Ammonia Synthesis Activity of Alumina-Supported Ruthenium Catalyst

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Ammonia Synthesis Activity of Alumina-Supported Ruthenium Catalyst Enhanced by Alumina Phase Transformation Bingyu Lin, Lan Heng, Biyun Fang, Haiyun Yin, Jun Ni, Xiuyun Wang, Jianxin Lin, and Lilong Jiang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03554 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Catalysis

Ammonia Synthesis Activity of Alumina-Supported Ruthenium Catalyst Enhanced by Alumina Phase Transformation Bingyu Lin*, Lan Heng, Biyun Fang, Haiyun Yin, Jun Ni, Xiuyun Wang, Jianxin Lin, Lilong Jiang* National Engineering Research Center of Chemical Fertilizer Catalyst, College of Chemical Engineering, Fuzhou University, Fuzhou 350002, Fujian, China E-mail: [email protected] (Bingyu Lin); [email protected] (Lilong Jiang); Fax: +86 0591-83738808; Tel: +86 0591-83731234

Abstract The increase of alumina calcination temperature from 800 to 1300 °C results in the transformation of γ-Al2O3 to α-Al2O3 phase accompanying with decrease of specific surface area and the amount of tetrahedral Al3+ sites. Over Ru-Ba/alumina catalysts, an increase in alumina calcination temperature would broaden the size distribution of Ru particles, enlarge the metal-to-oxide ratio of Ru, decrease the amount of surface hydroxyl groups, as well as lower the temperature for N2 desorption. As a result, the increase of alumina calcination temperature lessens the effect of hydrogen poisoning and decreases the activation energy for ammonia synthesis. The Ru-Ba/Al2O3 catalyst with alumina calcined at 980 °C having both θ-Al2O3 and α-Al2O3 shows ammonia synthesis rate three times higher than that with alumina calcined at 800 °C having γ-Al2O3 phase.

Keywords: Alumina; Phase Transformation; Ru Catalyst; Hydroxyl groups; Ammonia Synthesis

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1. Introduction Catalytic synthesis of ammonia from hydrogen and nitrogen has been claimed to be one of the most important inventions of the 20th century because ammonia is indispensable for the production of artificial fertilizer.1 Recently, ammonia is also considered as an energy carrier,2-3 and there is much interest in the development of environment-benign processes for ammonia synthesis. However, among options such as photocatalysis, electrocatalysis, plasma catalysis and enzyme catalysis,2,

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the heterogeneous Haber−Bosch process is the only one that is

commercialized. Currently, ammonia is produced at high temperature and pressure (400−500 °C, 20−30 MPa) using iron catalyst,9 consuming 1−2% of the world’s energy yearly.10-11 During the past decade, extensive studies have been focused on the exploration of Ru catalysts for ammonia synthesis under mild condition, and carbon-supported Ru catalysts have been successfully used industrially at pressure as low as 100 bar.12 However, the presence of Ru species would catalyze carbon methanation,13-14 which was usually considered to be the key reason for the deactivation of Ru/C catalysts. Iost et al. proposed that the oxygen-containing carbon species were much more easily gasified to form methane than other carbon species.15 Lin et al. reported that besides carbon methanation, carbon oxidation to form CO was also observed during heat treatment carbon-supported Ru catalysts, consequently affecting the activity and stability of Ru/C catalysts.16 Compared to alkali compounds, Ba is catalytically less active for carbon gasification (CH4 and CO),17-18 thus a Ru/C catalyst of high stability can be prepared by using Ba as promoter. On the other hand, the presence of oxygen surface groups not only facilitated the dispersion of Ru species,15 but also changes the properties of adsorbed hydrogen species.19 In such a case, the removal of oxygen surface groups could lead to conflict between catalytic activity and catalyst stability, and the excellence of carbon-supported Ru catalysts for ammonia synthesis cannot be fully realized. Despite much research efforts, It is very urgent to develop oxide-supported Ru catalysts those are high activity and stability in ammonia synthesis. It is well known that the rate-determining step of ammonia synthesis is the dissociation of N2,20-22 which can be significantly promoted by having electrons donated to Ru.22-23 As a consequence, compounds with low electronegativity have been considered ideal supports for Ru catalysts, and MgO was claimed to be the most suitable.24 Nonetheless, beside N2 dissociation, other surface interactions would also exert a strong influence on the activity of a catalyst in ammonia synthesis. Theoretical result showed that the dissociation of N2 dissociation might follow a H-assisted N2 activation pathway on Ru(0001) surfaces.25 On the other hand, the reaction order of H2 over Ru catalysts could even be -1 because there is competition between the adsorption of H2 and that of N2.22, 26 In industrial applications, the active sites of Ru powder or Ru catalysts available for N2 dissociation and ammonia synthesis could be occupied by adsorbed 2


