Molecular Catalysis for Utilizing CO2 in Fuel Electro-Generation and in Chemical Feedstock

Processes for the conversion of CO2 to valuable chemicals are highly desired as a result of the increasing CO2 levels in the atmosphere and the subsequent elevating global temperature. However, CO2 is thermodynamically and kinetically inert to transformation and, therefore, many efforts were made in the last few decades. Reformation/hydrogenation of CO2 is widely used as a means to access valuable products such as acetic acids, CH4, CH3OH, and CO. The electrochemical reduction of CO2 using heteroand homogeneous catalysts recently attracted much attention. In particular, molecular CO2 reduction catalysts were widely studied using transition-metal complexes modified with various ligands to understand the relationship between various catalytic properties and the coordination spheres above the metal centers. Concurrently, the coupling of CO2 with various electrophiles under homogeneous conditions is also considered an important approach for recycling CO2 as a renewable C-1 substrate in the chemical industry. This review summarizes some recent advances in the conversion of CO2 into valuable chemicals with particular focus on the metal-catalyzed reductive conversion and functionalization of CO2.


Introduction
Many efforts were devoted in the recent decades to utilizing CO 2 as a source for renewable energy and materials. Direct gas-phase reactions of CO 2 with other readily accessible small molecules, e.g., via the reformation with CH 4 to produce syngas (CO/H 2 ) and acetic acid (Equations (1) and (2)), as well as hydrogenation to produce CH 4 , CH 3 OH, and CO (Equations (3)-(5)), were widely studied using mainly heterogeneous or supported metal catalysts [1][2][3][4][5][6][7]. Although these gaseous reactions convert CO 2 into useful products, the binary processes, which involve the reactions of CO 2 with explosive gases such as H 2 and CH 4 , are usually pressure-sensitive and, thus, require careful control over the reaction conditions. More handy chemical approaches via which diversified products can be derived directly from CO 2 are, therefore, highly desirable.

3d Metal CO 2 Reduction Electrocatalysts
Although the 4d and 5d CO 2 reduction catalysts of bidentate ligands, especially bipyridyl ligands, attracted much attention, the relatively low abundancy of these metals prompted the development of cost-efficient catalysts and, subsequently, the study of 3d transition-metal catalysts for CO 2 reduction. Probably as a result of their better stability in the reduced state, a number of 3d metal complexes bearing macrocyclic and tetradentate chelating ligands were investigated for activity toward CO 2 electroreduction. Cobalt, iron, and nickel tetraaza macrocyclic complexes were reported to catalyze the electrochemical reduction of CO 2 . Eisenberg first reported the use of cobalt and nickel tetraaza macrocyclic complexes as electrocatalysts for CO 2 reduction (at −1.3 to −1.6 V vs. SCE) to CO or CO/H 2 mixture in aqueous acetonitrile at nearly quantitative FE (Scheme 2) [38]. Mechanistic studies for catalytic CO 2 reduction were also reported using Co complexes of similar ligands [39][40][41][42].

3d Metal CO2 Reduction Electrocatalysts
Although the 4d and 5d CO2 reduction catalysts of bidentate ligands, especially bipyridyl ligands, attracted much attention, the relatively low abundancy of these metals prompted the development of cost-efficient catalysts and, subsequently, the study of 3d transition-metal catalysts for CO2 reduction. Probably as a result of their better stability in the reduced state, a number of 3d metal complexes bearing macrocyclic and tetradentate chelating ligands were investigated for activity toward CO2 electroreduction. Cobalt, iron, and nickel tetraaza macrocyclic complexes were reported to catalyze the electrochemical reduction of CO2. Eisenberg first reported the use of cobalt and nickel tetraaza macrocyclic complexes as electrocatalysts for CO2 reduction (at −1.3 to −1.6 V vs. SCE) to CO or CO/H2 mixture in aqueous acetonitrile at nearly quantitative FE (Scheme 2) [38]. Mechanistic studies for catalytic CO2 reduction were also reported using Co complexes of similar ligands [39][40][41][42].
