Achieving High CO ₂ Photoreduction Activity by Conductive Crosslinks of Metal–Organic Framework

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES30 Aug 2023Achieving High CO2 Photoreduction Activity by Conductive Crosslinks of Metal–Organic Framework Ning Li†, Gui-Qi Lai†, Lai-Hon Chung†, Fei Yu, Jun He and Ya-Qian Lan Ning Li† Department School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong 510006 School of Chemistry, South China Normal University, Guangzhou 510006 , Gui-Qi Lai† Department School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong 510006 , Lai-Hon Chung† Department School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong 510006 , Fei Yu School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023 , Jun He *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong 510006 and Ya-Qian Lan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry, South China Normal University, Guangzhou 510006 https://doi.org/10.31635/ccschem.023.202303055 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Fast photogenerated charge migration is crucial for the improvement of photocatalytic performance, but its deliberate modulation is difficult. This work presents two Zr-based metal–organic framework catalysts, GDUT-8 and GDUT-8-Ox, for photocatalytic CO2 reduction. Specifically, thiophene pendants in GDUT-8 were coupled covalently via Scholl reaction to give GDUT-8-Ox, a catalyst with at least two orders of magnitude (up to 6.1 × 10−3 S cm−1) enhanced conductivity and faster transport of photogenerated carriers during photocatalysis. Furthermore, from GDUT-8 to GDUT-8-Ox, stronger ligand-to-cluster charge transfer with pronounced light absorption extension was observed. As a result, GDUT-8-Ox exhibited the highest photocatalytic CO2-to-HCOO− conversion rate (1725 μmol g−1 h−1) to date, in the absence of photosensitizer, as well as turnover number, turnover frequency, and quantum efficiency much higher than GDUT-8. This work presents an unprecedented strategy to accelerate the photogenerated carrier transport of photocatalysts. Download figure Download PowerPoint Introduction Solar-driven photochemical reduction of CO2 to usable chemicals is recognized as one of the most promising technologies for sustainable carbon recycling.1–7 By the effort of past decades, it was found that parameters governing photocatalytic performance should be considered for design of photocatalysts to achieve highly selective and effective CO2 conversion.8–10 Specifically, competent photocatalysts should possess the following features: (1) effective active site (for adsorption and activation of CO2);11–15 (2) good light-harvesting capability (dictating solar energy utilization efficiency);16,17 (3) tunable functionalities [for modulation of band structure and highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy level distribution];18–21 and (4) prompt transfer of photogenerated charges (attaining high catalytic rate by suppression of electron–hole, e−–h+, pair recombination).9,15,22 Out of various materials, metal–organic frameworks (MOFs) arise as ideal candidates for photocatalysis owing to their unambiguous structure, tailorable functionalities, and so on, and have been proven to be outstanding in modulation of photocatalytic activity via rich structural regulation.23–27 Numerous studies on light-driven CO2 reduction reaction (CO2RR) by MOF catalysts reveal that the right combination of metal nodes and functionalized linkers leads to a framework with effective active sites and decent light-harvesting capability.28,29 Upon a broad spectrum of irradiation, the MOF catalysts exhibit higher selectivity of CO2RR to specific products (e.g., CO and HCOOH). However, generally inherent poor electrical conductivity in MOF catalysts always results in slow transfer of photogenerated charge carriers and hence limited quantum efficiency as well as low CO2RR rate.30,31 Unfortunately, there is still a lack of solutions to tackle this issue. Of note, oxidative coupling of thiophene and its derivatives to polythiophenes represents one of the most commonly used strategies to generate materials with high