# Boosting Effect of Encapsulated Polyoxometalates in the Photocatalytic CO 2 Reduction by MOF-545

Khadija Talbi, Amanda Lyn Robinson, Albert Solé-Daura, Hongmei Chen, Jérémy Delafoulhouze, Youven Benseghir, Sandeep Kumar, Zakaria Halime, Ferdaous Ben Romdhane, Mathieu Frégnaux, et al.

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Khadija Talbi, Amanda Lyn Robinson, Albert Solé-Daura, Hongmei Chen, Jérémy Delafoulhouze, et al. Boosting Effect of Encapsulated Polyoxometalates in the Photocatalytic CO 2 Reduction by MOF-545. Applied Catalysis B: Environmental, 2025, 378, pp.125644. 10.1016/j.apcatb.2025.125644. hal-05357908

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# Boosting Effect of Encapsulated Polyoxometalates in the Photocatalytic CO₂ Reduction by MOF-545

Khadija Talbiª, Amanda L. Robinsonᵇ, Albert Solé-Dauraᶜ, Hongmei Chenᵇ, Jérémy Delafoulhouzeª, Youven Benseghirᵇ, Sandeep Kumarª,ᵈ, Zakaria Halimeᵈ, Ferdaous Ben Romdhaneᵉ, Mathieu Frégnauxª, Maria Gomez-Mingotᵇ, Pierre Mialaneª, Caroline Mellot-Draznieksᵇ,*, Anne Dolbecqª,\*

**Keywords:** Polyoxometalates, Metal-organic frameworks, photocatalysis, CO₂ reduction, DFT calculations

## ABSTRACT

Achieving efficient photocatalytic CO₂ reduction is a current complex challenge, requiring the development of strategies that optimize not only the capture of photons but also the photoinduced charge separation and electron transfer processes. In this pursuit, we have immobilized polyoxometalates (POMs), specifically [SiW₁₂O₄₀]⁴⁻ (SiW₁₂) and [W₁₀O₃₂]⁴⁻ (W₁₀), within the Zr-based porphyrinic metal-organic framework (MOF) MOF-545 catalytic material with the purpose of maximizing its CO₂ photoreduction activity. The resulting SiW₁₂@MOF-545 and W₁₀@MOF-545 composites were fully characterized by various techniques (IR spectroscopy, powder X-ray diffraction, N₂ adsorption isotherms, HADDF-STEM) to confirm the POM's incorporation *via* impregnation. These characterizations were complemented by simulations in order to locate the POM into the MOF's cavities and identify host/guest interactions. In photocatalytic conditions, i.e. under visible-light irradiation and in CH₃CN/TEOA 20:1 solution, the two SiW₁₂@MOF-545 and W₁₀@MOF-545 composites reduced CO₂ to formate with 100% selectivity at rates of 669 and 1239 µmol gMOF⁻¹ h⁻¹, respectively. Remarkably, W₁₀@MOF-545 showed around a 3-fold increase in activity compared to its POM-free counterpart. DFT calculations suggest that both POM guests can accept photoexcited electrons from the porphyrin linkers of MOF-545, allowing increased lifetime of the photogenerated holes in the MOF upon illumination, thus boosting TEOA oxidation by the porphyrinic MOF for subsequent CO₂ reduction. Moreover, the calculations unveil the origin of the observed superior overall catalytic activity of W₁₀@MOF-545 over SiW₁₂@MOF-545 due to stronger thermodynamic driving force for charge separation, providing rational guidelines for future design of efficient photocatalysts.

## 1. Introduction

Photocatalytic transformation of CO₂ into added-value carbon-based compounds (e.g., CO, HCO₂H, CH₄ ...) is a promising approach for the valorization of anthropogenic CO₂. Semiconductors [1], molecular complexes [2,3] and metal-organic frameworks (MOFs) [4–7] have attracted an increasing research interest as platforms for CO₂ conversion. In the photocatalytic reduction process, there are three key components, i.e. the catalyst, the photosensitizer (PS) and the sacrificial electron donor (SED), that can operate in homogeneous or heterogeneous conditions. As heterogeneous catalysts, MOFs have the advantage of being porous materials which allow for the diffusion of reactants and products. Additionally, MOFs are modulable materials that can be exploited for immobilizing

guest species. For instance, molecular catalysts [8–11] or metallic nanoparticles [12] can be immobilized in the pores of MOFs, leading to the so-called Guest@MOF composites. Such composites are particularly attractive for CO₂ photoreduction [13] and H₂ evolution [14]. Moreover, MOFs can be modified to include catalytic sites that can be (i) part of the MOF's metal nodes, (ii) grafted on these nodes or (iii) introduced as constitutive linkers, typically in the form of metallic complexes with bipyridine dicarboxylate ligands, wherein the dicarboxylate moieties bind to the metallic nodes [15,16]. Likewise, in addition to being catalytically active when suspended in a solution containing an external PS (typically Ru(bpy)₃), MOFs can also incorporate photosensitizers immobilized in their cavities [17] or as constitutive linkers [16]. For example, MOFs with amino-substituted linkers such as NH₂-MIL-

<footnote>ªUniversité Paris-Saclay, UVSQ, CNRS, Institut Lavoisier de Versailles, 78000 Versailles, France</footnote>

<footnote>ᵇLaboratoire de Chimie des Processus Biologiques, CNRS UMR 8229, Collège de France, Sorbonne Université, PSL Research University, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France</footnote>

<footnote>ᶜInstitute of Chemical Research of Catalonia (ICIQ-CERCA), The Barcelona Institute of Science and Technology, Avgda. Països Catalans, 16, 43007 Tarragona, Spain</footnote>

<footnote>ᵈUniversité Paris-Saclay, CNRS, Institut de Chimie Moléculaire et des Matériaux d'Orsay, ICMMO, 91400 Orsay, France</footnote>

<footnote>ᵉFédération de Chimie et Matériaux de Paris-Centre, 4 Place Jussieu, Paris 75005, France</footnote>

<footnote>* Corresponding authors. E-mail addresses: caroline.mellot-draznieks@college-de-france.fr (C. Mellot-Draznieks), anne.dolbecq@uvsq.fr (A. Dolbecq)</footnote>
125(Ti) [18] and NH₂-UiO-66(Zr) [19] were reported to be photocatalytically active for CO₂ reduction into formate upon visible light illumination in the absence of external PS. Also, MOFs containing photosensitive porphyrin linkers constitute a particularly attractive sub-family of porous solids for photocatalysis and in particular for CO₂ reduction [20–23]. Among them, MOF-545 (also known as PCN-222), is built from the connection of Zr₆ oxoclusters with tetrakis(4-carboxyphenyl) porphyrin (TCPP) linkers (Fig. 1), and has been particularly studied, showing 100% selectivity towards formate production [24,25]. Since the first report of its photocatalytic activity for CO₂ reduction in 2015 [24], functionalization of MOF-545 with CH₃NH₃PbI₃ [26] or CsPbBr₃ [27] perovskite quantum dots or grafted Re¹(CO)₃ complexes [28,29] (orienting the reaction towards CO production) has been described to obtain enhanced catalytic performances. Recently, our group has shown that two complementary strategies can further be combined to improve the photocatalytic activity of MOF-545 for formate production: (i) the metalation of the porphyrin linkers, and (ii) the synthesis of nanosized particles of MOF-545 [30]. Furthermore, through a combination of spectroscopic, photophysical, and computational investigations, we proposed a novel reaction mechanism for CO₂ conversion into formate. This mechanism highlights the key role of the triethanolamine (TEOA) SED, which undergoes photo-oxidation by the photoexcited porphyrin linkers of the MOF forming radical species. These radicals were found to be capable of transferring a formal hydride to an activated CO₂ molecule at a Zr⁴⁺ center (Fig. S1). Notably, this finding contrasts with the prevailing belief that Zr(III) ions in the nodes, generated via a proposed photoinduced linker-to-metal charge transfer that was not supported by our investigations, serve as the active sites for CO₂ reduction.

