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Photocatalytic Conversion of Carbon Dioxide into Valuable Chemicals in “Artificial Photosynthesis” Systems: Recent Developments and Patents Review Ángel Martín, Alexander Navarrete and María D. Bermejo* High Pressure Processes Group, Department of Chemical Engineering and Environmental Technology, University of Valladolid, Valladolid, Spain Received: March 20, 2015; Revised: June 10, 2015; Accepted: June 12, 2015

Abstract: Our current economic and industrial growth relies on fossil fuels that can be easily and economically converted into energy and useful chemicals, but this economic model is endangered by the depletion of these resources and by environmental problems as the emission of CO 2 and other greenhouse gases. To solve this dilemma, we can find inspiration in natural photosynthetic organisms that consider CO2 as a carbon source and not as a waste, and power its transformation into valuable chemicals with renewable solar energy. To transform this vision into a practical and affordable reality, several challenging scientific and technical questions must be solved, including: the cost effective production of stable photocatalysts, the elucidation of the mechanisms of formation of useful compounds from CO2, or the implementation of the technology using scalable reactor concepts. This article presents a review of recent developments and patents related to “artificial photosynthesis” systems, covering key aspects such as the transition from macro-sized photo-electrochemical cells to nanostructured catalyst, the combination of these catalysts with organic or inorganic light sensitizers in order to extend the light wavelength range in which they are active, and the design of artificial photosynthesis reactors.

Keywords: Artificial photosynthesis, carbon dioxide, light sensitizer, methanol economy, nanomaterial, photo electrochemical cell, solar fuel. 1. INTRODUCTION During the last two centuries, our industrial and economic growth has been driven by the availability of fossil fuels that could be easily and economically extracted, stored, transported and converted into a huge variety of useful chemical compounds and into energy. However, the nonrenewable nature of these fossil fuels has given rise to a growing concern about their depletion. Finding alternative energy sources is only one part of the problem, because fossil fuels also are the main source of chemicals used for the manufacture of myriad products: polymers, solvents, pharmaceuticals, fertilizers... Indeed, about 10% of the oil, coal and natural gas consumed are employed as sources of compounds for chemical synthesis and not as fuels [1]. Moreover, as a consequence of the growing use of fossil resources and the associated pollution of the environment, carbon dioxide has been stigmatized as a useless contaminant gas that must be get rid of. However, looking into nature, carbon dioxide is the basic chemical resource used by living organisms to synthesize the organic compounds that they need by photosynthesis. Thus, CO2 should not be considered as a pollutant or as a residue, but as a renewable, *Address correspondence to this author at the Department of Chemical Engineering and Environmental Technology, University of Valladolid. c/Doctor Mergelina s/n 47011 Valladolid (Spain); Tel: +34 983184077; E-mail: [email protected]

2212-4047/15 $100.00+.00

widespread resource that can contribute to a sustainable growth, closing the carbon cycle [2]. Despite the low energy state of carbon dioxide and, therefore, its low reactivity, recent developments demonstrate that CO2 can be used as a raw material for the chemical industry for many applications, including the synthesis of new chemical compounds, fuels or polymers, and decisive advancements in basic chemistry, catalysis and process engineering have made it possible to develop innovative CO 2 conversion processes. Among other options, four key CO 2 conversion technologies are being intensively studied: (1) catalytic copolymerization of CO2, particularly with highly reactive epoxides for the synthesis of polycarbonates [3, 4]; (2) thermo-catalytic CO2 conversion processes [5, 6]; (3) CO2 fixation in photo-bioreactors [7]; and (4) photocatalytic CO2 conversion processes, also known as “artificial photosynthesis” methods. In the latter case, the term “artificial photosynthesis” reflects that these methods are inspired in natural photosynthetic organisms. There is a huge variety of photosynthetic systems in nature, but they share some common basic features. Photosynthetic organisms universally employ antenna systems, constituted by pigment molecules organized in an energy hierarchy, to absorb light at wavelengths available in the environment, and funnel the excitation energy to photosynthetic reaction center proteins. These are nano-scale devices that use this energy to transfer electrons from donors to © 2015 Bentham Science Publishers

