Metal Oxide Semiconductors for Photothermal Catalytic CO2 Hydrogenation Reactions: Recent Progress and Perspectives

Yutong Wan,Fan Fang ,Ruixue Sun,Jie Zhang,Kun Chang

College of Materials Science and Technology,Nanjing University of Aeronautics and Astronautics,Nanjing 210016,China.

Abstract: Owing to the accelerated growth of the human economy and society,the increasing concentration of CO2 in the atmosphere has caused serious ecological and environmental problems because of the greenhouse effect.In response to the challenges posed by climate change,China has made a significant commitment to “peak carbon emissions by 2030 and achieve carbon neutrality by 2060”.Ideally,converting CO2 into carbon-based energy and chemicals is supposed to be the best strategy of both worlds,mitigating the greenhouse effect while also addressing the shortage of energy supply.Among the proposed concepts for the above strategy,the scheme of reducing CO2 using renewable green H2 to produce chemicals is preferred,because it can stimulate the potential of clean energy while also reducing CO2 emission.To accelerate this reduction process,many catalytic reactions,including photocatalysis,have been designed and investigated.Owing to its high catalytic efficiency and extensive use of solar energy,photothermal catalytic CO2 hydrogenation in photocatalysis is desirable for increasing sun-to-fuel efficiency.There are two main interpretations of photothermal catalytic hydrogenation: (1) only sunlight is used as the energy source to drive the catalyst,which generates heat to promote CO2 conversion.In this case,the reaction still proceeds in the form of thermocatalysis,whereas photocatalysis has a limited effect.(2) Solar and heat energy are coupled to participate in the catalytic reaction,which has a synergistic effect.Therefore,according to the catalytic mode,the rational design and successful synthesis of photothermal catalysts are very important.Metal oxide semiconductors,owing to their unique energy band structure and chemical properties,high stability,and environmental friendliness,are widely used in the research of photothermal catalytic hydrogenation reactions.This paper reviews the research progress on metal oxide materials used in the CO2 hydrogenation reaction by photothermal catalysis.In particular,the most significant results of research in the last five years have been performed mainly from three different catalyst modulation strategies,such as supporting catalysts,applying microstructure engineering,and defect engineering.The mechanisms of these modulation strategies are summarized and presented for further understanding.In addition,this study introduces different types of photothermal hydrogenation reactors,accompanied by the effects of some key parameters on the reactions.Finally,design strategies for metal oxide catalysts are suggested,and an outlook of photothermal abatement technology is presented.

Key Words: CO2 reduction; Catalytic hydrogenation; Photothermal synergy; Metal oxide; Modulation strategy

1 Introduction

The continuous development of the human economy and society has contributed to the massive use of fossil fuels,resulting in the annual increase of CO2concentration in the air1which has reached the alert value of 37.12 billion T shown in Fig.1.The excessive emission of CO2has brought out an increasingly serious greenhouse effect,seriously affecting the survival of human beings and even the equilibrium of the whole ecosystem2,3.In response to the challenges caused by climate change,China,the world’s second-largest economy and one of the signatories to the Paris Agreement,has made a significant commitment to “carbon emissions peak by 2030 and achieve carbon neutrality by 2060”.The “two-carbon” development strategy has become one of the major tasks to protect the ecological environment in China in the future.On the one hand,the development and utilization of renewable energy such as solar energy,wind energy,geothermal energy,and biomass energy4to reduce carbon dioxide emissions at source have shown great potential for achieving the “carbon emissions peak” target.On the other hand,for the goal of “carbon neutrality”,in addition to the CO2capture and sequestration5,the most striking measure is the recycling of carbon dioxide,that is,the conversion of CO2into higher-value chemicals such as methane,methanol,and ethylene through thermal catalysis6,electrocatalysis7,photocatalysis8,and other means.

Fig.1 Fossil fuel and industrial carbon dioxide (CO2)emissions; Data from https://ourworldindata.org.

CO2catalytic conversion reaction is a process in which old bonds are destroyed and new bonds are created in essence9.From the analysis of the CO2molecular structure,it is clear that the length of the C＝O bond is short,contributing to a higher energy10.In addition,the delocalization resonance between two C＝O bonds aroused by the lone pair of electrons on the oxygen atom,can further strengthen the C＝O bond.Therefore,breaking the C＝O bond in CO2is difficult,which is unfavorable to CO2conversion both in kinetic and thermodynamic,resulting in stable chemical properties and limited conversion,requiring huge driven energy11,12.Currently,thermocatalytic techniques can realize the CO2hydrogenation with high activity and selectivity,having been widely applied in industrial production.By controlling the temperature and pressure of the hydrogenation reaction,it is possible to convert CO2into a highvalue-added carbonaceous compound.However,this catalytic process consumes a huge amount of thermal energy for exciting the electrons13,14,and the energy source is always not clean.Hence,it will increase the additional energy consumption and CO2generation,limiting the application of this technology.Photocatalytic technology used in CO2hydrogenation has attracted much attention for its clean and sustainable utilization of the abundant solar energy in the earth’s atmosphere,but the light energy utilization is low because only ultraviolet (UV) or visible light can be used,extremely easy recombination of photogenerated carriers,and uncontrollable product selectivity,leading to the low-level CO2conversion.In contrast,photothermal catalytic CO2hydrogenation technology driven by both solar and thermal energy is an effective alternative.Photothermal catalytic CO2hydrogenation refers to the integration of light and thermal energy in a reaction process that uses mainly sunlight as the energy source.UV and visible light serve to motivate carriers,and the photothermal effect of infrared light increasing the catalyst surface temperature,can both accelerate the reaction.As a result,the photothermal catalysis process is achieved under the irradiation of the complete solar spectrum15,16.The introduction of light reduces the reaction activation energy barrier and overcomes the limitations of thermodynamic17,and the existence of heat supplies energy for breaking through the activation energy barrier and kinetic bottleneck.The synergistic effect of heat and light effectively improves the productivity of CO2conversion in the catalytic reaction18,19.Since photothermal catalytic CO2hydrogenation is simultaneously efficient,green,and sustainable,this novel catalytic approach is favored by more researchers.

The following are the major categories for photothermal catalytic CO2hydrogenation reactions and the details are exhibited in Table 1: (1) reverse water gas reaction (RWGS) for the synthesis of CO.This reaction is endothermic and therefore needs to be carried out at a higher temperature.Moreover,this reaction is an intermediate step in the synthesis of methanol as well as other hydrocarbons,thus having great research value.In recent years,numerous CO-selective photothermal catalysts have emerged,such as Pt-based catalyst20,Ni-based catalyst21,In2O322,etc.(2) Sabatier reaction for the synthesis of CH4.The reaction is thermodynamically favorable,but the conversion of CO2to CH4requires the transfer of 8 electrons and the reaction kinetics is severely limited23.However,the reaction is expected to be applied to the synthesis of natural gas and potentially to the modulation of gas composition in the Martian atmosphere24.The current research on photothermal catalytic CO2hydrogenation reaction is mainly focused on the synthesis of CH4and CO; (3) methanol synthesis by carbon dioxide hydrogenation.Due to the advantages of methanol such as convenient transport and extensive usage,methanol production by CO2reduction is greatly significant.However,the direct conversion efficiency of CO2to methanol through photothermal catalysis is still at a low level; (4) CO2hydrogenation for the synthesis of C2+hydrocarbons and alcohols,etc.Due to their importance in the chemical industry,C2+products are widely used in industrial production25.But due to the complex reaction mechanism and low product selectivity,it still has a long way to realize the efficient one-step conversion of CO2to C2+hydrocarbons by photothermal catalysis26.

Table 1 The reaction free energy and heat of CO2 hydrogenation reactions.

Fig.2 shows some remarkable works reported for photothermal CO2hydrogenation.In 2014,Prof.Ye pioneered the use of group VIII metals for photothermal catalyticmethanation of CO2,and found that they can fully absorb visible and infrared light with a good photothermal conversion performance27.Compared to pure photocatalytic reactions,the CO2conversion rate of the catalyst loaded with group VIII metals is six orders of magnitude higher.This important discovery has attracted more research into this field,and since then,giving rise to enhanced advances in the photothermal catalytic CO2hydrogenation reaction.In 2017,Prof.Ozin’s group compounded size-controllable Pd nanocrystals with Nb2O5nanorods with CO yields over 18.8 mol·gPd-1·h-1,CO selectivity of 99.5%,which is a milestone in the field of photothermal catalysis28.Prof.Zhang’s group obtained Al2O3-supported CoFe alloy catalysts by reducing CoFeAl LDHs nanocrystals in H2/Ar with 35% selectivity for C2+hydrocarbons29.In 2019,some scholars prepared two-dimensional black In2O3-xnanosheets,which demonstrated that the existence of oxygen vacancies substantially improved the catalytic activity of indium oxide30.In addition,some special structures31,MOF materials32,etc.are used for the study of photothermal catalytic CO2hydrogenation reaction due to their unique structural properties.The continuous development of photothermal catalytic technology in CO2hydrogenation provides a new way to prepare high-value hydrocarbons from CO2using abundant solar energy.

Fig.2 Timeline of significant reported works for photothermal CO2 hydrogenation.

In addition to the common synthesis of C1/C2products,the Tianjin Institute of Industrial Biotechnology of the Chinese Academy of Sciences has made world-renowned achievements in the artificial synthesis of starch.They first achieved the total synthesis from CO2to starch molecule in the laboratory without relying on photosynthesis,using CO2and hydrogen produced by electrolysis as raw materials33.Exciting progress has also been made in extraterrestrial artificial photosynthesis.Recently,Chinese scientists used Chang’e-5 lunar soil (CE-5) as a catalyst to convert human respiratory waste gas (CO2and H2O) into various high-value-added hydrocarbon fuels (CH4,CH3OH,etc.),laying the material basis for a “zero-energy” lunar life support system34.In extraterrestrial artificial photosynthesis,CO2and H2O are not directly reacted,but larger quantities of H2are produced from H2O by photovoltaic electrolytic cell PV-EC(Photovoltaic-Electrocatalysis) technology.Then,high-valueadded carbon fuels are generated by photothermal catalytic CO2hydrogenation technology (Fig.3a,b),which further confirms the great significance of the research on this technology.

Fig.3 (a) Material of CE-5 sample compared with Apollo-12 sample (No.12041)13; (b) diagram of extraterrestrial photosynthesis on the moon 34.

Metal oxide materials are widely used in the research of photothermal catalytic hydrogenation reactions due to their particular band structure and chemical properties,high stability,and environmental friendliness35.ZnO,CeO2,WO3,In2O3,and other metal oxides have been proven to be favorable catalysts for photothermal catalytic CO2hydrogenation reactions.Among them,ZnO materials are more widely used due to their strong adsorption ability of CO2and higher catalytic conversion ability.CeO2materials have excellent redox capacity and oxygen storage capacity,which can provide additional oxygen release without changing their structure,and the oxygen vacancies on the surface of CeO2can also promote catalytic reactions.However,the metal oxide materials are generally difficult to absorb most of the sunlight because of their wide band gap,the photogenerated carriers are easy to recombine and the reactive sites are limited,resulting in unsatisfactory CO2conversion efficiency.

