Yakubu Adekunle

Peter Olusakin Oladoye
email: poladoye@fiu.edu

Onome Ejeromedoghene

Owolabi Mutolib Bankole

Oyekunle Azeez Alimi

Elizabeth Oyinkansola Omotola

Clement Ajibade Olanrewaju

Karine Philippot

Adeyemi S Adeleye

Adeniyi Sunday Ogunlaja

Yakubu Adekunle Alli
email: yakubu.alli@lcc-toulouse.fr

Peter Olusakin Oladoye

Elizabeth Oyinkansola Omotola

Sunday Ogunlaja

Nanomaterials as catalysts for CO2 transformation into value-added products: A review

Keywords: CO2 based fuels, nanomaterials, global warming, catalysts

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

One of the primary environmental concerns today is climate change. Indicators of climate change include global warming, increased frequency and intensity of natural disasters and extreme weather events such as wildfires which was responsible for the loss of ~$650 billion between 2016 to 2018, extreme droughts in Europe, heatwaves in USA, and hurricanes in other parts of the world [START_REF] Mustafa | Current technology development for CO2 utilization into solar fuels and chemicals: A review[END_REF]. Climate change is mainly attributed to increased emission of greenhouse gases into the atmosphere [START_REF] Cassia | Climate change and the impact of greenhouse gasses: CO2 and NO, friends and foes of plant oxidative stress[END_REF][START_REF] Solomon | Persistence of climate changes due to a range of greenhouse gases[END_REF][START_REF] Solomon | Irreversible climate change due to carbon dioxide emissions[END_REF], which has led to the global surface temperature of 0.88°C (1.58°F) in September 2022 which is above the average for the 20th century of 15.0°C (59.0°F), tying September 2021 as the sixth warmest for September in the 143-year record. In addition, global CO2 levels was 280 ppm in the pre-industrial era; it has now increased to 417.86 ppm as at October 19, 2022 according to the Global Monitoring Laboratory (daily global CO2 trend) and it is projected to reach 570 ppm by the end of the 21 st century [START_REF] Mustafa | Current technology development for CO2 utilization into solar fuels and chemicals: A review[END_REF].

In order to ameliorate global warming and other climate change indicators, it is imperative that emission of CO2 is controlled, and the excess atmospheric CO2 is captured. Several strategies exist for achieving atmospheric CO2 reduction. For instance, afforestation is effective for CO2 emission reduction and sequestration [START_REF] Halder | Assessment of biomass energy resources and related technologies practice in Bangladesh[END_REF][START_REF] Jansen | Pre-combustion CO2 capture[END_REF][START_REF] Li | Advances in CO2 capture technology: A patent review[END_REF][START_REF] Mondal | Progress and trends in CO2 capture/separation technologies: A review[END_REF]. More so, various governmental and inter-governmental policies have been implemented to also reduce the emissions of CO2 by putting taxes on carbon sources and fossil fuels [START_REF] Atsbha | A review on the catalytic conversion of CO2using H2for synthesis of CO, methanol, and hydrocarbons[END_REF][START_REF] Herzer | The impact on domestic CO2 emissions of domestic government-funded clean energy R&D and of spillovers from foreign government-funded clean energy R&amp[END_REF][START_REF] Omoregbe | Carbon capture technologies for climate change mitigation: A bibliometric analysis of the scientific discourse during 1998-2018[END_REF][START_REF] Peiseler | How can current German and EU policies be improved to enhance the reduction of CO2 emissions of road transport? Revising policies on electric vehicles informed by stakeholder and technical assessments[END_REF].

Chemical CO2 conversion is also effective but is a herculean task due to the characteristic thermodynamic stability of CO2 molecule. The stability of CO2 arises from its structure (O=C=O) and possession of double covalent bonds with two oxygen atoms (Ateka et al., 2022). The carbon in CO2 is extremely inactive and is kinetically inert owing to its occurrence at the highest oxidation state in CO2 (Wang et al., 2022).

As a result, the transformation of CO2 to useful fuels and/or chemicals demands the input of substantial amount of energy to cleave the C-O bond [START_REF] Pawelec | Catalysts for the conversion of co2 to low molecular weight olefins-a review[END_REF][START_REF] Sha | Hydrogenation of Carbon Dioxide to Methanol over Non-Cu-based Heterogeneous Catalysts[END_REF] The energy requirement for CO2 conversion is met through three major routes: (i) the use of (nanomaterial-based) catalysts with active sites, (ii) the application of high pressure and/or temperature, and (iii) application of energy from carbon-neutral and renewable energy sources [START_REF] Kamkeng | Transformation technologies for CO2 utilisation: Current status, challenges and future prospects[END_REF]. There are a number of technological approaches that can be employed to execute transformation of CO2. In terms of using catalysts for CO2 transformation, the processes and/or mechanisms involved may be photocatalytic, electrocatalytic, thermal or non-thermal etc., to lower the energy barrier and create alternative pathway for the catalytic CO2 reduction (Ateka et al., 2022). For example, in photo-catalyzed transformation using titanium dioxide (TiO2; a common photo-catalyst) as a catalyst, upon irradiation, a photo hole-electron pair is generated, which interacts with CO2 and H2O molecules to produce hydrocarbons and other valueadded products. This is feasible only by appropriate catalyst selection, doping and conditions [START_REF] Low | Surface modification and enhanced photocatalytic CO 2 reduction performance of TiO 2 : a review[END_REF]. Scheme 1 presents the various value added-products of CO2 transformation covered in this synthesis.

Scheme 1: Products obtained from the transformation of CO2

A number of comprehensive, systematic and/or critical reviews on the transformation of CO2 have been published [START_REF] Kamkeng | Transformation technologies for CO2 utilisation: Current status, challenges and future prospects[END_REF][START_REF] Low | Surface modification and enhanced photocatalytic CO 2 reduction performance of TiO 2 : a review[END_REF][START_REF] Mustafa | Current technology development for CO2 utilization into solar fuels and chemicals: A review[END_REF][START_REF] Pawelec | Catalysts for the conversion of co2 to low molecular weight olefins-a review[END_REF][START_REF] Sha | Hydrogenation of Carbon Dioxide to Methanol over Non-Cu-based Heterogeneous Catalysts[END_REF]Wang et al., 2011;[START_REF] Wu | CO2 Reduction: From the Electrochemical to Photochemical Approach[END_REF].

Although a large proportion of them are detailed with respect to application and transformation of CO2, recent advances in technologies for CO2 conversion have not been sufficiently covered by the existing reviews. In addition, the technical challenges facing the real-world implementation of these technologies are yet to the comprehensively explored and presented. Therefore, this synthesis on CO2 conversion to valued-added products attempts to provide current and comprehensive understanding on nano-enabled CO2 conversion, routes of CO2 conversion, fuels generated from CO2 conversion, application, technical issues and economic implications of transformation of CO2 into hydrocarbon fuels. The review also elaborates the research gaps that need to be filled.

Nanomaterials for CO2 conversion

Nanomaterials have gotten a lot of attention in the research world because of their fascinating small size (1-100 nm), which is responsible for their improved functionality [START_REF] Afolabi | A selective and efficient chemosensor for the rapid detection of arsenic ions in aqueous medium[END_REF][START_REF] Alli | Compressed Hydrogen-Induced Synthesis of Quaternary Trimethyl Chitosan-Silver Nanoparticles with Dual Antibacterial and Antifungal Activities[END_REF][START_REF] Alli | Quaternary Trimethyl Chitosan Chloride Capped Bismuth Nanoparticles with Positive Surface Charges: Catalytic and Antibacterial Activities[END_REF]. In this regard, the use of nanomaterials for direct transformation of CO2, to renewable organic fuels and other feedstocks has become a promising solution to simultaneously mitigate the global energy crisis and climatic change. Due to the thermodynamic stability, fully oxidized nature, and inertness of CO2, various nanoscale catalysts have been developed to activate CO2 molecules into low (C1) and high hydrocarbon (C1-C9) molecules. Figure 1 depicts possible catalytic pathways for CO2 transformation to fuels and chemical products.

They include thermocatalytic reduction of CO2/H2 to produce CH4/CO, electrocatalytic transformation of CO2/H2/electron to C1-C9 hydrocarbons, cycloaddition of CO2 with epoxide to form cyclic and polycarbonates, and the emerging photocatalytic reduction of CO2/H2O to CH4, CHOOH, CH3OH, etc. The nature and kinds of nanomaterials for conversion of CO2 also play a vital role in product selectivity to get carbon-containing fuels, in addition to process parameters such as solvent effects, reaction temperature, H2/CO2 ratios, and partial pressure condition of CO2. The nanomaterials for catalytic conversion of CO2 are range from plasmonic metal nanoparticles and metal oxidesbased composites to metal-free nanoporous materials including carbon nanostructures such as grapheme oxide (GO) reduced grapheme oxide (rGO) and carbon nanotubes (CNT), silica, alumina, covalent organic polymers, etc.

Figure 1: Pathways for catalytic conversions of CO2 to various feedstocks

Routes of CO2 conversion

Photocatalytic transformation of CO2

The natural process of photosynthesis, in which plants change CO2 and H2O into oxygen and carbohydrates when exposed to sunlight, served as the inspiration for photocatalytic CO2 reduction. This process transforms solar energy into chemical bonds, which are then stored (carbohydrates). It combines light and dark reactions with water oxidation and carbon dioxide reduction (or CO2 fixation) reactions. The process of photocatalytic transformation of CO2 into value added particularly using semiconductor or metal complex as catalyst can be divided into four steps: (i) absorption of light greater or equal to the band gap (ii) Separation of valence band (CB) and conduction band (CB) (iii) migration of electrons from the VB to CB and migration of electrons to the surface of cocatalyst (iv) adsorption and activation of CO2 by the photogenerated electrons [START_REF] Kovačič | Photocatalytic CO2 Reduction: A Review of Ab Initio Mechanism, Kinetics, and Multiscale Modeling Simulations[END_REF](Figure 2).

To understand the kinetic and thermodynamic point of view, the formal reduction potentials for the reactions connected to the photoreduction of CO2 (at pH 7) is presented in Figure 12a. Due to their extremely high negative reduction potentials [E(CO2/CO2 •-) = -1.85 V vs NHE] in relation to the CB edges of many semiconductors, one-electron reduction processes of CO2, such as the generation of CO2 •, are not practical [START_REF] Lingampalli | Recent Progress in the Photocatalytic Reduction of Carbon Dioxide[END_REF]. A highly negative reduction potential results from the conversion of C's sp 2 to sp 3 hybridization. Reduction potentials for HCOOH, HCHO, CH3OH, and CH4 production are minimal (-0.81, -0.48, -0.38, and -0.246 V) (Figure 12b) and positive relative to CB edges of various semiconductors. Proton coupled electron transfer (PCET), in which electron transfer to CO2 is accompanied with proton transfer, is thus preferable [START_REF] Kovačič | Photocatalytic CO2 Reduction: A Review of Ab Initio Mechanism, Kinetics, and Multiscale Modeling Simulations[END_REF][START_REF] Lingampalli | Recent Progress in the Photocatalytic Reduction of Carbon Dioxide[END_REF].

A good number of works has been done recently to functionalize semiconductors and it's like as novel and effective photocatalysts. Various single and composites nanomaterial have been reported for photocatalytic transformation of CO2. This section critically survey the scattered literatures Reproduced with the permission from [START_REF] Shen | Photocatalytic Reduction of CO 2 by Metal-Free-Based Materials: Recent Advances and Future Perspective[END_REF]. Copy right 2020 Willey Library

Single metal nanoparticles

Plasmonic metal nanoparticles (NPs) such as Au, Ag, and Cu have a large optical extinction cross section and the ability to absorb significant visible light which accounts for up to 40% of solar irradiation to catalyze a wide range of photochemical reactions.