[Ni(cyclam)] 2+ and other structurally related Ni complexes (Scheme 3) exhibit remarkable efficiency and selectivity for reduction of CO2 to CO on a mercury electrode (at −0.86 V vs. SCE) in purely aqueous solution (pH 4-5) [43][44][45]. Product selectivity depends on the pH and potential at which the controlled potential electrolysis was performed [46,47]. Electrolysis of a CO2 saturated solution (pH 4.1) containing [Ni(cyclam)] 2+ at −1.00 V vs. SHE (overpotential = 640 mV) on an Hg electrode yields CO almost quantitatively at 96% FE and a TON of 116 [43,44]. N-substituted Ni cyclams exhibit similar activity and produce CO at high FE of 84-92% at pH 5 and 550 mV overpotential. Formic acid is obtained similarly, with CO as the byproduct, using the dimeric {[Ni(cyclam)]2} 4+ and related derivative (Scheme 3) at 75% and 68% FE, respectively, in DMF on an Hg electrode at −1.4 V vs. SCE (460 mV overpotential) [48]. [Ni(cyclam)] 2+ and other structurally related Ni complexes (Scheme 3) exhibit remarkable efficiency and selectivity for reduction of CO 2 to CO on a mercury electrode (at −0.86 V vs. SCE) in purely aqueous solution (pH 4-5) [43][44][45]. Product selectivity depends on the pH and potential at which the controlled potential electrolysis was performed [46,47]. Electrolysis of a CO 2 saturated solution (pH 4.1) containing [Ni(cyclam)] 2+ at −1.00 V vs. SHE (overpotential = 640 mV) on an Hg electrode yields CO almost quantitatively at 96% FE and a TON of 116 [43,44]. N-substituted Ni cyclams exhibit similar activity and produce CO at high FE of 84-92% at pH 5 and 550 mV overpotential. Formic acid is obtained similarly, with CO as the byproduct, using the dimeric {[Ni(cyclam)] 2 } 4+ and related derivative (Scheme 3) at 75% and 68% FE, respectively, in DMF on an Hg electrode at −1.4 V vs. SCE (460 mV overpotential) [48]. solution (pH 4.1) containing [Ni(cyclam)] 2+ at −1.00 V vs. SHE (overpotential = 640 mV) on an Hg electrode yields CO almost quantitatively at 96% FE and a TON of 116 [43,44]. N-substituted Ni cyclams exhibit similar activity and produce CO at high FE of 84-92% at pH 5 and 550 mV overpotential. Formic acid is obtained similarly, with CO as the byproduct, using the dimeric {[Ni(cyclam)]2} 4+ and related derivative (Scheme 3) at 75% and 68% FE, respectively, in DMF on an Hg electrode at −1.4 V vs. SCE (460 mV overpotential) [48].   Iron(0) porphyrin complexes were widely studied for electrocatalytic reduction of CO2 to CO by Savéant et al. and are among the most efficient homogeneous molecular catalysts in aprotic solvent (DMF and acetonitrile (can)) [52][53][54][55]. Fe(TPP) was found to selectively catalyze the reduction of CO2 to CO at −1.8 V vs. SCE in DMF (Scheme 5, R = H) [56][57][58][59]. The catalysis is significantly enhanced by the presence of weak Brönsted acids (water, trifluoroethanol, and phenol), as well as Lewis acids. For example, electrolysis using Fe(TPP) at −1.46 V vs. SHE on a mercury pool electrode in DMF, with phenol as the added acid, yields CO between 100% and 94% at varied phenol concentrations (0.1 to 1 M). When 1-propanol (6.7 M) is added instead, formic acid (35% yield) and CO (60% yield) were obtained on a mercury pool at −1.7 V vs. SCE, along with minor byproducts such as H2 and oxalate [59]. Fujita et al. later reported that iron(IV) and cobalt(III) corrole complexes similarly catalyze the reduction of CO2 to CO at −1.7 V vs. SCE under homogeneous conditions (Scheme 5) [60,61]. Cobalt porphyrin (CoTPP) and phtalocyanine (CoPc) complexes were also found to be catalytically active in water on carbon electrodes film-coated with the complexes [24,[62][63][64][65]. By using the Co(TPP) (Scheme 6) deposited on a carbon black gas-diffusion electrode, CO is produced at −0.76 V vs. SHE (overpotential = 230 mV) at 97% FE in a 0.5 M KHCO3 solution under a high CO2 pressure Iron(0) porphyrin complexes were widely studied for electrocatalytic reduction of CO 2 to CO by Savéant et al. and are among the most efficient homogeneous molecular