Barium Hydride-Mediated Nitrogen Transfer and Hydrogenation for

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Barium Hydride Mediated Nitrogen Transfer and Hydrogenation for Ammonia Synthesis: A Case Study of Cobalt Wenbo Gao, Peikun Wang, Jianping Guo, Fei Chang, Teng He, Qian-Ru Wang, Guotao Wu, and Ping Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00284 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017

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ACS Catalysis

Barium Hydride Mediated Nitrogen Transfer and Hydrogenation for Ammonia Synthesis: A Case Study of Cobalt Wenbo Gao1,2, Peikun Wang1,2, Jianping Guo1,4*, Fei Chang1,2, Teng He1, Qianru Wang1,2, Guotao Wu1 and Ping Chen1,3,4* 1

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian 116023, China. 2

University of Chinese Academy of Sciences, Beijing 100049, China.

3

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, Dalian 116023, China. 4

Collaborative Innovation Center of Chemistry for Energy Materials, Dalian 116023, China.

KEYWORDS: ammonia synthesis, barium hydride, cobalt, barium imide, promoter, heterogeneous catalysis

ACS Paragon Plus Environment

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ABSTRACT

Industrial ammonia synthesis catalyzed by Fe- and Ru-based catalysts is an energy-consuming process. The development of low-temperature active catalyst has been pursued for a century. Herein, we report that barium hydride (BaH2) can synergize with Co leading to a much better low-temperature activity, i.e., the BaH2-Co/CNTs catalyst exhibits ammonia synthesis activity right above 150 °C; at 300 °C, it is two orders of magnitude higher than the BaO-Co/CNTs and more than 2.5 times higher than the Cs-promoted Ru/MgO. Kinetic analyses reveal that the dissociative adsorption of N2 on Co-BaH2 catalyst may not be the rate-determining step, evidenced by the much smaller reaction order of N2 (0.43) and the lower apparent activation energy (58 kJ mol-1) compared with those of the unpromoted and BaO-promoted Co-based catalysts. BaH2, with a negative hydride ion, may act as a strong reducing agent removing activated N from Co surface and forming a BaNH species. The hydrogenation of the BaNH species to NH3 and BaH2 can be facilely carried out at 150 °C. The relayed catalysis by Co and BaH2 sites creates an energy-favored pathway that allows ammonia synthesis under milder condition.

1. INTRODUCTION Ammonia is the main source of nitrogenous fertilizer which is vital for crop growth. Industrial ammonia synthesis is carried out over a fused iron or supported Ru catalyst under high temperatures and pressures, consuming 1-2% of annual global energy production.1 Therefore, it is highly desirable to develop active catalyst for ammonia synthesis under milder conditions.2, 3 Ammonia synthesis from N2 and H2 is a mild exothermic reaction which is thermodynamically allowed at ambient temperature and pressure. However, there are severe kinetic barriers in the


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activation of N2 and hydrogenation of the chemisorbed N species over most of transition metal (TM)-based catalysts.4, 5 An ideal ammonia synthesis catalyst should have a strong activation on the reactants (N2 and H2), but weak bindings to the intermediate NHx (x=0, 1, 2) and H species.5 Unfortunately, this ideal scenario is inaccessible on transition metal-only catalyst because of the intrinsic scaling relations between the adsorption energies of reacting species in transition and intermediate states (kinetic scaling relations of Ea and ENHx)6 and among adsorption energies of the intermediate NHx species on transition metal surfaces (adsorption-energy scaling relations of ENHx).7, 8 As a consequence, early or late transition metals (such as Co), having adsorption energies that are either too high or too low, could not effectively catalyze ammonia synthesis. How to break or circumvent the scaling relations is the forefront of modern catalysis.9 We recently demonstrated that a relayed two-active center (LiH-TM) catalysis could break the scaling relations because LiH functions as a reductant removing chemisorbed N from 3d metals to form Li-N-H species so that the energy state of NHx can be leveraged. Such a function leads to universal activities in ammonia synthesis among 3d transition metals spanning from V to Ni. Among which, Co-LiH has the best activity that is 3.4 times of the Cs-promoted Ru catalyst at 300 °C.10 Neat cobalt exhibits negligible activity in catalyzing ammonia synthesis because of its strong affinity with H resulting in the inhibition of N2 adsorption.11,

