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carbon catalysts: The importance of the structure of carbon support
Applied Catalysis A: General 320 (2007) 166–172 www.elsevier.com/locate/apcata
Catalytic ammonia decomposition over Ru/carbon catalysts: The importance of the structure of carbon support L. Li a,b, Z.H. Zhu b,*, Z.F. Yan a, G.Q. Lu b, L. Rintoul c a b
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 257061, China ARC Centre for Functional Nanomaterials, The University of Queensland, Qld 4072, Australia c School of Physical and Chemical Sciences, Queensland University of Technology, Australia
Received 11 September 2006; received in revised form 24 December 2006; accepted 15 January 2007 Available online 18 January 2007
Abstract Catalytic ammonia decomposition has been of increasing interests as a means of supplying pure hydrogen for fuel cells. In this study, Ru catalysts supported on different carbons were used for catalytic ammonia decomposition. The influences of the porous and graphitic structures of carbon supports on the activities of the catalysts were examined. The catalytic activity over supported Ru catalysts is ranked as Ru/GC (graphitic carbon) > Ru/CNTs (carbon nanotube) > Ru/CB-S (carbon black) > Ru/CB-C > Ru/CMK-3 Ru/AC. The samples are characterized by XRD, N2 adsorption, Raman spectra and H2 chemisorption. Ru particles are highly dispersed on carbon supports. The optimum range of Ru particle sizes is around 3–4 nm. On the support side, the graphitic structure of the carbons is critical to the activity of the supported Ru catalyst, while the surface area of carbons is less important. # 2007 Elsevier B.V. All rights reserved. Keywords: Ammonia decomposition; Catalysts; Carbon support; Graphitic structure; Pore structure
1. Introduction With the increasing demand for high efficient and low emission energy alternatives to conventional energy sources, more and more attentions have been paid to the fuel cells. Onsite hydrogen generation from ammonia draws increasing interests because of production of COx-free hydrogen, high energy density (3000 Wh/kg) and hydrogen capacity (17.7%) [1–8]. Moreover, ammonia storage and delivery can be easily handled. Ammonia decomposition is more economical to provide H2 for fuel cells compared to methanol. Many metals such as Fe, Ni, Ru, Ir, Pt, Co and Rh [9–24], alloys (Zr1xTixM1M2, M1, M2 = Cr, Mn, Fe, Co, Ni; x = 0–1) [25] and compounds of nitrides and carbides (MoNx, VNx,VCx and MoCx) [26–29] have been tested as catalysts in ammonia decomposition, and Ru has shown the highest catalytic activity. It is also found that the catalytic performance of Ru catalyst is support-dependent [2,8,15,6,30–32]. Choudhary et al. [8]
* Corresponding author. Fax: +61 7 3365 4199. E-mail address: [email protected] (Z.H. Zhu). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.01.029
compared the catalytic activity of Ru or Ir catalysts on Al2O3 and SiO2 and found that the catalytic activity of Al2O3supported Ru or Ir catalysts was lower than that of those supported on SiO2. The TOF (turnover frequency) over Ni/ HZSM-5 was much lower than that over Ni/HY and Ni/SiO2. Yin et al. studied the effect of supports on catalytic activity of Ru-supported catalysts in ammonia decomposition [15]. The order of activities of the catalysts was ranked as Ru/ CNTs > Ru/MgO > Ru/TiO2 > Ru/Al2O3 > Ru/ZrO2 > Ru/ AC > Ru/ZrO2-BD, and the excellent catalytic activity of Ru/ CNTs was ascribed to the high dispersion of Ru particles, the high graphitization and the high purity of the CNTs. However, they did not further explain how the graphitization of carbon affected NH3 conversion. In addition, Raro´g-Pilecka et al. [33] examined the role of the carbon pre-treatment and concluded that ammonia decomposition on Ru catalysts is a structuresensitivity reaction and the catalytic activity of Ru surfaces is strongly affected by the crystallite size of carbon. However, no systematic studies have been conducted on the comparison of the roles of the porous and graphitic structures of carbon materials as catalyst support for catalytic ammonia decomposition, no quantitative results have shown how
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important the graphitic structure of carbons is for the activity of the carbon-supported catalyst. In this paper, we investigate the effects of the porous and graphitic structures of carbon materials on the catalytic activities of carbon-supported Ru catalysts in ammonia decomposition. A (semi-quantitative) correlation between the activity of the catalysts and the graphitic degree of carbon support has been established. 2. Experimental 2.1. Catalyst synthesis The employed mesoporous carbon (CMK-3) was synthesized following the method reported by Ryoo [34] using SBA-15 as template [35]. The as-synthesis carbon black (CBS) was prepared by CVD method using Ni/MgO catalyst and acetylene was used as carbon source. Other carbon supports in this study included activated carbon (AC) from Norit Company, commercial carbon black (CB-C), graphite carbon (GC) from Aldrich Company and carbon nanotubes (CNTs) from Tsinghua University. All the catalysts were prepared by wetness incipient impregnation using RuCl3xH2O as active component precursor with a nominal metal loading of 5%. Then the samples were calcined in Ar flow at 500 8C for 2 h. 2.2. Catalyst testing Catalytic ammonia decomposition reaction was performed in a vertical fixed-bed flow reactor under atmospheric pressure. A 0.2 g (60–80 mesh) catalyst was placed in the central section of reactor. Prior to the reaction, the catalyst was heated to 500 8C in Ar flow with a heating rate of 10 8C/min, followed by reduction in pure H2 flow at 500 8C for 2 h. Catalytic testing was carried out in pure NH3 (gas flow rate = 100 ml/min) at 550 8C. Gas product analysis was performed on a gas chromatograph (Shimadzu) equipped with TCD, using Ar as carrier gas. 2.3. Catalyst characterization The XRD patterns of the carbon supports and Ru catalysts were performed on a Bruker D8 advance research XRD with Cu Ka radiation at a scanning rate of 28 min1 in the 2u range from 108 to 808. The particle size was calculated from the Scherrer’s equation: L¼
Kl b cos u
where l is the wavelength of the X-rays, u the diffraction angle, K the shape factor, and b is the peak width at half-maximum intensity. The value of K = 0.9 and 1.84 were used for Lc and La determination, respectively. Lc, the layer dimension perpendicular to the basal plane, is obtained from the (0 0 2) diffraction. La, the layer dimension parallel to the basal plane, is calculated by the (1 0 1) diffraction.
