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Kinetics, Catalysis, and Reaction Engineering

Morphology Effect of Ceria on the Catalytic Performances of Ru/CeO2 Catalysts for Ammonia Synthesis Bingyu Lin, Yi Liu, Lan Heng, Xiuyun Wang, Jun Ni, Jianxin Lin, and Lilong Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02126 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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Industrial & Engineering Chemistry Research

Morphology Effect of Ceria on the Catalytic Performances of Ru/CeO2 Catalysts for Ammonia Synthesis

Bingyu Lin, Yi Liu, Lan Heng, Xiuyun Wang, Jun Ni, Jianxin Lin, Lilong Jiang*

National Engineering Research Center of Chemical Fertilizer Catalyst, College of Chemical Engineering, Fuzhou University, Fuzhou 350002, Fujian, China Email: [email protected]

Fax: +86 0591-83738808; Tel: +86 0591-83731234

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Abstract Ru/CeO2 catalysts with different amount of surface oxygen vacancies were prepared by changing the morphology of CeO2. The conversion of Ce4+ to Ce3+ and the formation of Ru−O−Ce bonds led to enhancement of the amount of oxygen vacancies. Ru species of low crystallinity enriched with Ru4+ ions exist on the surface of CeO2 nanorods, while metallic Ru particles exist on CeO2 nanocubes. The low crystallinity of Ru species and high concentration of oxygen vacancies enhanced the adsorption of hydrogen and nitrogen, and also led to desorption of surface hydrogen in the form of H2. Therefore, Ru/CeO2 nanorods showed high ammonia synthesis activities. On the contrary, lower catalytic activity was observed over Ru/CeO2 nanocubes catalyst because H2 and N2 adsorption was less favorable plausibly due to the large particle size of Ru species and low concentration of oxygen vacancies, and most of the hydrogen species were consumed in H2O formation.

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1. Introduction Catalytic ammonia synthesis from hydrogen and nitrogen is one of the most important inventions in the 20th century because ammonia (NH3) is the detonator of the population explosion.1 Recently, ammonia has also been found to be an important energy storage intermediate and carbon-free energy carrier.2 As a consequence, management of the nitrogen cycle has been identified as one of the 14 Grand Challenges for Engineering in the 21st Century.3 Multiple efforts have attempted to develop a sustainable process for ammonia production through electrocatalysis, photocatalysis, plasma catalysis or enzyme catalysis.2, 4-9 However, the efficiencies are far below commercially viable levels. Up to now, ammonia is generally produced by means of the Haber − Bosch process with iron catalysts under the extreme temperature and pressure (450 °C, 200 bar) reaction conditions, and 1−2% of the global energy has been consumed to produce more than 130 million tons of ammonia per year.3, 10 Currently, carbon-supported Ru catalyst is the only one which can be used in commercial application of the low-pressure ammonia synthesis.11 However, their application prospect still have a dispute because of the poor stability of Ru/C catalysts.12,

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There is an urgent need for the active and long-lived catalysts for

sustainable ammonia synthesis. CeO2 has attracted much attention in the field of catalysis as a support, a promoter or a catalyst. The material was used in water-gas shift reaction,14-16 CO2 methanation,17, 18 CO oxidation,19-23 selective reduction of NO with NH3,24 oxygen reduction reaction,25 and so on. For all these applications, the 3



essence of CeO2 is its Ce4+/Ce3+ redox ability, oxygen storage capacity, thermal stability and unique electronic properties. Recently, Ma et al. prepared Ru/CeO2 catalysts (4% Ru metal) with different CeO2 morphology (nanocubes, nanorods, and nanoparticles), and simply attributed the differences in the ammonia synthesis activity to the surface composition/electronic structure and the exposed crystal planes of the CeO2 support.26 However, the interactions among the CeO2 morphology, oxygen vacancies, adsorption properties and ammonia synthesis activity of Ru/CeO2 have not been fully established. Recently, Li et al. have proposed that oxygen vacancies in BiOBr nanosheets could activate the adsorbed nitrogen molecule and reduce N2 to ammonia.27 Hirakawa et al. have found that the Ti3+ species on the oxygen vacancies were the adsorption sites for N2.28 Li et al. also have illustrated that surface oxygen vacancies in a amorphous TiO2 layer on rutile TiO2/Au nanorods enhanced the adsorption and activation of N2, and thus promote N2 reduction to NH3.29 These studies have confirmed that the surface oxygen vacancies exert a strong influence on N2 photofixation, and a similar phenomenon might be observed on a thermal process of ammonia synthesis. In the present work, CeO2 rods and CeO2 cubes (herein denoted as CeO2-r and CeO2-c, respectively) were prepared to support Ru, and the as-prepared Ru/CeO2 catalysts with high Ru loading (10 wt%) were tested in ammonia synthesis. It was observed that there is difference in terms of hydrogen and nitrogen adsorption, hydroxyl reaction pathway, and ammonia synthesis activity for Ru/CeO2-r and Ru/CeO2-c catalysts. 4


