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Ceria as a catalyst for hydrogen iodide decomposition in sulfur–iodine cycle for hydrogen production
international journal of hydrogen energy 34 (2009) 1688–1695
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Ceria as a catalyst for hydrogen iodide decomposition in sulfur–iodine cycle for hydrogen production Yanwei Zhang, Zhihua Wang*, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China
article info
abstract
Article history:
In this work, ceria (CeO2) prepared with different methods and at various calcination
Received 7 October 2008
temperatures have been tested to evaluate their effect on hydrogen iodide (HI) decompo-
Received in revised form
sition in sulfur–iodine (SI) cycle at various temperatures. The CeO2 catalysts’ strongly
20 November 2008
enhance the HI decomposition by comparison with blank test, especially gel CeO2 300.
Accepted 20 November 2008
TG–FTIR, BET, XRD, TEM and TPR were performed for catalysts’ characterization. The
Available online 6 January 2009
results show that the CeO2 catalyst synthesized by citric-aided sol–gel method and calcined at low temperature (<500 C) shows more lattice defects, smaller crystallites, larger surface
Keywords:
area and better reducibility. Oxygen can promote the significantly rapid surface reaction,
SI thermochemical cycle
but simultaneously consume hydrogen species (H) contained in HI. Lattice defects, espe-
Hydrogen production
cially the reduced surface sites, i.e., Ce3þ and oxygen vacancy, play the dominant role in
Hydrogen iodide
surface reactions of HI decomposition. A new reaction mechanism for HI catalytic decomposition over CeO2 catalyst is proposed.
Ceria
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen has ideal characteristics as an energy carrier. In order to develop a hydrogen energy system, for which huge hydrogen demand is expected, the development of an efficient hydrogen production system is an urgent issue. Among the large scale, cost effective and environmentally attractive hydrogen production processes, the sulfur–iodine (SI) thermochemical cycle is a quite promising one. The potential of the SI process for large-scale hydrogen production and the experimental works on the SI process have been indicated by many researchers [1–14]. This cycle consists of the following three reactions: I2 þ SO2 þ 2H2 O/2HI þ H2 SO4
(a)
2HI/I2 þ H2
(b)
H2 SO4 /SO2 þ H2 O 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 of I2. The two acids separate into upper and lower phases 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 fully recycled and H2O is decomposed into H2 and O2. The thermal reaction (b) is one of the most significant reactions not only in this cycle but also in other thermochemical water-splitting cycles using iodine [15]. The use of catalyst allows a substantial temperature reduction to achieve workable reaction rates. With regard to heterogeneous catalytic decomposition of HI, many studies were reported [7,16– 23] in which different kinds of catalysts, especially noble metal catalysts, were employed to decompose HI and catalytic decomposition mechanisms were also investigated.
* Corresponding author. Tel./fax: þ86 571 87953162. E-mail address: [email protected] (Zh.H.Wang). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.11.089
international journal of hydrogen energy 34 (2009) 1688–1695
The purpose of this work was to investigate the performance of ceria (CeO2) prepared with different methods and at various calcination temperatures for HI decomposition to produce hydrogen. The choice of CeO2 was prompted by the fact that among the metal oxides we tested, CeO2 has interesting economical and physicochemical properties. CeO2 is abundant, nontoxic and inexpensive material that has potential use in a number of applications mainly due to its oxygen storage capacity and particular interaction with noble metals [19,20,24,25]. On the other hand, 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 [26–31]. Such a catalytic action can significantly reduce the quantities of expensive noble metals. Despite the fact that CeO2 has been used in a number of applications, its use in thermochemical water-splitting cycles for HI decomposition has not been widely explored.
2.
Experimental
2.1.
catalysts’ preparation
Pump
Fig. 2 – The thermokinetics characteristic of the gel CeO2.
temperatures 300 C, 500 C, 700 C and 900 C and cooled to room temperature in the furnace. The gel CeO2 catalysts calcined at different temperatures were called gel CeO2 300, gel CeO2 500, gel CeO2 700 and gel CeO2 900.
The catalyst called Commerce CeO2 was provided by a commercial source. A precipitation method involving the use of Ce(NO3)3$6H2O and ammonia solution as precipitating agent was used to prepare ceria catalyst denoted as Precipitation CeO2. Appropriate Ce(NO3)3$6H2O was dissolved in 100 ml of deionized water. The resulting solution was continuously stirred at room temperature and ammonia solution was discontinuously added until pH > 10. The solid obtained was filtered and dried at 110 C for 12 h. The resulting yellow solid was calcined in static air at 300 C for 3 h. The gel CeO2 catalysts were synthesized by citric-aided sol–gel method. The molar ratio of citric acid to nitrate 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 yellow gel remained. The gel CeO2 was prepared by the spongy yellow gel dried at 110 C for 12 h. Then the gel CeO2 was decomposed at 300 C for 1 h and subsequently calcined for 3 h at various
HI solution
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2.2.
catalysts’ characterization
The thermal behavior of the gel CeO2 was investigated by a thermo-gravimetric apparatus with an online infrared spectrum analyzed (TG–FTIR). The specific surface area was 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 of 20 2q 90 with 0.3 s count accumulation per step. The transmission electron microscopy (TEM) was
Evaporator Gas chromatograph
Reactor
Spiral condenser
Scrubber
N2
NaOH
H 2O
Fig. 1 – Schematic of experimental catalyst activity test system.
