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Ruthenium catalyst supported on Ba modified ZrO2 for ammonia decomposition to COx-free hydrogen
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Ruthenium catalyst supported on Ba modified ZrO2 for ammonia decomposition to COx-free hydrogen Ziqing Wang a,b,*, Yingmin Qu b, Xiaolong Shen b, Zhifeng Cai b a
School of Chemistry and Chemical Engineering/Key Laboratory for Green Process of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi, 832003, China b Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, 610041, Sichuan, China
article info
abstract
Article history:
The Ba modified ZrO2 materials were prepared and loaded Ru nanoparticles for ammonia
Received 20 August 2018
decomposition to COx-free hydrogen reaction. The catalytic activity of Ru supported on
Received in revised form
BaeZrO2 derived from sol-gel process (Ru/BaeZrO2) is found to be several times of those for
11 January 2019
Ru/ZrO2 without any promoter and RueBa/ZrO2 catalysts prepared by conventional im-
Accepted 25 January 2019
mersion method at the identical conditions. It is found that the formation of BaZrO3 phase
Available online 16 February 2019
in BaeZrO2 can enhance the electron-donating ability of the support and Ru nanoparticles dispersion. Therefore, mobile electrons would be transferred from BaZrO3 to the surface Ru
Keywords:
particles, facilitating the recombinative desorption of N over Ru particles, leading to the
NH3 decomposition
increase of activity for ammonia decomposition sufficiently. Additionally, the suitable size
COx-free hydrogen
of spherical Ru particles with average size of 2.4 nm for the formation of active sites are
Ba modified ZrO2
also responsible factors for the higher activity of this catalyst. The catalytic performance of
Ru catalyst
Ru/BaeZrO2 catalyst can also be further improved by introducing of K and Cs promoters,
BaZrO3
and the apparent activation energies over Ru/BaeZrO2 of 94.1 kJ/mol decrease to 70.7 kJ/
Basic sites
mol and 64.2 kJ/mol for K and Cs promoted Ru/BaeZrO2, respectively. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Ammonia is being considered as a promising energy carrier used for small-scale fuel cell for its large hydrogen content of 17.8 wt% compared with other hydrogen storage mediums, as well as low cost and ease in liquefaction under mild conditions [1,2]. The chemistry for NH3 as a fuel lies in only H2 and N2 released from the decomposition of NH3 without COx emission, which can be used as on-site H2 production for some niche applications [3e5]. To decompose NH3 completely at low temperatures, highly active catalyst is necessary. So far, Fe [6], Co [7], Ni [8], Ru [9,10], Li2NH [11], NaNH2 [12], Co3Mo3N
[13], Zr7O11N2 [14] and even Cu/ZnO/Al2O3 [15] all have been tested for this reaction. Among them, supported Ru nanoparticles have been thought as the most active ones. As reported in previous works, the activity of Ru catalysts for NH3 decomposition is support-dependent and an ideal support for Ru nanoparticles should concomitantly possess basicity and conductivity [16e20]. Therefore, Ru supported on nitrogen-doped graphitized carbon materials are proven to be the best choice, such as nitrogen-doped ordered mesoporous carbon (NOMC), carbon nanotubes (NCNT) and carbon nanofiber (NCNF) [21e23]. Unfortunately, the methanation of supports with product H2 at temperatures of greater than 425 C would shorten the life of these carbon materials loaded Ru
* Corresponding author. School of Chemistry and Chemical Engineering/Key Laboratory for Green Process of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi, 832003, China. E-mail address: [email protected] (Z. Wang). https://doi.org/10.1016/j.ijhydene.2019.01.235 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 7 3 0 0 e7 3 0 7
catalysts [16]. Recently, the development of a stable and highly active catalyst by loading Ru nanoparticles on oxides has attracted serious concern of researchers. Among all the tested Ru catalysts, MgO [18], Pr6O11 [24,25], Cs2Ti6O13 [26] and C12A7:e [17] were found to be the promising support materials, which are all rich in basic sites. Therefore, ZrO2 seems not to be an ideal support for NH3 decomposition due to the acidic nature of that [10]. However, the properties of ZrO2 can also be easily modified via a simple chemical or physical route. For instance, the highly active Ru/ZrO2 catalyst can be prepared by reducing RuCl3 precursor with NaBH4 as reducing agent, Furusawa et al. ascribed the higher activity for NH3 decomposition to no Cl residual and appropriate size of Ru particle [27]. Additionally, as reported in previous literature that reflux-digestion of ZrO(OH)2 gel in an alkaline solution leads to a superbasicity ZrO2 with surface area of as high as 267 m2/g, which showed excellent promotional effect to Ru nanoparticles in the reaction of ammonia decomposition [28]. Using lanthanum-stabilized ZrO2 embedded Ru nanoparticles (Ru@LSZ) as catalyst, Lorenzut et al. [29] ascribed its excellent and stable catalytic performance for ammonia decomposition to the minimization of the undesirable agglomeration of the Ru nanoparticles via this embedded strategy. Na2ZrO3 was also found to be an excellent promoter for this reaction in the view of its strong electron-donating power. Most recently, a series of highly effective Ru/ZrO2 catalysts can also be obtained by co-doping ZrO2 support with alkaline earth metals (Mg, Ca and Sr) and rare earth element La via a coprecipitation method [30]. Among them, the highest ammonia conversion over Ru/Ca(1)La(7)ZrO2 is 86.7% at 773 K, which increase by 43.5% compared with that over Ru/ZrO2 catalyst. Miyamoto et al. [30]. thought the formation of rich medium and strong basic sites was the main reason for the improvement of catalytic performance upon the Ca and La doping, but no information about alkaline earth metal Ba as promoter was reported in this literature. Ba has been proven to be an excellent promoter for carbon material and oxide supported Ru-based catalysts in the reaction of NH3 decomposition [20,31,32]. Nevertheless, to my knowledge, no work concerning Ba promoted ZrO2 loaded Ru nanoparticles is reported for NH3 decomposition in previous literature. In the present manuscript, the Ba modified ZrO2 materials were prepared by sol-gel as well as impregnation route and used as support for Ru catalyst, the relationships between the structure and the catalytic performance for NH3 decomposition were discussed in detail based on the results of various characterization techniques.