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hydrogen species.27-31 As a result, the activity of Ru catalysts does not increase with the increase of reaction pressure, because hydrogen poisoning of this kind becomes more significant at high pressure.32-35 Recently, it was demonstrated that Ru catalysts supported on oxides of high electronegativity show catalytic activity higher than that of Cs-Ru/MgO as a result of the participation of support materials in the adsorption/desorption of reactant gases. For example, Kitano et al. proposed that Ru/12CaO•7Al2O3 showed higher ammonia synthesis activities because of the presence of reversible storage–release of hydrogen atoms on the surface of 12CaO•7Al2O3 electride near the Ru particles.36 We found that the shapes of ceria strongly affected the activities of Ru catalysts in ammonia synthesis by changing the amount of adsorbed hydrogen and nitrogen as well as altering the desorption pathway of adsorbed hydrogen species.37 Li et al. suggested that the presence of TiO2 layer on TiO2/Au nanorods created surface oxygen vacancies, which enhanced the ammonia synthesis rate by promoting the adsorption and activation of N2.4 It is therefore envisioned that certain oxides which were considered to be unsuitable for ammonia synthesis could be used as supports to prepare high-efficient Ru catalysts if, by so doing, the adsorption properties of H2 and N2 can be appropriately tuned. γ-Al2O3 is often used as a support because of its high specific surface area and adjustable physicochemical properties. However, γ-Al2O3 was reported to be an inferior support for Ru catalysts in ammonia synthesis because of its high electronegativity.23 Nonetheless, further exploration on the application of alumina in ammonia synthesis is still necessary because alumina has many crystalline phases (γ, η, κ, δ, θ, α, etc.).38 In the present work, alumina was calcined at different temperatures for phase alteration before being used as support for Ru-Ba catalysts. The results reveal that catalytic activity of Ru-Ba/Al2O3-980 (with alumina calcined at 980 °C) is three times higher than that of Ru-Ba/Al2O3-800 (with alumina calcined at 800 °C). Furthermore, the calcination of alumina at 980 or 1300 °C could effectively prevent Ru metal from hydrogen poisoning, and such a discovery is important for the development of efficient Ru catalysts for ammonia synthesis. 2. Experimental Section 2.1 Preparation of ruthenium catalysts Commercial alumina (WYA-251, Wenzhou Jingjing Alumina Co., Ltd.) was calcined in air for 4 h at different temperatures (i.e., 800, 900, 980 and 1300 °C), and the samples are herein named as Al2O3-x, where x stands for the calcination temperature. Ruthenium(III) nitrosyl nitrate solution (1.5% w/v, Aldrich) was introduced onto the

calcined alumina by incipient wetness impregnation to prepare Ru/Al2O3, and the weight ratio 3


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of Ru to Al2O3 was ca. 5 wt%. The as-prepared samples were reduced in hydrogen at 450 °C for 6 h, and then impregnated with aqueous solution of barium nitrate to prepare the Ba-promoted Ru catalysts. The weight ratio of Ba to Al2O3 was 6 wt%, and the samples are herein denoted as Ru-Ba/Al2O3-x. 2.2 Characterization XRD patterns of the samples were recorded on a PANalytical X'Pert3 Powder diffractometer. GSAS software was used to perform Rietveld refinement. The textural properties of alumina were measured from the nitrogen isotherms at −196 °C using in a Micromeritics ASAP 2020 apparatus. Raman spectra of the catalysts were acquired on an InVia Reflex Raman microscope equipped with a 532 nm laser. A Bruker Avance III 500 MHz was used to obtain NMR data of Ru catalysts. Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 F20 microscope. The average size of Ru particles (dTEM) and the dispersion (DTEM) were calculated as: 𝑑𝑇𝐸𝑀 =

𝐷𝑇𝐸𝑀 = 1.23

𝑑𝑎𝑡 ∙ 3.32 𝑑𝑇𝐸𝑀

𝐷𝑇𝐸𝑀 =

∑𝑖𝑛𝑖 ∙ 𝑑3𝑖 ∑𝑖𝑛𝑖 ∙ 𝑑2𝑖 for 0.2 ≤ 𝐷𝑇𝐸𝑀 ≤ 0.92

d𝑎𝑡 ∙ 5.01 𝑑𝑇𝐸𝑀

for 𝐷𝑇𝐸𝑀 < 0.2

Where dat is the atomic diameter of Ru (dat = 0.269 nm) and ni is the number of particles with diameter di.39 Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was carried out on a Nicolet 6700 spectrometer. Following H2 reduction at 450 °C for 4 h, the sample was purged with He for 30 min and then cooled down to 50 °C in flowing He. Afterward, the background spectrum was taken, and then 5% CO/He (50 mL/min) was introduced into the sample for 10 min. Finally, the sample was purged with He for 10 min and the DRIFTS spectra were recorded. X-ray phototoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB 250Xi photoelectron spectrometer (Thermo Fisher Scientific). Prior to measurements, each Ru-Ba catalyst was reduced in a pretreatment chamber attached to the spectrometer at 450 °C for 4 h in a flow of 5%H2/Ar mixture (30 mL/min). The XPS binding energy was calibrated against the C1s peak at 284.6 eV of adventitious carbon. Temperature-programmed reduction of carbon monoxide (CO-TPR) was carried out on a Micromeritics AutoChem II 2920. The sample (100 mg, 0.30–0.56 mm) was treated in hydrogen at 450 °C for 4 h, and then purged with Ar and cooled down to 50 °C. Afterward, the sample was heated in flowing CO with a rate of 10 °C/min, and the signals were recorded by a mass 4