catalysts in aprotic solvent (DMF and acetonitrile (can)) [52][53][54][55]. Fe(TPP) was found to selectively catalyze the reduction of CO 2 to CO at −1.8 V vs. SCE in DMF (Scheme 5, R = H) [56][57][58][59]. The catalysis is significantly enhanced by the presence of weak Brönsted acids (water, trifluoroethanol, and phenol), as well as Lewis acids. For example, electrolysis using Fe(TPP) at −1.46 V vs. SHE on a mercury pool electrode in DMF, with phenol as the added acid, yields CO between 100% and 94% at varied phenol concentrations (0.1 to 1 M). When 1-propanol (6.7 M) is added instead, formic acid (35% yield) and CO (60% yield) were obtained on a mercury pool at −1.7 V vs. SCE, along with minor byproducts such as H 2 and oxalate [59]. Fujita et al. later reported that iron(IV) and cobalt(III) corrole complexes similarly catalyze the reduction of CO 2 to CO at −1.7 V vs. SCE under homogeneous conditions (Scheme 5) [60,61].  Iron(0) porphyrin complexes were widely studied for electrocatalytic reduction of CO2 to CO by Savéant et al. and are among the most efficient homogeneous molecular catalysts in aprotic solvent (DMF and acetonitrile (can)) [52][53][54][55]. Fe(TPP) was found to selectively catalyze the reduction of CO2 to CO at −1.8 V vs. SCE in DMF (Scheme 5, R = H) [56][57][58][59]. The catalysis is significantly enhanced by the presence of weak Brönsted acids (water, trifluoroethanol, and phenol), as well as Lewis acids. For example, electrolysis using Fe(TPP) at −1.46 V vs. SHE on a mercury pool electrode in DMF, with phenol as the added acid, yields CO between 100% and 94% at varied phenol concentrations (0.1 to 1 M). When 1-propanol (6.7 M) is added instead, formic acid (35% yield) and CO (60% yield) were obtained on a mercury pool at −1.7 V vs. SCE, along with minor byproducts such as H2 and oxalate [59]. Fujita et al. later reported that iron(IV) and cobalt(III) corrole complexes similarly catalyze the reduction of CO2 to CO at −1.7 V vs. SCE under homogeneous conditions (Scheme 5) [60,61]. Cobalt porphyrin (CoTPP) and phtalocyanine (CoPc) complexes were also found to be catalytically active in water on carbon electrodes film-coated with the complexes [24,[62][63][64][65]. By using the Co(TPP) (Scheme 6) deposited on a carbon black gas-diffusion electrode, CO is produced at −0.76 V vs. SHE (overpotential = 230 mV) at 97% FE in a 0.5 M KHCO3 solution under a high CO2 pressure  Cobalt porphyrin (CoTPP) and phtalocyanine (CoPc) complexes were also found to be catalytically active in water on carbon electrodes film-coated with the complexes [24,[62][63][64][65]. By using the Co(TPP) (Scheme 6) deposited on a carbon black gas-diffusion electrode, CO is produced at −0.76 V vs. SHE (overpotential = 230 mV) at 97% FE in a 0.5 M KHCO 3 solution under a high CO 2 pressure of 20 atm. Similarly, CO is produced in 80% FE at pH 4.6 with TOF = 140 h −1 and TON = 1100 using cobalt chlorin (CoCh) deposited on carbon nanotubes (700 mV overpotential) [66]. Cobalt protoporphyrin (CoPP) deposited on pyrolytic graphite also catalyzes the CO 2 -to-CO conversion at a low pH of 3 at 60% FE and 500 mV overpotential under 10 atm of CO 2 , while 2.5% CH 4 was obtained when the pH was lowered to 1 [24]. The introduction of intramolecular phenolic protons at the ortho and ortho' positions of Fe(TPP) was found to be effective in enhancing the efficiency of CO2 electroreduction to CO and the catalyst durability (Scheme 7) [53,54]. The catalyst (FeTDMPP) operates at a lower overpotential (450 mV) compared with FeTPP (600 mV), while the substitution of a trimethylammonio group (Fe-p-TMA) not only increases the CO conversion efficiency and selectivity, but also catalyzes the reduction of CO2 to methane [55,67,68]. Heterogeneous catalytic materials based on solid-support or supramolecular porphyrin catalysts of Co, Cu, and Fe for the reduction of CO2 in aqueous solutions were explored. A catalytic electrode constructed from two-dimensional (2D) covalent organic frameworks (COFs) of cobalt