Effects of Alkaline Earth Metal Amides on Ru in ... - ACS Publications

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Effects of Alkaline Earth Metal Amides on Ru in Catalytic Ammonia Decomposition Pei Yu,†,∥ Jianping Guo,*,† Lin Liu,† Peikun Wang,†,∥ Fei Chang,†,∥ Han Wang,†,∥ Xiaohua Ju,† and Ping Chen*,†,‡,§ †

Dalian Institute of Chemical Physics, ‡State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, and §Collaborative Innovation Center of Chemistry for Energy Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ∥ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: The effects of alkaline earth metal amides (Mg(NH2)2, Ca(NH2)2, and Ba(NH2)2) on Ru in catalyzing NH3 decomposition were investigated. The catalytic activities rank in the order of Ru−Ba(NH2)2 > Ru−Ca(NH2)2 > Ru−Mg(NH2)2, among which Ru−Ba(NH2)2 and Ru−Ca(NH2)2 catalysts have higher intrinsic activities (TOF) and lower apparent activation energies than those of Ru−Mg(NH2)2 and Ru/MgO catalysts, indicating that Ca(NH2)2 and Ba(NH2)2 may have different roles from those of Mg(NH2)2 and MgO. The TPR (temperature-programmed reaction) results show that Ca(NH2)2 or Ba(NH2)2 decomposes to N2 and H2 rather than NH3 in the presence of Ru. Ru may promote the NHx (x = 1, 2) coupling to H2 and N2 and change the decomposition pathways of Ca(NH2)2 and Ba(NH2)2. Kinetic analyses reveal that the Ru promoted NHx coupling to H2 and N2 may be the rate-determining step for catalytic ammonia decomposition. We suggest that the catalysis is very likely fulfilled via (1) Ru catalyzes the decomposition of amides to form H2, N2, and imides through an energy more favorable pathway and (2) imides react with NH3 to regenerate amides. The presence of Ca(NH2)2 or Ba(NH2)2 creates a NHx-rich environment, and Ru mediates the electron transfer from NHx to facilitate NHx coupling to N2 and H2.

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INTRODUCTION NH3 decomposition catalyzed by transition metals is one of the thoroughly investigated reactions in heterogeneous catalysis.1 The works before the 1990s on NH3 decomposition were conducted mainly to get insights into its reverse reaction, NH3 synthesis.2,3 Until recently, NH3 has been proposed as a promising COx-free (x = 1, 2) hydrogen carrier because of its abundance based on the well-established Haber−Bosch ammonia production process, high hydrogen content (17 wt %), high energy density (3 kWh/kg), and facile storage and transportation.4−7 To realize the practical utilization of NH3 as a hydrogen carrier, the development of highly active catalyst is of great importance. To date, various kinds of transition metals,8−10 alloys,11,12 metal carbides, and nitrides13−15 have been evaluated, among which Ru-based catalyst is the most active.16 Carbon8,17−19 and N-modified carbon20−23 materials are generally believed to be better supports than the oxide materials for Ru because they may have the ability to donate electrons and thus can weaken the metal−N bond and facilitate the recombinative desorption of N atoms adsorbed on Ru surfaces, which is a rate-determining step. A recent work showing effective electron donation from the unique inorganic electride of [Ca24Al28O64]4+(e−)4 to Ru leading to enhanced catalytic activity has been reported.24 It is a common practice to add small amount of promoters, such as alkali or alkaline earth © 2016 American Chemical Society

metal oxides and hydroxides, to enhance the catalytic activities of transition metals.25−28 However, the promotional capabilities and promoting mechanisms of alkali or alkaline earth metals or compounds are still controversial. The alkalis have usually been regarded as electronic promoters, with similar roles as the electride. Mg and Ca are considered to be structural promoters, whereas Ba has been proposed to be either electronic29,30 or structural promoter.31 Yin et al. reported that the order of promoting effect can be ranked as K > Na > Li > Ba > Ca on the Ru/CNTs catalysts,32 whereas Zhu et al. found that the promoting effect follows the order K > Na > Ca > Li on the Ru/CMK-3 catalysts.33 Nagaoka reported that Ru/Pr6O11 doped with alkali metal oxides, except for Li2O, exhibited higher NH3 conversions than bare Ru/Pr6O11. In contrast, Ru samples doped with alkaline earth metal oxides showed lower NH3 conversions than the bare Ru/Pr6O11.34 Recently we reported that the promoting effects of alkalis depend strongly on their chemical forms. Li, commonly regarded as the least promoting alkali, when in the form of lithium amide (LiNH2), can synergize with Ru leading to extraordinarily high catalytic activity that is even superior to Received: December 1, 2015 Revised: January 18, 2016 Published: January 19, 2016 2822