The immobilization of polyoxometalates (POMs) near catalytic centers of MOFs has been identified as a promising strategy to enhance their photocatalytic activity for CO₂ reduction. POMs are molecular anionic oxides built from the connection of MOₓ polyhedra where M is a metal in high oxidation states such as W⁵⁺, Mo⁵,⁵I or V⁴,⁴V [31]. A large majority of POMs can be reversibly reduced [32,33] and exhibit acid-o basic properties, and can thus be considered as proton and electron reservoirs [34]. POM-containing metal-organic frameworks (POM@MOFs) have emerged as excellent composites in photocatalysis, allowing not only to achieve the heterogenisation of POMs but also combining the electronic properties of the POM with those of the MOF guest. For example, non-catalytic POMs have been shown to play the role of electron relay in photocatalytic CO₂ reduction in POM@MOF composites, such as ([PTi₂W₁₀O₄₀],AuNP)@HKHUST-1 (Au@NENU-10) [35] and XM₁₂@CdMOF (X = Si, P; M = Mo, W) [36]. Our contribution to this field lies in the report of the (PW₁₂,Cp*Rh)@UiO-67 material where the Keggin-type POM [PW₁₂O₄₀]³⁻ was co-immobilized with a catalytic Cp*Rh complex in the Zr-based UiO-67 MOF [37]. The reduction of CO₂ into formate was

significantly enhanced when compared to that of the POM-free Cp*Rh@UiO-67 catalyst. DFT calculations further allowed us to elucidate how such non-catalytic POMs influence the catalytic activity of MOFs in CO₂ reduction [38]. In (POM,Cp*Rh)@UiO-67, POMs act as electron reservoirs, transferring electrons to the Rh catalyst and also quenching the photoreduced form of the [Ru(bpy)₃]²⁺ PS, thus limiting its degradation. This theoretical investigation has identified the decatungstate [W₁₀O₃₂]⁴⁻ (W₁₀) POM as one of the best candidates for this system [38].

Inspired by the above findings, we thus decided to encapsulate W₁₀ POMs in MOF-545 (Fig. 1) in order to enhance its photocatalytic activity for CO₂ reduction. For comparison, we also encapsulated the [SiW₁₂O₄₀]⁴⁻ (SiW₁₂) Keggin-type POM (Fig. 1), which despite having same charge as W₁₀ is reduced at a different potential [38]. We report herein the synthesis, characterizations and docking simulations for these two new composites, namely W₁₀@MOF-545 and SiW₁₂@MOF-545, and their photocatalytic properties under two different catalytic conditions. These composites represent rare examples of model systems with earth-abundant, easy to synthesize POMs that enhance the photocatalytic activity of MOF-based materials for CO₂ conversion, this effect being rationalized thanks to DFT calculations, providing a robust reasoning strategy for designing more efficient catalysts.

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Representation of the constituting units of the W₁₀@MOF-545 and SiW₁₂@MOF-545 composites; orange polyhedra: ZrO₂, green octahedra: WO₆, yellow tetrahedron: SiO₄, black spheres C, blue spheres N, red spheres O.</figcaption></figure>

## 2. Experimental section

### 2.1 Synthesis and Characterizations

All reagents were purchased from commercial sources and used as received. MOF-545 [30], H₄SiW₁₂O₄₀•nH₂O (SiW₁₂) [39], Na₄W₁₀O₃₂•nH₂O (W₁₀) [40] and ZnA₃BCOOH [41] (see Supplementary Information) were synthesized according to reported procedures.

**W₁₀@MOF-545.** The composite was synthesized by the impregnation method. A suspension was prepared by
adding 40 mg of MOF-545, previously activated for 18 h under vacuum at 130 °C, to 8 mL of a 5 mM acetonitrile solution of W₁₀. The suspension was stirred gently in a water bath at 60 °C for 6 h. After cooling to room temperature, the powder was collected by centrifugation and washed twice with acetonitrile, twice with ethanol and twice with acetone and then dried at 100 °C, yielding a red solid (45 mg). The supernatant was colorless (see UV-vis spectrum Fig. S2), consistent with the absence of porphyrin linkers' leaching.

**SiW₁₂@MOF-545.** The composite was synthesized following the same protocol but replacing W₁₀ by SiW₁₂, yielding a green solid (42 mg). The supernatant was colorless (see UV-vis spectrum Fig. S2), consistent with the absence of porphyrin linkers' leaching.

## 2.2 Materials and instrumentation

Infrared (IR) spectra were recorded on a Nicolet 30 ATR 6700 FT spectrometer. Diffuse reflectance UV-vis spectra in the range of 200–800 nm were collected using a PerkinElmer Lambda 2 UV-vis spectrophotometer. Powder X-ray diffraction data were collected on a Brüker D5000 diffractometer using Cu radiation (1.54059 Å). Thermogravimetry analyses (TGA) were performed on a Mettler Toledo TGA/DSC 3, STARe System apparatus under oxygen flow (50 mL min⁻¹) at a heating rate of 5 °C min⁻¹ up to 700 °C. N₂ adsorption isotherms (BET) were obtained at 77 K using a BELsorp Mini (Bel, Japan). Scanning electron microscopy (SEM) measurements were performed on a JEOL JSM 7001F apparatus. Scanning transmission electron microscopy (STEM) images using a high-angle annular dark-field (HAADF) detector were acquired using a JEOL 2100 Plus UHR microscope operating at 200 kV. The powdered samples were dispersed in ethanol, and then a drop was evaporated on a carbon-coated copper grid or a carbon-coated nickel grid. Analytic investigations were performed with an energy dispersive X-ray (EDX, Oxford Instruments X-Max 80 SDD) spectrometer attached to the microscope column. The W₁₀@MOF-545 material was used for analysis by ultramicrotomy. A few mg of powder were deposited in the bottom of a BEEM capsule. Embedding resin (AGAR 100) was added and polymerized at 70°C for 48 hours. The polymerized block was cut using a diamond knife. Ultrathin sections of ~80 nm thickness were cut with a Leica ULTRACUT UCT ultramicrotome and collected onto lacey carbon film-coated copper grids.

The amount of Zr and W leached in solution were analyzed by inductively coupled plasma coupled atomic emission spectrometry (ICP-AES) (PerkinElmer NexION 2000 ICP mass spectrometer). The filtrates were diluted into 2 wt.% HNO₃ aqueous solution for the measurements (See Table S1).

**XPS measurements.** Few milligrams of MOF-545, SiW₁₂@MOF-545 and W₁₀@MOF-545 powders were deposited on conductive copper tape and mounted on the XPS sample holder. Regular XPS analyses were conducted

on a Thermofisher Scientific Escalab 250xi equipped with a monochromated Al-Kα anode (1486.6 eV) and a dual flood gun (low energy electron and ion). High energy resolution spectral windows of interest were recorded with a 650 μm spot size. The photoelectron detection was performed perpendicularly to the surface using a constant analyzer energy (CAE) mode (20 eV pass energy) and a 0.1 eV energy step. The photopeaks Zr 3d, W 4f and Si 2p were reconstructed after a Shirley type background subtraction on the Thermofisher Scientific Avantage© software.

**Photocatalytic experiments.** Samples were prepared in a 1 cm path length quartz sealed cuvette (Hellma). The photocatalytic experiments where run with 2 mg of composite dispersed in 2 mL of CH₃CN/TEOA 10:1 or 20:1 (v/v). Photochemical reactions were performed using an Oriel® MIniSol LSH-7320 LED Solar Simulator, equipped with LED lamps across 400 to 1100 nm that are matched to an AM1.5G. Samples were irradiated with 1 SUN (100 mW/m²). Samples were saturated with CO₂ *via* directly bubbling CO₂ through the solution mixture for 20 min. During irradiation, the samples were vigorously stirred. Formate in the liquid phase was analyzed using a Metrohm 883 Basic IC plus ionic exchange chromatography instrument, using a Metrosep A Supp 5 column and a conductivity detector. The sample was 100-fold diluted in deionized 18 MΩ water and 20 μL were injected into the IC chromatograph. Calibration curve for HCOOH in water (0-30 μM) was performed before analysis. Aliquots of gas were also analyzed. H₂, CO and CH₄ measurements (aliquots of 100 μL of the headspace) were performed by gas chromatography on a SRI instruments (8610 GC) with Ar as the carrier gas equipped with a HaySepD precolumn attached to a 3 Å molecular sieve column to separate the different gases. Detection of H₂ was permitted by a thermal conductivity detector, and carbon-based products were detected by a flame-ionization detector fitted with a methanizer. The GC was calibrated using a custom standard gas mixture in CO₂ periodically.

## 2.3 Computational Details

A simulated annealing (SA) Monte Carlo-based methodology was used to explore the positioning of SiW₁₂ and W₁₀ POMs in MOF-545 as previously reported on similar Guest@MOF systems [37,42,43]. In short, each annealing cycle consisted of 50000 independent Monte Carlo steps, initiating each energy minimization at 1000 K followed by system-cooling to 300 K. Low energy sites were thus identified by searching the configurational space of the {POM, MOF} system as the temperature slowly decreased, while treating the docked POM as a rigid body and the MOF unit-cell as the fixed-atom host. The POM-MOF host-guest interactions were described using the *uff* forcefield [44,45] with a real-space cutoff of 16.5 Å. The long-range electrostatic interactions were handled with explicit charges and the Ewald summation technique with a relative precision of 10⁻⁶. The atomic charges were calculated using
the charge-equilibration method [46] and imposing a total
charge of +4 to the MOF host to compensate the POM's
negative charge. The structural models were built starting
with the primitive cell of MOF-545 building a 1x1x2 3D-
periodic supercell (α = b = 42.68 Å, c = 33.32 Å, α = β = 90°,
γ = 120°) consisting of 1278 atoms in addition to one POM
cluster (either SiW12 or W10). Cartesian coordinates of the
calculated POM@MOF systems can be found in the SI.
DFT calculations were further performed at the ωB97X-D
level [47] using the Gaussian 16 (rev. C.01) [48] quantum
chemistry package. The LANL2DZ[49] basis set and
associated pseudopotentials were used to describe Zr and
W atoms. This basis set was supplemented by a shell of f-
type polarization functions with the exponents provided by
Frenking and coworkers [50]. The 6-31G(d,p) basis set
[51,52] was used for the remaining atoms. Solvent effects
of acetonitrile were included in both geometry
optimizations and energy calculations by means of the IEF-
PCM solvent model [53] as implemented in Gaussian 16.
MOF-545 was modeled as a H-capped cluster model
consisting of a chromophoric TCPP linker connected to a Zr6
node. The positions of C atoms connected to the capping
groups were fixed throughout geometry optimizations to
reproduce the rigidity imposed by the MOF structure. A
dataset collection of the optimized structures is available in
the ioChem-BD repository [54] and can be accessed via DOI:
10.19061/iochem-bd-2-71.

