Alkali and alkaline earth hydrides-driven N2 activation and

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Alkali and alkaline earth hydrides-driven N2 activation and transformation over Mn nitride catalyst Fei Chang, Yeqin Guan, Xinghua Chang, Jianping Guo, Peikun Wang, Wenbo Gao, Guotao Wu, Jie Zheng, Xingguo Li, and Ping Chen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08334 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Journal of the American Chemical Society

Alkali and alkaline earth hydrides-driven N2 activation and transformation over Mn nitride catalyst Fei Chang,†‡& Yeqin Guan,†‡ Xinghua Chang,§ Jianping Guo,*†ǁ Peikun Wang,†‡ Wenbo Gao,†‡ Guotao Wu,† Jie Zheng,§ Xingguo Li,§ and Ping Chen*†ǁ † Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China. ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China. ∥ Collaborative Innovation Center of Chemistry for Energy Materials, Dalian 116023, P. R. China. § Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China. Dedication to 70th anniversary of Dalian Institute of Chemical Physics, Chinese Academy of Sciences

KEYWORDS. Alkali hydrides, Alkaline earth hydrides, Early transition metals, Manganese nitride, Ammonia synthesis

ABSTRACT: Early 3d transition metals are not focal catalytic candidates for many chemical processes because they have strong affinities to O, N, C or H etc., which would hinder the conversion of those species to products. Metallic Mn, as a representative, undergoes nitridation under ammonia synthesis conditions forming bulk phase nitride, and unfortunately exhibits negligible catalytic activity. Here we show that alkali or alkaline earth metal hydrides (i.e., LiH, NaH, KH, CaH2 and BaH2, AHs for short) promotes the catalytic activity of Mn nitride by orders of magnitude. The sequence of promotion is BaH2 > LiH > KH > CaH2 > NaH, which is different from the order observed in conventional oxide or hydroxide promoters. AHs, featured by bearing negatively charged hydrogen atoms, have chemical potentials in removing N from Mn nitride and thus lead to significant enhancement of N2 activation and subsequent conversion to NH3. Detailed investigations on Mn-LiH catalytic system disclosed that the active phase and kinetic behavior depend strongly on reaction conditions. Based on the understanding of the synergy between AHs and Mn nitride a strategy in the design and development of early transition metals as effective catalysts for ammonia synthesis and other chemical processes is proposed.

1.

Introduction

The activities of transition metals (TM) in catalyzing many reactions usually exhibit a volcano-type plot. The Sabatier principle, proposed at the start of the 20th century, provides a qualitative description on the correlation of the reaction rate and the bond strength of adsorbates on catalyst. With the advance of computational chemistry, such a correlation can be quantitatively expressed as the scaling relations of adsorption energies of intermediates on TM surfaces.1 It nicely explains that owing to their moderate bonding to N-, O-, C-, or H-containing species, the group VIII metals (Fe, Ni, Ru, Pt etc.) are extensively employed as catalysts in numerous chemical processes including ammonia synthesis, Fischer-Tropsch synthesis, hydrogenation and oxygen reduction reaction.2-4 Early 3d transition metals (ETMs, i.e., from Sc to Mn), on the other hand, have strong affinities towards those species resulting in significant changes in their surface or even bulk phase compositions and electronic structures and thus are of less catalytic capabilities.5 A typical case showing this TM-

dependent catalysis can be referred to ammonia synthesis reaction. The conversion of N2 to NH3 is vital for the production of fertilizers and for the utilization of NH3 as an energy carrier.6-7 Compared with the well-established and intensively studied Fe-, Ru-, and Co-based catalysts, ETMs received much less attention. Mittasch et al. reported that Mo, W and Mn were more or less active under harsh reaction conditions (823 K and 100 bar) and they all formed nitrides during reaction.8 Specifically, since Mn has strong affinity to N (the calculated adsorption energy of N on Mn (211) surface is ca. -3.5 eV),9 the initial dissociation of N2 into surface Nad on clean metallic Mn surface is highly thermodynamically and kinetically favored (kinetic scaling relations). Some of those Nad species diffuse into the bulk at elevated temperatures and pressures forming stable Mn nitrides. Further activation of N2 on the N-rich surfaces would be difficult because of the downshift of the d-band center that causes increased energy barrier for N2 activation and weakened bonding with Nad.10-11 The hydrogenation of ETM nitrides (ETMNs), such as Mn5N2 or

