Facilitating green ammonia manufacture under milder conditions: what do heterogeneous catalyst formulations have to offer?

Abstract

Ammonia production is one of the largest industrial processes, and is currently responsible for over 1.5% of global greenhouse gas emissions. Decarbonising this process, yielding 'green ammonia', is critical not only for sustainable fertilizer production, but also to unlocking ammonia's potential as a zero-carbon fuel and hydrogen store. In this perspective, we critically assess the role of cutting-edge heterogeneous catalysts to facilitate milder ammonia synthesis conditions that will help unlock cheaper, smaller-scale, renewables-coupled ammonia production. The highly-optimised performance of catalysts under the high temperatures and pressures of the Haber-Bosch process stands in contrast to the largely mediocre activity levels reported at lower temperatures and pressures. We identify the recent advances in catalyst design that help overcome the sluggish kinetics of nitrogen activation under these conditions and undertake a categorized analysis of improved activity achieved in a range of heterogeneous catalysts. Building on these observations, we develop a 'catalyst efficiency' analysis which helps uncover the success of a holistic approach - one that addresses the issues of nitrogen activation, hydrogenation of adsorbed nitrogen species, and engineering of materials to maximize the utilization of active sites - for achieving the elusive combination of high-activity, low-temperature formulations. Furthermore, we present a discussion on the industrial considerations to catalyst development, emphasising the importance of catalyst lifetime in addition to catalyst activity. This assessment is critical to ensuring that high productivities can translate into real advances in commercial ammonia synthesis.

Fig. 1. Ammonia synthesis and utilization pathways. Pathway for ‘green ammonia’ synthesis is shown using green arrows.

Fig. 2. (a) Catalyst efficiencies of different iron-based industrial HB catalysts at pressures above 100 bar and temperatures above 748 K (blue scatter points) and temperatures between 673 K and 723 K (green scatter points). For comparison, the efficiency of the ruthenium catalyst (KAAP HB process) at 698 K is shown in red. A106, KM-I(II) and ICI74-1 catalysts are Fe₃O₄-based; A301, ZA-5 and FA400 are Fe_1−xO-based. Data for catalyst efficiencies collated from the following ref. and ; (b) volcano-type relationship between catalyst activity and nitrogen adsorption energy for different transition metals. Reproduced from ref. with permission from Elsevier, copyright 2015; (c) catalyst activity as a function of nitrogen adsorption energy and N₂ transition-state energy with the linear scaling relation exhibited by transition metals shown as a dashed line. Reproduced from ref. with permission from Elsevier, copyright 2015.

Fig. 3. (a) Schematic of multi-site reaction mechanism over transition metal-loaded LiH; reproduced from ref. with permission from Springer Nature, copyright 2020; (b) schematic of Mars–van Krevelen reaction mechanism over transition metal-loaded LaN; reproduced from ref. with permission from Springer Nature, copyright 2020; (c) evolution in the complexity of heterogeneous catalyst design for ammonia synthesis: from a simple metal surface to multi-site approaches.

Fig. 4. Scatter plot showing the efficiencies of various types of heterogeneous catalysts as a function of temperature for ammonia synthesis at pressures no greater than 10 bar. The open circles and filled diamonds are used to represent catalyst productivities lower and greater than 10 mmol NH₃ per g_cat per h, respectively. For categorization of catalyst types and references for catalytic activity data, please refer to Table 1.

Fig. 5. Scatter plot showing the ammonia productivities of various heterogeneous catalysts as a function of temperature for ammonia synthesis at pressures no greater than 10 bar. The same catalyst systems were represented in terms of their efficiency in Fig. 4. References for activity data of the individual catalyst systems can be found in Table 1.

Fig. 6. Catalyst costs per tonne of ammonia produced assuming a catalyst lifetime of 6 months. The catalyst activity data was taken from the following references: Ru/Ca(NH₂)₂, Fe–LiH/MgO, Co–BaH₂/CNT, and Ni/CeN. The cost of the materials per kg were taken as $400, $8, $15 and $10 respectively, which are assumed from current element costs and previously reported costs of similar materials.

Manoj Ravi

Joshua W. Makepeace