Photocatalytic Hydrogen Evolution Coupled with Efficient Selective

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Photocatalytic Hydrogen Evolution Coupled with Efficient Selective Benzaldehyde Production from Benzyl Alcohol Aqueous Solution over ZnS-NixSy Composites Hongchang Hao, Ling Zhang, Wenzhong Wang, Simeng Qiao, and Xuechen Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01017 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Sustainable Chemistry & Engineering

Photocatalytic Hydrogen Evolution Coupled with Efficient Selective Benzaldehyde Production from Benzyl Alcohol Aqueous Solution over ZnS-NixSy Composites Hongchang Haoa,b, Ling Zhang*a,c, Wenzhong Wang*a,c, Simeng Qiao a and Xuechen Liu a a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China b

University of Chinese Academy of Sciences, No.19(A) Yuquan Road, Shijingshan

District, Beijing 100049, P. R. China c

Center of Materials Science and Optoelectronics Engineering, University of Chinese

Academy of Sciences, No.19(A) Yuquan Road, Shijingshan District, Beijing 100049, P. R. China

*Corresponding Authors: Ling Zhang, [email protected]; Wenzhong Wang, [email protected] KEYWORDS: selective oxidation, benzyl alcohol, H2 production, dehydrogenation, photocatalyst

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ABSTRACT: Photocatalytic hydroxy group transformation to carbonyl is an important application in organic chemistry. Whereas it seems challenging to acquire the great selectivity towards carbonyl products in green water solution, especially for the widebandgap semiconductors which always need to seek assist from organic media like acetonitrile. Herein, we cast off the dependence on such organic solvents and develop an efficient NixSy modification on ZnS nanorods, for selective oxidation of benzyl alcohols coupled with H2 evolution in anaerobic water. The ternary augments in H2 production, alcohol conversion and more strikingly, benzaldehyde selectivity (from 8.1% to 90.5%) were simultaneously achieved by loading NixSy co-catalysts. Mechanism investigations indicated a striking transformation from hole-predominant oxidation on ZnS surface to an electron-initiating dehydrogenation of alcohols on NixSy nanoparticles. Besides, NixSy nanoparticles tend to swiftly desorb the produced benzaldehyde, thereby resulting in promoted selectivity. Particularly, the catalysts performed the much greater benzyl alcohol transformation efficiency in corresponding aqueous solution than they did in acetonitrile, where little aldehyde remained. Overall, the study reveals the inspiring roles of NixSy as a selective dehydrogenation unit and a hydrogen evolution co-catalyst, therefore enabling ZnS in efficient selective benzyl alcohol transformation coupled with hydrogen evolution in green aqueous solution.


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Introduction The selective oxidation of alcohols into corresponding carbonyl compounds is a momentous approach in organic chemistry. Traditional procedures inevitably involve the use of strong oxidants, including chromate, permanganate, and hypervalent iodine, frequently followed by the substantial generation of waste water.1-2 In contrast, photocatalysis for selectively oxidizing alcohols has been invested with a string of attention, due to the characteristics of cleanness and viability of operating under mild conditions.3 Specifically, the photo-induced holes and diverse reactive oxygen species (•O2−/•HO2/H2O2/•OH/1O2) derived from green oxygen/water can serve as oxidants for driving the alcohols transformations. However, the photocatalytic functional group transformation is largely constrained by the tendency that carbonyl compounds are apt to be further decomposed by the plethoric oxidants, especially for nonselective •OH and robust holes. Therefore, researchers always attempt to selectively control either the kinds or participation degrees of various photo-induced active species, for shunning the undesired product mineralization and subsequent poor selectivity.4 Researchers frequently resort to the organic solvents such as acetonitrile (CH3CN) or benzotrifluoride (BTF) as reaction media, which efficiently inhibit the generation of •OH. Moreover, Zhang et.al established a connection between the valence band position of semiconductors and the selectivity of the photocatalytic benzyl alcohol oxidation in acetonitrile, highlighting the crucial role of photogenerated holes towards organic conversion and selectivity.5 Therefore visible light is always pressed into service in the heterogeneous



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photocatalytic systems where holes are endowed with moderate oxidizing ability, such as the applications over C3N4,6 Bi2MoO6,7 Bi12O17Cl2,5 WO3,8 CdS9-10 and so forth.11 Nevertheless, the great expense of organic solvents and potential harm on environment are inevitable and visible light-driven catalysts tend to exhibit the decrepit oxidation ability which followingly brings about sluggish transformation rate (Table 1). Therefore, developing a novel strategy for alcohol transformation with satisfying aldehyde productivity and constituent green media like water appears all the more appealing.