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CrN, to form N-poor nitrides (Mn4N or Cr2N) and NH3, on the other side, is unfavorable at 1 bar of H2.12 The apparent activation energy of ammonia formation from the hydrogenation of Mn5N2 was measured to be 98 kJ mol-1 in the temperature range of 823-973 K. Theoretical calculations showed that although H2 prefers to dissociate forming Had species on the Mn2N (0001) surface, those Had species bond so strongly that would “poison” the surface from generating NHx (x = 1, 2, 3) species.12 Our experimental data also evidence that the hydrogenation of Mn2N to NH3 can only take place at temperatures higher than 573 K (vide infra). All the results mentioned above reveal that the creation of N-poor (or N vacancy-rich) Mn nitride via direct hydrogenation is difficult. Such energy unfavorable N2 activation and hydrogenation renders the limited activities of manganese nitride and perhaps other ETMNs in catalyzing ammonia synthesis. As a matter of fact, Mn4N alone shows very poor ammonia synthesis activity even under 673 K and 30 bar of syngas (see Figure S1); Laassiri et al. also reported that Mn3N2 as the starting material exhibited negligible activity at 673 K and 1 bar.13 The way to circumvent this “dead end” of ETMNs in catalyzing ammonia synthesis we proposed is to introduce a second component that can remove N from the surfaces of ETMNs facilely to create N vacancies or can form a new center for N2 activation, and meanwhile can mediate the hydrogenation of the activated N to NH3. We have recently reported that LiH can abstract N from Mn2N forming Li-NH (LiNH2/Li2NH) species and Mn4N. The Li-N-H can undergo hydrogenation giving off NH3 and regenerate LiH. The synergy between LiH and 3d TMs circumvents the energy scaling relations on TM-only catalysts and hence enables LiH-TM unprecedentedly high catalytic activities for ammonia synthesis.14 It is of scientific importance to find out whether other alkali or alkaline earth metal hydrides (AHs), e.g., NaH, KH, CaH2 and BaH2, who also bear hydridic H, would resemble LiH in synergizing with Mn nitride to achieve catalytic enhancement, and whether a general approach in “activating” ETM for effective catalysis can be derived from the understanding of such a synergy. In the present study, the influence of AHs on N2 activation and hydrogenation over Mn nitride, as a representative of ETMNs, was investigated by using combined techniques, such as thermogravimetry (TG), Temperature-Programmed Reaction (TPR), X-ray diffraction spectroscopy (XRD), X-ray absorption spectroscopy (XAS) and kinetic analyses. The experimental results show that BaH2 and LiH exhibit the most pronounced promotion effect on Mn4N followed by KH, CaH2 and NaH, which is different from the order of conventional alkali and alkaline earth metal oxide or hydroxide promoters. The active phases and kinetic behaviors of Mn-AH catalyst depend strongly on reaction conditions. Those insights inspire us to propose a strategy for the design and development of more efficient ETMs catalysts for other chemical reactions. 2.

Experimental section

2.1 Catalyst Preparation. MnN was prepared following the procedure described previously.15 Mn2N was prepared by heating MnN to 673 K and held for 6 hours under 1 bar of hydrogen. Mn4N was prepared by heating MnN to 673 K