Fig. 1. The model of local graphitic structure of hexagonal graphite.
In order to quantitatively characterize the volume size of each quasi-graphitic crystallite, each graphitic crystallite is assumed to be a nano-size cylinder as shown in Fig. 1. The basal plane is supposed as the plane of cylinder and the layer dimension perpendicular to the basal plane is supposed as the height of cylinder. The cylindrical volume of each nanocrystallite was designated as Vnano [36]: V nano ¼
3:14La 2 Lc 4
The pore structures were measured by nitrogen adsorption– desorption isotherms using an NOVA1200 adsorption analyser (Quantachrome, USA). Prior to N2 adsorption measurement, the samples were degassed at 200 8C for 8 h. Surface areas of samples (SBET) were obtained from the BET equation, which was applied in relative pressure range from 0.05 to 0.25. The total pore volume is derived from the amount of vapour adsorbed at a relative pressure of 0.98, by assuming that the pores are then filled with liquid N2. The pore size distribution of carbon samples were evaluated by the desorption branch of the isotherm using BJH method. Ru dispersion on carbon support was analysed by H2 chemisorption. Static volumetric H2 chemisorption measurements were carried out with an Autosorb 2010 adsorption analyser (Quantachrome, USA). For the measurement, a catalyst sample was reduced in H2 at 500 8C and then evacuated at 400 8C. After cooling to 80 8C in He flow, a H2 chemisorption isotherm was measured between 3 and 35 kPa. The amount of total and reversible hydrogen was determined by extrapolating the flat portion of the isotherm to zero pressure. A spherical model for the metallic particles and an H/Ru adsorption stoichiometry = 1 was assumed in calculating metal dispersion. Raman spectra were measured by Renishaw Raman microprobe with laser sources at 633 nm (He–Ne) and 785 nm (semiconductor laser).
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3. Results 3.1. Pore and surface area of carbon supports and Ru catalysts The textural structures of the carbon supports and the carbon-supported catalysts are summarised in Table 1. AC, CMK-3 and CB-C carbons have a high surface area and welldeveloped porosity (>1000 m2/g, >0.8 ml/g), while the CB-S, CNTs and GC carbons have much lower surface area and pore volume (<200 m2/g, <0.43 ml/g). From the pore size distributions (PSD) of carbon supports as shown in Fig. 2, AC carbon possesses microporous structure; CB-S and CMK-3 carbons have mesoporous structures and high porosity; and the pores of CB-S and GC are more disordered. The average pore sizes of carbon supports are in the range of 1.8–8.3 nm. As shown in Table 1, upon catalyst loading, there is a significant decrease in surface area and pore volume due to pore blockage by Ru particles except for 5%Ru/CMK-3 catalysts. A comparison of Figs. 2 and 3 show that the shapes of the pore size distributions before and after catalyst loadings are quite similar, which indicates that the Ru particles are reasonably uniformly dispersed in both micro- and meso-pores of carbons. We also notice that the micropores in as-synthesized CMK-3 are negligible, but the doping of Ru catalyst produced some new micropores. This may be the reason for the increased surface area and pore volume of CMK-3 after catalyst loading. 3.2. XRD studies of supports and Ru catalysts The XRD patterns of different carbon supports are presented in Fig. 4. AC and CMK-3 carbons have an amorphous structure. The other carbons possess two diffraction peaks at around 258 and 438, which are assigned to the (0 0 2) and (1 0 1) diffraction of the carbon, respectively. This indicates that CB-C, CB-S and CNTs have the quasi-graphitic structure. In Fig. 4, we also see that besides (0 0 2) and (1 0 1) diffraction, the (0 0 4) diffraction at 538 also exits in GC carbon, which means that GC has an even higher graphitic degree. The quantitative values of interplannar distance (d0 0 2), the layer dimension perpendicular to the basal plane (Lc) and the layer dimension parallel to the basal plane (La) Table 1 The pore structure of carbon supports and Ru-supported catalysts
Fig. 2. The pore size distribution of carbon supports.
are shown in Table 2. The d0 0 2 values of carbon supports are ranked as CB-C > CB-S CNTs > GC, indicating that compared to the graphite carbon, the CB-C, CB-S and CNTs carbons have a poor alignment of the basal planes and have defects, stacking faults and dislocations in these carbons. In addition, based upon the crystallite size (La) and alignment parameters (Lc) of carbons, the order of the graphitic degree of these samples is GC > CNTs > CB-S > CB-C > CMK-3 AC. The XRD patterns of Ru-supported catalysts are shown in Fig. 5. 5%Ru-supported catalysts show no clear peaks attributed to Ru particles, indicating that the Ru particles are highly dispersed on the carbon supports.
Samples
SBET (m2/g)
Vtot (cm3/g)
Pore diameter (nm)
AC CMK-3 CB-C CB-S CNTs GC
1476.4 1424.0 1048.0 78.8 192.4 119.0
0.833 1.277 2.593 0.178 0.431 0.250
1.8 3.2 2.2 2.2 2.1 2.4
Table 2 Crystallographic parameters of carbon supports Sample
d0 0 2 (nm)
La (nm)
Lc (nm)
Vnano (nm3)
5%Ru/AC 5%Ru/CMK-3 5%Ru/CB-C 5%Ru/CB-S 5%Ru/CNTs 5%Ru/GC
1080.8 1555.0 768.1 48.7 157.2 92.6
0.596 1.568 1.854 0.137 0.331 0.186
– 3.7 3.7 2.5 – 2.4
AC CMK-3 CB-C CB-S CNTs GC
– – 0.357 0.342 0.343 0.338
– – 6.50 5.80 6.07 7.08
– – 3.65 6.84 7.06 33.1
– – 121.1 180.6 204.0 1309.0
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Fig. 3. The pore size distribution of Ru-support catalysts.
