MoâBiâCd Ternary Metal Chalcogenides: Highly Efficient

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Mo-Bi-Cd Ternary Metal Chalcogenides: Highly Efficient Photocatalyst for CO2 Reduction to Formic Acid Under Visible Light Baowen Zhou, Jinliang Song, Chao Xie, Chunjun Chen, Qingli Qian, and Buxing Han ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00956 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Sustainable Chemistry & Engineering

Mo-Bi-Cd Ternary Metal Chalcogenides: Highly Efficient Photocatalyst for CO2 Reduction to Formic Acid Under Visible Light Baowen Zhou,† Jinliang Song,† Chao Xie,†,‡ Chunjun Chen,†,‡ Qingli Qian,† and Buxing Han†,‡* †

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of

Sciences, 1st North Street Zhongguancun, Beijing 100190, P.R.China ‡

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Yuquan

Road 19, Beijing 100049, P.R.China E-mail: [email protected]

ABSTRACT: The exploration of efficient and cheap photocatalysts for the transformation of CO2 into value-added chemicals is of great importance. Herein, we carried out the work on CO2 reduction using ternary metal chalcogenides as photocatalysts. It was found that the ternary metal chalcogenides of (Mo-Bi)Sx supported on mesoporous CdS ((Mo-Bi)Sx/Meso CdS) was very active and selective for photocatalytic reduction of CO2 to HCOOH in 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4)-acetonitrile (MeCN) mixed solvent under visible light. HCOOH was the only carbonaceous product and H2 was the only byproduct. The rate of HCOOH formation could be as high as 208 µmol·g-1·h-1 with a 72% faradaic efficiency. The synergistic effect of the components played a key role for the outstanding performance of the catalyst; the catalyst and [Bmim]BF4 further cooperated in promoting the reaction. KEYWORDS: Mo-Bi-Cd chalcogenides, visible-light-driven CO2 reduction, formic acid, ionic liquid, photocatalysis

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INTRODUCTION Transformation of CO2 into value-added chemicals and fuels through artificial photosynthesis is a promising strategy, which has attracted increasing interest like solar-driven water splitting and environmental remediation.1-7 Generally,

photocatalytic

reduction of CO2 involves multiple

proton-coupled electrons transfer to generate various products, which makes the products separation complex. Meanwhile, due to the thermodynamic stability and kinetic inertness, reduction of CO2 is more difficult than proton reduction to H2. Thus, photocatalytic reduction of CO2 with high efficiency and selectivity is challenging.8 Formic acid (HCOOH) is a very useful chemical. For the photocatalytic reduction of CO2 into HCOOH, homogeneous catalytic systems based on molecular complexes in combination with semiconductors or enzymes have been primarily researched.9-10 Particularly, complexes with expensive and rare metals, such as Rh and Ru, have been dominant as catalysts.11-14 As such, large-scale application of these systems has been stymied by high cost and poor stability. For practical application, earth-abundant heterogeneous photocatalysts are preferred. Although much effort has been devoted to developing heterogeneous photocatalysts for CO2 reduction, reports on conversion of CO2 to HCOOH have been limited; moreover, these photocatalysts generally suffer from low efficiency, poor selectivity, and/or they employ UV-light or precious metal cocatalysts to drive the reaction.15-17 Therefore, designing efficient and low-cost heterogeneous catalysts for the selective reduction of CO2 towards HCOOH under visible light is highly desired. Transition metal chalcogenides have attracted tremendous attention owing to their outstanding mechanical, electrical, and optical properties.18-20 Due to their unique structures, metal chalcogenides have exceptional interaction with CO2 and its reducing inter-mediates,21 and show promising CO2 catalytic activity.20 Meanwhile, they are inexpensive, have a tunable energy bandgap, and a high density ACS Paragon Plus Environment