3. Results and discussion

3.1. *Synthesis and Characterizations.*

There are two main methods to synthesize POM@MOF
composites: synthetic encapsulation and impregnation [8].
In the synthetic encapsulation method, the POM is added
as a reactant during the MOF synthesis, while in the
impregnation method, the preformed MOF is stirred in a
solution containing the POM. The large hexagonal channels
of MOF-545 (Fig. 1) allow the use of both methods for the
POM's incorporation since SiW₁₂ and W₁₀ are small enough
to diffuse into the pores. We initially attempted the
synthetic encapsulation method. However, the presence of
POMs in the MOF reaction medium inhibited the formation
of a crystalline phase. Consequently, we further adopted
the impregnation method as a milder route, whereby a
crystalline powder of MOF-545 was suspended in an
acetonitrile solution containing POMs in large excess. This
method successfully allowed to isolate W₁₀@MOF-545 and
SiW₁₂@MOF-545 composites in good yields and good
crystallinity.

For both composites, the powder X-ray diffraction (PXRD)
patterns showed no loss of crystallinity but a reduction in
the relative intensity of the low angle peak (2θ = 4.8°)
compared to the pristine MOF (Fig. S3). Such decrease in

intensity can be attributed to the presence of species in the
MOF cavities [8]. Additionally, infrared (IR) spectra (Fig. S4)
exhibited the characteristic peaks of both the MOF's
carboxylates (between 1350 and 1650 cm⁻¹) and the POM's
Si-O, W=O and W-O-W bands (between 1050 and 750 cm⁻¹),
therefore suggesting that the materials contain both the
MOF-545 and the respective POMs. Moreover, for both
composites, the high similarities between the IR spectra of
the POM and of the POM@MOF compounds in the 1100-
850 cm⁻¹ range strongly suggest that the structures of the
POMs have been conserved after the impregnation process.
In addition, nitrogen adsorption measurements were also
performed to evaluate the impact of the immobilization of
the POMs on the porosity of the composite materials (Fig.
S5). A significant decrease of the BET (Brunauer–Emmett–
Teller) surface area of the W₁₀@MOF-545 (793 m² g⁻¹) and
SiW₁₂@MOF-545 (1157 m² g⁻¹) composites compared to
MOF-545 (1581 m² g⁻¹) was observed. Also, the higher
reduction of the surface area as well as of the mesopore
size and volume observed for W₁₀@MOF-545 than for
SiW₁₂@MOF-545 suggested a higher POM loading for the
former. Together, the PXRD, IR, and BET data suggest that
the POMs were successfully incorporated in the pores of
MOF-545 without altering the crystallinity of the host
material.

STEM-EDS (STEM = Scanning Transmission Electron
Microscopy, EDS = Energy Dispersive X-ray spectroscopy)
and SEM-EDS (SEM = Scanning Electron Microscopy) were
then used to evaluate the distribution of the POMs in the
composite materials. Ultramicrotomy was performed to
obtain ~80 nm thick sections of W₁₀@MOF-545
perpendicular to the rod axis, thereby enabling access to
the internal structure of the MOF. High-resolution TEM
images (Fig. 2a) evidence the presence of spherical dark
spots that can be attributed to W₁₀ POMs, indicating that
POMs are distributed not only on the surface of the crystals
but also within their bulk. Furthermore, the mapping of the
W₁₀@MOF-545 crystallites suggests a rather uniform
distribution of the W₁₀ POM in the MOF (Fig. 2b), and the
presence of Na indicates that the counter-ions of the POMs
are co-immobilized with the W₁₀ polyoxometalate.
Likewise, the STEM-EDS mapping for SiW₁₂@MOF-545 also
suggests a uniform distribution of the SiW₁₂ POM in the
MOF matrix (Fig. S6). Additionally, the green color of the
composite (Fig. S2a) indicates a protonation of the
porphyrin, the protons acting as counter-ions for the SiW₁₂
POMs in the composite. The solid-state UV-vis spectra of
W₁₀@MOF-545 and SiW₁₂@MOF-545 were recorded and
compared to that of MOF-545 and of the POM precursors
(Fig. S2c). The main difference between the spectra of MOF-
545 and of the composite counterparts lies in the 350 and
450 nm range with a contribution of POM absorption in this
region.
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** a) TEM images of W<sub>10</sub>@MOF-545 (prepared by ultramicrotomy), showing the distribution of POM within the MOF structure; b) HADDF-STEM image with EDS chemical mapping.</figcaption></figure>

The formulae of the composite materials were assessed by a combination of SEM-EDS and thermogravimetric analyses (TGA) (Fig. S7, Tables S2 and S3), which indicated that the POM/Zr<sub>6</sub> ratios were of 0.30 for W<sub>10</sub>@MOF-545 and of 0.15 for SiW<sub>12</sub>@MOF-545, in line with a lower BET surface area for the W<sub>10</sub>@MOF-545 sample (see above). Additionally, the POM loadings (22.3 wt% for W<sub>10</sub>@MOF-545 and 15.9 wt% for SiW<sub>12</sub>@MOF-545) are comparable to those found in other MOF-545 composites containing similar POMs, such as H<sub>3</sub>PMo<sub>12</sub>O<sub>40</sub> [55], H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub> [56], Co-substituted monomeric Keggin type POMs [57], or Co- [58], Mn-, Ni- and Zn- [59] sandwiched type POMs. Furthermore, it can be noted that increasing the POM concentration in the impregnation solution did not lead to higher POM loadings in the final composite material, suggesting that maximum loadings were reached in these conditions.

Finally, X-ray photoelectron spectroscopy (XPS) was
performed to confirm the chemical compositions and
determine the elemental states of the as-prepared
samples. Fig. 3a compares the XPS spectral lines of Zr 3d
respectively in MOF-545, SiW<sub>12</sub>@MOF-545 and W<sub>10</sub>@MOF-

545. For the three compounds, the spectrum evidences
only one doublet whose main component Zr 3d<sub>5/2</sub> was
centered between 182.6 and 182.8 eV. This energy position
corresponds to the Zr<sup>IV</sup> oxidation state, as the characteristic
signal for stoichiometric ZrO<sub>2</sub> is typically observed at 182.7
eV [60,61]. This suggests that the Zr centers in MOF-545 and
MOF-545-based composites have similar chemical
environment. The chemical environment of W was also
investigated for SiW<sub>12</sub>@MOF-545 and W<sub>10</sub>@MOF-545 (Fig.
3b). In both cases, a doublet with well-resolved spin-orbit
splitting (2.1 eV) is observed with W 4f<sub>7/2</sub> peak positions
measured at 35.5 eV. This energy position can be assigned
to W<sup>VI</sup> and is consistent with that of the pristine POM
structures [62,63]. The high-resolution Si 2p XPS spectrum
collected on the SiW<sub>12</sub>@MOF-545 composite (Fig. 3c) can
be reconstructed with a doublet with a low spin-orbit
splitting (0.63 eV). The Si 2p<sub>3/2</sub> peak at 101.7 eV is consistent
with the binding energy found in the literature for the
Keggin-type SiW<sub>12</sub> POM [63]. Overall, the XPS results show
no alteration in the chemical or electronic state of either
POM or MOF after POM incorporation.
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** High energy resolution spectra of (a) Zr 3d region for MOF-545, SiW<sub>12</sub>@MOF-545 and W<sub>10</sub>@MOF-545, (b) W 4f core level for SiW<sub>12</sub>@MOF-545 and W<sub>10</sub>@MOF-545 and (c) Si 2p photopeak for SiW<sub>12</sub>@MOF-545.</figcaption></figure>