DOI: 10.1021/acs.jpcc.5b11768 J. Phys. Chem. C 2016, 120, 2822−2828

Article

The Journal of Physical Chemistry C

Activity was measured at 25 °C interval. Ammonia conversion data reported here were collected after 20 min time-on-stream at the corresponding temperatures. The flow rates of 5 vol % NH3/Ar (99.999%) and pure NH3 (99.999%) both were 30 mL/min regulated by a mass flow controller (Brooks instrument, model 5850 E). The exhaust gas composition including the unconverted NH3 and produced N2 was analyzed by using an online gas chromatograph (GC-2014C, Shimadzu) equipped with a Porapak N column and a thermal conductivity detector. Catalyst Characterization. ICP-OES (inductive coupled plasma optical emission spectrometer) measurements: ICPOES (PerkinElmer ICP-OES 7300DV) was used to determine the content of Ru in the catalyst. All of the catalysts contain ca. 5 wt % Ru. XRD (X-ray diffraction) measurements: Phase identification of samples was performed by using a PANalytical X’Pert diffractometer with Cu Kα radiation at 40 kV and 40 mA. A self-made sample cell covered with KAPTON film was used to protect samples from air or moisture contamination. Gaseous products analysis: The gaseous products after ballmilling RuCl3 with Mg(NH2)2, Ca(NH2)2, and Ba(NH2)2 samples were identified by a mass spectrometer (Hiden HPR20) adopting bar mode, which recorded the signals of m/z from 1 to 45 simultaneously. BET (Brunauer, Emmett, and Teller) method: A Micromeritics ASAP 2010 automated physisorption instrument was used to measure the N2 adsorption isotherm of self-made MgO at liquid N2 temperature (−196 °C). The specific surface area was determined from the linear portion of the BET plot. Before measurement, the sample was heated to 300 °C and kept at this temperature for 5 h. The specific surface area of MgO is ca. 350 m2/g. HRTEM (high resolution transmission electron microscopy) measurements: To identify the chemical state, morphology, and particle size of Ru, HRTEM (JEM-2100) was performed on these catalysts at 200 kV. Typically, the catalyst powder was dispersed in tetrahydrofuran (THF) and dropped on a carboncoated copper TEM grid. TPR (temperature-programmed-reaction) measurements: The gaseous products from samples during the heating process were measured by using a homemade TPR system equipped with an online mass spectrometer (Hiden HPR-20), which recorded the signals of H2 (m/z = 2), N2 (m/z = 28), and NH3 (m/z = 17) simultaneously. Pure Ar was used as the carrier gas (40 mL/min). In each test, ca. 10 mg quartz fiber was loaded in the middle of the straight quartz reactor (diameter: 0.6 cm; length: 25 cm) to hold the catalyst powder. 30 mg of sample was loaded and heated from room temperature to 500 °C at a ramping rate of 5 °C/min. Kinetic analysis: Kissinger’s approach36−38 was employed to determine the apparent activation energy (Ea) of gas release from the Ru−Ca(NH2)2 and Ru−Ba(NH2)2 samples. The Kissinger equation is described as

that of K-promoted Ru/MgO for NH3 decomposition.35 The role of Li was suggested to stabilize the NHx (x = 1, 2) species, and Ru mediates the electron transfer facilitating the NHx coupling to form N2 and H2. It is therefore of interest to find whether amides of other alkali or alkaline earth metals can have similar roles in catalytic NH3 decomposition. In the present work, we aim to investigate the effects of alkaline earth metal (Mg, Ca, and Ba) amides on the catalytic behavior of Ru for NH3 decomposition. Our experimental results show that the catalytic activities rank in the order of Ru−Ba(NH2)2 > Ru−Ca(NH2)2 > Ru−Mg(NH2)2, among which Ru−Ba(NH2)2 and Ru−Ca(NH2)2 catalysts have higher intrinsic activities (TOF) and lower apparent activation energies than those of Ru−Mg(NH2)2 and reference Ru/ MgO catalysts, indicating that Ca(NH2)2 and Ba(NH2)2 may have different roles from those of Mg(NH2)2 and MgO.

