Photocatalytic CO2 Conversion to Ethanol: A Concise Review

Abstract

Photo-catalytically converting the greenhouse gas CO₂ into ethanol is an important avenue for the mitigation of climate issues and the utilization of renewable energies. Catalysts play critical roles in the reaction of photocatalytic CO₂ conversion to ethanol, and a number of catalysts have been investigated, including semiconductors and plasmonic metal-based catalysts, as well as several other catalysts. In this review, the progress in the development of each category of catalysts is summarized, the current status is reviewed, the remaining challenges are pointed out, and the future research directions are prospected, with the aim being to pave pathways for the rational design of better catalysts.

1. Introduction

With the proposal of the concept of “emission peaking” and “carbon neutralization”, the conversion and utilization of CO₂ have been put on the agenda [1,2,3,4,5]. Photo-catalytically converting CO₂ into valuable fuels is a promising approach, since it could mitigate the climate issues caused by greenhouse gas CO₂ and store the renewable solar energy as chemical energy simultaneously [6,7,8,9]. The products of CO₂ photocatalytic conversion reactions include CO [10,11,12], CH₄ [13,14], CH₃OH [15,16], C₂H₅OH [17,18,19,20], HCOOH [21,22], etc. Noteworthily, C₂H₅OH (ethanol) is a chemical with wide applications in the chemical industry, medical and healthcare industries, food industry, agriculture production, and so on. Therefore, photocatalytic CO₂ conversion to ethanol has recently become a research hotspot.

Catalysts play an essential role in the reaction of photocatalytic CO₂ conversion to ethanol. Up to now, a great number of photocatalysts have been developed, such as TiO₂ [23], Bi₂MoO₆ [24], g-C₃N₄ [25], Cu/TiO₂ [26], and AuCu/g-C₃N₄ [27]. Based on the nature of the developed catalysts, they could roughly be divided into semiconductors, plasmonic metal-based catalysts, and several others (Scheme 1).

Up to the present, there have been many excellent reviews on CO₂ photocatalytic reactions. However, some of them focused on a special type of catalyst, such as a semiconductor [28,29] or Mexene [30], while some of them focused on the conversion of CO₂ to CH₄ [31] or other products [32]. To the best of our knowledge, there have been no reviews on catalysts for photocatalytic CO₂ reduction to ethanol. In this paper, the progress of each category of catalysts (Scheme 1) for photocatalytic CO₂ conversion to ethanol is summarized, and the current research status and the future prospect are reviewed, with the aim being to give the readers a clear picture and to inspire more studies to further advance this research area.

2. Semiconductor Based Catalysts

Semiconductors describe a category of materials which can harness solar light. Upon the irradiation of solar light with photon energy, hν matches or exceeds the bandgap energy of the semiconductor, and an electron jumps from the valence band (VB) to the conduction band (CB), leaving a hole. The electrons and holes can combine and dissipate the input energy as heat or transfer it to the catalyst surface. In the case that the position of CB is lower than the potential required for CO₂ conversion to ethanol, the electron reacts with the adsorbed species and participates in a CO₂ reduction reaction to produce ethanol (Figure 1). According to this principle, several semiconductors have been verified to be active in photocatalytic CO₂ conversion to ethanol.

There are several factors affecting the efficiency of photocatalytic CO₂ conversion to ethanol, as follows: (1) Light absorption region and efficiency. Solar light mainly consists of a large amount of infrared light, visible light, and a small amount of ultraviolet light. Absorbing more light means more energy can be utilized to promote the reaction. The bandgap of the catalysts is an important factor influencing the light absorption region. Meanwhile, the energy levels of the catalysts should meet the requirement of the reaction. Therefore, upon suitable energy levels, the bandgap width of the catalysts should be as small as possible to absorb more sunlight to improve the photocatalytic activity and conversion efficiency. (2) The separation and transfer of photogenerated electron–hole pairs. As catalysis is a surface reaction process, photogenerated electrons and holes must be separated and transferred to the surface to react with the adsorbates. However, photogenerated electron–hole pairs are unstable and easy to recombine during transfer. Therefore, facilitating the electron–hole pairs’ separation and transfer is effective for accelerating the reaction. (3) Photogenerated electrons and holes react with the surface adsorbates, respectively, to give products. Not all the photogenerated electrons and holes can react with the adsorbates. ① The positions of the conduction band and valence band must correspond to the positions of the corresponding reaction levels in order to have sufficient redox capacity. ② For different reactants, their ability to adsorb electrons and holes is also different. In order to ensure the smooth progress of the surface reaction, it is usually required that the reactants have sufficient adsorption on the catalyst surface, and can receive the electrons and holes on the surface smoothly.

2.1. Pristine Semiconductors

Pristine semiconductors, such as TiO₂, Bi₂MoO₆, BiOCl, and TaON, have been reported to be active in photocatalytic CO₂ conversion to ethanol. For instance, self-organized TiO₂ nanotube arrays could serve as a photocatalyst for the conversion of CO₂ to alcohols under xenon lamp irradiation, with the methanol and ethanol formation rates are ~10.0 nmol cm⁻² h⁻¹ and ~9.0 nmol cm⁻² h⁻¹, respectively [23]. The large specific surface area and the one-dimensional nanotubular structure of TiO₂ nanotube arrays accounted for catalytic activities (Figure 2a). The mechanism is proposed as follows (Figure 2b). Upon light irradiation, electrons and holes are generated over TiO₂ nanotube arrays. The holes react with adsorbed H₂O to form hydroxyl radicals and hydrogen ions. The interaction between hydrogen ions and the excited electrons leads to hydrogen radicals. Meanwhile, the photoexcited electrons transfer to the conduction band and react with CO₂ to produce ·CO₂⁻. However, ·CO₂⁻ is not stable and will be transformed to chemisorbed ·CO, which is subsequently reduced to ·CH₂ and eventually yields methanol and ethylidene by reaction with ·OH and ·H. Ethanol is formed via the reaction of ethylidene, ·OH, and ·H (Figure 2b).

