Accepted Manuscript

Title: Highly effective perovskite-type BaZrO3 supported Rucatalyst for ammonia synthesis

Author: Ziqing Wang Benyao Liu Jianxin Lin

PII: S0926-860X(13)00179-8DOI: http://dx.doi.org/doi:10.1016/j.apcata.2013.03.037Reference: APCATA 14159

To appear in: Applied Catalysis A: General

Received date: 25-12-2012Revised date: 5-3-2013Accepted date: 28-3-2013

Please cite this article as: Z. Wang, B. Liu, J. *lin3jx@fzu.edu.cn Lin, Highlyeffective perovskite-type BaZrO3 supported Ru catalyst for ammonia synthesis, AppliedCatalysis A, General (2013), http://dx.doi.org/10.1016/j.apcata.2013.03.037

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Graphical Abstract

Highly effective perovskite-type BaZrO3 supported Ru catalyst for

ammonia synthesis

Ziqing Wang, Jianxin Lin

National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou

350002, Fujian, China

Corresponding author: Tel.:+86-0591-83731234-8102

Email: lin3jx@fzu.edu.cn

1 2 3 4 5 6

0

2

4

6

8

10

12

Am

mo

nia

co

nce

ntr

atio

n (

%)

Pressure (MPa)

T = 673 K

GHSV = 10000 h-1

N2 + 3H

2 = 2NH

3

*Graphical Abstract (for review)

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Highlights

Perovskite type BaZrO3 is an excellent support of Ru catalyst for NH3

synthesis.

The presence of SMSI effect in Ru/BaZrO3 is advantageous to NH3

ammonia.

Pure phase Ru/BaZrO3 can be obtained by calcination of the precursor

in H2 atmosphere at 973 K.

The Ru particles can be highly dispersed on BaZrO3, despite the

surface area is only 7.0 m2/g.

The Ru/BaZrO3 catalyst is more attractive for industrial applications.

*Highlights (for review)

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Highly effective perovskite-type BaZrO3 supported Ru catalyst for

ammonia synthesis

Ziqing Wang, Benyao Liu, Jianxin Lin

National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University,

Fuzhou, Fujian, 350002 China

Corresponding author. Tel.:+86-591-83731234-8102

E-mail address:lin3jx@fzu.edu.cn

Abstract

The ammonia synthesis was performed over Ru catalyst supported on perovskite-type

BaZrO3. Physiochemical properties of this catalyst were also characterized by means of

various techniques to show the relationships between catalyst structure and catalytic

performance. It was found that the ammonia concentration in the effluent of Ru/BaZrO3

catalyst was 6.36% at 673 K under 3.0 MPa, which was much higher than that of Ru catalyst

supported on other supports including carbon material systems under the similar conditions,

and the rate did not decrease even if the reaction was executed for 120 h. This might be

attributed to the facts that the optimum size of Ru particles has more B5-type sites, strong

electron-donating ability of the BaZrO3 support and the strong metal-support interaction

provides an efficient channel for electron transfer. On the other hand, the adsorption of H2 was

inhibited over the surface of this catalyst and the number of active sites for activation of N2

was also increased.

Keywords: Barium zirconate; strong metal-support interaction; Ru catalyst; ammonia

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synthesis; H2 poisoning

1. Introduction

Catalytic synthesis of ammonia was considered to be a never-ending story not only because

NH3 is a very important feedstock for the chemical industry but also because the activation of

N2 is still one of the frontiers for inorganic chemistry [1]. In the past more than 100 years, lots

of approaches have been continuously proposed for synthesizing ammonia, such as transition

metal-based catalyst[2-4], metal complex catalyst[5], photocatalyst[6]，and electrocatalyst

[7,8]. However, only Haber-Bosch process based on fused iron catalyst have been

successfully applied in the ammonia synthesis in plant.

It is well known that the Haber-Bosch process must be carried out at a higher pressure,

which is connected with remarkable energy consumption for the syngas compression [9].

Therefore, to develop a completely new catalyst with high activity under lower pressure and

temperature is still a challenge. Carbon supported Ru catalyst was known to be the

second-generation catalyst and has been used in a commercial ammonia plant since 1992.