Table 1 Selected examples and this work of photocatalytic selective oxidation of benzyl alcohol to benzaldehyde. Photocatalyst

Light Source

Solvent

Atmosphere

Time [h]

Conv. [%]

Sel. [%]

HER Rate [mmol g-1 h-1]

Ref.

mpg-C3N4

λ > 420 nm

C6H5CF3 (80 °C )

O2 (8 bar)

3

57

> 99

a

6

g-C3N4

λ > 420 nm

CH3CN (50 °C)

O2

8

22.1

> 99

a

5

Bi3O4Br

λ > 420 nm

CH3CN (50 °C)

O2

8

36.0

> 99

a

5

Bi12O17Cl12

λ > 420 nm

CH3CN (50 °C)

O2

8

44

> 99

a

5

Bi2MoO6

λ > 400 nm

C6H5CF3 (25 °C)

O2

4

26.5

> 99

a

7

7% BN/In2S3

λ > 420 nm

C6H5CF3 (25 °C)

O2

3

60

99

a

12

0.3rGO−C3N3S3

λ > 420 nm

C6H5CF3 (25 °C)

O2

4

51.5

100

a

13

CdS/graphene

λ > 420 nm

C6H5CF3 (25 °C)

O2

4

45

90

a

9

HNb3O8 nanosheet

λ > 400 nm

C6H5CF3 (25 °C)

O2

4

20

> 99

a

14

WO3(7.6)-TiO2

λ > 350 nm

H2O (25 °C)

O2

5

50

56

a

15

TiO2 (P25)

λ = 365 nm

CH3CN (30 °C)

Air

4

> 99

32

a

16

TiO2 (P25)

λ = 365 nm

H2O (30 °C)

Air

4

> 99

8

a

16

WO3-PdOx

300 < λ < 500 nm

H2O (5 °C)

Air

3

30.6

63.1

a

17

WO3-Pt

300 < λ < 500 nm

H2O (5 °C)

Air

3

55.8

55.7

a

17

TiO2-Pd

300 < λ < 500 nm

H2O (5 °C)

Air

3

84.7

5.1

a

17


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TiO2-Pt

300 < λ < 500 nm

H2O (5 °C)

Air

3

96.7

3.7

a

17

Ni-CdS

λ > 420 nm

CH3CN (20 °C)

Ar

20

96

96

a

18

Co-CdS

λ > 420 nm

CH3CN

N2

3

a

93.9

8.940

19

Pt-g-C3N4

λ > 400 nm

H2O

Ar

20

40

90

0.255

20

ZnS

λ > 200 nm

H2O (25 °C)

N2

3

21.8

8.1

0.712

ZnS-NixSy (2)

λ > 200 nm

H2O (25 °C)

N2

3

49.3

80.4

3.648

ZnS-NixSy (3)

λ > 200 nm

H2O (25 °C)

N2

3

42.1

90.5

2.943

a

Not measured or mentioned Since the transformation from alcohols to corresponding aldehydes substantially

represents as a dehydrogenation process, it seems valid to annex a functional dehydrogenation unit on photocatalysts surface for assisting selective oxidation, meanwhile palliating the over-dependence on photo-induced holes and reactive oxygen species which always uncontrollably engage in the overoxidation. Moreover, such a functional unit can further upgrade the overall products value from biomass by simultaneously reducing protons into hydrogen in anaerobic environment. For instance, Dongsheng et.al firstly reported a Ni-Modified CdS for highly-selective alcohols split into the corresponding carbonyl compounds and hydrogen in acetonitrile.18 Among the catalysts, Ni nanoparticle simultaneously served as a hydrogen evolution unit and dehydrogenation unit that contributes to the high aldehyde selectivity. Besides the metallic Ni specie, nickel sulfide (NiS, Ni3S2, etc.) are such kinds of dual-functional units,21-23 therefore potentially feasible to promote the H2 yield and dehydrogenative efficiency of alcohols, with hole-induced overoxidation evaded in anaerobic media. In order to apply the dual-function cocatalysts extensively to green water medium, herein we integrated zinc sulfide (ZnS) nanorods with nickel sulfide (NixSy)


