Uncovering the synergistic photocatalytic behavior of bimetallic molecular catalysts

Lin Yun,Lei Zhng,Xio-Xin Li,Jing Liu,Jing-Jing Liu,Long-Zhng Dong,Dong-Sheng Li,Shun-Li Li,*,Y-Qin Ln

a Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials,Jiangsu Key Laboratory of New Power Batteries,College of Chemistry and Materials Science,Nanjing Normal University,Nanjing 210023,China

b School of Chemistry,South China Normal University,Guangzhou 510006,China

c School of Chemistry and Chemical Engineering,Southeast University,Nanjing 211189,China

d Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials,China Three Gorges University,Yichang 443002,China

Keywords:Dinuclear metal compounds Crystalline photocatalyst Synergistic effect Photocatalytic CO2RR Artificial photosynthesis

ABSTRACT Bimetallic catalysts usually exhibit better performance than monometallic catalysts due to synergistic effect.However,there is a lack of exploring the synergistic effect on catalytic performance caused by the introduction of inactive metal ion.In this work,we design a molecular model system that can precisely regulate the metal site number and catalytic property.When these molecular metal compounds are used as homogeneous catalysts for photocatalytic CO2 reduction,the dinuclear heterometallic CuNi-L2 shows the highest CO2-to-CO conversion,which is 2.1 and 3.0 times higher than that of dinuclear homometallic Ni2-L2 and mononuclear Ni-L1.Density functional theory calculations demonstrate that,in CuNi-L2,the introduction of inactive CuII is easier to promote the photo-generated electrons transferring to the coupled active NiII site to achieve the highest activity.In addition,this work also provides insights to design and construct more efficient bimetallic catalysts in future.

Photocatalytic CO2reduction reaction (CO2RR) is one of the most promising and green methods of CO2conversion and utilization [1–4] for the reason that it can exploit solar energy as a driving force to convert low-value CO2into high-value carbonbased chemicals like CH4,CO,CH3OH,etc.[5–9].At present,molecular metal compounds usually exhibit excellent photocatalytic performance such as high turnover number (TON) and turnover frequency (TOF) among the reported CO2-photoreduction catalysts[10–12] due to the high active site utilization rate and the essence of homogeneous catalysis [13–16].Particularly,the vast majority of molecular metal compounds can provide a clear single-crystal structure [17–19],allowing the catalytic active sites to be identified accurately and studying the reaction mechanism [20–22].Based on these advantages [23],molecular metal compounds have the great potential to be used as model catalyst systems to understand the corresponding structure-property relationship [24–27],and thus guide for designing and synthesizing high-efficiency CO2RR catalysts [28,29].

Generally,compared to monometallic catalysts,bimetallic catalysts always tend to show better performance in many catalytic reactions because of the synergistic effect of two interacted metal sites [30–34].Unfortunately,only few suitable model systems were reported to specifically investigate the synergistic effect of two adjacent metal sites by means of second metal (activity or inactive) introduction on photocatalytic CO2RR performance [35,36].In many cases,metal ion-doping is the most common strategy used for surveying the influence of synergistic effect of two metal sites on the catalytic performance [37–39].Although the ratio of two metal ions can be effectively adjusted by this method [40,41],it is hard to accurately control the position and coordination environment of the two specific metal ions [42,43],which affects the precise evaluation for the structure-property relationship [37].In order to overcome this issue,the best way is to design and synthesize a well-defined model catalyst system that can accurately identify each catalytic site [44].In the previous reports,only one molecular metal compound catalyst investigated the effect of the introduction of an inactive metal ion nearby active metal site on photocatalytic CO2RR performance,but there are no direct interactions between the two involved metal sites [35].If there are direct bonding interactions existed between two metal sites,the introduction of the secondary inactive metal site may significantly change the molecular orbitals and charge transfer characteristics of the active metal site,and thus affecting the final catalytic performance [45].