Unfortunately, carbon support is prone to formation of methanation in reaction under the

required temperature and pressure, which limits the catalyst lifetime. To overcome this

significant drawback, the stable oxides-supported ruthenium catalysts have been extensively

studied with the aim of preparing a high efficient Ru-based catalyst [10-14]. Up to now, the

catalytic activity of Ru catalysts supported on most of all oxides was much lower than that of

Ru/AC catalysts. On the other hand, the relationships between catalysts structure and activity

of Ru-based catalysts in the reaction of ammonia synthesis are still to be investigated.

ABO3 perovskite-type oxides are particular interesting because they exhibit high proton and

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oxygen ion conductivities, high electron conductivity, and excellent chemical stability over a

wide range of temperature, which has been used as catalyst widely [15]. Using

perovskite-type SrCe0.95Yb0.05O3 as a catalyst, Marnellos et al. [7] reported that the

measurable ammonia could be synthesized by an electrochemical process at a temperature as

high as 570 ℃. Yiokari et al. [16] found that the rate of ammonia synthesis over the

commercial Fe-based catalyst supported with In-doped perovskite-type CaZrO3 could be

enhanced by up to 1300%, which was known to be resulted from the nonfaradaic

electrochemical promotion modification of catalytic activity (NEMCA effect). At present, the

electrochemical synthesis of ammonia using perovskite-type oxides as proton-conducting

electrolyte has also been an important research topic in electric catalytic field [8,17].

However, the ruthenium catalyst directly supported on perovskite-type oxides for ammonia

synthesis has not been paid attention to extensively in Haber-Bosch process due to the low

surface area (< 10 m2/g).

Most recently, Hosono et al. have revealed that ruthenium particles supported on a stable

electride Ca24Al28O64 is the most efficient catalytic system for ammonia synthesis in

Haber-Bosch process [18]. It was found that the catalytic activity and TOFs over such a novel

catalyst was much higher than these of Ru catalyst supported on other supports. This result

was believed to be related with the high electron-donating power and reversible

hydrogen-storage property of this support. Our group has also synthesized a series of

Ba-doped ZrO2 materials and used them as supports of Ru catalyst for ammonia synthesis

[19]. It was found that these catalysts exhibited higher activity due to the presence of

perovskite-type BaZrO3. However, the dependency of the catalytic properties on the nature of

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BaZrO3 is still unclear. In the present work, the relationships between the physicochemical

properties and the catalytic behaviors of the Ru/BaZrO3 catalyst were investigated in detail.

2. Experimental

2.1 Support and Catalysts preparation

The BaZrO3 was prepared by the citric acid (CA) process according to the work reported

by Zou et al. as follows[20]. Stoichiometry amounts of nitrates, Ba(NO3)2 and

Zr(NO3)4· 5H2O were dissolved in 100 ml deionized water. Then, a mixture of 60 wt% citric

acid monohydrate and 40 wt% ethylene glycol was added to the cation solution. The obtained

mixture was homogenized by stirring at room temperature for 1 h. The obtained solution was

slowly evaporated at 353 K in the water bath until a clear yellow gel and then a brown resin

was formed. Then the obtained resin was thermally treated at 973 K for 3 h in air atmosphere.

The resulting white powder was labeled as BaZrO3 support.

Ruthenium catalyst was prepared by incipient-wet impregnating the BaZrO3 using a water

solution containing K2RuO4 for 6 h, after being dried in air at 373 K for 5h, the samples were

firstly reduced with absolute ethylalcohol, then treated with the feedstock gas (mixture of N2

and H2 at mole ratios is 1:3) at 823 K for 5 h, followed by cooling to room temperature in the

mixture atmosphere, which was labeled as Ru/BaZrO3. For comparison, ZrO2, MgO and CeO2

supported Ru catalysts were also prepared by the same way. The Ru content of all the samples

was set 4.0 wt%.