This work


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nanoparticles for selective oxidation of benzyl alcohols coupled with H2 evolution in anaerobic water. As ZnS is a typical wide-bandgap catalyst and only excited by ultraviolet, the photo-induced holes are correspondingly endowed with robust oxidizing ability, therefore exhibiting poor aldehyde selectivity. In contrast to alone ZnS, however, ZnS/NixSy composites were granted with the ternary augments in H2 yield, alcohol conversion and more strikingly, benzaldehyde selectivity (from 8.1% to 90.5%). Experimental results demonstrated that the transformation from holepredominant

oxidation

mechanism

over

ZnS

alone

to

electron-initiating

dehydrogenation of alcohols on NixSy was responsible for the great product selectivity over composites. Particularly, when comparing the photocatalytic performances of ZnS-NixSy in aqueous and CH3CN mediums, we gained a superior efficiency in water which was inferred to be attributed by the better benzaldehyde desorption capacity correspondingly. This is the first time ultraviolet-absorbing semiconductor is applied in the efficient selective oxidation of benzyl alcohols and concerted H2 evolution in green water (Table 1). Experimental Materials Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), thiourea (NH2CSNH2), ethylenediamine (C2H4(NH2)2,

en),

nickel

acetate

tetrahydrate

(NiAc2·4H2O),

thioacetamide

(CH3CSNH2), benzyl alcohol (C7H8O), benzaldehyde (C7H6O), absolute ethanol (C2H6O), acetonitrile (C2H3N), tetrachloromethane (CCl4), methyl alcohol (CH4O), isopropanol (C3H8O) ； terephthalic acid (C8H6O4), sodium hydroxide (NaOH) were


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purchased from Sinopharm chemical reagent Co., Ltd. (Shanghai, China). The deionized water was obtained from local sources. Preparation of ZnS nanorods Hydrothermal method was employed to synthesis ZnS nanorods, with Zn(NO3)2·6H2O and NH2CSNH2 as zinc and sulfur source, respectively.24 Specifically, 1 mM of Zn(NO3)2·6H2O and 5 mM NH2CSNH2 were dispersed in 35 mL of mixture solution of ethylenediamine (C2H4(NH2)2, en) and deionized water in 1:1 volume ratio. The fully stirred solution was placed in a 50 mL Teflon-lined stainless steel autoclave and maintained at 200 °C for 12 h. After natural cooling to room temperature, the inorganic products were washed with ethanol and deionized water for several times, finally dried in air at 60 °C. Preparation of ZnS-NixSy composites NixSy nanoparticles were loaded on ZnS nanorods with an adaptive method reported in the literature.25 In a typical synthesis, 200 mg of ZnS and 51.2 mg of thioacetamide (CH3CSNH2) were fully dissolved in 50 mL of absolute ethanol. Then 10 mL of ethanol in which 16.9 mg of nickel acetate tetrahydrate (NiAc2·4H2O, account for 2 wt.% Ni content of ZnS) were pre-dissolved were dropwise injected into the aforementioned solution. Subsequently, the mixture was transferred to a 100 mL three-necked flask and degassed with N2, followed by heat at 80 °C in oil bath and continuous magnetic stirring for 2 hours. After cooling down to room temperature naturally, the products were collected and repeatedly washed with ethanol and deionized water for 3 times. The final products were dried in vacuum drying oven at 60 °C, and labeled as ZnS-NixSy (2). A range of various loading amounts were achieved by using different amounts of nickel acetate (8.5, 25.4 mg) and thioacetamide (25.6, 76.8 mg), respectively.



Correspondingly, the obtained samples were labelled as ZnS-NixSy (1) and ZnS-NixSy (3), based on nickel content. Characterization In order to analysis the crystalline structure, X-ray powder diffraction (XRD) was recorded on a Rigaku powder diffractometer using Cu Kα radiation. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of the samples were collected on transmission electron microscopy (TEM, FEI Tecnai G2 F30). UV–vis diffuse reflectance spectroscopy (DRS) measurements were carried out using a Hitachi U-3010 spectrophotometer equipped with an integrating sphere, by using BaSO4 as the reflectance standard. The prepared samples were pressed onto X-ray photoelectron spectroscopy (XPS) analysis (Thermo Scientific Escalab 250) equipped with monochromated aluminum Kα X-rays at 1253.6 eV under ultrahigh-vacuum conditions, to record the corresponding chemical element states. The C 1s signal was used to correct the charge effects. Zeta potential experiments were recorded on Zeta Potential Analyzer (Zetaplus, Brookhaven, USA). The photoelectrochemical experiments were performed in a three-electrode cell with 0.1 M of Na2SO4 solution (pH 6.8) as electrolyte, and recorded with CHI660D electrochemical workstation (Shanghai Chenhua, China). Among the quartz cell, the saturated calomel electrode (SCE) and platinum wire served as reference electrode and counter electrode, respectively. While the working electrodes were prepared by uniformly coating catalysts powders on a 15 × 25 mm fluorine-doped tin oxide (FTO) glass electrode, followed by the calcination at 120 °C for 1 h, in an Ar-filling tube furnace. The current-time curves were collected at open-circuit voltages under the illumination of 500-W Xe lamp.