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Two strategies have been

employed to enhance its activity, i.e., 1) Alloying Co with other transition metals that have stronger chemisorption of N, such as Fe13, 14, 15 or Mo;16, 17, 18 2) Modifying Co catalysts with promoters such as BaO. Hagen et al. reported that BaO had a significant promoting effect on carbon supported Co catalyst (Co/C) by increasing the NH3 synthesis activity by more than two


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orders of magnitude.15,

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The Ba-Co/C catalyst also has a less inhibition by NH3 than the

commercial Fe-based catalyst, which manifests its practical interests.15, 20 In addition to Co, Ba was also found effective in promoting Ru-based catalysts.21, 22, 23, 24 There are a number of detailed investigations with regards to the chemical form and promoting mechanism of Ba. Zeng et al. proposed that the active form of Ba was BaO and/or Ba(OH)2.25 Through atomic-resolution in-situ transmission electron microscopy observation, Hansen et al. found that the active Ba exists as a two-dimensional Ba-O overlayer on the surface of Ru.26 Truszkiewicz et al., on the other hand, proposed that Ba may exist as a mixture of Ba0 and BaO based on in-situ XRD and temperature-programmed reduction (TPR-MS) measurements.27 Ba has been debated as an electronic promoter or structural promoter to transition metals for years. Hagen et al. observed that the addition of Ba resulted in a significant reduction of the apparent activation energy of Co/C catalyst from 149 to 102 kJ mol-1 suggesting its electronic promoting role.15 Bielawa et al., on the other hand, suggested that the Ba+O coadsorbate plays the role of a structural promoter that increases the number of active B5-type sites of Ru.23, 28 It is known that the properties of a material are determined by its chemical composition and structure. Unlike BaO and Ba(OH)2, BaH2 possesses some unique properties. For examples, BaH2, bearing a H- ion, is a strong reductant;29 recent report shows that BaH2 has a much higher H- ion conductivity than those of oxide ion conductors at high temperatures;30 more importantly, BaH2 can reversibly react with NH3 to form BaNH/Ba(NH2)2 and H2 under mild conditions through the reaction R1, where Ba plays closely with the key elements (N and H) in NH3 synthesis. Therefore, it is of great interest to find out whether BaH2 would exhibit different role from BaO/Ba(OH)2 in catalyzing ammonia synthesis.


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BaH2 + 2NH3 = BaNH + 2H2 + NH3 = Ba(NH2)2 + 2H2

(R1)

The present work serves as the first investigation on the effect of BaH2 on Co in catalyzing ammonia synthesis. Our experimental results reveal that the Co-BaH2 composite could activate N2 and accommodate the N atoms in BaH2 to form a Ba-N-H species. And the further hydrogenation of Ba-N-H species leads to the formation of NH3 and BaH2. The synergy between Co and BaH2 results in a superior catalytic performance to the BaO-promoted Co and the Cspromoted Ru catalysts. Kinetic analyses disclose that the role of BaH2 is distinctly different from that of BaO or Ba(OH)2. It is a co-catalyst that participates in the hydrogenation steps rather than an electronic or structural promoter.

2.

EXPERIMENTAL

2.1 Catalyst Preparation. The purities of chemical reagents and gases used in this work were listed in Table S1. Ba(NH2)2 was prepared by reacting barium metal (Aldrich, 99%, shot diameter: ca. 2 cm) with liquid ammonia (99.999%) in a closed system at room temperature for several weeks. The synthesized sample was collected and confirmed by powder X-ray diffraction (PXRD) to be Ba(NH2)2. BaH2 was prepared by gradually heating Ba(NH2)2 to 300 °C under 1 bar of H2 (99.999%) flow until no ammonia is detected by a conductivity meter (Mettler Toledo SevenMulti). Co metal was prepared via ball milling anhydrous cobalt chloride (Sigma-Aldrich, 97%) and LiH (SigmaAldrich, 97%) in a molar ratio of 1 : 2 on a Retsch planetary ball mill (PM 400) at 150 rpm for 3 h, where the reaction of CoCl2 + 2LiH = Co + 2LiCl + H2 takes place. The black solid residue


