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Effect of carbon and chlorine on the performance of carbon-covered alumina supported Ru catalyst for ammonia synthesis
Catalysis Communications 12 (2011) 1452–1457
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Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Effect of carbon and chlorine on the performance of carbon-covered alumina supported Ru catalyst for ammonia synthesis Bingyu Lin, Rong Wang, Jianxin Lin, Jun Ni, Kemei Wei ⁎ National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou 350002, PR China
a r t i c l e
i n f o
Article history: Received 15 April 2011 Received in revised form 23 May 2011 Accepted 27 May 2011 Available online 6 June 2011 Keywords: Carbon-covered alumina Carbon Chlorine Ru catalyst Ammonia synthesis
a b s t r a c t Uniformly carbon-covered alumina (CCA), which was prepared by pyrolysis of sucrose, was used as support of ruthenium catalyst. Carbon did not significantly influence on the ammonia synthesis activities of Ru catalysts by changing their Ru particle sizes or Ru 3d5/2 binding energies. Residual chlorine severely suppressed ammonia synthesis by decreasing the amount of hydrogen with the desorption peak at medium temperatures. Carbon also inhibited the adsorption of this hydrogen species for chlorine-free Ru catalysts, but did not change their activities. On the other hand, carbon can increase the ammonia synthesis activities of containing-chlorine Ru catalysts by decreasing the disadvantageous effect of chlorine on H2 adsorption. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Supported ruthenium catalyst represents the next generation of catalyst for ammonia synthesis after the iron-based catalyst [1–4]. Ru catalysts supported on the thermally modified active carbon have been proved to be much more active than fused iron catalysts [5,6]. However, the unavoidable methanation of carbon support in Ru catalyst under the ammonia synthesis condition [5,6] is still a problem for its extensive use in industry. It is thus desirable to develop a stable Ru catalyst that overcomes these issues, and oxides supported ruthenium catalysts have been investigated frequently. Aika et al. [1] claimed that the rate of ammonia synthesis was relative with the basicity of support materials (Ru/CaO N RuMgO N Ru/BeO N Raney Ru N Ru/A12O3 N Ru powder N Ru/AC) because basic supports can donate electrons to Ru atoms. This idea has been used widely for accounting for the difference in ammonia synthesis activities for Ru catalysts supported on various oxides [7–9], and thus Al2O3 was not considered to be an ideal support material for Ru catalyst because of the acidic nature of alumina [1]. Rao et al. [10,11] prepared carbon coated alumina by pyrolysis of an alkene on Al2O3. They claimed that this carbon coated alumina not only can eliminate the disadvantages of the low strength of carbon and the acidity of Al2O3, but also offered the advantages of the electron withdrawing capacity of carbon and the stability of alumina, therefore, the high activity for supported Ru catalyst can be obtained.
⁎ Corresponding author. Tel.: + 86 591 83731234; fax: +86 591 83738808. E-mail addresses: [email protected] (B. Lin), [email protected] (K. Wei). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.05.029
However, recently the high efficient Ru catalysts supported on Al2O3 have been prepared by some appropriate methods. Miyazaki et al. [12] found that the ammonia formation rate of Ru/Al2O3 catalyst obtained from a metal colloid with ethylene glycol as reducing agent was about 12 times higher than those of non-promoted Ru/Al2O3 catalysts prepared by conventional methods. Seetharamulu et al. [13] presented this polyol reduction method can be used for preparing the highly active Ru catalysts supported on Al2O3, MgO or Mg-Al hydrotalcite. Furthermore, they found that the activity of Cs promoted Ru/Al2O3 with hydrogen reduction was higher than that of Cs-Ru/MgO. Miyazaki et al. [12] and Seetharamulu et al. [13] claimed that the high dispersion of Ru nano-particles was the main reason for the high activity of the catalysts with polyol reduction. We also have successfully prepared chlorine-free Ru/Al2O3 catalysts with high catalytic activity by hydrazine reduction [14,15] or precipitation method [16]. We found that residual chlorine inhibited CO chemisorption, and then Ru particle sizes based on CO chemisorption would be overestimated. Transmission Electron Microscopy (TEM) study showed the average metal particle sizes of chlorine-containing Ru/Al2O3 catalysts were close to the values of chlorine-free samples, which can rule out the possibility that the change of the activity was mainly depended on the difference in Ru dispersion. The hydrogen temperature-programmed desorption study (H2-TPD) for Ru catalyst showed that residual chlorine inhibited the hydrogen desorption peaks at medium temperature. Previously, these peaks have been temporarily assigned to hydrogen adsorbed at sites at the metalsupport interface, and then it can be assumed the interaction between Ru particles and alumina surface exerts a strong influence on hydrogen adsorption and catalytic activity.
