γ-Al2O3 catalyst with high surface area

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Hydrodechlorination of trichloroethylene over MoP/␥-Al2O3 catalyst with high surface area Qinghui Guo, Lili Ren ∗ School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, PR China

a r t i c l e

i n f o

Article history: Received 25 May 2015 Received in revised form 11 September 2015 Accepted 15 September 2015 Available online xxx Keywords: MoP/␥-Al2 O3 Sol–gel Hydrodechlorinaton Trichloroethylene

a b s t r a c t Alumina-supported molybdenum phosphide (MoP/␥-Al2 O3 ) with high surface area was successfully synthesized by combining sol–gel and temperature-programmed reduction (SG-TPR) method and was firstly used to study the hydrodechlorination (HDC) of trichloroethylene (TCE). The properties of the catalysts were evaluated by several different characterization techniques, such as Braunner–Emmet–Teller (BET) surface area, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The results prove that a well-dispersed and high BET surface area catalyst can be obtained by SG-TPR method, which shows higher catalytic activity than those prepared by impregnation and mechanical mixing method for the HDC of TCE. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Trichloroethylene (TCE) is one of the halogenated organic compounds that can be easily found in the emission from chemical processing, ground water, and soils due to its extensive uses in industries [1–3]. It has been pointed out that TCE is a volatile organic compound with potent carcinogenicity and resistant to natural degradation leading to long-term environmental and health risks [4]. Also, the emission of TCE into the environment is now strictly regulated [5]. As a result, there is substantial interest in the development of efficient, economic methods for remediating ground water, and soils contaminated with TCE. It has been recognized that the traditional remediation technologies, such as incineration and solidification, are not entirely satisfactory, because they require high capital, operation cost, and extremely long-time [5,6]. In addition, they can lead to the formation of carcinogenic byproducts [6]. Compared with the conventional methods, the catalytic hydrodechlorination (HDC) for treating TCE is a viable and nondestructive low-energy alternative, which transforms the chlorinated waste streams into nontoxic, easily degradable, or useful raw materials [7]. Therefore, as a safe method, the catalytic HDC of TCE has attracted more and more attention.

Over the past years, studies on HDC of TCE compounds have been carried out mainly using supported metal, notably Pt, Pd [4,8–10]. Although they exhibit good catalytic activities, they are expensive and easily deactivated due to HCl poisoning, metal sintering or carbon deposition, which limits their application in industry [11]. Recently, transition metal phosphides have been reported as a new class of high activity hydroprocessing catalysts that have substantial promise as next-generation catalysts [12]. The group of Nozaki first reported the catalytic activity of phosphides in 1980 [13]. In 1998, Oyama group found that MoP could be easily synthesized by temperature-programmed reduction (TPR) of a metal phosphate precursor [14], transition metal phosphides began to be widely used in catalytic reaction. Especially, they exhibited excellent reactivity in hydrodenitrogenation (HDN) [15] and hydrodesulfurization (HDS) [16]. Considering the similarity of HDN, HDS, and HDC, we synthesized the MoP catalysts used for HDC of TCE. However, the low BET surface area limits the performance of the catalysts [16]. In this paper, well-dispersed MoP/␥-Al2 O3 with high surface area was prepared by SG-TPR method for the first time, and the application of the catalyst in the HDC decomposition of TCE was compared with those prepared by impregnation and mechanical mixing methods. 2. Experimental 2.1. Preparations

∗ Corresponding author. Tel.: +86 25 83358312; fax: +86 25 83358312. E-mail address: [email protected] (L. Ren).

In this paper, s-MoP/␥-Al2 O3 represents the catalyst prepared by sol–gel method. i-MoP/␥-Al2 O3 represents the catalyst prepared

http://dx.doi.org/10.1016/j.cattod.2015.09.019 0920-5861/© 2015 Elsevier B.V. All rights reserved.