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was washed with dry tetrahydrofuran (THF) for 3 times to remove LiCl. The purified product was dried under vacuum overnight. The Co-BaH2 composite was prepared by ball milling a mixture of the as-prepared Co metal and BaH2 in a molar ratio of 1:3 for 2 hours on PM 400. The carbon nanotube (CNT) supported Co sample was obtained by impregnating CNT (Courtesy from Prof. Hongbin Zhang, Xiamen university) with ethyl alcohol solutions of Co(NO3)2·6H2O (Kermel, 99.0%). This mixture was stirred until ethanol evaporated completely and then dried at 100 °C for 10 h. Subsequently, the residue was reduced by H2 at 400 °C for 6 h. The BaH2Co/CNTs was prepared by impregnating the Co/CNTs in a barium-ammonia solution. Ba metal converted to barium amide (Ba(NH2)2) in the presence of Co. After removing NH3 the residue of Ba(NH2)2-Co/CNTs was treated by syngas (N2: H2 = 1 : 3, 99.999%) at 300 °C to convert Ba(NH2)2 to BaH2 according to the reaction of Ba(NH2)2 + 2H2 = BaH2 + 2NH3. Following this procedure, the catalysts thus obtained (“as-prepared”) were denoted as nBaH2-x%Co/CNTs (x%: a mass ratio of Co metal to CNTs, n is a molar ratio of Ba to Co). The BaO-Co/CNTs was prepared by impregnating 10%Co/CNTs (same batch with that of BaH2-Co/CNTs) with aqueous solution of Ba(O2CCH3)2 followed by drying the sample at 100 °C for 10 h. Subsequently, this sample was reduced at 400 °C for 6 h under pure H2 atmosphere. The molar ratio of Ba/Co is 1:1. 2.2 Catalyst Activity Test. The ammonia synthesis activities of as-prepared catalysts were measured on a quartz-lined stainless steel fix-bed reactor under a continuous-flow of syngas (N2 : H2 = 1 : 3, purified through a Na-NaCl solid mixture). 30 mg or 15 mg catalyst was loaded respectively to test the reproducibility of the catalytic activities and to ensure the absence of heat and mass-transfer limitations. Typically, 30 mg catalyst was loaded and the temperature was raised at a ramping


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rate of 5 °C min-1 under the given pressure and flow rate. The ammonia production rate was measured by a conductivity meter. The exhaust gas was conducted to a diluted sulfuric acid solution and the change in proton conductivity with time was monitored. The activity data at each temperature was recorded after a stable performance of catalyst was reached. N2 reaction order measurement was carried out with a flow of mixed gas (N2, H2, Ar) at a constant total pressure (10 bar) and H2 pressure (5 bar) while changing N2 and Ar partial pressures. The reaction order of H2 was measured at a constant total pressure (10 bar) and N2 pressure (2 bar) while changing H2 and Ar partial pressures. The reaction order of NH3 was obtained by changing the flow rate of syngas while keeping a constant N2 to H2 partial pressure.15 All the kinetic measurements were conducted under conditions far from equilibrium. 2.3 Catalyst Characterization. Powder X-ray diffraction (PXRD) patterns were recorded on a PANalytical X’pert diffractometer using a homemade sample cell covered with KAPTON film to avoid air and moisture contamination. Transmission electron microscopy (TEM) images were obtained on a Tecnai G2 F30 S-Twin (FEI Company). The catalyst powder was dispersed in tetrahydrofuran (THF) and dropped on a carbon-coated copper TEM grid. The morphology of the used BaH2Co/CNTs sample was examined by scanning electron microscopy (SEM) taken with a Quanta 200 FEG scanning electron microscope coupled with an energy-dispersive X-ray spectroscopy (EDX) detector for elemental analysis. Extended X-ray absorption fine structure (EXAFS) spectra at Co K-edge were recorded at the BL14W beamline of Shanghai Synchrotron Radiation Facility (SSRF, electron energy of 3.5 GeV and ring currents of 300 mA). Samples were pressed into pellets and then sealed with KAPTON film on both sides to avoid air contamination. The


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BET specific surface areas of catalysts were measured by N2 physisorption at -196 °C on a Micromeritics ASAP2010 instrument. Inductively coupled plasma atomic emission spectrometer (ICP-AES, PerkinElmer ICP-AES 7300DV) was used to determine the content of Co and Ba in the catalyst. Temperature-programmed decomposition (TPD) measurements were performed in a quartz-lined stainless steel reactor, and the effluent gases were analyzed by an on-line mass spectrometer (MS, Hiden HPR20). Samples were heated in a Ar (99.999%) flow (30 ml min-1) from room temperature to the desired temperatures at a ramping rate of 2 °C min-1, the signals of H2 (m/z=2), N2 (m/z=28) and NH3 (m/z=17) were recorded. Temperature-programmed reaction with H2 (H2-TPR) was performed using the same apparatus and similar procedure as that of TPD, except that the carrier gas was changed to a H2 flow (30 ml min-1). Thermogravimetry (TG) measurements were performed on a Netsch 449C TG unit. Dynamic flow mode was employed with N2 as the carrier gas, and the ramping rate was set at 2 °C min-1. The commercial grade N2 (99.999%) carrier gas was further purified through a Na/NaCl solid mixture. Fourier transform infrared (FTIR) spectra were recorded on a Varian 3100 infrared spectrometer in the Diffuse reflectance FT-IR (DRIFT) mode. 3.