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EXPERIMENTAL SECTION Catalyst Preparation. Mg(NH2)2 was synthesized by reacting metallic Mg powder (Sigma-Aldrich, 99%) with purified NH3 (Dalian CREDIT, 99.999%) at 300 °C for ca. 200 h. Ca(NH2)2 and Ba(NH2)2 were prepared by reacting calcium (Alfa-Aesar, 99%, shot diameter: ca. 1 cm) and barium (Sigma-Aldrich, 99%, shot diameter: ca. 2 cm) metals with liquid ammonia in closed systems at room temperature for 1 day. The synthesized samples were collected for XRD (X-ray diffraction) characterization as shown in Figure S1. Figure S1 shows no any other phases except the alkaline earth metal amides. RuCl3·4.7H2O was dried at 210 °C for 7 h under vacuum. TG (thermogravimetric) measurement was carried out on postdried sample showing no obvious weight loss until 500 °C, indicating that crystal water was removed completely. The Ru-alkaline earth metal amide composite catalysts were prepared by ball-milling self-made alkaline earth metal amides (1 g) with RuCl3 (114 mg) in an iron jar at 150 rpm for 3 h on a Retsch planetary ball mill (PM 400). The desired loadings of Ru in the catalysts were 5 mg per g catalyst. The obtained samples were denoted as Ru−Mg(NH2)2, Ru−Ca(NH2)2, and Ru−Ba(NH2)2, respectively. High surface area MgO was prepared by the precipitation method. C4H6O4Mg·4H2O (DAMAO, 99%) and H2C2O4 (Kermel, 99%) were used as starting materials. C4H6O4Mg·4H2O and H2C2O4 were dissolved in distilled water. The C4H6O4Mg aqueous solution was added to H2C2O4 aqueous solution with constant stirring for 3 h to ensure complete precipitation. After filtration, the precipitate was washed three times and dried in an oven at 100 °C for 2 h. The dried precursor was calcined at 540 °C for 4 h under argon flow, and then the high surface area MgO was obtained. MgO supported Ru catalyst was prepared by the incipient wetness impregnation of MgO with acetone solution of RuCl3. Prior to test, the sample was reduced in H2 (30 mL/ min) at 400 °C for 2 h. All catalysts were stored in a glovebox filled with Ar to protect samples from air or moisture contamination. Catalyst Testing. Ammonia decomposition reaction was performed on a continuous-flow fixed-bed straight quartz reactor (diameter: 0.6 cm; length: 25 cm) at atmospheric pressure. Typically, ca. 10 mg quartz fiber was loaded in the middle of the quartz reactor to hold the catalyst powder. 30 mg of catalyst powder was loaded, calcined in an argon flow at 250 °C for 2 h, and tested in the temperature range of 250−500 °C under 5 vol % NH3/Ar or 300−600 °C under pure NH3 flow. The temperature was raised at a ramping rate of 5 °C/min.

d[ln(β /Tm 2)/d(1/Tm)] = −Ea /R

where Tm is the peak temperature at which the maximum reaction rate is attained, β is the heating rate, Ea is the activation energy, and R is the gas constant. The TPR technique was employed to collect the peak temperatures of H2 at various heating rates (5, 6, 8, and 10 °C/min). The dependency of ln(β/Tm2) on 1/Tm was plotted, and the slope of the fitted line was used to determine the value of Ea/R. 2823


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RESULTS AND DISCUSSION The Ru−alkaline earth metal amide composite catalysts were prepared by ball-milling mixtures of RuCl3 and alkaline earth metal amides (Mg(NH2)2, Ca(NH2)2, and Ba(NH2)2). The gaseous products after ball-milling were analyzed by mass spectrometer. NH3 and N2 were detected for ball-milled RuCl3−Ca(NH2)2 and Ba(NH2)2 samples except for the RuCl3−Mg(NH2)2 sample (Figure S2). The formation of N2 indicates the occurrence of redox reaction between RuCl3 and Ca(NH2)2 (or Ba(NH2)2) during the ball-milling process, and it is expected that the Ru3+ should be reduced simultaneously. The solid products after ball-milling were collected for further XRD characterizations. However, no any of the crystalline phases related to Ru metal can be observed except for the unreacted alkaline earth metal amides (Figure 1), which may

Figure 2. HRTEM images of (a) Ru−Mg(NH2)2, (b) Ru−Ca(NH2)2, and (c) Ru−Ba(NH2)2 catalysts collected after heating at 500 °C under an Ar atmosphere.

