Catalytic Reduction Of CO2 Research Articles

Design and selective photocatalytic activity of highly concentrated C, N, O co-doped Zn, S co-defective ZnS particles mediated by ethylenediamine derivatives

ZnS particles are synthesized by adding ethylenediamine (en) and its derivatives, triethylenetetramine (trien) and ethylenediaminetetraacetate (EDTA), as capping agents. In the capped ZnS particles, Zn and S defects due to O, N-doping and loading of carbon dots (CDs) are confirmed. The degree of defects is suggested from photoluminescence (PL) spectra and density of states calculations. The intensities of PL are lowered in the order of ZnS-none> ZnS-EDTA> ZnS-en> ZnS-trien, while the photocurrent densities are increased in the opposite order. Time resolved PL shows extremely slow recombination of photo-excited charges in ZnS-trien. ZnS-trien is the most suitable for catalyzing CO2 reduction, and ZnS-en is the best for water-splitting. The excellent photocatalytic activity of ZnS-trien encapsulated with N-CDs shell maintains without photo-corrosion over more than five recycling experiments. Eventually, this study reveals that well-ordered ZnS with C, N, O co-doping-induced Zn, S co-defects is self-assembled using en-derivatives as capping agents, exhibits good photocatalytic activity without grafting with other particles.

Structurally designable Bi2S3/P-doped ZnO S-scheme photothermal metamaterial enhanced CO2 reduction

In this study, a novel 3D Bi2S3/P-ZnO S-scheme gyroid metamaterial was prepared by 3D printing and the hydrothermal method. The unique gyroid macrostructure and nanoarray microstructure offer a large volume of active sites for the photothermal catalytic reaction, thus improving the overall utilization rate of light. ZnO obtained by sintering the precursor produces oxygen vacancies, primarily due to P doping. This enables it to have a photothermal effect. After loading Bi2S3, the photothermal effect of P-ZnO and Bi2S3 is coupled, further increasing the surface temperature of composite structure and speeding up the reaction rate. P-ZnO and Bi2S3 formed a S-scheme heterojunction, promoting the separation and transfer of photogenerated carriers. Under simulated sunlight, the CO and CH4 yield of Bi2S3/P-ZnO S-scheme gyroid metamaterials are 8.87 and 1.49 μmol h−1. These are 3.45 times and 4.65 times of P-ZnO gyroid structure, respectively. Through SEM, TEM, XPS, ESR and in-situ FTIR characterizations, the transfer paths of photogenerated carriers between heterojunctions were revealed, and the reaction paths of photothermal catalytic CO2 reduction were explored. The co-design of 3D structure and material allows for a novel concept for further improving photothermal catalytic performance.

Diverse Cooperative Reactivity at a Square Planar Aluminium Complex and Catalytic Reduction of CO2.

The use of a sterically demanding pincer ligand to prepare an unusual square planar aluminium complex is reported. Due to the constrained geometry imposed by the ligand scaffold, this four-coordinate aluminium centre remains Lewis acidic and reacts via differing metal-ligand cooperative pathways for activating ketones and CO2 . It is also a rare example of a single-component aluminium system for the catalytic reduction of CO2 to a methanol equivalent at room temperature.

Hybrid Catalyst Coupling Zn Single Atoms and CuNx Clusters for Synergetic Catalytic Reduction of CO2

AbstractReverse water‐gas shift (RWGS) reaction is the initial and necessary step of CO2 hydrogenation to high value‐added products, and regulating the selectivity of CO is still a fundamental challenge. In the present study, an efficient catalyst (CuZnNx@C‐N) composed by Zn single atoms and Cu clusters stabilized by nitrogen sites is reported. It contains saturated four‐coordinate Zn‐N4 sites and low valence CuNx clusters. Monodisperse Zn induces the aggregation of pyridinic N to form Zn‐N4 and N4 structures, which show strong Lewis basicity and has strong adsorption for *CO2 and *COOH intermediates, but weak adsorption for *CO, thus greatly improves the CO2 conversion and CO selectivity. The catalyst calcined at 700 °C exhibits the highest CO2 conversion of 43.6% under atmospheric pressure, which is 18.33 times of Cu‐ZnO and close to the thermodynamic equilibrium conversion rate (49.9%) of CO2. In the catalytic process, CuNx not only adsorbs and activates H2, but also cooperates with the adjacent Zn‐N4 and N4 structures to jointly activate CO2 molecules and further promotes the hydrogenation of CO2. This synergistic mechanism will provide new insights for developing efficient hydrogenation catalysts.

