SCIENCE CHINA Chemistry • ARTICLES •

April 2010 Vol.53 No.4: 846–850 doi: 10.1007/s11426-010-0087-y

Ni supported on activated carbon as catalyst for flue gas desulfurization CHU YingHao1,2, GUO JiaXiu1,2, LIANG Juan1,2, ZHANG QiangBo1 & YIN HuaQiang1,2 1

2

Department of Environmental Science and Engineering, Sichuan University, Chengdu 610065, China; National Engineering Technology Research Center for Flue Gas Desulfurization, Sichuan University, Chengdu 610065, China Received August 31, 2009; accepted October 6, 2009; published online March 1, 2010

A series of Ni supported on activated carbon are prepared by excessive impregnation and the desulfurization activity is investigated. It has been shown that the activated carbon-supported Ni is an efficient solid catalyst for flue gas desulfurization. The activated carbon treated by HNO3 exhibits high desulfurization activity, and different amounts of loaded-Ni on activated carbon significantly influence the desulfurization activity. The catalysts are studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results of XRD and XPS indicate that the activated carbon treated by HNO3 can increase oxygen-containing functional groups. Ni on activated carbon after calcination at 800 °C shows major Ni phase and minor NiO phase, and with increasing Ni content on activated carbon, Ni phase increases and affects the desulfurization activity of the catalyst, which proves that Ni is the main active phase. activated carbon, Ni, desulfurization, XRD, XPS

1

Introduction

Much more energy-saving and whole removal of SO2 from flue gas has been expected for a better environment. Oxidative adsorption of SO2 and conversion to H2SO4 on activated carbon (AC) is a basis for a dry process. Activated carbon as catalyst support exhibits several advantages [1, 2] because it is an inert material with a large surface area and porous structure. The microcrystalline structure of AC is not of integrity, in addition to the existence of ash and other hetero-atoms [3, 4], resulting in producing unsaturated valence of defects and elements, which displays oxygen-containing functional groups and nitrogen-containing functional groups in the chemical structure. It is well known that carriers strongly influence the metal particle surface morphology and electronic structure, which can directly affect the stability and catalytic activity of catalysts [5]. Pre-treatment of *Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2010

the support or addition of promoters can significantly affect such interactions as well as the dispersion of the active species [6, 7]. The dispersion of specific supported metal is one of the main parameters related to its catalytic activity. When carbon is used as the support, metal dispersion can be more or less controlled by means of thermal and chemical modification of the support porous structure and surface oxygenated complexes [8]. It is reported that the SO2 adsorption characteristics of low ash active carbons are strongly influenced by the presence of certain transition metal derivatives [9]. Klinik et al. [10] also studied the removal ability of SO2 over the activated carbon loaded with Co, Ni, Mn and V and found that the generated Co(OH)2, Ni(OH)2, MnO2 and V2O3 microcrystalline could increase the desulfurization activity of activated carbon. Some authors have reported that Ca2+ and Mo6+ introduced to activated carbon surface can greatly promote SO2 conversion [11]. Therefore, in order to investigate the Ni active species on activated carbon, a series of Ni supported on activated carbon as catalyst for flue gas desulfurization are prepared. In this chem.scichina.com

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CHU YingHao, et al.

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paper, Ni(NO3)2 salt solution was impregnated on activated carbon. The desulfurization activity of the prepared catalyst was measured, and Ni chemical states were characterized by XRD and XPS.

2 Experimental 2.1

SO2 removal

SO2 removal was carried out at 90 °C in a fixed bed flow reactor by passing a flue gas mixture. Gases were controlled by a rotor flow meter before entering the blender. The catalyst was packed in a reactor tube of 15 mm diameter and the packed height was about 50 mm. The simulated flue gas contained 0.26% of SO2, 11% of O2, 14% of water vapor, and N2 (balance). The gas space velocity (SV) was 2000 h1. A relation curve about SO2 removal and reaction time was obtained. The flue gas before and after reactor passed through a solution containing H2O2 (5 vol%), and formed H2SO4 was determined by titrating with NaOH (0.01 M) solution [4]. The amount of H2SO4 was used to calculate the total sulfur amount on catalysts. 2.3

XPS experiments were carried out on a spectrometer (XSAM-800, KRATOS Co.) with Al K radiation under UHV and operating at 12 kV and 12 mA. Energy calibration was done by recording the core level spectra of Au 4f7/2 (84.0 eV) and Ag 3d5/2 (368.3 eV). Peak areas including satellites, were computed by a program which assumed Gaussian-line shapes and flat background subtraction.

