Polyoxometalate-Based Catalysts for CO₂ Conversion

Abstract

Polyoxometalates (POMs) are a diverse class of anionic metal-oxo clusters with intriguing chemical and physical properties. Owing to unrivaled versatility and structural variation, POMs have been extensively utilized for catalysis for a plethora of reactions. In this focused review, the applications of POMs as promising catalysts or co-catalysts for CO₂ conversion, including CO₂ photo/electro reduction and CO₂ as a carbonyl source for the carbonylation process are summarized. A brief perspective on the potentiality in this field is proposed.

1. Introduction

As the main greenhouse gas produced by human activity, about 10 ± 0.5 gigatons CO₂ were released in 2018, bringing the atmospheric CO₂ concentration level over the threshold of 400 ppm [1]. The large increase of global CO₂ levels has led to serious environment problem. Meanwhile, it is also the most abundant and nontoxic carbon resource for preparing useful compounds [2]. From a practical point of view, its catalytic conversion is of significance to the supply of renewable energy, chemicals, and mitigation of global warming.

Polyoxometalates with {MO_x} (x = 5, 6) as basic construction units have derived an enormous fraternity of inorganic molecular complexes [3,4,5,6]. To obtain various composite materials with specific function, the modification and decoration of POM can be attained by partially substituting {MO_x} units with different transition metal moieties or attaching organometallic complexes onto POM. Based on geometrical morphology, most of POMs can be classified as Keggin, Wells-Dawson, Anderson-Evans, Silverton, Waugh, Strandberg, Lindqvist, and Peacock-Weakley type structures, which are named after their corresponding discoverers [7]. Among these types, Keggin-type POMs ([XM₁₂O₄₀]ⁿ⁻) are comparatively well-studied. POMs discussed in the following sections are mostly Keggin type [8]. The original Keggin structure is designated α-, which consists of a tetrahedron central ion, [XO₄]^{n − 8}, caged by twelve MO₆ octahedron. The Keggin structure includes four additional isomers (β-, γ-, δ-, and ε-), each resulting from 60° rotations of the four {M₃O₁₃} units [9]. The lacunary species of Keggin-type POMs stem from removing a variable number of {MO₆} octahedra from the plenary polyanion, resulting in metal-oxide clusters with vacant addenda metal sites. Such vacant sites can be treated as inorganic multidentate ligands (Figure 1) [10], For example, sandwich-type POMs are usually synthesized by the reaction of transition metal ions with appropriate lacunary POM precursors [5,8]. The extraordinary diversity and synthetic accessibility of POMs have led to a wide spectrum of applications, ranging from catalysis, biochemistry/medicinal chemistry, to materials science [11,12,13,14]. In the field of catalysis, the development of POMs as oxidation and acid catalysts was flourish in the past few decades, with molybdic and vanadomolybdic clusters being more employed in the former case and tungstic ones in the latter [12,15]. The explorations of POMs as catalysts (or co-catalysts) for many other industrially important transformations were sought-after.

As early as 1988, Kozik and co-workers first reported the coordination of CO₂ with several POM derivatives [16]. Afterwards more researches about the interaction of POMs with CO₂ emerged [17,18,19,20,21,22]. Since the interaction between CO₂ and POM is one of the crucial steps for catalytic transformations of CO₂, it is necessary to clarify the mode of such interaction. However, considering the facts that CO₂ can be transformed to ${\mathrm{CO}}_3^{2-}$ or ${\mathrm{HCO}}_3^{-}$ with water and this transformation is both reversible and temperature-dependent, the explicit “real” form of dissolved CO₂ to interact with POM catalyst, was challenging to be justified. POMs displayed variable interacting modes. ¹³C NMR, UV/vis, IR with isotope-labelled CO₂ (¹³CO₂ and C¹⁸O₂), and X-ray crystallography are useful tools to solve this issue. As pointed in the seminal report of Kozik et al. [16], the IR spectra and thermochromic behavior of POM α-[SiW₁₁O₃₉Co]⁶⁻ in the presence of either ${\mathrm{CO}}_3^{2-}$ or ${\mathrm{HCO}}_3^{-}$ was in contrast to the scenario with CO₂. On ¹³C NMR spectra, the ¹³C chemical shifts of aqueous CO₂, ${\mathrm{HCO}}_3^{-}$, and ${\mathrm{CO}}_3^{2-}$ are respectively at 125, 160, and 162 ppm. After bubbling CO₂ into hydrous toluene solution of α-[SiW₁₁O₃₉Co]⁶⁻, the appearance of two signals at 792 and 596 ppm demonstrated the presence of two different kinds of paramagnetic CO₂ species. In their report, two patterns were suggested on the basis of all ¹³C NMR, IR observations: (i) the complexation between α-[SiW₁₁O₃₉Co]⁶⁻ and CO₂ was either via CO₂ complexes with a direct η¹ metal-carbon bond or bicarbonate complexes; (ii) the existence of H-bonding in the CO₂ complexes was plausible. Other modes of POM-CO₂ or POM-${\mathrm{CO}}_3^{2-}$ interaction were also discovered. Hill et al. reported the sandwich-type encapsulation of ${\mathrm{CO}}_3^{2-}$ via forming [(YOH₂)₃(CO₃)(A-α-PW₉O₃₄)₂]¹¹⁻ [20]. [SiMo₁₁CoO₃₈(CO₂)]_n polymeric chains reported by Xu et al. displayed the μ-η¹,η¹-OCO linear coordination mode of POM-CO₂ interaction [21]. Two kinds of POM-CO₂ interaction modes were observed on [(n-C₈H₁₇)₄N]₈[α₂-P₂W₁₇O₆₁Zn-(CO₂)] [21]. The stronger “side-on” binding of CO₂ by is predominant at higher temperatures (room temperature down to ca. 250 K) and the more weakly coordinated “end-on” POM-Zn-O-C-O structures were observable at lower temperatures (Figure 2). One particular case was uptaking 30 CO₂ molecules by one capsule of [{(Mo^VI)${\mathrm{Mo}}_5^{\mathrm{VI}}$ O₂₁(H₂O)₆}₁₂{${\mathrm{Mo}}_2^{\mathrm{V}}$ O₄(CH₃COO)}₃₀]⁴²⁻. In this case, CO₂ reacted directly with the H₂O ligands in {${\mathrm{Mo}}_2^{\mathrm{V}}$ O₄(H₂O)₂}²⁺ linkers to generate ${\mathrm{CO}}_3^{2-}$ at pH 7. Then the uptake of 30 CO₂ molecules was realized via the fast exchange of acetate by carbonate ligands [22]. Above cases indicated the abundance of POM-CO₂ interaction mode.

In this context, we have witnessed the leap in developing POMs-based catalysts for the photo- or electrocatalytic CO₂ reduction and CO₂ as C1 synthon in organic synthesis. Herein, in this review, we focused on the recent experimental and theoretical advances in CO₂ transformation with POMs-based catalyst or co-catalyst. The key factors for high-efficiency POMs-catalyzed CO₂ conversion are highlighted.

2. Photocatalytic CO₂ Reduction

Only high-energy vacuum ultraviolet laser (<200 nm) is able to directly excite and split CO₂ into CO and O fragments [23], therefore highly efficient photocatalyst is vital to motivate and accelerate photocatalytic CO₂ reductions with low-energy common visible light (>380 nm). This reduction process usually involves multi-electron transfer and the final products are carbon monoxide (CO), formic acid (HCOOH), formaldehyde (HCHO), methanol (CH₃OH), and methane (CH₄) [24,25]. The corresponding redox potentials for the possible CO₂ reduction at pH = 7 are listed in

Table 1. Redox potentials for CO₂ reduction.

