Nanoflower Bimetal CuInOS Oxysulfide Catalyst for the Reduction of

Research Article pubs.acs.org/journal/ascecg

Nanoflower Bimetal CuInOS Oxysulfide Catalyst for the Reduction of Cr(VI) in the Dark Xiaoyun Chen†,‡ and Dong-Hau Kuo*,† †

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Road, Taipei 10607, Taiwan ‡ College of Material Engineering, Fujian Agriculture and Forestry University, No. 15, Shangxiadian Road, Cangshan District, Fuzhou 350002, China S Supporting Information *

ABSTRACT: Bimetal CuInOS oxysulfide solid-solution catalysts were successfully synthesized by a feasible method. The reduction performance of CuInOS was investigated by the reduction of Cr(VI) aqueous solution in the dark without the light illumination and additional reagents. The results showed that CuInOS catalyst exhibited excellent reduction activity, with which the 100 mL Cr(VI) solution of 50 mg/L was completely reduced by 20 mg catalyst within 4 min. The CuInOS also performed relatively good stability and durability for the reusability tests of Cr(VI) reduction, in which 91.8% reduction was achieved in 4 min even after the sixth run. The end product of the Cr(VI) reduction was identified to be metallic Cr(0) with Cr3+ as an intermediate. CuInOS with extremely fast Cr(VI) reduction in the dark was well characterized for its bond and crystal structure, microstructure, composition, and physical properties. A reasonable Cr(VI) reduction mechanism was proposed. At last, CuInOS is recyclable and can be easily separated from solution due to its easy sedimentation. KEYWORDS: Bimetal oxysulfide, Catalysts, Cr(VI) reduction, Anion vacancy, Reduction mechanism

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INTRODUCTION Hexavalent chromium Cr(VI) is a highly toxic, carcinogenic, and nonbiodegradable pollutant. Most Cr(VI) sources have been discharged from leather tanning, chromium painting manufacturing, electroplating, chromate manufacturing, etc.1,2 Biological degradation will not eliminate hexavalent chromium, but its chemical states can be changed. Cr(VI) is a cause for the hazards of human health when it enters the food chain and the water supply system. Therefore, the World Health Organization strictly limits the concentration of Cr(VI) in drinking water to be less than 0.05 mg/L.3 In the environment, chromium mainly has two oxidation states, Cr(VI) and Cr(III), where Cr(VI) is considered to be more than 100 times as hazardous as Cr(III) for its enrichment and its potential to damage to DNA and RNA. Cr(VI) can be degraded to less harmful Cr(III) or unharmful Cr(0), but Cr(III) would be reoxidized back to Cr(VI) through certain natural processes.4−6 Except for chemical reduction and solvent extraction,7,8 traditional treatment for the Cr(VI) contaminated water was by means of adsorption of the porous materials such as activated carbon, SiO2, metal−organic frameworks (MOFs), biosorbents, etc.9−12 However, the adsorption of porous materials is slow, and the adsorption amount to fill the pores is limited. After reaching the adsorption saturation, porous materials need regeneration before reuse. Therefore, the adsorption technology based upon porous materials has a high cost. On the other hand, the chemical reduction and solvent extraction technology need the addition of © 2017 American Chemical Society

some other chemicals, so they will produce another pollution. Thus, many efforts are still devoted to developing special catalysts with high activity for chromium reduction.13,14 Recently, photocatalytic reduction has been demonstrated for the treatment of Cr(VI) pollution. Compared with the conventional reduction processes, the photocatalytic reduction of Cr(VI) over semiconductor materials has some significant advantages for its environmental friendliness without the need of another chemical.15,16 Many different photocatalysts such as TiO2, ZnO, ZnS, SnO2, and g-C3N4 have been studied for the reduction of Cr(VI). Due to the wide band gap values above 3.0 eV, these semiconductor photocatalysts can only be excited by ultraviolet light, which makes it difficult for them to utilize solar energy.17,18 In order to efficiently harvest visible light, many efforts have been focused on extending the spectral response of such semiconductor catalysts to the visible region by doping with metals19,20 and nonmetal metals,21−23 or by coupling with narrow-band gap semiconductors such as Fe2O3 on TiO2,24 SnS on SnO2,25 SnS2 on TiO2,26 Bi2O3 on TiO2,27 WO3 on TiO2,28 CuO on ZnO,29 CdS on graphene,30 etc. However, the catalytic activities in those studies were not satisfactory, and their reduction reactions needed the extra photo energy input from the high power visible light source. To develop a catalyst with Received: January 11, 2017 Revised: March 4, 2017 Published: March 22, 2017 4133

DOI: 10.1021/acssuschemeng.7b00107 ACS Sustainable Chem. Eng. 2017, 5, 4133−4143

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ACS Sustainable Chemistry & Engineering

washing by just adding the Cr(VI) solution. HNO3 and NaOH were used to adjust the solution pH, and the solution pH was determined by a PHS-3C pH meter. The effects of different anions and the real wastewater on the reduction of Cr(VI) were also investigated.

high activity for Cr(VI) reduction in the dark without the input, thermal, photo, or electrical energy is a great challenge. In this paper, we synthesize the novel catalyst system of bimetallic CuInOS oxysulfide catalyst to enable the extremely fast Cr(VI) reduction in the dark without thermal, electrical, and photo energies and additional reagents. The CuInOS system displays a great promise for industrial applications in the field of Cr(VI) wastewater treatment due to its high reduction activity, fast reduction rate, no need of excitation light, easy preparation, and low cost.