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and held for 24 hours under 20 bar of hydrogen. The formation of MnN, Mn2N and Mn4N were confirmed by XRay Diffraction (XRD), respectively. PECVD-MnNx sample was prepared by plasma-enhanced chemical vapor deposition method: the experiment setup consists of a quartz tube mounted in a tube furnace, an inductively coupled coil (driven by a 13.56 MHz, 500 W radio frequency power source), a pumping system and a gas flow control system. MnCl2 powder (1.0 g) was loaded in an alumina boat, which was inserted horizontally into the quartz tube 5 cm upstream the coil. Two small quartz tubes were served as substrates and placed right beside the MnCl2 powder. The system was evacuated to 0.1 Pa and was flushed with pure Ar several times to remove oxygen and moisture. Then the MnCl2 powder was heated under a mixture gas flow of Ar-NH3. The MnCl2 evaporation temperature was controlled at 723 K to ensure stable MnCl2 evaporation. The NH3 flow was 10 ml min-1 and the chamber pressure was regulated. After the temperature raised to 723 K, plasma was ignited and the radio frequency power was kept at 130 W for 90 min. After the reaction the MnNx deposition can be found on the inside of the small quartz tube. Mn-LiH composite catalyst was prepared via the reaction between MnCl2 and LiH (MnCl2 + 2LiH → Mn + 2LiCl + H2). Typically, 223 mg LiH and 500 mg MnCl2 (molar ratio of LiH to MnCl2 is 7:1) were mixed and ball-milled on a Retsch planetary ball mill (PM 400) at 200 rpm for 3 hours. The black solid residue was washed with dry tetrahydrofuran (THF) for 3 times to remove LiCl. The purified product was dried under vacuum for overnight. The weight of product is very close to the calculated value (ca. 95%) showing negligible loss of Mn or LiH during the preparation process. So according to the above stoichiometric reaction, the molar ratio of Mn to LiH in the product is thus ca. 1:5. PECVD-MnNx-LiH catalyst was prepared by ball-milling PECVD-MnNx and LiH for 3 hours at 200 rpm. Mn4N-LiH, Mn4N-NaH, Mn4N-KH, Mn4NCaH2 and Mn4N-BaH2 catalysts were prepared by ball milling the pristine Mn4N and corresponding alkali or alkaline earth metal hydrides with a molar ratio of 1:5 for 3 hours at 200 rpm. Mn2N-LiH, Mn2N-NaH, Mn2N-KH, Mn2N-CaH2, and Mn2N-BaH2 were prepared by ball milling the pristine Mn2N and corresponding alkali or alkaline metal hydrides with a molar ratio of 1:2.5. MgO was synthesized according to the method described in literature.16 Ru/MgO catalyst was prepared by the incipient wetness impregnation of MgO with a THF solution of Ru3(CO)12 precursor. After removing THF, the powder was reduced by a syngas flow (N2:H2=1:3) at a ramping rate of 2 K min-1 to 773 K and held for 2 hours. 5.4 wt% Ru was loaded as determined by inductively coupled plasma atomic emission spectroscopy. Cs-Ru/MgO catalyst was prepared by the incipient wetness impregnation of prepared Ru/MgO catalyst with an ethanol/water solution of CsNO3. After removing the solvent, the powder was reduced by the syngas (N2:H2=1:3) to 773 K at a ramping rate of 2 K min-1 and held for 2 hours. The weight ratio of Cs to Ru is 1:1. 2.2 Catalyst Activity Test. The ammonia synthesis activity was measured on a quartz-lined stainless steel fixbed reactor under a continuous-flow of syngas. Typically, 30 mg catalyst was loaded and the temperature was raised


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at a ramping rate of 5 K min-1 under the given pressure and flow rate. The ammonia production rate was measured by using a conductivity meter (Mettler Toledo SevenMulti). The exhaust gas was conducted to a diluted sulfuric acid solution and the change in proton conductivity with time was calculated. The stable activity data at each temperature point was recorded. N2 reaction order measurements were carried out with a flow of mixed gas (N2, H2, Ar) at a constant total pressure (10 bar) while changing the corresponding N2 pressures. The calculation method for N2 order was described briefly in the supporting information. All the kinetic measurements were conducted under conditions far from equilibrium. 2.3 Catalyst Characterization. XRD patterns were recorded on a PANalytical X’pert diffractometer using a homemade sample cell covered with KAPTON film to avoid air contamination. N2 adsorption analyses were carried out at 77 K using a QuadraSorb SI instrument. The specific surface area was calculated using the Brunauer, Emmett and Teller (BET) method. Temperatureprogrammed desorption or reaction (TPD or TPR) experiments were performed in a quartz-lined stainless steel reactor and the exhaust gases were analyzed by an on-line mass spectrometer (Hiden HPR20). Typically, 30 mg sample was heated in an Ar or H2 flow (30 ml min-1) from room temperature to desired temperature at a ramping rate of 5 K min-1. TG measurement was performed on a Netzsch 449C TG unit. Argon was used as carrier gas and the ramping rate was set on 0.5 K min-1. X-ray Adsorption Spectra (XAS) for Mn K-edge were recorded at the BL14W beamline of Shanghai Synchrotron Radiation Facility (electron energy of 3.5 GeV and ring currents of 300 mA). Samples were pressed into pellets and then sealed within the KAPTON film to avoid air contamination. 3.