RESULTS AND DISCUSSION

3.1 Introducing BaH2 as a second active center Advanced surface science and theoretical investigations revealed that NH3 synthesis over transition metal surfaces proceeds via the dissociation of N2 and H2 to form adsorbed N and H species respectively, followed by sequential hydrogenation of N species to give off NH3.4, 31 The adsorption energies of NHx (x=0, 1, and 2) and H species at transition and intermediate states are dependent on the electronic properties of transition metal sites rendering the existence of


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intrinsic scaling relations among those adsorption energies.7 Such scaling relations explain the volcano-type activities observed experimentally and imply that only those who have moderate binding energies to N, such as Ru and Fe, can catalyze NH3 formation well.5 One way to break the scaling relations is to intervene in the transition metal-governed catalysis by introducing a second active center which can effectively remove the activated N species from transition metals and carry on hydrogenation independently.10 Bearing hydridic H, BaH2 is a powerful reductant and may serve as the second center to synergize with transition metals such as Co to catalyze NH3 formation. Figure 1a shows the temperature-programmed thermogravimetric (TG) measurements of CoBaH2 composite (molar ratio of Co to BaH2 is 1 to 3) as well as the reference BaH2 and Co-BaO samples under a pure N2 flow. The weight gain of Co-BaH2 starts at ca. 250 °C and reaches the maximum rate at ca. 390 °C. Nearly 6.4 wt% weight gain is achieved at 475 °C. To test if the mass gain is due to possible contamination by moisture or oxygen in the N2 carrier gas, we performed the hydrogenation of Co-BaH2 sample collected after N2-TG measurement to quantify the amount of NH3 produced based on the reaction of BaNH + H2 → BaH2 +NH3. The result shown in Figure S1 shows that ca. 6.0 wt% of the mass gain should come from the absorption of N by Co-BaH2 sample, which is in agreement with the weight gain (6.4 wt%) observed in the N2TG measurement. Compared with the little weight change (ca. 1.1 wt%) of the Co-BaO sample, the significant mass gain that is equivalent to 0.74 nitrogen atom per Ba for Co-BaH2 should result from the accommodation of N atoms by BaH2. IR spectrum of the Co-BaH2 sample collected after the TG measurement (Figure 1b) shows two peaks centered at 3119 and 3000 cm1

, which suggests the formation of a barium imide (BaNH) species (see also Figure S2). XRD


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pattern of the Co-BaH2 sample pretreated under a pure N2 flow at 475 °C for a longer period corroborates the formation of crystalline BaNH phase (Figure S3). An interesting observation is that neat BaH2 undergoes nitridation at temperatures above 350 °C, which is ca. 100 °C higher than that of Co-BaH2, and has a weight gain of 5.9 wt% which is equivalent to 0.59 N per Ba. The IR spectrum of the BaH2 collected after TG measurement (Figure S2) reveals the presence of similar bands centered at 3118 and 3001 cm-1 as those observed in the post-TG Co-BaH2 sample, further verifies that the N-H species observed on postTG sample are bonded with Ba rather than Co. To understand this phenomenon, temperatureprogrammed decomposition (TPD) measurement was conducted on BaH2 (Figure S4), which evidenced that trace amount of H2 desorption occurred at temperatures above 300 °C, correlating well with its onset nitridation temperature (Figure 1a). This result shows that partial decomposition of BaH2 took place possibly forming a H-deficient BaH2-x that is capable of activating N2. We, therefore, suppose that the nitridation comes from the reaction of N2 and BaH2-x. It should be noted that the thermal decomposition of BaH2 is highly endothermic (BaH2 = Ba + H2, ∆H = 178.7 kJ mol-1-H2) and thus has a strong dependence on H2 pressure. If H2 back pressure is applied, the decomposition of BaH2 and the following nitridation are expected to be inhibited, which are corroborated by the negligible catalytic activity of BaH2 in ammonia formation (shown in Figure 2a). A much lower onset temperature of nitridation for Co-BaH2 than for neat BaH2 should be related to the presence of Co metal, because the dissociative activation of N2 to adsorbed N species can be carried out on Co32, 33, 34 and thus the nitridation of BaH2 by the activated N species may take place at lower temperatures. The BaNH species formed upon nitridation of the Co-BaH2 sample can further react with H2 to produce NH3 in the temperature range of 150-450 °C (shown in Figure 1c). The lower onset