In recent years,researchers have improved the photothermal catalytic performance by loading metal co-catalysts,modulating morphology,and constructing defects.Loading metal cocatalysts is the most efficient way to improve photothermal catalytic performance.In the metal/metal oxide system,metal nanoparticles are the main catalytic active phase,and metal oxide carriers provide such roles as stabilizing nanoparticles36,building high specific surfaces,oxygen donor,and acidic site donor.In addition,the adsorption of the catalysts to the reaction gas and the absorption of sunlight can also be effectively improved by modulating the microscopic morphology of the metal oxide,while the oxygen defect can provide catalytically active sites,lowering the reaction’s activation energy.In this review,we first discuss in depth the fundamental principles of the photothermal catalytic CO2hydrogenation reaction,including photocatalysis,thermocatalysis principles,and three main types of photothermal catalysis principles.On this basis,we mainly review the research progress of metal oxide materials for photothermal catalytic CO2hydrogenation reaction,especially the important study advances in the last five years,mainly from three different catalyst design strategies of loaded catalysts,microstructure engineering,and defect engineering.In addition,we further sort out and summarize the photothermal hydrogenation reactors used in most research,and analyze the advantages and disadvantages of different reactor types and the effects of different parameters on the reaction,aiming to provide reference and help for more perfect reactors design.Finally,the design strategies of metal oxide catalysts are suggested and the outlook on the development of photothermal abatement technology is presented shown in Fig.4.

Fig.4 Research advance of metal oxide materials for photothermal catalytic CO2 hydrogenation.

2 Basic principles

Photothermal catalytic CO2reduction is a relatively efficient CO2conversion scheme.On the one hand,capturing and converting CO2in the atmosphere can effectively reduce atmospheric CO2concentration and mitigate the greenhouse effect.On the other hand,the one-step conversion of CO2into carbon-based energy by using sunlight achieves an efficient carbon cycle under the green concept.In photothermal catalytic carbon dioxide reduction,the light promotes the quick activation of reactants and intermediates and changes the selectivity of products by adjusting the electron injection position37.The heat increases the mass transfer rate and provides energy for the carbon dioxide reduction reaction to cross the activation energy barrier.When the heat produced by light is not enough to drive the reaction,external heating is usually employed.Thus,light and heat simultaneously participate in and promote the CO2reduction reaction.On the basis of different action mechanisms of light and heat,photothermal catalysis can be divided into two types: photo-driven thermal catalysis and photothermal synergistic catalysis,and photothermal synergistic catalysis can be classified into photo-assisted thermal catalysis and thermalassisted photocatalysis shown in Fig.5.To accelerate photothermal catalysis from laboratory to industrial applications,it is essential to explore and summarize the basic principles.Therefore,the basic principles for photothermal catalytic CO2reduction will be reviewed in this part.In addition,we will also provide a concise review of the principles of thermal catalysis and photocatalysis.

2.1 Principles of thermal catalysis

Carbon dioxide is a 16-electron molecule that contains two polar C＝O bonds (C＝O in CO2has a bond enthalpy of 750 kJ·mol-1),resulting in an extremely stable chemical property.Therefore,directly cracking CO2is very difficult and requires large energy.Thence,designing suitable catalysts plays a key role in effectively solving this problem.As shown in Fig.6a,the CO2reduction process needs to overcome the huge activation energy barrierEa1without the presence of catalyst (solid line),and the addition of catalyst (dashed line) can effectively lower the activation energy and improve the CO2conversion rate16.The thermocatalytic CO2hydrogenation reaction generally consists of the following processes: firstly,the catalyst adsorbs H2and CO2,and the H2adsorbed on the catalyst surface will become *H.When the system energy exceeds the reaction’s activation energy (Ea),the C＝O bond of the CO2molecule opens and reacts with *H to generate the reaction intermediates,which will further transform into carbon-containing compounds by interacting with the catalyst surface,and finally the product molecules are desorbed from the catalyst surface38,39.Generally,catalytic reactions are composed of many meta-reaction steps,and the occurrence of each step of the meta-reaction requires the system energy to exceed a specificEavalue.The overall reaction’s rate-determining phase is often the production of intermediates.Whether the product molecules can desorb quickly from the catalyst surface so that the catalyst can be exposed to continue the reaction also affects the reaction rate to a certain extent.In addition,various catalysts have different effects on the intermediates,then contributing to the differences in product selectivity.The thermal catalytic CO2reduction technology driven by heat energy is often used in industry for the synthesis of fine chemicals in large batches and has great practicality.

Fig.6 (a) Energy diagram of thermocatalytic reaction process 16; Adapted from Angew.Chem.Int.Ed.,Wiley-VCH.(b) Schematic diagram of photocatalytic CO2 conversion mechanism on semiconductor photocatalyst 41;Adapted from Int.J.Hydrog.Energy,American Chemical Society.

2.2 Principles of photocatalysis

Photocatalytic CO2reduction is known as artificial photosynthesis40.It refers to the successful reduction of CO2to some carbon-based energy sources such as CO,methane,methanol,etc.by applying light to the surface of a semiconductor with a suitable conductivity band position (Fig.6b).Photocatalytic CO2reduction generally occurs on semiconductors,and the reaction can be roughly divided into three steps: (1) Electrons will be excited to jump from the valence band to the conduction band and produce electron/hole pairs when the semiconductor absorbs photons with energy higher than or equal to the semiconductor band-gap width.(2)Photogenerated holes and electrons migrate,part of which migrates to the surface active site of the semiconductor to take part in the reaction,while the other part undergoes secondary compounding.(3) The holes and electrons successfully migrated to the surface of the semiconductor take part in the redox reaction.Photogenerated electrons reduce CO2to carboncontaining compounds,and photogenerated holes oxidize H2O to O2.In the process of photocatalytic CO2reduction,the capacity of the catalyst to absorb light is a prerequisite for the reaction to occur,while the effective carriers separation and the smooth conduct of the surface catalytic reaction are the decisive factors.Although researchers have made great efforts in the area of photocatalytic CO2reduction,the yield of its products is still at the μmol stage and many problems need to be solved.

2.3 Principles of photothermal catalysis

2.3.1 Photo-driven thermocatalysis

Photo-driven thermocatalytic CO2reduction refers to using only sunlight as the energy source and utilizing the photothermal effect of the catalyst to raise the catalyst surface temperature to the reaction temperature of thermocatalytic CO2reduction,and then drive the reaction to occur,which is still a thermocatalytic reaction in nature41.There are generally three types of catalysts used for photo-driven thermocatalytic CO2reduction: (1) Plasma metals such as Ag and Au shift the energy of sunlight into the lattice through the localized surface plasmon resonance (LSPR)effect and dissipate it in the form of heat thereby increasing the catalyst local temperature.(2) Non-plasma metals such as Pd,Pt,Ru,Rh,Ni,etc.also have photothermal effects and can achieve strong photothermal conversion.(3) Some catalysts (especially black samples) have strong photothermal conversion ability,which can absorb a large amount of sunlight and transform it into thermal energy.The photo-driven thermocatalytic CO2reduction can make the reaction proceed under relatively mild conditions.Compared with thermocatalytic CO2reduction,solar energy replaces the energy-intensive external heat source and raises only the local reaction temperature of the catalyst without raising the temperature of the whole reaction device,which completely solves the problems of high energy cost and environmental pollution caused by the use of non-clean energy in thermocatalysis.There are two general types of catalysts used for light-driven thermocatalytic CO2reduction: plasma metal systems and other systems.

Some plasmas (such as Au,Ag,Cu,etc.) that existed on a certain material surface can excite the LSPR effect to raise the temperature of the catalyst system,which is called a plasma metal system.Fig.7a shows the schematic diagram of the LSPR effect42.There exist a large number of free electrons on the surface of metal nanoparticles.When the incident light irradiates the metal nanoparticles,it causes the collective oscillation of free electrons in the particles,and when the oscillation of the free electrons on the metal surface oscillates at the same frequency as the incident light,a strong resonance effect occurs on the plasma surface,which leads to a great increase in the absorption of light in this plasma metal system43,44.This physical process results in a substantial increase of the local electric field on the metal nanoparticle surface as well,which results in the generation of abundant high-energy carriers (hot electrons and holes) through non-radiative transitions excited within the band(between thespconduction band and the Fermi level) or between the bands (between thespconduction band and thed-band)shown in Fig.7b16,and these hot carriers are dissipated as heat in the surrounding medium within 100 ps–10 ns,causing the local temperature rise44.In addition to raising the temperature of the catalytic system,the LSPR effect also promotes the rapid activation of reactant molecules and intermediates and thus increases the reaction rate.The chemical composition,shape and size of the plasma as well as the semiconductor carrier can affect the hot carrier generation.Plasma metals are typically loaded on semiconductors to form photothermal catalysts.The plasma nanoparticles generate hot electrons and holes upon photoexcitation,and the hot electrons with energy higher than the Schottky barrier migrate to the valence band of the semiconductor for the reduction reaction45,and the holes remaining in the metal for the oxidation reaction,so the presence of plasma metals significantly improves the catalytic performance of the catalyst.

Fig.7 (a) Schematic illustration of the dynamics of an excited plasmonic nanoparticle 42; Adapted from Chem.Soc.Rev.,The Royal Society of Chemistry.(b) Non-radiative transitions excited within or between bands produce a large number of high-energy carriers (hot electrons and holes) 16;Adapted from Angew.Chem.,Int.Ed.,Wiley-VCH.(c) The mechanisms of the photocatalytic reactions with nonplasmonic metal NP photocatalysts 45;Adapted from Chem.Eng.J.,Wiley-VCH.(d) Monitoring the temperature of the catalyst bed of the Fe5C2 catalyst being exposed to light 47;Adapted from Angew.Chem,Elsevier.

Some non-plasma metals such as Pd,Pt,Ru,Rh,Ni,etc.also have photothermal effects.Yeetal.found that Group VIII metals have a great photothermal conversion ability and loaded them on Al2O3,and their surface temperature increased to 400 °C,reaching the temperature required for CO2dissociation,which makes Group VIII metals hold a vital role in the area of photothermal catalysis27.Sarinaetal.loaded non-plasmonic nanoparticles on ZrO2,and under UV light irradiation,highenergy hot electrons are generated in the non-plasmonic metals46,and the electrons can be transferred to higher energy levels by interband leap (Fig.7c) to react with reactant molecules,which is consistent with plasmonic metals.In addition,the reaction rate is enhanced by thermal effects under low-energy visible and infrared light.Group VIII metals can absorb all wavelengths of sunlight and can activate H2and CO2,which are widely used in the research of photothermal catalytic CO2hydrogenation reactions.

In addition to metal nanoparticles,a few non-metallic dark catalysts are also extremely excellent in photothermal conversion.Liuetal.used B for photothermal catalytic CO2reduction reaction and its surface temperature reached 462 °C under light irradiation of 456 mW·cm-2.The B catalyst showed strong absorption of light in the UV-Vis and even part of IR regions,and exhibited high CO2conversion activity47.Gaoet al.further studied the photothermal conversion of black nonmetallic catalysts and found that when sunlight irradiates the catalyst,the temperature increases continuously and eventually reaches an equilibrium temperature,which results from the dynamic equilibrium between the heat provided by sunlight and the heat dissipation of the catalyst to the environment48.In addition,they also found that the equilibrium temperature of the catalyst system under different wavelengths of light irradiation is from high to low in the order of full-spectrum > visible infrared light > infrared light > ultraviolet light (Fig.7d).Due to the remarkable light absorption ability of black catalysts,many scholars are devoted to the research of different black photothermal catalysts.

Besides,some metal oxide semiconductors,MOFs,sulfides,MXenes,and other carbon-based materials also have photothermal effects49.The reaction mechanism of their photothermal effect is still unknown,but these materials have been gradually applied in the study of photothermal catalytic CO2hydrogenation reaction50.Although some progress has been made in photo-driven thermocatalytic CO2reduction,its drawbacks are also obvious,the reaction still relies heavily on the thermocatalytic reaction alone,the role of light is more reflected in providing the heat source,and the light reaction and the thermal reaction are nearly independent of each other,and a more perfect coupling mechanism needs to be further explored.