Results and discussion

3.1 Alkali or alkaline earth hydride-Mn nitride composites for ammonia synthesis. To test the effect of alkali and alkaline earth hydrides on catalytic performance of Mn, we employed Mn4N-AHs composites prepared by ball milling Mn4N and AHs as model system to minimize variables that may be introduced during catalyst preparation. The temperature dependence of activities of those composite catalysts for ammonia synthesis is presented in Figure 1a, where all of the AHs can enhance the ammonia formation rates over Mn4N. Figure 1b compares the activities of Mn4N-based catalysts at 573 K and 10 bar of syngas (N2:H2=1:3). The activities of Ru/MgO and Cs-promoted Ru/MgO are also included as references. The ammonia synthesis rates over Mn4N-AHs are ca. 1 (NaH), 2 (KH and CaH2) and 3 (LiH and BaH2) orders of magnitude higher than that of neat Mn4N at 573 K, respectively. Specially, the Mn4N-LiH and Mn4N-BaH2 samples have ammonia formation rates of ca. 2250 and 1320 μmol gcat-1 h-1, which are significantly higher than the supported Ru (Ru/MgO) catalyst and are comparable to the highly active Cs-promoted Ru/MgO (as shown in the black bars) and reported Ru-based17-18 catalysts (Table S1). Noted that the Mn4N-AHs catalysts were made by simply ball milling Mn4N and AH powders, their activities can be enhanced significantly upon optimizing the sample preparation methods (i.e., up to 9.7 times increment can be

achieved for the Mn-LiH sample at 573 K as shown in Fig. S2), and thus the activities of Mn4N-BaH2 and Mn4N-LiH can considerably outperform Cs-promoted Ru/MgO catalyst especially at lower temperatures. The other important observation is that the activities of Mn4N-AHs are ranked in the order of Mn4N-BaH2 > Mn4NLiH > Mn4N-KH > Mn4N-CaH2 > Mn4N-NaH on the basis of per gram of Mn (as shown in the red bars in Figure 1b). Such an order of promoting capability is different from that of the conventional alkali or alkaline earth oxide or hydroxide promoters, where the higher is the electronegativity of alkali or alkaline earth metal, the lower is its promoting capability.19 For example, Aika et al. reported that the order of promoting effect to a Ru/MgO catalyst was Cs2O > K2O > Na2O > BaO > CaO.20 Such a difference in promoting order suggests that the function mechanism of alkali and alkaline earth metals depend strongly on their chemical forms. The molecular level understanding on the promoting mechanism of AHs on Mn4N is rather intriguing owing to the complex nature of the system (reaction likely occurs along the interface of two reactive solid phases). As a matter of fact, albeit considerable research efforts have been given to the clarification of promoting mechanisms of alkali and alkaline earth metal oxides and hydroxides on Fe or Ru catalyst in the past few decades, the interpretations are still controversial.21 It has been proposed that these alkali or alkaline earth metals, especially K, Cs, or Ba could donate electrons to the antibonding π orbitals of N2 through transition metal, thus offer a favorable pathway for the activation of nitrogen.22 On the other hand, alkali metals may induce electrostatic interaction with transition states or repulsive effect on the adsorbed species, thus facilitate either N2 activation23 or ammonia desorption.24 Different from their oxide or hydroxide counterparts, AHs bear hydride anions, which can provide lone electron pair and therefore are strong Lewis bases and reducing agents. In the present work, we designed and conducted a series of experiments over the Mn nitride-AH system to gain some indirect evidences to shed light on their synergy in catalyzing ammonia synthesis. 3.2 AHs-driven N2 activation and transfer over Mn nitride. Thermogravimetry (TG) technique was used to monitor the activation and conversion of N2 over Mn4N with and without AHs, respectively (Figure S3). Under a flow of N2, the weight gain over neat Mn4N sample is negligible even upon heating the sample to 661 K, meanwhile no phase change was observed (Figure S4). These facts reveal that the nitridation of neat Mn4N to a Nrich Mn nitride phase (e.g., Mn2N) via R1 is kinetically difficult, although it is thermodynamic favorable (Table S2 and S3). Comparatively, the Mn4N-LiH, Mn4N-CaH2 and Mn4N-BaH2 samples started to gain weight at ca. 400 K, and ca. 14.6, 6.4 and 4.7 wt% increases were achieved as temperature was increased to 660 K (Figure S3), which are equivalent to ca. 0.7, 0.4 and 1.1 mole of fixed N per mole of Mn, respectively (see Figure 2a). The Mn4N-NaH and Mn4N-KH samples, on the other hand, show very little weight change in the same temperature range. Because same Mn4N was used in all the samples (Figure S5), the different results of N2-TG are likely related to the interaction of Mn4N and AHs. As shown in Figure S4, the



