A review on the state-of-the-art advances for CO2 electro-chemical reduction using metal complex molecular catalysts

1 University of Chinese Academy of Sciences, National Centre for Nanoscience and Technology, CAS Centre for Excellence in Nanoscience, CAS Key Laboratory for Nanosystem and Hierarchical Fabrication, Beijing, China. 2 University of Calabar, Faculty of Physical Sciences, Department of Pure and Applied Chemistry, Calabar, Cross River State, Nigeria 3 University of Chinese Academy of Sciences, Ningbo Institute of Materials Technology and Engineering, Zhejiang, P.R. China 4 University of Ibadan, Faculty of Physical Sciences, Department of Chemistry, Ibadan, Nigeria


Introduction
The increasing level of atmospheric CO2 concentration and fossil resources diminishing are sources of global environmental problems, hence, restoring environmental paradigm requires: atmospheric CO2 level monitoring and renewable energy sources exploration for alternative uses CO2 to value-added products [1][2][3] .For instance, CO2 conversion for other uses (e.g., fuel or chemical feedstock compounds) could fundamentally reduce CO2 emissions and fossil fuel consumption causing climate change and other related environmental problems 1 .Moreover, the reliability of electrocatalytic reduction of CO2 method has been reported, because: (i) operates under ambient conditions and (ii) could exert electricity produced from renewable energy sources 2 .Selective photocatalytic CO2 reduction to CO is one promising process because CO is an essential feedstock chemical for liquid hydrocarbons and methanol production 4 .Several materials (e.g.

ABSTRACT:
Significantly, global warming which is caused by CO2 emission and energy shortage are global problems resulting from artificial photosynthesis because it required many functions (light harvesting, Z water, and oxidation scheme).Therefore, photocatalytic systems development for CO2 reduction is germane in this field.Metal complexes molecular catalyst have become prevalent homogeneous catalysts for carbon dioxide (CO2) photocatalytic reduction since it was initially known as CO2 reduction catalysts in the 70s and the 80s, while utmost part involved macrocyclic cobalt(II) and nickel(II) complexes.This review article presents a broad understanding on some active catalysts recently reported as a metal complex molecular catalytic schemes for CO2 reduction, alongside catalytic activity, stability, selectivity under electroreduction, and photoreduction circumstances.The progress of in situ spectroelectrochemical methods, typically supported via theoretical calculations, helped to access this know-how by providing information which enabled researchers to acquire more in-depth perception into unveiling the catalytic reaction and mechanisms intermediates.
Generally, two types of materials (homogeneous metal complexes [coordination and organometallic complexes] and solid materials) are revealed in recent CO2 reduction catalyst studies.At the same time, while acknowledging the significance of former approach, our interest in this review would address the mechanisms and benefits of metal complex molecular catalysts; a common model for fine-tuning reactivity by ligands synthetic modifications.Molecular catalysts attributes must include the ability to store multiple reducing equals over-mediated multielectron/multi-proton transformations essential for catalytic CO2 reduction.This can be achieved either by reducing the metal center, which then obliges a proficient ligand field to stabilize the reduced metal ions or by lessening the ligand scaffold, with the metal operating as a mediator for electron relay [25][26][27][28] .Nonetheless, it is challenging to transform a "conventional" catalyst into an electrochemical type owing to preconditions and design rules/set 26,27 .Notably, the catalyst conduction band edge requires a suitable range, while the catalyst reduced form is adequately stable, and the chemical step is obtainable under the desired electrolysis conditions.Expectedly, the operating state of homogeneous electrocatalyst should adjoin the thermodynamic reaction potential of the proposed catalysis, while fast chemical step kinetics is mandatory for rapid turnover frequency.For metal-organic compounds, these factors are adjustable consequent to anticipated electrolysis conditions of an appropriate metal choice and ligand tuning optimization.In the last few decades, remarkable advancement has been achieved both in the electrocatalytic performance optimization as well on mechanistic aspects.

Principles
Most electrochemical reactions are conducted in a pair, indicating that reaction at the anode (anodic reaction) is paired with a reaction occurring at the cathode (cathodic reaction) in the photoelectrochemical cell (PEC).The co-reaction of two electrochemical, separately provides valuable, important product or intermediate for chemical synthesis, and constitutes a universal sustainable production plan for chemicals 29 .Past studies have recorded weighty examples for paired electro-organic syntheses [29][30][31] .However, the most prevalent paired process is chloro alkali electrolysis.Such a method can be practical to recover the energy efficiency and the atom economy in comparison to distinctly conducted processes.Achieving a maximum Faradaic efficiency (200%) is realizable since the passed electric charge is primarily used twice.However, in CO2 reduction, the conventional approach for up-scaled reactors is the coupling to water oxidation (oxygen evolution reaction, OER) [32][33][34][35][36] .
