CeO2 catalytic properties and performance for hydrogen production in sulfur–iodine cycle

international journal of hydrogen energy 34 (2009) 5637–5644

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Experimental study of Ni/CeO2 catalytic properties and performance for hydrogen production in sulfur–iodine cycle Yanwei Zhang, Zhihua Wang*, Junhu Zhou, Jianzhong Liu, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

article info

abstract

Article history:

The Ni/CeO2 catalysts with different calcination temperatures have been tested for

Received 20 March 2009

hydrogen production in sulfur–iodine (SI or IS) cycle. TG-FTIR, BET, XRD, HRTEM and TPR

Received in revised form

were performed for catalyst characterization. It was found that the Ni2þ ions could be

8 May 2009

inserted into the ceria lattice. This brought about the strong interaction between Ni and

Accepted 14 May 2009

CeO2 and the generation of oxygen vacancies. Perfect crystallites were formed in the

Available online 21 June 2009

catalysts. It was evident that there was a change in particle size and morphology as the calcination temperature increased from 300 to 900  C. The Ni/CeO2 catalysts with different

Keywords:

calcination temperatures showed better catalytic activity by comparison with blank yield,

Sulfur–iodine cycle

especially Ni/Ce700. A hypothetic mechanism of HI catalytic decomposition on Ni/CeO2 has

Hydrogen production

been constructed. The two important reactive sites were assumed for HI catalytic

Hydrogen iodide

decomposition.

Nickel

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Ceria

1.

Introduction

Hydrogen is an attractive fuel for the future because it is renewable as an energy resource and also flexible as an energy carrier. One of the promising methods for large-scale hydrogen production is thermochemical water-splitting cycle. Hydrogen is obtained by decomposition of water by using heat energy through a chemical cycle process that consists of several reactions at much lower temperatures than direct thermal decomposition. Among the large-scale, cost effective and environmentally attractive thermochemical cycles, the sulfur–iodine (SI or IS) thermochemical cycle is a quite promising one. The potential of the SI process for hydrogen production has been indicated by many researchers [1–14]. This cycle consists of the following three reactions:

I2 þ SO2 þ 2H2O / 2HI þ H2SO4

(a)

2HI / I2 þ H2

(b)

H2SO4 / SO2 þ H2O þ 0.5O2

(c)

The Bunsen reaction (a) is an exothermic SO2 gas-absorbing reaction in an aqueous phase. The hydrogen iodide (HI) solution and the H2SO4 solution are separated by a liquid–liquid phase separation phenomenon that occurs in the presence of an excess I2. The two acids were divided into upper and lower solutions with a clear boundary. The separated HI solution and H2SO4 solution are purified, concentrated, vaporized and decomposed to produce H2 (b) and O2 (c). All chemicals in the cycle are recycled and H2O is decomposed into H2 and O2 in total. Among these reactions, HI decomposition presents a rather low homogeneous gas-phase conversion even at high operating temperatures. The use of catalyst allows a substantial temperature reduction to achieve workable reaction rates.

* Corresponding author. Tel./fax: þ86 571 87952205. E-mail address: [email protected] (Z. Wang). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.05.061

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The majority of work to identify active catalysts for this reaction was performed in the late 1970s and early 1980s [15– 18] and is best summarized by O’Keefe et al. [15]. Different catalytic systems were found effective, in particular the platinum group metals supported on activated carbon or g-Al2O3 showed very good performance. Recent work explored platinum–ceria catalysts for the hydrogen production reaction [19,20]. The limited availability of noble metal makes it necessary to develop less expensive catalysts based on nonprecious metals and supports. Some authors proposed activated carbon [21] and Ni-supported catalysts [15,22,23] for HI catalytic decomposition. The cost savings resulting from the use of non-precious metals may be an advantage in the design of a plant of large size. The purpose of this work has been to investigate the performance of the Ni/CeO2 catalysts with different calcination temperatures for HI decomposition. CeO2 can act as a catalyst itself, especially if its particles are in the nanometer range, because of the large number of lattice defects (grain and interphase boundaries, oxygen vacancies, dislocations) present at this scale [24–27]. Cerium-based catalysts in the presence of transition metals, especially nickel, have attracted increasing attention in recent years. Ni/CeO2 and Ni–Ce–ZrO2 catalysts have been studied in many reactions by taking advantage of their redox property, such as WGS, CH4 combustion and reforming of methane with CO2 and O2 [28– 32]. Such a catalytic action can significantly reduce the quantities of expensive noble metals. Despite the fact that the Ni/CeO2 catalyst has been used in a number of applications, but its use in thermochemical water-splitting cycles for HI decomposition has not been widely explored.