Experimental section Catalysts preparation Ba-modified ZrO2 (BaeZrO2) was synthesized by sol-gel process. The solution A was obtained by dissolving Zr(NO3)4.5H2O (8.58 g, 98%, Sinopharm Chemical Reagent Co., Ltd) and Ba(NO3)2 (0.52 g, 99.5%, Aldrich) in deionized water. Citric acid monohydrate was dissolved into ethylene glycol under continuous stirring to form solution B. Then, mixing solution B and solution A got a transparent solution. The mixed
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solution was firstly dried at 383 K to obtain a yellow gel. And then the resulting gel was heated under argon atmosphere at 673 K for 3.0 h and finally calcinated at 973 K for 6.0 h in air atmosphere to obtain a white powder, which is marked as Bae ZrO2. BaeZrO2 supported Ru catalyst was prepared by impregnation route using RuCl3 as precursor, then dried at 393 K for 10.0 h, and then reduced at 773 K under H2 after remove of Cl completely by distilled water washing. For comparison with other catalysts, ZrO2 was also prepared by the same way as Bae ZrO2, whereas Ba/ZrO2 was prepared by wet impregnating ZrO2 with Ba(NO3)2 solution. Then, the obtained resultant was calcined at the same condition as BaeZrO2 to decompose Ba(NO3)2 into Ba/ZrO2 support material, in which the molar ratio of Ba to Zr was ca.10%. ZrO2 and Ba/ZrO2 loaded Ru catalysts were also prepared via the same process mentioned above. The introduction of Cs and K promoters into supported Ru catalysts were also carried out by impregnation route as described in previous works with cesium hydroxide and potassium hydroxide as precursor [16,18], in which the molar ratio of promoter to Ru was set as 1:2. Theoretical content of Ru metal in all the samples is 3.0 wt%.
Catalyst characterization X-ray diffraction (XRD) patterns for all the samples were recorded on a PANalytical X'Pert Pro Diffractometer CoKa radiation. The BET surface area, pore volume and diameter of the samples were measured on the gas adsorption instrument Nova Win 2, BET surface area was calculated using the standard BET route and pore volume take P/P0 ¼ 0.99 corresponding adsorption volume, while pore diameter were recorded by BJH method. Temperature-programmed reduction (H2-TPR) of the samples were proceed on an TP5080 chemistry adsorption spectrometer. The sample was pretreated in Ar at 423 K for 1.0 h to clean the catalyst surface, then the H2-TPR was contracted from 425 to 1073 K at 10 K/min in the flow of 10 vol% H2/Ar at a flow rate of 50 ml min1. The temperatureprogrammed desorption of CO2 (CO2-TPD) was carried out in the same installations. After treatment the sample with N2 at 773 K for 1.5 h, CO2 adsorption was carried out at 323 K until the sample was saturated. After cooling to room temperature, the physisorbed CO2 was removed by a flushing with N2. Then, CO2-TPD was carried out under N2 flow of 20 ml/min with a heating rate of 10 K min1 from room temperature to 1100 K. CO chemisorption was conducted at ambient temperature with 5% CO/He gas stream. 100 mg of sample was firstly reduced with H2 at 773 K for 0.5 h, then purged with He to remove the remaining gas and cooled down to ambient temperature under the same gas flow. The CO consumption was monitored by TCD. Uptake of CO at monolayer coverage of Ru species was used to estimate Ru metal dispersion and particle size. The Ru dispersion and particles size were calculated by the methods proposed in literature [33]. The morphologies of samples were analyzed using TEM (Tecnai G2 F20) microscope. X-ray photoelectron spectroscopy (XPS) was performed in a Thermo Scientific ESCALAB 250 spectrometer, using Al Ka irradiation as the X-ray source.
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The C1s peak at 284.6 eV was used as an inner standard calibration peak.
Catalytic performance tests A certain amount of catalyst with particle size of 60e80 mesh was transferred to fixed-bed flow silica glass reactor, where it was reduced for 5.0 h at 773 K under hydrogen. Catalytic testing was carried out using pure NH3 as feedstock under given hourly space velocity (GHSV). A gas chromatograph equipped with TCD from Shimadzu was used to analysis gas product with Ar as carrier gas. NH3 conversion and H2 formation rate were calculated by the mole fraction of H2 in the products following the formula described in previous works [19,24].
Result and discussion Catalytic performance It had been found that the un-promoted Ru/ZrO2 catalyst prepared via traditional method showed poor activity for ammonia decomposition [9,27e29]. Indeedly, one can see from Fig. 1 that the Ru/ZrO2 catalyst after H2 reduction exhibited rather low activity. Surprisingly, no any increase in ammonia conversion can be observed for RueBa/ZrO2, and the ammonia conversion orders can be ranked as RueBa/ ZrO2
XRD The XRD patterns of Ru nanoparticles supported on various support materials after reduction under hydrogen at 773 K for
100
H2-TPR Plotted in Fig. 3 is the H2-TPR of various samples. One can see from Fig. 3a that only one sharp reduction peak at 498 K can be seen in the H2-TPR of Ru/ZrO2 for the reduction of welldispersed Ru species on ZrO2 [10]. The reduction of RueBa/ ZrO2 happens in two stages with peak temperatures at 453 and 546 K, the low temperature reduction peak often results from the reduction of surface exposure and bulk Ru species that has no interaction with the support. The slight peak at 546 K can be ascribed to the reduction of a few RuO2 dispersed on the support [37]. In terms of Ru/BaeZrO2, a wide reduction profile ranging from 424 to 700 K with a slight peak at 410 K can be seen in Fig. 3c. The low-temperature peak below to 473 K corresponds to the reduction of Ru species with weak interaction with supports phase, while high reduction temperature is due to the strong metal-support interaction (SMSI) of welldispersed Ru species with support [18,29,38].
Ru/ZrO 2 Ru-Ba/ZrO 2 Ru/Ba-ZrO 2
80
NH 3 conversion (%)
5.0 h are presented in Fig. 2. The profile for Ru/ZrO2 exhibited predominantly peaks of cubic crystalline phase ZrO2 (c-ZrO2, 2q ¼ 35.1 and 59.2 ), no Ru specie signal can be detected, implying that Ru species were well dispersed on the surface of c-ZrO2 [34]. Moreover, the crystal structure of profile b was also detected as pure c-ZrO2, besides new peak at 27.8 correspond to the presence of BaCO3 (JCPDS-ICDD files No.78-2057) [35]. That is to say, the crystal structure of ZrO2 could not be changed upon the introduction of Ba species. In terms of Ru/ BaeZrO2 sample, there are clear peaks for BaZrO3 besides the diffraction lines of c-ZrO2, implying the BaZrO3 solid solution is formed by incorporating Ba into ZrO2 lattice (JCPDS 06-0399). And the crystallite size of the obtained BaZrO3 was estimated to be 28.4 nm based on the Scherrer equation. The texture properties of different Ru-based catalysts are displayed in Table 1. The Ru/ZrO2 has the largest BET surface area of 38.6 m2/g, this value is much higher than that of RueBa/ZrO2. Obviously, the introduction of Ba(NO3)2 on the surface of ZrO2 leaded to a decrease in pore volume and surface area for Rue Ba/ZrO2, which is often thought to be resulted from the plugging of pores in previous works [36]. Classically, Ba/ZrO2 with lower surface area would decrease Ru dispersion, reducing the amount of active sites available for ammonia decomposition.