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RESULTS AND DISCUSSION XRD Analysis. The XRD diffraction pattern of a representative CuInOS-2 is shown in Figure 1. All three CuInOS

EXPERIMENTAL METHODS

Synthesis of CuInOS. Under magnetic stirring, 1.5 g thioacetamide (CH3CSNH2) was added into a 500 mL solution containing cupric nitrate (Cu(NO3)2·2.5H2O) and indium(III) chloride (InCl3) precursor with the molar ratios of n(In):n(Cu) = 2:3, 3:3, and 4:3. After 30 min stirring, the mixture solution was heated to 90 °C with a heating rate of 2 °C/min, then added 0.15 mL hydrazine (N2H4). After further 2 h stirring, the precipitate solid was collected by centrifugation, washed with deionized water until pH = 7, then further washed with absolute ethanol two times. Finally, the solid was vacuum-dried at 90 °C for 24 h. The obtained three catalysts were correspondingly labeled as CuInOS-1, CuInOS-2, and CuInOS-3. For comparison, the CuOS was synthesized with the same procedure except for the InCl3. Characterizations of CuInOS. Surface composition, chemical state, and elemental content of the CuInOS catalysts were investigated by XPS (VG Scientific ESCALAB 250) photoelectron spectrometry under Al Kα X-rays (hv = 1486.6 eV) radiation and calibrated with carbon C1s (Ea = 284.62 eV). The crystal structure of samples was characterized using X-ray diffractometry (Bruker D2 phaser, Japan) under Cu Kα radiation. Particle size and morphology of CuInOS powders were examined by JSM-7610F field-emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM). The element mapping of catalysts was performed by scanning transmission electron microscopy (STEM, Tecnai G2 F20, Philips). N2 adsorption− desorption experiments were performed on ASAP2020 porosity and specific surface area analyzer with the sample degassed at 100 °C for 2 h before the test. Specific surface area was calculated according to Brunauer−Emmett−Teller (BET). Interphase development was studied on Nicolet-380 Fourier transform infrared spectrometer (FTIR) with samples embedded in a KBr pellet. The energy threshold structure and light absorption of samples were analyzed by JASCD V670 ultraviolet−visible spectrophotometer equipped with an integrating sphere using BaSO4 as a reference. The photoluminescence (PL) emission spectra was measured on JASCD FB-8500 fluorescence spectrophotometer with 330 nm emission wavelength at room temperature. Reduction of Cr(VI). Under magnetic stirring, 5−60 mg catalyst was added into the reactor which was filled with 100 mL Cr(VI) solution of 12.5−200 mg/L. The reactor was wrapped with aluminum foil in order to avoid photo excitation, which can make the explanations for the reduction reaction different. After reaction for 1 min, approximately 4 mL sample was taken out with a 0.45 μm membrane filter syringe to immediately separate catalysts from the solution. Using the diphenylcarbazide (DPC) colorimetric method31 with JASCD V-670 spectrophotometer and ion chromatography (IC) method with Thermo ICS-5000 spectrophotometer to determine the Cr(VI) concentration in filtrate. The reduction ratio of Cr(VI) was calculated using the following expression:

Figure 1. XRD pattern of CuInOS-2 catalyst.