tetrakis(4-aminophenyl)porphyrin (Co(TAP)) on porous conductive carbon fabric was found to demonstrate significantly improved (26-fold) catalytic activity for CO2-to-CO conversion at an overpotential of −0.55 V (FE of 90%, TON up to 290,000, and initial TOF of 9400 h −1 ) at pH 7 with respect to the molecular cobalt complex under the same condition [69]. A similar catalytic electrode was prepared by depositing a porous organic cage (Fe-PB), which bears six Fe(TPP) centers, onto a glassy carbon electrode coated with carbon nanotubes (CNTS/GCE) [70]. Both electrochemically active surface area  The introduction of intramolecular phenolic protons at the ortho and ortho' positions of Fe(TPP) was found to be effective in enhancing the efficiency of CO 2 electroreduction to CO and the catalyst durability (Scheme 7) [53,54]. The catalyst (FeTDMPP) operates at a lower overpotential (450 mV) compared with FeTPP (600 mV), while the substitution of a trimethylammonio group (Fe-p-TMA) not only increases the CO conversion efficiency and selectivity, but also catalyzes the reduction of CO 2 to methane [55,67,68]. The introduction of intramolecular phenolic protons at the ortho and ortho' positions of Fe(TPP) was found to be effective in enhancing the efficiency of CO2 electroreduction to CO and the catalyst durability (Scheme 7) [53,54]. The catalyst (FeTDMPP) operates at a lower overpotential (450 mV) compared with FeTPP (600 mV), while the substitution of a trimethylammonio group (Fe-p-TMA) not only increases the CO conversion efficiency and selectivity, but also catalyzes the reduction of CO2 to methane [55,67,68]. Heterogeneous catalytic materials based on solid-support or supramolecular porphyrin catalysts of Co, Cu, and Fe for the reduction of CO2 in aqueous solutions were explored. A catalytic electrode constructed from two-dimensional (2D) covalent organic frameworks (COFs) of cobalt tetrakis(4-aminophenyl)porphyrin (Co(TAP)) on porous conductive carbon fabric was found to demonstrate significantly improved (26-fold) catalytic activity for CO2-to-CO conversion at an overpotential of −0.55 V (FE of 90%, TON up to 290,000, and initial TOF of 9400 h −1 ) at pH 7 with respect to the molecular cobalt complex under the same condition [69]. A similar catalytic electrode was prepared by depositing a porous organic cage (Fe-PB), which bears six Fe(TPP) centers, onto a glassy carbon electrode coated with carbon nanotubes (CNTS/GCE) [70]. Both electrochemically active surface area Heterogeneous catalytic materials based on solid-support or supramolecular porphyrin catalysts of Co, Cu, and Fe for the reduction of CO 2 in aqueous solutions were explored. A catalytic electrode constructed from two-dimensional (2D) covalent organic frameworks (COFs) of cobalt tetrakis(4-aminophenyl)porphyrin (Co(TAP)) on porous conductive carbon fabric was found to demonstrate significantly improved (26-fold) catalytic activity for CO 2 -to-CO conversion at an overpotential of −0.55 V (FE of 90%, TON up to 290,000, and initial TOF of 9400 h −1 ) at pH 7 with respect to the molecular cobalt complex under the same condition [69]. A similar catalytic electrode was prepared by depositing a porous organic cage (Fe-PB), which bears six Fe(TPP) centers, onto a glassy carbon electrode coated with carbon nanotubes (CNT S /GCE) [70]. Both electrochemically active surface area (3.7 nmol EA-Fe·cm −2 vs. 2.5 nmol EA-Fe·cm −2 ) and mass transport were increased for Fe-PB/CNTs/GCE with respect to the Fe(TPP)/CNTs/GCE containing an equivalent amount of Fe(TPP), resulting in an enhanced catalytic current response, CO/H 2 selectivity, and product turnover (TON = 55,250 after 24 h and TOF = 0.64 s −1 vs. TON = 32,770 after 24 h and TOF = 0.38 s −1 ) for controlled potential electrolysis at −0.63 V vs. RHE. Cu(II)-5,10,15,20-tetrakis-(2,6-dihydroxyphenyl)porphyrin) Cu(TDMPP) deposited on a commercial porous polytetrafluoroethylene-treated carbon fiber paper reduces CO 2 to hydrocarbons (methane and ethane) at −0.976 V vs. RHE at 44% FE [71].