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2. Experimental 2.1 Catalyst preparation The cerium oxides were prepared following the hydrothermal method reported elsewhere.30,

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For CeO2-r (and CeO2-c), a mixed solution (60 mL) of 3 mmol

Ce(NO3)·6H2O and 360 mmol NaOH was hydrothermally heated at 100 °C (and 180 °C) for 24 h. Then the CeO2 precipitate was collected by centrifugation, washed with deionized water and ethanol until pH 7. Subsequently, the sample was dried overnight at 60 °C, and calcined in air at 550 °C for 4 h. For the synthesis of the Ru/CeO2 catalysts, ruthenium nitrosyl nitrate solution (1.5% w/v, Aladdin) was heated at 70 °C for 6 h, and then mixed with methanol. The as-obtained solution was used as Ru precursor, and Ru/CeO2 catalysts were prepared by impregnation. The catalysts were reduced in hydrogen at 450 °C for 6 h, and the loading of Ru was calculated to be 10 wt% (weight ratio) for the Ru catalysts. 2.2 Catalyst characterization XRD patterns were recorded on a PANalytical X'Pert3 Powder diffractometer with Cu Kα radiation at 45 kV and 40 mA. BET surface area and pore size distribution of Ru catalysts were measured by means of N2 adsorption-desorption at -196 °C using a Micromeritics ASAP 2020 apparatus. X-ray phototoelectron spectroscopy (XPS) analyses were performed on a Thermo Fisher Scientific ESCALAB 250 photoelectron spectrometer. Raman spectra were obtained from an InVia Reflex Raman microscope equipped with a 532 nm laser. Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 F20 microscope. Temperature programmed reduction 5



(TPR) studies were carried out using a Micromeritics AutoChem 2920 instrument equipped with a thermal conductivity detector, in which about 0.1 g catalyst (sieve fraction 0.30–0.56 mm) was heated to 600 °C at a rate of 10 °C/min in a flow of 10% H2/Ar mixture (30 mL/min). Subsequently, the sample was cooled down to 300 °C in H2/Ar mixture, then oxygen was injected (in pulses) until surface saturation. After cooled down to 50 °C, the catalyst was heated to 600 °C at a rate of 10 °C/min in a H2/Ar mixture to obtain the TPR profile of the catalyst pre-exposed to oxygen. Temperature programmed desorption (TPD) measurements were carried out on an Autochem 2920 instrument. In a typical experiment, the catalyst (100 mg) was reduced in hydrogen (30 mL/min) at 430 °C for 2 h, then flushed with Ar (30 mL/min, purity >99.99%) for 2 h. The sample was then exposed to hydrogen (H2-TPD), nitrogen (N2-TPD) or 2.5%N2-7.5%H2-90%Ar gas mixture (H2+N2-TPD) at 400 °C for 1 h and then cooled down to 50 °C. The sample was then flushed with Ar for 1 h and heated to 600 °C at a rate of 10 °C/min; the desorption signals were monitored by mass spectrometry (Hiden Analytical HPR-20). 2.3. Catalytic testing The ammonia synthesis activity was tested using a stoichiometric H2–N2 gas mixture in a stainless steel reactor, and these calculation equations have been given elsewhere 13, 32. Briefly, the samples (0.30 g, 32–60 meshes) were diluted with quartz powder (quartz powder/catalyst = 30 v/v) and reduced in a H2–N2 gas flow at 500 °C for 30 h. Under the adopted reaction condition (T = 400–430 °C, p = 10 MPa, and Sv = 70 dm3/h), the ammonia concentration in the outlet was obtained using a known 6


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amount of diluted H2SO4 solution (0.015 mol/L), and then the rate of ammonia synthesis was calculated.

3. Results 3.1 Structure and morphology XRD analyses were performed to study the structure and morphology of the CeO2 samples and Ru/CeO2 catalysts (Figure 1). As shown in Figure 1a, the peaks of the CeO2 samples can be indexed to those of face-centered cubic (fcc) structure (JCPDS 34-0394). The average crystallite size was 13.7 nm for CeO2-r, and 29.0 nm for CeO2-c (Table 1). For the Ru/CeO2 catalysts without undergone hydrogen reduction (Figure 1b), the diffraction patterns are similar to those of ceria (fcc), and there is no detection of signals assignable to Ru, Ce(OH)3 or Ce(OH)CO3. After hydrogen reduction at 450 °C for 6 h (Figures 1c and 1d), signals of metallic Ru (Ru0) can be found for the three catalysts, and the weak signals suggest that the Ru0 particles are well dispersed. The Ru0 peak intensity of Ru/CeO2-c was the highest, but the Ru0 crystalline size could not be estimated because of the low signal intensity. According to Huang et al.