In light of these thoughts,a molecular metal compound model catalyst system including mononuclear [NiL1(CH3OH)] (labeled as Ni-L1),[CuL1(H2O)] (labeled as Cu-L1),dinuclear homometallic[Ni2L2(CH3OH)2] and [Cu2L2(CH3OH)] (labeled as Ni2-L2and Cu2-L2),and dinuclear heterometallic [CuNiL2] (labeled as CuNi-L2) was designed and synthesized.Single crystal X-ray diffraction result shows that the Cu-L1and Ni-L1are isostructural,Ni2-L2,Cu2-L2and CuNi-L2are isostructural.Importantly,these five molecular metal compounds can be regarded as a model catalyst system to systematically explore the influences of active monometallic site,bimetallic sites and heterometallic sites on the photocatalytic performance of CO2RR in homogeneous conditions.The photocatalytic CO2RR measurement results showed that both Ni-L1and Ni2-L2exhibited high photocatalytic CO2-to-CO activity,but neither Cu-L1nor Cu2-L2showed catalytic activity,indicating that NiIIis the photocatalytic active center while CuIIis the inactive site.Moreover,under 8 h of visible light irradiation and the same concentration of NiII(0.8 μmol/L),the photocatalytic performance of Ni2-L2(2.18 μmol) is better than that of Ni-L1(1.52 μmol),manifesting the dual active NiIIsites show more advantageous catalytic activity owing to the synergistic effect.It is worth noting that for dinuclear heterometallic CuNi-L2,the single active NiIIsite is coupled with inactive CuIIion,under the same photocatalytic conditions as Ni-L1and Ni2-L2,the CO yield of the photoreduced product is calculated to be 4.60 μmol (CO selectivity 93.5%),which is 2.1 and 3.0 times higher than that of Ni2-L2and Ni-L1,respectively.It reveals that the introduction of the secondary inactive CuIIsite can further promote the photocatalytic performance of the adjacent interacted active NiIIion.The density functional theory (DFT) calculation results declared that the electron cloud distribution around catalytic active site NiIIion is effectively enhanced owing to the introduction of relatively catalytic inactive CuIIion,which increases the electron-acquiring ability of active NiIIsite and significantly accelerates the photocatalytic CO2RR.More importantly,this work represents a well-defined model catalyst system to effectively reveal the important influence of the bimetallic synergy between active and inactive metal ions on the photocatalytic CO2RR performance by adjusting the species,number and location of metal active centers in catalysts.

The CuNi-L2was synthesized through the precursor CuNa2-L2.A 50 mL EtOH solution of copper acetate (2.00 g,0.01 mol) was slowly added to a 50 mL H2O solution of H4L2(3.84 g,0.01 mol)and NaOH (0.8 g,0.02 mol) [46,47].The mixture was heated under reflux under N2atmosphere for 30 min followed by concentrating the resulting mixture to give a brown product CuNa2-L2with quantitative yield.After acidification and recrystallization,crystal CuH2-L2was obtained which further proved CuNa2-L2synthesized successfully.Ni(ClO4)2·6H2O (50 mg) and CuNa2-L2(10 mg)was dissolved in 8 mLN,N-dimethylformamide (DMF) ultrasonically,then 2 mL EtOH was added into the solution.This solution was transferred into a 10 mL glass vial for 72 h at 353 K under autogenous pressure.After cooling down to room temperature,brown crystals were collected by filtration and fully washed with MeCN.The crystals were soaked in 20 mL of CH3CN for 2 days replacing the solvent every 12 h.Finally,filtered and dried under vacuum.See supporting information for the specific synthesis methods of the other compounds.

Photocatalytic reduction of CO2was performed in a 50 mL quartz reactor with as-prepared crystals.Photocatalyst (0.02 μmol,Ni-L1,Ni2-L2and CuNi-L2based on the NiIIsite,Cu-L1and Cu2-L2based on the CuIIsite),[Ru(bpy)3]Cl2·6H2O (bpy=2,2′-bipyridine,0.02 mmol) were added into the mixed solution which contained CH3CN (20 mL),H2O (5 mL) and triisopropanolamine (TIPA,5 mL)as an electron donor.After degassing with high-purity CO2to remove the dissolved O2for 30 min,the reaction was performed under the irradiation of a 300 W Xe lamp with UV-cut to keep the wavelengths in the range of exceeding 420 nm.The reaction temperature was kept at 298 K by using the cooling water circulation.In order to detect the content of reduction product produced by the reaction mixture,500 μL of gas-product was extracted from the reactor with a syringe and injected into the GC equipped a flame ionization detector (FID) with a methanizer and a thermal conductivity detector (TCD),using nitrogen as the carrier gas.By comparing the integrated area of the gas-phase product with the calibration curve,the volume of carbon monoxide and hydrogen can be calculated.Each photocatalytic reaction was repeated at least three times to confirm the reliability of the data.