Products	Products Reaction E0 (V)	E0 (V) pH = 7
-	HO + 2 h+ → 1/2O2 + 2H+	+0.82
-	CO2 + e− → ${\mathrm{CO}}_2^{-}$	−1.9
HCOOH	CO2 + 2 H+ + 2 e− → HCOOH	−0.61
CO	CO2 + 2 H+ + 2 e− → CO + H2O	−0.53
HCHO	CO2 + 2 H+ + 4 e− → HCHO + H2O	−0.48
CH3OH	CO2 + 6 H+ + 6 e− → CH3OH + H2O	−0.38
CH4	CO2 + 8 H+ + 8 e− → CH4 + 2H2O	−0.24
-	2 H+ + 2 e− → H2	–0.41

.

Since the pioneering application of Honda–Fujishima effect in TiO₂-catalyzed CO₂ photoreduction was reported [26], multitudinous different type of photocatalysts, including simple metal oxides [25], perovskite oxides [27,28], C₃N₄ [29], MOFs [30], conjugated polymers [31,32], and POMs [33,34] have been developed. Since the seminal study on the photochemistry of molybdates and tungstates for analytical purposes over half century ago [35,36], the photoredox chemistry of POMs has been verified for a long time. The metals in POMs are fully oxidized with d⁰ electron configuration. Light absorption is mainly attributed to O→M ligand to-metal charge transfer (LMCT) bands in the wide range of the electronic spectra. Consequently, an electron is promoted from a spin-paired, doubly occupied bonding orbital (HOMO) to an empty, antibonding orbital (LUMO), thus an oxo-centered radical is generated. This photo-excited POMs are more reactive both in oxidation and reduction than the non-excited species [37]. In the photocatalysis by POM with LMCT mode, forming lower-energy O→M LMCT transitions is vital to more efficiently absorb visible light. Substituting addenda centers with different metals has been well demonstrated as one of the effectual tactics to tune the photochemical properties of POMs and optimize their visible light absorption via narrowing the band gap between valence band and conduction band. For instance, Streb et al. discovered that the vanadium substitution effectively improved the visible light absorption and photocatalytic activity of molybdate [Mo₆O₁₉]²⁻ [38]. TD-DFT calculations disclosed the corresponding contribution of O→V LMCT on absorption profile in the visible range. The progress of such strategy on POM-catalyzed selective photooxidation driven by visible light has been included in the recent review [15]. Besides altering the addenda centers on POMs, various other strategies have also been established in order to foster POMs as effective photocatalyst and most of their utilization were focused on photooxidation of water, photodegradation of organic dyes, as well as H₂ evolution [37,39]. Here the progress of photoreduction of CO₂ with POMs-based catalyst was summarized.

2.1. CO₂ to CO

2.1.1. Homogeneous Catalysts

Neumann and co-workers reported that Ru-substituted POM ([Ru^III(H₂O)SiW₁₁O₃₉]⁵⁻) in combination with tertiary amines exhibited photoactivity for CO₂ reduction Figure 3 [40]. Triethyl amine acted as the sacrificial agent. CO was the major product (~50 μmol after 20 h irradiation with ca. 2% quantum yield). None of HCOOH, MeOH, and CH₄ was detected. UV/Vis, EPR, ¹³C NMR, and isotope labelling demonstrated that coordinated CO₂ ([Ru^III(CO₂)SiW₁₁O₃₉]⁵⁻) was easily formed by substituting H₂O in [Ru^III(H₂O)SiW₁₁O₃₉]⁵⁻. DFT results demonstrated that CO₂ tended to coordinate to Ru^III by forming a Ru-O bond in an “end-on” manner. Based on these experimental and computational results, the authors concluded that the Ru^III site was responsible for activating CO₂ via the coordination and the lacunary [SiW₁₁O₃₉]⁸⁻ site acted as photocatalyst in this process.

By employing H₂ instead of tertiary amines as the sacrificial agent, a viable CO₂ reduction approach with Pt/C and Re^I(L)(CO)₃-${\mathrm{MHPW}}_{12}^{\mathrm{VI}}$ O₄₀ (L = 5,6-(15-crown-5)-1,10-phenanthroline) as synergistic catalysts was developed. The grafting of Re^I(L)(CO)₃ complexes onto POMs was achieved via the complexation of crown ether moiety on L with the sodium cation binding two PW₁₂O₄₀ moieties (Figure 4) [41]. H₂ was oxidized with the facilitation of Pt(0) on the Pt/C surface to afford two protons. In the following step, protons and electrons retained by Pt were simultaneously transferred onto Re^I(L)(CO)₃(CH₃CN)-${\mathrm{MHPW}}_{12}^{\mathrm{VI}}$ O₄₀ (${\mathrm{MHPW}}_{12}^{\mathrm{VI}}$ O₄₀ + 2H⁺ + 2e⁻ → MH₃ ${\mathrm{PW}}_2^{\mathrm{V}}$ ${\mathrm{W}}_{10}^{\mathrm{VI}}$ O₄₀) (Scheme 1). Re^I(L)(CO)₃-MH₃${\mathrm{PW}}_2^{\mathrm{V}}$ ${\mathrm{W}}_{10}^{\mathrm{VI}}$ O₄₀ possessed the photoreductive activity for converting CO₂ to CO, because its transition to excited CO₂-reduction active state was allowed by absorbing visible light. CO were almost exclusively produced with turnover of 22.6% and 1.1% quantum yield. Besides CO, only trace CH₄ but no further CO reduction products (e.g., CH₃OH, HCHO, and HCOOH) was detected on GC-MS after 14 h irradiation.

The TD-DFT calculations disclosed a more detailed mechanism for the photoreduction of CO₂ to CO with this POM-Re^I complex catalyst [42]. Simulated absorption spectrum of Re^I-POM complex was calculated at different functional level (i.e., X3LYP, M06X, B3LYP, and CAMB3LYP) and the computed results were found to be basically consistent with the experimental absorption spectrum (two major bands with maxima at 500 and 656 nm). In the ground state without any light irradiation, bonding CO₂ with Re^I center to afford the reactive POM-Re^I-${\mathrm{CO}}_2^{-}$ complex had to overcome a high energy barrier (≥38 kcal mol⁻¹), therefore, the direct CO₂ reduction with Re^I-POM occurred less likely (Figure 5). In the conditions with photoexcitation, the excited Re^I-POM was allowed to interact with CO₂ to form POM-Re^I-${\mathrm{CO}}_2^{-}$. The subsequently protonation and further electron reduction process were exothermic by more than 78 kcal mol⁻¹. These energy profiles revealed that the role of POM as photosensitizer in the reduced state, electron “shuttles” as well as electron/proton reservoirs in this process. Plausible CO₂ photoreduction mechanism consists of five steps: (1) photoexcitation and charge transfer; (2) disassociation of solvent molecule; (3) CO₂ coordination; (4) proton and electron transfer; and (5) release of CO and recoordination of solvent.