Figure 1. XRD patterns of as-prepared (a) Ru−Mg(NH2)2, (b) Ru− Ca(NH2)2, and (c) Ru−Ba(NH2)2 samples.

due to the high dispersion of Ru particles on the Ca(NH2)2 (or Ba(NH2)2) or unreduced RuCl3 on the Mg(NH2)2. After calcination at 500 °C in Ar flow, samples were collected and characterized by HRTEM. As shown in Figure 2, nanoparticles show lattice spacing of the (100), (002), or (101) crystallographic planes of metallic Ru, and no particle with lattice spacing corresponding to ruthenium oxide was detected in all of these three composite catalysts, indicating that metallic Ru formed after calcination. Statistical analyses by counting 100 Ru nanoparticles provide the information on particle size distributions of Ru−Mg(NH2)2, Ru−Ca(NH2)2, Ru−Ba(NH2)2, and Ru/MgO samples (Figure 3), and the mean particle sizes and dispersions of Ru are summarized in Table 1. The average particle sizes of Ru in all samples are all around 2− 4 nm. It has been widely recognized that both NH3 synthesis and decomposition are structure-sensitive reactions.39 Structure sensitivity of the reaction indicates the need for a polynuclear cluster as an active site. B5-type site consists of an arrangement of three Ru atoms in one layer and two further Ru in the layer directly above this at a monatomic step on an Ru(0001) terrace and was proposed as Ru active sites for the ammonia synthesis process.40 Based on this model, the concentration of B5 sites has been estimated to be the highest for Ru particles with size around 2−4 nm.39,41 The similar particle size distributions of Ru−alkaline earth metal amides allow the comparison of the effects of alkaline earth metal amides on catalytic behaviors of Ru nanoparticles. Shown in Figure 4a is the temperature dependence of NH3 conversion over a series of Ru-based catalysts under a flow of

Figure 3. TEM images of (a) Ru−Mg(NH2)2, (b) Ru−Ca(NH2)2, (c) Ru−Ba(NH2)2, and (d) Ru/MgO catalysts after catalytic testing at 600 °C under a pure NH3 atmosphere. Insets: Ru particle size distributions.

pure NH3. Alkaline earth metal amides affect the catalytic activities of Ru dramatically. The order of NH3 conversion can be ranked as Ru−Ba(NH2)2 > Ru−Ca(NH2)2 > Ru−Mg(NH2)2 below 525 °C. Under diluted NH3 atmosphere, the same sequence of activity is shown in Figure S3. It is worth pointing out that the activity of Ru−Ba(NH2)2 has an obvious drop above 350 °C under 5 vol % NH3/Ar (Figure S3) and above 475 °C under pure NH3 atmosphere, which may be due to the fact that Ba(NH2)2 has a relatively lower melting point of 280 °C, and the separation of metallic Ru from Ba(NH2)2 and the aggregation of metallic Ru may occur. The Arrhenius plots of all catalysts are compared in Figure 4b. The apparent activation energy (Ea) of Ru/MgO is 99.3 ± 8.6 kJ/mol, which is very similar to that of Ru−Mg(NH2)2 2824


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The Journal of Physical Chemistry C Table 1. Properties and Activities of Ru-Based Catalysts catalyst

Ru loadinga (wt %)

av particle sizeb (nm)

dispersionc (%)

H2 formation rated (mmolH2/gcat/min)

TOFH2 (s−1)

Ru−Mg(NH2)2 Ru−Ca(NH2)2 Ru−Ba(NH2)2 Ru/MgO Ru/CNTse

5.0 4.6 4.4 4.7 4.8

2.6 3.2 3.7 2.7 2

50.1 41.0 35.2 48.3 21.1

1.21 4.60 8.07 4.22 5.7

0.14 0.42 1.29 0.33 1

a

Ru actual loading was determined by ICP-OES. bStatistical analyses by counting 100 Ru nanoparticles from TEM images. cRu dispersion was calculated employing model proposed by Anderson;42 the calculation process is available in the Supporting Information. dH2 formation rate was measured under pure NH3 flow at 400 °C. WHSV (weight hourly space velocity) = 60 000 mLNH3/gcat/h. eThe properties and activities of Ru/CNTs are available from ref 43.