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of active sites. These characteristics make them an ideal alternative to precious catalysts for photocatalytic fixation of CO2.23 Ionic liquids (ILs), which have emerged as a new class of green solvents, are crucial to the fixation of CO2 as well as the pho-tocatalysts.24 High CO2 capture25 and activation ability, and inherent ionic conductivity make ILs excellent solvents and catalysts for thermo- and electrochemical conversion of CO2.26-27 However, to the best of our knowledge, there is only one work that ILs promoted CO2 photoreduction to CO using [Ru(bpy)3]Cl2 as the catalyst, but HCOOH was not obtained.28 Herein, we report the work on photocatalytic reduction of CO2 over ternary metal chalcogenides. It was demonstrated that (Mo-Bi)Sx/Meso CdS had outstanding performance for the reduction of CO2 to HCOOH in [Bmim]BF4-MeCN under visible light. RESULTS AND DISCUSSIONS TEM images at low magnification (Figure 1a and Figure S1a) and scanning electron microscopy (SEM) image (Figure S1b) show that the diameter of the synthesized (Mo-Bi)Sx/Meso CdS is about 50 nm. In the high-resolution TEM image in Figure 1b, the crystal lattices of CdS are observed; average lattice spacing of ca. 0.35 nm corresponds to the (0001) plane of CdS.29 The lattice fringes of Bi2S3 are also measured. The lattice spacing of 0.33 nm is assigned to the (130) plane of Bi2S3 (Figure 1c).30 Figure 1d illustrates the merged elemental mappings of (Mo-Bi)Sx/Meso CdS. From Figure 1e to 1h, it is discovered the coexistence of Mo, Bi, Cd, and S elements. The intensity of Cd (Figure 1e) and S (Figure 1f) is much higher than that of Bi (Figure 1g) and Mo (Figure 1h), attributing to the low depositing ratio of Mo and Bi. The ICP-AES analysis (VISTAL-MPX) indicated that the molar ratio of Mo and Bi based on CdS was 3% and 6%, respectively. Despite of the low intensity, Mo and Bi sites are dispersed on CdS uniformly. The binding energies of (Mo-Bi)Sx/Meso CdS were characterized to study the oxidation states of the elements.



The high resolution Mo 3d spectra in Figure 1.i demonstrates two peaks at 232.3 eV (Mo 3d5/2) and 235.4 eV (Mo 3d3/2) that are credited to Mo6+. The peaks of Cd 3d at 405.4 eV (Cd 3d5/2) and 412.1 eV (Cd 3d3/2) reveal that Cd2+ is the dominant oxidation state (Figure 1j).29 The typical peaks of Bi 4f at 164.2 eV (Bi 4f5/2) and 158.8 eV (Bi 4f7/2) are assigned to Bi3+ while the peaks appearing at 162.3 eV (S 2p1/2) and 161.2 eV (S 2p3/2) signify the presence of S2- (Figure 1k).30 X-ray diffraction (XRD) analysis was performed to study the as-synthesized photo-catalyst (Figure S2). As shown in Figure S2, the diffraction peaks agree well with the standard pattern of CdS (JCPDS card No. 47-1179).

Figure 1. Structure and composite characterization of the (Mo-Bi)Sx/Meso CdS with a Mo to Bi molar ratio of 1:2. Transmission electron microscopy (TEM) images of (Mo-Bi)Sx/Meso CdS at low (a) and high (b and c) magnification, respectively. The elemental mappings of (Mo-Bi)Sx/Meso CdS: Merged (d), Cd (e), S (f), Bi (g), and Mo (h). High-resolution X-ray photoelectron spectrum (XPS) of Mo 3d (i), Cd 3d (j), S 2p and Bi 4f (k).

The performance of three ternary metal chalcogenides and (Mo-Bi)Sx/TiO2 was examined (Table 1). In the four tested catalysts, (Mo-Bi)Sx/Meso CdS showed the highest activity for HCOOH formation


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(Entry 1), and HCOOH was the only carbonaceous product. However, the formation rate of H2 (180 µmol·g-1·h-1) was about four times larger than that of HCOOH (44 µmol·g-1·h-1), and the faradaic efficiency of HCOOH was only 19.6%, which could be attributed to that protons reduction towards hydrogen was much easier than the reduction of CO2 to HCOOH in pure MeCN. In contrast, when (Mo-Au)Sx/Meso CdS was used, CO was the major product with a rate of 60 µmol·g-1·h-1, while the rate of HCOOH production was only 10 µmol·g-1·h-1 with a faradaic efficiency less than 6% (Entry 2). Similarly, CO was obtained but HCOOH was not detected when the reaction was performed over (Mo-Cu)Sx/Meso CdS (Entry 3). The low activity of (Mo-Au)Sx/Meso CdS and (Mo-Cu)Sx/Meso CdS for HCOOH ascribed to Au and Cu being prone to produce CO from CO2.31,32 In addition, the reaction did not occur over (Mo-Bi)Sx/TiO2 because TiO2 could not be excited by visible light (Entry 4, and Figure S3). These results indicate that the photogeneration of electron-hole pairs is a crucial step for the subsequent photoredox reactions; the synergistic effect of the components of Mo, Bi, and Cd is also essential for the selective production of HCOOH. Moreover, we found that the solvent has a strong effect on the photocatalytic performance of (Mo-Bi)Sx/Meso CdS for HCOOH formation (Entries 1, 5 and 6); the highest HCOOH formation rate is achieved in MeCN. Meanwhile, we have conducted control experiment for CO2 photoreduction in the absence of TEOA (Entry 7). It is discovered that neither hydrogen evolution nor CO2 reduction happened without TEOA, indicating that TEOA was crucial for the reaction by eliminating the photogenerated holes and providing hydrogen source. Furthermore, the activity of (Mo-Bi)Sx/bulk CdS with small surface area (0.6 m2/g) was much inferior to that of (Mo-Bi)Sx/Meso CdS (120 m2/g), indicating the unique mesoporous structure was highly beneficial for the photoactivity (Entry 8).



