Conjugation Effect Contributes to the CO2-to-CO Conversion Driven

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Conjugation Effect Contributes to the CO2to-CO Conversion Driven by Visible-Light Dongcheng Liu, Hong-Juan Wang, Ting Ouyang, JiaWei Wang, Long Jiang, Di-Chang Zhong, and Tong-Bu Lu ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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ACS Applied Energy Materials

Conjugation Effect Contributes to the CO2-to-CO Conversion Driven by Visible-Light Dong-Cheng Liu,†§ Hong-Juan Wang,‡§ Ting Ouyang,† Jia-Wei Wang,† Long Jiang,† Di-Chang Zhong,*‡ and Tong-Bu Lu*†‡ †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-

Sen University, Guangzhou 510275, China. ‡

Institute for New Energy Materials & Low Carbon Technologies, School of Materials Science

& Engineering, Tianjin University of Technology, Tianjin 300384, China.

ABSTRACT

Structural modification of ligand is an effective way to improve the catalytic activity of molecular catalysts for photocatalytic CO2 reduction. In this study, we designed and synthesized three

tripodal

ligands

with

different

conjugate

groups

(L1

=

tris{2-[(9'

-

anthrylmethyl)amino)ethyl}amine, L2 = tris{2-[(1' -naphthylmethyl)amino]ethyl}amine, L3 = tris[2-(benzylamino)ethyl]amine), and their corresponding mononuclear cobalt complexes, [CoL1(OH)]ClO4 (1), [CoL2(OH)]ClO4 (2), and [CoL3(OH)]ClO4 (3). Control experiments showed that 1 and 2 possess higher efficiency than 3 for the photocatalytic CO2-to-CO conversion, with TON and TOF for CO of 58000 and 1.61 s-1 for 1, and 49200 and 1.37 s-1 for 2, respectively, greatly higher than those of 3. 1 and 2 also display higher CO selectivity (≥ 97%)

ACS Paragon Plus Environment

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than 3. Control experiments and DFT calculations revealed that the excellent catalytic performances of 1 and 2 can be ascribed to the extended conjugation substituent in L1 and L2, which endows the CoII catalytic centre with low reduction potential, accelerates the intermolecular electron transfer, and thus dramatically boosts the CO2-to-CO conversion. This study demonstrates that the improvement of the electron transfer between photosensitizer and catalysts is the key for enhancing the activity of catalyst for CO2-to-CO conversion.

KEYWORDS: photocatalytic CO2 reduction, monouclear cobalt complex, homogeneous, ligand modification, extended conjugation 1. INTRODUCTION Global energy shortage and climate deterioration underpin broad interest in converting CO2 into value-added carbon products/fuels.1-3 The sunlight-driven CO2 reduction is considered as a sustainable approach, by which both of the above-mentioned problems would be solved in a carbon-neutral fashion.4-11 The bottleneck is to design highly-efficient and highly-active catalysts as CO2 is an extremely stable linear molecule.12-15 Homogeneous molecular catalysts, with the advantages in structural design and modification, as well as mechanistic analysis, have been widely used for investigations on photocatalytic CO2 reduction.12, 16-18 Noble metal complexes (e.g., Re,19-23 Ru,24-25 and Ir26) are a type of molecular catalysts used in many photocatalytic CO2 reduction systems for their high activity and stability. This kind of catalysts do not facilitate broad practical application.27 In recent years, a number of earthabundant metal complexes (e.g., Co,28-32 Ni,12, 18, 33-35 Mn,36-37 Fe,1, 38-41 and Cu42) have been extensively used as catalysts for photocatalytic CO2 reduction, which evidence the much progress in CO2 reduction. However, most of these earth-abundant catalysts only exhibit high


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efficiency and selectivity in organic solvent.6, 18 Due to the fact that the proton reduction of water kinetically favored over CO2 reduction,43-44 water participation would dramatically decrease the selectivity of photocatalytic CO2 reduction.6,

13, 45-48

Therefore, the development of high

efficiency and selectivity earth-abundant catalysts for photochemical CO2 reduction in watercontaining systems is still a great challenge. The tetradentate tripodal ligands, such as tris-(2-aminoethyl)amine (tren) derivatives, usually impose C3 symmetry and bond to a five-coordinated metal center to form mononuclear metal complex, leaving one position available on the trigonal-bibyramidal metal center to combine with a solvent molecule.49-51 This solvent molecule is readily exchanged by a substrate molecule, facilitating the activation and catalytic transformation of the substrate molecule. Therefore, complexes with this kind of ligands are potentially active catalysts for CO2 reduction. For example, [CoII(TPA)Cl]Cl (TPA = tris(2pyridylmethyl)amine) can reduce CO2 to produce CO in CH3CN with a TON of 953 and a selectivity

of

methyl)amine)