Photocatalytic Conversion of Carbon Dioxide into Valuable Chemicals

Recent Patents on Engineering, 2015, Vol. 9, No. 2

acceptors, creating charge-separated states. This energy is used to oxidize water or other electron donors, generating electrons at a sufficiently negative redox potential to be used for CO2 reduction at catalytic centers. The global reaction involves the transformation of carbon dioxide and water into carbohydrates and oxygen by fixation of solar energy. Artificial photosynthesis technologies seek to mimic these natural systems [8]. The basic step of artificial photosynthesis is the dissociation of water into hydrogen and oxygen, but hydrogen (or hydrogen radicals) generated from photocatalytic water splitting reactions can also be used for CO2 hydrogenation. This conversion pathway is commonly represented as a sequence of gradual hydrogenation reactions from CO2, to formic acid, formaldehyde, methanol and, finally, methane, although the mechanism of this reaction is poorly understood, and recent developments indicate a complex mechanism that proceeds through several two-carbon species as intermediates [9]: +2e "

+2e "

+2e "

+2e "

+2H

+2H " H 2O

+2H

+2H " H 2O

CO 2 !+ HCO 2 H !+ H 2 CO !+ CH 3OH !+ CH 4

Most researchers have focused on the formation of methanol by this reaction [10], with the aim of obtaining a "solar", carbon-neutral fuel that could power a so-called "methanol economy", as presented in Fig. (1) [2], or alternatively in the production of methane also for application as a fuel, but the potential applications of this reaction pathway for the generation of platform chemicals for the organic synthesis industry are also clear. The great interest of these potential applications of artificial photosynthesis has driven research efforts for the development of this technology. Frequently, the main aspect consider in the investigations is the development of suitable photocatalyst materials. Starting with TiO2 as the basic element [11], many researchers have attempted to improve the characteristics of this material (and, particulary, the band gap) doping it with other compounds such as metal oxides, metals or carbn or combining it with organic dyes. Additionally, other materials have been studied such as other semiconductor compounds (e.g. gallium or tungsten oxides), carbon nitride, carbon nanotubes, plasmonic metal nanoparticles or metal-organic frameworks [12, 13]. However, it must not be forgotten that artificial photosynthesis is a complex process involving different steps such as CO2 adsorption on the catalyst, product desorption, light harvesting and charge separation and mass transfer phenomena, which not only depend on the composition of the catalyst, but also in its morphological and textural properties and on the design of the reactor [13]. This article presents of revision of recent developments and patents related to the chemical conversion of CO2 into valuable products by photocatalytic, “artificial photosynthesis” methods. The discussion is organized around the main challenges faced during the development of this technology: the production of cheap, active and robust photocatalytic materials, capable of withstanding the long operation times required for an economically feasible implementation of the process while maintaining a high catalytic activity; the sensitization of catalysts to light wavelengths in the visible range,

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Solar Energy

CO2 sequestration

Artificial Photosynthesis

Chemicals

CO2

Methanol

CO2 capture

Atmospheric CO2

Fuel Cell

CO2 from fossil fuel

Fig. (1). The role of Artificial Photosynthesis in the Methanol Economy.

as required for a high energy efficiency; and the implementation of these catalytic materials into practical reaction devices, tailored for different applications. 2. CO2 CONVERSION IN PHOTO ELECTROCHEMICAL CELLS The development of methods for the chemical conversion of light energy can be traced back to the seminal work of Fujishima and Honda, who in 1972 demonstrated the photoelectrochemical splitting of water using a single-crystal TiO 2 photoanode and a Pt cathode with an external electrical bias [11]. In the 70s, different patents described photoelectrochemical cells based on semiconductor electrodes [14] or in doped semiconductors [15] for the production of methanol, formic acid or formaldehyde. Fig. (2) presents the basic features of such photoelectrochemical cells. Still today, some of the most successful designs are based on similar photoelectrochemical cells with a physical separation between electrodes and an externally applied voltage in the reaction. There are mainly two kinds of devices that are commonly used for water splitting by means of artificial photosynthesis. The first device is based on the use of photoactive species that are either molecules or semiconductor particles where both oxygen and hydrogen evolve in the same place. The second device is based on the separation of the evolution places using two different electrodes [16]. These devices are generally known as “artificial leaves” and are constituted by [17]: 1.

An anode exposed to sunlight, where electrons oxidize water or in lack of the anode, an electrocatalyst can supply electrons. In any case, sensitizers are usually applied to the materials in order to make a more efficient use of the solar spectrum.

2.

A cathode where CO2 is chemisorbed and transformed into hydrocarbons or valuable compounds.

3.

A membrane that allows the proton transfer to the cathode. This membrane can be made of carbon nanotubes, TiO2 or polymeric materials.

