Direct Conversion of CO₂ into Hydrocarbon Solar Fuels by a Synergistic Photothermal Catalysis

Abstract

Photothermal coupling catalysis technology has been widely studied in recent years and may be a promising method for CO₂ reduction. Photothermal coupling catalysis can improve chemical reaction rates and realize the controllability of reaction pathways and products, even in a relatively moderate reaction condition. It has inestimable value in the current energy and global environmental crisis. This review describes the application of photothermal catalysis in CO₂ reduction from different aspects. Firstly, the definition and advantages of photothermal catalysis are briefly described. Then, different photothermal catalytic reductions of CO₂ products and catalysts are introduced. Finally, several strategies to improve the activity of photothermal catalytic reduction of CO₂ are described and we present our views on the future development and challenges of photothermal coupling. Ultimately, the purpose of this review is to bring more researchers’ attention to this promising technology and promote this technology in solar fuels and chemicals production, to realize the value of the technology and provide a better path for its development.

1. Introduction

With the development of industrialization, energy consumption is also increasing. In the current situation, 85% of primary energy or non-renewable energy resource is still dependent on fossil fuels [1]. The heavy consumption of fossil fuels caused an energy crisis, at the same time, tremendous CO₂ growth in the atmosphere has affected the climate system. The annual growth rate of CO₂ from 1970 to 2020 is getting faster and CO₂ concentration is getting higher [2]. The International Panel on Climate Chang predicted that the global temperature will rise by 1.9 °C in mean value if the concentration of CO₂ reaches about 590 ppm by 2100, which will cause serious climate impact [3]. Therefore, climate change’s harm, which threatens both humanity and the environment, urgently requires us to shift away from our current fossil fuel economy toward renewable and carbon-neutral energy technologies [4]. Converting carbon dioxide into valuable chemicals and fuels in the carbon cycle is regarded as the most promising solution to energy and environmental problem [5].

There are at least three ways to reduce CO₂ emissions. The first one is to improve the fuel efficiency of existing fossil energy sources or replace fossil fuels with renewables. The second one is to capture CO₂ and sequestration. The last is to utilize CO₂ as the carbon source to produce value-added chemicals/fuels. Up to now, photocatalytic reduction [6,7,8,9], electrochemical reduction [10,11,12,13], photoelectrochemical reduction [14,15,16,17], thermal chemical reduction [18,19,20,21] and photothermal reduction [22,23,24,25,26] have been proven to be good methods for CO₂-conversion. It is well known that CO₂-conversion involves multi-electron processes which can lead to different products and selectivity, for example, CO, CH₄, CH₃OH, HCOOH and so on, single carbon compounds. In addition, multi-carbon compounds formed by the C–C coupling reaction has also been reported [27,28,29]. Therefore, converting CO₂ into valuable chemicals and fuels seems to be a crucial path for mitigating global energy shortages and environmental pollution matters.

Compared with electric energy, solar energy has certain advantages, because solar energy is the Earth’s inexhaustible natural energy source, which has the potential to become the ideal renewable source for fuels in the future [30]. Meanwhile, solar fuels have higher energy density and are easier to store and transport [31]. Therefore, researchers use the energy of sunlight to convert CO₂ into fuels directly through photocatalyst, but the shortcomings are the limited absorption range of sunlight and the poor utilization rate, limited electron transfer kinetic process, the poor catalytic activity of photocatalyst lead to low conversion rate and poor product selectivity, which impede the large-scale application and remain significant challenges for photocatalytic CO₂ reduction [30]. For the above limiting factors, it is necessary to ameliorate a more effective method for CO₂ reduction by solar power.

CO₂ is an extremely stable molecule with a high bond energy of C=O (750 kJ mol⁻¹) [32]. The implementation of CO₂ reduction always requires sufficient energy inputs and appropriate catalysts [33]. Photothermal catalysis can be induced by coupling both the photo and thermal, photo-irradiation can induce photo-generated charge carriers and heating can drive the reaction and accelerate the reaction rate to further enhance the catalytic reaction, which is a promising method for CO₂ reduction [34]. Compared with pure-photo or pure-thermal, photothermal remedies the low conversion rate, poor selectivity, slow electrons or hole transfer and fast electron-hole pair recombination of photocatalysis, and substantial heat energy input from thermal catalysis. In other words, photothermal has the characteristics of both photocatalysis and thermal catalysis. Light promotes the generation of electron-hole pairs, while thermal energy provides activation energy which can promote the breaking of old bonds and the formation of new bonds. Meanwhile, thermal energy can increase the speed and kinetic energy of reactant molecules, improving the probability of collisions and energy between molecules. Therefore, photothermal coupling can effectively alleviate the high energy consumption and catalyst deactivation caused by pure thermocatalysis [35]. Our previous works have proved that photothermal coupling can effectively improve the activity of CO₂ reduction reactions [36,37,38,39,40]. Therefore, whether from the current research state or long-term development, the photothermal coupling technology to realize the carbon cycle of CO₂ is undoubtedly the most potential research and development technology.

Herein, we make this review of photothermal catalysis strategy. The whole review is roughly divided into four parts. The first part will discuss the concept and principle of photothermal catalysis. The second part summarizes some photothermal catalysts for CO₂ reduction. Then, main CO₂ reduction products are introduced and strategies for controlling products are discussed. Finally, a personal opinion on the developments and application perspectives of photothermal catalytic CO₂ reduction into fuels. The purpose of this review is for more researchers to continue studying and improving the photothermal catalytic reduction of CO₂ technology. Using photothermal catalysis reduces CO₂ technology to achieve carbon neutralization can immediately solve energy shortages and environmental problems.

2. Fundamentals of the Photothermal Catalytic CO₂ Reduction Reaction

2.1. Photocatalytic CO₂ Reduction

Photocatalytic CO₂ reduction is also known as “artificial photosynthesis”. It is similar to photosynthesis in plants. Chlorophyll captures sunlight and uses solar energy to drive H₂O and CO₂ into O₂ and glucose. Photocatalysis is just like the natural photosynthesis process. The overall process of photocatalytic CO₂ reduction includes three major steps: (i) absorbing light energy to form photogenerated electron-hole pairs; (ii) photogenerated electron-hole pairs separate and migrate; and (iii) photogenerated electrons or holes react with the surface adsorbed CO₂ and H₂O at the reactive sites to generate hydrocarbons and oxygen, H⁺. Firstly, semiconductor materials have an appropriate band structure, which can absorb photons to generate electron-hole pairs by ultraviolet (UV) or Vis, so it is usually used as photocatalysts [41]. Secondly, photogenerated electron-hole pair separation involving electrons will be excited from the valence band (VB) to the conduction band (CB), and there are the same number of holes in the VB; those electrons and holes migrate to the catalyst surface, respectively. During the process of migration, if some electrons and holes are not transferred rapidly, they will recombine or be captured by internal defect traps leading to inactivation [33,42], which is the main factor affecting photocatalytic efficiency. Lastly, surface reaction includes adsorption and activation of CO₂ and H₂O, CO₂ reduction and H₂O oxidation and product desorption. Adsorption and activation of the reactants are prerequisites for the succedent redox reaction. Product desorption is also crucial for surface reaction, if untimely desorption of the product occupies the active site, thus terminating the subsequent reaction, and eventually causing the reaction speed to slow down or stop, that is, catalyst deactivation [43]. Therefore, increasing the rapid separation and transfer rates of electron-hole pairs is the main point to improve the efficiency of photocatalytic reduction of CO₂ [44]. At the same time, improving the adsorption of reactants, promoting the activation of reactants and accelerating the desorption of products are also essential for CO₂ reduction.

2.2. Thermocatalytic CO₂ Reduction

Thermocatalytic reduction of CO₂ requires an input of heat energy to promote the reaction. For certain chemical reactions, the activation energy is constant, and the higher the temperature, the faster the reaction, which means that for certain chemical reactions, the activation energy (Ea) usually can be driven by thermal energy [33]. Gibbs free energy is a thermal dynamic function for judging the progress of a reaction in chemical thermal dynamics. Thermocatalysis reflects the maximum thermal dynamic tractive force for redox reaction between electrons and holes and reactants [44]. Whether it is thermocatalysis or photocatalysis, the whole process contains the adsorption of reactants, formation of intermediates and desorption of products. The difference is that thermal energy provided by thermocatalysis can activate chemical bonds to facilitate the breaking of old bonds and the formation of new bonds during the reaction process under the activation of appropriate catalysts, which determine the form of the intermediates. Moreover, different intermediates have different reaction paths, which dynamically influence the products and product selectivity [45]. Therefore, intermediates are decisive for the whole reaction. Thermocatalysis also provides more advantages of favorable kinetics for CO₂ reduction [46].

2.3. Photothermal Catalytic CO₂ Reduction

Photothermal catalysis is one of the typical new catalytic methods and a current focus of photothermal technology research; as a highly efficient catalytic technology integrating photochemistry and thermochemistry, it has received extensive attention in the field of CO₂ conversion [47]. Photothermal catalytic CO₂ reduction can be understood as the product of the unity between the thermal acceleration of photocatalysis and the photo enhancement of thermocatalysis [44]. The uniqueness of photothermal catalysis is that it is not a simple superposition effect with conventional solar-driven photocatalytic and thermodynamics-driven thermocatalytic activity, but a synergistic effect of the two reactions promoting each other. This synergistic effect between light energy and thermal energy can effectively improve the solar energy harvesting ability, including low-energy visible light and near-infrared light; improve the catalytic activity and product selectivity for the reaction; and promote certain reactions that can only react at high temperature and be carried out at relatively low temperature [48]. In summary, compared with a single photocatalytic system, photothermal catalysis provides a more efficient conversion pathway to convert CO₂ into carbon-containing fuels, because the existence of the thermal effect broadens the absorption range of the solar energy spectrum and accelerates the separation and transfer efficiency of photogenerated electron-hole pairs as well as solves the problem of slow reaction speed in photocatalytic process. At the same time, compared with single thermocatalysis, the introduction of light energy can greatly reduce the harsh conditions of high energy consumption and high temperature and high pressure in the thermocatalysis process, and can also reduce material costs and alleviate catalyst deactivation. Due to the different subjects driving the whole reaction, the photothermal catalytic reaction can be divided into three categories: photo-assisted thermocatalysis, thermal-assisted photocatalysis and photothermal co-catalysis. In addition, the heat sources in photothermal catalysis can also be divided into three types: external heating device; the local heating caused by the absorption of photon energy by the catalyst through lattice vibration or electronic oscillation; and plasmon resonance effect. These will be described in detail in the following sections.