Fig. 3 – FTIR spectra of the products from thermogravimetric apparatus (a–d correspond to A–D in Fig. 2).
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international journal of hydrogen energy 34 (2009) 1688–1695
Table 1 – Routine sample characterization. Sample
BET surface area (m2 g1)
Pore volumea (ml g1)
Average pore diameter (nm)
Crystallite sizeb (nm)
Lattice ˚) constantsc (A
9.24 30.92 80.94 35.75 11.93 4.11
3.14 102 1.01 101 1.68 101 1.06 101 4.89 102 1.57 102
13.61 12.96 8.32 11.88 16.41 15.25
151.8 14.8 5.2 12.4 42.8 145.6
5.4128 5.4118 5.4149 5.4129 5.4124 5.4124
Commerce CeO2 Precipitation CeO2 gel CeO2 300 gel CeO2 500 gel CeO2 700 gel CeO2 900
a Micropore volume calculated from N2 adsorption data. b Calculated by the Scherrer equation due to the (111), (200), (220) and (311) planes. c Calculated by the Bragg’s law 2d sin q ¼ kl due to the (111), (200), (220) and (311) planes.
performed on a JEM-2010 (HR). Temperature-programmed reduction (TPR) experiment was carried out on a TPR catalytic surface analyzer. Typically 0.1 g of sample was employed. The samples were heated under flowing H2 (5% in N2, 20 ml/min) from room temperature up to 800 C (10 C/min).
2.3.
Activity measurement
The catalytic decomposition of hydrogen iodide (HI) was performed at 300–550 C in a quartz reaction tube with the diameter of 18 mm. About 1 g of catalyst was homogeneously mixed with an appropriate volume of coarse quartz particles. The mixture was loaded in the tube. Fig. 1 shows the experimental apparatus to investigate HI catalytic decomposition. The starting material, 55 wt% hydriodic acid (HI solution), was pumped using a BT00-50M peristaltic pump into an evaporator where the solution vaporized. Nitrogen gas was used as carrier gas. The gaseous mixture (HI, H2O and N2) was then introduced into the quartz reaction tube. Flow rate of the nitrogen gas was maintained at 60 ml/min. The reaction was carried out at atmospheric pressure. All the products (HI, I2, H2O, H2 and N2) from the reaction tube, except hydrogen and nitrogen gas, were trapped in a spiral condenser and the residual HI and I2 were sequentially trapped in two scrubbers. Hydrogen was analyzed by gas chromatography.
3.
Results and discussion
3.1.
catalysts’ analysis
The TG, DTG and SDTA curves for the gel CeO2 are shown in Fig. 2. The three curves represent the sample weight loss, the differential coefficient of TG curve (weight loss rate) and the exchanged heat, respectively. Four different steps can be found on the weight loss curve and are called A, B, C and D, respectively. According to the DTG and SDTA curves, the gel CeO2 is primarily decomposed during B phase and D phase. The gel CeO2 can be completely decomposed after 310 C. FTIR spectra of the gaseous products obtained during thermal decomposition are shown in Fig. 3. FTIR spectra a, b, c and d were detected in the four steps A, B, C and D during the thermo-gravimetric experiment. The peak intensities of CO2 are obviously larger in B and D steps. CO can be seen in B and C steps, which implies that residual organic material cannot be
completely decomposed to CO2 at the temperature below 275 C. Residual organic material can be completely decomposed to CO2 in the D step. The specific surface area, average pore diameter, and pore volume as measured by N2 adsorption using the BET method are shown in Table 1. The specific surface area of Commerce CeO2 (9.24 m2/g) is comprised between the gel CeO2 700 (11.93 m2/g) and gel CeO2 900 (4.11 m2/g) areas. The average pore diameter of Commerce CeO2 (13.61 nm) is similar to the average pore diameter of gel CeO2 900 (15.25 nm). It is interesting to note that Precipitation CeO2 calcined at 300 C (30.92 m2/g, 1.01 101 ml/g and 12.96 nm) has similar surface area, pore volume, and average pore diameter of gel CeO2 500 (35.75 m2/g, 1.06 101 ml/g and 11.88 nm) and different from gel CeO2 300 (80.94 m2/g, 1.68 101 ml/g and 8.32 nm). The catalyst prepared by sol–gel method shows better thermal stability. From Table 1, it is possible to observe as there is a decrease in the surface area as the calcination temperature increased from 300 to 900 C. X-ray diffraction patterns of Commerce CeO2, Precipitation CeO2, gel CeO2 300, gel CeO2 500, gel CeO2 700 and gel CeO2 900 are shown in Fig. 4. All samples display the XRD pattern corresponding to the cubic fluorite structure of pure CeO2 with
Fig. 4 – XRD patterns of different CeO2 catalysts.