electrical conductivity.32–34 Thus, constructing polythiophene within a framework may be a viable strategy to enhance the inherent electrical conductivity of MOF catalysts and hence the transport of photogenerated charge carriers for boosting CO2 photoreduction. As a proof-of-concept study, we designed and constructed a Zr-based MOF catalyst, GDUT-8, assembled from [Zr6O8] nodes and 2,2′,5,5′-tetrafluoro-3,3′,6,6′-tetrakis(2-thiophenethio)-4,4′-biphenyl dicarboxylic acid (H2BPD-4F4TS) linkers, for light-driven CO2 reduction. GDUT-8 shows high structural stability, and the large ionic radius and high coordination number of the Zr4+ center in the Zr-O cluster can create ample space and a catalytic site for the adsorption and subsequent photoactivation of CO2 molecules. Notably, adjacent thiophene pendants in GDUT-8 can be bonded through Scholl reaction to give a crystalline framework bearing coupled thiophene linkers, GDUT-8-Ox. Importantly, GDUT-8-Ox exhibits electrical conductivity of 6.1 × 10−3 S cm−1, better than GDUT-8 (2.3 × 10−5 S cm−1) by at least two orders of magnitude. Furthermore, GDUT-8-Ox showed more pronounced ligand-to-cluster charge transfer (LCCT) with extension of absorption profile from ending at 400 nm (UV region) to ending at 800 nm (covering the whole visible spectral region) when compared with GDUT-8. Based on the above improvement by thiophene coupling, GDUT-8-Ox achieved highly selective CO2-to-HCOO− conversion (i.e., selectivity to HCOO− of 100%) together with improved catalytic activity compared with primitive GDUT-8 as reflected by higher conversion rate of HCOOH (from 553 to 1078 μmol g−1 h−1), turnover numbers (TONs, from 21.88 to 50.93), turnover frequencies (TOFs, from 10.94 to 25.47 h−1), and quantum efficiency (from 0.009% to 0.018%) under visible-light irradiation. Theoretical calculation reveals that the kinetic barrier for the rate-determining *COOH intermediate formation observed in GDUT-8-Ox is lower than that of GDUT-8 and, therefore, contributes to faster catalysis along with prompt HCOOH production. The better photocatalytic performance by GDUT-8-Ox probably originates from the elevated mobility of photogenerated charge carriers. More importantly, this work represents the first example of boosting photocatalytic CO2RR by improving the transport rate of photogenerated charge carriers and suppressing electron–hole (e−–h+) pair recombination of MOFs via electrical conductivity enhancement. Experimental Methods Materials All starting materials, reagents, and solvents used in experiments were commercially available, high-grade-purity materials and used without further purification. 2-Thiophene thiol and nitromethane were purchased from Aladdin (Shanghai, China). ZrCl4 was purchased from Acros (Guangzhou, China). Trifluoroacetic acid, I2, K2CO3, NaOH, ethanol (EtOH), and N,N-dimethylformamide (DMF) were purchased from Admas (Guangzhou, China). Toluene and acetone were purchased from Guangzhou Chemical reagents (Guangzhou, China). Synthesis method of GDUT-8 H2BPD-4F4TS (540 mg, 0.71 mmol), ZrCl4 (400 mg, 1.72 mmol), trifluoroacetic acid (245.6 mg, 2.15 mmol), and dry DMF (4 mL) were added to a glass vial (10 mL) with an aluminum cap. After sonication for 5 min to form a light-yellow solution, the reaction was carried out at 120 °C for 4 days. After cooling to room temperature, the solution was washed with DMF and acetone and filtered to obtain colorless polyhedral crystals. Elemental analysis found [C, 36.85; H, 1.86; S, 23.05; N, 0.82], and a fitting formula was determined to be Zr6O4(OH)4(C30H12F4O4S8)2.9(HCOO)6.2(H2O)8(DMF)2 (MW 3230). Synthesis method of GDUT-8-Ox GDUT-8 crystals (10 mg), I2 (31 mg, 0.12 mmol), and n-hexane (5 mL) were added to a 50 mL Schlenk tube. After sonication and dispersion, the reaction solution was heated through an oil bath (90 °C) for 12 h. After the reaction, the crystals were filtered and washed several times with hexane and acetone until the solution became colorless. The oxidized crystals were then further subjected to acetone Soxhlet extraction for 1 day and dried to give brown crystals of the as-made GDUT-8-Ox. Photocatalytic CO2 