2.2 Measurements of catalytic activities

The catalyst activity for ammonia synthesis was measured in a stainless steel reactor. Before

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testing, the catalysts were activated in a flowing gas mixture of 75% H2 and 25% N2 from

ambient temperature to 723 K at 1 K/min (kept at 473 K, 573 K, 673 K, and 723 K for 2 h,

respectively) and then stabilized under the reaction conditions (i.e. 3 MPa, 673 K, 10 000 h−1

,

and H2/N2 volume ratio of 3) for more than 2 h. The ammonia concentration in the effluent

was determined by a chemical titration method [19].

2.3 Characterization of catalysts

XRD analysis was carried out on an X’Pert PRO diffractometer at 40 kV and 40 mA using

CoKα radiation (=0.1789 nm). N2 physisorption was measured on an ASAP 2010

(Micromeritics, USA). Then the surface area was obtained by BET equation. CO

chemisorption was carried out on an AutoChem 2910 instrument. Ru dispersion was

calculated from the cumulative volume of CO adsorbed during the pulse, assuming a

chemisorption stoichiometry CO/Ru=1:1[18]. H2 chemisorption was also carried out to

confirm the size and dispersion of Ru particles on the BaZrO3. The process was described

everywhere [21]. The H2-TPR were carried out with the same instrument with a 10%H2/N2

mixture and heated at a rate 10 K/min. A ca. 100 mg sample was used, with H2 consumption

monitored by TCD [11].

The H2-TPD was carried out using the same instrument as H2-TPR experiment. The

catalyst (100 mg) was treated at 773 K for 2 h under a flow of hydrogen. Then the gas flow

was switched to pure argon for purging at 773 K for 1h, and the gas flow was switched to H2

and the sample was cooled slowly to room temperature for 1 h. Then, the gas flow was

switched to pure argon (40 ml·min-1

). A stable baseline was established after a flow of argon

at room temperature for 1 h, and the catalyst was heated and the H2-TPD curve was recorded

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every 0.2 second. The effluent gas was analyzed with TCD.

The basicity of the supports was evaluated by temperature-programmed desorption of

carbon dioxide (CO2-TPD). The sample was treated at 773 K for 2 h under a flow of He

atmosphere (flow rate of 30 ml·min-1

). After being cooled down to room temperature, pure

CO2 was adsorbed at room temperature for 1.5 h. Then the physisorbed CO2 was removed by

a flushing with He at room temperature for 1.5 h. TPD was carried out in the stream of He (20

ml·min-1

) at a heating rate of 10 K up to 1073 K (all the samples were 100 mg).

TEM images were obtained by a Tecnai G2 F20 S-TWIN field emission transmission

electron microscope working at 200 kV. The Ru dispersion was calculated by the methods

proposed by Borodziński [22].

3 Results and discussion

3.1 Catalytic activity for ammonia synthesis of Ru/BaZrO3

Table 1 compares the activity for ammonia synthesis over Ru catalyst supported on

various supports in the present work and previous literatures under the given conditions.

Evidently, the activity of Ru/BaZrO3 is comparable to that of Ru supported on MgO, ZrO2 and

CeO2, despite the lowest surface area of BaZrO3 among these supports. The NH3

concentration in effluent (6.36%) of 673 K at 3 MPa is also almost similar to that of Ru/CeO2

(6.40%) at 10 MPa [12]. This activity over Ru/BaZrO3 is also remarkable higher than that of

other Ru catalysts at 10 MPa, such as Sm-Ru/Al2O3, Sm-Ru/CCA and rare-earth oxides

systems [10,11]. It also can be seen from Table 1 that the NH3 concentration has a distinct

advantage over carbon material systems under the same conditions including activated carbon

(AC), carbon molecular sieve (CMS) and active carbon fiber (ACF) [23,24]. Thus, it can be

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considered that the perovskite type BaZrO3 is also an outstanding support for Ru-based

catalyst in the reaction of ammonia synthesis.

