be used, such as carbon nitride family (CNX, where x is in the range of 1 to 2.33 and most commonly about 1.33). It is particularly preferred that the photocatalyst is a titanium-containing photocatalyst.

[0092] It is particularly preferred that the photocatalyst comprises or consists of TiO2, e.g. that the photocatalyst is a TiO2 photocatalyst. The photocatalyst may optionally be doped, for example doped TiC

[0093] The catalyst system also comprises a molecular catalyst. It will be understood that the molecular catalyst contributes to the active site at which CO2 conversion takes place. Such a reaction will comprise the reduction of CO2 to CO. In the present invention, the nature of the molecular catalyst need not be particularly limited, and certain examples will be known to those skilled in the art. It will be understood that the molecular catalyst should be suitable for the conversion of CO2 to syngas, or, said another way, suitable for the reduction of C02to CO.

[0094] In the present invention, the molecular catalyst is preferably a transition metal catalyst, more preferably a first-or second-row transition metal catalyst, still more preferably a first-row transition metal catalyst. Preferred transition metals are those selected from Co, Rh, Ir, Ni, Re, Mn, more preferably Co. The preferred oxidation state for the transition metal is (II).

[0095] It is preferred that the molecular catalyst comprises aromatic ligands, which includes heteroaromatic ligands, which may be substituted or unsubstituted. It is particularly preferred that the molecular catalyst comprises pyridine-containing ligands, examples of which include pyridine, bipyridine, and / or terpyridine, each of which may be substituted or unsubstituted. Pyridine groups in pyridine-containing ligands are preferably substituted with anionic groups, preferably phosphorus-containing anionic groups, more preferably wherein the pyridine is phosphonated. Other preferred ligands include phthalocyanine, porphyrin, and carbonyl ligands. A particularly preferred ligand is tpyP.

[0096] It is particularly preferred that the molecular catalyst is selected from Co(tpyP)2 and Ni(tpyP)2, more preferably Co(tpyP)2.

[0097] The catalyst system also comprises a solid support. It has been found that the use of the solid support allows for efficient gas transport through the catalyst system, and thus improved catalytic activity. As explained in the Examples, a significant decrease in CO formation was observed when a catalyst system without a solid support was used. Preferably, the photocatalyst and the molecular catalyst are immobilised and / or loaded on the solid support. Preferred solid supports are inorganic solid supports, more preferably solid supports selected from alumina solid supports and silica solid supports, preferred examples of which are mM2O3(m-alumina), AI2O3 ( / -alumina), SiC>2, and nSiC>2 (n-silica). The choice of the solid support allows tuning of the ratios of syngas products and the rate of production thereof.

[0098] It is particularly preferred that the solid support is an alumina solid support, more preferably mM2O3or AI2O3, most preferably gAI2C>3. It was found that gAI2C>3 had particularly high conversion rates, with a H2:CO ratio particularly suited to certain downstream applications.

[0099] It is preferred that the photocatalyst, the molecular catalyst, and the solid support are distinct entities. For example, it is preferred that the solid support is not the same entity as the molecular catalyst or as the photocatalyst. Similarly, it is preferred that the photocatalyst is not the same entity as the molecular catalyst.

[0100] It is preferred that the catalyst system further comprises a reducing agent. In other words, it is preferred that the second unit comprises a reducing agent, or, said another way, that a reducing agent is present in said second unit. The conversion of CO2 to syngas is characterised by a reduction of CO2 to CO. An oxidation of the reducing agent is the counter reaction to this reduction step.

[0101] In the present invention, the nature of the reducing agent need not be particularly limited, and certain examples will be known to those skilled in the art. It will be understood that the free energy change associated with oxidation of the reducing agent must be sufficiently low so as not to prohibit the counter reaction of CO2 reduction.

[0102] However, preferred reducing agents are selected from water, alcohols, amines or mixtures thereof. The reducing agent may be supplied in and / or be present in the second gas and / or the carrier gas. In such as case, rather than (or in addition to) the catalyst system / second unit comprising a reducing agent, preferably the second gas and / or the carrier gas comprises a reducing agent and / or is a wet gas, wherein the gas is wet with a reducing agent.

[0103] It has been found that the favourable energetics of alcohol oxidation facilitate effective CO2 conversion. As described in the examples, it has also been found that oxidation products can comprise useful platform chemicals, such as formate and glyceraldehyde, which may preferably be separated from the second unit. The reducing agent is therefore preferably an alcohol.

[0104] Suitable alcohols can advantageously be recovered from different waste streams, contributing to the improved environmental impact of the present invention. The reducing agent, preferably an alcohol, is thus preferably derived from (e.g. a product of) plastic recycling (a preferred example of which is ethylene glycol), bio refining (a preferred example of which is glycerol), and / or biomass (preferred examples of which are sugars).

[0105] Preferred alcohols include polyols, such as diols and / or triols and / or sugars, preferred examples of which are ethylene glycol (EG) and / or glycerol. As shown in the Examples, it has been found that the use of EG leads to particularly high conversion rates. The reducing agent is therefore preferably EG.

[0106] EG can also be, and preferably is, obtained or derived from recycling of polyethylene terephthalate (PET) plastic waste (such as plastic bottle waste). For example, as shown in the Examples herein, PET waste can be employed as a reducing agent, preferably following lysis pre-treatment to produce EG, preferably base-mediated lysis pre-treatment, more preferably hydroxide-mediated lysis pre-treatment, still more preferably KOH-mediated lysis pre-treatment. Plastic waste is preferably subjected to a shredding step or similar prior to conversion to EG, which may allow for improved conversion. It has been found that using PET-derived EG in this manner advantageously does not sacrifice the syngas production activity of the system or the CO selectivity.

[0107] Other features of the second unit of the present invention in the method and apparatus for the conversion of CO2 to syngas as well as in the reactor unit per se are now discussed.

[0108] High light intensities may be preferred for the second unit. This can be rationalised by considering an increase in activity of the photocatalyst. Preferably, the light intensity is at least 1 sun, more preferably greater than 1 or 2 suns, still more preferably greater than or equal to 3 suns. Higher light intensity can be achieved by artificial lighting means and / or by light intensifying means as discussed above, preferably through the use of a reflector, more preferably a parabolic reflector, still more preferably a parabolic trough reflector.

[0109] As described in the Examples, it has been found that the CO2 conversion reaction proceeds better under lower temperatures. Without wishing to be bound by any theory, this may be due to a partial deactivation of the molecular catalyst at very high temperatures. Preferably, the second unit is held at a temperature of less than 56 °C, preferably less than 43 °C, more preferably less than 30 °C, still more preferably less than or equal to 25 °C. Lower temperatures can be achieved using environmental or artificial control, preferably without compromising light intensity, preferred values for which are discussed above. Preferably, the method comprises cooling the reactor, optionally using cooling means. Preferably, the apparatus comprises cooling means. Preferred cooling means include a water bath (e.g. set to an appropriate temperature) or a water jacket, preferably with a supply of running water, or the application of ice / an ice bath.

[0110] It will be understood that the purpose of the second unit, and the method steps associated therewith, is the conversion of CO2 to syngas. Preferably, the syngas comprises, consists essentially of, or consists of H2and CO.

[0111] It has been found that the molar ratio of H2to CO can be controlled by various factors, such as the choice of solid support. Different ratios will be suitable for different downstream applications, so the values of the ratio need not be particularly limited. However, in some methods and apparatuses, the preferred molar ratio of H2to CO is between 1 :99 to 99:1 , preferably 10:90 to 90:10, more preferably 25:75 to 75:25, still more preferably 40:60 to 60:40, e.g. 45:55 to 55:45. Most preferably, the molar ratio is about 1 :1 .

[0112] As alluded to already, the use of the gas phase has advantages over traditional solution methods. The second unit provides conversion rates comparable with the reported activities of the catalytic system in solution, without the need for any organic solvent or buffer solution, highlighting the advantages of a gas-phase reaction setup described herein. Preferably, the steps associated with the second unit are conducted in the gas phase. Preferably, the second unit is a gas phase unit. Said another way, the conversion of C02to syngas is preferably a gas phase conversion of C02to syngas.

[0113] Said another way, the steps associated with the second unit are preferably not conducted in the liquid or solution phase. Preferably, the second unit is not a liquid or solution phase unit. Said another way, the conversion of C02to syngas is preferably not a liquid or solution phase conversion of C02to syngas. For example, CO2is preferably not provided to the second unit in solution. Preferably, the reactor unit does not comprise, or substantially does not comprise, liquid or solution. The catalyst system is preferably not provided in solution. It will also be understood that the steps associated with the second unit are preferably not electrochemical steps, for example involving an electrochemical cell. For example, the conversion of CO2to syngas is preferably not an electrochemical conversion of C02to syngas, for example involving an electrochemical cell.

