Direct Activation of Cobalt Catalyst by 12CaOÂ·7Al2O3 Electride for

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Direct Activation of Cobalt Catalyst by 12CaO·7AlO Electride for Ammonia Synthesis 2

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Yasunori Inoue, Masaaki Kitano, Mai Tokunari, Teppei Taniguchi, Kayato Ooya, Hitoshi Abe, Yasuhiro Niwa, Masato Sasase, Michikazu Hara, and Hideo Hosono ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03650 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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ACS Catalysis

Direct Activation of Cobalt Catalyst by 12CaOꞏ7Al2O3 Electride for Ammonia Synthesis Yasunori Inoue1, ‡, Masaaki Kitano2, ‡, Mai Tokunari1, Teppei Taniguchi1, Kayato Ooya2, Hitoshi Abe3,4, Yasuhiro Niwa3, Masato Sasase2, Michikazu Hara1,*, Hideo Hosono1,2,* 1

Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta,

Midori-ku, Yokohama, 226−8503, Japan 2

Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259

Nagatsuta, Midori-ku, Yokohama, 226−8503, Japan 3

Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1−1,

Oho, Tsukuba, Ibaraki, 305−0801, Japan 4

Department of Materials Structure Science, School of High Energy Accelerator Science,

SOKENDAI (The Graduate University for Advanced Studies), 1−1 Oho, Tsukuba, Ibaraki, 3050801, Japan

ACS Paragon Plus Environment

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ABSTRACT

Cobalt is well known as an active component of heterogeneous solid catalysts or metal-complexes for the reduction of dinitrogen into ammonia. However, the activity of bare Co metal itself is not high due to its low nitrogen adsorption energy. Here, we show that the ammonia synthesis activity of a Co catalyst can be significantly boosted by 12CaOꞏ7Al2O3 electride (C12A7:e−) with a low work function of 2.4eV at low reaction temperatures (200−400°C) compared with that for typical Co catalysts. The ammonia formation reaction is initiated at 200°C for Co/C12A7:e−, whereas the reaction initiation temperature for Co/C12A7:O2− without electrons is 400°C. Therefore, Co/C12A7:e− has a much lower activation energy (ca. 50 kJ mol−1) than Co/C12A7:O2− (113 kJ mol−1), which is a similar level to that of the state-of-the-art Co-based catalysts such as Co3Mo3N and LiH-Co, in which ammonia formation over the Co catalyst is mediated by the formation of nitrides such as MoNx and LiNH. On the other hand, C12A7:e− directly enhances the activity of the Co catalyst for the N2 dissociation reaction without the aid of nitride formation, which results in the highest turnover frequency with low activation energy for ammonia synthesis at low reaction temperatures.

KEYWORDS: 12CaOꞏ7Al2O3, electride, cobalt catalyst, ammonia synthesis, electronic promotion

INTRODUCTION Some d-block transition metals, such as Re, Mo, Fe, Ru, Os, Co and Rh are known as catalysts for ammonia synthesis.1-2 Nørskov and colleagues have theoretically demonstrated that the catalytic activities of these transition metals are correlated with the chemisorption energy of nitrogen to the


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surface.3 Metals with high nitrogen adsorption energy have a low activation energy for N2 dissociation, while conversely, the energy barrier for NH3 desorption becomes large. This correlation leads to a volcano-type correlation between ammonia synthesis reactivity and the adsorption energy of nitrogen.3-5 On the basis of this relationship, Ru is regarded as the best catalyst among these metals because it has an intermediate nitrogen adsorption energy. Ozaki and Aika et al. experimentally demonstrated that Ru catalysts exhibit higher activity than other transition metal catalysts.6-7 On the other hand, Co is located at the right-hand side of the volcano plot, which means that the nitrogen adsorption energy of Co is much lower than that of Ru, and the energy barrier for N2 dissociation is large.5 As a result, the intrinsic activity of Co catalysts for ammonia synthesis is very low. Therefore, it is indispensable to add an electronic promoter, such as K2O, Cs2O, BaO to a Co catalyst in a heterogeneous solid catalyst system.8-11 Although bariumpromoted Co catalysts supported on carbon function as efficient catalysts for ammonia synthesis, these catalysts require high reaction temperatures (≥ 400°C) as well as high pressures (1.0-9.0 MPa), and exhibit high activation energies (90-110 kJ mol−1). On the other hand, it was recently reported that composite catalysts containing Co and metal hydrides (LiH and BaH2) exhibit high activity at low reaction temperatures, and have low activation energies (52-58 kJ mol-1),12-13 although these materials are not stable in the ambient atmosphere. Co catalysts are also promoted by the formation of bimetallic nitrides such as Co3Mo3N because Co-Mo bimetallic catalyst binds nitrogen species neither too strongly nor too weakly as compared with monometallic Co and Mo catalysts.14-17 While Co-based metal complexes exhibit ammonia synthesis activity even at room temperature, the use of strong reducing reagents and proton sources are necessary to render the nitrogen reduction to ammonia.18-19