3.3. Raman spectrum of carbon supports Raman spectroscopy is an effective tool to study the properties of the different carbon materials, including crystalline, nanocrystalline and amorphous carbons. It can provide useful information on the hybridization state, the size of the graphite crystallites and the degree of ordering of the materials. Fig. 6 shows the Raman spectra of carbon supports. All spectra of the carbon samples have the two Raman-active bands at 1580 and 1340 cm1. The peaks at 1580 cm1 is the G band, characteristic feature of ordered graphite carbon, attributed to the vibration of sp2-bonded carbon atoms. Another peak at 1340 cm1 is the D band, associated with vibrations of carbon atoms with dangling bonds in plane terminations of the disordered graphite and related to the defects and disorders of structures in carbon materials [37,38]. The commercial graphite (GC) exhibits a high-intensity sharp G band at 1582 cm1, D band at 1342.9 cm1 and a shoulder D0 band at 1615 cm1, signifying the graphitic nature. Compared with the peaks and band positions of GC carbon, G band peak of CNTs, CB-S and CB-C carbons is shifted to higher wavelength numbers (1587 cm1) and the CNTs carbon has a narrower peaks than CB-S, CB-C carbons, suggesting that the CNTs, CB-S and CB-C carbons are composed of small graphite sheets with a structural imperfection and disorder of the graphite structure and the graphite degree of CNTs is higher than that of carbon
Fig. 4. The XRD patterns of carbon materials (a) AC, (b) CMK-3, (c) CB-C, (d) CB-S, (e) CNTs and (f) GC.
black samples, in accordance with the XRD results of carbon supports as shown in Fig. 4. The bands of CMK-3 are even broader, also in agreement with the XRD results that CMK-3 is an amorphous carbon. The intensity ratio of D and G bands (ID/IG) is related to the orderliness of the graphite structure and is inversely proportional to the microcrystalline planar size La, which corresponds to the in-plane dimension of single microcrystallite in graphite. It is well know that both the band positions and bandwidths of G- and D-mode are sensitive to La, and the downshifting of the G-band position has been correlated with the presence of bondangle disorder. The quantitative results of Raman spectra of different carbon supports are shown in Table 3. It is observed that the ID/IG ratio of carbon supports is ranked as AC CMK3 > CB-C > CB-S CNTs > GC, indicating a decrease in graphitization degree of carbon supports with the increase in the ID/IG ratio in accordance with the XRD results. 3.4. Ru dispersion The catalyst dispersion results of Ru catalysts characterized by H2 chemisorption are summarised in Table 4. The average
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L. Li et al. / Applied Catalysis A: General 320 (2007) 166–172 Table 3 Raman spectroscopic parameters of carbon supports Sample
AC CMK-3 CB-C CB-S CNTs GC
Peak position (cm1)
Intensity (a.u.)
D band
G band
D band
G band
– 1345.8 1343.4 1343.6 1340.4 1342.9
– 1588.8 1587.8 1587.4 1586.6 1582.9
283,276 261,399 784,252 501,237 726,902 768,999
82884.3 72878.3 243463 238287 314679 620758
ID/IG
3.4 3.6 3.2 2.1 2.3 1.2
Table 4 The results of H2 chemisorption of Ru-supported catalysts
Fig. 5. The XRD patterns of Ru-support catalysts (a) 5%Ru/CMK-3, (b) 5%Ru/ CB-C, (c) 5%Ru/CB-S, (d) 5%Ru/CNTs and (e) 5%Ru/GC.
Samples
Dispersion (%)
Ru (nm)
5%Ru/AC 5%Ru/CMK-3 5%Ru/CB-C 5%Ru/CB-S 5%Ru/CNTs 5%Ru/GC
49.6 53.4 24.1 27.8 85.6 41.8
2.6 2.5 5.5 4.8 1.6 3.2
tube structure of the support. In addition, the catalyst dispersion is also closely related to the surface chemistry of the carbon. The effects of surface chemistry on catalyst dispersion have been extensively investigated in the previous studies, and will not be detailed in this work. 3.5. Catalytic activity of Ru
size of particles was calculated base on the dispersion by assuming spherical particles [39]. The Ru particles are highly dispersed on the carbon supports with size about 2–5 nm. This is the reason why Ru peaks are invisible in the XRD spectra in Fig. 5. However, the Ru dispersion on CNTs is the highest despite the very low surface area of CNTs (less than 200 m2/g), while the CB-C sample has a very high surface area (over 1000 m2/g) but the supported catalyst has the worst dispersion. The highest Ru dispersion on CNTs may be due to the special
Table 5 shows the NH3 conversions and H2 formation rates of all Ru catalysts. At 550 8C, NH3 conversions over Ru/CMK3, Ru/AC and Ru/CB-C catalysts are only 14.4–26.1%, and the H2 formation rate is less than 8.0 mmol/(min gcat); while NH3 conversion over Ru/GC catalyst is up to 95.0%, and the H2 formation rate is 29.1 mmol/(min gcat). The catalytic activities over Ru-supported catalysts can be ranked as 5%Ru/ GC > 5%Ru/CNTS > 5%Ru/CB-S > 5%Ru/CB-C > 5%Ru/ CMK-3 5%Ru/AC, signifying that GC is the most suitable support for Ru catalyst. However, the catalytic activity on the present Ru/C catalysts is slight lower than that reported by Yin and coworkers [1].
Table 5 NH3 conversion and H2 formation rate over supported Ru catalysts (GHSVNH3 = 30,000 ml/(h gcat), 550 8C)
Fig. 6. Raman spectra of carbon supports (a) AC, (b) CMK-3, (c) CNTs, (d) GC, (e) CB-C and (f) CB-S.