It is very important for investigating the behavior of these catalysts under a high pressure,

because the industrial ammonia synthesis is often carried out. Therefore, the NH3

concentration was measured at various pressures over Ru/BaZrO3 at 673 K. As depicted in

Fig.1, It was found that the ammonia concentration is almost linearly with total pressure from

1.0 MPa to 6.0 MPa. This is very different from the reported catalysts Ru/MgO and

Ru/Al2O3 [13,25], demonstrating that the hydrogen poisoning phenomenon did not appeared

even under a higher pressure. Moreover, the ammonia concentration of Ru/BaZrO3 at 6 MPa,

673 K and 10000 h-1

was higher than that of K-Ru/C-SiC (10 MPa, 683 K and 10000 h-1

) [14],

indicating this catalyst also showed an excellent activity in the high-pressure conditions. The

catalyst stability of 4.0wt% Ru/BaZrO3 was tested under different conditions and result was

shown in Fig.2. It can be found that the activity of Ba/BaZrO3 catalyst did not decrease. It

notices that the activity of the catalyst treated at 723 K in feedstock gas for 20 h was almost

similar to that over untreated catalyst, implying that Ru/BaZrO3 has an excellent thermal

stability.

3.2 XRD

Fig.3 shows the XRD patterns of the BaZrO3 support, as-prepared and used catalyst

Ru/BaZrO3. All diffraction peaks in Fig. 3a can be indexed to the mixture of perovskite-type

BaZrO3 (JCPDS 06-0399) and an intermediate phase BaCO3 (JCPDS 005-0378). According to

the previous reports, it was a necessary condition for the formation of pure BaZrO3, in which

the treatment of the precursors was executed at a high temperature above 1273 K for several

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hours [20,26]. Thus, the pure BaZrO3 could not be obtained in the present study, because the

thermal treatment temperature was too low to decompose the BaCO3 completely. However, it

can also be found from Fig. 3b that the calcination in H2 atmosphere was favorable to the

decomposition of BaCO3 and decrease the temperature for BaZrO3 formation. This may be

resulted from the presence of H+

due to the dissociated H2 on the Ru atom surface [27]. The

similar results have also been found in Ru/Ba-MgO catalyst [28]. It notices that the diffraction

patterns of the used Ru/BaZrO3 catalyst remained almost unchanged compared with that of

the fresh catalyst, demonstrating the high stability of Ru/BaZrO3 catalyst under the reaction

conditions. On the other hand, the diffraction peaks ascribed to Ru species in support can not

be detected, which may be resulted from the good dispersion of Ru particles.

3.3 H2-TPR

The TPR profiles shown in Fig.4 suggest the reduction behavior of Ru catalysts supported

on BaZrO3 and ZrO2 as well as the interactions between Ru and supports. There was an

obvious reduction peak concentrated at 750 K in the profile of BaZrO3 support, which is

probably due to the reduction of the oxygen species on the surface or lattices [29]. In addition,

only one reduction peak at 504 K can be observed in the H2-TPR profile for Ru/ZrO2 catalyst,

which was usually attributed to the reduction of well-dispersed Ru species over the catalyst.

For Ru/BaZrO3 catalyst, a high intensity wide reduction profile range from 410 to 750 K

appeared at 556 K with a slight shoulder at 401 K, which is clearly different from that of

Ru/ZrO2 catalyst. According to report [30], the low-temperature peak at 401 K has been

assigned to the reduction of surface Ru species with a weak interaction with support.

Compared with the reduction temperature of Ru/ZrO3, BaZrO3 supported Ru showed the

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biggest peak at a higher temperature. Simultaneously, the reduction peaks at 750 K for

BaZrO3 support disappeared after Ru loading, implying that the presence of Ru metal on the

surface favored the reduction of the support. Thus, it was reasonable to propose that such a

higher reduction temperature for Ru/BaZrO3 would be resulted from the strong metal-support

interaction (SMSI) existing in the interface of Ru mental and reduced support. The similar

results have also been reported in many literatures [31,32].