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spectrometer (Hiden Analytical HPR-20). For H2-TPR, the sample was treated in 9% O2/Ar mixture at 150 °C for 0.5 h, and then heated in a 10% H2/Ar flow from 50 to 900 °C at a rate of 10 °C/min. Temperature-programmed desorption (TPD) experiments were performed using the same apparatus. Following H2 reduction at 450 °C for 4 h, the sample was cooled to 400 °C in flowing Ar. Afterward, the sample was exposed to nitrogen (N2-TPD) or 3.3%N2-10%H2-Ar mixture for 1 h and then cooled down 50 °C. Subsequently, the sample was purged with Ar gas, and then heated in an Ar flow to 900 °C at a rate of 10 °C/min. The mass signals corresponded to N2, H2 and H2O were monitored by mass spectrometry. Before NH3-TPD, alumina adsorbs NH3 at 120 °C for 1 h, and then purged with Ar gas, subsequently, the sample was heated in He to 500 °C (10 °C/min). For H2-O2 titration experiment, a certain amount of the reduced catalyst was pretreated at 100 °C in flowing 9% O2/Ar mixture (30 mL/min) for 1 h in order to remove surface hydrogen species that were adsorbed on Ru. Afterward, Ar was used as carried gas (30 mL/min), and H2 was pulsed into the Ar flow. The titration would end when the area of the H2 peaks came to a constant. 2.3 Activity measurements The catalytic activities of Ru catalysts for ammonia synthesis were measured in a stainless steel reactor as previously described.16, 40 Typically, 0.3 g of a sample (32–60 meshes) was diluted with quartz powder and reduced in a stoichiometric H2–N2 gas mixture at 450 °C for 6 h. After the reactor was cooled to 400 °C under the H2–N2 flow, the reaction pressure was adjusted to 1 MPa. The ammonia in the effluent with a space velocity of 60000 mL g-1 h-1 was trapped by a known amount of diluted H2SO4 solution (0.015 mol/L), and the NH3 synthesis rate was calculated. Turnover frequency (TOF) was calculated having the ammonia synthesis rate divided by the total number of Ru atoms. 3 Results 3.1 Characterization of alumina samples As shown in Table 1, the BET surface area of Al2O3-x decreases with increase of calcination temperature, which is in consistent with previous reports.41-44 The XRD patterns of Al2O3-x samples are shown in Figure 1. All the peaks attributable to γ-Al2O3 are observed in the XRD pattern of Al2O3-800. The observation is in accord with previous results.41, 45-46 Over Al2O3-900, peaks at 31.8°, 32.9°, 56.7°, 59.9° and 64.5°assignable to θ-Al2O3 phase were also detected.42 In the case of Al2O3-980, the characteristic diffraction peaks of α-Al2O3 are observed together with the peaks of θ-Al2O3 phase, indicating the coexistence of the θ-Al2O3 and α-Al2O3 phases. With increasing calcination temperature, the intensities of the α-Al2O3 peaks grow while those of θ-Al2O3 significantly decrease. Ultimately only the diffraction peaks of α-Al2O3 are identified on 5


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Al2O3-1300, indicating that alumina has been transformed to α-Al2O3 at 1300 °C, which is in good agreement with reported results.41-44 A similar conclusion on the phase transformation of alumina can also be drawn from Rietveld analysis (Figures S1-S4 and Table S1). Table 1 BET surface areas, pore volumes and pore size of the alumina samples Samples

BET surface area (m2/g)

Pore Volume (cm3/g)

Pore Size (nm)

Al2O3-800

130

0.56

13

Al2O3-900

89

0.54

19

Al2O3-980

46

0.38

28

Al2O3-1300

11

0.04

15

Al2O3 (80-0786) Al2O3 (79-1559) Al2O3 (79-1558)

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

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Al2O3-1300 Al2O3-980 Al2O3-900 Al2O3-800

10

20

30

40

50

60

70

80

90

2 Theta (degree)

Figure 1 XRD patterns of Al2O3-x samples. 27Al

magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of alumina are

presented in Figure 2. Two resonances at approximately 10 and 67 ppm assignable to octahedral and tetrahedral Al, respectively,42, 45-55 could be observed in the spectrum of Al2O3-800. Referring to the 27Al MAS NMR studies have reported the pentahedrally coordinated aluminium,45-46, 48, 52-53 there is no detection of any signals that can be assigned to pentahedral Al over Al2O3-800. The result suggests that in the commercial alumina used in the present study, the amount of pentacoordinate Al3+ sites is low, which is in line with previous works.49-51 The tetrahedral/octahedral peak area ratio reflects the difference in the occupancy of cationic sites in alumina,47, 50 and such ratio for Al2O3-800 is approximately 0.35. As shown in Figure 2, there is no significant change in the NMR spectra when the calcination temperature was raised from 800 to 900 °C. Nonetheless, when calcination temperature was changed to 980 °C, the position of the octahedral Al3+ peak shifts toward ∼15 ppm, and there is an increase of peak intensity, indicating the formation of α-Al2O3 phase.48, 54 Moreover, the presence of resonance corresponding to tetrahedral Al3+ in the NMR spectrum of Al2O3-980 suggests the co-existence of other phase(s) (most probably θ-phase). The result is in harmony with that of XRD 6