2.

Experimental

2.1.

Catalyst preparation

The Ni/CeO2 catalysts were synthesized by citric-aided sol–gel method. The solution of Ce(NO3)3$6H2O and Ni(NO3)2$6H2O fulfilled the requirement that the mass percentage of Ni in the catalysts is 3 wt.%. Citric acid was used as the complexing agent. The molar ratio of citric acid with nitrates Ce(NO3)3$6H2O was 1.1:1. Appropriate glycol (10 wt.% of citric acid) was added as the dispersant followed by evaporation and peptization. The mixture was stirred at 80  C until a spongy gel remained. The Ni/CeO2 gel prepared by desiccation of the spongy gel at 110  C for 12 h was decomposed at 300  C for 1 h and subsequently calcined at different temperatures 300  C, 500  C, 700  C and 900  C for 3 h. The Ni/CeO2 catalysts with different calcination temperatures called Ni/Ce300, Ni/Ce500, Ni/Ce700 and Ni/Ce900 were successfully prepared. The Ni/ CeO2 catalysts with different Ni contents were prepared by the same synthesis process of Ni/Ce700.

2.2.

Catalysts characterization

The thermokinetics characteristic of the Ni/CeO2 gel was investigated on a thermo-gravimetric apparatus (air atmosphere, 30 ml/min and 30  C/min) with an online infrared spectrum analyzed (TG-FTIR). The specific surface area,

average pore diameter and pore volume were determined by Brunauer–Emmett–Teller (BET) method with a Quantachrome NOVA instrument using N2 as adsorbent. The X-ray diffraction analysis (XRD) was performed on a D/max 2550PC. The X-ray tube was operated at 40 kV and 200 mA. The X-ray powder diffractogram was recorded at 0.02 intervals in the range 20  2q  90 with 0.3 s count accumulation per step. The High Resolution Transmission Electron Microscopy (HRTEM) picture was performed on a JEM-2010 (HR). Temperatureprogrammed reduction (TPR) experiment was carried out on a TPR catalytic surfaces analyzer. The samples were heated under flowing H2 (5% in N2, 20 ml/min) from room temperature to 800  C (10  C/min).

2.3.

Activity measurement

The catalytic decomposition of HI was performed at 300– 550  C in a quartz tube with the diameter of 18 mm. The mixture composed of 1 g catalyst powder and appropriate volume of coarse quartz particles was loaded in the tube. As shown in Fig. 1, the 55 wt.% hydriodic acid (HI solution) was pumped using a BT00-50M peristaltic pump into an evaporator where the acid vaporized and mixed with nitrogen gas and then the mixture was introduced into the quartz tube. Flow rate of the nitrogen gas and HI was maintained at 60 ml/min and 0.7 ml/min. The reaction was carried out at atmospheric pressure. All the gases from the quartz tube, except hydrogen and nitrogen gas, were trapped in a spiral condenser and few residual HI and I2 were sequentially trapped in two scrubbers. Hydrogen was analyzed by a gas chromatograph.

3.

Results and discussion

3.1.

Catalysts analysis

The measured thermo-gravimetric TG, DTG and SDTA curves for the Ni/CeO2 gel are shown in Fig. 2. The three curves respectively represent the weight loss of the sample, the differential coefficient of TG curve (weight loss rate) and exothermic curve. The curve of weight loss can be mainly divided into four phases, called A, B, C and D, but the peaks in weight loss rate and exothermic curves can only be observed during B and D phases. This indicates that the Ni/CeO2 gel is primarily decomposed during B and D phases and completely decomposed after 300  C. FTIR patterns of the products from the thermo-gravimetric apparatus are shown in Fig. 3. FTIR patterns of a, b, c and d were detected in the four phases A, B, C and D during the thermo-gravimetric experiment. The peak intensities of CO2 in B and D phases are obviously larger than that in A and C phases. CO can be seen in B and C phases, which implies that residual organic materiel can not be completely decomposed to CO2 at the temperature below 265  C. But residual organic materiel can be completely decomposed to CO2 in D phase. NO can also be appreciably found in D phase. The specific surface area, average pore diameter and pore volume are shown in Table 1. The BET surface area is 66.31 m2/ g for Ni/Ce300, 33.92 m2/g for Ni/Ce500, 18.69 m2/g for Ni/ Ce700 and 4.19 m2/g for Ni/Ce900. It is noted that the surface