Catalyst
NH3 conversion (%)
H2 formation rate (mmol/(min gcat))
5%Ru/AC 5%Ru/CMK-3 5%Ru/CB-C 5%Ru/CB-S 5%Ru/CNTs 5%Ru/GC
14.4 22.7 26.1 52.7 84.7 95.0
4.4 7.0 8.0 16.2 26.0 29.1
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4. Discussion 4.1. Effect of pore structure of supports on catalytic activity of ammonia decomposition The pore structures of catalyst and/or support are known as a critical factor in adsorption and catalytic reaction but it is not always dominant. A high surface area is normally favourable for the dispersion of active component. Due to the high surface area and large pore volume of the carbon materials, Ru particles are so highly dispersed on the carbon supports that the Ru species are not detectable by the XRD patterns. From Tables 1 and 4, one can see that the higher the surface area of the carbon supports, the higher dispersion the Ru particles on the supports except CNTs and CB-C. As mentioned above, the poor dispersion of Ru on CB-C may be caused by the contained impurities such as Cl, oxygen groups, and metal ions. The fine particles of Ru on CNTs may be related to the tube structure of carbon nanotubes. Despite that the surface areas of CMK-3 and AC carbons are much higher than that of GC carbon (as shown in Table 1), the catalytic activity of Ru/GC is much higher than that of Ru/CMK-3 and Ru/AC, signifying that the surface area is not a determining factor for the activity of the supported Ru catalysts. 4.2. Effect of Ru particle size on catalytic activity Many studies have revealed that the metal dispersion strongly influences the activity towards NH3 decomposition [18,32,40–42], and the ammonia decomposition can be expressed as the following steps: NH3 ðgÞ $ NH3 ðadÞ NH3 ðadÞ ! NH2 ðadÞ þ HðadÞ NH2 ðadÞ $ NHðadÞ þ HðadÞ NHðadÞ $ NðadÞ þ HðadÞ 2HðadÞ $ H2 ðadÞ H2 ðadÞ $ H2 ðgÞ 2NðadÞ ! N2 ðadÞ N2 ðadÞ $ N2 ðgÞ Among these steps, it is proposed that the N–H bond cleavage and recombinative N2 desorption are the rate-limiting steps. Based on these results, Zhang et al. further reported that Ru crystallites with mean particle size of about 2.2 nm are extremely active for ammonia decomposition [43]. Meanwhile, some studies indicated that for Ru, the maximum probability for the most active B-5 sites appeared for particles of 1.5– 2.5 nm [44]. As shown in Tables 4 and 5, the larger Ru particles on 5%Ru/CB-S and 5%Ru-GC catalysts result in higher catalytic activity, the smaller Ru particles on 5%Ru/CMK-3 and 5%Ru/AC show lower activity. This means better metal dispersion does not necessarily improve the activity of catalyst. The over-dispersed catalyst may result in too small catalyst particles on the support, which may not provide enough space to accommodate the recombination of N atoms to form N2 molecules (the size of one N2 molecule is 0.36 nm). Jacobsen et al. simulated the ammonia synthesis using DFT method [44],
171
and indicated that Ru clusters need to expose a three-fold hollow site and bridge site close together and part of the Ru atoms have to be low coordinated surface atoms such as edge atoms on small crystals. Therefore, a relatively large catalyst particle would provide enough space for N–N recombination to form N2. This may be an important reason why large Ru particles are beneficial to ammonia decomposition for hydrogen production in our study. However, an over-large Ru particle size will not help the catalytic activity either. For example, the Ru size on 5%Ru/CB-C is the biggest, but the activity is very low. The reason may be due to the less active site caused by the too large metal particles. Consequently, there is an optimum range of metal particle for ammonia decomposition, which is ca. 3–4 nm according to our study, in accordance with Sergey’s results [43]. 4.3. Effect of carbon graphitization and electronic structure on catalytic activity of ammonia decomposition As mentioned before, recombinative desorption of surface nitrogen atoms may be the rate-limiting steps in ammonia decomposition. It was also reported that the electron transfer from support to Ru facilities the recombinative desorption of surface N atoms. Kowalczyk et al. [4,7,23,31,33] reported that high temperature treatment of carbon materials results in higher degree of graphitization, and the use of this material as support yielded a more active Ru/C catalyst in NH3 synthesis. This is consistent with our results. As shown in Tables 2, 3 and 5, ammonia decomposition over Ru catalysts is support-dependent and the order of catalytic activity is Ru/CMK-3 < Ru/ AC < Ru/CB-C < Ru/CB-S < Ru/CNTs < Ru/GC, in good agreement with the graphitic degrees as characterized by XRD and Raman spectra. The cylindrical volume (Vnano) of each nano-crystallite of carbon materials is shown in the fifth column of Table 2. It is clear that the activity of the catalyst is almost proportional to the size of the crystallites of the carbon support. Among these carbon materials, GC carbon has the highest graphitic structure than other of carbons, which makes electron transfer easier between the support and the metal particles and consequently results in the best catalytic activity in ammonia decomposition. A lower graphitic degree results in the poor electron conductivity of the carbon support, thus resulting in the lower activity of the carbon supported catalyst as observed in this study. However, there are no quantitative reports so far about the correlation between the carbon graphitic structure and the activity of the corresponding catalyst. In this study, we provide a semi-quantitative correlation between the graphitic degree between the carbon graphitic degree and the activity of the carbon supported catalyst. The Vnano as shown in Table 2 may be used to indicate the carbon graphitic degree, but the AC and CMK-3 are too amorphous, and such Vnano values are not available. Therefore Raman spectra are used here. In Fig. 7, the graphitic degree of the carbon is presented as 1/(ID/IG) value. The higher the 1/(ID/IG) value, the higher the graphitic degree. Fig. 7(a) shows that the activity of catalyst (mmol/(min gcat)) is almost linearly related to the graphitic degree of the carbon, but such a correlation does
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References
Fig. 7. The relationship between the carbon graphitic degree and the activity of the catalyst (a) the activity is in the form of mmol/(min gcat); (b) the activity is in the form of turnover frequency (1/s).
not take the catalyst active sites into account. Fig. 7(b) shows that the turnover frequency of the catalyst, which is calculated as the activity (mmol/(min gcat)) divided by the H2 update of catalyst (mmol/gcat) in H2 chemisorption, linearly changes with the graphitic degree. The physical meaning of the semiquantitative relation as shown in Fig. 7 is still not clear. To investigate the more quantitative relation between the graphitic degree of the carbon support and the activity of the supported catalyst is an ongoing project in our group. 5. Conclusion The catalytic activity of Ru catalysts supported on different carbons in ammonia decomposition has been investigated. It is found that the GC carbon is an excellent support for Ru catalyst. For ammonia decomposition reaction, catalytic activity follows this sequence: 5%Ru/GC > 5%Ru/CNTS > 5%Ru/CBS > 5%Ru/CB-C > 5%Ru/CMK-3 5%Ru/AC. The effects of carbon structures including pore structures and graphitic degree of carbons are studied. Surface area of carbon is not a crucial factor in the NH3 decomposition process. The graphitic structure of the carbon support nature is more important for the catalytic activity of the carbon-supported catalyst compared to the pore structures of the carbon supports. Acknowledgements The financial supports from ARC Centre for Functional Nanomaterials and ARC discovery project are appreciated.