60 40 20
0 620 640 660 680 700 720 740 760 780
Temperature (K) Fig. 1 e The NH3 conversion over various Ru-based catalyst in the reaction of NH3 decomposition. Conditions: 200 mg of catalysts and 10.0 mL.mim¡1 NH3.
Fig. 2 e XRD patterns of (a) Ru/ZrO2 (b) RueBa/ZrO2 and (c) Ru/BaeZrO2 catalysts after reduction.
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Table 1 e Texture properties of Ru supported on different support. Sample
BET surface area (m2/g)
Pore volume (ml/g)
Mean pore diameter (nm)
ZrO2 crystallite size/(nm)a
38.6 25.4 6.3
0.15 0.10 0.03
24.2 19.4 30.8
15.8 14.2 15.2
Ru/ZrO2 Ru/BaeZrO2 RueBa/ZrO2
Estimated by the Scherrer equation.
H 2 Consumption (a.u.)
a
(a)
(b) (c) 400
500
600
700
800
900
1000
Temperature (K) Fig. 3 e H2-TPR profiles for (a) Ru/ZrO2 (b) RueBa/ZrO2 and (c) Ru/BaeZrO2 catalysts.
TEM TEM images of various Ru catalysts after reduction are shown in Fig. 4, the average size and dispersion of Ru particles are listed in Table 2. Apparently, the nano-size spheroidal Ru particles with 1.5e3.8 nm and 1.0e6.2 nm can be seen on the surface of BaeZrO2 and ZrO2, respectively. The average sizes of Ru nanoparticles were estimated to be 2.4 nm for Ru/Bae ZrO2 and 3.5 nm for Ru/ZrO2, while the average sizes of Ru nanoparticles for RueBa/ZrO2 was estimated to be 9.8 nm according to the formula reported in literature [10,28]. As reported in many previous works, NH3 decomposition on Ru catalyst has been proven to be a typical structure-sensitive reaction, and the real active sites, B5 sites, are strong connections with the particle size and shape of Ru nanoparticles [39,40]. Karim et al. [41] found that the hemispherical Ru nanoparticles with size ranging 1.8e3 nm are prone to form so-called B5 sites, and not only shape but also the size of Ru
particles over BaeZrO2 are obviously optimum for the formation of active sites for ammonia decomposition [39e41]. CO chemisorption was also conducted to further accessible surface Ru nanoparticles, and the results are also displayed in Table 2 using a CO/Ru stoichiometry. One can see from Table 2 that the Ru particles size determined by CO chemisorption has no difference with that from TEM images for RueBa/ZrO2 and Ru/ZrO2 catalysts. However, as for Ru/BaeZrO2 catalyst, the average size of Ru particle size evaluated by a cubical approximation model based on CO chemisorption is much higher than that observed by TEM. This discrepancy can be explained by the fact that the presence of SMSI effect would suppress CO chemisorption, and then the amount of adsorbed CO decreased [38,42]. In such a case, the value of particle size based on CO chemisorption was overestimated. Such difference in particle sizes can be ascribed to different surface area and interaction strength of support with Ru particle. Therefore, the best utilization ratio of Ru and the optimum particle sizes for B5 sites formation over Ru/BaeZrO2 should be the key factors for its excellent performance.
CO2-TPD The catalytic performance is strongly correlated to the basic nature of support materials for loaded Ru-based catalysts in the reaction of NH3 decomposition [20e26]. The surface basicity of the present support materials based on CO2-TPD is shown in Fig. 5. As reported in literature [38], the desorption curves appeared in the range of 373e698 K could be accredited as CO2 desorption from weak basic sites, and hightemperature peak (more than 823 K) corresponds the presence of strong basic site. And the amount of basic sites could be represented by the area for the desorbed CO2 in corresponding scope. One can see from Fig. 5 that compared with ZrO2, the addition and introduction of alkaline-earth metal Ba
Fig. 4 e TEM images of (a) Ru/ZrO2 (b) RueBa/ZrO2 and (c) Ru/BaeZrO2 catalysts.
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Table 2 e Mental size and dispersion of Ru-based catalysts derived from CO chemisorption and TEM. Catalysts
Ru/ZrO2 Ru/BaeZrO2 RueBa/ZrO2 a b c
CO adsorption umol/gcata
0.56 0.65 0.60
Dispersion (%)
Ru particles size (nm)
CO adsorptionb
TEMc
CO adsorptiond
TEM
28.1 0.7 5.2
29.7 44.8 13.6
3.2 128.6 17.5
3.5 2.4 9.8
78.7 2.4 15.6
Calculated from H2-TPR. Calculated by the equation in literature [33]: dispersion (%) ¼ CO adsorption/(H2consumption/2) 100%. Calculated by the equation in literature [34]. Calculated by the equation: particles size ¼ 0.9/dispersion [33].
Ru3d3/2+C1S
CO 2 desorption singnal (a.u.)
d
H2consumption (mmol/gcata)a
(c) Intensity
(c)
(b) (a)
Ru3d5/2
(b) (a)
300 400 500 600 700 800 900 1000 1100
Temperature (K) Fig. 5 e CO2-TPD of (a) Ba/ZrO2, (b) ZrO2 and (c) BaeZrO2 support materials.
can significantly alter the number and strength of basic sites. Obviously, the number of weak basic sites increased as the order of Ba/ZrO2
XPS The XPS spectrums of Ru3d measured from different catalysts are shown in Fig. 6. The spectra of Ru3d includes Ru3d3/2 and Ru3d5/2 peaks for the spin-orbital splitting. Because the peak for Ru3d3/2 is superimposed with that for C1s, hence only the Ru3d5/2 is measured for analysis. One can see in Fig. 6a that the Eb value of Ru3d5/2 in Ru/ZrO2 sample is about 280.6 eV, identical with that for Ru catalyst supported on acid support, in which this phenomenon is often explained by the withdraw
278
280
282
284
286
288
290
292
Binding energy (eV) Fig. 6 e XPS patterns of (a) Ru/ZrO2 (b) RueBa/ZrO2 and (c) Ru/BaeZrO2 catalysts after reduction. electron effect of acid sites [34]. In terms of RueBa/ZrO2 catalyst, it is analyzed into two Ru3d doublets, one at Eb ¼ 280.6, which represents metallic Ru, and the second one with much stronger intensity at Eb ¼ 281.7 for RuO2 [43]. As reported in literature [44], the major control factors for Ru electronic state should be the support material, particle size, support-metal interaction and even the reduction procedure. In term of RueBa/ZrO2 catalyst, the aggregation of Ru particles and weaker support-metal interaction were thought to the major reason for the oxidation of metallic Ru. In fact, this result also can be further confirmed by the H2-TPR profile and TEM of RueBa/ZrO2, in which most Ru species were exposed on the surface as bulk. In contrast, the Eb of Ru3d5/2 for Ru/Bae ZrO2 is 279.5 eV, which is the characteristic of metallic Ru nanoparticles with d-band center upward shift resulted from electron-donating effect of basic support materials [34,45]. These Ru nanoparticles with electron-rich state is beneficial to the recombinative desorption of N species that is thought to be the rate-determining step of ammonia decomposition [28]. This result is also line with the conclusions of CO2-TPD and H2-TPR, in which BaeZrO2 support can transfer its electron to the surface of Ru nanoparticles through the SMSI [38].