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, the incorporation of Ru species (rRu4+ = 0.62 Å) into the

fluorite-like lattice (rCe4+ = 0.94 Å) would lead to decrease of lattice parameters and formation of Ru–O–Ce structure. Phenomenon of such nature was also observed by Hu et al. over Pd/CeO2.34 As shown in Table 1, two Ru catalysts do not show any decrease of CeO2 lattice parameter, indicating that the incorporation of Ru species into the fluorite-like lattice of CeO2 should not be significant. Compared to the CeO2 samples, the corresponding Ru/CeO2 catalysts are lower in specific surface area and 7



pore volume, plausibly a result of partial plugging of CeO2 pores by the Ru particles (Table 1). It is noted that with the loading of Ru on the CeO2 samples, there is a shift of pore size distribution to lower value (Figure S1). The TEM images of the Ru/CeO2 catalysts are shown in Figure 2. As shown in Figure 2a and 2b, the Ru/CeO2-r catalyst is composed of CeO2 nanorods 8–12 nm in diameter and 60–130 nm in length. Two lattice fringes attributable to (111) and (200) planes of CeO2 are detected in the HRTEM image of Ru/CeO2-r (Figure 2b), indicating the presence of CeO2 (110) and (100) facets, which is consistent with literature results.26, 30, 34, 35 In the HRTEM image of Ru/CeO2-c (Figure 2d), lattice fringe of 0.27 nm interplanar spacing attributable to (200) planes can be seen, which is a clear indication of CeO2 (100) facets.30, 36 A large number of Ru particles with size ranging from 1 to 5 nm (average size: about 2.0 nm) are found on the surface of Ru/CeO2-c. In contrast, the presence of Ru particles on the CeO2 supports of Ru/CeO2-p and Ru/CeO2-r is less apparent, suggesting high dispersion and low crystallinity of the Ru species. The average Ru particle sizes of Ru/CeO2 catalysts estimated using the results of H2 and CO chemisorption are depicted in Table 2. The sizes of Ru/CeO2-r particles are much smaller than that of Ru/CeO2-c, but are higher than that revealed in the TEM study of Ru/CeO2-c. The results suggest that certain amount of Ru metal is unavailable for H2 or CO chemisorption over the Ru/CeO2 catalysts. The percentage of Ru metal accessible to gas phase H2 or CO molecules might be dependent on the specific surface area and/or morphology of the CeO2 supports. However, Ru particles 8


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with size ranging from 1 to 5 nm can be clearly observed in the TEM image of Ru/CeO2-c with 3 wt% Ru loading (Figure S2 of SI), which has Ru surface density similar to that of Ru/CeO2-r. The results evident that the morphology of CeO2 supports has significant effect on the percentage of Ru metal accessible to gas phase molecules. Hence despite Ru particles are more apparent on the (200) planes of CeO2 as revealed in the TEM images, a higher percentage of Ru metal is available for gas adsorption in the case of Ru/CeO2-r catalyst. Based on the results of XRD analysis, the possibility of having the Ru species incorporated into the CeO2 lattice can be discarded. In other words, the Ru species are highly dispersed on the CeO2-r surfaces, indicating higher percentage of Ru metal accessible to gas phase molecules. The observation is in good agreement with the Ru/Pr2O3 results reported by Sato et al.37 They found that low-crystalline Ru nano-layers rather than Ru particles are present on the surface of Pr2O3. It is hence deduced that the distribution of Ru species in the three Ru/CeO2 catalysts is governed by the morphology of CeO2 supports. Similar deduction was made by Jones et al.38 who found that polyhedral ceria and ceria nanorods were more effective in the anchoring of platinum than ceria cubes when CeO2 was mixed with a Pt/La-Al2O3 catalyst followed by ageing in air. 3.2 Ceria defects We conducted Raman experiments to study the defects of the CeO2 samples and Ru/CeO2 catalysts (Figure 3). Four peaks are observed in the Raman spectra of the CeO2 samples: a strong one at 462 cm-1 and three weak ones at 258, 600, and 1175 cm-1, which can be related to F2g mode, second-order transverse acoustic (2TA) mode, 9



defect-induced (D) mode, and second-order longitudinal optical (2LO) mode of fluorite phase, respectively.33,

34, 36

Over the Ru/CeO2 catalysts, new peaks are

observed at 695 cm−1 and 975 cm−1, plausibly a result of interaction between CeO2 and Ru. In Raman spectroscopic studies on Ru interaction with CeO2, Satsuma et al. 39

and Huang et al.

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attributed these peaks to the formation of Ru−O−Ce bonds.