[0114] The present invention also provides a reactor unit, preferably a gas phase reactor unit, comprising a catalyst system for the conversion of C02to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support, preferably a solid alumina or silica support, more preferably a solid alumina support. Preferred embodiments for each of the features, as well as additional features, of such a reactor unit are as discussed in relation to the method and apparatus for producing syngas and the catalyst system as discussed above.

[0115] As mentioned above, the photocatalyst is preferably selected from metal oxides, preferably transition metal oxides, examples being SrTiOs and TiO2. It is particularly preferred that the photocatalyst is a titanium-containing photocatalyst. The molecular catalyst is preferably a transition metal catalyst and / or preferably comprises aromatic ligands.

[0116] As mentioned above, preferred solid supports are inorganic solid supports, more preferably solid supports selected from alumina solid supports and silica solid supports, preferred examples of which are mAI2O3 (m-alumina), gAI2C>3 ( / -alumina), SiO2, and nSiO2(n-silica). It is particularly preferred that the solid support is an alumina solid support, more preferably mAI2C>3 or gAI2C>3, most preferably gAI2C>3.

[0117] As mentioned above, it is preferred that that the catalyst system further comprises a reducing agent. In other words, it is preferred that the reactor unit comprises a reducing agent, or, said another way, that a reducing agent is present in said reactor unit. The reducing agent is preferably an alcohol, more preferably EG, which may be derived from recycling of polyethylene terephthalate (PET) plastic waste.

[0118] It is preferred that the reactor unit is a gas phase reactor unit. Preferably, the reactor unit does not comprise, or substantially does not comprise, liquid or solution. The catalyst system is preferably not provided in solution.

[0119] The present invention also provides a method of converting CO2to syngas, the method comprising: supplying a gas comprising C02to a reactor unit as hereinbefore defined; and converting said CO2to syngas.

[0120] Preferred embodiments for each of the features, as well as additional features, of such a reactor unit are as discussed in relation to the method and apparatus for producing syngas as discussed above, in addition to the below.

[0121] In the method of converting C02to syngas, it is preferred that the gas comprises CO2at a concentration greater than in air. Preferred concentrations are as discussed above in relation to the second gas. The gas may consist, or consist essentially, of CO2. It has been found that higher concentrations of CO2are conducive to more efficient CO2conversion. Preferably, the gas is from an exhaust or waste stream, e.g. flue gas.

[0122] The present invention also provides a catalyst system for the conversion of C02to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support, preferably a solid alumina or a silica support, more preferably a solid alumina support.

[0123] Preferred embodiments for each of the features, as well as additional features, of such a reactor unit are as discussed in relation to the methods and apparatuses for producing syngas, and the reactor unit comprising a catalyst system for the conversion of CC>2to syngas, as discussed above, in addition to the below.

[0124] As mentioned above, the photocatalyst is preferably selected from metal oxides, preferably transition metal oxides, examples being SrTiOs and TiC>2. It is particularly preferred that the photocatalyst is a titanium-containing photocatalyst. The molecular catalyst is preferably a transition metal catalyst and / or preferably comprises aromatic ligands.

[0125] As mentioned above, preferred solid supports are inorganic solid supports, more preferably solid supports selected from alumina solid supports and silica solid supports, preferred examples of which are mAI2C>3 (m-alumina or mesoporous alumina), gAI2C>3 (y- alumina or gamma-alumina), SiC>2, and nSiC>2 (n-silica or silica nanoparticles). It is particularly preferred that the solid support is an alumina solid support, more preferably mAI2C>3 or gAI2C>3, most preferably gAI2C>3.

[0126] As mentioned above, it is preferred that the catalyst system further comprises a reducing agent. The reducing agent is preferably an alcohol, more preferably EG, which may be derived from recycling of polyethylene terephthalate (PET) plastic waste.

[0127] It is preferred that the catalyst system is not provided in solution. Instead, it is preferred that the catalyst system is for the gas phase conversion of CO2 to syngas. Preferably, the reactor unit does not comprise, or substantially does not comprise, liquid or solution. The catalyst system is preferably not provided in solution.

[0128] It is preferred that the photocatalyst, the molecular catalyst, and the solid support are distinct entities. For example, it is preferred that the solid support is not the same entity as the molecular catalyst or as the photocatalyst. Similarly, it is preferred that the photocatalyst is not the same entity as the molecular catalyst. Preferably, the photocatalyst and the molecular catalyst are immobilised and / or loaded on the solid support.

[0129] Further Features

[0130] Further features of the method and apparatus for producing syngas are described below. The methods and apparatuses may preferably comprise at least one further downstream unit in fluid communication with said first and second units. Such downstream units are preferably selected from: a unit for the capture and release of CO2; and / or a unit for reaction of syngas.

[0131] The method may thus comprise the following downstream steps: capture and release of CO2 in a unit for the capture and release of CO2; and / or reacting syngas in a unit for the reaction of syngas.

[0132] An advantage of a downstream unit for the capture and release of CO2 is that unreacted CO2 produced in the second unit can be put to further use. This may compensate for any imperfect conversion in the second unit. Such a unit may preferably be followed by a downstream unit for the conversion of CO2 to syngas. A downstream unit for the capture and release of CO2 may preferably be a unit which matches the description of the first unit hereinbefore described. A downstream unit for the conversion of CO2 to syngas may preferably be a unit which matches the description of the second unit hereinbefore described.

[0133] Rather than incorporating distinct downstream units, it is alternatively (or additionally) envisioned that the second unit may comprise means for recycling a gas stream exiting said second unit back to said first unit. Preferably, said gas stream comprises CO2 which was unconverted by the second unit.

[0134] A downstream unit for the reaction of syngas has the advantage of usefully exploiting the product of the second unit. Syngas is a known feedstock for many reactions, and the skilled person will be able to contemplate a number of such reactions and how they can be accommodated in a reaction unit.

[0135] The methods and apparatuses may preferably comprise at least one further upstream unit in fluid communication with said first and second units. The method may thus comprise corresponding method steps. Example upstream units include those capable of producing CO2 - e.g. from any chemical, physical, and / or natural process -, and those capable of purifying a gas to provide the first gas. For example, an upstream unit may be suitable for removing oxygen from a gas.

[0136] Preferably, the apparatus for producing syngas is modular. This may advantageously facilitate the addition of various downstream units, and / or the recycling of different gas streams to earlier units. The apparatus is preferably a tube reactor, which may permit ease of assembly. It is preferred that the apparatus is a flow reactor, more preferably a gas phase flow reactor. It will therefore be understood that the units of the apparatus and the reactor units of the invention are preferably flow reactor units, more preferably gas-phase flow reactor units.

[0137] Uses

[0138] The present invention also relates to the uses of the different aspects of the invention hereinbefore described. For example, the present invention relates to the use of the methods, apparatuses, reactor units and / or catalyst system hereinbefore described for the capture, preferably direct air capture, and / or utilisation (e.g. conversion to syngas) of CO2, preferably wherein the CO2 is from air, from waste, and / or from exhaust gas mixtures, more preferably from air.

[0139] Preferred embodiments

[0140] Some preferred embodiments of the present invention are given below.

[0141] A method of producing syngas, the method comprising: supplying air to a first unit for the direct air capture and release of CO2, wherein said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; capturing CC^from said air on said adsorber material at said first condition; releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2; supplying said second gas to a second unit, wherein said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support; and converting CO2 to syngas.

[0142] A method of producing syngas, the method comprising: supplying a first gas comprising CO2 to a first unit for the capture and release of CO2, wherein said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; capturing CC^from said first gas on said adsorber material at said first condition; changing from said first condition to said second condition by photothermal heating, preferably using a reflector, preferably a parabolic trough reflector, and / or a thermal absorbent material; and releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2; supplying said second gas to a second unit, wherein said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support; and converting CO2 to syngas, wherein: said second condition is a higher temperature than said first condition , preferably wherein: said first condition is an ambient temperature, more preferably a temperature of less than 50 °C, preferably less than 40 °C, more preferably less than or equal to 35 °C; and said second condition is a temperature of greater than 50 °C, preferably greater than 70 °C, more preferably greater than 90 °C, still more preferably at least 100 °C.