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observation. The tetrahedral/octahedral ratio for Al2O3-900 and Al2O3-980 is 0.29 and 0.24, respectively, indicating that the increase of calcination temperature results in decrease of tetrahedral/octahedral vacancy proportion in the Al2O3-x samples. On the other hand, no NMR signal of tetrahedral aluminum ions could be observed for Al2O3-1300, suggesting the full establishment of the α-phase. A broad ammonia desorption peaks at 130−400 °C can be found in the NH3-TPD profiles for all Al2O3-x, and the total peak intensities decrease with increase of alumina calcination temperature (Figure S5). This result indicates that heat treatment of alumina at high temperautre might lead to the depletion of acid sites, which might be directly correlate with the loss of hydroxyl groups. Octahedral

Tetrahedral Al2O3-1300 Al2O3-980 Al2O3-900 Al2O3-800 100

80

60

40

20

0

-20

Chemical shift (ppm)

Figure 2 27Al MAS-NMR spectra of alumina samples. 3.2 Characterization of Ru-Ba catalysts The TEM and HRTEM images of alumina-supported Ru-Ba catalysts are shown in Figure 3 and Figure S5, respectively. As shown in Figure 3, the alumina of Ru-Ba/Al2O3-800 shows a morphology of agglomerated crystallites with irregular shapes, and the rise of alumina calcination temperature leads to the agglomeration of small alumina crystallites to large crystals, which is consistent with previous result.54 Most of the Ru particles of Ru-Ba/Al2O3-800 and Ru-Ba/Al2O3-900 are round shaped (Figure 3 and Figure S6), whereas ellipse shaped Ru particles are detected in the TEM images of Ru-Ba/Al2O3-980 and Ru-Ba/Al2O3-1300. Over Ru-Ba/Al2O3-1300, fairly round nanoparticles can still be seen at the edge of large alumina crystals (inset figure of Figure 3d), and the size distribution of Ru particles is broadened. Over all, the mean sizes of Ru particles increase significantly with the increase of alumina calcination temperature. Temperature-programmed reduction experiments of the oxidized catalysts (Figure S7) show that the temperature of maximum hydrogen consumption decreases with increasing alumina calcination temperature. Therefore, it is envisioned that the interaction between Ru particles and alumina weakens with the rise of alumina calcination temperature, especially in the 7


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case of Ru-Ba/Al2O3-1300. Similar to the deduction made over Pt/Al2O3 catalyst,53 we envisage that weak interaction between Ru species and alumina would result in an increase of Ru particle

size and enhanced formation of metallic Ru upon H2 reduction. Figure 3 TEM images of alumina-supported Ru-Ba catalysts (a) Ru-Ba/Al2O3-800, (b) Ru-Ba/Al2O3-900, (c) Ru-Ba/Al2O3-980 and (d) Ru-Ba/Al2O3-1300. Figure 4 displays the CO-DRIFTS spectra of the reduced Ru-Ba catalysts, and IR signals at 1356, 1579, 1972, 2048 and 2169 cm−1 are detected for all catalysts. The peaks at 1356 and 1586 cm−1 are characteristic of formate-associated surface hydroxyl on alumina,56-58 and the intensity of these IR peaks decrease with increase of alumina calcination temperature. Furthermore, the bands corresponding to hydroxyl groups,59 which are located in the 2800−3750 cm−1 range, also decrease in intensity. It has been reported that the surface hydroxyl groups react with CO to from H2 (CO + OH ↔ CO2 + H2).40, 60-63 In view of that, we performed CO-TPR over the Ru-Ba/Al2O3-x catalysts (Figure S8), and observed that with the increase of alumina calcination temperature, there is decrease in the amount of H2 evolution. It is hence deduced that the calcination of alumina at high temperatures would result in the depletion of hydroxyl groups for Ru catalysts.

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1579

2048 1972 2169

1356

Absorbance (a.u.)

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ACS Catalysis

Ru-Ba/Al2O3-800

Ru-Ba/Al2O3-900

Ru-Ba/Al2O3-980 Ru-Ba/Al2O3-1300 1200

1400

1600

1800

2000

2200 2800

3500

Wavenumber (cm-1)

Figure 4 DRIFTS spectra of Ru catalysts obtained after exposing the reduced samples to CO at 50 °C for 10 min followed by He purging for 10 min. Three peaks at 1972, 2048 and 2169 cm−1 ascribable to multicarbonyl CO species on Ru0 and partially oxidized Ru (Run+),58,

