CO2 methanation is a well-known reaction that is of interest as a capture and storage (CCS) process and as a renewable energy storage system based on a power-to-gas conversion process by substitute or synthetic natural gas (SNG) production. Integrating water electrolysis and CO2 methanation is a highly effective way to store energy produced by renewables sources. The conversion of electricity into methane takes place via two steps: hydrogen is produced by electrolysis and converted to methane by CO2 methanation. The effectiveness and efficiency of power-to-gas plants strongly depend on the CO2 methanation process. For this reason, research on CO2 methanation has intensified over the last 10 years. The rise of active, selective, and stable catalysts is the core of the CO2 methanation process. Novel, heterogeneous catalysts have been tested and tuned such that the CO2 methanation process increases their productivity. The present work aims to give a critical overview of CO2 methanation catalyst production and research carried out in the last 50 years. The fundamentals of reaction mechanism, catalyst deactivation, and catalyst promoters, as well as a discussion of current and future developments in CO2 methanation, are also included.

Supported Catalysts for CO2 Methanation: A Review

The high power density, ease of transportation and storage and many years of development of internal combustion engine technologies have put liquid hydrocarbon fuels at a privileged position in our energy mix. Therefore processes that use renewable energy sources to produce liquid hydrocarbon fuels from H2O and CO2 are of crucial importance. Concentrated Solar Power (CSP) can be employed as the only energy source for the renewable production of hydrogen from water either indirectly, e.g. by supplying the electricity for electrolysis, or directly by supplying the necessary heat for thermochemically producing hydrogen. Among the various thermochemical cycles tested so far for CSP-driven hydrogen production via water splitting (WS), those based on redox-pair oxide systems, are directly adaptable to carbon dioxide splitting (CDS) and/or combined CO2/H2O splitting for the production of CO or syngas, respectively. The acknowledgement of this fact has recently revived the interest of the scientific community on such technologies. The current article presents the development, evolution and current status of CSP-aided syngas production via such redox-pair-based thermochemical cycles. At first the various redox oxide material compositions tested for water/carbon dioxide splitting are presented and their redox chemistries are discussed. Then the selection of suitable solar reactors is addressed in conjunction with the boundary conditions imposed by the redox systems as well as the heat demands, technical peculiarities and requirements of the cycle steps. The various solar reactor concepts proposed and employed for such reactions and their current status of development are presented. Finally, topics where further work is needed for commercialization of the technology are identified and discussed.

A review on solar thermal syngas production via redox pair-based water/carbon dioxide splitting thermochemical cycles

Spherical mesoporous magnetite (Fe3O4) aggregates with a wormhole-like pore structure were successfully synthesized for the first time using a single iron precursor (iron(III) ethoxide) and an amphiphilic poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer (PEO100–PPO65–PEO100) as a soft template In this synthesis, the interaction between the iron precursor and the triblock copolymer self-assemblies in ethanol leads to the assembly of magnetite nanocrystals into spherical mesoporous aggregates These aggregates were characterized using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, standard and high-resolution transmission electron microscopy, 57Fe Mossbauer spectroscopy, and X-ray diffraction, confirming the formation of pure-phase Fe3O4 particles with monodisperse morphology (about 130 nm in diameter), three-dimensional wormhole-like mesopores, and highly crystalline spinel structure In addition, a formation mechanism for this material in the present system is proposed, based on the analysis of results The mesoporous magnetite has a high specific surface area of 1656 m2 g−1, and relatively large pores with a mean size of 52 nm The magnetic susceptibility data demonstrate that this material exhibits superparamagnetic behavior

Assembly of magnetite nanocrystals into spherical mesoporous aggregates with a 3-D wormhole-like pore structure