Catalyst preparation

Catalysts were prepared by excessive impregnation. The original activated carbon (AC) was pretreated by nitric acid (labeled as NAC) and immersed into excessive nickel nitrate aqueous solution, followed by pouring excessive liquid and drying. The Ni content in the catalysts was 1, 3, 5, and 10 wt%. The prepared catalysts were calcined at 800 °C in nitrogen atmosphere. These catalysts were written as Ni/NAC1, Ni/NAC3, Ni/NAC5 and Ni/NAC10, respectively. Using the same preparation method, 1 wt% Ni loading on AC was used as comparative sample and written as Ni/AC1. 2.2

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3

Results and discussion

3.1

Desulfurization performance of Ni/NAC catalysts

The desulfurization performance of Ni/NAC catalysts, including the relation between SO2 removal and reaction time and sulfur capacity (SO2 removal rate decreases to 90% corresponding to the total amount of SO2 removal), is presented in Figure 1 and Table 1. Compared with NAC, Ni/AC1 catalysts exhibit good desulfurization activity and high sulfur capacity, showing that Ni component supported on AC can significantly improve desulfurization performance of NAC. At the same time, it is also found that the desulfurization performance of Ni/NAC is better than Ni/AC1, indicating that the supports treated by HNO3 can further enhance the desulfurization efficiency. From Figure 1 and Table 1, it is noticed that the sulfur capacity increases with time prolonging. Under the same experiment conditions, when SO2 removal rate decreases to 90%, Ni/AC1 catalyst needs 6.59 h while Ni/NAC10 catalyst needs longer time (7.69 h) and they correspond to the sulfur capacity of 206 and 263 mg/g, respectively. Furthermore, it can be seen that the desulfurization performance of Ni/NAC catalysts with different loading amounts of Ni seems almost the same, which could be due to similar physical and chemical

Catalyst characterization

The crystal structures of the samples were determined by a power X-ray diffraction on a DX-1000 diffractometer using Cu K radiation ( = 0.15406 nm) and operating at 40 kV and 25 mA. The XRD data were recorded for 2 values between 10° and 90° with an interval of 0.03°. The crystalline phases were identified by comparison with the reference data from the International Center for Diffraction Data (ICDD).

Figure 1 The curves about the relation between SO2 removal rate and time.

Table 1 The sulfur capacity of Ni/NAC catalysts at 90 °C Catalysts

NAC

Ni/AC1

Ni/NAC1

Ni/NAC3

Ni/NAC5

Ni/NAC10

Sulfur capacity (mg/g)

127

206

258

265

270

263

Time (h)

4.29

6.59

7.54

7.77

8.01

7.69

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properties of activated carbon treated by HNO3. According to the results, the desulfurization activity of catalysts is in the following sequence from poor to excellent: NAC< Ni/AC1
XRD and XPS study

Surface oxygen on carbon consists of inorganic and organic matters. Acid treatment mainly removes the inorganic oxides and increases the organic oxygen-containing materials, which can influence catalyst activity. Figure 2 shows the C1s spectra of Ni/AC1 and Ni/NAC1. The band energy (BE) values obtained from the deconvolution are listed in Table 2. It is found that Ni/AC1 has one major peak at 284.78 eV with three shoulders at 285.86, 287.45, and 290.1 eV, respectively, which can be assigned to graphitic carbon, alcohol or ester carbon, carbonyl or ketone carbon, and car-

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boxylic acid carbon, respectively [12–14]. Meanwhile, it is found that the C1s BE values of Ni/NAC1 are close to the C1s BE values of Ni/AC1, indicating that the original activated carbon after treated by HNO3 does not change carbon properties, but oxygen-containing functional groups significantly increase. This is in agreement with the reference [8]. The O1s XPS patterns of Ni/AC1 and Ni/NAC1 are shown in Figure 3. For the Ni/AC1 catalyst, the peak at 533.85 eV is assigned to OH oxygen, and the peak at 532.66 eV is attributed to C=O or CO oxygen [15]. For the Ni/ NAC1 catalyst, there are two major peaks at 533.55 and 532.17 eV, which belong to OH oxygen and C=O or CO oxygen, and the minor peak at 529.6 eV corresponds to an oxide oxygen, which is assigned to NiO [16], indicating that a relatively strong nickel-oxygen bond has been formed. In Table 2, it can be seen that the content of oxygen-containing functional groups on Ni/NAC1 is higher than that of Ni/AC1, suggesting that AC treatment by HNO3 could increase oxygen-containing functional groups. Karatepe [17] et al. have found that the phenolic and lactone groups present on the ACs seem to play an important role in SO2 adsorption. In Figure 1, it is also found that the desulfurization performance of Ni/NAC1 is better than that of Ni/AC1, indicating that oxygen-containing functional groups play a key role in SO2 removal. Since SO2 is an acid gas, active sites responsible for adsorption should have a basic character. The basic sites have been related to surface oxygen [18]. The catalytic