**3.2. Simulated annealing (SA) calculations**

SA calculations were performed to determine the possible
locations of the guest POMs in W₁₀@MOF-545 and
SiW₁₂@MOF-545 and to investigate the nature of the host-
guest interactions (see experimental section for
computational details). In both cases, the most favorable
positions for the POMs were exclusively located within the
hexagonal channels of MOF-545 as illustrated in Fig. 4, the
triangular channels remaining unvisited during the
simulations. Such positions are stabilized by short-range
and electrostatic interactions between the charged POMs
and the MOF framework. For the SiW₁₂ POM (Fig. 4, left), a
typical favorable location was found near Zr₆-oxoclusters
where stabilizing hydrogen-bond interactions are found at
distances ranging from 2.6 Å to 3 Å at the host-guest
interface. These interactions involve the terminal oxygens
of the SiW₁₂ POM and hydrogen atoms from water
molecules coordinated to the Zr centers, a feature observed
in other POM@MOF composites [58]. Similar interactions
are found for W₁₀@MOF-545 (Fig. 4, right). Still, the ovoid
shape of W₁₀ allows this POM to nest between two Zr₆-
oxoclusters, thus benefiting from stabilizing hydrogen-bond
type interactions with apical oxygen atoms of the POM in
the 2.7 – 3 Å distance range at both ends of the POM.
Notably, both SiW₁₂ and W₁₀ POMs were found in the
vicinity of at least two porphyrin linkers, with typical
distances of 9.2 Å for W₁₀ and 9.9 Å for SiW₁₂, (expressed as
the distance between the POM's and the TCCP's centers of
mass). While the chemical environments around the POMs
(porphyrins and Zr₆-oxoclusters) are quite similar in MOF-
545, the SA calculations suggest that the unique shape of
W₁₀ leads to more stabilizing interactions with the MOF-545
host.

<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Simulated positions of the SiW<sub>12</sub> (left) and W<sub>10</sub> (right) POMs in the hexagonal channels of MOF-545 as extracted from SA calculations. H-bond type interactions and distances (Å) to the closest porphyrin center of mass are indicated in dashed lines. Connolly surface of the MOF (bottom) is represented in blue for each POM@MOF composite. A polyhedral representation is adopted for Zr (orange), W (green) and Si (yellow) atoms, and ball-and-stick for the linker: C (grey), N (blue) and O (red).</figcaption></figure>

**3.3. Photocatalytic measurements in CH<sub>3</sub>CN/TEOA (10:1) solutions**

The photocatalytic activities of the composites were first
tested under conditions previously reported for MOF-545,
by our group [30] and others [24,25,27], i.e. in a CO2-
saturated CH3CN/TEOA (10:1) mixture (Fig. 5a and Table
S4). For MOF-545 and its composite counterparts, formate
(HCOO-) was the only detected liquid product, and no
gaseous products such as CO, H2 or CH4 were observed. All
three catalysts thus exhibited 100% selectivity for formate
production. After 2 h, W10@MOF-545 and SiW12@MOF-545
produced HCOO- at rates of 1110 and 815 µmol g-1,
respectively, which is around 2 times and 1.4 times higher
than the production rate of pristine MOF-545
photocatalyst. When normalized to the amount of MOF-545 in the composite, the production rates were 1432 and 968 µmol g<sub>MOF</sub><sup>-1</sup> for W<sub>10</sub>@MOF-545 and SiW<sub>12</sub>@MOF-545, respectively, showing 2.5- and 1.7-times higher activity than pristine MOF-545. This indicates a boosting effect of both SiW<sub>12</sub> and W<sub>10</sub> POMs for the photocatalytic activity of the MOF.

Control experiments (Table S5) confirmed a very low formate production when one component of the photocatalytic system (CO₂, catalyst, light or TEOA) was absent. However, unexpectedly, recyclability measurements gave poor results, suggesting a potential degradation of the composites. We thus further investigated their stability in the photocatalytic medium. A suspension of W₁₀@MOF-545 was stirred in CH₃CN/TEOA (10:1) in the absence of light for 1 h and filtered. The filtrate was colorless, suggesting the absence of porphyrin linker in solution. Its photocatalytic activity was then evaluated upon addition of an external photosensitizer (such as [Ru(bpy)₃]²⁺ or [Ru(phen)₃]²⁺) and CO₂ bubbling (Fig. 5b, Table S6). When [Ru(bpy)₃]²⁺ or [Ru(phen)₃]²⁺ were added

to the filtrate, 2.60 and 2.72 µmol of HCOO⁻ were produced after 1 h, respectively. These experiments suggest that the degradation of the POM@MOF may occur in the absence of light and produce catalytically active species in solution. We thus hypothesized that catalytically active Zr-based species were released in solution due to the action of TEOA when the POM@MOF was suspended in CH₃CN/TEOA (10:1), in agreement with two recent studies on Zr-based MOFs. In one study by Saouma and coworkers, Zr leaching from UiO-66 Zr-MOF derivatives in CH₃CN/TEOA (5:1), under UV-vis irradiation (λ ≥ 320 nm) was detected by ICP analysis. The Zr-containing filtrates still produced formate even when no additional PS was added, suggesting that the linker was also released and remained connected to the Zr ions [64]. Similarly, Fang's group reported TEOA-induced dissolution of UiO-66(NH₄OH) (an UiO-66 type MOF prepared with NH₄OH as reactant) [65]. In this case, the authors proposed that the degradation of the MOF resulted in soluble Zr₆ clusters with benzenedicarboxylate linkers and TEOA molecules coordinated to the Zr ions.

<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Formate production after 2 h irradiation for 2 mg of MOF-545, W<sub>10</sub>@MOF-545 or SiW<sub>12</sub>@MOF-545 solids suspended in 2 mL of CO<sub>2</sub>-saturated CH<sub>3</sub>CN/TEOA a) 10:1 and c) 20:1 solutions; green bars represent the formate production normalized by the percentage of the catalyst's weight associated to the MOF; b) scheme showing the protocol to obtain the solutions which formate production is given in c); c) formate production after 1 h irradiation of the solutions obtained by filtration of a suspension of 2 mg of W<sub>10</sub>@MOF-545 stirred for 1 h in 2 mL of CH<sub>3</sub>CN/TEOA (10:1) with or without additional photosensitizer (3.5 mmol); e) kinetics of formate production for MOF-545 and POM@MOF composites suspended in 2 mL of CH<sub>3</sub>CN/TEOA (20:1) solutions.</figcaption></figure>
ICP-AES measurements were performed in order to quantify the amount of Zr and W elements leached in the above filtrate solutions. The quantification of W allowed to determine if POMs were released concomitantly to Zr species. W and Zr concentrations were measured in the solutions obtained after filtration of the suspensions of POM@MOF stirred in CH3CN/TEOA (10:1) and the percentage of each element was calculated (Table S1). These measurements showed that the percentage of leached Zr was higher in SiW12@MOF-545 than in W10@MOF-545 (1.4% and 1.1% respectively), in line with the photocatalytic activities. Likewise, a non-negligible amount of POM was also released in solution (5.6% and 4.4% of leached W detected for SiW12@MOF-545 and W10@MOF-545, respectively), with thus more POM released for SiW12@MOF-545 than for W10@MOF-545. We hypothesized that TEOA might be responsible for the observed leaching and MOF degradation. Powder X-ray diffraction measurements confirmed also a loss of MOF crystallinity following the photocatalytic experiments conducted in CH3CN/TEOA (10:1) solutions (Fig. S3). We thus lowered the TEOA concentration by using a 20:1 CH3CN/TEOA ratio. ICP measurements confirmed that the percentage of leached Zr and W in the photocatalytic solution were much lower under these new conditions, showing that the stability is greatly improved when the TEOA concentration is half as high (Table S1). These new conditions were thus used for the further photocatalytic tests reported in section 3.4.

In addition, we aimed to confirm that porphyrin molecules
can act as photosensitizers to promote the photooxidation of
TEOA and generate TEOA\* radicals, which can then
hydrogenate CO₂ molecules bound to the leached Zr
species [30]. In that purpose, we introduced a
monocarboxylate Zn-porphyrin (ZnA₃BCOOH, Fig. 5b) into
the filtrate obtained with the W₁₀@MOF-545 composite
rather than TCPP (the linker of MOF-545) which has a low
solubility in CH₃CN; the Zn-metallated porphyrin was used
for its known ability to enhance light harvesting [66–68].
Notably, a significant formate production (1.25 µmol) was
observed in this experiment (Table S6, entry 5), although
still lower than that observed with Ru-based PS (Fig. 5b).
Similar experiments were performed with the filtrate
obtained with SiW₁₂@MOF-545 composite (Table S7). The
addition of ZnA₃BCOOH to the filtrate also promoted
photocatalytic CO₂ reduction, and the formate production
was higher with SiW₁₂@MOF-545 (Table S7, entry 2) than
with the W₁₀@MOF-545 composite (Table S6, entry 5). This
suggests greater degradation of SiW₁₂@MOF-545 in the
CH₃CN/TEOA (10:1) solution, in line with the ICP
measurements (see above). Control experiments with no PS
and either the SiW₁₂ or the W₁₀ POM alone (without any Zr
species in solution) confirmed that the POMs alone are non-
catalytic and that the presence of both Zr species and PS are
required for the catalysis.