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Catalytic ammonia decomposition over Ru/carbon catalysts: The importance of the structure of carbon support L. Li a,b, Z.H. Zhu b,*, Z.F. Yan a, G.Q. Lu b, L. Rintoul c a b
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 257061, China ARC Centre for Functional Nanomaterials, The University of Queensland, Qld 4072, Australia c School of Physical and Chemical Sciences, Queensland University of Technology, Australia
Received 11 September 2006; received in revised form 24 December 2006; accepted 15 January 2007 Available online 18 January 2007
Abstract Catalytic ammonia decomposition has been of increasing interests as a means of supplying pure hydrogen for fuel cells. In this study, Ru catalysts supported on different carbons were used for catalytic ammonia decomposition. The influences of the porous and graphitic structures of carbon supports on the activities of the catalysts were examined. The catalytic activity over supported Ru catalysts is ranked as Ru/GC (graphitic carbon) > Ru/CNTs (carbon nanotube) > Ru/CB-S (carbon black) > Ru/CB-C > Ru/CMK-3 Ru/AC. The samples are characterized by XRD, N2 adsorption, Raman spectra and H2 chemisorption. Ru particles are highly dispersed on carbon supports. The optimum range of Ru particle sizes is around 3–4 nm. On the support side, the graphitic structure of the carbons is critical to the activity of the supported Ru catalyst, while the surface area of carbons is less important. # 2007 Elsevier B.V. All rights reserved. Keywords: Ammonia decomposition; Catalysts; Carbon support; Graphitic structure; Pore structure
1. Introduction With the increasing demand for high efficient and low emission energy alternatives to conventional energy sources, more and more attentions have been paid to the fuel cells. Onsite hydrogen generation from ammonia draws increasing interests because of production of COx-free hydrogen, high energy density (3000 Wh/kg) and hydrogen capacity (17.7%) [1–8]. Moreover, ammonia storage and delivery can be easily handled. Ammonia decomposition is more economical to provide H2 for fuel cells compared to methanol. Many metals such as Fe, Ni, Ru, Ir, Pt, Co and Rh [9–24], alloys (Zr1xTixM1M2, M1, M2 = Cr, Mn, Fe, Co, Ni; x = 0–1) [25] and compounds of nitrides and carbides (MoNx, VNx,VCx and MoCx) [26–29] have been tested as catalysts in ammonia decomposition, and Ru has shown the highest catalytic activity. It is also found that the catalytic performance of Ru catalyst is support-dependent [2,8,15,6,30–32]. Choudhary et al. [8]
* Corresponding author. Fax: +61 7 3365 4199. E-mail address: [email protected] (Z.H. Zhu). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.01.029
compared the catalytic activity of Ru or Ir catalysts on Al2O3 and SiO2 and found that the catalytic activity of Al2O3supported Ru or Ir catalysts was lower than that of those supported on SiO2. The TOF (turnover frequency) over Ni/ HZSM-5 was much lower than that over Ni/HY and Ni/SiO2. Yin et al. studied the effect of supports on catalytic activity of Ru-supported catalysts in ammonia decomposition [15]. The order of activities of the catalysts was ranked as Ru/ CNTs > Ru/MgO > Ru/TiO2 > Ru/Al2O3 > Ru/ZrO2 > Ru/ AC > Ru/ZrO2-BD, and the excellent catalytic activity of Ru/ CNTs was ascribed to the high dispersion of Ru particles, the high graphitization and the high purity of the CNTs. However, they did not further explain how the graphitization of carbon affected NH3 conversion. In addition, Raro´g-Pilecka et al. [33] examined the role of the carbon pre-treatment and concluded that ammonia decomposition on Ru catalysts is a structuresensitivity reaction and the catalytic activity of Ru surfaces is strongly affected by the crystallite size of carbon. However, no systematic studies have been conducted on the comparison of the roles of the porous and graphitic structures of carbon materials as catalyst support for catalytic ammonia decomposition, no quantitative results have shown how
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important the graphitic structure of carbons is for the activity of the carbon-supported catalyst. In this paper, we investigate the effects of the porous and graphitic structures of carbon materials on the catalytic activities of carbon-supported Ru catalysts in ammonia decomposition. A (semi-quantitative) correlation between the activity of the catalysts and the graphitic degree of carbon support has been established. 2. Experimental 2.1. Catalyst synthesis The employed mesoporous carbon (CMK-3) was synthesized following the method reported by Ryoo [34] using SBA-15 as template [35]. The as-synthesis carbon black (CBS) was prepared by CVD method using Ni/MgO catalyst and acetylene was used as carbon source. Other carbon supports in this study included activated carbon (AC) from Norit Company, commercial carbon black (CB-C), graphite carbon (GC) from Aldrich Company and carbon nanotubes (CNTs) from Tsinghua University. All the catalysts were prepared by wetness incipient impregnation using RuCl3xH2O as active component precursor with a nominal metal loading of 5%. Then the samples were calcined in Ar flow at 500 8C for 2 h. 2.2. Catalyst testing Catalytic ammonia decomposition reaction was performed in a vertical fixed-bed flow reactor under atmospheric pressure. A 0.2 g (60–80 mesh) catalyst was placed in the central section of reactor. Prior to the reaction, the catalyst was heated to 500 8C in Ar flow with a heating rate of 10 8C/min, followed by reduction in pure H2 flow at 500 8C for 2 h. Catalytic testing was carried out in pure NH3 (gas flow rate = 100 ml/min) at 550 8C. Gas product analysis was performed on a gas chromatograph (Shimadzu) equipped with TCD, using Ar as carrier gas. 2.3. Catalyst characterization The XRD patterns of the carbon supports and Ru catalysts were performed on a Bruker D8 advance research XRD with Cu Ka radiation at a scanning rate of 28 min1 in the 2u range from 108 to 808. The particle size was calculated from the Scherrer’s equation: L¼
Kl b cos u
where l is the wavelength of the X-rays, u the diffraction angle, K the shape factor, and b is the peak width at half-maximum intensity. The value of K = 0.9 and 1.84 were used for Lc and La determination, respectively. Lc, the layer dimension perpendicular to the basal plane, is obtained from the (0 0 2) diffraction. La, the layer dimension parallel to the basal plane, is calculated by the (1 0 1) diffraction.
