Unique Catalytic Mechanism for Ru-Loaded Ternary Intermetallic Electrides for Ammonia Synthesis

Abstract

Intermetallic electrides have recently shown their priority as catalyst components in ammonia synthesis and CO2 activation. However, their function mechanism has been elusive since its inception, which hinders the further development of such catalysts. In this work, ternary intermetallic electrides La–TM–Si (TM = Co, Fe, and Mn) were synthesized as hosts of ruthenium (Ru) particles for ammonia synthesis catalysis. Although they have the same crystal structure and possess low work functions commonly, the promotion effects on Ru particles rather differ from each other. The catalytic activity follows the sequence of Ru/LaCoSi > Ru/LaFeSi > Ru/LaMnSi. Furthermore, Ru/LaCoSi exhibits much better catalytic durability than the other two. A combination of experiments and first-principles calculations shows that apparent N2 activation energy on each catalyst is much lower than that over conventional Ru-based catalysts, which suggests that N2 dissociation can be conspicuously promoted by the concerted actions of the specific electronic structure and atomic configuration of intermetallic electride-supported catalysts. The NH x formations proceeded on La are energetically favored, which makes it possible to bypass the scaling relations based on only Ru as the active site. The rate-determining step of Ru/La–TM–Si was identified to be NH2 formation. The transition metal (TM) in La–TM–Si electrides has a significant influence on the metal–support interaction of Ru and La–TM–Si. These findings provide a guide for the development of new and effective catalyst hosts for ammonia synthesis and other hydrogenation reactions.

Localization Mechanism of Interstitial Electronic States in Electride Mayenite

“…Because the mayenite framework is quite rigid, removing oxygen atoms from the cavities does not destroy it. Moreover, these cavities can embed, for example, ions of gold, platinum, and chlorine as well as rare-earth elements, leading to a wide variety of technological applications, including promising field-emission properties , and prominent catalytic activity. − Mayenite, which is self-doped with electrons placed in positively charged cavities, can exhibit interesting physical properties such as low work function, a state of phonon glass and an electron crystal state, a metal–insulator transition with a change in the concentration of interstitial electrons, and superconductivity. − These properties of mayenite originate from the nesting of electride electrons in the cages weakly coupled to the lattice. These electrons form the energy states lying in the vicinity of the Fermi level.…”

Insight into rare-earth-incorporated catalysts: The chance for a more efficient ammonia synthesis

“…Therefore, the presence of RE is indispensable for the outstanding supporting manner of ternary intermetallic electrides. Very recently, we found that the REs in the electrides are more than an electron donor [16,85]. La in LaTMSi (TM = Co, Fe, Ru, or Mn) is a better active site for the NH x formation and NH 3 desorption than the Ru site, which can help break the scaling relations on the Ru/LaTMSi catalysts.…”

“…Besides, they can alleviate or even avoid the H 2 poisoning effect, which is benefited from their strong H affinity that shunts H from Ru sites. Furthermore, the construction of the multi-sites (e.g., the Ru site or nitrogen-vacancy site for the N 2 activation [14,16], the Ni site for the H 2 activation [14], the RE site for the NH x formation [85]) would settle the competition of different elementary steps on one specific site. Recent studies have also revealed that the ammonia synthesis via an associative mechanism is preferable to the dissociative mechanism for a high-performance catalyst.…”

“…Different from the traditional alkaline earth oxide supports, RE-involving supports, such as RE nitrides [14] and oxynitride-hydrides [15], can promise a more energy-favourable N 2 activation (Mars-van Krevelen mechanism), which dramatically decreases the energy barrier for the ammonia synthesis. In some cases, the REs act as a favourable active site for the combination of N and H [16]. The combination of the RE sites with the TM sites could liberate the ammonia synthesis from the scaling relations that trouble the conventional Ru and Fe materials, allowing the ammonia synthesis to proceed via a low-barrier path.…”

On the Nature of Three-Atom Metal Cluster Catalysis for N2 Reduction to Ammonia

“…As shown in Figure 8, the calculated activation barriers for N 2 dissociation are 0.75 and 0.31 eV on Y 3 /G and Nb 3 /G, respectively, which are exothermic in total due to the large energy gain of N 2 adsorption, indicating that N 2 is prone to dissociate on such catalysts. 72 After nitrogen dissociation, the first hydrogenation step is to form N* + NH*; next, the two reaction pathways (distal and alternative) are considered as well. 37 In the alternative pathway, N* + NH* is hydrogenated to 2NH*, followed by alternate hydrogenation on two NH* to form 2NH 3 ultimately.…”