Turning Waste to Resource: An Example of Dehydrogenation Catalyst

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Kinetics, Catalysis, and Reaction Engineering

Turning Waste to Resource: an example of dehydrogenation catalyst Cr/ZrO2 derived from Photoreduction Treatment of Chromium-containing Wastewater with ZrO2 Changjun Liu, Yingming Zhu, Kequan Mu, Qiang Liu, Hairong Yue, and Wei Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05861 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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Industrial & Engineering Chemistry Research

Turning Waste to Resource: an example of dehydrogenation catalyst Cr/ZrO2 derived from Photoreduction Treatment of Chromiumcontaining Wastewater with ZrO2 Changjun Liu1, Yingming Zhu2, Kequan Mu1, Qiang Liu1, Hairong Yue1, Wei Jiang1, 1

1Low-carbon

Technology and Chemical Reaction Engineering Laboratory, School of

Chemical Engineering, Sichuan University, Chengdu, 610065, P.R.China

2Insitute

of New Energy and Low-carbon Technology, Sichuan University, Chengdu,

610065, China

Abstract This work presents a strategy to convert the Cr(VI) pollutant in waste water into active Cr catalysts in one step by simultaneous photoreduction and deposition using ZrO2 nanoparticles as the photocatalyst and support. Both highly dispersed Cr(0) and Cr2O3 were found on the surface of the resultant Cr/ZrO2 catalyst after UV irradiation. After photoreduction treatment, no residual chromium was detected in post-treated water. The resultant Cr/ZrO2 was found to be an active catalyst for selective

1

Corresponding author. Tel.: +86-28-85990133; fax: +86-28-85460556.

E-mail address: [email protected]

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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dehydrogenation of ethane. It is slightly more stable and active than that was prepared by impregnation method. The interconversion between the structures of Cr-O-Cr and Cr=O should be the active site for oxidative dehydrogenation of C2H6 with CO2. An ethylene yield as high as 17.0% was achieved at 650°C. This work proved that producing supported metal catalysts from the corresponding metal contaminated waste water via proper one-step procedure, such as photoreduction treatment, is a feasible strategy to meet the ever-increasing stringent environmental requirements with better economic efficiency.

KEY WORDS: Wastewater; Photoreduction; Zirconium dioxide; Dehydrogenation; Chromium.


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1. Introduction Heavy metal contamination of water and soil is a grave threat for the environment and human health. Various methods have been developed to remove these heavy metal contaminants from water and soil1-3. However, most of the existing approaches that meets the strictest discharge criteria are costly. Tuning the waste into resource could improve the economic feasibility of the waste treatment technics. However, converting the useful substances in the waste into valuable products in a simple process is still a great challenge.

Chromium contamination is a major threat to our water source and soil due to its carcinogenic and toxic nature4, 5. Nowadays the total chromium limit in discharging water should be less than 1.5 mg/L and Cr(VI) content should be less than 0.5 mg/L according to current integrated wastewater discharge standard GB8978-1996. Total chromium limitation for drinking water is more stringent and restricted to less than 0.05 mg/L according to Chinese standard GB 5749-2006 and the recommendation of World Health Organization (WHO). On the other hand, the reservation of chromium in China is almost negligible. The production of chromium in China is only about 1% of its consumption6. Various methods such as adsorption7-9, chemical precipitation10, bacterial reduction11-13, ion exchange resin14, 15, photocatalytic reduction11, 16-18 have been developed to remove and recycle chromium contaminant from the waste stream. However, few of them is economically available and can lead to discharging water stream which meets with the stringent regulation. The resultant chromium-concentrated 3 ACS Paragon Plus Environment


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products need further treatment to avoid secondary pollution. It is desirable to directly remove chromium from the waste water and simultaneously convert it into valuable chromium-containing products in an economical and convenient approach.