international journal of hydrogen energy 34 (2009) 1688–1695
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Fig. 5 – TEM analysis of (a) gel CeO2 300; (b) gel CeO2 500; (c) gel CeO2 700; (d) gel CeO2 900.
intense bands due to the (111), (200), (220) and (311) planes. The XRD peaks for the samples with low calcination temperatures are rather broad indicating the presence of small crystallites. The calculated crystallite sizes using the Scherrer
Fig. 6 – TEM analysis of Precipitation CeO2.
equation are shown in Table 1. Gel CeO2 300 has an average crystallite size of 5.2 nm. The crystallite size of Commerce CeO2 (151.8 nm) is consistent with gel CeO2 900 (145.6 nm). It is interesting that Precipitation CeO2 (14.8 nm) calcined at 300 C has a crystallite size similar to gel CeO2 500 (12.4 nm) and different from gel CeO2 300 (5.2 nm). The catalyst prepared by sol–gel method shows better thermal stability. The mean lattice constants d of the samples shown in Table 1 are calculated using the Bragg’s law 2d sin q ¼ kl. The lattice ˚ ) is obviously larger than constant of gel CeO2 300 (5.4149 A other samples. This implies that gel CeO2 300 has not been ˚) completely decomposed and that residual Ce3þ ions (1.14 A 4þ ˚ which are larger than Ce ions (0.97 A) can also occur in gel CeO2 300. The sintering of CeO2 crystallites is particularly evident at temperatures above 700 C. Samples sintered at temperature higher than 700 C show narrow diffraction peaks, indicating an increase in particle dimensions. This is also in agreement with the decrease of surface area observed in BET. The peak sharpness and intensity increase with temperature, indicating an improvement in ceria crystallinity. Contrarily, the ceria catalysts calcined at lower temperatures show more lattice defects (grain and interphase boundaries, oxygen vacancies, dislocations, etc.). The TEM pictures of all samples are shown in Figs. 5–7. The catalyst particle sizes can be seen distinctly in the TEM pictures. Fig. 5(a–d) shows the particle size and morphology variations of gel CeO2 as a function of calcination temperatures. A change in the particle size and sample morphology with the increase in the
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international journal of hydrogen energy 34 (2009) 1688–1695
Fig. 9 – HI conversion for CeO2 catalysts with different calcination temperatures.
Fig. 7 – TEM analysis of Commerce CeO2.
calcination temperature from 300 to 900 C is observed. Ceria samples calcined at 300 and 500 C show individual irregular particles, whereas those of catalysts calcined at 700 and 900 C tend to agglomerate, forming larger blocks. The particle size of Commerce CeO2 shown in Fig. 7 is consistent with gel CeO2 900 in Fig. 5(d). It is also interesting to note that Precipitation CeO2 calcined at 300 C and shown in Fig. 6 has the same particle size of gel CeO2 500 in Fig. 5(b) and different from gel CeO2 300 shown in Fig. 5(a). 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. 8. TPR assays mainly show three reduction bands between ambient temperature and 800 C. A small, low temperature reduction peak (only for gel CeO2 300)
Fig. 8 – TPR profiles of different CeO2 catalysts.
centered at about 260 C (LT peak), a wide reduction band at middle temperature, from 300 to 600 C (MT peak) and an incomplete reduction peak at higher temperature, above 700 C (HT peak). The low reduction band is related to highly reducible surface ceria species, whereas middle temperature reduction peaks can be attributed to the surface reduction of oxygen [31,32]. It must be noted that the bulk reduction of CeO2 does not take place in the middle temperature interval, since it occurs only above 700 C [31,32]. The effect of calcination temperature on the reduction of profiles of gel CeO2 300, gel CeO2 500, gel CeO2 700 and gel CeO2 900 is shown in Fig. 8. All four catalysts showed the middle temperature reduction peak in the range 300–600 C. The peaks intensity and width of gel CeO2 300 and gel CeO2 500 are obviously larger than gel CeO2 700 and gel CeO2 900. The peak intensity and width of Precipitation CeO2 are larger than that of Commerce CeO2, but smaller than that of gel CeO2 300 and gel
Fig. 10 – HI conversion for CeO2 catalysts with different preparation methods.
international journal of hydrogen energy 34 (2009) 1688–1695
Fig. 11 – Operating lifetime evaluation of the gel CeO2 300 at 500 8C.
CeO2 500. This variation in hydrogen consumption can be related to the formation of subtly different surface structures of CeO2. This also implies that the ceria catalyst prepared by sol–gel method and calcined at 300 C shows better surface activity and oxygen migration ability.
3.2.