As a result, , they offer several advantages over other conventional catalysts like Pt, Pd, Ru, and Rh, whose localized surface plasmon resonances (LSPRs) fall in the UV region of the solar spectrum. A single silver nanoparticles (AgNPs) photocatalyst was excited in a microfluidic reaction cell filled with CO2 led to CO2 reduction reaction CO2RR, to CO and HCOOH. The synergy between photoexcited CO2 and catalytic active sites, which is not clearly understood at the molecular level, was evidenced by in-situ surface-enhanced Raman spectroscopy (SERS) [START_REF] Kumari | Watching visible light-driven CO2 reduction on a plasmonic nanoparticle catalyst[END_REF]. The SERS was used to physi-sorb CO2 on nanoparticles, LSPR-triggered CO2 activation to HOCO * (intermediate), followed by transformation of HOCO * to multiple products (H2, CO, HCOOH, HCHO and CH3OH), (see Figure 3). The real-time monitoring of CO2RR activity by SERS shows that plasmonic AgNPs are selective for CO and HCOOH, with CO being the predominant product. On polyvinylpyrrolidone (PVP)-capped plasmonic AuNP-photocatalyst, photoexcited LSPR induced a multi-electron and multi-proton reduction of CO2 to C1 (methane) and C2 (ethane) hydrocarbons, without formation of CO [START_REF] Yu | Plasmonic Control of Multi-Electron Transfer and C-C Coupling in Visible-Light-Driven CO2 Reduction on Au Nanoparticles[END_REF]. The CO2RR was conducted in isopropyl alcohol (IPA) saturated CO2-aqueous solution to prevent h + /e -recombination during photoredox reaction, and the CO2RR selectively produced CH4 (6.8 NP -1 ) and C2H6 (5.6 NP -1 ) after 10 h of light irradiation. The results show that catalytic and selectivity of plasmonic gold nanoparticles (AuNPs) for CO2 transformation to C1 and C2 is modulated by laser excitation wavelength (photon energy), and photon flux (light intensity). Thus, the CH4 production rate increased with decreasing excitation wavelength and with overall 8e - process (8H + + CO2 → CH4 + 2H2O), while C2H6 production was only observed at lower excitation wavelength with 14e -process (14H + + 2CO2 → C2H6 + 4H2O). Zeng et al. [START_REF] Zeng | Plasmonic Cu NPs-embedded phenothiazine benzene with tunable bonding units for superior photocatalytic CO2 reduction[END_REF] embedded uniformly distributed plasmonic CuNPs into a triphenothiazine benzene ring (TPB) for photocatalytic CO2 reduction to CO, H2 and CH4. CO evolution rates over TPB-Cu increased as the wt% of Cu bonded to TBP units increased, and CuNPs with 1wt% content yielded a CO evolution rate 3.2 times higher than bare CuNPs. Interestingly, the evolution rates of H2 and CH4 on TPB-Cu remained relatively constant, implying that incorporating plasmonic CuNPs into TPB increased the directional selectivity for CO formation over H2 and CH4. Plasmonic cupper NPs conjugated with TBP could provide a large contact area for hot electron migration, opening new doors for energy and carbon cycle processes. NPs on which each type of species was observed. Reproduced with permission from the American Chemical Society from ref. [START_REF] Kumari | Watching visible light-driven CO2 reduction on a plasmonic nanoparticle catalyst[END_REF].

Metal/metal oxide-based composites

In order to overcome the limitations of single-component plasmonic metal photocatalysts and to expand their light harvesting to UV region, researchers have turned to multi-component semiconductors for a variety of applications. Decoration of metal oxide semiconductors with transition or plasmonic metals has been a huge success. Both plasmonic and transition metals on semiconductors can help harness UV and visible light by reducing energy band gaps, which play key roles in CO2 activation and intermediates stabilization for CO2RR.

For many years, the selective absolute conversion of CO2 to CO has been a source of contention. In 2020, Nguyen et al. (Nguyen et al., 2020) constructed air-stable Zscheme photocatalysts composed of Ag-decorated reduced titanium oxide/tungsten hybrid nanoparticles (TiO2/WO3-AgHNPs) for CO2 reduction to CO (100%, without byproducts) and with an evolution rate of 1166.72 μmol g -1 h -1 after 7 h of light illumination. On one hand, the plasmonic Ag (1wt% Ag) increased the photocatalytic electron reaction rate up to 2333.44 μmol g -1 h -1 , and facilitated intense absorption light in the visible region of the catalysts due to its strong LSPR effect. The presence of Ti 3+

states in the catalysts, on the other hand, facilitated the physisorption, stabilization, and reorientation of the intermediates formed during CO2 reduction. Other gas products such as CH4 and H2 were produced in the absence of Ag deposition, suggesting the importance of using plasmonic Ag to achieve the formation of only CO.

Zhang et al. (Zhang et al., 2021) Wang and co-workers demonstrated LSPR photo-assisted electron migration from plasmonic CuNPs to ZnO via metal-support interfaces in Cu/ZnO catalysts for CO2 reduction to MeOH [START_REF] Wang | Photo-assisted methanol synthesis via CO2 reduction under ambient pressure over plasmonic Cu/ZnO catalysts[END_REF]. In the photothermal process, the rate of MeOH formation was 2.13 μmol/g. min, compared to 1.38 μmol/g. min recorded in pure thermal process, and the apparent activation energy decreased from 82.4 to 49.4 kJ mol - 1 . The rate-determining step in the thermal process was the formation of reaction intermediates ( * HCOO, * HCOOH, and * H2COOH species), followed by the hydrogenation of * HCOOH to MeOH. Formation of MeOH in photothermal process followed the same reaction pathway as thermal process, with the exception that induced hot electrons on CuNPs accelerated the conversion of intermediates to product (MeOH). Under visible light irradiation, the product selectivity over Cu/ZnO remained relatively constant and the decreased activation energy had no effect. Small amounts of Pt nanoparticles have also been used as co-catalyst on ZnMn2O4 nanorods as visiblelight-driven catalysts for promoting CO2 reduction to produce methane, as was the case in other studies (Alhaddad and Shawky, 2021) (see Figure 4a). The wt% of Pt species on ZMO nanorods influenced the catalytic activity and selectivity of Pt/ZMO catalysts, with 1.5% Pt/ZMO exhibiting the best photocatalytic performance for CO2RR activity (Figure 4b). Band connection and surficial interactions between Pt species and ZMO nanorods with optimal Pt/ZMO at 1.5% Pt enhanced selective reduction of CO2 to

MeOH with an evolution rate of 1,906 μmol/g within 9 h. V, Cr, and Co loaded on TiO2 were studied for CO2RR to produce H2, C1 and C2+.

Higher photoconversion rates were observed compared to pristine TiO2, which were found to be sensitive to the metal concentrations and TiO2 crystallite size modification [START_REF] Ola | Transition metal oxide based TiO2 nanoparticles for visible light induced CO2 photoreduction[END_REF]. After 4 h of illumination, 1wt% Co-TiO2 demonstrated the highest photoconversion rates of 62.91 μmol/gcath for H2 and 26.12 μmol/gcath for MeOH (Figure 5). Ni-modified CeO2, MgO, α-Al2O3 and TiO2 NPs were studied for CO2 methanation, and the effects of the solid supports were elucidated [START_REF] Tada | Ni/CeO2 catalysts with high CO2 methanation activity and high CH4 selectivity at low temperatures[END_REF]). CO2 methanation pathways followed two steps: (i) reaction of CO2 and H2 at the meta/metal oxide interfaces to afford CO through reverse water gas shift reaction (CO2 + H2 → CO + H2O), and (ii) prompt methanation of CO with H2 (CO + 3H2 → CH4 + H2O) at low temperature via the Fisher-Tropsch synthesis (FTs). CeO2 has the highest number of CO2 adsorption sites and exhibits the highest CO2 methanation with nearunity CH4 selectivity. Li and co-workers showed that partially oxidized metallic Fe NPs (Fe and FeOx) deposited on MgO-Al2O3 and then reduced under H2 at different temperatures photothermally hydrogenated CO2 into C1 and C2+ hydrocarbons [START_REF] Li | Fe-Based Catalysts for the Direct Photohydrogenation of CO2 to Value-Added Hydrocarbons[END_REF]. The Fe-500 sample (reduction performed at 500°C) showed the highest CO2 conversion and excellent selectivity for C2+ hydrocarbons with 0% CO. The FeOx phase promotes CO2 reduction to CO via RGSW reaction and modulates the electronic properties of Fe phase to suppress CH2 and CH3 hydrogenation, thus enhancing C2+ hydrocarbon formation. Reproduced from [START_REF] Ola | Transition metal oxide based TiO2 nanoparticles for visible light induced CO2 photoreduction[END_REF] with permission.

The conjoining of metal, e.g. Ag and narrow band semiconductor e.g. InS2 has been reported to improve photocatalytic reduction of CO2 activity. Recently, a quaternary nanocomposite (AgCuInS2) deposited on a graphene-TiO2 based material presented excellent photoreduction activity of carbon dioxide in H2O with the formation of mostly methanol, formaldehyde and formic acid. AgCuInS2 combined with TiO2 semiconductors (AgCuInS2-TiO2) absorb photon energy, which led to the excitation of electrons from valence band (VB) to conduction band (CB), creating electron-hole pair, and the graphene act as electron assimilates (Figure 6).

Figure 6: Mechanism of photocatalytic CO2 reduction. Re-used with permission from [START_REF] Otgonbayar | Novel Micro and Nanostructure of a AgCuInS2-Graphene-TiO2 Ternary Composite for Photocatalytic CO2 Reduction for Methanol Fuel[END_REF]. Copyright (2021) American Chemical Society.

Metal oxide/mesoporous oxides

Many metal-functionalized mesoporous aluminas and silicas have been studied as catalysts for CO2 hydrogenation to CO via RWGS or CO methanation by Fischer Tropsch (FT) reactions. In terms of activity and selectivity, mesoporous materials with high Brunauer-Emett-Teller (BET) surface areas are important features for CO2 adsorption and conversion [START_REF] Petrovic | Influence of surface modification on selective CO2 adsorption: A technical review on mechanisms and methods[END_REF]. The nature of metal species immobilized on porous oxides also influences the CO2RR selectivity for either CO or CH4. Deposition of Ni on CaO-Al2O3 showed CO2 conversion with 81% selectivity to CH4; however, lower performance of the Ni-based catalyst in the subsequent methanation reaction (due to partial oxidation of Ni fractions) suggests that a better approach for CO2RR on metal/mesoporous materials is required [START_REF] Mutz | Methanation of CO2: Structural response of a Ni-based catalyst under fluctuating reaction conditions unraveled by operando spectroscopy[END_REF].

Addition of La as a second metal to Ni/-Al2O3, enhanced direct CO2 methanation (CO2 + H2 → CO + H2O) via the Sabatier reaction, increasing the CO2 conversion rate to CH4 from 42% to 80.3% [START_REF] Rahmani | Preparation of Ni-M (M: La, Co, Ce, and Fe) catalysts supported on mesoporous nanocrystalline γ-Al2O3 for CO2 methanation[END_REF]. CO2 methanation over mono and bimetallic catalysts was temperature, feedstock (H2/CO2 ratio), and gas hourly space velocity (GHSV) sensitive. CO2 was transformed into CO as a result of the RWGS reaction which occurred at higher GHSV values (between 6000-18,000 mL/(gcath)), 350 o C, and 3.5 H2/CO2 ratio, resulting in decreased CH4 selectivity. In contrast, Bashiri and coworkers demonstrated that lowering the GHSV from 2000 to 1250 nml/min over a Ni-promoted iron catalyst on HZSM5-Al2O3 (Fe-Ni/HZSM5-Al2O3) increased CO2 methanation significantly [START_REF] Bashiri | The catalytic performance of different promoted iron catalysts on combined supports Al2O3 for carbon dioxide hydrogenation[END_REF]. The results also show that adding Mesoporous siliceous materials are valuable supports because of their large number of active sites for supporting various metal ions to catalyze CO2RR to renewable feedstocks. Single Cu-based catalytic systems are not suitable for direct CO2 hydrogenation to either CH4 or CH3OH because they lead preferentially to the formation of CO via RWGS reaction. For example, CuNi/fumed-SiO2 showed enhanced activity and selectivity for CO, as well as a tendency for methanation to CH4 via interaction of CO and neighboring H, whereas Cu/fumed-SiO2 showed only selectivity to CO due to decomposition of adsorbed formate intermediates to CO, Figure 7a [START_REF] Kumar | Effect of nickel on combustion synthesized copper/fumed-SiO2 catalyst for selective reduction of CO2 to CO[END_REF]. The incorporation of Ni increases the system's catalytic active sites and lowers the activation barriers for CO2RR to CO and CH4, Figure 7b.

Mesoporous Cu/SiO2 doped with Ga and/or Zn had greater catalytic activity for direct CO2 reduction to MeOH than undoped Cu/SiO2 [START_REF] Paris | CO2 Hydrogenation to Methanol with Ga-and Zn-Doped Mesoporous Cu/SiO2 Catalysts Prepared by the Aerosol-Assisted Sol-Gel Process*[END_REF].

The strong promoting effects of dopants restricted the occurrence of RWGS reaction for CO formation while increasing selectivity for MeOH one. Both dopants provide more active sites on the catalysts, lowering the activation barrier of MeOH synthesis compared to CO formation, so methanol formation was preferred over CO formation.

When mesoporous TiO2/SiO2 was irradiated with visible light without Cu species, CO was the main product of photo-transformation of CO2/H2O [START_REF] Li | Photocatalytic reduction of CO2 with H2O on mesoporous silica supported Cu/TiO2 catalysts[END_REF]. Cu deposition on TiO2/SiO2 improved photoreduction of CO2 to CO, and CH4 selectivity increased as well, peaking at 0.5wt% optimum Cu specie on TiO2/SiO2 (Figure 8). XPS allowed to identify the deposited Cu species as Cu2O, which acts as a photoexcited electron sink, limiting the recombination of photoinduced holes and excited electrons and thus enhancing multi-electron photoredox reactions for CO2 to CO and CH4. nanocomposites in photoreduction of CO2 [START_REF] Crake | Titanium dioxide/carbon nitride nanosheet nanocomposites for gas phase CO2 photoreduction under UV-visible irradiation[END_REF]. They found that TiO2/CNNS possessed improved CO2 conversion and adsorption when compared to the individual materials used to make the composites (> 10 times). In addition, it was reported that the composite material with more {001} TiO2 facets was the most catalytically active and this is attributed to enhanced charge transfer at the TiO2/CNNS heterojunction interface. Previously, Crake et al [START_REF] Crake | CO2 capture and photocatalytic reduction using bifunctional TiO2/MOF nanocomposites under UV-vis irradiation[END_REF]) coupled NH2-UiO-66 onto pre-formed TiO2 nanosheets via in situ process to develop TiO2/NH2-UiO-66 heterojunction with permanent pore sizes and exposed catalytic actives sites for enhance CO2 uptake and photoreduction of adsorbed CO2 to CO under UV-visible light. The evolution rate of CO2-to-CO conversion over the bound nanocomposite was 1.9 times more than that of pure TiO2. Interestingly, the photoreduction efficiency of mechanically blended TiO2 and NH2-UiO-66 is comparable to pure TiO2, proving that in situ growth method was necessary for the enhanced catalytic activity. Transient absorption spectroscopy confirmed that the increased photoactivity of TiO2/NH2-UiO-66 was due to the efficient charge transfer at the interface of heterojunction. Reduction efficiency of Fe-N-TiO2/CPO-Cu can be tuned by varying the amount of CPO-Cu-27 (CPO) in composites from 8 to 65wt.%, and the Fe-N-Ti@50CPO sample with 50wt% CPO yields the highest conversion of CO2 to methane. The performance achieved is ca. 1.7 times higher than in the case of CO2 methanation over Fe-NTiO2.