Photoreduction deposition has been developed as an efficient method to prepare supported catalyst19-22. Photoreduction is able to reduce Cr(VI) anions into Cr(III) cations and turn it into chromium-contained products without extra energy demand. Titanium dioxide and some other semiconductors have been tested as the photocatalyst for the reduction of metallic ions including Cr(VI) in water by light23-26. First, the aqueous Cr(VI) species were transformed into Cr(III) cation,

then Cr(III) was

precipitated in the form of Cr(OH)3 by introducing the basic reactant11. The introduction of base lengthens the treatment process and increases the cost, which somehow compromise the advantages of photoreduction process. It is more attractive to achieve deposit product via photoreduction only, such as reducing Cr(VI) straight forward to metallic Cr(0) deposit. However, proper photocatalyst which has the potential to reduce Cr(VI) to Cr(0) via Cr(III) is the key to realize this concept.

The standard redox potential of Cr(VI)/Cr(III) and Cr(III)/Cr(0) are known to be 1.36 eV and -0.74 eV27, respectively. Therefore, a photocatalyst with a wide band gap over 2.10 eV, and high conduction band over -0.74 eV, is necessary to reduce Cr(VI) into Cr(0) upon light radiation. ZrO2 is a widely used catalyst and support in many thermal catalytic processes such as hydrogenation, dehydrogenation, alkylation, isomerization, and esterification28-32, due to its high thermal stability and unique 4 ACS Paragon Plus Environment

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acid/base properties. In addition. ZrO2 is a readily available photocatalyst with a wide bandgap of 5.0 eV and high conduction band of -1.09 eV (vs SHE). Thus ZrO2 is capable to catalyze the photoreduction of Cr(VI) to Cr(III) or even to Cr(0). The resultant Cr/ZrO2 can be used as catalyst for the oxidative dehydrogenation of ethane to ethylene with CO2 33-35.

In this work, chromium and chromium contaminated water was taken as the typical metallic pollutant treating system, and successfully converted chromium pollutant into supported catalyst for ethane dehydrogenation by developing a ZrO2-based one-step photoreduction-precipitation process. This photoreduction catalyst and process could also be efficient for other heavy metal polluted waste water treatment.

2. Experimental 2.1 Materials Methanol, sulfuric acetic acid, potassium dichromate and chromic nitrate of analytical grade were purchased from Chengdu Kelong Chemical Reagent Co. Ltd. Analytical grade Zr(OC4H9)4 were purchased from Shanghai Aladdin Biochemical Reagent Co. Ltd. Ethane (99.99%), ethylene (99.99%), carbon dioxide (99.9%), and compressed air were purchased from Chengdu Keyuan Gas Co. Ltd. All the reagents were used as received.



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2.2 Preparation of ZrO2 nanoparticles Zirconia photocatalyst was prepared by a sol–gel method using zirconium n-butylate as the zirconium source36, 37. Generally, 1.9095 g (98wt%) concentrated sulfuric acid and 10.0 mL 80 wt.% zirconium n-butylate butanol solution were dissolved in 35.0 mL isopropanol, the resultant mixture is marked as solution A. Then a solution of 17.0 mL isopropanol and 4.0 mL deionized water was dropped into the solution A at a rate of one drop per second under vigorous stirring. The resultant gel was aged at 80 °C for 2 hours without stirring, and then calcined at 775 °C for 3 hours to obtain the ZrO2 photocatalyst.

2.3 Photoreduction of Cr(VI) with ZrO2 under UV light irradiation Dilute K2Cr2O7 solutions were used to simulate the waste water with various hexavalent chromium concentrations. Typically, the photoreduction test was carried out in a 50 mL quartz tube with 20 mL K2Cr2O7 solution, 100 mg ZrO2 and 2 mL methanol or other hole scavengers. A 500 W mercury lamp was employed as the UV light source. All materials were simultaneously added into the quartz tube, and kept in darkness for 40 min to achieve the adsorption equilibrium. Then the suspension was exposed to UV light irradiation, and sampled at a given time interval. The Cr(VI) absorptive intensity of samples was detected with a Shimadzu UV-1500PC ultraviolet-visible (UV-vis) spectrophotometer and the total chromium species in water was determined by a


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Thermo Fisher iCap 7000 inductively coupled plasma-optical emission spectroscopy (ICP-OES).

