Recovery and Reduction of Spent Nickel Oxide Catalyst via Plasma ...

Plasma Chem Plasma Process (2006) 26:585–595 DOI 10.1007/s11090-006-9035-1 ORIGINAL ARTICLE

Recovery and Reduction of Spent Nickel Oxide Catalyst via Plasma Sintering Technique Fung Fuh Wong Æ Chun-Min Lin Æ Chih-Ping Chang Æ Jun-Rong Huang Æ Mou-Yung Yeh Æ Jiann-Jyh Huang

Received: 7 October 2005 / Accepted: 16 June 2006 / Published online: 21 September 2006 Springer Science+Business Media, Inc. 2006

Abstract A thermal plasma process for the recovery and reduction of nickel oxide in the spent nickel-based catalyst (NiO/SiO2) was developed. The spent catalyst was sintered at >1,500 C under plasma condition and the nickel oxide therein was reduced to nickel, which was proven by XRD and EDX. By application of SEM and GC technique, the organic tar on the surface of the spent catalyst was found to decomposed and converted to syngas (CO, CO2, and H2), which might be the reducing agents in the process. A gasification mechanism for the generation of syngas and the reduction of nickel oxide under plasma conditions was proposed. Keywords Plasma

Recovery Æ Reduction Æ Nickel oxide Æ Nickel Æ Catalyst Æ

F. F. Wong (&) Graduate Institute of Pharmaceutical Chemistry, China Medical University, No. 91 Hsueh-Shih Rd., Taichung, Taiwan 40402, R.O.C e-mail: [email protected] C.-P. Chang Æ J.-R. Huang Sustainable Environment Research Center, National Cheng Kung University, No. 500, Sec. 3, An-ming Rd., Tainan City 709, Taiwan, R.O.C C.-M. Lin Æ M.-Y. Yeh Department of Chemistry, National Cheng Kung University, No. 1, Ta Hsueh Rd., Tainan 701, Taiwan, R.O.C M.-Y. Yeh Nan Jeon Institute of Technology, No. 178, Chaocin Rd., Yanshuei Township, Tainan County 737, Taiwan, R.O.C J.-J. Huang Development Center for Biotechnology, No. 101, Lane 169, Kangning St., Xizhi City, Taipei County 221, Taiwan, R.O.C

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1. Introduction Supported nickel catalysts were widely used in industry for hydrogenation, hydrotreating, and steam-reforming reaction [1, 2]. The metal nickel was supported on the ceramic substrates (i.e., silica and alumina) to increase its surface area for achieving better catalytic efficiency. After multiple of hydrotreating reactions, the surface of NiO-supported catalyst was completely covered with organic tar and loses its catalytic efficiency. In the reforming reaction, deactivation of nickel-based catalysts may also occur due to the coke deposition on the surface of the catalysts [3, 4]. The reduction of nickel oxide to nickel is of great interest, owing to nickel being an important electronactive material in electrochemical systems such as batteries, fuelcells and alkaline electrlizers. Many studies have been concerned the reduction of oxidized nickel [5–7] and some recent work dealt with heating the NiO surface [8–10] and nickel-supported catalysts [10–15] by the different reduced source and sintering technologies. A.C. plasma has been widely used in a number of fields such as ceramics and the metallurgy industry because of the characteristics of high temperature (usually up to 5,000 C) with high densities of ions and electrons [16, 17]. These diverse active species with the high energy radiation capability of the A.C. plasma can help to enhance the chemical reactions substantially and to make some reactions possible. As for gasification, it is commonly applied to convert coal [18], biomass [19], and waste materials [14, 20–22] to syngas and useful chemicals in industries. In this paper, we reported the utilization of plasma and gasification technique for the reduction of nickel oxide in the spent nickel-based catalyst (NiO/SiO2) to nickel. The syngas (CO + H2), generated from the partial oxidation of organic tar, was served as the reducing agents. 2. Experimental 2.1. Sample preparation The spent nickel-based catalyst (NiO/SiO2), obtained from Chinese Petroleum Corp., was ground in an agate mortar to the particle size of 300–500 mesh with a mean particle size of 400 meshes. The sample was added into a graphite crucible with a diameter of 5.0 cm and a thickness of 1.0 cm. The outside wall of graphite crucible was coated with refractory material and treated in an industry coupled thermal plasma torch in the plasma furnace. 2.2. Sintering of the sample by plasma The sintering of the sample by plasma was performed on a plasma furnace with an effective volume of 1.0 L and equipped with an A.C. transferred-type torch. The masses of the bricks consisted of 30 wt% graphite and iron, which ranged from 1.0 to 2.5 kg and 0.25 to 1.0 kg, respectively. A high-voltage A.C. power supply for the plasma was used, with a maximum voltage of 10 kV, a frequency of 10–20 kHz, and a power of 10–15 kW. Constant flow of N2 was used as the central plasma gas (7 L min–1) and sample was heated in the graphite crucible at >1,500 C for 2–3 h. The residual sample was separated from the furnace and weighted.