Fig. 1. The model of local graphitic structure of hexagonal graphite.
In order to quantitatively characterize the volume size of each quasi-graphitic crystallite, each graphitic crystallite is assumed to be a nano-size cylinder as shown in Fig. 1. The basal plane is supposed as the plane of cylinder and the layer dimension perpendicular to the basal plane is supposed as the height of cylinder. The cylindrical volume of each nanocrystallite was designated as Vnano [36]: V nano ¼
3:14La 2 Lc 4
The pore structures were measured by nitrogen adsorption– desorption isotherms using an NOVA1200 adsorption analyser (Quantachrome, USA). Prior to N2 adsorption measurement, the samples were degassed at 200 8C for 8 h. Surface areas of samples (SBET) were obtained from the BET equation, which was applied in relative pressure range from 0.05 to 0.25. The total pore volume is derived from the amount of vapour adsorbed at a relative pressure of 0.98, by assuming that the pores are then filled with liquid N2. The pore size distribution of carbon samples were evaluated by the desorption branch of the isotherm using BJH method. Ru dispersion on carbon support was analysed by H2 chemisorption. Static volumetric H2 chemisorption measurements were carried out with an Autosorb 2010 adsorption analyser (Quantachrome, USA). For the measurement, a catalyst sample was reduced in H2 at 500 8C and then evacuated at 400 8C. After cooling to 80 8C in He flow, a H2 chemisorption isotherm was measured between 3 and 35 kPa. The amount of total and reversible hydrogen was determined by extrapolating the flat portion of the isotherm to zero pressure. A spherical model for the metallic particles and an H/Ru adsorption stoichiometry = 1 was assumed in calculating metal dispersion. Raman spectra were measured by Renishaw Raman microprobe with laser sources at 633 nm (He–Ne) and 785 nm (semiconductor laser).
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3. Results 3.1. Pore and surface area of carbon supports and Ru catalysts The textural structures of the carbon supports and the carbon-supported catalysts are summarised in Table 1. AC, CMK-3 and CB-C carbons have a high surface area and welldeveloped porosity (>1000 m2/g, >0.8 ml/g), while the CB-S, CNTs and GC carbons have much lower surface area and pore volume (<200 m2/g, <0.43 ml/g). From the pore size distributions (PSD) of carbon supports as shown in Fig. 2, AC carbon possesses microporous structure; CB-S and CMK-3 carbons have mesoporous structures and high porosity; and the pores of CB-S and GC are more disordered. The average pore sizes of carbon supports are in the range of 1.8–8.3 nm. As shown in Table 1, upon catalyst loading, there is a significant decrease in surface area and pore volume due to pore blockage by Ru particles except for 5%Ru/CMK-3 catalysts. A comparison of Figs. 2 and 3 show that the shapes of the pore size distributions before and after catalyst loadings are quite similar, which indicates that the Ru particles are reasonably uniformly dispersed in both micro- and meso-pores of carbons. We also notice that the micropores in as-synthesized CMK-3 are negligible, but the doping of Ru catalyst produced some new micropores. This may be the reason for the increased surface area and pore volume of CMK-3 after catalyst loading. 3.2. XRD studies of supports and Ru catalysts The XRD patterns of different carbon supports are presented in Fig. 4. AC and CMK-3 carbons have an amorphous structure. The other carbons possess two diffraction peaks at around 258 and 438, which are assigned to the (0 0 2) and (1 0 1) diffraction of the carbon, respectively. This indicates that CB-C, CB-S and CNTs have the quasi-graphitic structure. In Fig. 4, we also see that besides (0 0 2) and (1 0 1) diffraction, the (0 0 4) diffraction at 538 also exits in GC carbon, which means that GC has an even higher graphitic degree. The quantitative values of interplannar distance (d0 0 2), the layer dimension perpendicular to the basal plane (Lc) and the layer dimension parallel to the basal plane (La) Table 1 The pore structure of carbon supports and Ru-supported catalysts
Fig. 2. The pore size distribution of carbon supports.
are shown in Table 2. The d0 0 2 values of carbon supports are ranked as CB-C > CB-S CNTs > GC, indicating that compared to the graphite carbon, the CB-C, CB-S and CNTs carbons have a poor alignment of the basal planes and have defects, stacking faults and dislocations in these carbons. In addition, based upon the crystallite size (La) and alignment parameters (Lc) of carbons, the order of the graphitic degree of these samples is GC > CNTs > CB-S > CB-C > CMK-3 AC. The XRD patterns of Ru-supported catalysts are shown in Fig. 5. 5%Ru-supported catalysts show no clear peaks attributed to Ru particles, indicating that the Ru particles are highly dispersed on the carbon supports.
Samples
SBET (m2/g)
Vtot (cm3/g)
Pore diameter (nm)
AC CMK-3 CB-C CB-S CNTs GC
1476.4 1424.0 1048.0 78.8 192.4 119.0
0.833 1.277 2.593 0.178 0.431 0.250
1.8 3.2 2.2 2.2 2.1 2.4
Table 2 Crystallographic parameters of carbon supports Sample
d0 0 2 (nm)
La (nm)
Lc (nm)
Vnano (nm3)
5%Ru/AC 5%Ru/CMK-3 5%Ru/CB-C 5%Ru/CB-S 5%Ru/CNTs 5%Ru/GC
1080.8 1555.0 768.1 48.7 157.2 92.6
0.596 1.568 1.854 0.137 0.331 0.186
– 3.7 3.7 2.5 – 2.4
AC CMK-3 CB-C CB-S CNTs GC
– – 0.357 0.342 0.343 0.338
– – 6.50 5.80 6.07 7.08
– – 3.65 6.84 7.06 33.1
– – 121.1 180.6 204.0 1309.0
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Fig. 3. The pore size distribution of Ru-support catalysts.