2.4. Thermal Classification of Photothermal Catalytic CO₂ Reduction

Nowadays, the thermal/heat sources in photothermal catalysis can be divided into three different types. One type is heated from the external heating device;the other is the local heating caused by the absorption of photon energy by the catalyst through lattice vibration or electronic oscillation; and the the third type is heated by plasmon-induced heating.

The external heating device is the most direct and easiest way to heat. Generally, the input of heat energy is directly realized by installing a heating table at the bottom of the reactor or wrapping the reactor with a heating jacket/heating belt and adjusting the temperature according to the actual reaction. In a study on thermal coupled photocatalysis to enhance CO₂ reduction activities on Ag-loaded g-C₃N₄ catalysts [49], the photochemical-thermal CO₂ reduction activities of the catalysts were directly evaluated in a stainless steel reactor equipped with a quartz window and a device for controlling the reaction temperature.

A semiconductor absorbs light energy close to or greater than its band gap to generate electron-hole pairs. The exciting high-energy electrons will return from a higher energy state to a lower energy state in two forms. One form is that photogenerated carriers recombine through radiative relaxation, where the process will cause energy loss. The other is that the excited electrons in the form of photons increase the temperature through non-radiative relaxation, while local heating is generated by lattice vibrations. The thermal vibrations of the molecules also cause the molecules to generate heat. In one study, an asymmetric Zn-O-Ge triple atom site was designed to trigger CO₂ reduction using light radiation [50]. The results showed that inducing light increases the thermal vibration of the molecule, and the heat generated by the thermal vibration of the molecule promotes the C–C coupling and the evolution of hydrogen species, which greatly speeds up the rate-determining step.

Localized surface plasmon resonance (LSPR) means that when light irradiates on the surface of plasmonic nanomaterials; if the incident photon frequency matches the overall vibration frequency of electrons on the surface of the material, the plasmonic material will have a strong absorption effect on the photon energy, which will cause localized surface plasmon resonance [51,52]. The plasmon resonance phenomenon can obtain heat energy from sunlight. Plasma catalysts can induce localized heating and provide heat carriers to initiate CO₂ reduction reactions and/or modulate reaction pathways, leading to high solar-to-fuel conversion efficiency [53,54]. In addition, the photocatalytic performance of photocatalyst can be further increased by enhancing light scattering, hot electron injection, inducing a strong local electric field and heating the surrounding environment to increase the oxidation-reduction reaction rate, mass transport and polarize adsorbed molecules on the surface of photocatalytic materials [55,56]. The localized surface plasmon resonance effect can control the spectral response range of the photocatalytic system by adjusting the composition, morphology and medium environment of nanoparticles [57]. Lian et al. observed a broadened absorption peak in the UV-visible spectrum for Au/CdSe nanorod composite, which suggested a strong orbital hybridization between both Au and CdSe components and transient absorption spectroscopy (TAS) data revealed fast electron transfer efficiency [58]. Ag/TiO₂, in a photocatalytic process, converted CO₂ and CH₄ into C₂H₄ and CO. The success relies on hot electrons and holes that can be generated by a visible light-induced SPR effect over Ag and UV light-induced photoelectric effect over TiO₂. Hot electrons can overcome the Schottky barrier and inject into TiO₂ to combine with photogenerated holes; the hot holes on Ag could be captured by CH₄. The synergy achieved by the Ag-SPR effect and selective adsorption of CH₄ on Ag indicates that ethylene is generated on the surface of Ag [59].

2.5. Classification of Photothermal Catalytic CO₂ Reduction

The photo-assisted thermocatalytic system is mainly based on thermocatalysis. Light energy promotes thermocatalysis by increasing the local temperature of the catalyst surface through illumination, rather than through heat conduction. Bai reported on P-Mo₂C applied to light-driven thermocatalytic CO₂ reduction with H₂O. The surface temperature of the catalyst was found to be as high as 140 °C when the catalyst was irradiated by light. Compared with the thermocatalytic activity without light irradiation, the catalytic stability of the P-Mo₂C in the light-driven thermocatalytic activity was significantly improved and exhibited good photothermal conversion effect and catalytic activity, which could be attributed to the light-promoted removal of surface hydroxyls that derived from the reaction of CO₂ with H₂O [60]. m-CN@CsPbBr₃ can effectively drive the thermocatalytic reduction of CO₂ with H₂O. By coupling light into the system, the activity for CO₂ to CO reduction is further improved under lower temperatures (150 °C) compared to pure photocatalytic and thermocatalytic activity. This result showed that light excitation can facilitate the thermocatalytic CO₂ reduction process, adjust the adsorption of the reaction intermediates of m-CO₃²⁻ and promote the transformation of b-HCO₃⁻ to CO₂⁻ on the surface. Finally, CO₂⁻ releases CO when it encounters H_ads and free electrons. It can be concluded that photo-assisted thermocatalytic activity can effectively improve the stability of the catalyst, alleviating catalyst deactivation, promoting the transformation of reactants or intermediate and reducing the temperature of thermal catalytic reaction [61].

Thermal-assisted photocatalysis is mainly based on photocatalysis; light is the primary driving force for the reaction, while it is helpful to promote a photocatalytic reaction rate rather than drive thermocatalysis. The input of thermal energy can effectively improve the utilization rate of sunlight, promote the excitation and separation of carriers, accelerate the diffusion of reaction molecules and increase the reaction rate [62]. Pt/TiO₂ serving as a photothermal catalyst has shown relatively high performance and selectivity for CH₄ when using the thermally-assisted photocatalytic reduction of CO₂ with H₂O. The reason is that thermal energy with the photocatalytic process can enhance the generation of electrons and holes and increase charge carrier transfers to the Pt nanoparticles at the surface of the supports, resulting in the increased formation rates of the products. Moreover, the thermal energy in the photothermal catalytic process assists in the splitting of H₂ molecules by the Pt nanoparticles, which subsequently facilitates the reaction between monatomic H· with CO [63].

Photothermal co-catalytic activity is the co-participation of light energy and thermal energy in a catalytic reaction, and the mutual synergy promotes the occurrence of the reaction. In this system, single light energy or thermal energy can drive the reaction, but the photothermal coupling can accelerate the reaction and improve the catalytic activity. Pr³⁺-doped La_1−xPr_xMn_0.6Ni_0.4O_3−δ is an efficient artificial photosynthesis catalyst for solar CH₃OH; La_0.4Pr_0.6Mn_0.6Ni_0.4O_3−δ presents the best catalytic activity. Under the pure photocatalytic mode, although the yield of CH₃OH was not enough high, a study found it to have good selectivity (80%) for CH₃OH. Under the pure thermocatalytic activity, the yield and selectivity for CH₃OH were higher than pure photocatalytic, and the product yield and selectivity increased with the increase in temperature. When the temperature reached 300 °C, CH₃OH had the highest selectivity (87.8%) and yield (1.85 mmol). However, under photothermal coupling conditions, the yield of methanol was up to 3.97 mmol, which was about two times that of pure heat, and the selectivity reached 98%. The results show that photothermal co-catalytic activity can improve catalytic performance [64].

3. Basic Products of Photothermal Catalytic

Photothermal catalytic CO₂ reduction is a complex multistep reaction, which involves multiple electron transfers and reactions. CO₂ is a linear molecule with the highest oxidation state of carbon (C⁴⁺), which can generate various products in the presence of reducing agents by obtaining different numbers of electrons. Compared with H₂ or CH₄, H₂O may be a good candidate for reducing agents, because of its richness, no toxicity and effectiveness, which can be an H⁺ source to provide protons [65]. The most possible CO₂ reaction reduction in the aqueous medium with potentials versus normal hydrogen electrodes are mentioned below:

CO₂ + 2H⁺ + 2e⁻ → CO + H₂O E₀ = −0.53 V

CO₂ + 2H⁺ + 2e⁻ → HCOOH E₀ = −0.61 V

CO₂ + 4H⁺ + 4e⁻ → HCHO + H₂O E₀ = −0.48 V

CO₂ + 6H⁺ + 6e⁻ → CH₃OH + 2H₂O E₀ = 0.38 V

CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O E₀ = −0.24 V

CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O  E₀ = −0.24 V

2CO₂ + 12H⁺ + 12e⁻ → C₂H₅OH + 3H₂O E₀ = −0.33 V

2CO₂ + 14H⁺ + 14e⁻ → C₂H₆ + 4H₂O E₀ = −0.27 V

3.1. CO

Carbon oxide (CO) is both oxidizing and reducing in terms of chemical properties, so it is often chosen as a model for product research. From the point of chemical transformation, CO is preferred because it is an unsaturated metastable molecule, which can be converted into other organic compounds as reactants or primary products, or intermediates under the action of catalysts under different reaction conditions. Therefore CO has a certain application potential and value [66]. Ag@Cr/CaO/CaGa₄O₇/Ga₂O₃ photocatalyst has been successfully synthesized and has demonstrated a satisfying formation rate of CO (>835 mol·h⁻¹) and excellent selectivity (95%) by artificial photosynthesis for the conversion of CO₂ with H₂O [67]. The high formation rate and selectivity for CO are related to the increasing concentration of CO₂-related species around the active sites of Ag and the heterojunction formed by Ga₂O₃/CaGa₄O₇, which can promote the spatial separation efficiency of photogenerated carriers. Ultrathin two-dimensional Bi₄O₅Br₂ nanosheet (Bi₄O₅Br₂-UN) catalyst has demonstrated an outstanding photocatalytic activity and selectivity for CO₂; the formation of CO reaching 63.13 µmol·g⁻¹ and selectivity over 99.5% in the whole catalytic process, which is 2.3 times higher than bulk Bi₄O₅Br₂ (27.56 µmol·g⁻¹); so, changing catalyst structure has significance in improving catalytic activity [68]. In other research, Ag/SrNb₂O₆ nanorods exhibited a better CO formation rate (287 µmol·g⁻¹) and selectivity (94.1%) in aqueous solutions when NH₄HCO₃ was used as an additive. From previous experimental results and theoretical analysis, it can be seen that the actual CO₂ reactant in the photocatalytic reaction comes from the dissociation of HCO₃⁻, and the existence of HCO₃⁻ can facilitate the conversion of CO₂ in aqueous solutions [69].

