γ-Al2O3 catalyst via high-frequency cold plasma direct reduction process

Journal of Natural Gas Chemistry 18(2009)35–38

Preparation of novel Ni-Ir/γ-Al2O3 catalyst via high-frequency cold plasma direct reduction process Liqiong Huang1 ,

Wei Chu1∗ , Tao Zhang2 ,

Yongxiang Yin1 , Xumei Tao1

1. Department of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China; 2. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China [ Manuscript received September 27, 2008; revised November 3, 2008 ]

Abstract The novel Ni-Ir/γ-Al2 O3 catalyst, denoted as NIA-P, was prepared by high-frequency cold plasma direct reduction method under ambient conditions without thermal treatment, and the conventional sample, denoted as NIA-CR, was prepared by impregnation, thermal calcination, and then by H2 reduction method. The effects of reduction methods on the catalysts for ammonia decomposition were studied, and they were characterized by XRD, N2 adsorption, XPS, and H2 -TPD. It was found that the plasma-reduced NIA-P sample showed a better catalytic performance, over which ammonia conversion was 68.9%, at T = 450 ◦ C, P = 1 atm, and GHSV = 30, 000 h−1 . It was 31.7% higher than that of the conventional NIA-CR sample. XRD results showed that the crystallite size decreased for the sample with plasma reduction, and the dispersion of active components was improved. There were more active components on the surface of the NIA-P sample from the XPS results. This effect resulted in the higher activity for decomposition of ammonia. Meanwhile, the plasma process significantly decreased the time of preparing catalyst. Key words: high-frequency cold plasma jet; Ni-Ir catalyst; direct reduction; ammonia decomposition; hydrogen production

1. Introduction Proton exchange membrane fuel cell (PEMFC) gives a relatively high efficiency of energy conversion and no pollution problem by using hydrogen as fuel source [1]. The process of decomposition of NH3 was believed to be an ideal on-site source of COx -free hydrogen and the unconverted NH3 could be reduced to less than 200 ppb level by means of an adsorber for pure hydrogen production [2]. Various catalysts, like supported noble metal catalysts, have been studied for their role in decomposition of ammonia [3−10]. Among them, supported Ni-based catalysts showed good catalytic performances, next to those of Ru and Ir samples. In practice, the Ni-based catalysts were more promising than the noble metal catalysts. However, the content of the nickel metal was relatively high at 30% to 40%, and the prepared catalysts presented low catalytic activities at lower temperatures [10]. Therefore, there was a need for further researches on the development of novel catalysts with low metal content for high NH3 conversion at lower temperatures. In recent years, plasma techniques had attracted consid∗

erable attention in the field of preparing effective catalysts [11−17]. Plasma treatment could increase the dispersion of the metals on supports to a large extent [11,12]. He et al. used cold H2 plasma to synthesize reduced noble metal nanoparticles supported on TiO2 -gel films and controlled the particle sizes between 2 and 10 nm [11]. Zhang et al. reported a novel Ni/α-Al2 O3 catalyst activated by H2 plasma [12], improving the catalytic performances for methane partial oxidation to syngas. Chen et al. prepared a Pd/α-Al2 O3 catalyst via H2 plasma reduction [14], showed a higher conversion, a better selectivity, and a good stability for the selective hydrogenation of acetylene. Among these, H2 plasma had excellent abilities in reducing metal ions to lower valences because of the strong reducing nature of the H radicals or atoms which were produced in the plasma [13,15]. Conventionally, the reduction process of catalysts was a two-step procedure, decomposing the precursors in air first and then thermally reducing the obtained samples in hydrogen. In our previous study [17], the cold plasma jet was successfully applied to prepare the reduced Ni/SiO2 sample directly, instead of the thermal reduction in hydrogen for

Corresponding author. Tel: +86-28-85403836; Fax: +86-28-85461108; E-mail: [email protected] and [email protected] Foundation item: National Natural Science Foundation of China (20590360) and New Century Excellent Talent Project of China (NCET-05-0783).