2.4 Oxidative dehydrogenation of ethane on Cr/ZrO2 The catalytic dehydrogenation of ethane was carried out in a tubular fixed bed reactor of an inner diameter of 4 mm. About 100 mg sieved catalyst (125~149 μm) was filled into the fixed bed reactor for each test. A given mixture of ethane and CO2 was fed into the reactor at a flow rate of 17 mL/min-1. The reaction temperature ranges from 500 ºC to 650 ºC. The product stream was analyzed by an online gas chromatograph (Fuli Instruments, FL 9790II).

Basically, three Cr/ZrO2 catalysts prepared by different procedure were tested. One is the Cr/ZrO2 from the photoreduction process, which was separated centrifugally from aqueous solution, and dried in air at 110 ºC for 5 hours. Another one is the Cr/ZrO2 from the photoreduction process but calcined at 650 ºC for 2 hours in air. The third one is the Cr/ZrO2 that prepared by impregnation method using chromium nitrate as chromium source. The ZrO2 support and Cr loading of the three catalyst are the same. The impregnated catalyst was also dried at 110 ºC for 5 hours and calcined at 650 ºC for 2 hours before activity test.



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2.5 Characterization X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Fisher Scientific ESCALAB 250XI with monochrome Al Kα (hv = 1486.6 eV). X-ray diffraction (XRD) patterns were collected on X'Pert pro MPD with a Cu Kα 40 kV/40 mA X-ray source. Scanning electron microscopy (SEM) images were taken with JEOL JSM-7500F, transmission electron microscopy images (TEM) and highresolution TEM (HRTEM) images were obtained with FEI Tecnai G2 F20. Photoluminescence (PL) spectra were recorded with a fluorescence spectrometer (F7000, Hitachi, Japan). The chromium content in the ZrO2 was determined by an inductively coupled plasma optical emission spectrometry (ICP-OES) (Vista Axial, Varian). Temperature programmed desorption (TPD) was carried out in a Quantachrome Instrument Chemstar-TPx. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis were conducted by a Netzsch STA 449 F3 Jupiter. in situ-Raman analysis was conducted by a Thermo Fisher DXR Microscope.

3. Results and discussion 3.1 Dynamic of photoreduction process Our previous work36 has found that the hexvalent Cr pollutant can be efficiently reduced (Eqs. 1-2) and removed by UV irradiation with the presence of ZrO2 and methanol. The simultaneous presence of ZrO2, methanol and UV irradiation is critical for the rapid removal of Cr(VI). 8 ACS Paragon Plus Environment

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UV HCrO-4 +H +   Cr 3+ +H 2 O

UV CrO 2-4  H 2 O   Cr(OH)3

E o  1.35 V/SHE

E o  0.13 V/SHE

(1)

(2)

Figure 1. Cr(VI) and Cr(III) residue changing in photoreduction process. (20 mL, 20 mg/L K2Cr2O7, 0.100 g ZrO2)

The remaining Cr(VI) concentration in treated waste water is beyond the detection limitation of ICP-OES method (less than 0.05 mg/L total chromium substance). The concentration changes of the Cr(VI) and the total Cr species were determined with UVvis spectrophotometer and ICP-OES, respectively. The concentration of Cr(III) was deduced from the difference between the total Cr concentration and Cr(VI) concentration. The results (Figure 1) show that Cr(VI) was rapidly reduced and removed from the solution. The concentration of Cr(III) slightly increases in the initial 40 minutes and then decreases. About 37% of the reduced Cr(VI) remained in the solution in the initial 20 minutes. This ratio also decreases gradually to 20% in 60



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minutes and to 7.6% in 80 minutes. The dissolution equilibrium of Cr(III) cation in aqueous solution didn’t limit the removal efficiency of chromium. This suggests that Cr(OH)3 or Cr2O3 may not be the primary form of the chromium species that precipitated by photoreduction. This is also consistent with the observation that acidic condition favors the removal of chromium in photoreduction process although it is still not impossible to determine the actual mechanism of this process36, 38-40. Under acidic conditions, Cr(VI) mainly exists as HCrO4- which can be more easily reduced (Eqs.12)38. Under such conditions, the surface of ZrO2 is positively charged since the point of zero charge of ZrO2 is about 6.5 41, which facilitates the adsorption and reduction of HCrO4-.