A number of 3d metal complexes (Co, Fe, and Ni) bearing tetradentate chelating ligands of N, C, and O donor atoms were reported to be active electrocatalysts for the selective reduction of CO 2 to CO. The N-heterocyclic carbene-isoquinoline complexes [Ni( Pr bimiq1)] 2+ ( Pr bimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane) (Scheme 8) were also reported to catalyze CO 2 -to-CO electroreduction, yielding CO at an optimal FE of 90% (overpotential = 840 mV) on a glassy carbon electrode; however, the catalytic efficiency was found to decrease significantly upon prolonged electrolysis [72]. Co-azacalix [4](2,6)pyridine catalysts (Scheme 8) containing methyl-and allylsubstituted macrocyclic aminopyridine ligands were reported to selectively produce (optimal FE = 98%) CO in a CO 2 -saturated DMF solution with 1.2 M trifluoroethanol on a glassy carbon electrode (680 mV overpotential) at a TON of 6.2, and the pendant NH moiety was suggested to stabilize the Co I -CO 2 adduct [73].
Recently, Lau and Robert reported Co and Fe quaterpyridine complexes (Scheme 10), [M(qpy)(OH2)2] 2+ (qpy = 2,2′:6′,2":6",2‴-quaterpyridine), as catalysts for CO2-to-CO electroreduction in acetonitrile with a selectivity of >95% in the presence of phenol at low overpotentials of 140 and 240 mV, respectively, and an impressive turnover frequency (TOF) of 3.3 × 10 4 s −1 was reported for the Fe catalyst [76]. Multinuclear catalysts of first-row transition metals draw much attention, as a their multiple reaction centers are considered potentially more effective for mediating the simultaneous reduction of more than one CO2, thus leading to the formation of C-2 (or higher) products. Kubiak reported a number of multinuclear nickel and copper phosphine complexes as electrocatalysts for CO2 reduction (Scheme 12 When pendent proton sources are introduced in proximity to the metal center on Mn(L)(CO) 3 Br, i.e., L = 4-phenyl-6-(1,3-dihydroxybenzen-2-yl)-2,2 -bipyridine (Scheme 11), the catalyst is active for CO 2 reduction in acetonitrile at −1.8 V vs. SCE without added acids, producing both CO (70% FE) and HCOOH (22% FE) [80,81]. A reaction pathway involving the formation of Mn hydride via intramolecular proton transfer from the phenolic moiety was suggested. Interestingly, a similar Mn(L)(CO) 3 Br with a single pendent hydroxyphenyl substituent on the bpy ligand selectively produces CO at 76% FE (at 540 mV overpotential) in electrolysis in acetonitrile containing 5% H 2 O [82]. The reactivity of the similarly structured Mn(I)-tricarbonyl catalysts bearing two and three acidic hydroxyphenyl functions was also compared in the presence of added acids (H 2 O, 2,2,2-trifluoroethanol (TFE), and phenol), and the product (CO, HCOOH, and H 2 ) distribution was dependent on the strength of the added acid. [81,83]. Recently, a series of fac-Mn(CO) 3 catalysts bearing imidazolium-functionalized bipyridine, {Mn[bpyMe(lmMe)](CO) 3 Br} + (Scheme 11), were found to demonstrate a superior reactivity (FE = approximately 70%) toward CO 2 reduction in comparison to the dimesityl analogue (FE = 49.6%) at a mild potential (−1.44 V vs. SCE) in acetonitrile with 9.25 M H 2 O [84]. The imidazolium moiety is suggested to favor the formation of a local hydration shell which promotes more efficient protonation of the reaction intermediates [84].
Multinuclear catalysts of first-row transition metals draw much attention, as a their multiple reaction centers are considered potentially more effective for mediating the simultaneous reduction of more than one CO 2 , thus leading to the formation of C-2 (or higher) products. Kubiak reported a number of multinuclear nickel and copper phosphine complexes as electrocatalysts for CO 2 reduction (Scheme 12). The catalyst [Ni 2 (µ-CNMe)(CNMe) 2 (dppm) 2 ] (dppm = 1,1-bis(diphenylphosphino)methane) operates at −0.87 V vs. SCE [88]. The similar dinuclear nickel complex [Ni 2 (µ-CNR)(CNR) 2 (µ-dppa) 2 ] (dppa = bis(diphenylphosphino)amine; CNR = isocyanide ligand) also catalyzes CO 2 reduction [89]. However, these catalysts suffer from carbonylation upon extended cycles of catalysis [88]. Trinuclear nickel clusters [Ni 3 (µ 3 -I)(µ 3 -CNR)(µ 2 -dppm) 3 ] were also found to catalyze CO 2 selective reduction to CO [93]. Controlled-potential electrolysis using the catalyst under optimized conditions produces HCOOH at a good Faradaic yield of 88% as the major product (selectivity ≈ 81%), together with a small amount of CO (selectivity ≈ 11%) and H 2 (selectivity ≈ 8%) [93]. Despite the formation of C-2 or longer hydrocarbons being reported using electrode modified with nanocatalysts, C2 products are less common for the electroreduction of