Moreover, the introduction of Ru species leads to red shift as well as broadening of the F2g peaks. The red shift to 449 cm-1 together with band broadening indicate change of Ce–O bond property as a result of Ru interaction, which is in line with the work of Wang et al. 41 In Raman investigation, the I(1175+600)/I462 ratios reflect the concentration of intrinsic defects such as oxygen vacancies.33, 34, 36 In the present study, the I(1175+600)/I462 ratios are small in value and are not significantly different across the CeO2 samples (CeO2-r: 0.07 and CeO2-c: 0.04), indicating that the number of defects are low in the CeO2 samples. However, the two Ru/CeO2 catalysts show I(1175+600)/I449 ratios of about 0.40 and 0.31 for Ru/CeO2-r and Ru/CeO2-c, respectively. The results suggest that the presence of Ru species leads to increase of ceria defects such as oxygen vacancies, which is consistent with literature results.33, 42 Furthermore, between the two catalysts, Ru/CeO2-c is the lowest in terms of defect concentration. As shown in Figure 4, the XPS Ce 3d spectra are composed of five pairs of 3d5/2 and 3d3/2 features. The three pairs with peaks at 882.5, 889.1, and 898.5 eV individually accompanied by peaks at 901.1, 907.6 and 916.6 eV are ascribed to Ce4+ species, and the other two pairs are attributed to Ce3+ species. Thus the Ce3+ 10


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concentration, which correlates with the amount of oxygen vacancies on ceria surfaces, can be estimated by calculating the area ratio of the Ce3+ peaks to all peaks,33, 41, 43-45 giving a ratio value of 0.18 and 0.20 for Ru/CeO2-r and Ru/CeO2-c, respectively. The similarity in Ce3+ concentration across the three Ru/CeO2 catalysts indicates that the chemical states of Ce species on the Ru/CeO2 catalysts are not obviously shape dependent, which is in good agreement with the results of Huang et al. 33 The Ru 3d5/2 peaks at 279.9 eV can be assigned to metallic Ru, while that at 281.2 and 282.2 eV is characteristic of ruthenium oxides.26, 33, 46, 47 As shown in Figure 5, the three Ru/CeO2 catalysts exhibit three Ru 3d5/2 peaks, which are characteristic of Ru0 and ruthenium oxides. The ratios of Ru0/( Ru0 + Ru4+) are approximately 27.3% and 32.5% for Ru/CeO2-r and Ru/CeO2-c, respectively. The results indicate that there are more Ru particles and/or clusters on the surface of CeO2-c, which is in accord with the TEM observation. On the other hand, in the cases of Ru/CeO2-r, there are more Ru4+ ions to interact with CeO2 to form Ru−O−Ce bonds, and thus a higher percentage of ruthenium oxides would be observed. On the other hand, the O 1s spectra of the three Ru/CeO2 catalysts are significantly different (Figure 6). The peaks 529.2–529.4 eV in binding energy can be assigned to lattice oxygen (Oα), those at 531.2–531.4 eV are characteristic of oxygen vacancies (Oβ) while those at 533−534 eV can be attributed to hydroxyl species. It is common to evaluate the concentration of oxygen vacancies using the Oβ/Oα ratio;33, 41 the Oβ/Oα ratios are 0.63 and 0.42 for Ru/CeO2-r and Ru/CeO2-c, respectively. The result 11



suggests that the concentration of oxygen defects on the surface of ceria in the case of Ru/CeO2-r is higher than that in the case of Ru/CeO2-c, which is consistent with the result of Raman spectroscopic investigation (Figure 3). However, there is no significant difference in Ce3+ concentration between the two Ru/CeO2 catalysts. The result indicates that besides the transformation of Ce4+ to Ce3+, there are other factors that contribute to the generation of oxygen vacancies. Li et al.48 found that the introduction of other ions (Gd, Zr, La, Sm, Y, Lu, and Pr) into the CeO2 lattice led to enhancement of defect sites (oxygen vacancies and MO8-type complex). Aranda et al.49 proposed that there was increase in concentration of surface oxygen defects on the surface of ceria due to copper incorporation. In this work, the incorporation of Ru species into ceria to form the Ru–O–Ce structure, which has been confirmed by XRD, Raman and XPS analyses, could also result in increase of oxygen vacancies. 3.3 Reducibility of Ru/CeO2 catalysts H2-TPR was deployed to investigate the reducibility of the Ru/CeO2 catalysts, and the results are given in Figure 7. For the Ru/CeO2 catalysts without undergone hydrogen reduction (Figure 7a), a broad hydrogen consumption peak in the temperature range of 75–250 °C is observed, showing maximum consumption at around 158 and 153 °C for Ru/CeO2-r and Ru/CeO2-c, respectively. Based on the peak areas, the corresponding hydrogen consumptions were estimated to be 9.3 and 7.2 mmol/g, and these values are much higher than the theoretical one for the reduction of RuO2 to Ru0 (1.8 mmol/g). The results suggest that over the Ru/CeO2 catalysts, there is the reduction of ruthenium nitrosyl nitrate as well as the reduction 12