Effect of promoters It is well known that the catalytic activity of Ru-based catalysts for ammonia decomposition can be remarkablely enhanced by the introduction of alkali metals, K and Cs, as
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electronic promoters [9,10,18,19,46]. Therefore, KOH and CsOH.xH2O were added into the Ru/BaeZrO2 catalyst to further improve its catalytic performance for the synergetic effect of promoter and support [47]. One can see from Fig. 7a that the addition of K and Cs lead to an obvious improvement in NH3 conversion for Ru/BaeZrO2 catalyst, and the catalytic performance can be ranked as the follow of Ru/BaeZrO2
(a)
2.4
Ln r(mmolH2/gcata/min)
NH3 conversion (%)
80
Ru/MgO-DP (K/Ru ¼ 1/2) catalysts, respectively. Hill et al. [46,47] also found the Ea value decreased from 96.7 kJ/mol to 49e78 kJ/mol upon the addition of Cs promoter to Ru catalyst supported CNT. Various modified ZrO2 supported Ru catalysts have been reported in previous work for ammonia decomposition to COx-free H2, Table 3 summarizes the catalytic performance of Ru/BaeZrO2 and literature results with regard to NH3 conversation, H2 production rate and Ea value at 723 K. For the comparison, the performance evaluation was performed under a condition of high GHSV of 30 000 ml. g1cat.h1. Obviously, Ru/BaeZrO2 has remarkable advantages in H2 production rate compared with other modified ZrO2 supported Ru catalysts, such as Ru/ZrO2-BD [10], Ru/ZrO2-lpr [27], Ru/LSZ-NaBH4 [29] and Ru/ZrO2 co-doped with alkaline earth and La [30]. The current NH3 decomposition rate over Ru/ZrO2-BD is also comparable to those of Ru/CNT[22,48], Ru/NCNT [21], Ru/NOMC [22] and Ru/MgO-IM [18]. Although the catalytic performance of CseRu/BaeZrO2 did not reach that for KeRu/MgO-DP [18], CseRu/Pr6O11 [24,25] and Ru/ ZrO2eKOH [28] reported in recent years, the Ru content in CseRu/BaeZrO2 is approximately half of that for the mentioned catalysts.
60
40
20
(b)
2.0 1.6 1.2 0.8 0.4
0 575
600
625
650
675
700
725
750
775
0.0 1.36
1.40
1.44
1.48
1.52
1.56
1.60
1.64
-1
Temperature (K)
1000/T(K )
Fig. 7 e (a) NH3 conversion and (b) Arrhenius plots for (-) Ru/BaeZrO2, (C) KeRu/BaeZrO2 and (:) CseRu/BaeZrO2. Conditions: 100 mg of catalysts and 50.0 mL.mim¡1 of NH3.
Table 3 e Catalytic performance of K and Cs promoted Ru/BaeZrO2 catalysts for ammonia decomposition at 723 K. Catalysts Ru/BaeZrO2 KeRu/BaeZrO2 CseRu/BaeZrO2 Ru/ZrO2-BD Ru/LaeZrO2 Ru@LaeZrO2 Ru/CNTs ZrO2-lpr a Ru/Mg(1)La(7)ZrO2 a Ru/Ca(1)La(7)ZrO2 a Ru/Sr(1)La(7)ZrO2 Ru/AC Ru/NOMC Ru/CNTs Ru/MgO-IM a
Tested at 773 K.
GHSV (mL.g1 h1)
NH3 conversion (%)
H2 production rate (mmol H2/gcat.min)
Ea (KJ/mol)
Ref.
30 000 30 000 30 000 150 000 30 000 30 000 30000 2000 4335 4335 4335 6000 6000 6000 36000
23.6 32.5 37.8 1.9 ca.6.0 ca.40.0 ca.20.0 ca.99.5 83.9 86.7 75.9 8.7 84.5 69.5 17.5
7.9 10.9 12.7 3.2 2.0 13.4 6.7 2.2 4.1 4.2 3.7 0.6 5.7 4.7 7.0
92.1 70.7 64.2 80.4 _ _ 87.3 _ _ _ _ _ 37.3 _ 102.7
This work This work This work [10] [29] [29] [48] [27] [30] [30] [30] [22] [22] [22] [18]
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Conclusion In this work, the barium modified ZrO2 materials was found to be an ideal support material for Ru nanoparticles to catalytic NH3 decomposition reaction. The obtained Ru/BaeZrO2 catalyst has appropriate Ru particle size, strong electron-donating ability and enhanced metal-support interaction due to the presence of BaZrO3. As a result of the characteristics mentioned above, Ru/BaeZrO2 catalyst exhibits excellent catalytic activity for NH3 decomposition. Furthermore, the NH3 conversion over Ru/BaeZrO2 can also be remarkablely improved by the addition of K and Cs promoters, which also show comparable catalytic performance with those of other effective Ru catalysts reported in earlier literature.
Acknowledgements This work was financially supported by National High Technology Research and Development Program of China (2008AA062601).
