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2.4.1.2 Methanol (CH3OH)

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Methanol, also known as wood alcohol, is considered an alternative to conventional transportation fuel due to its lower production costs, improved safety, and increased energy security. In general, methanol is cheaper than other fossil fuels with lower flammability risk. Also, methanol can be produced from a variety of carbon‐based feedstocks. Unlike in the United States with abandoned blending methanol into gasoline, China allows around 12% of methanol used in fuel. As a fuel substitute, methanol does not have to compete for food consumption, such as ethanol production. In industrial production, methanol is primarily produced from the methane component of natural gas, while natural gas is an unsustainable source. Based on this requirement, chemists expect to produce methanol through greener, milder, and more efficient way. In 1980s, Halmann and coworkers have attempted to reduce CO2 into methanol and other organic fuels over several semiconductor catalysts, such as SrTiO3, WO3, and TiO2, with the photoelectron‐assisted conditions, while the conversion efficiency was very low and time consuming [32]. After that, because of industrial production of methanol, the photoreduction of CO2 to methanol is stagnant. In the twenty‐first century, global warming is considered as a severe environmental problem, so ways of reducing CO2 have been widely studied, including photoreduction. Wu [33] and Gondal and coworkers [34] tried to improve the photocatalytic conversion of CO2 into methanol over traditional TiO2 and modified ones by using novel light source, laser, and photoreactor, optical fiber, which to some degree enhanced the CO2 conversion efficiencies and selectivity. However, the conversion rate was still at micromole level per photocatalyst weight hour, and by‐product H2 accounted for large proportion of reductive products.

With the rapid development of nanotechnology and nanocatalyst, morphologies, surficial properties, and heterojunction have been extensively investigated in the past, which extremely promote the design and fabrication of highly efficient semiconductor‐based photocatalysts for CO2 photoreduction. Zhang and coworkers synthesized Ag‐based plasmonic photocatalysts with different constitutions and shapes for enhanced reduction of CO2 to methanol, where Ag nanoparticles can accumulate photogenerated electrons and benefit for reducing the potential barriers of CO2 reduction [35]. Under visible‐light irradiation, with the assistance of anisotropic AgCl/Ag nanoparticles, the yield rate of methanol reached as high as 188 μmol gcat−1, and the activity of the photocatalysts has no obvious loss in the recycling reactions. Recently, Liu et al. prepared facet‐dependent Cu2O that was coupled with reduced graphene, which was used to investigate the effects of Cu2O‐exposed facet on the photoreduction of CO2 to methanol [36]. The rhombic dodecahedral Cu2O/rGO exhibited the highest methanol yield (355.3 μmol gcat−1), which was ca. 4.1–80.8 times superior to cubic octahedral Cu2O/rGO and CuO/rGO after 20 hours of visible‐light illumination.

To further improve the yield and selectivity of methanol, Sun and coworkers prepared a single‐unit‐cell Bi2WO6 layers that exhibited enhanced CO2 photoreduction because the density of states at the CB from the surface atomic layer is significantly increased upon reducing the 3D bulk Bi2WO6 to the atomically thin Bi2WO6 layers [37]. The reduced dimension is beneficial for rapid movement of the photogenerated electrons and holes to the surface and thereby improves the solar CO2 reduction. What's more, the increased surface charge density can effectively facilitate the two‐dimensional (2D) conductivity, leading to the increase of CO2 adsorption amount. Therefore, over single‐unit‐cell Bi2WO6, the predominant product of CO2 photoreduction is methanol, and yield of CH3OH reaches to 75 μmol h−1 g−1 under a 300 W Xe lamp irradiation, which is 125 times higher than that of bulk Bi2WO6 (0.6 μmol h−1 g−1). Meanwhile, Mi and coworkers revealed the potential of III‐nitride semiconductor nanostructures in solar‐powered reduction of CO2 into hydrocarbon fuels [38]. It was demonstrated that CO2 molecules can be spontaneously activated on the clean nonpolar surfaces of wurtzite metal nitrides, InGaN, based on the results of ab initio calculations, in which the photoreduction conversion rate of CO2 into methanol is ∼0.5 mmol gcat−1 h−1 under visible‐light illumination (>400 nm). Moreover, it was found that upon incorporating a small amount of Mg dopant into InGaN, the photocatalytic performance of CO2 reduction can be drastically enhanced. It is well known that the separation and transport of photogenerated electrons and holes are largely governed by the presence of surface band bending, for instance, upward and downward band bending. For Mg‐doped InGaN nanowires, the introduction of Mg can make the surface potential of InGaN nanowires upward (up to 2 eV), leading to the near‐flat surface band structure, which can apparently improve the photocatalytic performance in CO2 reduction.

In addition, in 2018, Gong and his group reported 0D/2D materials with polymeric C3N4 nanosheets and CdSe quantum dots to enhance the separation and reduce the diffusion length of charge carriers [39]. The rapid outflow of carriers also restrains self‐corrosion and consequently enhances the stability. Owing to quantum confinement effects of the QDs, the energy of the electrons could be adjusted to a level that inhibits the hydrogen evolution reaction (HER) (the main competitive reaction to the photocatalytic CO2 reduction reaction) and improves the selectivity and activity for CH3OH production from the photocatalytic CO2 reduction reaction. With an optimal CdSe particle size of 2.2 nm, high selectivity (73%) and activity (186.4 mmol h−1 g−1) for CH3OH production with an apparent quantum efficiency (AQE) of 0.91% are achieved, higher than most multicomponent CO2 reduction reaction photocatalysts for CH3OH production (generally 0.02–75 mmol h−1 g−1).

Besides, inspired by biocatalytic reaction, based on high selectivity of enzyme, a series of enzyme‐based inorganic/organic systems have been developed in the past. For example, Baeg and coworkers attached isatin–porphyrin to chemical‐converted graphene for constructing a newly developed photocatalyst that was used sequentially to couple with enzymes in the presence of nicotinamide cofactor (NADH) under visible‐light irradiation [40]. In this system, the selectivity of MeOH reached to almost 100%, while the yield (∼5 μM h−1) is lower than that of efficient semiconductor‐based photocatalysts. Besides, in 2017, Park and coworkers further took advantage of enzyme‐cascade system to improve the transferring of photoinduced electrons into a multienzyme for biocatalyzed reduction of CO2 to methanol in a photoelectrocatalytic system [41]. In this tandem photoelectronic cell with an integrated enzyme cascade (TPIEC) system, hematite, and bismuth ferrite were used as photoanode and photocathode, respectively, and NADH and three different enzymes (formate dehydrogenase, formaldehyde dehydrogenase, and alcohol dehydrogenase) as specific reaction enzyme catalyzed CO2 into formate, formaldehyde, and finally methanol, as shown in Figure 2.10. The average rate of methanol formation per reaction volume (∼220 μM h−1) is much higher than the pure photocatalytic system mentioned above.


Figure 2.10 (a) Schematic illustration of the solar‐assisted production of methanol from CO2 and water through a three‐enzyme cascade. (b) Methanol production as a function of time in multienzymatic systems. (c) Photoelectrochemical production of methanol by TPIEC under various applied potentials under visible‐light illumination. (d) Methanol production of the TPIEC system in various multienzymatic systems with an external voltage of 0.8 V.

Source: Kuk et al. [41].

Solar-to-Chemical Conversion

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