Electrocatalysts sensu lato are perfect electron transfer agents that function near the thermodynamic potential of the intended driven (products/substrates).Because, the overpotential is the difference between the applied potential (Vapplied) necessary to produce certain current density at a time t, and the thermodynamic potential (E o ) of the studied system, direct electrochemical CO2 reduction on virtually all electrode surfaces requires a tremendous overpotential, which consequently decreases the conversion efficiency because the amount of voltage applied is greater as compared to the thermodynamic required voltage.Interestingly, at this state, both thermodynamic and kinetic considerations are essential.Apparently, the function of catalysts is to minimize overpotentials and for that, their development is based on the following requirements: (a) to possess formal potentials (E o (C atn+/0 ), well matched to reactions [E o ] (products/substrates), and (c) appreciable constants rate, for substrates chemical reduction to products at the present state.In addition, the heterogeneous rate constant for electrocatalyst reduction at the electrode surface must be high 37 .A general method for an electrocatalytic system is presented in Figure 1.

Electronic structure of CO2
The electrocatalytic CO2 reduction is activated by the interactions of the active form of the catalyst and the substrate molecule through a process of activation.Carbon dioxide (16e -molecule), belongs to D∞h symmetry group with linear geometry at the ground state or gaseous phase.Regardless the molecular nature (nonpolar molecule and chemical stability), it encompasses two polar bonds, with two orthogonal orbital (π) sets.Previously CO2 is assumed to be a poor ligand, however topical scientific studies on CO2 has improved this understanding which are the display of various coordination mode and several coordination sites in its complexes.The carbon atom (LUMO orbital), has a Lewis acid character and can be described as an electrophilic center, while the oxygen atoms (HOMO orbitals) are weak Lewis bases and defined as nucleophiles 39 .Noteworthy, most CO2 catalytic reactions involve a concurrent acid-base activation, with the carbon atom and one of the oxygen atoms tangled in the interface with the metal 40 .The two double bonds include π electrons that can interrelate per transition metals electrons in a Dewar -Chatt -Duncanson bonding scheme Figure 2 41 .When the CO2 LUMO orbitals are engaged (through electron transfer), the least energy state and a bent geometry are equivalent, reflected in the bond length increase, bond angle decrease, and a negative charge determined by the degree of CO2 adsorption and activation.

Carbon monoxide binding modes
There are four basic CO2 coordination approaches [(C, O), and binding modes [Figure 3 and the antibonding CO2 orbital.This bonding mode is made easier via additional weak interaction between one or two CO2 oxygen atoms with a Lewis acid center positioned in the metal coordination domain and more partial with nucleophilic (electron-rich) metals.The ɳ 1 -CO2 complexes are not strong; mostly, their isolation entails glove box or Schlenk techniques, exclusion of oxygen and water [40][41][42] .Following past records of Herskovitz 43,44 on ɳ 1 complexes, it becomes requisite to pressurize the system with CO2 in order to attain complexes (iridium and rhodium).Nevertheless, ligand displacement does not occur in the forming process of these compounds.In the (C, O) bonding mode, there is a double bonding structure involving bond from the CO2 orbital to an empty metal orbital, alongside a "back-bonding" from a filled dxy metal orbital to the empty CO2 orbital.Aresta and Nobile 45 reported the ɳ 2 complex [Ni(CO2)(PCy3)2] made by reaction of Ni(PCy3)3 or [Ni(PCy3)3]N2, in toluene, with CO2 at atmospheric pressure and obtained as toluene solvate.The ɳ 1 (O) end-on coordination mode is ideal with electron poor metals, and the CO2 molecule may persist as linear or bent weakly.The (O, O) coordination mode can be defined as a metal carboxylate with an ionic bond and regularly encountered with alkali or alkaline-earth metals or, with metal surfaces in the CO2 adsorption, for instance.Also, ɳ 2 -CO2 complex synthesis was conveyed by Karsch 46 from the reaction of Fe(PMe3)4 with CO2 in pentane; a second product, Fe(PMe3)3(CO)(CO3) was likewise attained.Recently, features of the complex Fe(CO2)(depe)2, fully characterized by Komiya et al. 47 , buttressed the formulation of the first compound (Karsch 46 ).

Iron-based Molecular Catalyst
As previously explained, electrochemical CO2 reduction to hydrocarbon fuel plays a significant part in climate change and energy cycle 48 .For instance, in the Fischer-Tropsch process carbon monoxide and hydrogen can be converted to liquid fuels.Molecular catalysts (homogenous or heterogeneous) in electrochemical or photochemical conditions are typically useful in the field 49 .In past years, iron porphyrins and metalorganic structures have received vast attention for electrocatalytic and photo electro-catalytic CO2 reduction 50 .Applications of various modern methods and modification through synthesis to enhance the reactivity and product selectivity afforded homogeneous electro-catalysts to be one of the preferred approaches.Past results of Taheri et al. 51 on "an iron electrocatalyst for selective reduction of CO2 to formate in the water" stated that with low applied overpotential, formate is produced with a high current density and Faradaic efficiency (96%).Besides, those studies explicated catalysis mechanism by means of cyclic voltammetry, and structurally categorized a key reaction intermediate, that is the reduced hydride.