Furthermore, Bi₂MoO₆ is another pristine semiconductor for photocatalytic CO₂ conversion to ethanol. Dai et al. reported that hierarchical flower-like Bi₂MoO₆ exhibited high catalytic activity for the photocatalytic reduction of CO₂ under visible light irradiation, with methanol and ethanol yields of 6.2 and 4.7 μmol g⁻¹ h⁻¹, respectively [34]. Ribeiro et al. fabricated Bi₂MoO₆ catalysts by a simple hydrothermal or solvothermal method and investigated the effects of synthesis parameters on their performance in CO₂ photoreduction in an aqueous medium under visible light irradiation, with the aim to pave pathways for the rational design of better catalysts in the future [35]. It was discovered that the pH value of the precursor suspensions was a key factor in determining the properties (such as zeta potential, crystallinity, and morphology) and performance of Bi₂MoO₆ catalysts. The more acidic the pH values, the higher ethanol production rates. The Bi₂MoO₆ synthesized with H₂O as the solvent and pH = 2 gave the highest ethanol yield, reaching 34.4 μmol g⁻¹ h⁻¹ [35].

Several other Bi-based pristine semiconductors, such as BiVO₄ [36], Bi₂WO₆ [24], and BiOCl [37], have also been successfully applied in photocatalytic CO₂ reduction to ethanol. Taking BiVO₄ as an example, Huang et al. reported that a large number of C₁ intermediates could be generated on the surface of BiVO₄ under highly intensive light irradiation, which dimerized to produce ethanol [36]. Monoclinic BiVO₄ was more efficient than tetragonal BiVO₄ for ethanol production, recording an ethanol production rate of 2033.0 μmol g⁻¹ h⁻¹ under a 300 W Xe-arc lamp irradiation, without the detection of methanol as a byproduct [36].

Additionally, TaON [38] and SrZrO₃ [39] are also promising in photocatalytic CO₂ reduction to ethanol. Here, SrZrO₃ is taken as a representative example for elaboration. He et al. prepared SrZrO₃ nanoparticles via a sonochemical method and employed it in a photocatalytic CO₂ reduction reaction [39]. Ethanol, methane, and carbon monoxide were detected as the main products, with an ethanol production rate of 10.2 μmol g⁻¹ h⁻¹ under the irradiation of a 300 W xenon lamp. Characterization results suggested that the position of CB of SrZrO₃ was 1.37 eV vs. vacuum and −3.13 eV vs. NHE, which lies above the redox potential of methane, ethanol, and carbon monoxide, indicating all of them are possible products of CO₂ reduction by SrZrO₃. Upon light irradiation, electron–hole pairs were generated. The electrons activated CO₂ on the catalyst surface to form ·CO²⁻ and reacted with H⁺ in the solution to produce ·H. The interaction between ·CO²⁻ and ·H gave CO. The resultant CO could also be converted into ·C, followed by the formation of ·CH, ·CH₂, and ·CH₃ through successive reactions, which then reacted with H₂O, H⁺ or OH to produce ethanol or methane [39].

In spite of the fact that several pristine semiconductors have been successfully applied in the reaction of photocatalytic CO₂ reduction to ethanol, their efficiencies are generally low, due to their weak light absorption capacity, low photon utilization efficiency, and so on [40,41]. In this regard, several approaches have been adopted to modify the semiconductors, for example, by delicately introducing vacancy sites and constructing a heterojunction or hybrid catalyst with another semiconductor or non-semiconductor material, with the aim being to further improve their catalytic performance. In the following several sub-sections, we will review the progress of modified semiconductors in photocatalytic CO₂ reduction to ethanol.

2.2. Semiconductors with Vacancy Sites

Delicately introducing vacancy sites into semiconductors is an important approach to extend the light absorption spectrum, narrow the bandgap, and regulate the electronic structure of pristine semiconductors. Semiconductors with vacancy sites have also been studied in photocatalytic CO₂ reduction to ethanol.

Yang et al.’s work is a typical example [42]. They synthesized a Bi₂MoO₆ catalyst by assembling two-dimensional ultra-thin Bi₂MoO₆ nanoflakes into three-dimensional nanospherical Bi₂MoO₆. During the assemble process, abundant oxygen vacancies were created, resulting in two primary sites, namely the oxygen vacancies and the exposed molybdenum atoms (Figure 3). The two primary sites served as dual binding sites to trap CO₂ for its activation into electronic CO* species, which were subject to accepting electrons and holes, realizing the selective reduction of CO₂ into methanol and ethanol. Under visible light irradiation, the as-prepared Bi₂MoO₆ catalyst afforded methanol and ethanol production rates of 26.6 μmol g⁻¹ h⁻¹ and 2.6 μmol g⁻¹ h⁻¹, respectively, far surpassing those of bulk Bi₂MoO₆ [42].

Do et al.’s work is another example that falls into this category [43]. The authors reduced a HCa₂Ta₃O₁₀ nanosheet and used it as a catalyst for photocatalytic CO₂ reduction with H₂O. It was discovered that the reduction process induced a considerable amount of Ta⁴⁺ and oxygen vacancies, which significantly improved the visible light harvesting capacity of HCa₂Ta₃O₁₀ [43]. Introducing CuO onto reduced HCa₂Ta₃O₁₀ further enhanced its performance in photocatalytic CO₂ reduction to alcohols, with ethanol and methanol production rates of 113.0 μmol g⁻¹ h⁻¹ and 7.4 μmol g⁻¹ h⁻¹, respectively. The enhanced performance was ascribed to the facilitated separation of photogenerated electron–hole pairs due to the formation of p–n junctions as well as the boosted CO₂ adsorption and stabilization of C₁ intermediates by CuO [43].