Figure 4. Temperature dependence of NH3 conversion over Ru-based catalysts (a) and the corresponding Arrhenius plots (b). Reaction conditions: sample loading, 30 mg; flow rate, 30 mL/min, pure NH3.

(101.4 ± 7.7 kJ/mol). Compared with Ru/MgO and Ru− Mg(NH2)2, a significant drop in apparent activation energies (ca. 25 kJ/mol) can be achieved for Ru−Ca(NH2)2 and Ru− Ba(NH2)2, which are 76.7 ± 2.0 and 73.1 ± 1.7 kJ/mol, respectively. Table 1 summarizes the TOF values calculated from the dispersions of Ru nanoparticles. The TOF values of Ru−Ba(NH2)2 and Ru−Ca(NH2)2 are 1.29 and 0.42 s−1, which are 8.2 and 2 times higher than that of Ru−Mg(NH2)2 (0.14 s−1) at 400 °C, respectively. It is worth pointing out that TOF value of Ru−Ba(NH2)2 is even superior to that of highly active Ru/CNTs catalyst (1.1 s−1, 400 °C, WHSV = 150 000 mLNH3/ gcat/h).6 Considering the similar particle size distributions of Ru on all of the catalysts, the different apparent activation energies and intrinsic activities (TOF) reflect that Ru−Ca(NH2)2 and Ru−Ba(NH2)2 may have different reaction pathway from Ru/ MgO and Ru−Mg(NH2)2 in catalytic ammonia decomposition. To investigate the interactions of Ru and alkaline earth metal amides, TPR measurements were carried out. As shown in Figures 5a and 5b, the onset and peak temperatures of NH3 over Ru−Mg(NH2)2 are ca. 300 and 375 °C, respectively, which exhibit no significant difference from those of neat Mg(NH2)2, indicating the decomposition pathway of Mg-

Figure 5. TPR profiles of (a) ball-milled Mg(NH2)2, (b) Ru− Mg(NH2)2, (c) ball-milled Ca(NH2)2, (d) Ru−Ca(NH2)2, (e) ballmilled Ba(NH 2 ) 2 , and (f) Ru−Ba(NH 2 ) 2 samples. Reaction conditions: sample loading, 30 mg; Ar flow rate, 40 mL/min; ramping rate, 5 °C/min.

(NH2)2 to release NH3 is not altered in the presence of Ru. However, the formation of H2 and N2 above 300 °C can be observed for Ru−Mg(NH2)2 showing the main difference from that of neat Mg(NH2)2. The fact that the peak temperatures of H2 and N2 coincide very well with the NH3 peak temperature (ca. 375 °C) is interesting. It can be proposed that Mg(NH2)2 decomposes to release NH3,44 which further decomposes to release N2 and H2 over Ru nanoparticles. Above results 2825


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that Ru may also has a catalytic function that directly decomposes Ba(NH2)2 to form H2, N2, and BaNH through an energy more favorable path (R4).

evidence that there is no obvious interaction between Mg(NH2)2 and Ru, particularly in the temperature range of 250−500 °C. For the Ru−Ca(NH2)2 sample, however, the presence of Ru affects the decomposition behavior of Ca(NH2)2 significantly. As shown in Figure 5c, two NH3 peaks are observed for the neat Ca(NH2)2, which can be ascribed to 3Ca(NH2)2 → (CaNH)2·Ca(NH2)2 + 2NH3 and (CaNH)2·Ca(NH2)2 → 3CaNH + NH3 (R1), respectively.45 The first NH3 peak temperature almost does not change after the addition of Ru, whereas the second NH3 peak which is observed in the neat Ca(NH2)2 (T = 429 °C) disappears in the Ru−Ca(NH2)2 sample (Figure 5d). Instead, H2 and N2 are observed with the peak temperatures at 388 °C, which are 41 °C lower than the second NH3 peak of neat Ca(NH2)2. We propose here that direct decomposition of (CaNH)2·Ca(NH2)2 to from N2, H2, and CaNH (R2) in the presence of Ru may occur, similar to the case of Ru−LiNH2 that we reported previously.46 We deduce that Ru should have a catalytic function that directly promote the decomposition of (CaNH)2·Ca(NH2)2 to release H2 and N2 through an energy more favorable path. The following two reactions may represent the thermal decomposition paths, respectively: (CaNH)2 Ca(NH 2)2 → 3CaNH + NH3 (CaNH)2 Ca(NH 2)2 → 3CaNH +

without Ru

3 1 H 2 + N2 2 2

1 3 2Ba(NH 2)2 → 2BaNH + NH3 + ( N2 + H 2) 2 2

without Ru (R3)