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temperature compared with that of neat BaNH (ca. 250 °C) suggests that Co may catalyze the hydrogenation of BaNH to release NH3. The FTIR and XRD characterizations of the post-TPR sample evidences the disappearance of N-H vibration (Figure 1b) and the appearance of BaH2 phase (Figure S5), evidencing the occurrence of the reaction R1. Based on these N2-TG and H2-TPR results, a catalytic cycle of ammonia synthesis on the CoBaH2 composite through the following steps (I to III) is thus proposed, where * denotes a site on the Co metal surface. Noted that this cycle resembles to that proposed for the LiH-transition metal composite,10 indicating the similar behavior of BaH2 as that of LiH. N2 + 2* = 2N*

(Step-I)

N* + BaH2 → * + BaNH

(Step-II)

BaNH + H2 → BaH2 + NH3

(Step-III)

3.2 Catalytic performance A series of Co-based catalysts were thus prepared and examined to verify the function of BaH2. Carbon nanotubes (CNTs) were used as supports to increase the Co dispersions. The catalytic performances of BaH2-Co/CNTs with various Co and Ba loadings were evaluated under 10 bar of syngas and a weight hourly space velocity (WHSV) of 60000 ml gcat-1 h-1. To compare the effect of the chemical state of Ba, a BaO-Co/CNTs catalyst was also tested as a reference catalyst. The specific surface area, metal loading, mean particle size, Co metal dispersion and turnover frequency (TOF) values of catalyst are summarized in Table 1. Moreover, Ru/MgO with and without Cs promotion were also tested as references. The activities of these reference


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Ru/MgO and Cs-Ru/MgO catalysts measured in this work are similar to those reported in literature.35 Figure 2a summarizes the catalytic activities of Co- and Ba-based samples. The maximum activity is achieved on 3BaH2-10%Co/CNTs, which exhibits an activity of 480 µmolNH3 gcat-1 h-1 at 200 °C. It is worthy of highlighting that NH3 synthesis could be realized on the BaH2Co/CNTs catalysts at a temperature as low as 150 °C (ca. 58 µmolNH3 gcat-1 h-1), which is ca. 150 °C lower than the onset temperature of BaO-promoted Co/CNTs. At 350 °C, the activity of 3BaH2-10%Co/CNTs is more than 1200 and 30 times higher than those of reference Co/CNTs and BaO-Co/CNTs, respectively. The TOF of the BaH2-Co/CNTs, calculated on the basis of the reaction rate and the Co content, is more than two orders of magnitude higher than the BaOCo/CNTs under 300 °C and 10 bar. Moreover, if taking the number of exposed surface Co atoms derived from the TEM images (Figure 3) in consideration, the calculated TOF of the BaH2Co/CNTs would be even larger, i.e., more than 200 times higher than that of the BaO-Co/CNTs (Table 1). Such a result reflects the chemical state of Ba could make a big difference in “promoting” Co for ammonia synthesis. We further compare the catalytic activities of 3BaH2-10%Co/CNTs catalyst with the Co- and Ru-based catalysts at 250 and 300 °C (see Figure 2b). The activity of self-made BaO-Co/CNTs is lower than that of reported one15 (Figure 2b). Nonetheless, the 3BaH2-10%Co/CNTs outperforms those Co- and Ru-based catalysts, especially at lower temperatures, e.g., its activity is ca. 20 (250 °C) and 2.5 (300 °C) times higher than that of Cs-Ru/MgO, one of the most active noble metal catalysts.35, 36