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It can be expected that the presence of carbon on alumina surface would change the interaction between Ru and alumina, herein CCA was used as support material for Ru catalyst to further understand the relationship among ruthenium particles, support material and chlorine. The samples were characterized by X-ray fluorescence (XRF), CO chemisorption, X-ray diffraction (XRD), TEM, X-ray photoelectron spectroscopy (XPS) and H2-TPD. The aim of this work was also to further confirm the influence of chlorine on hydrogen adsorption and the catalytic activity of Ru catalysts. The influence of carbon on the performance of Ru catalyst supported on CCA for ammonia synthesis also has been reconsidered. 2. Experimental 2.1. Preparation of CCA and ruthenium catalysts Uniformly CCA was prepared according to the procedure described by Lin et al. [17,18], briefly, γ-Al2O3 (WYA-251, 175 m 2/g, Wenzhou Jingjing Alumina Co., Ltd.) was sieved into 12–16 meshes, baked at 500 °C for 4 h and impregnated with aqueous solutions of sucrose. After dried at 120 °C for 1 h, the samples were heated at 600 °C in N2 (100 ml/min) for 0.5 h. The weight ratios of sucrose to alumina for CCA3 and CCA6 were 0.3 and 0.6, respectively. The carbon content was estimated by burning the organic deposit in the oven at 800 °C for 4 h. Ru catalysts (about 0.04 g Ru metal per gram of alumina) were prepared by impregnating aqueous of RuCl3·nH2O (37 wt.% Ru, SinoPlatinum Metals Co. Ltd.) on CCAx. One part of the impregnated solids was treated by precipitation method [16]. After washing until no chlorine ions were detected (AgNO3 titration), the sample was heated in hydrogen gas at 450 °C for 6 h. Sm was introduced to the asobtained Ru/CCAx by incipient wet impregnation method with an aqueous solution of samarium nitrate, and the weight ratio of Sm to Al2O3 was ca. 20%. The catalyst was labeled as SmRu/CCAx. Another part of the sample was treated with only hydrogen reduction, and Sm promoted catalyst was named as SmRu/CCAx-H. 2.2. Catalyst characterization N2 physisorption measurements were performed on an ASAP 2020 apparatus (Micromeritics, USA). The surface area was calculated by means of the Brunauer–Emmett–Teller (BET) method. The total pore volume was obtained from the amount of adsorbed N2 at a relative pressure of 0.98. The pore size distribution curves were determined from the N2 desorption isotherms using the Barrett–Joyner–Halenda (BJH) method. The micropore volume and micropore surface area were calculated by t-plot method. The composition concentrations of catalyst were obtained using a PANalytical Axios XRF spectrometer under a vacuum atmosphere. The result was reported based on a calibration curves. The surface morphology was examined with a field-emission scanning electron microscopy (FE-SEM; Hitachi S-4800). TEM was performed on a FEI Tecnai G2 F20 field emission transmission electron microscope operated at 200 kV. XPS analysis was performed on an ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scientific) at 3.0 × 10 − 10 mbar using Al Kα X-ray beam (1486.6 eV). XPS peak fitting were performed with XPSPEAK software (version 4.1, Raymund WM Kwok). The C1s peak at 284.6 eV was selected as an inner standard calibration peak. A Shirley background and an 20:80 Lorentzian/Gaussian peak shape were assumed. H2-TPD and CO chemisorption were performed on an AutoChem 2910 instrument (Micromeritics), according to the procedure from Lin et al. [16]. 2.3. Activity studies Ammonia synthesis was measured in a stainless steel reactor. Before activity testing, the catalysts (2 ml) were activated in a
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stoichiometric H2 and N2 mixture (3:1) at different temperatures (200, 300, 400, 450 and 500 °C) for 2 h. The ammonia concentration in the effluent was determined by neutralizing a known amount of diluted H2SO4 with Congo red as indicator after the catalysts were stabilized under the reaction conditions (i.e. 10 MPa, 10000 h –1, 450 °C) [19]. 3. Results and discussion The carbon content and the texture properties of Al2O3, CCA3 and CCA6 are listed in Table 1. Fig. 1 presents the adsorption-desorption nitrogen isotherms and pore size distribution curves of Al2O3, CCA3 and CCA6. XRD study shows the patterns of Al2O3, CCA3 and CCA6 were similar (see Fig. S1 in Supplementary material), indicating that carbon was highly dispersed on Al2O3 or the deposited carbon was amorphous. As can be seen in Table 1, the double sucrose loading led to the double increase of carbon content, which agrees well with the result of Lin et al. [18]. CCA3 had lower surface area (SBET) than that of Al2O3, while CCA6 showed highest specific surface area. Obviously, the increase of carbon content led to the decrease of total pore volume and pore size, but increased the micropore surface and micropore volume. The results were due to the change of the pore structures after the introduction of carbon. The presence of carbon would cover the surface of alumina and fill in the mesopores of alumina, which have been proved by pore size distributions of samples (Fig. 1) and SEM images (Fig. S2), and then decreased its surface area, total pore volume and pore size. However, a new step at a lower partial pressure, 0.4–0.7 can be clearly observed in the nitrogen adsorption/desorption isotherms of CCA6, and a new peak at a pore diameter of 3–4 nm also appeared in its pore size distribution curve. Obviously, the presence of larger amount of carbon decreased the total pore volume and pore size, but were advantageous to the generation of micropore, and thus the micropore volume and micropore surface area both increased. CCA6 even showed much higher value of specific surface area than that of Al2O3 as a result of large amounts of micropores. SEM images of samples (see Fig. S2) also show that the morphologies of carbon were quite different with those of CNTs [20], indicating that the as-obtained samples obtained by pyrolysis of sucrose mainly were amorphous carbon. The compositions, CO uptake, Ru metal dispersion and particle size of the Sm-promoted Ru catalysts are collected in Table 2. Rather large number of chlorine remained on the Ru catalysts with only hydrogen reduction, while only trace of chlorine was detected in samples with precipitation method. This observation is good accordance with our previous result [16]. The presence of chlorine severely decreased the values of particle size based on CO chemisorption. However, TEM images with particle size distribution histograms (Fig. 2) clearly exhibited that SmRu/CCA3 and SmRu/CCA3–H both had the particle size distribution in the range of 1.0–4.0 nm, the average sizes for SmRu/CCA3 and SmRu/CCA3–H were 2.2 and 2.5 nm, respectively. The discrepancy between the mean metal particle sizes determined from TEM and from CO chemisorption was due to the presence of chlorine would suppress CO chemisorption [21], and then the amount of adsorbed CO decreased. In such a case, the value of particle size based on CO chemisorption was overestimated [16]. From Table 2 and Fig. 2, it also can conclude that residual chlorine and carbon all had a Table 1 Carbon content and texture properties of Al2O3 and CCA samples. Total pore Micropore Sample Ws:WAa C content SBET Micropore Pore (m2/g) volume (wt.%) surface area volume size (cm3/g) (m2/g) (cm3/g) (nm) Al2O3 CCA3 CCA6 a
– 0.3:1 0.6:1
– 7.8 14.8
175 168 189
Weight ratio of sucrose to alumina.
0.53 0.43 0.37
– 26.2 34.9
– 0.01 0.03
11.5 9.7 7.4
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a 360 320 Al2O3 CCA3 CCA6
Quantity Adsorbed (cm3/g)
280 240 200 160 120 80 40 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
b
0.12 Al2O3
Pore Volume (cm3/(g*nm))
0.10
CCA3 CCA6
0.08
0.06
0.04
0.02
0.00 2
3
4
5 6 7 8 910
20
30
40
Pore Diameter (nm) Fig. 1. (a) Nitrogen adsorption/desorption isotherms and (b) pore size distributions of Al2O3, CCA3 and CCA6.
limited impact on the Ru particle size, but the high amount of carbon can decrease the overestimation of Ru particle size determined from CO chemisorption for containing-chlorine samples. Table 2 Compositions, dispersion and particle size of ruthenium for Sm-promoted Ru catalysts. Samples
SmRu/Al2O3 SmRu/Al2O3–H SmRu/CCA3 SmRu/CCA3–H SmRu/CCA6 SmRu/CCA6–H a b c d
Compositions (wt.%)a Ru
Cl
Sm
Al2O3
3.12 3.01 2.83 3.12 2.99 2.93
– 0.96 – 0.78 0.02 0.83
16.14 15.48 15.51 16.34 15.81 15.47
80.02 80.22 80.55 81.50 81.52 80.30
CO uptake (ml/gcat)
Particle size (nm)b
Particle size (nm)c
3.56 1.29 3.19 1.68 3.05 1.98
2.1d 6.2d 2.4 5.3 2.6 4.3
2.1d 2.2d 2.2 2.5 – –
Obtained by XRF analysis. Based on CO chemisorption and an assumption of CO/Ru = 1. Obtained by TEM. Data obtained from Ref. [16].