Mn phase changes from Mn4N to Mn2N after the N2-TG treatment for the Mn4N-LiH sample. Provided all Mn4N converts to Mn2N, i.e., absorption of 0.25 equiv. N, the extra absorbed N (0.45 equiv. N) should be bounded with Li forming Li-N-H compound(s). Indeed, the crystalline phase of Li2NH was clearly observed on a Mn4N-LiH sample heated in N2 (Figure S6). Similarly, there are stable Ca-N-H (Ca2NH and CaNH) and Ba-N-H (Ba2NH and BaNH) species,25 which allow CaH2 or BaH2 to accommodate some N absorbed by Mn4N-CaH2 and Mn4N-BaH2 samples. For Na or K, however, there are no stable imide phases25 because of their small lattice enthalpies (see Table S2). The similar diffraction peaks of Mn4N (Figure S5) and specific surface areas of the Mn4N-AHs samples (Table S4) indicate the dispersion/active site number may not be the main reason for different activities. Furthermore, it is intriguing to figure out whether a specific active site could form at the interface of Mn4N and LiH/CaH2/BaH2 that is responsible for N2 activation and conversion. Further investigation should be performed. Mn4N + 1/2N2 → 2Mn2N

(R1)

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Our previous results showed that Mn2N can be reduced by LiH to form Li-N-H, H2 and N-deficient Mn4N (XAS data in Figure S7 also supports this).14 Mn2N can also be reduced by CaH2 and BaH2, which is characterized by the formation of H2 far below the decomposition temperatures of these hydrides (ca. 930 K26)(Figure 2b). While, the ArTPD profiles for the Mn2N-NaH and Mn2N-KH are quite similar to the neat NaH and KH, respectively (Figure S8). It indicates that Mn2N can not be reduced by NaH/KH under the applied reaction conditions even though they are also strong reducing agents. The interaction of Mn2N and AHs under a flow of H2 (Figure 2c), however, shows some distinct features from that observed in Ar-TPD. All the Mn2N-AHs except Mn2NCaH2 produce NH3 during the H2-TPR treatment showing the removal of N from Mn2N. Compared with the neat Mn2N, the NH3 evolution is much faster with the addition of LiH, NaH, KH and BaH2 which is characterized by ca. 20110 K down shift of onset temperature. Although the reaction of Mn2N and NaH or KH does not take place in a Ar flow, the co-existence of H2 and NaH or KH can boost the hydrogenation of Mn2N to NH3 likely following the reactions R2 and R3 (thermodynamic analyses are shown in Table S2, S3 and Figure S9). In the case of CaH2, the formation of N-containing compound, such as CaNH or Ca2NH, is thermodynamically feasible (Figure S9). However, they are known to be difficult to be hydrogenated to NH3 under the applied condition.27-28 2Mn2N + Na(K)H + 1/2H2 → Mn4N + Na(K)NH2 (R2) Na(K)NH2 + H2 → NH3 + Na(K)H (R3)

Figure 1. (a) Temperature dependence of the ammonia synthesis rates of Mn4N, Mn4N-LiH, Mn4N-NaH, Mn4N-KH, Mn4N-CaH2 and Mn4N-BaH2 catalysts. Reaction conditions: syn-gas (N2:H2 = 1:3) with a WHSV of 60000 ml gcat-1 h-1; pressure, 10 bar. (b) NH3 production rates of various catalysts as well as Ru/MgO and Cs-Ru/MgO catalysts as references under the conditions of WHSV, 60000 ml gcat-1 h-1; pressure, 10 bar; temperature, 573 K. The activity data of black bars is calculated based on per gram catalyst weight. The activity data of red bars is calculated based on per gram Mn weight.


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Figure 2. (a) TG curves of Mn4N and Mn4N-AHs samples in a N2 flow. Reaction conditions: sample loading 5 mg, N2 flow rate 30 ml min-1, ramping rate 0.5 K min-1. (b) H2 signals (m/z = 2) during the temperature-programmed heating of the Mn2N-AHs samples in a Ar flow. (c) NH3 signals (m/z = 17) during the H2 temperature programmed reaction of Mn2N and Mn2N-AHs samples under a H2 flow of 7.5 bar.

Our previous investigation suggested that ammonia synthesis catalyzed by the TM-LiH composites may go through three steps, i.e., N2 activation, N transfer and the following hydrogenation to NH3.14 Based on this proposal, the activity of TM-AH depends on the thermodynamic and kinetic properties of all steps. Figure S9 shows the temperature dependence of free energy changes (∆G) for N2 activation, N transfer and hydrogenation reactions. It is seen that LiH and BaH2 have appropriate thermodynamics