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Figure S6 shows the stability of the BaH2-Co/CNTs catalyst monitored under 10 bar of syngas at 300 °C. It should be mentioned that the total amount of ammonia produced reached 4.32 mmol during this period, which is 25 times more than the total nitrogen amount in Ba(NH2)2 (170 µmol) which is the initial form of barium in the precursor of BaH2-Co/CNTs catalyst. This result indicates that the ammonia produced is derived from the catalytic ammonia synthesis by BaH2Co/CNTs rather the hydrogenation of Ba(NH2)2 in the precursor. As shown in Figure S6, ca. 10% of activity loss was found on the sample during a testing period of 30 h, whereas a relatively slower activity loss (less than 3%) was observed from the time of 20 to 30 h on the stream. The morphologies of the Co-based samples were investigated by transmission/scanning electron microscopy (TEM and SEM). Figure 3 shows the TEM images of BaH2- and BaO-Co/CNTs samples collected after catalytic testing. As shown in Figure 3(a) and (b), all of the Co particles are dispersed in the nanoscale on the CNTs supports. The statistical analyses of the Co particle sizes result in a mean particle size of 42.0 and 18.5 nm with wide distributions for the BaH2Co/CNTs and BaO-Co/CNTs catalysts, respectively. Figure 3c and d show that the lattice spacing of Co particles (2.05 Å) is in good agreement with that of metallic Co (2.046 Å for Co(111)), indicating that metallic Co is formed after catalytic testing. SEM image (Figure S7) shows that the BaH2-Co/CNTs sample collected after catalytic testing comprises irregular particles of up to 100 nm in size. The Co K-edge EXAFS characterization results further reveal that Co is present in the metallic form in the BaH2-promoted samples before and after catalytic testing (Figure S8). XRD patterns of the fresh and used BaH2-Co/CNTs samples (Figure 4a) do not show any reflections of Ba-containing compounds, indicating the amorphous nature of Ba phase. In order to ascertain the chemical state of Ba in the BaH2-Co/CNTs catalyst under reaction, the H2-TPR of used BaH2-Co/CNTs sample was conducted. As shown in Figure 4b, the


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ammonia signal was observed by MS in the temperature range of 150-550 °C. The nitrogen content in the used BaH2-Co/CNTs catalyst was also determined quantitatively through measuring the amount of ammonia released from H2-TPR by conductivity meter, which is ca. 0.084 mmolN gcat-1. Because the amount of N adsorbed on Co surface can be negligible, most of the N should bond to Ba. The N to Ba ratio is ca. 0.1 showing that the BaH2 and BaNH co-exist in the catalyst under the reaction condition applied in the present work. 3.3 Kinetic studies Figure 5a displays the Arrhenius plots of activities of 3BaH2-10%Co/CNTs (250-350 °C) and BaO-Co/CNTs (300-400 °C). The apparent activation energy (Ea) of 3BaH2-10%Co/CNTs is calculated to be 58 kJ mol-1, which is significantly smaller than those of the self-made BaOCo/CNTs (136 kJ mol-1) and reported Ba-Co/C catalyst (102 kJ mol-1).15 Such a low Ea value corresponds well with its superior low-temperature performance and implies a different catalytic mechanism from that of the BaO-promoted one. It has been commonly regarded that the dissociative adsorption of N2 is the rate-determining step in the conventional transition metal-catalyzed ammonia synthesis and the reaction order of N2 is usually close to unity,15, 35, 37 e.g., the reaction order of N2 (β value) over BaO-Co/CNTs is 1.02 (shown in Figure 5b), which is consistent with the reported value of BaO-Co/C (β = 0.9)15 and other conventional ammonia synthesis catalysts.15, 35, 37 The reaction order of N2 for the 3BaH2-10%Co/CNTs, however, is only 0.43 (Figure 5b). Such a low β value suggests that the dissociative adsorption of N2 may not be the rate-determining step. The significant difference in N2 reaction order between BaH2-Co/CNTs and BaO-Co/CNTs manifests the different functions of BaH2 and BaO on the Co catalysts. The above TG results have shown that BaH2 is able to