Liu et al. reported visible-light-driven CO2 to CO conversion on hollow transition

metal NPs-based MOFs [START_REF] Liu | Phase-enabled metal-organic framework homojunction for highly selective CO2 photoreduction[END_REF]. Their results show that Co-MOF-3 with two stacked piezoelectric phononic crystal nanoplates had a 2.5-fold higher CO2 photoreduction efficiency than Co-MOF-1 with single nanoplate ( Figure 9). The Co-MOF-3 homojunction system demonstrated a high CO2 to CO conversion rate with 100% selectivity, outperforming most of the previously reported semiconductors for this reaction.

Polyoxometalates-based MOFs (POMOF) composites have also been studied for CO2RR due to POM's unique ability to act as nodes or pillars, and more importantly, as high-efficiency catalytic sites in the MOF cages to adsorb reactants and improve MOF acidity, stability, and redox activity. Li et al. (Li et al., 2019) process. Reproduced with permission from ref. [START_REF] Liu | Phase-enabled metal-organic framework homojunction for highly selective CO2 photoreduction[END_REF]. Copyright 2021 Nature.

Metal-free nanomaterials

The high cost of production of metal-based nanomaterials, their susceptibility to deactivation in catalytic systems, environmental concerns about toxicity [START_REF] Fall | Removal of Toxic Chromium Ions in Aqueous Medium Using a New Sorbent Based on rGO @ CNT @ Fe 2 O 3[END_REF][START_REF] Keller | Comparative environmental fate and toxicity of copper nanomaterials[END_REF], and more often than not, their moderate catalytic performance in terms of activity and selectivity, have limited their use in CCUs.

Graphene oxides, fullerenes, and graphitic carbon nitrides have recently emerged as valuable metal-free platforms for CO2RR preparation, chemical inertness, and natural abundance (Yang et al., 2022). These materials are frequently used as a standalone material or in combination with other non-metal semiconducting materials to improve their CO2RR performance [START_REF] Shen | Photocatalytic Reduction of CO 2 by Metal-Free-Based Materials: Recent Advances and Future Perspective[END_REF]. Several reports have shown that GO can act by itself as a photocatalyst for CO2 reduction either by adapting its method of preparation or through UV-Vis light-induced activation of pristine GO [START_REF] Kuang | Photoactivated Graphene Oxide to Enhance Photocatalytic Reduction of CO2[END_REF]. Hsu et al. [START_REF] Hsu | Graphene oxide as a promising photocatalyst for CO2 to methanol conversion[END_REF] studied the effect of volume of H3PO4 used as a mild oxidizer to prevent overoxidation of pristine GO on the CO2RR performance.

Their results show that modified GOs have increased visible light absorption and improved CO2 photoreduction to MeOH compared to pristine GO. The CO2-to-MeOH conversion rate of GO-3 with excess H3PO4 was found 6-fold higher than with pure TiO2. [START_REF] Kuang | Photoactivated Graphene Oxide to Enhance Photocatalytic Reduction of CO2[END_REF] reported on the photoreduction of CO2 to CO over GO activated either by UV or simulated sunlight. GO activated by simulatedsunlight provided the highest CO2 reduction efficiency, with a CO yield of 1.3-fold higher than the UV-activated GO and nearly 3 times higher than with pristine GO. The observed efficiency is due to light-induced increased defects and restoration of large πconjugative systems. Compared to GO, reduced-GO (rGO) are superior solid supports for other semiconducting materials due to their high thermal and electrical conductivity.

Yang et al. supported quinacridone (QA) NPs on rGO nanosheets through H-bonding and with different rGO contents and observed that this metal-free photocatalysts allowed photoreduction of CO2 to CO and CH4 under UV light [START_REF] Yang | Photocatalytic Reduction of Carbon Dioxide over Quinacridone Nanoparticles Supported on Reduced Graphene Oxide[END_REF]. The sample containing 2% rGO had the highest CO2 reduction efficiency and selectivity for CO and CH4 due to the synergistic effects of QA as a photosensitizer and rGO as an electron acceptor to promote effective charge separation.

Based on the number of published articles on photocatalysis, graphitic carbon nitride (gCN) materials is the highest promising among the reported carbon-based materials for photoreduction of CO2, due to its facile tunable synthetic routes, structural maneuverability, and tailorable morphology. However, they are limited due to their narrow visible light absorption window, low specific surface area, poor electrical conductivity, and rapid recombination of photoexcited excitons [START_REF] Shi | Photoassisted Construction of Holey Defective g-C3N4 Photocatalysts for Efficient Visible-Light-Driven H2O2 Production[END_REF] [START_REF] Huang | Efficient photocatalytic reduction of CO2 by amine-functionalized g-C 3 N 4[END_REF] successfully photoreduced CO2 to CH4 and MeOH over bulk g-C3N4 and aminefunctionalized g-C3N4, which was prepared by heating g-C3N4 in monoethanolamine (MEA) solution. Both pure and functionalized g-C3N4 have a 2D lamellar structure with equal porosity, indicating that the amine functionalization has no effect on g-C3N4 morphology. The functionalized g-C3N4 heated in an oil bath for 5 hours had the highest CO2 to fuel conversion rate. The CO2 photoreduction efficiency over aminefunctionalized g-C3N4 was significantly higher than pristine g-C3N4 due to the enhanced adsorption and destabilization of CO2 by amine functional groups. According to this study, m CD/CN photoreduces CO2 to 6e -product MeOH (Figure 11a), while s CD/CN favors formation of 2e -product CO (Figure 11b). The authors attributed the improved m CD/CN reduction efficiency to the hole-accepting role of m CD, which led to electron accumulation on CN surface and facilitated a multi-electron reduction process to produce MeOH with near-unity selectivity (Figure 11c). Apart from all the aforementioned examples, this section also surveyed other recent/relevant research works on photocatalytic transformation of CO2 (see Table S1 in the supplementary information)

Electrochemical transformation of CO2

The electrochemical transformation of CO2 to value added fuels has received considerable attention in recent years owing to its plethora benefits including energy production, storage, and others. The conversion process is located at the interface between the solid catalyst surface and the electrolyte, as is the case with all electrochemical reactions. A compact layer of ions next to the electrode, followed by a diffusion layer of solvated ions, can be used to describe the interface's structure [START_REF] Rosen | Tunable Molecular-Scale Materials for Catalyzing the Low-Overpotential Electrochemical Conversion of CO2[END_REF]). The more the potential an electroactive species has, the more the distance it can travel to the interface, and its relative orientation to the interface are all influenced by the bilayer structure which is thus essential for the rate and efficiency of the electrochemical CO2RR. Electrochemical CO2RR can produce useful chemicals and fuels including formaldehyde, CO, formic acid, ethylene, ethanol, methane, and methanol, depending on the electrical voltage supplied to the electrode (Chen et al., 2020).

The methods most frequently used for electroreduction of CO2 are cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The catalytic performance can also be evaluated using chronoamperometry. These CO2 conversion operations are typically carried out at a specific reaction rate and ambient conditions of temperature and pressure [START_REF] Nielsen | Chemically and electrochemically catalysed conversion of CO2 to CO with follow-up utilization to value-added chemicals[END_REF]. A coupled proton electron transfer that results in the protonation of the oxygen atom O (eq 1) or the carbon atom C (eq 2) triggers the adsorption of CO2 on the electrode surface (eq 4) [START_REF] Nielsen | Chemically and electrochemically catalysed conversion of CO2 to CO with follow-up utilization to value-added chemicals[END_REF][START_REF] Zhu | Active and selective conversion of CO2 to CO on ultrathin Au nanowires[END_REF]. As such, this process requires the use of a large potential, which results in competing reactions such as the hydrogen evolution reaction (HER). In lieu of the above, integration of a catalyst in the system allows to reduce the overpotential and activate CO2 conversion. In general, a heterogeneous catalyst deposited on the electrode surface or a homogeneous catalyst dissolved in solution are used in electrocatalytic CO2 transformation. These catalysts will allow for a faster electron transfer coupled with protons, resulting in the observed products HCOOH (eq 2 and eq

3) or CO (eq 5 and eq 6) [START_REF] Yin | Electrochemical Reduction of CO2 Catalyzed by Metal Nanocatalysts[END_REF]. In contrast, the conversion of CO2 to CO and formate is less difficult than CO2RR to C-H hydrocarbons, which are more significant scientifically and technologically but are more difficult to manage. Concerning the synthesis of ethylene, Cu and Cu2O are regarded as the most common and effective electrocatalysts for CO2RR. Multi-carbon oxygenates, such as ethanol (the primary product), acetate, and n-propanol, can also be produced in an alkaline electrolyte at -0.25 to -0.5 V vs RHE with a total EF of 57 % [START_REF] Li | Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper[END_REF].

* + CO2 + H + + e -→
In order to achieve a lofty ultra-high selectivity of 82.4% for ethylene in an H-cell system with good catalytic stability and material durability of 100 h, Sultan et al. This lacks of product selectivity is a significant barrier to the use of these catalysts for selective CO2RR, and more research on the application of these substitute catalysts is required for increased hydrocarbon yield.

In a related piece of study, Liu and coworkers noted the formation of exceptional oxygen-rich ultrathin CuO nanoplate arrays that spontaneously transform into Cu/Cu2O heterogeneous surfaces after CO2RR (Liu et al., 2022) (Wang et al., 2020). DFT studies showed that alloying Cu with Bi lowers the energy required for the rate-determining phase of * COH production, which provides an additional explanation for the enhanced performance.

In addition to the aforementioned systems, other metal-based nanomaterials have been investigated towards the production of hydrocarbon via the electrochemical reduction of CO2, which are outlined in Table 1 below. reduction occurs during the synthesis of ethanol and acetate at 0.5 0.6 V (vs. RHE), with ethanol accounting for 51.6% -56.0% of the total reduction. Due to the combined effects of sp 3 /sp 2 carbon and B/N doping, there is a strong ethanol selectivity.

To perform electrocatalytic CO2 reduction to liquid ethanol and acetone, Yuan and colleagues presented their findings. Yuan and colleagues used five pyridine derivatives, including pyridoxine, 4-hydroxypyridine, 4-aminopyridine, 8-hydroxyquionline, and 5amino-1,10-phenanthroline, to functionalize graphene oxide (GRO) surfaces [START_REF] Yuan | Electrochemical reduction of CO2 at metal-free N-functionalized graphene oxide electrodes[END_REF]. Surprisingly, the greatest catalyst for electrochemical CO2 conversion is pyridoxine-modified GRO sheets with a pyridinic N concentration of 2.32% and an overall FE of 45.8%. Furthermore, the CO2 reduction potentials of the other four pyridine derivatives functionalized GO varied with respect to ethanol and acetone. Li and colleagues described non-metal nanoporous S-doped and S,N-codoped carbons as catalysts for electrochemical transformation CO2 [START_REF] Li | Metal-free Nanoporous Carbon as a Catalyst for Electrochemical Reduction of CO2 to CO and CH4[END_REF]. The team found that S,N-doped carbon has greater FE for conversion to CO and CH4 compared to S-doped counterpart. The former revealed a maximum FE of 11.3% and 0.18%. for CO and CH4 production, respectively. The S,N-nanoporous carbon was better at decreasing the overpotential of the reduction process.

Two-dimensional (2D) boron nitride nanosheets or meshes (BNs) doped with beryllium have been the subject of research by Azofra and coworkerss [START_REF] Azofra | An intensified π-hole in beryllium-doped boron nitride meshes: Its determinant role in CO2 conversion into hydrocarbon fuels[END_REF].

According to their mechanistic study, adding electron-deficient Be atoms to the 2D network significantly lowered the energy barriers for CO2 fixation and the initial steps in hydrogenation. As a result of the exceptionally large hole created on the Be-doped surface environment, CO2 was spontaneously absorbed (in terms of the Gibbs free binding energy).

Despite all the accomplishments in CO2RR, it is worth to mention that high progress is still needed in the design of catalysts based on metal and non-metal for achieving a more performant CO2 electrocatalytic transformation. The main challenge being the energy requirement for reducing CO2 (due to its stability), an electrochemical potential

[E°] of 1.9 V vs normal hydrogen electrode (NHE) at pH. Furthermore, due to the high reactivity of the one-electron reduction product (CO2 •-), regulating product selectivity is difficult. When a multi-electron process is combined with proton-coupled electron transfer, the potential for CO2 reduction is greatly reduced, resulting in a variety of products depending on the number of electrons involved [START_REF] Maeda | Metal-Complex/Semiconductor Hybrid Photocatalysts and Photoelectrodes for CO2 Reduction Driven by Visible Light[END_REF]. Finally, the quantum yield and current density reported thus far are extremely low, particularly for C2 + products, making commercial production extremely difficult. As a result, extensive and intensive research is required to overcome all of the aforementioned challenges.