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Table1. Photocatalytic reduction of CO2 with different catalysts and solvents.a Formation rate (µmol·g-1·h-1) Entry

a

Photocatalyst

Solvent HCOOH

CO

H2

1

(Mo-Bi)Sx/Meso CdS

MeCN

44±3

0

180±5

2

(Mo-Au)Sx/Meso CdS

MeCN

10±1

60±3

106±5

3

(Mo-Cu)Sx/Meso CdS

MeCN

N.D

17±1

10±1

4

(Mo-Bi)Sx/TiO2

MeCN

0

0

0

5

(Mo-Bi)Sx/Meso CdS

Ethanol

30±3

N.D

222±5

6

(Mo-Bi)Sx/Meso CdS

H2 O

10±2

N.D

55±2

7b

(Mo-Bi)Sx/Meso CdS

MeCN

0

0

0

8

(Mo-Bi)Sx/bulk CdS

MeCN

1

0

10

Reaction conditions: catalyst, 25 mg; solvent, 20 mL; visible-light (λ=420-780 nm, 0.3 W/cm2);

TEOA, 1 g; time, 10 h; illuminated area, 5 cm2; CO2 pressure, 3 MPa; temperature, 25 oC. bWithout TEOA.

To explain the role of the components of (Mo-Bi)Sx/Meso CdS, we examined the activity of different metal chalcogenides (Figure 2). As shown in Figure 2, H2 evolution over MoSx/CdS was superior to that over bare CdS. Meanwhile, control experiments were conducted using MoSx/Meso CdS as the photocatalyst in the absence/presence of 2,2’,6,6’-tetramethylpiperidine-N-oxyl (TEMPO) as an active hydrogen scavenger.33 It was discovered that no hydrogen was formed over MoSx/Meso CdS in the presence of TEMPO (100 mg) while the hydrogen evolution rate over MoSx/Meso CdS was as high as 151 µmol·g-1·h-1 without TEMPO. These results indicated that MoSx was an efficient cocatalyst for hydrogen evolution reaction.32 It should be noted that Mo sites can bind the intermediate COOH*,21,22


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which favors the formation of HCOOH. Bare CdS was not active for CO2 reduction and its activity for hydrogen evolution was also lower than of (Mo-Bi)Sx/Meso CdS (Figure 2). By contrast, Bi2S3/CdS exhibited slight activity for CO2-into-HCOOH conversion while pure CdS and MoSx/Meso CdS were inactive, indicating the Bi species played an important in activating CO2. In addition, (Mo-Bi)Sx/Meso CdS showed higher activity than the physical mixture of MoSx/CdS and Bi2S3/CdS. Therefore, (Mo-Bi)Sx not only promoted the hydrogen evolution due to its enhancement of charge carriers separation, but also offered catalytic active sites for photocatalytic reduction of CO2. However, (Mo-Bi)Sx without CdS was not active for CO2 reduction because CdS was essential to act as the photoabsorber for light harvesting to form charge carriers. These results supported the synergistic effect of the components in (Mo-Bi)Sx/Meso CdS on CO2 reduction. In the catalytic cycle, Mo sites served as a promoter for forming active hydrogen and an anchor for the reducing intermediate COOH*. Bi species were further involved in catalyzing the reduction of CO2 to HCOOH.35 This was attested by the fact that the amount of Bi affected the photocatalytic performance significantly (Figure S4). The HCOOH formation rate increased with the increasing amount of Bi when keeping Mo content unchanged. However, with excessive loading of Bi (molar ratio of Bi to Mo = 3.5), the incident light absorption of CdS was retarded, resulting in lower performance.36 Moreover, the binding energy of Cd 3d and S 2p of (Mo-Bi)Sx/Meso CdS showed mild shift as compared to the pure CdS (Figure S5), which implied the unique interaction between CdS and Mo-Bi metal chalcogenides. From these results, it is reasonable to consider that the cooperative effect of the components in (Mo-Bi)Sx/Meso CdS is crucial for the CO2 reduction.