3.2. CH₄

Methane (CH₄) is widely distributed in nature and is one of the main components of natural gas and biogas. It can be used as fuel and raw materials for the production of hydrogen, carbon monoxide, carbon black, acetylene, hydrofluoric acid and formaldehyde. Ulmer reported four methods for the application of photocatalytic CO₂ methanation, which were photothermal and plasmon-driven, biophoton-catalytic, heterogeneous photoredox and homogeneous photoredox catalysis (Figure 1a) [70]. It should be noted that H₂O and CO₂ also have been definitively demonstrated in photothermal or plasmon-driven CO₂ methanation. Herein, photothermal or plasmon-driven CO₂ methanation will only be discussed. Two possible pathways for CO₂ to CH₄ were also listed in Figure 1b [71]. For the formaldehyde pathway, carboxyl radical recombines with a hydrogen radical H· to form HCOOH and then HCOOH accepts H· to form a dihydroxymethyl radical, which dehydrates upon attachment of another H· to form HCOH. Two more steps will lead to CH₃OH, which is further reduced in another two steps to CH₄ [72]. For the carbene pathway, H· attached to the oxygen atom of the CO₂⁻ radical, caused an immediate cleavage of the bond between this oxygen atom and the carbon atom to form CO. CO accepts two electrons to form the C atom as with the above step [73]. These radicals can then combine with H· to form CH· radical, CH₂· radical, CH₃· radical and eventually, CH₄. If the CH₃· radical combines with the OH· radical, CH₃OH will form. There are two important steps in these two methods. The first step is that HCOOH, HCHO and CH₃OH are the intermediate steps in the formaldehyde pathway, and CO and C are the intermediate steps in the carbene pathway. The second step is that CH₃OH is intermediate in the formaldehyde pathway, but a product in the carbene pathway. Pd-loaded WN-WO₃ heterostructure as a direct Z-scheme catalyst broadens the absorption solar spectrum for photothermal assisted conversion CO₂-H₂O into CH₄. The conversion activity reaches 40.6 ± 0.7 mmol·h⁻¹·g⁻¹ for CH₄. The main reason for increasing catalytic activity is that the unique carrier transfer mode of Z-scheme catalyst. Under sunlight illumination, WN-WO₃ heterostructure can separate and migrate photoinduced carrier efficiency, resulting in more effective electrons involved in the reaction. The local photothermal effect accelerates the shifting of gas molecules and carriers, which improves the CO₂ conversion rate [74]. In this study, Ni NPs supported on barium titanate perovskite were prepared for photothermal conversion of CO₂. Under the optimum reaction conditions (UV-visible light irradiation), CO₂ with H₂ was converted into CH₄ with yield rates of 103.7 mmol·h⁻¹·g⁻¹ and the selectivity for CH₄ is nearly 100%. Mechanism studies indicated that a nonthermal hot-electron-driven pathway dominates reactivity in coexistence with a minor thermal contribution to the photothermal process [75]. The other article reported that strong interaction between metal and semiconductor can alter the reaction pathways of intermediate species during CO₂ conversion, leading to dramatic changes in product selectivity. The authors hypothesize that it is reasonable to add hydrogen one by one to CO₂ and suggest several possible pathways, which are shown in Figure 2. The hydrogenation on the C atom of CO₂ has four possible paths. It can be seen that the initial reaction CO₂ → CO₂* → HCOO* → H₂COO* → H₂COOH* was the same for P1, P2, P3 and P4 four reaction paths. The whole reaction path for P1 and P4 are similar, but the difference is that the intermediate state in the previous step before CH₄ generation is different. P1 was H₃C* → CH₄, while P4 was H₄COH* → CH₄. This was the same for P2 and P3. Meanwhile, P1 and P3, P2 and P4 have different reaction pathways but the similar point is that they have the same intermediate state before producing methane. The paths for P5 and P6 are different from the H₃COOH* → CH₄ pathway, which are the cross-hydrogenation of O and C atoms. P5 has the second dehydroxylation process [76].

3.3. CH₃OH

Methanol (CH₃OH) is one of the basic raw materials of many organic chemical products and has important commercial value. Photothermal catalytic CO₂ to CH₃OH has been a research hotspot in recent years. There are two ways for CH₃OH synthesis to occur from CO₂: direct hydrogenation and CO₂ with H₂O into CH₃OH [77,78,79,80,81].

2CO₂ + 4H₂ → 2CH₃OH + O₂

4CO₂ + 2H₂O → 4CH₃OH + 3O₂

Direct hydrogenation using heterogeneous catalysis has been studied extensively and seems to be much easier. CO₂ with H₂O is one of the most tempting routes. For example, the CoO/Co/TiO₂ catalyst used in catalyzing CO₂ to CH₃OH under photothermal conditions, it has been shown to be as high as 99.9% for selectivity and catalytic activity (39.6 μmol g_cat⁻¹·h⁻¹) [82]. From these results, a Z-scheme heterojunction photothermal catalytic system showed high activity for CH₃OH formation and a mechanism for photothermal catalytic CO₂ over CoO/Co/TiO₂ catalyst was proposed (Figure 3a). It proposed that the entire photothermal catalytic reduction of CO₂ to CH₃OH was at the CB of CoO instead of TiO₂ because the potential of the CB of TiO₂ (−0.14 eV) is more negative than the standard redox potential E_(CO2/CH3OH) = −0.38 eV, so the conversion of CO₂ to CH₃OH cannot occur. However, TiO₂ also had a certain promoting effect on the reaction because one part of the electrons involved in the reaction was provided by CoO and the other part of electrons was provided by TiO₂. TiO₂ can successfully transfer electrons to the VB of CoO and the surface of Co through the Co⁰ species. It can indirectly inhibit the recombination of CoO and TiO₂ electron-hole pairs and promote the occurrence of CO₂ reduction reaction.Meanwhile, it also can promote the thermal reduction of CO₂ to CH₃OH on Co, thus improving the efficiency of CO₂ conversion to CH₃OH, based on the good catalytic performances for CH₃OH formation originated from the special Z-scheme structure and the high temperature of the reaction. Wu reported a series of hexagonal tungsten bronze M_0.33WO₃ (M = K, Rb, Cs) photocatalyst showed unexpected photocatalytic CO₂ activity directly from air at ambient pressure under full spectrum (UV, visible and NIR lights). After a 4-h reaction, Cs_0.33WO₃, Rb_0.33WO₃ and K_0.33WO₃ exhibited the production of CH₃OH to be 17.50, 15.10 and 7.70 μmol·g⁻¹, respectively (Figure 3b). Both Cs_0.33WO₃ and Rb_0.33WO₃ exhibited outstanding CH₃OH production rates under NIR light irradiation (Figure 3c). After the reaction, the new signals of C−H and O−H functional groups can be clearly seen through the FTIR spectrum, which can prove that the existence of CH₃OH really comes from the photocatalytic reduction of carbon dioxide, and the production rate of methanol increases with the increase in temperature (10 °C, 60 °C, 150 °C). Combined with experiments and DFT analysis, it can be concluded that the {010} facets are more conducive to the adsorption of CO₂, and the introduction of alkali metals promotes the adsorption capacity of CO₂ and inhibits the adsorption of O₂. At the same time, the introduction of alkali metals into hexagonal tungsten bronze crystal regularly optimized and reconstructed the electronic structure, particle size, morphology and light capture ability of the M_0.33WO₃ series, which can provide a large number of free electrons to the crystal. In addition, it also enhanced the transition of polarized electrons, changed the energy band structure of the material and reduced the activation energy of CO₂ reaction. Finally, it promotes the conversion of CO₂ to CH₃OH in the air as a reality [83]. Recently, the Cu/Zn/Al₂O₃ catalyst achieved higher activity and selectivity for CO₂-CH₃OH under simultaneous heat (225 °C) and UV-Vis light (350–800 nm) than the thermal catalytic system alone (275 °C); this result showed that light can promote the catalytic reduction of CO₂-CH₃OH over Cu/Zn/Al₂O₃ catalyst. Further research found that CO₂ initially preferentially adsorbed on ZnO, and ZnO as the main active site promoted the conversion and activation of CO₂. The presence of Cu could facilitate the conversion of ZnO-HCOO* to CH₃OH because the hydrogenation of HCOO* → H₃COO* was mainly driven by Cu. The results further indicate that Cu/ZnO interface adsorption is the key to synthesis of CH₃OH. Figure 3d delineates the roles of the ZnO surface, Cu surface and ZnO-Cu interface in the Cu/ZnO/Al₂O₃ catalyst and the mechanism for the photo-driven CO₂ hydrogenation process [84].