Photo-electrochemical (PEC) reduction of CO2

Photoelectrochemical (PEC) conversion of CO2 can be described as a procedure mimicking the artificial photosynthesis technique. This method has been employed for the conversion of CO2 to formate, formaldehyde, formic acid, methane, methanol, and ethanol [START_REF] Kumaravel | Photoelectrochemical conversion of carbon dioxide (CO2) into fuels and value-added products[END_REF].This technique combines electrocatalytic and photocatalytic systems in which photogenerated electrons are transferred to the electrode surface under an applied electric field, thus reducing CO2 in the process [START_REF] Nandal | CO2 to ethanol: A selective photoelectrochemical conversion using a ternary composite consisting of graphene oxide/copper oxide and a copper-based metal-organic framework[END_REF].

During the 80s, Fisher et al. [START_REF] Fisher | Electrocatalytic reduction of carbon dioxide by using macrocycles of nickel and cobalt[END_REF]and Inoue et al. [START_REF] Inoue | Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders[END_REF] studied the electrocatalytic reduction reactions that gave rise to studies of photoelectrocatalytic reduction of CO2 on selected semiconductors. However, an extensive study using various metals and metal oxides requires large over potentials and suffers from hydrogen evolution [START_REF] Benson | Electrocatalytic and homogeneous approaches to conversion of CO2 to liquid fuels[END_REF][START_REF] Qiao | A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels[END_REF][START_REF] Shen | High-Yield and Selective Photoelectrocatalytic Reduction of CO2 to Formate by Metallic Copper Decorated Co3O4 Nanotube Arrays[END_REF].

Since then, it has become a promising technique for CO2 conversion. In 2020, Arai et al. [START_REF] Arai | Photoelectrochemical reduction of CO 2 in water under visible-light irradiation by a p-type InP photocathode modified with an electropolymerized ruthenium complex[END_REF] reported the PEC conversion of CO2 into HCOOH using a p-InP-Zn photocathode which has been modified with a Ru polymer. The conversion was achieved in the presence of visible light/ solar energy. Similarly, Kumaravel and his coleagues [START_REF] Kumaravel | Photoelectrochemical conversion of carbon dioxide (CO2) into fuels and value-added products[END_REF]) also synthesized a range of fuels and chemicals (Formic acid, methanol, ethanol, methane and ethane) from CO2. They employed a catalyst-embedded microfluidic PEC reactor for the synthesis. The catalyst was activated in the presence of visible light irradiation. The schematic of the conversion is presented in Figure 13 Figure 13: PEC Conversion of CO2 into value-added product using solar irradiation.

Adapted with permission from reference [START_REF] Kumaravel | Photoelectrochemical conversion of carbon dioxide (CO2) into fuels and value-added products[END_REF]. Copyright [2020],

[American Chemical Society].

According to a recent study by [START_REF] Karim | Photoelectrochemical reduction of carbon dioxide to methanol on p-type CuFe2O4 under visible light irradiation[END_REF], single electron reduction of CO2 to • CO2 occurs at -1.90 V vs NHE at pH = 7 in aqueous solution at 25 ο C under 1 atm pressure. The 2e-pathway of CO2 reduction produces CO (E = -0.53 V vs NHE) and formate (E = -0.61 V vs NHE) while the 6e-pathway leads to the production of methanol (E= -0.38 V vs NHE) [START_REF] Qiao | A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels[END_REF][START_REF] Karim | Photoelectrochemical reduction of carbon dioxide to methanol on p-type CuFe2O4 under visible light irradiation[END_REF]. The band varies depending on the type of metal coordination as shown in 2 depending on the bandgap of the photocatalysts [START_REF] Ekambaram | Combustion synthesis and photocatalytic properties of transition metal-incorporated ZnO[END_REF]. Metal oxides present desirable bandgap and bandgap edge positions, and their electronic structure plays a major role where the VB is fully filled by electrons and the CB is empty. Therefore, the integration of electrocatalysis and photocatalysis generates a unique synergistic photo-electrochemical catalysis system [START_REF] Huang | Synthesis of GOmodified Cu2O nanosphere and the photocatalytic mechanism of water splitting for hydrogen production[END_REF][START_REF] Karim | Photoelectrochemical reduction of carbon dioxide to methanol on p-type CuFe2O4 under visible light irradiation[END_REF]. The catalytic activity of a photocatalyst towards CO2 reduction depends on its band gap and appropriate positioning of the conduction and valence bands [START_REF] Handoko | Controllable proton and CO2 photoreduction over Cu2O with various morphologies[END_REF]. Doping semiconductors have extensively been explored in the literature to enable visible-light-activated photocatalysis and the usage efficiency of solar light. Several published studies have used titanium for photocatalytic applications for organic compound decompositions due to its ability to oxidize organic and inorganic substances in water and air through a redox reaction [START_REF] Xie | Enhanced photocatalytic activity of Se-doped TiO2 under visible light irradiation[END_REF]. TiO2

and ZnO absorb ultraviolet light of the solar spectrum due to their large bandgap's energy (~380 nm and ~360 nm respectively) [START_REF] Liu | ZnO-based ultraviolet photodetectors[END_REF]. The bandgap (the distance between the valence band and conduction band) of each semiconductor determines are shown in Figure 14, it shows the relationship between the bandgap and EMF at pH 7. Several alternative semiconductors such as Fe2O3, ZnO, WO3, SrTiO3, NaTaO3, CdS, Ag3PO4, BiPO4 and g-C3N4 have been reported to have promising electro-photocatalytic performance [START_REF] Kubacka | Advanced Nanoarchitectures for Solar Photocatalytic Applications[END_REF]L. White et al., 2015;[START_REF] Liu | ZnO-based ultraviolet photodetectors[END_REF][START_REF] Wang | A metal-free polymeric photocatalyst for hydrogen production from water under visible light[END_REF]. They reported an ethanol yield of 162 µM cm -1 . Most researchers believe that the PEC reduction technique is not only efficient, but it is also greener and sustainable. Other PEC for CO2 reduction surveyed in this section are listed in the supplementary file (Table S2)

Figure 15: PEC reduction of CO2 to ethanol. Adapted with permission from reference [START_REF] Nandal | CO2 to ethanol: A selective photoelectrochemical conversion using a ternary composite consisting of graphene oxide/copper oxide and a copper-based metal-organic framework[END_REF]. Copyright [2022], [Elsevier].

Photothermal heterogeneous catalysts for the hydrogenation of CO2

The hydrogenation of CO2 is gaining significant impetus in scientific investigations considering the large amount of CO2 generated by the industrial processes and the associated environmental degradation. It facilitates the production of clean fuels and hydrocarbons such as CO, CH4, and C2 + products [START_REF] Fan | Recent developments in photothermal reactors with understanding on the role of light/heat for CO2 hydrogenation to fuels: A review[END_REF]. The hydrogenation reaction can proceed via thermal catalysis, photocatalysis, and photothermal catalysis. While thermal and photocatalytic processes require the action of heat and light energies respectively, the photothermal catalytic approach combines both in a single reaction system. In photothermal catalytic systems, the CO2 transformation reaction is driven by light energy into a thermally inspired catalytic reaction system or utilizing the photoinduced thermal effect of a catalyst. The light energy initiates a photoinduced redox reaction and a photoinduced thermally driven reaction [START_REF] Oladoye | Remediation potentials of composite metal-organic frameworks (MOFs) for dyes as water contaminants: A comprehensive review of recent literatures[END_REF]. For instance, the exposure of a transition metal oxidebased catalyst to a sufficient amount of light energy causes an excitation of photons that produces electron-hole pairs in the conduction band (CB) and valence band (VB) (Figure 16). These highly energetic electrons enter the anti-bonding orbitals of the reactants and activate the reactant molecules to stimulate a redox reaction. In other instances, the absorbed photon could be converted into heat energy (Liu et al., 2022).

At an activated temperature, the active sites on the surface of the photothermal catalytic material could induce a thermally driven catalytic reaction. Several semiconducting metal oxides with tunable porosity and defective active sites have been designed for the hydrogenation of CO2 into different hydrocarbon-based fuels (Table 3).

Thermochemical transformation of CO2

The thermochemical transformation of CO2 is an interesting strategy that has been gainfully explored for the reduction, splitting, or reforming of CO2 to fuels/products such as carbon monoxide, methanol, methane, and dimethyl ether in the presence of suitable catalysts [START_REF] Fuqiang | Energy storage efficiency analyses of CO2 reforming of methane in metal foam solar thermochemical reactor[END_REF][START_REF] Roy | Thermochemical CO2 Hydrogenation to Single Carbon Products: Scientific and Technological Challenges[END_REF]. Although, there are many high-temperature thermochemical energy conversion processes for energy storage, energy conversion and water splitting reactions [START_REF] Ali | Thermochemical and electrochemical aspects of carbon dioxide methanation: A sustainable approach to generate fuel via waste to energy theme[END_REF][START_REF] Cheng | The levelized cost of negative CO2 emissions from thermochemical conversion of biomass coupled with carbon capture and storage[END_REF][START_REF] Lorentzou | Thermochemical CO2 and CO2/H2O splitting over NiFe2O4 for solar fuels synthesis[END_REF]Wu et al., 2018), the solar-induced thermochemical cyclic method is been emphasized as an outstanding approach for the efficient conversion of CO2 (Table 4). This approach has produced intriguing results from several the experimental tests/designs conducted in different solar reactor models, which has given rise to more intensified research and technology development in this field [START_REF] Guene Lougou | Thermochemical CO2 reduction over NiFe2O4@alumina filled reactor heated by highflux solar simulator[END_REF].

Many studies have shown that the solar thermochemical CO2-transformation is a promising solution for solar energy harvesting and storage using materials like Fe-Ni alloy embedded in a perovskite [START_REF] Hu | Intensified solar thermochemical CO2 splitting over iron-based redox materials via perovskite-mediated dealloying-exsolution cycles[END_REF], Ca-and Al-doped nSmMnO3 perovskite [START_REF] Gao | Direct solar thermochemical CO2 splitting based on Ca-and Al-doped SmMnO3 perovskites: Ultrahigh CO yield within small temperature swing[END_REF], and ZrO2-supported NiFe2O4 [START_REF] Shuai | Solar-driven thermochemical redox cycles of ZrO2 supported NiFe2O4 for CO2 reduction into chemical energy[END_REF], to convert CO2 into CO with a 99% conversion rate of 381 mL g -1 min -1 at 850°C, high conversion of 595.56 mmol/g, and 37.89 mL of CO yield in 46 min, respectively. Meanwhile, Ni-doped CaCO3 transformed 86.6% of CO2 into syngas along with CH4 in a fixed bed photo-reactor [START_REF] Teng | Direct solar-driven reduction of greenhouse gases into hydrocarbon fuels incorporating thermochemical energy storage via modified calcium looping[END_REF]. The solar thermochemical process follows a two-step dissociation approach wherein the first step is an endothermic process that involves the production of O2 by partially reducing a perovskite or metal oxide (MO) to a nonstoichiometric oxide with the assistance of high-temperature solar heat (Eq. 7). The second step usually occurs at low temperature via an exothermic process. At this stage, the reduced perovskite or MO is oxidized with H2O and/or CO2 giving rise to the formation of H2 and/or CO (Eq. 8-9), together with the stoichiometric oxide [START_REF] Chen | Platinum Group Metal Catalyst (RuO x , PtO x , and IrO x )-Decorated Ceria-Zirconia Solid Solution as High Active Oxygen Carriers for Solar Thermochemical CO2 Splitting[END_REF][START_REF] Nair | Experimental screening of perovskite oxides as efficient redox materials for solar thermochemical CO 2 conversion[END_REF].

MO + solar heat → MOred + O2 (7)

MOred + CO2 → MOox + CO (8) CO2 + solar heat → CO + ½O2 (9)
In the same vein, the photo-thermochemical (PTC) model produces photoinduced oxygen vacancies on the surface of the MO catalyst after exposure to UV-light energy (Eq. 10). Further, the oxygen vacancies are used to reduce CO2 at temperatures below 873 K, thereby producing CO and O2 (Eq. 5) [START_REF] Xu | Guiding effective nanostructure design for photo-thermochemical CO2 conversion: From DFT calculations to experimental verifications[END_REF][START_REF] Xu | Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction[END_REF].