Significantly enhanced photothermal catalytic CO2 reduction over TiO2/g-C3N4 composite with full spectrum solar light

Using solar energy to drive catalytic conversion of CO2 into value-added chemicals has great potential to alleviate the global energy shortage and anthropogenic climate change. Herein, a “hitting three birds with one stone” strategy was reported to prepared boron-doped g-C3N4/TiO2-x composite (BCT) by a one-step thermal reduction process. A series of characterizations showed that the composite catalyst has extended full-spectrum absorption, rapid photogenerated charge separation, and outstanding CO2 photoreduction performance (265.2 μmol g-1h−1), which is 7.5 and 9.2 times higher than that of pure TiO2 and g-C3N4, respectively. In addition, the CO2 conversion rate can be further increased to 345.1 μmol g-1h−1 at 70 °C due to its excellent photothermal conversion. Mechanistic studies reveal that synergistic effects alter the charge density distribution, thereby lowering the energy barrier for CO2 conversion by adsorbing and activating CO2 molecules. This work provides a novel three-in-one integrated strategy for fabricating high-efficiency catalysts.

Two Novel Schiff Base Manganese Complexes as Bifunctional Electrocatalysts for CO2 Reduction and Water Oxidation.

One mononuclear Mn(III) complex [MnIIIL(H2O)(MeCN)](ClO4) (1) and one hetero-binuclear complex [(CuIILMnII(H2O)3)(CuIIL)2](ClO4)2·CH3OH (2) have been synthesized with the Schiff base ligand (H2L = N,N'-bis(3-methoxysalicylidene)-1,2-phenylenediamine). Single crystal X-ray structural analysis manifests that the Mn(III) ion in 1 has an octahedral coordination structure, whereas the Mn(II) ion in 2 possesses a trigonal bipyramidal configuration and the Cu(II) ion in 2 is four-coordinated with a square-planar geometry. Electrochimerical catalytic investigation demonstrates that the two complexes can electrochemically catalyze water oxidation and CO2 reduction simultaneously. The coordination environments of the Mn(III), Mn(II), and Cu(II) ions in 1 and 2 were provided by the Schiff base ligand (L) and labile solvent molecules. The coordinately unsaturated environment of the Cu(II) center in 2 can perfectly facilitate the catalytic performance of 2. Complexes 1 and 2 display that the over potentials for water oxidation are 728 mV and 216 mV, faradaic efficiencies (FEs) are 88% and 92%, respectively, as well as the turnover frequency (TOF) values for the catalytic reduction of CO2 to CO are 0.38 s-1 at -1.65 V and 15.97 s-1 at -1.60 V, respectively. Complex 2 shows much better catalytic performance for both water oxidation and CO2 reduction than that of complex 1, which could be owing to a structural reason which is attributed to the synergistic catalytic action of the neighboring Mn(III) and Cu(II) active sites in 2. Complexes 1 and 2 are the first two compounds coordinated with Schiff base ligand for both water oxidation and CO2 reduction. The finding in this work can offer significant inspiration for the future development of electrocatalysis in this area.

First principles investigations of structural and electronic properties of Ga-doped ZnZrOx solid solutions for catalytic reduction of CO2

ZnZrOx and gallium-doped ZnZrOx solid solutions exhibit excellent catalytic properties for the hydronation of CO2 to methanol. However, the details of the structures and catalytic properties are not fully understood. Herein, density functional theory has been used to explore the structural, electronic and catalytic properties of ZnZrOx and gallium-doped ZnZrOx catalysts. We used zinc-doped Zr sites of tetragonal ZrO2 with different Zn concentrations as a model to simulate the ZnZrOx solid solution. For the Ga-doped ZnZrOx catalysts, Zr atoms were substituted by Ga atoms. The doping of Zn atoms in ZrO2 reduce the coordination numbers of Zr atoms. Heavily Zn-doped ZrO2 systems do not maintain the tetragonal structure and, hence, deviate from the perfect tetragonal phase. At ∼29.16% Zn concentration, the bandgap of ZnZrOx has been reduced from 4.45 to 2.45 eV. The insertion of Ga in ZnZrOx solid solutions shifts the conduction band edges back, increasing the bandgap of the solid solutions. The Ga doping improves the CO2 adsorption ability of ZnZrOx solid solutions and efficiently dissociates the H2 molecules. On the surface of Ga-Zn doped ZrO2(101), the hydrogenation of CO2 to methanol via the formate pathway is more favorable than the CO pathway.