3.4. *Photocatalytic measurements in CH<sub>3</sub>CN/TEOA (20:1)*
*solutions*

A new set of photocatalytic experiments was performed using the 20:1 CH3CN/TEOA ratio, shown to considerably lower the degradation of the POM@MOF composite. The formate productions after 2 h of irradiation, using W10@MOF-545, SiW12@MOF-545 or the pristine MOF-545, under these new conditions, are compared in Fig. 5c and (Table S4, entries 4-6). These results show that all three materials exhibit significantly higher formate productions in CH3CN/TEOA (20:1) (Fig. 5c) compared to CH3CN/TEOA (10:1) (Fig. 5a). We attribute the improved photocatalytic activity in CH3CN/TEOA (20:1) to the better preservation of the MOF-545 crystallinity under these conditions, as evidenced in Fig. S3. The structural integrity of MOFs is known to play a crucial role in facilitating the diffusion of reactants and products to and from the active sites through their porous framework, which is essential for maintaining high catalytic performance. In addition, the superior performance of W10@MOF-545 when compared to that of SiW12@MOF-545 for formate production is maintained, reaching 1925 µmol g⁻¹ or 2476 µmol gMOF⁻¹, respectively, after 2 h. This represents one of the highest values reported for MOF derivatives (Table S8). Notably, this photocatalytic system does not require an external PS, as the chromophoric porphyrinic groups are intrinsic components of the MOF framework.

Kinetic measurements were also performed as illustrated in
Fig. 5d. The formate production increases linearly for the
first 2 h but levels off after a few hours. This plateau can be
attributed to an accumulation of formate ions inside the
MOF's cavities, as a result of the strong affinity of the
formate ions for the undercoordinated Zr(IV) centers of the
MOF-545. This accumulation of formate thus poisons the Zr
centers required to fix and activate CO₂, as evidenced in the
reported mechanistic study [30]. The MOF activity can be
retrieved upon washing the MOF with water at 60°C, as
shown by our recyclability tests, in the present study (see
below) and in our previous report [30]. Recyclability
experiments performed after 1 h illumination (Tables S9
and S10, Fig. S8) revealed a slight decrease in activity, which
could be attributed to the difficulty of recovering the very
small amount (2 mg) of composites after each run. The
stability of the materials was further confirmed by the
similarity of the XRD patterns (Fig. S3), SEM images and
TEM-EDS elemental mapping measurements collected
before and after the catalytic tests (Fig. S6).

3.5. *DFT calculations*

Finally, DFT calculations using cluster models were
performed to obtain further insights into the origin of the
enhanced photocatalytic activity of MOF-545 granted by
the immobilization of POM guests. Previously, our group
reported the mechanism governing the photocatalytic
reduction of CO₂ into formate by MOF-545 [30], which is
schematically summarized in Fig. 6, left (blue and black
cycles). In the first step, light absorption induces the
formation of an excited state centered on a TCPF linker (1),
which possesses the ability to oxidize TEOA. This results in
the formation of a neutral TEOA· radical through
spontaneous deprotonation, facilitated by a second TEOA
molecule acting as a Brönsted base (2). The TEOA· radicals
were found to play a crucial role as hydride donors,
promoting the thermally activated hydrogenation of CO2
molecules activated on the Zr6 nodes of the MOF (3), where
ZrIV sites function as Lewis acid centers. This reaction leads
to the formation of the formate product, along with a
dehydrogenated TEOA radical (ald'), which recovers an

electron from the linker to produce a TEOA-derived
aldehyde (4), that we identified by NMR [30]. Importantly,
the catalytic activity was found to be tightly related to the
ability of the catalyst to promote the photo-oxidation of
TEOA. On these grounds, we herein analyzed how the
presence of SiW12 and W10 within the pores of the
porphyrinic MOF-545 can assist this process and now
propose an unprecedented POM-assisted mechanism in
Fig. 6, right (green and black cycles).

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Schematic representation of the proposed photocatalytic CO₂ reduction mechanism promoted by MOF-545 in the presence of TEOA (see main text for details). The blue cycle (left) illustrates the previously reported mechanism for TEOA photo-oxidation at the TCPF linkers of MOF-545, [30] while the green cycle (right) represents the proposed novel mechanism for POM@MOF-545 catalysts. This new pathway might be responsible for the enhanced photocatalytic activity, attributed to the formation of charge-separated states involving TCPF linkers and the POM guests. Black arrows (top cycle) indicate the thermally-activated CO₂ reduction cycle by TEOA*, which is common to both POM-free and POM-containing materials. The frontier molecular orbital diagram of the rightmost panel compares the energy levels of TCPF linkers, SiW₁₂ and W₁₀ POMs. Energies (eV) obtained at the ωB97X-D level with a double-ζ quality basis set, including implicit solvent effects of acetonitrile. CT stands for charge-transfer events.</figcaption></figure>

The rightmost panel of Fig. 6 compares the energy levels of
the frontier molecular orbitals (MOs) of a Zr₆-TCPP cluster
model of MOF-545 with those of the POM guests. TCPF
linkers are known to undergo π-π* transitions under visible
light irradiation, which involve the HOMO and LUMO
orbitals depicted in Fig. 6. Notably, our DFT calculations
indicate that the LUMOs of both POMs of interest, W₁₀ and
SiW₁₂, lie lower in energy than that of the TCPF linker. This
suggests that upon photoexcitation, the TCPF linkers of the
MOF are poised to transfer an electron to neighboring POM
guests as illustrated in the green cycle of Fig. 6, forming a
charge-separated state that populates the empty d(W)-

type orbitals. This is expected to increase the lifetime of
photogenerated holes in the MOF, enhancing the efficiency
of TEOA photo-oxidation and, in turn, the overall catalytic
activity. Conspicuously, the LUMO of W10 was found to be
lower in energy compared to that of SiW12. This indicates
that W10 provides a stronger thermodynamic driving force
for the photoinduced charge transfer (CT) event and
consequently, may allow a more efficient charge
separation, in line with the experimentally observed higher
activity of W10@MOF-545, compared to the SiW12@MOF-
545 material.

4. Conclusions
We successfully synthesized two composite photosystems, SiW<sub>12</sub>@MOF-545 and W<sub>10</sub>@MOF-545, by immobilizing two different POMs in the Zr-based porphyrinic MOF-545. The photocatalytic activities of these materials for CO<sub>2</sub> reduction were investigated under visible light irradiation. We show that in CH<sub>3</sub>CN/TEOA 20:1 solution, the POM@MOFs composites are stable and exhibit formate productions that surpass that of the POM-free MOF-545, with W<sub>10</sub>@MOF-545 exhibiting superior performances. Notably, this fully noble-metal free catalyst has one of the highest formate production reported for MOF derivatives, is 100% selective and requires a particularly low TEOA concentration. DFT calculations provided a rational explanation for the enhanced photocatalytic activities of these POM@MOF catalysts over the POM-free counterpart. LUMO alignment between POMs and TCPP is expected to promote charge transfer from the excited TCPP linkers to the POM guests, extending the lifetime of photogenerated holes, which boosts TEOA photo-oxidation and in turn the catalytic activity, a very specific role that has never been considered so far for CO<sub>2</sub> photocatalytic conversion. The lower energy of the LUMO of W<sub>10</sub> compared to that of SiW<sub>12</sub> makes electron transfer thermodynamically more favorable with W<sub>10</sub>, explaining its greater impact in experimentally observed photocatalytic performances. This work highlights the broader potential of our method for developing more efficient photocatalysts for CO<sub>2</sub> reduction and in advancing photochemical applications.

**Credit authorship contribution statement**

**Khadija Talbi:** Investigation, Formal - analysis, Data curation, Visualization. **Amanda L. Robinson:** Investigation, Formal - analysis, Writing - review & editing. **Albert Solé-Daura:** Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing. **Hongmei Chen:** Investigation, Formal - analysis, Data curation, Visualization. **Jérémy Delafoulhouze:** Investigation, Formal - analysis, Data curation, Visualization. **Youven Benseghir:** Investigation, Formal - analysis, Data curation, Visualization. **Sandeep Kumar:** Investigation, Formal - analysis, Data curation, Visualization. **Zakaria Halime:** Validation, Supervision, Writing - review & editing. **Ferdaous Ben Romdhane:** Formal - analysis, Data curation, Visualization. **Mathieu Frégnaux:** Investigation, Formal - analysis, Data curation, Visualization. **Maria Gomez-Mingot:** Validation, Writing - review & editing. **Pierre Mialane:** Conceptualization, Supervision, Writing - review & editing. **Caroline Mellot-Draznieks:** Conceptualization, Methodology, Supervision, Funding acquisition, Writing - original draft, Writing - review & editing. **Anne Dolbecq:** Conceptualization, Supervision, Project administration, Funding acquisition, Writing - original draft, Writing - review & editing.