[0143] A method of producing syngas, the method comprising: supplying a first gas comprising CO2 to a first unit for the capture and release of CO2, wherein said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; at night-time, capturing CC^from said first gas on said adsorber material at said first condition; at day-time, releasing CC^ from said adsorber material at said second condition to provide a second gas comprising CO2; supplying said second gas to a second unit, wherein said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support; and converting CO2 to syngas wherein: said first condition is a night-time condition, preferably an ambient night-time temperature; and said second condition is a day-time condition, preferably achieved by photothermal heating, preferably using a preferably a parabolic trough reflector and / or a thermal absorbent material, preferably wherein the second condition is a temperature of greater than 50 °C, preferably greater than 70 °C, more preferably greater than 90 °C, still more preferably at least 100 °C.

[0144] FIGURES

[0145] Figure 1 is a schematic of a preferred, and non-limiting, embodiment of the present invention, showing direct air carbon capture and utilization (DACCU) through a dual bed flow reactor consisting of direct air capture (DAC) and CO2 utilization (CO2U) units, (a-b) Schematics of the overall system, (c) The carbon capture unit with chemical CO2 capture and release equations, and (d) The solar-driven CO2 utilization unit. MFC, PEI, and EG stand for mass flow controller, polyethyleneimine, and ethylene glycol, respectively.

[0146] Figure 2(a) shows a parabolic trough reflector used in the Examples, along with the dimensions. A sample tube reactor is shown mounted on the axis. Figure 2(b) shows design of the endpiece of the reflector.

[0147] Figure 3(a) shows a TGA curve of the solid adsorbent (n?SiO2|PEI), as described in the Examples. For the calculation of organic content, the loss of weight beyond 150 °C was considered. Figure 3(b) shows the BET isotherm plot during N2physisorption of the CO2 adsorbent at 77 K.

[0148] Figure 4 shows SEM images of the bare mSiO2 support (a-c) and the mSiO2|PEI adsorbent (d-f), as discussed in the Examples. The images show similar morphological structures with uniform PEI coatings onto the support after impregnation.

[0149] Figure 5 shows SEM-EDS analysis of the CO2 adsorbent, as discussed in the Examples. Images show uniform deposition of PEI (containing C, N) on the support (containing Si, O).

[0150] Figure 6 shows the in-flow CO2 capture (a) and desorption (b) setup, as discussed in the Examples.

[0151] Figure 7(a) shows CO2 levels in the outflow during DAC and the adsorbed CO2 amount over time, in accordance with one of the Examples of the present invention. Figure 7 (b) shows a photothermal CO2 desorption setup with photothermal coating and parabolic trough reflector (note that the light source is for demonstration purposes only and different from the actual solar simulator used in the Examples). Figure 7(c) shows CO2 concentration in outflow gas stream during release with different flowrates, in accordance with one of the Examples of the present invention.

[0152] Figure 8 shows thermal analysis of a system described in the Examples, (a) Without light concentration at 1 sun (the reflector was covered with a white paper) or infrared absorbing photothermal layer, and (b) With light concentration (3 sun) and photothermal layer. Panel (c) shows an optical image of the system corresponding to panel (b).

[0153] Figure 9 shows SEM (a-d) and EDS (e) analysis of the adsorbent after one cycle of CO2 capture and desorption, as described in the Examples. No significant changes in the morphology or composition were observed.

[0154] Figure 10 shows HAADF-STEM image and EDS maps of a composite, as described in the Examples.

[0155] Figure 1 1 shows a TEM image (a) and HAADF-STEM image (b) of a composite, as described in the Examples, showing close proximity of silica and titania particles. The dark spherical particles are TiC>2, whereas amorphous silica is seen as a translucent layer.

[0156] Figure 12 shows (a) Attenuated Total Reflection Infra-red (ATR-FTIR) spectra and (b) solid state UV-Vis Diffuse Reflectance Spectra (DRS) of the nSi02|Ti02|CotpyP composite and its separate components, as described in the Examples. Both spectra show the presence of TiC>2 and the molecular complex CotpyP in the final as prepared composite.

[0157] Figure 13 shows The BET isotherm plot during N2 physisorption of the nSi02|Ti02|CotpyP composite at 77 K, as described in the Examples.

[0158] Figure 14 shows a schematic diagram of the setup for a gas-phase CO2 conversion in flow, as described in the Examples.

[0159] Figure 15 shows the effect of N2vs CO2 carrier gas on performance of the CO2 utilization unit, as described in the Examples.

[0160] Figure 16 shows an FTIR spectrum of the product gas at the headspace when using13CO2as the reactant, as described in the Examples. The spectrum is zoomed in to show the CO stretching window and shows the selective formation of13C labelled CO. The shift in vibrational stretches is due to the higher reduced mass of the13C labelled C=O bond.

[0161] Figure 17 shows product formation profile with time when using nSi02|Ti02|CotpyP (solid line; with CotpyP) and nSiO2|TiO2 (dashed line, no CotpyP) as the CO2U composites, as described in the Examples. The figure shows the need of CotpyP for CO formation.

[0162] Figure 18 shows mass spectra of the headspace gases (zoomed into the low molecular weight region) of control air (a), when EG-6H is used as reactant (b), and when EG-D6 is used as reactant (c), as described in the Examples. The overwhelming presence of H2when using EG-D6 indicates the dissolved water as the likely proton source.

[0163] Figure 19 shows STEM-EDS analysis of the CO2 conversion composite after CO2U catalysis including the HAADF-STEM (a) and BF-STEM (b) images, and the EDS maps (c), as described in the Examples. The images and maps show similar morphological structures as pre-catalysis samples, without any observed Co cluster formation. Figure 20 shows powder X-ray Diffraction (PXRD) pattern of the nSi02|Ti02|CotpyP composite before (a) and after (b) catalysis, as described in the Examples. A broad peak at 22° is observed for amorphous silica. No peaks corresponding to clustered Cobalt metal were observed, indicating the retention of the molecular structure of CotpyP in the material after catalysis.

[0164] Figure 21 shows the effect of different supports on performance of the CO2 utilization unit, as described in the Examples.

[0165] Figure 22 shows time dependent product formation over time using different supports, as described in the Examples, (a) none (b) SiC>2 nanoparticles (nSiC>2) (c) mesoporous silica SBA-15 (mSiC ) (d) gamma AI2O3 nanoparticles (gAhOs) (e) mesoporous alumina (mAhOs), and (f) CeC>2 nanoparticles (nCeC>2).

[0166] Figure 23 shows an image of the parabolic trough reflector with the tubular fixed- bed reactor.

[0167] Figure 24 shows the effect of increasing solar intensity via concentrator without temperature control on CO formation, as described in the Examples.

[0168] Figure 25 shows thermal images of the CO2 conversion setup without (a) and with (b) the solar reflector, as described in the Examples. The CO2 conversion bed temperature reaches up to 43 °C without the reflector, whereas using the reflector, the temperatures reach up to 56 °C.

[0169] Figure 26 shows the effect of temperature regulation on performance of the CO2 utilization unit, as described in the Examples.

[0170] Figure 27 shows H2and CO production with time at 25 °C under concentrated solar light using gAI203|Ti02|CotpyP composite, as described in the Examples. The observed CO selectivity is shown in the right Y axis.

[0171] Figure 28 shows a HPLC trace of the oxidation product analysis after extraction from the conversion bed with water, as described in the Examples. Formate and glycolaldehyde dimer were observed as the major products.

[0172] Figure 29 shows the effect of CO2 dilution on CO generation, as described in the Examples.

[0173] Figure 30 illustrates integrated direct air carbon capture and utilization (DACCU) to solar syngas, as described in the Examples, (a) Designed reactor for in -flow direct air capture and conversion, (b) CO2 levels and CO and H2formation rates in the outflow during DACCU, (c) Cumulative H2and CO yield over time during DACCU, (d) Recapture and rerouting of unreacted CO2 for increased conversion and low carbon emission, and (e) Cumulative CO formation with time during first and second pass conversion during daytime operation. Shaded areas represent the standard deviations.

[0174] Figure 31 shows a conceptual schematic diagram for recapture of unreacted CO2 (a) followed by second pass conversion (b).

[0175] Figure 32 shows the modified reactor mounted on the axis of the parabolic trough reflector, as described in the Examples. The DAC and CO2U chamber are exposed to concentrated solar irradiation whereas the downstream conversion unit can be illuminated or kept in the dark as per the needed conditions.

[0176] Figure 33 shows product formation during the second cycle of CO2 capture and conversion, as described in the Examples. The CO2 release concentration is shown in the right Y axis.

[0177] Figure 34 shows (a) theoretical recirculation of unreacted CO2 using a pump and (b) possible downstream conversion to produced syngas to liquid fuels, as described in the Examples.