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Recently, we have demonstrated that the 12CaOꞏ7Al2O3 electride (C12A7:e−) acts as an strong electronic promoter for Ru catalysts in ammonia synthesis when it is used as a catalyst support.2022

C12A7:e− consists of a nanocage (ca. 5 Å) framework with a positive charge, and electrons are

accommodated in the nanocages as counter anions.23 While C12A7:e− has a very low work function (2.4 eV) similar to metallic potassium, it is inert in the ambient atmosphere because electrons as counter anions are accommodated inside the subnanometer-sized stable Ca-O-Al cage framework with a small opening.24 The N2 dissociation over Ru is effectively boosted by electron injection from C12A7:e−.21 In addition, the electronic promoting ability of C12A7:e− in ammonia synthesis is outstanding compared with that of alkali promoters such as Cs-oxides, i.e., the high density of electrons in C12A7:e− boost N2 dissociation on the Ru surface and shift the bottleneck in ammonia synthesis from N2 dissociation to the formation of N-H species.21-22 Therefore, it may be expected that the dissociative adsorption of N2 on Co catalysts would be significantly enhanced by C12A7:e− due to its low work function (2.4 eV) compared with that of Co metal (5.0 eV).25 Herein we demonstrate that air-stable C12A7:e− is a highly efficient electronic promoter for Co catalysts in ammonia synthesis under low temperatures and low pressures. The effect of electronic promotion on the activity of the Co catalyst is considered based on results from scanning transmission electron microscopy (STEM), in situ X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), and N2 temperature programmed desorption (N2-TPD) measurements. In addition, we present the difference in the reaction mechanism between Coloaded C12A7:e− and highly active Co-based catalysts such as Co3Mo3N and LiH-Co. RESULTS AND DISCUSSION


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Figure 1A shows the temperature dependence of the ammonia synthesis rate over various Co catalysts at ambient pressure. Co(6 wt%)/MgO exhibits much lower activity for ammonia synthesis than Ru(6 wt%)/MgO, which is attributed to the lower nitrogen adsorption energy of Co than that of Ru. This result was consistent with the prediction by Nørskov and colleagues.3 The activity of Co/MgO was enhanced by the addition of Cs-oxide, which is a typical electronic promoter. The initiation temperature of ammonia synthesis was obviously reduced for Cs-Co(6 wt%)/MgO (340°C) compared with that for Co/MgO (400°C). When C12A7:O2− without electrons in the cages is used as a support for Co catalyst (2.9 wt%), the activity is slightly higher than that of Co/MgO. The activity of Ba-Co(10 wt%)/activated carbon (AC), which is well reported as a highly active Co catalyst,8-11 is similar to that of Cs-Co/MgO. The Co-based catalysts exhibited low ammonia synthesis rates (90 kJ mol−1)8,

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However, Co(2.6 wt%)/C12A7:e−

exhibits catalytic activity an order of magnitude higher than the other Co catalysts at 400°C, despite a low amount of Co-loading. The ammonia synthesis rates reach near thermodynamic equilibrium at 400°C but are far from thermodynamic equilibrium at low reaction temperatures (Figure S1). Ammonia synthesis over Co/C12A7:e− unexpectedly started at 200°C, which is 200°C lower than that for the Co/C12A7:O2− catalyst (Figure 1A). The activation energy of the Co/C12A7:e− catalyst was estimated to be ca. 50 kJ mol−1 from the Arrhenius plots for ammonia synthesis at 200-340°C (Figure S2), which is comparable to that of Cs-Co3Mo3N (47-58 kJ mol−1).15-16 Cs-Co3Mo3N was reported to have a higher nitrogen adsorption energy than other Co