2MxOy → 2MxOy-1 + O2 (Photochemical step) (10)
2MxOy-1 + CO2 (g) → MxOy + CO (Thermochemical step) (11)

By exploring the Fisher Tropsh (F-T) technology, solar-assisted MOs can also afford carbon-free alternative fuels i.e. production of H2 that can be used directly as a fuel or converted into other value-added chemicals (Figure 17) (G. Takalkar and Bhosale, 2019). Besides, other heterogeneous catalysts that change dynamically during the thermochemical reactions as well as possessing catalytic active sites and definite reaction pathways have been identified for CO2 transformation [START_REF] Feng | Insitu/operando techniques to identify active sites for thermochemical conversion of CO2 over heterogeneous catalysts[END_REF]. For example, the loading of IrOx onto LaFeO3 almost double the maximal CO release rate and increases the maximal O2 evolution rate 0.5 times i.e. a 5-fold CO production (Jiang et al., 2016); meanwhile, the sol-gel synthesized CoxFe3-xO achieved higher CO generation rate via CO2 splitting (Takalkar and Bhosale, 2019). rate of 40 sccm

Non-thermal plasma (NTP) catalysis

Non-thermal plasma (NTP) catalysis is an auspicious technology that is gaining significance in C1 chemistry. It has been postulated as an effective alternative to conventional thermal catalysis due to the unique interplay between the plasma and heterogeneous catalyst. Thus, this approach could overcome the kinetic limitation of conventional thermal catalysis without the need for any external heat sources [START_REF] Parastaev | Temperature-programmed plasma surface reaction: An approach to determine plasmacatalytic performance[END_REF]. Typically, this process employs an electric discharge of partially ionized gas made up of electrons possessing average electron energy in the range of 1-10 eV (Chen et al., 2020c). These electrons have the capacity to activate stable C1 molecules as well as denature their bonding structures in the gaseous phase under moderate conditions, thereby producing atoms, ions, and vibrationally-excited reactive species [START_REF] Mehta | Catalysis Enabled by Plasma Activation of Strong Chemical Bonds: A Review[END_REF]. The NTP catalytic approach has the potential of accelerating many energy-intensive and kinetically and/or thermodynamically limited heterogeneous catalytic reactions such as CO2 hydrogenation [START_REF] Song | Synergetic Effect of Nonthermal Plasma and Supported Cobalt Catalyst in Plasma-Enhanced Co2 Hydrogenation[END_REF], water-gas shift reaction [START_REF] Xu | Sustaining metal-organic frameworks for water-gas shift catalysis by non-thermal plasma[END_REF], hydrocarbon reformation (Wu et al., 2018) and CO2 splitting [START_REF] Chen | Coupling non-thermal plasma with Ni catalysts supported on BETA zeolite for catalytic CO2 methanation[END_REF], amongst others.

In Chen et al. (Chen et al., 2020a) Ni NPs supported on silicalite-1 zeolites with suitable active sites and pore structures were comparably investigated in CO2 hydrogenation under the thermal and NTP conditions. The comparative investigation showed that the hierarchical mesomicroporous structures and the associated well-dispersion of Ni species were highly beneficial to the catalysis under the NTP conditions, especially at relatively high voltages. Using a dielectric barrier discharge (DBD) reactor at 150°C, Zeng and Tu [START_REF] Zeng | Plasma-catalytic hydrogenation of CO2 for the cogeneration of CO and CH4 in a dielectric barrier discharge reactor: Effect of argon addition[END_REF] showed that the presence of Ni in the reactor and injection of argon (up to 60%) in the reaction promotes the conversion of CO2 to CO and CH4 (selectively, 85%) and the energy efficiency of the plasma process. In the reactor, Ar decelerates the breakdown voltage of the feed gas and accelerates charge transfer through the reactor. Furthermore, the NTP-activated hydrogenation of CO2 in the presence of Ru/MgAl layered double hydroxide catalysts (having low activation energy) provided ∼84 % of CH4 yield at relatively low temperatures [START_REF] Xu | Mechanistic study of non-thermal plasma assisted CO2 hydrogenation over Ru supported on MgAl layered double hydroxide[END_REF]. In another approach, a MOF-supported Ni catalyst (15Ni/UiO-66) presented 99% selectively towards the conversion of CO2 to CH4 under NTP conditions and the catalyst was still stable after 20h of reaction (H. Chen et al., 2020b). The experimental investigations of Nizio et al. [START_REF] Nizio | Low temperature hybrid plasma-catalytic methanation over Ni-Ce-Zr hydrotalcite-derived catalysts[END_REF] revealed that ceria and zirconia-promoted Ni-containing hydrotalcite-derived catalysts could yield high CH4 under NTP conditions and temperatures < 330 °C.

Fuels generated from CO2 conversion

The discovery and development of efficient technologies for the catalytic transformation of CO2 is an exciting area of research [START_REF] Ramírez-Valencia | From CO2 to value-added products: a review about carbon-based materials for electrochemical CO2 conversion[END_REF] as, in parallele of lowering the atmospheric CO2, this would allow the access to essential chemicals and fuels such as formic acid (HCOOH), methanol (CH3OH), carbon monoxide (CO), ethanol (CH3CH2OH) and methane (CH4), among others, that are presently produced from fossil resources. Figure 18 describes the potential products that could be derived from CO2 conversion.

Formic acid

Among the myriad of products accessible from CO2, formic acid is a valuable intermediate chemical and also used in the domain of energy for reversible hydrogen storage (Figure 19) [START_REF] Moret | Direct synthesis of formic acid from carbon dioxide by hydrogenation in acidic media[END_REF]. Formic acid is of great importance in textile industries, pharmaceuticals and food companies dues to its acidic nature and reducing properties [START_REF] Afshar | Chemical profile: formic acid[END_REF]. In particular, the leather and tanning industries are one of the largest global consumers of formic acid [START_REF] Pérez-Fortes | Formic acid synthesis using CO2 as raw material: Techno-economic and environmental evaluation and market potential[END_REF]. Other critical applications of formic acid include preservatives and antibacterial agents in animal feeds [START_REF] Anadón | Probiotics for animal nutrition in the European Union. Regulation and safety assessment[END_REF]. Therefore, formic acid is a highly valued product with little or no alternative.

Figure 19: Reversible hydrogenation of CO2 to formic acid [START_REF] Li | Properties of hydrogen storage alloy Mg~ 2Ni produced by hydriding combustion synthesis[END_REF] Several synthetic methods for the conversion of CO2 to formic acid have been explored. This includes biochemical, photochemical, electrochemical, microwave-assisted, photosynthetic and catalytic reduction (Asiri and Lichtfouse, 2020;[START_REF] Hu | Thermal, electrochemical, and photochemical conversion of CO2 to fuels and value-added products[END_REF]Lu et al., 2014;[START_REF] Scibioh | Electrochemical reduction of carbon dioxide: a status report[END_REF][START_REF] Yadav | A photocatalyst-enzyme coupled artificial photosynthesis system for solar energy in production of formic acid from CO2[END_REF]. Catalytic conversion of CO2 to formic acid was first reported by Inoue [START_REF] Inoue | Catalytic fixation of carbon dioxide to formic acid by transition-metal complexes under mild conditions[END_REF] in 1976. Since then, CO2 has become an attractive starting molecule for the synthesis of formic acid. Compared to other methodologies, the catalytic reduction method offers higher selectivity to targeted product, thus minimizing the risk of unwanted CO as a byproduct that may poison the fuel. However, the selectivity of the preferred product depends on the reduction method and the nature of the catalyst used.

Weilhard et al. reported the synthesis of formic acid from CO2 feedstock using an ionic liquid as an alternative to expensive precious metal-based catalytic systems [START_REF] Weilhard | Efficient carbon dioxide hydrogenation to formic acid with buffering ionic liquids[END_REF]. As shown in Scheme 3, the synthetic strategy consists of a mixture of CO2 and H2 in 1:1 molar ratio.

The catalytic system consists of a homogeneous mixture of 1-butyl-2,3-dimethylimidazolium acetate (BMMI.OAc) and Scandium(III) trifluoromethanesulfonate (Sc(OTf)3. They reported a very high catalytic efficiency with turnover numbers (TON) and turnover frequency (TOF) of 8 x 10 5 and 2.1 x 10 4 h -1 , respectively. They reported that the catalytic system is highly sustainable than the existing routes. In addition, it is highly recyclable and gives a higher yield of the target product. al. [START_REF] Mubarak | Efficient photoelectrocatalytic conversion of CO2 to formic acid using Ag-TiO2 nanoparticles formed on the surface of nanoporous structured Ti foil[END_REF] published a greener approach via photocatalytic transformation of CO2 into formic acid. The catalytic procedure involves the breaking down of CO2 into valuable chemicals in the presence of sunlight and Ag-doped TiO2 NPs. This catalytic system iss highly selective with HCOOH as the single product, formic acid being obtained at a yield of 193 μM cm -2 h -1 .

Carbon monoxide

In the series of products derived from CO2, carbon monoxide (CO) is one of the most critical chemicals and feedstock. It is a primary raw material along with H2 gas for syngas production via Fischer-Tropsch synthesis to produce liquid fuels [START_REF] Zeng | Advanced Cu-Sn foam for selectively converting CO2 to CO in aqueous solution[END_REF]. It is also valuable for the metallurgical industries (Liu et al., 2017). Most of the efforts concerning CO2 reduction into CO efforts can be listed in two major categories, namely thermochemical reduction and electrochemical reduction [START_REF] Porosoff | Catalytic reduction of CO 2 by H 2 for synthesis of CO, methanol and hydrocarbons: challenges and opportunities[END_REF]. CO2 being a very stable molecule, it requires a very high temperature (> 1200 °C) to decompose to CO (Equation 13). Therefore, this procedure is not thermodynamically favorable and must be catalytically driven.

CO2(g) → CO(g) + 1 2 O2(g) ∆𝐻 = 279.8 𝑘𝐽/𝑚𝑜𝑙 (13)

Various catalytic materials have been employed to reduce the energy cost of the catalytic conversion of CO2 to CO. For example, Au nanocatalysts have shown tremendous potential in converting CO2 to CO with a remarkable selectivity [START_REF] Qi | Surface oxide-derived nanoporous gold catalysts for electrochemical CO2-to-CO reduction[END_REF]. Qi et al. [START_REF] Qi | Electrochemical CO2 to CO reduction at high current densities using a nanoporous gold catalyst[END_REF] reported a successful electrochemical conversion of CO2 to CO over Au nanocatalyst. Wen et al. [START_REF] Wen | A reassembled nanoporous gold leaf electrocatalyst for efficient CO2 reduction towards CO[END_REF] used a nanoporous Au leave electrocatalyst prepared by a de-alloying technique to reduce CO2 to CO, achieving a very high catalytic efficiency to CO (90% FE).

While gold-based catalysts are the most successful catalysts for the conversion of CO2 to CO, the high cost of Au may prevent large-scale industrial applications (Ma et al., 2016b) which requires A challenge often characterized by this synthetic procedure is that it is performed at very high temperature and pressure [START_REF] Bertau | Methanol: the basic chemical and energy feedstock of the future[END_REF]. In addition, it also suffers from poor selectivity and low yield [START_REF] Jadhav | Catalytic carbon dioxide hydrogenation to methanol: A review of recent studies[END_REF].

To improve this procedure, various homogeneous and heterogeneous catalytic hydrogenation reactions as well as thermocatalysis, photocatalysis, electrocatalysis, and enzymatic catalysis have been explored with promising results [START_REF] Navarro-Jaén | Highlights and challenges in the selective reduction of carbon dioxide to methanol[END_REF].

CO2 + 3H2 ⇌ CH3OH + H2O → ΔH = -40.9 kJ mol -1 ( 14)

CO2 + H2 ⇌ CO + H2O → ΔH = +49.8 kJ mol -1 (15) CO + 2H2 ⇌ CH3OH → ΔH = -90.7 kJ mol -1 (16)
Conventionally, methanol production from syngas is usually catalyzed by a Cu-based heterogeneous catalyst at 513-533K temperature and 50-100 bar pressure [START_REF] Bart | Copper-zinc oxide-alumina methanol catalysts revisited[END_REF]. It is worth mentioning that Cu alone exhibits a very low activity; to enhance the activity, the Cu catalyst is usually tethered on support or promoters such as Al2O3, ZnO, and ZrO2. In most cases, the synergy between the Cu catalyst and the promoters only achieves a conversion of approximately 30% due to a fast deactivation of the Cu catalyst [START_REF] Navarro-Jaén | Highlights and challenges in the selective reduction of carbon dioxide to methanol[END_REF].

Although the activity is low, Cu-based catalysts are very cheap thus justifying their predominance in methanol synthesis. However, the growing demand for methanol induced emergence of new ideas for converting CO2 to methanol.

For instance, Pd-based heterogeneous catalytic systems have been used to improve the synthesis of methanol. [START_REF] Díez-Ramírez | CO2 hydrogenation to methanol at atmospheric pressure: influence of the preparation method of Pd/ZnO catalysts[END_REF] reported the application of Pd/ZnO catalyst to convert CO2 to methanol under atmospheric conditions, observing a low conversion rate due to the side reactions from coke formation and water from the (RWGS) reaction (eq 15).

Various attempts have been made to suppress side reactions from the (RWGS) reaction.

Fiordaliso et al. [START_REF] Fiordaliso | Intermetallic GaPd2 nanoparticles on SiO2 for low-pressure CO2 hydrogenation to methanol: Catalytic performance and in situ characterization[END_REF] reported the use of supported Pd-Ga bimetallic catalysts, specifically, GaPd2/SiO2 for converting CO2 to methanol, at low CO2 pressure, and with a MeOH production rate of 0.12 mol%. Although the catalytic activity of this GaPd2/SiO2 was better than in previous studies, the major drawback is the Pd cost, thus limiting practical applications.

