Multistep Electron Transfer Research Articles

Mechanistic modeling of electrochemical charge transfer in HT-PEM fuel cells

An electrochemical model for H2 oxidation and O2 reduction in HT-PEM fuel cells derived from multi-step single electron transfer elementary reactions is presented. The exchange current density expressions are free from arbitrary order dependency with respect to the concentration of reactants and products. The reaction mechanism also considers CO adsorption and desorption, which enables the prediction of cell performance drop due to CO poisoning. The electrochemical model is coupled with a quasi two dimensional model of the cell, which resolves the flow channel in the axial direction and the electrodes along its length and thickness. A 3 cm long cell is simulated and the results are compared with experimental measurements. Very good agreement is obtained between the model predictions and experimental measurements for cell operating on pure H2 and with 1–16% CO in the temperature range of 150–200 °C.

Behaviors of the Interfacial Consecutive Multistep Electron Transfer Controlled by Varied Transition Metal Ions in Porphyrin Cores.

Almost all life activities involve the process of multistep electron transfer (ET) which occurs on biomembrane. Metalloporphyrins (MTPPs) are a class of molecules which are closely related to life course. Here, the n-step (n = 1, 2) ET behaviors controlled by different metal ions in porphyrin cores were investigated by thin-layer cyclic voltammetry (TLCV). The bimolecular ET was reacted between the MTPP (M = Fe, Zn, Co, Cu, Ni) and Fe(CN)64- in nitrobenzene and aqueous phase, respectively, and the interface between nitrobenzene and aqueous phase was considered as a bionic membrane. The thin-layer theory, which has been revised, was used to calculate the kinetic constants for each step electron transfer reactions. It was shown that the kinetic data were affected dramatically by the different coordinated ions in porphyrin complexes.

Model of multistep electron transfer in a single-mode polar medium

A mathematical model of multistep photoinduced electron transfer (PET) in a polar medium with a single relaxation time (Debye solvent) is developed. The model includes the polarization nonequilibrity formed in the vicinity of the donor–acceptor molecular system at the initial steps of photoreaction and its influence on the subsequent steps of PET. It is established that the results from numerical simulation of transient luminescence spectra of photoexcited donor–acceptor complexes (DAC) conform to calculated data obtained on the basis of the familiar experimental technique used to measure the relaxation function of solvent polarization in the vicinity of DAC in the picosecond and subpicosecond ranges.

Manipulating the Energetics and Rates of Electron Transfer in Rhodobacter capsulatus Reaction Centers with Asymmetric Pigment Content

Seemingly redundant parallel pathways for electron transfer (ET), composed of identical sets of cofactors, are a cornerstone feature of photosynthetic reaction centers (RCs) involved in light-energy conversion. In native bacterial RCs, both A and B branches house one bacteriochlorophyll (BChl) and one bacteriopheophytin (BPh), but the A branch is used exclusively. Described herein are the results obtained for two Rhodobacter capsulatus RCs with an unnaturally high degree of cofactor asymmetry, two BPh on the RC's B side and two BChl on the A side. These pigment changes derive, respectively, from the His(M180)Leu mutation [a BPh (ΦB) replaces the B-side BChl (BB)], and the Leu(M212)His mutation [a BChl (βA) replaces the A-side BPh (HA)]. Additionally, Tyr(M208)Phe was employed to disfavor ET to the A branch; in one mutant, Val(M131)Glu creates a hydrogen bond to HB to enhance ET to HB. In both ΦB mutants, the decay kinetics of the excited primary ET donor (P*) resolve three populations with lifetimes of ∼9 ps (50-60%), ∼40 ps (10-20%), and ∼200 ps (20-30%), with P+ΦB- formed predominantly from the 9 ps fraction. The 50-60% yield of P+ΦB- is the highest yet observed for a ΦB-containing RC. The results provide insight into factors needed for efficient multistep ET.