[0178] Figure 35 shows H2and CO formation rates when using air as the carrier gas during release and conversion of captured atmospheric CO2. The low CO rates are likely due to the competing oxygen reduction reaction.

[0179] Figure 36 shows background CO formations observed when using air as the carrier gas due to photooxidation of surface impurities in the presence of molecular oxygen, as described in the Examples. The background CO was minimal when CO2 or N2 was used as the carrier gas.

[0180] Figure 37 shows the effect of different alcohol electron donors on performance of the CO2 utilization unit, as described in the Examples.

[0181] Figure 38 shows the results of using real-world PET-waste as an electron donor, as described in the Examples, (a) images from different steps of the PET alkaline treatment in MeOH / THF to obtain pure EG, (b)1H and13C NMR of the distillate in D2O (700 MHz and 176 MHz, respectively at 298 K) showing high purity EG, (c) the H2, CO, formate and GAD dimer formation with time using PET-derived EG as reductant, along with CO selectivity, and (d) syngas and oxidation product formation with time with EG obtained from alternate PET pretreatment involving KOH-mediated lysis in EG solvent at 150 °C.

[0182] EXAMPLES

[0183] Materials Silica nanopowder (nSiC>2, 5-20 nm, >99%, Sigma), mesoporous silica SBA-15 (mSiC>2, <150 |im particle size, pore size 8 nm, Sigma), gamma alumina nanoparticles (gAhOs, <50 nm, >99%, Sigma), mesoporous alumina (mAhOs, SBA-15, pore type MSLI-X, average pore size 3.8 nm, Sigma), cerium(IV) oxide nanopowder (nCeC>2, <25 nm, Sigma), titania nanopowder (TiC>2, particle size <21 nm, Sigma), branched polyethyleneimine (PEI, Mw25k, Sigma), methanol (>99%, Sigma), ethanol (>99%, Sigma), ethylene glycol (EG, >99%, Sigma), and glycerol (>99%, Sigma) were used as received. The phosphonated cobalt bis(terpyridine) complex (CotpyP) was synthesized following a previously reported procedure (Leung, J. J. et al. Solar-driven reduction of aqueous CO2 with a cobalt bis(terpyridine)-based photocathode. Nat. Catal. 2, 354-365 (2019)). CO2 (CP grade BOC), N2(99.99%, BOC), simulated air (400 ppm CO2 in 20% O2 and 80% N2, BOC), carbon-13C dioxide (13CO2, 99.0 atom%13C, Sigma-Aldrich), and ethylene glycol-de (98% D, Sigma) were used without further purification unless otherwise stated.

[0184] The parabolic trough reflector was constructed from two 3D printed parabolic endpieces combined with highly reflective (90%+ solar reflectance) Alanod MIRO-SUN 20901 L aluminium sheeting to create a parabolic trough solar concentrator. The parabolic endpieces were designed using Autodesk Inventor, with the curve of the parabola being defined in Cartesian coordinates by the equation y = (xA2) / 32 with the focus and point of light concentration at (x,y) = (0,8) (Fig. 2). The reactor was built to be 32 cm wide (due to the size of available simulated solar light sources), making the focus of the parabolic trough concentrator at a height equal to the height of the parabolic endpieces, with the focus line running down the centre of the symmetrical parabolic trough. Three 1 .0 cm diameter holes were designed in each endpiece, so the two sides of the reactor could be connected using 1 .0 cm diameter derlin rods, which were press-fit to connect the two reactor endpieces. A piece of Alanod sheeting was then attached using high-strength double-sided fastening tape. Two sample holder crossbeams (see Fig. 2) were also 3D printed to hold a borosilicate glass reaction tube with an 11 mm outer diameter at the focus of the parabolic trough concentrator. When constructed, the reactor had a length of 25 cm, including the two endpieces. All 3D printed reactor parts were printed using Ultimaker Tough PLA material using an Ultimaker S5 printer.

[0185] Material and product characterization

[0186] The gas products were analyzed with a Shimadzu GC-2010 Plus gas chromatogram with ultrapure Helium (CP Grade) as the carrier gas. The chromatographic separations for the oxidation products were conducted using a Waters HPLC system equipped with a Phenomenex Rezex 8% H+column at a column temperature of 60 °C. The samples were analyzed in the isocratic flow mode (flow rate: 0.5 mL min"1, 0.0025 M aqueous H2SO4) using a Waters breeze HPLC system equipped with refractive index (RIS-2414) and diode array UV-Vis ( / \ = 254 nm) detectors. For the13C-isotope labelling experiments,13CO was detected using infrared spectroscopy (Thermo Scientific Nicolet iS50 infrared spectrometer) in the gas-phase transmission mode. The headspace from the cell was transferred to an air-tight evacuated infrared cell (path length, 10 cm; equipped with KBr windows) after the experiment for the detection of13CO (data resolution, 0.125; data spacing, 0.060 cm"1). Attenuated total reflectance (ATR) FTIR spectra were recorded in a Thermo Scientific Nicolet iS50 IR spectrometer in reflectance mode. UV-Vis diffused reflectance spectra were recorded using a Bruker Cary 60 UV-Vis spectrophotometer. The PXRD measurements of the samples were performed using a Panalytical X’Pert Pro (Cu Ka radiation) diffractometer with a 20 range from 10 to 80° at a scan rate of 1 ° min-1. The SEM images and EDX maps were acquired using a TESCAN MIRA3 FEG-SEM instrument (operated at 5 kV) equipped with an Oxford Instruments Aztec Energy X-maxN 80 EDX system. The Transmission Electron Microscopy (TEM), bright-field (BF) STEM, high angle annular dark field (HAADF) STEM images, and EDX maps were acquired using a Thermo Scientific Talos F200X G2 TEM (operating voltage 200 kV). TEM images were acquired using a Thermo Scientific Ceta CMOS camera. STEM images were collected using a Thermo Scientific BF detector and Fischione HAADF detector at a camera length of 98 mm and EDX maps using a Super-X detector system. Samples were prepared by drop-casting a dilute composite solution on holey-carbon-coated Cu grids or lacey carbon-coated Ni grids followed by evaporation of the solvent. The ICP-OES measurements were performed on a Thermo Scientific iCAP 7400 ICP-OES DUO spectrometer at the Microanalysis Service, Yusuf Hamied Department of Chemistry, University of Cambridge. CO2 was detected using a Gas Sensing Solutions CozlR®-LP (0-2000 ppm) sensor during air capture and using a SprintlR®-6S CO2 sensor (0-100%) during desorption. Nitrogen physisorption isotherms were measured using Micromeritics 3Flex Adsorption Analyzer. All the materials were degassed for a few hours under vacuum prior to measurement. Nitrogen adsorption and desorption isotherms were measured at 77 K. The specific surface area was calculated by the multi-point Brunauer- Emmett-Teller (MBET) method using the adsorption branch of the physisorption isotherm. The total pore volume (PV) was determined at P / Po close to 1. TGA measurements were taken in a Mettler Toledo Thermogravimetric Analyser under an airflow of 100 mL min-1in a temperature range from 25 to 800 °C with a heating rate of 5 °C min-1(temperature holding time at 800 °C = 10 min). The initial weight loss observed below 150 °C was attributed to the loss of adsorbed moisture and CO2. The weight loss from 150 to 800 °C was counted towards the organic content. The thermal images were taken using a FLIR ONE® Gen 3 thermal camera. MS was recorded using Hiden Analytical HPR-20 benchtop gas analysis system to a HAL 101 RC electron impact quadrupole mass spectrometer with a Faraday detector.

[0187] Example 1: synthesis of CO2 adsorber material

[0188] The purpose of this example was to synthesise a CO2 adsorber material for the capture and concentration of atmospheric CO2.

[0189] Polyethyleneimine (PEI, weight average molecular weight 25,000 g / mol) was impregnated as an active CO2-capturing chemical onto a porous solid support of mesoporous silica (mSiC>2, particle size <150 |im) in equal weight via wet impregnation to obtain a solid CO2 adsorbent (mSiO2|PEI).

[0190] Detailed Method

[0191] 3 g of PEI was dissolved in 20 mL methanol and was subsequently added dropwise to a suspension of 3 g mSiC>2 (SBA-15) silica on 200 mL methanol under vigorous stirring. The resulting solution was stirred overnight. The solvent was subsequently removed under reduced pressure (85 mbar) at 40 °C to afford a solid powder composite (mSiO2|PEI) which was analyzed by SEM, EDS, BET, TGA and directly used for direct air capture in the subsequent examples, unless specified otherwise.

