Review pubs.acs.org/acscatalysis
Ru-Loaded C12A7:e− Electride as a Catalyst for Ammonia Synthesis Michikazu Hara,*,†,§ Masaaki Kitano,‡ and Hideo Hosono*,†,‡,§ †
Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan § ACCEL, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡
ABSTRACT: The artificial mass production of ammonia has supported the increase in human population and modern civilization for over 100 years. However, more efficient ammonia production is now a significant concern for society. Here we show that Ru-loaded C12A7:e− electride derived from 12CaO·7Al2O3 (C12A7:O2−) acts as an efficient and stable catalyst for ammonia synthesis. Ammonia synthesis over Ru nanoparticles-loaded C12A7:e− (Ru/C12A7:e−) is distinct from other conventional catalysts in both mechanism and properties. The dissociative adsorption of N2 molecules, which is the largest energy barrier in ammonia synthesis for conventional catalysts, is no longer the rate-limiting step for the Ru/C12A7:e− catalyst. In addition, Ru on the electride prevents the inhibition of ammonia synthesis by hydrogen adatoms, known as hydrogen poisoning, which is a common and serious drawback of Ru catalysts. This characteristic results in the highly efficient formation of ammonia over the Ru/C12A7:e− catalyst. KEYWORDS: heterogeneous catalysis, ammonia, electride, ruthenium, hydride ions already been attempting nitrogen fixation, which had resulted in several methods such as the Frank−Caro process through calcium cyanamide and the Birkeland−Eyde process with an electric arc. However, these nitrogen fixation methods consumed large amounts of energy, and as such, they were not applicable to the mass production of fertilizer.2 At the beginning of the 20th century under this situation, Fritz Haber and Setsuro Tamaru clarified the correlation of equilibrium with catalysis in ammonia synthesis,3−6 Alwin Mittasch found the most effective catalyst for ammonia synthesis,7 and Carl Bosch established a high pressure chemical process, all of which contributed to the successful mass production of ammonia by artificial means, the so-called Haber−Bosch process. Their great achievement largely contributed not only to science, engineering, and industry but also to society and civilization. Since then, nitrogen fertilizers derived from ammonia produced by this process have supported an increase in human population; approximately 5 billion people are currently supported by the Haber−Bosch process. In addition, this process has produced significant food surpluses, which from the perspective of cultural anthropology, has decreased the amount of people required for food production and led to a diversification of society and the developing civilization. Today, many people benefit from this
1. INTRODUCTION The Agricultural Revolution, which originated from innovative crop rotation, helped to drive the Industrial Revolution from the 18th to the 19th century, which resulted in a rapidly increasing world population. People immediately knew that sufficient crops to support the increasing population could not be produced with conventional organic fertilizers alone, and thus, they began to mine inorganic fertilizers such as nitratine. This mining was draining mineral resources, and the emergency of the situation was sensed after the first half of the 19th century. Sir William Crookes, President of the British Association for the Advancement of Science, investigated the growth in population, wheat acreage, and consumption of nitrogen-containing mineral resources, and he warned in his presidential address on September 10, 1898, that humans would face starvation in several decades, especially due to the exhaustion of nitrogen resources; therefore, it would be necessary to establish nitrogen fixation methods.1,2 He recognized that all civilized nations stood in peril of not having enough to eat, and said: “It is the chemist who must come to the rescue of the threatened communities. It is through the laboratory that starvation may ultimately be turned into plenty. Before we are in the grip of actual dearth, the chemist will step in and postpone the day of famine to so distant a period that we and our sons and grandsons may legitimately live without undue solicitude for the future”. He further indicated: “The fixation of atmospheric nitrogen is one of the great discoveries awaiting the ingenuity of chemists.”1,2 Many researchers had © 2017 American Chemical Society
Received: November 25, 2016 Revised: February 13, 2017 Published: February 15, 2017 2313
DOI: 10.1021/acscatal.6b03357 ACS Catal. 2017, 7, 2313−2324
Review
ACS Catalysis
Figure 1. Schematic structure of [Ca24Al28O64]4+(O2−)2−x(e−)2x (0 ≤ x ≤ 2).