64-67

could be detected in the DRIFTS spectra for all

Ru-Ba/Al2O3-x catalysts. The intensity of these bands decreases with increase of alumina calcination temperature, indicating sintering of Ru particles, and hence diminution of Ru active sites available for CO adsorption. The results also suggest that Ru exists in oxidized as well as metallic form, which is in harmony with the results of Raman analysis (Figure S9). As shown in the Raman spectra, Ru-Ba/Al2O3-800 shows three relatively weak and broad bands at 499, 616 and 694 cm−1, corresponding to Eg, A1g, and B2g modes of RuO2, respectively. 68-70 The increase of alumina calcination temperature leads to the disappearance of the Raman peak at 499 cm−1 along with the appearance of a new peak at 401 cm−1. The latter was observed in the Raman spectra of Ru/TiO2 upon in-situ hydrogen reduction,69 but the adscription of this band is still uncertain. The coexistence of metallic Ru and oxidized Ru in the Ru-Ba/Al2O3-x catalysts can also be verified by XPS analysis. It is found in Figure S10 that the as-prepared Ru catalysts exhibit two Ru 3d5/2 peaks at 281.1 and 282.5 eV, which can be attributed to RuO2 and RuO3, respectively.37, 71-73

Over Ru-Ba/Al2O3-800, there is the detection of an additional Ru 3d5/2 peak at 279.7 eV

(Figure 5). The binding energy of this peak decreases with increase of alumina calcination temperature, and is 279.3 eV for Ru-Ba/Al2O3-1300. Referring to the work of Elmasides et al.,

72

we assign this peak to Ru0 clusters of low nuclearity. According to the Ru 3d5/2 peaks, the Ru0/(Ru0+Run+)

ratios

for

Ru-Ba/Al2O3-800,

Ru-Ba/Al2O3-900,

Ru-Ba/Al2O3-980

and

Ru-Ba/Al2O3-1300 are ca. 37.8%, 52.4%, 68.9% and 77.1%, respectively. As mentioned before, the increase of alumina calcination temperature promotes the formation of metallic Ru, which is due to the weakened interaction between Ru particles and alumina.

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279.3 Ru3d metal oxide

Intensity (a.u.)

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C1s

Ru-Ba/Al2O3-1300

279.4

Ru-Ba/Al2O3-980

279.6

Ru-Ba/Al2O3-900 279.7 276

279

Ru-Ba/Al2O3-800 282

285

288

291

294

Binding Energy (eV)

Figure 5 XPS Ru 3d spectra of Ru-Ba/Al2O3-x catalysts. 3.3 Ammonia synthesis over Ru-Ba/Al2O3-x catalysts The ammonia synthesis rates, turnover frequencies (TOFs) and activation energies are depicted in Table 2. The increase of alumina calcination temperature from 800 to 980 °C leads to the increase of ammonia synthesis rate from 2245 to 7217 μmol g−1 h−1. However, there is decrease of catalytic activity over Ru-Ba/Al2O3-1300 of which ammonia synthesis rate is only 3539 μmol g−1 h−1. The dispersion of Ru particles estimated by CO chemisorption is very low (Table S2), which may be due to (i) strong metal–support interaction between Ru species and alumina, (ii) the formation of oxidized Ru, and (iii) the blockage of Ru particles available for CO chemisorption by Ba. In the case of Ru dispersion evaluation by TEM measurement, there could be significant error because it is difficult to detect and/or identify Ru species those are too small. Because of these uncertainties in estimating the number of Ru atoms by chemisorption or TEM measurement, the TOFs based on total number of Ru atoms were calculated. The maximum TOF of Ru-Ba/Al2O3-980 is 4.5×10−3 Ru atom−1 s−1, which is three-fold higher than that of Ru-Ba/Al2O3-800. The TOF value of Ru-Ba/Al2O3-980 is comparable to those reported for Ru-Ba/AC, Ru/CeO2 and Ru/C12A7:e- (Table S3).32,

74-75

No significant decrease of ammonia

synthesis rates are observed at 400 °C for 200 h (Figure 6a), indicating that all Ru-Ba catalysts show long-term stability. As showed in Figure S11 and Table S4, the low estimated N2 reaction orders (0.54–0.96) of Ru-Ba catalysts indicate that N2 dissociation is sufficiently fast, and the ammonia synthesis reaction would not be limited by N2 cleavage.32, 36 The reaction orders of H2 and NH3 are -0.87 and -0.76 for Ru-Ba/Al2O3-800, and these values increase with the rise of the alumina calcination temperature. This result suggests that the increase of the alumina calcination temperature results in the drop of the adsorption strengths for H2 or NHx species. Table 2 Catalytic performance of Ru-Ba catalysts at 400 °C under pressure of 1.0MPa. Samples

Rate (μmol g−1 h−1)

TOF a (atom−1 s−1) 10


Ea (kJ mol−1)

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Ru-Ba/Al2O3-800

2245

1.4×10−3

116

Ru-Ba/Al2O3-900

6005

3.7×10−3

115

Ru-Ba/Al2O3-980

7217

4.5×10−3

103

Ru-Ba/Al2O3-1300

3539

2.2×10−3

102

aTurnover

frequency (TOF) of Ru-Ba catalysts based on total number of Ru atoms. 8

10

(a)