Ahmad et al. [START_REF] Ahmad | Conversion of the greenhouse gas CO 2 to methanol over supported intermetallic Ga-Ni catalysts at atmospheric pressure: thermodynamic modeling and experimental study[END_REF], developed an efficient, cheaper bimetallic catalyst Ga3Ni5 for CO2 conversion and compared their results with those of Fiordaliso et al. [START_REF] Fiordaliso | Intermetallic GaPd2 nanoparticles on SiO2 for low-pressure CO2 hydrogenation to methanol: Catalytic performance and in situ characterization[END_REF]. The activity of the Ga3Ni5 catalyst was found to be higher under the same experimental conditions, showing a MeOH conversion rate of 38.85 µmol gcat -1 min 1 coupled to an impressive selectivity of 94 %.

While Ga, Zn, Cu, Al, Pd, Ni, Pt-based catalysts have been widely tested for MeOH synthesis,

To motivate a cost-effective catalyst, scientists are considering an alternative to Pt and noble metals. For instance, CeO2-based catalyst is getting attention due to its abundant oxygen and reversible valence charges (Ce 4+ and Ce 3+ ) (F. Wang et al., 2016). This potential was demonstrated by [START_REF] Chang | Effect of the Zn/Ce ratio in Cu/ZnO-CeO 2 catalysts on CO 2 hydrogenation for methanol synthesis[END_REF], who applied a CeO2-based catalyst Cu/ZnO-CeO2 to convert CO2 to methanol.

Perovskites have been also identified as low-cost approach for CO2 conversion to methanol. This is described by [START_REF] Zhan | Influence of element doping on La-Mn-Cu-O based perovskite precursors for methanol synthesis from CO 2/H 2[END_REF], who employed a perovskite-based catalyst to produce methanol from CO2. They reported an excellent catalytic efficiency with impressive stability of the prepared catalyst.

Apart from heterogeneous-based catalytic systems, homogeneous catalysts have been also

reported for methanol synthesis from CO2. [START_REF] Kothandaraman | Conversion of CO2 from air into methanol using a polyamine and a homogeneous ruthenium catalyst[END_REF] developed a Ru-based homogeneous catalytic system that showed a very good yield (79 %) in methanol. In addition, this catalyst also shows excellent recyclability with over five runs without a significant loss in activity. [START_REF] Zhou | A theoretical study on the hydrogenation of CO 2 to methanol catalyzed by ruthenium pincer complexes[END_REF] published the use of a ruthenium pincer complex [RuH2(Me2PCH2SiMe2)2NH(CO)] which led to a good catalytic efficiency from the density functional theory (DFT) data.

Application of enzymes to reduce CO2 to methanol is also getting considerable attention. Zhang et al. (Z. Zhang et al., 2021) reported the enzymatic conversion of CO2 to methanol in the presence of nicotinamide adenine dinucleotide (NADH). The enzyme was immobilized on porous a MOF (ZIF-8) to improve CO2 concentration. The prepared NADH/ZIF-8 composite was placed in the bio-electrochemical cell. The methanol conversion rate was 822 μmol g-1 h-1, and the catalytic system was found stable and environmentally friendly.

Ethanol

Ethanol is a valuable chemical in our daily life. In contrast to methanol, ethanol has some distinct advantages, such as higher energy density, lower toxicity, and transportation safety (D. Wang et al., 2016). As a result of these advantages, it intervenes in a wide range of domestic and industrial applications, including chemical raw material for chemical synthesis, in disinfectants, pharmaceuticals and cosmetics, plasticizers, polishes and dyes (An et al., 2021). In addition to its domestic and industrial applications, ethanol is a necessary fuel and hydrogen carrier (Pang et al., 2019). The United States are the world largest ethanol producer, accounting for 59% of global production. This is followed by brazil with 28% while the rest of the world accounts for 13% (Figure 22). The high energy density and the compatibility of ethanol with existing and modern combustion engines make it a vital fuel. Ethanol is usually produced from the fermentation of starch and cellulose feedstock [START_REF] Ali | A review on CO2 hydrogenation to ethanol: Reaction mechanism and experimental studies[END_REF]Wang et al., 2021). With the population and increasing demand for food, the search for an alternative strategy for ethanol synthesis is highly urgent.

That's why hydrogenation of CO2 to ethanol has drawn a lot of attention.

Several catalytic systems have been used for CO2 conversion to ethanol, notably Pt, Co, Fe, and Cu systems [START_REF] Zheng | Direct synthesis of ethanol via CO2 hydrogenation over the Co/La-Ga-O composite oxide catalyst[END_REF]. Cu-based catalysts have been used for hydrogenating CO2 to ethanol due to their success in converting CO2 to methanol; however, the selectivity to ethanol is very low [START_REF] Cao | Growing layered double hydroxides on CNTs and their catalytic performance for higher alcohol synthesis from syngas[END_REF]. Recently, it has been found out that ethanol selectivity can be improved by using Cu 1 instead of Cu o . Despite this success, the Cu-catalyst suffers from longterm use due to instability under the reaction conditions and operating conditions.

Other catalytic systems have been tested to improve the selectivity and yield of ethanol from CO2, including Pt-based catalysts and their alloys; while good yield was obtained with these catalysts, the high cost of the catalytic materials is an issue that limits their practical application [START_REF] Choi | Mechanism of Ethanol Synthesis from Syngas on Rh(111)[END_REF]. Lou et al. (Y. Lou et al., 2021) successfully applied a Pd/CeO2 catalyst to convert CO2 to ethanol. They reported a space yield time of 45.6 g ethanol gPd -1 h -1 . Together with an excellent selectivity of 99.2 %. [START_REF] Zhang | Selective Hydrogenation of CO2 to Ethanol over Sodium-Modified Rhodium Nanoparticles Embedded in Zeolite Silicalite-1[END_REF] reported the application of supported sodium-modified Rh NPs in zeolite to hydrogenate CO2 to ethanol that showed a CO2 conversion rate of 10 % and a selectivity of 24% to ethanol. Zheng et al. [START_REF] Zheng | Direct synthesis of ethanol via CO2 hydrogenation over the Co/La-Ga-O composite oxide catalyst[END_REF] reported the conversion of CO2 to ethanol using a LaCo1-xGaxO3 composite perovskite catalyst with a very good catalytic CO2 conversion of 9.8% and a 88.1% selectivity to ethanol.

Another challenge is catalyst separation from the product and recyclability; therefore, extensive research efforts are is devoted to develop effective and highly reusable catalysts in order to address this challenge.

Methane

Methane (CH4) is a C1 compound primarily used as fuel. Moreover, it is also used as a feedstock for chemical synthesis, especially as a synthetic natural gas and is also a major source of hydrogen storage [START_REF] Koytsoumpa | Equilibrium and kinetic aspects for catalytic methanation focusing on CO2 derived Substitute Natural Gas (SNG)[END_REF]. Therefore, the hydrogenation of CO2 to methane is an effective strategy for CO2 capture, utilization and conversion to valuable chemicals. Catalytic hydrogenation is the most widely studied route for CO2 conversion to methane [START_REF] Koytsoumpa | Equilibrium and kinetic aspects for catalytic methanation focusing on CO2 derived Substitute Natural Gas (SNG)[END_REF]. This procedure is called methanation or Sabatier reaction [START_REF] Lu | Metal (Fe, Co, Ce or La) doped nickel catalyst supported on ZrO2 modified mesoporous clays for CO and CO2 methanation[END_REF] as it was first reported in 1902 by Sabatier et al. [START_REF] Sabatier | New synthesis of methane[END_REF].

According to the literature, two possible mechanisms have been proposed. The first mechanism is direct methanation (eq 17) which involves the reaction of CO2 and H2 with a nominal ratio of 1:4, respectively, while the second mechanism is the reaction of CO and H2 in a ratio of 1:3 respectively (equation 2) [START_REF] Dias | CO2 conversion to methane using Ni/SiO2 catalysts promoted by Fe, Co and Zn[END_REF]. As shown in equations 17 and 18, the conversion of CO2 to methane is a highly exothermic reaction which can proceed at a relatively lower temperature. However, given the thermodynamic stability of CO2 molecules due to the presence of delocalized  bonds, the conversion of CO2 to methane remains a serious challenge at a lower temperature (Álvarez et al., 2017).

CO2 + 4H2 ⇌ CH4 + 2H2O ΔH = -165 kJ mol -1 (17) CO + 3H2 ⇌ CH4 + H2O ΔH = -206 kJ mol -1 (18)
To overcome this challenge and permit activation of CO2 at lower temperatures, highly active catalytic materials with good selectivity to methane and low deactivation must be developed.

CO2 methanation has been performed with transition metals such as Co, Ni, Fe, Cu, Ru, Rh, and Pd (Y.-T. Li et al., 2022). However, due to the cost and limited availability, these noble metals are not widely employed for methane synthesis, especially on an industrial scale. Therefore, a new generation of low-cost and efficient catalysts have been investigated for methanation reaction. Among them, Ni, Fe, and Co-based catalysts have been widely studied because of their low cost and catalytic efficiency for CO2 methanation. Ni-based catalytic materials are the most studied for CO2 methanation due to their low cost and availability [START_REF] Fukuhara | A novel nickelbased structured catalyst for CO2 methanation: A honeycomb-type Ni/CeO2 catalyst to transform greenhouse gas into useful resources[END_REF]

Formaldehyde

The ease of availability of CO2 has provided an opportunity for energy production and commodity products for the economy. Among other compounds which could be generated from CO2 is formaldehyde (FMD). Formaldehyde, with the chemical formula HCHO, is a substance that has gained wide usage by millions of people, directly or indirectly, across the globe due to its versatility [START_REF] Rauch | Selective Conversion of Carbon Dioxide to Formaldehyde via a Bis(silyl)acetal: Incorporation of Isotopically Labeled C1 Moieties Derived from Carbon Dioxide into Organic Molecules[END_REF][START_REF] Subasi | Formaldehyde Advantages and Disadvantages: Usage Areas and Harmful Effects on Human Beings[END_REF][START_REF] Usanmaz | Neurotoxic effects of acute and subacute formaldehyde exposures in mice[END_REF]. Generally, FMD is produced industrially via three stages. These include the production of synthesis gas (a mixture of water gas, carbon(iv) oxide and hydrocarbons) by steam reforming natural gas, synthesis of MeOH, and the generation of FMD through the partial oxidation of MeOH [START_REF] Bahmanpour | Formaldehyde production via hydrogenation of carbon monoxide in the aqueous phase[END_REF]. All these methods are energy-intensive procedures [START_REF] Heim | Future perspectives for formaldehyde: pathways for reductive synthesis and energy storage[END_REF]Nguyen et al., 2020). Moreover, during these processes, CO2 was contained in most of the products formed [START_REF] Lee | Selective formation of formaldehyde from carbon dioxide and hydrogen over PtCu/SiO2[END_REF], leading to increased pollution.

eq 19

In a bid to curb the emission of CO2, scientists have developed methods that could possibly convert CO2 into useful products. Of these methods is the production of FMD [START_REF] Friedlingstein | Carbon cycle feedbacks and future climate change[END_REF][START_REF] Sümbelli | Synergistic effect of binanoenzyme and cryogel column on the production of formic acid from carbondioxide[END_REF]. Although the generation of FMD from CO2 is quite challenging, frantic efforts are being made to overcome this fall-out as CO2 is resistant to chemical changes [START_REF] Rauch | Selective Conversion of Carbon Dioxide to Formaldehyde via a Bis(silyl)acetal: Incorporation of Isotopically Labeled C1 Moieties Derived from Carbon Dioxide into Organic Molecules[END_REF]. One of those efforts is the conversion of CO2 at ambient temperature yielding the formation of FMD. The CO2 reacts with triphenylsilane in the presence of a magnesium catalyst, resulting in an intermediate product formed, known as bis(silyl)acetal. This product is then converted to FMD in the presence of cesium fluoride (Eq 19) [START_REF] Rauch | Selective Conversion of Carbon Dioxide to Formaldehyde via a Bis(silyl)acetal: Incorporation of Isotopically Labeled C1 Moieties Derived from Carbon Dioxide into Organic Molecules[END_REF].

Another way that allows to convert CO2 into FMD is the hydrogenation process, which different researchers have debated to be dependent on the catalytic system (heterogeneous or homogeneous) involved (Dang et al., 2019;Nguyen et al., 2020;[START_REF] Studt | The Mechanism of CO and CO2 Hydrogenation to Methanol over Cu-Based Catalysts[END_REF]. The most prevalent catalysts used for this reaction are heterogeneous Cu-based, such as Cu-ZnO-Al2O3 [START_REF] Kobl | Power-law kinetics of methanol synthesis from carbon dioxide and hydrogen on copper-zinc oxide catalysts with alumina or zirconia supports[END_REF] or Pt-promoted Cu/SiO2 [START_REF] Lee | Selective formation of formaldehyde from carbon dioxide and hydrogen over PtCu/SiO2[END_REF]. This latter converts CO2 to FMD at 150 o C for 120 min at 600 kPa. However, Lee and co-partners indicated some issues encountered with this catalyst [START_REF] Lee | Selective formation of formaldehyde from carbon dioxide and hydrogen over PtCu/SiO2[END_REF]. Moreso, a mesoporous graphitic carbon nitride (g-C3N4)

material with specific crystallinity and surface area, has been shown to have a significant effect on the conversion of CO2 to FMD. Since this discovery other photocatalytic mesoporous nanomaterials have been explored for the photocatalytic reduction of CO2 [START_REF] Park | Highly porous gallium oxide with a high CO2 affinity for the photocatalytic conversion of carbon dioxide into methane[END_REF][START_REF] Xu | High-Active Anatase TiO2 Nanosheets Exposed with 95% {100} Facets Toward Efficient H2 Evolution and CO2 Photoreduction[END_REF].