[0192] Thermogravimetric analysis (TGA) of the adsorbent in air showed 50 wt% PEI loading onto the silica support (Fig. 3a).

[0193] Effective deposition of the amine throughout the silica pores was verified by Brunauer-Emmett-Teller (BET) isotherm analysis, which showed a reduced surface area upon PEI incorporation onto the support (9.2 m2g"1vs 300 m2g"1for the parent silica support, Fig. 3b).

[0194] Uniform deposition of PEI on silica was also observed by scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDS) mapping of the adsorbent (Fig. 4-5).

[0195] Example 1 demonstrates the effective synthesis of an adsorber material. Effective and uniform deposition of PEI on silica was observed, with high loading.

[0196] Example 2: direct air capture of CO2

[0197] The purpose of this example was to investigate direct air capture of CO2 using the adsorber synthesised in Example 1. DAC experiments with the synthesized solid CO2 adsorber (mSiO2|PEI) were carried out with a fixed bed of mSiC>2|PEI (600 mg, length 5 cm) inside a glass tube reactor (inner diameter 0.6 cm) by flowing air (400 ppm CO2, flowrate 90 mL min"1) through it at ambient temperature (Fig. 6a).

[0198] Detailed method

[0199] In a tubular glass reactor, 600 mg of solid CO2 adsorbent was loaded and was fixed inside the reactor using glass wool plugins. The tube was then closed, and wet air was passed through the adsorber bed at 90 mL min"1flow-rate under ambient temperature. The CO2 levels in the outlet stream were recorded using a CoZIR CO2 sensor from Gas Sensing Solutions (0-2000 ppm). The sensor was calibrated before each experiment using nitrogen (0 ppm) and air (400 ppm). The time-profile of CO2 loadings onto the adsorbent was calculated by integrating the removed CO2 concentration with time.

[0200] Analysis of the CO2 concentration in the outflow stream showed complete CO2 removal for around 9 h of operation (Fig. 7a), followed by a slow increase in CO2 level consistent with the onset of saturation of the capture material. Complete CO2 saturation was reached after 18 h at which time the total CO2 intake of the composite by DAC was around 87±4 mgCo2gadsorbent (0.17±0.01 molCo2molamine) as determined from the CO2concentration-time curve integral (Fig. 7a).

[0201] The observed time scale for capture (10-16 h) is optimized for night-time operation and complements the average daily sunlight exposure of many tropical and subtropical regions. Furthermore, the complete capture duration (tonset) is proportional to the mass of adsorbent (mads) and inversely proportional to the flowrate (fr) and thus can be tuned by changing these two parameters accordingly.

[0202] Example 2 thus demonstrated effective direct air capture of CO2 using the adsorber material of Example 1. The capture is desirably optimised for night-time operation and can be tuned.

[0203] Example 3: photothermal release of CO2

[0204] The purpose of this example was to investigate the solar-driven photothermal release of CO2 from the adsorber material following the direct air capture described in Example 2.

[0205] Detailed Method

[0206] For the desorption studies, the adsorber bed was wrapped with an infra-red absorbing photothermal material on top of the glass reactor. For this purpose, a blackcolored gaffer tape was used. The tube was then placed at the focal axis of the parabolic trough solar reflector and placed inside the solar simulator. Air was flown subsequently through the adsorber bed as carrier gas (0.5 - 3 mL min"1) and the solar simulator light (Newport Oriel, AM 1 ,5G, 100 mW cm"2) was turned on. The CO2 levels of the output stream were detected using a GSS Sprint-IR CO2 sensor (0-100%) which was calibrated before each experiment using N2(0%) and pure CO2 (100%).

[0207] Solar-driven photothermal release of CO2 was investigated by irradiating the adsorber bed with simulated sunlight (100 mW cm"2, AM1 ,5G) under carrier air flow (1 mL min"1) at ambient pressure. Under these standardized conditions, no CO2 release in the outflow was detected. Thermal image analysis of the system showed a temperature of 35 °C at the adsorber bed upon light irradiation, which was insufficient for the thermal desorption of the captured CO2.

[0208] However, increasing the reactor bed temperature via 1 ) light concentration using a parabolic trough reflector (to 3 suns) and 2) covering the outside of the adsorber bed with an infrared absorbing photothermal black tape resulted in efficient CO2 release (Fig. 2, 6, and 7b-7c). Under these conditions, the temperature of the adsorbent reached up to 100 °C at 1 mL min"1flow rate (Fig. 8) while the CO2 concentration in the outflow reached up to 30% (v / v) within 30 min of solar irradiation (Fig. 7c). The overall solar-driven photothermal desorption was almost complete (>90%) after approximately 2 h.

[0209] Higher CO2 concentration in the outflow stream (45%) for a longer duration was obtained by decreasing the flowrate to 0.5 mL min"1, whereas increasing the flowrate of the carrier gas results in a quicker desorption process with lower CO2 concentrations in the outflow (Fig. 7c).

[0210] SEM and EDS analysis of the post capture-release adsorbent showed comparable morphological structures and organic content to the pre-capture material, reflecting minimal changes during the cycle (Fig. 9). This means the regenerated CO2 adsorber material can again be employed for another cycle of DAC.

[0211] Example 3 thus demonstrated the effective photothermal release of captured CO2 from the adsorber material. The concentration of CO2 in the outflow was tunable using the flow rate. The structure of the absorbent was also found to be desirably maintained following the capture and release process making it reusable.

[0212] Example 4: Synthesis of CO2U material

[0213] The purpose of this example was the development of an efficient, inexpensive gas phase C02-to-fuel conversion system.

[0214] Detailed method 0.5 mg of CotpyP was dissolved in 5 mL methanol and was subsequently added dropwise to a suspension of 50 mg TiC>2 (P25) on 10 mL methanol under stirring. The resulting suspension was stirred for 2 h. Subsequently, the suspension was added dropwise to a suspension of 1 g of support (nSiC>2 / mSiC>2 / AI2O3 / mM2O31 nCeC>2) in 100 mL methanol. The resulting solution was stirred overnight. The solvent was subsequently removed under reduced pressure (85 mbar) at 40 °C to afford a solid powder composite that was analyzed by SEM, TEM, EDS, BET, XRD, IR, UV-Vis, and was used directly in subsequent examples unless specified otherwise.

[0215] Commentary

[0216] For the solar-driven conversion, TiC>2 (P25) is used as an inexpensive photocatalyst, along with a chemically immobilized first-row transition metal cobalt(ll) based molecular catalyst (CotpyP) as the CO2 reduction co-catalyst to bring down the required overpotential.

[0217] For our gas phase study, we distributed the Ti02|CotpyP over a porous high surface area solid support (silica or alumina) to facilitate mass transport of the gases to enhance CO2U rates (Fig. 1 d). The gas-phase CO2U material was synthesized by first stirring a solution of CotpyP (0.5 mg) and TiC>2 nanoparticles (50 mg, particle size 21 nm) in MeOH (for immobilization of CotpyP via its phosphonic acid linkers on the TiC>2 surface) followed by the addition of SiC>2 nanoparticles (nSiC>2, 1 g, particle size 5-20 nm) as a high surface area support. Removal of the solvent under vacuum after overnight stirring afforded a beigecolored solid powder of the hybrid composite (nSi02|Ti02|CotpyP) for CO2 conversion.

[0218] Scanning transmission electron microscopy - energy dispersive X-ray spectroscopy (STEM-EDS) mapping of the composite showed uniform distribution of TiC>2 P25 nanoparticles with amorphous phase SiC>2 throughout (Figs. 10 and 1 1 ).

[0219] Although the loading of molecular cobalt catalyst in the composite was expectedly below the detection limit by elemental mapping, the uniformly immobilized CotpyP on TiC>2 surfaces was indirectly observed by C, N EDS mapping.

[0220] Inductively coupled plasma - optical emission spectroscopy (ICP-OES) analysis confirmed the presence of Co and showed around 0.6 Co |imol g"1catalyst loading.

[0221] The infra-red (IR) spectra showed clear vibrational frequencies (aromatic stretches) of the molecular complex (1400-1700 cm-1), which were slightly shifted due to interaction with TiC>2 particles (Fig. 12a).

[0222] The solid-state UV-Vis diffuse reflectance spectra (DRS) of the composite further showed characteristic absorption peaks of both TiC>2 and CotpyP molecular complex (Fig. 12b). BET isotherm revealed a high surface area (494 m2g1) and pore volume (0.502 cm3g"1) of the nSi02|Ti02|CotpyP composite, suggesting the ease of gas penetration (Fig. 13)

[0223] Example 4 thus demonstrated an effective synthesis of a CO2II catalyst system, having desirably high surface area and pore volume, as well as uniform immobilisation on the solid support.