(b)

Ru-Ba/Al2O3-980

7 Ru-Ba/Al2O3-900

6 5 4

Ru-Ba/Al2O3-1300

3

Ru-Ba/Al2O3-800

8

rate (mmolNH3g-1 h-1) cat

rate (mmolNH3g-1 h-1) cat

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

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Ru-Ba/Al2O3-900 Ru-Ba/Al2O3-980 Ru-Ba/Al2O3-1300

6

4

Ru-Ba/Al2O3-800 2 1

2 0

20

40

60

80

100 120 140 160 180 200

1

2

3

4

5

6

7

8

9

10

Pressure (MPa)

Time on stream (h)

Figure 6 (a) Time dependence of ammonia synthesis rate at 400 °C and 1.0 MPa, and (b) pressure dependence of ammonia synthesis rate over the Ru-Ba catalysts at 400 °C. Figure 6b shows the pressure dependence of ammonia synthesis rate over the Ru-Ba catalysts. The catalytic activities of Ru-Ba/Al2O3-800 and Ru-Ba/Al2O3-900 are independent of pressure within the pressure range of 1 to 10 MPa, indicating the possibility of hydrogen poisoning.32-35 In contrast, ammonia synthesis rate increases significantly with the increase of reaction pressure for Ru-Ba/Al2O3-980 and Ru-Ba/Al2O3-1300, indicating that the problem of hydrogen poisoning has been eliminated to a certain extent for these two catalysts. Mass transport and heat transfer calculations were carried out for the best-performing Ru-Ba/Al2O3-980 (see supplementary information).76-77 It was found that the activation energies for ammonia synthesis over the Ru-Ba/Al2O3-x catalysts decrease from 116 to 102 kJ/mol with the increase of alumina calcination temperature from 800 to 1300 °C (Table 2 and Figure S12). A similar phenomenon was observed by Inoue et al. over Ru catalysts that were supported on 12CaO•7Al2O3 electride.78 The researchers reported that with the increase of 12CaO•7Al2O3 calcination temperature, there is decrease of activation energies for ammonia synthesis over the Ru catalysts. The researchers attributed the phenomenon to the high electron donating efficiency and high surface area of the 12CaO•7Al2O3 electride that was calcined at elevated temperature. 3.4 Adsorption properties of Ru catalysts The adsorption properties of the catalysts were investigated to elucidate the cause of discrepancy in catalytic behavior. N2-TPD profiles obtained after nitrogen pre-adsorption on the samples are shown in Figure 7 and Figure S13. A broad N2 desorption peak above 300 °C can be 11


ACS Catalysis

seen in the N2-TPD profile of Ru-Ba/Al2O3-800. As for Ru-Ba/Al2O3-900, two desorption peaks centered at ca. 356 and 362 °C can be detected. Further increase of the alumina calcination temperature leads to a shift of the desorption peak maximum of N2 desorption peak to lower temperatures. Furthermore, the amount of N2 evolution over the Ru-Ba/Al2O3-1300 catalyst is significant in comparison to the other catalysts. It is worth pointing out that there is pronounced desorption of water in the desorption profiles of Ru-Ba/Al2O3-x catalysts (Figure S13), indicating that

a large amount of Ru adsorption sites have been taken up by hydrogen species.37 Similar results of N2 desorption are found when the catalysts were exposed to a H2–N2 mixture (Figure S14). 115

N2 (m/z=28)

503

Ru-Ba/Al2O3-1300

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

520

332 Ru-Ba/Al2O3-980

562

356 Ru-Ba/Al2O3-900

600

Ru-Ba/Al2O3-800 100

200

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Figure 7 N2-TPD profiles of Ru-Ba/Al2O3-x catalysts. It was reported that H2 molecules dissociate on the surface of oxide-supported transition metals (e.g., Ru and Pt), and the hydrogen atoms migrate to the surface of oxides and form hydroxyl groups by reacting with framework oxygen.36

79-81

As a consequence, the hydrogen species on

oxide-supported Ru catalysts may desorb in the form of H2 molecules (2H→H2) or water (OH+H→H2O) with the rise of temperature.37, 82-83 In the present study, the strong mass signals of H2O together with the weak signals of H2 in the TPD profiles of Ru-Ba/Al2O3-x catalysts exposed to a H2–N2 mixture (Figure 8) indicates that most of the surface hydrogen species desorb following the H2O-formation pathway. The amount of hydrogen evolution decreases only slightly with the increase of alumina calcination temperature, while the amount of water evolution decreases significantly. The results of H2–O2 titration, which involves O2 treatment of freshly prepared samples followed by H2 titration at 100 °C (Figure S15), show that Ru-Ba/Al2O3-800 consumes 0.88 equivalent H2 per mole of Ru, significantly higher than the other alumina-supported counterparts (0.04–0.81 equivalent H2 per mole of Ru). Overall, the results indicate that a larger amount of hydrogen species exists on the surface of Ru-Ba/Al2O3-x catalysts when alumina is calcined at 800 or 900 °C, plausibly a result of stabilization of atomic hydrogen in the presence of surface hydroxyls.80, 84

12


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Intensity (a.u.)

