Fundamentals and applications of photocatalytic CO2 methanation

Abstract

The extraction and combustion of fossil natural gas, consisting primarily of methane, generates vast amounts of greenhouse gases that contribute to climate change. However, as a result of recent research efforts, "solar methane" can now be produced through the photocatalytic 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.

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

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

Fig. 3

Key concepts and examples of photothermal methanation architectures. a Schematic representation of the light absorption spectra of semiconductors and plasmonic metals in comparison to the solar emission spectrum. b Light-to-heat conversion: photo-excited electrons (e⁻) interact with atomic nuclei, possibly generating phonons (ph). c Mechanisms of heat transfer within a particulate catalyst. Heat can be localized in catalyst nanoparticles by inhibiting phonon and electron transfer from the nanoparticle to its support. Structural defects, as well as phase boundaries between the catalyst and support phases, inhibit phonon and electron transfer and are hence beneficial for heat localization. Properties such as thermal and electronic conductivity, size and shape of the catalyst/support systems govern nanoscale heat transfer. d Selected potential photothermal catalyst architectures

Fig. 4

Biomethanation reaction systems. a A simplified scheme describing the methanation of CO₂ catalyzed by archaea. Reproduced from ref. (Copyright [2002], Elsevier). b A hybrid system for photomethanation utilizing CO₂-metabolizing archaea in the cathode compartment as adapted with permission from ref.

Fig. 5

Band energy diagram of selected semiconductors. These materials are commonly used for photoelectrochemical water splitting and CO₂ reduction. Redox potentials of key CO₂ reduction reactions are also included. In principle, water splitting and CO₂ reduction can take place on the same semiconductor material if the conduction band energy level is aligned with, or more negative than, the energy level of the targeted CO₂ methanation reaction (−0.24 V_NHE) and the valence band energy level is aligned with, or more positive than, the oxygen evolution reaction energy level (1.23 V_NHE). This is indicated by the position of each material relative to the vertical bar dividing the figure. The materials exhibiting unfavorable band alignment are included in the figure, as they are commonly used as light-absorbers in photoelectrochemical cells^, —adapted from ref. —Published by Wiley-VCH; and ref. —Published by The Royal Society of Chemistry

Fig. 6

Band energy diagrams and device architectures of proposed heterogeneous photoredox methanation systems. a A single semiconductor photocatalyst that drives both the water oxidation and CO₂ reduction reactions at its valence (VB) and conduction band (CB) sites. b A semiconductor photocatalyst with co-catalysts added to facilitate reduction and oxidation half-reactions. Electrons and holes can be transferred to the co-catalysts to initiate the associated half-reactions. c A Z-scheme consisting of two semiconductor photocatalysts, in which the water oxidation reaction is occurring at VB 1, and the CO₂ reduction reaction occurs at CB 2. Electron transfer between the two semiconductors can be facilitated through the appropriate choice of semiconductors. Photoelectrochemical device architectures of (d) a monolithic device, in which protons and electrons are transferred from the anode to the cathode through an electrolyte or via conduction, respectively; (e) a wired device, in which protons and electrons are transferred from anode to cathode via a membrane and an external circuit, respectively; and (f) a photoelectrochemical cell, in which the anode and cathode are separated by a proton-conducting membrane with integrated electron-conducting material. Adapted from ref. —Published by the Royal Society of Chemistry

Fig. 7

a A proposed sequence of steps in the photomethanation of CO₂, as catalyzed by a metal porphyrin in solution. b Models of the catalysts and photosensitizers (some hydrogen atoms and counterions have been omitted for clarity). Adapted with permission from Nature Publishing Group: Nature 548, 74–77, Visible-light-driven methane formation from CO₂ with a molecular iron catalyst. Rao, H., Schmidt, L. C., Bonin, J. & Robert, M. (2017)