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Photocatalytic performances tests The photocatalytic reactions were performed in a sealed vitreous reactor (capacity: 600 mL) with a quartz window, and a double-walled jacket. Typically, 20 mg of catalyst was dispersed in 100 mL of 5 mM benzyl alcohol aqueous solution, via 5-minute ultrasonication. Then the reactor was purged with N2 to dissipate the inside air. After adsorption equilibrium in dark, the suspension was illuminated by a 500-W Xe lamp (λ > 200 nm) for 3 h. The temperature of reaction systems was sustained at 25 °C with circulating water. The generated H2 amount was quantified using an online gas chromatography (Tianmei, GC-7890, TCD, N2 carrier) with thermal conductivity detector (TCD). When reaction finished, the solution was filtered away from solid catalysts, and then injected into a high-performance liquid chromatography (HPLC, C18 column, injection volume: 20 μL, water: acetonitrile=30:70, column temperature: 30 °C, flow rate: 0.6 mL/min, detection wavelength: 254 nm) for alcohol and aldehyde amounts quantification. The conversion and selectivity efficiency were calculated according to Eqs. (1) and (2)

𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) =

𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 (%) =

𝑛𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑏𝑒𝑛𝑧𝑦𝑙 𝑎𝑙𝑐𝑜ℎ𝑜𝑙 ― 𝑛𝑟𝑒𝑠𝑖𝑑𝑢𝑎𝑙 𝑏𝑒𝑛𝑧𝑦𝑙 𝑎𝑙𝑐𝑜ℎ𝑜𝑙 𝑛𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑏𝑒𝑛𝑧𝑦𝑙 𝑎𝑙𝑐𝑜ℎ𝑜𝑙

𝑛𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑏𝑒𝑛𝑧𝑎𝑙𝑑𝑒ℎ𝑦𝑑𝑒 𝑛𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑏𝑒𝑛𝑧𝑦𝑙 𝑎𝑙𝑐𝑜ℎ𝑜𝑙 ― 𝑛𝑟𝑒𝑠𝑖𝑑𝑢𝑎𝑙 𝑏𝑒𝑛𝑧𝑦𝑙 𝑎𝑙𝑐𝑜ℎ𝑜𝑙

× 100 % Eq. (1)

× 100 % Eq. (2)

Analysis of photocatalysis mechanism A range of active species trapping experiments were conducted, with carbon tetrachloride (CCl4), methanol (MA), isopropyl alcohol (IPA) as electrons, holes and hydroxide radicals trapping agents, respectively. Moreover, control experiments in various media (pure H2O, 5 mM benzyl alcohol CH3CN solution and 5 mM



benzaldehyde aqueous solution) were also performed to further investigate the catalysis mechanism, while keeping other parameters constant. The adsorption capacities of catalysts for benzyl alcohol and benzaldehyde were recorded by dispersing 50 mg of catalysts in corresponding aqueous solutions (5 mM, in the sealed vitreous reactor) and stirring for 0.5 h. Then test the substrates concentration of the solutions after adsorption, for getting the adsorption amounts on catalysts surface. The photoluminescence (PL) technique was employed to detect hydroxyl radicals formation during the irradiation, by using terephthalic acid (TA) as a probe molecule, which are prone to react with •OH and then exhibit high fluorescent.26 The experiment was carried out with the same procedure as photocatalytic process, except the replacement of benzyl alcohol solution with 5 × 10-4 M terephthalic acid solution in 2 × 10-3 M NaOH. Then the filtered solution was placed into the Hitachi F-4500 spectrophotometer and excited at 315 nm for recording the fluorescence spectra.