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abstract the activated N species from Co to form a BaNH species, thus may create a N-poor or even N-free metal surface that favors further dissociation of N2 molecules. In this context, the presence of BaH2 accelerates the activation of N2, and thus alters the reaction pathway including the rate-determining step. As proposed previously, the catalytic cycle for producing ammonia contains three steps, i.e., Step-I, II and III. Therefore, the overall reaction rate would be determined by the kinetics of all the three steps. Under this hypothesis, the rate-determining step may vary from step I to step II or III. The reaction order of H2 (γ value) over 3BaH2-10%Co/CNTs is 0.58 (Figure 5c), which means that hydrogen poisoning effect over the unpromoted Co catalyst (γ = -0.4)15 is improved significantly by BaH2. The adsorption of H on Co metal surface is relatively strong, as supported by the surface science results showing that the complete desorption of adsorbed H species from Co(0001) surface occurs above 200 °C in a high vacuum system,38 which explains the negative reaction order of H2 (γ = -0.4) observed on the unpromoted Co catalysts.15 The stronger interaction of H2 with Co will inhibit the adsorption of N2 molecules, which partly account for the low activity of unpromoted Co catalyst. The presence of BaH2 alters the H2 reaction order to a positive value, i.e., 0.58 (Figure 5c), which eliminates the poisoning effect of H2 on N2 activation over Co surface and is beneficial for the activation of N2 at lower temperatures. A possible explanation for the positive H2 reaction order is the fast consumption of adsorbed H on Co surface by the BaNH species to refresh the Co active sites. However, the reaction order of NH3 (α value) for 3BaH2-10%Co/CNTs (α = -1.27) is negatively greater than that of self-made BaO-Co/CNTs sample (α = -0.4) (Figure 5d), but comparable to


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the reported value of the BaO-Co/C sample (α = -0.9).15 Deduced from the reaction orders of N2, H2 and NH3, we suppose that the overall reaction rate would be determined by the third step in the proposed catalytic cycle, i.e., the hydrogenation of BaNH to BaH2 and NH3, where the influences of partial pressures of NH3 and H2 can be reflected. 3.4 Discussion Alkali and alkaline-earth metals have been demonstrated to be efficient promoters to enhance the activity, selectivity and stability of catalyst in many important heterogeneous catalytic processes including ammonia synthesis and decomposition, Fisher-Tropsch synthesis, water-gas shift, and so on.39 Alkali and alkaline-earth metals are usually introduced in the form of nitrate, carbonate or hydroxide during catalyst preparation process, so the most likely chemical form of alkali and alkaline-earth metals under the reaction condition of ammonia synthesis or decomposition is oxide or hydroxide, e.g., the chemical state of Ba in Ru and Co-based catalysts has been suggested to be BaO or Ba(OH)2 under the working condition of ammonia synthesis.25 The significant differences in catalytic activities and kinetic parameters between BaH2- and BaOCo/CNTs catalysts indicate the chemical state of Ba has a strong impact on its promoting efficiency as well as the promotional mechanism. BaH2 is a strong reducing agent and can accommodate activated nitrogen from the surface of Co to form BaNH species, but BaO does not possess such a function. Similar to LiH, BaH2 participates in the catalytic cycle as a second active center rather than an electronic or structural promoter. Its chemical form would be hydride, amide or imide or the mixture of them depending on the reaction conditions applied, i.e., the equilibrium of reaction R1 under different conditions. The details of N transfer from Co to Ba and the function of hydridic H of BaH2 in the activation of N2, however, remain open questions that are worthy of detailed investigation.


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In view of the prominent role of BaH2 on Co in catalyzing ammonia synthesis, one may rationally deduce that, through a similar two-active center mechanism, BaH2 may exert a similar effect on other transition metals. Furthermore, Co may also synergize with other alkaline earth metal hydrides, such as MgH2, CaH2, or SrH2, leading to an enhanced catalytic activity in ammonia synthesis. These would be our following work. 4.

CONCLUSIONS

A highly active Co-BaH2 composite catalyst for ammonia synthesis has been developed. The synergy between BaH2 and Co through a relayed two-active center catalysis breaks the transition metal-exerted scaling relations and leads to a remarkable low-temperature catalytic activity, i.e., it is ca. 20 and 2.5 times higher than that of highly active Cs-Ru/MgO at 250 and 300 °C, respectively. We found that the reaction pathway over BaH2-Co/CNTs catalyst is different from the conventional Co-based catalysts, e.g., the dissociative adsorption of N2 may not be the ratedetermining step. And the chemical form of alkaline earth metal has a strong impact on its catalytic function.


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Figure 1. (a) TG measurements of Co-3BaH2, Co-3BaO and BaH2 under a pure N2 flow. (b) FTIR spectra of BaH2, the Co-3BaH2 samples after TG (Co-BaH2-N2) and H2-TPR (Co-BaH2N2-TPR) measurements. (c) H2-TPR profiles of as-prepared and post-TG Co-3BaH2 samples as well as neat BaNH.