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catalysts.3 In addition, Co/C12A7:e− has almost the same activation energy as those of the Co-LiH and BaH2-Co/CNTs catalysts (52-58 kJ mol−1).12-13 Co(3.0 wt%)/CeO2, which is known as an efficient ammonia synthesis catalyst at low reaction pressure,26 also shows high catalytic activity below 400°C (Figure 1). However, its activation energy (85 kJ mol−1) is much larger than that of Co/C12A7:e−, revealing that ammonia synthesis rate of Co/CeO2 is distinctly inferior to that of Co/C12A7:e− at low reaction temperatures. The high catalytic activity and low activation energy of Co/C12A7:e− are attributed to efficient electron-donation from C12A7:e−, which has a low work function (2.4 eV), to Co metal (5.0 eV), which leads to largely facile N2 dissociation at low reaction temperatures. The kinetic analysis was conducted to investigate the mechanism of ammonia synthesis over Co/C12A7:e− catalyst. As shown in Fig. S3, the reaction orders with respect to N2, H2, and NH3 are 1.08, 1.40, and -1.18, respectively. It is reported that carbon-supported bare Co catalyst exhibits negative H2 reaction order,9 indicating a strong interaction of hydrogen species with Co surface. This result may be related to higher chemisorption energy for hydrogen than that for nitrogen.27 This hydrogen poisoning on Co catalyst can be overcome by surface modification with Ba or Sr species.9 On the other hand, Co/C12A7:e− has large positive H2 order even though there is no additive on the surface of Co nanoparticles. As we reported previously, C12A7:e− has reversible exchangeability between H− ions and electrons (H0 + e− ↔ H−), which prevents hydrogen poisoning on Ru surface.20-21 Co/C12A7:e− also has hydrogen storage ability as demonstrated by H2-TPD result (Figure S4), i.e., hydrogen spillover takes place over Co/C12A7:e− and the excess hydrogen atoms are incorporated into the cages on the surface and near surface region of C12A7:e− during ammonia synthesis. Thus, the reaction mechanism is an analog of that for Ru/C12A7:e−. As expected from the positive reaction order for N2 and H2, the ammonia synthesis rate increased


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with reaction pressure up to 0.9 MPa (Figure 1B). The stability of Co/C12A7:e− was investigated in a continuous flow of the reactant gas for 60 h at 400°C and 0.9 MPa (Figure 1C). The catalyst continuously produced ammonia with a constant reaction rate over 60 h, indicating the high durability of the catalyst. We further investigated the effect of the reduction temperature on the catalytic performance of Co/C12A7:e−. Before the reaction, the Co/C12A7:e− catalyst was reduced at various temperatures under the same gas flow condition as ammonia synthesis reaction. As shown in Figure 1D, the activity is decreased slightly with the reduction temperature, but the decrement remained only 7% after the reduction at 600°C. This result shows the excellent thermal stability of the Co/C12A7:e− catalyst. Powder XRD patterns of Co-loaded C12A7:e− with various amounts of Co after reaction (400°C) are shown in Figure S5. Co/C12A7:e− consists mainly of the mayenite phase, although impurity phases such as CaO and CaAl2 were also observed in each sample; these two impurity phases are produced from an excess amount of CaH2 during the reduction process. The CaO phase cannot function as an effective electronic promoter because the Co/CaO catalyst exhibited much low catalytic activity (17 μmol g−1 h−1 at 400°C) in the same temperature range. No XRD peaks due to Co3O4 or Co were observed, which indicates that fine Co nanoparticles are dispersed on the surface of C12A7:e−. XAFS analyses were conducted to investigate the interaction between the Co precursor and C12A7:e− support during preparation of the catalyst. Figure 2A shows in situ Co K-edge X-ray absorption near-edge structure (XANES) spectra of the mixture of C12A7:e− and Co2(CO)8 at elevated temperatures under N2 and H2 gas flow. The spectrum for the catalyst precursor is almost identical to that of Co2(CO)8 (Figure S6A). The absorption edge was shifted to the lower-energy side and the peak above the edge decreased with an increase in the reduction temperature (Figures