[0224] Example 5: CO2-to-fuel utilization in flow

[0225] The purpose of this example was to investigate the use of the catalyst system prepared in Example 4 in solar CO2 conversion to syngas.

[0226] Detailed Method

[0227] For initial batch experiments (Example 5a), a glass photo vial was loaded with 100 mg of solid CO2 reduction material (npSi02|Ti02|CotpyP) which was subsequently wetted with 0.3 mL EG. The headspace of the vial was then purged with wet CO2 (containing 2% CH4as internal standard) and the vial was irradiated with simulated solar light (Newport Oriel, 100 mW cm-2, AM 1.5G) for 20 h keeping the temperature at 25 °C. Afterward, an aliquot of the headspace gas was analyzed by GC via manual injection to determine the H2and CO yield. The residual solid was then suspended in 1 mL water, and the resulting mixture was filtered. The filtrate was analyzed by HPLC for oxidation product determination.

[0228] For flow studies (all other sub-examples), in a tubular glass reactor (length 28 cm, inner diameter 0.6 cm) 250 mg of CO2 conversion composite (support|Ti02|CotpyP) was loaded. Subsequently, the composite was manually wetted with ethylene glycol (EG) by adding 0.75 mL EG dropwise using a pipette. The tube was subsequently sealed with septa at both ends and placed into the focal axis of the parabolic trough reflector. Wet nitrogen or CO2 was subsequently flowed through the tube at a constant flowrate using a mass flow controller (Brooks GF040) while keeping the reactor inside a simulated solar simulator (lamp off). The outlet of the reactor was connected to an online GC through a 1 mL loop which injected approximately every 4.25 min into the GC. The flow rate at the GC outlet was verified prior to the experiment with an Alicat gas flow meter to ensure no gas leakage. The initial 45 minutes of the experiments were run in the dark to obtain a stable GC baseline after which the solar simulator lamp (Newport Oriel, AM 1.5G, 100 mW cm-2) was turned on for 12 h. The momentary rates of CO and H2 evolution (ngas) from individual injections were determined from the GC responses by subtracting the baseline under dark conditions and using a previously described procedure with the same setup using equation S1 :

[0229] (equation 1 ) where p is the pressure in the flow reactor (ambient pressure, 101 ,325 bar), V is the flowrate, R is the universal gas constant, T is the temperature prior to injection (298 K), and fi is the response factor of each gas determined by a calibration procedure. The GC calibration was performed with a known standard for H2, CO, and CH4(4000 ppm H2 / 4000 ppm CO / 1000 ppm CH4in balance gas CO2, BOC) by diluting the mixture with pure CO2. The total amount of evolved product with time was calculated using trapezoidal integration of the product evolution rates using MATLAB software.

[0230] For the reactions under 1 sun, the reflector surface of the concentrator was covered with a non-reflective material (white paper). For reactions under 3 suns, the paper cover was removed to ensure solar concentration. For the reactions at 25 °C, water was circulated through a water jacket on the reactor using a chiller to ensure a steady temperature.

[0231] For HPLC analysis of the oxidation products, the glass wool plugs were carefully removed from the reactor after the reaction. The composite was then recovered in a vial and sonicated after adding 2 mL of deionized water. The resulting mixture was then filtered using a syringe filter and the liquid was analyzed by HPLC.

[0232] For the isotope labelling study with13CO2, the tube was filled with13CO2and sealed instead of continually passing13CO2. The sealed tube was then irradiated under 1 sun for 20 h, after which the inside gas was analyzed with infra-red spectroscopy in transmission mode, which showed the formation of13C labelled CO.

[0233] For the reactions with real-world PET waste as a reductant, PET plastic from a plastic bottle was shredded. Following that, 100 g of PET was suspended in a solvent mixture of methanol and THF (400 mL and 100 mL, respectively). Subsequently, 56 g of KOH was added, and the resultant mixture was stirred at 60 °C for 24 h. Afterwards, the solution was filtered to remove the dipotassium terephthalate, and the THF and MeOH were removed from the filtrate under reduced pressure. EG was extracted from the remaining mixture through vacuum distillation (yield 19.4 g, 60%), which was then directly used in the DACCU reactor. For the PET breakdown in EG, 5 g of PET was suspended in 25 mL of EG, followed by the addition of 2.8 g KOH. The resultant mixture was stirred at 150 °C for 4 h. Subsequently, the solution was left undisturbed for 24 hours (to settle down the dipotassium terephthalate). The EG was then decanted from the mixture, containing both solvent EG and PET-derived EG, and was used in the reaction. The breakdown of PET plastic was verified in this case by characterising and quantifying the dipotassium terephthalate product by1H and13C NMR (5.1 g, 81 %). Example 5a: effect of electron donor

[0234] The purpose of this sub-example was to investigate the effect of different electron donors on the production of syngas.

[0235] Alcohol oxidation reaction (DG ~ 0 kJ mol"1) was used as the counter reaction. This enhanced the CO2 reduction rates due to the favourable thermodynamic conditions. Suitable alcohols can advantageously be recovered from different waste streams including depolymerized discarded plastics (ethylene glycol), biorefineries (glycerol), and biomass (sugars), and are upgraded in the process of Example 5 to value-added chemicals.

[0236] Initial batch experiments showed that several alcohols including methanol, ethanol, ethylene glycol (EG), and glycerol can act as efficient electron donors in the system for syngas formation by CO2 reduction, consistent with the high photoactivity of Ti©2 towards alcohol oxidation (Fig. 37). EG was found to yield highest CO formation rates and can be obtained from polyethylene terephthalate plastic waste and was used for all subsequent examples.

[0237] Real-world PET waste can also be employed as a reductant following KOH- mediated lysis pre-treatment (to produce EG) without sacrificing the syngas production activity of the system and is transformed into formate and glycolaldehyde (GAD) dimer in the process over a long time (96 h) with syngas formation reaching over 1500 gmol at 80% CO selectivity (Fig. 38).

[0238] Example 5b: effect of carrier gas

[0239] The purpose of this sub-example was to investigate the production of syngas from gas-phase CO2 and to investigate the effect of different carrier gases on the production thereof.

[0240] The solid composite (250 mg, containing 12.5 mg TO2) was loaded inside a glass tube (inner diameter 0.6 cm) to prepare a fixed bed flow photoreactor (bed length ~ 2 cm). The bed was manually wetted with EG prior to the experiments, which acted as an electron donor.

[0241] For the experiments, wet CO2 or N2was passed through the tube reactor as the carrier gas at a flowrate of 1 mL min"1(gas residence time in the CO2U reactor bed ~ 35 s) while keeping the catalytic bed irradiated under 1 sun (100 mW cm-2, AM1.5G), and the outflow gas was analyzed by gas chromatography (GC) (Fig. 14).

[0242] Under these conditions, CO and H2 formation were observed in the outflow when CO2 was used as the carrier gas, with respective yields of 105±8 gmol and 363±35 |imol gTiQ2after 12 h (Fig. 15). These values are comparable with the reported activities of the catalytic system in solution, without the need for any organic solvent or buffer solution, highlighting the advantages of a gas-phase reaction setup. Further, these results also highlight the excellent promise of molecular catalysts immobilized onto a semiconductor surface in gas-phase CO2 reduction for the first time.

[0243] A control experiment with wet N2 as a carrier gas showed only H2 formation (885±41 limol with minor CO generation, reflecting CO2 reduction to be the source of CO (Fig. 15). CO2 as the CO source was further verified by isotopic labelling, where using13CO2 as the reactant selectively produced13C-labelled CO as the product (Fig. 16).

[0244] Another control experiment with nSiO2|TiO2 composite without CotpyP yielded only H2and no CO, confirming the role of the molecular catalyst in CO2 reduction (Fig. 17). The major proton source in the system is likely the dissolved moisture in the wet carrier gas, although EG can also act as a minor proton source as suggested by an isotopic labelling study with EG-D6(Fig. 18). STEM-EDS and powder X-ray diffraction (PXRD) analysis of the post-catalysis composite did not show any trace of Co aggregates, suggesting the immobilized CotpyP as the sole active CO2U catalyst (Fig. 19-20).

[0245] This sub-example thus demonstrated the effective production of syngas from gasphase CO2. It also showed the essential role of both CO2 and the molecular catalyst in yielding CO as part of a syngas mixture.

[0246] Example 5c: effect of solid support

[0247] The purpose of this sub-example was to investigate the effect of different solid supports on the production of syn gas (Fig. 21 and 22).