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Figure 2. (a) Temperature dependences of the catalytic performances. The inset is the enlarged portion of the rates in the temperature range of 150-200 °C for the 3BaH2-10%Co/CNTs sample. (b) Activity comparison of Co- and Ru-based catalysts at 250 and 300 °C, respectively. The data of BaO-Co/C is taken from the reference15. An error analysis was performed for the activity data and the error range was ca. ±5%.


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Figure 3. TEM images of (a) BaH2-Co/CNTs and (b) BaO-Co/CNTs catalysts collected after catalytic testing at 300 °C and 10 bar of syngas. Insets: Co particle size distributions. (c) and (d) Enlarged image of Co particles in BaH2-Co/CNTs and BaO-Co/CNTs catalysts, respectively.


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Figure 4. (a) XRD patterns of Co-based catalysts. (b) Conductivity (top) and H2-TPR-MS (bottom) profiles of the used 3BaH2-10%Co/CNTs sample collected after catalytic test at 400 °C and 10 bar.


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Figure 5. Kinetic parameters of the 3BaH2-10%Co/CNTs and BaO-Co/CNTs catalysts for NH3 synthesis. (a) Arrhenius plots. Reaction conditions, N2 : H2 = 1 : 3 with a weight hourly space velocity (WHSV) of 60000 ml g-1 h-1; pressure, 10 bar. (b) Dependence of ammonia synthesis rates on the partial pressures of N2 over catalysts at 300 °C for BaH2-Co/CNTs and 400 °C for BaO-Co/CNTs, WHSV=60000 ml g-1 h-1 and a constant total pressure of 10 bar. (c) Dependence of ammonia synthesis rates on the partial pressures of H2 over catalysts at 300 °C for BaH2Co/CNTs and 400 °C for BaO-Co/CNTs, WHSV=60000 ml g-1 h-1 and a constant total pressure of 10 bar. (d) Dependence of ammonia partial pressures on the mixed gas flow rates over catalysts at 300 °C for BaH2-Co/CNTs and 400 °C for BaO-Co/CNTs and 10 bar. Slope = 1/(1 − α).


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Table 1 Properties of the Co-based catalysts. Catalyst

Specific surface area (m2 g-1)

Co loading (wt%)*

Ba loading (wt%)*

Mean particle size (nm)ǁ

Dispersion (%)ǁ

Number of exposed surface Co atoms (µmol g-1)

NH3 synthesis rate (µmol g-1 h-1)

BaH2-

TOF (×103 s-1)

‡

TOF (×103 s-1)§

†

53

5.20

20.1

42.0

3.0

106

4800

50.4

1.51

48

3.25

4.5

18.5

6.8

136

29

0.217

0.015

Co/CNTs BaOCo/CNTs *

Co and Ba contents were determined by inductively coupled plasma atomic emission ǁ

spectroscopy (ICP-AES). Mean particle size and dispersion were calculated based on TEM data assuming spherical metal particles.40 †NH3 synthesis conditions: catalyst loading, 30 mg; syngas, N2 : H2 = 1 : 3 with a flow rate of 30 ml min-1; temperature, 300 °C; pressure, 10 bar. ‡TOF was calculated from the rate of ammonia synthesis divided by the number of exposed surface Co atoms on the catalysts. §TOF was calculated from the rate of ammonia synthesis divided by the total number of Co atoms on the catalysts. ASSOCIATED CONTENT Supporting information. Quantification of NH3 during the H2 treatment of Co-BaH2 after N2TG measurement, FTIR spectra of BaH2 collected after TG measurement in N2, Ar-TPD profile of BaH2 sample, XRD pattern of solid product collected after hydrogenation of the post-TG CoBaH2, Time-dependences of catalytic activities of BaH2-Co/CNTs catalysts, SEM image of the used BaH2-Co/CNTs catalyst, EXAFS spectra of fresh and used Co-based catalysts. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to acknowledge financial support from the projects of National Natural Science Foundation of China (21633011, 21603220), Dalian Institute of Chemical Physics (DICP DMTO201504), the Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChEM), Science and Technology Project of Liaoning Province (201601374), and the Shanghai Synchrotron Radiation Facility (SSRF) for providing the beam time. REFERENCES 1.

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Table of Contents Graphic


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Barium Hydride-Mediated Nitrogen Transfer and Hydrogenation for

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