While fundamental research clearly identify materials science aspects as the key element for the successful

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V

V

H2

02

CH3OH

02 + H2

e-

e-

H20

CO2 + H+

H+ H+

H20

electrolyte

H+

H+

Fig. (2). The basic elements of a water splitting photoelectrochemical cell.

Fig. (3). The membrane photoelectrocatalytic cell patented by Ji et al. [22].

development of such photoelectrochemical cells, it can be difficult to include such elements in the claims of new patents, as frequently novel, improved materials design may be considered as covered by the general claims of the early patents in this field, or as already presented in the open scientific literature. Therefore, most patents center their claims on elements of the physical implementation of the cells, which may seem of minor relevance in comparison with the materials science aspects of the electrodes, but still can make a difference in terms of the practical application of the systems.

splitting systems available today are based on such semiconductor systems combined with a suitable cocatalyst to provide an active redox site [26]. It has been shown that if these semiconductor photocatalyst/metal cocatalyst systems (e.g., Pt/TiO2) are directly coupled without using an external electrical circuit, as shown in Fig. (4), this composite material can be used to run the water-splitting reaction [27], constituting a more convenient implementation that a macro-sized cell with external circuit, especially for the scale-up of the process and the reduction of costs [28]. The simplest implementation of this concept is in the form of a powder composite [29, 30], but some authors as Ono et al. from Toshiba [31] claimed a design based on two catalyst layers deposited over the sides of a semiconductor layer, installed in a cell with an external ion transfer path, similar to the cell of Ji et al. described in Section 2 [22], or the use of stacked PEM cells to generate hydrogen by water splitting [32]. Employing those systems, overall water splitting absorbing light in narrow wavelength ranges (typically, 400-450 nm) has been described by several authors, both in liquid and gas phases. As the reduced range of wavelengths in which such inorganic photocatalysts are active is one of their main limitations, many researchers and inventions deal with the development of materials with band gaps that provide light sensitivity in suitable wavelength ranges.

Many recent designs are strongly based on Proton Exchange Membrane (PEM) fuel cells and benefit from the advances in the development of these cells [18]. In 2010, Rajh et al. [19] patented a cell where anode and catode compartments are separated by a PEM membrane. Other recent examples include the patent of Deguchi et al. from Panasonic [20], that describes the use of a solid electrolyte membrane to separate the electrode chambers, employing nitride semiconductors as photoelectrodes, or the patent of He from Honda [21] that uses a PEM photelectrochemical cell to hydrogenate carbon dioxide or hydrocarbons. In another patent, Ji et al. from Samsung [22] describes a cell where the catalysts are deposited on each side of the separating membrane, and the system counts on a micropump (such as a salt bridge, an electroosmotic pump or an electric pump) to facilitate a one-directional electrolyte flow between chambers, as depicted in Fig. (3). In another group of patents [23-25], Bocarsly and Cole describe the use of photoelectrochemical cells based on aromatic amines as catalysts. 3. COMPOSITE PHOTOCATALYSTS As described in Section 2, the earliest artificial photosynthesis catalysts were based on a metal-semiconductor pair, where the semiconductor captured the energy of light and the metal acted as an electron trap to stabilize the charge separation. Some of the most promising photocatalytic water

To mention some examples, Sonoda et al. [33] describe a photocatalyst based on Al/Ga nitrides doped with transition metals. Similarly, in the patent of Landry [34], a semiconductor colloidal nanocrystal is combined with a photocatalytic capping agent allowing to expand and tune the band gaps thus increasing the range of wavelengths of sunlight usable by the material. Subramanian and Murgesan [35] describe the application of bismuth titanate nanocubes loaded with catalyst nanoparticles, prepared by hydrothermal synthesis. He et al. [36] developed composites based on silver halides which were then loaded onto a conductive or semiconductive material such TiO2 allowing to absorb light in the visible spectrum. A nickel/nickel oxide/nickel borate