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2.3. Analytical apparatus Thermodynamical analysis was performed on a Perkin Elemer Pyris Diamond differential thermogravimeteric analyzer with a heating rate at 10 C min–1 in N2. X-ray diffraction (XRD) spectra were measured on a Bruker D8A Series XRD spectrometer. EDX and scanning electron microscopy (SEM) spectra were measured on a Hitachi S-3000N spectrophotometer. 2.4. Gas chromatography Gas samples were collected by a gas sampling port and then injected into Hewlett Packard GC 6850A series equipped with thermal couple detector for characterization. The analyses of H2, N2, CO and CH4 were conducted at 50 C with a 2 m ˚ molecular sieve and argon (99.99 vol%) as the column packed with granular 5A carrier gas. The analyses of CO2 were conducted at 40 C with a 2 m column packed with GDX-104 molecular sieve and hydrogen (99.99 vol%) as the carrier gas. The gas standards for comparison were purchased from San Fu Chemical Co.

3. Results and discussion 3.1. Thermodynamical analysis The water content in the sample was determined by direct heating the sample at 150 C for 24 h and calculating the weight loss. The weight change of the sample was 2.5%. The spent catalyst (4.29 mg) was then measured by TGA. The TGA results (Fig. 1) indicated the onset of weight loss at 40 C and an acceleration of weight loss at 450 C. No significant change of weight was observed at the temperature >1,200 C.

Fig. 1 TGA data of the weight loss of the spent NiO catalyst (10 C/min–1)

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From the TGA result, a reasonable content of water and organic tar in the spent catalyst was considered as 20.2% (Fig. 1). When the same batch of model sample (15.4 g) was sintered in high temperature oven at >1,200 C under atmospheric pressure for 2–3 h, the weight of the sintered product was 12.1 g and the weight loss was 21.4%. The weight loss of the sample in high temperature oven at atmospheric pressure is consistent with that from TGA. For the study of spent catalyst upon sintered by plasma, model sample (NiO/SiO2, 16.3 g) was put in the graphite crucible and sintered in N2 medium under plasma condition at >1,500 C for 2–3 h. The weight of the residual product was found to be 11.8 g and the weight loss was 27.6%. Compared with the weight loss by that from heating without plasma, additional weight loss (6.2%) was observed. We considered this difference was resulted from the reduction of NiO to metallic Ni with a loss of oxygen atom. 3.2. Characterization of the sintered products by XRD For identification of the products formed without the presence of plasma, we performed X-ray power diffraction (XRD) technique for the analysis. The XRD spectrum of the model spent catalyst before and after sintering without plasma showed approximately identical peaks, which possessed the main reflection peak characterized as NiO at 43.3 (2h) (Figs. 2 and 3). We considered the reduction of NiO to metallic Ni did not take place, or was not significant, for the spent catalyst upon sintering without plasma. The sample before and after sintering both showed the characteristic peaks at 37.3, 43.3, 62.9, 75.4, and 79.4 (2h), which was consistence with the data of NiO reported [12, 23]. Results of XRD with NiO characteristic peaks were summarized in Table 1. The XRD of the sintered sample with plasma showed a main and large reflection peak at 44.5 (2h, Fig. 4). The peak positions corresponding to 2h angles of the Ni

Fig. 2 XRD spectrum of the spent catalyst before sintering without plasma

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Fig. 3 XRD spectrum of the spent catalyst after sintering without plasma

Table 1 XRD peaks for NiO of the spent catalyst [12, 23]

2h (NiO)

[hkI]

I/I0

37.3 43.3 62.9 75.4 79.4

111 200 220 311 222

63 100 48 17 12

Fig. 4 XRD patterns of the sintered sample in N2 medium under plasma condition

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2h (Ni)

[hkI]

I/I0

44.5 51.9 76.4

111 200 220

100 44 19

phase (with the relative intensities in the parentheses) are 44.5 (100), 51.9 (44), and 76.4 (19) and the 2h peaks are assigned to the faces (111), (200), and (220), respectively (Table 2) [10, 19]. The XRD data indicated that NiO in the spent catalyst was reduced to metallic nickel under plasma conditions. 3.3. Quantitative analysis Scanning electronmicrographs of the samples after sintered at high temperature with or without plasma conditions were shown in Figs. 5–7. In Fig. 5, the surface of the untreated NiO-supported catalyst was completely covered with organic tar. After sintering at >1,200 C under atmospheric pressure for 2–3 h, the SEM results indicated that most of the small particle of organic tar was vaporized. However, some big particle is still observed in the surface of the spent catalyst (see Fig. 6). When the sintering process was carried out in A.C. plasma conditions under nitrogen atmosphere for 2–3 h, silica supporting material was clearly observed (see Fig. 7). The particles showed homogenous size with an average diameter (~1.5 lm). The NiO in the spent catalyst was believed to be reduced to Ni under such condition. The EDX results of spent NiO catalyst and the sintered samples with or without plasma were shown in Figs. 8–10, and their quantitative analyses were summered in Table 3. In the samples after sintering either with or without plasma, most carbon contents vanished. For the samples treated under plasma conditions, its oxygen