Catalytic activity
The decomposition efficiency was strongly affected by the temperature as can be seen in Figs. 9 and 10 which show the HI conversion between 300 and 550 C. In these two figures the
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thermodynamic tests and an uncatalyzed (blank) tests are also plotted. Thermodynamic test A is obtained from Ref. [16], and thermodynamic test B is investigated by FactSage which is a commercial software for thermodynamic calculation. As shown in Fig. 9, the gel CeO2 catalysts with different calcination temperatures show better catalytic activity by comparison with blank test. For temperatures lower than 450 C and except for gel CeO2 900, all the gel CeO2 catalysts showed similar catalytic activity. By increasing the temperature, the gel CeO2 300 and gel CeO2 500 showed better catalytic activity, especially gel CeO2 300, achieving 18.4% and 19.4% at 550 C, respectively. As shown in Fig. 10, the gel CeO2 300 and Precipitation CeO2 showed better catalytic activities by comparison with Commerce CeO2. At temperatures above 400 C, the HI conversion of gel CeO2 300 becomes better than that of Precipitation CeO2, achieving 19.4% and 17% at 550 C, respectively. In Fig. 11, the catalytic activity of gel CeO2 300 is reported as a function of the lifetime. These data have been obtained in subsequent experiments that were performed at 500 C. Although the study of the catalysts lifetime is beyond the scope of this paper and the collected data do not allow for conclusive evaluation, it is remarkable that gel CeO2 300 showed a relatively stable activity after approximately 200 min of operating lifetime.
3.3.
Hypothetic mechanism analysis
Detailed kinetic modeling and sensitivity analysis for HI homogeneous decomposition have already been reported by our lab [10]. The result shows that the reactions HI þ HI ¼ H2 þ I2 and HI þ I ¼ I2 þ H play a major role in the hydrogen production process. Detailed kinetic modeling and sensitivity analysis for HI homogeneous decomposition in the
Fig. 12 – Reaction mechanism for HI catalytic decomposition over CeO2 catalyst.
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international journal of hydrogen energy 34 (2009) 1688–1695
presence of oxygen species (O) have also been investigated recently by our lab. According to the results, the reactions HI þ I ¼ I2 þ H and HI þ O ¼ OH þ I are the most important steps for HI homogeneous decomposition in the presence of O, especially the reaction HI þ I ¼ I2 þ H plays the dominant role in HI decomposition process. The presence of O can obviously promote the HI decomposition reaction rate, but simultaneously it consumes hydrogen species (H) contained in HI. The mechanism studies of CeO2 as a catalyst or an oxidant during the reforming of methane and the oxidation of CO have already been reported by many researchers [26,29,30]. The reduced surface sites, i.e., Ce3þ, oxygen vacancy or oxygen diffusion from the bulk of the solid particles to CeO2 surfaces play the very important role in CeO2 surface reaction. According to the above discussion, a new detailed reaction mechanism of HI catalytic decomposition over CeO2 catalyst is constructed and shown in Fig. 12. Firstly, HI is adsorbed and activated on the surface of CeO2 catalyst through the reduced surface sites, i.e., Ce3þ and oxygen vacancy causing the cleavage of H–I. The hydrogen and iodine atoms thus formed might be adsorbed in the vacancies near Ce3þ. Then there are two possibilities for the next step depending on the absence and presence of O. In the absence of O, as shown in A area of Fig. 12, H2 and I2 are produced and released from the surface of CeO2. In the presence of O, as shown in C area of Fig. 12, the reaction between O and H causes the production of H2O and I2. According to the results of detailed kinetic modeling and sensitivity analysis for HI homogeneous decomposition in the absence and presence of O, the reaction HI þ I ¼ I2 þ H plays a major role in the hydrogen production process. On the other hand, the amount of HI is obviously larger than I around the surface of the catalyst, and the reaction opportunity for I with HI may be larger than I with I. So B area in Fig. 12 involving the direct reaction between adsorbed I and HI on the surface of CeO2 is also considered to be a very important part of HI catalytic decomposition process. Here, it should be stressed that CeO2 during HI catalytic decomposition process acts not only as a catalyst but also as an oxidant. Lattice defects, especially the reduced surface sites, i.e., Ce3þ and oxygen vacancy, play the dominant role in surface reactions of HI decomposition. According to the catalysts’ analysis, the CeO2 catalyst calcined at low temperature shows more lattice defects, especially gel CeO2 300. That is one important reason why gel CeO2 300 shows the best catalytic activity on HI decomposition.
4.
Conclusions
The HI conversion is rather low. The use of a suitable catalyst allows a considerable decrease in the decomposition temperature to achieve workable reaction rates. The CeO2 catalysts, especially gel CeO2 300, strongly enhance the decomposition rate of HI to H2. The results of BET, XRD, TEM and TPR studies show that the CeO2 catalyst synthesized by citric-aided sol–gel method and calcined at 300 C shows more lattice defects, smaller crystallites, larger surface area and better reducibility. These provide this material with a potential to be used in sulfur–iodine cycle for hydrogen production.