Photocatalytic reduction of CO2 by two-dimensional Zn-MOF-NH2/Cu heterojunctions

The two-dimensional (2D) Zn-MOF-NH2/Cu heterojunctions were prepared via in situ photodeposition of Cu NPs on the surface of 2D Zn-MOF-NH2. The 2D Zn-MOF-NH2/Cu (1 wt%) heterojunction exhibited the excellent photocatalytic activity with a CO production rate of 0.76 μmol/h under irradiation of full spectrum light only using H2O as a hole sacrificial agent, which is 16.2 and 2.7 times that of bulk Zn-MOF-NH2 and 2D Zn-MOF-NH2 nanosheets, respectively. This work provides an insight into design of a highly efficient 2D MOFs-based photocatalysts in catalytic CO2 reduction.

Catalytic activity of silicene biflakes for CO2 reduction: a theoretical approach.

The use of renewable energy sources to reduce carbon dioxide (CO2) emissions has gained significant attention in recent years. The catalytic reduction of CO2 into value-added products is a promising approach to achieve this goal, and silicene biflakes (2Si) have been identified as potential candidates for this task. In this study, we explored the catalytic activity of these structures using density functional theory calculations. Our results show that the reaction pathway involves the adsorption of CO2 onto the silicene surface, followed by the addition of hydrogen molecules to form products such as formic acid, methanol, methane, carbon monoxide, and formaldehyde. Our proposed mechanism indicates that silicene biflakes exhibit a higher affinity for CO2 than single-layer silicon. We also found that the hydrogenation with H2 occurs by adding one hydrogen atom to the absorbed CO2 and another to the surface of 2Si. Intermediate species are reduced by systematically adding hydrogen atoms and removing water molecules, forming formic acid as the most probable product. The rate-controlling step for this reaction has an energy of 32.9 kcal mol-1. In contrast, the process without a catalyst shows an energy of 74.6 kcal mol-1, suggesting that the silicon bilayer is a structure with outstanding potential to capture and reduce CO2. Our study provides important insights into the fundamental mechanisms underlying the silicene-mediated CO2 reduction and could facilitate the development of more efficient catalysts for this process.

1,2-Azolylamidino ruthenium(II) complexes with DMSO ligands: electro- and photocatalysts for CO2 reduction.

New 1,2-azolylamidino complexes fac-[RuCl(DMSO)3(NHC(R)az*-κ2N,N)]OTf [R = Me (2), Ph (3); az* = pz (pyrazolyl, a), indz (indazolyl, b)] are synthesized via chloride abstraction from their corresponding precursors cis,fac-[RuCl2(DMSO)3(az*H)] (1) after subsequent base-catalyzed coupling of the appropriate nitrile with the 1,2-azole previously coordinated. All the compounds are characterized by 1H NMR, 13C NMR and IR spectroscopy. Those derived from MeCN are also characterized by X-ray diffraction. Electrochemical studies showed several reduction waves in the range of -1.5 to -3 V. The electrochemical behavior in CO2 media is consistent with CO2 electrocatalytic reduction. The catalytic activity expressed as [icat(CO2)/ip(Ar)] ranged from 1.7 to 3.7 for the 1,2-azolylamidino complexes at voltages of ca. -2.7 to -3 V vs. ferrocene/ferrocenium. Controlled potential electrolysis showed rapid decomposition of the Ru catalysts. Photocatalytic CO2 reduction experiments using compounds 1b, 2b and 3b carried out in a CO2-saturated MeCN/TEOA (4 : 1 v/v) solution containing a mixture of the catalyst and [Ru(bipy)3]2+ as the photosensitizer under continuous irradiation (light intensity of 150 mW cm-2 at 25 °C, λ > 300 nm) show that compounds 1b, 2b and 3b allowed CO2 reduction catalysis, producing CO and trace amounts of formate. The combined turnover number for the production of formate and CO is ca. 100 after 8 h and follows the order 1b < 2b ≈ 3b.

Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis.

Cyclic iron tetracarbenes are an emerging class of macrocyclic iron N-heterocyclic carbene (NHC) complexes. They can be considered as an organometallic compound class inspired by their heme analogs, however, their electronic properties differ, e.g. due to the very strong σ-donation of the four combined NHCs in equatorial coordination. The ligand framework of iron tetracarbenes can be readily modified, allowing fine-tuning of the structural and electronic properties of the complexes. The properties of iron tetracarbene complexes are discussed quantitatively and correlations are established. The electronic nature of the tetracarbene ligand allows the isolation of uncommon iron(III) and iron(IV) species and reveals a unique reactivity. Iron tetracarbenes are successfully applied in C-H activation, CO2 reduction, aziridination and epoxidation catalysis and mechanisms as well as decomposition pathways are described. This review will help researchers evaluate the structural and electronic properties of their complexes and target their catalyst properties through ligand design.

Continuously Flow Photothermal Catalysis Efficiently CO2 Reduction Over S-Scheme 2D/0D Bi5 O7 I-OVs/Cd0.5 Zn0.5 S Heterojunction with Strong Interfacial Electric Field.

Using CO2 , water, and sunlight to produce solar fuel is a very attractive process, which can synchronously reduce carbon and convert solar energy into hydrocarbons. However, photocatalytic CO2 reduction is often limited by the low selectivity of reduction products and poor photocatalytic activity. In this study, S-scheme Bi5 O7 I-OVs/Cd0.5 Zn0.5 S (Bi5 O7 I-OVs/CZS-0.5) heterojunction with strong interfacial electric field (IEF) is prepared by in situ growth method. The performance of reduction CO2 to CO is studied by continuous flow photothermal catalytic (PTC) CO2 reduction platform. 12.5% Bi5 O7 I-OVs/CZS-0.5 shows excellent CO yield of 58.6µmolg-1 h-1 and selectivity of 98.4%, which are 35.1 times than that of CZS-0.5 under visible light. The charge transfer path of the S-scheme through theoretical calculation (DFT), in situ irradiation Kelvin probe force microscope (ISI-KPFM) and in situ irradiation X-ray photoelectron spectroscopy (ISI-XPS) analysis, is verified. The study can provide useful guidance and reference for improving activity by oxygen vacancy induced strong IEF and the development of a continuous flow PTC CO2 reduction system.

Guiding catalytic CO2 reduction to ethanol with copper grain boundaries.

The grain boundaries (GBs) in copper (Cu) electrocatalysts have been suggested as active sites for CO2 electroreduction to ethanol. Nevertheless, the mechanisms are still elusive. Herein, we describe how GBs tune the activity and selectivity for ethanol on two representative Cu-GB models, namely Cu∑3/(111) GB and Cu∑5/(100) GB, using joint first-principles calculations and experiments. The unique geometric structures on the GBs facilitate the adsorption of bidentate intermediates, *COOH and *CHO, which are crucial for CO2 activation and CO protonation. The decreased CO-CHO coupling barriers on the GBs can be rationalized via kinetics analysis. Furthermore, when introducing GBs into Cu (100), the product is selectively switched from ethylene to ethanol, due to the stabilization effect for *CH3CHO and inapposite geometric structure for *O adsorption, which are validated by experimental trends. An overall 12.5 A current and a single-pass conversion of 5.18% for ethanol can be achieved over the synthesized Cu-GB catalyst by scaling up the electrode into a 25 cm2 membrane electrode assembly system.

Oxygen-deficient MoO3−x evoked synergistic photo-thermal catalytic CO2 reduction over g-C3N4

In situ photodeposition of MoO3−x with oxygen vacancies evoked synergistic photo-thermal catalytic CO2 reduction over g-C3N4.