Chemical looping combustion (CLC) is a clean energy technology for CO 2 capture that uses periodic oxidation and reduction of an oxygen carrier with air and a fuel, respectively, to achieve flameless combustion and yield sequestration-ready CO 2 streams. While CLC allows for highly efficient CO 2 capture, it does not, however, provide a solution for CO 2 sequestration. Here, we propose chemical looping dry reforming (CLDR) as an alternative to CLC by replacing air with CO 2 as the oxidant. CLDR extends the chemical looping principle to achieve CO 2 reduction to CO, which opens a pathway to CO 2 utilization as an alternative to sequestration. The feasibility of CLDR is studied through thermodynamic screening calculations for oxygen carrier selection, synthesis and kinetic experiments of nanostructured carriers using cyclic thermogravimetric analysis (TGA) and fixed-bed reactor studies, and a brief model-based analysis of the thermal aspects of a fixed-bed CLDR process. Overall, our results indicate that it is indeed possible to reduce CO 2 to CO with high reaction rates through the use of appropriately designed nanostructured carriers, and to integrate this reaction into a cyclic redox (“looping”) process.

Carbon capture and utilization via chemical looping dry reforming

Double perovskites LaSrCoFeO6−δ (LSCF) and LaSrCoFeO6−δ with a three-dimensionally ordered macroporous structure (3DOM-LSCF) were successfully synthesized by a facile combustion process. Their crystal structure, morphology, BET surface area, band gap and catalytic properties were characterized in detail. Phase pure double perovskites LSCF and 3DOM-LSCF can be obtained by calcination at 550–950 °C for 4 h. The ordered and interconnected pore structure generated by using a PMMA template can be maintained successfully in the 3DOM-LSCF catalyst. Both catalysts had good catalytic performance in either CH4 selectivity or total yield. Production of CH4 from CO2 and H2O can reach 351.32 μmol g−1 for LSCF and 557.88 μmol g−1 for 3DOM-LSCF under photothermal conditions (350 °C + vis-light) in 8 h. The high solar-to-methane (STM) energy conversion efficiency was 1.217% for LSCF and 1.933% for 3DOM-LSCF in the photothermal mode. The results also show that the yield of CH4 in the photothermal mode is 5 times that in the thermal reduction mode. The double perovskites LSCF and 3DOM-LSCF are promising catalytic materials for the photothermal reduction of CO2 to hydrocarbon fuels.

3DOM-LaSrCoFeO6−δ as a highly active catalyst for the thermal and photothermal reduction of CO2 with H2O to CH4

The CO2 decomposition into carbon with the rhodium-bearing activated magnetite (Rh-AM) was studied in comparison with the activated magnetite (AM). The Rh-AM and the AM were prepared by flowing hydrogen gas through the rhodium-bearing magnetite (Rh-M) and the magnetite (M), respectively. The rate of activation of the Rh-M to the Rh-AM was about three times higher than that of the M to the AM at 300 °C. The reactivity for the CO2 decomposition into carbon with the Rh-AM (70% CO2 was decomposed in 40 min) was higher than that with the AM (30% in 40 min) at 300 °C. The Rh-M was activated to the Rh-AM at a lower temperature of 250 °C, and the Rh-AM decomposed CO2 into carbon at 250 °C. On the other hand, the M was little activated at 250 °C.

Carbon dioxide decomposition into carbon with the rhodium-bearing magnetite activated by H2-reduction

The reduction of CO2 to carbon was studied in oxygen-deficient Mn(II)-bearing ferrites (MnxFe3-xO4-δ, O⩽x⩽1, δ>0) at 300 °C. They were prepared by reducing Mn(II)-bearing ferrites with H2 gas at 300°C. The oxygen-deficient Mn(II)-bearing ferrites showed a single phase with a spinel structure having an oxygen deficiency. The decomposition reaction of CO2 to carbon was accompanied by oxidation of the oxygen-deficient Mn(II)-bearing ferrites. The decomposition rate slowed when the Mn(II) content in the Mn(II)-bearing ferrites increased. A Mossbauer study of the phase changes of the solid samples during the H2 reduction and CO2 decomposition indicated the following. Increases in the Mn(II) content lowered the electron conductivity of the Mn(II)-bearing ferrites. Increases in the oxygen deficiency, δ, contributed to an increase in electron conductivity and suggested that electron conduction due to the electron hopping determines the reductivity of CO2 to carbon by the donation of an electron at adsorption sites.