3.3. Raman spectrum of carbon supports Raman spectroscopy is an effective tool to study the properties of the different carbon materials, including crystalline, nanocrystalline and amorphous carbons. It can provide useful information on the hybridization state, the size of the graphite crystallites and the degree of ordering of the materials. Fig. 6 shows the Raman spectra of carbon supports. All spectra of the carbon samples have the two Raman-active bands at 1580 and 1340 cm1. The peaks at 1580 cm1 is the G band, characteristic feature of ordered graphite carbon, attributed to the vibration of sp2-bonded carbon atoms. Another peak at 1340 cm1 is the D band, associated with vibrations of carbon atoms with dangling bonds in plane terminations of the disordered graphite and related to the defects and disorders of structures in carbon materials [37,38]. The commercial graphite (GC) exhibits a high-intensity sharp G band at 1582 cm1, D band at 1342.9 cm1 and a shoulder D0 band at 1615 cm1, signifying the graphitic nature. Compared with the peaks and band positions of GC carbon, G band peak of CNTs, CB-S and CB-C carbons is shifted to higher wavelength numbers (1587 cm1) and the CNTs carbon has a narrower peaks than CB-S, CB-C carbons, suggesting that the CNTs, CB-S and CB-C carbons are composed of small graphite sheets with a structural imperfection and disorder of the graphite structure and the graphite degree of CNTs is higher than that of carbon
Fig. 4. The XRD patterns of carbon materials (a) AC, (b) CMK-3, (c) CB-C, (d) CB-S, (e) CNTs and (f) GC.
black samples, in accordance with the XRD results of carbon supports as shown in Fig. 4. The bands of CMK-3 are even broader, also in agreement with the XRD results that CMK-3 is an amorphous carbon. The intensity ratio of D and G bands (ID/IG) is related to the orderliness of the graphite structure and is inversely proportional to the microcrystalline planar size La, which corresponds to the in-plane dimension of single microcrystallite in graphite. It is well know that both the band positions and bandwidths of G- and D-mode are sensitive to La, and the downshifting of the G-band position has been correlated with the presence of bondangle disorder. The quantitative results of Raman spectra of different carbon supports are shown in Table 3. It is observed that the ID/IG ratio of carbon supports is ranked as AC CMK3 > CB-C > CB-S CNTs > GC, indicating a decrease in graphitization degree of carbon supports with the increase in the ID/IG ratio in accordance with the XRD results. 3.4. Ru dispersion The catalyst dispersion results of Ru catalysts characterized by H2 chemisorption are summarised in Table 4. The average
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L. Li et al. / Applied Catalysis A: General 320 (2007) 166–172 Table 3 Raman spectroscopic parameters of carbon supports Sample
AC CMK-3 CB-C CB-S CNTs GC
Peak position (cm1)
Intensity (a.u.)
D band
G band
D band
G band
– 1345.8 1343.4 1343.6 1340.4 1342.9
– 1588.8 1587.8 1587.4 1586.6 1582.9
283,276 261,399 784,252 501,237 726,902 768,999
82884.3 72878.3 243463 238287 314679 620758
ID/IG
3.4 3.6 3.2 2.1 2.3 1.2
Table 4 The results of H2 chemisorption of Ru-supported catalysts
Fig. 5. The XRD patterns of Ru-support catalysts (a) 5%Ru/CMK-3, (b) 5%Ru/ CB-C, (c) 5%Ru/CB-S, (d) 5%Ru/CNTs and (e) 5%Ru/GC.
Samples
Dispersion (%)
Ru (nm)
5%Ru/AC 5%Ru/CMK-3 5%Ru/CB-C 5%Ru/CB-S 5%Ru/CNTs 5%Ru/GC
49.6 53.4 24.1 27.8 85.6 41.8
2.6 2.5 5.5 4.8 1.6 3.2
tube structure of the support. In addition, the catalyst dispersion is also closely related to the surface chemistry of the carbon. The effects of surface chemistry on catalyst dispersion have been extensively investigated in the previous studies, and will not be detailed in this work. 3.5. Catalytic activity of Ru
size of particles was calculated base on the dispersion by assuming spherical particles [39]. The Ru particles are highly dispersed on the carbon supports with size about 2–5 nm. This is the reason why Ru peaks are invisible in the XRD spectra in Fig. 5. However, the Ru dispersion on CNTs is the highest despite the very low surface area of CNTs (less than 200 m2/g), while the CB-C sample has a very high surface area (over 1000 m2/g) but the supported catalyst has the worst dispersion. The highest Ru dispersion on CNTs may be due to the special
Table 5 shows the NH3 conversions and H2 formation rates of all Ru catalysts. At 550 8C, NH3 conversions over Ru/CMK3, Ru/AC and Ru/CB-C catalysts are only 14.4–26.1%, and the H2 formation rate is less than 8.0 mmol/(min gcat); while NH3 conversion over Ru/GC catalyst is up to 95.0%, and the H2 formation rate is 29.1 mmol/(min gcat). The catalytic activities over Ru-supported catalysts can be ranked as 5%Ru/ GC > 5%Ru/CNTS > 5%Ru/CB-S > 5%Ru/CB-C > 5%Ru/ CMK-3 5%Ru/AC, signifying that GC is the most suitable support for Ru catalyst. However, the catalytic activity on the present Ru/C catalysts is slight lower than that reported by Yin and coworkers [1].
Table 5 NH3 conversion and H2 formation rate over supported Ru catalysts (GHSVNH3 = 30,000 ml/(h gcat), 550 8C)
Fig. 6. Raman spectra of carbon supports (a) AC, (b) CMK-3, (c) CNTs, (d) GC, (e) CB-C and (f) CB-S.