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2A and S6B). Finally, the XANES spectrum of the catalyst coincided with that of Co foil. The fractions of Co and Co2(CO)8 in the catalyst were determined by fitting analysis using standard Co and Co2(CO)8 XANES spectra, and are shown as a function of the reduction temperature in Fig. 2B. The Co fraction gradually increased with the reduction temperature and Co2(CO)8 was fully converted into Co metal above 300°C. Figure 2C shows Fourier transforms (FTs) of the k2weighted extended XAFS (EXAFS) spectra (these spectra are not corrected for phase shift) at the Co K-edge. Co2(CO)8 has three peaks in the range of 1.0-3.0 Å. As shown in Figure S7, one molecule of Co2(CO)8 contains two bridging CO ligands and six terminal CO ligands.28 The mean Co-C, Co-Co, and C-O distances are reported to be 1.85, 2.52, and 1.17Å, respectively.29 Accordingly, these three peaks are assigned to Co-C, Co-Co, and Co-O bonds. When the reduction temperature is increased above 122°C, the intense Co−Co metal bond signal is observed at ca. 2.1 Å. After reduction at 400°C, the intensity of the Co-Co bond for Co/C12A7:e− is comparable to that for Co/C12A7:O2− (Figure 2D). These two FTs were fitted well with a theoretical EXAFS spectrum for Co metal (Figure S8) and the structural parameters determined by the curve-fitting analysis are summarized in Table S1. The coordination number (CN) of the Co-Co bond for Co/C12A7:e− and Co/C12A7:O2− were determined to be 6.21 and 6.49, respectively, which are much smaller than that of bulk Co (CN = 12). In addition, there is no additional peak, such as that for a Co-N bond,30 in the spectrum of Co/C12A7:e− (Figure S8A), which indicates that Co nanoparticles on C12A7:e− are maintained in the metallic form during ammonia synthesis. These results indicate that small sized Co metal nanoparticles are present on the surface of C12A7:e− during the reduction treatment and the Co particle size is almost identical to that of Co/C12A7:O2−. Accordingly, the difference in activity between Co/C12A7:e− and Co/C12A7:O2− is not due to the number of Co active sites.


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The local structures of these catalysts were further investigated by STEM analysis. Figure 3 shows STEM images of Co-loaded C12A7:e− and C12A7:O2− catalysts. STEM images of Coloaded C12A7:e− with various amounts of Co are also summarized in Figure S9. The nanosized Co particles were immobilized on both C12A7:e− and C12A7:O2−. In both cases, the average Co particle sizes were determined to be ca. 4.0 nm (Figure 3), which is consistent with the XAFS results. The shape of the Co nanoparticles was spherical for both samples. Therefore, the high catalytic performance of Co/C12A7:e− cannot be attributed to a structural promotion effect of Co particles themselves. Furthermore, we investigated the Co particles of Co/C12A7:e− catalyst after the stability test. Although the average Co particle size is slightly increased (ca. 6.2 nm) after 60 h of the reaction, most Co particles are distributed at ca. 4.0 nm (Figure S10). The catalyst surface was characterized by X-ray photoelectron spectroscopy (XPS) to evaluate the electronic and chemical state of Co nanoparticles on C12A7:e‒. As shown in Figure 4A, the main peak of Co 2p (778.1 eV) of Co/C12A7:e‒ catalyst is almost identical to the zero-valent cobalt (Co0) and additional peaks due to satellite structure of Co and CoO.31 The content of metallic Co is determined to be 99%. Thus, combined result of XAFS and XPS analyses revealed that both surface and bulk of Co nanoparticles on C12A7:e‒ are maintained in metallic state during ammonia synthesis. For the N 1s spectrum in Figure 4B, the broad peak is observed at 397.0 eV, which can be assigned to nitrogen atoms adsorbed on the catalyst surface. The N 1s peaks of Co-based nitride materials are observed at 397.7-397.9 eV,17, 32 while the chemisorbed nitrogen species appear at lower energy region (~397.0 eV).17, 33 This observation indicates the nitrogen is adsorbed on the Co-surfaces without forming nitride. N2-TPD measurement of Co/C12A7:e‒ were carried out to investigate the nitrogen adsorption ability of the catalyst. In the case of Co/C12A7:O2‒, the signal with m/z = 28 increased above