Acknowledgements This work has been supported by Doctor foundation of the Ministry of Education in China (20050335046), Zhejiang Provincial Natural Science Foundation of China (Y106538), National High Technology Research and Development Program of China (863 Program) (no. 2008AA05Z103) and National Science Foundation for Distinguished Young Scholars (50525620).
references
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Ceria as a catalyst for hydrogen iodide decomposition in sulfur–iodine cycle for hydrogen production Yanwei Zhang, Zhihua Wang*, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China
article info
abstract
Article history:
In this work, ceria (CeO2) prepared with different methods and at various calcination
Received 7 October 2008
temperatures have been tested to evaluate their effect on hydrogen iodide (HI) decompo-
Received in revised form
sition in sulfur–iodine (SI) cycle at various temperatures. The CeO2 catalysts’ strongly
20 November 2008
enhance the HI decomposition by comparison with blank test, especially gel CeO2 300.
Accepted 20 November 2008
TG–FTIR, BET, XRD, TEM and TPR were performed for catalysts’ characterization. The
Available online 6 January 2009
results show that the CeO2 catalyst synthesized by citric-aided sol–gel method and calcined at low temperature (<500 C) shows more lattice defects, smaller crystallites, larger surface
Keywords:
area and better reducibility. Oxygen can promote the significantly rapid surface reaction,
SI thermochemical cycle
but simultaneously consume hydrogen species (H) contained in HI. Lattice defects, espe-
Hydrogen production
cially the reduced surface sites, i.e., Ce3þ and oxygen vacancy, play the dominant role in
Hydrogen iodide
surface reactions of HI decomposition. A new reaction mechanism for HI catalytic decomposition over CeO2 catalyst is proposed.
Ceria
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen has ideal characteristics as an energy carrier. In order to develop a hydrogen energy system, for which huge hydrogen demand is expected, the development of an efficient hydrogen production system is an urgent issue. Among the large scale, cost effective and environmentally attractive hydrogen production processes, the sulfur–iodine (SI) thermochemical cycle is a quite promising one. The potential of the SI process for large-scale hydrogen production and the experimental works on the SI process have been indicated by many researchers [1–14]. This cycle consists of the following three reactions: I2 þ SO2 þ 2H2 O/2HI þ H2 SO4
(a)
2HI/I2 þ H2
(b)
H2 SO4 /SO2 þ H2 O 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 of I2. The two acids separate into upper and lower phases 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 fully recycled and H2O is decomposed into H2 and O2. The thermal reaction (b) is one of the most significant reactions not only in this cycle but also in other thermochemical water-splitting cycles using iodine [15]. The use of catalyst allows a substantial temperature reduction to achieve workable reaction rates. With regard to heterogeneous catalytic decomposition of HI, many studies were reported [7,16– 23] in which different kinds of catalysts, especially noble metal catalysts, were employed to decompose HI and catalytic decomposition mechanisms were also investigated.
* Corresponding author. Tel./fax: þ86 571 87953162. E-mail address: [email protected] (Zh.H.Wang). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.11.089
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The purpose of this work was to investigate the performance of ceria (CeO2) prepared with different methods and at various calcination temperatures for HI decomposition to produce hydrogen. The choice of CeO2 was prompted by the fact that among the metal oxides we tested, CeO2 has interesting economical and physicochemical properties. CeO2 is abundant, nontoxic and inexpensive material that has potential use in a number of applications mainly due to its oxygen storage capacity and particular interaction with noble metals [19,20,24,25]. On the other hand, 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 [26–31]. Such a catalytic action can significantly reduce the quantities of expensive noble metals. Despite the fact that CeO2 has been used in a number of applications, its use in thermochemical water-splitting cycles for HI decomposition has not been widely explored.
2.
Experimental
2.1.
catalysts’ preparation
Pump
Fig. 2 – The thermokinetics characteristic of the gel CeO2.
temperatures 300 C, 500 C, 700 C and 900 C and cooled to room temperature in the furnace. The gel CeO2 catalysts calcined at different temperatures were called gel CeO2 300, gel CeO2 500, gel CeO2 700 and gel CeO2 900.
The catalyst called Commerce CeO2 was provided by a commercial source. A precipitation method involving the use of Ce(NO3)3$6H2O and ammonia solution as precipitating agent was used to prepare ceria catalyst denoted as Precipitation CeO2. Appropriate Ce(NO3)3$6H2O was dissolved in 100 ml of deionized water. The resulting solution was continuously stirred at room temperature and ammonia solution was discontinuously added until pH > 10. The solid obtained was filtered and dried at 110 C for 12 h. The resulting yellow solid was calcined in static air at 300 C for 3 h. The gel CeO2 catalysts were synthesized by citric-aided sol–gel method. The molar ratio of citric acid to nitrate 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 yellow gel remained. The gel CeO2 was prepared by the spongy yellow gel dried at 110 C for 12 h. Then the gel CeO2 was decomposed at 300 C for 1 h and subsequently calcined for 3 h at various
HI solution
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2.2.
catalysts’ characterization
The thermal behavior of the gel CeO2 was investigated by a thermo-gravimetric apparatus with an online infrared spectrum analyzed (TG–FTIR). The specific surface area was 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 of 20 2q 90 with 0.3 s count accumulation per step. The transmission electron microscopy (TEM) was
Evaporator Gas chromatograph
Reactor
Spiral condenser
Scrubber
N2
NaOH
H 2O
Fig. 1 – Schematic of experimental catalyst activity test system.