showed the exactly the same XRD pattern. The XRD diffraction patterns of CuInOS-1 and CuInOS-3 are shown in Figure S1 in the Supporting Information (SI). Notice that the XRD peaks of CuInOS is well matched to the JCPDS No. 65-3561 of the hexagonal CuS covellite structure. The main peaks located at 28.17°, 29.55°, 31.98°, 32.80°, 48.26°, and 52.96° are attributed to the (101), (102), (103), (006), (110), and (206) crystal planes, respectively. The weak CuInOS peaks indicate that the crystallinity of CuInOS is not strong. The XRD diffraction pattern did not find the second phases, which is evidence for solid solution behavior. XPS Analysis. CuInOS-2 was chosen for the XPS analysis. Figure 2a shows the high resolution Cu2p XPS spectra of CuInOS. The asymmetric shape of the Cu2p peak can be related to the existence of different chemical states of Cu in CuInOS. The peaks of Cu2p3/2 and Cu2p1/2 located at 932.8 and 952.6 eV, respectively, with a separation of 19.8 eV indicates that copper belongs to the monovalent Cu+.32 The peaks of 2p3/2 and 2p1/2 located at 935.1 and 955.2 eV, respectively, are attributed to the spin−orbit splitting of the bivalent Cu2+.33,34 The peak positions at 943.8 and 963.5 eV corresponded to the Cu2p3/2 and Cu2p1/2 satellite peaks, respectively. According to the peak area, the molar contents of Cu+ and Cu2+ were calculated to be 71.6% and 28.4%, respectively. It is important to note that CuInOS has a much higher Cu+ content than Cu2+ in a CuS structure. Figure 2b shows the high resolution In3d XPS spectra of CuInOS. The spectrum exhibited two peaks located at 444.6 and 452.2 eV, which corresponded to In3+ 3d5/2 and 3d3/ 2, respectively. According to the previous report, the appearance of these peaks indicates that indium exists in the form of In3+.35,36 The Cu content was much higher than In. CuInOS was actually the Cu-based oxysulfide. Figure 2c shows the high resolution O1s XPS spectra of CuInOS. The O1s peak with asymmetric shape indicates that different chemical states of oxygen exist in CuInOS. Three convoluted peaks were observed at binding energy values of 531.5, 530.6, and 529.8 eV. It is assumed that the peak at 531.5 eV originates from the hydroxyl oxygen,37 that at 530.6 eV is from the In−O and monovalent Cu−O,38,39 and that at 529.8 eV is from the bivalent Cu−O.40 The higher monovalent

reduction ratio of Cr(VI) = (C0 − Ct )/C0 × 100% Three tests were conducted for each catalyst to obtain the average reduction ratio. Due to the small deviations for each test, the error bars in expressing data were small and omitted. To evaluate the reusability of CuInOS catalyst, 20 mg catalyst was added into 100 mL Cr(VI) solution of 50 mg/L to react for 4 min in the dark. After the first run, the supernatant solution was discarded by decantation and the remaining catalyst was reused for the next reduction reaction without further 4134


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Figure 2. High resolution (a) Cu2p, (b) In3d, (c) O1s, and (d) S2p XPS spectra of CuInOS.

Table 1. XPS Composition Analyses of CuOS and CuInOS Cu+/Cu2+ molar ratio

molar percentage catalyst CuOS CuInOS CuInOS sixth runs

O molar percentage

S molar percentage

Cu

In

O

S

Cu+

Cu2+

Cu molar percentage

O−H

Olattice

S6+

S2−

S6+/ S2− molar ratio

Olattice/S2− molar ratio

46.93 40.98 41.00

0 2.75 2.77

24.43 28.78 28.71

28.68 27.49 27.52

75.5 71.6 71.5

24.5 28.4 28.5

3.09 2.52 2.51

25.6 26.3 26.5

74.4 73.7 73.5

20.9 19.0 19.0

79.1 81.0 81.0

0.264 0.235 0.235

0.802 0.953 0.947

stability of CuInOS in the covellite structure is strongly related to the S6+ formation and the Cu2+-to-Cu+ transition. The 5.2% S6+ is such a high content that CuInOS is expected to be special in performance. SEM and TEM Analyses. Figure 3a and b shows FE-SEM images of CuInOS-2 at low and high magnifications, respectively. All CuInOS looks like the petal-gathered flowers at the size of 300−500 nm and has displayed superior particle dispersion. The FE-SEM images of other CuInOS-1 and CuInOS-3 are shown in Figure S2 of the SI. Figure 3c shows the TEM image of CuInOS2 to further verify its microstructure. Figure 3d shows HR-TEM image of CuInOS-2. Different lattice fringes belonging to different grains were observed, which indicates the nature of nanoparticles. Figure 3e shows the selected area electron diffraction (SAED) pattern of CuInOS-2. The ring patterns from the (102), (103), and (110) planes explain its polycrystalline nature. The scattered ring pattern explains the solid solution nature of CuInOS. Figure 3f gives the HAADF-STEM image, which reveals many pores with different sizes inside the flowerlike CuInOS particles. Figure 3g shows the HAADFSTEM-EDS spectrum, which verifies that aggregates are composed of Cu, In, S, and O with the Ni peak from the nickel grid. Figures 3h−k show the HAADF-STEM-EDX elemental maps of Cu, In, O, and S. From these element mappings, we can confirm the composition uniformity in samples. FTIR Analysis. FTIR spectra of CuOS and CuInOS are shown in Figure 4. The peaks at 3450 and 1615 cm−1 were