is essential for efficient N2 cleavage. Sixty years after the birth of the Haber−Bosch process, Ozaki and Aika found that ruthenium, located just below iron on the periodic table, acts as a more effective catalyst for ammonia synthesis than a promoted iron catalyst at around atmospheric pressure, and Aika succeeded in improvement of the catalytic activity by the addition of alkali metal oxides such as Cs oxides to Ru-based catalysts.14−17 This enhancement is observed even by the addition of alkaline earth metal oxides such Ba oxides, which can also be attributed to an enhancement in electron-donating power to N2 π* orbitals.15−18 The performance of promoted iron and Ru-based catalysts clearly indicates that transition metals function as the leading catalysts for ammonia synthesis and addition of electrondonating components are also effective for efficient ammonia synthesis. Although transition metals have been extensively investigated as ammonia synthesis catalysts since Haber studied osmium, catalyst development with respect to the electron donor may also be a promising way to further improve the Haber−Bosch process. The electron-donating powers of alkaline and alkaline earth metal oxides used in promoted iron and Ru-based catalysts are not so strong because metal oxides do not have a small work function. On the other hand, alkali metals exhibit a small work function and thus have strong electron-donating power. For example, the deposition of metallic K onto a Fe(100) surface significantly promotes N2 dissociative adsorption.19 However, alkali metals readily react with ammonia under the reaction conditions for catalysis, and the resulting alkali metal amides are volatilized from the catalyst surfaces. Another drawback of an alkali metal as electron donor is its low melting point which causes evaporation at the reaction temperature. Low work function materials with high melting temperature, comparable to alkali metals, that do not react with ammonia, nitrogen, or hydrogen are thus required to develop new types of ammonia synthesis catalysts on the basis of this strategy.
modern civilization, and the annual amount of ammonia production is thus unparalleled by other chemicals. Ammonia production reached 150 million tons/year in 2015 and is predicted to exceed 200 million tons/year up to 2020 due to the very rapid growth in population in developing countries. At present, the population is predicted to exceed 8−10.5 billion by the 2040−2050 period in the not so distant future, thereby requiring a further increase in ammonia production. Furthermore, ammonia has received considerable attention as a promising energy carrier for H2 produced from natural gas and renewable energy. As a result, efficient ammonia production will have become one of our greatest concerns. The Haber− Bosch process, based on a catalyst that was discovered over 100 years ago, is today one of the most established processes with high efficiency. Even so, the development of new catalysts to further improve the efficiency of ammonia production remains a challenge.
2. PROMOTED IRON AND RUTHENIUM-BASED CATALYSTS FOR AMMONIA SYNTHESIS Low reaction temperatures are favorable for high ammonia yield in exothermic ammonia formation (46.1 kJ mol−1) from nitrogen and hydrogen molecules from an equilibrium perspective.3 An increase in the rate of ammonia formation at low temperatures is the basic strategy for efficient ammonia production. However, this is not so easily accomplished because the rate-determining step in ammonia synthesis is the dissociative adsorption of N2 molecules due to cleavage of the NN bond with a large bond energy that reaches to 945 kJ mol−1.8,9 The use of transition-metal catalysts based on equilibrium by Haber led to a major breakthrough in this problem, and Mittasch found that some iron ores, such as mixtures of Fe3O4, Al2O3, CaO and K2O, act as effective catalysts for ammonia synthesis. Catalysts based on iron ores, known as promoted iron catalysts, have since then been used as the main catalyst for the Haber−Bosch process. The working principle of the promoted iron catalyst remains a significant breakthrough, even today. A N2 molecule is fixed to form a bond with a transition metal by the donation of electrons from its bonding orbitals and acceptance of electrons by its antibonding π-orbitals (π*), that is, back-donation.10 However, the transition metal alone does not affect efficient N2 cleavage. The back-donation is enhanced by electron donors such as K2O, which further weakens the NN bond and results in the cleavage of N2.11,12 While the reaction mechanism for ammonia formation over the catalyst has been extensively discussed by many researchers, and still requires further discussion,11−13 there is a general agreement that an electron-donating material
3. [CA24AL28O64]4+(E−)4: C12A7:E− ELECTRIDE We have focused on [Ca 24 Al 28 O 64 ] 4+ (e − ) 4 (C12A7:e − hereafter), a stable solid electride with strong electron-donating capability, as a first step toward catalyst development with respect to electron-donating materials. An electride is an ionic compound composed of cations and electrons instead of anions, and it was first synthesized as Cs+(18-crown-6)2e− where Cs+ cations are sandwiched between crown ethers and electrons.20−22 Such electrides are applicable in various fields, including organic synthesis and the preparation of metal nanoparticles, because of their unique electronic properties. 2314
DOI: 10.1021/acscatal.6b03357 ACS Catal. 2017, 7, 2313−2324
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ACS Catalysis
Figure 2. Schematic band structures of CaO and [Ca24Al28O64]4+(O2−)2−x(e−)2x (0 ≤ x ≤ 2).