3.4 CO2-TPD

According to the reports, the activity of Ru-based catalysts for ammonia formation was

closely related with the basicity of support [11,19,30,33]. Generally, the more basic is the

support, the higher is the activity of Ru catalyst for ammonia synthesis. Thus, the strong

basicity has been considered to be an indispensable property for an outstanding support all the

time. Fig.5 shows the TPD profiles of CO2, and the amount of weak, medium and strong basic

sites, expressed in μmol CO2.m-2

desorbed over the range of 330-400, 500-700, and >700 K,

respectively, was calculated and the results are listed in Table 2. Obviously, a small peak at

about 360 K was traced in the profile of ZrO2, which is ascribed to CO2 desorption on weak

basic sites. This may be the major reason why ZrO2 was not considered to be an outstanding

support of Ru-based catalyst for ammonia synthesis. As for BaZrO3 support, a sharp peak at

900 K can be observed, indicating the perovskite-type BaZrO3 is also an excellent solid base.

The amount of CO2 adsorbed on BaZrO3 was approximate 30.57 μmol/m2 , about 16 times of

that adsorbed on MgO (1.94 μmol/m2), indicating that the number density of basic sites on

BaZrO3 was higher than on MgO. The latter supported Ru is still known to be one of the most

active catalysts for ammonia synthesis for its basicity [21]. It was well accepted that the weak

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basic sites were probably associated with Brönsted basicity and most likely with the

lattice-bond OH groups, while the medium and strong sites were probably associated with

Lewis basicity, with the three- and four-fold-coordinated O2-

anions representing the strongest

among these sites [34]. In a word, only medium and strong basic sites have electron-donating

ability, which can provide Ru atoms with electrons and the activity can be enhanced

significantly. Thus, the order of electron-donating ability can be ranked as the amount of

medium and strong basic sites: BaZrO3 (30.50 μmol/m2)>MgO (1.44 μmol/m

2) >ZrO2 (0.39

μmol/m2) >CeO2 (0.21 μmol/m

2).These indicated that BaZrO3 support has a lot of basic sites

and high electron-donating ability

3.5 TEM

Fig.6 shows the typical TEM images for the used Ru/BaZrO3 with 4.0 wt% of Ru. It can be

seen that the used catalst showed the uniform particle size dispersion with a size of 1.5-5.8 nm.

Based on the observation of 30 particles, the average value of particles size was calculated to

be ca. 3.0 nm. High-resolution TEM image indicates that some of Ru particles are partially or

fully encapsulated by a thin amorphous overlayer.This is often attributed to the well-known

strong metal-support interaction (SMSI), which results in partially covering of Ru particles by

the reduced support species[32,35]. It was also found that Ru nanoscale particles could also

be well dispersed on the surface of BaZrO3 support. In fact, this has also been observed in

other low surface area materials supported metal systems, such as Ni/BaTiO3 (< 1.0 nm)[36],

Ru/BaCeO3 (1.6 nm)[31], Ru/TiO2 (1.6 nm)[37] and so on. The mentioned above are all

ascribed to the presence of SMSI effect, which can outweigh the cohesive forces in the metal

aggregates and inhibit the aggregation of Ru particles effectively. This maybe a main reason

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that the activity did not decrease at all during the reaction even treated at a high temperature.

To further confirm the size and dispersion of Ru particles as well as the SMSI effect, the

H2- and CO-chemisorption characterizations were also carried out. It can be found from

Table.3 that the Ru particle size observed by HRTEM is much smaller than that evaluated by

a cubical approximation model based on the H2- and CO-chemisorption for Ru/BaZrO3

catalyst, which is different from Ru/MgO and Ru/Al2O3 catalysts [10,21]. The similar result

was also reported in the most recent literature for Ru/C12A7 system [18]. Therefore, H2- and

CO chemisorption can not be used to determine the Ru dispersion in Ru/BaZrO3 system

because the chemisorption of H2 and CO did not appear in such a catalyst. Simultaneously,

the result also indicates the SMSI effect is well attested in this catalyst. Therefore, the

turnover frequencies (TOF) for ammonia formation at active sites should be calculated based

on the TEM image rather than chemisorption methods. Obviously, the TOF of Ru/BaZrO3 of

673 K at 3.0 MPa reaches 2.9×10-2

s-1

, which is still larger than that (1.1×10-2

and 2.3×10-2

s-1

)

of reported for K-Ru/AC and Ba-Ru/AC at the similar conditions[23].