3.4. C₂₊

Although photothermal catalytic CO₂ reduction to hydrocarbons has been successful, the obtained products mainly focused on C₁ chemicals with low added value. The synthesis of higher value-added C₂₊ hydrocarbons products via a photothermal route is also important. Up to now, there have been many researchers who have successfully obtained C₁-C₃ hydrocarbons from CO₂ hydrogenation and CO₂ and H₂O. Zhang and colleagues reported a dual-function Au-Ru/TiO₂ catalyst for the simultaneous decomposition of H₂O to H₂ and CO₂ hydrogenation, which can convert CO₂ and H₂O to fuels by the photothermal coupling method (Figure 4a). In this paper, the thermocatalytic CO₂ hydrogenation, photothermal catalytic CO₂ hydrogenation, photothermal catalytic H₂O splitting to H₂ and photothermal catalytic reduction of CO₂ and H₂O were studied. Au/TiO₂ showed a high CO selectivity and production rate in thermocatalytic CO₂ hydrogenation. Ru/TiO₂ showed a high formation rate of CH₄ and C₂H₆. The same as Au-Ru/TiO₂, but its activity was not as good as Ru/TiO₂ due to the partial blocking effect of Au. However, the production rates of CH₄ and C₂H₆ increased with the increase in temperature. Au-Ru/TiO₂ revealed much higher activity under photothermal catalytic conversion of CO₂ + H₂. The results indicated that light illumination can improve thermal catalytic activity. In addition, Au-Ru/TiO₂ can also decompose H₂O to H₂ under similar conditions as CO₂ hydrogenation, but the difference is that the production rate of H₂ is greatly affected by temperature. The optimal thermal temperature can achieve the maximum rate of H₂ production, and the reaction rate of CO₂ hydrogenation is higher than that of H₂O to H₂. Finally, combined with the above experiments, the photothermal coupling catalytic reduction of CO₂ and H₂O over Au-Ru/TiO₂ catalyst was studied directly. The results showed that the efficient thermocatalytic CO₂ hydrogenation can be realized on Au-Ru/TiO₂, so H₂ generated from H₂O by photocatalysis can directly react with CO₂ to generate alkanes. The whole reaction process can greatly accelerate the migration of photogenerated electrons and avoid the recombination of photogenerated electron-hole pairs, and ultimately improve the collaborative reduction of CO₂ and H₂O by photocatalytic and thermocatalytic activity [85]. At present, most of the catalysts to generate multi-carbons need the support of noble metals or through the combination of metal elements and various oxide catalysts or the construction of a complex structure of the catalyst to achieve this. Our group synthesized a proton conductor BZCY532, under the photothermal coupling conditions successfully converted CO₂ and H₂O into CH₄, C₂H₆, and C₃H₈ [39]. Although the yield of C₂H₆ and C₃H₈ were not high and the catalytic mechanism was not very clear, the realization of multi-carbon conversion of CO₂ through cheap materials and simple structure is worth further study in terms of economy and cost. Deng introduced WO_3−x, which contains oxygen vacancies and exhibits an excellent photothermal conversion efficiency [86]. WO_3−x produced 5.30 and 0.93 μmol·g⁻¹ C₂H₄ and C₂H₆, respectively, after 4 h. The results showed that oxygen defects can expand the light absorption range, improve the carrier separation efficiency and enhance the H₂O activation and hydrogenation of adsorbed CO. Under UV-Vis-IR irradiation, solar-energy-driven photothermal catalytic C–C coupling occurs from CH₂/CH₃ intermediates (Figure 4b).

Table 1. Photothermal catalytic products.

Catalyst	Reactant	Reaction Conditions	Products	Yield/Distribution/Selectivity	Ref
Bi2O3−x	CO2 and H2	420 nm LED light 200 °C	CO	16.15 μmol·g−1·h−1	[87]
Pd-TiO2	CO2	Hg lamp 500 W 500 °C	CO	11.05 μmol·g−1·h−1	[25]
Cs3Sb2I9	CO2 and H2O	200 mW cm−2 Xenon lamp Visible light 235 °C	CO CH4 (less)	95.7 μmol·g−1·h−1	[88]
Pt0.01Fe0.05-g-C3N4	CO2 and H2	300 W Xe lamp 360 °C	CO	7.36 mmol·h−1·gcat−1	[89]
Oxygen vacancies TiO2	CO2 and H2	UV light 120 °C	CO	22.5 ppm	[26]
TiO2−x/CoOx	CO2 and H2O steam	150 W UV lamp 120 °C	CH4 CO	16.464 μmol·g−1·h−1 16.275 μmol·g−1·h−1	[90]
Ru/TiNT nanotubes	CO2 and H2	150 mW/cm2 simulated sunlight 210 °C	CH4	12.4 mmol·gcat−1·h−1	[91]
TiO2 nanotube	CO2 and H2O	300 W Xenon lamp	CH4	20.67 μmol·g−1·h−1	[92]
Au NP colloid in 10% (v/v) aqueous isopropyl alcohol solution	CO2 and H2O	Visible light 50 °C	CH4 C2H6	6.8 NP−1 5.6 NP−1	[93]
Pt-sensitized graphene-wrapped blue-colored titania	CO2 and H2O	Solar spectrum (AM 1.5 G) illumination	CH4 C2H6	259 μmol·g−1 77 μmol·g−1	[94]
Au NPs	CO2 and H2O (ionic liquid medium 5%)	UV-Vis extinction	CH4 C2H4 C2H2 C3H6 C3H8	4.53 NP−1·h−1 1.10 NP−1·h−1 0.99 NP−1·h−1 0.93 NP−1·h−1 0.56 NP−1·h−1	[95]
ex-Ti-oxide/Y-zeolite (1.1 wt% as TiO2)	CO2 with H2O	UV irradiation 53 °C	CH4 CH3OH	7.25 μmol·g−1-TiO2·h−1 4.89 μmol·g−1-TiO2·h−1	[81]
Pt-loaded ex-Ti-oxide/Y-zeolite				12.25 μmol·g−1-TiO2·h−1 1.38 μmol·g−1-TiO2·h−1
Nf/Pd-TiO2	CO2 with H2O	UV irradiated	CH4 C2H6	11.5 μmol 10.2 μmol	[29]
CdS/(Cu-NaxH2-xTi3O7)	CO2 with H2O	450 W Xe lamp 25 °C	CH4 C2H6 C3H8 C2H4 C3H6	28 μL·g−1·h−1 17 μL·g−1·h−1 10 μL·g−1·h−1 2 (×10) μL·g−1·h−1 8 (×10) μL·g−1·h−1	[96]
CoO/Co/TiO2	CO2 and H2O	300 W Xe lamp 120 °C	CH3OH	39.6 μmol·gcat−1·h−1	[82]
BiVO4/Bi4Ti3O12	CO2 and H2O	300W Xe lamp 298 K	CH3OH CO	16. 6μmol·g−1·h−1 13.29 μmol·g−1·h−1	[79]
Bi2S3/CeO2	CO2 and H2O	Visible light 15 °C	CH3OH CH4	1346.8 μmol·g−1 15.84 μmol·g−1	[97]
CeO2-Pt@mSiO2-Co	CO2 and H2 (H2/CO2 = 3)	250 °C	CH4 C2-C4	40% 60%	[27]
α-Fe2O3 impregnated with Zn	CO2 and H2	230 °C	CO CH4 C2-C4 C5+	38.1 mol % 13.1 mol % 17.7 C mol % 31.1 mol %	[98]
γ-Fe2O3 impregnated with K				27.0 mol % 11.0 mol % 21.1 mol % 40.9 mol %
α-Fe2O3 impregnated with Cu				23.4 mol % 10.1 mol % 19.5 mol % 47.0 mol %
BaZr0.5Ce0.3Y0.2O3	CO2 and H2O	UV light 350 °C	CH4 C2H6 C3H8	39.13 umol·g−1 8.64 umol·g−1 3.22 umol·g−1	[39]
Ba(Zr0.5Ce0.3Y0.2)0.05Co0.05O3				266.08 umol·g−1 122.69 umol·g−1 21.57 umol·g−1
Ba(Zr0.5Ce0.3Y0.2)0.05Ni0.05O3				219.13 umol·g−1 82.95 umol·g−1 12.56 umol·g−1
AuCu/g-C3N4	CO2 and H2O	300 W Xenon lamp (λ > 420 nm) Vis 120°C	CH3CH2OH	0.89 mmol·g−1·h−1	[99]
Cu2O/g-C3N4	CO2 and H2O (with ionic liquids)	300 W Xenon lamp (λ > 420 nm) Vis 100°C	CH3CH2OH	0.71mmol⋅g−1⋅h−1	[100]

summarizes the photothermal catalytic product.

4. Photothermal Catalysts

The photothermal catalysts are the key to achieving high-efficiency photothermal catalytic activity and selectivity. An efficient photothermal catalyst should possess the capacity of both catalytic properties: (1) harvesting the full sunlight spectrum, especially for visible light and even for infrared light; (2) promoting the effective separation and migration of electron-hole pairs; and (3) reducing the activation energy (Ea) for chemical reaction progresses. So far, there have been many different types of photothermal catalysis, but all photothermal catalysts can be systematically classified into three categories, plasmon-mediated (Pt, Au, Ag and Cu) catalysts, not-plasmon-mediated and other common catalysts.