**Declaration of Competing Interest**

There are no conflicts of interest to declare.

**Acknowledgments**

This work was supported by CNRS, UVSQ, the Ministère de l'Enseignement Supérieur, de la Recherche et de l'Innovation, the French National Research Agency (ANR) as part of the 'Investissements d'Avenir' program No. ANR-11-IDEX-0003-02 and CHARMMMAT ANR-11-LABX-0039 and a grant of the Agence Nationale de la Recherche (ANR-21-CE50-0024). The calculations have been performed using the HPC national resources from GENCI (CINES/TGCC/) through Grant 2016-097343. Solène Béchu is gratefully acknowledged for SEM measurements. A. S.-D. acknowledges financial support from the "la Caixa" Foundation (ID 100010434) under the fellowship code LCF/BQ/PI24/12040012. We thank Marc Fontecave for insightful discussions.

**Appendix A. Supporting information**

Supplementary data associated with this article can be found in the online version.

**References**

[1] Y. Cui, P. Ge, M. Chen, L. Xu, Research Progress in Semiconductor Materials with Application in the Photocatalytic Reduction of CO2, Catalysts 12 (2022) 372. https://doi.org/10.3390/catal12040372.

[2] E. Boutin, L. Merakeb, B. Ma, B. Boudy, M. Wang, J. Bonin, E. Anxolabéhère-Mallart, M. Robert, Molecular catalysis of CO2 reduction: recent advances and perspectives in electrochemical and light-driven processes with selected Fe, Ni and Co aza macrocyclic and polypyridine complexes, Chem. Soc. Rev. 49 (2020) 5772-5809. https://doi.org/10.1039/D0CS00218F.

[3] Y.-H. Luo, L.-Z. Dong, J. Liu, S.-L. Li, Y.-Q. Lan, From molecular metal complex to metal-organic framework: The CO2 reduction photocatalysts with clear and tunable structure, Coord. Chem. Rev. 390 (2019) 86-126. https://doi.org/10.1016/j.ccr.2019.03.019.

[4] J. Canivet, F.M. Wisser, Metal-Organic Framework Catalysts for Solar Fuels: Light-Driven Conversion of Carbon Dioxide into Formic Acid, ACS Appl. Energy Mater. 6 (2023) 9027-9043. https://doi.org/10.1021/acsaem.2c03731.

[5] Y.-C. Hao, L.-W. Chen, J. Li, Y. Guo, X. Su, M. Shu, Q. Zhang, W.-Y. Gao, S. Li, Z.-L. Yu, L. Gu, X. Feng, A.-X. Yin, R. Si, Y.-W.
Zhang, B. Wang, C.-H. Yan, Metal-organic framework membranes with single-atomic centers for photocatalytic CO2 and O2 reduction, Nat. Commun. 12 (2021) 2682. https://doi.org/10.1038/s41467-021-22991-7.

[6] M.-M. Yang, J.-M. Cao, G.-D. Qi, X.-Y. Shen, G.-Y. Yan, Y. Wang, W.-W. Dong, J. Zhao, D.-S. Li, Q. Zhang, Construction of Low-Cost Z-Scheme Heterojunction Cu2O/PCN-250 Photocatalysts Simultaneously for the Enhanced Photoreduction of CO2 to Alcohols and Photooxidation of Water, Inorg. Chem. 62 (2023) 15963-15970. https://doi.org/10.1021/acs.inorgchem.3c02026.

[7] W.-W. Dong, J. Jia, Y. Wang, J.-R. An, O.-Y. Yang, X.-J. Gao, Y.-L. Liu, J. Zhao, D.-S. Li, Visible-light-driven solvent-free photocatalytic CO2 reduction to CO by Co-MOF/Cu2O heterojunction with superior selectivity, Chem. Eng. J. 438 (2022) 135622. https://doi.org/10.1016/j.cej.2022.135622.

[8] P. Mialane, C. Mellot-Draznieks, P. Gairola, M. Duguet, Y. Benseghir, O. Oms, A. Dolbecq, Heterogenisation of polyoxometalates and other metal-based complexes in metal-organic frameworks: from synthesis to characterisation and applications in catalysis, Chem. Soc. Rev. 50 (2021) 6152-6220. https://doi.org/10.1039/D0CS00323A.

[9] C.T. Buru, O.K. Farha, Strategies for Incorporating Catalytically Active Polyoxometalates in Metal-Organic Frameworks for Organic Transformations, ACS Appl. Mater. Interfaces 12 (2020) 5345-5360. https://doi.org/10.1021/acsami.9b19785.

[10] M. Samaniyan, M. Mirzaei, R. Khajavian, H. Eshtiagh-Hosseini, C. Streb, Heterogeneous Catalysis by Polyoxometalates in Metal-Organic Frameworks, ACS Catal. 9 (2019) 10174-10191. https://doi.org/10.1021/acscatal.9b03439.

[11] J. Gosch, E. Svensson Grape, C. Atzori, F. Steinke, K.A. Lomachenko, A.K. Inge, N. Stock, Solubility and Stability of

Hexanuclear Ce(IV)-O Clusters, Chem. Mater. 35 (2023) 5876-5885. https://doi.org/10.1021/acs.chemmater.3c00636.

[12] Y. Liu, Z. Liu, D. Huang, M. Cheng, G. Zeng, C. Lai, C. Zhang, C. Zhou, W. Wang, D. Jiang, H. Wang, B. Shao, Metal or metal-containing nanoparticle@MOF nanocomposites as a promising type of photocatalyst, Coord. Chem. Rev. 388 (2019) 63-78. https://doi.org/10.1016/j.ccr.2019.02.031.

[13] H.-Q. Yin, Z.-M. Zhang, T.-B. Lu, Ordered Integration and Heterogenization of Catalysts and Photosensitizers in Metal-Covalent-Organic Frameworks for Boosting CO2 Photoreduction, Acc. Chem. Res. 56 (2023) 2676-2687. https://doi.org/10.1021/acs.accounts.3c00380.

[14] S. Guo, C.-W. Pan, M. Hou, Y.-T. Hou, S. Yao, T.-B. Lu, Z.-M. Zhang, Dual Regulation of Sensitizers and Cluster Catalysts in Metal-Organic Frameworks to Boost H2 Evolution, Angew. Chem. Int. Ed. 64 (2025) e202420398. https://doi.org/10.1002/anie.202420398.

[15] M.B. Chambers, X. Wang, N. Elgrishi, C.H. Hendon, A. Walsh, J. Bonnefoy, J. Canivet, E.A. Quadrelli, D. Farrusseng, C. Mellot-Draznieks, M. Fontecave, Photocatalytic Carbon Dioxide Reduction with Rhodium-based Catalysts in Solution and Heterogenized within Metal-Organic Frameworks, ChemSusChem 8 (2015) 603-608. https://doi.org/10.1002/cssc.201403345.

[16] P.M. Stanley, J. Haimerl, C. Thomas, A. Urstoeger, M. Schuster, N.B. Shustova, A. Casini, B. Rieger, J. Warnan, R.A. Fischer, Host-Guest Interactions in a Metal-Organic Framework Isoreticular Series for Molecular Photocatalytic CO2 Reduction, Angew. Chem. Int. Ed. 60 (2021) 17854-17860. https://doi.org/10.1002/anie.202102729.

[17] X. Wang, F.M. Wisser, J. Canivet, M. Fontecave, C. Mellot-Draznieks, Immobilization of a Full Photosystem in the Large-Pore MIL-101 Metal-Organic Framework for CO₂ reduction,
ChemSusChem 11 (2018) 3315–3322.
https://doi.org/10.1002/cssc.201801066.

[18] Y. Fu, D. Sun, Y. Chen, R. Huang, Z. Ding, X. Fu, Z. Li, An Amine-Functionalized Titanium Metal–Organic Framework Photocatalyst with Visible-Light-Induced Activity for CO2 Reduction, Angew. Chem. Int. Ed. 51 (2012) 3364–3367.
https://doi.org/10.1002/anie.201108357.

[19] D. Sun, Y. Fu, W. Liu, L. Ye, D. Wang, L. Yang, X. Fu, Z. Li, Studies on Photocatalytic CO2 Reduction over NH2-Uio-66(Zr) and Its Derivatives: Towards a Better Understanding of Photocatalysis on Metal–Organic Frameworks, Chem. – Eur. J. 19 (2013) 14279–14285.
https://doi.org/10.1002/chem.201301728.