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by impregnation method. m-MoP/␥-Al2 O3 represents the catalyst prepared by mechanical mixture method. The detailed preparation methods are as follows: For s-MoP/␥-Al2 O3 catalyst, firstly, a mixture of 1.25 g (NH4 )6 Mo7 O24 ·4H2 O (AR, Hefei Kehua Fine Chemical Research Institute) and 0.94 g (NH4 )2 HPO4 (AR, Xilong Chemical Co., LTD., China) was dissolved in 15 mL deionized water and stirred for 3 h under 80 ◦ C. Secondly, a solution was made by mixing AlCl3 . 6H2 O (24.43 g, 0.1 mol) (AR, Sinopharm Chemical Reagent limited corporation), 24 mL water with 56 mL ethanol, and heated to 80 ◦ C, while stirred for 0.5 h. After that, the mixture prepared in the first step was poured into the solution. Then, propylene oxide, as gelation inducing agent, was added to the solution, cooled in ice water bath. Gel formation typically occurred within 40 min at the temperature of 30 ◦ C. The gel parts were then soaked in a bath of absolute ethanol for 24 h to exchange the water and reaction byproducts from the pores of the material. After the dryness at 120 ◦ C, the sample was calcined in air at 500 ◦ C for 5 h, pressed, and sieved to particles between 420 and 840 ␮m in diameter for use. The particles obtained above were reduced by heating from room temperature to 300 ◦ C at a rate of 10 ◦ C/min, then from 300 to 650 ◦ C at a rate of 1 ◦ C/min, and kept at 650 ◦ C for 2 h in flowing H2 (50 mL/min), followed by cooling to room temperature. Bulk MoP, i-MoP/␥-Al2 O3 , m-MoP/␥-Al2 O3 , and Mo/␥-Al2 O3 catalysts were prepared as the procedure reported references [14] and [17]. Bulk MoP catalyst was prepared as the procedure: a mixture of 4.0 g ammonium heptamolybdate (NH4 )6 Mo7 O24 ·4H2 O and 3.0 g diammonium hydrogen phosphate (NH4 )2 HPO4 was dissolved in 30 mL deionized water. A white solid obtained following evaporation of the water. Then the sample was calcined in air at 773 K for 5 h to give a dark blue solid, subsequently pressed, broken, and sieved to particles between 420 and 840 ␮m in diameter for use (mesh size 20–40). And then the sample was reduced by H2 as the same as the preparation method of s-MoP/␥-Al2 O3 catalyst.iMoP/␥-Al2 O3 catalyst was prepared as the procedure: a commercial aluminum oxide was used as the support. Impregnating solutions were prepared by dissolving 2.45 g (NH4 )6 Mo7 O24 ·4H2 O and 1.83 g (NH4 )2 HPO4 in approximately 60 mL water. Then 10 g Aluminum oxide was added to it. The mixture was stirred 12 h at the room temperature. After evaporation of the water at 80 ◦ C, the following operation was the same as the preparation method of unsupported MoP. Mo/␥-Al2 O3 catalyst was prepared as the same method as i-MoP/␥-Al2 O3 catalysts except for the absence of P. MoP loadings for all the MoP/␥-Al2 O3 catalysts in this paper equal to 15 wt%. 2.2. Characterizations BET surface areas were determined from the nitrogen adsorption-desorption curves by the conventional multipoint technique with a Belsorp Mini 2. XRD patterns of the samples were acquired on a Bruker D8 Discover X-ray diffractometer using Cu K␣ ˚ at a respective voltage radiation and a nickel filter ( = 1.5406 A)