Results and discussion Characterization of ZnS and ZnS-NixSy catalysts In order to analysis the crystalline structure, Figure S1 displays the XRD patterns of the as-prepared ZnS and ZnS-NixSy composites with different amounts of nickel. The diffraction peaks of ZnS nanorods are indexed into the pure wurtzite ZnS. Moreover, the greatly protruding peak located in 28.5° is assigned to its (002) face and indicates a preferential growth direction of the (002) plane. The XRD patterns of ZnS-NixSy composites display the analogous profiles to bare ZnS whereas invisible peaks corresponding to nickel sulfide, which may be ascribed to the low loading amounts or high dispersion of nanoparticles.


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Figure 1. TEM images of (a) ZnS nanorods and (b) ZnS-NixSy (3) composite. HRTEM images of (c) ZnS-NixSy (3) composite and (d) NixSy nanoparticles.

Transmission electron microscope (TEM) images of the ZnS and ZnS-NixSy (3) are provided in Figure 1a and Figure 1b, respectively, which indicate ZnS nanorods have general diameters in the range of 10-20 nm. The high-resolution TEM (HRTEM) images of ZnS-NixSy (3) confirm the formation of the intimate contact between NixSy and ZnS lattices (Figure 1 c and Figure 1d). The measured lattice distances of ZnS nanorods are 0.30 and 0.28 nm, which belong to (002) and (101) faces of wurtzite form and match well with XRD profiles. Moreover, hexagonal NiS nanoparticles are identified from the interplanar lattice spacings (d) of 0.18 nm and 0.30 nm which correspond to (110) and (100) planes, as well as the measured angel which is close to theoretical value of the angle (30°) between them. The deposition of NiS nanoparticles can be further proofed by a Ni 2p3/2 peak at 853.3 eV and the corresponding satellite



peak around 861.0 eV in high-resolution XPS spectrum of Ni 2p (Figure 2), which is in good agreement with the previously reported values for NiS.23 Besides, an another main peak centered at 855.6 eV suggests the different chemical states of Ni, which may be attributed to other possible nickel sulfide like Ni3S2, that can serve as H2 evolution co-catalyst as well.27-28 The overall XPS spectrum of ZnS-NixSy (3) is shown in Figure S2.

Figure 2. The high-resolution Ni 2p XPS spectrum of ZnS-NixSy (3).

The diffuse reflectance UV/Vis spectra of ZnS (Figure S3a) reveals the ultraviolet-absorbing character of it. Corresponding bandgap was determined to be 3.4 eV based on Kubelka-Munk function (Figure S3b). Furthermore, the conduction band (CB) edge position was estimated to be - 0.94 eV, according to the formula 𝐸𝐶𝐵 = 𝜒 ― 𝐸𝑒 ― 1 2𝐸𝑔, where 𝜒 and 𝐸𝑒 represent as the electronegativity (5.27 eV) of ZnS and free electrons energy on hydrogen scale (4.5 eV), respectively.29-30 Consequently, the valence band (VB) edge was deduced to be located around 2.46 eV, indicating such a


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robust oxidizing ability that grants holes the gorgeous potential for propelling benzyl alcohol oxidation, but the wretched aldehyde selectivity which requires strategical amelioration. After NixSy loading, the catalysts strongly absorbed the light in visible and near-infrared regions, and the energy gaps of ZnS were narrowed to some degrees (Figure S3b), but still not enough to the visible region. Therefore, the photocatalytic processes over ZnS and composites would be still motivated by ultraviolet light. The photon-to-electron conversion efficiencies of various catalysts were evaluated via recording the transient current–time (I-t) curves (Figure S4). Low loading amount of NixSy could increase photocurrent which means the superior charge separation and transfer efficiency to pristine ZnS nanorods, matching the metallic character and outstanding electronic conductivity of some nickel sulfide (Ni3S2).31-32 Whereas things went athwart when ZnS was excessively equipped with NixSy nanoparticles, reflected by the weaker photocurrent response over ZnS-NixSy (2) which may be due to the photon scattering effect derived from NixSy nanoparticles.33-35 Dehydrogenation performance in benzyl alcohol aqueous solution Photocatalytic performances were performed in 5 mM benzyl alcohol-contained water at 25 °C using a Xe-lamp as light source (Table S1, entries 1-4). The conversion of alcohol and corresponding selectivity towards benzaldehyde after 3-hour photocatalysis over various catalysts were diagrammed in Figure 3a, which showed a conversion of 21.8% but a poor product selectivity of only 8.1% over ZnS catalyst alone. In contrast, all of ZnS-NixSy samples exhibited the promoted substrate conversion efficiency and more strikingly, the progressively upward product bias to benzaldehyde as NixSy loading amount augmented. Specifically, ZnS-NixSy (3) performed an optimized aldehyde selectivity valued of 90.5%, which was 11-fold higher than that over ZnS,