Combination of investigational data in MeCN/H2O (95:5) and aqueous solution point to a mechanism for CO2 reduction where the electrocatalyst (Fig. 4) is reduced from 1 − to 1 2− and then protonated to produce (H−1) − .A further selective reaction of (H−1) − with CO2 to yield a C−H bond resulted in formate.Ambre et al. 52 explored the "molecular engineering for effectual and discriminating iron porphyrin catalysts for the electrochemical reduction of CO2 to CO".Here, ester groups (para, meta, and ortho positions) of Fe-porphyrin were introduced and set iron porphyrins Fe-pE, Fe-mE, and Fe-oE.The electrochemical reduction of CO2 to CO by these catalysts was studied.The Faradaic efficiency [(FE) -exclusive 65% and quasiexclusive 98%] for CO was accomplished by Fe-mE and Fe-oE in CO2 saturated electrolyte with addition of 2 mol L -1 H2O, after 2 h bulk electrolysis.Also, the meta-substituted derived (FE-mE) is extremely selective for CO production giving FE (65%) lacking competitive H2 production while the para substituted (Fe-pE) generated only H2 (FE, 84%) as a key product of bulk electrolysis.
The cyclic voltammograms (CVs) results of these Fe porphyrins specify that Fe-pE, Fe-mE, and Fe-oE in DMF exhibited distinctive iron porphyrins redox behaviors.Notably, the E(Fe I/0 ) of Fe-oE is apparently more negative than those of Fe-pE and Fe-mE, perhaps consequence of its dipole.Therefore, the Fe 0 species of Fe-oE is the most robust reducing catalyst between these four Fe porphyrins.Congruently, Savéant and coworkers also discussed that the reduction of CO2 is simplified by Fe(0) species at Fe(I)/Fe(0) redox wave under CO2 atmosphere [53][54][55] .
Further, Rao et al. 56 studied the "visible-lightdriven methane formation from CO2 with a molecular iron catalyst".They observed functionalization of iron tetraphenylporphyrin complex with trimethylammonio groups, which is the most common, competent and selective molecular electrocatalyst for CO2 conversion to CO [57][58][59] .This catalyst is also able to catalyze the eight-electron CO2 reduction to methane upon distinguishable light radioactivity at ambient temperature and pressure.In addition, the catalytic system functioned in an acetonitrile solution covering a photosensitizer and sacrificial electron donor and works stably over several days.Direct CO2 photoreduction reaction produces CO as the principal product, nonetheless, a two-pot method that first reduces CO2 to CO and then reduces CO to methane with a selectivity (ca.82%) and quantum yield (0.18%). Figure 5 presents conceivable mechanism illustration based on obtained results and experimental considerations, including the assumption of formyl intermediate 60,61 , which could be steadied through space interactions between the trimethylammonium group's positive charges and fractional negative charge on the CHO species bounded to the metal.The positional secondary organization dependence sphere groups on the reactivity of a conserved chief iron porphyrin core for electrochemical CO2 reduction were investigated by Nichols et al. 63 .Thereby, four positional isomers (Figure 7) were synthesized, changing the position of the second-sphere amide group orthoand para-, likewise proximal and distal, to the porphyrin plane.
In an atmosphere of CO2 and in existence of phenol (acid source), CO2 reduction catalytic responses suggestive is examined using cyclic voltammetry for all catalysts.Comparatively, when phenol (100 mmol L -1 ) was used, Fe-ortho-1amide display a catalytic onset that is somewhat more positive than Fe-ortho-2-amide, while both evinces meaningfully higher catalytic responses than the equivalent para-functionalized positional un-functionalized Fe-TPP.
For a better understanding of CO2 reduction catalyzed by this series of amide-functionalized porphyrins, the pragmatic constants rate determined by FOWA were reviewed as a function of phenol and CO2 concentration.All four functionalized catalysts reveal first-order dependence on phenol concentration under pseudofirst order circumstances.Fe-ortho-2-amide has the most momentous experimental constants rate of all catalysts studied.Nonetheless, displays nonlinearity at higher phenol concentrations, probably as a result of catalyst inhibition or local depletion of CO2.The Fe-ortho-1-amide has the next highest values (Kobs), trailed by Fe-para-2-amide while Fe-para-1-amide is the least.To appraise the catalytic efficacy, it is mandatory to examine the overpotential (ɳ) vs. log (TOF) relationships exhibited by the catalytic Tafel plot 64 .Operative catalysts function with higher TOFs at lower overpotentials (upper left portion of such plots.Feortho-2-amide demonstrate higher TOFs above all over potential values than Fe-ortho-1-amide that exhibits higher TOFs compared to Fe-TPP and both para-substituted porphyrins.The values at the peak of the curves depict the maximum turnover frequency realizable at large over potential.It crucial to emphasize the difference in E0cat values amongst catalysts when aiming to resolute optimal second-sphere pendant location queries.Previously 65,66 , the electrochemical CO2 reduction driving force is habitually pretentious by electron retreating or bequeathing substituents on the porphyrin aryl rings (E0cat).In such event, a linear scaling relationship among log(TOFmax) and its observations (E0cat), moreover, larger TOFs is achieved wherever there are catalysts with high negative values (E0cat).Nonconformity arises when second-sphere interactions either inhibit or promote catalysis, as previously recorded for electrostatic effects 67 .evolution was ascertained in the mass spectra acquired before and after bulk electrolysis.The electrolysis cell headspace was determined for gas composition in an airtight syringe and injected into a GC/MS instrument.The attained spectrum prior to controlled potential electrolysis (CPE), showed the superseding peak (44 m/z) assigned to CO2 with the other negligible peaks in the spectrum result.Besides, the CO2 obtained was an insignificant product in the mass spectrum and topmost (28 m/z is leading) after 4 hours of CPE.Summarily, it is evident that dichloro[diphenyl-(2pyridyl)phosphine-κ 1 -N]zinc(II) complex is the first organometallic Zn complex that improves the CO2 electrochemical conversion to CO and sinking the required overpotential (approximately 0.6 V).Moreover, this transformation is earned at a glassy carbon electrode; more desirable compared to expensive Pt or Pd alternatives often used for electrocatalysis of this kind.