of 40 kV and current of 40 mA. TEM was carried out on a FEI-G202010 electron microscope operated at an acceleration voltage of 200 kV. XPS measurements were carried out using a Thermo Scientific K-Alpha instrument. Binding energies were corrected for sample charging using the C (1s) peak at 284.6 eV for adventitious carbon as a reference. 2.3. Catalysts activity measurement Trichloroethylene HDC reactions were carried out using a continuous fixed-bed quartz reactor (id. = 10 mm) at 400–600 ◦ C and atmospheric pressure. A total of 8 mL catalyst was put in the middle of the quartz reactor. The hydrogen flow rate was 100 mL/min. Using a bubbler for TCE supply, the amount of evaporated TCE corresponds to the weight loss of the bubbler. The flow ratio of H2 to TCE was adjusted approximately 14:1. The produced HCl was trapped in a water bubbler and the amount of formed HCl can be determined very accurately by NaOH titration with a pH-indicator (e.g. phenolphthalein). So the decomposition rate of C Cl bonds, which reflects the catalyst activity, can be calculated as the following formula: R C − Cl bonds decomposition =

n (HCl) × 100% 3n (C2 HCl3 )

3. Results and discussion The BET surface area, pore volume, and average pore diameter for the s-MoP/␥-Al2 O3 , i-MoP/␥-Al2 O3 , m-MoP/␥-Al2 O3 , bulk MoP catalysts, and ␥-Al2 O3 support are listed in Table 1. The low BET surface area of the bulk MoP (<10 m2 /g) limits the application of MoP as catalyst. Cheng et al. [18,19] synthesized bulk MoP and 8 wt% MoP/Al2 O3 with high surface area by combining citric acid and temperature-programmed reduction (CA-TPR) method. However, in this communication, we successfully improved the MoP/␥-Al2 O3 catalyst surface area to 417.8 m2 /g by SG-TPR method for the first time. As presented in Table 1, s-MoP/␥-Al2 O3 catalyst has a much higher BET surface area than i-MoP/␥-Al2 O3 (104.6 m2 /g), m-MoP/␥-Al2 O3 (111.2 m2 /g) and bulk MoP (7.2 m2 /g). Fig. 1 shows the XRD patterns for s-MoP/␥-Al2 O3 , i-MoP/␥Al2 O3 , m-MoP/␥-Al2 O3 catalysts as well as for the ␥-Al2 O3 support and the bulk MoP. The XRD pattern of bulk MoP shows peaks at 27.91◦ , 32.08◦ , 43.07◦ , 57.14◦ , 64.75◦ , 67.49◦ , and 74.19◦ , which can be attributed to the (001), (100), (101), (110), (111), (102), and (201) crystal planes of the MoP phase [20]. The XRD patterns for the mMoP/␥-Al2 O3 catalyst shows the same peaks as those observed for bulk MoP, confirming that the MoP crystallites are present on the ␥-Al2 O3 support. In addition, the i-MoP/␥-Al2 O3 catalyst exhibits one weak peak at 74.19◦ for (201) crystal planes of the MoP phase except the characteristic peaks of ␥-Al2 O3 support. Zaman [21] once discovered that it was difficult to identify a MoP phase on silica supported catalysts prepared by impregnation method at loadings below 15 wt% and the MoP/SiO2 catalyst only showed weak characteristic peaks of MoP at 15 wt% loading. Based on this research

Table 1 Physicochemical properties of the studied catalysts. Sample

BET (m2 /g)

Pore volume (cm3 /g)

Pore diameter (nm)

Ref

MoP ␥-Al2 O3 m-MoP/␥-Al2 O3 i-MoP/␥-Al2 O3 s-MoP/␥-Al2 O3 MoP MoP(CA-TPR) 8wt%MoP/Al2 O3 (CA-TPR)

7.2 133.3 111.2 104.6 417.8 8.2 122 161.9

0.05 1.10 0.67 0.41 1.34 – – –

26.6 33.1 24.2 15.6 12.8 – – –

[18] [18] [19]

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(101) (100) (110) (102) (111) (201)

Intensity/a.u.