[0248] The support is an important component in the catalytic system that provides a solid matrix to the cobalt catalytic centres and allows for efficient gas transport through the material. Reflecting this, a significant decrease in CO formation was observed (17±6 limol gyi102after 12 h) when Ti02|CotpyP was used in the catalytic bed without a support matrix (Fig. 21 ).

[0249] The support also influences the composition of the produced syngas by altering the local catalytic environment. Thus, an increase in H2formation was noticed (by 2-3 times) when mesoporous silica (SBA-15) was used as support instead of silica nanoparticles (Fig. 21 and 22). In contrast, the use of alumina support suppressed the H2formation while enhancing the CO production, likely due to the altered surface chemistry involving aluminium hydroxyl groups of the more alkaline alumina. From screening several supports, activated g-alumina nanoparticles (gAhOs, particle size <50 nm) were found to be the most suitable support material for the system due to its highest CO2 reduction rates (126±17 gmol after 12 h). The resultant syngas was rich in CO (H2:CO 4:5) which is ideal for downstream applications ranging from liquid fuel production to chemical syntheses.

[0250] This sub-example showed the importance of the solid support in syngas production, in particular its importance in tuning the ratios of syngas products and the rate of production thereof.

[0251] Example 5d: effect of concentrated solar light

[0252] The purpose of this sub-example was to investigate the effect of concentrated solar light on the production of syngas.

[0253] The effect of concentrated solar light on the system was explored with the g I203|Ti02|CotpyP composite by placing the tube reactor at the focal axis of a parabolic trough reflector where the light intensity reached up to 300 mW cm-2during the experiment (Fig. 23).

[0254] Under the concentrated light, high initial rates of H2and CO formation were observed for 0.5 h (up to 50 and 100 |imol g-^ h-1, respectively), followed by a decrease in CO production (Fig. 24). The decrease is likely due to the increased temperature of the reactor bed, reaching 56 °C during the reaction (Fig. 25) and causing partial deactivation of the molecular catalyst. A similar effect is also noticeable under 1 sun, although less prominent, owing to lower reactor temperature (43 °C).

[0255] To circumvent this, a water jacket was introduced around the CO2U chamber, and the temperature was kept constant at 25 °C. Under these conditions, high and steady CO formation was observed with CO yields reaching 255±16 gmol g^ after 12 h (Fig. 26 and 27). The produced syngas was found to be CO rich due to the suppression of H2formation at this temperature (CO:H24:1 ).

[0256] High performance liquid chromatography (HPLC) analysis of the post-catalysis reactor bed detected the presence of formate and glycolaldehyde dimer (molar ratio ~5:2) as the major EG oxidation products (accounting for >80% of syngas electrons), which could be utilized as platform chemicals following separation from the CO2U chamber (Fig. 28).

[0257] This sub-example thus showed the effect of temperature in tuning the syngas products, in particular showing the advantages of providing cooling means. The production of useful oxidation products was also demonstrated. Example 5e: effect of CO2 concentration

[0258] The purpose of this sub-example was to investigate the effect of CO2 concentration on the production of syngas.

[0259] Investigations towards the response of the fixed-bed flow photoreactor in dilute CO2 streams revealed a CO production rate roughly proportionate to the CO2 concentration (Fig. 29). Thus, moving from 100% CO2 to 50% CO2 in N2(v / v) as the carrier gas, the CO yield nearly halved (255±16 after 12 h) whereas the H2production remained similar.

[0260] The system remained active even when air was used as the carrier gas (50% CO2), albeit with a notable activity drop possibly due to the competing oxygen reduction reaction (ORR) over titania surfaces.

[0261] A steady CO production with time was observed even after increasing the dilution to 20% CO2 in the air, reaching 53±10|imol g^ of CO after 12 h (Fig. 29) and demonstrating the ability of the developed reactor to operate at CO2 levels typically found in post-combustion exhaust streams.

[0262] A control experiment with air as both the carrier gas and CO2 source (400 ppm) produced negligible CO as a CO2 reduction product, reflecting the advantage of an upstream CO2 concentrator unit for effective photoconversion of the atmospheric CO2.

[0263] It is noted that in all experiments with air as the carrier gas, a background CO production was observed (~2 gmol g^ h-1) due to photoinduced surface organic impurity oxidation (Figure 36), which was subtracted in all calculations.

[0264] This sub-example thus showed the effect of CO2 concentration on the production of syngas products, in particular showing the advantages of maximising CO2 concentration.

[0265] Example 6: Direct air carbon capture and utilization to solar fuel

[0266] Following the development of separate DAC and CO2U units in the preceding examples, the purpose of this example was to explore integrated capture and utilization of atmospheric CO2 by combining the capture and utilization bed into a custom-designed tube reactor (Fig. 30a).

[0267] Detailed Method

[0268] The modified tubular reactor as shown in Fig. 30a was used for integrated capture and conversion. First, the tube was loaded with 600 mg of CO2 adsorbent in the capture compartment and 250 mg of CO2 conversion composite in the downstream conversion chamber. Air was then flown through the capture bed for 12 h and an alternate outlet was used to bypass the flow around the conversion chamber during this time. Following the capture, the CO2 conversion was wetted with 0.75 mL EG, and the reactor was then sealed and placed inside a solar simulator (lamp off). Wet nitrogen or air was then passed through the capture and conversion unit as carrier gas (1 mL min"1) and the CO2, CO and H2levels of the outlet stream were measured using a CO2 sensor (CO2) or GC (CO, H2). The first 45 mins of conversion was done without light to obtain a stable baseline after which the solar simulator lamp was turned on for 12 h. The temperature of the conversion unit was kept at 25 °C throughout the experiment using a chiller.

[0269] For the reaction with multiple capture and conversion cycles, the first cycle of capture and conversion was carried out as described. Subsequently, air was flown again through the CO2 absorbent and the outlet gas stream was bypassed around the conversion unit using an alternate outlet and a placed septum before the conversion unit. After 12 h flowing the capture, the septum was removed, and the reloaded CO2 adsorbent was used for subsequent conversion following similar procedures of the first cycle.

[0270] For the recapture of unreacted CO2 followed by double pass conversion, CO2 capture was first carried out using the procedure previously stated in the capture chamber. Subsequently, the conversion chamber was loaded with the CO2 conversion composite, wetted with EG, and another layer of fresh CO2 adsorbent was loaded in the downstream additional chamber. The resulting reactor was then placed in the focal axis of the solar concentrator. The reactor was then placed inside the solar simulator, wet N2was flown through it as carrier gas, and the output CO2, CO, and H2levels were detected using the CO2 sensor and GC, respectively. The solar light was then turned on, irradiating only the capture and conversion unit while keeping the downstream unreacted CO2 capture unit in the dark. After about 6 h, as the CO2 and CO levels in the outlet almost subsided, the inlet and outlet of the reactor were switched, and concentrated light was selectively shone in the conversion unit and the captured unreacted CO2 unit (Fig. 31 ). The experiment was continued for another 6 h after which the light was turned off.

[0271] Commentary

[0272] The reactor contained an upstream DAC chamber, followed by a downstream utilization chamber with an alternate outlet between the two to divert the flow around the utilization unit during DAC.

[0273] Downstream of the utilization, an additional chamber was installed that can be utilized for different downstream processing including 1 ) capture of unreacted CO2 and / or 2) further conversion of the generated syngas. In a standard experiment, the reactor was loaded with both the mSiO2|PEI (for DAC) and g I203|Ti02|CotpyP composite (for CO2II) in their respective compartments and mounted on the axis of a parabolic trough reflector (Fig. 32). Wet air (CO2 level 400 ppm) was flown through the tube for 12 h (flowrate 90 mL min-1) at ambient temperature for DAC in the dark, mimicking night-time operation. During this time, the CO2 level in the outflow remained zero for most of the time reflecting effective CO2 removal by the adsorber (Fig. 30b).

[0274] Following capture, the utilization bed of the CO2U unit was wetted with EG, the flow was changed to N2(1 mL min-1), and the solar simulator with concentrator was turned on to mimic daytime operation. The presence of elevated CO2 levels in the outflow was detected within 10 min due to effective desorption at elevated temperatures (100 °C) via photothermal heating (Fig. 30b). GC analysis of the stream showed the initiation of CO and H2formation by conversion of the released CO2 and locally available protons, respectively (Fig. 30b). The CO formation rate followed a similar pattern as the released CO2 concentration, peaking at 1 h of operation at 8.5±0.8 h-1with high CO2 levels, and decreasing afterwards with diminished CO2 availability. This is consistent with the linear CO production rate dependency on CO2 concentration observed above (Fig. 29). During this time, the H2production remained stable and independent of the CO2 level.