Ba(NH 2)2 → BaNH +

3 1 H 2 + N2 2 2

with Ru

(R4)

The effects of Ru on Ca(NH2)2 and Ba(NH2)2 are quite similar to the case of Ru−LiNH2,46 in which Ru promotes the NHx coupling and changes the decomposition pathways of Ca(NH2)2 and Ba(NH2)2. As shown in Figure 6, the activation

(R1) Figure 6. Kissinger’s plots of Ru−Ca(NH2)2 (red) and Ru−Ba(NH2)2 (blue) samples.

with Ru (R2)

The possibility of an alternative pathway of self-decomposition of (CaNH)2·Ca(NH2)2 to form CaNH and NH3 which further decomposes to release N2 and H2 over Ru nanoparticles cannot be excluded. However, if this path is the possible pathway, the production rates of N2 and H2 should be expected to depend on the formation rate of NH3 from the decomposition of (CaNH)2·Ca(NH2)2; then the H2 peak temperature of Ru− Ca(NH2)2 should be the same as the second NH3 peak temperature of pure Ca(NH2)2. However, as shown in Figure 5d, H2 and N2 peak temperatures are 388 °C, which are 41 °C lower than the second NH3 peak temperature of neat Ca(NH2)2. Hence, the former pathway, i.e., direct formation of H2 and N2 from (CaNH)2·Ca(NH2)2 (R2) may prevail in this case. The decomposition behavior of Ba(NH 2 ) 2 is more complicated than those of Mg(NH2)2 and Ca(NH2)2. Jacobs et al. reported that Ba(NH2)2 decomposes to Ba(NH)1−xN2/3x (x = 0.1) and NH3 after heating to 370 °C.47 With the aid of mass spectrometer (MS), we observed the evolution of considerable amount of H2 and N2 besides NH3 as shown in Figure 5e. XRD pattern shows that BaNH forms after TPR treatment to 500 °C (Figure S4). Combined with TPR and XRD results, reaction R3 may occur during the TPR process. The reaction pathway of formation of N2 and H2 during the decomposition process of neat Ba(NH2)2 is not clear yet; however, it indicates that the NHx coupling over Ba(NH2)2 to release N2 and H2 (R3) has a much lower kinetic barrier than those of neat Ca(NH2)2 and Mg(NH2)2 and even than LiNH2. For the Ru−Ba(NH2)2 sample, as shown in Figure 5f, the evolution of NH3 was significantly reduced. Similar to the case of Ru−Ca(NH2)2, the difference between NH3 peak temperature of neat Ba(NH2)2 and N2 or H2 peak temperature of Ru− Ba(NH2)2 suggests that the direct route of decomposition of Ba(NH2)2 to release N2 and H2 (R4) may prevail. We deduce

energies of reactions R2 and R4 were obtained by using Kissinger’s method. The activation energies of reactions R2 (72.3 kJ/mol) and R4 (67.3 kJ/mol) are consistent with the apparent activation energies of NH3 decomposition over Ru− Ca(NH2)2 (76.7 kJ/mol, Figure 4b) and Ru−Ba(NH2)2 (73.1 kJ/mol, Figure 4b) very well, respectively, evidencing that Ru catalyzing the decomposition of amides to form H2 and N2 may be the rate-determining step for catalytic NH3 decomposition. We suggest that the catalysis over Ru−Ca(NH2)2 and Ru− Ba(NH2)2 catalysts is very likely fulfilled via two steps: (1) Ru catalyzes the decomposition of amides to form H2, N2, and imides, and (2) imides react with NH3 to regenerate amides. That is to say, the combination of reaction R2 (or R4) and reverse reaction R1 (or R3) allows the fulfillment of the catalytic cycle of NH3 decomposition, among which Ru catalyzing NHx coupling to N2 and H2 is the rate-determining step for catalytic NH3 decomposition. This case is very different from that NH3 decomposition over transition metals. On the surfaces of transition metals, it is widely accepted that NH3 decomposition proceeds consecutive dehydrogenation followed by recombinative desorption of N and H atoms, among which the recombinative desorption of N atoms is the ratedetermining step.6 Although the NHx coupling over neat Ca(NH2)2 is very difficult (Figure 5c shows minor amount of H2 and N2) because of the large repulsive interaction between these two negatively charged NHx groups, the presence of Ru may decrease the negative charge of NHx groups through electron transfer leading to a reduced kinetic barrier of NHx coupling. The function of Ru together with the easiness of formation of N2 and H2 for neat Ba(NH2)2 (Figure 5e shows large amount of H2 and considerable amount of N2) indicates the kinetic barrier of NHx coupling on Ru−Ba(NH2)2 is much lower than that of Ru−Ca(NH2)2, which may account for the higher intrinsic 2826