international journal of hydrogen energy 34 (2009) 5637–5644

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Fig. 1 – Schematic of experimental catalyst activity test system.

areas of the Ni/CeO2 catalysts decreased obviously as calcination temperatures increased. The pore volume shows the same trend. The average pore diameters of Ni/Ce900 (15.56 nm) and Ni/Ce700(15.98 nm) are obviously larger than that of Ni/Ce300 (6.55 nm) and Ni/Ce500(8.51 nm). The X-ray diffraction patterns of Ni/Ce300, Ni/Ce500, Ni/ Ce700 and Ni/Ce900 are shown in Fig. 4. All samples display the XRD pattern corresponding to the cubic fluorite structure of pure CeO2 with intense bands due to the (111), (200), (220) and (311) planes. The XRD bands for the samples with low calcination temperatures were rather broad indicating the presence of small crystallites. The sintering of CeO2 crystallites was particularly evident at temperatures above 700  C. This is also in agreement with the loss of surface area observed in BET. As shown in Table 1, the crystallite size of CeO2 calculated by the Scherrer equation is 3.8 nm for Ni/ Ce300, 7.1 nm for Ni/Ce500, 32.1 nm for Ni/Ce700 and 121.9 nm for Ni/Ce900. It is noted that the crystallite sizes increased obviously as calcination temperatures increased. The lattice

constant d calculated by Bragg’s law 2d sin q ¼ kl is 5.4093, ˚ for Ni/Ce300, Ni/Ce500, Ni/Ce700 5.4089, 5.4106 and 5.4119 A and Ni/Ce900, respectively. According to our recent experimental results [33], the lattice constant of pure CeO2 was about 5.4124 nm. The lattice constants of all Ni/CeO2 catalysts were obviously smaller than that of pure CeO2. The ionic ˚ ) is smaller than that of Ce4þ (0.97 A ˚ ). radius of Ni2þ (0.72 A Thus, the decrease of the lattice constant indicated that the Ni2þ ions have dissolved into the ceria lattice instead of the Ce4þ ions during the sol–gel synthesis process [28]. It is interesting that the lattice constants of Ni/Ce700 and Ni/Ce900 were appreciably larger than that of Ni/Ce300 and Ni/Ce500. We believe that it may be related to the agglomeration and growth of NiO crystallites at high temperature which result in the decrease of Ni2þ ions dissolved into the ceria lattice.

Fig. 2 – The thermokinetics characteristic of the Ni/CeO2 gel.

Fig. 3 – FTIR patterns of the products from thermogravimetric apparatus (a–d correspond to A–D in Fig. 2).

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Table 1 – Basic sample characterizaton. Sample

BET surface area (m2 g1)

Pore volume (ml g1)

Average pore diameter (nm)

Crystallite size (CeO2) (nm)