electrons occupy the delocalized cage conduction band states, giving band conduction. When Ne reaches 1.4 mmol g−1, the theoretical maximum concentration, all encaged O2− anions are replaced with electrons, which results in [Ca24Al28O64]4+(e−)4 (C12A7:e−) with a work function of 2.4 eV. It is reasonable that materials with a small work function are subject to oxidation, whereas C12A7:e− is chemically stable and exhibits high resistance toward reactants. In air without moisture, C12A7:e− is even stable at 673 K because the small size of an opening in the latticework prevents reaction with molecules in the outer atmosphere. Thus, C12A7:e− with a low work function and high stability, which are generally incompatible properties, may be available as a strong electron-donating material to accelerate N2 cleavage for ammonia formation; therefore, C12A7:e− deposited with transition-metal nanoparticles would be expected to exhibit much higher catalytic performance for ammonia synthesis than conventional catalysts, such as metallic potassium, by strong electron donation into the antibonding π-orbitals of N2 through pushing up the Fermi level of the deposited transition metals.
However, the instability of electrides has meant they have had limited utility; organic electrides are not thermally or chemically stable and easily decompose above 230 K. After two decades, the first room-temperature-stable electride derived from 12CaO·7Al2O3 (C12A7:O2−), a constituent of commercial alumina cement, was realized in 2003.23 The unit cell of C12A7:O2− can be expressed as [Ca24Al28O64]4+(O2−)2, where O2− anions are accommodated in subnanocages composed of Ca, Al and O as the counteranion as shown in Figure 1. The cage O2− anions, which are loosely bounded by 6 Ca2+ ion constituting the cage wall, can be replaced without destruction of the framework structure by electrons under reducing conditions at high temperatures, and the resulting materials with different electron concentrations (Ne) are denoted by [Ca24Al28O64]4+(O2−)2−x(e−)2x (0 ≤ x ≤ 2). In the theoretical maximum exchange (x = 2, Ne = 2.3 × 1021 cm−3 (1428 μmol g−1)), [Ca24Al28O64]4+(e−)4 (C12A7:e−) has a high electrical conductivity (1500 S cm−1) and a small work function (ca. 2.4 eV) comparable to metallic K.24 Such electronic properties are attributed to the unique band structure of the material. Figure 2 illustrates the schematic energy band structures of CaO and [Ca24Al28O64]4+(O2−)2−x(e−)2x. There is no fundamental difference in the positions of valence and conduction bands between CaO and [Ca24Al28O64]4+(O2−)2 (C12A7:O2−) without any cage electrons (x = 0, Ne = 0) because the top of the framework valence band and the bottom of the framework conduction band in both materials are composed of the 2p orbitals of the framework O2− ions and the 4s orbitals of the framework Ca2+ ions, respectively. 25 However, C12A7:O 2− has a cage conduction band (CCB) just below the conduction band bottom.26 This cage conduction band characterizing C12A7 originates from the three-dimensionally connected cages by sharing one oxide monolayer. The 2p levels of O2− ions in the cages are located in an energy region ca. 1 eV above the top of the framework valence band. Furthermore, a cage conduction band derived from electron tunneling among the threedimensionally connected cages with positive charges is formed in the band gap and is located 1−2 eV below the framework conduction band. In the case of 0 < Ne < 0.6 mmol g−1 (0 < x < ca. 1), electrons are confined in the cages and form F+-like centers of which the energy level is located 0.4 eV below the bottom of the cage conduction band.29 In this state, electron conduction occurs via hopping. The Fermi level of [Ca24Al28O64]4+(O2−)2−x(e−)2x at x ≥ ca. 1 (Ne ≥ 0.6 mmol g−1) vary from 0.15 to 0.5 eV above the cage conduction band minimum with increasing Ne, which indicates that most