(001)

a

*

b c d e

0

10

20

30

40

50

60

70

80

90

2θ/degree Fig. 1. X-ray diffraction patterns of (a) bulk MoP; (b) m-MoP/␥-Al2 O3 ; (c) i- MoP/␥Al2 O3 ; (d) s- MoP/␥-Al2 O3 ; (e) ␥-Al2 O3 .

achievement, it can be inferred that the weak characteristic peaks of MoP were due to the low MoP loadings. While no XRD characteristic peaks of MoP were observed in s-MoP/␥-Al2 O3 . So we did the XRD experiments for S-MoP/Al2 O3 with different MoP contents, supplied in the supplementary information (SI) file (Fig. SI-1). From the figure we can find there is no MoP peak until the contents of MoP increased to 40%. But for other metal phosphide catalyst prepared through the same SG-TPR method, for example S-Ni2 P/Al2 O3 , its peak can be easily found, see Fig.SI-2. We consulted a lot of literature, and found that other scholars have encountered the same

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situation. Only for supported MoP catalysts, there is no reflection peak until the content of MoP excess 20%, which is generally considered that the high dispersion of MoP on support lead to this phenomenon [21–23]. So we carried out the TEM measurements on these samples to further investigate the dispersion of MoP on Al2 O3 support. Fig. 2 shows the TEM images of m-MoP/␥-Al2 O3 , i-MoP/␥-Al2 O3 , and s-MoP/␥-Al2 O3 catalysts, respectively. Fig. 2a indicates that the presence of MoP particles dispersed on the ␥-Al2 O3 support is in mass, while a smaller particle size of MoP can be observed in Fig. 2b. We did the energy-dispersive X-ray spectra copy (EDS) analysis on individual dark particle for m-MoP/Al2 O3 (Fig. SI-3), which reveals Mo and P peaks, confirming the formation of molybdenum phosphide. There is no peak of Al, which hints mixed uneven through mechanical mixture method. But we still can tell that the dark particles are MoP. So we can say that the dispersibility of MoP in s-MoP/␥-Al2 O3 catalyst is the best among these three. It is obvious that MoP catalyst with high dispersion can be obtained by sol–gel method. Fig. 3 shows the XPS spectra of the s-MoP/␥-Al2 O3 catalyst. The XPS spectra indicate the presence Mo and P species with one or more states at the surface of the material. The Mo 3d5/2 peak positions for the catalyst is located at electron binding energies of about 227.8, 229.2, 230.1, 231.2, and 232.7 eV, and these are with the same as those of Mo0 , Mo3+ , Mo4+ , Mo5+ , and Mo6+ in a multiphase environment. These results are in agreement with previously reported results [24,25]. So we think that Mo exists as Mo6+ , Mo5+ , Mo4+ , Mo3+ , Mo0 in the s-MoP/␥-Al2 O3 catalyst. The Fig. 3b shows

Fig. 2. TEM images of (a) m-MoP/␥-Al2 O3 (b) i-MoP/␥-Al2 O3 (c) s-MoP/␥-Al2 O3 .

Fig. 3. XPS spectra in the Mo 3d (a) and P 2p (b) regions for the s-MoP/␥-Al2 O3 ;.

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4

0

198 C

30

0

25

s-MoP/γ-Al2O3

c 20 d

15 10

i-MoP/γ-Al2O3

0

571 C

0

0

320 C

183 C

e

5 0

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278 C

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a b TCD signals

C-Cl bonds decomposition ratio/%

35

MoP

f 100

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450

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200

300

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0

Temperature/ C

Temperature/ºC Fig. 5. NH3 -TPD graph of s-MoP/␥-Al2 O3 , i-MoP/␥-Al2 O3 , and MoP catalyst. Fig. 4. C Cl bonds decomposition ratio at different temperatures over (a) s- MoP/␥Al2 O3 ; (b) Mo/␥-Al2 O3 ; (c) i-MoP/␥-Al2 O3 ; (d) m -MoP/␥-Al2 O3 ; (e) ␥-Al2 O3 ; (f) bulk MoP; (8 mL catalyst under 100 mL/min hydrogen flow-rate).