2.3. Heterojunctions

Heterojunctions constructed by two or more semiconductors generally exhibit stronger light absorption capacity and a narrower bandgap than their corresponding single semiconductor counterparts. A number of heterojunctions have been adopted as catalysts for photocatalytic CO₂ reduction to ethanol, including g-C₃N₄/ZnTe [44], Cu₂O/g-C₃N₄ [45], Co₃O₄/CeO₂ [46], MoS₂/Bi₂WO₆ [47], TiO₂/Ni(OH)₂ [48], Bi/Bi₂MoO₆ [49], TiO₂/Ti₃C₂ [50], CuO/TiO₂ [51], and AgBr/TiO₂ [52].

Here, the applications of TiO₂/Ti₃C₂ [50] and P25 (heterojunction between anatase and rutile TiO₂) [53] in photocatalytic CO₂ reduction to ethanol are chosen as representatives for elaboration. The TiO₂/Ti₃C₂, synthesized by a facile hydrothermal oxidation method, exhibited a narrowed band gap and enhanced light harvesting capacity [50]. The ratio between TiO₂ and Ti₃C₂ affected the optical properties and performance of the heterojunctions. After the functionalization by imine ligands and Pd nanoparticles, the performance of the catalysts in CO₂ activation and water splitting was further promoted. The TiO₂/Ti₃C₂ with an optimal TiO₂:Ti₃C₂ ratio recorded an ethanol production rate of ~10.0 μmol cm⁻² h⁻¹ at -0.6 V [50]. In case that P25 was used as a photocatalyst for CO₂ conversion with H₂O, multiple products, including O₂, H₂, C₁-C₄ hydrocarbons, methanol, ethanol, and acetone were detected, with an ethanol yield of 0.14 μmol g⁻¹ h⁻¹, under the illumination of a 100 W UV-LED in a wavelength range of 355–385 nm and a light intensity of 120 mW cm⁻² [53]. The specific structure and the intensive light illumination accounted for the high ethanol yield over P25 [53].

A Z-scheme is a special category that falls into the class of heterojunctions. Catalysts with a Z-scheme structure have also been investigated in photocatalytic CO₂ reduction to ethanol. For instance, Seeharaj et al. constructed TiO₂/rGO/CeO₂ (rGO is reduced graphene oxide) catalysts by combining surface-modified TiO₂ nanoparticles with rGO and CeO₂ [54]. The TiO₂ surface was initially modified via the sono-assisted exfoliation method in 10 M NaOH for 1 h, which led to increased specific surface area, enhanced light absorption, and a decreased recombination rate of photoinduced electron–hole pairs. The incorporation of rGO and CeO₂ further boosted the separation and transfer of photogenerated charges, electron mobility, and CO₂ absorptivity. The high interfacial contact area and strong interaction between modified TiO₂, rGO, and CeO₂ resulted in a high photocatalytic CO₂ reduction rate, with methanol and ethanol production rates of 641.0 μmol g⁻¹ h⁻¹ and 271.0 μmol g⁻¹ h⁻¹, respectively [54]. The reaction mechanism is proposed with a schematic illustration in Figure 4. The photocatalytic CO₂ reduction reaction is a two-step process, involving water splitting and CO₂ photoreduction. Upon light irradiation, both modified TiO₂ and CeO₂ were excited, forming electrons in CB and holes in VB. Then, the holes from the modified TiO₂ VB transferred to CeO₂ VB and subsequently oxidized H₂O into OH·, H⁺, and O₂. Meanwhile, the electrons at CeO₂ CB transferred to modified TiO₂ CB and then to the rGO sheet. The multiple electrons were collected and transported along the rGO sheet to reduce the adsorbed CO₂ to form intermediates, such as ·CO₂ and ·CO. Eventually ·CO₂ and ·CO reacted with H⁺ to obtain methanol and ethanol [54].

2.4. Hybrid Catalysts Constructed between a Semiconductor and a Non-Semiconductor Material

Fabricating a hybrid catalyst by combining a semiconductor with a non-semiconductor material is another avenue to tailor the physicochemical and optical properties of semiconductors. Quantum dots (QD), metal organic frameworks (MOFs), conducting materials, and isolators have been adopted as modifiers to construct this type of hybrid catalyst.

(1) QD–semiconductor hybrid catalysts. The QDs are nanoparticles of semiconductors and describe a category of nanoscale crystals that can transport electrons [55,56]. In this regard, QD–semiconductor hybrid catalysts generally exhibit extraordinary properties and performance. A number of QD–semiconductor hybrid catalysts have been constructed and applied in photocatalytic CO₂ reduction to ethanol, such as WS₂ QD/Bi₂S₃ [57], and Bi₂MoO₆ QD/rGO [58]. Taking WS₂ QD/Bi₂S₃ as an example, WS₂ QD/Bi₂S₃ constructed by decorating WS₂ QD onto Bi₂S₃ nanotubes by seed-mediated strategy was sensitive to visible/near-infrared light and displayed an excellent CO₂ photoreduction activity, with methanol and ethanol production rates of 9.6 μmol g⁻¹ h⁻¹ and 7.0 μmol g⁻¹ h⁻¹, respectively [57]. Characterization results revealed that in WS₂ QD/Bi₂S₃, the exposed S atoms in WS₂ QD coordinated to Bi³⁺ to form a Bi–S bond, enabling the sharing of S atoms between WS₂ QD and Bi₂S₃ (Figure 5). The junction interface between WS₂ QD and Bi₂S₃ facilitated the separation and transfer of electron–hole pairs and consequently accounted for its enhanced catalytic performance [57]. Cheng et al.’s study is another example [59]. They prepared a CdS-Cu²⁺/TiO₂ nanorod array film photocatalyst, in which a TiO₂ nanorod array was synthesized by a hydrothermal method, and CdS and Cu²⁺ were deposited on TiO₂ by a cation adsorption method and successive ion layer adsorption reaction (SILAR). Its performance in photocatalytic reduction of CO₂ under visible light irradiation was measured under visible-near infrared light. The results showed that the yield of ethanol reached the maximum value (109.1 μmol g-cat⁻¹ h⁻¹) when SILAR was deposited twice, at a flow rate of 4 mL min⁻¹ and a reaction temperature of 80 °C. The high catalytic activity of CdS-Cu²⁺/TiO₂ was attributed to the combination of one-dimensional nanostructure with Cu²⁺ ions and CdS quantum dots, which restrained the recombination of the electron–hole pairs and broadened the visible light responsive region [59].