Lattice strain and interfacial engineering of a Bi-based electrocatalyst for highly selective CO2 electroreduction to formate

Surface strain tuning in a coupled heterostructure efficiently engineers the catalytic performance of heterogeneous catalysts by altering the electronic structures and boosting electron transport. Generally, Bi-based catalysts are more favorable than ZnO for CO2 electroreduction to formate, but Bi is much more costly than Zn. Herein, a new Bi2O2CO3/ZnO heterojunction catalyst with porous nanoplate morphology is synthesized through a hexadecyl trimethyl ammonium bromide-templated hydrothermal reaction for a highly efficient catalytic CO2 reduction reaction (CO2RR) to produce formate. The Bi2O2CO3/ZnO catalyst shows a maximum Faradaic efficiency of 92% for formate production at −1.0 V vs. reversible hydrogen electrode (RHE) and a large partial current density of −200 mA mgBi−1 at −1.2 V vs. RHE. More importantly, the mass activity of Bi2O2CO3/ZnO normalized by Bi mass is an approximately 3.1-fold enhancement over that of the pristine Bi2O2CO3 at −1.2 V vs. RHE. By coupling X-ray photoelectron spectroscopy and adsorption spectroscopy measurements, the charge transfer from the Zn atom to the Bi atom through a heterogeneous interface results in an electron-enriched Bi2O2CO3 surface, which facilitates CO2 capture and activation. Meanwhile, compressive stress produced on the catalyst surface helps optimize the adsorption energy of the reaction intermediate, synergistically enhancing the catalytic selectivity and activity of Bi2O2CO3/ZnO for electrochemical CO2 reduction to formate.

Photothermal Catalytic Reduction of CO2 by Cobalt Silicate Heterojunction Constructed from Clay Minerals

The coupled utilization of solar and thermal energy is considered an efficient way to improve the efficiency of CO2 reduction. Herein, palygorskite (Pal) clay is as a silicon source, while Co2+ is introduced to prepare two-dimensional Co2SiO4 nanosheets, and the excess of Co2+ leads to the growth of Co3O4 on the surface of Co2SiO4 to obtain an S-scheme Co2SiO4/Co3O4−x heterojunction, which facilitates the charge transfer and maintains higher redox potentials. Benefiting from black color and a narrow band gap, the cobalt oxide on the surface can increase the light absorption and produce a local photothermal effect. Under proper thermal activation conditions, the photoelectrons captured by the abundant oxygen vacancies can obtain a secondary leap to the semiconductor conduction band (CB), suppressing the recombination of electron-hole pairs, thus favoring the electron transfer on Co2SiO4/Co3O4−x. The composites not only have abundant oxygen vacancies, but also have a large specific surface area for the adsorption and activation of CO2. The yields of CH3OH on Co2SiO4/Co3O4−5% reach as high as 48.9 μmol·g−1·h−1 under simulated sunlight irradiation. In situ DRIFTS is used to explore the photocatalytic reduction CO2 mechanism. It is found that the thermal effect facilitates the generation of the key intermediate COOH* species. This work provides a new strategy for photothermal catalytic CO2 reduction by taking advantage of natural clay and solar energy.

Architecture Design and Catalytic Activity: Non‐Noble Bimetallic CoFe/fe3O4 Core–Shell Structures for CO2 Hydrogenation

Non-noble metal catalysts now play a key role in promoting efficiently and economically catalytic reduction of CO2 into clean energy, which is an important strategy to ameliorate global warming and resource shortage issues. Here, a non-noble bimetallic catalyst of CoFe/Fe3 O4 nanoparticles is successfully designed with a core-shell structure that is well dispersed on the defect-rich carbon substrate for the hydrogenation of CO2 under mild conditions. The catalysts exhibit a high CO2 conversion activity with the rate of 30% and CO selectivity of 99%, and extremely robust stability without performance decay over 90 h in the reverse water gas shift reaction process. Notably, it is found that the reversible exsolution/dissolution of cobalt in the Fe3 O4 shell will lead to a dynamic and reversible deactivation/regeneration of the catalysts, accompanying by shell thickness breathing during the repeated cycles, via atomic structure study of the catalysts at different reaction stages. Combined with density functional theory calculations, the catalytic activity reversible regeneration mechanism is proposed. This work reveals the structure-property relationship for rational structure design of the advanced non-noble metallic catalyst materials with much improved performance.