Wu et al. 75 reported heterogeneous zinc−porphyrin complex (zinc (II) 5,10,15,20tetramesitylporphyrin) as an electrocatalyst that consigns a relatively high turnover frequency (14.4 sites −1 s −1 ) and a Faradaic efficiency (95%) for CO2 electro-reduction to CO at −1.7 V vs. usual hydrogen electrode in an organic/water mixed electrolyte.The PorZn electrodes were studied for electrochemical CO2 reduction at different potentials in a CO2-saturated solvent system including 0.1 mol L -1 tetrabutylammonium hexafluorophosphate hexafluorophosphate (TBAPF6) in DMF/H2O.The fractional current CO production densities and matching Faradaic efficiencies at various potentials are represented (Figure 8A, left).Positive potentials above −1.4V vs. SHE, resulted in H2 as the only product.CO2 conversion to CO observation begins at −1.4 V, with a CO 22% Faradaic efficiency.Notably, applications of more negative potentials are corresponded to increase in both the current density and CO Faradaic efficiency.The supreme CO Faradaic efficiency (95%, −1.7 V vs. SHE) with a current density (ca.2.1 mA/cm 2 ).Equally, the CO Faradaic efficiency and total current density can be reserved (at least 4 hours at −1.7 V, Figure 8B, left).Certain that Zn metal is an active recognized catalytic for electrochemical CO2 reduction to CO [76][77][78] , it is important to ignore the metallic Zn formation, and its catalyzing reaction.
To observe oxidation state vagaries and Zn center electronic structure in electrochemical CO2 reaction situations, in situ and operando XAS extents was conducted with the reaction cell previously reported 79 .This method has been magnificently used to scrutinize the Co oxidation state in a solid-state (cobalt−porphyrin-based catalyst material) in electrocatalysis 80 .No obvious variations in the Zn K-edge X-ray absorption close to edge structure (XANES) spectra are detected as the active electrode potential is tuned from open circuit voltage (OCV, −1.7 V vs. SHE) (Figure 8A, right) and thereafter to +0.2 V vs. SHE, depicting no changes of Zn center oxidation state in the PorZn catalyst at the tested potential range.The high-quality data afforded us a clearer understanding of the PorZn local structures.As presented in Figure 8B (right), slight variations in the Zn coordination number and bond distances are recorded at the examined potentials and during electrolysis.The inconsequential alterations in the Zn local structure could be attributed to the reduction of the porphyrin ligand or binding of molecules on the Zn site.This shows that the Zn center is influential to the PorZn catalytic activity, despite that it is still redox-innocent during electrocatalysis.
The PorZn porphyrin ligand is therefore likely to be the impetus for the two-electron reduction of CO2 to CO.The PorZn CV in saturated electrolyte shows a reduction wave that begins near −1.4 V vs. SHE, corresponding with the CO2 reduction to CO observation at the potential.The cyclic voltammograms in Ar-saturated electrolyte exhibit a comparable wave linked with PorZn reduction, perhaps joined with protonation [81][82][83][84] .The utmost surprising alteration among the two CVs is the reversing wave pattern.The CV in CO2-saturated electrolyte presents anodic wave that is weak (about −1.45 V) equivalent to the reduction wave (about −1.6 V).In contrast, the CV in Ar-saturated electrolyte presents three anodic waves (about −1.53 V, −1.17 V, and −0.43 V vs. SHE).Similar event (three anodic waves) was recorded in an investigation of zinc(II) 5,10,15,20tetraphenylporphyrin studied at the same scan rate 83 .Following precedent literature 83 , these anodic waves are presumed to conform with subsequent oxidation and deprotonation of the reduced porphyrin ring.For deeper understanding in PorZn-catalyzed CO2 reduction, Wu and coworkers conducted PorZn chemical reduction using solution under inert situations and adopt one or two equals of sodium naphthalene in tetrahydrofuran (NaNap, ca.−2.4 V vs. SHE in THF 85 ) as the reducing agent.species display absorption bands at 710, 820, and 920 nm in the UV−Vis spectra, which are features of transiently generated zinc−porphyrin compounds with reduced ligands 81,86 .Upon exposure of the reduced PorZn species to air, the rapid PorZn renaissance was reported in the UV−Vis spectroscopy.Thus far, the wavering of these reduced species has precluded full characterization.This experiment discloses the first molecularly structured Zn-based catalyst for electrochemical CO2 reduction to CO with significant product selectivity.The Zn(II) center, implies redox-inactive though intrinsic to the catalysis, which separates the studied catalyst from transition metal-based molecular catalysts previously reported.distribution of product and the FE.The potential utilized current between -1.4 V and -1.9 V vs. SCE using similar electrode.For gaseous analysis and liquid phase products, gas chromatography (GC) and nuclear magnetic resonance spectroscopy (NMR) was applied.Three discoveries were reported: (i) all ZIF-8 materials, (ii) important products (H2 and CO) and (iii) only a small amount of formate in the liquid phase was discovered.The used potential increase the FE for CO primarily increases till maximum attainment, and then lessening with further utilized potentials Various CO Faradaic efficiency were obtained (highest = ZIF-8 SO4 [65.5%],ZIF-8 NO3 [69.8%] & ZIF-8 AC [57.7%] at -1.8 V vs. SCE, however, ZIF-8 SO4 displays the most extensive potential range appropriate for CO production (-1.5 to -1.9 V vs. SCE.The current densities of H2 and CO products for ZIF-8 materials at -1.8 V vs. SCE has been also investigated 87 .The heightened partial CO current density justified that ZIF-8 SO4 is a highly efficient catalyst for CO2 reduction compared to ZIF-8 NO3 and ZIF-8 AC .Many synthesized ZIF-8 nanocomposites were proven to be impressive CO2 reduction catalysts.Through zinc sources regulation, ZIF-8 chemical reactivity can be modulated, and ZIF-8 SO4 generates excellent CO selectivity.Likewise, the electrolyte performs a vital role for high CO selectivity.The Cl -anion improves and yields the best CO2 reduction reactivity, probably due to superficial anion exchange and small hydrated range.These observances propound ZIF-8s as effective electrocatalysts for CO2 reduction.