CO, CH3OH… CO2 + H2O e-

02 H20 Fig. (4). Semiconductor – co-catalyst composites.

composite photocatalyst was synthesized by Yaan, Limei and Xiaodan, using the sol-gel method [37], and was applied for the production of methane from CO2. Formic acid and formaldehyde were synthesized from hydrogen and carbon dioxide using a nickel/perovskite (LaNiO3) composite [38] which was synthesized using infrared radiation during the gelification stage. Composite-based electrodes have also been developed using copper-based films, to obtain methane and ethylene [39]. Gold (20 nm) deposited onto TiO2 accumulates electrons which results in an increase in the reductive capacity of the composite that is capable to transform CO2 into oxalic acid through dimerization under light irradiation [40]. 4. VISIBLE-LIGHT SENSITIZATION OF PHOTOCATALYSTS The development of composite photocatalysts suitable for light absorption in a wide wavelength band, as required for application of artificial photosynthesis systems under natural light conditions, has proved to be very challenging, as light absorption depends on many factors, including the chemical composition of the catalysts, its crystalline structure and density of defects, the particle size, or the interactions at the interface between the photocatalyst and the oxidation catalyst. As inorganic systems show limitations in their capacity to absorb light in a wide wavelength range, and particularly in the visible range, researchers have sought to improve them by incorporating pigments into the semiconductor material that sensitize them toward visible light. One of the most representative results of this approach are the so-called DyeSensitized Solar Cells (DSSC), that have achieved an incipient commercialization. Currently, DSSCs can show overall conversion efficiencies of light into electricity of over 10%. In general, these efficiencies have been achieved using pigments based on metals that are rather expensive and difficult to manufacture (e.g. functional ruthenium - polypyridyl complexes [41]). Similar materials can be used in water splitting systems aimed at hydrogen production, as for example with the ruthenium complexes with quinone moieties patented by Norden [42]. Recently, water splitting has been achieved by researchers from the Royal Institute of Technology of Stockholm using molecular ruthenium catalysts, reaching high efficiencies and reaction rates, with catalyst


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turnover numbers similar or even higher than those of natural systems [43, 44]. Alternative organic dyes have been proposed, which are cheaper but still show limited efficiencies [45, 46]. Some of them are based on a phorphyrin reaction centre. Porphyrins are naturally occurring biological pigments (e.g., haem, chlorophyll...) that act as cofactors of numerous enzymatic reactions. Porphyrins themselves strongly absorb light at wavelengths near 410 nm (violet). Different patents describe the use of aqueous solutions of iron and manganese metalloporphyrins [47, 48], pyridinium metalloporphyrins [49] and melanins [50]. However, a common limitation of these systems is the low stability of the sensitizers that easily undergo degradation in aqueous media. Thus different researchers have sought to enhance their stability by immobilizing them, for example attaching them to the membrane used to separate cell electrolytes as described by Ang and Sammells [51], or incorporating them into monomers with the objective of developing a polymeric material with light-sensitizing properties [52]. Experiences in the application of porphyrinbased sensitized photocatalysts in DSSCs or artificial photosynthesis systems are limited. Recently, zinc metalloporphyrins could be used as solar cell sensitizers, reaching remarkable overall light conversion efficiencies of over 12% [53]. Porphyrin sensitized photoelectrodes have also been used for the reforming of biofuels for the generation of hydrogen gas [54]. These organic or metallorganic light sensitizers can also be attached to the metal-semiconductor photocatalytic pairs. For example, Sato et al. [55] describe a photocatalyst where the sensitizer is directly linked with the semiconductor in order to enable an exchange of electrons, particularly when the sensitizer is a rhenium or a ruthenium complex. Hayase et al. [56] described a method of enhancing the contact between a light sensitizer and a semiconductor substrate, based on impregnation of the dye by treatment with supercritical carbon dioxide, and with a particular focus on Dye-Sensitized Solar Cells. Another approach suggested by Fukuzumi [57] consists of synthesizing a porphyrin dendrimer. This author prepared such dendritic structures using zinc porphyring, and laminated them over a platinum catalyst and a glass substrate. Besides increasing the light absorption range of the photocatalyst, systems where an electron exchange is possible can also enhance the catalytic activity of the material by stabilizing the charge separation, which is transferred in cascade from the light sensitizers, to the semiconductor substrates and finally to the metal catalysts that act as charge traps. An alternative successful embodiment of this concept is a three-component system or triad constituted by a carotenoid, phorphyrin and fullerene [58]. In these systems, fullerenes contribute to stabilize the charge separation by transfer of the porphyrin photoexcited state to the fullerene [59]. Besides attaching organic sensitizers to inorganic materials in order to enhance charge separation and improve the stability of the material, some authors propose to develop materials with a combination of light sensitizers that are active in different wavelength ranges and are linked in order to create a cascade transference of light-induced charge separation. Following this idea, complex light antennas have been developed, as for example a molecular hexad constituted by a zinc porphyrin attached to BPEA (bis phenylthynyl anthracene) and BPDY (borondipyrromethene), where the zinc