Recently, by replacing crown ether-affiliated 1,10-phenanthroline with 6-hydrazinyl-2,2′-bipyridine on the Re complex, the Re₂(CO)₆Cl₂L₂-H₃PW₁₂O₄₀ (L = 6-hydrazinyl-2,2′-bipyridine) hybrid pair and related relay process for CO₂ reduction by photocatalysis and electrocatalysis was developed [43]. Initially, [${\mathrm{PW}}_{12}^{\mathrm{VI}}$ O₄₀]³⁻ was reduced to [${\mathrm{PW}}_2^{\mathrm{V}}$ ${\mathrm{W}}_{10}^{\mathrm{VI}}$ O₄₀]⁵⁻ by two electrons at −1.302 V (versus Fc/Fc⁺, Fc= ferrocene). Then the reduced POM undertook the functions of photoreductant and electron/proton acceptors. Photo irradiation promoted the generation of Re^I(L⁻)(CO)₃ via the electron transfer from [${\mathrm{PW}}_2^{\mathrm{V}}$ ${\mathrm{W}}_{10}^{\mathrm{VI}}$ O₄₀]⁵⁻ to Re^I(L)(CO)₃. This reductive Re^I intermediate possessed reactivity for the selective CO₂ reduction. Only CO was observed in this catalytic electro-photochemical reduction process. The result in the dark conditions indicated that light facilitated the CO₂ reduction. The formation of dirhenium complex-H₃PW₁₂O₄₀ hybrid pair via acid-base interaction was indispensable to this catalytic electro/photochemical transformation.

2.1.2. Heterogeneous Catalysts

Photocatalytic CO₂ reduction by POMs-based heterogeneous catalyst has received intensive research interests. As a POM-stabilized multi–Co-oxide clusters, [Co₄(PW₉O₃₄)₂]¹⁰⁻ is comprising a Co₄O₄ core stabilized by two oxidatively resistant polytungstate ligands (Figure 6a). The Na₁₀[Co₄(H₂O)₂(PW₉O₃₄)₂ @graphitic carbon nitride hybrid material (Co₄@g-C₃N₄) was reported for the efficient photocatalytic CO₂ reduction [44]. It manifested the advantages of convenient recovery, steady reuse, simple preparation and flexible composition. The Co₄@g-C₃N₄ photocatalyst with 43 wt% Co₄ content showed high CO yield (107 μmol g⁻¹ h⁻¹) and excellent selectivity (94%) (Figure 6b). After 10 h reaction, the production of CO reached 896 μmol g⁻¹ (Figure 6c). After 5 runs, the activity still remained (Figure 6d). Experimental and characterization results revealed that the Co₄ unit both facilitated the charge transfer of g-C₃N₄ and significantly enhanced the surface catalytic oxidative activity.

NENU-10 and NENU-3 are respectively Ti-substituted Keggin-type POM [PTi₂W₁₀O₄₀]⁷⁻ (This Ti-disubstituted Keggin-type POM contains the Ti centers in relative 1,5 positions according to the IUPAC nomenclature. This anion is one of the few cases in the literature in which a salt of a disubstituted Keggin species displays a predominant isomer (ca. 75% of the α (1,5) isomer)) and Keggin-type POM [PW₁₂O₄₀]³⁻ encaged into MOF Cu₃(BTC)₂ (BTC: benzene-1,3,5-tricarboxylate; Cu₃(BTC)₂ = HKUST-1) developed by Liu et al. Efficient photoreduction of CO₂ catalyzed by hybrid material Au@NENU-10 (Both Au@NENU-10 and Au@NENU-3 were prepared by one-pot method. The Au NPs were in-situ deposited during the assembly of NENU-10 and NENU-3 as shown in Figure 7.) was recently achieved [45].

Table 2. Catalytic performance of CO, H₂, and CH₄ from CO₂ photoreduction ^a.

Entry	Catalysts	Products (μmol g−1 h−1)
		CO	CH4	H2
1	Au@NENU-10	12.8	2.1	2.6
2	Au@NENU-3	0.5	-	0.15
3	Au/Na3PW12 O40)	-	-	0.45
4	Au/K7(PTi2W10O40)	2.1	0.35	0.29
5	NENU-10	-	-	-
6	NENU-3	-	-	-
7	HKUST-1	-	-	-

^a. Reaction conditions: 100 mL quartz reactor, 15 mL H₂O, 100 mg catalyst, 40 °C, 5 h, 300 W Xe lamp (λ > 420 nm). Solid catalyst was in the atmosphere of water vapor and CO₂ instead of being immersed in water.

illustrated the catalytic performance of as-prepared hybrid in the reductive CO₂ transformation to CO and CH₄ under visible-light irradiation. Compared to Ti-free Au@NENU-3, Au@NENU-10 presented both higher activity and selectivity in the CO₂ photoreduction. Similarly, Au/K₇(PTi₂W₁₀O₄₀) also had better performance than Au/Na₃(PW₁₂O₄₀). Results of control experiments implied the indispensability of Au nanoparticles to the CO reduction activity and precursors was inactive. Combing these patterns, authors deduced the following steps for the overall process: Firstly, Au nanoparticles produced electrons and holes where the holes oxidized H₂O to generate two electrons and two protons. Then the electrons and protons directly transferred to [PTi₂W₁₀O₄₀]⁷⁻ to form [P(Ti^III-H)₂ ${\mathrm{W}}_{10}^{\mathrm{VI}}$ O₄₀]⁷⁻. Some of [P(Ti^III-H)₂ ${\mathrm{W}}_{10}^{\mathrm{VI}}$ O₄₀]⁷⁻ species reduced CO₂ to CO and H₂, and others further obtained electrons and protons to form [P(Ti^III-H₂)₂ ${\mathrm{W}}_2^{\mathrm{V}}$ ${\mathrm{W}}_8^{\mathrm{VI}}$ O₄₀]⁷⁻ intermediates. Finally, [P(Ti^III-H₂)₂ ${\mathrm{W}}_2^{\mathrm{V}}$ ${\mathrm{W}}_8^{\mathrm{VI}}$ O₄₀]⁷⁻ reduced CO₂ to CH₄ and simultaneously returned to initial state. Ti-O-W in [PTi₂W₁₀O₄₀]⁷⁻ may responsible to absorb CO₂. Compared with [PTi₂W₁₀O₄₀]⁷⁻ and [PW₁₂O₄₀]³⁻, [PTi₂W₁₀O₄₀]⁷⁻ had stronger electron-coupling protons ability.

2.2. CO₂ to HCOOH

HCOOH is one of the promising hydrogen carriers and the important C1 source for organic synthesis. The generation of HCOOH in CO₂ photoreduction consumes the same amount of protons and electrons as that for CO formation, but it requires slightly higher reduction potential.