4.1. Metal Oxides Photothermal Catalyst

Metal oxides as catalysts have been widely studied in photocatalytic and photothermal catalytic processes for CO₂ due to their facile preparation, low cost and tunability, such as TiO₂, ZnO, NiO, Fe₂O₃, V₂O₃, MnO₂, WO₃ and MoO₃. The catalytic principle of metal oxide catalyst and its influence on CORR can be referred to in the literature [101,102,103]. The lattice structure, electronic structure, surface properties, volume defects and metal-oxygen bond strength of metal oxides directly affect the catalytic performance [104]. The poor catalytic activity of single metal oxides is due to the low efficiency of photogenerated charge separation and transfer and the lack of active sites on the surface for adsorption and activation of reactant molecules [101]. Therefore, the catalytic activity and product selectivity for the catalysts can be changed by modifying metal oxide catalysts, including introducing oxygen vacancies [105], doping [106], loading noble metal [107], coupling with other metal oxides [108] to adjust the material lattice structures, electronic structures, lattice defects and the diffusion. In our former work, a series of mesoporous WO₃ (m-WO_3−x) with different oxygen vacancies were synthesized after hydrogenation treatment at different temperatures (350 °C, 450 °C and 550 °C). Under the same photothermal coupling conditions (300 W Xe lamp with a UV-light filter (λ > 420 nm), 250 °C, 25 Kpa), the CH₄ evolution rate reached 25.77 μmol·g⁻¹ by m-WO₃-H550, which is 36 and 22 times higher than com-WO₃ (1.17 μmol·g⁻¹) and m-WO₃ (1.17 μmol·g⁻¹), so the good photothermal catalytic activity is closely related to the formation and concentration of oxygen vacancies (Figure 5). A mechanism for the catalytic reduction of CO₂ by hydrogenated m-WO_3−x was proposed; a high oxygen vacancy concentration of m-WO_3−x can reduce CO₂ to C, which is deposited on the catalyst surface and reacts with H to generate CH₄, but a low oxygen vacancy concentration can reduce CO₂ to CO, which is dispersed in the system and reacts with H₂ to generate CH₃OH. Therefore, the formation of oxygen vacancy can effectively improve the photothermal coupling efficiency and promote the reduction of CO₂ to generate hydrocarbons [37]. Li reported a variety of W-doped TiO₂ (W doping concentrations ranging from 2 to 10%) for photothermal catalytic CO₂ reduction (150 W UV lamp, 393 K); the results showed that doping can effectively enhance catalytic activities. However, appropriate doping is beneficial to improve the catalytic activity, because doping too low is not conducive to the formation of oxygen vacancies and thus lacking active sites required for CO₂ reduction; doping too high may cause the undoped ions to react with the anions on the catalyst to generate other substances, thus inhibiting the occurrence of CO₂ reduction [109]. In this research, when the W doping concentration reached 4%, it exhibited 3.5 times higher catalytic activity than pure TiO₂, which further proved that doping concentration influences the catalytic activity of the catalyst. Although the photothermal catalytic activity of single metal oxide is not high, the catalytic activity can be improved by loading, doping or multi-catalyst combination.

Table 2. Metal oxide photothermal catalysts for CO₂ reduction and products.

Promoter	Catalyst	Reaction Conditions	Products	Ref
In2O3	In2O3−x/ In2O3	Stainless steel batch reactor 300 W Xe lamp (Local temperature 261 °C)	CO (1874.62 μmol·h−1·m−2) CH3OH (1.48 μmol h−1·m−2)	[110]
	1/1-In2O3	Visible light irradiation 200 °C	H2 (5.32 µmol·gcat·h−1) CO (8.25 µmol·gcat−1·h−1) CH4 (27.19 µmol·gcat·h)	[111]
TiO2	CoO/Co/TiO2	300 W Xe lamp 120 °C	CH3OH (39.6 µmol·gcat·h)	[82]
	TiO2-graphene	300 W Xe lamp	CH4 (26.7 μmol·g−1·h−1) CO (5.2 μmol·g−1·h−1)	[112]
	2% CuS/TiO2	a 300 W xenon lamp 138 °C	CO (25.97 μmol·g−1·h−1)	[22]
	Pt/TiO2−x	Xe lamp (150 W) 120 °C	CH4 (0.3412 μmol·h−1)	[63]
ZnO	Pd/ZnO	Light irradiation 250 °C	CH3OH (3.8 mmol·g−1·h−1) CO (3.6 mmol·g−1·h−1)	[113]
NiO	NiO/Ni-G	UV-Vis light from a 300 W Xe lamp 200 °C	CH4 (642 µmol·gNi−1·h−1)	[114]
Al2O3	Ni/Mg-Al2O3	500 W Xe lamp 450 °C	H2 (69.71 mmol·min−1·g−1) CO (74.57 mmol·min−1·g−1)	[115]
Fe2O3	Fe2O3 film	Solar light 500 °C	CH4 (1470.7 µmol·gcat−1) C2H4 (736.2 µmol·gcat−1) C2H6 (277.2 µmol·gcat−1)	[116]
FeO-CeO2	FeCe-300	Xe lamp 419 °C	CO (19.61 mmol·h−1·gcat−1)	[117]
Bi2O3	α/β-Bi2O3	Visible light	HCOOH (1932 μmol·g−1) CH3OH (6 μmol·g−1)	[118]
	γ-Bi2O3	Xe lamp 300 mW cm−2	CO (48.10 μmol·h−1·g−1)	[119]
MoO3	MoO3−x	UV-Vis-IR 160 °C	CH4 (2.08 μmol·g−1·h−1) CO (10.3 μmol·g−1·h−1)	[120]
Nb2O5	Pd@Nb2O5	300 W Xe lamp 470 °C (local temperature)	CH4 (0.11 μmol·gPd−1·h−1) CO (0.75 mol·gPd−1·h−1)	[34]
	BBN/Nb2O5		CO (2.8 μmol·g−1·h−1) C2H4 (0.1 μmol·g−1·h−1)	[121]

summarizes the metal oxide photothermal catalysts and products.

4.2. Plasmonic-Metal Photothermal Catalyst

In plasma catalysis, the free electrons of the metal catalyst resonate with the electric field formed by the incident light to generate collective oscillation, thus promoting the migration of electrons in the metal catalyst to the catalyst surface to participate in the catalytic reaction [122]. Surface plasmon resonance (SPR) is widely used in CO₂ reduction. Based on noble metals, the SPR effect has a great improvement in catalytic activity. Noble metals such as Au, Ag, Cu and Pd nanoparticles are promising for harvesting photon energy for chemical reactions due to their extraordinary and tailorable LSPR properties [48,123]. Metal nanostructured catalysts absorb light energy through in-band or inter-band transitions to generate abundant energetic charge carriers (hot carriers or electron-hole pairs). In one study, electron-electron or electron-photon caused local heating compared with traditional thermal catalytic heating, which offers a relatively mild condition and consumes less energy [122]. It has also been shown that the hot carriers can help the activation of chemical bonds and the transformation of intermediate species through excited transient electrons, which can influence the product’s selectivity and expedite redox reactions by whittling certain chemical bonds [124]. Plasmon resonance as an attractive alternative to conventional heating reaction with full-spectrum absorption has attracted attention in the study on photothermal catalytic reduction of CO₂. With certain illumination, light can cooperate with the thermal and non-thermal mechanisms for conventional thermal catalysts or specific plasma photocatalysts to improve the overall chemical reaction rate and product selectivity at relatively low temperatures. To understand the catalytic properties for the plasma resonance effect of noble metal in driving catalytic reaction, Rh/TiO₂ photocatalyst was used to demonstrate and illustrate the application of non-thermal effects in plasmon to improve CO₂ conversion into CH₄. It was found that Rh/TiO₂ enhanced the process of CO₂ methanation due to the plasma resonance effect, and non-thermal CH₄ production rate and apparent quantum efficiency are significantly correlated with top-surface temperature. Under medium and low temperatures, the synergistic effects of light and heat can accelerate the generation of CH₄, but when the temperature rises to 350 °C, begins to act as the dominant reaction to affect the occurrence of light-induced reaction because light enhances the occurrence of the reverse reaction of CO₂ reduction reaction [125]. Zhao et al. [76] used plasmonic photocatalysis (Au, Ag and Pd/3D porous ZnO nanosheets). To effectively promote chemical polarization and activation of inert molecules, which was designed specifically to expose the polar {001} facet, based on the physical prototype of field-field coupling, to benefit chemical polarization and activation of the inert molecule (Figure 6). The characterization results indicated that the M/m-ZnO-x NSs array catalysts show a good photocatalytic performance of the CO₂-to-hydrocarbon (Figure 7) with CO, CH₄ and C₂H₆ as hydrocarbon products. However, Au/m-ZnO-4.6 photocatalyst generated a large number of C₂H₆ besides CH₄ and CO, Ag/m-ZnO-x and Pd/m-ZnO-x photocatalysts which predominantly generated CO and CH₄, respectively. The thermal dynamic analysis with DFT calculations found that the loading of plasmonic metal cocatalysts can reduce the thermal dynamic barrier and alter the molecular pathways; consequently, the product selectivity and the polarization of the adsorbent surface caused the O=C=O bond to be bent and negatively charging, while the electrophilic attack by the dehydroxylation from COOH* and H₂COOH* led to the formation of CO and CH₄. The enhancement of polarization results from the field-field coupling showed that plasmonic metals in the photothermal catalytic reduction of CO₂ to carbon fuel also have a good application.

Table 3. Plasmonic-metal photothermal catalysts for CO₂ reduction and products.