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porphyrin has strong absorption at orange and red wavelengths, BPEA absorbs light in the blue region, and BPDY in green, thus spanning the visible spectral region from blue to red wavelengths [60]. In a similar way, the patent of Ramasamy [61] also proposed to use a combination of light reaction centres and pigments. Working with purely inorganic catalysts, it has been found that titanium oxide nanoclusters dispersed within a silica glass framework show a broadened light sensitivity due to the inhomogeneity of nanoclusters, and are able to operate under visible light [62]. Another approach is the "Z-Scheme" water splitting [63] that uses two different semiconductor photocatalysts coupled through a reversible donor/acceptor pair to utilize visible light more efficiently, in an approach inspired in the coupled photosystems I and II of green plants. Another method of tuning the light sensitivity of photocatalysts is to deposit noble metal nanoparticles in the catalyst with a carefully controlled size and crystallinity. This approach increases the light absorption by a plasmon resonance effect [64]. Such plasmonics have been widely used in biosensors [65] and biomedicine applications [66], but they have also found applications in photovoltaics [67]. In the field of carbon dioxide reduction, Hill et al. [68] describe the use of metal nanoparticles (particularly, made of silver) to perform the catalytic reduction of carbon dioxide to formic acid driven by a plasmon resonance effect. In this invention, particles are applied over a dielectric substrate (made, for example, of silica, aluminia or titania) and protected with a polymer film. They also describe the use of metal coated silica nanoparticles that exhibit the plasmon resonance effect. On the other hand, Boyd [69] describes the application of an aqueous suspension of gold nanoparticles to achieve water splitting by plasmon resonance. In order to stabilize such powderous catalysts, Smolyakov and Osinski [70] describe a core-shell composite presented in Fig. (5), where the core metal nanoparticle exhibiting the plasmon effect is covered by a semiconductor shell, which can also capture the excitation electrons and deliver them to catalyst metal particles deposited on its surface, where the water splitting reaction takes place. Hyde [71] describes a method where plasmons on a waveguide deliver energy to photocatalize a reaction. In addition, Jenning and Landry developed a photoactive material assembled using photocatalytically capped colloidal nanocrystal located between plasmonic nanoparticles for CO2 reduction to methane [72]. 5. NANO-STRUCTURED PHOTOCATALYSTS When using composite catalysts, a fundamental aspect determining the photocatalytic activity of the material is the efficiency of stabilization of the charge separation induced by light excitation. Among other factors, charge separation can be stabilized by geometrical factors, producing catalysts with appropriate and controlled sizes and shapes [73]. In recent years, the development of such nano-structured catalysts has been one of the most important and active fields of research in artificial photosynthesis systems. Figure 6 presents schematic diagrams of some patented nanostructured catalysts. A group of patents describe

Martín et al.

H+

H2

Co-catalyst hv1

hv2

eeCore

H20 + 02

Shell OH-

Fig. (5). The core-shell catalyst of Solyakov and Osinski [70].

(a) TiO2, ZnO… Substrate (b) Nano-wires Membrane Nickel/Molibdenum Catalysts (c)

Titanium nanotubes (with dispersed co-catalyst) Substrate Fig. (6). some patented nanostructured catalysts: (a) semiconductor nanorods patented by Yi and An [76]. (b) Si nanowires doublelayer structure patented by McKone et al. [77]. (c) Metal-loaded titanium nanotubes patented by Mohapatra and Misra [79].

nano-structured materials with a high aspect ratio (e.g. fibers or rods). Muuhtanen et al. [74] propose to use nitrogendoped TiO2 nanofibers combined with p-type semiconductors or with metal catalysts, deposited over a film or a membrane by a solvent drying method. Besides the morphology of the photocatalytic nanomaterial, an ordered structure can also improve the activity, and different authors have patented designs based on ordered arrays of nanofibers: Shen et al. [75] proposed a catalyst based on structural elements with high aspect ratio forming a pattern over a metal or metal oxide substrate, so they can focus light on the catalytic