The POM microions are the molecules denoted as {POM} in dilute solutions, they tend to afford suprestructural macroions denoted as {POM}_n (POM macroions). Three POM macroions catalysts, Na₁₅[${\mathrm{Mo}}_{126}^{\mathrm{VI}}$ ${\mathrm{Mo}}_{28}^{\mathrm{V}}$ O₄₆₂H₁₄(H₂O)₇₀]_0.5[${\mathrm{Mo}}_{124}^{\mathrm{VI}}$ ${\mathrm{Mo}}_{28}^{\mathrm{V}}$ O₄₅₇H₁₄(H₂O)₆₈]_0.5 ({Mo₁₅₄}₁₁₅₆), Na₁₇[Mn₆P₃W₂₄O₉₄(H₂O)₂] ({Mn₆P₃W₂₄}₉₃₁) and (NH₄)₄₂[${\mathrm{Mo}}_{72}^{\mathrm{VI}}$ ${\mathrm{Mo}}_{60}^{\mathrm{V}}$ O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]@RGO hybrid ({Mo₁₃₂}₁₀₆₄@RGO; RGO = reduced graphene oxide) featured with peculiar structures, were respectively employed in the photocatalytic CO₂ reduction coupling with water oxidation [46]. The enormous spherical superstructures in these three catalysts in dilute dispersion ({Mo₁₅₄}₁₁₅₆, {Mn₆P₃W₂₄}₉₃₁, {Mo₁₃₂}₁₀₆₄@RGO) are respectively made up by wheel-shaped {Mo₁₅₄} rings, bent rod {Mn₆P₃W₂₄} units and “Keplerate” {Mo₁₃₂} spheres (Figure 8). The numbers of metal oxide cluster units in these superstructures were calculated by following equation: n = (4πR²)/(72.006σ²) × 60. (R, representing the hydrodynamic radius of POM vesicles formed in the dispersion, was obtained by dynamic light scattering analysis. σ, representing the diameter of isolated single cluster, was determined by the van der Waals radii of the constituent atoms.) The results showed that although HCOOH is the main product of CO₂ reduction with these three catalysts, a considerable amount of HCHO was also obtained when {Mo₁₅₄}₁₁₅₆ and {Mo₁₃₂}₁₀₆₄@RGO catalysts were applied. In the case of {Mn₆P₃W₂₄}₉₃₁, CO₂ was exclusively reduced to HCOOH. In terms of formic acid production, {Mo₁₅₄}₁₁₅₆ and {Mo₁₃₂}₁₀₆₄@RGO displayed better performance than {Mn₆P₃W₂₄}₉₃₁ (

Table 3. The photocatalytic performance of the different catalysts ^a.

Catalyst	HCOOH Yield (μmol)	TON	TOF (s−1)
{Mo154}1165	116.7	778	377
{Mn6P3W24}931	40.6	270	56
{Mo132}1064@RGO	205	1366	610

^a. Catalyst containing 0.15 μmol of {Mo₁₅₄}, {Mn₆P₃W₂₄} or {Mo₁₃₂} units. The reaction mixtures were kept in a photo-reactor under UV-light lamp with 373 nm wavelength and 19 mW cm⁻² energy density.

). This phenomenon can be rationally explained by that the simultaneous excitation of multitudinous photoactive clusters on the surface of {Mo₁₅₄} and {Mo₁₃₂} vesicles under light irradiation. For {Mo₁₃₂}₁₀₆₄@RGO, good conductivity of RGO may help the facile electrons transfer.

Afterwards, POM macroions for the water oxidization-coupled CO₂ photoreduction to HCOOH was extend to {Cu-PW₁₂}_{n=1348−2024} ({Cu-PW₁₂}_n = [(K_6.5Cu(OH)_8.5(H₂O)_7.5)_0.5@(K₃PW₁₂O₄₀)]_n) [47] and gigantic oxo-molybdate catalyst Na₄₈[H_xMo₃₆₈O₁₀₃₂(H₂O)₂₄₀(SO₄)₄₈] ({Mo₃₆₈}) [48]. In the former case, the max TON per mole was 613. Both FT-IR and Raman spectroscopies showed that the structure remained integral after reaction and the activity was kept after ten cycles. In the latter one, {Mo₃₆₈}, which consisted of a central ball shaped unit {Mo₂₈₈} and two {Mo₄₀} capping units, showed excellent selectivity for HCOOH (95.73%) with impressive TON (27666) and TOF (4419 h⁻¹). Its external quantum efficiency reached 0.6%. It is worth noting that in all above cases external photosensitizer was unessential. Because intervalence charge transfers (IVCT) of Mo^V to Mo^VI and W^V to W^VI have appropriate gap between conduction band and valence band, which can give rise to absorbance maxima in the region of visible light.

2.3. CO₂ to CH₄

Compared to the photoreductive conversion of CO₂ to CO or HCOOH, photocatalytic methanation of CO₂ involving an eight-electron transfer process was much more challenging. [PTi₂W₁₀O₄₀]⁷⁻ was reported as the first POM-based photocatalyst for reducing CO₂ to CH₄ with CH₃OH as electron donator [49]. Although [PTi₂W₁₀O₄₀]⁷⁻ had poor activity and limited efficiency in photoreduction of CO₂, this result suggested that POM could be an active catalyst in the CO₂ photoreduction.

Since this time, few works focusing on photocatalytic CO₂ reduction to CH₄ during past decades. Recently, a remarkable achievement in photocatalytic CO₂ reduction to CH₄ with two hydrothermal-synthesized POM catalysts H[[Na₂K₄Mn₄(PO₄) (H₂O)₄]₃[[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₄[Mn₆(H₂O)₄]] (NENU-605) and H[[Na₆CoMn₃(PO₄)(H₂O)₄]₃[[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₄[Co_1.5Mn_4.5]] (NENU-606) was reported [50]. These two water-insoluble POMs showed good structural stability and extended solar spectrum absorption range in aqueous solutions.

Under CO₂ atmosphere with triethanolamine (TEOA) as sacrificial agent and [Ru(bpy)₃]Cl as photosensitizer, CH₄ and CO were the main gaseous photoreduction products, only a trace amount of HCOOH was detected in the aqueous phase. The productivity of CH₄ for NENU-605 and NENU-606 reached up to 170 nmol (894.7 nmol g⁻¹ h⁻¹) and 402 nmol (1747.8 nmol g⁻¹ h⁻¹) respectively. Good CH₄ selectivity of 76.6% (NENU-605) and 85.5% (NENU-606) was achieved (

Table 4. The photocatalytic reduction of CO₂ to CH₄ using different catalysts ^a.

Entry	Catalyst	CO (nmol/g)	CH4 (nmol/g)	CH4-TON (10−3)	CH4-TOF (10−3 h−1)	All-TON (10−3)	All-TOF (10−3 h−1)
1	NENU-605	52	170	104.1	5.5	135.9	7.2
2	NENU-606	68	402	241.4	10.5	282.2	12.3
3	NENU-607	47	70	15.2	0.75	25.4	1.3
4 b	NENU-606	n.d.	n.d.	-	-	-	-
5 c	NENU-606	n.d.	n.d.	-	-	-	-
6 d	NENU-606	n.d.	n.d.	-	-	-	-
7	blank	n.d.	n.d.	-	-	-	-

^a. Reaction conditions: PS = [Ru(bpy)₃]Cl₂.6H₂O (0.01 mmol, ) H₂O (28 mL), SD = TEOA (2 mL), CO₂ (1 atm), λ ≥420 nm, 20 °C; ^b. without PS; ^c. In dark; ^d. Altering H₂O with dry MeCN. n.d. = not detected.