Single crystal X-ray diffraction result shows that the mononuclear Ni-L1crystallizes in triclinic system with theP-1 space group(Table S1 in Supporting information),and the asymmetric unit contains a transition metal ion NiII,a deprotonated Schiff base ligand (L1)2-and a free methanol molecule.The only crystallographic NiIIcenter is captured by the N2O2chelating pocket (two N atoms from two C=N bonds and two O atoms from two phenolic hydroxyl groups) to form a planar quadrilateral coordination configuration.It is noteworthy that the NiIIion and its coordination atoms in Ni-L1are totally in the same plane.This means that there is sufficient coordination space in axial position of the metal ion to allow small molecules to attack.Moreover,the NiIIion in Ni-L1can be replaced by CuIIion,namely the isostructural Cu-L1,which crystallizes in the orthorhombic system with aPnmaspace group (Table S1).In addition,we further introduced carboxyl groups in theorthopositions of the two phenolic hydroxyl groups of H2L1to synthesize the H4L2ligand containing two metal ion chelating pockets (N2O2and O4coordination modes).The ligand can catch hold of two identical or different metal ions effectively.Furthermore,just like the original N2O2cavity,all coordination atoms of the constructed parallel secondary O4coordination mode are also composed of a planar quadrilateral geometry.When the H4L2ligand captures two NiIIions,a dinuclear homometallic compound Ni2-L2is synthesized and it crystallizes in triclinic system with the space groupP-1(Table S2 in Supporting information).Ni1 atom is located in N2O2chelating pocket,while Ni2 atom is captured by another O4chelating pocket.Ni1 and Ni2 atoms are connected and interacted with each other by sharing two phenolic hydroxyl O atoms.Similarly,the H4L2ligand can also clamp two CuIIions to form an isostructural homometallic Cu2-L2,but which crystallizes in monoclinic system with theP21/cspace group (Table S2).More importantly,we can fix CuIIand NiIIions simultaneously by these two kinds of different chelating pockets of H4L2ligand though stepwise synthesis method to construct isostructural dinuclear heterometallic compound CuNi-L2,which crystallizes in monoclinic system withC2/cspace group (Table S2).For CuNi-L2,Cu1 atom is trapped into the N2O2chelating pocket,and Ni1 atom is fastened by the O4coordination mode.It is worth noting that due to the similar configuration of these compounds,this series of compounds Ni-L1,Cu-L1,Ni2-L2,Cu2-L2and CuNi-L2(Fig.1) can serve as model systems to study the effects of monometallic,bimetallic (homometallic and heterometallic) compounds on the catalytic performance for specific small molecule activity.More importantly,the performance impact produced by the synergistic effect between dinuclear metal ions can be explored systematically.

Fig.1.Structures of the molecular complexes.(a) mononuclear Ni-L1; (b) mononuclear Cu-L1; (c) dinuclear Ni2-L2; (d) dinuclear Cu2-L2; (e) dinuclear CuNi-L2.All hydrogen atoms are omitted for clarity.Cu,yellow; Ni,green; O,red; N,blue; C,gray.

Fig.2.(a) High resolution Ni 2p XPS spectra of Ni-L1,Ni2-L2 and CuNi-L2 in survey scan.(b) Cyclic voltammogram of CuNi-L2 in acetonitrile containing 0.1 mol/L Bu4NPF6 at a scan rate of 50 mV/s.(c) UV–vis absorption spectra of Ni-L1,Cu-L1,Ni2-L2,Cu2-L2 and CuNi-L2 in the solution state (chloroform).(d) Band structure for Ni-L1,Ni2-L2 and CuNi-L2.