Visible Light-Driven Water Oxidation with Porphyrin Sensitizers and Water Oxidation Catalysts

Finding renewable energy sources is a crucial task in making our society sustainable. In this regard, exploitation of sunlight as an infinite energy source is attractive. Specifically realizing artificial photosynthesis, i.e., integration of light-harvesting, multi-step electron and proton transfer, and water oxidation for the efficient production of fuels, is a great challenge in chemistry. For the purpose, dye-sensitized photoelectrosynthesis cells (DSPSC) have been investigated, as the heterogeneous water splitting on inorganic semiconductor is promising for the upcoming large scale device operation. In DSPSC a molecular sensitizer adsorbed on a semiconducting electrode harvests visible light and injects an electron from the sensitizer excited-state (S*) to a conduction band (CB) of the electrode. Then, the sensitizer radical cation (S• +) extracts an electron from a water oxidation catalyst (WOC) to regenerate the sensitizer and one-electron oxidized WOC. After repeating the cycle, high oxidation states of the WOC are generated, eventually transforming two water molecules into four protons and one oxygen molecule. As the sensitizer bis(2,2’-bipyridine)(4,4’-diphosphonato-2,2’-bipyridine)ruthenium(II) (RuP) has been frequently employed for the construction of molecular artificial photosynthetic systems, due to its sufficient first oxidation potential for water oxidation and a long lifetime of its excited state for electron injection. However, the light-harvesting ability of RuP is rather low in visible region beyond 500 nm. Given that yellow to red photons mainly shower down on the earth from sun, utilization of photons in visible region is important for efficient chemical conversion by sunlight. Along this line, porphyrins are promising as the sensitizer because of their excellent light-harvesting in visible region and facile tuning of their excited-states and redox properties by their peripheral functionalization. Nevertheless, molecular artificial photosynthetic systems with porphyrins as the sensitizer are so far limited owing to their low performance. One plausible reason is the occurrence of fast charge recombination (CR) between the electron injected into the CB of TiO2 (denoted as TiO2(e−)) and S• +. CR from TiO2(e−) to the oxidized WOC is also reported to take place within a few microsecond. Undesirable CR from TiO2(e−) to water is also suggested. Therefore, to overcome the drawbacks, it is crucial to modulate the electron transfer (ET) properties at the interfaces. In this talk, I will give an overview of our recent initiatives on visible light-driven water oxidation with porphyrin sensitizers and water oxidation catalysts. [1] M. Yamamoto, L. Wang, F. Li, T. Fukushima, K. Tanaka, L. Sun and H. Imahori, Chem. Sci., 7, 1430-1439 (2016). [2] M. Yamamoto, Y. Nishizawa, P. Chábera, F. Li, T. Pascher, V. Sundström, L. Sun, and H. Imahori, Chem. Commun., 52, 13702-13705 (2016).

A Ruthenium Complex-Porphyrin-Fullerene-Linked Molecular Pentad as an Integrative Photosynthetic Model.

A ruthenium complex, porphyrin sensitizer, fullerene acceptor molecular pentad has been synthesized and a long-lived hole-electron pair was achieved in aqueous solution by photoinduced multistep electron transfer: Upon irradiation by visible light, the excited-state of a zinc porphyrin (1 ZnP*) was quenched by fullerene (C60 ) to afford a radical ion pair, 1,3 (ZnP.+ -C60.- ). This was followed by the subsequent electron transfer from a water oxidation catalyst unit (RuII ) to ZnP.+ to give the long-lived charge-separated state, RuIII -ZnP-C60.- , with a lifetime of 14 μs. The ZnP worked as a visible-light-harvesting antenna, while the C60 acted as an excellent electron acceptor. As a consequence, visible-light-driven water oxidation by this integrated photosynthetic model compound was achieved in the presence of sacrificial oxidant and redox mediator.

A Ruthenium Complex–Porphyrin–Fullerene‐Linked Molecular Pentad as an Integrative Photosynthetic Model

AbstractA ruthenium complex, porphyrin sensitizer, fullerene acceptor molecular pentad has been synthesized and a long‐lived hole–electron pair was achieved in aqueous solution by photoinduced multistep electron transfer: Upon irradiation by visible light, the excited‐state of a zinc porphyrin (1ZnP*) was quenched by fullerene (C60) to afford a radical ion pair, 1,3(ZnP.+‐C60.−). This was followed by the subsequent electron transfer from a water oxidation catalyst unit (RuII) to ZnP.+ to give the long‐lived charge‐separated state, RuIII‐ZnP‐C60.−, with a lifetime of 14 μs. The ZnP worked as a visible‐light‐harvesting antenna, while the C60 acted as an excellent electron acceptor. As a consequence, visible‐light‐driven water oxidation by this integrated photosynthetic model compound was achieved in the presence of sacrificial oxidant and redox mediator.