3.6 H2-TPD

Although the dissociation of chemisorbed N2 over Ru catalysts usually was recognized as

the rate-limiting step of ammonia formation, the strong adsorption of H2 has also been found

to be a unique feature of Ru-based catalyst in the reaction of ammonia synthesis [10-12]. Thus,

the activation of N2 would be retarded by H2 adsorption on the surface of Ru, which could

also be a serious problem with Ru catalyst used at a high pressure under the industrial

conditions. Many investigations also found the H2 inhibition differs among the Ru catalyst

loaded on different supports or promoters [11,38]. Herein, H2-TPD was conducted to

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investigate the effect of supports on the H2 desorption. In our previous study, the high

temperature peak above 773 K could be assigned to spillover H2 associated with support,

which has no impact on the activity for ammonia synthesis [38]. Moreover, these hydrogen

species would not be desorbed at the reaction temperature range.

Fig.7 shows the H2-TPD profiles of the Ru catalysts supported on various supports. There

is an obvious peak for absorption of H2 centered at low temperature for all the samples, which

was usually assigned to chemisorption H2 on the surface of Ru particles [39]. In the present

work, it was found that the adsorption of H2 over Ru/BaZrO3 catalyst is similar to the support

BaZrO3, suggesting that this peak may be resulted from the reduced BaZrO3 or the physical

sorption H2 on support rather than the Ru phase. The reason for the appearance of peak at low

temperature is unclear at present, but the fact that the reaction activity was not influenced at

such a temperature. In addition, the broad peaks in the temperature range of 400 -750 K can

be observed in the profiles for all the catalysts except Ru/BaZrO3. Generally, this peak was

known as the adsorbed H2 on active sites at metal/support interface, which would exert a

strong impact on the catalytic activity of ammonia synthesis [38,39]. The strong adsorption of

H2 would possess lots of active sites and has negative effect to the activation of N2 which was

known to be the rate-limiting step for ammonia synthesis [2,10-12]. Based on the facts

mentioned above, it seems that the promotion effect of support in ammonia synthesis is

mainly due to releasing hydrogen poisoning for the activation of N2, which is good accord

with the result of the activity test at a high pressure, as shown in Fig.1. From the viewpoint of

suppressing adsorption of H2, the perovskite-type BaZrO3 can also be considered as an

outstanding support material for Ru-based catalyst in the reaction of ammonia synthesis.

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As described in many studies, the major control factors for ammonia synthesis should be

the number of active sites, the nature of Ru/support interface, and the electron-donating effect

of support [35]. The ammonia synthesis over Ru catalyst is a structure sensitive, and the

so-called B5-type site was regarded as the real active site. Previous investigations found that

the maximum probability for B5-type was found in the particles of 1.8 -2.5 nm [40-42]. In

our study, the TEM images of the used Ru/BaZrO3 catalyst showed an average Ru particle

size of 3.0 nm, approximated the optimum crystal size possessing more B5-sites, implying a

rather high utilization ratio of Ru and creating much more active sites for ammonia synthesis.

It is well known that the promoting effect of support is in terms of its electron-donating

capacity. Higher activity of Ru catalyst for ammonia synthesis was usually explained by

electron donation theory that electrons were transferred from their support or promoter to the

surface of Ru, leading to a decreased ionization potential of Ru, therefore allowing the

electron transfer from Ru metal to the anti-bonding orbits of N atom and reducing the

activation energy for the dissociative adsorption of N2 molecules [30]. Thus, MgO, as an

outstanding support, was widely researched in the past 40 years, because it was regarded as a

typical solid base with strong electron donation ability. However, Moroz et al [43] recently

found that the electrons could not be transferred directly from MgO to the surface of Ru and

modified the electronic state of ruthenium. In fact, MgO-supported Ru catalysts without

promoters showed a poor activity for ammonia synthesis, because it is an insulator [11,21].