Fig. 3 – FTIR spectra of the products from thermogravimetric apparatus (a–d correspond to A–D in Fig. 2).
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Table 1 – Routine sample characterization. Sample
BET surface area (m2 g1)
Pore volumea (ml g1)
Average pore diameter (nm)
Crystallite sizeb (nm)
Lattice ˚) constantsc (A
9.24 30.92 80.94 35.75 11.93 4.11
3.14 102 1.01 101 1.68 101 1.06 101 4.89 102 1.57 102
13.61 12.96 8.32 11.88 16.41 15.25
151.8 14.8 5.2 12.4 42.8 145.6
5.4128 5.4118 5.4149 5.4129 5.4124 5.4124
Commerce CeO2 Precipitation CeO2 gel CeO2 300 gel CeO2 500 gel CeO2 700 gel CeO2 900
a Micropore volume calculated from N2 adsorption data. b Calculated by the Scherrer equation due to the (111), (200), (220) and (311) planes. c Calculated by the Bragg’s law 2d sin q ¼ kl due to the (111), (200), (220) and (311) planes.
performed on a JEM-2010 (HR). Temperature-programmed reduction (TPR) experiment was carried out on a TPR catalytic surface analyzer. Typically 0.1 g of sample was employed. The samples were heated under flowing H2 (5% in N2, 20 ml/min) from room temperature up to 800 C (10 C/min).
2.3.
Activity measurement
The catalytic decomposition of hydrogen iodide (HI) was performed at 300–550 C in a quartz reaction tube with the diameter of 18 mm. About 1 g of catalyst was homogeneously mixed with an appropriate volume of coarse quartz particles. The mixture was loaded in the tube. Fig. 1 shows the experimental apparatus to investigate HI catalytic decomposition. The starting material, 55 wt% hydriodic acid (HI solution), was pumped using a BT00-50M peristaltic pump into an evaporator where the solution vaporized. Nitrogen gas was used as carrier gas. The gaseous mixture (HI, H2O and N2) was then introduced into the quartz reaction tube. Flow rate of the nitrogen gas was maintained at 60 ml/min. The reaction was carried out at atmospheric pressure. All the products (HI, I2, H2O, H2 and N2) from the reaction tube, except hydrogen and nitrogen gas, were trapped in a spiral condenser and the residual HI and I2 were sequentially trapped in two scrubbers. Hydrogen was analyzed by gas chromatography.
3.
Results and discussion
3.1.
catalysts’ analysis
The TG, DTG and SDTA curves for the gel CeO2 are shown in Fig. 2. The three curves represent the sample weight loss, the differential coefficient of TG curve (weight loss rate) and the exchanged heat, respectively. Four different steps can be found on the weight loss curve and are called A, B, C and D, respectively. According to the DTG and SDTA curves, the gel CeO2 is primarily decomposed during B phase and D phase. The gel CeO2 can be completely decomposed after 310 C. FTIR spectra of the gaseous products obtained during thermal decomposition are shown in Fig. 3. FTIR spectra a, b, c and d were detected in the four steps A, B, C and D during the thermo-gravimetric experiment. The peak intensities of CO2 are obviously larger in B and D steps. CO can be seen in B and C steps, which implies that residual organic material cannot be
completely decomposed to CO2 at the temperature below 275 C. Residual organic material can be completely decomposed to CO2 in the D step. The specific surface area, average pore diameter, and pore volume as measured by N2 adsorption using the BET method are shown in Table 1. The specific surface area of Commerce CeO2 (9.24 m2/g) is comprised between the gel CeO2 700 (11.93 m2/g) and gel CeO2 900 (4.11 m2/g) areas. The average pore diameter of Commerce CeO2 (13.61 nm) is similar to the average pore diameter of gel CeO2 900 (15.25 nm). It is interesting to note that Precipitation CeO2 calcined at 300 C (30.92 m2/g, 1.01 101 ml/g and 12.96 nm) has similar surface area, pore volume, and average pore diameter of gel CeO2 500 (35.75 m2/g, 1.06 101 ml/g and 11.88 nm) and different from gel CeO2 300 (80.94 m2/g, 1.68 101 ml/g and 8.32 nm). The catalyst prepared by sol–gel method shows better thermal stability. From Table 1, it is possible to observe as there is a decrease in the surface area as the calcination temperature increased from 300 to 900 C. X-ray diffraction patterns of Commerce CeO2, Precipitation CeO2, gel CeO2 300, gel CeO2 500, gel CeO2 700 and gel CeO2 900 are shown in Fig. 4. All samples display the XRD pattern corresponding to the cubic fluorite structure of pure CeO2 with
Fig. 4 – XRD patterns of different CeO2 catalysts.