Enhancement of the Oxygen Evolution and the Oxygen Reduction Reactions in Mn3+ based Electrocatalysts

The material design of highly active catalysts for energy storage applications (e.g., metal-air batteries and fuel cells) is crucial for solving global energy problems. The oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is used for charging and discharging rechargeable metal-air batteries, respectively. Although precious metal-based catalysts facilitate these reactions, the kinetics of the OER and ORR are rather sluggish due to their multistep electron transfer. It is therefore important to explore the mechanism of the OER and ORR to discover the key factors for highly active catalysts. As demonstrated by Suntivich et al. (2011)[1],[2], when the number of electrons in the eg orbital is close to unity for transition metals, perovskite oxides exhibit maximum OER and ORR activities. In other words, Mn3+ (t2g 3 eg 1 for both surface and bulk), Co3+ (t2g 5 eg 1 for surface), and Ni3+ (t2g 6 eg 1 for both surface and bulk) become OER and ORR active sites for Mn3+, Co3+ and Ni3+ based compounds . However, LaMnO3 exhibits a significantly lower specific OER activity[3] compared with LaCoO3 and LaNiO3 (~6% of LaNiO3 at 1.8 V vs. RHE). Also, Mn3O4 exhibits a low specific OER activity[4] (40% of Mn2O3 at 1.8 V vs. RHE) similar to LaMnO3. It is therefore important to explore what causes degradation of the OER activity of Mn3+-based (t2g 3 eg 1) compounds to improve their catalytic activity. We attempt to improve the performance of Mn3+-based oxides by controlling the degradation factors of their OER activities. To directly compare the OER activities of Mn3+-based oxides containing more than two cation sites, Mn3+ concentrations at the octahedral site and their initial crystal structures were maintained. Mn3-x Co x O4 (0 ≤ x < 1), a series of tetragonally distorted spinel compounds, satisfies this condition, as their octahedral sites remain occupied by only Mn3+ ions . For Mn3+-based oxides with a single cation site, Mn3+ was directly substituted by Co3+.We systematically studied the OER and ORR performances of Mn3+-based oxides by Co3+-substitution. Nanoparticles were prepared for the evaluation of catalytic activity to minimize the influence of geometric factors. Electrochemical measurements were conducted using a rotating ring disk electrode rotator (RRDE-3A, BAS Inc., Japan) at 1600 rpm, in combination with a bipotentiostat (ALS Co., Ltd, Japan). In addition, a Pt wire counter electrode, and an Hg/HgO reference electrode (International Chemistry Co., Ltd., Japan) filled with 0.10 M KOH (Nacalai Tesque, Inc., Japan) were used. Electrochemical measurements were conducted with O2 saturation (30 min bubbling O2 gas through the solution) at ~25 °C, where the equilibrium potential of the O2/H2O redox couple was fixed at 0.304 V vs. Hg/HgO (or 1.23 V vs. RHE). During OER current density measurements for each sample, the potential of the sample-modified GC was controlled from 0.3-0.9 V vs. Hg/HgO (1.226-1.826 V vs. RHE) at 10 mV/s. For all measurements, the OER current density was iR-corrected (R = ~43 Ω) using the measured solution resistance, and capacitance-corrected by averaging the anodic and cathodic scans to remove the influence of the current related to the formation of an electrical double layer. With an increase in Co content, signicant improvements in the OER and ORR activities were observed for Mn3+-based oxides. For example, Mn2.1Co0.9O4 exhibited a high specific OER activity (1700 % of Mn3O4 at 1.76 V vs. RHE) and a long-term stability over 100 cycles [5]. The OER activities of Mn3+-based oxides increased linearly with the suppression of the Jahn–Teller distortion. We examined whether the electronic state of Mn3+ changes in favor of enhancing the OER and ORR activities. When the Jahn-Teller distortion of Mn3+O6 octahedra is suppressed due to the increase in Co content, the splitting of the Mn3+ eg orbitals becomes smaller and the electron occupying the Mn3+ eg orbital shifts to a higher energy level. Thus, the overlap of the antibonding Mn3+ eg orbitals with the O 2p orbitals of the oxygen adsorbate becomes stronger. The OER and ORR activities of Mn3+-based oxides should therefore be enhanced due to the stronger binding of OER intermediates to the catalytic surface. We therefore conclude that the suppression of Jahn–Teller distortion enhances the OER and ORR activities of Mn3+-based oxides. Our result suggests a future application of Mn3+-based oxides as bifunctional catalysts for metal-air batteries. [1] J. Suntivich et al., Science, 2011, 334, 1383. [2] J. Suntivich et al., Nature Chemistry, 2011, 3, 546. [3] B. Han et al., Phys. Chem. Chem. Phys., 2015, 17, 22576. [4] A. Ramirez et al., J. Phys. Chem. C, 2014, 118, 14073. [5] S. Hirai et al., RSC Adv., 2016, 6, 2019.