85%.30 displays

[Co(NTB)CH3CN](ClO4)2 photocatalytic

activity

(NTB for

=

tris-(benzimidazolyl-2-

CO2-to-CO

conversion

in

water/acetonitrile solution with a TON of 1179 and a selectivity of 96%.49 [CoL3(OH)]ClO4 (3, Figure 1) is also active for photochemical CO2-to-CO conversion, with a TON of 1600 and selectivity of 85%.29 Combining two tripodal parts together formed a cryptand ligand, based on which a dinuclear cobalt cryptate complex [Co2(OH)L](ClO4)3 (L = N[(CH2)2NHCH2(m-C6H4)CH2-NH-(CH2)2]3N) was found to be a highly efficient and selective homogeneous catalyst for the photocatalytic reduction of CO2 to CO, with the TON of 16896 and the selectivity of 98%.29


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Structural modification is an effective approach to further improve the catalytic performances of molecular catalysts,50, 52 which was widely used in enhancing the activity of catalysts for photocatalytic53-56 and electrocatalytic57-59 hydrogen evolution, as well as photocatalytic CO2 reduction.10, 16, 18 By this method, Chang group found that a complex [Ni(Prbimiq1)]2+ (Prbimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane) is favorable for photoinduced conversion of CO2 to CO in CH3CN, with a unprecedented TON value of 98 000.18 Encouraged by these excellent works, we initiated a research work aiming at ligand modification to improve activity of catalysts through gradually increasing the conjugation effect of the group attached to the tetradentate tripodal tren unit.

Figure 1. Chemical structures of 1, 2, and 3. Based on the complex [CoL3(OH)]ClO4 (3, Figure 1) bearing a phenylmethyl-substituted tren derivative,29 we further synthesized another two mononuclear cobalt complexes of two tren derivatives

with

extended

conjugation,

anthracenylmethyl)amino)ethyl}amine)

and

[CoL1(OH)]ClO4

(1,

L1

=

Tris{2-[(9'

-

[CoL2(OH)]ClO4

(2,

L2

=

Tris{2-[(1'

-

naphthylmethyl)amino]ethyl}amine) for photocatalytic conversion of CO2 to CO (Figure 1). The results of photocatalytic experiments show that 1 and 2 possess excellent activity for photoreduction of CO2 to CO. Electrochemical investigations, steady-state and time-resolved fluorescence quenching experiments, as well as DFT calculations demonstrate that the good


4

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performence for photochemical CO2-to-CO conversion of 1 and 2 can be attributed to the extended conjugation substituents of L1 and L2, which endow the CoII catalytic centers with low reduction potentials and accelerate the intermolecular electron transfer, and thus dramatically promote the CO2-to-CO conversion. 2. EXPERIMENTAL SECTION 2.1. Materials Milli-Q ultrapure water (> 18 MΩ) was utilized unless otherwise stated. Acetonitrile was distilled three times over P2O5 prior to use. All other chemical reagents and solvents were commercially available and used without further purification. The purities of Argon and

12

CO2

are 99.999%, and 13CO2 is 99%, respectively. 2.2. Synthesis of ligands and photocatalysts The ligands L1 and L2 were synthesized by literature methods.60-61 [CoL1(OH)]ClO4 (1). In a schlenk flask, Co(ClO4)2·6H2O (36.6 mg, 0.10 mmol) and L1 (71.6 mg, 0.10 mmol) were dissolved in deoxygenated CH3CN (15 mL). The resulting red solution was refluxed for 4 h and then cooled to room temperature under an argon atmosphere. The red brown precipitate obtained by diffusion of ethyl ether into the mixture solution was filtered off, washed with ethyl ether and dried in vacuum. Yield: 65 mg (70%). ESI-MS (CH3CN + H2O): m/z calcd for [CoL1 - H+]+ 774.3, [CoL1(OH) + H2O]+ 810.3, [CoL1 + HCOO−]+ 820.3. Found: 774.1, 810.2, 820.2. Anal. Calcd for C51H53N4CoClO7 (1·2H2O): C 65.98, H 5.75, N 6.03%; found: C 65.49, H 5.72, N 6.06%.