Lattice constants ˚) (CeO2) (A

Ni/Ce300 Ni/Ce500 Ni/Ce700 Ni/Ce900

66.31 36.92 18.69 4.19

1.09  101 7.85  102 4.27  102 1.51  102

6.55 8.51 15.98 15.56

3.8 7.1 32.1 121.9

5.4093 5.4089 5.4106 5.4119

In Fig. 4, the weak peaks of nickel oxide (2q z 37 and 43 ) are observed in Ni/Ce700 and Ni/Ce900, but not in Ni/Ce300 and Ni/Ce500. It is likely because the NiO highly dispersed as smaller crystallites which were too small to be detected by XRD techniques. The peak intensity of NiO increased as calcination temperatures increased, indicating the agglomeration and growth of NiO crystallites. No peaks of the metallic Ni were observed in the XRD patterns for all Ni/CeO2 catalysts. The HRTEM pictures of Ni/Ce300, Ni/Ce500, Ni/Ce700 [23] and the TEM picture of Ni/Ce900 are shown in Fig. 5(a–d). The CeO2 microcrystals in Ni/Ce300, Ni/Ce500 and Ni/Ce700 are faceted well in HRTEM Fig. 5(a–c). Perfect crystallites are formed in the catalysts. It is evident that there was a change in particle size and morphology as the calcination temperature increased from 300 to 900  C. Particles of ceria calcined at lower temperatures (300 and 500  C) existed more frequently as individual irregular particles, whereas particles of catalysts calcined at higher temperatures (700 and 900  C) tended to agglomerate, forming larger blocks of irregular particles. The NiO particles can not be seen directly in HRTEM Fig. 5(a) and (b). This indicates that the NiO highly dispersed as smaller NiO crystallites which were too small to be observed. The NiO nanoparticles for Ni/Ce700 and Ni/Ce900 (illustrated by black arrows) can be seen distinctly in Fig. 5(c) and (d). The NiO particles dispersed within CeO2 support tended to agglomerate and form analogously spherical nanoparticles at high calcination temperatures. It is proposed that the Ni–Ce interphase, where interfacial Ni sites were located and the Ni–Ce interaction occurred, played an important role in catalytic

surface reaction. All the TEM results are consistent with the XRD data. The reducibility of all samples was determined by TPR and the results are shown in Fig. 6. According to the results of XRD and TEM, there are three types of NiO phase: highly dispersed NiO crystallite, crystallized NiO and solid solution NiO. Solid solution NiO existed in all Ni/CeO2 catalysts, whereas highly dispersed NiO crystallite mainly formed in Ni/Ce300 and Ni/ Ce500, and crystallized NiO mainly occurred in Ni/Ce700 and Ni/Ce900. As shown in Fig. 6, Ni/Ce300 and Ni/Ce500 show two broad peaks at the temperature 200–450  C and 500–700  C. The broad peak (200–450  C) was related to easily reducible surface ceria species and highly dispersed NiO crystallites, and the other broad peak (500–700  C) was assigned to surface reduction of capping oxygen [26,27]. It must be noted that the reduction of the bulk of CeO2 did not take place in the interval of such temperature, since it only occurred above 700  C [26,27]. TPR results indicate that Ni/Ce700 and Ni/Ce900 exhibit one reduction peak assigned to the combined reduction of surface ceria species and crystallized NiO particles at about 500  C. However, the Ni2þ ions dissolved into the ceria lattice (solid solution NiO) are more difficult to be reduced. So there are no hydrogen consumption peaks corresponding to the reduction of solid solution NiO [28]. It is noted that the peaks of Ni/Ce700 and Ni/Ce900 corresponding to the broad peak (500–700  C) of Ni/Ce300 and Ni/Ce500 shift downward and overlap the reduction peak of crystallized NiO particles. This indicates that a strong interaction between Ni and CeO2 or Ni incorporation in CeO2 made ceria more reducible, which

Fig. 4 – XRD patterns of the Ni/CeO2 catalysts with different calcination temperatures.

international journal of hydrogen energy 34 (2009) 5637–5644

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Fig. 5 – TEM pictures of (a) Ni/Ce300, (b) Ni/Ce500, (c) Ni/Ce700 [23] and (d) Ni/Ce900.

helped to produce mobile oxygen during the HI decomposition reaction [30].

3.2.

Catalytic performance

The decomposition efficiency was strongly affected by the temperature as can be seen in Fig. 7. In this figure, the different thermodynamic yields and an uncatalyzed (blank) yield are also plotted. Thermodynamic yield A was obtained from reference [15], and thermodynamic yield B was investigated by FactSage [34] which is a commercial software for thermodynamic calculation. As shown in Fig. 7, the Ni/CeO2 catalysts with different calcination temperatures show better catalytic activity by comparison with blank yield, especially Ni/Ce700. The HI conversions of Ni/Ce700 achieve 16.5% at 450  C and 24.7% at 550  C. The HI conversions of Ni/Ce500 are similar to that of Ni/Ce900.