Mn-based Metal Complex Molecular Catalyst
The most persistent manganese oxidation state occurs in +2, +4, and +7 of the compounds complete range formed by manganese.The Mn +7 oxidation state is sturdy, frequently lessened to Mn +2 .The oxidation state of Mn +1 is less common, but normally ensues within manganese-based organometallic complexes; d 6 Mn I tricarbonyl complexes have become important in the catalysis field 88,89 .Typically, complexes like the archetypal fac-[Mn(bpy) (CO)3Br] uncover a HOMO controlled by a Mn 3d-orbital and a diimine-based LUMO.The electronic and photophysical character state of analogous Mn I complexes can be better adjusted through diimine ligand chemical structure modification hence adapting the LUMO energy.Application of manganese as a catalyst is due to chemical similarities with rhenium (same group, oxidation states, and geometries).Nonetheless, manganese is 1.3 million times more abundant in the Earth's crust than rhenium 90 .
There are several reviews on transition metals and manganese-based systems for CO2 [91][92][93][94] until recent review (Sinopoli et al. 90 ) which unveiled the efficiency, feature and strategy of Mn carbonyl schemes for CO2 electro-and photoreduction.In the same way, Stanbury and colleagues 95 reported in a comprehensive survey of all the Mn carbonyl systems reported as being active catalysts for CO2 reduction, based on activity, stability, and selectivity under electro-reduction and photoreduction circumstances.In their review, they uncovered that Mn-based carbonyl complexes display an elevated capacity for CO2 catalysis through electro-, photo-or photo electro-reduction, with activities (TON, TOF) that can contend with the Re analogs.
Fei et al. 96 reported the post-synthetic metallation of a robust Zr(IV)-based metal-organic framework (MOF) with open bpy metal-chelating linkers to obtain isolated Mn(bpy)-(CO)3Br moieties in the MOF.Most significantly, in conjunction with [Ru(dmb)3] 2+ as a redox photosensitizer and 1-benzyl-1,4dihydronicotinamide (BNAH) as a propitiatory reducing agent, the resultant [UiO-67 Mn(bpy)(CO)3Br] was realized to be immensely active and selective for the photocatalytic reduction of CO2 to formate with a turnover number (TON) of 110 through 18 h of catalysis.
The suggested mechanism for the photocatalytic reaction is presented in this review (Figure 9).In these reactions, BNAH assists as the conciliatory reducing agent, reducing the excited Ru(II) photosensitizer and starting the photocatalytic reaction.There is an electron transmission from the reduced photosensitizer to the Mn catalyst, forming an absorbable Mn(0) during catalysis.The UiO-67 [97][98][99] large pores, are copious to allow electron transfer amongst the Ru(II) photosensitizer and the Mn complex within the MOF, as the Ru(II) photosensitizer have the strength to allow the interior of UiO-67.TEOA plausibly improves the reaction by contributing a yielded proton and electron (i.e., a hydrogen atom) for the time of catalysis through a Hofmann-type degradation process (Figure 9) 100  of CO2 with the metal center is aided by TEOA, constituting an O-bound Re−OC(O)OCH2CH2NR2 complex, this was made known in past studies with Re bipyridine catalysts 101 .Fei and co-workers propose TEOA giving one proton and one electron to the catalytic reaction, devising a Mn(I)−H complex.CO2 can insert into the Mn−H bond, making a Mn(I)−OC(O)H complex.Formate (or formic acid after further protonation) can then be liberated from the Mn center renewing the starting Mn(I) complex.These conclusions are from the bulk of previously published work on photosensitized catalysis impelled by sacrificial reducing agents [102][103][104][105][106][107] .According to Franco et al. 108 , the first pure organometallic fac-[MnI(CO)3(bis-MeNHC)Br] complex with the revolutionary operation for selective electrocatalytic CO2-to-CO reduction, overreaching 100 turnovers CO with outstanding FE yields (ηCO~95%) in anhydrous CH3CN.During similar state, CV was used determined maximum turnover frequency (TOFmax) of 2100 s - 1 , thus, evidently greater than the values described for other Mn-based catalysts.
