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. 2015 Mar 30:6:6731.
doi: 10.1038/ncomms7731.

Electride support boosts nitrogen dissociation over ruthenium catalyst and shifts the bottleneck in ammonia synthesis

Masaaki Kitano  1 , Shinji Kanbara  2 , Yasunori Inoue  2 , Navaratnarajah Kuganathan  3 , Peter V Sushko  4   5 , Toshiharu Yokoyama  1   5 , Michikazu Hara  2   5   6 , Hideo Hosono  1   2   5   6
Affiliations

Electride support boosts nitrogen dissociation over ruthenium catalyst and shifts the bottleneck in ammonia synthesis

Masaaki Kitano et al. Nat Commun. .

Abstract

Novel approaches to efficient ammonia synthesis at an ambient pressure are actively sought out so as to reduce the cost of ammonia production and to allow for compact production facilities. It is accepted that the key is the development of a high-performance catalyst that significantly enhances dissociation of the nitrogen-nitrogen triple bond, which is generally considered a rate-determining step. Here we examine kinetics of nitrogen and hydrogen isotope exchange and hydrogen adsorption/desorption reactions for a recently discovered efficient catalyst for ammonia synthesis--ruthenium-loaded 12CaO·7Al2O3 electride (Ru/C12A7:e(-))--and find that the rate controlling step of ammonia synthesis over Ru/C12A7:e(-) is not dissociation of the nitrogen-nitrogen triple bond but the subsequent formation of N-Hn species. A mechanism of ammonia synthesis involving reversible storage and release of hydrogen atoms on the Ru/C12A7:e(-) surface is proposed on the basis of observed hydrogen absorption/desorption kinetics.

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Figures

Figure 1
Figure 1. Energy profile of N2 dissociation.
Potential energy profile for dissociative adsorption of N2 and associative desorption of N2 on Ru/C12A7:e− and Ru/C12A7:O2−. These values were estimated from the results of N2 exchange and ammonia synthesis reactions. N2(g) and N(ad) represent N2 in gas phase and adsorbed nitrogen atom, respectively.
Figure 2
Figure 2. Ab initio simulations of N2 interaction with the Ru/C12A7 catalysts.
Character of the charge redistribution between C12A7 substrate and deposited Ru clusters for the stoichiometric (a) and electride (b) C12A7. (c,d) Adsorption energies of N2 on C12A7-supported Ru, charge transfer in the process of N2 dissociation (N2(g)+Ru→2N(ad)+Ru) and the corresponding energy gain (ΔE). In Ru/C12A7:O2– system (c), N2 and N accept electron charge from the Ru cluster, making it positively charged. In Ru/C12A7:e– (d), the electron charge is transferred from the substrate, leaving the Ru cluster nearly neutral. N2(g), N2(ad) and N(ad) represent N2 in gas phase, adsorbed N2, and adsorbed nitrogen atom, respectively. (e) Electronic structure: the Fermi level (Ef) of Ru on C12A7:O2– is similar to that of bulk Ru (4.7 eV) and that of the Ru/C12A7:e– is determined by the charge transfer from the cage conduction-band electrons of C12A7:e– (2.4 eV). Evac denotes vacuum level.
Figure 3
Figure 3. Kinetic analysis of ammonia synthesis over Ru/C12A7:e− catalysts.
(a) Temperature dependence of the rate of ammonia synthesis over Ru/C12A7:e− catalysts at an atmospheric pressure (catalyst=0.025 g, H2:N2=3:1, flow rate=60 ml min−1) (b,c) Dependence of NH3 synthesis rate on the partial pressures of (b) N2 and (c) H2 at 573 (open circles) and 633 K (filled circles) under atmospheric pressure. α And β represent the reaction orders for N2 and H2 in equation 2, respectively.
Figure 4
Figure 4. Hydrogen incorporation into C12A7:e−.
(a) H2 TPA profiles of Ru/C12A7:e−, Ru/C12A7:O2− and Ru/CA. The TPA experiment was performed with a dilute mixture of H2 (5%) in Ar using a total flow of 10 ml min−1. (b) H2 TPD profiles of Ru/C12A7:e−, Ru/C12A7:O2− and Ru/CA. The TPD experiment was performed with Ar using a total flow of 10 ml min−1. (c) Amount of incorporated H− ions in Ru/C12A7:e− after heat treatment in H2 atmosphere; black circle: Ru/C12A7:e− heated in H2 (75 kPa) and Ar (25 kPa) gas flow at 633 K. Red diamond: Ru/C12A7:e− heated in H2 (75 kPa) and N2 (25 kPa) gas flow at 633 K. Inset shows the enlarged profiles. Neutral hydrogen species such as H0 and H2 are metastable C12A7 because the cage wall is positively charged.
Figure 5
Figure 5. Enthalpy changes for extra-framework species in 12CaO·7Al2O3 (C12A7).
C12A7 has two chemical formula units/cubic unit cell. The extra-framework O2− ions are loosely bound to the positively charged framework [Ca24Al28O64]4+ to keep electroneutrality. The O2− ions can be partially or completely replaced by e− and H− ions. The enthalpies (ΔH) for e− or H− ions formation in the cage of C12A7:O2− are 318 and −367 kJ mol−1, respectively. C12A7:e− easily reacts with hydrogen gas to form H− ions in the cage (ΔH=−434 kJ mol−1) as compared with C12A7:O2−. ‘c’ and ‘g’ denote the species in a cage and gas phase, respectively.
Figure 6
Figure 6. Proposed reaction mechanism and energy profile for ammonia synthesis.
Reaction mechanism and energy profile for ammonia synthesis over (a) conventional catalyst and (b) Ru/C12A7:e−. (a) N2 and H2 react on the catalyst surface through a Langmuir–Hinshelwood mechanism to form NH3 in which N2 dissociation is the RDS. The energy barrier (Edis) for this step corresponds to the apparent activation energy (Ea) for ammonia synthesis. As for Ru/C12A7:e− (b), the rate-limiting step is not N2 dissociation but the formation of N–Hn species. NH3 is formed through the Langmuir–Hinshelwood mechanism (route 1) and the direct reaction of N adatoms with H radicals (nascent hydrogen) derived from cage H− anions (route 2). Ea is determined by the difference between the top of the barrier for N–Hn formation and the energy level of reactant molecules (N2 and H2).
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