, entry 1 and 2). Moreover, H₂ evolution as the side reaction was not detected. Regarding to the higher CH₄ selectivity of NENU-606 than NENU-605, the heterometallic Mn^II/Co^II ions in NENU-606 might be more favorable to the adsorption and activation of CO₂ than the homometallic Mn^II ions in NENU-605. In contrast, Mn[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₂ (NENU-607), the dimer containing only one Mn^II atom sandwiched between two P₄Mo₆ unit and displaying a similar connection mode to NENU-605 and NENU-606 (Figure 9), had much lower activity (Table 4, entry 3). Outcomes from control experiments demonstrated the necessities of both photosensitizer and light irradiation as well as the contribution from solvent effect (Table 4, entries 4–6). ¹³C-isotope labelling verified that CO₂ was the carbon source of CO and CH₄. Deduced from these patterns, a plausible multi-step mechanism for the photocatalytic CO₂ methanation was proposed (Figure 10). Initially, the photosensitizer absorbs light to produce photo-excited electrons from its HOMO and then transfers electrons to the P₄${\mathrm{Mo}}_6^{\mathrm{VI}}$ unit through the matched LUMO positions (P₄${\mathrm{Mo}}_6^{\mathrm{VI}}$ + 6e⁻ → P₄ ${\mathrm{Mo}}_6^{\mathrm{V}}$). Simultaneously, the electron holes produced in the valence band of ruthenium complex was consumed by TEOA (TEOA + h⁺ → TEOA⁺). Subsequently, strongly reductive P₄${\mathrm{Mo}}_6^{\mathrm{V}}$ unit further transfer electrons to the active metal center (M^II + e⁻ → M^I). Then the adsorbed CO₂ (M^I → M^I-CO₂) obtains electrons from active metal sites (M^I-CO₂ → M^II-${\mathrm{CO}}_2^{-}$) with the help of H₂O as a proton source (M^II-${\mathrm{CO}}_2^{-}$ + H⁺ → M^II-COOH). M^II-CO was formed via the proton- and electron-assisted dehydroxylation (M^II-CO₂H + H⁺ + e⁻ → M^II-CO + H₂O). CH₄ is eventually produced with further six-electron transfer process (M^II-CO + 6H⁺ + 6e⁻ → M^II + CH₄ + H₂O).

3. Electrocatalytic CO₂ Reduction

Electrocatalytic CO₂ reduction to produce various C1 and C2 molecules, for instance, CO, HCOOH, HCHO, CH₃OH, CH₄, oxalic acid, ethylene, and ethanol, is regarded as a promising proctol for the production of liquid fuels or bulk chemicals. The corresponding reactions and standard potentials were list in

Table 5. CO₂ reduction potentials vs. SHE ^a.

Products	Half-Electrochemical Thermodynamic Reactions	V vs. SHE
C	CO2(g) + 4 H+ + 4 e− = C(s) + 2 H2O(l)	−0.210
C	CO2(g) + 2 H2O(l) + 4 e− = C(s) + 4 OH−	−0.627
HCOOH	CO2(g) + 2 H+ + 2e− = HCOOH(l)	−0.250
HCOO−	CO2(g) + 2 H2O(l) + 2 e− = HCOO− (aq) + OH−	−1.078
CO	CO2(g) + 2 H+ + 2 e− = CO(g) + H2O(l)	−0.106
CO	CO2(g) + 2 H2O(l) + 2 e− = CO(g) + 2 OH−	−0.934
HCHO	CO2(g) + 4 H+ + 4e− = HCHO(l) + H2O(l)	−0.070
HCHO	CO2(g) + 3 H2O(l) + 4 e− = HCHO(l) + 4 OH−	−0.898
CH3OH	CO2(g) + 6 H+ + 6 e− = CH3OH(l) + H2O(l)	0.016
CH3OH	CO2(g) + 5 H2O(l) + 6 e− = CH3OH(l) + 6 OH−	−0.812
CH4	CO2(g) + 8 H+ + 8 e− = CH4(g) + 2 H2O (l)	0.169
CH4	CO2(g) + 6 H2O(l) + 8 e− = CH4(g) + 8 OH−	−0.659
oxalic acid	2 CO2(g) + 2 H+ + 2 e− = (COOH)2(aq)	−0.500
oxalate	2 CO2(g) + 2 e− = C2${\mathrm{O}}_4^{2-}$(aq)	−0.590
ethylene	2 CO2(g) + 12 H+ + 12 e− = CH2CH2(g) + 4 H2O(l)	0.064
ethylene	2 CO2(g) + 8 H2O(l) + 12 e− = CH2CH2(g) + 12 OH−	−0.764
ethanol	2 CO2(g) + 12 H+ + 12 e− = CH3CH2OH(l) + 3 H2O(l)	0.084
ethanol	2 CO2(g) + 9 H2O(l) + 12 e− = CH3CH2OH(l) + 12 OH−	−0.744

^a. Reprinted with permission from [51]. Copyright © 2014 Royal Society of Chemistry.

[51].

Electrocatalytic reduction of CO₂ with POM was first reported by Kozik et al. [52]. Cyclic voltammograms (CV) of several POMs with or without the presence of CO₂ in various nonpolar solvents were compared. For α-[SiW₁₁O₃₉Co]⁶⁻, a large change on its CV was recorded after CO₂ was bubbled. Meanwhile for α-[P₂W₁₈O₆₂]⁶⁻, almost no change was observed before and after bubbling CO₂. The evidence from CV hinted that α-[SiW₁₁O₃₉Co]⁶⁻ might be active to CO₂ reduction. Further investigations on reduced α-[SiW₁₁O₃₉Co]⁶⁻ with CV indeed confirmed that α-[SiW₁₁O₃₉Co]⁶⁻ showed the electrocatalytic for CO₂ reduction. However, the final CO₂ reduction product was unstated.

More than decade later, Proust et al. reinvestigated this reaction in more detail by using [α-SiW₁₁O₃₉Co]⁶⁻ in the electro-assisted reduction of CO₂ (Figure 11) [53]. The [α-SiW₁₁O₃₉Co]⁶⁻ contained a Co^II in place of a W^VI. The square-pyramidal CoO₅ with a vacant site was generated by losing a coordinated water molecule from CoO₅(H₂O) octahedral when the POM was extracted from aqueous to organic media. Except CO and HCHO, neither H₂ nor other CO₂ reduction products were detected. This indicated that the unique selectivity of [α-SiW₁₁O₃₉Co]⁶⁻ POM catalyst in the electroreduction of CO₂. The turnover of CO₂ to CO reached to 3.7 with the faradic efficiency of 13%. HCHO was the other detectable product. Its amount varied from 2.1 × 10⁻⁷ to 2.2 × 10⁻⁶ mol with the faradic yield varying from 25% (high HCHO content with low electrolysis charge) to 0.8% (low HCHO content with high electrolysis charge).

Inspired by organometallic CO₂ reduction catalyst [Cp*Rh^III(bpy)Cl]⁺ (Cp* = pentamethyl-cyclopentadienyl anion; bpy = 2,2′-bipyridine), [α-H₂PW₁₁O₃₉{Rh^IIICp*(OH₂)}]³⁻ with analogous coordination structure via grafting a {Cp*Rh^III}²⁺ fragment on the monovacant [PW₁₁O₃₉]⁷⁻ anion was prepared (Figure 12) and tested as the electrocatalyst for CO₂ reduction [54]. Compared to previously reported [CoSiW₁₁O₃₉]⁶⁻ catalyst, its electrochemical behavior in the presence of CO₂ exhibited a clear improvement, strongly suggesting some interaction with the POM derivative despite the presence of a coordinating solvent. But H₂ was still as major product (68% faradic yield) with HCOO⁻ as minor reduction product (4.5% faradic yield).