reduction test Photocatalytic reduction of CO2 was performed in a 50 mL Schlenk tube with as-prepared crystal. Photocatalyst (5 mg) was added into the mixed solution that contained H2O (28 mL) and triethanolamine (TEOA, 2 mL) as an electron donor. After degassing for 30 min with high-purity CO2 to remove dissolved O2, the reaction was performed under the irradiation of a 300 W Xe lamp with UV and IR-cut to keep the wavelengths in the range of 420 to 800 nm. The reaction temperature was controlled at 303 K by using cooling water circulation. The photocatalytic gas-phase products (CO, CH4, H2, and O2) were analyzed by gas chromatography (GC, Agilent, Guangzhou, China) equipped with a flame ionization detector (FID) with methanizer and a thermal conductivity detector. Using a syringe, 500 μL of gas-product was extracted from the reactor and injected into the gas chromatograph system with an FID, using nitrogen as the carrier gas and reference gas. The liquid products (HCOO¯) in the liquid phase of the reactor were measured using ion chromatography (IC). The flow velocity of the IC mobile phase was 1 mL/min, and the sampling volume of the automatic injection syringe was 25 μL (the injection needle was washed three times with deionized water between different standards, and the needle was rinsed in water before it withdrew samples). The solution obtained after photocatalysis was filtered with a filter head (0.22 μm), and a syringe was used to obtain a clear solution of the filtrate; 1 mL of the filtrate was collected and diluted 100 times with a 100 mL volumetric flask. By comparing the peak area of the liquid-phase product with the standard curve, the volume of the liquid-phase product HCOO¯ was calculated. Electrochemistry measurements All electrochemical measurements [photocurrent and electrochemical impedance spectroscopy (EIS)] were carried out with a CHI 660E electrochemical workstation via a conventional three-electrode system in a 0.5 M Na2SO4 aqueous solution. The working electrode was indium tin oxide (ITO) glass plates coated with a catalyst slurry, the counter electrode was a platinum foil, and the reference electrode was a saturated Ag/AgCl electrode. EIS measurements were recorded over a frequency range of 100 kHz-0.1 Hz, and 0.5 M Na2SO4 aqueous solution was used as the supporting electrolyte. For preparation of the working electrode, 2 mg photocatalysts were dispersed in a mixed solution of 990 μL EtOH and 10 μL Nafion D-520 dispersion solutions to generate a homogeneous slurry. Subsequently, 100 μL of slurry was transferred and coated on ITO glass plates (1 cm × 1 cm) then dried at room temperature. The Ag/AgCl electrode was employed as the reference electrode, and platinum plate was used as the counter electrode. Results and Discussion Structures and characterizations of GDUT-8 and GDUT-8-Ox Solvothermal reaction between ZrCl4 and H2BPD-4F4TS ( Supporting Information Figures S1–S7) in a sealed glass bottle at 120 °C for 72 h gave GDUT-8 as colorless octahedral crystals ( Supporting Information Figure S8). The single crystal obtained has a UiO-67 type network structure ( Supporting Information Figures S9–S12 and Tables S1–S3). With optimized self-assembly conditions, GDUT-8 crystals of good quality were isolated and taken for single-crystal X-ray diffraction (SCXRD) data collection. SCXRD analysis shows that GDUT-8 adopts the cubic space group P m 3 ¯ m (No. 221), consisting of a face-centered-cubic array of [Zr6O4(OH)4] clusters connected to carboxylate units of the BPD-4F4TS2 − linker to yield the fcu topology of the prototype UiO-67 framework.35 It is noteworthy that the thiophene groups on the linkers within the Zr6 cluster-based octahedral cage in GDUT-8 were difficult to couple due to the long distance and inappropriate spatial arrangement. In contrast, the proximity between thiophene groups on adjacent linkers from two neighboring octahedral cages highlights the possibility of oxidative thiophene coupling to give conjugated crosslinks (Figure 1a–f). The free solvent was excluded, and the porosity of GDUT-8 was obtained by PLATON calculations to be about 48.4%. Upon oxidation