2.3.2 Photothermal synergistic catalysis

When light and heat energy operating on the same reaction site are connected to cooperatively promote the reaction,this is referred to as photothermal synergistic catalysis37.Unlike photo-driven thermocatalytic reactions,the catalytic performance of the photothermal synergistic catalysis exceeds the simple sum of the single thermal catalytic reaction and the single photocatalytic reaction.Considering the complexity of the photothermal coupling mechanism,researchers commonly divide photothermal co-catalysis into photo-assisted thermal catalysis and thermally assisted photocatalysis according to the dominant driving force of the reaction,study them separately,and then investigate the coupling process.The main driving force of the photo-assisted thermocatalytic reaction originates from heat,but light plays a significant role in it.Besides increasing the catalyst’s temperature,the role of light is mainly reflected in the activation of intermediates and reactants.Yeet al.found that under irradiation of visible light,the high-energy hot electrons excited by Au can effectively activate the nonpolar molecule CO2,which makes the chemical bond of CO2break easily and significantly improves the catalytic activity50.Luet al.performed photothermal catalytic CO2methanation over Au/CeO2catalyst and discovered that the rate of a photothermal reaction was 10 times higher than the pure thermal reaction rate,showing a strong photothermal synergy (Fig.8a,b)51.Further investigation into the character of light in the catalytic reaction byinsituinfrared spectroscopy revealed that the presence of light caused the appearance of Au-H species during the reaction,which was absent in the thermocatalytic reaction.The reason may be the LSPR effect of Au nanoparticles stimulated by light,which generates lots of high-energy hot electrons,and these electrons are transferred into H2molecules to promote the quick activation and dissociation of hydrogen molecules,and the H*dissociated reacts with CO2(Fig.8c).Zhaoetal.performed CO2hydrogenation reaction by Pt/Al2O3and found that CO was discovered only when the temperature got to 200 °C in the pure thermal catalytic reaction,while the CO yield reached 50 μmol·g-1·min-1at 120 °C after the introduction of light.Density functional theory (DFT) andinsitudiffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) on the catalyst were used to shed light on the reaction mechanism.Since the CO*adsorption energy on Ptterracewas much lower than that on Ptstep,it was discovered that CO* was preferentially desorbed from the Ptterracesite when heated in the dark.The hydrogenation reaction of CO2was accelerated by light irradiation,which also accelerated the transfer of CO* from the Ptstepto the Ptterracesite and the desorption from that location (Fig.8e)52.According to Upadhyeetal.,the presence of light boosted the rate of hydroxyl hydrogenation and decomposition while lowering the activation energy of the CO2reduction reaction from 47 to 35 kJ·mol-153.In conclusion,light can encourage the activation of reactant molecules and intermediates,lower the energy barrier,and thus promote the thermocatalytic process at high temperatures.

Fig.8 (a) Catalytic performance on Au/CeO2 during different situations; (b) the CO2 conversion and CO selectivity under different conditions;(c) illustration of enhanced the catalytic performance of CO2 hydrogenation in photothermal method on Au/CeO2 51; Adapted from Catal.Commun.,Elsevier.(d) Schematic illustration of Ov-TiO2 of the thermally coupled photoconductivity,(RT is room temperature) 55; Adapted from Chin.J.Catal.,Elsevier.(e) Effect of different factors during CO2 hydrogenation 52; Adapted from Chin.J.Catal.,Elsevier.

Thermally-assisted photocatalysis mainly relies on photocatalytic reactions to generate intermediates to form new chemical bonds,and the efficiency of the reaction can be considerably increased by thermal energy by accelerating some photocatalytic processes that are strongly influenced by temperature54.The change in reaction temperature is the most intuitive manifestation of thermal energy accumulation.The promotion effect of temperature increase on photocatalytic reactions is mainly manifested in two aspects: (1) Photoelectrons will undergo a secondary transition under the effect of thermal activation,which effectually inhibits the recombination of photogenerated electron-hole pairs.(2) Sufficient heat can also accelerate carrier migration.Lietal.found that thermal energy can activate CO2molecules rapidly after the reaction temperature is increased under light,and when the light stopped,a higher-level CO2reduction could be achieved only by heat energy (Fig.8d).This is explained by the fact that after illumination,many electrons are stored at intermediate energy levels.Thermal energy can supply energy for these electrons to keep up transitioning through the guide band,which inspires the quick migration and isolation of photogenerated carriers and leads to a faster rate of CO2consumption55.In addition to facilitating the photoreaction steps,the temperature can also regulate the surface redox reaction.From the perspective of reaction thermodynamics,an increase in temperature facilitates endothermic photoreaction,and the thermal energy helps to provide enough energy for the reaction to cross the activation energy barrier.From the perspective of reaction kinetics,an increase in temperature promotes the rate of both the endothermic reaction and exothermic reaction.In addition,the presence of thermal energy enhances the thermal motion of molecules,accelerating the diffusion of product and reactant molecules and increasing the mass transfer rate.

In summary,in photothermal synergistic catalytic reactions,light can inject electrons into the reaction active sites,promote the activation of intermediates and reactant molecules,and lower the activation energy barrier of the reaction.Heat can promote the rapid separation and migration of photo-induced carriers to provide energy for the reaction to cross the activation energy barrier.Instead of promoting the photoreaction process and thermal reaction process independently,light and thermal energy are coupled with each other and synergistically promote the photothermal catalytic reaction.However,due to the complicated way that light and heat are coupled,our current understanding of photothermal synergistic catalytic reactions is not comprehensive enough,and the carrier migration mechanism under photothermal synergy is yet unknown,along with the photothermal synergistic sites,and more research questions need to be solved.Unquestionably,photothermal synergistic catalytic CO2reduction is highly significant to overcome the bottleneck problems of low energy utilization and low CO2reduction efficiency,which is anticipated to realize large-scale industrial applications.

3 Catalyst design strategies

Photothermal CO2hydrogenation is one of the most prospective strategies for reducing atmospheric CO2concentrations.The reaction can achieve the conversion of CO2to other carbon-containing chemicals under relatively mild conditions.In the past five years,research achievements in this field have emerged intensively,for example,Prof.Ye50,Prof.Zhang29,56,57and Prof.Ozin22,58have made outstanding contributions.The main catalysts used for photothermal catalytic CO2hydrogenation are metal oxides,metal sulfides,and some emerging functional materials such as graphene,black phosphorus59,and MXenes60.Metal oxides are widely used in a variety of catalytic reactions due to their unique energy band structure,especially in photothermal catalytic CO2hydrogenation reactions.ZnO is one of the most widely studied photothermal catalysts,and Yeetal.prepared Cu/ZnO catalysts by co-precipitation method,which realized the reduction of CO2to liquid fuel methanol under the stimulation of visible light and low temperature (220 °C) and atmospheric pressure.The methanol yield under light is 1.54 times that under dark conditions,and 40% lower apparent activation energy75.Zhangetal.prepared FeO/CeO2nanocomposite catalysts,which showed excellent catalytic performance in CO2hydrogenation to CO,and the presence of FeO effectively promoted the photothermal RWGS reaction108.However,metal oxide materials tend to have a wide band gap,poor absorption of light,and lack active sites for reactant molecules (H2,CO2) adsorption and activation,so modification of metal oxides generally required to improve their photothermal catalytic activity.In this section,metal oxide materials for photothermal catalytic CO2hydrogenation reaction are reviewed from three different catalyst design perspectives: supported catalysts,microstructure engineering,and defect engineering,especially the research results in the last five years,to offer a reference for the design of more high activity metal oxide photothermal catalysts.

3.1 Supported catalysts

Some precious metals and transition metals show excellent catalytic performance due to their unique electronic structure.Abundant studies have demonstrated that loading metal cocatalysts on metal oxides is an efficient strategy to improve CO2conversion.By depositing metal nanoparticles on the metal oxides,not only the selectivity of CO2reduction products can be adjusted,but also the ability of metal oxides to activate CO2can be enhanced and the migration efficiency of photo-induced electron-hole pairs can be improved.In this metal/metal oxide catalyst system,the metal oxide does not simply act as an inert carrier,but has an impact on each step of the catalytic reaction process,thus affecting the product selectivity.Reactive centers are also generated between the metal nanoparticles and the metal oxide interface,which promote the capture of CO2and charge redistribution at the metal-carrier interface20.According to the different supported metals,these catalysts are divided into precious metal/metal oxide system,non-precious metal/metal oxide system,and polymetal/metal oxide system,which are reviewed respectively.

3.1.1 Precious metal/metal oxide systems

Ru-based catalysts are considered to be the best catalysts for photothermal catalytic CO2methanation.In 2014,Yeetal.found that Ru-based catalysts exhibited the highest catalytic activity among all Group VIII metals27.In addition,Ru nanoparticles showed excellent recovery performance with no significant change in their particle size after the photothermal reaction.Buskensetal.reported a rod-shaped Ru metal catalyst with Al2O3as the carrier for CO2methanation in the full spectrum using sunlight as the only energy source61.The photon-to-methane conversion efficiencies of Ru/Al2O3catalyst are about 22.7% and 54.8% at a higher light intensity of 5.7 and 8.5 sunlight.This is because this rod-shaped Ru metal nanoparticle has a strong photothermal conversion ability and exhibits strong plasma absorption under sunlight irradiation(Fig.9a).The Ru/TiO2catalyst was also found to absorb visible and infrared light sufficiently by Wangetal.62.The methane yield of 1% (wt,mass fraction) Ru/TiO2reached 1.72 mmol·g-1·h-1at weaker light intensity (one sunlight) as well as low temperature (150 °C) and atmospheric pressure.Kimetal.investigated the catalytic principle of Ru-based catalysts and found that the hydrogenation of CO2on the Ru surface was considerably improved under light irradiation63.The CO2conversion on the ruthenium surface was 1.6% in the absence of light but increased to 32.6% in the presence of light at 150 °C.DFT calculations showed that CO2and Ru were strongly bound and that light promoted the dissociation of CO2into active intermediates CO on the Ru surface,reducing the activation energy of the catalytic reaction.In addition,unlike conventional thermal reactions,the catalytic reduction of CO2can be switched on or off instantaneously with light irradiation,which is advantageous for industrial applications.

Fig.9 (a) The excellent sunlight-harvesting ability of the Ru/Al2O3 61; Copyright 2019,American Chemical Society.(b) reaction mechanism on a Rh nanocube; (c) Production rates of CH4 and CO on Rh/Al2O3 and Al2O3 at 623 K,(d) production rates of CH4 and CO on Au/Al2O3 at 623 K 64.Copyright 2017,Springer Nature.(e) Pictures of pure Nb2O5 and Pd@Nb2O5; (f) CO production rate and (g) CH4 selectivity on Pd@Nb2O5 under 300 W xenon lamp irradiation 28; Copyright 2017,Wiley-VCH.