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of surface oxygen species on CeO2. For the Ru/CeO2 catalysts with oxygen chemisorbed at 300 °C (Figure 7b) the reduction starts at 60 °C and maximizes at 98.5 °C in the cases of Ru/CeO2-r, while in the case of Ru/CeO2-c, it maximizes at 113 °C. The results suggest that the ruthenium oxides of Ru/CeO2-r can be more easily reduced than that of Ru/CeO2-c, which may be due to the high dispersion and low crystallinity of the Ru species on the former two catalysts. The amount of hydrogen consumption for Ru/CeO2-c (1.5 mmol/g) is far below the theoretical value for the reduction of RuO2, indicating that certain portion of Ru remains metallic during exposure to oxygen at 300 °C. On the other hand, the presence of Ru on CeO2-r can significantly enhance the surface reduction of CeO2 as well as the formation of hydroxyls, consuming more hydrogen as a consequence. The H2 consumptions is about 2.7 mmol/g for Ru/CeO2-r. 3.4 Adsorption properties of Ru/CeO2 catalysts The adsorption properties of the Ru/CeO2 catalysts were investigated by temperature programmed desorption/mass spectroscopy (TPD-MS). Figure 8 shows the TPD-MS profiles obtained after hydrogen (H2-TPD) or nitrogen (N2-TPD) pre-adsorption on the catalysts. Using AgO as standard, the amount of desorbed H2 and H2O was calculated based on the desorption signals of H2 (m/z=2) and H2O (m/z=18), and the results are depicted in Table 3. Only a trace amount of hydrogen was detected in H2-TPD study of Ru/CeO2-c (Figure 8a), while over Ru/CeO2-r, desorption of hydrogen is significant in the range of 55–400 °C. It is noted that a broad water desorption peak is also observed in this temperature range in the H2-TPD 13



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profiles of the three catalysts. By comparing the extents of hydrogen and water desorption (Table 3), we realize that the formation of water is the major way for hydrogen consumption over the Ru/CeO2 catalysts. No desorption of hydrogen or nitrogen can be detected in the N2-TPD profiles of the catalysts, but pronounced water desorption peaks are observed in the temperature range of 55–400 °C. However, unlike the H2-TPD results, the amount of water evolution in N2-TPD studies of the Ru/CeO2 catalysts decreases with a shift of H2O peak maximum to lower temperatures (Figure 8b). These results suggest that with hydrogen pre-reduction, the adsorbed sites of Ru/CeO2 catalysts have been taken up by the strong adsorbed H2, and these hydrogen species that would desorb in the form of water. It is well known that H2 molecules dissociate on the surface of CeO2-supported transition metals such as Ru and Pt, and the hydrogen atoms migrate to the surface of support 50 and react with Ce−O bonds to form hydroxyl groups.17, 51, 52 It is hence possible that the adsorbed hydrogen may exist as hydrogen atoms or in the form of hydroxyls. With the rise of temperature, the hydrogen atoms on Ru particles could either desorb in the form of H2 molecules (2H→H2) or react with hydroxyl groups to form water (OH+H→H2O). On the other hand, hydroxyl groups on Ru/CeO2 catalysts may interact to form water (2OH→H2O+Olattice+Ovacancy) or hydrogen molecules (2OH→H2+2Olattice).45,

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The

results of N2-TPD experiments show that most hydrogen leaves the catalyst in the form of water. It is largely so in the cases of H2-TPD studies because H2 desorption is much lower than water desorption. As shown in Figure 9, with the pre-exposure of the Ru/CeO2 catalysts to a gas mixture 14


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of 2.5%N2-7.5%H2-90%Ar, water was detected below 250 °C in the corresponding TPD profiles. Moreover, distinct desorption peaks corresponding to N2 and H2 appear simultaneously with the rise of temperature. The results clearly indicate that there is a certain synergistic effect between hydrogen and nitrogen during adsorption as well as desorption. Therefore, more hydrogen species and nitrogen molecule would desorb during TPD study. A comparison of the amount of hydrogen desorption in TPD studies with or without nitrogen adsorption reveals that the presence of nitrogen greatly enhances the amount of hydrogen species on the catalyst surfaces, which desorb mainly in the form of H2. Furthermore, the presence of more oxygen vacancies in Ru/CeO2-p and Ru/CeO2-r catalysts also lead to enhancement of the adsorbed nitrogen, which is in line with previous works over BiOBr nanosheets or TiO2.27-29 3.5 Catalytic activity Figure 10a shows the activities of the Ru/CeO2 catalysts, and it was observed that the rate of ammonia formation increases with increasing reaction temperature (400– 430 °C) over the three Ru catalysts. Under the same reaction conditions, the rates over Ru/CeO2-r are much higher than those over Ru/CeO2-c. The results clearly indicate that the condition for the preparation of CeO2 supports has a notable influence on the performance of the Ru/CeO2 catalysts in ammonia synthesis. No significant loss of activity is observed over Ru/CeO2-r and Ru/CeO2-c during a reaction time of 64 h at 430 °C. For the Ru/CeO2-r catalyst with further thermal treatment of 30 h at 530 °C, the activities increase initially and then decrease with time until a plateau is reached (Figure 10b). The increase of initial activity after heat treatment could be due to the 15