a strong band at 133.4 eV and a tiny band at 128.8 eV, which cannot match with the value for element P (129.9 eV). However, they are consistent with other groups’ findings. Oyama et al. [26] reported XPS spectra for a tungsten phosphide (WP) catalyst and observed peaks in the P 2p region at 130.0 and 134.6 eV. Phillips et al. [27] studied the silica-supported molybdenum phosphide catalyst and concluded that the XPS peak in the P 2p3/2 region at 129.6 and 133.9 eV were assigned to P bonded to Mo and to PO4 3− species. So, we think that no XRD peaks of MoP observed in the s-MoP/␥Al2 O3 catalyst was due to the well dispensability of MoP on the ␥-Al2 O3 support. To study the activity of the catalysts, all catalysts were employed in the trichloroethylene HDC reaction. The catalytic activity for the HDC decomposition of TCE was studied by comparing the C Cl bonds decomposition ratio at different temperatures for each catalyst. In Fig. 4, we compared the catalytic activity of s-MoP/␥-Al2 O3 , Mo/␥-Al2 O3 , i-MoP/␥-Al2 O3 , m-MoP/␥-Al2 O3 bulk MoP catalysts, and the ␥-Al2 O3 support. Low catalytic activity over ␥-Al2 O3 support was observed at 500, 550, and 600 ◦ C, while no activity was detected at the temperature below 500 ◦ C. It was obvious that the activity of the Mo/␥-Al2 O3 catalyst raised with the increase of the temperature. In marked contrast, the catalytic activity of all MoP-catalysts, including s-MoP/␥-Al2 O3 , i-MoP/␥-Al2 O3 , m-MoP/␥-Al2 O3 , and bulk MoP increased at first and then decreased and kept higher activity at 450, 500, and 550 ◦ C than that at 400 and 600 ◦ C. This phenomenon showed that lower temperature cannot activate the MoP-catalysts, while hightemperature results in catalyst deactivation, which means that the

dechlorination reaction needs an optimum temperature in order to avoid the deposition of carbon on the surface of the active phase. Besides, the supported types of MoP-catalysts outperformed the pure MoP at all temperatures. And among those supported MoP catalysts, the s-MoP/␥-Al2 O3 catalyst exhibited the highest activity. The s-MoP/␥-Al2 O3 catalyst also showed higher activity than Mo/␥-Al2 O3 , especially at low temperature. As shown in Fig. 4, at 450 ◦ C, the decomposition rate of C Cl bonds is 29.80% over the sMoP/␥-Al2 O3 catalyst, while only 17.26% over Mo/␥-Al2 O3 . Hideaki Takashima [8] studied the catalytic decomposition of trichloroethylene over Pt-/Ni-catalyst and discovered the decomposition ratio of C Cl bonds is 10.4, 46.9, and 35.2% over 4% Pt/Al2 O3 , 10% Pt/SiO2 and 10% Ni/SiO2 catalysts at 300 ◦ C. It can be inferred that the activity of our new catalyst can approach or even exceed some noble metal HDC catalysts. That is to say, MoP/␥-Al2 O3 prepared through sol–gel method is an effective catalyst for HDC of TCE. In order to further investigate the factors, which attribute the enhanced catalytic activity during HDC by S-MoP/Al2 O3 , we carried out the NH3 -TPD experiment to investigate the effect of acids (Fig. 5). As we could see, MoP catalyst shows a NH3 desorption peak at 183 ◦ C, corresponding to weak acid center, and a broad peak at 320 ◦ C, corresponding to medium strong acid center. For i-MoP/␥-Al2 O3 catalyst, there are a strong desorption peak of weak acid center at 320 ◦ C and a weak desorption peak of strong acid center at 571 ◦ C. The two obvious NH3 desorption peaks of s-MoP/␥-Al2 O3 are at 198 and 278 ◦ C, corresponding to weak acid center and medium strong acid center, respectively, and after 300 ◦ C, the desorption amount is quite little. This graph clearly indicates that all three of these catalysts present weak acid centers, and the signal response values are at following order: s-MoP/␥-Al2 O3 > i-MoP/␥-Al2 O3 > MoP. This order is in accordance with the order of the hydrodechlorination 60