The remarkable capability of storing and donating electrons endows POMs the feature of reversibly transferring multi-electrons. Hence, selecting proper co-catalyst to establish synergistic catalytic systems containing POM catalyst is reasonable to achieve the efficient CO₂ reduction. On one side, by taking the advantage of POMs on reversible transfer of multi-electrons or protons, co-catalysts can efficiently overcome the barrier of CO₂ activation. On the other side, co-catalysts with good conductivity and adsorbing CO₂ capability can facilitate the electroreduction and improve the activity of catalytic system. Poor electrical conductivity and electron-donating capability are the major drawbacks for MOFs being as efficient electrocatalysts. Therefore, the integrated use of POM and MOF would combine the strengths of each other and generate electrocatalysts with excellent activity [55,56,57].

Following the above intention, Lan et al. prepared various Polyoxometalate-Metalloporphyrin Organic Frameworks (PMOFs) by assembling Zn-ε-Keggin cluster ε-${\mathrm{PMo}}_8^{\mathrm{V}}$ ${\mathrm{Mo}}_4^{\mathrm{VI}}$ O₄₀Zn₄ with M-TCPP (M-TCPP = tetrakis[4-carboxyphenyl]-porphyrinato-M; M = Co, Fe, Ni, Zn) via the links between Zn²⁺ ions of POM and carboxylates of porphyrin in hydrothermal conditions (Figure 13) and measured their catalytic performances (

Table 6. CO₂ electroreduction performances ^a.

Catalysts	E V vs. SHE	Onset Potential V	jCO mA cm−2	Tafel Slope (mV dec−1)	ECSA mF cm−2	EIS Ω	FEco %	TOF H−1
Co-PMOF	−0.8	−0.35	18.08	98	12.17	9.83	98.7	1656
Fe-PMOF	−0.7	−0.53	0.47	211	10.26	10.26	28.8	17.45
Ni-PMOF	−0.8	−0.58	0.27	675	10.16	10.70	18.5	8.11
Zn-PMOF	−0.9	−0.60	0.02	206	9.83	12.17	0.95	0.005
Co-TMCP	−0.9	−0.53	NA b	151	NA	NA	40	NA
TMCP	−0.6	−0.67	NA	552	NA	NA	0.77	NA
NNU-12	−0.6	−0.6	NA	413	NA	NA	1.8	NA

^a. Electrolyte and pH 0.5 M KHCO₃, pH = 7.2; ^b. NA means not mentioned in the [58].

) [58]. In the skeleton of these M-PMOFs, the moieties of Zn-ε-Keggin cluster, as electron reservoirs, potently helped the electron transfer to CO₂ reduction catalyst M-TCPP and improve its performance of CO₂ reduction. The best results were obtained on Co-PMOF with lower onset potential, tafel slope, and electrochemical impedance as well as higher partial CO current density (j_CO), electrochemical active surface area (ECSA), faradic efficiency for CO (FE_CO) and turnover frequency (TOF)_. Co-PMOF converted CO₂ to CO with a superior faradic efficiency of 99%. The value of TOF was elevated to 1656 h⁻¹ at −0.8 V. No evident activity attenuation was observed during the 36 h stability test, which indicated the robustness of Co-PMOF.

DFT calculations was employed to explicitly understand the excellent performance of Co-MPOF and synergism between Zn-ε-Keggin cluster and Co-TCPP. Owing to rather high Gibbs free energies of 0.96 eV (ΔG₁ in Figure 14a), the formation of adsorbed intermediates * COOH was regarded as the rate-determining step (RDS) for CO₂ reduction on Zn-ε-Keggin cluster. On the contrary, the formation of adsorbed intermediates * CO on Co-TCPP was RDS with high Gibbs free energies of 0.53 eV (ΔG₂ in Figure 14a). In accordance with expectation, remarkable drops of both ΔG₁ and ΔG₂, particularly the much lower ΔG₁ = 0.34 eV, were revealed. For Co-PMOF, the more favorable CO₂ reduction active site is Co on Co-TCPP instead of Zn on POM. In fact, this Zn POM was inactive in CO₂ reduction, and the synergistic effect was derived from the intramolecular electron transfer between the electron mediator of POM and Co-TCPP. The effect of different metal center in porphyrin was also computed (Figure 14b). The energy profile of reaction on different metal centers was well consistent with the outcomes of experiments. Based on the experimental results and theoretical calculations, the possible mechanism about reducing CO₂ to CO on Co-PMOF was suggested. Firstly, the POM captures and transfers electrons from the electrode to the Co^II center. Subsequently, Co^II centers was reduced to Co^I. Then Co^I interacts with CO₂ to afford Co^II-* COO⁻, which was converted to Co^II-* CO via the proton-coupled electron transfer. Finally, CO is desorbed and released (Figure 14c,d).

The design and application of [α-SiW₁₂O₄₀]⁴⁻-modified AgNC@BSA catalyst (AgNC@BSA = silver nanoclusters capped with bovine serum albumin; BSA is both the stabilizer and reducing reagent for the synthesis of AgNC) in the electroreduction of CO₂ to CO also exhibited the same concept of synergism [59]. Similar to the case of Co-PMOF, although [α-SiW₁₂O₄₀]⁴⁻ had little CO₂ reduction activity, it played the role of electron transfer mediator and assisted the CO₂ reduction through its strong interaction with CO₂ on the surface of AgNC. It had excellent faradic efficiency (>75%) in DMF containing 1% (v/v) H₂O. The overpotential of [α-SiW₁₂O₄₀]⁴⁻-modified AgNC@BSA electrode was about 0.7 V. The onset potential for this POM-decorated AgNC electrocatalyst was about 400 mV more positive than that on bulk Ag.

Metal nanoparticles stabilized by POMs was extensively researched [60,61]. By replacing BSA with POM as the stabilizer of AgNC, three Ag-POM nanocomposites respectively with [PMo₁₂O₄₀]³⁻, [α-SiW₁₂O₄₀]⁴⁻ and [PW₁₂O₄₀]³⁻ were synthesized by electrodeposition method [62]. POM is the promoter for the CO₂ reduction. Due to the higher charge density and stronger basicity associated with [PMo₁₂O₄₀]³⁻, the activity of Ag-[PMo₁₂O₄₀]³⁻ nanocomposite was better than Ag-[PW₁₂O₄₀]³⁻ and Ag-[α-SiW₁₂O₄₀]⁴⁻. This nanocomposite exhibited efficient and sustained CO₂ reduction at a wide potential range with faradic efficiency of 90 ± 5%. The Tafel slope, an inherent property of electrocatalyst determined by the rate-limiting step (formation of ${\mathrm{CO}}_2^{-}$ or protonation of ${\mathrm{CO}}_2^{-}$), calculated for Ag-[PMo₁₂O₄₀]³⁻ (60 mV dec⁻¹) was very close to the theoretical value (59 mV dec⁻¹), which indicated the faster formation of ${\mathrm{CO}}_2^{-}$ on the Ag-[PMo₁₂O₄₀]³⁻ nanocomposite-modified electrode

4. Electromicrobial Conversion of CO₂

Since the introduction of lithoautotrophic bacterium Ralstonia eutropha H16 as the production host for biological formate conversion as well as the coupling between this biological formate conversion method and the electroreductive conversion of CO₂ to formate on indium cathode was employed to produce fuels for internal combustion engines from CO₂ [63], the integration of electrocatalytic reduction with microbial conversion represents an edge-cutting strategy for the direct but non-photosynthetic CO₂ conversion to important molecules.