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[31] Sørensen RZ, Klerke A, Quaade U, Jensen S, Hansen O, Christensen CH. Promoted Ru on high-surface area graphite for efficient miniaturized production of hydrogen from ammonia. Catal Lett 2006;112:77e81. g-Pilecka W, Szmigiel D, Kowalczyk Z, Jodzis S, [32] Raro Zielinski J. Ammonia decomposition over the carbon-based ruthenium catalyst promoted with barium or cesium. J Catal 2003;218:465e9. [33] Li D, Li RL, Lu MM, Lin XY, Zhan YY, Jiang LL. Carbon dioxide reforming of methane over Ru catalysts supported on Mg-Al oxides: a highly dispersed and stable Ru/Mg(Al)O catalyst. Appl Catal B Environ 2017;200:566e77. [34] Wang ZQ, Ma YC, Lin JX. Ruthenium catalyst supported on high-surface-area basic ZrO2 for ammonia synthesis. J Mole Catal A :Chem 2013;378:307e13. [35] Pradhan S, Swarnima K, Mishra BG. Ba/ZrO2 nanoparticles as efficient heterogeneous base catalyst for the synthesis of bnitro alcohols and 2-amino 2-chromenes. J Chem Sci 2016;128:1119e30. [36] Okura K, Okanishi T, Muroyama H, Matsui T, Eguchi K. Promotion effect of rare-earth elements on the catalytic decomposition of ammonia over Ni/Al2O3 catalyst. Appl Catal Gen 2015;505:77e85. [37] Whang HS, Choi MS, Lim J, Kim C, Heo I, Chang TS, et al. Enhanced activity and durability of Ru catalyst dispersed on zirconia for dry reforming of methane. Catal Today 2017;293e294:122e8. [38] Wang Z, Liu B, Lin J. Highly effective perovskite-type BaZrO3 supported Ru catalyst for ammonia synthesis. Appl Catal Gen 2013;458:130e6. [39] Garcı´a-Garcı´a FR, Guerrero-Ruiz A, Rodrı´guez-Ramos I. Role of B5-type sites in Ru catalysts used for the NH3 decomposition reaction. Top Catal 2009;52:758e64.
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Ruthenium catalyst supported on Ba modified ZrO2 for ammonia decomposition to COx-free hydrogen Ziqing Wang a,b,*, Yingmin Qu b, Xiaolong Shen b, Zhifeng Cai b a
School of Chemistry and Chemical Engineering/Key Laboratory for Green Process of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi, 832003, China b Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, 610041, Sichuan, China
article info
abstract
Article history:
The Ba modified ZrO2 materials were prepared and loaded Ru nanoparticles for ammonia
Received 20 August 2018
decomposition to COx-free hydrogen reaction. The catalytic activity of Ru supported on
Received in revised form
BaeZrO2 derived from sol-gel process (Ru/BaeZrO2) is found to be several times of those for
11 January 2019
Ru/ZrO2 without any promoter and RueBa/ZrO2 catalysts prepared by conventional im-
Accepted 25 January 2019
mersion method at the identical conditions. It is found that the formation of BaZrO3 phase
Available online 16 February 2019
in BaeZrO2 can enhance the electron-donating ability of the support and Ru nanoparticles dispersion. Therefore, mobile electrons would be transferred from BaZrO3 to the surface Ru
Keywords:
particles, facilitating the recombinative desorption of N over Ru particles, leading to the
NH3 decomposition
increase of activity for ammonia decomposition sufficiently. Additionally, the suitable size
COx-free hydrogen
of spherical Ru particles with average size of 2.4 nm for the formation of active sites are
Ba modified ZrO2
also responsible factors for the higher activity of this catalyst. The catalytic performance of
Ru catalyst
Ru/BaeZrO2 catalyst can also be further improved by introducing of K and Cs promoters,
BaZrO3
and the apparent activation energies over Ru/BaeZrO2 of 94.1 kJ/mol decrease to 70.7 kJ/
Basic sites
mol and 64.2 kJ/mol for K and Cs promoted Ru/BaeZrO2, respectively. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Ammonia is being considered as a promising energy carrier used for small-scale fuel cell for its large hydrogen content of 17.8 wt% compared with other hydrogen storage mediums, as well as low cost and ease in liquefaction under mild conditions [1,2]. The chemistry for NH3 as a fuel lies in only H2 and N2 released from the decomposition of NH3 without COx emission, which can be used as on-site H2 production for some niche applications [3e5]. To decompose NH3 completely at low temperatures, highly active catalyst is necessary. So far, Fe [6], Co [7], Ni [8], Ru [9,10], Li2NH [11], NaNH2 [12], Co3Mo3N
[13], Zr7O11N2 [14] and even Cu/ZnO/Al2O3 [15] all have been tested for this reaction. Among them, supported Ru nanoparticles have been thought as the most active ones. As reported in previous works, the activity of Ru catalysts for NH3 decomposition is support-dependent and an ideal support for Ru nanoparticles should concomitantly possess basicity and conductivity [16e20]. Therefore, Ru supported on nitrogen-doped graphitized carbon materials are proven to be the best choice, such as nitrogen-doped ordered mesoporous carbon (NOMC), carbon nanotubes (NCNT) and carbon nanofiber (NCNF) [21e23]. Unfortunately, the methanation of supports with product H2 at temperatures of greater than 425 C would shorten the life of these carbon materials loaded Ru
* Corresponding author. School of Chemistry and Chemical Engineering/Key Laboratory for Green Process of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi, 832003, China. E-mail address: [email protected] (Z. Wang). https://doi.org/10.1016/j.ijhydene.2019.01.235 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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catalysts [16]. Recently, the development of a stable and highly active catalyst by loading Ru nanoparticles on oxides has attracted serious concern of researchers. Among all the tested Ru catalysts, MgO [18], Pr6O11 [24,25], Cs2Ti6O13 [26] and C12A7:e [17] were found to be the promising support materials, which are all rich in basic sites. Therefore, ZrO2 seems not to be an ideal support for NH3 decomposition due to the acidic nature of that [10]. However, the properties of ZrO2 can also be easily modified via a simple chemical or physical route. For instance, the highly active Ru/ZrO2 catalyst can be prepared by reducing RuCl3 precursor with NaBH4 as reducing agent, Furusawa et al. ascribed the higher activity for NH3 decomposition to no Cl residual and appropriate size of Ru particle [27]. Additionally, as reported in previous literature that reflux-digestion of ZrO(OH)2 gel in an alkaline solution leads to a superbasicity ZrO2 with surface area of as high as 267 m2/g, which showed excellent promotional effect to Ru nanoparticles in the reaction of ammonia decomposition [28]. Using lanthanum-stabilized ZrO2 embedded Ru nanoparticles (Ru@LSZ) as catalyst, Lorenzut et al. [29] ascribed its excellent and stable catalytic performance for ammonia decomposition to the minimization of the undesirable agglomeration of the Ru nanoparticles via this embedded strategy. Na2ZrO3 was also found to be an excellent promoter for this reaction in the view of its strong electron-donating power. Most recently, a series of highly effective Ru/ZrO2 catalysts can also be obtained by co-doping ZrO2 support with alkaline earth metals (Mg, Ca and Sr) and rare earth element La via a coprecipitation method [30]. Among them, the highest ammonia conversion over Ru/Ca(1)La(7)ZrO2 is 86.7% at 773 K, which increase by 43.5% compared with that over Ru/ZrO2 catalyst. Miyamoto et al. [30]. thought the formation of rich medium and strong basic sites was the main reason for the improvement of catalytic performance upon the Ca and La doping, but no information about alkaline earth metal Ba as promoter was reported in this literature. Ba has been proven to be an excellent promoter for carbon material and oxide supported Ru-based catalysts in the reaction of NH3 decomposition [20,31,32]. Nevertheless, to my knowledge, no work concerning Ba promoted ZrO2 loaded Ru nanoparticles is reported for NH3 decomposition in previous literature. In the present manuscript, the Ba modified ZrO2 materials were prepared by sol-gel as well as impregnation route and used as support for Ru catalyst, the relationships between the structure and the catalytic performance for NH3 decomposition were discussed in detail based on the results of various characterization techniques.