(2) The MOF–semiconductor hybrid catalysts. The MOFs are a class of porous polymeric materials, in which metal ions are linked together by organic bridging ligands. These MOFs usually have the advantages of highly porous structure, large specific surface area, and adjustable pore size, which endow them special properties as modifiers or catalysts [60,61,62]. For instance, Liu et al. encapsulated CuO QDs in the pores of MIL-125(Ti) (MIL-125(Ti) is a type of MOF) and further combined it with g-C₃N₄ to fabricate a g-C₃N₄/CuO@MIL-125(Ti) catalyst, which exhibited a high catalytic activity for photocatalytic CO₂ reduction in the presence of H₂O, with yields of CO, methanol, acetaldehyde, and ethanol up to 60.0 μmol g⁻¹ h⁻¹, 332.4 μmol g⁻¹ h⁻¹, 177.2 μmol g⁻¹ h⁻¹, and 501.9 μmol g⁻¹ h⁻¹, respectively [63]. A mechanism study revealed that, under light irradiation, electrons and holes were generated and separated (Figure 6). Due to the positions of the energy levels of g-C₃N₄, CuO QDs, and MIL-125(Ti), the electrons remained at CB of CuO QDs, and the holes remained at VB of g-C₃N₄. The potential energy of electrons on CB of CuO QDs met the requirements for CO₂ reduction to CO, methanol, acetaldehyde, and ethanol, and led to the generation of these products. The valence band of g-C₃N₄ was more positive than the oxidation potential of H₂O, resulting in the oxidation of H₂O to O₂ [63]. Cardoso et al. prepared a hybrid catalyst via growing MOF-based nanoparticles (ZIF-8) on Ti/TiO₂ nanotubes and adopted the as-prepared Ti/TiO₂-ZIF-8 catalyst in the photocatalytic CO₂ reduction reaction. The Ti/TiO₂-ZIF-8 can produce ethanol up to 10.0 mmol L⁻¹. The increased photocurrent (ZIF-8 acted as a cocatalyst to interact with Ti/TiO₂ nanotubes) and promoted electron transfer accelerated CO₂ photocatalytic reduction to ethanol [64].

(3) Conducting material–semiconductor hybrid catalysts. Integrating a semiconductor with a conducting material is an avenue to facilitate the electron transfer and prohibit the recombination of photoexcited electron–hole pairs [65,66]. For instance, modifying semiconductor Bi₂WO₆ with conducting polymers tailored the photoelectronic properties (band gap, charge mobility, etc.) and promoted the photocatalytic performance in photocatalytic CO₂ reduction [65]. Under visible light irradiation, the as-fabricated catalyst demonstrated methanol and ethanol yields of 14.1 μmol g⁻¹ h⁻¹ and 5.1 μmol g⁻¹ h⁻¹, respectively [65]. Similarly, graphitic-supported multiple functionalized TiO₂ nanowire (denoted as R-TiO₂@Gs) recorded an ethanol yield of 124.2 μM in CO₂ reduction with water after light irradiation for 6 h. The graphitic support accelerated the electron transfer, while the ligands in functionalized TiO₂ enabled the catalyst to capture CO₂ more efficiently and facilitated C–C coupling to produce ethanol [67].

(4) Isolator–semiconductor hybrid catalysts. Loading a semiconductor onto an isolator with a large specific surface area could increase the number of active sites and enhance the photocatalytic activity. Du and co-author’s work is representative of this [68]. They constructed a TPS/g-C₃N₄ (TPS is trimodal porous silica) composite catalyst via a two-step hydrothermal synthesis method. The TPS/g-C₃N₄ catalysts were of hollow tubular shapes, with a large specific surface area, high CO₂ adsorption capacity, and more active sites. Consequently, TPS/g-C₃N₄ exhibited a high activity in photocatalytic CO₂ reduction reaction to ethanol, with an ethanol yield of 196.0 μmol g⁻¹ h⁻¹ and an ethanol selectivity of ~100% [68].

2.5. Doped Semiconductors

Doped semiconductors generally exhibit engineered energy levels and bandgaps, which improve the light absorption and facilitate the separation and transfer of electron–hole pairs.

For example, Maimaitizi et al. prepared hollow-graded BiOCl microspheres co-doped with N and Pt by an in situ hydrothermal method and explored its performance in CO₂ photoreduction to ethanol [69]. Under visible light irradiation, the ethanol yield reached 14.15 μmol g_cat⁻¹ h⁻¹. Results suggested that the scattering effect and surface reflection caused by the special layered structure of the catalyst, the narrowing of the bandgap caused by N doping, and the Schottky barrier caused by the existence of Pt accelerated the charge separation and transfer, and consequently accounted for the high catalytic performance [69]. Li et al. successfully synthesized a Zn-doped g-C₃N₄ catalyst by a one-step calcination method and investigated the effects of operational conditions on its performance in CO₂ photoreduction under ultraviolet or visible light irradiation [70]. Notably, the optimized 0.5%Ru/Zn-g-C₃N₄-1/20 catalyst gave the best photocatalytic activity, with the yield of ethanol reaching 1442.9 μmol g⁻¹. A mechanism study revealed that electrons were transferred to Ru through Zn–N bonds and reacted with adsorbed CO₂ during light irradiation. At the same time, CH₄ combined with holes to form methyl, which can be attracted by Ru and connects with *CHO to form acetaldehyde intermediate. When some of the intermediates were converted to acetaldehyde, most of them were further hydrogenated to form ethanol [70].