Reactivity of oxygen-deficient Mn(II)-bearing ferrites (MnxFe3-xO4-δ, O⩽x⩽1, δ>0) toward CO2 decomposition to carbon

Oxygen-deficient ZnII-bearing ferrites (ZnxFe3 –xO4 –δ, 0 ⩽x⩽ 1, δ > 0) have been synthesized and studied for their reactivity in the decomposition of CO2 to carbon at 300 °C. They were prepared by reducing ZnII-bearing ferrites with H2 gas at 300 °C. The oxygen-deficient ZnII-bearing ferrites consisted of a single phase of a spinel-type structure which was oxygen-deficient compared with stoichiometric composition. Their lattice constants were larger than those of the corresponding stoichiometric spinel. Decomposition of CO2 to carbon was accompanied by an oxidation of the oxygen-deficient ZnII-bearing ferrite. The amount of carbon deposited on the solid decreased when the Zn content in the ZnII-bearing ferrite increased. The decrease in decomposition rate is due to changes in the electron conductivity according to the Zn content in the ZnII-bearing ferrite. These changes may contribute to its reactivity for decomposition of the CO2 to carbon.

Mössbauer study of oxygen-deficient ZnII-bearing ferrites (ZnxFe3 –xO4 –δ, 0 ⩽x⩽ 1) and their reactivity toward CO2 decomposition to carbon

Oxygen-deficient magnetite (ODM) powder has been reacted with CO2 at 300 °C; black particles of carbon separated from the ODM were studied by Raman, energy-dispersive X-ray (EDX) and wave-dispersive X-ray (WDX) spectroscopies. The carbon was a mixture of graphite and amorphous carbon in at least two levels of crystallization. Temperature-programmed desorption–mass spectrometry (TPD-MS) showed that a large portion of the carbon deposited on the ODM was released as CO and CO2 gas in a flow of Ar at ca. 520 °C, indicating that the carbon particles formed on the ODM are reactive. X-Ray photoelectron spectra of the carbon-deposited ODM showed no indication of carbide (Fe3C) or α-Fe phase formation.

Characterization of carbon deposited from carbon dioxide on oxygen-deficient magnetites

Synthesis of a carbon-iron(II) oxide layer on the surface of magnetite and its reactivity with H2O for hydrogen generation reaction have been studied. X-ray diffractometry and chemical analysis showed that the carbon-bearing magnetite synthesized by the carbon-deposition reaction from CO2 gas with the hydrogen-reduced magnetite, was magnetite with a carbon-iron(II) oxide layer (CIO layer-M; M is stoichiometric magnetite) represented by (Fe3O4)1−δ (Fe3O3)δCτ. The TG-MS spectra also showed the evidence for the formation of the CIO layer on the bulk stoichiometric magnetite. Some amorphous phase was formed in the CIO layer during the activation step (in vacuo at 300 °C for 30 min). This amorphous phase reacted with H2O and evolved H2 gas at 350 °C. This H2 generation reaction resulted in the oxidation of the CIO layer into γ-Fe2O3 component and the release of a part of carbon in the CIO layer as CO2. The X-ray diffractometry and Mossbauer spectroscopy indicated that the solid solution of Fe3O4-γ-Fe2O3 was formed in the solid phase after the H2 generation reaction. This shows the cation movement in the B site of the bulk magnetite of the activated CIO layer-M during the H2 generation reaction. The TG-MS spectra also supported the above estimation.

Kazuhiro Akanuma

Papers

Carbon dioxide decomposition into carbon with the rhodium-bearing magnetite activated by H2-reduction

Reactivity of oxygen-deficient Mn(II)-bearing ferrites (MnxFe3-xO4-δ, O⩽x⩽1, δ>0) toward CO2 decomposition to carbon

Mössbauer study of oxygen-deficient ZnII-bearing ferrites (ZnxFe3 –xO4 –δ, 0 ⩽x⩽ 1) and their reactivity toward CO2 decomposition to carbon

Characterization of carbon deposited from carbon dioxide on oxygen-deficient magnetites

Synthesis of carbon-iron(II) oxide layer on the surface of magnetite and its reactivity with H2O for hydrogen generation