Experimental results account for the direct transformation of the [MnI(CO)3(bis-MeNHC)Br] species (Figure 10), into five-coordinate [Mn(CO)3(bis-MeNHC)] -, the core product formed upon reduction.Importantly, the energy of the experimental CO stretching of [Mn(CO)3(bis-MeNHC)] -show a powerfully localized negative charge over the Mn atom persistent with the bis-MeNHC ligand redox innocence.In fact, Kohn-Sham orbitals of [Mn(CO)3(bis-MeNHC)] + to [Mn(CO)3(bis-MeNHC)]• and [Mn(CO)3(bis-MeNHC)] -indicate that the reduction occurs particularly over the metal center.The HOMO orbital geometry ([Mn(CO)3(bis-MeNHC)] -) is openly accessible to undertake a nucleophilic attack in contrast with [Mn(CO)3(py-MeNHC)] -.To summarize, the first family of organometallic NHC-based tricarbonyl Mn I complexes active for electrocatalytic CO2 reduction to CO was reported by Franco and co-workers 108 .Here, bis-MeNHC = methylene bis(N-methylimidazolium) ligand and py-MeNHC = N-methyl-N'-2-pyridilimidazolium ligand.Pyridine ring replacement with a NHC unit exceptionally influences the catalytic operation, to improve the TOFmax and selectivity for CO production of well-established C^N ligand-based Mn systems significantly.Additionally, the unique bis-NHC catalyst successfully and selectively adapts CO2 to CO in an anhydrous aprotic organic solvent; differentially, the traditional bpy-based systems mainstream are unveiled to be inert without regard to definite proton source [109][110][111][112][113][114] .The complex [MnBr(bpy)(CO)3] (bpy = 2,2'bipyridine) is a noble metal-free model catalyst for CO2 reduction consequent to the all-around and direct structure of the bpy ligand 30 .This catalyst displayed high activity (turnover frequency up = 480 s −1 ) in MeCN, and its catalytic mechanism has been studied greatly by wavering the nature of the substituents on the bpy ligand [116][117][118][119][120][121][122] .The Mn catalyst was presently integrated onto CNTs utilizing Nafion 123 , onto p-Si through polymerization 124 , and onto TiO2 via a phosphonate anchoring group 125  record turnover number (TON) for the Mn catalyst of 112 in MeCN 125 .The grafted Mn catalyst form a dimer on the electrode surface 125 , as indicated by the UV−Vis spectra electrochemistry (SEC).It has also been enumerated in solution upon electrochemical reduction for this class of catalyst 126 .Despite the observations of these studies, yet, the reported activity is limited to organic solvents and low TONs (maximum of 101) in aqueous conditions 123 .The pyrene unit allowed stable immobilization onto CNTs.The compound, electrocatalytic activity, was the first study approaching the CO2 reduction in the homogeneous organic solution (MeCN + 5% H2O) and then in completely aqueous solutions after being heterogenized on the CNT surface.Using CPE and CV, the Mn catalyst modified electrodes were explored.The Mn complex surface loading was disclosed to have a unique effect on the selectivity toward CO or HCOO − production (Fig. 11).The different catalytic intermediates involved were investigated in situ through the application of transmission UV-VIS and surface-sensitive IR SEC in the impaired total reflection (ATR) mode.Selectivity toward either CO or HCOO − at various surface loadings, accurate formation assignment of one or the other catalytic intermediate is important 116,122,124 .

Ni-based Molecular Catalyst
Nickel, a non-precious metal (group VIIIB), is considered the best possible alternative instead of palladium or platinum for molecular catalysts 127,128 owing to its simply achievable oxidation states (e.g., Ni 0 , Ni I , Ni II , Ni III , and Ni IV ).The CO2 reduction performed by nickel catalysts repeatedly required the Ni II reduction to Ni I , which is linked with the distortion of geometrical from a tetradentate, square planar coordination mode fitting for Ni II to a tetrahedral one model for Ni I 129-131 .Similarly, OER, the Ni II oxidation to Ni III typically allows the structural shift from a square planar to a tetragonal or octahedral geometry 132,133 .Therefore, the rich redox properties connected with many coordination geometries concede the ligand logical form scaffolds to harmonize with nickel (II) centers, contributing to the nickel complexes with precise catalyst operations [133][134][135][136][137][138] .Moreover, nickel is an active center in natural enzymes, the wellknown {NiFe} hydrogenases for the reversible conversion hydrogen, proton 139,140 , and the {NiFe} CO dehydrogenases (CODHs) for the reversible transformation between CO2 and CO 141,142 .These findings further compelled the scientist (mostly energy conversion catalysis), to develop molecular catalysts based on nickel complexes.In spite many insightful reviews on spotlighted earth-abundant metal complexes as catalysts for the HER [143][144][145][146][147][148][149][150][151] , OER [152][153][154] and CO2 reduction [155][156][157][158][159][160][161][162] , however, details on the Ni-based molecular catalyst is yet to be available in the literature.More recently, Wang, Jia-Wei et al. 163 reported a systematical review on the recent developments in the utilization of nickel complexes as molecular catalysts for water splitting and CO2 reduction.In summary, nickel cyclam complexes display high efficiency and selectivity.Nonetheless, other nickel-based catalysts display moderate activity, and selectivity, while the solvent used, is restricted to non-aqueous solvents.