[20] X. Zhang, M.C. Wasson, M. Shayan, E.K. Berdichevsky, J. Ricardo-Noordberg, Z. Singh, E.K. Papazyan, A.J. Castro, P. Marino, Z. Ajoyan, Z. Chen, T. Islamoglu, A.J. Howarth, Y. Liu, M.B. Majewski, M.J. Katz, J.E. Mondloch, O.K. Farha, A historical perspective on porphyrin-based metal–organic frameworks and their applications, Coord. Chem. Rev. 429 (2021) 213615.
https://doi.org/10.1016/j.ccr.2020.213615.

[21] F. Liu, I. Rincón, H.G. Baldoví, A. Dhakshinamoorthy, P. Horcajada, S. Rojas, S. Navalón, A. Fateeva, Porphyrin-based MOFs for photocatalysis in water: advancements in solar fuels generation and pollutants degradation, Inorg. Chem. Front. 11 (2024) 2212–2245.
https://doi.org/10.1039/D3QI01996A.

[22] Z. Liang, H.-Y. Wang, H. Zheng, W. Zhang, R. Cao, Porphyrin-based frameworks for oxygen electrocatalysis and catalytic reduction of carbon dioxide, Chem. Soc. Rev. 50 (2021) 2540–2581.
https://doi.org/10.1039/D0CS01482F.

[23] P. Asselin, P.D. Harvey, Visible-Light-Driven Production of Solar Fuels Catalyzed by Nanosized Porphyrin-Based Metal–Organic Frameworks and Covalent–Organic Frameworks: A Review, ACS Appl. Nano Mater. 5 (2022) 6055–6082.
https://doi.org/10.1021/acsanm.2c00831.

[24] H.-Q. Xu, J. Hu, D. Wang, Z. Li, Q. Zhang, Y. Luo, S.-H. Yu, H.-L. Jiang, Visible-Light Photoreduction of CO2 in a Metal–Organic Framework: Boosting Electron–Hole Separation via Electron Trap States, J. Am. Chem. Soc. 137 (2015) 13440–13443.
https://doi.org/10.1021/jacs.5b08773.

[25] R. Hariri, S. Dehghanpour, Effective visible-light CO2 photoreduction over (metallo)porphyrin-based metal–organic frameworks to achieve useful hydrocarbons, Appl. Organomet. Chem. 35 (2021) e6422.
https://doi.org/10.1002/aoc.6422.

[26] L.-Y. Wu, Y.-F. Mu, X.-X. Guo, W. Zhang, Z.-M. Zhang, M. Zhang, T.-B. Lu, Encapsulating Perovskite Quantum Dots in Iron-Based Metal–Organic Frameworks (MOFs) for Efficient Photocatalytic CO2 Reduction, Angew. Chem. Int. Ed. 58 (2019) 9491–9495.
https://doi.org/10.1002/anie.201904537.

[27] P. Wang, X. Ba, X. Zhang, H. Gao, M. Han, Z. Zhao, X. Chen, L. Wang, X. Diao, G. Wang, Direct Z-scheme heterojunction of PCN-222/CsPbBr3 for boosting photocatalytic CO2 reduction to HCOOH, Chem. Eng. J. 457 (2023) 141248.
https://doi.org/10.1016/j.cej.2022.141248.

[28] S. Choi, W.-J. Jung, K. Park, S.-Y. Kim, J.-O. Baeg, C.H. Kim, H.-J. Son, C. Pac, S.O. Kang, Rapid Exciton Migration and Amplified Funneling Effects of Multi-Porphyrin Arrays in a Re(I)/Porphyrinic MOF Hybrid for Photocatalytic CO2 Reduction, Acs Appl. Mater. Interfaces 13 (2021) 2710–2722.
https://doi.org/10.1021/acsami.0c19856.

[29] P.M. Stanley, K. Hemmer, M. Hegelmann, A. Schulz, M. Park, M. Elsner, M. Cokoja, J. Warnan, Topology- and wavelength-governed CO2 reduction photocatalysis in molecular catalyst-metal–organic framework assemblies, Chem. Sci. 13 (2022) 12164–12174.
https://doi.org/10.1039/D2SC03097G.

[30] Y. Benseghir, A. Solé-Daura, D.R. Cairnie, A.L. Robinson, M. Duguet, P. Mialane, P. Gairola, M. Gomez-Mingot, M. Fontecave, D. Iovan, B. Bonnett, A.J. Morris, A. Dolbecq, C. Mellot-Draznieks, Unveiling the
mechanism of the photocatalytic reduction of CO2 to formate promoted by porphyrinic Zr-based metal-organic frameworks, J. Mater. Chem. A 10 (2022) 18103-18115.
https://doi.org/10.1039/D2TA04164B.

[31] M.T. Pope, A. Müller, Polyoxometalate Chemistry: An Old Field with New Dimensions in Several Disciplines, Angew. Chem. Int. Ed. Engl. 30 (1991) 34–48.
https://doi.org/10.1002/anie.199100341.

[32] N.I. Gumerova, A. Rompel, Synthesis, structures and applications of electron-rich polyoxometalates, Nat. Rev. Chem. 2 (2018) 0112. https://doi.org/10.1038/s41570-018-0112-1.

[33] M. Sadakane, E. Steckhan, Electrochemical Properties of Polyoxometalates as Electrocatalysts, Chem. Rev. 98 (1998) 219–238. https://doi.org/10.1021/cr960403a.

[34] Z. Lang, J. Miao, Y. Lan, J. Cheng, X. Xu, C. Cheng, Polyoxometalates as electron and proton reservoir assist electrochemical CO2 reduction, APL Mater. 8 (2020) 120702.
https://doi.org/10.1063/5.0031374.

[35] S.-M. Liu, Z. Zhang, X. Li, H. Jia, M. Ren, S. Liu, Ti-Substituted Keggin-Type Polyoxotungstate as Proton and Electron Reservoir Encaged into Metal-Organic Framework for Carbon Dioxide Photoreduction, Adv. Mater. Interfaces 5 (2018) 1801062.
https://doi.org/10.1002/admi.201801062.

[36] J. Du, Y.-Y. Ma, W.-J. Cui, S.-M. Zhang, Z.-G. Han, R.-H. Li, X.-Q. Han, W. Guan, Y.-H. Wang, Y.-Q. Li, Y. Liu, F.-Y. Yu, K.-Q. Wei, H.-Q. Tan, Z.-H. Kang, Y.-G. Li, Unraveling photocatalytic electron transfer mechanism in polyoxometalate-encapsulated metal-organic frameworks for high-efficient CO2 reduction reaction, Appl. Catal. B Environ. 318 (2022) 121812.
https://doi.org/10.1016/j.apcatb.2022.121812.

[37] Y. Benseghir, A. Lemarchand, M. Duguet, P. Mialane, M. Gomez-Mingot, C. Roch-Marchal, T. Pino, M.-H. Ha-Thi, M. Haouas, M. Fontecave, A. Dolbecq, C. Sassoye, C. Mellot-Draznieks, Co-immobilization of a Rh Catalyst and a Keggin Polyoxometalate in the UiO-67 Zr-Based Metal-Organic

Framework: In Depth Structural Characterization and Photocatalytic Properties for CO2 Reduction, J. Am. Chem. Soc. 142 (2020) 9428–9438.
https://doi.org/10.1021/jacs.0c02425.

[38] A. Solé-Daura, Y. Benseghir, M.-H. Ha-Thi, M. Fontecave, P. Mialane, A. Dolbecq, C. Mellot-Draznieks, Origin of the Boosting Effect of Polyoxometalates in Photocatalysis: The Case of CO2 Reduction by a Rh-Containing Metal–Organic Framework, ACS Catal. 12 (2022) 9244–9255.
https://doi.org/10.1021/acscatal.2c02088.

[39] A. Téazéa, G. Hervéa, R.G. Finke, D.K. Lyon, α-, β-, and γ-Dodecatungstosilicic Acids: Isomers and Related Lacunary Compounds, in: Inorg. Synth., John Wiley & Sons, Ltd, 1990: pp. 85–96.
https://doi.org/10.1002/9780470132586.ch16.

[40] D.C. Duncan, T.L. Netzel, C.L. Hill, Early-Time Dynamics and Reactivity of Polyoxometalate Excited States. Identification of a Short-Lived LMCT Excited State and a Reactive Long-Lived Charge-Transfer Intermediate following Picosecond Flash Excitation of [W10O32]4-in Acetonitrile, Inorg. Chem. 34 (1995) 4640–4646.
https://doi.org/10.1021/ic00122a021.

[41] J.B. Oh, K.L. Paik, J.-W. Ka, S.-G. Roh, M.-K. Nah, H.K. Kim, New synthetic methodology and luminescent properties of lanthanide-cored supramolecular complexes based on metalloporphyrins for optical amplification, Mater. Sci. Eng. C 24 (2004) 257–260.
https://doi.org/10.1016/j.msec.2003.09.054.