CO 2 on molecular catalysts. Apart from the abovementioned Cu(TDMPP) supported on porous polytetrafluoroethylene-treated carbon fiber paper, which reduces CO 2 to hydrocarbons (methane and ethane) [71], so far, only a dinuclear copper(I) complex, [Cu 2 (L)] 2+ (HL = [N-(2-mercaptopropyl)-N,N-bis(2-pyridylmethyl)amine]), was reported to produce a tetranuclear copper(II) complex bearing two CO 2 -derived oxalate groups upon reaction with atmospheric CO 2 [94]. Treatment of the oxalate-bridged copper(II) complex with a soluble lithium salt in acetonitrile quantitatively produces lithium oxalate, and a subsequent electrochemical reduction of the copper(II) complex regenerates the initial dinuclear copper(I) compound, which reacts again with CO 2 and demonstrates six turnovers (producing 12 equivalents of oxalate) at an applied potential of −0.03 V vs. NHE in 7 h [94]. , was reported to produce a tetranuclear copper(II) complex bearing two CO2-derived oxalate groups upon reaction with atmospheric CO2 [94]. Treatment of the oxalatebridged copper(II) complex with a soluble lithium salt in acetonitrile quantitatively produces lithium oxalate, and a subsequent electrochemical reduction of the copper(II) complex regenerates the initial dinuclear copper(I) compound, which reacts again with CO2 and demonstrates six turnovers (producing 12 equivalents of oxalate) at an applied potential of −0.03 V vs. NHE in 7 h [94].

Diagonal (Reductive) CO2 Functionalization
While, in the above "horizontal" transformations, the CO2 carbon is functionalized with its formal oxidation state unchanged, the "vertical" reduction of CO2 very often results in a lower oxidation state and, thus, higher-energy products, such as CO, formic acid/formate, methanol, and methane, in which no new bonds, other than C-H, are formed to the CO2 carbon (Scheme 18) [19,[23][24][25][26]. For CO2 functionalization processes to be more versatile and widely applied, the spectrum of products directly obtained from CO2 has to be broadened in terms of functionalities and energy. Thus, a "diagonal" reductive approach, where CO2 is reacted in a concerted manner with a nucleophilic functionalization reagent and a reducing agent, was proposed and explored [98,169,170].

Diagonal (Reductive) CO 2 Functionalization
While, in the above "horizontal" transformations, the CO 2 carbon is functionalized with its formal oxidation state unchanged, the "vertical" reduction of CO 2 very often results in a lower oxidation state and, thus, higher-energy products, such as CO, formic acid/formate, methanol, and methane, in which no new bonds, other than C-H, are formed to the CO 2 carbon (Scheme 18) [19,[23][24][25][26]. For CO 2 functionalization processes to be more versatile and widely applied, the spectrum of products directly obtained from CO 2 has to be broadened in terms of functionalities and energy. Thus, a "diagonal" reductive approach, where CO 2 is reacted in a concerted manner with a nucleophilic functionalization reagent and a reducing agent, was proposed and explored [98,169,170].

Summary and Outlook
While CO 2 utilization remains a major scientific challenge, it is also a promising means for providing petroleum substitutes, thus achieving sustainable and carbon-neutral resource utilization, in face of the current rising atmospheric CO 2 level. The direct reduction and the concerted reductive functionalization of CO 2 enable us to assess a broad spectrum of products which are higher in energy and varied in functionality, in comparison with the "horizontal" pathways. However, the search of novel, economic, and environmentally friendly catalysts, which circumvent the current reaction bottlenecks, will be highly important in terms of the access to desirable products, as well as the enhancement of catalyst stability and energy efficiency. Much is to be done for molecular catalysts with respect to understanding how the coordination spheres will facilitate yielding the desired products of varied oxidation states on the CO 2 -derived carbon, particularly in obtaining C-2 or higher hydrocarbons [30,[53][54][55]67,68,71,[80][81][82][83][84][85][86][87]94,98,. For the reductive transformations of CO 2 to be sustainably driven by renewable energy sources, the exploration of direct photo-driven redox catalysis using systems of photosensitizer-catalyst combinations without precious metals [17][18][19][20] and the development of catalytic electrodes bearing the immobilized macromolecular or molecular catalysts [24,[62][63][64][65][69][70][71][85][86][87] will be essential, such that the technology will be further advanced and materialized in the form of photoelectrochemical cells [31].