Niu et al. 164 has designed and synthesized a spongy nickel-organic heterogeneous catalyst through the photochemical pathway.The catalyst possesses crystalline network architecture with a high imperfections concentration and active in CO2 conversion to CO, with ~1.6 × 10 4 mol h −1 g −1 production rate.During the reaction, no measurable H2 is generated, resulting to approx.100% selective CO production over the evolution of H2 and the evolution of CO from these five Ni-organic catalysts in a photocatalytic 6 h response.The spongy Ni(TPA/TEG) (L) composite results to be the highest activity, and the CO amount is 95.2 mmol after a 2 h reaction, yielding to a CO production rate (15,866 mol h −1 g −1 ), that is many times greater than other samples.The total CO volume produced on the spongy Ni(TPA/TEG) catalyst in 6 h attains 136.9 mol, giving a turnover of 11.5 for the 6 h reaction.By investigating the CO yield in 2 h on different amounts of Ni(TPA/TEG) catalyst, a roughly linear relationship was obtained between the number of CO evolved and the catalyst amount.However, kinetically, it was found that the CO production rate decrease is proportional to increase in the catalyst amount, where 1.0 mg of the Ni(TPA/TEG) catalyst results to CO production rate of ~26,620 mol h −1 g −1 in a similar solution.Thus, implies that more electrons generated from the photosensitizer molecules may perhaps have been transferred to the active catalytic sites.Also, the reusability of the spongy Ni(TPA/TEG) catalyst upon each 2 h of photocatalysis have been tested, where the catalyst retained its activity and selectivity after recycling.In addition, it also displays outstanding architectural stability with no apparent detectable structural change after 24 h of photocatalysis.For further confirmation of the origin of as-produced CO, isotopic 13 CO2 was used as feedstock gas for the photocatalytic reduction and the products were examined by gas chromatography-mass spectrometry (GC-MS).The study confirms that the detected CO originates from the CO2 gas source; with the important signal at a mass/charge ratio (29) on the mass spectrum corresponding to 13 CO.
Consequence of the previous result, Niu and coworkers proposed the mechanism for the photocatalytic CO2 reduction reactions on the spongy Ni(TPA/TEG) catalyst (Fig. 12).Based on visible light irradiation, the [Ru(bpy)3] 2+ (photosensitizer) is excited and then reductively snuffed out by the sacrificial electron donor [TEOA, 65-167], resulting to the reduced species of [Ru(bpy)3] 2+ (Fig. 12).Afterward, the [Ru(bpy)3] 2+ reduced species could maybe transfer an electron to the spongy Ni(TPA/TEG) catalyst, which later engages in CO2 molecules reducing fixed on the catalyst (Fig. 12).The yield tests of CO production in the solution with different Ni(TPA/TEG) amounts for 2 h, revealed that the CO production rate decreases with increase in the catalyst amount, indicating that the electron transfer from [Ru(bpy)3] 2+ to the catalyst may be a rate-determining step for the CO2 reduction reaction, however, limited diffusion scenario may have occurred in this heterogeneous catalytic system 168 .Thereafter, the derivative CO can be reduced to liquid fuels by means of proton-coupled multi-electron reaction processes (Fig. 12).In the electrolyte with a pH value of ~8, it was also suggested conversion pathways resulting to the HCOOH, CH3COOH, and CH3CH2OH formation through proton-coupled one-, four-, and eightelectron steps, respectively.