ChemInform Abstract: Attractive Aerobic Access to the α,β‐Unsaturated Acyl Azolium Intermediate: Oxidative NHC Catalysis via Multistep Electron Transfer.

AbstractOxidative esterification and oxidative Michael addition are accomplished using air as oxidative agent and oxidative N‐heterocyclic carbene (NHC) catalysis together with several additives.

Multistep Electron Transfer Systems Containing [2.2]- or [3.3]Paracyclophane.

Paracyclophanes (PCPs), which exhibit interesting properties due to their transannular interactions, have been employed as a spacer in various electron transfer (ET) systems. In the present work, we investigated ET processes in dyads and triads containing [2.2]PCP or [3.3]PCP as donors to study their properties in multistep ET processes. The dyad molecules of PCP and 1,8-naphthalimide (NI) as a photosensitizing electron acceptor exhibited charge separation (CS) upon excitation of NI. In addition, triads of NI, PCP, and carbazole showed charge shift after an initial CS, thus confirming multistep ET. In this study, we demonstrated that use of [3.3]PCP in place of [2.2]PCP enhanced the initial CS rate. Lower oxidation potentials and a smaller reorganization energy for [3.3]PCP are shown to be key factors for this enhanced CS rate. Both of these properties are closely related to the strained structure of PCP; hence, the present results demonstrate the importance of strain in ET chemistry.

Charge-Transfer Dynamics at the α/β Subunit Interface of a Photochemical Ribonucleotide Reductase.

Ribonucleotide reductase (RNR) catalyzes the conversion of ribonucleotides to deoxyribonucleotides to provide the monomeric building blocks for DNA replication and repair. Nucleotide reduction occurs by way of multistep proton-coupled electron transfer (PCET) over a pathway of redox active amino acids spanning ∼35 Å and two subunits (α2 and β2). Despite the fact that PCET in RNR is rapid, slow conformational changes mask examination of the kinetics of these steps. As such, we have pioneered methodology in which site-specific incorporation of a [Re(I)] photooxidant on the surface of the β2 subunit (photoβ2) allows photochemical oxidation of the adjacent PCET pathway residue β-Y356 and time-resolved spectroscopic observation of the ensuing reactivity. A series of photoβ2s capable of performing photoinitiated substrate turnover have been prepared in which four different fluorotyrosines (FnYs) are incorporated in place of β-Y356. The FnYs are deprotonated under biological conditions, undergo oxidation by electron transfer (ET), and provide a means by which to vary the ET driving force (ΔG°) with minimal additional perturbations across the series. We have used these features to map the correlation between ΔG° and kET both with and without the fully assembled photoRNR complex. The photooxidation of FnY356 within the α/β subunit interface occurs within the Marcus inverted region with a reorganization energy of λ ≈ 1 eV. We also observe enhanced electronic coupling between donor and acceptor (HDA) in the presence of an intact PCET pathway. Additionally, we have investigated the dynamics of proton transfer (PT) by a variety of methods including dependencies on solvent isotopic composition, buffer concentration, and pH. We present evidence for the role of α2 in facilitating PT during β-Y356 photooxidation; PT occurs by way of readily exchangeable positions and within a relatively "tight" subunit interface. These findings show that RNR controls ET by lowering λ, raising HDA, and directing PT both within and between individual polypeptide subunits.

Attractive aerobic access to the α,β-unsaturated acyl azolium intermediate: oxidative NHC catalysis via multistep electron transfer

Employing a system of electron transfer mediators (ETMs) enables the use of oxygen as the terminal oxidant for oxidative NHC-catalyzed reactions.

Titanium deposition from ionic liquids - appropriate choice of electrolyte and precursor.