In addition to Ru,Rh nanoparticles are also great catalysts for CO2methanation.Zhangetal.found that Rh nanoparticles can reduce the activation energy of photocatalytic reactions and show strong product photo-selectivity64.When loading Rh nanoparticles on the surface of Al2O3,the catalyst can convert CO2to CH4and CO under dark conditions.In turn,once the light is turned on,the Rh surface will excite the plasma effect to produce hot electrons,which will be injected into the antibonding orbitals of the key intermediates to turn the catalytic products into methane with nearly 100% selectivity,while the Au/Al2O3catalyst catalyzes the production of CO regardless of whether it is illuminated as shown in Fig.9c,d.In addition,the interaction between light and Rh nanoparticles reduces the activation energy of the CO2hydrogenation reaction by 35% compared to the thermocatalytic reaction (Fig.9b).Lietal.deposited Rh nanospheres on a TiO2carrier65,and the CO2hydrogenation product of this catalyst was methane both in the dark and under the light.Surface spectroscopy analysis shows that in the process of CO2methanation,CO2firstly dissociates into CO and O,CO hydrogenates to CHO,and then CH―O will further separate into CH and O can be considered as the ratedetermining step (RDS),followed by rapid CH hydrogenation to CH4.Meanwhile,the hot electrons generated by light will transfer to the anti-bonding orbital of CHO,promoting CHO dissociation,thus significantly increasing the methanation rate.

The Pt-based catalyst can significantly promote the RWGS reaction.Geetal.prepared Pt/HxMoO3-ynanosheet catalysts for the photothermal catalytic RWGS reaction,and the CO yield of the catalyst under visible light irradiation was nearly 4 times higher than that under dark conditions,showing good photothermal synergistic catalytic activity66.H2adsorption on the Pt surface leads to H atom insertion into Pt/HxMoO3-ynanosheets and generates O vacancies in the catalyst through H spillover,thus improving the catalyst’s capacity to capture CO2.Zhaoetal.examined how light irradiation affected the RWGS reaction over Pt/Al2O3catalysts and discovered that light facilitated the transport of CO* from Ptstepto Ptterracesites,thus facilitating the production of CO molecules as well as their resolution from the catalyst surface52.

Pd-based catalysts are also widely used for photothermal catalytic RWGS reactions.Ozinetal.deposited sizecontrollable Pd nanocrystals on Nb2O5nanorods,and the catalytic activity and selectivity of the conversion of CO2to CH4and CO could be adjusted by changing the size of Pd nanocrystals28.When using larger particles of Pd nanocrystals,CO and CH4generation rates reached 0.75 and 0.11 mol·gPd-1·h-1,while the generation rates of CO exceeded 18.8 mol·gPd-1·h-1with 99.5% CO selectivity when using smaller Pd nanocrystals (Fig.9e–g).As known,Pd nanocrystals perform a significant photothermal effect,and Pd can act as a “nano heater” to increase the local reaction temperature of the catalyst and thus improve the photothermal catalytic activity.Besides,electrons on Pd nanocrystals are transferred to Nb2O5carriers,resulting in positively charged Pd nanocrystals,which are more pronounced on smaller Pd nanocrystals.During methane synthesis,CO2will first be hydrogenated at the Pd/Nb2O5interface to form CO,and the presence of a positive surface charge facilitates CO desorption.The more positive charge on the surface of Pd,the less likely CO will be adsorbed on the catalyst to enter the next step,so the small size of Pd nanoparticles facilitates the highly selective production of CO.Hongetal.used the conventional impregnation method to prepare Pd/ZnO catalysts for photothermal catalytic CO2hydrogenation at 12 bar (1 bar =105Pa) and 190–250 °C,and the methanol and CO yields got to 4.0 and 8.3 mmol·h-1·g-1,respectively67.LSPR effect will occur on the Pd nanoparticles to generate hot electrons,which migrate to the Pd-ZnO interface,leading to an improvement in the electron density at the Pd-ZnO interface.It is generally believed that the Pd-ZnO interface is the active site to synthesize methanol,and the interfacial electrons are injected into the antibonding orbitals of CO2molecules to promote the dissociation of CO2.Pd/TiO2catalysts were synthesized by Lietal.and exhibited high methane selectivity68.The existence of Pd nanocrystals can greatly accelerate electron transfer and increase the catalytic reaction rate69.

Luetal.loaded Au nanoparticles on CeO2nanorods,which after prolonged light exposure showed persistent catalytic activity and CO selectivity51.The CO yield is 10 times higher than that in the absence of light at the same temperature.They found that an Au-H species was produced upon illumination,which could promote the activation and dissociation of CO2.Wang’s group investigated the reaction mechanism of the Au/TiO2catalyst used in photothermal RWGS reaction70.They found that the oxygen vacancies were the reactive sites for CO2conversion,and Au would produce the LSPR effect under light irradiation thus generating a large number of hot electrons,and hot electrons injected into TiO2carriers will promote the formation of oxygen defects and thus CO2reduction.

In conclusion,loading noble metals has become an effective means to increase the rate of photothermal catalytic reactions.There are great differences in the interactions between noble metals and metal oxides,which lead to different selectivity of their products.In addition,precious metals are difficult to be industrially produced due to their relatively low reserves and expensive prices,so the deactivation analysis of catalysts and the search for relatively cheap substitutes should be the focus of future research.

3.1.2 Non-precious metal/metal oxide systems

Although precious metals have excellent catalytic activity,their low earthly reserves and high prices make them difficult to achieve large-scale applications,so it is crucial to find a relatively inexpensive alternative.Some non-precious metal catalysts have shown superior performance in CO2hydrogenation.Yeetal.found that Ni-based catalysts have even higher photothermal catalytic performance than some noble metals (Pd,Pt,and Ir).The activity of the 7.3% (wt) Ni/ Al2O3catalyst was equivalent to that of the 2.6% (wt) Rh/ Al2O3catalyst,and the CO2conversion reached 90% in 15 min27.This is due to the narrow band gap of nickel-based catalysts which can absorb most of the sunlight.Santamariaetal.chose a nickelbased commercial catalyst (Ni-Al2O3/SiO2) for CO2hydrogenation to produce methane under an LED lamp and achieved a CO2conversion of up to 76%,a methane yield of up to 35 mmol·h-1·g-1and a methane selectivity of more than 99.7%71.Golovanovaetal.synthesized Ni/CeO2catalysts with high specific surface area using mesoporous SBA-15 silica templates (Fig.10c)72.The catalyst has high CO2conversion efficiency (80%) and methane selectivity (95%),and the reaction rate of this catalyst under sunlight irradiation is 2.4 times higher than that obtained at dark (Fig.10a,b).On the one hand,the LSPR effect induced by Ni nanoparticles under light,and the consequent heat generated can increase the catalyst surface temperature and thus improve the reaction efficiency.On the other hand,the CeO2carrier has photocatalytic activity,and the light-excited carriers promote the production of formic acid species thus changing the reaction path and improving the methane selectivity.Amaletal.used different ratios of Ce and Ti mixed oxides (Ni/CexTiyO2) as carriers for Ni nanoparticles to investigate the role of carriers in photothermal CO2methanation73.In addition to the broad absorption of sunlight in the wavelength range of 200–2000 nm by Ni-based catalysts,the authors found that a “snowball effect” occurs during the reaction.Ni metal is passivated and a NiO shell is formed on its surface before the reaction.Under light irradiation,the catalyst surface temperature reaches above 200 °C within 30 min due to the photothermal effect,at which time the NiO shell layer is reduced by H2,and the degree of reduction is influenced by different Ti/Ce ratios.Subsequently,CO2and activated H will methane on the bare Ni.Furthermore,the methanation reaction will release heat to further increase the temperature,thus promoting the reduction of more NiO (Fig.10d).As a result,the methane yield reached 80 mL·h-1under a 300 W Xe lamp.Lzumi’s group prepared a Ni/ZrO2catalyst that can stably convert CO2to methane using only UV and visible light without any external heating74.The production rate of methane was 130 μmol·g-1·h-1,and the CO2conversion rate under UV or visible light was 15 and 5.8 times higher than that under dark conditions,respectively.According to the mass spectra,insituFT-IR spectra,and X-ray absorption fine structure spectra (EXAFS),the mechanism of CO2methanation on this catalyst was derived.CO2was firstly adsorbed on the ZrO2surface in the form of bicarbonate,and the ZrO2molecules were activated to generate electrons to reduce CO2into CO and formate species.CO and formate were transferred to the Ni surface,and CO was first reduced to methyl,and then hydrogenated to CH4on the Ni0surface.The charge separation of light on the ZrO2surface and the photothermal effect of Ni to raise the catalyst temperature are the keys to the smooth methanation reaction.

Fig.10 (a) The catalytic activity of CO2 methanation under different light irradiation at different temperatures; (b) relative increase of Ni/CeO2 catalyst at different light intensities 72; Copyright 2021,Elsevier.(c) The mechanism of CO2 methanation in the presence of sunlight 73;Copyright 2018,Elsevier.(d) A speculation on possible events in the photothermal methanation process over Ni/CexTiyO2 catalyst 25;Copyright 2022,Elsevier.(e) Proposed light-assisted CO2 hydrogenation mechanism; (f) the space-time yield (STY) and selectivity of MeOH under different conditions 76.Copyright 2020,Springer Nature.

In addition to Ni-based catalysts,Cu nanoparticles are often used as photothermal catalysts due to their LSPR effect.Cu/ZnO was used to reduce CO2to methanol at atmospheric pressure75.The photo-induced thermoelectrons on the surface of Cu nanoparticles and near the Cu-ZnO interface synergistically promoted the activation of the intermediates,thus lowering the reaction activation energy barrier,and improving the methanol yield.The Cu/ZnO/Al2O3(CZA) thermal catalyst for industrial methanol production was used for the photothermal catalytic CO2hydrogenation reaction by Scottetal.76.The photothermal catalytic system was 50 °C cooler than the thermal system,but was able to produce methanol more efficiently and more selectively.They proposed an interaction mechanism between Cu/ZnO electron excitation and surface chemistry reactions.ZnO is excited by UV light to produce electrons and Cu plasma is excited by visible light to produce hot electrons,which contribute to the direct dissociation of CO2to CO.In addition,the dissociation of H2from the Cu surface and the migration of H* to the ZnO surface as well as the HCOO* hydrogenation reaction (reaction decisive step) on the ZnO surface are promoted,which significantly increased the reaction rate (Fig.10e,f).Co and La2O3were coupled on TiO2carriers for CO2methanation by Amaletal.,where Co10/La10-TiO2had the strongest photoresponse and the highest catalytic activity,while the introduction of La increased the CO2conversion by 80% and reduced the reaction activation energy by 20%77.By DFT calculations andinsituDRIFTS analysis,HCOO* species are key intermediates in the catalytic reaction,and the higher number of HCOO* species on the surface of Co10/La10-TiO2is mainly due to the enhanced CO2adsorption.As a promoter,La was found to have the ability to promote CO2adsorption on the TiO2carrier and the conversion of CO2into the active intermediate HCOO*.In addition,the introduction of La reduces the particle size of Co crystals and generates more oxygen defects on the TiO2surface.Co promotes methane production by activating H2molecules to generate H* to react with HCOO*.The smaller-size Co nanocrystals exhibit a wide range of absorption in the visible range,which is similar to Ni.Zhanget al.prepared a MnO-Co catalyst that exhibited the 3 times and 2 times increase in photothermal catalytic activity and C2+product selectivity,respectively78,over conventional thermo-catalytic conditions (150 °C) under light irradiation of 1.0 mW·cm-2.The CO2hydrogenation process was shown to be a photo-assisted thermocatalytic system,and theinsituDRIFTS and APXPS results indicated that the photo-enhancement between MnO and Co could enrich the electrons of Co,allowing the hydrogenation reaction to being carried out through various reaction routes.The CO2hydrogenation on MnO-Co was a formate (HCOO*)mechanism pathway under thermocatalytic conditions,while it was an RWGS-CO hydrogenation mechanism pathway under photothermal conditions.The change of the photoinduced route lowers the activation energy barrier and enhances the activity of CO2hydrogenation.In addition,Fe-based catalysts have also been used in the research of photothermal catalytic hydrogenation,and Prof.Zhang’s group has made great contributions in this regard.Zhangetal.synthesized MgO/Al2O3-loaded Fe-based catalysts using MgFeAl-layered double hydroxide nanosheets as precursors79.The catalyst (Fe-500) obtained by hydrogenation reduction at 500 °C exhibited high CO2conversion and high C2+hydrocarbon selectivity of 52.9% in CO2hydrogenation test.The structure characterization showed that the Fe-500 catalyst was composed of Fe/FeOx,FeOxsuppressed the hydrogenation of ―CH2and ―CH3on Fe0nanoparticles,which effectively enhanced the C―C coupling for the synthesis of high value-added hydrocarbons.Maetal.prepared a series of Fe-based catalysts80.The Fe3O4catalysts with complete selectivity for CO (～100%) and pure phase θ-Fe3C with very high selectivity for hydrocarbon products (> 97%).The DFT calculations and TP-H/D and CO-TPD exchange experiments confirmed that there were significant differences in the adsorption ability of different Fe-based catalysts for CO and the dissociation ability of H2,and the selectivity of hydrocarbon products could be adjusted according to the degree of hydrocarbonization of Fe3O4.