Catalyst	Reactants	Reaction Conditions	Products	Yield	Ref
Ag (5.0wt%) ZrO2	CO2 and H2	UV−visible light 119 °C	CO	0.57 μmol·h−1·gcat−1	[126]
	CO2 and H2O			0.0031 μmol·h−1·gcat−1
Au-Ru/TiO2	CO2 and H2O	Hg lamp 85 °C	CH4	27.1 μmol·g−1·h−1	[85]
Au&Pt@ZIF	CO2 and H2	Xe lamp 150 °C	CH3OH	9.1 mmol	[127]
Au-ZnO	CO2 and H2	Visible light 600 °C	CO	4.22 mmol·g−1·h−1	[128]
Ru/TiO2	CO2 and H2	1.5 G sunlight 300 °C	CH4	69.49 mmol·gcat−1·h−1	[129]
Ru/RuOx/TiO2	CO2 and H2	Simulated sunlight 80 mW cm−2 46 °C	CH4	49 mmol·gcat−1·h−1	[130]
Ru/Si nanowire	CO2 and H2	Xe lamp simulated sunlight (14.5 suns) irradiation 150 °C	CH4	0.8 mmol·g−1·h−1	[131]
Au/ZrO2	CO2 and H2O	Visible light Room temperature	CO CH4	25.6 μmol·g−1·h−1 5.1 μmol·g−1·h−1	[132]
Pt/0.15Sr-C3N4	CO2 and H2O	Visible light	CH4 CO	48.55 μmol·h−1·g−1 74.54 μmol·h−1·g−1	[133]
Au/TiO2	CO2 and H2O	UV and visible light 278K	CH4	36 ppm	[134]
Ag/TiO2			CH4	11 ppm
Pd@Nb2O5	CO2 and H2	300 W Xe lamp at 25 kW m−2 160 °C	CO	4.9 mmol·gcat−1·h−1	[135]
Pt/HxMoO3-y(Sheet)	CO2 and H2	Visible light 200 °C	CO	120 mmol	[136]
Ru/Al2O3	CO2 and H2	6.2 suns 220 °C	CH4	5.09 mol gRu−1·h−1	[137]

summarizes plasmonic-metal photothermal catalysts and products.

4.3. Perovskite Photothermal Catalyst

Perovskite oxides (ABO₃) are widely studied for various photocatalytic applications owing to their high stability, structural tunability and excellent photocatalytic activity [138,139,140]. The composition of elements at A and B sites will directly affect band structure and photophysical properties for materials [141]. Figure 8 shows the corresponding structures and compositions of these different classes of perovskites, as well as the various elements commonly used to date to construct stable perovskite-type catalysts. The most notable advantage of perovskite-type catalysts in photocatalysis is their flexibility in A and B-site elements’ composition and structure. The different perovskite-type catalysts exhibit significant distinction in reduction activity and product selectivity for CO₂, which has been demonstrated in many studies [142,143,144]. In our previous study, double perovskites LaSrCoFeO_6−δ (LSCF) and LaSrCoFeO_6−δ (3DOM-LSCF) catalysts with three-dimensionally ordered macro-porous structure showed good catalytic activity and product selectivity for CO₂ and H₂O into CH₄ under the photothermal coupling condition. 3DOM-LSCF catalyst showed the production of CH₄ reached (557.88 μmol·g⁻¹), which was 1.6 times higher than LSCF (351.32 μmol·g⁻¹) at the same condition. The excellent catalytic performance of 3DOM-LSCF because of 3DOM structure with highly porous structure, high surface areas, small crystallite size, mixed ions, mixed-valence, self-formed heterostructures and self-formed oxygen vacancies can improve CO₂ and H₂O absorption and reaction on the catalyst surface [36]. The other research on LaCo_xFe_1−xO₃ [38] and LaNi_xFe_1−xO₃ [40] perovskite catalysts was also investigated. The results showed that catalytic activity varied with the amount of Co and Ni-doping. When Co = 0.6 and Ni = 0.6, the CH₄ and CH₃OH reached 437.28 μmol·g⁻¹, 13.75 μmol·g⁻¹ and 471.39·μmol·g⁻¹, 15.50 μmol·g⁻¹ under photothermal coupling with CO₂ and H₂O, which were 3.2, 4.0 and 3.5, 4.1 times than pure LaFeO₃, respectively. It was found that doping-Co and Ni in LaFeO₃ can effectively change the positions of VB and CB of semiconductor materials, the gap width decreases and the valence band and conduction band are more prone to oxidizing reaction, promoting the generation of CH₄ and CH₃OH (0 ≤ x ≤1). Except for the perovskite oxide photocatalysts, halide perovskites (ABX₃, X = Cl, Br and I) are of interest for photocatalysis, especially for CO₂ reduction into hydrocarbons, which have been studied extensively. As with perovskite oxides, halide perovskite has a strong light absorption capacity and its band gap value, light absorption range, catalytic activity and product selectivity can be adjusted by changing the element composition and structure of halide [145,146]. The toxicity issue, poor stability and inadequate active sites of pristine metal halide perovskite limit the application in photocatalytic reactions, which raise an intense recent research effort to develop various engineering strategies for improving their performance (e.g., selectivity, activity, stability, recyclability and environmental compatibility) [147]. Fe(II)-doped CsPbBr₃ perovskite nanocrystals have shown enhanced catalytic activity, and also predominantly leading to the evolution of CH₄ instead of CO. The reason for this finding was that Fe(II)-doped CsPbBr₃ nanocrystals, the adsorption energy for CH₄, became more positive, indicating active CH₄ molecules once formed were faster desorbed from the catalyst surfaces and as a better photoconductor to show a superior photo-response compared to undoped CsPbBr₃ nanocrystals [148]. In general, whether it be perovskite oxide or halide perovskite, improving the activity of photothermal catalytic reduction of CO₂ through structural modification is worth further studying.

Table 4. Perovskite photothermal catalyst for CO₂ reduction and products.

Catalyst	Reactants	Reaction Conditions	Products	Yield or Selectivity	Ref
LaCo0.6Fe0.4O3	CO2 and H2O	Visible light 350 °C	CH4 CH3OH	437.28 μmol·g−1 13.75 μmol·g−1	[38]
LaNi0.6Fe0.4O3	CO2 and H2O	Visible light 350 °C	CH4 CH3OH	471.39 μmol·g−1 15.50 μmol·g−1	[40]
LaSrCoFeO6−δ	CO2 and H2O	Visible light 350 °C	CH4	351.32 μmol·g−1	[36]
3DOM-LaSrCoFeO6−δ				557.88 μmol·g−1
LaNi0.4Co0.6O3	CO2 and H2O	Visible light 350 °C	CH4 CH3OH	678.5 μmol·g−1 20.83 μmol·g−1	[151]
RuO2/SrTiO3	CO2 and H2	UV-Vis Xe lamp 150 °C	CH4	14.6 mmol·h−1·g−1	[142]
SrTiO3	CO2	Hg lamp 500 W 500 °C	CO	1.04 μmol·g−1·h−1	[152]
1.5Au-SrTiO3				7.28 μmol·g−1·h−1
self-doped SrTiO3	CO2 and H2O	Visible light	CH4	0.25 mmol m−2cata h−1	[153]
BaZr0.8Y0.16Zn0.04O3	CO2 and H2	600 °C	CO	0.97	[154]
NaNbO3	CO2 and H2O	UV fluorescent lamps 50 °C	CO H2 CH4 CH3OH	75.50 μmol·gcat−1 26.35 μmol·gcat−1 1.65 μmol·gcat−1 3.36 μmol·gcat−1	[155]
NaTaO3			CO H2 CH4 CH3OH	74.51 μmol·gcat−1 16.45 μmol·gcat−1 2.35 μmol·gcat−1 3.26 μmol·gcat−1
BF@PbTiO3	CO2 and H2O	Hg lamp UV 303 K	CH4	290 μmol·gcat−1·L−1	[156]
Ag-BaZrO3	CO2 and H2O	300W Xe lamp UV light	CH4	0.57 μmol·g−1·h−1	[157]
Au-CaTiO3	CO2 and H2O	UV light	CH4	0.029 μmol·g−1·h−1	[158]
Ag-BaCeO3	CO2 and H2O	300-W Xe lamp UV light	CH4	0.55 μmol·g−1·h−1	[159]

summarizes perovskite photothermal catalysts and products.

4.4. Other Photothermal Catalysts

Apart from these three important photothermal catalysts, other photothermal catalysts with strong light and thermal absorption capacity, high catalytic activity and product selectivities, such as g-C₃N₄, graphene and carbon nanotubes/nanowires also show the potential for photothermal catalytic reduction CO₂ into fuel [160,161,162,163]. Cui and Liu et al. [99] reported that photothermal catalytic reduction of CO₂ to produce CH₃CH₂OH using Au-Cu alloy nanoparticles loaded with ultra-thin porous g-C₃N₄ nanosheets. The positive charge on the surface of Au is conducive to enhancing the adsorption of CO₂ molecules on its surface, while the negative charge transferred from the surface of Au to Cu in the alloy, which makes the Cu surface enriched with a negative charge and promotes intermediate species (CO₂⁻ and *CO) formation on the surface. The close connection and strong interaction between g-C₃N₄ and alloy are favorable for the conduction of photogenic carriers. In the process of reaction, increasing the reaction temperature can accelerate the thermal movement of molecules and promote the synergistic effect of photocatalysis and thermocatalysis, which was favorable for the C–C coupling of *CO polymerization and increased the generation of ethanol. The ethanol yield of Au-Cu (1.0%)/g-C₃N₄ catalyst reached 0.89 mmol·g⁻¹·h⁻¹ under the condition of heating to 120 °C, which was 4.2 and 7.6 times higher than single photocatalysis and thermocatalysis, respectively. TiO₂-G (30g) composite showed CH₄ and CO production rates of 26.7 μmol·g⁻¹·h⁻¹ and 5.2 μmol·g⁻¹·h⁻¹, which were 5.1 and 2.8 times higher than pure TiO₂. The reason was that graphene acted as storage in TiO₂-based photocatalyst that can efficiently collect and accept photogenerated electrons from TiO₂, which can sharply inhibit the recombination and enhance the separation efficiency of carriers. Upon light illumination, graphene can transform the absorbed light energy into heat energy to induce a local photothermal effect. The increase in graphene surface temperature is beneficial to improving the conversion of CO₂ and also establishes a balance with TiO₂ light absorption [112].