3. Plasmonic Metal-Based Catalysts

Plasmonic metals, such as Cu, Ag, Au, and their alloys, are sensitive to visible light and could act as active sites for photocatalytic reactions [71,72]. Plasmonic metal-based catalysts have also been widely applied in photocatalytic CO₂ reduction to ethanol [73,74]. Generally speaking, plasmonic metal-based catalysts give higher activities than semiconductors for CO₂ photoreduction. In this section, the progress of plasmonic metals-based catalysts for the photocatalytic conversion of CO₂ to ethanol is reviewed.

3.1. Cu-Based Catalysts

(1) Cu nanoparticle-based catalysts. These Cu nanoparticles are of plasmonic properties and have been studied in photocatalytic CO₂ reduction to ethanol. For example, Xuan et al. took the advantages of the plasmonic effect of Cu and the chemical absorption capacity of CO₂ by Cu@Ni to fabricate a SrTiO₃/Cu@Ni/TiN catalyst [75]. The as-prepared SrTiO₃/Cu@Ni/TiN could capture full-spectrum solar energy and activate CO₂ efficiently, and consequently exhibited an ethanol evolution rate of 21.3 μmol g⁻¹ h⁻¹ and an ethanol selectivity of 79% under the irradiation of a 600 mW cm⁻² Xe lamp [75]. Density functional theory calculation suggested that CO₂ activation was the rate-determining step and that CO₂* was easier to absorb on the interface of Cu (100) and Ni (111) (Figure 7). In addition, CO* was difficult to desorb at the interface of Cu (100) and Ni (111), which facilitated the dimerization of CO to produce ethanol (Figure 7) [75]. Similarly, Cu-TiO₂/GO (GO = graphene oxide) synthesized via a one-step hydrothermal method was effective for photocatalytic CO₂ reduction to ethanol, with an ethanol production rate of 233 μmol g⁻¹ h⁻¹ [76]. The high specific surface area, the narrowed band gap, and the plasmonic properties of Cu accounted for its performance [76].

(2) Cu ion-based catalysts. Here, Cu^I could selectively catalyze CO₂ conversion to ethanol; however, the catalytic sites of Cu^I are not stable. Incorporating Cu^I into the cavities of MOFs or decorating Cu single atoms onto MOFs could retain the chemical state of Cu^I [77,78]. In this regard, several light responsive Cu–MOFs catalysts have been designed for photocatalytic CO₂ reduction to ethanol. For instance, Lin et al. used low intensity light to activate an in situ Cu^II(H_xPO₄)_y@Ru-Uio catalyst to generate Cu^I species in the cavities of Uio-67 [77]. Upon light irradiation, one single electron transferred from photoexcited [Ru(bpy)₃]²⁺-based ligands on Uio-67 to Cu^II centers in the cavities and one single hole transferred from Cu⁰ to [Ru(bpy)₃]²⁺-based ligands for the generation of Cu^I (Figure 8). The Cu^I then served as the active centers for photocatalytic CO₂ reduction to ethanol, with an activity of 9650.0 μmol g_Cu⁻¹ h⁻¹ at 150 °C [77].

The Cu²⁺ incorporated into semiconductors can also serve as a catalyst to drive the reaction of photocatalytic CO₂ reduction to ethanol, such as Cu doped into TiO₂ [26,79]. The preparation method, as well as the morphology of TiO₂, strongly affected the properties and performance of the as-prepared Cu-TiO₂ catalysts. The Cu-doped TiO₂ nanorod, which was synthesized via the combination of the hydrothermal method and ultrasonic assisted sequential adsorption method, exhibited improved photon transfer due to the one-dimensional nanostructure of TiO₂ and the incorporation of Cu²⁺, and resulted in methanol and ethanol yields of 36.2 μmol g⁻¹ h⁻¹ and 79.1 μmol g⁻¹ h⁻¹ at 80 °C and UV light irradiation [26]. The Cu-TiO₂ nanocatalyst fabricated by the sol-gel method possessed a large specific surface area, increased number of oxygen vacancies, and enhanced atomic mobility, which improved CO₂ photoreduction by H₂O, with methane, hydrogen, methanol, ethanol, and acetaldehyde as products [79].

(3) CuO semiconductor-based catalyst. In recent years, CuO has attracted extensive attention in the field of photocatalytic CO₂ reduction due to its strong absorption capacity towards solar energy. In addition, the combination of CuO with other semiconductors could reduce the rapid recombination of photogenerated electron–hole pairs and produce ethanol under light irradiation. For example, Lu et al. prepared a Re-doped CuO/TiO₂-NTs catalyst by doping rhenium into CuO/TiO₂ nanotube arrays, which gave methanol and ethanol as the main products in photocatalytic CO₂ reduction reaction. With the increase in Re, the proportion of ethanol in the product increased (Figure 9), with the optimized yield of ethanol reaching 7.5 µmol over 6wt% re-doped CuO/TiO₂-NTs after applying an external voltage of 0.4 V under simulated solar light illumination. The remarkable result might have originated from the tuned interface characteristics of re-doped CuO/TiO₂-NTs, which promoted the selectivity towards alcohols and accelerated the occurrence of the C–C coupling reaction [80].