The Ni and Co nanoparticles exhibit strong light absorption ability due to their narrow band gap,the surface LSPR effect of Cu nanoparticles can excite hot electrons effectively and promote the activation of reaction intermediates,exhibiting strong photothermal catalytic performance,and the Fe-based catalysts exhibit higher selectivity for C2+hydrocarbons.In conclusion,the Ni-based,Cu-based,Fe-based,and other nonprecious metal catalysts exhibit catalytic activities not inferior to those of precious metal-based catalysts,but they are prone to agglomeration and deactivation at high temperatures,and more excellent catalyst systems need to be further explored.

3.1.3 Polymetallic/metal oxide systems

Compared with monometallic,polymetallic catalysts have higher catalytic activity and C2+product selectivity due to their unique electronic structures.The polymetallic structure not only combines the properties of individual metals,but also exhibits a certain synergistic effect between different metals.Amal’s group prepared a Ni-Au/SiO2bimetallic catalyst,and the CO2conversion of the catalyst was increased by 79% under the irradiation of visible light at 520 nm compared with the dark condition81.Based on the XPS analysis,it is known that the enhanced light action at 520 nm may be due to the LSPR effect of Au generating hot electron transfer to the Ni surface.However,the LSPR effect of Au was not sufficient to change the selectivity of the catalytic reaction.The products were still dominated by CH4,and the introduction of Au led to a decrease in the catalytic activity of the catalyst compared with Ni/SiO2.Ozinetal.used a metal-anchored method to encapsulate Cu atoms on the Pd/HyWO3-xsurface (Fig.11a),which is unique in that it enables atomically precise modification of the metal oxide surface82.With the increase of Cu loading amount,it was observed that the CO production first increased and was followed by a decrease.Cu promoted the formation of carboxylic acid on the CO2surface,and a certain amount of Cu doping could significantly improve the CO2hydrogenation performance of the catalyst.However,when the Cu content exceeded a certain value,the Cu nanoclusters would cover the reactive sites on the surface of Pd and WO3and hinder the reaction (Fig.11b).The Al-loaded CoFe catalysts were obtained by the reduction of CoFeAl-LHD nanosheets by Zhang’s group in H2/Ar atmosphere at 200–700 °C29.When the temperature of the reduction exceeds 300 °C,Fe species moved to the collapsing nanosheets’ surface and formed FeOxnanoparticles,and the FeOxnanoparticles acted as catalytically active substances to promote the hydrogenation of CO2to CO.When the reduction temperature was in the range of 450 to 550 °C,FeOx/CoOxmixtures were obtained and loaded on amorphous Al2O3nanosheets.It can be known that CoOxhad a high selectivity for CH4and FeOxfavoring the generation of CO,while the main products of this catalyst were CH4and CO.Moreover,when the reduction temperature was increased to 600 °C,the Al-loaded CoFe alloy nanoparticles were obtained,which had high CO2hydrogenation activity,and the selectivity of C2+hydrocarbons was increased from 3.0% to 36.3% shown in Fig.11e.The CoFe-650 catalyst demonstrated outstanding catalytic activity under visible light irradiation as well as extremely strong operational stability during cycle testing.The unique electronic structure of CoFe alloy nanoparticles was found by DFT calculations to promote the coupling reaction of C―C bonds in the CO2hydrogenation reaction,thus facilitating the formation of C2+hydrocarbons (Fig.11c,d).Besides,the great photothermal conversion activity is also responsible for the excellent performance of CoFe alloys.This work provides a promising beginning for the synthesis of highly selective C2+hydrocarbons by photo-induced CO2hydrogenation reaction.

Fig.11 (a) Deposition method of Cu1 on Pd/WO3 and the corresponding photos; (b) in a flow reactor and batch reactor,the production of CO on Pd/WO3 in dark (brown) and light (orange) with different Cu loadings 82; Copyright 2019,American Chemical Society.(c) Reaction paths on the surfaces of Fe2O3 (110),Fe3O4 (110) and Co3O4 (100); (d) the possible C-C coupling paths; (e) the different CoFe-x catalysts formed at different temperatures and the CO2 hydrogenation selectivity of each CoFe-x catalyst are illustrated 29.Adapted from Adv.Mater.,Wiley-VCH.

The complex metals/metal oxide systems can provide abundant vacancies and active sites for the reaction.A series of Co-Cu-Mn trimetal oxide catalysts were prepared for photothermal catalytic CO2reduction by Heetal.83.Among them,the Co7Cu1Mn1Oxcatalyst gave a yield of 7.5 mmol·g-1·h-1C2+hydrocarbons at the CO2and hydrogen concentrations of 75% and 25%,respectively.Co and Mn increased the local reaction temperature through the photothermal effect,Cu promoted the reduction of Co and Mn oxides and the coupling of C―C bonds to generate C2+products,and Mn also enhanced the adsorption of CO2and H2by the catalyst.The construction of this polymetallic system provides an effective strategy for the synthesis of multi-carbon products.

Polymetallic catalysts exhibit surprising catalytic performance in the field of photothermal catalysis due to their unique electronic structures,with significantly higher selectivity of C2+products compared to monometallic catalysts.However,the interaction principle between different metals is more complicated and the selectivity of the products is difficult to control,which needs to be investigated more thoroughly.

Loading catalysts have been widely used in the study of photothermal catalytic CO2hydrogenation reactions due to their high conversion and selectivity.However,the absorption of sunlight by loaded catalysts is mainly dependent on metal nanoparticles,which cannot yet fully and efficiently utilize the entire solar spectrum.The interaction between metal and metal oxide carriers is more complex and needs to be further explored.In addition,the thermal stability of the loaded catalysts is generally poor,and the metal nanoparticles will agglomerate under strong light irradiation or high-temperature conditions,which affects the catalytic activity.

3.2 Micro-structure engineering

In recent years,most of the researches on photothermal catalytic reactions have focused on loaded metal particles.However,the catalytic activity of the loaded catalysts is also affected by the dispersion and stability of the loaded metal,highly associated with the supports,therefore,the choice of which is important.A favorable structure design can contribute to the uniform dispersion of metal nanoparticles,beneficial to expose more reactive sites.Meanwhile,it can improve the absorption of sunlight by the catalyst and inhibit the recombination of photoexcited carriers.In addition,some metal oxide materials with special structures can also exhibit better photothermal catalytic activity without the presence of cocatalysts.Therefore,in this section,metal oxide materials with different morphological structures for photothermal catalytic CO2hydrogenation reactions are reviewed in terms of nanoarray structures,core-shell porous structures and metal-organic framework (MOF) based catalysts,and their catalytic action principles are explored to provide ideas and references for more perfect photothermal catalyst design.

3.2.1 Nanoarray structures

When light strikes a macroscopic structure,most of the light is scattered or reflected.When light falls on a nanoarray structure whose size is smaller than its wavelength,the light will be trapped in the cracks and produce multiple internal reflections.The array structure,as the best trapped light absorber,has a stronger solar-light absorption capacity84.Therefore,some scholars have applied nanoarray structures into photothermal catalytic CO2hydrogenation reactions.

Silicon nanowires (SiNW) are good carriers for solar energy catalysis because their band gap is only 1.1 eV,which can absorb 85% of the sunlight.In 2014,Ozin’s group first constructed array structure catalysts by sputtering Ru nanoparticles onto black silicon nanowires and the methane yields reached the mmol·g-1·h-1level85.They discovered that the nanoarray structure could facilitate the quick rise in photothermal temperature while also enhancing carrier transport rate and reactant adsorption diffusion in photothermal catalysis.Subsequently,they uniformly coated the In2O3-x(OH)ycatalyst on silicon nanowires (SiNW),and the light capture efficiency of this In2O3-x(OH)y/SiNW film was significantly improved due to the reduction of light loss caused by scattering and reflection,and the CO2conversion is about 6 times higher than that of the same In2O3-x(OH)yfilm deposited on a glass substrate shown in Fig.12a–c86.However,the high loading amount of indium oxide,calculated to be about 0.232 mg·cm-2of Si nanowires,was not favorable for the exposure of active sites.Ozin’s team developed a method,depositing a TiO2layer on the surface of periodic-array TiN nanotubes as a substrate,following the uniform coating of a layer of In2O3-x(OH)ynanoshells thus forming ntTiN@ncTiO2@ncIn2O3-x(OH)yternary heterojunction structure catalysts (Fig.12d)87.Among them,TiN nanotube arrays provide the photothermal driving force for the catalytic reaction due to their unique light absorption properties and can be used as the main support to make the indium oxide uniformly dispersed.Meanwhile,TiO2can be excited by solar light to generate electron-hole pairs and migrate to the In2O3-x(OH)ysurface to participate in the reaction,thus increasing the catalytic reaction rate.In addition,the nanoarray structure can build “multiple electron channels” through the heterojunction structure to improve the charge transfer rate.The experimental results show that this ternary catalyst structure design significantly improves the reaction rate of photothermal catalytic CO2hydrogenation.In addition,the team also prepared a silicon inverse opal photonic crystal by template method and loaded highly dispersed RuO2on its surface88.The methane yield reached 4.4 mmol·g-1·h-1under the light irradiation of 22 kW·m-2,and the methanation reaction process was found by DFT calculations to be the hydroxyl groups formed when H2interacts with RuO2,which then further interact with CO2to produce methane.The high light absorption property and high specific surface area of silicon micro-nano structures are mainly responsible for the significant increase in methane yield.Therefore,the nanoarray materials provide a new idea and breakthrough in the design of photothermal catalysts for CO2hydrogenation.

Fig.12 Absorption spectra of In2O3-x(OH)y nanoparticles compared with photon utilization in solar irradiance of (a) In2O3-x(OH)y/SiNW materials and (b) In2O3-x(OH)y/glass films,(c) diagram of the effect of nanostructure 86; Copyright 2016,American Chemical Society.(d) Formation of ntTiN@ncTiO2@ncIn2O3-x (OH)y 87; Copyright 2021,American Chemical Society.