Microorganisms, represented by R. eutropha and Clostridium spp, have been well harnessed to transform CO₂ and H₂ to energy-dense liquid fuels [64]. As potent hydrogen-oxidizing bacterium, R. eutropha has exhibited its versatility in synthesize poly[R-(−)-3-hydroxybutyrate], which is one of the ingredients to manufacture biodegradable plastics [65]. On the other aspect, many POMs-based catalysts have presented excellent activity for hydrogen evolution reaction (HER) [12,66]. Searching biocompatible POM-based HER catalysts and utilizing suitable microorganism to catalytically reduce CO₂ by consuming H₂ generated from HER can effectually accomplish the electromicrobial conversion of CO₂.

Recently, Zhang and co-workers demonstrated the feasibility of such reckoning. The combination of a heterometallic Co/Cu-containing polyoxometalate/carbon cloth (Cu₆Co₇/CC; Cu₆Co₇ = [Cu(en)₂]₆[((PW₉O₃₄)Co₃(OH)(H₂O)₂(O₃PC(O)(C₃H₆NH₃)PO₃))₂Co]²⁻, en = ethanediamine; see the structure of Cu₆Co₇ POM in Figure 15; CC = carbon cloth) precatalyst with bacterium R. eutropha H16 was developed to achieved the electricity-driven bioconversion of CO₂ to biomass in neutral water [67]. Cu₆Co₇/CC hybrid material possessed good biocompatibility in this CO₂ conversion system. It was evidenced that most of the H₂ produced on the Cu₆Co₇/CC cathode was consumed by the bacteria for living and growth. In excess of half input electrical energy was transported into biomass. The authors assumed that the electricity would be supplied by a photovoltaic device with an efficiency of 18%, the expected overall solar-to-biomass efficiency could reach 10%, which would be nearly 10 times higher than the natural photosynthesis. This Cu₆Co₇/CC-R. eutropha hybrid system exemplified the potential to explore non-photosynthetic but highly efficient CO₂-fixation methods by using POMs-based catalysts and solar electricity.

5. Non-Reductive CO₂ Conversion to Carbonyl-Contained Organic Chemicals

Non-reductive chemical CO₂ conversion provides diverse alternatives to achieve both the synthesis of practical chemicals and CO₂ fixation at the same time [68]. Currently, the industrial manufacture of several chemicals demands CO₂ as starting material [69]. Predominantly, due to the basic properties of POM-based materials, the capture and transformation of CO₂ with these materials has been widely explored. Therefore, the versatility of POMs-based materials in non-reductive chemical CO₂ fixation has been fully presented. Various useful chemicals, including cyclic carbonate [19], dimethyl carbonate [70,71,72,73,74], urea [74], quinazoline-2,4-(1H,3H)-diones [74,75], 2-benzimidazolone [74], 2-oxazolidinones [18], α-methylene cyclic carbonate [74], and methacrylic acid [76,77] were able to be synthesized with POM-based catalyst (Scheme 2). Considering that several reviews have already given comprehensive summaries of representative progress on non-reductive CO₂ fixation by POM catalysts [69,78,79], herein we briefly introduced the recent advances about non-reductive CO₂ conversion to organic chemicals, which were not collected in the very recent review published in 2018 [79].

Various important applications of cyclic carbonates make the research on POM or other material-catalyzed synthesis of cyclic carbonates always receive long-lasting interests [80]. By integrating POM components, chiral organic catalysts and CO₂ activator in the skeleton of a single MOF, the polyoxometallate-organocatalyst-metal organic framework (POMOF) was designed by Duan group [81]. Chiral cyclic carbonates were produced efficiently from olefins and CO₂ on the POMOFs tandem catalyst. By ingeniously connecting Keggin-type POM anions α-[ZnW₁₂O₄₀]⁶⁻, l-proline-derived asymmetric organocatalysts pyrrolidine-2-yl-imidazole (PYI) and NH₂-functionalized bridging links 2-amino-4,4′-bipyridine via the six-coordinated Zn^II nodes in these POMOFs (α-[ZnW₁₂O₄₀]⁶⁻-PYIs), a clear division of catalytic working for oxidation-coupled CO₂ conversion was established that POM as an oxidation catalyst, pyrrolidine moiety as a chiral organocatalyst, Zn^II ions as Lewis acid catalyst to activate the epoxide intermediate and amino groups on 4,4′-bipyridines as renewable CO₂ absorption reagent in this tandem process (Figure 16). The multi-catalytic sites were orderly distributed and spatially matched in the framework. The captured CO₂ molecules are synergistically fixed and activated by well-positioned pyrrolidine and amine groups, providing further compatibility with the terminal W=O activated epoxidation intermediate and driving the tandem catalytic process in a single workup stage and an asymmetric fashion.

In conjunction with ionic liquid co-catalyst, two kinds of POMs attached with metal carbonyl {P₂W₁₅O₅₆Co₃(H₂O)₃(OH)₃Mn(CO)₃}⁸⁻ [82] and {(Se₂W₁₁O₄₃)(Mn(CO)₃)₄}⁸⁻ [83] were reported as efficient catalysts for the cycloaddition of CO₂ with epoxides under mild conditions. A rare three-dimensional CO₂-linked POM polymer {PMo₁₂O₄₀Zn₄(CO₂)}²⁻ exhibited superior performance in the cycloaddition of CO₂ with epoxides [19]. The structural feature of catalyst is that the CO₂ ligand connects with two Zn-ε-Keggin cores in a linear and symmetrical μ₂-η²o,o coordination pattern(Figure 17). Polyoxoniobates (DBUH)₃(NbO₅), (TBA)₆[Nb₁₀O₂₈] and Na₁₆[SiNb₁₂O₄₀] were applied as Lewis base-type catalysts for the cycloaddition of CO₂ with epoxides under halide-free conditions [84,85,86]. Carbon nanotubes-supported Fe_1.5PMo₁₂O₄₀ (theoretical formulas) obtained 57.7% propylene oxide conversion and 99.0% propylene carbonate selectivity, both activity and selectivity were higher than Fe_1.5PMo₁₂O₄₀, Co_1.5PMo₁₂O₄₀, Cu_1.5PMo₁₂O₄₀, and Zn_1.5PMo₁₂O₄₀ (theoretical formulas). The good activity can be attributed to the well dispersion of the Fe_1.5PMo₁₂O₄₀ on the CNTs [87].