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600ºC (Figures 5 and S11A), but there is no signal with m/z = 14 and the signals with m/z = 44 and 12 appeared. Therefore, the observed peaks are attributed to CO2 and CO desorption rather than N2 desorption. The CO2 desorption is derived from CO2 adsorbed on the C12A7 support when it is exposed in an ambient atmosphere. The desorbed CO comes from the decomposition of CO2 into CO accompanied by the incorporation of O2‒ ions into the nanocages of C12A7 support.34 On the other hand, only N2 desorption was observed at 300-800ºC for Co/C12A7:e‒ catalyst (Figures 5 and S11B), implying that dissociative adsorption of nitrogen takes place effectively over Co/C12A7:e‒, making a contrast with Co/C12A7:O2‒. C12A7:e‒ without Co loading also shows small N2 desorption peak at 700ºC, which is attributed to nitrogen species such as N3- ions incorporated into the cages of C12A7:e‒ during the pretreatment of TPD measurement. The amounts of N2 desorption are determined to be 24.8 μmol g‒1 for Co/C12A7:e‒ and 2.5 μmol g‒1 for C12A7:e‒. Accordingly, the N2 desorption from Co/C12A7:e‒ catalyst is mainly derived from chemisorbed nitrogen species on the surface of Co catalyst. These results reveal that 24% of the surface Co sites on Co/C12A7:e‒ are covered with N adatoms on the basis of mean Co particle size of 4.0 nm. Thus, we can conclude that nitrogen adsorption ability of Co nanoptaricles is significantly strengthened by electron donation from C12A7:e‒ support. The catalytic activity was evaluated on the basis of the surface Co sites. As summarized in Table 1, the turnover frequency (TOF) of Co/C12A7:e− is an order of magnitude higher than those of other conventional Co catalysts. This result clearly indicates that electron donation from C12A7:e− significantly enhances the catalytic activity of Co nanoparticles. We have previously reported that the activities of Ru nanoparticle catalysts supported on C12A7:e− are dependent on the electron concentration (Ne) of C12A7:e−, i.e., the ammonia synthesis activity of Ru/C12A7:e− increases significantly at the metal-insulator transition point (Ne = 1.0×1021 cm−3).22 While the Co catalyst


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has intrinsically low activity for N2 dissociation because of its low binding energy with nitrogen, C12A7:e− with a high electron concentration (Ne = 1.0×1021 cm−3) accelerates N2 dissociation over the Co catalyst, which results in a high TOF with a low activation energy for ammonia synthesis. Figure 6 shows the dependence of the Co-loading amount on the catalytic activity of Co/C12A7:e−. The catalytic activity increased gradually with the Co-loading to reach a maximum at 3.6 wt% Co. The increase in activity is related to the increase in the number of surface Co sites (Ns), e.g., Ns for the 1 wt% and 2.6 wt% samples were estimated to be 5.7×1019 and 7.2×1019, respectively. It should be noted that the activity of Co (1 wt%)/C12A7:e− reaches approximately 80% of the maximum and still surpasses the activities of the other Co catalysts (Figure 1A). Table 2 summarizes the comparison of the catalytic performance of Co/C12A7:e− with those of the state-of-the-art Co catalysts reported to date. The results for Ru/C12A7:e− are also listed in Table 2. Although the ammonia synthesis rate over Co/C12A7:e− was almost half that of Ru/C12A7:e−, the activation energies of both catalysts were comparable. It was demonstrated that the electron-rich Co catalyst on C12A7:e− has sufficient potential to lower the energy barrier for N2 dissociation. The difference in the ammonia synthesis rates between these catalysts is tentatively attributed to the frequency factor. It can be deduced that Co atoms near the CoC12A7:e− interface work as active sites and the number is much less than that of Ru/C12A7:e−. As summarized in Table 2, Co/C12A7:e− exhibits low activation energies (ca. 50-51 kJ mol-1) comparable to Co3Mo3N, Co-LiH, and LaCoSi catalysts (ca. 42-57 kJ mol-1). However, the reaction mechanism for ammonia synthesis over the Co3Mo3N and Co-LiH catalysts is completely different from that over Co/C12A7:e−. In these reported Co-based catalysts, ammonia formation is mediated by the formation of nitrides. For instance, N2 molecules are activated at 3-fold hollow Mo sites on Co3Mo3N to form Mo3N and then hydrogen reacts with the activated nitrogen species