The phase purity of Ni-L1,Cu-L1,Ni2-L2,Cu2-L2and CuNi-L2were confirmed by well-matched powder X-ray diffraction (PXRD)patterns (Figs.S1-S5 in Supporting information).According to the X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectrometer (ICP-OES) results (Table S3 in Supporting information),the valence of Ni and Cu is +2 (Fig.2a and Fig.S6 in Supporting information) and the stoichiometric ratio of Cu/Ni in CuNi-L2is 1:1.Furthermore,the thermogravimetric analysis (TGA) curves of Ni-L1,Cu-L1,Ni2-L2,Cu2-L2and CuNi-L2showed that their structures can be maintained well below 300°C under air atmosphere (Fig.S7 in Supporting information).To evaluate the potential of the five compounds as photocatalysts,we performed a series of characterizations to assess their band structures.The highest occupied molecular orbital (HOMO) positions of the five molecular compounds were evaluated approximately by cyclic voltammetry (CV) (Fig.2b,Figs.S13-S16 in Supporting information).Thus,the HOMO energy levels of the compounds were estimated to be 6.59 eV (1.74 Vvs.NHE),6.73 eV (1.88 Vvs.NHE),6.63 eV (1.78 Vvs.NHE),6.77 eV (1.92 Vvs.NHE),6.40 eV (1.55 Vvs.NHE) for Ni-L1,Cu-L1,Ni2-L2,Cu2-L2and CuNi-L2,respectively.The bandgaps (Eg) of Ni-L1,Cu-L1,Ni2-L2,Cu2-L2and CuNi-L2were estimated by the UV–vis absorption spectra (Fig.2c).TheEgvalues were calculated to be 2.36,2.51,2.46,2.51 and 2.17 eV,respectively.Then the lowest unoccupied molecular orbital (LUMO) positions of Ni-L1,Cu-L1,Ni2-L2,Cu2-L2and CuNi-L2were calculated to be-0.62 V,-0.63 V,-0.68 V,-0.59 V,-0.62 Vvs.NHE,pH 7.As the LUMO positions of these compounds are more negative than the redox potentials of most photocatalytic CO2RR products such as HCOOH (-0.58 Vvs.NHE,pH 7),CO (-0.51 Vvs.NHE,pH 7) or CH4(-0.24 Vvs.NHE,pH 7) (Fig.2d),these compounds have potential for CO2RR as catalysts [48].

Taking all the above features into consideration,the photocatalytic CO2RR of these compounds as homogeneous catalysts were tested under a pure atmosphere of CO2(1.0 atm,25 °C) in a 30 mL CH3CN/H2O solution (v/v=4:1) with a certain amount (0.8 μmol/L) of catalysts,triisopropanolamine (TIPA) as electron donor and [Ru(bpy)3]Cl2·6H2O as auxiliary photosensitizer (PS) under visible light irradiation (λ≥420 nm) [11,49].After being irradiated,the generated gasses products were analyzed by gas chromatography (GC) (Fig.S17 in Supporting information) and the liquid products were tested by NMR.The results showed that the main gaseous reductive products of these compounds were CO and H2,and no liquid reduction products were detected in the reaction system.As the irradiation time increased,the reduction products decreased gradually per unit time,and there is almost no product generating after 8 h.After one single photoredox cycle,1.52 μmol of CO and 0.20 μmol of H2were produced in accordance with a CO selectivity of 88.4% and a TONCOof 76 catalyzed by mononuclear compound Ni-L1(0.8 μmol/L) within 8 h.In contrast,as the catalyst replaced by another mononuclear compound Cu-L1,almost no reduction products were detected.This consequence implied that the metal ion CuIIis inactive in this catalytic system.Specifically,when the photocatalyst changed from the mononuclear complex Ni-L1to the bimetallic complex Ni2-L2(0.4 μmol/L) with the same NiIIion concentration and irradiation time,the yield of CO was increased to 2.18 μmol (TON=109),along with the selectivity of CO was also improved from 88.4% to 93.9% (Fig.3a).The results showed that both of the NiIIions in the two types of coordination environment (N2O2and O4) have effective photocatalytic CO2RR capabilities.There is no doubt that introducing the second metal has a certain positive effect on the product selectivity.However,the isomorphic compound Cu2-L2produced only trace amounts of gaseous products,which confirmed that CuIIion expresses negligible catalytic activity in this photocatalytic system one step closer.In other words,CuIIion did not show an effective photocatalytic CO2reduction ability whether it was in the N2O2coordination environment or the O4coordination environment.Interestingly,when CuNi-L2(0.8 μmol/L) acted as the catalyst,it showed the highest CO yield (4.60 μmol) among the nickel-containing catalysts (Ni-L1,Ni2-L2and CuNi-L2) under the identical photocatalytic condition.The TONCOwas up to 230,which was 2.1 and 3.0 times as much as that of Ni2-L2and Ni-L1respectively.Meanwhile,the selectivity for CO was 93.5%,which was comparable to that of Ni2-L2.A summary of the specific selectivities,TONs and TOFs of these photocatalytic systems were provided in Table S4 (Supporting information).