removal of strongly bonded hydrogen species during thermal treatment at 530 °C, in consistent with the results of TPD investigation. On the other hand, the insignificant change of catalytic activity over the catalysts with or without thermal treatment confirms that the as-prepared Ru/CeO2 catalysts are highly stable under the adopted conditions. In the absence of mass and heat transfer effects, the activation energy for ammonia synthesis was determined using the Arrhenius relationship (Figure S3). The activation energies of ammonia synthesis over Ru/CeO2-r and Ru/CeO2-c are 103.9 and 110.3 kJ/mol, respectively, indicating that the difference in reaction rates might be mainly related to the number of the accessible Ru sites. As shown in Table S1, under atmospheric pressure, our Ru/CeO2-r catalysts also show high ammonia synthesis rates, which are comparable to those of Ru-Ba/AC or Ru-Cs/MgO.53

4. Discussion With Ce4+/Ce3+ redox activity, oxygen storage capacity, and unique electronic properties, CeO2 is widely utilized in the field of catalysis. It is generally accepted that the catalytic performance of Ru/CeO2 catalysts for ammonia synthesis is closely related to the reduction of Ce species.26, 43, 54 However, in the present study, we investigated the shape effect of CeO2 on the performance of Ru/CeO2 catalysts in ammonia synthesis. The results suggest that there is no clear correlation between ammonia synthesis activity and Ce3+ concentration of the Ru/CeO2 catalysts that are different in CeO2 shape. Rather, it is the properties of CeO2 that have an effect on the dispersion of Ru particles and the chemical state of Ru species. The Ru species of and Ru/CeO2-r are low in crystallinity and high in oxide ratio, while Ru species larger in particle size and higher 16


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in Ru0 proportion are observed over Ru/CeO2-c. Due to property difference of the Ru species, there is variation in concentration of oxygen vacancies across the Ru/CeO2 catalysts. The TPD results showed that the formation of water is inevitable during heating of Ru/CeO2 catalysts in a hydrogen-containing gas flow (Figure 9). The presence of oxygen species on a Ru catalyst is beneficial to hydrogen adsorption through the formation of hydroxyl groups. However, the total amount of hydrogen accessible to Ru metal is fixed for a particular catalyst. There is competitive adsorption between hydrogen and nitrogen, and the active sites available for nitrogen adsorption could be occupied by hydrogen atoms

55-58

, which is consistent with the

observation of TPD study (Figure 9). As a consequence, the presence of oxygen vacancies enhances nitrogen adsorption, and exerts a strong influence on the reaction pathways of surface hydroxyls and the ammonia synthesis activities. Based on the above results and literature information,51,

52

reaction routes for the

formation of hydroxyl, water and ammonia on the Ru/CeO2 catalysts are proposed (Figure 11). For Ru/CeO2-r catalyst, dissociative adsorption of hydrogen is favored, and the adsorbed hydrogen desorbs as H2 at elevated temperature. Furthermore, it was reported that H2 may react with electropositive metal elements to form metal hydrides (H− ions), and react with metal oxides to form OH− ions (H+ state).59 The dissociation of H2 to H+ and H- ions has been found on the surface of oxides such as MgO,60 SrO,60 CaO60 and 12CaO·7Al2O3.61, 62 The H- ion can release two electrons and the proton reacts with an extra-framework oxide ion to form OH-. Hosono and coworkers61,