Fig. 2. TEM images with particle size distribution histograms for (a) SmRu/CCA3 and (b) SmRu/CCA3-H.
Fig. 3 presents XPS spectra in the Ru 3d and C 1s regions from SmRu/Al2O3, SmRu/Al2O3–H, SmRu/CCA6 and SmRu/CCA6–H. The Ru 3d spectra consist of the Ru 3d5/2 and Ru 3d3/2 peaks resulting from the spin–orbital splitting, and there is an overlap between Ru 3d3/2 and C 1s peak at ca. 285 eV. In view of this, a fixed spin-orbit splitting (4.2 eV [22,23]) and fixed area ratio ( 3:2 [23]) were used for the fit of Ru 3d peaks. As showed in Fig. 3, the binding energy values of Ru 3d5/2 binding energy of SmRu/Al2O3 and SmRu/Al2O3–H were ca. 279.6 eV, which were characteristic of metallic ruthenium [24,25]. On the other hand, Ru 3d5/2 binding energies for SmRu/CCA6 and SmRu/CCA6–H were about 280.6 eV, which agrees with the value of Ru 3d5/2 binding energy in Ru metal for Ru catalysts supported on hydrogen treated active carbon reported by Wu et al. [26]. This result can be accepted by considering that carbon contained a large number of acidic surface functional groups such as carboxylic and phenol groups [27,28], which can withdraw electron density from Ru atoms [29,30], and thus Ru 3d5/2 binding energy increased.
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Intensity (Counts/s)
Ru 3d + C 1s 280.6
SmRu/CCA6-H
280.6
SmRu/CCA6
279.6
SmRu/Al2O3-H
279.6
SmRu/Al2O3
278
280
282
284
286
288
290
292
Binding Energy (eV)
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high temperature both significantly decreased, which indicated that the presence of carbon had a disadvantageous impact on hydrogen adsorption on sites at the metal–alumina interface or hydrogen spillover associated with the alumina. This result can be accepted by considering that carbon on alumina surface changed the interaction between Ru and alumina, and then influenced on the hydrogen adsorption. A similar influence of carbon on the hydrogen desorption peaks at high temperature also can be observed over containingchlorine Ru catalysts supported on carbon-covered alumina. On the other hand, with a similar amount of residual chlorine (Table 2), the amount of adsorbed hydrogen corresponding to the hydrogen desorption peak at medium temperature increased with the increase of carbon content, indicating that the presence of carbon can decrease the inhibition effect of chlorine on hydrogen adsorption on sites at the metal–surface interface. Fig. 5a presents the temperature dependence of the ammonia concentration over various Ru catalysts at 10 MPa and 10,000 h − 1. The activities for all Ru catalysts increased with the increase of reaction temperature in the temperature range studied. For the catalysts with precipitation method, all catalysts showed high activities, and the catalytic activities of SmRu/CCA6 were higher
a
Fig. 3. XPS spectra of Sm-promoted Ru catalysts.
10
SmRu/Al2O3
Ammonia concentration (%)
Our previous result [15] has shown the high temperature peaks at above 500 °C in H2–TPD profiles for non-promoted Ru/Al2O3 samples were caused by the irreversible hydrogen. These types of hydrogen species would not desorb at the reaction temperature range studied, and thus herein H2–TPD profiles with the temperature range of room temperature to 500 °C were obtained. Fig. 4 shows the TPD profiles of Sm–promoted Ru catalysts. Hydrogen desorbed from SmRu/Al2O3 exhibiting three peaks centered at ca. 100, 260, and 375 °C. As has been discussed in our previous study [16], the low temperature peak was due to hydrogen chemisorbed on the surface of Ru particles [31,32], whereas the high temperature peak can be assigned to spillover hydrogen associated with the alumina. The medium temperature peak has been attributed to adsorbed hydrogen on sites at the metal–alumina interface [16]. This assignment is consistent with previous work obtained from Pt/TiO2 [32], Pt/LTL zeolite [33] and supported Rh catalysts [34]. Similar peaks also has been observed in the H2–TPD profiles over SmRu/CCA3 and SmRu/CCA6, but the peak intensities at medium temperature and
SmRu/Al2O3-H SmRu/CCA3-H SmRu/CCA3 SmRu/CCA6-H SmRu/CCA6
8
6
4
2
0 390
400
410
420
430
440
450
Temperature (°C)
b
1.0 VNH3(SmRu/Al2O3-H)/VNH3(SmRu/Al2O3)
Ammonia concentration ratio
TCD Signal (a.u.)
VNH3(SmRu/CCA3-H)/VNH3(SmRu/CCA3)
(a) (b) (c)
VNH3(SmRu/CCA6-H)/VNH3(SmRu/CCA6)
0.8
0.6
0.4
0.2
(d) Carbon content
(e) 0.0 390
(f) 50
100
150
200
250
300
350
400
450
500
Temperature (°C) Fig. 4. H2-TPD profiles of (a) SmRu/Al2O3, (b) SmRu/Al2O3-H, (c) SmRu/CCA3, (d) SmRu/CCA3-H, (e) SmRu/CCA6 and (f) SmRu/CCA6-H.
400
410
420
430
440
450
Temperature (°C) Fig. 5. (a) Ammonia concentration in outlet gas over various Ru catalysts in 3:1 mixture H2:N2 at temperatures from 390 to 450 °C, 10 MPa and 10,000 h− 1, and (b) the ammonia concentration ratio of the Ru catalyst with precipitation method to the corresponding one with hydrogen reduction.