3.2. Ag-Based Catalysts

Another plasmonic metal, Ag, has been utilized in photocatalytic CO₂ reduction to ethanol. For example, Shu et al. synthesized an Ag@AgBr/carbon nanotubes (CNT) nanocomposite catalyst by anchoring Ag@AgBr nanoparticles onto the surface of CNT, and investigated the effects of CNT length on the performance of Ag@AgBr/CNT in photocatalytic CO₂ reduction reaction under visible light irradiation [81]. It was discovered that CNT with longer length facilitated the separation of electron–hole pairs. Together with the plasmonic properties of Ag and the unique structure of Ag@AgBr/CNT nanocomposite, Ag@AgBr/CNT with longer CNT length exhibited a promoted activity in CO₂ reduction to methane, CO, methanol, and ethanol, with an ethanol yield of ~5.0 μmol g⁻¹ h⁻¹ [81]. This was not only limited to Ag@AgBr/CNT, as Ag@AgBr/AgCl also showed activity for CO₂ conversion to methanol and ethanol under visible light irradiation [82].

3.3. Au-Based Catalysts

The Au-based catalysts are another type of plasmonic metal for CO₂ reduction to ethanol. Do et al. deposited plasmonic Au nanoparticles onto ZIF-67 (ZIF-67 is a type of MOF) and investigated the performance of Au/ZIF-67 in a photocatalytic CO₂ reduction [83]. It was found that the loading of Au affected the size of Au nanoparticles, and Au nanoparticles with sizes in the range of 30–40 nm exhibited improved light harvesting capacity, enhanced charge separation, and played crucial roles in determining selectivity. Volcano relationships were obtained between the production rates of methanol/ethanol and the loading of Au, with an optimal ethanol production rate of 0.5 mmol g⁻¹ h⁻¹ (Figure 10a,b) [83]. The mechanism was proposed as follows: under light irradiation, plasmonic Au were excited and generated energetic electrons. These electrons overcame the Schottky barrier and injected into ZIF-67, which then participated in the activation and conversion of CO₂ to methanol and ethanol, which had already been adsorbed on the surface of ZIF-67 (Figure 10c) [83]. Ramis et al. probed the key intermediates and products over Au/TiO₂ in a photocatalytic CO₂ reduction reaction [84]. They revealed that several different CO₂ adsorption modes (i.e., CO₂, bicarbonate, and carbonate) could be observed depending on the loading of Au. The presence of H₂O promoted the formation of CO₂ radicals. Methanol mainly adsorbed over TiO₂ sites, forming methoxy-species, which could be converted into ethanol [84]. The probation of the intermediates and products provided insights for the mechanism study.

3.4. Plasmonic Alloy-Based Catalysts

Plasmonic alloys exhibit not only the plasmonic properties but also some specific properties [11], which empower their applicability in photocatalytic CO₂ reduction to ethanol. For instance, AuCu/g-C₃N₄ was a very promising catalyst, affording an ethanol yield and selectivity of 0.9 mmol g⁻¹ h⁻¹ and 93.1%, respectively [27]. In addition to the plasmonic properties, the alloy structure of AuCu, as well as the interactions between AuCu and g-C₃N₄, contributed to its photocatalytic performance. Over AuCu alloy, Au was positively charged, and Cu was negatively charged due to their electronegativity difference. The positive charge on Au promoted CO₂ adsorption and the negative charge on Cu facilitated the formation of the intermediates CO₂·⁻ and *CO. The interaction between AuCu and g-C₃N₄ facilitated the transfer of photogenerated charges [27]. Similarly, Pd₂Cu/TiO₂ catalyst gave an ethanol production rate of 4.1 mmol g⁻¹ h⁻¹ at 150 °C under visible light irradiation [85]. The plasmonic properties of Pd₂Cu, the CO₂ adsorption capacity of Cu, and the oxidation and C-C bond formation competence of Pd accounted for its performance [85].

4. Other Catalysts

In addition to semiconductor- and plasmonic metal-based catalysts, Co-based catalysts and Pd-based catalysts have also been studied in photocatalytic CO₂ reduction to ethanol. In this section, we will review the progress of these two types of photocatalysts for CO₂ reduction reactions.

4.1. Co-Based Catalysts

In thermal-driven reaction systems, Co is one of the active metals for C-C coupling reactions [86,87]. The competence of Co active sites for the formation of C-C bond endows Co-based catalysts applicability in the photocatalytic CO₂ reduction to ethanol [88,89]. For instance, Na-modified Co@C nanocomposite catalyst gave almost 100% selectivity to hydrocarbons and ~6% selectivity towards ethanol at 235 °C and under the irradiation of a solar simulator. Mechanism study revealed that, upon light irradiation, photoexcited charges were generated on Na-Co@C, which facilitated the formation of electron-rich carbon species. These species were further involved in CO₂ activation to CO₂^δ− and promoted the dissociation of CO₂ to CO. The CO was stabilized by the carbon layers on Na-Co@C, and produced ethanol via a CO insertion pathway [88].

4.2. Pd-Based Catalysts

The PdIn@N₃-COF (N₃-COF is a photosensitizing covalent organic framework) [90] and Pd/Mn-TiO₂ [91] catalysts have also been successfully utilized in photocatalytic CO₂ reduction to ethanol. Here, PdIn@N₃-COF is taken as an example for elaboration. Lu et al. confined bimetallic PdIn nanoclusters in N₃-COF to construct a PdIn@N₃-COF composite, and investigated its performance in photocatalytic CO₂ reduction to ethanol [90]. It revealed that PdIn@N₃-COF gave a total yield toward alcohols of 33.3 μmol g⁻¹ h⁻¹ and a selectivity to ethanol of 26%. On the one hand, the interaction between PdIn and N₃-COF facilitated the charge transfer; on the other hand, the bimetallic synergistic effect of PdIn-stabilized C₁ intimidates C-C coupling while retaining some of the C-O bonds. Both of these two factors contributed to the high conversion of CO₂ to ethanol [90].