3.2.2 Core-shell porous structures

Micro-hollow mesoporous catalysts and core-shell catalysts are widely used in many fields such as thermal catalysis and photocatalysis due to their high specific surface area and special morphology.In recent years,some scholars have also used these catalysts with unique morphology for photothermal catalytic CO2hydrogenation reactions.Their main advantages are as follows: (1) The shell layer and microporous structure can make light multiple reflections in the internal cavity to promote the absorption of light and provide sufficient channels for the adsorption of reactant gases.(2) The larger specific surface area can improve the dispersion of the co-catalyst and thus expose more reactive sites and promote carrier migration.(3) Some shell structures can protect the co-catalysts from agglomeration at high temperatures.

Louetal.prepared a core-shell structured Ni12P5@SiO2catalyst for the photothermal catalytic RWGS reaction.Under Xe lamp irradiation,the CO yield of the catalyst can reach 135 mmol·g-1·h-1,and the CO selectivity can reach 100%89.The core-shell structure can maintain good dispersion and stability at 30% (wt) Ni12P5loading.Meanwhile,Ni12P5still maintains a relatively small particle size at high temperatures above 600 °C,indicating that the SiO2shell layer keeps the catalyst well resistant to sintering.A Co@SiO2integrated photothermal nanoreactor catalyst with a core-shell structure was prepared by Shenetal.,in which large-sized Co nanoparticles act as “nanoheaters” due to their excellent photothermal conversion ability90.Moreover,small-sized Co nanoparticles can act as “nanoreactors” due to their better catalytic activity,and different sizes of Co nanoparticles show good synergistic effects with each other.In addition,the spatial confinement effect of the SiO2shell can ensure the excellent stability and dispersion of Co nanoparticles.Under the light irradiation of 1.7 W·cm-2,Co@SiO2showed a very stable CO yield and selectivity of 80 mmol·gCo-1·h-1within 8.5 h.Well-dispersed small-sized Co nanoparticles could still be observed in the TEM image of the catalyst after 8.5 h,and its shell structure still existed,exhibiting excellent thermal stability.Caietal.reported a supraphotothermal CO2catalysis Ni@p-SiO2,which consisted of a porous SiO2shell layer encapsulated with Ni nanocrystal core(Fig.13d),with a high methanation reaction and RWGS reaction activity31.This core-shell structure design is inspired by the natural “greenhouse effect”,where the non-radiative relaxation of inter- and intra-band electrons generated by light when sunlight irradiates the catalyst surface heats the Ni nanocrystal core,while the SiO2shell absorbs the infrared light radiated outward from the Ni core and feeds it back to the Ni nanocrystals shown in Fig.13a,b.This core-shell structure not only provides super photothermal effects through the thermal insulation and IR shielding of the SiO2nanoshells but improves the thermal stability of the Ni nanoparticles due to the spatial confinement effect of the SiO2shells.Besides,accurate measurement of microscopic local temperatures (Tlocal) of the nanoscale catalysts is crucial for the measurement of the photothermal effect.Based on the equilibrium components after the reaction reaches thermodynamic equilibrium,they derived the microscopic local temperatures of the catalysts by chemical calculations.The calculations show that the SiO2shell can reduce the total heat dissipation of Ni nanoparticles by 43%,increasing theTlocalof Ni to 852 K shown in Fig.13c.The enhanced thermal stability and photothermal effect can synergistically improve the photothermal catalytic CO2activity.The CO2conversion reached 0.344 mol·gNi-1·min-1at the light intensity of 2.8 W·cm-2(Fig.13e),and the catalyst still maintained a stable CO2conversion and constant CO selectivity after 10 consecutive cycles of measurement.

Fig.13 (a) Illustration of the Earth’s greenhouse effect,(b) illustration of the nanoscale greenhouse effect in Ni@p-SiO2-30,(c) evaluation of Tlocal of different catalysts under different intensity of light,(d) schematic illustration of the preparation process for Ni@p-SiO2,(e) CO2 conversion rate for photothermal CO2 hydrogenation of different Ni catalysts under different illuminations 31; Adapted from Nat.Energy.,Springer Nature.(f) SEM image and TEM image of the superstructure of In2O3-x(OH)y nanocrystals,(g) the methanol rate of NR-14h in Photothermal catalytic CO2 hydrogenation at atmospheric pressure 91; Copyright 2018,Elsevier.

Ozin’s team prepared a defective In2O3-x(OH)ycatalyst with a rod-like nanocrystal superstructure (Fig.13f),which can achieve 50% selective solar methanol production,which is an exciting development91.The methanol yield was stabilized at 0.06 mmol·g-1·h-1under solar irradiation (Fig.13g).The N2adsorption-desorption results showed that this In2O3-x(OH)ynanocrystal superstructure has a nanopore morphology on the surface,consisting of a three-dimensional nanolattice network with a specific surface area of about 161.21 m2·g-1.This unique surface structure significantly enhances the photothermal catalytic activity,which together with the nanocrystal superstructure of the catalyst can extend the lifetime of photoexcited carriers to facilitate the photocatalytic process,enabling a higher level of methanol conversion under ambient pressure.In addition to the modulation of the microscopic morphology,Ozin’s team has also made efforts in the macroscopic structure92.They coated such nanorods of In2O3-x(OH)yon treated nickel foam,which increased the CO yield by about an order of magnitude compared to the thin film structure.The interconnectivity and high thermal conductivity of the foam structure improve the uniformity of thermal distribution among catalyst nanoparticles,and its macroporous structure provides sufficient channels for gas adsorption and allows multiple scattering-reflection of light in its internal cavity to enhance light absorption.The excellent performance of the indium oxide nanorods coated in the oxidized nickel foam configuration demonstrates the great promise of structured supports in the field of photothermal catalysis.

In summary,the core-shell structure can effectively protect the metal catalysts,thus maintaining better dispersion and stability at high temperatures.The porous structure can improve the adsorption of sunlight and reaction gas.Microstructure or macrostructure modulation is an effective means to improve the performance in photothermal CO2hydrogenation.

3.2.3 Metal-organic framework (MOF) based catalysts

Metal-organic frameworks (MOFs) are a group of crystalline porous materials with a periodic network structure consisting of inorganic metal centers connected with organic linkers.In recent years,MOFs are mostly used for the synthesis of well-dispersed metal-based catalysts due to their great specific surface area,ordered structure,and high density of metal immobilization sites.In addition,MOFs also exhibit good gas adsorption ability and have great potential for CO2reduction.Relevant studies have confirmed that the metal oxide carriers derived from MOF precursors have better catalytic performance93,and such metal/metal oxide composites prepared from suitable MOF precursors have been used for photothermal catalytic CO2hydrogenation reactions to achieve better CO2conversion.

Co/Al2O3catalytic derived from ZIF-67 for photothermal catalytic CO2methanation was reported by Chenetal.94.The methane yield of the catalyst was up to 6036 μmol·h-1·g-1with 97.7% methane selectivity.The SEM image shows that the morphology of Co/Al2O3is a cubic structure,which is similar to the original ZIF-67.However,the surface of ZIF-67 is smoother,while the surface of Co/Al2O3is rather rough.This is because of that the aggregation of loosely arranged nanoparticles constructed a three-dimensional penetrating mesopore channel on the surface,exposing more reactive sites for the reaction.TPD test results show that the material has stronger adsorption on reactants (CO2and H2) and reaction intermediates CO,but weaker adsorption on CH4,which is conducive to methanation reaction.In addition,the uniform distribution of Co in the catalyst and the strong interaction between Co and Al2O3make the photothermal conversion more efficient.Ye’s team prepared a series of well-dispersed Ga-Cu/CeO2catalysts for photothermal catalytic CO2hydrogenation by direct cleavage of the Ce metal-organic framework containing Ga and Cu95.Due to the high dispersion of Ga and Cu in CeO2,and the prepared 10Cu5Ga/CeO2catalysts reached a CO yield of 112 mmol·g-1·h-1with a selectivity close to 100% (Fig.14b,c).The DRIFTS study shows that CO2is mainly adsorbed on the oxygen vacancies of CeO2in the form of carbonate,and the introduction of Ga can adjust the electronic structure of CeO2to generate more oxygen vacancies,further improving the adsorption of CO2.The Cu nanoparticles then act as reaction active sites,and H2decomposes on its surface to generate H substance overflow to react with CO32-to produce formate (HCOO―),which eventually decomposes into CO (Fig.14a).Light can not only promote the dissociation of H2into active H during the catalytic reaction,but also promote the decomposition of formate,thus effectively reducing the activation energy of the reaction.Liet al.reported a Ni/TiO2catalyst derived from Ti-based MOF(MIL-125(Ti)),and the methane yield of 8Ni/TiO2reached 271.9 mmol·gNi-1·h-1under IR irradiation with a selectivity close to 100% shown in Fig.14e,f32.The N2adsorption-desorption isotherm indicates that the catalyst is a mesoporous structure,and the TEM images also show that it presents a porous and sparse morphology,providing sufficient channels for gas adsorption.The element mapping clearly shows that all elements are uniformly distributed in the sample compared to Ni/P25 (Fig.14d).In addition,the size of metallic Ni calculated by the Scherrer equation is in the range of about 9.1–11.5 nm,which is quite smaller than that of Ni/P25 (17.7 nm).This is because Ni NPs have a stronger resistance to sintering and keep their small size.The LSPR effect of Ni nanoparticles results in significantly enhanced absorption in the visible-IR light region,and when the light intensity increases to 1530 mW·cm-2,the surface temperature of 8Ni/TiO2reached 390 °C.In summary,the small size,highly dispersed Ni nanoparticles,abundant oxygen vacancies,strong absorption of sunlight and sufficient adsorption of reactant gases are the major reasons for the excellent catalytic performance of Ni/TiO2catalysts.Although the research on MOF-based catalysts for photothermal CO2hydrogenation is limited,MOF materials as photothermal catalytic carriers with many advantages have been paid more and more attention by scientific scholars.

Fig.14 (a) Proposed mechanism over CuGa/CeO2 for photothermal RWGS reaction,(b) catalysts surface temperature at different light intensities,(c) solar-driven CO yields over CuGaCe catalysts under different light intensities 95; Adapted from Appl.Catal.,B,Elsevier.(d) TEM images of TiO2,5Ni/TiO2,8Ni/TiO2 and 10Ni/TiO2; HRTEM images of 5Ni/TiO2,8Ni/TiO2,10 Ni/TiO2 and 8Ni/P25;element mapping images of 8Ni/TiO2 and 8Ni/P25; Yield in the first 3 h (e) and stability test (f) 32; Adapted from Appl.Catal.,B,Elsevier.

3.3 Defect engineering

Due to the poor light absorption and lack of reactive sites in metal oxide materials,researchers have been focusing on the modification of metal oxide catalysts to achieve efficient CO2hydrogenation conversion.Defects such as surface hydroxyl groups and oxygen defects are effective in enhancing the efficiency of CO2reduction reactions.On the one hand,surface defects can improve the light absorption ability of the catalyst and promote the isolation of photoexcited carriers,then increasing the photothermal catalytic activity.On the other hand,it has been suggested that oxygen defects can promote the adsorption of CO2by the catalyst and offer active sites for the activation process and reduction reaction of CO2,which in turn affects its product selectivity.Defective In2O3-xmaterials have been widely used in photothermal catalytic CO2reduction,and both Ozin’s and Ye’s teams have made outstanding contributions in this region.In addition,metal oxides such as TiO2-x,HxMoO3-x,HyWO3-x,and Bi2O3-x,which contain abundant oxygen or hydroxyl defects,have also been used for the study of photothermal catalytic CO2hydrogenation reactions.