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than those of SmRu/Al2O3 at low reaction temperatures. This result could be accepted because carbon based Ru catalysts usually had much higher ammonia synthesis activities at low temperature than those of oxides supported Ru catalysts. For the samples with hydrogen treatment, the activities at different temperatures all increased with the increase of carbon content. The ammonia concentration ratio of the Ru catalyst with precipitation method to the corresponding one with hydrogen reduction is also presented in Fig. 5b. The ratios at different reaction temperatures all increased significantly with the increase in carbon content, which clearly confirmed that the presence of carbon can decrease the disadvantage influence of chlorine on ammonia synthesis, which corresponds to the result of H2–TPD study (Fig. 4). As for carbon-based Ru catalysts, the influence of chlorine on the activity for ammonia synthesis is much less [27], and thus RuCl3 usually was as Ru precursor for high efficient Ru catalysts supported on carbon materials [27,35]. The activity remained constant throughout the time-on-stream activity test carried out for 100 h (see Fig. S3), indicating that the catalysts could be stably used for more than 100 h. Rao et al. [10,11] claimed the high activities of the catalysts supported on carbon coated alumina should be attributed to (1) the better dispersion of ruthenium, (2) the beneficial effect of the graphite lattice by accelerating the transfer of electrons from promoter to Ru crystallites and (3) containing-carbon offered the advantages of the electron withdrawing capacity of carbon. However, here one can find that the presence of carbon in support material had a limited impact on Ru particle size. Carbon increased the Ru 3d5/2 binding energy, but there was not direct correlation between the value of Ru 3d5/2 binding energy and the catalytic activities for our CCA supported Ru catalysts. Furthermore, as confirmed in our previous study [16], the presence of chlorine almost completely inhibited hydrogen adsorption with medium desorption temperature, and then decreased the ammonia synthesis activity. Carbon also decreased the amount of adsorbed hydrogen corresponding to the H2 desorption peak at medium temperature for chlorine-free CCA supported Ru catalysts because carbon on alumina surface inhibited the close contact between alumina and Ru particles. But all chlorine-free Ru catalysts exhibited similar ammonia synthesis activities. This result indicated that little amount of active sites would be enough for obtaining high activity since ammonia synthesis reaction might be totally dominated by less than 1% atomic step sites [36,37]. Dahl et al. [37] also found that N2 dissociation rate drop by roughly nine orders of magnitude by depositing small amounts of Au (1–2% of a monolayer) a Ru(0001) surface. In such a case, appropriate amount of adsorbed hydrogen and active sites can be enough for high catalytic activity of chlorine-free samples. In our previous study about the influence of Sm promoter on ammonia synthesis, it also is found that Sm increased the catalytic activity by decreased the amount of adsorbed hydrogen with medium desorption temperatures [15]. On the other hand, the introduction of carbon decreased significantly the disadvantageous effect of chlorine on CO chemisorption and H2 adsorption, and then the activities of ammonia synthesis for containing-chlorine samples increased with the increase of carbon content since an appreciative amount of adsorbed hydrogen corresponding to desorption peak at medium temperature is required for high activity of Ru/Al2O3 catalyst [15]. Rao et al. [10,11] also prepared their Ru catalysts supported on carbon coated alumina from aqueous RuCl3 solution by hydrogen reduction, and thus the higher catalytic activities of ammonia synthesis can be obtained. Unfortunately, chlorine cannot be completely removed by hydrogen reduction [15,16]. They did not consider the effect of chlorine on the activity Ru catalyst, and thus the effect of carbon cannot be fully understood. However, the influence of the characters of carbon materials on ammonia synthesis for Ru supported on containing-carbon composites still cannot be fully ignored because the electronic effect of carbon had a strong impact on the catalytic activities [38,39]. Thus the performance of the containing-carbon composites supported Ru catalysts may can be improved
by the change of the characters of carbon. Furthermore, the measure by changing the hydrogen adsorption characters or the interaction between oxides and Ru particles might well be used for improving the performance for other oxides supported Ru catalysts, further studies for these issues are in progress. It is worthy noted that chlorine was a poison not only for iron catalysts in ammonia synthesis, but also for ruthenium catalysts. It is widely accepted that chlorine had a disadvantageous effect on iron catalysts by the formation of the volatile potassium chloride with potassium promoter, which resulted in the elimination of potassium [40]. However, There is still some controversy about the influence of chlorine on Ru catalysts for ammonia synthesis, two different ideas usually were used for explaining the effect of chlorine: One is that the presence of chlorine was disadvantage to the opening of nitrogennitrogen triple bonds because Cl decreased the local electron density of Ru surface [41,42]. The other is that Cl activated some sites available for hydrogen adsorption, and then retarded the nitrogen activation. However, Narita et al. [21] also found that chlorine inhibited CO and H2 adsorption by site blockage for ruthenium catalysts supported on SiO2 and Al2O3. Iyagba et al. [43] proposed that structural rearrangement, rather than site blocking or electronic interactions, should be the primary mechanism of chlorine modification of the activity of CO hydrogenation for silica-supported Ru catalyst. Herein it is quite obvious that chlorine did not lead to the significant change in Ru 3d5/2 binding energies for Ru catalysts supported on Al2O3 or carboncovered alumina. It is no doubt that site blocking is an important factor for the influence of chlorine on their ammonia synthesis activity of Ru catalysts. But it is also obvious that this site blocking effect is closely related with the property of support material because the effect of chlorine on the activity of Ru catalyst supported on Al2O3 is much more significant than that of carbon-covered alumina supported Ru catalysts. More studies are necessary for fully understanding this issue. 4. Conclusions Uniformly carbon-covered alumina was prepared by pyrolysis of sucrose. The total pore volume and pore size decreased with the increase of carbon content, but the micropore surface and micropore volume increased, and the specific surface area first decreased and then increased. The presence of carbon had a limited impact on Ru particle size for CCA supported Ru catalyst, but increased Ru 3d5/2 binding energy. However, the change in the values Ru 3d5/2 binding energy had a neglectable influence on ammonia synthesis. Residual chlorine significantly suppressed hydrogen adsorption with medium desorption temperature. Carbon also decreased the amount of this type of hydrogen species for chlorine-free Ru catalysts supported on CCA, but did not change their ammonia synthesis activities. On the other hand, carbon can decrease the disadvantageous effect of chlorine on CO chemisorption and H2 adsorption, thus the activities for containing-chlorine samples and the ammonia concentration ratios between the containing-chlorine Ru catalysts and chlorine-free samples both increased with increasing carbon content. This result further confirmed the influence of chlorine on hydrogen adsorption was an important factor for catalytic activity of ammonia synthesis, and carbon in support materials may had a strong impact on ammonia synthesis over supported Ru catalysts by change the hydrogen adsorption. Acknowledgements The authors are grateful for the financial support from the National Key Technology R&D Program of China (2007BAE08B02) and Science and Technology Development Foundation of Fuzhou University (2008-XY-7).