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Ru-Ba/Al2O3-1300

100

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Ru-Ba/Al2O3-800

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Ru-Ba/Al2O3-900 Ru-Ba/Al2O3-980 Ru-Ba/Al2O3-1300

100

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300

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Figure 8 MS signals of H2 (top) and water (bottom) during TPD studies of Ru-Ba/Al2O3-x after exposure to a H2–N2 mixture. 4. Discussion The calcination of alumina at different temperatures changes the phases (γ-to-α) and the properties of alumina, such as, specific surface area and the proportion of various Al sites. The alumina calcined at 800 °C mainly consists γ-Al2O3. Al2O3-900 has both γ-Al2O3 and θ-Al2O3. The calcination of alumina at 980 °C would result in θ-Al2O3 as well as α-Al2O3, while only α-Al2O3 can be identified over Al2O3-1300. The amount of tetrahedral Al3+ sites and the specific surface area of alumina both decrease with increasing alumina calcination temperature. The nature of the alumina sites where active metal particles can anchor has not been fully understood,53, 85-86 and Kwak et al. proposed that the pentacoordinate Al3+ sites were responsible for the anchor of Pt particles based on the difference in NMR peak areas between γ-Al2O3 and Pt/γ-Al2O3.53 However, herein hardly any pentacoordinate Al3+ sites can be detected in Al2O3-x, and loading Ru and Ba species onto Al2O3-x leads to a significant decrease in the number of octahedral and tetrahedral Al3+ sites, as evidenced by the large decrease in the intensities of the NMR peak areas (Figure S16). On the other hand, the elevation of alumina calcination temperature decreases the amount of hydroxyl groups and Al3+ sites as well as the surface area of Al2O3-x, resulting in lower the dispersion of Ru particles. However, the charge transfer between Ru species and Al2O3-x would not be inhibited by the abundant oxygen groups,87 and thus the binding energy of Ru species decreases with the rise of alumina calcination temperature. The ammonia synthesis rate of Ru-Ba/Al2O3-x catalysts increases initially and then decrease with the increase of alumina calcination temperature. Compared to Ru-Ba/Al2O3-800, 13


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Ru-Ba/Al2O3-980 is three times higher in TOF based on total number of Ru atoms. With the rise of reaction pressure, the ammonia synthesis rates over Ru-Ba/Al2O3-980 and Ru-Ba/Al2O3-1300 increase significantly. However, over Ru-Ba/Al2O3-800 and Ru-Ba/Al2O3-900, catalytic activities are almost independent of reaction pressure. The increase of alumina calcination temperature also results in lowering of activation energies for ammonia synthesis over the Ru-Ba/Al2O3-x catalysts. The discrepancy in the catalytic performance could be attributed to factors such as (i) difference in size distribution of Ru particles, (ii) property variation of alumina support, and (iii) change in H2 and N2 adsorption/desorption behaviors. Ru particles with a broader size distribution are found on the surface of alumina that is subject to calcination at 980 and 1300 °C, and the average size of Ru particle increase with the rise of alumina calcination temperature. Furthermore, a higher “metal-to-oxide” ratio of Ru is found on Ru catalysts supported on Al2O3-900 and Al2O3-1300. Murata et al. found that compared to the metal catalyst supported on γ-Al2O3, that supported on θ-Al2O3 or α-Al2O3 with weaker metal–support interaction has metal particles that are broader in size distribution.53 Similar phenomenon have been observed in the present study. Unlike Ru-Ba/Al2O3-800 and Ru-Ba/Al2O3-900, Ru-Ba/Al2O3-980 has the co-existence of θ-Al2O3 and α-Al2O3, while Ru-Ba/Al2O3-1300 the sole presence of α-Al2O3. Therefore, compared with the former two, the latter two are weaker in metal–support interaction. Compared with Ru-Ba/Al2O3-800 and Ru-Ba/Al2O3-900, Ru-Ba/Al2O3-980 and Ru-Ba/Al2O3-1300 are broader in size distribution of Ru particles, bigger in average size of Ru particles, and higher in “metal-to-oxide” ratio of Ru. The B5 sites of Ru particles have been proved to be the active sites for ammonia synthesis, and the Ru particles with a size of around 2.0 nm exhibit the highest number of active sites.88-89 Nonetheless, Fernández et al. reported that the dissociative adsorption of H2 on large Ru particles of Ru/γ-Al2O3 catalysts resulted in H atoms of high diffusivity, whereas on small Ru particles, the H atoms were strongly attached. On the a large Ru particle, H atoms could transfer to nearby small particles, releasing active sites for nitrogen desorption.90-91 In such a case, a broad size distribution of Ru particles would lead to enhancement of ammonia synthesis rate. Moreover, a higher

“metal-to-oxide” ratio of Ru would mean more active Ru species for N2 desorption and ammonia synthesis. Furthermore, through exchange with surface OH groups of alumina, hydrogen atoms can be stabilized on the catalyst surface.80,