50 40 30 20 10 0

a 1000 2000 3000 4000 5000 6000 Space velocity / h

-1

C-Cl bonds decomposition ratio/%

C-Cl bonds decomposition ratio/%

60

50 40 30 20

b

10 0 0

1

2

3

4

5

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8

Time / h

Fig. 6. (a) C Cl bonds decomposition ratio at different space velocity (keep hydrogen flow rate at 100 ml/min and the catalysts change from 1 to 12 mL) over the 15 wt% s-MoP/␥-Al2 O3 catalysts under 450 ◦ C, (b) C Cl bonds decomposition ratio at 1500 h−1 space velocity (4 mL catalysts under 100 ml/min hydrogen flow rate) over the 15 wt% s-MoP/␥-Al2 O3 catalysts under 450 ◦ C.

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activities of catalysts, which hints that except the surface area, the acid amount on catalyst surface also effect the activities of catalysts. Fig. 6a is the curve of the C Cl bonds decomposition ratio at different space velocity over the 15 wt% s-MoP/␥-Al2 O3 catalysts under 450 ◦ C. The activity of this catalyst increased from 500 to 1500 h−1 space velocity and decreased after 1500 until 2000 h−1 . The C Cl bonds decomposition ratio stabilized when the space velocity is beyond 2000 h−1 . We can obtain the highest activity at the 1500 h−1 space velocity. To study the stability of the catalyst, we carried out a long time experiment and the results were presented in Fig. 6b. Fig. 6b is the curve of the C Cl bonds decomposition ratio at 1500 h−1 space velocity over the 15 wt% s-MoP/␥-Al2 O3 catalyst under 450 ◦ C over 7 h. In Fig. 6b, C Cl bonds decomposition ratio of the catalyst keep around 51 ± 3%, and the fluctuations are tend to reduce along the time without decrease of the activity. It indicates that the catalyst will remain high HDC activity for a long time. 4. Conclusions We have successfully prepared MoP/␥-Al2 O3 catalyst with high surface area through SG-TPR method, which shows remarkably HDC catalytic activities. This method can be applied to prepare other phosphide catalysts and solve their common problem-low surface area, and accelerate the industrialization of such kind catalysts. Acknowledgments This work was financially supported by National Nature Science Foundation of China (21206018), Nature Science Foundation of Jiangsu Province of China (BK20151408), and the Chinese Fundamental Research Funds for the Central Universities, (3207045411). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.09. 019. References [1] C. Amorim, X.D. Wang, M.A. Keane, Application of hydrodechlorination in environmental pollution control: comparison of the performance of supported and unsupported Pd and Ni catalysts, Chinese J. Catal. 32 (2011) 746–755. [2] N. Barrabes, D. Cornado, K. Foettinger, A. Dafinov, J. Llorca, F. Medina, G. Rupprechter, Hydrodechlorination of trichloroethylene on noble metal promoted Cu-hydrotalcite-derived catalysts, J. Catal. 263 (2009) 239–246. [3] O. Orbay, S. Gao, B. Barbaris, E. Rupp, E. Saez, R.G. Arnold, E.A. Betterton, Catalytic dechlorination of gas-phase perchloroethylene under mixed redox conditions, Applied Catalysis B: Environmental 79 (2008) 43–52.

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Please cite this article in press as: Q. Guo, L. Ren, Hydrodechlorination of trichloroethylene over MoP/␥-Al2O3 catalyst with high surface area, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.019