3.2 Cr deposition capacity of ZrO2

Figure 2. Cr(VI) removal efficiency with various ZrO2 loading. (20mL, 20 mg/L K2Cr2O7)

Figure 2 shows the Cr contaminant removal efficiency of various ZrO2 loading. It is found that the Cr contaminants can be almost completely removed as long as the ZrO2 10 ACS Paragon Plus Environment

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loading is greater than 0.05 g, otherwise the Cr(VI) residue in solution can be observed. Increasing the ZrO2 to waste water ratio can accelerate the photoreduction and deposition process. The actual Cr(VI) deposition capacity of ZrO2 was calculated to be 0.603 mmol Cr per gram of ZrO2 or 0.01 mmol/m2 with the 0.010 g ZrO2 case. This number is close to the theoretic Cr deposition capacity of ZrO2, 0.016 mmol/m2, which is roughly estimated by assuming the effective Cr atom radius to be half of the lattice constant of metallic Cr crystalline (PDF# 88-2323). This also suggests that the termination of Cr(VI) photoreduction is likely due to the complete coverage of the ZrO2 surface by the reduced Cr species, which blocks the light irradiation and deactivates the active site of ZrO2. The deposition of reduced product of Cr(VI) leads to the separation of Cr species from aqueous system and a ZrO2 supported Cr product. Generally, excessive ZrO2 was employed to assure the complete removal of Cr contaminants. Figure 3 shows that the ZrO2 can be recycled and reused providing the Cr deposition capacity has not been reached.

Figure 3. The reusability of ZrO2, (0.100g ZrO2, 20 ml, 30 mg/L)



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3.3 Cr/ZrO2 catalyst from photoreduction

Figure 4.

XRD patterns of fresh and used ZrO2;

Figure 5. TEM-EDX mapping of

Cr/ZrO2 from photoreduction

Figure 4 shows the XRD patterns of the fresh ZrO2 catalyst and the resultant Cr/ZrO2 from the photoreduction process. It indicates that ZrO2 is a mixture of monoclinic (PDF#24-1165) and tetragonal (PDF#50-1089) phases. There is no significant difference was observed from XRD pattern, which suggests no crystalline change of 12 ACS Paragon Plus Environment

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ZrO2 took place. No Cr crystalline was observed in XRD patterns of spent ZrO2. This suggests that the deposited Cr species, if there is only, highly dispersed on the surface of ZrO2. TEM-EDX mapping (Figure 5) approves the presence and good dispersion of the Cr species on the used ZrO2.

Figure 6. Photoluminescence of the fresh and used ZrO2.

Although the electronic structure of ZrO2 is not distorted by the immobilized chromium species, its quantum efficiency declined sharply as evidenced by the notable weakening of the PL intensity (Figure 6). The significant abatement of luminous intensity of the spent ZrO2 indicates the declination of the recombination rate of photogenerated electron and hole pairs. In this case, such declination is likely due to the decrease of the effective light-absorbing surface which was covered by Cr2O3 or metallic Cr.



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3.3 Photoreduction deposition mechanism

Figure 7. Ratio of Cr(III) to Cr(0) in Cr/ZrO2 determined by XPS against time

both Cr2O3 and Cr(0) have been detected in the photoreduction deposition product36. The relative amount of Cr(III) and Cr(0) in the Cr/ZrO2 samples with various photoreduction time were determined by XPS (Figure 7). Figure 7 shows that Cr(0) is the dominant Cr species on ZrO2 at the initial stage. Then it drops from about 65% to about 50% in 40 minutes and levels off as the reaction proceeds. Meanwhile, the Cr(III) species increase accordingly from about 35% to 50%. This suggests that the aqueous Cr(VI) could have been directly reduced to Cr(0) and deposited on ZrO2 at the initial stage. The resultant Cr(0) is likely deposit in the form of single Cr atom. Those single metallic Cr atoms are amiable to oxidation by Cr(VI) , unannihilated holes or dissolved oxygen at its vicinity and lead to Cr2O3.Thus the following redox reactions (Eqs. 3-6) were purposed for the photoreduction process of Cr(VI) with ZrO2 under UV light:


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ZrO2 +ℎ𝑣

𝑈𝑉

h+ + e―

(3)

MeOH + h + →CO2 + H2O

(4)

HCrO4 ― +3e ― →Cr3 + +3e ― →Cr

HCrO4 ― + Cr→2Cr2O3

(5)

(6)

In the photoreduction process, first ZrO2 is excited by UV irradiation and generates hole and electron pairs (Eq.3). Holes are consumed up by the sacrificing reagent such as methanol and hydroxy anion leading to the release of CO2 or O2 (Eq.4). Meanwhile the photogenerated electrons effuse to the ZrO2 surface and reduce the adsorbed Cr(VI) species to Cr(0) via Cr(III) (Eq.5). However, Cr(0) would be oxidized by the Cr(VI) presents at its vicinity and directly lead to the formation of Cr2O3 deposit on ZrO2 (Eq.6).



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3.4 Oxidative dehydrogenation activity of Cr/ZrO2

Figure 8. Ethane dehydrogenation with Cr/ZrO2: a, b) Ethane conversion and ethylene yield under 650°C; c) Top ethylene yield against reaction temperature; d) Calculated TOF with different C2H6 to CO2 ratio.

Supported Cr catalysts have been extensively studied in dehydrogenation of alkanes42-46. The resultant Cr/ZrO2 products were investigated in oxidative dehydrogenation of ethane to ethylene by CO2. This reaction occurs according to Eq. 8.

16C2H6 +9CO2→14C2H4 + 12CO + 6H2O + CH4 + 12H2

(8)


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Cr/ZrO2 produced in photoreduction process was found to be active oxidative dehydrogenation catalyst at 650 oC (Figure 8). In comparison, ZrO2 itself leads to only marginal conversion of ethane. Calcination of the Cr/ZrO2 from photoreduction can significantly improve the oxidative dehydrogenation (ODH) activity in ODH of ethane with CO2 (Figure 8A). The catalytic activity of the calcined Cr/ZrO2 from photoreduction is even better than that of the Cr/ZrO2 prepared by impregnation method. The maximum ethylene yield of impregnated Cr/ZrO2 is 14% and that of the Cr/ZrO2 from photoreduction is about 13% (Figure 8B). However, if Cr/ZrO2 from photoreduction was calcined in situ at 650 oC under air atmosphere before test, the maximum yield can reach 17% and the life time is about doubled comparing to that without calcination. This performance improvement is due to the oxidization of immobilized Cr(0) by air to Cr2O3, which is confirmed by the successive XPS characterization. The activity of the calcined Cr/ZrO2 from photoreduction is better than that of Cr/ZrO2 by impregnation in a wide temperature range (Figure 8C). CO2 tends to competitively adsorb on the catalyst since excessive CO2 leads to the decrease of turnover frequency which is based on the Cr content determined by ICP-OES (Figure 8D).



Figure 9. Characterization of the Cr/ZrO2 catalysts:

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a, b) SEM of the fresh and spent Cr/ZrO2 by

impregnation; c, d) SEM of the fresh and spent Cr/ZrO2 by photoreduction and calcination; e, f) XPS spectra of the fresh and spent Cr/ZrO2 by photoreduction and calcination; g) TG analysis of the spent Cr/ZrO2; h) Regeneration of spent Cr/ZrO2 under 650°C;

The spent Cr/ZrO2 catalysts were characterized to understand their deactivation mechanism. The SEM pictures show no perceptible change of the surface morphology on

the fresh and spent Cr/ZrO2 (Figures 9A -9D). Simultaneous thermal analysis

shows about 4 wt.% weight loss at about 325 °C which indicates significant carbon


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deposition (Figure 9G). XPS results show that Cr(III) species are dominant on both fresh and spent Cr/ZrO2 catalysts, and no significant change was detected (Figure 9E and 9F) . This suggests that the activity of the spent Cr/ZrO2 could be recovered by burning off the coke. Oxygen was introduced to the reactor at the reaction temperature to regenerate the deactivated Cr/ZrO2. The activity of Cr/ZrO2 can be almost completely recovered even after three regeneration cycles (Figure 9H).