5. Summary and Outlook

Up to now, a number of catalysts have been designed for photocatalytic CO₂ reduction to ethanol, including semiconductors, plasmonic-metal based catalysts, and several other catalysts (a brief summary of some typical catalysts is shown in Table 1). Clearly, in spite of the rapid progress, several challenges remain.

(1) CO₂ conversion rates over most of the catalysts are still low.

At present, semiconductors in photocatalytic CO₂ conversion to ethanol mainly face the following challenges: ➀ most semiconductors have a relatively low response to light due to the limitation of their own electronic structures, and ➁ photogenerated electron–hole pairs in semiconductors combine relatively quickly. These challenges result in ineffective performance of semiconductors in CO₂ photoreduction to ethanol. In terms of plasmonic metal-based catalysts, the absorption of light by plasmonic metal nanoparticles is mainly concentrated in the range of the ultraviolet light and visible light region, which makes the light utilization efficiency very low and leads to poor performance.

Due to these reasons, over most of the studied catalysts, CO₂ conversion rates are in the magnitude of μmol g⁻¹ h⁻¹. Even though some catalysts could record CO₂ conversion rates up to mmol g⁻¹ h⁻¹, this is still far away from what is required for industrialization applications. Therefore, there is still a long way to accelerate CO₂ conversion rates. Developing catalysts with high efficiencies for CO₂ activation or establishing photothermal catalytic systems to enhance CO₂ conversion might be future research pathways. For instance, it has been reported that surface site engineering of semiconductors is beneficial to increase the absorption range of light and to enhance the separation of photogenerated electrons and holes, thereby promoting the surface redox reaction and improving the photocatalytic CO₂ reduction to methanol, methane, CO, and others [92,93]. Adopting the surface site engineering strategy to develop suitable catalysts for CO₂ photoreduction to ethanol might be promising approach to enhance the CO₂ conversion rates.

(2) The selectivity towards ethanol needs improvement.

Photocatalytic CO₂ reduction to ethanol requires multiple electrons of strong energies [94,95]. The requirements are more critical than CO₂ photoreduction to CO, methane, methanol, and so on, which makes it difficult to realize 100% selectivity towards ethanol. Therefore, developing catalysts with tailored properties which could selectively produce ethanol is one of the challenges in this study. It has been reported that single atom catalysts are of specific geometric and electronic structures, which limits the absorption geometry of reactants on the catalytic active sites and is beneficial to providing product selectively. Therefore, delicately designing single atom catalysts or catalysts with specified sizes might be avenue to improve the selectivity towards ethanol.

(3) Some plasmonic metal-based catalysts are expensive.

Currently, the plasmonic metal-based catalysts used in photocatalytic CO₂ conversion to ethanol are mainly focused on precious metals, such as gold and silver. These noble metals are expensive and deactivate easily due to sintering. Therefore, non-noble metal-based plasmonic catalysts should be developed, such as Cu, Al, and some transition oxides with plasmonic properties. With the help of the above two prospects to enhance the CO₂ conversion rate and improve the selectivity towards ethanol, if these non-noble metal-based plasmonic catalysts are of high catalytic performance in CO₂ photoreduction to ethanol, it will be of stronger practical significance.

(4) The reaction mechanism is still unclear.

Most of the current studies focus on catalyst design and the improvement of ethanol production, whereas the underlying reaction mechanisms are not extensively investigated. Making clear the reaction mechanism could provide guidance for the rational design of efficient catalysts in the future. Combining the advanced in situ techniques (such as high-angle annular dark field scanning transmission electron microscopy, extended X-ray absorption fine structure, X-ray absorption near-edge structure, diffuse reflectance infrared Fourier transform spectroscopy, atmospheric pressure X-ray photoelectron spectroscopy), and theoretical calculations (density functional theory), might be avenues to unravel the underlying mechanisms.

Author Contributions

Writing—original draft preparation, D.L.; writing—review and editing, C.H.; guidance, supervision and project administration, H.L.; visualization, Conceptualization, R.Z. and Y.L.; software, J.G. (Jiawen Guo); formal analysis, J.G. (Jiapeng Guo); resources, C.C.V.; All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

We can provide the data upon requirements.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures, Scheme and Table

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Scheme 1. Schematic illustration of the catalysts used for photocatalytic CO2 conversion to ethanol.

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Figure 1. Band structure of several typical semiconductors with respect to CO2 reduction potentials towards different products at pH = 7. Reproduced with permission from reference [33].

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Figure 2. (a) Schematic illustration of the photocatalytic CO2 reduction to alcohols over TiO2 nanotubes, and (b) proposed mechanism of photocatalytic CO2 reduction to methanol and ethanol over TiO2 nanotubes. Reproduced with permission from reference [23].

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Figure 3. Proposed reaction pathway of photocatalytic CO2 reduction to methanol and ethanol. Reproduced with permission from reference [42].

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Figure 4. Proposed mechanism for photocatalytic CO2 reduction to methanol and ethanol over a TiO2/rGO/CeO2 catalyst. Reproduced with permission from reference [54].

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Figure 5. Schematic illustration of the structure of WS2 QD/Bi2S3. Reproduced with permission from reference [57].

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Figure 6. Schematic illustration of photoexcited electron–hole separation process over g-C3N4/CuO@MIL-125(Ti). Reproduced with permission from reference [63].

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Figure 7. (a) Free energy diagram for CO2 reduction to ethanol and ethylene on the Cu@Ni interface, (b) free energy diagram for CO2 reduction to ethanol and ethylene on Cu (100) surface. The red line represents the lowest energy path. The Cu, Ni, C, O, and H atoms are shown in brown, blue, gray, red, and white, respectively. Reproduced with permission from reference [75].