In this study titanium isopropoxide was dissolved in 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (BMITFSI) and further in a custom-made guanidinium-based ionic liquid (N11N11NpipGuaTFSI). Electrochemical investigations were carried out by means of cyclic voltammetry (CV) and the initial stages of metal deposition were followed by in situ scanning tunneling microscopy (STM). For BMITFSI we found one large cathodic reduction peak at a potential of -1.2 V vs. Pt, corresponding to the growth of monoatomic high islands. The obtained deposit was identified as elemental titanium by Auger Electron Spectroscopy (AES). Furthermore, we found a corresponding anodic peak at -0.3 V vs. Pt, which is associated with the dissolution of the islands. This observation leads to the assumption that titanium deposition from the imidazolium-based room-temperature ionic liquid (RTIL) proceeds in a one-step electron transfer. In contrast, for the guanidinium-based RTIL we found several peaks during titanium reduction and oxidation, which indicates a multi-step electron transfer in this alternative electrolyte.

Visible light-driven water oxidation using a covalently-linked molecular catalyst-sensitizer dyad assembled on a TiO2 electrode.

The combination of porphyrin as a sensitizer and a ruthenium complex as a water oxidation catalyst (WOC) is promising to exploit highly efficient molecular artificial photosynthetic systems. A covalently-linked ruthenium-based WOC-zinc porphyrin (ZnP) sensitizer dyad was assembled on a TiO2 electrode for visible-light driven water oxidation. The water oxidation activity was found to be improved in comparison to the reference systems with the simple combination of the individual WOC and ZnP as well as with ZnP solely, demonstrating the advantage of the covalent linking approach over the non-covalent one. More importantly, via vectorial multi-step electron transfer triggered by visible light, the dye-sensitized photoelectrochemical cell (DSPEC) achieved a broader PEC response in the visible region than DSPECs with conventional ruthenium-based sensitizers. Initial incident photon-to-current efficiencies of 18% at 424 nm and 6.4% at 564 nm were attained under monochromatic illumination and an external bias of -0.2 V vs. NHE. Fast electron transfer from the WOC to the photogenerated radical cation of the sensitizer through the covalent linkage may suppress undesirable charge recombination, realizing the moderate performance of water oxidation. X-ray photoelectron spectroscopic analysis of the photoanodes before and after the DSPEC operation suggested that most of the ruthenium species exist at higher oxidation states, implying that the insufficient oxidation potential of the ZnP moiety for further oxidizing the intermediate ruthenium species at the photoanode is at least the bottleneck of the system.

Asymmetric aerobic oxidative NHC-catalysed synthesis of dihydropyranones utilising a system of electron transfer mediators.

In the context of green chemistry, the replacement of high molecular weight stoichiometric oxidants with O2 is most desirable but difficult. Here, we report the asymmetric aerobic oxidative synthesis of dihydropyranones. The oxidation is aided by a system of electron transfer mediators and is selective toward the homoenolate. The dihydropyranones can be isolated in high to excellent yields, with high ee (up to 95%).

Photo-initiated multi-step electron transfer in donor–acceptor systems using a novel bi-functionalized perylene chromophore

The excited state and redox properties of a new bi-functional perylene redox chromophore, 2,3-dihydro-1-azabenzo[cd]perylene (DABP), are described. Perylene has been widely used in electron donor–acceptor molecules in fields ranging from artificial photosynthesis to molecular spintronics. However, attaching multiple redox components to perylene to carry out multi-step electron transfer reactions often produces hard to separate regioisomers, which complicate data analysis. The use of DABP provides a strategy to retain the electronic properties of perylene, yet eliminate regioisomers. Ultrafast photo-initiated single- and two-step electron transfer reactions in three linear electron donor–acceptor systems incorporating DABP are described to illustrate its utility.

Photogeneration and Quenching of Tryptophan Radical in Azurin.

Tryptophan and tyrosine can form radical intermediates that enable long-range, multistep electron transfer (ET) reactions in proteins. This report describes the mechanisms of formation and quenching of a neutral tryptophan radical in azurin, a blue-copper protein that contains native tyrosine (Y108 and Y72) and tryptophan (W48) residues. A long-lived neutral tryptophan radical W48• is formed upon UV-photoexcitation of a zinc(II)-substituted azurin mutant in the presence of an external electron acceptor. The quantum yield of W48• formation (Φ) depends upon the tyrosine residues in the protein. A tyrosine-deficient mutant, Zn(II)Az48W, exhibited a value of Φ = 0.080 with a Co(III) electron acceptor. A nearly identical quantum yield was observed when the electron acceptor was the analogous tyrosine-free, copper(II) mutant; this result for the Zn(II)Az48W:Cu(II)Az48W mixture suggests there is an interprotein ET path. A single tyrosine residue at one of the native positions reduced the quantum yield to 0.062 (Y108) or 0.067 (Y72). Wild-type azurin with two tyrosine residues exhibited a quantum yield of Φ = 0.045. These data indicate that tyrosine is able to quench the tryptophan radical in azurin.