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Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Effect of carbon and chlorine on the performance of carbon-covered alumina supported Ru catalyst for ammonia synthesis Bingyu Lin, Rong Wang, Jianxin Lin, Jun Ni, Kemei Wei ⁎ National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou 350002, PR China
a r t i c l e
i n f o
Article history: Received 15 April 2011 Received in revised form 23 May 2011 Accepted 27 May 2011 Available online 6 June 2011 Keywords: Carbon-covered alumina Carbon Chlorine Ru catalyst Ammonia synthesis
a b s t r a c t Uniformly carbon-covered alumina (CCA), which was prepared by pyrolysis of sucrose, was used as support of ruthenium catalyst. Carbon did not significantly influence on the ammonia synthesis activities of Ru catalysts by changing their Ru particle sizes or Ru 3d5/2 binding energies. Residual chlorine severely suppressed ammonia synthesis by decreasing the amount of hydrogen with the desorption peak at medium temperatures. Carbon also inhibited the adsorption of this hydrogen species for chlorine-free Ru catalysts, but did not change their activities. On the other hand, carbon can increase the ammonia synthesis activities of containing-chlorine Ru catalysts by decreasing the disadvantageous effect of chlorine on H2 adsorption. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Supported ruthenium catalyst represents the next generation of catalyst for ammonia synthesis after the iron-based catalyst [1–4]. Ru catalysts supported on the thermally modified active carbon have been proved to be much more active than fused iron catalysts [5,6]. However, the unavoidable methanation of carbon support in Ru catalyst under the ammonia synthesis condition [5,6] is still a problem for its extensive use in industry. It is thus desirable to develop a stable Ru catalyst that overcomes these issues, and oxides supported ruthenium catalysts have been investigated frequently. Aika et al. [1] claimed that the rate of ammonia synthesis was relative with the basicity of support materials (Ru/CaO N RuMgO N Ru/BeO N Raney Ru N Ru/A12O3 N Ru powder N Ru/AC) because basic supports can donate electrons to Ru atoms. This idea has been used widely for accounting for the difference in ammonia synthesis activities for Ru catalysts supported on various oxides [7–9], and thus Al2O3 was not considered to be an ideal support material for Ru catalyst because of the acidic nature of alumina [1]. Rao et al. [10,11] prepared carbon coated alumina by pyrolysis of an alkene on Al2O3. They claimed that this carbon coated alumina not only can eliminate the disadvantages of the low strength of carbon and the acidity of Al2O3, but also offered the advantages of the electron withdrawing capacity of carbon and the stability of alumina, therefore, the high activity for supported Ru catalyst can be obtained.
⁎ Corresponding author. Tel.: + 86 591 83731234; fax: +86 591 83738808. E-mail addresses: [email protected] (B. Lin), [email protected] (K. Wei). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.05.029
However, recently the high efficient Ru catalysts supported on Al2O3 have been prepared by some appropriate methods. Miyazaki et al. [12] found that the ammonia formation rate of Ru/Al2O3 catalyst obtained from a metal colloid with ethylene glycol as reducing agent was about 12 times higher than those of non-promoted Ru/Al2O3 catalysts prepared by conventional methods. Seetharamulu et al. [13] presented this polyol reduction method can be used for preparing the highly active Ru catalysts supported on Al2O3, MgO or Mg-Al hydrotalcite. Furthermore, they found that the activity of Cs promoted Ru/Al2O3 with hydrogen reduction was higher than that of Cs-Ru/MgO. Miyazaki et al. [12] and Seetharamulu et al. [13] claimed that the high dispersion of Ru nano-particles was the main reason for the high activity of the catalysts with polyol reduction. We also have successfully prepared chlorine-free Ru/Al2O3 catalysts with high catalytic activity by hydrazine reduction [14,15] or precipitation method [16]. We found that residual chlorine inhibited CO chemisorption, and then Ru particle sizes based on CO chemisorption would be overestimated. Transmission Electron Microscopy (TEM) study showed the average metal particle sizes of chlorine-containing Ru/Al2O3 catalysts were close to the values of chlorine-free samples, which can rule out the possibility that the change of the activity was mainly depended on the difference in Ru dispersion. The hydrogen temperature-programmed desorption study (H2-TPD) for Ru catalyst showed that residual chlorine inhibited the hydrogen desorption peaks at medium temperature. Previously, these peaks have been temporarily assigned to hydrogen adsorbed at sites at the metalsupport interface, and then it can be assumed the interaction between Ru particles and alumina surface exerts a strong influence on hydrogen adsorption and catalytic activity.