Therefore, an ideal support for the Ru catalyst not only has an electron-donating ability, but

also has an electron transfer capacity. In a word, the strong basicity for an outstanding support

is only a necessary condition, but not a sufficient one. Fortunately, the SMSI effect itself is

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efficient channel for electron transfer from basic support to Ru in Ru/BaZrO3 system and

greatly enhanced the activity, which is good accord with the results in many reports

[2,31,44-47]. The electron-transfer processes were also described in terms of Fermi Energies

by Hosono et al. in Ru/Ca24Al28O64 system [18]. The interface on metal-semiconductor has

special electrical properties, which was known as advantageous to electron-transfer [48].

Moreover, the process for an electron transfer from base support to the supported mental has

been described in Pb/SrTiO3 system by Chung et al. via an electron energy-loss spectroscopy

[49].

The important limiting factors for industrial ammonia synthesis with supported Ru catalyst

are hydrogen poisoning and the thermal stability under high pressure conditions. Fortunately,

one of the most remarkable support effects for the perovskite-type BaZrO3 is suppression of

the H2 adsorption. Taking into the excellent activity and high stability, it can be concluded that

the novel Ru/BaZrO3 catalyst has a greatly promise to be used in commercial ammonia

synthesis in the near future. Of course, the activity especially under the high pressure should

be further improved.

4. Conclusions

We found that the perovskite-type BaZrO3 was an outstanding support material of Ru-based

catalyst for ammonia synthesis. The activity of Ru catalyst supported on this support was

much higher than that of other Ru catalysts supported on MgO, ZrO2, CeO2 and activated

carbon, especially at lower temperature and pressure. The strong metal-support interaction

(SMSI) was observed when the catalyst was reduced at a high temperature under the reducing

atmosphere, and the electrons can be transferred from the base BaZrO3 to the surface of Ru by

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means of the SMSI effect, which can promote the rate of N2 dissocation by weakening the N≡

N triple bond, resulting in significant enhancement of activity. The result of the H2-TPD

showed that H2 can not be adsorbed on the surface of Ru metal supported on BaZrO3, which

would increase the number of active sites for activation of N2. In a word, Ru/BaZrO3

catalyst has higher activity and stability under a high pressure and will also attract more

attention for industrial application in the future.

Acknowledgements

We would like to express our appreciation to Prof. Mingdeng Wei for his insightful remarks

and language editing. This work was supported by the Innovation Fund of China National

Petroleum Corporation under grant No. 2010D-5006-0502 and the National Science and

Technology Support Program of China under Grant No. 2007BAE08B02.

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Table captions

Table 1 The activity of Ru-based catalyst supported on various supports

Table 2 The results of CO2-TPD of various supports

Table.3 Mental size and dispersion of Ru/BaZrO3 derived based on chemisorption

measurement and TEM observation.

Figure captions

Fig.1 Effect of total pressure on the catalytic performance of Ru/BaZrO3 catalyst for ammonia

synthesis. Measurements were performed using a mixing of H2/N2=3 and STP of 10000 h-1

at

673 K

Fig.2 Rate of ammonia synthesis over Ru/BaZrO3 vs. time on stream. Measurements were

performed using a mixing of H2/N2=3, STP of 10000 h-1

and 3.0 MPa

Fig.3 The XRD patterns of (a) BaZrO3 support, (b) fresh Ru/BaZrO3, and (c) used Ru/

BaZrO3.

Fig.4 The H2-TPR patterns of (a) BaZrO3, (b) Ru/BaZrO3, and (c) Ru/ZrO2.

Fig.5 the CO2-TPD patterns of various supports (a) ZrO2, (b) MgO, (c) CeO2, and (d) BaZrO3.

Fig.6 TEM images of used Ru/BaZrO3 catalyst at 648 -698 K.

Fig.7 The H2-TPD patterns of (a) unsupported BaZrO3, (b) Ru/BaZrO3, (c) Ru/MgO, (d)

Ru/ZrO2, and (e) Ru/CeO2.

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Tables

Table 1 the activity of Ru-based catalysts supported on various supports

Catalysts Pressure

(MPa)

NH3concentrationa

(vol.%)

Reaction rate

(mmol/(gcatal.h))

Ref.