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[CoL2(OH)]ClO4 (2). This complex was obtained as red-violet precipitate, by the same procedure as that for 1 except using L2 (56.6 mg, 0.10 mmol) instead of L1. Yield: 51 mg (66%). ESI-MS (CH3CN + H2O): m/z calcd for [CoL2 - H+]+ 624.2, [CoL2(OH) + H2O]+ 660.2, [CoL2 + HCOO−]+ 670.2. Found: 624.2, 660.2, 670.0. Anal. Calcd for C39H47N4CoClO7 (2·2H2O): C 60.19, H 6.08, N 7.19%; found: C 59.91, H 5.66, N 7.25%. [CoL3(OH)]ClO4 (3). [CoL3(OH)]ClO4 was synthesized according to a literature method.29 Yield: 6.5 mg (73%). ESI-MS (CH3CN + H2O): m/z calcd for [CoL3 - H+]+ 474.2, [CoL3(OH) + H2O]+ 510.2, [CoL3 + HCOO−]+ 520.2. Found: 474.1, 510.2, 520.2. 2.3. Characterization method UV-Vis spectra were determined on a Shimadzu UV-3600 spectrophotometer. A spectrofluorimeter (Shimadzu RF-5301 PC) was used for steady-state fluorescence measurements. Time-resolved photoluminescence was collected with an LP980 laser flash photolysis system from Edinburgh Instruments. The excitation pump source was the Vibrant LD 355 II Nd:YAG/OPO system (OPOTEK). The photocatalytic experiments were conducted with a blue LED light (Zolix, MLED4, λ = 450 nm, irradiation area 0.8 cm2, 100 mW·cm−2). Electrochemical experiments were carried out using an electrochemical workstation CHI 620E. The electrolyte was 0.1 M NBu4PF6 H2O/CH3CN solution (1:4 v/v). Pt foil was used as the counter electrode in the three-electrode system. For cyclic voltammetric (CV) measurements, the reference electrode is 0.1 M Ag/AgNO3 electrode, and the working electrode is glassy carbon (GC) electrode. The potential of Ag/AgNO3 reference electrode was calibrated using ferrocene/ferrocenium (Fc0/+) as an external standard. The gas samples of the photocatalytic reaction were analyzed by an Agilent 7820A gas chromatography. The solution after photoirradiation was analyzed using an ion chromatograph (DX-600, Dionex). Particle size analysis


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was performed using a Brookhaven Elite Sizer zeta-potential and particle size analyzer. The products of the 13CO2 isotopic experiment were analysed by GC-MS (Agilent 7890A-5975C). 2.4. Photocatalytic experiments Photocatalytic CO2 reduction was performed on a 16 mL reactor containing a catalyst, TEOA (0.3 M), [Ru(phen)3](PF6)2 (Ru-PS) (0.4 mM), and 5 mL H2O/CH3CN mixed solution (1:4 v/v) under 1 atm CO2 atmosphere at 293±2 K. After the reaction system purged with CO2 for 15 min, the reaction mixture was continuously stirred with a magnetic bar and irradiated under a blue LED light (λ = 450 nm, irradiation area = 0.8 cm2, light intensity = 100 mW.cm−2). Then the gas samples were detected using gas chromatography (GC, Agilent 7820A). And the possible products in the solution were analyzed by a ion chromatograph (DX-600). All the photocatalytic reactions were repeated three times to confirm the reliability of the data. The reported turnover numbers (TON) are based on the Co catalysts. 2.5. The determination of quantum yield The number of incident photons was measured by a standard method using a potassium ferrioxalate (K3[FeIII(C2O4)3]) actinometer at 25 °C. And the photon flux was determined to be 3.02 × 10−7 einstein/s.62 The quantum yield of the overall catalytic photoredox cycle for CO2 reduction to CO was calculated according to the following equation: ΦCO = [2 × (number of the produced molecule) / (number of photons)] 100%18 For 1: CO2 → CO, 2e−, 10.12 µmol of CO was produced within 10 h. ΦCO = 0.19% For 2: CO2 → CO, 2e−, 8.89 µmol of CO was produced within 10 h. ΦCO = 0.17%


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For 3: CO2 → CO, 2e−, 2.02 µmol of CO was produced within 10 h. ΦCO = 0.04% 2.6. Computational methods All the calculations in the present study were carried out using the hybrid meta exchange correlation functionals M06 as implemented in the Gaussian 09 program.63 For the optimizations of all stationary points, the 6-31G(d, p) basis set was applied to the C, H, O, N atoms, while the SDD pseudopotential and its corresponding basis set was used for the Co atom. In order to obtain more accurate relative energies, single point calculations using the 6-311+G(d, p) basis set were performed based on the M06/6-31G(d, p) optimized molucular geometries. The solvation effects of acetonitrile were simulated by the SMD continuum solvation model with the larger basis set in the single point calculations. Frequency analysis calculations were performed at the same level of theory as the structural optimizations to obtain the thermal correction to Gibbs free energy and to confirm the structures as the minima or transition states. The corresponding intrinsic reaction coordinate (IRC) computations were also conducted to confirm the connectivity of the transition state and its two relevant minimum. To construct the energy profile for the whole catalytic process, a concentration correction of 1.9 kcal/mol at room temperature was added to the free energy term, which derived from the free energy change of 1 mol of ideal gas to the standard state of solution. The solvation free energy of -260.2 kcal/mol was used for a proton.64 All the computed free energy results have been summarized in the Figure S17 and optimized structures of the intermediates and transition states were given in Figure S18. The 3D optimized structures were drawn by CYLview visualization program.65