despite a slightly inferior reactant conversion (42.1%) to two other composites (54.7% and 49.3%, respectively) derived from the excessive loading. The TON and TOF for benzyl alcohol dehydrogenation (3 h) were calculated to be 21.2 mol benzyl alcohol mol-1 Ni, and 7.1 mol benzyl alcohol mol-1 Ni h-1, respectively. For benzaldehyde production, the corresponding values were 19.2 mol benzaldehyde mol-1 Ni and 6.4 mol benzaldehyde mol-1 Ni h-1, respectively. After 3-hour catalysis in benzyl alcohol aqueous solution, the characteristic peaks assigned to NiS and Ni3S2 were still identifiable in Ni 2p XPS spectrum (Figure S5), indicating the steady chemical states of Ni during catalysis.

Figure 3. (a) Benzyl alcohol conversion efficiencies over various catalysts after 3-hour Xe-lamp irradiation in 5 mM benzyl alcohol aqueous solution. (b) Time-dependent photocatalytic H2 production over various catalysts under Xe-lamp irradiation in 5 mM benzyl alcohol aqueous solution.


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Moreover, as shown in Figure 3b, light-driven H2 production was also facilitated arising from the NixSy loading, which reached the highest H2 amount (218.9 umol) over ZnS-NixSy (2), 5 times as much as it did over ZnS. And due to the highly promoted selectivity towards benzaldehyde, the H2/aldehyde ratio was reduced from 4.8 over ZnS to 0.9 over ZnS-NixSy (3). Overall, the results revealed the ternary effects of NixSy nanoparticles in augmenting H2 yield, alcohol conversion and especially benzaldehyde selectivity during the photocatalysis. Besides, despite the increased light absorption in visible and near-infrared regions, the void activity of ZnS-NixSy (2) under visible-light irradiation (Table S1, entry 5) revealed the only motivation by ultraviolet light during photocatalysis. However in previous reports, ultraviolet-driven selective benzyl alcohol transformation over such kinds of wide-bandgap catalysts was vastly dependent on organic solvent for retaining the preferential selectivity towards aldehyde.16,

36

Water were found to have a

detrimental effect, such as the report that reaction efficiency over Pt-P25 drastically decreased in the presence of only a small amount (1%) of water dissolved in CH3CN and even completely faded away when 10% of water was employed.36 Herein, the great benzaldehyde selectivity over ZnS-NixSy (3), reveals a vital impact granted by NixSy which could assist in averting the drastic oxidation of produced aldehyde, though in an ultraviolet-driven aqueous system.



Figure 4. Durability of ZnS-NixSy (2) in photocatalytic benzyl alcohol conversion and H2 production under Xe-lamp irradiation for 3 h*4 runs in aqueous solution.

Then we examined the durability of ZnS-NixSy (2) by using it repeatedly for 4 times in photocatalyzing benzyl alcohol aqueous solution. Figure 4 shows that the composite was reusable without significant loss of activity. Photocatalytic hydrogen production from pure water Photocatalytic performances of ZnS and ZnS-NixSy (2) in pure water (Table S1, entries 9, 10) were also recorded. Despite the inferior photon-to-electron performance to ZnS, ZnS-NixSy (2) still produced more H2 which was approximately valued of 625.0 µmol h-1 g-1, therefore showing the validity of NixSy as H2 evolution co-catalysts. Nontheless, both performances in water represented the comparatively overshadowed H2 outputs than in benzyl alcohol solution. The results were possibly attributed by the absent contribution from alcohol, for affording extra protons or trapping photo-induced holes as a scavenger agent. Dehydrogenation performance in benzyl alcohol acetonitrile solution Switching reaction medium into benzyl alcohol acetonitrile solution contributed to the deteriorated photocatalytic performances of both ZnS and ZnS-NixSy (2) (Table S1, entries 7, 8), which were dually reflected in the aldehyde selectivity as well as H2