Fig.3.The photocatalytic CO2RR was carried out under the conditions of a 30 mL CO2-saturated CH3CN/H2O solution (v/v=4:1) at 25 °C with the same concentration of NiII (0.8 μmol/L),TIPA (0.8 mol/L) and [Ru(bpy)3]Cl2·6H2O (0.85 mmol/L) under visible light irradiation (λ ≥420 nm,irradiation area,7.06 cm2).(a) Photocatalytic production of CO and H2 catalyzed by Ni-L1 (0.8 μmol/L),Ni2-L2 (0.4 μmol/L)and CuNi-L2 (0.8 μmol/L).(b) Mounts of CO and H2 produced as a function of the visible-light irradiation time over CuNi-L2 (0.8 μmol/L).(c) Durability test for CuNi-L2 (0.8 μmol/L).(d) The mass spectra of 13CO recorded under a 13CO2 atmosphere.

Considering that CuNi-L2presented the highest photocatalytic activity and superior selectivity for CO2-to-CO conversion,a collection of control tests with CuNi-L2as the representative example were carried out to identify the roles of catalyst and other influence factors in photocatalytic CO2RR.The corresponding results were summarized in Table S4.No gas phase product was detected when the photocatalytic reaction was performed without CO2(replaced by N2) (Table S4,entry 4),which indicated the detected CO originated from CO2.The absence of gas production in the dark indicated that it is the truly photoexcited states to initiate the reaction (Table S4,entry 5).Moreover,the reaction system in the absence of TIPA exhibited trace evolution amount of CO,suggesting that the photoexcited states of CuNi-L2were reductively quenched by TIPA (Table S4,entry 6).When the photocatalytic system without CuNi-L2or photosensitizer,a trace amount of CO was detected (Table S4,entries 7 and 8).The above experimental results showed that the catalyst,photosensitizer,visible light,CO2atmosphere and TIPA are all the indispensable reaction conditions in this photocatalytic CO2RR model system.At the same time,noting that CO production was also very low in the absence of H2O (Table S4,entry 9),we conducted a series of experiments to explore the influence of different proportions of H2O on CO2RR.When the ratio of H2O/CH3CN was slightly increased to 1:9,the yield of CO (3.7 μmol) was increased significantly,indicating that H2O acted as an important proton source in the reaction process.When the H2O/CH3CN ratio further increased to 1:4,the amount of CO was increased to 4.6 μmol,and the selectivity was also risen from 77.1% to 91.3% at the same time.However,as the H2O/CH3CN ratio reached to 1:1,the yield of CO decreased instead,and only a few CO produced in pure H2O (Table S5 in Supporting information).