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Fig. 5 – TEM analysis of (a) gel CeO2 300; (b) gel CeO2 500; (c) gel CeO2 700; (d) gel CeO2 900.
intense bands due to the (111), (200), (220) and (311) planes. The XRD peaks for the samples with low calcination temperatures are rather broad indicating the presence of small crystallites. The calculated crystallite sizes using the Scherrer
Fig. 6 – TEM analysis of Precipitation CeO2.
equation are shown in Table 1. Gel CeO2 300 has an average crystallite size of 5.2 nm. The crystallite size of Commerce CeO2 (151.8 nm) is consistent with gel CeO2 900 (145.6 nm). It is interesting that Precipitation CeO2 (14.8 nm) calcined at 300 C has a crystallite size similar to gel CeO2 500 (12.4 nm) and different from gel CeO2 300 (5.2 nm). The catalyst prepared by sol–gel method shows better thermal stability. The mean lattice constants d of the samples shown in Table 1 are calculated using the Bragg’s law 2d sin q ¼ kl. The lattice ˚ ) is obviously larger than constant of gel CeO2 300 (5.4149 A other samples. This implies that gel CeO2 300 has not been ˚) completely decomposed and that residual Ce3þ ions (1.14 A 4þ ˚ which are larger than Ce ions (0.97 A) can also occur in gel CeO2 300. The sintering of CeO2 crystallites is particularly evident at temperatures above 700 C. Samples sintered at temperature higher than 700 C show narrow diffraction peaks, indicating an increase in particle dimensions. This is also in agreement with the decrease of surface area observed in BET. The peak sharpness and intensity increase with temperature, indicating an improvement in ceria crystallinity. Contrarily, the ceria catalysts calcined at lower temperatures show more lattice defects (grain and interphase boundaries, oxygen vacancies, dislocations, etc.). The TEM pictures of all samples are shown in Figs. 5–7. The catalyst particle sizes can be seen distinctly in the TEM pictures. Fig. 5(a–d) shows the particle size and morphology variations of gel CeO2 as a function of calcination temperatures. A change in the particle size and sample morphology with the increase in the
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Fig. 9 – HI conversion for CeO2 catalysts with different calcination temperatures.
Fig. 7 – TEM analysis of Commerce CeO2.
calcination temperature from 300 to 900 C is observed. Ceria samples calcined at 300 and 500 C show individual irregular particles, whereas those of catalysts calcined at 700 and 900 C tend to agglomerate, forming larger blocks. The particle size of Commerce CeO2 shown in Fig. 7 is consistent with gel CeO2 900 in Fig. 5(d). It is also interesting to note that Precipitation CeO2 calcined at 300 C and shown in Fig. 6 has the same particle size of gel CeO2 500 in Fig. 5(b) and different from gel CeO2 300 shown in Fig. 5(a). 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. 8. TPR assays mainly show three reduction bands between ambient temperature and 800 C. A small, low temperature reduction peak (only for gel CeO2 300)
Fig. 8 – TPR profiles of different CeO2 catalysts.
centered at about 260 C (LT peak), a wide reduction band at middle temperature, from 300 to 600 C (MT peak) and an incomplete reduction peak at higher temperature, above 700 C (HT peak). The low reduction band is related to highly reducible surface ceria species, whereas middle temperature reduction peaks can be attributed to the surface reduction of oxygen [31,32]. It must be noted that the bulk reduction of CeO2 does not take place in the middle temperature interval, since it occurs only above 700 C [31,32]. The effect of calcination temperature on the reduction of profiles of gel CeO2 300, gel CeO2 500, gel CeO2 700 and gel CeO2 900 is shown in Fig. 8. All four catalysts showed the middle temperature reduction peak in the range 300–600 C. The peaks intensity and width of gel CeO2 300 and gel CeO2 500 are obviously larger than gel CeO2 700 and gel CeO2 900. The peak intensity and width of Precipitation CeO2 are larger than that of Commerce CeO2, but smaller than that of gel CeO2 300 and gel
Fig. 10 – HI conversion for CeO2 catalysts with different preparation methods.
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Fig. 11 – Operating lifetime evaluation of the gel CeO2 300 at 500 8C.
CeO2 500. This variation in hydrogen consumption can be related to the formation of subtly different surface structures of CeO2. This also implies that the ceria catalyst prepared by sol–gel method and calcined at 300 C shows better surface activity and oxygen migration ability.
3.2.