5. Challenges and Strategies to Boost CO₂ Photothermal Conversion

5.1. Enlarge and Enhance Light Absorption and Photothermal Conversion

The wavelength range of sunlight is mainly between 250 and 2500 nm, in which ultraviolet only accounts for 5%, visible light accounts for 43% and near-infrared light accounts for 52% of the entire solar spectrum [164]. The first step for any photothermal catalysis conversion process is light absorption. Most of the traditional semiconductor catalysis has a wide bandgap, such as TiO₂, which can only absorb ultraviolet light to generate electron-hole pairs to achieve catalytic ability [92,165]. Therefore, broadening the spectral response range aims to improve the utilization rate of solar energy and it is one of the main ways to enhance the catalytic performance of semiconductors. In general, regulating the band structure of semiconductors is a common method to broaden the spectral response range. It is well known that surface defects (such as the existence of oxygen vacancies) can effectively enhance light absorption and the LSPR effect leads to plasmonic excitations being observed across the visible, near-infrared and mid-infrared regions of the electromagnetic spectrum that can enhance light absorption [166,167]. Oxygen vacancies cannot only serve as electron capture centers but also enhance the ability of light capture to improve the catalytic reaction activity. A catalyst with oxygen vacancy confined in PO₄ oxoanion-doped Bi₂WO₆ atom layers (V_o-PO₄-BWO) has been proposed. Combined with theoretical and experimental results, it can be concluded that the light absorption of the Vo-PO₄-BWO atom layer obviously shifted to visible light and effectively narrowed the band gap. The conversion of CO₂ to CH₃OH reached 157 μmol·g⁻¹ h⁻¹. Doping of PO₄ oxoanion promoted the increase of carriers, and the existence of oxygen vacancy caused the emergence of a new defect level, which enabled electrons to be easily excited by sunlight and quickly transferred to the surface of the catalyst. This work was an effective approach for efficient solar energy conversion using spatially positioned material designs to expand catalyst light absorption and enhance charge transport at the atomic level and demonstrates the importance of well-positioned material designs for CO₂ reduction [168]. Loading Cu, Ag, Au, Pt and doping non-metallic elements such as S and N seem to be promising techniques to effectively enhance the light response range of semiconductor catalytic [169,170]. Khalid et al. [171] achieved metal ions (Ag and Cu) loaded and N doped TiO₂ nanometer photocatalyst. In this study, the light absorption range and catalytic activity of TiO₂, N/TiO₂, Ag-N/TiO₂ and Cu-N/TiO₂ were studied. The results showed that N-doping would generate an intermediate energy level between the bandgap of TiO₂, Ag and Cu loading would promote the interaction between metal and non-metal, leading to a significant decrease in the energy band, thus improving the visible light absorption of the catalyst. In terms of catalytic activity, Ag-N/TiO₂ had the highest catalytic activity, because Ag loading not only enhances the absorption of visible light of the catalyst, but also acts as the separation center of electron trap and electron-hole pair to improve the activity of photocatalytic reduction of CO₂ and H₂O.

5.2. Improving Carrier Separation and Migration Efficiency

In the process of photothermal catalytic CO₂ reduction, photogenerated electron-hole pair separation and migration are the rate-limiting step in the whole reaction, which determines photothermal catalytic CO₂ reduction activity. The rapid and effective separation of electron-hole pairs can generate more active electrons CO₂ reduction and the rapid migration can avoid the recombination of photogenerated electrons and holes, thus the number of effective electrons increases and the catalytic activity improves. To facilitate the separation and migration of photogenerated electrons and holes, the modification of catalysts has been extensively studied, such as combining with wide bandgap and narrow bandgap semiconductors, noble metal loading to build heterojunction, doping to change the bandgap value and introducing level defects in the bulk of the catalyst or near the CB, forming shell structure and so on.

5.2.1. Building Heterojunction

To enhance the low activity of semiconductor catalysts, many strategies have been studied. A well-defined heterojunction structure can accelerate the separation of photogenerated e⁻/h⁺ pairs and suppress the recombination of e⁻/h⁺ pairs, thereby considerably promoting the activity during the photocatalytic reduction of CO₂ into solar fuels. There are three types of heterojunctions: p-n heterojunctions [172], no heterojunction and Z-scheme heterojunction [173]. A p-n heterojunction can be formed when n-type semiconductors couple with an appropriate p-type semiconductor with matching electronic band structures; non-p-n heterojunction semiconductor A (SA) and semiconductor B (SB) are tightly bonded to form a heterojunction, e⁻ can transfer from the CB of SA to the CB of SB, whereas h⁺ transfers from the VB of SB to the VB of SA under light irradiation, because of the staggered alignment of the energy levels; the Z-scheme heterojunction system is typically made up of two semiconductors with the staggered alignment of band structures, but the reduction and oxidation reaction occur on CB and VB of the two semiconductors and the whole reaction cannot be maintained independently [174]. Co₃O₄@CdIn₂S₄ p-n heterojunction has been shown to exhibit a high CO₂ convert ratio with visible light. In this study, the high CO₂ reduction performance of Co₃O₄@CdIn₂S₄ p-n heterojunction photocatalyst was due to the strong coupling interface formed by the composition of hybrid materials. There was an internal electric field between the interfaces. When visible light is irradiated, the electron-hole pairs generated by both Co₃O₄ and CdIn₂S₄ and rapidly transferred from CB of Co₃O₄ to CB of CdIn₂S₄, and the hole rapidly transferred from VB of CdIn₂S₄ to VB of Co₃O₄ under the action of the internal electric field, which can effectively avoid the recombination of carriers. In addition, the Co₃O₄@CdIn₂S₄ composite catalyst had more catalytic active sites to promote the adsorption of CO₂. The main reason for promoting solar-driven CO₂ is to generate carbon fuel. This study also provides enlightenment for the further rational design of heterojunction photocatalysts [175]. In another study, BiPO₄-BiOBr_xI_1−x p-n heterojunction catalysts were successfully exploited for photocatalytic reduction of CO₂ to CH₄ and CO, because the formation of the p-n junction can reduce the recombination of photogenerated electrons and holes, improve the separation efficiency and enhance the absorption of visible light. Loading (5%) of BiPO₄ in BiPO₄-BiOBr_0.75I_0.25 showed the highest CO (24.9umol·g⁻¹) and CH₄ (9.4umol·g⁻¹) yield rates, which showed the optimal amount loading result higher conversion efficiency. This report demonstrated that the construction and design of BiPO₄-BiOBr_xI_1−x p-n heterojunction provides a novel approach for efficient catalytic reduction of CO₂ [176]. TiO₂/ SrTiO₃ heterojunction structure films were fabricated and applied for CO₂ photoreduction. TiO₂/SrTiO₃ reflected a good catalytic activity of CO₂ to CH₄. The reason is attributed to the low recombination of charge carriers, the fast separation of photogenerated e⁻-h⁺ pairs, large BET surface, strong photo conversion ability and adsorption capacity for CO₂ of TiO₂/SrTiO₃ heterojunction catalyst, which promote the efficiency of CO₂ and H₂O conversion into CH₄ and CO [177]. A direct Z-scheme Sn-In₂O₃/In₂S₃ heterogeneous photocatalyst effectively enhanced photocatalytic CO₂ reduction activity and expanded the range of light absorption. There are few defect traps in the heterojunction, which could promote the separation and transmission of e⁻ and h⁺ pairs without sacrificing the redox capacity of the carriers due to the good lattice match [178]. All these results indicate that construct heterojunction structure photocatalysts can rapidly inhibit the recombination and accelerate the separation and migration of carriers to strengthen the activity of CO₂ photocatalytic reduction.

5.2.2. Doping

Regulating semiconductor bandgap is one of the effective methods to improve photothermal catalytic activity. Doping is the simplest and most effective way to change the band structure of semiconductor-based photothermal catalysts. After doping, impurity levels are formed in the energy bandgap. Generally, the introduction of impurity level can shorten the bandgap, inducing the change of CB position and light with lower energy can also excite electrons, which broaden the range of light response. Tuning the CB can obtain appropriate reduction potentials of electrons and control the reaction from a thermal dynamic point of view. Doping results in lattice defects, which are favorable for the formation of more active centers. Moreover, doped ions can be used as capture centers to separate electron-hole pairs. Wei et al. synthesized a series of LaNi_xCo_1−xO₃ perovskites. Under photothermal reaction conditions, Ni doping can effectively improve the catalytic performance of CO₂ plus H₂O reduction to CH₄/CH₃OH compared with LaCoO₃. LaNi_0.4Co_0.6O₃ showed the highest catalytic activity; the accumulated yield of CH₄ and CH₃OH reached 678.57 μmol·g⁻¹ and 20.83 μmol·g⁻¹, which were 3.4 and 3.8 times higher than LaCoO₃ under the same condition. The reason was that the introduction of Ni can change the positions of the CB and VB, reduce the bandgap value and improve the absorption capacity of visible light. In addition, the introduced Ni also formed additional oxygen vacancies to promote the adsorption of reactants, leading to the distortion of CO₂ and H₂O molecules, accelerating the decomposition of molecules, and can capture electrons to reduce the recombination rate of carriers and promote the occurrence of carbon dioxide reduction reaction as the active site of the reaction [151]. In another research [39], proton conductor BaZr_0.5Ce_0.3Y_0.2CoO₃ (BZCY 532) as a new photothermal catalyst can convert CO₂ into CH₄ C₂H₆ and C₃H₈ under photothermal coupling. Introducing Ni and Co effectively improves the photocatalytic efficiency, especially for Co-doping, because doping elements can change the original ion space of the catalyst and the structure of the catalyst is deformed, which could promote the absorption of light, increase the surface oxygen vacancies, produce more reactive sites to promote the CO₂ absorption and activation and stabilize the adsorption of intermediate ·CH₃ to promote C–C and CCC coupling.