Hydrogen

Aside from FMD, a critical commodity product is hydrogen, being also a fuel, that could be generated from CO2 via a process called carbon dioxide reforming. This process, represented on equation 20, consumes CO2 and methane (CH4), two greenhouse gases. This process requires high temperature, and as such, it is highly endothermic and also favored by low pressure [START_REF] Wang | Carbon Dioxide Reforming of Methane To Produce Synthesis Gas over Metal-Supported Catalysts: State of the Art[END_REF].

CO2 + CH4 → 2CO + 2H2 ∆𝐻 (298) = 247 𝑘𝐽/𝑚𝑜𝑙 eq 20

Fan and coworkers reported this process has been catalyzed over a Ni-Co/MgO-ZrO2 nanomaterial, [START_REF] Fan | Hydrogen production from carbon dioxide reforming of methane over Ni-Co/MgO-ZrO2 catalyst: Process optimization[END_REF]. The effect of Ni-based catalysts on the CO2 reforming of methane had earlier been investigated by [START_REF] Takano | Carbon dioxide reforming of methane on supported nickel catalysts[END_REF]. This process produces green hydrogen a quality of hydrogen which is otherwise generated via the electrolysis of water using a renewable energy source [START_REF] Acar | Selection criteria and ranking for sustainable hydrogen production options[END_REF][START_REF] Luo | Hydrogen production from offshore wind power in South China[END_REF]. This process could solve the problem of CO2 emission during hydrogen production from classical techniques [START_REF] Madadi Avargani | A comprehensive review on hydrogen production and utilization in North America: Prospects and challenges[END_REF]. However, there is a fall-out associated with the production of green hydrogen via the method mentioned above; it involves the release of carbon(ii)oxide as a byproduct. Although CO has a significant value as a fuel and is crucial in manufacturing other chemicals, it has been known over the years for its highly hazardous nature [START_REF] Prockop | Carbon monoxide intoxication: An updated review[END_REF][START_REF] Rose | Carbon Monoxide Poisoning: Pathogenesis, Management, and Future Directions of Therapy[END_REF][START_REF] Wu | Carbon monoxide poisoning[END_REF] and toxicity. Studies should focus more on the production of hydrogen from CO2, as this process is not well explored. This will go a long way in tackling the issue of global warming.

Olefins and gasoline

The hydrogenation of CO2 into high molecular weight hydrocarbons such as lower olefins and gasoline has become increasingly popular due to the necessity to use non-fossil carbon resources

In recent years, significant progress in the development of new catalysts that increase the selectivity of product formed has been made. For instance bifunctional catalysts such as oxide/zeolite have been shown to increase selectivity for lower olefins (C2 = -C4 = ) with significant inhibition of CH4 formation (Jiao et al., 2016;Li et al., 2019;Liu et al., 2017). Bifunctional catalysts typically consist of oxides and zeolites, which activate CO2 and catalyze C-C coupling, respectively. In a recent work Gao et al. obtained 80% C2 = -C4 = selectivity with ~ 4% CH4 inhibition among all hydrocarbons using In-Zr composite based oxides and SAPO-34 zeolites [START_REF] Wen | A reassembled nanoporous gold leaf electrocatalyst for efficient CO2 reduction towards CO[END_REF]Dang et al., 2018). In addition, CO selectivity over the bifunctional catalysts could be jacked up to 85 % during CO2 reduction to lower olefins at high temperature reversed water-gas shift (RWGS).

Moreover, the CO selectivity could be reduced if In2O3 is further modified by Zr and Zn promoters [START_REF] Gao | Direct conversion of CO2 into liquid fuels with high selectivity over a bifunctional catalyst[END_REF]. The reduction of CO2 to hydrocarbons over bifunctional catalysts involves hydrogenation of CO2 to CHxO species or intermediates on the oxygenated sites of the oxide component as the first step, and the migration of the intermediates to the zeolite component where they are transformed into various hydrocarbons at the acid sites of the zeolite [START_REF] Cheng | Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity and Stability[END_REF][START_REF] Gao | Direct conversion of CO2 into liquid fuels with high selectivity over a bifunctional catalyst[END_REF]. The transport of the CHxO intermediates plays a crucial role in suppressing the undesired RWGS side reaction and achieving a high selectivity and it can be improved by acid treatment of the zeolite crystals. The performance of bifunctional catalysts with and without acid treatment for In2O3-ZnZrOx/SAPO-34 composite is summarized in Table 5. The maximum of C2 = -C4 = selectivity is 85 % among all hydrocarbons with 1.6 % CH4.

Moreover, the decrease of zeolite crystal size (Dang et al., 2019) and shortening of the diffusion path of the methanol intermediates should favor the formation of lower olefins over the composite catalyst [START_REF] Jiao | Catalytic Function of IrOx in the Two-Step Thermochemical CO2-Splitting Reaction at High Temperatures[END_REF][START_REF] Yang | Direct conversion of syngas to aromatics[END_REF].

Conclusions and future outlook

Information gathered from recent literature confirms that continuous efforts are being made to reduce CO2 in the environment. However, most articles are focused on its conversion to hydrocarbons, mainly methane, and other organic compounds, such as ethanol, methanol, and methanoic acid. Only limited articles were published on CO2 reduction to either FMD (mostly as intermediates) or hydrogen. Thus, more reaction pathways that yield these products should be investigated. There are some challenges involved in CO2 conversion into useful products, which should be considered. These include but are not limited to the production of harmful gases like CO, technological feasibilities in terms of more research as regards more-efficient conversion techniques, instability and low selectivity of the conversion products, and insufficient basic understanding of the catalytic processes [START_REF] Sharma | Nanocatalysts as potential candidates in transforming CO2 into valuable fuels and chemicals: A review[END_REF]. Having pointed out these fall-outs, it is worth mentioning that scientists and other stakeholders should consider the project of CO2 conversion as germane because it technically solves the problem of global warming and, in turn, impacts the economy of the worldwide system.

The use of nanomaterials-based technologies for the transformation of CO2 is a relatively new area that scientists have to continue to explore. Unarguably, the demonstrated suitability, selectivity and applicability of nanomaterials for producing hydrocarbons and valued-added chemicals (methanol, ethanol, formic acid, formaldehyde, etc) have made nanomaterials promising systems for the transformation of CO2 into various fuels. The research findings so far are quite intriguing at the laboratory scale. However, the extension of these findings to industrial scale would be highly phenomenal. It is therefore recommended and/or suggested industrially suitable, and applicable catalyst (nanomaterials) be developed to selectively and efficiently transformed atmospheric CO2 to value-added products.

Figure 2 :

 2 Figure 2: (a) Energy diagram for CO2 reduction and water oxidation on a

Figure 3 :

 3 Figure 3: Representative SERS spectra that capture specific adsorbates or products

  reported p-type Cu2O photocatalysts with phase structures of 100 (100Cu2O) and 111 (100Cu2O) modified with Pd nanoparticles for CO2RR activities. The well-matched band alignments between Pd and 100Cu2O increased the work functions (Ws) of the 100Cu2O-Pd composite, resulting in an enhanced photocatalytic CO generation at a production rate of 0.13 μmol/g.h, which is 3.0 times higher than bare 100Cu2O nanoparticles. Pd nanoparticles (in 100Cu2O-Pd) promoted efficient photoexcited electrons by acting as a photoinduced hole scavenger, and also improved photostability of 100Cu2O-Pd by reducing photocorrosion. However, 111Cu2O-Pd composites lacked electron-hole mobility due to mismatched band structures between 111Cu2O and Pd species, resulting in lower work function and poor photocatalytic CO2RR compared to 100Cu2O-Pd composites, emphasizing the importance of crystal surface structure and matched band structure construction for selective CO2RR to CO.

Figure 4 :

 4 Figure 4: (a) Photocatalytic generation of methanol over Pt/ZMO nanocomposites at

Figure 5 :

 5 Figure 5: Irradiation time profile of 1 wt%Co-TiO2 for CO2 reduction to fuels.

  HZSM5 to Al2O3 increases the rate of CO2 conversion to light hydrocarbons, CO, and CH4 by 15.53%, indicating that combined supports have the potential to improve CO2 conversion activity and selectivity.

Figure 7 :

 7 Figure 7: (a) Proposed reaction pathways on Cu/fumed-SiO2 and CuNi/fumed-SiO2

Figure 8 :

 8 Figure 8: (a) Peak production rates of CO and CH4 on various TiO2-containing

Scheme 2 :

 2 Scheme 2: Dual excitation pathways over amino-functionalized Fe-MOFs for

  photoreduction. Pristine POM and MOF have negligible CO2-to-CO activity compared

Figure 9 :

 9 Figure 9: (a) Schematic view for the hollow Au/Ag directed formation of Co-MOF-3

Figure 10 :

 10 Figure 10: Schematic view and mechasism of CN/CTF heterostructure for

Figure 11 :

 11 Figure 11: Photocatalytic activity of (a) m CD/CN for methanol formation, and (b) s CD/CN for CO formation measured under visible light (λ > 420 nm). (c) Schematic diagram of photocatalytic CO2 reduction by the m CD/CN and s CD/CN. Reproduced

(

  [START_REF] Sultan | Interface rich CuO/Al2CuO4 surface for selective ethylene production from electrochemical CO2 conversion[END_REF] presented a completely redesigned electrocatalyst consisting of Al2CuO4 nanosheets uniformly coated with CuO nanoparticles (CuAl-1: CuO/Al2 CuO 4 -23) by phase. They found that Cu (CuAl-1) permitted high surface CO intermediate coverages and improved CO adsorption for C-C coupling to *OCCO, a generation of CO intermediate, and Cu (CuAl-1) also enabled high surface coverages of CO intermediates.Cu catalysts for CO2RR can produce a wide range of hydrocarbons at a high catalytic activity. For selective CO2RR, Yin et al.[START_REF] Yin | Electrochemical Reduction of CO2 Catalyzed by Metal Nanocatalysts[END_REF] used a catalyst based on copper nitride (Cu3N) nanocubes (NC). With a FE of 60%, a mass activity of 34 A/g, a C2H4/CH4 molar ratio over than 2000, and strong selectivity and stability of CO2RR towards ethylene at -1.6 V vs. RHE, their 25 nm Cu3N NCs showed good performance.

Figure 14 .

 14 Figure 14.

Figure 14 :

 14 Figure14: Band gap energy of various semiconductors[START_REF] Wang | A metal-free polymeric photocatalyst for hydrogen production from water under visible light[END_REF]. Copy right

Figure 16 :

 16 Figure 16: Scheme of the photothermal catalytic CO2 reduction

Figure 17 :

 17 Figure 17: Solar-assisted thermochemical process for the production of hydrogen and syn gas by

Figure 18 :

 18 Figure 18: Potential products from CO2 conversion (Ramírez-Valencia et al., 2021).

  et al.[START_REF] Moret | Direct synthesis of formic acid from carbon dioxide by hydrogenation in acidic media[END_REF]) applied a ruthenium-based homogeneous catalytic system to convert CO2 feedstock into formic acid directly. The catalytic conversion was performed following the catalytic life cycle described in Figure20. The reaction was performed at 40 °C using a mixture of water and dimethyl sulphoxide (DMSO) as the solvent.

Figure 20 :

 20 Figure 20: Catalytic cycle and hydrogenation of CO2 into formic acid over [HRu(PTA)4Cl]

Figure 22 :

 22 Figure22: Ethanol production in various countries for the year 2020[START_REF] Cooper | Ethanol industry outlook[END_REF].

  As shown in Figure12, various studies using different metal electrodes modified with catalysts such as Sn, In, Au, Ag, Pd, Bi, Co, Mo, and others have been reported. As the other side reactions (i.e. HER) can be almost suppressed, these materials exhibit significant catalytic properties for the CO2 transformation(Lu et al., 2019).Figure12: Overview of some typical Au, Ag, Pd, Sn, and Ni-based nanocatalysts for electrochemical reduction of CO2 to CO. All potentials are contrasted with an electrode made of reversible hydrogen (RHE). NP stands for nanoparticle, whereas FE stands for faradic efficiency. Reprinted with the permission from[START_REF] Yin | Electrochemical Reduction of CO2 Catalyzed by Metal Nanocatalysts[END_REF]. Copyright (2019) Elsevier.Yang et al also investigated the CO2RR on Ag, Pd, and Ag@Pd nanocubes. Pd nanocubes led to CO with high selectivity as a result of Pd turning into Pd hydride (PdH) during CO2RR(Yang et al., 2022). In contrast, Ag@Pd makes the formation of PdH more difficult due to inhibited production of CO from the intermediate 2 and an astonishingly high faradaic efficiency of roughly 94.8%. The production of formate at a remarkable rate of 634.3 mol/cm 2 h -significantly higher than most rates previously reported-is noteworthy. The quick electron transfer of BiSn nanomaterials also contributed in the creation of the CO2 •-intermediate. Additionally, they produced a metastable metal oxide/metal interface that helped to stabilize the CO2 •-intermediate and suppress the HER side reaction. The pathway connected to the OCHO• intermediate was improved because of the optimized electronic structure of BiSn, promoting the conversion of CO2 to formate. According to these results, Bi/Sn bimetallic catalysts are a viable option for very effective CO2 reduction.