3.5 Role of Cr/ZrO2 in ODH of ethane with CO2

Figure 10. C2H4 and C2H6 TPD profile of Cr/ZrO2

Temperature programmed desorption (TPD) of C2H6 and C2H4 on Cr/ZrO2 were performed to understand the interaction between the catalyst and reactants. The C2H6TPD (Figure 10) shows two broad peaks, centered at 220 °C and 500°C respectively, which suggests a strong interaction between C2H6 and Cr/ZrO2. The C2H4-TPD profile



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is similar with that of C2H6-TPD (Figure 10). The high desorption temperature peak shifts to about 535 oC.

This suggests that an oxidation dehydrogenation temperature

higher than 535 oC is preferable since it favors the desorption of the product.

Figure 11. In situ-Raman spectrum of Cr/ZrO2 under various atmosphere, λ=455 nm, a) 10 ml/min N2, b) 10 ml/min CO2, c) 10 ml/min C2H6, d) 10 ml/min C2H6 and 7 ml/min CO2

In situ Raman analysis was carried out to understand how CO2 and C2H6 interacts with Cr species. The Raman spectra of Cr/ZrO2 in pure nitrogen at various temperature (Figure 11A) shows the presence of both tetragonal ZrO2 (143 and 263 cm-1) and monoclinic ZrO2 (178, 187, 617, and 639 cm-1)47-49. Chromium oxides were identified by the presence of Cr-O-Cr (850 cm-1), monochromate Cr=O (1031 cm-1) and 20 ACS Paragon Plus Environment

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polychromate Cr=O (1008 cm-1)49, 50. Under the CO2 atmosphere, the peaks at 850 cm-1, 1008 cm-1 and 1031 cm-1 of the chromates decrease gradually with increasing temperature and disappear above 500 °C (Figure 11B). Under the C2H6 atmosphere, significant carbon deposition (Figure 11C) was observed at temperatures above 200 oC as evidenced by peaks at 1340 and 1580 cm-1 which were ascribed to amorphous carbon (D peak) and graphite carbon (G peak) 51. The ratio of the two type of carbon (IG/ID) decreased from 3.01 to 1.09 while the temperature increases from 200 to 800 °C. This suggests that a reaction temperature higher than 500 oC can delay the severe carbon deposition 52-54. With the presence of both CO2 and C2H6, the Raman spectra is similar to that under C2H6 atmosphere except less severe carbon deposition peaks (Figure 11D). This suggests that the presence of CO2 can somehow alleviate the coke formation.

4. Conclusion ZrO2 has been proved to an efficient photocatalyst to remove Cr(VI) contaminant from the waste water by photoreduction process. It successfully removes the Cr(VI) pollutant and tunes it into valuable catalyst. The waste water treated by this method can meet the stringent discharging regulations on heavy metal contents. The direct photoreduction and deposition is mainly responsible for the rapid and efficient removing of chromium contaminants from waste water. The resultant Cr/ZrO2 shows high oxidative dehydrogenation activity in the ODH of ethane with CO2.

An ethylene

yield as high as 17.0% was achieved at 650°C on the Cr/ZrO2 catalyst obtained from the waste water treatment. 21 ACS Paragon Plus Environment


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Acknowledgements

We appreciated the financial support from the National Natural Science Foundation of China (No. 21676168 & No. 21476146). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Changjun Liu, Yingming Zhu, Kequan Mu and Qiang Liu conducted the experiments and characterizations; Hairong Yue, Wei Jiang gave supports on experiment guides and facilities.

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