Figure 2. Activity of different metal chalcogenides for photocatalytic reduction of CO2 to HCOOH. Conditions: photocatalyst, 25 mg; visible-light (λ=420-780 nm, 0.3 W/cm2); MeCN, 20 mL; TEOA, 1 g; time, 10 h; illuminated area, 5 cm2; CO2 pressure, 3 MPa; temperature, 25 oC.

Imidazolium-based ILs can interact with CO2 to decrease the reaction energy barrier due to their unique structures.37 We first tested [Bmim]+-based ionic liquids with different anions (Figure 3a). [Bmim]BF4 showed the best performance among the ILs examined; it accelerated the formation of HCOOH and prohibited the H2 evolution simultaneously. In contrast, [Bmim]NO3 and [Bmim]OAc mainly inhibited H2 evolution but did not promote the HCOOH yield. KBF4 and NaBF4 yielded the similar results (Figure 3b). The results indicated that both [Bmim]+ and BF4- were indispensable for the reaction because they are both essential for reducing the activation energy of CO2 by forming the weak complex of (Bmim-CO2-BF4).38,39 Further study demonstrated that the content of [Bmim]BF4 in the solution also affected the reaction (Figure 3c); 10 Vol% [Bmim]BF4 yielded the optimal result. After introducing 10 Vol% [Bmim]BF4 into MeCN, the H2 evolution rate decreased from 180 to 43 µmol·g-1·h-1 whereas the HCOOH formation rate sharply increased from 44 µmol·g-1·h-1 in pure MeCN to 108 µmol·g-1·h-1. Correspondingly, the faradaic efficiency of HCOOH increased from 19.6% to 71%. To


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explain this phenomenon, we studied the dependence of the conductivity of the solution and the photoluminescence (PL) intensity of (Mo-Bi)Sx/Meso CdS on the content of [Bmim]BF4. The conductivity of the solution increased with increasing the IL content, which followed a similar trend to the generation rate of HCOOH (Figure S6). In contrast, the PL intensity of (Mo-Bi)Sx/MesoCdS continuously declined with the increase of [Bmim]BF4 content (Figure S7). The results suggested that [Bmim]BF4 enhanced the charge carriers separation, and thus promoted the reaction.40

Figure 3. Dependence of the catalytic activity on the IL structure (a and b) and the content of the IL [Bmim]BF4 (c). Reaction conditions: (Mo-Bi)Sx/Meso CdS, 25 mg; visible-light (λ=420-780 nm, 0.3 W/cm2); MeCN, 20 mL; TEOA, 1 g; time, 10 h; illuminated area, 5 cm2; CO2 pressure, 3 MPa; temperature, 25 oC.

As illustrated in Figure 4a, an increase in light intensity led to a proportional enhancement of HCOOH generation. The formation rate of HCOOH reached a maximum of 208 µmol·g-1·h-1 with a 72% faradaic efficiency (H2 evolution: 80 µmol·g-1·h-1) when the light intensity approached to 0.6 W/cm2.



Correspondingly, the conversion efficiency of solar energy to chemical energy was 0.017%, calculated by dividing the combustion heat of formic acid and hydrogen produced by the integrated energy of the irradiation.11 The reduction of CO2 to HCOOH did not occur without irradiation. Furthermore, the performance of (Mo-Bi)Sx/Meso CdS was consistent with its absorption behavior (Figure 4b). These results confirmed that the reaction proceeded via photocatalysis. Meanwhile, Figure 4c showed that CO2 reduction occurred at an atmospheric pressure, and the HCOOH formation rate further increased with increasing CO2 pressure because high pressure favored its absorption and activation on the catalyst.41 However, at higher pressure (>3 MPa), the rate was less sensitive to CO2 pressure. Furthermore, HCOOH was not produced without CO2. Isotopic 13CO2 was used to confirm the carbon source of HCOOH in the reaction. The product was identified by

13

C NMR spectroscopy and all of the HCOOH was H13COOH

whereas no corresponding signals were detected from the reaction with 12CO2 (Figure S8), indicating that HCOOH originated from CO2. It is worth mentioning that HCOOH was the exclusive carbonaceous product from CO2 reduction in above experiments. The reusability experiments indicated that (Mo-Bi)Sx/Meso CdS showed mild but not obvious decrease in activity after four cycles (Figure S9). TEM (Figure S10) examinations illustrated that the morphology of (Mo-Bi)Sx/Meso CdS did not change notably before and after reusability experiments. XPS (Figure S11) spectrum showed that the oxidation states exhibited slight change after recycling experiments, which might be due to the photocorrosion of the support.42,43