After an 8-h photocatalysis,the total amount of gaseous products produced hardly increased (Fig.3b),which may attribute to the inactivation of the photosensitizer.In order to confirm that,recycling experiments were carried out by taking CuNi-L2as an example.When almost no CO was produced after being illuminated for 8 h,the equal amounts of fresh PS was added into the system and pure CO2gas was reintroduced.As expected,the stagnated reaction was activated again,which indicated that the termination of the system results from the deactivation of the photosensitizer rather than the poisoning of CuNi-L2catalyst.Moreover,CuNi-L2completed at least three cycles of 8 h reaction with a high total CO output (9.23 μmol) and fine selectivity (91.6%) to CO (Fig.3c).Additionally,the apparent quantum efficiency (AQE) of CuNi-L2,Ni2-L2and Ni-L1were also measured under the monochromatic light of 420 nm (see Supporting information).The corresponding results showed that theφCOof CuNi-L2was calculated to be 0.018%,which was 2-fold higher than Ni2-L2(0.0086%) and around 3-fold higher than Ni-L1(0.0060%).In order to confirm that the produced CO indeed comes from CO2reduction,isotopic tracing experiment was performed with representative CuNi-L2catalyst in13CO2atmosphere.After irradiation under visible light for 4 h,gas chromatography-mass spectrometry (GC–MS) was used to analyze the reaction products.The detected peak ofm/z29 was assigned to13CO,the peaks ofm/z13 and 16 belong to the debris of13C and16O (Fig.3d).It is clear from the result that13CO was converted from the13CO2atmosphere,which indicated that CO is derived from the photocatalytic CO2reduction instead of the compounds decomposing (catalyst/PS/TIPA) in the catalytic system.At the same time,the13C NMR spectrum indicated that there were no liquid reduction products detected in a series of reactions above (Fig.S20 in Supporting information) [50,51].As a result,this fact proved that the catalysts had good selectivity,and it also demonstrated that these compounds did have the CO2RR activity.In addition,the steady-state photoluminescence (PL) spectroscopy of CuNi-L2was further carried out to provide insight into the excited states recombination behavior (Fig.S21 in Supporting information).It can be observed that the fluorescence intensity decreases gradually as the catalyst concentration increases,indicating that CuNi-L2has a good separation efficiency of photogenerated carriers.Based on the above photocatalytic tests,both the mononuclear compound Ni-L1and the dinuclear compound Ni2-L2showed high CO2RR performance,indicating that the NiIIions in the two coordination environments (N2O2and O4) possess effective catalytic activities.Moreover,Ni2-L2exhibits better catalytic ability than Ni-L1under the same NiIIion concentration,we guess that it may be caused by the certain metal synergistic catalysis between the two NiIIsites in different coordination environments.However,when the catalyst was replaced by the isomorphic Cu-L1and Cu2-L2respectively,they did not show effective photocatalytic activity.That is to say,the CuIIions in the two coordination environments hardly express photocatalytic performance.It is worth noting CuNi-L2involves one active site NiII,but it shows the highest catalytic activity among these five catalysts.We considered that the inactive CuIIion probably exerts a certain effect on improving the photocatalytic activity of neighboring active NiIIsite.

The experimental results demonstrated that the photocatalytic CO2RR activities of the five monomolecular compounds gradually decreased by CuNi-L2,Ni2-L2,Ni-L1,Cu2-L2and Cu-L1successively.DFT calculation was employed to get insight into the activity of the photocatalytic conversion of CO2-to-CO.Then the Gibbs free energy (G) of the CO2RR paths are compared to explore the differences in photocatalytic activity of the five catalysts (Fig.4a).The CO2-to-CO reduction pathway is primarily composed of three steps: CO2activation and hydrogenation (CO2→*COOH),dehydration (*COOH →*CO),and CO desorption (*CO →CO),where the rate-determining step is the first path.The energy change from CO2to*COOH on CuNi-L2and Ni2-L2are the lowest (0.81 eV and 0.85 eV),Ni-L1(1.44 eV),Cu2-L2(1.57 eV) and Cu-L1(1.97 eV),suggesting the highest activity of dual TM centers (CuNi-L2and Ni2-L2) for CO2reduction.For the two investigated transitional metal ions,CuIIsites apparently present an inactive behavior for the CO2RR due to large energy increase for the formation of*COOH.Although the above results are consistent with the photocatalytic activity of the catalysts,the energy change from CO2to*COOH of CuNi-L2is not far different from Ni2-L2,while the CO production of CuNi-L2is about 1.5 times than that of Ni2-L2in experiment.The dual TM sites were expected to stabilize the*COOH intermediate with a bridge structure,but the adsorbate returns to a single NiIIsite after geometry optimization (Fig.S22 in Supporting information).That is to say,the coordination environment of the NiIIions is crucial during the photocatalytic possess.In order to further explore the structure-activity relationship between CuNi-L2and Ni2-L2,time-dependent DFT calculations were performed to investigate the excitation properties of CuNi-L2and Ni2-L2systems.Fig.4b illustrates the electron contributions of the first excited state,which is basically contributed by HOMO-LUMO transition,the charge density difference between the ground state (S0)and the first excited state (S1).It is clear from the result above that the excited electron of Ni2-L2is concentrated around two NiIIions and the coordinated N,O atoms,while the electron of CuNi-L2is mainly concentrated on the NiIIion and O atoms,no electron accumulates around the CuIIion and N atoms.In addition,the charge analysis reveals that the charge transfer from NiIIions to N2O2and O4coordination are 0.21 and 0.47 respectively (Fig.4c),indicating that NiIIions in the two coordination environments both have good reduction ability,of which the NiIIin O4coordination possesses better reduction activity.Meanwhile,comparing the free energy change between the two NiIIsites,the NiIIin O4-coordination expresses a lower energy change from CO2to*COOH than NiIIin N2O2coordination environment,which further proves the result above.The charge transfer in CuNi-L2for metal ions are 0.65 (NiII) and 0.01 (CuII) respectively.It can be found that the existence of inactive CuIIion promotes the accumulation of electrons on NiII,which causes the NiIIwith O4coordination environment in CuNi-L2expressing better reduction ability than that of Ni2-L2,and then significantly accelerates the photocatalytic reduction reaction of CO2to CO.