center functioning as a nano-optical lens. These authors mention two-carbon species and other complex organics as their target products. Yi and An [76] described a nanomaterial consisting in ZnO nanorods coated with TiO2 and deposited over a silicon, glass or plastic substrate. McKone et al. [77] presented a double-layer structure, formed by Si nanowires vertically disposed over the two sides of a PEM membrane. Nickel or nickel/molybdenum nanoparticles are deposited over the Si wires and act as catalyst. One of the sides acts as anode and the other as cathode, thus stabilizing the charge separation. When immersed in water and exposed to light, the material performs water splitting, producing hydrogen gas bubbles in one side of the membrane and oxygen bubbles on the other side. Hoertz [78] patented a composite structure consisting of aligned nanoparticles in form of nanopillars and coated by a p-type semiconducting material which was attached to the catalyst through a chromophore as porphyrin. Another group of successful nano-structured catalysts are based on a nanotube configuration. Grimes et al. [79] patented a catalyst based on nitrogen-doped titania nanotubes, that could be co-doped with co-catalyst nanoparticles, and acted as photo-catalysts for the conversion of carbon dioxide into hydrocarbons. Mohapatra and Misra [80] also proposed using an electrode based on an array of titanium nanotubes with a metal co-catalyst dispersed on their surface, submerging this electrode in an electrically conductive liquid. Shelnutt et al. [81] proposed using a porphyrin nanotube deposited over a gold nanoparticle and connected to a semiconductor surface, with Pt nanoparticles deposited over the surface of the nanotube acting as catalyst. 6. IMPLEMENTATION OF ARTIFICIAL PHOTOSYNTESIS REACTORS While the main efforts in the development of artificial photosynthesis systems currently are devoted to the elucidation of the mechanisms of the process and to the development of photocatalytic materials, as reviewed in the previous sections, the design of reaction systems specifically tailored for the requirements of this technology also is an important research need. Some incipient, promising results can be found in the research literature. Recently, researchers from the Technical University of Denmark succeeded in the application of such composite catalysts for overall water splitting, employing silicon based gas phase microreactors [82]. Another important milestone was achieved by researchers of the U.S. Joint Center for Artificial Photosynthesis, who developed standardized cells and protocols for the characterization of photoelectrochemical catalysts [83]. As results in this area of development of the technology still are incipient, there are very few examples of designs specifically patented for artificial photosynthesis applications and, in general, most patented reactor designs are based on common solar collectors. For example, some early patents that describe reactor designs based on solar collectors with very minor modifications were presented Porter [84] and Halmann et al. [85]. A more recent patent by Ahmed et al. [86] describes a panel where the cover is permeable to atmospheric gases and impermeable to the products, thus providing a combination of a reactor with a passive CO2 capture


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method, although current photocatalysts still cannot operate efficiently with such diluted feed streams. A multifunctional photocatalytic reactor was disclosed by Fan et al. [87] which includes a gas circulation system which allows the continuous transformation of CO2 in a quartz reactor. 7. CURRENT & FUTURE DEVELOPMENTS As described in the previous sections, in the last years the development of artificial photosynthesis systems has been strongly linked to the design of novel catalyst materials with enhanced activity. The creation of composite semiconductor/co-catalyst materials and the light sensitization of these materials with organic or inorganic additives have been very important research fields, which are at the core of many recent patents. More recently, the improvement of the activity of the catalysts through the stabilization of charge separation in nano-structured materials has become a crucial field of research, and most probably the activity in this topic will continue growing in the near future. While with these developments the activity of catalysts is approaching the requirements of practical, commercial applications, the next challenge is to maintain this catalytic activity during prolonged operation periods. This will require significant improvements in the stability of the catalysts, especially in the case of materials based in organic sensitizers that currently are limited by their high sensitivity to degradation processes. Another important research need is the development of novel reaction designs, optimized for the synthesis of specific products. The recent developments in carbon dioxide sequestration technologies, and the legislation initiatives that in many countries require the implementation of such technologies in new thermal power plants, create conditions that can greatly facilitate the initial implementation of artificial photosynthesis reactors: localized and concentrated CO2 feed streams with controlled purity, and a favorable economic balance, when CO2 sequestration technologies that only generate a cost are compared with CO2 utilization techniques that, as artificial photosynthesis, can generate revenue. With such installations at localized CO2 production centers serving as stepping stones, in the middle term artificial photosynthesis can become a significant renewable source of chemicals and energy. CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest. ACKNOWLEDGEMENTS All the authors contributed equally to the work. Á. Martín and M. D. Bermejo thank the Spanish Ministry of Economy and Competitiveness for a “Ramón y Cajal” research fellowship. A. Navarrete thank Junta de Castilla y León for a postdoctoral fellowship. REFERENCES [1]

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