In the mechanistic CH3COOH formation route, CO is perpetually hydrated to CHO→CHOH→CH2OH→CH3OH, which bonds with the adsorbed CO to form CH3COOH.With a focus on the CH3CH2OH formation, the dehydroxylation of the as-formed CHO may possibly be a crucial rate-limiting step to produce additional C that can be protonated CH→CH2→CH3 169 , and the C-C coupling in the space separating CH3 and multi protonated CO could result in the CH3CH2OH formation 170 .Frequently, the hydroxyl ions (OH−) are obtained for the CHO hydroxylation at pH 13, which select the HCOOH formation.Whereas, the facilitated CHO hydroxylation may suppress the kinetics of the CO multi protonation and CHO dihydroxylation, yielding a lower amount of CH3COOH and CH3CH2OH at pH 13.The CH3OH appearance may present a weak C-C coupling in the space separating CH3OH and CO at pH 13 (resulting to CH3COOH at pH 8), hence, may be considered for the next CO2/CO reduction catalyst design 171 .Kuehnel et al. 172 investigated a series of selfassembled nickel terpyridine complexes as catalysts for the CO2 reduction to CO in organic media.Immobilization on CdS quantum dots allows these catalysts to be functional in purely aqueous solution, and photo catalytically reduces CO2 with > 90% selectivity in UV-filtered simulated solar light irradiation (AM 1.5G, 100 mW cm -2 , λ > 400 nm, pH 6.7).The QD-BF4 adjustments were performed in -situ through an additional stock solution of a self-assembled Ni complex to a suspension of QDs in TEOA aqueous solution (0.1 mol L -1 ).This finally results into H2O:CH3CN solution composition including 99:1 for [Ni(terpy)2] 2+ and [Ni(terpyS)2] 2+ , and 99.5:1 for [Ni(terpyC)2] 2+ and [Ni(terpyP)2] 2+ .Catalyst attachment was affirmed by UV-vis spectroscopy with the anchoring group that involves dependent catalyst loading (Figure 13A).Considerably, the highest loading was accomplished with the thiol derivative, [Ni(terpyS)2] 2+ , while other anchors were measured with lower affinity (Figure 13B).The determination of main catalyst peaks in the UV-vis and ATR-IR spectra of [Ni(terpyS)2] 2+adjusted CdS QDs immobilized on a mesoporous SnO2 electrode show that the catalyst maintained its intact chemical structure on the QD surface (Figures 13C).Cyclic voltammetry showed that the anchored catalyst retained its electrochemical response, additional supporting functional integrity on the QD surface (Figure 13D).Therefore, transmission electron microscopy unveiled that anchoring of the catalyst does not affect the particle morphology.Surprisingly, the selectivity observed does not exhibit the electrocatalytic activity in the homogeneous phase, however, correlates with the adsorption efficiency of each complex to CdS.This behavior affirms that molecular catalyst interfacing with the nanoparticle is important to all-inclusive photocatalytic activity in aqueous solution.At the optimized state, about 20 Ni-based turnovers were obtained with CdS-[Ni(terpyS)2] 2+ in 24 h visible light illumination.The CO selectivity continues to exist (> 90%) for the first 8 h prior to slowly decrease that mostly produce H2 after 24 h.Ioncoupled plasma optical emission spectroscopy (ICP-OES) of QDs isolated from the reaction medium iterates the reducing selectivity co-occurs with a gradual [Ni(terpyS)2] 2+ loss of the QD surface, differentially, the CdSBF4 particles remain unchanged.UV-Vis spectra display a slight redshift of the first absorption, presenting limited particle aggregation with no significant photocorrosion.In addition, fresh catalyst after 20 h recovers the CO generation activity and subdue H2 evolution, while the Ni(BF4)2 addition only promotes H2 evolution.Conversely, lessening the initial catalyst:QD ratio reduced the CO selectivity, moreover, it did not significantly affect the maximum TONCO (with respect to [Ni(terpyS)2] 2+ ), where it can be inferred that a TONCO of ~20 represents the catalyst stability limit.

Conclusions and future perspectives
This general overview of the recent research on metal complex molecular catalysts for CO2 reduction shows their contributions to the growing and dynamic field that gaining new insights into science at an ever-increasing rate.In order to increase metal complexes molecular catalysts application, it is imperative to overcome their poor stability which is a significant challenge.We suggest that one of the possible means to achieve this stability could be to ensure complexes heterogeneous, through their substrate's immobilization.Notwithstanding, the limited examples we have presented in this review, (e.g., porous structures, or covalent grafting on electrodes through ligand functionalization), suggest that the concept is realizable.Succinctly, the advancing knowledge acquired in several reduction mechanisms might give-way for more efficient selective and prolong metal complex molecular catalysts development.Additionally, most metal complexes molecular catalysts are light sensitive and suffer from photostability issues in any of these two ways; (i) by ligand photodissociation reactions and (ii) photoisomerization.This particular problem is an important obstacle towards the efficient metal complex photocatalysts for CO2 reduction advancement.Unarguably, more improvements are still required, and other pathways for CO2 catalytic reduction may perhaps offer ample fruitful opportunities.

Figure 1 .
Figure 1.Diagrammatic illustration of a paired electrolysis relating cathodic CO2-to-CO conversion catalyzed with a Re complex and anodic synthesis of a 2-substituted benzimidazole mediated by ceric ammonium nitrate 38 .Modified by the authors.

Figure 6 .
Figure 6.Atomic structure of M−N4−C10 and M−N2+2−C8 (M=Fe or Co) active sites.(b) Calculated free energy evolution of CO2 reduction to CO on M−N2+2−C8 sites under useful electrode potential (U) of 0 V and −0.6 V. (c) The initial and final state for the COOH dissociation reaction on M−N4−C10 and M−N2+2−C8 sites.In the figure, the gray, blue, yellow, red, and white balls represent C, N, M, O, and H atoms, respectively 62 .Reprinted with permission from Pan et al., Unveiling Active Sites of CO2 Reduction on Nitrogen-Coordinated and Atomically Dispersed Iron and Cobalt Catalysts, ACS Catal.8 (4) (2018) 3116-3122.Copyright (2018) American Chemical Society.

Figure 12 .
Figure 12.Proposed mechanisms for the photocatalytic reduction of CO2 to CO and of CO to other liquid products.Visible light reduction of the photosensitizer [Ru(bpy)3] 2+ , which transfers an electron to the Ni(TPA/TEG) catalyst to convert CO2 to CO and to Ni(TPA/TEG)-(Ag/Rh) catalysts for the generation of HCOOH, CH3COOH, and CH3CH2OH from further reduction of CO 164 .