Figure 2 The binding energy patterns of C1s for Ni/AC1 (a) and Ni/NAC1 (b). Table 2 The binding energy results of C1s for Ni/AC1 and Ni/NAC Samples

Ni/AC1

Ni/NAC1

BE (eV)

area (%)

BE (eV)

area (%)

CC

284.78

68

284.78

55

CO

285.86

20

286.42

31

C=O

287.45

7

287.83

8

O=COH

290.1

4

290.07

6

Figure 3 The binding energy patterns of O1s for Ni/AC1 (a) and Ni/NAC1 (b).

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activity depends on oxygen-containing surface functional groups with Brønsted basicity properties [19]. SO2 is firstly absorbed on the catalyst and then reacts with oxygen species [20], forming absorbed SO3, followed by generating H2SO4 with H2O. In order to investigate the chemical state of Ni on NAC, Ni/NAC1and Ni/NAC5 catalysts are characterized by XRD. The XRD patterns of samples are shown in Figure 4. It is reported that the 2 angles of Ni phase are 44.51°, 51.85° and 76.36° while those of NiO phase are 37.29°, 43.30°, 62.91° and 75.43° [21]. For Ni/NAC1, minor peaks characteristic of NiO at 2 = 37.3°, 43.3° and 62.9° and major peaks characteristic of Ni at 2 = 44.5°, 51.8° and 76.5° are observed, indicating that Ni and NiO phases coexist. For Ni/NAC5 catalyst, it is very similar to that of the Ni/NAC1. Ni and NiO phases are observed, and peaks characteristic of Ni obviously increase, showing that nickel nitrate on activated carbon is decomposed and generates metal Ni with increasing the Ni loading. The reported nickel nitrate can form nickel oxide at 300 °C and metal Ni is formed at 500 °C [15]. In this paper, all catalysts are calcined at 800 °C, and metal Ni should be formed, but minor NiO is detected, which could be due to the interaction between metal Ni and surface oxygen-containing functional groups. Combined with SO2 removal results, the Ni/NAC5 with 5 wt% Ni content also shows high catalytic activity, which is due to high dispersion of Ni. When both cost-saving and catalytic activities are taken into consideration, it is suggested that 5 wt% Ni loading content is a better loading amount. In order to further clarify the valence state of Ni species on NAC, Ni/NAC1 catalyst is characterized by XPS. The state of Ni species in catalyst samples is evaluated by deconvolution of the Ni2p region spectra, using a multiple Gaussian fitting function, as shown in Figure 5. It is reported that the Ni2p3/2 binding energy of Ni0 appears at 852.6 eV [22]; for NiO, binding energies of Ni2p3/2 in the range of 853.9–854.9 eV for the main peak and accompanying shake-up satellite peak at 860.8–862.8 eV have

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Figure 5 Ni 2p XPS spectra of Ni/NAC1.

been assigned to Ni2+ species [16, 21, 23–25], while the Ni2p3/2 binding energies of 855.7–856.9 eV and 860.1– 864.5 eV are assigned to the NiO bond of Ni2O3 [26–29]. For Ni/NAC1, the Ni2p3/2 binding energy at 854.50 and 860.98 eV and the Ni2p1/2 binding energy at 872.03 and 878.91 eV are attributed to NiO and Ni0, which is in agreement with the reference [30]; the Ni2p3/2 binding energy at 854.65 and 861.16 eV and the Ni2p1/2 binding energy at 872.12 and 878.39 eV fit very well the previously reported results for zerovalent metallic nickel [31]. It is also confirmed that Ni and NiO species coexist on the activated carbon, which favors the flue gas desulfurization.

4

Conclusions

The original activated carbon treated by HNO3 can significantly increase the surface oxygen-containing functional groups. Ni loading on activated carbon can enhance SO2 removal ability. Chemical states of Ni on activated carbon are minor NiO and major Ni, and with increasing Ni content, the Ni phase on activated carbon is significantly enhanced and the catalytic activity remarkably increases. Considering two respects of the cost-saving and catalytic activity, it is suggested that 5 wt% Ni loading content is an optimal loading amount. The authors express their great thanks for the support from Sichuan Province Science and Technology Agency Public Research Projects (Grant No. 2008SG0014). They also thank Analytical & Testing Center of Sichuan University for assistance in XPS and XRD measurements. 1

2 Figure 4 XRD patterns of Ni/NAC1 (a) and Ni/NAC5 (b).

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