5.2.3. Core-Shell Structure

Core-shell structure has attracted the attention of researchers because of its unique structural characteristics, which integrate the properties for inner and outer materials and complement each other. Its high specific surface area and special shape are favorable for application in photothermal catalysis. The core usually exhibits different properties from the shell and can chemically contact the shell in three dimensions, leading to enhanced properties such as catalysis and interfacial charge transfer. Moreover, the shell with a large bandgap may improve the photostability of the core catalyst with a narrow bandgap [179]. Based on structure and morphology, core-shell structures were classified into the core-shell, yolk-shell/hollow structures and sandwiched core-shell structures, as shown in Figure 9. Au@TiO₂-Au catalysts with core-shell morphology were found to be superior for CO₂ conversion. The thickest TiO₂ semiconductor structure is the reaction. Plasmonic field effects or a “hot electron transfer” from the gold core may affect the reactivity of the small gold nanoparticles on the outer TiO₂ shell. The efficiency of CO₂ converted into CH₄ and CO has been shown to improve because of the synergistic effect between photogenerated electrons, hot electrons and gold core, titanium shell and Au NPs exposed on the outer three structure features [180]. In another study, Au@TiO₂ yolk-shell hollow spheres were created for CO₂ reduction under UV-Vis light irradiation, and were found to not only generate CH₄ but also produce C₂H₆. Experimental and calculated analysis elucidated that Au NPS plays an important role in improving photocatalytic efficiency and promoting multi-electron reactions to achieve C–C coupling. The LSPR of Au NPs contributes to the generation of electron hole pairs on the TiO₂ shell, and collective oscillation caused by LSPR can cause local electromagnetic field and photocurrent, which can greatly promote the generation and separation of electron-hole pairs and increase the number of carriers. The closer the TiO₂ shell was to the local electromagnetic field of Au NPs, the more favorable it was for the accumulation of ⋅CH₃ radicals and dimerization ⋅CH₃ radicals to form C₂H₆. The farther from Au NPs, the more beneficial it was for the formation of CH₄ [181].

5.3. Photothermal Reactor

To improve the activity of photothermal catalytic reduction of CO₂ into valuable chemicals or fuels, one method, described above, is to modify catalysts based on different aspects to improve the reactivity; another way is to design a photothermal catalytic reactor to improve the photothermal catalytic conversion efficiency [182]. Up to now, there are many types of reactors designed for the application of photothermal catalytic CO₂ reduction. A novel multi-channel mini-reactor with 1.17–2 mm channel internal diameter was reported (Figure 10a). Taking Ni on the alumina (25% Ni) catalyst as an example to study the difference in product yield and selectivity in catalytic CO₂ hydrogenation between the multi-channel micro-reactor and the traditional fixed-bed reaction at different temperatures (Figure 10b,c). It was observed that the multi-channel mini-reactor had higher CO₂ conversion and CH₄ selectivity than the fixed-bed reactor at the same reaction condition. The reason is that the multi-channel microreactor has a very high specific surface, which not only is conducive to the uniform distribution of the catalyst in the reactor but also promote the reaction gas and catalyst contact to be more adequate to improve the thermal conversion efficiency and eliminate the formation of hot spots [183]. Another study reported a “HI-light” (glass waveguide-based ‘‘shell-and-tube’’ photoreactor platform) was applied to photothermal catalytic CO₂ reduction (Figure 10d). The reactor design is characterized by flexibility in both diameter and length. The reaction activity of In₂O_3−x (OH) _Y nanocrystals as a photothermal catalyst for CO₂ hydrogenation to CO on the HI-light platform was studied from the reaction temperature, light intensity and residence time of reactants in the reactor. Through the experimental results, the photothermal coupling (UV light, 300 °C) catalytic CO₂-CO production rate was 20 times higher than the cubic In₂O_3−x(OH)_Y catalyst, which has reported the highest catalytic activity under similar conditions, and the CO₂ conversion and CO production rate increased with the increase in reaction temperature under HI-light platform reactor [184]. This study further proved that reactor design can be a useful approach to improving the catalytic activity of catalysts.

The photothermal coupling catalytic reaction consists of photoexcitation and thermal promotion. There are various ways where the temperature can be manipulated inside the photothermal reactor, either by changing the distance of the lamp with the reactor, or thermal energy will generally be produced by the irradiation of light on the catalyst, which will convert solar energy to heat energy by absorbing the incident photons or by using an external heater. The first way is that light provides heat. The main factors affecting heat are the category of light, the light intensity and the height of the light source. In the research on the Co nanostructure catalyst, by adjusting the distance of a lamp from the reactor, the temperature elevated to 158 °C and found that Co catalysts are efficiently heated due to their full absorption of UV-Vis-NIR light [185]. Li studied the light-driven temperature and CO₂ conversion rates of the Ni/Y₂O₃ nanosheets with (Ni/Y₂O₃ + S) and without (Ni/Y₂O₃) the selective light absorber-assisted photothermal system, respectively, under different intensities of sunlight irradiation [186]. The result showed that with the light intensity increased, both temperature and CO₂ conversion rate increased (Figure 11a,b). Time-dependent surface temperature profiles of different catalyst films upon laser illumination were researched. The results showed that the surface temperature of the catalyst increased with the irradiation time, and CO selectivity increased with reaction temperature [24]. Figure 11c shows the surface temperature and product yield of the catalyst under different light sources. It is clear that the surface temperature of the catalyst is the highest under VU-VIS-IR light and the production of the products over WO_3−x is higher than under UV-Vis and IR irradiation. This suggested that the catalytic activity is not only related to the reaction temperature but also indicates that different light sources will produce different thermal effects in photothermal catalysis [86].

The other study found that catalysts can absorb light and convert solar energy into heat energy, the increase in local temperature can help the whole reaction occur more easily and promote CO₂ conversion rapidly. Generally, the main reason for the temperature rise is the LSPR effects of plasmonic metals or photon vibrations of non-plasmonic metals, which were introduced in the former. The last way is to configure heating devices outside the reactor when the catalyst is unable to efficiently generate heat by absorbing solar energy or the heat generated is not high enough to drive the chemical reaction. External heating devices are used to ensure that the reactor can be maintained at a fixed temperature to provide steady heat energy for the progression of reaction over the catalyst. Usually, the external heating device is a heating plate, electrical furnace, heating sleeve or heating belt [40,187,188].

6. Conclusions and Outlook

In conclusion, photothermal catalysis technology is a combination of photocatalysis and thermocatalysis, which is an improvement on traditional catalytic technology. Carbon dioxide is an abundant source of C that can be converted into high-value-added chemicals or fuels through chemical methods. Low activity and selectivity are the main factors that limit wide use of single catalytic technology. The photothermal catalysis of CO₂ reduction can effectively promote the synergistic effect of light energy and heat energy, which can make the system have higher catalytic activity and selectivity to target products even under mild conditions. Until now, the photothermal catalytic CO₂ reduction has made great progress, both in the selection of photothermal catalysts and in the study of the photothermal catalytic reaction mechanisms. In this paper, different types of photothermal catalysts, photothermal catalytic products and ways to improve the catalytic activity of photothermal catalysts are described. While photothermal catalysis opens new avenues for the widespread use of renewable energy sources such as solar energy to reduce greenhouse gases in the atmosphere while generating useful green energy, fuels and chemicals, it is of inestimable value in both the energy and environmental fields. The reduction and effective utilization of carbon dioxide, regardless of whether in the current state or long-term development and the realization of the carbon cycle of CO₂ through photothermal coupling is undoubtedly the most promising research and development technology. However, much of the research and exploration are still in the most basic state and cannot be industrialized in practical application. Therefore, further research in this field still faces many challenges. Some typical issues are listed below:

(1)

Catalysts: Although various types of photothermal catalysts have been studied so far, poor light absorption conversion ability, low activity and low stability are still the biggest problems. Photothermal catalysts with high catalytic activity supported by noble metals have been extensively studied, but their wide application is limited by the high cost. It still recommends using low-cost and highly active materials. At the same time, the stability of photothermal catalysts also needs to be constantly improved, if a true sense of industrialization is to be achieved, low-cost, effective light absorption, high activity, high selectivity, and strong catalyst stability are essential.

(2)

Reactants: There are several applications for photothermal catalytic reduction of CO₂ into hydrocarbon. Reducing agents are mainly H₂, CH₄ and H₂O, which can also be divided into CO₂ dry reforming, CO₂ hydrogenation reaction and artificial photosynthesis. In hydrogenation and dry reforming, the advantage of reducing agents is that target products such as carbon CO, CH₄ and CH₃OH are easy to obtain. However, the reducing agent is water in artificial light synthesis. In the process of photothermal catalytic CO₂ reduction, at first, H₂O needs to be hydrolyzed in the valence band to provide a hydrogen source and then CO₂ reduction reaction occurs in the conduction band, so the reaction difficulty is increased. H₂O is inexhaustible clean energy and a good candidate for photocatalytic or photothermal catalytic CO₂ reduction. If the reaction activity is greatly improved in the photothermal catalytic reduction of H₂O with CO₂, it will accelerate the realization of clean energy in the real sense.

(3)

Products: At present, photothermal catalytic reduction of CO₂ has a high selectivity for low value-added single-carbon products, such as CO or CH₄, but high value-added multi-carbon products are more valuable, such as alcohols or C₂₊ hydrocarbons. If more value-added products can be obtained by catalyst design or adjusting reaction selectivity, then carbon cycling using CO₂ will be more valuable.

(4)

Mechanism: There are few studies on the mechanism for the photothermal catalytic reduction of CO₂. In the intermediate process of reaction, the process and intermediate that is promoted by light and heat and the roles played in each intermediate process are all worthy of further studying.

(5)

Reactor: By considering the structure of the reactor, the materials, temperature resistance, strength, volume, type of light source, external heating configuration and the operation mode of the reaction, the reactor is designed to be easy to operate and effectively promote the reaction, which can effectively improve the efficiency of photothermal catalytic reduction of CO₂ to hydrocarbons.

Photothermal catalysis opens up a new way for the widespread use of renewable energy sources such as solar energy to reduce greenhouse gases in the atmosphere while generating useful green energy, fuels and chemicals, which have inestimable value in both the energy and environmental fields. The reduction and effective utilization of CO₂, regardless of whether in the current state or long-term development and the realization of the carbon cycle of CO₂ through photothermal coupling is undoubtedly the most promising research and development technology.