Copyright ©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(08)60082-1

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methane reforming reaction. And the methane conversion and the CO2 conversion could reach 93% and 76%, respectively, over the Ni-SiO2 catalyst prepared by this novel method. In this study, the high-frequency cold plasma jet direct reduction was operated for the preparation of novel Ni-Ir/Al2 O3 catalyst for ammonia decomposition to hydrogen. The novel catalyst and conventional sample were studied by XPS, XRD, N2 adsorption, and H2 -TPD measurements. 2. Experimental 2.1. Catalyst preparation The γ-Al2 O3 support was calcined at 500 ◦ C for 3 h before use. The conventional Ni-Ir/γ-Al2O3 catalyst was prepared by the impregnation method (Ni: 10 wt%; Ir: 1.8 wt%). The γ-Al2 O3 support was impregnated with an aqueous solution containing the metal salts (H2 IrCl6 and Ni(NO3 )2 ) for 1 h, evaporated at 80 ◦ C water bath, and then dried in oven at 110 ◦ C for 12 h. The obtained sample was divided into two parts. One part was decomposed at 600 ◦ C in air for 3 h and then reduced in pure hydrogen for 2 h in situ (marked as NIA-CR). The other part was directly reduced using the atmospheric high-frequency plasma jet in a mixture of argon and hydrogen for only 10 min, with the source power value of 175 V and the current value of 2 A (marked as NIA-P). The process of plasma treatment and the apparatus had been illustrated in detail in our previous study [17]. 2.2. Catalyst characterization X-ray photoelectron spectroscopy (XPS) analysis [18] was performed on a XSAM 800 spectrometer with an Al K α (hν = 1486.6 eV) X-ray source. The binding energies (BE) were corrected by the calibration of BE of C 1s peak at 284.6 eV. X-ray diffraction (XRD) patterns were obtained from the DX-1000 diffractometer using Cu K α radiation from 20o to 90o at a scanning speed of 0.06o /s in a continuous mode. The X-ray source was operated at 40 kV and 25 mA. H2 -TPD experiments were carried out on a TP5000 apparatus. The samples were placed in a vertical quartz tube. Before hydrogen adsorption, the NIA-CR sample was reduced in situ at 600 ◦ C in hydrogen for 1 h and cooled down to room temperature in the same atmosphere. The NIA-P sample was tested as such, without further reduction treatment. Then, the tube was swept with the nitrogen flow at a rate of 30 ml/min until the chromatogram baseline became stable (2 h). Finally, the sample was heated at a rate of 10 ◦ C/min in nitrogen from 50 ◦ C to 700 ◦ C, and the TPD spectra were recorded. The specific surface areas and the pore sizes of the catalysts were determined by the nitrogen adsorption method using a NOVA 1000e apparatus operating at −196 ◦ C. The samples were degassed in vacuum at 300 ◦ C for 3 h before tests. 2.3. Catalytic activity test The decomposition of NH3 was carried out in a contin-

uous flow quartz reactor by feeding a gas mixture of 5.9% NH3 /Ar (GHSV = 30000 h−1 ) over 100 mg catalyst. Before reaction, the NIA-CR sample was reduced in situ in pure H2 flow at 600 ◦ C for 2 h, and then purged with a flow of pure argon until it was cooled down to 350 ◦ C. The reaction temperature ranged from 350 ◦ C to 600 ◦ C. The product analysis was performed on an SC-200 online gas chromatograph equipped with a thermal conductivity detector and a TDX-01 column, using argon as the carrier gas. The volume content of ammonia and hydrogen was determined by the peak area normalization method. The ammonia conversion was calculated according to the hydrogen balance using the equation as follows: X NH3 = 2CH2 /(2CH2 + 3CNH3 ). 3. Results and discussion 3.1. Surface element analysis of catalysts The results of surface element status in catalysts are presented in Table 1. The BE of Ni 2p3/2 was ca. 852.6±0.1 eV over both two catalysts, corresponding to the zero-valent nickel species. The BE of Ir 4f7/2 were 60.8 eV and 61.3 eV for NIA-P and NIA-CR samples, respectively, which indicated the existence of Ir0 . The BE of Ir 4f7/2 of NIA-P sample was 0.5 eV lower than that of NIA-CR sample. The catalyst sample had been better reduced by the plasma direct reduction process than by conventional method. According to the XPS analysis results, the atomic ratios of Ni/Al and Ir/Al over the above catalysts, obtained through the semi-quantitative computation, are also shown in Table 1. The amount of surface Ni species over the NIA-CR catalyst was less than that of the NIA-P sample, the ratio of Ni/Al increased from 5.8×10−2 to 6.9×10−2. The amount of surface Ir species also had an increase, the Ir/Al increased from 1.2×10−2 to 2.0×10−2. An enrichment of metal species on the surface of the NIAP sample suggested a higher dispersion, compared with that of the NIA-CR sample. The number of active sites increased. To sum up from the XPS results, the plasma direct reduction gave better reduction than that of the conventional reduction process and provided more active sites for the catalytic reaction. Table 1. Results of binding energy and surface atomic composition of typical catalysts from XPS analysis Binding energy (eV) Ni 2p Ir 4f Al 2p O 1s NIA-CR 852.7 61.3 74.4 531.1 NIA-P 852.6 60.8 74.4 531.1 Catalyst