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It can be expected that the presence of carbon on alumina surface would change the interaction between Ru and alumina, herein CCA was used as support material for Ru catalyst to further understand the relationship among ruthenium particles, support material and chlorine. The samples were characterized by X-ray fluorescence (XRF), CO chemisorption, X-ray diffraction (XRD), TEM, X-ray photoelectron spectroscopy (XPS) and H2-TPD. The aim of this work was also to further confirm the influence of chlorine on hydrogen adsorption and the catalytic activity of Ru catalysts. The influence of carbon on the performance of Ru catalyst supported on CCA for ammonia synthesis also has been reconsidered. 2. Experimental 2.1. Preparation of CCA and ruthenium catalysts Uniformly CCA was prepared according to the procedure described by Lin et al. [17,18], briefly, γ-Al2O3 (WYA-251, 175 m 2/g, Wenzhou Jingjing Alumina Co., Ltd.) was sieved into 12–16 meshes, baked at 500 °C for 4 h and impregnated with aqueous solutions of sucrose. After dried at 120 °C for 1 h, the samples were heated at 600 °C in N2 (100 ml/min) for 0.5 h. The weight ratios of sucrose to alumina for CCA3 and CCA6 were 0.3 and 0.6, respectively. The carbon content was estimated by burning the organic deposit in the oven at 800 °C for 4 h. Ru catalysts (about 0.04 g Ru metal per gram of alumina) were prepared by impregnating aqueous of RuCl3·nH2O (37 wt.% Ru, SinoPlatinum Metals Co. Ltd.) on CCAx. One part of the impregnated solids was treated by precipitation method [16]. After washing until no chlorine ions were detected (AgNO3 titration), the sample was heated in hydrogen gas at 450 °C for 6 h. Sm was introduced to the asobtained Ru/CCAx by incipient wet impregnation method with an aqueous solution of samarium nitrate, and the weight ratio of Sm to Al2O3 was ca. 20%. The catalyst was labeled as SmRu/CCAx. Another part of the sample was treated with only hydrogen reduction, and Sm promoted catalyst was named as SmRu/CCAx-H. 2.2. Catalyst characterization N2 physisorption measurements were performed on an ASAP 2020 apparatus (Micromeritics, USA). The surface area was calculated by means of the Brunauer–Emmett–Teller (BET) method. The total pore volume was obtained from the amount of adsorbed N2 at a relative pressure of 0.98. The pore size distribution curves were determined from the N2 desorption isotherms using the Barrett–Joyner–Halenda (BJH) method. The micropore volume and micropore surface area were calculated by t-plot method. The composition concentrations of catalyst were obtained using a PANalytical Axios XRF spectrometer under a vacuum atmosphere. The result was reported based on a calibration curves. The surface morphology was examined with a field-emission scanning electron microscopy (FE-SEM; Hitachi S-4800). TEM was performed on a FEI Tecnai G2 F20 field emission transmission electron microscope operated at 200 kV. XPS analysis was performed on an ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scientific) at 3.0 × 10 − 10 mbar using Al Kα X-ray beam (1486.6 eV). XPS peak fitting were performed with XPSPEAK software (version 4.1, Raymund WM Kwok). The C1s peak at 284.6 eV was selected as an inner standard calibration peak. A Shirley background and an 20:80 Lorentzian/Gaussian peak shape were assumed. H2-TPD and CO chemisorption were performed on an AutoChem 2910 instrument (Micromeritics), according to the procedure from Lin et al. [16]. 2.3. Activity studies Ammonia synthesis was measured in a stainless steel reactor. Before activity testing, the catalysts (2 ml) were activated in a
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stoichiometric H2 and N2 mixture (3:1) at different temperatures (200, 300, 400, 450 and 500 °C) for 2 h. The ammonia concentration in the effluent was determined by neutralizing a known amount of diluted H2SO4 with Congo red as indicator after the catalysts were stabilized under the reaction conditions (i.e. 10 MPa, 10000 h –1, 450 °C) [19]. 3. Results and discussion The carbon content and the texture properties of Al2O3, CCA3 and CCA6 are listed in Table 1. Fig. 1 presents the adsorption-desorption nitrogen isotherms and pore size distribution curves of Al2O3, CCA3 and CCA6. XRD study shows the patterns of Al2O3, CCA3 and CCA6 were similar (see Fig. S1 in Supplementary material), indicating that carbon was highly dispersed on Al2O3 or the deposited carbon was amorphous. As can be seen in Table 1, the double sucrose loading led to the double increase of carbon content, which agrees well with the result of Lin et al. [18]. CCA3 had lower surface area (SBET) than that of Al2O3, while CCA6 showed highest specific surface area. Obviously, the increase of carbon content led to the decrease of total pore volume and pore size, but increased the micropore surface and micropore volume. The results were due to the change of the pore structures after the introduction of carbon. The presence of carbon would cover the surface of alumina and fill in the mesopores of alumina, which have been proved by pore size distributions of samples (Fig. 1) and SEM images (Fig. S2), and then decreased its surface area, total pore volume and pore size. However, a new step at a lower partial pressure, 0.4–0.7 can be clearly observed in the nitrogen adsorption/desorption isotherms of CCA6, and a new peak at a pore diameter of 3–4 nm also appeared in its pore size distribution curve. Obviously, the presence of larger amount of carbon decreased the total pore volume and pore size, but were advantageous to the generation of micropore, and thus the micropore volume and micropore surface area both increased. CCA6 even showed much higher value of specific surface area than that of Al2O3 as a result of large amounts of micropores. SEM images of samples (see Fig. S2) also show that the morphologies of carbon were quite different with those of CNTs [20], indicating that the as-obtained samples obtained by pyrolysis of sucrose mainly were amorphous carbon. The compositions, CO uptake, Ru metal dispersion and particle size of the Sm-promoted Ru catalysts are collected in Table 2. Rather large number of chlorine remained on the Ru catalysts with only hydrogen reduction, while only trace of chlorine was detected in samples with precipitation method. This observation is good accordance with our previous result [16]. The presence of chlorine severely decreased the values of particle size based on CO chemisorption. However, TEM images with particle size distribution histograms (Fig. 2) clearly exhibited that SmRu/CCA3 and SmRu/CCA3–H both had the particle size distribution in the range of 1.0–4.0 nm, the average sizes for SmRu/CCA3 and SmRu/CCA3–H were 2.2 and 2.5 nm, respectively. The discrepancy between the mean metal particle sizes determined from TEM and from CO chemisorption was due to the presence of chlorine would suppress CO chemisorption [21], and then the amount of adsorbed CO decreased. In such a case, the value of particle size based on CO chemisorption was overestimated [16]. From Table 2 and Fig. 2, it also can conclude that residual chlorine and carbon all had a Table 1 Carbon content and texture properties of Al2O3 and CCA samples. Total pore Micropore Sample Ws:WAa C content SBET Micropore Pore (m2/g) volume (wt.%) surface area volume size (cm3/g) (m2/g) (cm3/g) (nm) Al2O3 CCA3 CCA6 a
– 0.3:1 0.6:1
– 7.8 14.8
175 168 189
Weight ratio of sucrose to alumina.
0.53 0.43 0.37
– 26.2 34.9
– 0.01 0.03
11.5 9.7 7.4
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a 360 320 Al2O3 CCA3 CCA6
Quantity Adsorbed (cm3/g)
280 240 200 160 120 80 40 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
b
0.12 Al2O3
Pore Volume (cm3/(g*nm))
0.10
CCA3 CCA6
0.08
0.06
0.04
0.02
0.00 2
3
4
5 6 7 8 910
20
30
40
Pore Diameter (nm) Fig. 1. (a) Nitrogen adsorption/desorption isotherms and (b) pore size distributions of Al2O3, CCA3 and CCA6.
limited impact on the Ru particle size, but the high amount of carbon can decrease the overestimation of Ru particle size determined from CO chemisorption for containing-chlorine samples. Table 2 Compositions, dispersion and particle size of ruthenium for Sm-promoted Ru catalysts. Samples
SmRu/Al2O3 SmRu/Al2O3–H SmRu/CCA3 SmRu/CCA3–H SmRu/CCA6 SmRu/CCA6–H a b c d
Compositions (wt.%)a Ru
Cl
Sm
Al2O3
3.12 3.01 2.83 3.12 2.99 2.93
– 0.96 – 0.78 0.02 0.83
16.14 15.48 15.51 16.34 15.81 15.47
80.02 80.22 80.55 81.50 81.52 80.30
CO uptake (ml/gcat)
Particle size (nm)b
Particle size (nm)c
3.56 1.29 3.19 1.68 3.05 1.98
2.1d 6.2d 2.4 5.3 2.6 4.3
2.1d 2.2d 2.2 2.5 – –
Obtained by XRF analysis. Based on CO chemisorption and an assumption of CO/Ru = 1. Obtained by TEM. Data obtained from Ref. [16].