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Figure 4. Dependence of the catalytic activity of (Mo-Bi)Sx/Meso CdS for the selective generation of HCOOH on light intensity (a), irradiation wavelength (b), and CO2 pressure (c). Reaction conditions: catalyst, 25 mg; MeCN, 20 mL; [Bmim]BF4, 2 mL; TEOA, 1 g; time, 10 h; illuminated area, 5 cm2; temperature, 25 oC, visible light(λ=420-780 nm, 0.3 W/cm2) except for (a).

Based on the experimental results and the related knowledge in literatures,44-46 a possible mechanism for visible-light-driven production of HCOOH from CO2 over (Mo-Bi)Sx/MesoCdS in the presence of [Bmim]BF4 is proposed (Scheme 1). Upon light irradiation, the electron-hole pairs are firstly generated in CdS. The TEOA is consumed to eliminate the positive holes and release the protons. The photogenerated electrons migrate to MoSx and react with H+ to form the active hydrogen. A weak complex (Bmim-CO2-BF4) is formed between CO2 and IL and is subsequently reduced. Mo sites bind with the intermediate COOH*, favoring the formation of HCOOH. Meanwhile, Bi species further catalyze CO2 reduction to HCOOH by subtracting the active hydrogen from MoSx. In brief, the components of (Mo-Bi)Sx/Meso CdS work synergistically for producing HCOOH. However, the positive holes can



induce photocorrosion of CdS, resulting in a slight decreasing activity during the process.

Scheme 1. Possible mechanism for visible-light-driven selective production of HCOOH from CO2 reduction over (Mo-Bi)Sx/Meso CdS in the presence of [Bmim]BF4.

CONCLUSIONS In summary, we have discovered that inexpensive Mo-Bi-Cd ternary metal chalcogenides have outstanding performance for the conversion of CO2 into HCOOH under visible light. The efficiency of the reaction depends on the composition of the chalcogenides, solvents, CO2 pressure, and light intensity. In [Bmim]BF4-MeCN, HCOOH was the only carbonaceous product, and the formation rate of HCOOH reached 208 µmol·g-1·h-1 with a 72% faradaic efficiency. The components in the (Mo-Bi)Sx/Meso CdS also have excellent synergistic effect on the conversion of CO2 into HCOOH. [Bmim]BF4 contributed to improving the CO2 reduction by reducing the activation energy of CO2, acting as an electron shuttle to favor charge carrier separation and inhibit hydrogen evolution. The catalyst and [Bmim]BF4 cooperated well in promoting the reaction. This work presents a new low-cost and efficient catalytic system for the transformation of CO2 into value-added chemicals using solar energy.


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ASSOCIATED CONTENT Supporting Information Experimental Section, TEM and SEM images of (Mo-Bi)Sx/Meso CdS (Figure S1), X-ray diffraction spectral of (Mo-Bi)Sx/Meso CdS (Figure S2), Diffuse reflectance UV-Vis spectra of the different photocatalysts (Figure S3), Effect of the Mo to Bi molar ratio on the HCOOH formation (Figure S4), High resolution XPS of the bare Meso CdS and (Mo-Bi)Sx/Meso CdS (Figure S5), Dependence of the HCOOH formation rate and the solution conductivity on the [Bmim]BF4 volume ratio in the solution (Figure S6), Dependence of the PL intensity of (Mo-Bi)Sx/Meso CdS on the volume ratio of [Bmim]BF4 in the solution (Figure S7),

13

C NMR spectra for the obtained products under CO2 and

13

isotopic CO2 atmosphere (Figure S8), The reusability of (Mo-Bi)Sx/Meso CdS in photocatalytic production of HCOOH from CO2 (Figure S9), XPS spectra of the fresh (Mo-Bi)Sx/Meso CdS and the reused (Mo-Bi)Sx/Meso CdS (Figure S10), TEM images of the fresh (Mo-Bi)Sx/Meso CdS (a) and the recovered (Mo-Bi)Sx/Meso CdS after four cycles reuse (b) (Figure S11). These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mails: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank National Natural Science Foundation of China (21533011, 21673249), and Chinese Academy of Sciences (QYZDY-SSW-SLH013).

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Mo-Bi-Cd Ternary metal chalcogenides was very active for visible-light-driven of CO2 to HCOOH in [Bmim]BF4-MeCN mixed solvent.


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