in all of the three steps. While, either nitridation of Mn4NK(Na)H (Figure S9a and b) or hydrogenation of CaNH (Figure S9c) is thermodynamically less favorable than that of LiH or BaH2. From the kinetic point of view, the relatively better catalytic performances of Mn4N-LiH and Mn4N-BaH2 may be correlated to the positive roles of LiH and BaH2 in all the three steps, as reflected at least partially from the designed N2-TG, Ar-TPD and H2-TPR experimental results (Figure 2) and relatively small apparent activation energies (ca. 60 and 64 kJ mol-1 for Mn4N-LiH and Mn4N-BaH2, respectively) and N2 reaction orders shown in Figure 3c and S10. CaH2, on the other hand, may help on N2 activation and N transfer, while NaH and KH promote N removal and NH3 formation under H2, therefore, they also synergize with Mn4N to catalyze NH3 formation at higher rates than neat Mn4N, although with a lower efficiency than the LiH or BaH2. Moreover, by selecting ternary or multi-nary hydrides, it is possible to tune the thermodynamics as well as the kinetics of each step, and thus enhance further the overall ammonia synthesis activity of Mn nitride, which will be the focus of future work. In the present, apart from the relatively general information about the synergy of AHs and Mn nitride in catalyzing NH3 synthesis discussed above, we would like to look into the catalysis in details. The chemistry of Li, N and H is relatively well understood,25, 29 so the Mn-LiH system was chosen as a representative in the following investigation. 3.3 Effect of MnNx crystallite size on ammonia synthesis activity. Three Mn (nitride)-LiH composites, i.e., Mn4N-LiH, PECVD-MnNx-LiH and Mn-LiH with Mn to Li molar ratio of 1:5, were prepared. The XRD patterns of the fresh made catalysts are shown in Figure S11. Mn4N phase in Mn4N-LiH sample has sharp diffraction peaks, while PECVD-MnNx-LiH and Mn-LiH samples contain much broader peaks of metallic Mn (the N content of the PECVDMnNx sample is low, only metallic Mn was observed). The crystallite sizes of Mn4N (26.1 nm) and Mn (8.9 and 6.7 nm) in these catalysts were estimated according to Scherrer equation and listed in Table S5, which is in the order of Mn4N-LiH > PECVD-MnNx-LiH > Mn-LiH. All of the composites are catalytic active for ammonia synthesis under the conditions of 10 bar and WHSV of 60000 ml gcat-1 h-1 and have activities in the opposite order of the crystalline size of Mn, i.e., at 573 K, the activity of Mn-LiH is ca. 2.7 times of PECVD-MnNx-LiH and 9.7 times of Mn4NLiH (Figure S2). These results manifest that the synergistic effect between Mn nitride and LiH has strong dependence on interfacial contact of these two components. Preparation method that can maximize the contact between Mn and AHs would lead to even better catalytic performance. The ammonia synthesis rates over the Mn-LiH catalyst increase by ca. 2.2 times with the raising of the reaction pressure from 10 bar to 30 bar (Figure S12). The stability test of the catalyst was conducted under the conditions of WHSV = 60000 ml gcat-1 h-1, 573 K and 30 bar (Figure S13). The ammonia formation rate was maintained during a period of 10 hours.



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3.4 Condition-dependent active phase and kinetic behavior. As discussed in our previous investigation, NH3 production catalyzed by the 3d TM-LiH composites may undergo a “three-step” cycle involving N2 activation (StepR1), “activated N” transfer (Step-R2) and Li-N-H hydrogenation (Step-R3) (Scheme S1). The rates of these steps are intra-dependent and also of reaction conditiondependent, which may have some influences on the active phases of the catalyst. More specifically, if the step-R1 is kinetically slow, N-poor nitride (e.g., Mn4N) and LiH are likely the active phases because once the activated N species are formed, it will be removed quickly by LiH to form Li-N-H that will also be quickly hydrogenated to NH3. If the step-R2 is the slowest step, some activated N species will be accumulated on Mn leading to N-rich Mn nitride but LiH remains the main Li-containing phase. Under conditions where the step-R3 becomes the slowest, activated N would be accommodated by both Mn and Li forming the N-rich Mn nitride and Li-N-H (LiNH2 and/or Li2NH) phases. In the following part, by measuring the dynamic changes of bulk phases under different reaction conditions, e.g., temperature and space velocity, we endeavored to correlate the changes in bulk phases with the kinetic behaviors. A. Temperature. The ammonia synthesis activities of the Mn-LiH composite in the temperature range of 423673 K were recorded and converted into the Arrhenius plot (Figure 3a and b). The reaction orders of N2 at 498 K and 573 K were determined (as shown in Figure 3c).