84

Therefore, for a Ru-Ba/Al2O3 catalyst rich in hydroxyl

groups, there would be large amount of hydrogen species (Figure 8 and Figure S13), which would lead to severe hydrogen poisoning. With the rise of the alumina calcination temperature, the number of Ru sites available for gas adsorption and ammonia synthesis would decrease, along with the enhancement of Ru electronic density and the dissociation capability of H2 and N2. 14


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Furthermore, the adsorption strengths for H2 or NHx species, which might be directly correlate with hydroxyl groups and acid sites of the samples, also decrease with the increase of the calcination temperature (Figure S11 and Table S4). As a result, Ru catalysts supported on alumina calcined at high temperature can show high ammonia synthesis rates even the Ru dispersion is low. The results suggest that alumina is not just a simple spectator in the interaction between H species and Ru. The deduction is in line with the work of Lisitsyn and Yakovina which demonstrated that any change in the property of a support would alter the hydrogen adsorption behavior of the supported metal particles.92 On the other hand, more studies are necessary to fully elucidate the effect of alumina phase on the catalytic performances. For example, the nature of the metal-support interaction and the size-dependent ammonia synthesis activity should be studied separately over Ru/Al2O3 catalysts with single-phase alumina. Overall, the calcination of alumina leads to transformation of γ-Al2O3, through the metastable θ-Al2O3 phase, to α-Al2O3. The presence of θ-Al2O3 and/or α-Al2O3 not only results in broader size distribution of Ru particles and higher proportion of Ru metal, but also leads to lower amount of hydroxyl groups and lower temperature of N2 desorption, and thus the Ru-Ba/Al2O3-980 catalyst is the highest in ammonia synthesis activity. Although Ru-Ba/Al2O3-1300 is the highest in TOF based on TEM or CO chemisorption (Table S3), the corresponding productivity based on the weight of Ru metal is lower because of the low specific surface area of alumina and poor dispersion of Ru particles. Therefore, Ru-Ba/Al2O3-980, which has both θ-Al2O3 and α-Al2O3, shows the best catalytic performance for ammonia synthesis.

5. Conclusions Alumina is calcined at high temperatures for the change of crystalline phase. Only γ-Al2O3 is observed in the alumina calcined at 800 °C. In the alumina calcined at 900 °C, there is the co-presence of γ-Al2O3 and θ-Al2O3. At a calcination temperature of 980 °C, θ-Al2O3 and α-Al2O3 co-exist, whereas calcination at 1300 °C would result in α-Al2O3 being the only phase. With the increase of alumina calcination temperature, there is decrease of specific surface area and the amount

of

tetrahedral

Al3+

sites.

Among

the

Ru-Ba/Al2O3-800,

Ru-Ba/Al2O3-900,

Ru-Ba/Al2O3-980, and Ru-Ba/Al2O3-1300 catalysts, the increase of alumina calcination temperature results in broader size distribution of Ru particles, larger average particle size and higher metal-to-oxide ratio of Ru. It is deduced that the use of alumina calcined at 980 or 1300 °C as support results in weaker metal–support interaction between Ru and alumina in comparison with the case of using alumina calcined at 800 or 900 °C. It is considered that the hydrogen atoms resulted in H2 dissociative adsorption on Ru have higher diffusivity on large Ru particles in comparison to those on small Ru particles, and in the latter case, the H atoms are more strongly 15


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adsorbed. A higher percentage of surface Ru metal would mean more Ru active centers for nitrogen desorption and ammonia synthesis. Nonetheless, hydrogen atoms could migrate to the surface of alumina and interact with surface OH groups, resulting in significant presence of adsorbed hydrogen species, which might lead to severe hydrogen poisoning. With such understandings, it is considered that the use of alumina calcined at 980 °C as support can prevent hydrogen poisoning and decrease activation energy for ammonia synthesis. The Ru-Ba/Al2O3-980 catalyst (with both θ-Al2O3 and α-Al2O3 phase) shows ammonia synthesis activity three times that of Ru-Ba/Al2O3-800 (γ-Al2O3 phase). We have demonstrated that the alumina support not only has an effect on the dispersion of Ru species, but also participates in the adsorption/desorption of H2 and N2. Overall, it is possible to tailor the ammonia synthesis activity of supported Ru catalysts by regulating the property of support, ultimately enabling the fabrication of suitable catalysts for industrial application. Supporting Information Rietveld refinement results, NH3-TPD profiles, HRTEM images, H2-TPR profiles, CO-TPR profiles, Raman spectra, XPS Ru 3d spectra of as-prepared Ru-Ba catalysts, Arrhenius plots, TPD measurements, H2–O2 titration curves,

27Al

MAS-NMR spectra, Graphs for calculating reaction

orders, Rietveld refinement data, Particle sizes, Catalytic performance of various Ru catalysts reported elsewhere, Reaction orders and related references. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21776047, 21825801, 21203028) and the Program for Qishan Scholar of Fuzhou University (XRC-18033). The authors thank to Prof. C. T. Au for helpful suggestions. References 1.

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Ammonia Synthesis Activity of Alumina-Supported Ruthenium Catalyst

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