treatment, colorless GDUT-8 darkened (Scheme 1 and Supporting Information Figure S8), and the resultant black solids were denoted as GDUT-8-Ox. As expected, GDUT-8-Ox crystallizes in the same cubic space group, P m 3 ¯ m , and keeps the pristine GDUT-8 framework but with oxidative coupling between thiophene moieties on adjacent linkers from two neighboring octahedral cages. Figure 1 | Construction of (c) GDUT-8 from (a) [Zr6O4(OH)4] clusters linked by (b) organic linker H2BPD-4F4TS. (d) Structure view of GDUT-8-Ox converted from GDUT-8. (e, f) Enlarged view of the organic linkers in GDUT-8 and GDUT-8-Ox. Download figure Download PowerPoint Crystal morphology of these two MOFs can be observed from scanning electron microscopy characterization ( Supporting Information Figures S13–S15). Consistent powder X-ray diffraction (PXRD) patterns of GDUT-8-Ox and GDUT-8 also suggest that the parent MOF remains intact throughout oxidation ( Supporting Information Figure S16). The N2 sorption isotherm of GDUT-8 at 77 K features a type-II isotherm accompanied by a hysteresis loop ( Supporting Information Figures S17–S20). The Brunauer–Emmett–Teller surface area of GDUT-8 calculated from the sorption isotherms is 531.60 m² g−1. In contrast, the N2 sorption isotherm of GDUT-8-Ox at 77 K is characteristic of a type-III isotherm and shows a hysteresis loop ( Supporting Information Figures S21–S24). The hysteresis loops may stem from the blockage of N2 by the thiophene ring in the pore channel during N2 desorption. Enhanced thermal stability of GDUT-8-Ox over GDUT-8 as reflected by their thermogravimetric analysis profiles (i.e., steep weight loss in GDUT-8 at ∼300 °C versus weight loss in GDUT-8-Ox starting at >350 °C, Supporting Information Figures S25 and S26). We have also further verified the solubility decrease of linker from GDUT-8 to GDUT-8-Ox. The activated crystals of GDUT-8 digested in DCl/NaF/DMSO-d6 for 1H and 19F nuclear magnetic resonance (NMR) spectroscopy and indicated the presence of H2BPD-4F4TS, ( Supporting Information Figures S27 and S28), whereas GDUT-8-Ox failed to digest at the same concentration in DCl/NaF/DMSO-d6 unless the concentration of HF was higher and the mixture was stirred at 85 °C for 4 h. Comparison between the 1H NMR spectra of digested GDUT-8 and GDUT-8-Ox ( Supporting Information Figures S29–S31) shows that the characteristic peaks of GDUT-8-Ox are broader, and the H-atom ratio is not consistent with that of the ligand compared to the almost indistinguishable sharp peaks of GDUT-8, indicating reduced solubility of the linkers in GDUT-8-Ox. Also, the broadening of the shoulder peak of the C atom from the coupling site in the solid 13C NMR spectrum of the oxidized GDUT-8-Ox reveals that some C–H bonds joined to give C–C bonds through oxidative coupling ( Supporting Information Figure S32). All the above results suggest that the structural robustness of linkers in GDUT-8 improves after oxidation. More importantly, the enhancement of electrical conductivity by two orders of magnitude from GDUT-8 (2.3 × 10−5 S cm−1) to GDUT-8-Ox (6.1 × 10−3 S cm−1) also supports that coupling of adjacent thiophenes gives electronically more conjugated moieties ( Supporting Information Figure S33 and Table S4). Scheme 1 | Preparation of GDUT-8 and its conversion into GDUT-8-Ox through the couplings among near-neighboring thiophenes. Download figure Download PowerPoint GDUT-8-Ox with darkened appearance ( Supporting Information Figure S8) shows a redshift in absorption. As shown in Figure 2a, GDUT-8 absorbs strongly in the UV region, 200–400 nm, with negligible absorption in the visible spectral region, whereas GDUT-8-Ox exhibits absorption almost covering the whole UV–vis spectral region (tailing until 800 nm). The optical absorption of GDUT-8 and GDUT-8-Ox is mainly from the electron transition, from the thiophene chromophore to the Zr-O clusters, that is LCCT.36–38 Sensibly, the enhanced electronic conjugation after thiophene coupling narrows the π → π* gap and accelerates the rate of electron transfer during LCCT, which results in the redshift of the GDUT-8-Ox absorption spectrum after thiophene