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3. RESULTS AND DISCUSSION 3.1. UV/vis absorption spectroscopy UV-vis absorption spectra of 1-3 in H2O/CH3CN solution (1:4 v/v) were initially studied. The results clearly show that the absorption band shape and intensity varies with the change of extending π-conjugated system of the tripodal ligands (Figure S1). The absorption of 1 exhibits a maximum peak at 254 nm (εmax = 249588 M−1·cm−1) and other six peaks between 200 and 405 nm, which result from the π → π* transitions of the anthrylmethyl-substituted ligand. For 2, a sharp peak at 222 nm (εmax = 189500 M−1·cm−1) and a broad band between 260-305 nm. For 3, the UV-vis absorption bands appear at 206 nm (εmax = 53794 M−1·cm−1) and 254 nm, which also result from π → π* transitions of the ligands. The absorption intensity of 1-3 are markedly different. From 1 to 3, with the decrease of the extended conjugation in the tripodal ligands, the absorption intensity obviously decreases. 1 and 2 also display much larger molar absorptivity than 3. The strong absorption intensity and large molar absorptivity of 1 and 2 can be ascribed to the extended conjugation groups attached to tren units in L1 and L2. Further UV-vis measurements for 1 and 2 in H2O/CH3CN solution (1:4 v/v) under an Ar atmosphere showed almost no change after irradiation for 10 hours (Figure S2), illustrating that they are stable in H2O/CH3CN solution. 3.2. Photocatalytic CO2 reduction The photocatalytic experiments for CO2 reduction was performed in a mixture of 5 mL CO2saturated H2O/CH3CN solution (1:4 v/v) using 1/2/3 as catalysts, [Ru(phen)3](PF6)2 (Ru-PS) as a photosensitizer, and triethylolamine (TEOA) as a sacrificial reductant. A 450 nm LED light was employed for CO2 photoreduction, as Ru-PS exhibits a broad absorption band in the visible


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region and a maximum peak at 448 nm (Figure S3). The results of photocatalytic experiments show that both anthrylmethyl-substituted 1 and naphthylmethyl-substituted 2 display excellent photocatalytic activity for CO2-to-CO conversion (Table 1). For 1, the visible-light photoredox cycle produced 3.17 µmol CO and 0.062 µmol H2 along with trace amount of formate within 10 h when 0.05 µM of 1 was used (Table 1, Entry 1). The corresponding TON and TOF values are 12680 and 0.35 s-1, respectively, and the selectivity to CO is 98%. For 2, 2.82 µmol CO and 0.076 µmol H2 were produced, with the TON, TOF and selectivity to CO values of 11280, 0.31 s−1 and 97%, respectively (Table 1, Entry 2 and Figure 2). These values are much higher than the corresponding values of phenylmethyl-substituted 3 under the same conditions (TON and TOF are 1600 and 0.04 s−1, respectively; selectivity to CO is 85%. Table 1, Entry 3),29 suggesting that the efficiency and selectively of 1 and 2 were greatly higher than 3. These results demonstrate that the environments around the catalytic center CoII are vital to its catalytic activity, and the conjugation effect of the extended π-system attached to the tren unit appears to be beneficial to the photocatalytic CO2 reduction.


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Table 1. Photoinduced CO2 reduction to CO by 1-3. Entry

Cat

1

1

2

2

3

3

4

Blank

5

1

6

1

7 8

2

CO

[µM]

[µmol]

0.05

H2 [µmol]

CO [%]

TON (CO)

TOF [s-1]

3.17

0.062

98

12680

0.35

2.82

0.076

97

11280

0.31

0.4

0.070

85

1600

0.04

0

0.049

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0.052

0

0

0

0

420 nm, 100 mW·cm−2). Control catalytic experiments showed that the replacement of the LED light with a Xe lamp resulted in larger TON values of 15080 for 1 and 13360 for 2 (Table S1) than the corresponding values of 12680 and 11280 (Table 1) at the same concentration of 1 and 2 (0.05 µM). But the selectivity to CO decreased from 98% to 95%. These results indicate that light source used for photocatalytic CO2 reduction strongly affects the performance of the catalytic system. Table 2. Photoinduced CO2 reduction to CO by 1-3 at different concentrations. Cat 1

2

3

Cat [µM]

CO [µmol]

H2 [µmol]

CO [%]

TON (CO)

TOF [s−1]

2.0

10.12

0.17

98

1012

0.028

0.01

2.90

0.09

97

58000

1.61

2.0

8.89

0.14

98

889

0.025

0.01

2.46

0.06

98

49200

1.37

2.0

2.02

0.32

86

202

0.006

0.01

NDa

Conjugation Effect Contributes to the CO2-to-CO Conversion Driven

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