[0275] These results clearly demonstrate the ability of the developed system to capture, concentrate and convert atmospheric CO2 into synthesis gas, driven completely by sunlight and utilizing ethylene glycol as the electron donor, in a gas-phase flow reactor setup.

[0276] The total CO formation of the system reached around 24±2 jimol after 12 h as shown in Fig. 30c. Following conversion, air was flown again through the reactor (90 mL min-1) for a second cycle of capture following a similar procedure, and the syngas formation activity of the gAI203|Ti02|CotpyP composite was partially retained (-50%) during subsequent second cycle conversion (Fig. 33). This demonstrates the potential suitability of the reactor design system for continuous operation between carbon capture and conversion cycles.

[0277] It is notable that air can also be used as carrier gas during the conversion step, although with a drop in CO formation activity compared to N2as carrier gas, likely due to the competing oxygen reduction reaction at the photoexicted TiO2surface (Figure 35).

[0278] The flow system displays high modularity and system flexibility. For example, we were able to recapture the unreacted CO2 from the outflow by using a second layer of CO2 adsorber downstream of the CO2 utilization unit. The CO2 emission during daytime operation from such a modified reactor reaches almost zero while keeping the solar syngas production rate unaffected (Fig. 30d).

[0279] Additionally, we could also reroute the recaptured CO2 to the utilization unit by switching the flow direction, increasing the daytime CO2 conversion (17 vs 27 jimol g in single or double pass reaction, respectively; Fig. 30e and 31 ). Single-pass CO2 conversion can also be improved in theory by employing a larger utilization unit, improved light management, and optimized reactor design.

[0280] The produced syngas can also be converted into liquid fuels in an additional downstream syngas conversion unit, or used in carbonylative hydrogenation reactions (Fig. 34), utilizing the benefits of a flow system for easy process integration.

[0281] In summary, it has been demonstrated an integrated solar-powered direct air carbon capture and utilization flow reactor that captures and concentrates atmospheric CO2 and converts it into renewable synthesis gas. The process follows a diurnal rhythm where CO2 is captured upstream during night-time operation and subsequently released and converted in a downstream conversion unit during daytime operation using the power of incident sunlight. The approach enables direct on-site utilization of CO2 at DAC plants using sunlight rather than the current cost-intensive practice of releasing, transporting, and storing the CO2. Moreover, the produced synthesis gas can be used as a fuel or a liquid fuel precursor that can directly compete in the fossil fuel markets, providing an avenue to transition away from the use of fossil fuels.

[0282] The CO2 adsorbent used is comparable to the industrially used DAC materials, in terms of efficiency (100% removal in initial hours) and capacity (~10 wt%). At the same time, the newly developed CO2 conversion hybrid material uses abundantly available oxides (e.g. titania, alumina) and metals (e.g. Co) which are ideal for scale-up while displaying promising gas-phase CO2 photoconversion activities (up to 0.1 mmol gw2h-1initial rates under concentrated light).

[0283] The CO2II unit is based on a molecular-semiconductor hybrid material, which is a novel approach in gas-phase CO2 conversion. The use of the CO2II unit allows for easy process integration with the upstream gas-phase DAC unit that also resolves the limitation of pure CO2 requirement as feed and overall represents a better process design for carbon capture and conversion than solution-phase counterparts suffering from low free CO2 availability. REFERENCES

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Claims

CLAIMS:1 . A method of producing syngas, the method comprising: supplying a first gas comprising CO2, preferably air, to a first unit for the capture, preferably direct air capture, and release of CO2, wherein said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; capturing CC^from said first gas on said adsorber material at said first condition; releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2; supplying said second gas to a second unit, wherein said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support; and converting CO2 to syngas.

2. A method according to claim 1 , wherein said method comprises changing from said first condition to second condition, preferably by photothermal heating.

3. A method according to claim 1 or claim 2, wherein said first gas is exhaust gas, gas from a waste stream, or air, preferably air.

4. A method according to any preceding claim, wherein said second gas exhibits a peak concentration of CO2 which is greater than the concentration of CO2 in the first gas.

5. A method according to any preceding claim, wherein said second unit is held at a temperature of less than 56 °C, preferably less than 43 °C, more preferably less than 30 °C, still more preferably less than or equal to 25 °C.

6. A method according to any preceding claim, wherein said syngas comprises CO and H2in a molar ratio of about between 1 :99 to 99:1 , preferably 10:90 to 90:10, more preferably 25:75 to 75:25, still more preferably 40:60 to 60:40, e.g. about 1 :1 .

7. An apparatus for producing syngas, the apparatus comprising: a first unit for the capture, preferably direct air capture, and release of CO2; and a second unit for the conversion of gaseous C02to syngas,wherein: said first unit has an inlet for a first gas, preferably air, comprising CO2 and an outlet for a second gas comprising CO2 in fluid connection with an inlet of said second unit, and said second unit has an outlet for syngas; said first unit comprises an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; and said second unit comprises a catalyst system for the conversion of C02 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support.

8. A method or apparatus according to any preceding claim, wherein said first and second conditions are different temperatures.

9. A method or apparatus according to any preceding claim , wherein said first condition corresponds to a night-time condition and said second condition corresponds to a day-time condition.

10. A method or apparatus according to any preceding claim , wherein said first condition is less than or equal to ambient temperature, preferably equal to ambient temperature.1 1. A method or apparatus according to any preceding claim, wherein said second condition is a temperature of greater than 50 °C, preferably greater than 70 °C, more preferably greater than 90 °C, still more preferably at least 100 °C.

12. An apparatus according to any one of claims 7 to 11 , wherein said first unit comprises means for achieving said second condition, preferably wherein said means is a reflector, preferably a parabolic trough reflector, and / or a thermal absorbent material.

13. A method or apparatus according to any preceding claim wherein said adsorber material comprises: a solid support; and a molecular CO2 capturing agent.

14. A method or apparatus according to any claim 13, wherein said adsorber material comprises: an alumina or silica solid support, preferably a silica solid support; and / ora nucleophilic molecular CO2 capturing agent, preferably an amine molecular CO2 capturing agent, more preferably a polyamine molecular CO2 capturing agent, still more preferably a PEI molecular CO2 capturing agent.

15. A method or apparatus according to any preceding claim, wherein said catalyst system comprises: a titanium-containing photocatalyst, preferably a TiO2 photocatalyst; and / or a CotpyP molecular catalyst.

16. A method or apparatus according to any preceding claim, wherein said solid support is g-alumina or m-alumina, preferably g-alumina.

17. A method or apparatus according to any preceding claim, wherein a reducing agent is present in said second reactor unit, preferably wherein said reducing agent comprises water, alcohol, or mixtures thereof, more preferably wherein said reducing agent is ethylene glycol.

18. An apparatus according to any one of claims 7 to 17, comprising at least one further downstream unit in fluid communication with said first and second units, preferably selected from: a unit for the capture and release of CO2; and / or a unit for reaction of syngas.

19. An apparatus according to any one of claims 7 to 18, wherein said apparatus comprises means for recycling a gas stream exiting said second unit back to said first unit.

20. A reactor unit for the capture, preferably direct air capture, and release of CO2, the unit comprising: an adsorber material which captures CO2 under a first condition and releases CO2 under a second condition; and a means for achieving said second condition, preferably wherein said means for achieving said second condition is a reflector, preferably a parabolic trough reflector, and / or a thermal absorbent material.21 . A method for the capture, preferably direct air capture, and release of CO2, the method comprising: supplying a first gas, preferably air, to the reactor unit of claim 20; capturing CC^from said first gas on said adsorber material at said first condition; releasing CO2 from said adsorber material at said second condition to provide a second gas comprising CO2.

22. A reactor unit, preferably a gas phase reactor unit, comprising a catalyst system for the conversion of CC^to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support, preferably a solid alumina or silica support, more preferably a solid alumina support.

23. A method of converting C02to syngas, the method comprising: supplying a gas comprising CO2 to a reactor unit as defined in claim 22; and converting said CO2 to syngas.

24. A catalyst system for the conversion of CO2 to syngas, the catalyst system comprising: a photocatalyst; a molecular catalyst; and a solid support, preferably a solid alumina or a silica support, more preferably a solid alumina support.

25. Use of the methods, apparatuses, reactor units, and / or catalyst system of any preceding claim in the conversion to syngas of CO2 from air, from waste, and / or from exhaust gas mixtures, preferably from air.

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