Cu−O content at 530.6 eV is consistent with the data for the dominant Cu+ in Figure 2a. Figure 2d shows the high resolution S2p XPS spectra of CuInOS. The S2p peak indicates that S existed in different chemical states. The two peaks of S2p3/2 and S2p1/2 at 161.9 and 163.1, respectively, belonged to the S2−,41,42 and the peaks of S2p3/2 and S2p1/2 at 168.5 and 169.3 eV were ascribed to S6+.43 The S2− and S6+ contents were quantitatively calculated to be 81.0% and 19.0%, respectively. It is rare to see a metal oxysulfide with such a high content of S6+. CuInOS has the lattice oxygen/sulfur ratio of 0.953, indicating CuInOS is slightly sulfur-rich. Table 1 lists the data of XPS composition analyses for CuOS and CuInOS. From the comparison, CuOS has higher Cu+ and S6+ contents than CuInOS, indicating that the In substitution is helpful in lowering the Cu+ and S6+ contents or increasing the Cu2+ and S2− content. With the composition ratios from XPS analysis, the molecular formula of CuInOS can be expressed in eq 1 below: + + + − − (Cu+0.2934Cu 20.1164 In 20.0275 S60.0522 )(O20.2121 S20.2227 )

(1)

If the molecular formula is to hold the total cation lattice to be 1.0, the formula in eq 2 can be rewritten as + + + − − (Cu+0.599Cu 20.238 In 20.056 S60.107 )(O20.433 S20.455 )

(2)

This compound has a cation/anion ratio above 1.0. The total anion ratio was 0.888, and there was an anion vacancy ratio of 11.2%. Every 10 anions can have one removed. This compound can be viewed as the heavily anion deficient CuInOS. The 4135


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Figure 3. Microstructural and structural characterizations of CuInOS-2. (a, b) FE-SEM image, (c) TEM image, (d) HR-TEM image, (e) SAED pattern, (f) HAADF-STEM image, (g) HAADF-STEM-EDS spectrum, (h−k) EDX elemental mapping of Cu, In, O, and S, respectively.

attributed to hydroxyl stretching and bending vibration, respectively, due to the adsorbed water or surface hydroxyl groups on CuInOS. The peak at 1121 cm−1 was correspond to the S−O stretching vibrations in CuInOS.44−46 The observation of S−O bonds confirms the incorporation of S6+ cation, which is also certified by XPS results shown in Figure 2d. The presence of vibrational peak around 617 cm−1 indicates the presence of Cu− S stretching vibration.47 The 781 cm−1 peak was attributed to the Cu−O stretching vibration, the 681 cm−1 peak, to the In−S bond, and the 851 cm−1 peak, to the In−O bond. It was observed that the peak intensities at 1121, 851, 781, 681, and 617 cm−1 changed with increase in the In content of CuInOS with the higher intensity at 1121 cm−1. Together with the data in XPS, the Cu+- and S6+-rich CuOS have strong orbital interactions

Figure 4. FTIR spectra of CuOS and CuInOS. 4136


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ACS Sustainable Chemistry & Engineering contributed from S6+, to lead to a special FTIR peak pattern with weak peak intensities. The Cu+- and S6+-less CuInOS has the weaker bonding contributed from S6+; therefore, it leads to stronger vibration peaks in FTIR. UV−vis Absorption Spectra, PL Spectra, and Surface Charge Analyses. The optical absorption property of CuInOS was characterized by UV−vis absorption spectra. With increasing the In content, CuInOS had the better visible light absorbance than CuOS (Figure 5a). From the UV−vis spectra, the direct

(αhn)2 = k(hν − Eg )

(3)