Fig. 2. TEM images with particle size distribution histograms for (a) SmRu/CCA3 and (b) SmRu/CCA3-H.
Fig. 3 presents XPS spectra in the Ru 3d and C 1s regions from SmRu/Al2O3, SmRu/Al2O3–H, SmRu/CCA6 and SmRu/CCA6–H. The Ru 3d spectra consist of the Ru 3d5/2 and Ru 3d3/2 peaks resulting from the spin–orbital splitting, and there is an overlap between Ru 3d3/2 and C 1s peak at ca. 285 eV. In view of this, a fixed spin-orbit splitting (4.2 eV [22,23]) and fixed area ratio ( 3:2 [23]) were used for the fit of Ru 3d peaks. As showed in Fig. 3, the binding energy values of Ru 3d5/2 binding energy of SmRu/Al2O3 and SmRu/Al2O3–H were ca. 279.6 eV, which were characteristic of metallic ruthenium [24,25]. On the other hand, Ru 3d5/2 binding energies for SmRu/CCA6 and SmRu/CCA6–H were about 280.6 eV, which agrees with the value of Ru 3d5/2 binding energy in Ru metal for Ru catalysts supported on hydrogen treated active carbon reported by Wu et al. [26]. This result can be accepted by considering that carbon contained a large number of acidic surface functional groups such as carboxylic and phenol groups [27,28], which can withdraw electron density from Ru atoms [29,30], and thus Ru 3d5/2 binding energy increased.
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Intensity (Counts/s)
Ru 3d + C 1s 280.6
SmRu/CCA6-H
280.6
SmRu/CCA6
279.6
SmRu/Al2O3-H
279.6
SmRu/Al2O3
278
280
282
284
286
288
290
292
Binding Energy (eV)
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high temperature both significantly decreased, which indicated that the presence of carbon had a disadvantageous impact on hydrogen adsorption on sites at the metal–alumina interface or hydrogen spillover associated with the alumina. This result can be accepted by considering that carbon on alumina surface changed the interaction between Ru and alumina, and then influenced on the hydrogen adsorption. A similar influence of carbon on the hydrogen desorption peaks at high temperature also can be observed over containingchlorine Ru catalysts supported on carbon-covered alumina. On the other hand, with a similar amount of residual chlorine (Table 2), the amount of adsorbed hydrogen corresponding to the hydrogen desorption peak at medium temperature increased with the increase of carbon content, indicating that the presence of carbon can decrease the inhibition effect of chlorine on hydrogen adsorption on sites at the metal–surface interface. Fig. 5a presents the temperature dependence of the ammonia concentration over various Ru catalysts at 10 MPa and 10,000 h − 1. The activities for all Ru catalysts increased with the increase of reaction temperature in the temperature range studied. For the catalysts with precipitation method, all catalysts showed high activities, and the catalytic activities of SmRu/CCA6 were higher
a
Fig. 3. XPS spectra of Sm-promoted Ru catalysts.
10
SmRu/Al2O3
Ammonia concentration (%)
Our previous result [15] has shown the high temperature peaks at above 500 °C in H2–TPD profiles for non-promoted Ru/Al2O3 samples were caused by the irreversible hydrogen. These types of hydrogen species would not desorb at the reaction temperature range studied, and thus herein H2–TPD profiles with the temperature range of room temperature to 500 °C were obtained. Fig. 4 shows the TPD profiles of Sm–promoted Ru catalysts. Hydrogen desorbed from SmRu/Al2O3 exhibiting three peaks centered at ca. 100, 260, and 375 °C. As has been discussed in our previous study [16], the low temperature peak was due to hydrogen chemisorbed on the surface of Ru particles [31,32], whereas the high temperature peak can be assigned to spillover hydrogen associated with the alumina. The medium temperature peak has been attributed to adsorbed hydrogen on sites at the metal–alumina interface [16]. This assignment is consistent with previous work obtained from Pt/TiO2 [32], Pt/LTL zeolite [33] and supported Rh catalysts [34]. Similar peaks also has been observed in the H2–TPD profiles over SmRu/CCA3 and SmRu/CCA6, but the peak intensities at medium temperature and
SmRu/Al2O3-H SmRu/CCA3-H SmRu/CCA3 SmRu/CCA6-H SmRu/CCA6
8
6
4
2
0 390
400
410
420
430
440
450
Temperature (°C)
b
1.0 VNH3(SmRu/Al2O3-H)/VNH3(SmRu/Al2O3)
Ammonia concentration ratio
TCD Signal (a.u.)
VNH3(SmRu/CCA3-H)/VNH3(SmRu/CCA3)
(a) (b) (c)
VNH3(SmRu/CCA6-H)/VNH3(SmRu/CCA6)
0.8
0.6
0.4
0.2
(d) Carbon content
(e) 0.0 390
(f) 50
100
150
200
250
300
350
400
450
500
Temperature (°C) Fig. 4. H2-TPD profiles of (a) SmRu/Al2O3, (b) SmRu/Al2O3-H, (c) SmRu/CCA3, (d) SmRu/CCA3-H, (e) SmRu/CCA6 and (f) SmRu/CCA6-H.
400
410
420
430
440
450
Temperature (°C) Fig. 5. (a) Ammonia concentration in outlet gas over various Ru catalysts in 3:1 mixture H2:N2 at temperatures from 390 to 450 °C, 10 MPa and 10,000 h− 1, and (b) the ammonia concentration ratio of the Ru catalyst with precipitation method to the corresponding one with hydrogen reduction.