Fig. 5 Scanning electron micrographs taken from the surface of the spent catalyst (magnification · 1,000)

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Fig. 6 Scanning electron micrographs taken from the surface of the sintered sample in high temperature oven (magnification · 1,000)

Fig. 7 Scanning electron micrographs taken from the surface of the sintered sample in N2 medium under plasma condition (magnification · 1,000)

content significantly decreased from 34.4% to 25.0% (wt%). However, the oxygen content of the sintered samples without plasma only decreased from 34.4% to 30.6% (wt%). As a result, the nickel oxide in the spent catalyst under plasma treatment was reduced to nickel. Some additional atoms found after the samples were treated with plasma possibly come form the plasma furnace or refractory material.

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Fig. 8 EDX spectra of the spent catalyst (a: Au atom)

Fig. 9 EDX spectra of the sintered sample in the high temperature oven (a: Au atom)

Fig. 10 EDX spectra of the sintered sample product in N2 medium under plasma condition (a: Au atom)

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Plasma Chem Plasma Process (2006) 26:585–595 Table 3 Chemical composition of the spent catalyst and the sintered sample by XRD

Element

a

The extra content of atoms (Ca, K, and S) were identified after sintering process

C O Al Si Fe Ni Total

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Weight (%) The spent catalyst

The sintered product in high temperature oven

The sintered product in N2 medium under plasma condition

28.5 34.4 0.6 9.2 0.3 27.0 100

– 30.6 1.3 19.4 0.6 48.0 99.9a

– 25.0 1.6 23.9 0.8 47.0 98.3a

The first stage (a) Active species in plasma at the initial step (1) N2 + e∗

N2+ + 2e

(2) N2 + e∗

2N + e

(b) Active species in the presence of water (3) H2O + e∗ (4) N + H

OH + H + e NH H2O + O

(5) OH + OH The second stage

(c) Active species formed with the organic waste in the spent catalysts Char + Volatile

(6) Organic waste material (7) Char/Volatile + e∗

CmHn + H + C + C2 + e

The third stage (d) Reduction of NiO and gases formed in the system (8) C + OH

CO + H

(9) H + H + M

H2 + M

(10) CO + OH

CO2 + H CO2 + M

(11) CO + O + M (12) H + CH

H2 + C

(13) H + C + H2 + e + NiO

HO + CO + Ni

Scheme 1 A possible mechanism for the reduction of NiO in nitrogen medium under plasma conditions

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3.4. Gasification and reduction mechanism To realize the mechanism of the reduction process for the spent catalyst under plasma conditions, the gases produced were analyzed by GC and peaks in the chromatogram were compared with the standards with and without spike. Except for the N2 gas from the furnace, three additional peaks were found and assigned as CO, H2, and CO2. Syngas (CO + H2) was found in sintering process, which might serve as the reducing agent for the reduction of NiO to Ni in the spent catalyst. The major component of tar was carbon and hydrocarbons in high molecular weight [24–26]. They were found to produce stable, excited and radical intermediate (C atoms, H atoms, CH radical, OH radical, NH radical, CN radical, CO+ ion, O+ ion and N+2 ion) species in the plasma [27]. There species would react with H2O to produce syngas in radical or atomic type reactions. Accordingly, we proposed a mechanism of the process into three-stage reactions as shown in Scheme 1. At the first stage, nitrogen gas was partly electrified to form active species such as N+2 and N intermediates, and water was dissociated into OH radicals and H radicals upon A.C. plasma (from (1) to (3)). The reaction of the active species might take place and result in the formation of new active intermediates such as NH and O atoms ((4) and (5)). The second stage is the vaporization of tar on the spent catalyst to form char and some volatile, which would generate CmHn hydrocarbon [28–30], exited C and H atoms, etc ((6) and (7)). The interaction of the active species from the first two stages was displayed in the third stage, which contained the formation of CO, H2 and CO2 in parallel- or chain-type reactions (from (8) to (13)). The net reaction of the three stages was the formation of syngas which contained H2, CO, and CO2. The NiO in the spent catalyst was thus reduced to Ni by H2 [31], carbon [13], H radicals, and electrons under plasma sintering process.

4. Conclusions We have developed a new method to reduce the spent nickel oxide catalyst (NiO/ SiO2) to nickel under A.C. plasma conditions. The organic tar in the spent catalyst was converted to syngas (CO and H2) to serve as a reducing agent in thermal plasma reactor. This technique might be applicable for the reduction and recovery of spent metallic oxide catalyst in industries. Acknowledgment We are grateful to the National Science Council of Republic of China for financial support.

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