Atomic concentration Ni/Al Ir/Al O/Al 5.8×10−2 1.2×10−2 2.5 6.9×10−2 2.0×10−2 2.5

3.2. Structure and texture analysis From the results of N2 adsorption analysis, shown in Table 2, the specific surface area of the NIA-P catalyst was 163.1 m2 /g, and it was 149.1 m2 /g over the NIA-CR sample. The specific surface area of the NIA-P catalyst increased by 14 m2 /g, compared with that of the NIA-CR sample. The increase of surface area facilitated the dispersion of metals. The average pore size was slightly smaller for the catalyst prepared by the cold plasma jet.

Journal of Natural Gas Chemistry Vol. 18 No. 1 2009

Table 2. Texture properties of the catalysts Catalyst NIA-CR NIA-P

Specific surface area (m2 /g) 149.1 163.1

Average pore diameter (nm) 5.94 5.43

Pore volume (ml/g) 0.221 0.221

XRD diagrams of NIA-CR and NIA-P samples are shown in Figure 1. There was no appearance of diffraction peaks of iridium. It indicated good dispersions of iridium in both catalysts. The characteristic peaks of Ni0 appeared at 2θ = 44o , 51o , and 76o , beside the peaks attributed to Al2 O3 (2θ = 37o , 43o , and 67o ) over the NIA-CR sample. There were also peaks of NiO at 2θ = 37o , 43o, and 61o which resulted from the exsitu XRD characterization. However, the diffraction peaks of Ni0 over the novel NIA-P catalyst were much broader and of lower intensity, nearly not visible in the XRD diagram. It was suggested that the crystallite size of Ni0 was smaller for the novel plasma catalyst, compared with that of the conventional counterpart. The plasma treatment would lead to a production of much smaller active Ni species [15]. No spinel phase of nickel was found in both catalysts, which was in agreement with the result of reference [17]. The zero-valent Ni metal peak position appeared beside that of γ-Al2 O3 [20]. As pointed out in reference [21] , smaller crystallite size of Ni0 could result in higher ammonia conversion. Zero-valent Ni metal was responsible for the high activity of ammonia decomposition.

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surface (ascribed to nondissociation hydrogen species). There was another low-temperature hydrogen desorption peak at ca. 300 ◦ C over the NIA-P sample, whereas there was no similar peak over the NIA-CR sample. The peak located at ca. 700 ◦ C could be originated from strongly chemisorbed hydrogen. Cui et al. had confirmed that it was due to the hydrogen desorption of Ni-H species [19], formed at higher temperature on the Ni/α-Al2 O3 catalyst. They believed that the Ni-H species at higher temperatures were favorable for the reforming reaction.

Figure 2. H2 -TPD profiles of catalysts. (1) NIA-CR, (2) NIA-P

3.4. Catalytic activity test results and analysis The catalytic activities of two typical Ir-Ni bimetallic catalysts are shown in Figure 3, at the temperature ranging from 350 ◦ C to 600 ◦ C. The conversion of ammonia increased with the elevating reaction temperature. It could be ascribed to that the decomposition of ammonia was endothermic and higher temperature was favorable for the forward reaction. The ammonia conversion over the NIA-P sample was 68.9% which was 31.7% higher than that of the NIA-CR sample, under the conditions at P = 1 atm, GHSV = 30000 h−1 , and T = 450 ◦ C. The NIA-P sample showed much better low temperature activities.

Figure 1. XRD patterns of the two typical catalysts. (1) NIA-CR, (2) NIA-P

3.3. H 2 -TPD study of catalysts The H2 -TPD profiles of two catalysts are shown in Figure 2. They both exhibited main peaks in lower temperature (<400 ◦ C) and higher temperature (>400 ◦ C) ranges, respectively. The lower temperature peak corresponded to the desorption of hydrogen which was weakly adsorbed on the metal

Figure 3. NH3 conversion versus reaction temperature on typical catalysts. Reaction conditions: 100 mg catalyst, P = 1 atm and GHSV = 30000 h−1

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4. Conclusions The catalytic performance of the Ni-Ir/γ-Al2O3 catalyst was improved via cold plasma jet direct reduction. The NIAP sample showed a distinctively higher ammonia conversion than that of the NIA-CR sample in low temperature range. TPD, XRD, and XPS results indicated that the plasma direct reduction improved the metal dispersion and provided more active sites, which was responsible for the high activity of ammonia decomposition. Acknowledgements The authors would like to thank Dr. Wang X D, Prof. Dai X Y, and Dr. Chen M H for their helpful discussions.

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