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than those of SmRu/Al2O3 at low reaction temperatures. This result could be accepted because carbon based Ru catalysts usually had much higher ammonia synthesis activities at low temperature than those of oxides supported Ru catalysts. For the samples with hydrogen treatment, the activities at different temperatures all increased with the increase of carbon content. The ammonia concentration ratio of the Ru catalyst with precipitation method to the corresponding one with hydrogen reduction is also presented in Fig. 5b. The ratios at different reaction temperatures all increased significantly with the increase in carbon content, which clearly confirmed that the presence of carbon can decrease the disadvantage influence of chlorine on ammonia synthesis, which corresponds to the result of H2–TPD study (Fig. 4). As for carbon-based Ru catalysts, the influence of chlorine on the activity for ammonia synthesis is much less [27], and thus RuCl3 usually was as Ru precursor for high efficient Ru catalysts supported on carbon materials [27,35]. The activity remained constant throughout the time-on-stream activity test carried out for 100 h (see Fig. S3), indicating that the catalysts could be stably used for more than 100 h. Rao et al. [10,11] claimed the high activities of the catalysts supported on carbon coated alumina should be attributed to (1) the better dispersion of ruthenium, (2) the beneficial effect of the graphite lattice by accelerating the transfer of electrons from promoter to Ru crystallites and (3) containing-carbon offered the advantages of the electron withdrawing capacity of carbon. However, here one can find that the presence of carbon in support material had a limited impact on Ru particle size. Carbon increased the Ru 3d5/2 binding energy, but there was not direct correlation between the value of Ru 3d5/2 binding energy and the catalytic activities for our CCA supported Ru catalysts. Furthermore, as confirmed in our previous study [16], the presence of chlorine almost completely inhibited hydrogen adsorption with medium desorption temperature, and then decreased the ammonia synthesis activity. Carbon also decreased the amount of adsorbed hydrogen corresponding to the H2 desorption peak at medium temperature for chlorine-free CCA supported Ru catalysts because carbon on alumina surface inhibited the close contact between alumina and Ru particles. But all chlorine-free Ru catalysts exhibited similar ammonia synthesis activities. This result indicated that little amount of active sites would be enough for obtaining high activity since ammonia synthesis reaction might be totally dominated by less than 1% atomic step sites [36,37]. Dahl et al. [37] also found that N2 dissociation rate drop by roughly nine orders of magnitude by depositing small amounts of Au (1–2% of a monolayer) a Ru(0001) surface. In such a case, appropriate amount of adsorbed hydrogen and active sites can be enough for high catalytic activity of chlorine-free samples. In our previous study about the influence of Sm promoter on ammonia synthesis, it also is found that Sm increased the catalytic activity by decreased the amount of adsorbed hydrogen with medium desorption temperatures [15]. On the other hand, the introduction of carbon decreased significantly the disadvantageous effect of chlorine on CO chemisorption and H2 adsorption, and then the activities of ammonia synthesis for containing-chlorine samples increased with the increase of carbon content since an appreciative amount of adsorbed hydrogen corresponding to desorption peak at medium temperature is required for high activity of Ru/Al2O3 catalyst [15]. Rao et al. [10,11] also prepared their Ru catalysts supported on carbon coated alumina from aqueous RuCl3 solution by hydrogen reduction, and thus the higher catalytic activities of ammonia synthesis can be obtained. Unfortunately, chlorine cannot be completely removed by hydrogen reduction [15,16]. They did not consider the effect of chlorine on the activity Ru catalyst, and thus the effect of carbon cannot be fully understood. However, the influence of the characters of carbon materials on ammonia synthesis for Ru supported on containing-carbon composites still cannot be fully ignored because the electronic effect of carbon had a strong impact on the catalytic activities [38,39]. Thus the performance of the containing-carbon composites supported Ru catalysts may can be improved
by the change of the characters of carbon. Furthermore, the measure by changing the hydrogen adsorption characters or the interaction between oxides and Ru particles might well be used for improving the performance for other oxides supported Ru catalysts, further studies for these issues are in progress. It is worthy noted that chlorine was a poison not only for iron catalysts in ammonia synthesis, but also for ruthenium catalysts. It is widely accepted that chlorine had a disadvantageous effect on iron catalysts by the formation of the volatile potassium chloride with potassium promoter, which resulted in the elimination of potassium [40]. However, There is still some controversy about the influence of chlorine on Ru catalysts for ammonia synthesis, two different ideas usually were used for explaining the effect of chlorine: One is that the presence of chlorine was disadvantage to the opening of nitrogennitrogen triple bonds because Cl decreased the local electron density of Ru surface [41,42]. The other is that Cl activated some sites available for hydrogen adsorption, and then retarded the nitrogen activation. However, Narita et al. [21] also found that chlorine inhibited CO and H2 adsorption by site blockage for ruthenium catalysts supported on SiO2 and Al2O3. Iyagba et al. [43] proposed that structural rearrangement, rather than site blocking or electronic interactions, should be the primary mechanism of chlorine modification of the activity of CO hydrogenation for silica-supported Ru catalyst. Herein it is quite obvious that chlorine did not lead to the significant change in Ru 3d5/2 binding energies for Ru catalysts supported on Al2O3 or carboncovered alumina. It is no doubt that site blocking is an important factor for the influence of chlorine on their ammonia synthesis activity of Ru catalysts. But it is also obvious that this site blocking effect is closely related with the property of support material because the effect of chlorine on the activity of Ru catalyst supported on Al2O3 is much more significant than that of carbon-covered alumina supported Ru catalysts. More studies are necessary for fully understanding this issue. 4. Conclusions Uniformly carbon-covered alumina was prepared by pyrolysis of sucrose. The total pore volume and pore size decreased with the increase of carbon content, but the micropore surface and micropore volume increased, and the specific surface area first decreased and then increased. The presence of carbon had a limited impact on Ru particle size for CCA supported Ru catalyst, but increased Ru 3d5/2 binding energy. However, the change in the values Ru 3d5/2 binding energy had a neglectable influence on ammonia synthesis. Residual chlorine significantly suppressed hydrogen adsorption with medium desorption temperature. Carbon also decreased the amount of this type of hydrogen species for chlorine-free Ru catalysts supported on CCA, but did not change their ammonia synthesis activities. On the other hand, carbon can decrease the disadvantageous effect of chlorine on CO chemisorption and H2 adsorption, thus the activities for containing-chlorine samples and the ammonia concentration ratios between the containing-chlorine Ru catalysts and chlorine-free samples both increased with increasing carbon content. This result further confirmed the influence of chlorine on hydrogen adsorption was an important factor for catalytic activity of ammonia synthesis, and carbon in support materials may had a strong impact on ammonia synthesis over supported Ru catalysts by change the hydrogen adsorption. Acknowledgements The authors are grateful for the financial support from the National Key Technology R&D Program of China (2007BAE08B02) and Science and Technology Development Foundation of Fuzhou University (2008-XY-7).
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