Multistep energy and electron transfer processes in novel rotaxane donor-acceptor hybrids generating microsecond-lived charge separated states.

A new set of [Cu(phen)2]+ based rotaxanes, featuring [60]-fullerene as an electron acceptor and a variety of electron donating moieties, namely zinc porphyrin (ZnP), zinc phthalocyanine (ZnPc) and ferrocene (Fc), has been synthesized and fully characterized with respect to electrochemical and photophysical properties. The assembly of the rotaxanes has been achieved using a slight variation of our previously reported synthetic strategy that combines the Cu(i)-catalyzed azide-alkyne cycloaddition reaction (the "click" or CuAAC reaction) with Sauvage's metal-template protocol. To underline our results, complementary model rotaxanes and catenanes have been prepared using the same strategy and their electrochemistry and photo-induced processes have been investigated. Insights into excited state interactions have been afforded from steady state and time resolved emission spectroscopy as well as transient absorption spectroscopy. It has been found that photo-excitation of the present rotaxanes triggers a cascade of multi-step energy and electron transfer events that ultimately leads to remarkably long-lived charge separated states featuring one-electron reduced C60 radical anion (C60˙-) and either one-electron oxidized porphyrin (ZnP˙+) or one-electron oxidized ferrocene (Fc˙+) with lifetimes up to 61 microseconds. In addition, shorter-lived charge separated states involving one-electron oxidized copper complexes ([Cu(phen)2]2+ (τ < 100 ns)), one-electron oxidized zinc phthalocyanine (ZnPc˙+; τ = 380-560 ns), or ZnP˙+ (τ = 2.3-8.4 μs), and C60˙- have been identified as intermediates during the sequence. Detailed energy diagrams illustrate the sequence and rate constants of the photophysical events occurring with the mechanically-linked chromophores. This work pioneers the exploration of mechanically-linked systems as platforms to position three distinct chromophores, which are able to absorb light over a very wide range of the visible region, triggering a cascade of short-range energy and electron transfer processes to afford long-lived charge separated states.

Fractional transfer of a free unpaired electron to overcome energy barriers in the formation of Fe(4+) from Fe(3+) during the core contraction of macrocycles: implication for heme distortion.

The free unpaired electron in Fe(3+) ions cannot be directly removed, and needs a transfer pathway with at least four steps to overcome the high energy barriers to form Fe(4+) ions. Fine changes in the electronic structure of Fe(3+) ions on spin conversion were identified through a deeper analysis of the diffraction, spectral and electrochemical data for six non-planar iron porphyrins. Fe(3+) ions can form four d electron tautomers as the compression of the central ion is increased. This indicates that the Fe(3+) ion undergoes a multistep electron transfer where the total energy gap of electron transfer is split into several smaller gaps to form high-valent Fe(4+) ions. We find that the interchange of these four electron tautomers is clearly related to the core size of the macrocycle in the current series. The large energy barrier to produce iron(iv) complexes is overcome through a gradient effect of multiple energy levels. In addition, a possible porphyrin Fe(3+)˙ radical may be formed from its stable isoelectronic form, porphyrin Fe(3+), under strong core contraction. These results indicate the important role of heme distortion in its catalytic oxidation functions.

Photocatalytic Reduction of CO2 into Fuels: A Short Review

The photocatalytic reduction of CO2 with water vapour and catalysts under UV irradiation to yield hydrocarbons is a potential way of decreasing greenhouse gas and it represents an attractive alternative energy source to fossil fuels. However, this process still has to overcome several hurdles, because it involves the activation of two stable molecules, H2O and CO2, and simultaneous conversion through a multi-step electron transfer reaction. The problem of CO2 emission and the possibility of exploiting CO2 as a raw material reaction is first reported in this short review. Subsequently, the fundamentals of photocatalysis are described. Finally, TiO2-based photocatalysts are reviewed, taking into consideration the optimization methods that can be adopted to improve performances. The information gained from this analysis will help to contribute towards a better understanding of the main parameters that affect the activity of photocatalysts and will ultimately lead to the optimized synthesis of more efficient photocatalytic material for the photocatalytic reduction of CO2 to fuels.