4. SYNTHESIS OF C12A7:E−: PREPARATION AND CATALYTIC PERFORMANCE OF RU-DEPOSITED C12A7:E− (RU/C12A7:E−) CATALYST C12A7:e− can be synthesized by replacing cage O2− anions in C12A7:O2− with electrons. Various synthesis methods based on this principle have been reported, which cover various types of C12A7:e−, such as single crystals, films, and powders.23,27−29 C12A7:e− powder for catalyst applications is synthesized by calcination of a mixture of CaCO3 and α-Al2O3 (ratio of the amount of substance 11:7) above 1573 K, followed by reaction of metallic Ca with the resultant oxide precursor at 1073 K.30 X-ray diffraction (XRD) patterns for the oxide precursor and C12A7:e− are shown in Figure 3. It should be noted that the Ca content in the mixture of CaCO3 and α-Al2O3 is somewhat smaller than the stoichiometry of C12A7:O2− ([Ca24Al28O64]4+(O2−)2). As a result, the precursor contains monocalcium aluminate (CaO·Al 2 O 3 ) in addition to C12A7:O2−, as shown in Figure 3. However, heating the precursor with metallic Ca under vacuum to compensate for the stoichiometric deficit results in single-phase C12A7:e− because the oxidation of metallic Ca exchanges cage O2− anions with electrons and the resultant Ca2+ forms the [Ca24Al28O64]4+ cage frame with CaO·Al2O3. Ru was selected as a transition metal for deposition onto the C12A7:e− surface because Ru acts as an 2315
DOI: 10.1021/acscatal.6b03357 ACS Catal. 2017, 7, 2313−2324
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ACS Catalysis
Figure 3. XRD patterns for C12A7:e−, C12A7:O2−, and CaO·Al2O3.
efficient transition-metal catalyst for ammonia synthesis under atmospheric pressure.14−17 C12A7:e− deposited with Ru nanoparticles (Ru/C12A7:e−) was obtained by the thermal decomposition of Ru3(CO)12 at 523 K.30 Figure 4 shows transmission electron microscopic (TEM) images of Ru/ C12A7:e−. Ru particles distributing from several nm to 10 nm in diameter are observed in the TEM images for 0.1 wt %Ru/ C12A7:e−. In the case of 1.2 wt %Ru/C12A7:e−, large-sized Ru particles are formed by aggregation. The specific surface area of Ru/C12A7:e− is only 1 m2 g−1 because the synthesis of crystalline C12A7:e− requires high temperatures, at least above 1000 K. The catalytic activities of various Ru-loaded catalysts for ammonia synthesis are summarized in Table 1. Dispersion, average Ru particle size, and TOF in the table were estimated by CO-pulse chemisorption.30 The results for Ru−Cs/MgO and Ru−Ba/activated carbon (Ru−Ba/AC) are also shown for comparison. Ru−Cs/MgO is one of the most active Ru catalysts for ammonia synthesis, and Ru−Ba/AC has been used for commercial ammonia production. 18,31 Ru/C12A7:e − exhibits much higher catalytic activity than Ru/Al2O3, Ru/ CaO, Ru/C12A7:O2−, and Ru−Ba/AC, despite the lower surface area, and the ammonia effluent mole fractions for Ru/ C12A7:e− and Ru−Cs/MgO reach almost thermodynamic equilibrium (ca. 0.5%). In addition, Ru/C12A7:e− is superior to all tested catalysts with respect to turnover frequency (TOF) of the effective surface Ru atoms. The TOFs of conventional Rubased catalysts are lower than 0.01 s−1 and are not significantly dependent on the amount of Ru loading. On the other hand, the TOF for Ru/C12A7:e− increases with the Ru content and reaches a maximum (0.27 s−1) at 0.3 wt % Ru. Further Ruloading somewhat decreases the TOF on Ru/C12A7:e−; however, the TOF of Ru/C12A7:e− is still much higher than those of conventional Ru-based catalysts, despite the smaller surface area, large Ru particles, and lower Ru dispersion. Table 1 summarizes the apparent activation energies for various Rucatalysts. It is evident that Ru/C12A7:e− has much lower activation energies than the other catalysts; the activation
Figure 4. TEM images of 0.1 and 1.2 wt %Ru/C12A7:e−.