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on the Mo sites to form NH3 (Mars-van Krevelen type mechanism).35-36 As for the Co-LiH catalysts, N2 molecules are dissociated on the Co surface and the resultant nitrogen species is transferred to the hydride material (LiH) to form Li2NH, after which these species are hydrogenated to NH3.12 In both cases, nitride species are formed as intermediates. In contrast, N2 and H2 molecules are directly activated to form NH3 over Co nanoparticles on C12A7:e− by efficient electron donation. Thus, the Co nanoparticles on C12A7:e− is maintained in the metallic form after ammonia synthesis and any nitride phases are not formed on Co/C12A7:e−, as demonstrated by the XRD (Figure S5), XPS (Figure 4) and XAFS analyses (Figures 2 and S8). N2-TPD results revealed that N2 cleavage on Co catalyst is accelerated by electron transfer from C12A7:e− support (Figure 5). It is reported that the presence of Ce3+ in Co/CeO2 increases the electron density of Co particles and promotes the N2 dissociation.26 However, Co/CeO2 exhibits low TOF values with high activation energy than that for Co/C12A7:e− (Table 2), indicating that the electron donating power by Ce3+ is inferior to C12A7:e− for enhancement of the N2 activation at low reaction temperatures. The activation of Co catalyst by electron donation is also observed for a LaCoSi catalyst, in which negatively charged Co sites are formed by electron transfer from lattice La atoms.37 However, Co/C12A7:e− is substantially superior to LaCoSi with respect to TOF per total Co atoms. Furthermore, the TOF of Co/C12A7:e− also surpasses those of Cs-Co3Mo3N and Co-LiH catalysts, even though ammonia synthesis over Co/C12A7:e− is conducted at lower temperature and pressure. The present study demonstrates that the use of Co metal can be effectively reduced by maximizing the activity of the Co catalyst through electron injection from the C12A7:e− support to the Co nanoparticles. CONCLUSIONS


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Co nanoparticles supported on C12A7:e− have been demonstrated to function as an efficient catalyst for ammonia synthesis under low temperature and pressure conditions. The operation temperature of the Co/C12A7:e− catalyst is 200°C lower than the Co/C12A7:O2− catalyst, even though the Co particle size of these two catalysts was almost identical. This high catalytic activity is attributed to the enhancement of N2 dissociation by electron injection from C12A7:e− to the nanosized Co particles. The activation energy of Co/C12A7:e− is comparable to those of Co3Mo3N and LiH-Co composite catalysts, in which ammonia formation over Co catalyst is mediated by the formation of nitrides such as Mo-N or Li2NH. This study has revealed that the activity of a Co catalyst can be directly boosted by electron donation from a low-work function electride material.

EXPERIMENTAL DETAILS Synthesis of Co/C12A7:e−. C12A7:e− powder was prepared in a similar manner to the previously reported procedure.38 Briefly, a stoichiometric mixture (Ca:Al=12:14) of Ca(OH)2 and Al(OH)3 was heated in a Teflon-lined stainless autoclave at 150°C for 6 h. The resultant Ca−Al mixed hydroxide (Ca3Al2(OH)12) was heated at 600°C for 5 h in air, resulting in the formation of C12A7 including OH− ions in the cages (C12A7:OH−). The C12A7:OH− was converted into C12A7:O2− after heat treatment at 900°C for 5 h in vacuo. The mixture of C12A7:O2− powder and an excess amount of CaH2 was heated under vacuum at 700°C for 15 h, which resulted in C12A7:e− with surface area of ca. 20 m2 g−1. The electron density of C12A7:e− was determined to be 1.0×1021 cm−3 from the peak position of the characteristic band in the optical absorption spectrum.39 Co loading was performed according to the following procedure. C12A7:e− and Co2(CO)8 were mixed in an Ar-filled glove box, and the mixture (0.2 g) was set in a fixed-bed flow reactor without