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Figure 8. Generation of CuI via light irradiation over a CuII(HxPO4)y@Ru-Uio catalyst (* represents the active site). Reproduced with permission from reference [77].

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Figure 9. Yields of products over different de-doped CuO/TiO2-NTs. Reproduced with permission from reference [80].

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Figure 10. (a,b) Effects of loading of Au on the photocatalytic activity of Au/ZIF-67 in a photocatalytic CO2 reduction. (c) Proposed mechanism for photocatalytic CO2 reduction to methanol and ethanol over Au/ZIF-67. Reproduced with permission from reference [83].

Table 1

Brief summary of some typical catalysts for photocatalytic CO₂ reduction to ethanol.

Catalyst Category	Catalyst	Reaction Condition	Performance	Ref.
Pristine semiconductors	TiO2	Reactor—home-made glass reactor, 50 mm in diameter and 100 mm in height;Reactant—10 mL deionized water and water saturated CO2; Light source—100 W Xe lamp, 35 mW cm−2.	Ethanol formation rate of ~9.0 nmol cm−2 h−1.	[23]
Pristine semiconductors	Bi2MoO6	Reactor—closed vessel; reactant—50 mL deionized water and saturated CO2; Light source—300 W Xe arc lamp (PLS-SXE300) with an ultraviolet cutoff filter (λ ≥ 420 nm).	Ethanol yield of 4.7 μmol g−1 h−1.	[34]
Semiconductors with vacancy sites	Reduced HCa2Ta3O10	Reactor—an in situ closed circulation system;Reactant—CO2 saturated with H2O vapor;Light source—150 W Xe arc lamp, 100 mW cm−2.	Ethanol yield of 113.0 μmol g−1 h−1.	[43]
Heterojunctions	TiO2/Ti3C2	Reactor—two electrode system;Reactant—0.1 M KHCO3 aqueous solution (pH = 6.8, 50 mL) saturated by CO2;Light source—300 W xenon lamp (PLS-SXE300/300UV) with 200 mW cm−2 light intensity;External bias potential—-0.6 V.	Ethanol formation rate of ~10.0 μmol cm−2 h−1.	[50]
Heterojunctions	TiO2/rGO/CeO2	Reactor—sealed photocatalytic reactor;Reactant—150 mL distilled water with saturated CO2;Light source—UV light (a 15 W UV-C mercury lamp, peak light intensity 254 nm);Catalyst—0.15 g.	Ethanol yield of 271.0 μmol g−1 h−1.	[54]
Hybrid catalysts constructed between a semiconductor and a non-semiconductor material	WS2 QD/Bi2S3	Reactor—closed 200 mL quartz glass reactor;Reactant—50 mL of ultrapure water with saturated CO2;Light source—300 W Xe arc lamp (PLS-SXE300).	Ethanol yield of 7.0 μmol g−1 h−1.	[57]
Hybrid catalysts constructed between a semiconductor and a non-semiconductor material	g-C3N4/CuO@MIL-125(Ti)	Reactor—visual micro autoclave lined with 100 mL polytetrafluoroethylene;Reactant—1.0 mL water and 0.3% CO2;Pressure—1.0 MPa; Light source—300 W Xe lamp, 326.1W m−2.	Ethanol yield of 501.9 μmol g−1 h−1.	[63]
Cu-based catalysts	SrTiO3/Cu@Ni/TiN	Reactor—Labsolar 6 A system (Beijing Perfectlight Technology Co., Ltd.);Reactant—10 mL ultrapure water and saturated CO2;Light source—300 W Xe lamp, 600 mW cm−2.	Ethanol yield of 21.3 μmol g−1 h−1 and an ethanol selectivity of 79%.	[75]
Ag-based catalysts	Ag@AgBr/CNT	Reactor—stainless steel vessel; Reactant—100 mL 0.2 M KHCO3 solution, pure CO2 (99.99%) with a pressure of 7.5 MPa;Light source—A 150 W Xe lamp (Shanghai Aojia Lighting Appliance Co. Ltd.) with UV cutoff filter (λ > 420 nm).	Ethanol yield of 5.0 μmol g−1 h−1.	[81]
Au-based catalysts	Au/ZIF-67	Reactor—horizontal-glass-type photoreactor;Reactant—10 mL of aqueous solution with 10 wt % triethanolamine, 67 mg NaHCO3, purged with CO2;Light source—Abet 103 with light intensity fixed at 150 mW cm−2.	Ethanol yield of 0.5 mmol g−1 h−1.	[83]
Plasmonic alloy-based catalysts	AuCu/g-C3N4	Reactor—high-temperature and-high pressure CEL-HPR reactor with a volume of 250 mL (Beijing Zhongjiao Jinyuan Technology Co., Ltd.);Reactant—100 mL ultrapure water, high-purity CO2 (99.999%), 0.8 MPa; Light source—300 W Xe lamp (λ > 420 nm),Temperature—80–160 °C.	An ethanol yield and selectivity of 0.9 mmol g−1 h−1 and 93.1%, respectively.	[27]
Co-based catalysts	Na-Co@C	Reactor—quartz cell reactor;Reactant—CO2, N2, and H2 of 20, 20, and 100 cm3 at standard conditions, with a final pressure of ~2.8 bar; Light source—Xe lamp (1000 W) coupled with an AM1.5 filter;Temperature—235 °C.	~6% selectivity towards ethanol.	[88]
Pd-based catalysts	PdIn@N3-COF	Reactor—double-walled 80 mL quartz photoreactor;Reactant—10 mL ultrapure water with saturated CO2;Light source—300 W Xe lamp (CEL-HXF300, CEAULICHT) with a 400 nm filter.	A total yield toward alcohols of 33.3 μmol g−1 h−1 and a selectivity to ethanol of 26%.	[90]

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