where α is the absorbance coefficient, h the Planck constant, k is the absorption constant for a direct transition, hν is the absorption energy, and Eg is the band gap energy. Figure 5b shows the hν−(αhν)2 curves of CuOS and CuInOS. The band gap (Eg) values of CuOS and CuInOS were determined to be ∼2.0 eV and 1.58−1.63 eV, respectively. Such an energy band gap further indicates that CuInOS is a bimetal oxysulfide solid solution instead of monocrystalline CuO with band gap of Eg = 1.2−1.4 eV, Cu2O of 2.0−2.2 eV, CuS of 2.15−2.36 eV, and Cu2S of 1.2−1.25 eV. Figure 5c shows PL spectra of the CuOS and CuInOS. Under a laser beam at wavelength of 330 nm, catalysts were excited with photoluminescence spectra at about 588 nm, accompanied by the peak at 659 nm from the laser contribution. The observed photoluminescence and absorption bands in Figure 5a are in good agreement. The occurrence of PL spectra is related to the radiative decay process for the charge transfer from the excited to the ground states. The increased PL peak intensity for CuInOS is proportional to the decreases in the Cu+ and S6+ contents due to its smaller Eg value. BET and Pore Size Analysis. Figure 6a shows the N2 adsorption−desorption isotherm of CuInOS-2 with the type IV isotherm at relative pressure (P/P0) between 0.2 and 1.0, indicating its mesoporous feature.49 The mesoporous structure of CuInOS is considered to form due to the aggregation of its petal-gathered particles. Based upon the NLDFT (nonlocalized density functional theory) and GCMC (grand canonical Monte Carlo method) from the computer simulation for the pore volume versus mean micropore width plot in Figure 6b, the average pore diameter was calculated to be 4.99 nm. The surface area (SBET) of the synthesized CuInOS-2 is determined to be 20.9 m2/g, and the pore volume is 0.032 m3/g; the CuOS SBET, pore volume, and average pore diameter are 26.3 m2/g, 0.0348 m3/g, and 5.29 nm, respectively. Reduction Activity of CuInOS. Figure 7a shows the Cr(VI) reduction for CuInOS and CuOS catalysts in the dark. It is noted that the CuInOS performed excellent Cr(VI) reduction activity. The CuInOS-2 catalysts completed the Cr(VI) reduction in 4 min, while CuOS only did less than 8% in 10 min. According to the kinetic analysis (Figure 7b), Cr(VI) reduction fitted pseudofirst-order kinetics well. The first-order reaction rate constant (k) followed the sequence: CuInOS-2 (k = 1.615 min−1) > CuInOS1 (k = 0.947 min−1) > CuInOS-3 (k = 0.475 min−1) > CuOS (k = 0.011 min−1). The effect of initial Cr(VI) concentration from 12.5 to 200 ppm on the reduction performance in the presence of 20 mg/L of CuInOS-2 is shown in Figure 7c. The reduction of

Figure 5. (a) Optical absorption spectra. (b) (αhν)2−hν plot for determining the bandgap. (c) PL spectra of CuOS and CuInOS.

band gap was measured with (αhν)2 versus photon energy (h), as shown in eq 3:48

Figure 6. (a) Nitrogen adsorption−desorption isotherm. (b) Pore size distribution curve of CuInOS. 4137


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Figure 7. (a) Cr(VI) reduction for CuInOS and CuOS. (b) Plots of ln(C0/Ct) versus reaction time (min) for CuInOS and CuOS. (c) Effect of Cr(VI) concentration on its reduction. (d) Effect of catalytic concentration on the reduction of Cr(VI). (e) Reusability of CuInOS-2 for Cr(VI) reduction in the dark. (f) Cr(VI) reduction for CuInOS-2 at different pH values in the dark.

Table 2. Data Comparison on the Cr(VI) Reduction over Different Catalysts testing condition catalyst

catalyst (mg)

K2Cr2O7 (ppm)

solution (mL)

reaction time (min)

light type

K2Cr2O7 /catal (mg/mg)

ref

CuO + tartaric acid SnS2 Fe2O3/g-C3N4 + citric acid SnS2/TiO2 In-SnS2 + HCOOH AC-adsorption Active GO AgI/BiOI/Bi2O3 TiO2−CNTs MoSe2 TiO2/MoSe2 g-C3N4 + citric acid CuInOS

20 300 100 300 100 200 50 50 30 96 80 300 20

29.4 50 28.2 50 20 50 500 23.5 132.4 80 20 50 50

50 300 50 300 250 100 50 50 30 80 80 300 100

70 120 15 100 60 180 90 90 40 180 120 180 4

Xe-lamp 500 W Xe-lamp 250 W Xe-lamp 300 W Xe-lamp 250 W LED light 48 W adsorption adsorption-reaction Xe-lamp 300 W UV-light Xe-lamp 300 W Xe-lamp 400 W visible-light (λ > 420 nm) dark

0.0735 0.05 0.0141 0.05 0.05 0.025 0.5 0.0235 0.132 0.0667 0.02 0.05 0.25

33 50 24 26 51 52 53 54 55 56 57 58 this work

Cr(VI) decreased with increase in the Cr(VI) concentration. At the extremely high Cr(VI) concentration of 200 ppm, the 20 mg CuInOS catalyst can reduce 84.5% Cr(VI) in 10 min. Figure 7d shows the influence of catalyst amount on the reduction of 50 mg/L Cr(VI) by varying the CuInOS-2 amount from 50 to 600 mg/L. The reduction efficiency of Cr(VI) increased with the increase in catalyst amount. As the CuInOS amounts increased to 40 and 60 mg, the reduction of Cr(VI) took only 2 and 1 min to complete. In order to test the CuInOS catalyst reusability, the

data for CuInOS-2 which was continuously tested 4 min for six runs is shown in Figure 7e. After the sixth run, the CuInOS-2 catalysts still maintained good reduction activity of 91.8%. To perform the regeneration test, the catalyst after the first run in the completely reduced solution was deactivated by continuously adding the fresh Cr(VI) solution of 1 g/L until the solution color showed the light yellow. The deactivated CuInOS was centrifuged, decanted, and washed for three times, followed by heating at 95 °C for 2 h in deionized water, 4138