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*(P.C.) E-mail [email protected]; Tel 86-411-84379905; Fax 86-411-84379583.

activity of Ru−Ba(NH2)2 for NH3 decomposition. The presence of Ca(NH2)2 or Ba(NH2)2 creates a NHx-rich environment and Ru mediates the electron transfer from NHx to facilitate NHx coupling to N2 and H2. However, for the Ru−Mg(NH2)2 composite, Mg(NH2)2 decomposes to Mg3N2 (Figure S5) rather than MgNH (unstable above 300 °C), resulting in the less chances of interaction of Mg(NH2)2 with Ru; thus, no significantly synergistic effect of Mg(NH2)2 on Ru can be obtained, and the catalytic activity of Ru−Mg(NH2)2 mainly comes from metallic Ru. In that case, magnesium amide, imide, or nitride can only function as a support.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support from the Project of National Science Funds for Distinguished Young Scholars (51225206) and National Natural Science Foundation of China (51472237 and 21473181).

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CONCLUSION In summary, the effects of alkaline earth metal amides (Mg(NH2)2, Ca(NH2)2, and Ba(NH2)2) on Ru in catalyzing NH3 decomposition have been evaluated. NH3 conversions rank in the order of Ru−Ba(NH2)2 > Ru−Ca(NH2)2 > Ru− Mg(NH2)2, among which Ru−Ba(NH2)2 and Ru−Ca(NH2)2 catalysts have higher intrinsic activities (TOF) and lower apparent activation energies than those of Ru−Mg(NH2)2 and reference Ru/MgO catalysts, indicating that Ca(NH2)2 and Ba(NH2)2 may have different roles from those of Mg(NH2)2 and MgO. The TPR results show that Ca(NH2)2 and Ba(NH2)2 decompose to N2 and H2 rather than NH3 in the presence of Ru. Ru may promote the NHx coupling to H2 and N2 and change the decomposition pathways of Ca(NH2)2 and Ba(NH2)2. The facts that the activation energies of (R2) and (R4) are consistent with the apparent activation energies of NH3 decomposition over Ru−Ca(NH2)2 and Ru−Ba(NH2)2 very well, respectively, suggest that Ru catalyzing the decomposition of amides to form H2 and N2 may be the rate-determining step for catalytic NH3 decomposition. We suggest that the catalysis over Ru−Ca(NH2)2 and Ru− Ba(NH2)2 catalysts is very likely fulfilled via (1) Ru catalyzes the decomposition of amides to form H2, N2, and imides through an energy more favorable pathway and (2) imides react with NH3 to regenerate the amides. In this context, Ba(NH2)2 and Ca(NH2)2 are neither electronic nor structural promoters. They participate in the catalytic reaction. The presence of Ca(NH2)2 or Ba(NH2)2 creates a NHx-rich environment, and Ru mediates the electron transfer from NHx to facilitate NHx coupling to release N2 and H2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11768. XRD patterns of self-made alkaline earth metal amide samples; MS (mass spectrum) analysis of collected gaseous products after ball-milling RuCl3 with Ca(NH2)2; temperature dependence of NH3 conversion over Ru-based catalysts under 5 vol % NH3/Ar; XRD pattern of Ba(NH2)2 sample collected after TPR to 500 °C; XRD pattern of decomposition products of Mg(NH2)2 at 500 °C; Ru dispersion calculation method (PDF)

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*(J.G.) E-mail [email protected]; Tel 86-411-84379583; Fax 86-411-84379583. 2827


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