62

suggested that the formation of transient H- ions would alter the 17



rate-controlling step of ammonia synthesis, and the formation of N-Hn species rather than the dissociation of nitrogen-nitrogen triple bond is rate determining. It is envisioned that reactions of these kinds may take place during ammonia synthesis on the Ru/CeO2-r surfaces, resulting in enrichment of hydroxyl groups on the surface. The formation of −OH groups, which is inevitable in hydrogen-involved reactions over ceria-supported metals, may have a specific influence on catalytic performance. The presence of oxygen vacancies in CeO2 promotes the desorption of surface hydroxyls following the H2-formation pathway, which has been proven by Chen et al.51 Consequently, a noteworthy amount of H2 was detected in TPD study of Ru/CeO2-r catalyst that were exposed to a flow of “N2 + H2” mixture. With the enrichment of surface hydrogen species and the reaction pathway that involves transient H-, there is significant enhancement of ammonia synthesis activity over Ru/CeO2-r catalyst. For the Ru/CeO2-c catalyst that is large in Ru particle size and low in oxygen vacancy concentration, the amounts of adsorbed hydrogen and nitrogen species are low, and most of the surface hydrogen species are consumed in H2O formation. As a consequence, Ru/CeO2-c is inferior in ammonia synthesis. Nonetheless, in the case of using ruthenium carbonyl as precursor to prepare Ru/CeO2 catalysts (4% Ru metal) with different CeO2 morphology (i.e., nanocubes, nanorods, and nanoparticles), Ma et al. found that the one with CeO2 nanoparticles was the most inferior in ammonia synthesis.26 It is apparent that there is discrepancy between the results of Ma et al. and those of the present study. However, our catalysts show much higher ammonia synthesis rates, even under a high WHSV of 60000 mL gcat-1 h-1 over the samples with low 18


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Ru loading (Table S1). It is envisioned that besides CeO2 morphology, other factors such as, for example, Ru loading, promoter addition, and method for catalyst preparation, would have an effect on the performance of Ru/CeO2 catalysts. Investigation of such is ongoing in our laboratory.

5. Conclusions Ru species of low crystallinity enriched with Ru4+ ions exist on the surface of CeO2 polyhedrons and CeO2 nanorods. On CeO2 nanocubes, it is metallic Ru particles with size largely in the range of 1 nm to 4 nm. The presence of Ru species increases the amount of oxygen vacancies not only through the conversion of Ce4+ to Ce3+, but also via the formation of Ru−O−Ce bonds. The Ru4+ ions react with surface oxygen atoms to form Ru-O bonds and induce high concentration of oxygen vacancies. Overall, Ru/CeO2-r show a larger number of oxygen vacancies than Ru/CeO2-c. The low crystallinity of Ru species and high concentration of oxygen vacancies in the cases of Ru/CeO2-p and Ru/CeO2-r lead to enhancement of H2 and N2 adsorption, and enable the conversion of surface hydroxyls following a H2-formation pathway. On the other hand, lower catalytic performance was observed over the Ru/CeO2-c catalyst. It is because H2 and N2 adsorption is less favorable plausibly due to the large particle size of Ru species and the low concentration of oxygen vacancies, and most of the hydrogen species are consumed in H2O formation. With the behavior of CeO2-supported Ru catalysts in reactions that involve hydrogen understood, it is feasible to design active catalysts suitable for low-temperature ammonia synthesis. Supporting Information 19



Pore size distribution of samples, TEM image of Ru/CeO2-c catalyst with 3 wt% Ru loading and Arrhenius plots of reaction rate. Acknowledgements This work was supported by the National Natural Science Foundation of China (21776047, 21203028).

The authors declare no competing financial interest.

20


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Tables 24


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Table 1. Structure and physical parameters of samples

Samples

SBET (m2 g-1) a

Crystalline size (nm) b

Lattice parameters (nm)

CeO2-r

76

13.7

0.5415

CeO2-c

23

29.0

0.5413

Ru/CeO2-r

68

12.9

0.5421

Ru/CeO2-c

19

29.6

0.5416

a

Determined by BET method.

b

Estimated by Scherrer equation based on (111) reflection of CeO2.

Table 2. Average Ru particle size of Ru/CeO2 catalysts and percentage of Ru metal 25



Page 26 of 35

available for H2 or CO chemisorption Samples

dTEM (nm)

H/Ru

dH (nm)a

CO/Ru

dCO (nm)b

Ru/CeO2-r

-

0.31

3.6

0.20

5.6

Ru/CeO2-c

2.0

0.002

463

0.01

246

a

calculated by H2 chemisorption assuming H:Ru stoichiometry of 1:1.

b

calculated by CO chemisorption assuming CO:Ru stoichiometry of 1:1.

Table 3. TPD data of Ru catalysts Samples

H2-TPD H2 (mmol/g)

N2-TPD H2 O

H2+N2-TPD H2O

H2

(mmol/g)

(mmol/g) (mmol/g)

H2

H2O

(mmol/g) (mmol/g)

Ru/CeO2-r

0.02

0.47

--

0.34

0.29

0.48

Ru/CeO2-c

--

0.22

--

0.12

0.01

0.25

Figures 26


Page 27 of 35

(a)

(b)

CeO2 (34-0394)

Intensity (a.u.)

Intensity (a.u.)