Ye’s team found that the oxygen vacancies on the In2O3(100)surface could be used as the reactive sites for methanol synthesis through periodic DFT modeling and could promote the formation of HCOO*,a key intermediate in the methanol synthesis process96.Subsequently,a large number of scholars used defective In2O3-xfor CO2reduction reactions.Ozin’s team prepared a black In2O3-x/In2O3heterojunction structure catalyst for photothermal catalytic RWGS reaction by controlling the degree of non-stoichiometry of In2O358.Compared with yellow indium oxide (0.78 μmol·h-1·m-2),the black indium oxide with abundant oxygen vacancies achieved a CO yield of 1874.62 μmol·m-2·h-1and a 100% selectivity of CO at 300 °C under the light.Stronger solar absorption and higher photothermal conversion are the major reasons for the higher photothermal performance of black indium oxide.Ye’s group used a photoinduced defect technique to activate inert In(OH)3to twodimensional In2O3-xnanosheets30.The simulation calculations and test characterization results confirmed the presence of bifunctional oxygen vacancies in two-dimensional In2O3-xnanosheets shown in Fig.15c–e.Using only a 300 W Xe lamp as the energy source,the CO yield of the catalyst could reach 103.2 mmol·g-1·h-1with a CO selectivity close to 100%,while having good stability.(Fig.15a,b) The two-dimensional In2O3-xnanosheets appear black and can produce strong light absorption in the full spectrum from 250 to 2400 nm.After turning on the light,the catalyst surface temperature reaches 280 °C within 10 min,and RWGS is a heat-absorbing reaction,so the photothermal catalyst acts as a “nano-heat source” to provide enough energy for the smooth progress of the RWGS reaction,which resembles other metal oxides rich in oxygen vacancies.Temperature-programmed CO2desorption (CO2-TPD) analysis shows that CO2adsorption greatly decreases from 0.095 to 0.012 mmol·g-1when the vacancy concentration decreases (Fig.15f),which indicates that the presence of oxygen vacancies can enhance the adsorption of CO2.The oxygen vacancies also act as reactive sites to activate the reactant molecules CO2and H2and effectively stabilize the reaction intermediates bidentate carbonates (m-CO32-and b-CO32-) thereby reducing the reaction activation energy.In conclusion,the introduction of oxygen defects is an efficient means to improve photothermal CO2reduction.In addition to the generation of CO,Ozin’s team synthesized a black HzIn2O3-x(OH)ynanocrystal with abundant hydroxyl groups and oxygen vacancies by solid-state synthesis technique,and the catalyst can realize the hydrogenation of CO2to methanol under light22,in which the selectivity of methanol reaches 30%–50%.A tandem reaction pathway for methanol synthesis was explored through theoretical calculations and experimental verification.In which the frustrated Lewis pairs(SFLPs) as the active site for RWGS reaction and the oxygen vacancy [O] as the active site for methanol synthesis,CO is synthesized by CO2hydrogenation firstly,and then methanol is synthesized by further reaction with hydrogen using CO as the feedstock.This tandem reaction pathway overcomes the thermodynamic limitations of conventional methanol synthesis and achieves a breakthrough in the synthesis of methanol at atmospheric pressure.

Fig.15 (a) CO yields of bulk In2O3-x(OH)y,commercial In2O3,In(OH)3 nanosheets and 2D black In2O3-x nanosheets; (b) XPS spectra of O 1s peak regions and (c) electron spin resonance spectra of In2O3-x nanosheets and bulk In2O3-x(OH)y; (d) CO2 selectivity and conversion of In(OH)3 nanosheets; (e) oxygen vacancy generation energies for the In2O3 surface and bottom surface;(f) UV-Vis–NIR absorption spectra 30; Adapted from Adv.Mate.,Wiley-VCH.

In addition to plasma metals,researchers have found that some metal oxides with oxygen vacancies can also induce LSPR effects.Zhouetal.found that MoO3-xhad stronger photothermal catalytic CO2reduction than MoO3,mainly because the oxygen vacancy induced LSPR effect enhanced the absorption of sunlight97.Geetal.synthesized α-MoO3with nanosheet structure using a solvothermal method,and obtained Ru/HxMoO3-ywith abundant oxygen vacancies by passing H21 h after loading 4% (wt) Ru on its surface98.The methane yield reached 20.8 mmol·h-1·g-1with 100% methane selectivity under IR-Vis light irradiation.This is due to the insertion of a large amount of H+in α-MoO3nanosheets,resulting in the production of many free electrons,which triggers the LSPR effect enabling them to effectively capture NIR photons.The decay of the hot electrons induced by the plasma motifs significantly increases the temperature of the catalytic system and enhances the rate of the CO2reduction reaction.InsituDRIFT spectra and Fourier Transform Infra-Red spectra show that the oxygen vacancies can effectively adsorb CO2and act as the reactive sites for CO2to CO conversion,and then the CO was transferred to the surface of Ru with the assistance of light for CO hydrogenation reaction to gradually generate CH4.In addition,they found that the oxygen vacancies were continuously consumed and regenerated,thus keeping the activity and stability of the photothermal catalysis99.Geetal.also synthesized a Pt/HxMoO3-ycatalyst with high RWGS activity due to surface oxygen vacancy and strong plasma absorption66.Ozinetal.loaded Pd nanocrystals on WO3nanowires and hydrogenated them to obtain Pd@HyWO3-x,which performs a high catalytic activity and CO selectivity under solar light irradiation100.As the reactive active site,oxygen vacancies (VO),Br?nsted acid hydroxyls OH,and W (V) sites promoted the capacity of the catalyst to capture CO2and changed the catalytic reaction pathway,causing the apparent activation energy to drop from 54 to 31 kJ·mol-1.Lietal.calcined commercial Bi powder under an atmosphere of 453.15K to form Bi2O3-xwith abundant oxygen vacancies101.Under 940 nm LED light irradiation,the catalytic activity of Bi2O3-xwas 4.6 μmol·h-1·g-1,and its AQY was about 0.113%.Although its catalytic activity is not high,its low cost,durability,and wide absorption of sunlight open new doors for photothermal catalytic CO2reduction.Lietal.found that Bi2O3-xhas a strong absorption of light between 600 and 1400 nm,which may be due to the oxygen vacancies generated by the incomplete oxidation of Bi powder during calcination,and the oxygen vacancies cause the LSPR effect thus enhancing the absorption of near-infrared light by Bi powder.Using the FTIR results,they proposed a possible reaction pathway for the CO2hydrogenation process(Mars-van Krevlen pathway): firstly,CO2molecules are adsorbed on the surface of oxygen vacancies and react with them to form free COO― groups,while H2is oxidized by vacancies to H+,and then COO― groups are rapidly converted to CO and H2O by the reaction of COO― groups with H+.During this process,the oxygen vacancies consumed on Bi2O3-xwill be regenerated,thus completing the next catalytic cycle.The COO― intermediates can be stabilized in the existence of oxygen vacancies,thereby lowering the reaction’s activation energy.

In conclusion,oxygen defects can increase the catalyst’s capacity to absorb light,provide adsorption and activation sites,effectively improve the stability of the reaction intermediates during CO2reduction,and have a significant influence on catalyst activity.

To summarize,the photothermal catalytic CO2hydrogenation technology has reached a higher level.The reported metal oxide semiconductors for photothermal catalytic CO2hydrogenation are shown in Table 2.The main products of photothermal abatement are still CH4,CO,and CH3OH,whose product yield as well as selectivity have reached a high level.Catalysts can improve the selectivity of C1products by modulating the adsorption behavior of the gas on the catalyst or by offering active sites with higher electron densities.In addition,higher value-added C2+products are more attractive.However,the complicacy of multiple electrons migrating and the great difficulty of C―C coupling predispose this to be a challenging task102.High charge density,high CO2adsorption/reduction,multi-electron migration,and strong suppression of reaction intermediate binding are all desirable characteristics for C2+photothermal catalysts.Fe79,Cu103,and their compounds exhibit efficient multi-electron transfer,which can stabilize reaction intermediates and increase photogenerated carrier abundance thus improving the selectivity of C2+products.It has been shown that Fe and Cu in different oxidation states also have a great influence on the selectivity of C2+products,and Wanget al.found that Cu+promoted the adsorption ofinsituproduced CO,that further coupled C―C by forming intermediate *OC―COH103.In addition,the polymetallic catalyst systems are also considered good catalysts for C2+products,and different metals can act as the reactive sites of different reaction steps to cooperatively catalyze the formation of C2+products.

Table 2 Reported metal oxide semiconductors for photothermal catalytic CO2 hydrogenation.

4 Photothermal reactors for CO2 hydrogenation

Designing and enhancing photothermal reactors is another crucial way to enhance the efficiency of photothermal catalytic hydrogenation,in addition to creating and modifying photothermal catalysts.In recent years,the reported reactors for photothermal catalytic CO2hydrogenation reaction can be divided into two types: structured reactors and fixed-bed reactors,among which fixed-bed reactors are the most widely used104.Fixed-bed reactors can be further divided into batch fixed-bed reactors and continuous flow fixed-bed reactors,which differ in that one is operated in batch mode and the other is operated continuously.The most important feature of fixed bed reactors is that the catalyst is dispersed on a fixed support(quartz or glass fiber) and light can be directed to the catalyst surface while the feed gas is in direct contact with and passes over the catalyst surface.Structured reactors are also known as monolithic reactors.Monolithic reactors have a unique structure,generally with a large number of channels,and the catalyst is coated on the walls of the channels,thus allowing the reaction gas to come into contact with the catalyst more fully105.In addition,temperature and light are two important external factors that affect the photothermal catalytic reaction.Therefore,this section gives a brief overview of these two different types of photothermal hydrogenation reactors and compares their advantages and disadvantages.The different effects of two external factors,temperature,and illumination,on the catalytic reaction will also be described.

4.1 Reactors type

4.1.1 Fixed bed photothermal reactors

Fixed bed reactors have been the first choice for photothermal hydrogenation catalytic reactions due to their simple structure,high safety and low cost.By dispersing the catalyst on quartz or glass fiber,the reaction gas generally flows over the surface of the catalyst,bringing the catalyst in full contact with the reactant molecules.And the fixed catalyst is not affected by the reaction gas flow,which effectively prevents the catalyst from being lost during the reaction or deactivated due to wear and tear.In a fixed bed reactor,photons reach the catalyst surface directly to excite carriers,and the particle size of the catalyst or the thickness of the coating affects the absorption of photons.In general,smaller catalyst sizes or highly dispersed films promote deeper access of light into the catalytic active site,thus facilitating more efficient photothermal conversion.In addition,fixed-bed reactors can generally be pressurized,meeting the needs of different catalytic reactions.Depending on the mode of operation,the fixed bed reactors are divided into two categories: batch fixed bed and continuous flow fixed bed.

The most often utilized reactors in photothermal catalytic CO2hydrogenation processes are batch fixed bed photothermal reactors.The schematic diagram of the reaction device is shown in the Fig.16a.The reaction gas flows in from one end,and after a period of reaction,the gas is collected periodically through a syringe or sampling bag at the sampling port of the reactor and then passed into gas chromatography (GC) for detection80.The photothermal performance of the Ni/CexTiyO2catalyst mentioned above was measured in the photothermal batch fixed bed reactor73.They used a 300 W Xe lamp as a simulated solar light source,dispersed 100 mg of catalyst in Milli-Q,dropped onto a glass fiber filter matrix with an area of 7 cm2,and injected 15 kPa CO2and 60 kPa H2respectively after evacuation,and collected samples at 30 min intervals for detection by GC.The GC was equipped with a thermal conductivity detector (TCD),a merchandiser and a flame ionization detector (FID)106.The Ni/CeO2sample with the best performance produced 80 mL of methane within 90 min,and the surface temperature of the catalyst reached 275 °C.However,such intermittent reactors are tended to cause reverse reactions,and may also cause Boudouard reactions or hydrocarbon cracking to produce carbon deposition.If Ni-based catalysts are used,when the temperature reaches above 230 °C,the product CO may react with Ni to produce highly toxic gases such as nickel tetracarbonyl107.