Figure 3. (a) Temperature dependence of the ammonia synthesis activities of the Mn-LiH catalyst. Reaction conditions: catalyst loading, 30 mg; syn-gas (N2:H2=1:3) flow rate, 30 ml min-1; pressure, 10 bar. (b) Arrhenius plots of MnLiH catalyst in the temperature ranges of 423 K-548 K and 548 K-623 K respectively. (c) Dependence of ammonia synthesis rates over Mn-LiH composite catalyst on the partial pressures of N2 under a total pressure of 10 bar at 498 K (black line) and 573 K (red line), respectively.

Meanwhile, the post-reaction catalysts at corresponding reaction temperatures were collected for XRD measurements to monitor the phase change (Figure 4). As shown in Figure 3b, there is a kink point (at ca. 548 K) in the Arrhenius plot, manifesting different apparent activation energies (Ea) in these two temperature ranges, i.e., the activation energies are 86.1±1.9 and 49.8±0.9 kJ mol-1 at temperatures below and above 548 K, respectively. And the N2 reaction order drops from ca. 1.6 at 498 K to ca. zero at 573 K (Figure 3c). The active phase of Mn-LiH in the low temperature range mainly contains Mn4N and LiH, while, N-rich Mn2N and Li2NH were observed in the high temperature range, as revealed by the XRD patterns shown in Figure 4. The higher Ea value, higher N2 reaction order and the presence of N-poor Mn nitride phase in the low temperature region suggest that the N2 dissociation (stepR1) is kinetically slower than the Step-R2 and Step-R3, which is similar to the conventional Fe or Ru catalysts


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where dissociative N2 activation has been widely regarded as the rate-determining step (RDS) regardless of the presence of alkali (hydr)oxide promoters. 20, 30-32 To support this hypothesis, kinetic studies by using N2-TG measurements with different ramping rates were investigated (Figure S14). By employing Kissinger’s equation,33 the apparent activation energy of nitridation of Mn4N-LiH sample was determined to be 89.5±5.1 kJ mol-1. The similar Ea for nitridation reaction and for the overall ammonia synthesis over Mn4N-LiH (86.1±1.9 kJ mol-1) provides further support showing that the nitridation is kinetically significant under lower temperature range. Based on these estimated Ea values and thermodynamic data from literature, a simplified energy profile of ammonia synthesis over Mn4N-LiH was constructed and shown in Scheme S1. At higher temperatures, however, a much reduced activation energy that is similar to the Ea of hydrogenation of neat LiNH2 (ca. 49-56 kJ mol-1),14, 34 nearzero reaction order of N2 and N-rich phases (Mn2N and Li2NH) are the characteristics of the composite catalyst, implying that hydrogenation of Li-N-H to form NH3 (R3) is relatively kinetically slow. These results show that the kinetic behavior of Mn-LiH for ammonia synthesis is highly temperature-dependent, i.e., the RDS switches from N2 activation on Mn nitride at lower temperatures to the hydrogenation of LiNHx species at higher temperatures where Li-N-H species play a role as a N buffer that weakens the influence of N2 partial pressures on the overall reaction rate. It should be noted here that although the surface composition and structure of a catalyst may vary from its bulk phase, the information derived from bulk phases may serve as an indicator of the kinetic behavior and reaction mechanism.

ml gcat-1 h-1, the phases changed from Mn4N and LiH to Mn4N/Mn2N, LiH and to Mn2N, Li2NH. Derived from the phase composition, the Step-R1 may be the slow step at high WHSV, while the Step-R3 is slower at low WHSV. More specifically, the ammonia partial pressure is higher under a lower WHSV, the hydrogenation of LiNH2 may be retarded by ammonia desorption because of a negative NH3 reaction order of -1.3 and the formation of N-rich Mn nitride is also thermodynamically favorable, which lead to the formation of Mn2N and Li2NH phases (Figure 5a). In contrast, under a high WHSV, the rapid removal of NH3 would favor the hydrogenation of LiNH2 to LiH, which is supported by the fact that the absence of Li2NH or LiNH2 phase and the appearance of LiH in the sample (Figure 5c). Higher LiH content in the composite would be beneficial for removing N from Mn2N to form N-deficient Mn4N phase and resulting in insufficient nitrogen supply, therefore, the N2 activation step would be rate-determining.

Figure 5. XRD patterns for Mn-LiH catalyst collected after testing under the conditions of a) WHSV = 20000 ml gcat-1 h-1, b) WHSV = 60000 ml gcat-1 h-1 and c) WHSV = 360000 ml gcat-1 h-1. Catalyst loading, 30 mg; temperature, 573 K; pressure, 10 bar.

Figure 4. XRD patterns of the Mn-LiH composite catalyst collected at different reaction temperatures. All samples were collected after cooling to room temperature rapidly and removal from the reactor.