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Figure 8. High resolution (a) Cu2p, (b) In3d, (c) O1s, (d) S2p, and (e) Cr2p XPS spectra, (f) XRD diffraction pattern, (g) SEM-EDS spectrum, (h−m) SEM elemental maps, and (n−p) TEM images of CuInOS-2 after six runs. 4139


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Reduction. It is very promising for industrial applications in Cr(VI) wastewater treatment to use CuInOS-2. Figure 8a shows the high resolution Cu2p XPS spectrum of CuInOS-2 after six runs. The monovalent Cu+ peaks of Cu2p3/2 and Cu2p1/2 located at 932.8 and 952.6 eV, respectively, and the bivalent Cu2+ peaks of 2p3/2 and 2p1/2 located at 935.3 and 955.0 eV, respectively. According to the peaks area, the molar contents of Cu+ and Cu2+ were calculated to be 71.1% and 28.9%, respectively. It can been found that the XPS Cu2p peaks position and the Cu+ and Cu2+ contents are similar to the fresh CuInOS catalyst. The results indicate that the CuInOS catalyst prepared in this work is quite stability. Figure 8b shows the high resolution In3d XPS spectrum of CuInOS after six runs. The spectrum exhibited two peaks located at 444.6 and 452.2 eV, which corresponded to In3+ 3d5/2 and 3d3/2, respectively. Figure 8c shows the high resolution O1s XPS spectrum of CuInOS after six runs. Three peaks were observed at 531.5, 530.6, and 529.8 eV, originating from the hydroxyl oxygen, the In−O and monovalent Cu−O, and the bivalent Cu−O. Figure 8d shows the high resolution S2p XPS spectrum of CuInOS after six runs. The two peaks of S2p3/2 and S2p1/2 at 161.8 and 163.2 eV, respectively, belonged to the S2− and the peaks of S2p3/2 and S2p1/2 at 169.8 and 170.8 eV were ascribed to S6+. The S2− and S6+ percentage contents were 81.0% and 19.0%, respectively. The element contents of the used CuInOS after six runs were listed in Table 1. From the comparison, the used CuInOS after six runs has its compositions unchanged, as compared to the fresh CuInOS before the Cr(VI) reduction test. XPS data further indicate that the CuInOS is quite stability and the structure has not changed after reduction Cr(IV). The detection of the Cr state by XPS is critical in understanding the reduction reaction. Figure 8e shows the high resolution Cr2p XPS spectra of CuInOS-2 after six runs. The peaks positions at 572.2, 575.9, and 580.0 eV corresponded to the Cr(0), Cr(III), and Cr(VI) peaks, respectively. There were 8.82% Cr(VI), 30.49% Cr(III), and 60.69% (0). The results indicate that over the CuInOS catalyst Cr(VI) was reduced into Cr(III) then to Cr(0). Figure 8f shows the XRD diffraction pattern of CuInOS-2 after six runs. The peaks located at 28.17°, 29.55°, 31.98°, 32.80°, 48.26°, and 52.96° attributed to the (101), (102), (103), (006), (110), and (206) crystal planes, respectively, were similar to the fresh CuInOS-2 catalyst, shown in Figure 1. The peaks located at 39.28°, 44.13°, and 48.49° matched well to the JCPDS No. 65-3316 of the cubic Cr structure, belonging to the (200), (210), and (211) crystal planes, respectively. Figure 8g shows the SEM-EDS spectrum of CuInOS-2 after six runs, which verifies the existence of Cu, In, O, S, and Cr. Figure 8h−m shows the SEM elemental maps of Cu, In, O, S, and Cr. From these element mappings, we can confirm the composition uniformity in catalyst. The HAADF-STEMEDS spectrum of CuInOS after six runs also verifies the existence of Cu, In, O, S, and Cr. Figure 8n−p shows the TEM images of CuInOS after six runs. The d-space values of 2.28 and 2.06 Å in the TEM image were related to the cubic Cr (200) and (210) planes. From those XPS, XRD, and TEM analyses, we can confirm that Cr(VI) can be mainly converted to Cr(0) with Cr(III) an intermediate species. Therefore, our experiment was undergoing the reduction reaction instead of the Cr(VI) adsorption. The slight degradation of CuInOS in reducing Cr(VI) after the sixth run, as shown in Figure 7e, is strongly related to the continuous deposition of Cr species on its surface. Proposed Reaction Mechanism for Cr(VI) Reduction. Reduction activity of bimetal CuInOS oxysulfide was evaluated