3.3.

Fig. 6 – TPR profiles of the Ni/CeO2 catalysts with different calcination temperatures.

Hypothetic mechanism analysis

Detailed kinetic modeling and sensitivity analysis for HI homogeneous decomposition with and without oxygen species (O) have already been reported by our lab [10]. The

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international journal of hydrogen energy 34 (2009) 5637–5644

Fig. 7 – HI conversion with temperature for the Ni/CeO2 catalysts.

results showed that the reactions HI þ I ¼ I2 þ H and HI þ O ¼ OH þ I were the most important steps for HI decomposition, especially the reaction HI þ I ¼ I2 þ H. The presence of O can obviously promotes the HI decomposition reaction rate, but simultaneously consumes hydrogen species (H) contained in HI. Mechanism study for homogeneous HI

decomposition plays an important role to mechanism research for catalytic HI decomposition. A hypothetic reaction mechanism of HI decomposition on Ni/CeO2 is constructed in this paper. We believe that there are two important active sites for HI catalytic decomposition. One is the surface site [29–31] exhibited in the Ni–Ce interphase, where interfacial Ni sites are located and the strong interaction between Ni and CeO2 occurs. The other is oxygen vacancy related to the reduced surface sites of CeO2 support, i.e., Ce3þ [24–27] and Ni2þ ions dissolved into the ceria lattice instead of the Ce4þ ions [28–32]. Oxygen diffusion from the bulk to CeO2 surface is also very important. According to our recent results [33], a detailed reaction mechanism of HI decomposition is shown in Fig. 8. Firstly, HI is adsorbed and activated on the surface of Ni/CeO2 through oxygen vacancy or the surface site causing the cleavage H–I. The H and I might be adsorbed in the vacancies near Ce3þ or Ni2þ ions. The next step is almost same for the surface site and oxygen vacancy. In the absence of O, as shown in A area of Fig. 8, H2 and I2 are produced. In the presence of O, as shown in C area of Fig. 8, the reaction between O and H causes the production of H2O and I2. B area in Fig. 8 involving the direct reaction between adsorbed I and HI is also considered. At initial reaction in the presence of O, HI decomposition occurs almost with the oxygen from the surface and near-surface regions. After the surface and near-surface oxygen reduction, latter HI is almost decomposed by bulk oxygen.

Fig. 8 – Reaction mechanism for HI catalytic decomposition on the Ni/CeO2 catalyst.

international journal of hydrogen energy 34 (2009) 5637–5644

According to the catalyst analysis, the Ni/CeO2 catalyst calcined at low temperature showed more oxygen vacancies due to lattice defects of CeO2 support, especially the reduced surface sites, i.e., Ce3þ, and more Ni2þ ions dissolved into the ceria lattice. As calcination temperature increased, the number of oxygen vacancy decreased related to the improvement in crystallinity of ceria support and the agglomerate of NiO crystallites, but the strong interaction between Ni and CeO2 occurred. The surface site exhibited in the Ni–Ce interphase plays an important role in surface reaction. It can be deduced that oxygen vacancy and the surface site were well-balanced on Ni/Ce700 catalyst. The well-balanced number of active sites resulted in high ability to activate HI. As the result, Ni/Ce700 catalyst showed the best activity as well as good stability.

4.

Conclusions

In this study, the Ni/CeO2 catalysts with different calcination temperatures have been tested to evaluate their effect on HI decomposition in sulfur–iodine cycle. TG-FTIR, BET, XRD, HRTEM and TPR were performed for catalyst characterization. It was found that the Ni2þ ions could be inserted into the ceria lattice. This brought about the strong interaction between Ni and CeO2, especially at high calcination temperature and the generation of oxygen vacancies. Oxygen vacancy and the surface site played the important role in catalytic surface reaction. The two active sites were well-balanced on Ni/Ce700 catalyst, which resulted in the best activity as well as good stability. All the results provided this material with a potential to be used in sulfur–iodine cycle for hydrogen production.

Acknowledgements This work has been supported by Zhejiang Provincial Natural Science Foundation of China (Y106538) and National High Technology Research and Development Program of China (863 Program) (No. 2008AA05Z103).

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