Catalytic activity
The decomposition efficiency was strongly affected by the temperature as can be seen in Figs. 9 and 10 which show the HI conversion between 300 and 550 C. In these two figures the
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thermodynamic tests and an uncatalyzed (blank) tests are also plotted. Thermodynamic test A is obtained from Ref. [16], and thermodynamic test B is investigated by FactSage which is a commercial software for thermodynamic calculation. As shown in Fig. 9, the gel CeO2 catalysts with different calcination temperatures show better catalytic activity by comparison with blank test. For temperatures lower than 450 C and except for gel CeO2 900, all the gel CeO2 catalysts showed similar catalytic activity. By increasing the temperature, the gel CeO2 300 and gel CeO2 500 showed better catalytic activity, especially gel CeO2 300, achieving 18.4% and 19.4% at 550 C, respectively. As shown in Fig. 10, the gel CeO2 300 and Precipitation CeO2 showed better catalytic activities by comparison with Commerce CeO2. At temperatures above 400 C, the HI conversion of gel CeO2 300 becomes better than that of Precipitation CeO2, achieving 19.4% and 17% at 550 C, respectively. In Fig. 11, the catalytic activity of gel CeO2 300 is reported as a function of the lifetime. These data have been obtained in subsequent experiments that were performed at 500 C. Although the study of the catalysts lifetime is beyond the scope of this paper and the collected data do not allow for conclusive evaluation, it is remarkable that gel CeO2 300 showed a relatively stable activity after approximately 200 min of operating lifetime.
3.3.
Hypothetic mechanism analysis
Detailed kinetic modeling and sensitivity analysis for HI homogeneous decomposition have already been reported by our lab [10]. The result shows that the reactions HI þ HI ¼ H2 þ I2 and HI þ I ¼ I2 þ H play a major role in the hydrogen production process. Detailed kinetic modeling and sensitivity analysis for HI homogeneous decomposition in the
Fig. 12 – Reaction mechanism for HI catalytic decomposition over CeO2 catalyst.
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presence of oxygen species (O) have also been investigated recently by our lab. According to the results, the reactions HI þ I ¼ I2 þ H and HI þ O ¼ OH þ I are the most important steps for HI homogeneous decomposition in the presence of O, especially the reaction HI þ I ¼ I2 þ H plays the dominant role in HI decomposition process. The presence of O can obviously promote the HI decomposition reaction rate, but simultaneously it consumes hydrogen species (H) contained in HI. The mechanism studies of CeO2 as a catalyst or an oxidant during the reforming of methane and the oxidation of CO have already been reported by many researchers [26,29,30]. The reduced surface sites, i.e., Ce3þ, oxygen vacancy or oxygen diffusion from the bulk of the solid particles to CeO2 surfaces play the very important role in CeO2 surface reaction. According to the above discussion, a new detailed reaction mechanism of HI catalytic decomposition over CeO2 catalyst is constructed and shown in Fig. 12. Firstly, HI is adsorbed and activated on the surface of CeO2 catalyst through the reduced surface sites, i.e., Ce3þ and oxygen vacancy causing the cleavage of H–I. The hydrogen and iodine atoms thus formed might be adsorbed in the vacancies near Ce3þ. Then there are two possibilities for the next step depending on the absence and presence of O. In the absence of O, as shown in A area of Fig. 12, H2 and I2 are produced and released from the surface of CeO2. In the presence of O, as shown in C area of Fig. 12, the reaction between O and H causes the production of H2O and I2. According to the results of detailed kinetic modeling and sensitivity analysis for HI homogeneous decomposition in the absence and presence of O, the reaction HI þ I ¼ I2 þ H plays a major role in the hydrogen production process. On the other hand, the amount of HI is obviously larger than I around the surface of the catalyst, and the reaction opportunity for I with HI may be larger than I with I. So B area in Fig. 12 involving the direct reaction between adsorbed I and HI on the surface of CeO2 is also considered to be a very important part of HI catalytic decomposition process. Here, it should be stressed that CeO2 during HI catalytic decomposition process acts not only as a catalyst but also as an oxidant. Lattice defects, especially the reduced surface sites, i.e., Ce3þ and oxygen vacancy, play the dominant role in surface reactions of HI decomposition. According to the catalysts’ analysis, the CeO2 catalyst calcined at low temperature shows more lattice defects, especially gel CeO2 300. That is one important reason why gel CeO2 300 shows the best catalytic activity on HI decomposition.
4.
Conclusions
The HI conversion is rather low. The use of a suitable catalyst allows a considerable decrease in the decomposition temperature to achieve workable reaction rates. The CeO2 catalysts, especially gel CeO2 300, strongly enhance the decomposition rate of HI to H2. The results of BET, XRD, TEM and TPR studies show that the CeO2 catalyst synthesized by citric-aided sol–gel method and calcined at 300 C shows more lattice defects, smaller crystallites, larger surface area and better reducibility. These provide this material with a potential to be used in sulfur–iodine cycle for hydrogen production.
Acknowledgements This work has been supported by Doctor foundation of the Ministry of Education in China (20050335046), Zhejiang Provincial Natural Science Foundation of China (Y106538), National High Technology Research and Development Program of China (863 Program) (no. 2008AA05Z103) and National Science Foundation for Distinguished Young Scholars (50525620).
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