centrifuging, washing with DI water three times, washing with ethanol dehydrate two times, and vacuum drying at 85 °C for 24 h to complete the regeneration procedure. Figure S3 of the SI shows the reduction of Cr(VI) with the fresh and regenerated catalysts. The regenerated CuInOS being used to execute the second run in the 100 mL Cr(VI) solution of 50 ppm performed 100% reduction in 6 min. The effect of the catalyst storage on the Cr(VI) reduction for analyzing the catalyst stability in air was also investigated. The fresh CuInOS-2 prepared in May 2016 showed 99.3% reduction in 3 min under the 100 mL Cr(VI) solution of 50 ppm. After 10 months in storage, the catalyst, under the same test conditions, showed in February 2017 98.9% reduction in 3 min. The CuInOS catalyst can be viewed to have stability even after 10 months in storage. Figure 7f shows the CuInOS-2 for Cr(VI) reduction with different pH values. It is noted that the CuInOS-2 had good Cr(VI) reduction activity with pH from 4 to 7. To understand the effects of inorganic ions on the Cr(VI) reduction, we individually added different anions of NaCl, Na2SO4, NaHCO3, K2CO3, and Al(NO3)3 into our formal Cr(VI) reduction experiments, having 100 mL Cr(VI) solution of 50 ppm. The data is shown in Table S1 of the SI. After a 3 min reduction duration, the Cr(VI) reduction removals were 81.2, 90.1, 46.3, 86.1, and 99.8% under the influence of NaCl, Na2SO4, NaHCO3, K2CO3, and Al(NO3)3, respectively. We have tested real wastewater taken from the local electroplating industry, and this wastewater with an initial concentration of 33.1 mg/L and a pH value of 4.2 contains of Cr6+, Ni2+, Zn2+, Al3+, SO42−, and NO32−. After filtration to remove the particles and dispersants, the Cr in wastewater was removed 85.2% in 5 min. Some reported data in Cr(VI) reduction are summarized in Table 2. To compare the reduction activity, it is better to compare the tested systems in terms of the Cr(VI)/catalyst ratio, as the reduction is strongly related to the initial catalyst content. Our CuInOS catalyst had the second highest (K2Cr2O7 weight)/ (catalyst weight) ratio of 0.25 mg/mg and the highest value of 0.0625 mg/(mg min) for complete removal. For comparison, active graphene oxide with surface modification had the highest ratio of 0.5 mg/mg but the second highest ratio of 0.0056 mg/ (mg min) for 94.5% completion. Our catalyst system in CuInOS2 has more than a 10-fold removal rate of K2Cr2O7. Most of the catalysts except for the surface-modified graphene oxide need the photo energy to be excited. To evaluate the adsorption contribution in the reduction reaction, activated carbon with a specific surface area of 1180 m2/ g was added into the Cr(VI) solution. The adsorption amounts were 1.5% and 14.7% after immersion of activated carbon for 5 min and 24 h, respectively, as compared with 100% removal of CuInOS in 4 min. The excellent activity is not attributed to the Cr(VI) adsorption, because the surface area of 20.9 m2/g for CuInOS is not good for absorber. The results in the reduction of Cr(VI) indicate that the bimetal CuInOS oxysulfide catalyst prepared with an appropriate amount of hydrazine to form CuInOS-2 in this work show the best reduction activity and reduction rate without the needs of other chemicals and photo energy. The catalysts need to be used at pH = 5−7 to optimize the reduction ability. One important issue is the incorporation of indium into CuInOS for better reduction ability. As all the characterizations for CuInOS and CuOS are similar except for the small changes in XPS data (Table 1), we conclude here that CuInOS with a lower S6+ content can form weak bonding for easy interaction with Cr(VI), as shown later in the section Proposed Reaction Mechanism for Cr(VI) 4140