Fig.16 Scheme of (a) batch fixed bed photoreactor 80 and (b) continuous flow fixed bed photoreactor 108; Copyright 2020,American Chemical Society.Copyright 2020,Springer Nature.Pictures of (c) Zirconia foam monolith and (d) Tubular quartz monolith;(e) Schematic of reactor model with design components and describing its operation 109; Adapted from Solar Energy,Elsevier.(f) Schematic of the solar reactor for splitting H2O and CO2 through a thermochemical redox cycle based on CeO2 111.Copyright 2022,Elsevier.

Continuous flow fixed bed photothermal reactors can be used to avoid the occurrence of reverse reactions in a batch fixed bed.The schematic diagram of the continuous flow fixed bed photothermal reactors is shown in Fig.16b.The light shines on the surface of the catalyst through the quartz window,CO2and H2enter the reactor from one end,and some are adsorbed on the catalyst surface to participate in the reaction108.The reaction products flow out from the other end along with the reaction gas that is not involved in the reaction and is introduced into the gas chromatography.Compared with batch fixed bed photothermal reactors,continuous flow fixed bed photothermal reactors have the following advantages: (1) effectively inhibit the occurrence of the reverse reaction,(2) evenly distributed temperature,(3)reduce carbon deposition,and (4) lower pressure drop.Zhanget al.prepared FeO/CeO2composites for photothermal CO2hydrogenation in a continuous flow fixed bed reactor108.They uniformly distributed 50 mg FeCe catalyst in a quartz filter,and used thermocouples to test the temperature of the reaction system.The reaction mixture (CO2/H2/Ar = 15/60/25) is introduced into the reactor at a flow rate of 15 mL·min-1,and the system pressure is maintained at 0.18 MPa.Under the light irradiation of 2.2 mW·cm-2,the CO2conversion of the catalyst reached 43.63% and maintained stability for 50 h.The continuous flow fixed bed reactor has become the new favorite of many scholars studying the photothermal catalytic reaction.Because of its simple structure and high catalytic efficiency,it is expected to be used in industrial production.

4.1.2 Structured photothermal reactors

Structured reactors are widely used in photocatalytic and thermocatalytic CO2hydrogenation reactions.Structured reactors generally have a large specific surface area,and the catalyst coated on its surface can be in high contact with light and reaction gas,effectively increasing the photothermal catalytic reaction rate.In addition,it can provide efficient mass and heat transfer between the reaction gas and catalyst,can support high flow rates and relatively low-pressure drops,and is expected to be an alternative to conventional fixed-bed reactors104.Bhattaetal.subjected Cu/TiO2catalysts to photothermal catalytic CO2hydrogenation in two different monolithic reactors109.They formed zirconia foam monolith(ZrFM) and tubular quartz monolith (TQM) based on zirconia foam and transparent quartz tubes,respectively.ZrFM is composed of seven circular zirconia foam discs stacked together(Fig.16c),with about 10 air pores per inch,80% porosity,and a sinuous internal structure.The TQM was prepared from 217 transparent quartz tubes packed into a cylindrical reactor with 65% porosity as shown in Fig.16d,e.A 6.5 kW high-flux solar simulator is used as the reaction light source,and the reaction gas is fed into the monolithic reactors through a bubbler.To minimize product recombination,gas samples can be taken either before or after the quenching chamber.Finally,the treated gas passes through a catalytic conversion device for catalytic conversion.This structured photothermal reactor is 10–100 times more efficient than other catalyst carriers in terms of catalyst exposed area,catalytic reaction activity,and light usage110.Recently,Aldo Steinfeldetal.designed a tower production unit using H2O,CO2,and solar energy to make aviation kerosene111.The study achieved an experimental demonstration of the complete thermochemical production chain from water and carbon dioxide to kerosene and has reached a pilot scale.Their solar reactor schematic is shown in Fig.16f.The CeO2RPC (reticulated porous ceramic) cavity receiver is the most critical component of the solar reactor,providing efficient heat and mass transfer.The uniformity of the radiation flux distribution within the cavity can be further improved by the adjustment of the cavity geometry and the concentrating optical system,i.e.,the inclusion of a secondary composite parabolic concentrator (CPC),thereby relieving thermal stress.Solar energy is recycled through thermochemical REDOX to produce hydrogen and carbon monoxide syngas,which is fed to the gasliquid converter (GtL) at the bottom of the tower for kerosene production.The overall efficiency of the unit (as measured by the energy content of the syngas as a percentage of the total solar input) is only about 4%,but by recovering and recycling more heat and changing the structure of the cerium dioxide,the efficiency of the system will hopefully increase to over 20%.Therefore,this structured reactor can effectively improve the catalytic reaction efficiency and is the development direction for solar reactors in the future.However,the disadvantage of this structural design is that the effects of light and thermal cannot be studied independently112.

4.2 Operating parameters

4.2.1 Temperature

Thermocatalytic CO2hydrogenation conversion usually occurs in the range of 250–450 °C.Temperature is an important factor affecting catalytic reactions,and in general,the increasing temperature is beneficial for catalytic reactions113.When the temperature increases,the molecular thermal motion will accelerate,and the number of activated molecules in the system will increase,thus the number of effective collisions between molecules per unit of time will increase,making the reaction rate faster.However,hydrogenation of carbon dioxide is an exothermic reaction,and when the temperature of the system is too high,the equilibrium of the reaction will shift in the direction of the opposite reaction according to Le Chatelier’s principle,resulting in a dramatic reduction in the production of the product104.In addition,numerous studies have shown that high temperature decreases the activity and stability of the catalyst.Therefore,regulating the appropriate temperature is an important means to improve the catalytic reaction rate.

There are two ways to raise the temperature of a photothermal catalytic system: one is to use a light source to induce heat production in the catalyst,and the other is to heat the catalytic system by external heating.The heat production induced by the light source mainly depends on the photothermal conversion capability of the catalyst itself,which is affected by the light intensity,the distance to the light source and the type of light.For the catalysts with poor photothermal conversion capability,the heat provided by the light source can only reach 110–160 °C,so an external heating source is necessary.The external heating source will generally use an electric furnace,electric heater,etc.The use of external heating makes it easier to control the system temperature and also helps to provide a stable thermal input to the system,which in turn maintains the system temperature at a fixed level.In addition,the introduction of an external heating source helps to independently investigate the effects of heat and light on the CO2hydrogenation process.Compared with conventional thermal catalytic reactions,photothermal catalysis only raises the local reaction temperature without raising the temperature of the whole catalytic system,thus it has the advantages of mild reaction conditions and low energy consumption.

In addition,accurate measurement of the local reaction temperature of nanoscale catalysts is a key challenge in photothermal catalysis.In most current studies,the catalyst bed temperature is generally monitored using a thermocouple placed in the middle of the sample surface.However,due to the locality of heat generation,there may be a considerable temperature gradient between the catalyst and the surroundings,which induced a large error.The catalyst surface temperature can be measured more accurately by the infrared imager,but the accuracy of the method is still controversial due to the different interpretations of catalyst emissivity and the parameter selection of thermal imaging camera42.Caietal.estimated theTlocal(microscopic local temperatures) of the catalyst based on the composition of different gases when the photothermal catalytic CO2hydrogenation reaction reaches equilibrium31.A scanning thermal microscope equipped with a nanoscale probe tip can measure the surface temperature with spatial resolutions in the range of 10 nm and ～10–50 mK precision.Local temperature maps with nanoscale resolution can also be obtained by analyzing the temperature-dependent energy shifts of the plasma peaks using scanning transmission electron microscopy(STEM).In conclusion,accurate monitoring of the temperature of photothermal catalytic reactions is crucial,and more accurate and simple temperature measurement techniques need to be further developed.

4.2.2 Illumination

The effect of light on the photothermal catalytic reaction can be categorized as the effect of light intensity and the effect of the solar spectrum.Yeetal.tested the photothermal performance of Ga-Cu/CeO2catalyst for CO2hydrogenation and found that the CO yield and light intensity varied almost linearly,and the catalyst surface temperature also varied linearly with the increase of light intensity95.This is because high-intensity light has higher energy,which contributes to the formation and migration of photo-induced carriers in the catalyst.In addition,there are various light sources used for photothermal catalysis,commonly including Xe lamps and light-emitting diodes(LEDs).Xe lamps are the most widely used light sources in photothermal catalysis,which can provide a wide range of emission spectra and simulate natural light.Infrared light can provide a heating effect,while ultraviolet light with high energy can excite carriers114.The LED lamps are monochromatic light sources,showing the wavelength of narrow-band emission.Green and blue LEDs tend to emit the wavelength of the visible spectrum,while deep red lamps emit the wavelength of the infrared spectrum,and UV-LEDs are often used in photocatalytic reactions.The catalysts with great photothermal conversion capability generally show absorption properties for the full spectrum of solar light.

5 Perspectives and challenges

CO2emission reduction has become one of the major tasks in ecological environmental protection in China,and the photothermal catalytic CO2hydrogenation technology has shown great potential in reducing atmospheric CO2concentration and achieving a high-quality carbon cycle due to its high efficiency,green and sustainable advantages.In this review,we first look into the great prospects of photothermal catalytic CO2hydrogenation.Then,a summary and overview of the photothermal catalysis principle is given,pointing out the difference between photo-driven thermal catalysis and photothermal synergistic catalysis.On this basis,the metal oxide materials used for photothermal catalysis in recent years are summarized in terms of different catalyst design strategies,especially the defect engineering that has emerged in recent years as an efficient means to improve the performance of photothermal hydrogenation.In addition,we also summarize the photothermal hydrogenation reactors to provide a reference for a more rational and efficient reactor design.Next,I will present some challenges and limitations that still exist in this field to point the way for further research in the future.

1.At present,the main products of photothermal emission reduction are still CH4and CO,and the higher value-added methanol and C2+products are more attractive.The future catalyst design direction should focus on the construction of active sites for methanol and C2+reactions and the research of such reaction mechanisms.According to the current research situation,Fe-based catalysts and Cu-based catalysts can be regarded as a suitable choice.

2.Studies on the long-term stability of photothermal catalysts are insufficient.The stability experiments reported in the literature are all maintained within 50 h,and there is a certain gap with the industrial application.Moreover,the research on catalyst deactivation analysis is not in-depth enough,so modification research should also be carried out for catalyst deactivation as well as the photothermal reactor should be optimized,to suppress the sintering,carbon accumulation and agglomeration of the catalyst at high temperatures.

3.In photo-driven catalytic reactions,it is difficult to separate the light effect from the thermal effect,and the principle of photothermal coupling needs further investigation.In addition,theinsitucharacterization facilities for photothermal CO2reduction are relatively backward,and it is urgent to develop customizedinsitucharacterization techniques to monitor the changes of reaction active sites and intermediates during the reaction process in real-time,and then explore the catalytic reaction mechanism.

4.The great improvement of the H2O splitting system provides the possibility of a sustainable and economically competitive supply of hydrogen as a reactant,and the two-step photochemical cycle reactor should be designed,thereby realizing an efficient and green reaction pathway for the conversion of CO2and H2O into hydrocarbon products.