648 K 673 K 648 K 673 K

Ru/BaZrO3 3.0 4.15 6.36 10.18 15.30 Present study

Ru/MgO 3.0 0.79 1.28 5.17 8.38 Present study

Ru/CeO2 3.0 0.92 2.42 3.25 8.43 Present study

Ru/ZrO2 3.0 0.81 1.45 2.56 4.58 Present study

K-Ru/AC 3.0 2.00 4.80 _ _ [23]

Ba-Ru/AC 3.0 _ 5.60 _ _ [23]

Sm-Ru/Al2O3 10.0 3.00 5.50 _ _ [39]

Ru/CeO2 10.0 _ 6.40 _ _ [12]

Sm-Ru/CCA 10.0 _ 5.20 _ _ [10]

a: Experimental error of activity of ammonia synthesis is ca. 0.2 vol.%.

Table 2 the results of CO2-TPD of various supports

Sample SBET (m2/g) Adsorption CO2 (μmol/g) Total basicity

weak Medium strong μmol/g μmol/m2

BaZrO3

7 22 - 183 214 30.57

MgO 96 48 86 52 186 1.94

ZrO2 43 62 17 - 79 1.84

CeO2 68 149 8 6 163 2.40

Table(s)

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Table.3 Mental size and dispersion of Ru/BaZrO3 derived from chemisorption and

TEM

Methods Adsorption (×10-3

ml/g) Ru size (nm) Dispersion (%) TOFa (s

-1)

CO chemisorption 16.2 580.5 0.19 5.6

H2 chemisorption 4.8 1959.0 0.06 17.7

TEMb _ 3.0 37.33 2.9×10

-2

a: the value for 673 K and 3 MPa; b: the dispersion of Ru particles was calculated according the

formula in Ref [22].

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1 2 3 4 5 6

0

2

4

6

8

10

12

Am

mo

nia

co

ncen

trati

on

(%

)

Pressure (MPa)

Fig.1 Effect of total pressure on the catalytic performance of Ru/BaZrO3 catalyst for

ammonia synthesis. Measurements were performed using a mixing of H2/N2=3 and

STP of 10000 h-1

at 673 K

Figure(s)

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0 20 40 60 80 100 120 140

0

2

4

6

8

10

12

14

16

18

773 K

673 K

Rate

of

NH

3 sy

nth

esi

s (m

mo

l/(g

.h))

Time on stream (h)

673 K

Fig.2 Rate of ammonia synthesis over Ru/BaZrO3 vs. time on stream. Measurements

were performed using a mixing of H2/N2=3, STP of 10000 h-1

and 3.0 MPa

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10 20 30 40 50 60 70 80 90

aCO3

Inte

nsi

ty

()

(a)

(b)

(c)

aZrO3

Fig.3 The XRD patterns of (a) BaZrO3 support, (b) fresh Ru/BaZrO3, and (c) used Ru/

BaZrO3.

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300 400 500 600 700 800 900

(c)

(b)

750 K

504 K

556 KTC

D s

ing

nal

(a.u

.)

Temperature (K)

401 K

(a)

Fig.4 The H2-TPR patterns of (a) BaZrO3, (b) Ru/BaZrO3, and (c) Ru/ZrO2.

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300 400 500 600 700 800 900 1000

(d)

(c)

(b)

TC

D s

ing

nal(

a.u

.)

Temperature (K)

(a)

Fig.5 the CO2-TPD patterns of various supports (a) ZrO2, (b) MgO, (c) CeO2, and (d)

BaZrO3.

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Fig.6 TEM images of used Ru/BaZrO3 catalyst at 648 -698 K.

10 nm10 nm

(a)

5 nm5 nm

(b)

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300 350 400 450 500 550 600 650 700 750

a

(e)

(d)

(c)

(b)

TC

D s

ing

nal(

a.u

.)

Temperature (K)

(a)

Fig.7 The H2-TPD patterns of (a) unsupported BaZrO3, (b) Ru/BaZrO3, (c) Ru/MgO,

(d) Ru/ZrO2, and (e) Ru/CeO2.