coupling and improves its light absorption in the visible region.13 The optical bandgaps of GDUT-8 and GDUT-8-Ox were calculated to be 2.98 and 1.93 eV, respectively, from the Tauc plot using the Kubelka–Munk function (Figure 2b). We performed UV photoelectron spectroscopy (UPS) to determine the HOMO energy levels of the MOFs (Figure 2c,d). The HOMO energy levels of GDUT-8 and GDUT-8-Ox are 6.24 [1.39 V versus normal hydrogen electrode (NHE)] and 5.74 eV (0.89 V versus NHE), respectively. Thus, the corresponding LUMO energy levels of GDUT-8 and GDUT-8-Ox can be obtained by Ev−Eg calculation as 3.26 (−1.59 V versus NHE) and 3.81 eV (−1.04 V versus NHE), respectively. LUMOs lying higher than the reduction potential required for reduction of CO2 to various products (HCOO−, -0.61 V; CO, -0.51 V; CH4, -0.24 V versus NHE at pH 7,39 Figure 2e) allows both GDUT-8 and GDUT-8-Ox to be promising candidates in CO2 reduction. As shown in the Nyquist plots of EIS, the smaller arc radius of GDUT-8-Ox than GDUT-8 (Figure 2f) indicates lower charge transfer resistance in GDUT-8-Ox than GDUT-8, that is, more efficient charge flow. The smaller charge transfer resistance of GDUT-8-Ox comes from the stronger electronic conjugation given by coupled thiophene moieties. This is one of the reasons why GDUT-8-Ox has a less negative LUMO energy level. Figure 2 | (a) Solid-state UV−vis absorption spectra of GDUT-8 and GDUT-8-Ox. (b) Tauc plots of GDUT-8 and GDUT-8-Ox. UPS spectra of GDUT-8 (c) and GDUT-8-Ox (d). (e) Energy band structure diagram for GDUT-8 and GDUT-8-Ox. (f) Electrochemical impedance spectra of GDUT-8 and GDUT-8-Ox. Download figure Download PowerPoint Photochemical CO2 reduction performance After oxidative coupling of thiophene, significant absorption redshift and electrical conductivity improvement make GDUT-8-Ox an ideal candidate for verifying how elevated mobility of photogenerated charge carriers promotes photocatalytic CO2RR. Therefore, GDUT-8 and GDUT-8-Ox were used as catalysts for photocatalytic CO2RR using H2O as solvent, triethanolamine (TEOA) as sacrificial agent, and a Xenon lamp as irradiation source without addition of external photosensitizers ( Supporting Information Figure S34). As shown in Figure 3a, the HCOO− yields of GDUT-8 and GDUT-8-Ox reach 858 and 1725 μmol g−1 h−1, highlighting GDUT-8-Ox as superior to GDUT-8 in terms of photocatalytic power. photocatalytic activity of GDUT-8-Ox most results from stronger in the visible spectral further that the enhanced photocatalytic performance is to mobility of photogenerated charge the photocatalytic performance of GDUT-8 and GDUT-8-Ox was by the same that UV light was by visible light the of stronger of visible light on the Figure shows the HCOO− yields of GDUT-8 and GDUT-8-Ox reach and μmol g−1 h−1, respectively, after 2 h of reaction under visible light irradiation. We that the improved photocatalytic performance of the MOF catalyst after oxidative coupling comes from mobility of photogenerated charge carriers and absorption covering the whole visible spectral Figure 3 | of HCOO− by GDUT-8 and GDUT-8-Ox under (a) UV light and (b) visible light over (c) of the liquid reaction products in photocatalytic system by ion (d) The of GDUT-8 and GDUT-8-Ox for CO2 photoreduction. (e) of GDUT-8 and GDUT-8-Ox under visible light irradiation. (f) spectra of GDUT-8-Ox. Download figure Download PowerPoint In this study, the yield of HCOO− was by IC (Figure and the gas phase products were by gas products from photocatalytic CO2RR by GDUT-8 and GDUT-8-Ox ( Supporting Information Figures and that the CO2RR through CO2-to-HCOO− conversion with In to the catalytic performance with to active in GDUT-8 and GDUT-8-Ox, the corresponding and were calculated ( Supporting Information Table The catalytic site is 21.88 for GDUT-8 and for GDUT-8-Ox GDUT-8-Ox as a better than GDUT-8. the photocatalytic reaction, the photogenerated flow to the Zr-O cluster via and the and results further that GDUT-8-Ox has more efficient transport of photogenerated charge which is as activity of the catalytic HCOO− was when the photocatalytic CO2RR was carried out under one of the following ( Supporting Information Table (1) (2) (3) linkers MOFs as and (4) in of results that these are for CO2RR to Also, experiments using as gas were to the source of carbon in 13C NMR spectra of the filtrate collected from CO2RR by both GDUT-8 and GDUT-8-Ox peaks of HCOO− at in ( Supporting Information Figures and which are negligible in the results using as reaction ( Supporting Information Figures and results that HCOO− comes from Furthermore, the UV–vis absorption profiles of the filtrate after photocatalytic CO2RR by GDUT-8 and GDUT-8-Ox not corresponding to linkers ( Supporting Information Figures and that both GDUT-8 and GDUT-8-Ox intact throughout the catalytic reaction their patterns collected before and after catalysis ( Supporting Information Figure The results showed that the spectra of GDUT-8 and GDUT-8-Ox are to before the reaction, which the possibility of of the catalyst during the reaction ( Supporting Information Figures and Of note, significant in HCOO− yield of CO2RR for three using both GDUT-8 and GDUT-8-Ox highlights the catalytic activity and of these MOFs (Figure further and the in photocatalytic activity between GDUT-8 and GDUT-8, various characterization were The results of the (Figure that in GDUT-8-Ox than GDUT-8 of charge carrier and more efficient of photogenerated pair in GDUT-8-Ox than GDUT-8. EIS characterization the results of and enhanced charge flow efficiency as the for higher photocatalytic In was also used to the of the photogenerated nm GDUT-8 at nm, GDUT-8-Ox shows a much peak at nm ( Supporting Information Figure more of photogenerated in GDUT-8-Ox than GDUT-8-Ox. The of GDUT-8-Ox than GDUT-8 ( Supporting Information Figure more in GDUT-8-Ox than GDUT-8. The of photogenerated is more for GDUT-8-Ox, in with the results of the is that the quantum efficiency of GDUT-8-Ox of the irradiation wavelengths ( Supporting Information Tables and S8) are higher than the of GDUT-8 The of GDUT-8-Ox is about times higher than that of GDUT-8 when at 420 nm. light utilization the photogenerated charge flow rate of GDUT-8-Ox after oxidation. irradiation of nm, the of GDUT-8-Ox is as high as that of GDUT-8 an enhancement of light utilization in GDUT-8-Ox is to its stronger in the visible spectral and in characterization resonance spectroscopy was employed to the of with GDUT-8-Ox as light irradiation under N2 the not (Figure Upon light irradiation under N2 sharp corresponding to in GDUT-8-Ox After in the Zr-O cluster from the linkers to form the This the flow of from the linkers to the Zr-O cluster through light In contrast, the of under light irradiation (1 in CO2 the of in the CO2RR. The can be as light irradiation, from the linker to the Zr-O cluster and the to the the as a After the of are transferred from the to CO2 in with the conversion of the to the as a of the Thus, a photocatalytic of CO2-to-HCOO− conversion is more the electron–hole energy in the first was calculated to the charge efficiency of GDUT-8 and GDUT-8-Ox ( Supporting Information Tables and The from the to the were calculated by calculations and and the results that the of GDUT-8-Ox are smaller than of GDUT-8 highlighting charge efficiency of GDUT-8-Ox than GDUT-8, which is consistent with the results of the of The in charge efficiency between GDUT-8 and GDUT-8-Ox mainly originates from higher electrical conductivity faster electron transfer of GDUT-8-Ox than GDUT-8. This as a in photocatalytic the of the photocatalytic reaction of further calculations were to determine the reaction of the CO2 reduction to HCOOH (Figure GDUT-8 and GDUT-8-Ox are mainly used for the reduction of CO2 to HCOOH via the CO2 is and on the catalyst and the free energy of GDUT-8 and GDUT-8-Ox to eV, indicating that CO2 adsorption is In the CO2 CO2 from the cluster to form an intermediate and this is the rate for the reduction of CO2 to HCOOH by GDUT-8 and GDUT-8-Ox. The results showed that the energy stability of the intermediate by GDUT-8-Ox is eV higher than that of GDUT-8, indicating that the acid is more CO2 then with and to further form which is from the catalyst Figure 4 | (a)

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