Fig.4.(a) The free-energy profile for the CO2RR pathway.(b) Charge density difference plot for the first excited state of CuNi-L2 (left) and Ni2-L2 (right).(c) 2D display of charge density of CuNi-L2 (left) and Ni2-L2 (right) systems.(d) Proposed photocatalytic mechanism of the molecular metal compounds for the CO2 to CO conversion.

According to the above DFT calculation results,the corresponding photocatalytic CO2RR mechanism can be proposed (Fig.4d).Firstly,[Ru(bpy)3]Cl2·6H2O is excited by the irradiation of visible light.Since the LUMO positions of the five compounds are lower than that of [Ru(bpy)3]Cl2·6H2O,the photo-generated electrons can be transferred to the catalysts.At the same time,CO2molecules are adsorbed and activated by the exposed metal sites of the catalysts.Secondly,the adsorbed CO2on the active metal center obtains an electron and a proton to form a*COOH intermediate.Next,the*COOH intermediate gets an electron and a proton,then the*CO species is generated after dehydration from the*COOH.Finally,CO is generated by the desorption of*CO separating from the surface of the catalysts.The sacrificial electron donor was performed by TIPA to fill the holes and reductive quench the excited photosensitizer.The catalysts regenerated and enter the next cycle of CO2-to-CO conversion.

In summary,we design and synthesize five molecular metal compounds,containing mononuclear Ni-L1and Cu-L1,dinuclear homometallic Ni2-L2,Cu2-L2and dinuclear heterometallic CuNi-L2.These molecular metal compounds can be regarded as a model catalyst system to systematically probe the influence on photocatalytic CO2RR performance caused by monometallic site,bimetallic sites and heterometallic sites by precisely controlling of the species,number and catalytic properties of metal ion centers.In particular,it explores the direct effect of introducing inactive metal ions into bimetallic catalysts on the photocatalytic CO2RR performance of two neighboring metal centers.When these molecular metal compounds act as homogeneous catalysts,the inactive Cu-introduced dinuclear heterometallic CuNi-L2(0.8 μmol/L) exhibits the highest photocatalytic selectivity (93.5%) and activity(4.60 μmol) after an 8-h photocatalysis,which is 2.1 and 3.0 times than that of dinuclear homometallic catalyst Ni2-L2(2.18 μmol)and the mononuclear catalyst Ni-L1(1.52 μmol) respectively with the same concentration of NiII(0.8 μmol/L).The related DFT calculation results also show that,compared with Ni2-L2,the photogenerated electrons in CuNi-L2are promoted transferring to the adjacent active NiIIsite due to the introduction of inactive CuIIion.This fact improves the utilization efficiency of photo-generated electrons,and then the photocatalytic CO2RR activity of NiIIions.Significantly,more insights were provided by this work for the future design and construction of high-efficiency bimetallic photocatalysts applied to photocatalytic CO2RR.

Declaration of competing interest

The authors report no declarations of interest.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (NSFC,Nos.92061101,21871141,21871142 and 22071109); Project funded by China Postdoctoral Science Foundation (No.2018M630572).

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.cclet.2022.01.039.