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production. At first, the meagre benzaldehyde selectivity in acetonitrile suggested an unfavourable desorption of aldehyde on catalysts surface, which involved the yielded benzaldehyde in being further decomposed, thus barely accumulating in the organic solution. As for H2 evolution half-reaction, the mediocre yields may be ascribed to the constraint that benzyl alcohol in acetonitrile solution served as the exclusive protons donor and gave skimpy protons concentration. But analogous to the cases in pure water and alcohol aqueous solution, ZnS-NixSy (2) still yielded much more H2 (79.2 µmol) than ZnS alone (0.4 µmol) in the absence of water as a second protons supplier. Not only did the result verified the potency of NixSy as HER co-catalysts again, it further enlightened us a strong interplay between alcohol protons and NixSy. Similar to the mechanisms of NixSy co-catalysts in photocatalysis and electrolysis that such sulfides favours both electron transfer (NixSy + H + + e - →HNixSy) and consequent desorption (HNixSy + H + + e - →NixSy + H2) for H2 evolution, it is proposed that NixSy cocatalysts are also conducive to the activation of benzyl alcohol by readily forming NixSy-H hydride species which are prone to generate H2. The process is substantially a NixSy-induced alcohol dehydrogenation course, akin to previously reported Ni-CdS in acetonitrile.18 Photocatalytic mechanisms investigation Due to the inconsistence between catalytic activities and corresponding photon-toelectron performances of prepared catalysts, it could be consequently ruled out that the tuned photocarriers separation process bore a dominant impact on elevating alcohol conversion efficiency. Then we carried out a series of experiments otherwise, for understanding the benzyl alcohol reaction pathways, and figuring out the mechanism about synchronously promoted conversion and selectivity over ZnS-NixSy samples.



At first, we tested the absorbed amounts of benzyl alcohol on ZnS and ZnS-NixSy (2), but both got the negligible values which suggest the poor adsorption capacity of ZnS for benzyl alcohol. Then we measured the zeta potential, and obtained the values of 14.8 mV and 8.7 mV for ZnS and ZnS-NixSy (2), respectively (Figure S6). The negatively shifted zeta potential of ZnS-NixSy (2) suggests a slightly increased affinity to benzyl alcohol, but may be still not favorable enough for benzyl alcohol conversion.8 Therefore, we focused on the dehydrogenation process of benzyl alcohol under illumination, which generally involves several kinds of active species. Hydroxyl radicals detection experiments (Figure S7) showed the inconspicuous difference between ZnS and ZnS-NixSy (2) in amount of hydroxyl radicals, which were therefore conjectured to bear little association with corresponding organic transformation process, let alone obvious conversion/selectivity discrepancies. The minimal contribution of hydroxyl radicals was further certified by corresponding trapping experiments (Figure 5), as the alcohol conversions both kept almost constant after capturing hydroxyl radicals with isopropanol.

Figure 5. The conversion of benzyl alcohol over ZnS and ZnS-NixSy (2) by adding different scavengers under Xe-lamp in water.


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Consequently, methanol and CCl4 were employed as well to distinguish the roles of other active species (Figure 5). When methanol were added for irreversibly consuming holes, the conversion of benzyl alcohols over ZnS and ZnS-NixSy (2) both decreased, revealing that photogenerated holes could drive the oxidation process to benzaldehyde, which could also account for the superior H2 yield in alcohol aqueous solution to pure water. However, the addition of electron scavenger CCl4 drove the substrate conversion evolved into different trends. For ZnS, the slight variation implied a feeble photoelectrons participation in selective transformation, and therefore an analogous mechanism to the reported catalysts like Bi12O17Cl2,5 Au-CeO237, carbon nitride38 and Co-CdS19 that holes are major oxidative species for selective oxidation. Specifically, benzyl alcohol molecule can be activated via spontaneously dissociating into alkoxide anion.5 The activated anion can react with the photogenerated hole to form the further deprotonated carbon radical or oxygen one, which is oxidized by hole into benzaldehyde afterwards. And the shed protons from alcohol and intrinsic ones of water are reduced to H2. However, the hole-predominant dehydrogenation on ZnS surface makes the adsorbed benzaldehyde under the great risk of further attack by ultravioletinduced holes with high energy, therefore worsening the selectivity. Nevertheless, ZnS-NixSy (2) performed a drastically degressive alcohol conversion after electrons elimination, highlighting the roles of electrons in transformation process. The significant dependence on electrons is well consistent with the aforementioned tendency that benzyl alcohol is apt to be dehydrogenated on NixSy surface under light illumination, following by the formation of NixSy-H hydride species. Therefore, a NixSy/electron-dedicating dehydrogenation mechanism is conceived to account for the high organic conversion and selectivity over NixSy-loading ZnS