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ACS Sustainable Chemistry & Engineering through reduction of Cr(VI). It is interesting that CuInOS can perform reduction reactions in the dark. CuInOS can completely reduce 100 mL Cr(VI) solution of 50 ppm in 4 min, even after 6 runs. The excellent reduction activity of Cr(VI) over CuInOS needs to be reasonably explained. From all the structure and property analyses, we suggest the defect-assisted interfacial reactions between catalyst and the Cr(VI) solution. CuInOS nanoparticle with a hexagonal CuS or covellite structure has a 2+ 6+ 2− 2− molecular formula of (Cu+0.599Cu2+ 0.238In0.056S0.107)(O0.433S0.455). CuInOS is Cu+- and S6+-less, as compared to the Cu+- and S6+rich CuOS. It is a heavily anion deficient compound with four kinds of anions, three kinds of cationic valence charge states, and two kinds of anions. The extrinsic and vacancy defects in CuInOS can contribute the lattice distortion, the internal strain energy, the multiple stress states, etc., and lead to its weak bonding. On the other hand, CuOS is Cu+- and S6+-rich. The rich S6+ can contribute the strong Columbic interaction to lead to the strong bonding. Therefore, CuInOS as a catalyst can be highly activated and its bonding is weakened. Here, we will propose an oxygen transfer model for the surrounding water to move in and out of the 11.2% anion vacancies of CuInOS. That is to say, the oxygen in the water molecule can be trapped by the anion vacancy to lower the potential energy of CuInOS and the trapped oxygen can be removed by the water oxidation to refresh the anion vacancies. The interaction between the catalyst and water in the dark has been reported in our previous publications, but here the anion vacancy viewpoint is proposed instead of the active lattice oxygen.59−61 On the other hand, CuOS with strong bonding cannot proceed the interaction reaction for the Cr(VI) reduction. The electron transport is also the important issue for the Cr(VI) reduction to occur. The generated Cu+ can be viewed as Cu2+ with the attachment of an electron to form the plasmon for electron hopping transport. This Cu+/Cu2+ electron hopping can continuously provide the electron flux for the Cr(VI) reduction reaction. The Cu+/Cu2+ effect with the metal-to-metal charge transfer mechanism had been mentioned.60,62−64 The Cr(VI) reduction mechanism has been frequently explained through the photoinduced electrons and holes by photocatalysis. However, the dark Cr(VI) reduction is rare and its mechanism needs to be different from photocatalysis. Our proposed reaction steps are shown with the following eqs 4−7. The Kröger−Vink notation developed to describe the defects in an ionic compound will be applied here. For the oxygen vacancy (V2+ O,lattice) as an example, the main body of V represents the vacancy, the subscript is the host lattice site or interstitial, and the superscript is for the relative charge. Here we adopted the positive charge of 2+ instead of the “••” symbol in the original invention. Equation 4 shows the water reduction by catalyst with its active anion vacancy to produce protons. The oxygen in water is exchanged into the anion vacancy site in CuInOS. With the electron hopping to transport conducting electrons and the water-reduced protons, the adsorbed Cr2O72− is reduced to Cr(III) in CrO2− (eq 5) and then to Cr(0) in metallic form (eq 6). The trapped lattice oxygen can react with water to refresh the anion vacancies for successive reactions (eq 7). With each electron hopping, one Cu+ converted to one Cu2+. In this model, the oxygen in water executes the oxygen transfer kinetics due to the active CuInOS with its highly distorted structure. The schematic illustration for the proposed reaction mechanism for Cr(VI) reduction is shown in Figure 9. The interaction between catalyst and water has been known for the thermochemical catalytic production for solar fuels at high

Figure 9. Schematic illustration of the reaction mechanism for Cr(VI) reduction over CuInOS.

temperature,65−67 but it is quite special to occur here at room temperature. + 2+ 0 H 2O(l) + VO,lattice = 2H(aq) + OO,lattice

(4)

Cr2O7 2 − + 6e− + 6H+ → 2CrO−2 + 3H 2O

(5)

CrO−2 + 3e− + 4H+ → Cr(0) + 2H 2O

(6)

0 2+ H 2O(l) + OO,lattice = 2OH−(adsorb) + VO,lattice

(7)

At last, we have to mention that CuInOS has performed the interesting reduction reaction of Cr(VI) through its anion vacancy. CuInOS, an ionically bonded ceramic, will not change its structural skeleton at room temperature. Equations 4 and 7 have shown the oxygen in and out through the interaction with the anion vacancy on the surface of CuInOS. Further, together with the fast degradation rate, we can identify CuInOS as a catalyst instead of a stoichiometric reductant for the Cr(VI) reduction.

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CONCLUSIONS A Cu-base bimetal CuInOS oxysulfide solid-solution catalyst was successfully synthesized. The CuInOS exhibited excellent reduction activity without thermal, electrical, and photo-energies and additional reagents. The 100 mL Cr(VI) solution of 50 mg/L was completely reduced by 20 mg hydrazine-added CuInOS catalyst within 4 min with a rate constant of 1.615 1/min or 80.75 1/(min g). Its turnover frequency was 0.0625 mg/(mg min), as compared to 0.0056 mg/(mg min) for surface-modified graphene oxide. Their reusability tests were performed for six runs without apparent degradation in catalyst activity. The major key characteristic in CuInOS is related to the lower Cu+ and S6+ contents after the indium substitution and the defective structure to distort the lattice, to weaken the lattice bonding, and to form the active anion vacancy for interface reactions among catalyst, water, and Cr(VI). Combining the transport electrons between Cu+ and Cu2+ and the generated protons through the oxygen exchange after the water reduction by the oxygen vacancies, the Cr(VI) reduction can continuously proceed until the formation of Cr(0) without light illumination. This study develops a novel CuInOS system, which provides possible practical applications 4141


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for Cr(VI)-containing wastewater treatment and other environmental issues.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00107. Figures S1−S3 and Table S1 as mentioned in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected] (D.-H.K.). ORCID

Xiaoyun Chen: 0000-0002-3973-7726 Dong-Hau Kuo: 0000-0001-9300-8551 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of the Republic of China (MOST 104-2221-E-011169-MY3).

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