	* + CO2 + H + + e -→ *COOH	(Eq4; a slow step)
	*COOH + H + + e -→ *CO + H2O	(Eq 5; a fast step)
	*CO → * + CO	(Eq6)
	*OCHO	(Eq 1; a slow step)
	*OCHO + H + + e -→ *HCOOH	(Eq 2; a fast step)
	*HCOOH → * + HCOOH	(Eq 3)

Furthermore, they present an extremely low energy barrier for the formation of the intermediate, which must then be protonated at the carbon or oxygen atom before being converted to formate or carbon monoxide. * HOCO which thus adjusts the reaction pathway towards HCOOH. The authors concluded that Ag nanocubes have high selectivity for H2 with no phase transition during CO2RR.

Li et al. (Z. Li et al., 2022)

studied electrochemical CO2RR to formate, which not only has the potential to reduce the issue of global warming brought on by CO2, but also has the potential to be exploited for energy storage as intermittent renewable energy. A bimetallic BiSn nanomaterials were electrodeposited on copper mesh and tested as electrocatalysts for the conversion of CO2 to formate. The resulting electrodes demonstrated excellent catalytic activity at 1.0 V vs. RHE, with a partial current density of 34.0 mA/cm

  . The electrocatalyst displayed a high ethylene energy efficiency of 84.5%, steady electrolysis for 55h, and a total cell ethylene energy efficiency of 27.6% at 200 mA.cm -2 in a flow cell with a neutral KCl electrolyte. A density functional theory (DFT) investigation of the mechanism, highlighted that stable nanostructures, stable Cu/Cu2O interfaces, and improved adsorption of the intermediate

* OCCOH can enhance selectivity and amount of C2H4 produced. Wang et al. developed CuBi NPs that showed a CH4 FE as high as 70.6% at 1.2 Vs RHE. According to their study, adding Bi NPs to the matrix (CuBi NPs) significantly boosted the FE when compared to regular Cu NPs by almost 25 times

Table 1 .

 1 Selected examples of electroactive metal nanoparticles for converting CO2 into hydrocarbon productsIn the quest for additional electro-active nanoparticles for CO2 reduction, non-metal NPs have been explored in order to speed up the reaction rate and increase the selectivity for products. For instance, boron phosphide NPs were shown byMou et al.

	716
	Nanoparticles	FE (%)	Current	overpotent	Products	Remarks	Reference
			Density	ial	vs
				RHE (V)
			(mA/c
			m 2 )
	Dibenzyldithioc	100 ± 1	-	-0.8		CO	The hydrophobic	(L. Souza and H.
	arbamate						nature	of	B. Lima, 2021)
	functionalized						dibenzyl moites
	AuNPs						permit CO2 to
							pass through
	C3N4/Cu2O-FeO	84.4	-	-0.24		CO	The interfacial	(Woyessa et al.,
	nanocomposites						interaction	2021)
							between g-C3N4

Table 2 :

 2 The oxidation-reduction potential at pH=7 vs NHE and half-cell reactions

	(Ekambaram et al., 2007)
	Reactants	Products	EMF
	CO2 + 2e -→	• CO2	-1.90
	• CO2 + 2H + + 2e -→	HCOOH	-0.61
	• CO2 + 2H + + 2e -→	CO + H2O	-0.53
	• CO2+ 4H + + 4e -→	HCHO + H2O	-0.48
	• CO2 + 6H + + 6e -→	CH3OH + H2O	-0.38
	• CO2 + 8H + + 8e -→	CH4+ 2H2O	-0.24
	2H2O + 4H + →	O2 + 4H +	+0.81
	H + + 2e -→	H2	-0.42

Table 4 :

 4 Some investigations on solar-assisted thermochemical transformation of CO2

	Fabricated	Product(s)	Condition		Conversion rate References
	thermochemical	of	the
	material	process
	NiFe2O4@SiC	CO		1073-1273 K	18.1 %, 410	(Jiang et al., 2022)
							μmol/g
	Ni-phyllosilicate	H2 and CO	100	h	of	90.4 %, 120.90	(Shi et al., 2023)
	@CeO2			irradiation		and 136.84 mmol

  More recently, Mubarak et

	Dioxane/water
	BMMI.OAc/ (Sc(OTf)3

  MOFsare highly porous coordination polymers that are characterized by high surface areas. As a result, they can enhance the catalytic activity in comparison to other traditional supports.[START_REF] Li | Iron promoted MOF-derived carbon encapsulated NiFe alloy nanoparticles core-shell catalyst for CO2 methanation[END_REF] obtained a very high CO2 conversion (72.3 %) and impressive selectivity (99.3 %) with the NiFe/MOF catalytic material. Similarly, Hu et al.[START_REF] Hu | Ni nanoparticles enclosed in highly mesoporous nanofibers with oxygen vacancies for efficient CO2 methanation[END_REF] also established the role of metal-support interaction using Ni supported on CeO2 nanofibers.The oxygen vacancy enhanced the catalytic efficiency of Ni in CeO2. The prepared catalyst recorded 82.3 % conversion of CO2 at low temperatures (250 o C -300 o C). In addition, they also reported impressive stability of the catalyst up to 60h. This result can be attributed to synergistic effects between the Ni NPs and the CeO2 nanofibers.

	. Dias
	et al. (Dias and Perez-Lopez, 2021) reported the conversion of CO2 to Methanol using Ni/SiO2
	catalyst promoted by Fe, Co, and Zn. They obtained a very high CO2 conversion (73 %) and
	excellent selectivity (98.5 %) to methane.
	It is generally believed that catalytic behavior depends on several factors, such as surface
	properties, metal support interaction, etc. Researchers are currently developing further catalytic
	supports to enhance the performance of catalytically active metal species. This was recently

demonstrated by

[START_REF] Li | Iron promoted MOF-derived carbon encapsulated NiFe alloy nanoparticles core-shell catalyst for CO2 methanation[END_REF]

who used a metal-organic framework (MOF) as a support to enhance the catalytic activity of NiFe for the hydrogenation of CO2 to methane.

Table 5 :

 5 Performance of In2O3-ZnZrOx and SAPO-34 as bifunctional catalysts prior to and after 1287 treatment with nitric acid(Dang et al., 2019)

	1288
	Entr	Catalyst type	Selectivity	CO2	Hydrocarbon distribution
	y				conversion
					(%)
			HC	CO		C2 = -	C2 o -C4 o C5+ CH4
						C4 =
	1.	In2O3-	17.0	44.2 55.8	85.0 11.1	2.3	1.6
		ZnZrOx/SAPO-
		34-S-a
	2.	In2O3-	17.9	45.0 55.0	79.6 13.9	5.2	1.3
		ZnZrOx/SAPO-
		34-S
	3.	In2O3-	17.0	46.6 53.4	84.5 11.0	3.3	1.2
		ZnZrOx/SAPO-
		34-S-H-a
	4.	In2O3-	17.4	45.9 54.1	79.0 16.5	3.3	1.2
		ZnZrOx/SAPO-
		34-H
	5.	In2O3-	17.2	46.2 53.8	80.6 15.2	2.9	1.3
		ZnZrOx/SAPO-

  Arumugam, M., Tahir, M., Praserthdam, P., 2022. Effect of nonmetals (B, O, P, and S) doped with porous g-C3N4 for improved electron transfer towards photocatalytic CO2 reduction ://doi.org/10.1016/j.chemosphere.2021.131765 Asiri, A.M., Lichtfouse, E., 2020. Conversion of Carbon Dioxide Into Hydrocarbons Vol. 2 Technology. Springer. Ateka, A., Rodriguez-Vega, P., Ereña, J., Aguayo, A.T., Bilbao, J., 2022. A review on the valorization of CO2. Focusing on the thermodynamics and catalyst design studies of the

	with	water	into	CH4.		Chemosphere	286,	131765.
	direct	synthesis	of	dimethyl	ether.	Fuel	Processing	Technology.

https

Acknowledgements

Alli, Y.A. would like to seize this opportunity to appreciate CNRS-Laboratory of Coordination Chemistry and Prof. Karine Philippot for providing him with laboratory space and enabling environment for postdoctoral research activities. Also, Oladoye, P.O. would like to appreciate Prof. Yong Cai for providing him with laboratory space and enabling environment for his doctoral research activities.

Funding

This research exercise received no external funding

to find low-cost alternative catalysts. Metallic Ag is gaining considerable attention for CO2 conversion because of its significantly lower cost compared to Au and also high selectivity to CO (Ma et al., 2016b). This has been demonstrated by Ma et al. (Ma et al., 2016b) who developed a Ag-based electrocatalyst comprising oxide-derived nanostructure silver (OD-Ag) and polycrystalline Ag. The proposed reduction mechanism is described in Scheme 3. The obtained catalytic efficiency was 80% Faradic efficiency which is comparable to the results obtained with Au electrocatalysts. Scheme 3: Mechanism of the conversion of CO2 to CO via electrochemical reduction proposed by (Ma et al., 2016b). [START_REF] Dong | ZnOHF nanorods for efficient electrocatalytic reduction of carbon dioxide to carbon monoxide[END_REF] reported the use of low-cost ZnOHF nanorods as a substitute to expensive Au and Ag metals for the reduction of CO2 to CO by electrochemical reduction. They obtained a CO Faraday efficiency of 76.4% with the CO current density of 57.53mA/Cm 2 . It is worth mentioning that the catalytic efficiency of this Zn-based catalyst is not as high as Au and Ag-based ones, but the price is significantly lower which may open avenues for a large-scale electrochemical reduction of CO2 to CO.

Methanol

The conversion of CO2 to MeOH has been drawing tremendous attention lately because of the vast applications of methanol [START_REF] Olah | Towards oil independence through renewable methanol chemistry[END_REF]. Methanol is a multipurpose chemical feedstock [START_REF] Offermanns | From Raw Materials to Methanol, Chemicals and Fuels[END_REF] for producing various value-added products and fuel. The current production rate of methanol is 98 million metric tons per year [START_REF] Kajaste | Methanol-Managing greenhouse gas emissions in the production chain by optimizing the resource base[END_REF]. It is also an important hydrogen carrier. The various applications and Carbon cycle of methanol are described in Figure 21.

Figure 21: Main applications of methanol in industry

Considering the vast application of methanol (Figure 16), conversion of CO2 to MeOH is a crucial research area. The production of methanol has been dominated by the conversion of syngas (a mixture of CO and H2) and CO2 [START_REF] Nieminen | CO2 hydrogenation to methanol by a liquid-phase process with alcoholic solvents: a techno-economic analysis[END_REF]. From the mechanistic point of view, the synthetic pathway may occur through direct CO2 hydrogenation (eq 14) or via reverse water gas shift (RWGS) (eq 15) followed by CO hydrogenation (eq 16) [START_REF] Gaikwad | From CO or CO2?: space-resolved insights into high-pressure CO2 hydrogenation to methanol over Cu/ZnO/Al2O3[END_REF]. as alternative to fossil oil for a sustainable production of liquid fuels and chemicals (Jiao et al., 2016;[START_REF] Yang | Direct conversion of syngas to aromatics[END_REF][START_REF] Zhong | Cobalt carbide nanoprisms for direct production of lower olefins from syngas[END_REF]. This process requires input energy to form C-H bonds and to break C=O bonds (Dang et al., 2019) and efficient catalysts.

Processes based on different catalysts and producing unique products distribution have been developed in the past decades. A commonly used process is Fischer-Tropsch synthesis (FTS) that yields typically straight-chain hydrocarbons controlled by the Anderson-Schulz-Flory (ASF) product distribution, which restricts to C2-C4 hydrocarbon fraction of about 56.7% selectivity for desired lower olefins (C2 = -C4 = ) with 29.2% of undesired CH4 selectivity (Scheme 4). If selective formation of lower olefins and low CH4 formation can be achieved by proper catalyst modification and optimization of reaction conditions [START_REF] Chen | N-doped graphene as an electron donor of iron catalysts for CO hydrogenation to light olefins[END_REF][START_REF] Liu | Manganese-modified Fe3O4 microsphere catalyst with effective active phase of forming light olefins from syngas[END_REF]Lu et al., 2014;[START_REF] Zhong | Cobalt carbide nanoprisms for direct production of lower olefins from syngas[END_REF][START_REF] Zhou | Hierarchical structured α-Al2O3 supported S-promoted Fe catalysts for direct conversion of syngas to lower olefins[END_REF] achieve high selectivity for lower olefins and high stability, requires to develop new catalysts that operate away from Anderson-Schulz-Flory (ASF) distribution [START_REF] Numpilai | Structure-activity relationships of Fe-Co/K-Al2O3 catalysts calcined at different temperatures for CO2 hydrogenation to light olefins[END_REF][START_REF] Visconti | CO2 hydrogenation to lower olefins on a high surface area K-promoted bulk Fe-catalyst[END_REF][START_REF] Zhang | Selective formation of light olefins from CO2 hydrogenation over Fe-Zn-K catalysts[END_REF]. Scheme 4. Typical Reaction processes related to reduction of Syngas to hydrocarbons through Fischer-Tropsch synthesis and methanol-to-Olefins Technology. (Adapted without change from [START_REF] Cheng | Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity and Stability[END_REF]