Experimental section Catalysts preparation Ba-modified ZrO2 (BaeZrO2) was synthesized by sol-gel process. The solution A was obtained by dissolving Zr(NO3)4.5H2O (8.58 g, 98%, Sinopharm Chemical Reagent Co., Ltd) and Ba(NO3)2 (0.52 g, 99.5%, Aldrich) in deionized water. Citric acid monohydrate was dissolved into ethylene glycol under continuous stirring to form solution B. Then, mixing solution B and solution A got a transparent solution. The mixed
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solution was firstly dried at 383 K to obtain a yellow gel. And then the resulting gel was heated under argon atmosphere at 673 K for 3.0 h and finally calcinated at 973 K for 6.0 h in air atmosphere to obtain a white powder, which is marked as Bae ZrO2. BaeZrO2 supported Ru catalyst was prepared by impregnation route using RuCl3 as precursor, then dried at 393 K for 10.0 h, and then reduced at 773 K under H2 after remove of Cl completely by distilled water washing. For comparison with other catalysts, ZrO2 was also prepared by the same way as Bae ZrO2, whereas Ba/ZrO2 was prepared by wet impregnating ZrO2 with Ba(NO3)2 solution. Then, the obtained resultant was calcined at the same condition as BaeZrO2 to decompose Ba(NO3)2 into Ba/ZrO2 support material, in which the molar ratio of Ba to Zr was ca.10%. ZrO2 and Ba/ZrO2 loaded Ru catalysts were also prepared via the same process mentioned above. The introduction of Cs and K promoters into supported Ru catalysts were also carried out by impregnation route as described in previous works with cesium hydroxide and potassium hydroxide as precursor [16,18], in which the molar ratio of promoter to Ru was set as 1:2. Theoretical content of Ru metal in all the samples is 3.0 wt%.
Catalyst characterization X-ray diffraction (XRD) patterns for all the samples were recorded on a PANalytical X'Pert Pro Diffractometer CoKa radiation. The BET surface area, pore volume and diameter of the samples were measured on the gas adsorption instrument Nova Win 2, BET surface area was calculated using the standard BET route and pore volume take P/P0 ¼ 0.99 corresponding adsorption volume, while pore diameter were recorded by BJH method. Temperature-programmed reduction (H2-TPR) of the samples were proceed on an TP5080 chemistry adsorption spectrometer. The sample was pretreated in Ar at 423 K for 1.0 h to clean the catalyst surface, then the H2-TPR was contracted from 425 to 1073 K at 10 K/min in the flow of 10 vol% H2/Ar at a flow rate of 50 ml min1. The temperatureprogrammed desorption of CO2 (CO2-TPD) was carried out in the same installations. After treatment the sample with N2 at 773 K for 1.5 h, CO2 adsorption was carried out at 323 K until the sample was saturated. After cooling to room temperature, the physisorbed CO2 was removed by a flushing with N2. Then, CO2-TPD was carried out under N2 flow of 20 ml/min with a heating rate of 10 K min1 from room temperature to 1100 K. CO chemisorption was conducted at ambient temperature with 5% CO/He gas stream. 100 mg of sample was firstly reduced with H2 at 773 K for 0.5 h, then purged with He to remove the remaining gas and cooled down to ambient temperature under the same gas flow. The CO consumption was monitored by TCD. Uptake of CO at monolayer coverage of Ru species was used to estimate Ru metal dispersion and particle size. The Ru dispersion and particles size were calculated by the methods proposed in literature [33]. The morphologies of samples were analyzed using TEM (Tecnai G2 F20) microscope. X-ray photoelectron spectroscopy (XPS) was performed in a Thermo Scientific ESCALAB 250 spectrometer, using Al Ka irradiation as the X-ray source.
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The C1s peak at 284.6 eV was used as an inner standard calibration peak.
Catalytic performance tests A certain amount of catalyst with particle size of 60e80 mesh was transferred to fixed-bed flow silica glass reactor, where it was reduced for 5.0 h at 773 K under hydrogen. Catalytic testing was carried out using pure NH3 as feedstock under given hourly space velocity (GHSV). A gas chromatograph equipped with TCD from Shimadzu was used to analysis gas product with Ar as carrier gas. NH3 conversion and H2 formation rate were calculated by the mole fraction of H2 in the products following the formula described in previous works [19,24].
Result and discussion Catalytic performance It had been found that the un-promoted Ru/ZrO2 catalyst prepared via traditional method showed poor activity for ammonia decomposition [9,27e29]. Indeedly, one can see from Fig. 1 that the Ru/ZrO2 catalyst after H2 reduction exhibited rather low activity. Surprisingly, no any increase in ammonia conversion can be observed for RueBa/ZrO2, and the ammonia conversion orders can be ranked as RueBa/ ZrO2
XRD The XRD patterns of Ru nanoparticles supported on various support materials after reduction under hydrogen at 773 K for
100
H2-TPR Plotted in Fig. 3 is the H2-TPR of various samples. One can see from Fig. 3a that only one sharp reduction peak at 498 K can be seen in the H2-TPR of Ru/ZrO2 for the reduction of welldispersed Ru species on ZrO2 [10]. The reduction of RueBa/ ZrO2 happens in two stages with peak temperatures at 453 and 546 K, the low temperature reduction peak often results from the reduction of surface exposure and bulk Ru species that has no interaction with the support. The slight peak at 546 K can be ascribed to the reduction of a few RuO2 dispersed on the support [37]. In terms of Ru/BaeZrO2, a wide reduction profile ranging from 424 to 700 K with a slight peak at 410 K can be seen in Fig. 3c. The low-temperature peak below to 473 K corresponds to the reduction of Ru species with weak interaction with supports phase, while high reduction temperature is due to the strong metal-support interaction (SMSI) of welldispersed Ru species with support [18,29,38].