Catalyst
NH3 conversion (%)
H2 formation rate (mmol/(min gcat))
5%Ru/AC 5%Ru/CMK-3 5%Ru/CB-C 5%Ru/CB-S 5%Ru/CNTs 5%Ru/GC
14.4 22.7 26.1 52.7 84.7 95.0
4.4 7.0 8.0 16.2 26.0 29.1
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4. Discussion 4.1. Effect of pore structure of supports on catalytic activity of ammonia decomposition The pore structures of catalyst and/or support are known as a critical factor in adsorption and catalytic reaction but it is not always dominant. A high surface area is normally favourable for the dispersion of active component. Due to the high surface area and large pore volume of the carbon materials, Ru particles are so highly dispersed on the carbon supports that the Ru species are not detectable by the XRD patterns. From Tables 1 and 4, one can see that the higher the surface area of the carbon supports, the higher dispersion the Ru particles on the supports except CNTs and CB-C. As mentioned above, the poor dispersion of Ru on CB-C may be caused by the contained impurities such as Cl, oxygen groups, and metal ions. The fine particles of Ru on CNTs may be related to the tube structure of carbon nanotubes. Despite that the surface areas of CMK-3 and AC carbons are much higher than that of GC carbon (as shown in Table 1), the catalytic activity of Ru/GC is much higher than that of Ru/CMK-3 and Ru/AC, signifying that the surface area is not a determining factor for the activity of the supported Ru catalysts. 4.2. Effect of Ru particle size on catalytic activity Many studies have revealed that the metal dispersion strongly influences the activity towards NH3 decomposition [18,32,40–42], and the ammonia decomposition can be expressed as the following steps: NH3 ðgÞ $ NH3 ðadÞ NH3 ðadÞ ! NH2 ðadÞ þ HðadÞ NH2 ðadÞ $ NHðadÞ þ HðadÞ NHðadÞ $ NðadÞ þ HðadÞ 2HðadÞ $ H2 ðadÞ H2 ðadÞ $ H2 ðgÞ 2NðadÞ ! N2 ðadÞ N2 ðadÞ $ N2 ðgÞ Among these steps, it is proposed that the N–H bond cleavage and recombinative N2 desorption are the rate-limiting steps. Based on these results, Zhang et al. further reported that Ru crystallites with mean particle size of about 2.2 nm are extremely active for ammonia decomposition [43]. Meanwhile, some studies indicated that for Ru, the maximum probability for the most active B-5 sites appeared for particles of 1.5– 2.5 nm [44]. As shown in Tables 4 and 5, the larger Ru particles on 5%Ru/CB-S and 5%Ru-GC catalysts result in higher catalytic activity, the smaller Ru particles on 5%Ru/CMK-3 and 5%Ru/AC show lower activity. This means better metal dispersion does not necessarily improve the activity of catalyst. The over-dispersed catalyst may result in too small catalyst particles on the support, which may not provide enough space to accommodate the recombination of N atoms to form N2 molecules (the size of one N2 molecule is 0.36 nm). Jacobsen et al. simulated the ammonia synthesis using DFT method [44],
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and indicated that Ru clusters need to expose a three-fold hollow site and bridge site close together and part of the Ru atoms have to be low coordinated surface atoms such as edge atoms on small crystals. Therefore, a relatively large catalyst particle would provide enough space for N–N recombination to form N2. This may be an important reason why large Ru particles are beneficial to ammonia decomposition for hydrogen production in our study. However, an over-large Ru particle size will not help the catalytic activity either. For example, the Ru size on 5%Ru/CB-C is the biggest, but the activity is very low. The reason may be due to the less active site caused by the too large metal particles. Consequently, there is an optimum range of metal particle for ammonia decomposition, which is ca. 3–4 nm according to our study, in accordance with Sergey’s results [43]. 4.3. Effect of carbon graphitization and electronic structure on catalytic activity of ammonia decomposition As mentioned before, recombinative desorption of surface nitrogen atoms may be the rate-limiting steps in ammonia decomposition. It was also reported that the electron transfer from support to Ru facilities the recombinative desorption of surface N atoms. Kowalczyk et al. [4,7,23,31,33] reported that high temperature treatment of carbon materials results in higher degree of graphitization, and the use of this material as support yielded a more active Ru/C catalyst in NH3 synthesis. This is consistent with our results. As shown in Tables 2, 3 and 5, ammonia decomposition over Ru catalysts is support-dependent and the order of catalytic activity is Ru/CMK-3 < Ru/ AC < Ru/CB-C < Ru/CB-S < Ru/CNTs < Ru/GC, in good agreement with the graphitic degrees as characterized by XRD and Raman spectra. The cylindrical volume (Vnano) of each nano-crystallite of carbon materials is shown in the fifth column of Table 2. It is clear that the activity of the catalyst is almost proportional to the size of the crystallites of the carbon support. Among these carbon materials, GC carbon has the highest graphitic structure than other of carbons, which makes electron transfer easier between the support and the metal particles and consequently results in the best catalytic activity in ammonia decomposition. A lower graphitic degree results in the poor electron conductivity of the carbon support, thus resulting in the lower activity of the carbon supported catalyst as observed in this study. However, there are no quantitative reports so far about the correlation between the carbon graphitic structure and the activity of the corresponding catalyst. In this study, we provide a semi-quantitative correlation between the graphitic degree between the carbon graphitic degree and the activity of the carbon supported catalyst. The Vnano as shown in Table 2 may be used to indicate the carbon graphitic degree, but the AC and CMK-3 are too amorphous, and such Vnano values are not available. Therefore Raman spectra are used here. In Fig. 7, the graphitic degree of the carbon is presented as 1/(ID/IG) value. The higher the 1/(ID/IG) value, the higher the graphitic degree. Fig. 7(a) shows that the activity of catalyst (mmol/(min gcat)) is almost linearly related to the graphitic degree of the carbon, but such a correlation does
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References
Fig. 7. The relationship between the carbon graphitic degree and the activity of the catalyst (a) the activity is in the form of mmol/(min gcat); (b) the activity is in the form of turnover frequency (1/s).
not take the catalyst active sites into account. Fig. 7(b) shows that the turnover frequency of the catalyst, which is calculated as the activity (mmol/(min gcat)) divided by the H2 update of catalyst (mmol/gcat) in H2 chemisorption, linearly changes with the graphitic degree. The physical meaning of the semiquantitative relation as shown in Fig. 7 is still not clear. To investigate the more quantitative relation between the graphitic degree of the carbon support and the activity of the supported catalyst is an ongoing project in our group. 5. Conclusion The catalytic activity of Ru catalysts supported on different carbons in ammonia decomposition has been investigated. It is found that the GC carbon is an excellent support for Ru catalyst. For ammonia decomposition reaction, catalytic activity follows this sequence: 5%Ru/GC > 5%Ru/CNTS > 5%Ru/CBS > 5%Ru/CB-C > 5%Ru/CMK-3 5%Ru/AC. The effects of carbon structures including pore structures and graphitic degree of carbons are studied. Surface area of carbon is not a crucial factor in the NH3 decomposition process. The graphitic structure of the carbon support nature is more important for the catalytic activity of the carbon-supported catalyst compared to the pore structures of the carbon supports. Acknowledgements The financial supports from ARC Centre for Functional Nanomaterials and ARC discovery project are appreciated.
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