REVIEW ARTICLE https://doi.org/10.1038/s41467-019-10996-2 OPEN Fundamentals and applications of photocatalytic CO2 methanation Ulrich Ulmer1, Thomas Dingle1,2, Paul N. Duchesne1, Robert H. Morris 1, Alexandra Tavasoli1,2, Thomas Wood1 & Geoffrey A. Ozin 1 The extraction and combustion of fossil natural gas, consisting primarily of methane, gen- erates vast amounts of greenhouse gases that contribute to climate change. However, as a 1234567890():,; result of recent research efforts, “solar methane” can now be produced through the photo- catalytic conversion of carbon dioxide and water to methane and oxygen. This approach could play an integral role in realizing a sustainable energy economy by closing the carbon cycle and enabling the efficient storage and transportation of intermittent solar energy within the chemical bonds of methane molecules. In this article, we explore the latest research and development activities involving the light-assisted conversion of carbon dioxide to methane. he combustion of natural gas (NG), which is used for heating, electricity generation and as 1 Ta chemical feedstock, accounts for 20 wt% of global CO2 emissions . While demand for NG is predicted to rise in the coming decades, known fossil fuel reserves are projected to 2 last for only another 60 years . Thus, a non-fossil-based, sustainable source of methane (CH4), the main component of NG, is needed in order to satisfy the growing demand for this important fuel and chemical feedstock while simultaneously reducing the environmental impact of its use. Sustainable production of “synthetic” natural gas (SNG) is possible through the conversion of CO2 and water (H2O) into CH4 and oxygen (O2) using renewable energy such as sunlight. This reaction proceeds as follows: þ ! þ ð Þ CO2 2H2O CH4 2O2 1 The advantages of this approach are threefold: 1. The infrastructure required for the storage, distribution, and use of (S)NG is already established and readily available. 2. The high energy density of SNG makes it an efficient storage medium for excess renewable energy. 3. The high abundance and relatively low cost of CO2 and H2O feedstocks make SNG a significantly value-added product. SNG production is already carried out through power-to-gas (PtG) technology. Solar PtG employs photovoltaic cells to electrolyze water and generate hydrogen (H2), which is subse- quently reacted with CO2 to form CH4 via heterogeneous catalysis or biocatalysis. The best-case 3 4,5 solar-to-methane efficiency is 13.0% (22.5% for solar cells ; 90% for CO2 capture from flue gas ; 6 7 80% for water electrolysis and 80% for CO2 methanation ). 1 Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, ON M5S 3H6, Canada. 2 Department of Material Science and Engineering, University of Toronto, 184 College Street, Toronto, ON M5S 3E4, Canada. Correspondence and requests for materials should be addressed to U.U. (email: [email protected]) or to G.A.O. (email: [email protected]) NATURE COMMUNICATIONS | (2019) 10:3169 | https://doi.org/10.1038/s41467-019-10996-2 | www.nature.com/naturecommunications 1 REVIEW ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-10996-2 Technologies capable of storing solar energy in the chemical Reaction energetics fi bonds of CH4 with greater ef ciency, relative to the aforemen- Thermodynamically, the reaction of CO2 and H2O to form CH4 8 tioned PtG method, are the subject of ongoing research . In this and O2 (Eq. (1)) is endergonic with a large positive change of fi Δ 0 = −1 review, we summarize the most recent such scienti c discoveries Gibbs energy ( G298 K 818 kJ mol or 1.037 VNHE) and that are paving the way towards a fully realized solar CO2-to- therefore does not occur spontaneously under standard condi- CH4 process. Here, the focus lies on the light-assisted CO2 tions. If this source of energy is sunlight, the reaction is described methanation reaction. In principle, sunlight can also be incor- as “artificial photosynthesis”, due to its conceptual analogy to porated into methanation processes by utilizing “natural photosynthesis”, the process by which certain organisms 9,10 11 12 photocatalytic or solar thermal water splitting to produce synthesize complex organic molecules from CO2 using light . the H2 consumed during CO2 methanation. The existing scien- Theoretically, if both the reactants and products are at their tific literature9–11 summarizes the concepts and research activ- respective thermodynamic standard states when entering and ities in these fields. leaving the solar methanation reaction, then the minimum energy Here, though, we begin by reviewing the reaction energetics for gap that has to be overcome is the energy difference between −1 the conversion of CO2 into CH4, then dive into a short summary them (i.e., 818 kJ mol ). In reality, though, this situation is more fi of the state-of-the-art in current CO2 methanation systems. The complex. When designing arti cial photosynthesis reactions, such most