6. Outlook

POMs are regarded as discrete metal oxide clusters with much smaller nanometric size than bulk metal oxides, therefore most of the distinctions on chemical properties, especially CO₂ reductive conversion-related redox property, between bulk metal oxides and POMs were originated from quantum size effect. Quantum size effect leads to the changes of electronic configurations. These changes arise through systematic transformations in the density of electronic energy levels as a function of the size. By plotting the correlation between redox potential and electron density per volume (Figure 18a) as well as the diagram of band structure (Figure 18b), the quantum size effect-resulted distinction on redox property among bulk WO₃, colloid WO₃ and [SiW₁₂O₄₀]⁴⁻ has been clearly illustrated [88]. Despite the prevalence of both POMs and bulk oxides as photocatalysts of harvesting light in recent years, the smaller size endows POMs with wider gap and lower reduction potentials. The case given by Hill and Geletii et al. involving water oxidation catalyst [Co₄(H₂O)₂(PW₉O₃₄)₂]¹⁰⁻ (Co₄POM) also manifested the distinction between POM and bulk metal oxide on redox property. In their report, (PW₉O₃₄)⁹⁻-sandwiched Co₄ unit in Co₄POM was unambiguously identified as the dominant active center for water splitting and not CoO_x. [89]

The prospering arising of new POM structures provides vast opportunities to excavate latent catalysts for chemical CO₂ conversion. Titanium oxo-clusters [90] and polyoxo-noble-metalates [91] are exemplified as two promising representatives among these new POM structures. Regarding titanium oxo-clusters (also named polyoxotitanates), there are diverse variants including titanium oxoalkoxides clusters Ti_nO_m(OR)_4n–2m, titanium oxo-carboxo-alkoxides clusters Ti_nO_{2n–x/2–y/2}(OR)_x(OOCR’)_y, and titanium oxocarboxo clusters [Ti_nO_m(OOCR)_p] (2m + p = 4n) [90]. The tunable wide polynuclearity (the number of titanium atoms n varies from 2 to 28) and facile fabrication of hybrid architectures with organic components by covalent bonding are the two major features of these clusters. MOF MIL-125 (MIL stands for Material from Insitut Lavoisier) consisting of 1,4-benzenedicarboxylate-connected titanium-oxo-hydroxo clusters has been reported to possess possible photocatalytic property when alcohols are adsorbed inside its framework [92]. Based on this indication and as the extensive applications of titanium oxide in CO₂ photocatalytic reduction [93,94,95], there are likely to be extensive catalytic application of hybrid materials containing titanium oxo-cluster cores in CO₂ reductive conversion. The emergance of various novel polyoxo-noble-metalates including polyoxopalladates, -platinates, and -aurates [91] also provides different prospects for catalytic CO₂ reductive conversion. On these polyoxo-noble-metalates, the noble metal atoms Pd, Pt, and Au act as “addenda” atoms rather than as heteroatoms [91]. Despite that there are no direct accessible active sites on noble metal centers with saturated coordination environments, the very recent report about the catalytic Suzuki-Miyaura cross-coupling application of polyoxopalladate-based MOF materials has implied that these polyoxo-noble-metalates may acquire catalytic activity via in situ partial reduction [96]. For polyoxoplatinate [Pt₁₂O₈(SO₄)₁₂]⁴⁻ [97], although it is insoluble in any media, its six dumbbell-shaped [Pt₂]⁶⁺ anionic units perhaps will attract electrocatalytic research interests, because they may be in-situ partially reduced to molecular mixed-valent metal clusters or platinum black-type “suboxides” [98]. Then these plausible in-situ generated species may be applied as HER catalysts for electromicrobial conversion of CO₂.

More than the mode of LMCT, developing POM-based catalysts with the intrinsic function of metal-to-metal charge transfer (MMCT) will also be beneficial to look for more efficient POM-based approach to harvest visible-near infrared emission of sunlight with higher quantum yield. The MMCT transition occurs when two metal centers with different valence states are coupled by bridging ligands (M^a+-O-M’^b+). The two metal centers are respectively reduced and oxidized (M^a+-O-M’^b+ → M^(a−1)+-O-M’^(b+1)+) during this transition. The MMCT transition often takes place in polynuclear complexes [99]. The oxo-bridged and all-inorganic heterobinuclear units such as Zr^IV-O-Co^II [100,101,102,103], Zr^IV-O-Cu^I [104], Ti^IV-O-Co^II [100], and Ti^IV-O-Mn^II [105] supported on silica material that display MMCT transitions can extend the optical absorption from UV to visible regions. Some of these materials have been applied as catalyst or cocatalyst for CO₂ photoreduction [101,102,103,104]. Several reported POM-based systems, such as the anchored polynuclear charge-transfer complexes consisting of Ce^III ions and Cu^II-substituted Keggin-type Cu^IIPW₁₁O₃₉ [106], the metal-oxide nanoclusters consisting of Ce^III or Co^II ions and Keggin-type PW₁₂O₄₀³⁻ [107], [Co^IIW₁₂O₄₀]⁶⁻ [108], and [Co^II(M^xOH_y)W₁₁O₃₉]^(12-x-y)− (M^xOH_y = V^IVO, Cr^III(OH₂), Mn^II(OH₂), Fe^III(OH₂), Co^II(OH₂), Ni^II(OH₂), Cu^II(OH₂), Zn^II(OH₂)) [109] have sufficiently demonstrated the feasibility that such molecular inorganic MMCT (or MPCT, metal-to-polyoxometalate charge transfer) [108,109] transition enable POM-based catalyst to function as efficient visible-light-driven multielectron-transfer catalysts. Regarding the metal-oxide nanoclusters consisting of Ce^III or Co^II ions and Keggin-type PW₁₂O₄₀³⁻, even the straightforward video guide of preparing catalytic material has been given [110]. These will further encourage to utilize MMCT of POM-based catalytic materials in CO₂ reduction. It is noteworthy that prolonging the picosecond-scale lifetimes of photogenerated states during MMCT transition is still an important issue to be tackled [111]. The works from Lan and Hill et al. has validated the effects of heterometal location [108] and different addenda substitution [109] on the lifetime of photo-excited state.

On the other hand, the tools of advanced operando spectral characterization techniques [112], electrochemical measuring methods [113] and theoretical calculations [114], imbibing more understandings and attaining insights on catalytic mechanisms and structure–reactivity relationships of those reported POM-based catalysts will be helpful to design better POM catalysts for CO₂ conversion with explicit destination. On CO₂ photo/electroreduction, the hybridation of POM with various novel materials can provide chances to fabricate catalysts with broader visible-light absorption spectrum region or better selectivity but lower overpotential. In those systems with hybrid electro- or photocaytalytic CO₂ catalysts, once the light-harvesting/electron-storage centers and catalytically active sites are designated in an integrated system, the key issue is the bridging of the two components—charge kinetics. Exploiting time-resolved spectroscopic techniques to characterize charge kinetics is useful to understand these hybrid catalysts and thus helps to improve their catalytic efficiencies [99].

In catalytic oxidation, POMs were looked upon as inorganic analogs of porphyrin [115,116,117]. The thriving development of transition metal porphyrin or phthalocyanine derivatives as heterogeneous molecular catalysts for electrochemical CO₂ reduction [118] makes us have such confidence that this analogy can be still established in electrochemical CO₂ reduction. Therefore, the accumulated understandings about electrochemical CO₂ reduction by transition metal porphyrin or phthalocyanine may help the development of POM-based catalysts for the electroreductive CO₂ conversion. By sunlight or solar electricity, producing fuels and chemicals via CO₂ conversion is a promising and reliable option for largely reducing fossil fuel consumption in the future [119]. Thus POM-catalyzed transformation of CO₂ with photo- or electrochemical methods will retain long-lasting interest of both academic and industrial research.

To the non-reductive CO₂ conversion, searching for POMs with stronger Lewis basicity will benefit the elevation of turnover efficiency and the mitigation of harsh conditions under halogen-free condition. To enhance Lewis basicity, high negative charge density on the terminal oxygen atoms on POMs is required. Calculating and comparing natural bond orbital (NBO) charges of oxygen atoms in POMs has been demonstrated as a useful access to gain more insights into the Lewis basicity on POMs [84,120]. Making full use of both structural traits and diversity of POMs would help to break through some ceilings of practical CO₂ chemical conversion.