B. Space velocity. The kinetic behaviors and active phases of Mn-LiH composite also depend on the weight hourly space velocity (WHSV). Figure 5 shows the XRD patterns of catalysts collected after test at different WHSV. We found that under the same temperature and pressure, along with the decrease of WHSV from 360000 to 20000

The effects of temperature and WHSV on the active phase and kinetic behavior of the composite catalyst discussed above disclose that the H and N contents in the active phase (and perhaps also at the active site) and the corresponding kinetic behavior (RDS, reaction order etc.) are sensitive to the reaction condition. Such a phenomenon was not observed or discussed in details in other conventional ammonia synthesis catalyst systems. For example, albeit γ-Mo2N was proposed to be the active phase under the ammonia synthesis reaction conditions of 673 K and 1 bar,35-36 the detailed phase information under different temperatures, pressures and gas compositions has not been disclosed. Alloying Mo with Co leads to a more active Co-Mo nitride catalyst for ammonia synthesis.21, 37 However, few investigation was given to the correlation between the composition of active phase and the kinetic behavior of the catalyst.38 The unique feature of the Mn-LiH composite can be ascribed to the rich chemistry of H2, N2 and NH3 with Li and Mn controlled by a set of kinetic and thermodynamic parameters. 3.5 Strategies to “activate” ETMs for catalysis. ETMs, such as Mn and Cr, show inferior activities of ammonia



synthesis. The present work proposed a strategy to activate Mn nitride by introducing alkali or alkaline earth metal hydrides. The role of hydrides is critical for the efficient ammonia synthesis. Recently, Kobayashi et al. reported on ammonia synthesis over the hydridecontaining Ti compounds (TiH2 and BaTiO2.5H0.5), where the complex interplay between hydride and nitride was discussed.39 Like the case of ETMs in ammonia synthesis, the strong adsorption of C, O or H species on some ETMs may hinder the further activation or conversion of reacting species, thus limit ETMs as effective catalysts in many heterogeneous catalytic processes. For example, Mn hydroxide shows inferior activity for electrochemical hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) due to the strong bond strength of OH-Mn.5 In another case, the inferior catalytic activities of V2O5, Cr2O3 and MnO2 for the H2 oxidation reaction have also been well correlated with their high metal-oxygen bond strengths.40 The roles of AHs on Mn in catalyzing ammonia synthesis disclosed by the experimental data of this work stimulates us to propose a possible strategy for the improvement of catalytic performance of ETMs. As shown in Scheme 1, a relayed two active center model for a bimolecular reaction is constructed. ETMs (active center 1) respond to the activation of N-, O- or C-containing reactant to form adsorbed species A’. The active center 2, on the other hand, has a strong driving force to abstract the species A’ from the center 1 forming more stable species A’’ and thus frees the center 1 for the continuous activation of A. The A’’ species may combine with another reactant B or activated species B’ to produce the final product. The relocation of reacting intermediates from ETM site and conversion of them to final product at the second active center disturb the intrinsic scaling relations existed on TM surfaces and adds freedom in the manipulation of the catalysis. The key point is the identification of second active center which can mediate the catalysis and thus enables ETM to realize their full catalytic potential.

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catalytic activity that is orders of magnitude higher than the neat manganese nitride. The understanding on the role of hydrides may shed light on the design and development of efficient early transition metal based heterogeneous catalysts for other chemical processes.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Calculation method for N2 order, Characterization results, stability test and catalytic cycle mechanism, including Table S1-5, Scheme S1 and Figures S1-S14 (PDF).

AUTHOR INFORMATION Corresponding Authors * [email protected] * [email protected]

ORCID Jianping Guo: 0000-0002-0229-8555

Present Addresses &F.C. Present affiliation: Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, David de Wiedgebouw, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank the financial support from National Natural Science Foundation of China (Grant Nos. 21633011 and 21603220), Sino-Japanese Research Cooperative Program of Ministry of science and technology (2016YFE0118300), DICP (DICP DMTO201504), Youth Innovation Promotion Association CAS (No. 2018213) and Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChEM). We also thank the Shanghai Synchrotron Radiation Facility (BL14W1) for providing beam time.

REFERENCES

Scheme 1 The schematic diagram for the design of more efficient ETMs catalysts.

4.

Conclusions

The strong N affinity of early transition metals limits their ammonia synthesis activities. The present work demonstrate that alkali or alkali earth hydrides, including LiH, BaH2, KH, CaH2 and NaH, can synergize with manganese nitride leading to significantly enhanced

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Table of Contents


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Alkali and alkaline earth hydrides-driven N2 activation and

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