[42] C. Viravaux, O. Oms, A. Dolbecq, E. Nassar, L. Busson, C. Mellot-Draznieks, R. Dessapt, H. Serier-Brault, P. Mialane, Temperature sensors based on europium polyoxometalate and mesoporous terbium metal-organic framework, J. Mater. Chem. C 9 (2021) 8323–8328.
https://doi.org/10.1039/d1tc01532j.

[43] S. Parshamoni, C. Viravaux, M. Robert, C. Mellot-Draznieks, G. Chen, P. Mialane, A. Dolbecq, J. Bonin, Heterogenization of
molecular cobalt catalysts in robust metal-organic frameworks for efficient photocatalytic CO₂ reduction, Catal. Sci. Technol. 12 (2022) 5418–5424.
https://doi.org/10.1039/d2cy01147f.

[44] A.K. Rappe, C.J. Casewit, K.S. Colwell, W.A. Goddard, W.M. Skiff, UFF, a full periodic table force field for molecular mechanics and molecular dynamics simulations, J. Am. Chem. Soc. 114 (1992) 10024–10035.
https://doi.org/10.1021/ja00051a040.

[45] A.K. Rappe, K.S. Colwell, C.J. Casewit, Application of a universal force field to metal complexes, Inorg. Chem. 32 (1993) 3438–3450.
https://doi.org/10.1021/ic00068a012.

[46] A.K. Rappe, W.A. Goddard, Charge equilibration for molecular dynamics simulations, J. Phys. Chem. 95 (1991) 3358–3363.
https://doi.org/10.1021/j100161a070.

[47] J.-D. Chai, M. Head-Gordon, Long-range corrected hybrid density functionals with damped atom-atom dispersion corrections, Phys. Chem. Chem. Phys. 10 (2008) 6615–6620.
https://doi.org/10.1039/B810189B.

[48] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, G.A. Petersson, H. Nakatsuji, Gaussian 16, revision B. 01; Wallingford, CT, 2016, Google Sch. There No Corresp. Rec. This Ref. (n.d.).

[49] P.J. Hay, W.R. Wadt, Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg, J. Chem. Phys. 82 (1985) 270–283.
https://doi.org/10.1063/1.448799.

[50] A.W. Ehlers, M. Böhme, S. Dapprich, A. Gobbi, A. Höllwarth, V. Jonas, K.F. Köhler, R. Stegmann, A. Veldkamp, G. Frenking, A set of f-polarization functions for pseudopotential basis sets of the transition metals Sc, Cu, Ag and Au, Chem. Phys. Lett. 208 (1993) 111–114.
https://doi.org/10.1016/0009-2614(93)80086-5.

[51] W.J. Hehre, R. Ditchfield, J.A. Pople, Self–Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian–Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules, J. Chem. Phys. 56

(1972) 2257–2261.
https://doi.org/10.1063/1.1677527.

[52] M.M. Francl, W.J. Pietro, W.J. Hehre, J.S. Binkley, M.S. Gordon, D.J. DeFrees, J.A. Pople, Self-consistent molecular orbital methods. XXIII. A polarization-type basis set for second-row elements, J. Chem. Phys. 77 (1982) 3654–3665.
https://doi.org/10.1063/1.444267.

[53] E. Cancès, B. Mennucci, J. Tomasi, A new integral equation formalism for the polarizable continuum model: Theoretical background and applications to isotropic and anisotropic dielectrics, J. Chem. Phys. 107 (1997) 3032–3041.
https://doi.org/10.1063/1.474659.

[54] M. Álvarez-Moreno, C. de Graaf, N. López, F. Maseras, J.M. Poblet, C. Bo, Managing the Computational Chemistry Big Data Problem: The ioChem-BD Platform, J. Chem. Inf. Model. 55 (2015) 95–103.
https://doi.org/10.1021/ci500593j.

[55] Y. Dong, J. Zhang, Y. Yang, J. Wang, B. Hu, W. Wang, W. Cao, S. Gai, D. Xia, K. Lin, R. Fan, Multifunctional nanostructured host-guest POM@MOF with lead sequestration capability induced stable and efficient perovskite solar cells, Nano Energy 97 (2022) 107184.
https://doi.org/10.1016/j.nanoen.2022.107184.

[56] S. Karamzadeh, E. Sanchooli, A.R. Oveisi, S. Daliran, R. Luque, Visible-LED-light-driven photocatalytic synthesis of N-heterocycles mediated by a polyoxometalate-containing mesoporous zirconium metal-organic framework, Appl. Catal. B Environ. 303 (2022) 120815.
https://doi.org/10.1016/j.apcatb.2021.120815.

[57] Efficient Electron Transfer from Electron-Sponge Polyoxometalate to Single-Metal Site Metal–Organic Frameworks for Highly Selective Electroreduction of Carbon Dioxide – Sun – 2021 – Small – Wiley Online Library, (n.d.).
https://onlinelibrary.wiley.com/doi/10.1002/smll.202100762 (accessed May 3, 2024).

[58] G. Paille, M. Gomez-Mingot, C. Roch-Marchal, B. Lassalle-Kaiser, P. Mialane, M.
Fontecave, C. Mellot-Draznieks, A. Dolbecq, A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal–Organic Framework for Water Oxidation, J. Am. Chem. Soc. 140 (2018) 3613–3618.
https://doi.org/10.1021/jacs.7b11788.

[59] M.-T. Peng, C. Chen, Y. Zhang, J.-Y. Xu, Y.-L. Teng, B.-X. Dong, Exploring the role of sandwich-type polyoxometalates in {K10(PW9O34)2M4(H2O)2}@PCN-222 (M = Mn, Ni, Zn) for electroreduction of CO2 to CO, Dalton Trans. 52 (2023) 10737–10743.
https://doi.org/10.1039/D3DT01535A.

[60] H. Chen, J.-B. Brubach, N.-H. Tran, A.L. Robinson, F. Ben Romdhane, M. Fregnaux, F. Penas-Hidalgo, A. Sole-Daura, P. Mialane, M. Fontecave, A. Dolbecq, C. Mellot-Draznieks, Zr-Based MOF-545 Metal-Organic Framework Loaded with Highly Dispersed Small Size Ni Nanoparticles for CO2 Methanation, ACS Appl. Mater. INTERFACES 16 (2024) 12509–12520.
https://doi.org/10.1021/acsami.3c18154.

[61] J.M. Manoli, C. Potvin, M. Muhler, U. Wild, G. Resofszki, T. Buchholz, Z. Paal, Evolution of the catalytic activity in Pt sulfated zirconia catalysts: Structure, composition, and catalytic properties of the catalyst precursor and the calcined catalyst, J. Catal. 178 (1998) 338–351.
https://doi.org/10.1006/jcat.1998.2104.

[62] Q. Zhu, H. An, T.-Q. Xu, Y. Chen, Y. Wei, H. Sun, Polyoxometalates Embedded into Covalent Triazine Frameworks Regulating Charge Transfer for Visible-Light-Driven Synthesis of Functionalized Sulfoxides and Detoxification of Mustard Gas Simulants, ACS Sustain. Chem. Eng. 12 (2024) 1655–1665.
https://doi.org/10.1021/acssuschemeng.3c07186.

[63] P. Niu, J. Hao, Photocatalytic degradation of methyl orange by titanium dioxide-decatungstate nanocomposite films supported on glass slides, COLLOIDS Surf. -Physicochem. Eng. Asp. 431 (2013) 127–132.
https://doi.org/10.1016/j.colsurfa.2013.04.033.

[64] M. Bhattacharya, K.J. Chandler, J. Geary, C.T. Saouma, The role of leached Zr in the photocatalytic reduction of CO2 to formate by derivatives of UiO-66 metal organic frameworks, Dalton Trans. 49 (2020) 4751–4757.
https://doi.org/10.1039/D0DT00524J.

[65] L. Tan, Y. Li, Q. Lv, Y. Gan, Y. Fang, Y. Tang, L. Wu, Y. Fang, Development of soluble UiO-66 to improve photocatalytic CO2 reduction, Catal. Today 410 (2023) 282–288.
https://doi.org/10.1016/j.cattod.2022.05.001.

[66] W.M. Campbell, A.K. Burrell, D.L. Officer, K.W. Jolley, Porphyrins as light harvesters in the dye-sensitised TiO2 solar cell, Coord. Chem. Rev. 248 (2004) 1363–1379.
https://doi.org/10.1016/j.ccr.2004.01.007.

[67] D. Chen, D. Yang, J. Geng, J. Zhu, Z. Jiang, Improving visible-light photocatalytic activity of N-doped TiO2 nanoparticles via sensitization by Zn porphyrin, Appl. Surf. Sci. 255 (2008) 2879–2884.
https://doi.org/10.1016/j.apsusc.2008.08.032.

[68] K. Chaitanya, X.-H. Ju, B.M. Heron, Theoretical study on the light harvesting efficiency of zinc porphyrin sensitizers for DSSCs, RSC Adv. 4 (2014) 26621–26634.
https://doi.org/10.1039/C4RA02473G.