Rational design of honeycomb-like APTES-TiO2/COF heterostructures: Promoted intramolecular charge transfer for visible-light-driven catalytic CO2 reduction

Utilizing organic structures to combine with TiO2 is regarded as a potent strategy for meliorating the photocatalytic performance of TiO2. Nonetheless, exploration towards the impact of modulation at organic molecular level for the COF construction (e.g., the types, substituent group and the arrangement of the building blocks), upon the optical or/and electronic features of TiO2-COF heterostructures is relatively scarce. Herein, a nitrogen-rich covalent organic framework (COF) which is employed to covalently integrate with aminopropyltriethoxysilane modified TiO2 (APTES-TiO2) through Schiff-base reaction. Due to the significant charge density disparity between the two building units, 2, 4, 6-tris (4-aminophenyl)-1, 3, 5-triazine (TAPT) and melem in the hybridized TiO2@TMP-COF with honeycomb-like morphology, the promoted intramolecular charge transfer (ICT) brings about accelerated charge delivery efficiency within the heterostructure by contrast to the structure-similar COF established hybrid, TiO2@TM-COF, which is built by the identical approach except for the involvement of TAPT monomer. Accordingly, the photocatalytic CO2 reduction reaction (CO2RR) dominated by TiO2@TMP-COF affords the CO production of 6.76 μmol·g−1·h−1, about 3 times that of TiO2@TM-COF; still, the enhanced catalytic efficiency than either the constituent TiO2 or TMP-COF owes much to the Z-scheme charge migration dynamics and augmented light-capturing capability within the hybrid. Our study opens a new landscape of heterostructure establishment for simultaneously ameliorating light absorption and accelerating charge delivery in photocatalysis.

Natural halloysite nanotubes supported Ru as highly active catalyst for photothermal catalytic CO2 reduction

Photothermal catalytic CO2 reduction has emerged as a promising approach to efficiently utilize solar energy and reduce greenhouse gas emissions. Designing novel catalyst with high activity and selectivity is essential and challenging for practical applications of photothermal CO2 reduction. Herein, we report that natural halloysite nanotubes-supported Ru nanoparticles can boost the photothermal catalytic activity and selectivity of CO2 methanation under continuous flow conditions. The photothermal catalytic performance of optimized catalyst is up to 1704 mmolCH4 gcat−1 h−1 with 93% CH4 selectivity and 68% CO2 conversion, outperforming any other Ru-based catalysts in photothermal CO2 reduction. Mechanistic studies show that the outstanding catalytic performance is mainly attributed to the unique mesoporous tubular structure, excellent ability of light-to-heat and interfacial interactions between the halloysite nanotubes and Ru. This method of utilizing natural minerals as support provides a facile avenue for rational design of abundant and low-cost catalysts for efficient photothermal catalytic CO2 reduction.

μ-Oxo Dimerization Effects on Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO2 Reduction Catalyst

Iron 5,10,15,20-tetra(para-N,N,N-trimethylanilinium)porphyrin (Fe-p-TMA) is a water-soluble catalyst capable of electrochemical and photochemical CO2 reduction. Although its catalytic ability has been thoroughly investigated, the mechanism and associated intermediates are largely unknown. Previous studies proposed that Fe-p-TMA enters catalytic cycles as a monomeric species. However, we demonstrate herein that, in aqueous solutions, Fe-p-TMA undergoes formation of a μ-oxo porphyrin dimer that exists in equilibrium with its monomeric form. The propensity for μ-oxo formation is highly dependent on the solution pH and ionic strength. Indeed, the μ-oxo form is stabilized in the presence of electrolytes that are key components of catalytically relevant conditions. By leveraging the ability to chemically control and spectrally address both species, we characterize their ground-state electronic structures and excited-state photodynamics. Global fitting of ultrafast transient absorption data reveals two distinct excited-state relaxation pathways: a three-component sequential model consistent with monomeric relaxation and a two-component sequential model for the μ-oxo species. Relaxation of the monomeric species is best described as a ligand-to-metal charge transfer (τ1 = ∼500 fs), an ionic strength-dependent metal-to-ligand charge transfer (τ2 = 2-4 ps), and finally relaxation of a ligand field excited state to the ground state (τ3 = 5 ps). Conversely, excited-state relaxation of the μ-oxo species proceeds via cleavage of an FeIII-O bond to generate transient FeIV═O and FeII porphyrin species (τ1 = 2 ps) that recombine to the ground-state μ-oxo species (τ2 = ∼1 ns). This latter lifetime extends to timescales relevant for chemical reactivity. It is therefore emphasized that further consideration of catalyst speciation and chemical microenvironments is necessary for elucidating the mechanisms of catalytic CO2 reduction reactions.