Review on catalytic roles of rare earth elements in ammonia synthesis: Development and perspective

doi:10.1016/j.jre.2021.06.014

Journal of Rare Earths

Volume 40, Issue 1, January 2022, Pages 1-10

https://doi.org/10.1016/j.jre.2021.06.014 Get rights and content

Abstract

Ammonia (NH₃) is mainly produced via the Haber–Bosch process. It was discovered that the performance of a wide variety of catalysts in NH₃ synthesis could be considerably enhanced by the addition of rare earth elements (REEs). As a result, catalysts promoted by REEs, especially the Ru-based ones have been extensively investigated. In this review, we summarize the progress of utilizing REEs for ammonia synthesis and outline the prospects of using them in the design and development of highly efficient and stable catalysts for ammonia synthesis.

Graphical abstract

This review summarizes the applications of rare earth elements as promoter, support and intermetallic catalyst in ammonia synthesis.

Introduction

Ammonia (NH₃) is not only vital for the production of fertilizers, but is also essential as a green-energy vector in hydrogen economy.¹ Currently, the industrial production of NH₃ (N₂+H₂→NH₃) is still dominated by the traditional Haber-Bosch process, giving a turnover of 500 million ton of NH₃ per year.² The main bottleneck of NH₃ synthesis at mild conditions lies in the activation of the N₂ molecule, which has an extremely strong bonding energy of 945 kJ/mol and large HOMO-LOMO gap (10.8 eV).³^,⁴ Researchers have found that transition metals (TMs) act as the electron donors, which are essential for the activation of N₂, plausibly due to the fact that these TMs could provide electrons to antibonding orbitals of N₂ molecule, which is beneficial to N₂ activation.5, 6, 7 Moreover, the activation of N₂ involves either the dissociative or associative route, where adsorbed N₂ directly dissociates or stepwise hydrogenates into NNH_x to form NH₃.8, 9, 10, 11 In the dissociative route (Table 1), electronic promoters such as electrides¹² and hydrides¹³ are employed to drive N₂ cleavage by enhancing electron transfer from transition metals to the π∗ antibonding orbitals of N₂, which is referred to as electronic promoting effect. On the other hand, N₂ can bypass direct dissociation and preferentially undergo hydrogenation via an associative route on some special catalysts.¹⁰^,¹¹ Interestingly, no matter N₂ activation in ammonia synthesis follows well-known dissociative or associative route, the adsorption and activation of N₂ molecule on TMs surfaces are the first step to be considered.

Additionally, previous theoretical calculations have demonstrated that the relationship between nitrogen adsorption energy and NH₃ synthesis rate on various TMs surfaces, and proposed the volcano plot, where Ru and Fe showed superior activity.¹⁴ According to the results of the typical volcano plot, various efficient Fe- and Ru-based catalysts were developed via the modification of electronic state of Fe and Ru, potentially because the electron-donating capability is the key factor for efficient catalytic NH₃ synthesis.

Before 1990, researchers focused on improving the catalytic activity of Ru- or Fe-based NH₃ synthesis catalysts via the addition of alkali metals, alkali earth metals or hydroxides of Cs, K or Ba, exploiting the positive effect of electron donation of the promoters.15, 16, 17 There are several excellent reviews addressing NH₃ synthesis reaction from different perspectives. For instance, Liu et al. concisely reviewed the development of typical ammonia synthesis catalysts with enlightenment.¹⁸ Chen et al. summarized the use of hydrides for the synthesis of ammonia at mild reaction conditions.¹³^,¹⁹ Most recently, Hosono et al. summarized a series of inorganic electrides materials for NH₃ synthesis.¹² However, to the best of our knowledge, there are no detail reviews on the roles of rare earth elements (REEs) and their participation in NH₃ synthesis. Since REEs were first applied to NH₃ synthesis in 1992,²⁰ the development of REE-containing NH₃ synthesis catalysts has gained extensive attention in the past few decades, involving structural or electronic promoters, which played an important role in the absorption and activation of N₂ molecules.21, 22, 23, 24 In this mini review, we delineate the development of utilizing REEs in the thermal catalytic processes of NH₃ synthesis. We first evaluate the roles of REEs for NH₃ synthesis, including that as promoter, support, intermetallic catalyst. Then, we conduct in-depth discussion to gain a fundamental understanding on the related mechanistic aspects. Finally, we outline the prospect of using REEs for the design of advanced catalysts for NH₃ synthesis with high efficacy and stability.

Section snippets

Rare earth elements as promoters

Rare earth elements such as La, Y, Nd, Ce, and Sm were employed to promote Ru-based catalysts for NH₃ synthesis. The NH₃ synthesis rate over La-promoted Ru/Al₂O₃ reached the highest when lanthanide/Ru molar ratio was 3 (Fig. 1(a)).²⁰ The apparent activation energies (E_a) for the La-promoted Ru catalysts are 46–63 kJ/mol, which are much lower than that of the Cs⁺-promoted one (100 kJ/mol) (Fig. 1(b)). In this scenario, the authors showed that the lanthanide entities were highly dispersed in the

Summary and outlook

In this work, we present a new perspective on the utilization of rare earth elements in the catalysis of NH₃ synthesis. Despite the superior catalytic activity as a result of REEs application NH₃ synthesis, there is the challenge of achieving excellent catalytic performance under mild conditions, especially at relatively low temperature and pressure. Most recently, it was reported that rare earth nitrides show high NH₃ synthesis rate at mild conditions, but there are still problems to be solved

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^☆: Foundation item: Project supported by the National Natural Science Foundation of China (22038002, 21972019).

^†: The two authors contributed equally to this work.

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Review on catalytic roles of rare earth elements in ammonia synthesis: Development and perspective☆

Abstract

Graphical abstract

Introduction

Section snippets

Rare earth elements as promoters

Summary and outlook

Surf Sci

Appl Catal Gen

J Catal

J Energy Chem

Chem Phys Lett

J Catal

Appl Catal

Chin J Catal

J Catal

Chem Sci

J Catal

J Catal

Int J Hydrogen Energy

J Catal

Catal Sci Technol

Appl Catal Gen

J Catal

J Catal

J Rare Earths

Carbon

Catal Commun

J Catal

Appl Catal Gen

Appl Catal Gen

Appl Catal Gen

Catal Today

Catal Commun

Catal Commun

Catal Commun

J Rare Earths

Nat Catal

J Catal

J Catal

J Catal

Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids

Energy Environ Sci

How a century of ammonia synthesis changed the world

Nat Geosci

Expanding boundaries: N2 cleavage and functionalization beyond early transition metals

Angew Chem Int Ed

Hydrogenation and cleavage of dinitrogen to ammonia with a zirconium complex

Nature

Catalyst design by interpolation in the periodic table: Bimetallic ammonia synthesis catalysts

J Am Chem Soc

Reaction mechanisms in catalysis by metals

Crit Rev Solid State

Low-T mechanisms of ammonia synthesis on Co3Mo3N

J Phys Chem C

Expanding boundaries: N₂ cleavage and functionalization beyond early transition metals

Low-T mechanisms of ammonia synthesis on Co₃Mo₃N