CeO2 (34-0394)

20

30

40

50

60

70

80

CeO2-c

Ru/CeO2-c

CeO2-r

Ru/CeO2-r

90

100

20

30

40

2 Theta (degree)

(c)

50

60

70

80

90

100

2 Theta (degree)

(d)

CeO2 (34-0394) Ru (89-4903)

Ru (89-4903)

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ru/CeO2-c

Ru/CeO2-c Ru/CeO2-r 20

30

40

50

60

70

80

90

Ru/CeO2-r 100

36

2 Theta (degree)

38

40

42

44

46

48

50

52

54

2 Theta (degree)

Figure 1. XRD patterns of (a) CeO2 samples, (b) Ru/CeO2 catalysts without hydrogen reduction, and (c, d) reduced Ru/CeO2 catalysts.

27



Figure 2. TEM images of (a,b) Ru/CeO2-r and (c,d) Ru/CeO2-c.

28


Page 28 of 35

Page 29 of 35

Intensity (a.u.)

462

258

(a)

600 1175 CeO2-c CeO2-r

200

400

600

800

1000

1200

1400

-1

Raman Shift (cm ) 449

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

600

695 975 1175

Ru/CeO2-c Ru/CeO2-r 200

400

600

800

1000

1200

1400

-1

Raman Shift (cm )

Figure 3. Raman spectra of (a) CeO2 samples and (b) Ru/CeO2 catalysts.

29



u'"

u0

Intensity (a.u.)

u

v'"

v

u"

u' v'

v"

Ru/CeO2-c

v0

Ru/CeO2-r

875

880

885

890

895

900

905

910

915

920

Binding Energy (eV)

Figure 4. XPS Ce 3d spectra of Ru/CeO2 catalysts.

C1s

C1s+Ru3d3/2 metal

oxide

Ru3d3/2 Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ru/CeO2-c

Ru/CeO2-r

276

279

282

285

288

291

Binding Energy (eV)

Figure 5. XPS Ru 3d spectra of Ru/CeO2 catalysts.

30


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Page 31 of 35

Oα

Intensity (a.u.)

Oβ

Oγ Ru/CeO2-c

Ru/CeO2-r 526

528

530

532

534

536

538

Binding Energy (eV)

Figure 6. XPS O 1s spectra of Ru/CeO2 catalysts.

Intensity (a.u.)

(a)

Ru/CeO2-r

Ru/CeO2-c 50

100

150

200

250

300

550

600

Temperature (oC) Ru/CeO2-c

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

Ru/CeO2-r

50

100

150

200

550

600

Temperature (oC)

Figure 7. H2-TPR profiles of Ru/CeO2 catalysts (a) without hydrogen reduction and (b) hydrogen-reduced and then subjected to oxygen chemisorption at 300 °C.

31



5E-10

(a)

Intensity (a.u.)

4E-10 Ru/CeO2-r 3E-10

2E-10 Ru/CeO2-c 1E-10

H2O (m/z=18)

Ru/CeO2-r H2 (m/z=2)

Ru/CeO2-c 0E+00 50

100 150 200 250 300 350 400 450 500 550 600 o

Temperature ( C)

5E-10

(b) Ru/CeO2-r

4E-10

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3E-10

2E-10 H2O (m/z=18)

Ru/CeO2-c

1E-10

N2 (m/z=28) H2 (m/z=2)

0E+00 50

100 150 200 250 300 350 400 450 500 550 600 o

Temperature ( C)

Figure 8. TPD-MS profiles of Ru/CeO2 catalysts (a) H2-TPD and (b) N2-TPD.

32


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Page 33 of 35

2.4E-09

Ru/CeO2-r Intensity (a.u.)

1.8E-09

H2O(m/z=18) 1.2E-09 Ru/CeO2-c

Ru/CeO2-r H2(m/z=2) Ru/CeO2-c

6.0E-10

Ru/CeO2-r

X3

N2(m/z=28)

Ru/CeO2-c

0.0E+00 50

100 150 200 250 300 350 400 450 500 550 600

Temperature (oC)

Figure

9.

TPD-MS

profiles

of

Ru/CeO2

catalysts

exposed

to

a

2.5%N2-7.5%H2-90%Ar gas mixture.

0.35

(a)

Rate (mol

0.30

0.20 0.15 0.10 0.05

500 o

430 oC

Ru/CeO2-c

h-1) Rate (molNH3g-1 cat

g h )

0.25

530 oC, 30 h

(b)

Ru/CeO2-r

430 C 400

0.25

Ru/CeO2-r

0.20

300

0.15 200 0.10

Ru/CeO2-c

100

0.05

0.00

0.00 400

410

420

430

0

Temperature (oC)

20

Temperature ( °C )

0.30

-1 -1 NH3 cat

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40

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0 140

Time on stream (h)

Figure 10. (a) Ammonia formation rates of Ru/CeO2 catalysts and (b) Time on stream of ammonia synthesis over Ru/CeO2 catalysts.

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Figure 11. Proposed reaction pathways for the formation of hydroxyl, hydrogen, water and ammonia on the Ru/CeO2 catalysts

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