energies for ammonia synthesis by Ru/C12A7:e− are ca. 30− 80% those of other Ru catalysts. It should be noted that the rate of ammonia formation over Ru/C12A7:e− increases with the pressure. Figure 5 shows the correlation of TOF with the total pressure over Ru/C12A7:e− and Ru−Cs/MgO at 633 K. Although an increase in reactant concentration generally increases the reaction rate, the TOF of Ru−Cs/MgO is independent of the pressure. This is due to hydrogen poisoning, a serious drawback that occurs for all Rubased catalysts.18,32 The dissociative adsorption of H2 occurs more easily than that of N2 and Ru surfaces prefer H adatoms to N adatoms under ammonia synthesis conditions. This preference is enhanced with an increase in total pressure; therefore, an increase in pressure cannot accelerate ammonia formation over conventional Ru-based catalysts. Industrial 2316
DOI: 10.1021/acscatal.6b03357 ACS Catal. 2017, 7, 2313−2324
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ACS Catalysis Table 1. Catalytic Performance of Ru Catalysts on Various Supportsa catalyst Ru/γ-Al2O3 Ru/CaO Ru−Ba/AC Ru−Cs/MgO Ru/C12A7:O2− Ru/C12A7:e‑
surface area (m2 g−1) 170 3 310 310 12 12 1−2 1−2
Ru-loading (%) 6.0 1.5 1.0 9.1 1.0 6.0 1.2 0.1 0.3 1.2 4.0
dispersion (%)
particle size (nm)
12.5 4.9 25.2 14.3 50.3 18.6 3.4 15.6 4.1 3.2 2.0
10.6 27.2 5.3 9.3 2.7 7.2 39.2 8.5 32.9 41.3 68.5
NH3 formation (μmol g−1 h−1) 50 160 150 2230 2260 3350 550 720 1030 2760 2120
TOF (s−1) −4
× 10 × 10−3 × 10−3 × 10−3 × 10−2 × 10−3 × 10−2 0.16 0.27 0.20 7.6 × 10‑2
2.0 6.0 3.0 3.0 1.3 8.0 3.8
Ea (kJ mol−1) 120 89 73 86 73 105 54 40 49 56
a Synthesis conditions: catalyst (0.2 g), synthesis gas (H2/N2 = 3, 60 mL min−1), reaction temperature (673 K), pressure (0.1 MPa). Ea is the apparent activation energy calculated from Arrhenius plots of the ammonia synthesis rate in the temperature range of 593−673 K.
Figure 6. Time course of ammonia formation over 1 wt % Ru/ C12A7:e−. Reaction conditions: catalyst, 0.2 g; synthesis gas, H2/N2 = 3 with a flow rate of 60 mL min−1; pressure, 1.0 MPa; reaction temperature, 633 K. Figure reproduced with permission from ref 30. Copyright the Nature Publishing Group 2012.
Figure 5. Correlation of ammonia production by 1 wt % Ru/ C12A7:e− and 6 wt % Ru−Cs/MgO with pressure. Reaction conditions: catalyst, 0.2 g; synthesis gas, H2/N2 = 3 with a flow rate of 60 mL min−1; temperature, 633 K. Figure reproduced with permission from ref 30. Copyright the Nature Publishing Group 2012.
the cage electrons were replaced with H− ions at the early stage of the reaction (