Ru/ZrO 2 Ru-Ba/ZrO 2 Ru/Ba-ZrO 2
80
NH 3 conversion (%)
5.0 h are presented in Fig. 2. The profile for Ru/ZrO2 exhibited predominantly peaks of cubic crystalline phase ZrO2 (c-ZrO2, 2q ¼ 35.1 and 59.2 ), no Ru specie signal can be detected, implying that Ru species were well dispersed on the surface of c-ZrO2 [34]. Moreover, the crystal structure of profile b was also detected as pure c-ZrO2, besides new peak at 27.8 correspond to the presence of BaCO3 (JCPDS-ICDD files No.78-2057) [35]. That is to say, the crystal structure of ZrO2 could not be changed upon the introduction of Ba species. In terms of Ru/ BaeZrO2 sample, there are clear peaks for BaZrO3 besides the diffraction lines of c-ZrO2, implying the BaZrO3 solid solution is formed by incorporating Ba into ZrO2 lattice (JCPDS 06-0399). And the crystallite size of the obtained BaZrO3 was estimated to be 28.4 nm based on the Scherrer equation. The texture properties of different Ru-based catalysts are displayed in Table 1. The Ru/ZrO2 has the largest BET surface area of 38.6 m2/g, this value is much higher than that of RueBa/ZrO2. Obviously, the introduction of Ba(NO3)2 on the surface of ZrO2 leaded to a decrease in pore volume and surface area for Rue Ba/ZrO2, which is often thought to be resulted from the plugging of pores in previous works [36]. Classically, Ba/ZrO2 with lower surface area would decrease Ru dispersion, reducing the amount of active sites available for ammonia decomposition.
60 40 20
0 620 640 660 680 700 720 740 760 780
Temperature (K) Fig. 1 e The NH3 conversion over various Ru-based catalyst in the reaction of NH3 decomposition. Conditions: 200 mg of catalysts and 10.0 mL.mim¡1 NH3.
Fig. 2 e XRD patterns of (a) Ru/ZrO2 (b) RueBa/ZrO2 and (c) Ru/BaeZrO2 catalysts after reduction.
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Table 1 e Texture properties of Ru supported on different support. Sample
BET surface area (m2/g)
Pore volume (ml/g)
Mean pore diameter (nm)
ZrO2 crystallite size/(nm)a
38.6 25.4 6.3
0.15 0.10 0.03
24.2 19.4 30.8
15.8 14.2 15.2
Ru/ZrO2 Ru/BaeZrO2 RueBa/ZrO2
Estimated by the Scherrer equation.
H 2 Consumption (a.u.)
a
(a)
(b) (c) 400
500
600
700
800
900
1000
Temperature (K) Fig. 3 e H2-TPR profiles for (a) Ru/ZrO2 (b) RueBa/ZrO2 and (c) Ru/BaeZrO2 catalysts.
TEM TEM images of various Ru catalysts after reduction are shown in Fig. 4, the average size and dispersion of Ru particles are listed in Table 2. Apparently, the nano-size spheroidal Ru particles with 1.5e3.8 nm and 1.0e6.2 nm can be seen on the surface of BaeZrO2 and ZrO2, respectively. The average sizes of Ru nanoparticles were estimated to be 2.4 nm for Ru/Bae ZrO2 and 3.5 nm for Ru/ZrO2, while the average sizes of Ru nanoparticles for RueBa/ZrO2 was estimated to be 9.8 nm according to the formula reported in literature [10,28]. As reported in many previous works, NH3 decomposition on Ru catalyst has been proven to be a typical structure-sensitive reaction, and the real active sites, B5 sites, are strong connections with the particle size and shape of Ru nanoparticles [39,40]. Karim et al. [41] found that the hemispherical Ru nanoparticles with size ranging 1.8e3 nm are prone to form so-called B5 sites, and not only shape but also the size of Ru
particles over BaeZrO2 are obviously optimum for the formation of active sites for ammonia decomposition [39e41]. CO chemisorption was also conducted to further accessible surface Ru nanoparticles, and the results are also displayed in Table 2 using a CO/Ru stoichiometry. One can see from Table 2 that the Ru particles size determined by CO chemisorption has no difference with that from TEM images for RueBa/ZrO2 and Ru/ZrO2 catalysts. However, as for Ru/BaeZrO2 catalyst, the average size of Ru particle size evaluated by a cubical approximation model based on CO chemisorption is much higher than that observed by TEM. This discrepancy can be explained by the fact that the presence of SMSI effect would suppress CO chemisorption, and then the amount of adsorbed CO decreased [38,42]. In such a case, the value of particle size based on CO chemisorption was overestimated. Such difference in particle sizes can be ascribed to different surface area and interaction strength of support with Ru particle. Therefore, the best utilization ratio of Ru and the optimum particle sizes for B5 sites formation over Ru/BaeZrO2 should be the key factors for its excellent performance.
CO2-TPD The catalytic performance is strongly correlated to the basic nature of support materials for loaded Ru-based catalysts in the reaction of NH3 decomposition [20e26]. The surface basicity of the present support materials based on CO2-TPD is shown in Fig. 5. As reported in literature [38], the desorption curves appeared in the range of 373e698 K could be accredited as CO2 desorption from weak basic sites, and hightemperature peak (more than 823 K) corresponds the presence of strong basic site. And the amount of basic sites could be represented by the area for the desorbed CO2 in corresponding scope. One can see from Fig. 5 that compared with ZrO2, the addition and introduction of alkaline-earth metal Ba
Fig. 4 e TEM images of (a) Ru/ZrO2 (b) RueBa/ZrO2 and (c) Ru/BaeZrO2 catalysts.
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Table 2 e Mental size and dispersion of Ru-based catalysts derived from CO chemisorption and TEM. Catalysts
Ru/ZrO2 Ru/BaeZrO2 RueBa/ZrO2 a b c
CO adsorption umol/gcata
0.56 0.65 0.60
Dispersion (%)
Ru particles size (nm)
CO adsorptionb
TEMc
CO adsorptiond
TEM
28.1 0.7 5.2
29.7 44.8 13.6
3.2 128.6 17.5
3.5 2.4 9.8
78.7 2.4 15.6
Calculated from H2-TPR. Calculated by the equation in literature [33]: dispersion (%) ¼ CO adsorption/(H2consumption/2) 100%. Calculated by the equation in literature [34]. Calculated by the equation: particles size ¼ 0.9/dispersion [33].