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exposure to the ambient atmosphere. The mixture was then heated in a stoichiometric mixture of H2 and N2 (H2/N2 = 3:1, purity > 6N) gases (60 mL min−1) at 400°C (heating rate: 3.1 K min−1) for 2 h under ambient pressure. During the heat treatment, the Co2(CO)8 complex was converted into Co metal particles on the surface of C12A7:e−. Synthesis of Ba-Co/AC. The method for the preparation of the Ba-promoted Co/AC catalyst was reported previously.11 To remove adsorbed water and oxygen functional groups, activated carbon (AC) (Aldrich Co. Darco KB-B) was heated at 1000°C for 24 h in N2 flow (50 mL min−1). The activated carbon was impregnated with a methanol solution of Co(NO3)2ꞏ6H2O, followed by drying in an oven at 60°C and calcination at 220°C for 24 h to decompose the nitrate species. The amount of Co loading was 10 or 25 wt%. The calcined precursor (Co3O4/AC) was also reduced at 350°C for 24 h in a H2 gas flow (50 mL min−1), followed by passivation with small air pulses was to stabilize the surface of Co particles. The passivated Co/AC was then calcined in air at 60°C for 24 h. The re-calcined Co/AC powder was impregnated with aqueous Ba(NO3)2 solution (Ba: 0.70 mmol g(Co+C)−1) at 90°C for 16 h, followed by evaporation of the water under vacuum. The resulting sample was dried in air at 110°C. Before catalysis tests, the Ba-Co/AC catalyst was reduced in a stoichiometric mixture of H2 and N2 (H2/N2 = 3:1) at a gas flow of 60 mL min−1 according to the following temperature program: 400°C for 20 h, 470°C for 24 h, and 520°C for 24 h. Synthesis of Cs-Co/MgO. The synthesis of Cs-promoted Co/MgO catalyst was conducted with reference to a previous report.40 MgO (Ube, 500A) was heated at 500°C under vacuum for 12 h, followed by impregnation with a dehydrated ethanol solution of Co(NO3)2ꞏ6H2O. The amount of Co loading was 6 wt%. After stirring for 3 h, the solvent was evaporated and the resultant sample


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ACS Catalysis

(Co(NO3)2/MgO) was heated at 400°C under vacuum to decompose the nitrate species. Both Cs2CO3 (Kanto Co. 99.9%) and Co/MgO (atomic ratio: Co/Cs = 1) were soaked in dehydrated ethanol and stirred for 3 h, after which the solvent was removed by evaporation and the catalyst was dried under vacuum overnight. Synthesis of Co/CeO2. The synthesis of Co/CeO2 catalyst was conducted using the same method as Co/C12A7:e−. The mixture of CeO2 (Aldrich, 99.9%) and Co2(CO)8 was set in a fixedbed flow reactor without exposure to the ambient atmosphere. The mixture was then heated in a stoichiometric mixture of H2 and N2 (H2/N2 = 3:1, purity > 6N) gases (60 mL min−1) at 400°C (heating rate: 3.1 K min−1) for 2 h under ambient pressure. Characterization. STEM (JEM-2100F, ARM-200F, Jeol) analysis was performed at 200 kV to determine the mean Co particle size. The resolution of the STEM apparatus is 2.0 Å. The powdered sample was suspended in ethanol. One drop of the suspension was deposited on a copper grid and then covered by a fine carbon membrane. X-ray powder diffraction (XRD) patterns of Co-loaded C12A7:e− with various amounts of Co were obtained using a diffractometer (D8 ADVANCE Bruker) with Co Kα radiation at 35 kV and 40 mA in the 2θ range from 15 to 70° with 0.03° resolution. The Co content was determined using X-ray fluorescent spectroscopy (XRF; ZSX100e, Rigaku), and the results are shown in table S2. The Brunauer-Emmett-Teller (BET) specific surface areas of the samples were measured from nitrogen adsorption-desorption isotherms at 77 K using an automatic analyzer (NOVA 4200e, Quantachrome). XAFS measurements were performed using a synchrotron radiation ring at BL-12C beamline (Photon Factory, KEK). In this beamline, a Si(111) double crystal monochromator and ionization chambers were used for transmission mode XAFS measurements. Co2(CO)8, C12A7:e− and BN (dried at 300°C) were mixed in an Ar-filled glove box and the obtained mixture was pressed into a pellet


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using a hand-press apparatus. The pellet sample was set in an in situ cell equipped with a heater, a water cooling system and a gas flow system to perform in situ XAFS measurements. Prior to measurement of the Co K-edge XAFS spectra, the sample was reduced in situ using a temperature program (range, 25−400°C; rate, 3.1°C min−1) with the same pretreatment conditions under ambient pressure. The Athena and Artemis software and the FEFF6 code were used to analyze XAFS data.41-42 XPS spectra were captured on an ESCA-3200 spectrometer (Shimadzu) using Mg Kradiation at

Direct Activation of Cobalt Catalyst by 12CaOÂ·7Al2O3 Electride for

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