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samples, shown in Scheme 1. When photocatalyst is illuminated, a photo-induced electron can readily transfer to NixSy and subsequently induce the dehydrogenation of OH group due to the strong adhesion of protons to NixSy, for affording NixSy-H specie, as well as the alkoxide anion which is still attached to NixSy with NixSy-H-C bond. Afterwards, a hole located at nearby ZnS surface oxidizes the alkoxide anion, accompanied with the homolytic cleavage of α C−H on NixSy, thereby forming benzaldehyde and an another NixSy-H hydride which finally evolves into H2 with the adjacent

NixSy-H

hydride.

The

facilitated

activation

process

induced

by

dehydrogenation unit and photogenerated electrons contribute to the increased alcohol conversion over composites.

Scheme 1. Proposed reaction mechanisms for the photocatalytic selective oxidation of benzyl alcohol and H2 evolution over ZnS-NixSy composites in aqueous solution.


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Moreover, we compared the benzaldehyde adsorption characteristics over ZnS and ZnS-NixSy (2), finding that ZnS-NixSy adsorbed less benzaldehyde, and it could leave more aldehyde remnants (73.9%) than ZnS (59.5%) after catalysing in corresponding aqueous solution for 3 hours (Figure S8). The results both indicate the lower adsorption/oxidation capacities of composite for aldehyde, which were derived from the less exposed ZnS surface, as well as the absence of OH group in benzaldehyde, which excludes aldehyde from being further dehydrogenated through the same route as alcohol on NixSy. Overall, since NixSy-ZnS interface owns the higher priority for dehydrogenating benzyl alcohol, and great trait of swiftly desorbing benzaldehyde, the aldehyde selectivity over composites could progressively grow with the increase of NixSy loading amount, although excessive modification attenuated the photon-toelectron conversion efficiency which was responsible for the inferior alcohol conversion and H2 yield over ZnS-NixSy (3).

Conclusions In summary, we present here the ternary augments in H2 production, alcohol conversion and more strikingly, benzaldehyde selectivity (from 8.1 % to 90.5 %) after loading NixSy nanoparticles on ZnS nanorods, thus achieving the H2 evolution coupled with efficient selective benzaldehyde production from benzyl alcohol aqueous solution under the ultraviolet light motivation. Hydroxyl radicals detection and a range of active species trapping experiments evidenced a mechanism transformation from holepredominant oxidation on ZnS surface to an electron-initiating dehydrogenation of alcohols on NixSy nanoparticles which have the great abilities of pulling H atoms of alcohol and desorbing yielded benzaldehyde. Moreover, the ZnS-NixSy composites



exhibit a superior transformation efficiency in benzyl alcohol aqueous solution to the corresponding acetonitrile system. This study reveals the feasibility of employing NixSy nanoparticles as both hydrogen evolution and dehydrogenation units, to ameliorate the performance of wide-bandgap catalysts in selective benzyl alcohol transformation coupled with hydrogen evolution in green aqueous solution.


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ASSOCIATED CONTENT Supporting Information. X-ray powder diffraction patterns of pristine ZnS and the prepared ZnSNixSy samples; Overall XPS spectrum of ZnS-NixSy (3); UV/Vis diffuse reflectance spectra of ZnS and ZnS-NixSy samples; I–t curves of ZnS and ZnS-NixSy electrodes; Photocatalytic performances of ZnS and ZnS-NixSy samples in various catalytic systems; Ni 2p XPS spectrum of ZnS-NixSy (3) before and after 3-hour catalysis; Zeta potentials of ZnS and ZnS-NixSy (2); Fluorescence spectral of terephthalic acid in NaOH solution over ZnS and ZnS-NixSy (2) under Xe-light for 3 hours, The desorption behaviors of ZnS and ZnS-NixSy (2) for benzaldehyde.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. L. Zhang. * Email: [email protected]. W. Z. Wang. Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the financial support from the National Natural Science Foundation of China (51772312, 51472260).


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Table of Content

Synopsis: NixSy-loaded ZnS nanorods achieve the H2 evolution coupled with efficient selective benzaldehyde production from benzyl alcohol aqueous solution via ultraviolet-light motivation.


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Photocatalytic Hydrogen Evolution Coupled with Efficient Selective

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