Ru3d3/2+C1S
CO 2 desorption singnal (a.u.)
d
H2consumption (mmol/gcata)a
(c) Intensity
(c)
(b) (a)
Ru3d5/2
(b) (a)
300 400 500 600 700 800 900 1000 1100
Temperature (K) Fig. 5 e CO2-TPD of (a) Ba/ZrO2, (b) ZrO2 and (c) BaeZrO2 support materials.
can significantly alter the number and strength of basic sites. Obviously, the number of weak basic sites increased as the order of Ba/ZrO2
XPS The XPS spectrums of Ru3d measured from different catalysts are shown in Fig. 6. The spectra of Ru3d includes Ru3d3/2 and Ru3d5/2 peaks for the spin-orbital splitting. Because the peak for Ru3d3/2 is superimposed with that for C1s, hence only the Ru3d5/2 is measured for analysis. One can see in Fig. 6a that the Eb value of Ru3d5/2 in Ru/ZrO2 sample is about 280.6 eV, identical with that for Ru catalyst supported on acid support, in which this phenomenon is often explained by the withdraw
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290
292
Binding energy (eV) Fig. 6 e XPS patterns of (a) Ru/ZrO2 (b) RueBa/ZrO2 and (c) Ru/BaeZrO2 catalysts after reduction. electron effect of acid sites [34]. In terms of RueBa/ZrO2 catalyst, it is analyzed into two Ru3d doublets, one at Eb ¼ 280.6, which represents metallic Ru, and the second one with much stronger intensity at Eb ¼ 281.7 for RuO2 [43]. As reported in literature [44], the major control factors for Ru electronic state should be the support material, particle size, support-metal interaction and even the reduction procedure. In term of RueBa/ZrO2 catalyst, the aggregation of Ru particles and weaker support-metal interaction were thought to the major reason for the oxidation of metallic Ru. In fact, this result also can be further confirmed by the H2-TPR profile and TEM of RueBa/ZrO2, in which most Ru species were exposed on the surface as bulk. In contrast, the Eb of Ru3d5/2 for Ru/Bae ZrO2 is 279.5 eV, which is the characteristic of metallic Ru nanoparticles with d-band center upward shift resulted from electron-donating effect of basic support materials [34,45]. These Ru nanoparticles with electron-rich state is beneficial to the recombinative desorption of N species that is thought to be the rate-determining step of ammonia decomposition [28]. This result is also line with the conclusions of CO2-TPD and H2-TPR, in which BaeZrO2 support can transfer its electron to the surface of Ru nanoparticles through the SMSI [38].
Effect of promoters It is well known that the catalytic activity of Ru-based catalysts for ammonia decomposition can be remarkablely enhanced by the introduction of alkali metals, K and Cs, as
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electronic promoters [9,10,18,19,46]. Therefore, KOH and CsOH.xH2O were added into the Ru/BaeZrO2 catalyst to further improve its catalytic performance for the synergetic effect of promoter and support [47]. One can see from Fig. 7a that the addition of K and Cs lead to an obvious improvement in NH3 conversion for Ru/BaeZrO2 catalyst, and the catalytic performance can be ranked as the follow of Ru/BaeZrO2
(a)
2.4
Ln r(mmolH2/gcata/min)
NH3 conversion (%)
80
Ru/MgO-DP (K/Ru ¼ 1/2) catalysts, respectively. Hill et al. [46,47] also found the Ea value decreased from 96.7 kJ/mol to 49e78 kJ/mol upon the addition of Cs promoter to Ru catalyst supported CNT. Various modified ZrO2 supported Ru catalysts have been reported in previous work for ammonia decomposition to COx-free H2, Table 3 summarizes the catalytic performance of Ru/BaeZrO2 and literature results with regard to NH3 conversation, H2 production rate and Ea value at 723 K. For the comparison, the performance evaluation was performed under a condition of high GHSV of 30 000 ml. g1cat.h1. Obviously, Ru/BaeZrO2 has remarkable advantages in H2 production rate compared with other modified ZrO2 supported Ru catalysts, such as Ru/ZrO2-BD [10], Ru/ZrO2-lpr [27], Ru/LSZ-NaBH4 [29] and Ru/ZrO2 co-doped with alkaline earth and La [30]. The current NH3 decomposition rate over Ru/ZrO2-BD is also comparable to those of Ru/CNT[22,48], Ru/NCNT [21], Ru/NOMC [22] and Ru/MgO-IM [18]. Although the catalytic performance of CseRu/BaeZrO2 did not reach that for KeRu/MgO-DP [18], CseRu/Pr6O11 [24,25] and Ru/ ZrO2eKOH [28] reported in recent years, the Ru content in CseRu/BaeZrO2 is approximately half of that for the mentioned catalysts.
60
40
20
(b)
2.0 1.6 1.2 0.8 0.4
0 575
600
625
650
675
700
725
750
775
0.0 1.36
1.40
1.44
1.48
1.52
1.56
1.60
1.64
-1
Temperature (K)
1000/T(K )
Fig. 7 e (a) NH3 conversion and (b) Arrhenius plots for (-) Ru/BaeZrO2, (C) KeRu/BaeZrO2 and (:) CseRu/BaeZrO2. Conditions: 100 mg of catalysts and 50.0 mL.mim¡1 of NH3.
Table 3 e Catalytic performance of K and Cs promoted Ru/BaeZrO2 catalysts for ammonia decomposition at 723 K. Catalysts Ru/BaeZrO2 KeRu/BaeZrO2 CseRu/BaeZrO2 Ru/ZrO2-BD Ru/LaeZrO2 Ru@LaeZrO2 Ru/CNTs ZrO2-lpr a Ru/Mg(1)La(7)ZrO2 a Ru/Ca(1)La(7)ZrO2 a Ru/Sr(1)La(7)ZrO2 Ru/AC Ru/NOMC Ru/CNTs Ru/MgO-IM a
Tested at 773 K.
GHSV (mL.g1 h1)
NH3 conversion (%)
H2 production rate (mmol H2/gcat.min)
Ea (KJ/mol)
Ref.
30 000 30 000 30 000 150 000 30 000 30 000 30000 2000 4335 4335 4335 6000 6000 6000 36000
23.6 32.5 37.8 1.9 ca.6.0 ca.40.0 ca.20.0 ca.99.5 83.9 86.7 75.9 8.7 84.5 69.5 17.5
7.9 10.9 12.7 3.2 2.0 13.4 6.7 2.2 4.1 4.2 3.7 0.6 5.7 4.7 7.0
92.1 70.7 64.2 80.4 _ _ 87.3 _ _ _ _ _ 37.3 _ 102.7
This work This work This work [10] [29] [29] [48] [27] [30] [30] [30] [22] [22] [22] [18]
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Conclusion In this work, the barium modified ZrO2 materials was found to be an ideal support material for Ru nanoparticles to catalytic NH3 decomposition reaction. The obtained Ru/BaeZrO2 catalyst has appropriate Ru particle size, strong electron-donating ability and enhanced metal-support interaction due to the presence of BaZrO3. As a result of the characteristics mentioned above, Ru/BaeZrO2 catalyst exhibits excellent catalytic activity for NH3 decomposition. Furthermore, the NH3 conversion over Ru/BaeZrO2 can also be remarkablely improved by the addition of K and Cs promoters, which also show comparable catalytic performance with those of other effective Ru catalysts reported in earlier literature.
Acknowledgements This work was financially supported by National High Technology Research and Development Program of China (2008AA062601).
references
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