recent developments regarding the various solar methana- as solar methanation, it is useful to consider natural photo- tion schemes covered in this review are discussed thereafter. The synthesis for inspiration. The reaction pathway for natural pho- article structure is illustrated graphically in Fig. 1. tosynthesis can be conceptually simplified into two separate We first consider “photothermal” methods in which photons reaction steps14: (1) light-driven splitting of water into “reducing 12,13 ” generate high catalyst temperatures , thereby driving the CO2 equivalents (i.e., [H], such as reduced nicotinamide adenine methanation reaction. “Biophotocatalytic” methods are then dinucleotide phosphate (NADPH)) and O2, and (2) CO2 con- discussed; these systems use microorganisms, or a combination of version into glucose: inorganic materials and microorganisms, to catalyze CO 2 !hv ½þ ð2Þ methanation. Finally, the latest developments regarding “photo- 12H2O 24 H 6O2 redox” methanation systems are presented. This section is further þ ½! þ ð Þ subdivided into heterogeneous and homogeneous categories. 6CO2 24 H C6H12O6 6H2O 3 These systems enable CO2 methanation via light-driven, photo- electrochemical reactions. hv À 6CO þ 6H O ! C H O þ 6O ; ΔG0 ¼ 2880 kJ mol 1 Under the IUPAC definition of photocatalysis12, all solar 2 2 6 12 6 2 298 K methanation schemes discussed herein are of photocatalytic ð4Þ nature, as they exhibit light absorption to achieve an excited state The majority of the energy required for photosynthesis is (“photoexcitation”), followed by interaction between the excited + consumed by the water splitting step. NADPH, H , and adeno- photocatalyst and reactant molecules to create products. While sine triphosphate (ATP) are then produced through a sequence of the light absorbed is restricted to ultraviolet, visible, and infrared − + − e - and H -transfer steps. These chemicals serve as e -transfer, wavelengths12, the excited state is not limited to electronic tran- + H -transfer, and energy-transfer shuttles for the CO conversion sitions and can include plasmonic, vibrational, or rotational 2 reaction, which is separated from the water splitting reaction both excitations12. spatially and temporally. CO2 + H2O Solar methanation CH4 + O2 Photothermal or plasmon-drivenBiophotocatalysis Heterogeneous photoredox Homogeneous photoredox CO + H CO + H O 2 2 CO2 + H2O CO2 + H2O 2 2 (Separate water splitting step necessary) hv Inorganic Microorganisms e–, 2H+ H+ CO nanostructures N N 2 hv hv M hv CO + H O O O 2– CH + H O N N 2 4 2 C N N O – 2 e CO M 2 – + 6 e + 6 H N N CH Photothermal/plasmonic H2O 4 hv Photosensitizer catalyst e–/H+ donor Anode Cathode Fig. 1 Schematic depiction of the solar methanation process and the various methods covered in this review paper. This includes the four main catalysis methods that are discussed: Photothermal or plasmon-driven, biophotocatalysis (hybrid bio-inorganic), heterogeneous photoredox and homogeneous photoredox 2 NATURE COMMUNICATIONS | (2019) 10:3169 | https://doi.org/10.1038/s41467-019-10996-2 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-10996-2 REVIEW ARTICLE a Solar methanation reaction scheme O2 System boundary hv hv H2O H2O Water splitting Methanation CH4 [H] CO2 b Thermodynamic states reduction CO S1: O , [H] 2 2 S2: CH4 Energy Water splitting S0: H2O, CO2 Process/reaction coordinate c Reactions “Molecular pathway” O2 CO2 [H] = H2 H2 –1 –1 H O ΔG° = 237 kJ mol ΔG° = –113 kJ mol 2 (1.23 V) (–0.24 V) CH4 2H+ + 2e– “Proton/electron pathway” O CO 2 [H] = H+ + e– 2 Fig. 2 Graphical representation of solar methanation reaction schemes and energetics. a In a first reaction step, water is split into reducing equivalents [H] + − + − and O2. [H] can be molecular H2 or a H /e pair. If [H] is molecular H2, then CO2 is reduced to CH4 via the “molecular pathway”. If [H] is a H /e pair, then CO2 is reduced to CH4 via the “proton/electron pathway”. b Graphical representation of the thermodynamic states of the solar methanation reaction. The energy level of the system is elevated from S0 to S1 during the water splitting step. Energy is released during CO2 reduction, reaching the energy level of the products, S2. c Reactions occurring during the solar methanation pathways shown in a. The “molecular” pathway represents the state-of-the-art of industrial methanation, where H2 is produced during water electrolysis, as discussed in the “State-of-the-art industrial CO2 methanation” section This example of natural photosynthesis illustrates that the State-of-the-art industrial methanation (discussed in the next complex synthesis of organic molecules from CO2 and H2Ois section) produces molecular H2 via an electrocatalytic water- most easily achieved by separating the water splitting and CO2 splitting approach, as do both the photothermal/plasmonic and reduction reaction steps. A similar approach can be taken when biophotocatalytic methods described in this review.