Thermodynamic and kinetic considerations of nitrogen carriers for chemical looping ammonia synthesis

Abstract

Ammonia (NH3) is a promising clean energy carrier, provided that its production is driven by renewable energy rather than fossil fuel-based Haber–Bosch (H–B) process. Chemical looping ammonia synthesis (denoted as CLAS) can intervene in the ubiquitous scaling relations in catalytic ammonia synthesis by separately feeding reactants to a nitrogen carrier to achieve atmospheric operation, which provides an alternative synthetic route to the H-B process. The key of CLAS is to develop efficient N carrier materials with suitable thermodynamic and kinetic properties. Metal nitrides and metal imides are two kinds of N carrier materials for the CLAS process, and H2 and H2O are commonly used as the hydrogen sources of NH3. Here, we first analyze the thermodynamic properties of the reactions of various metal nitrides and imides with water or hydrogen to produce NH3, N2 fixation on metals or metal hydrides, and the regeneration of metals from metal oxides, respectively. The thermodynamic calculation results display the reduction of main group metal hydroxide, early transition metal oxides, and rare earth metal oxides to the corresponding metallic state or hydrides, the nitridation of late transition metals to the corresponding nitrides, are the thermodynamic limiting steps for the metal nitride carriers. The metal imides, such as lithium imide and barium imide, have the relatively proper thermodynamics for two-step chemical looping reactions, however, their performance is limited by the thermodynamics of hydrogenation reaction. Moreover, for the thermodynamically unfavorable steps in the CLAS, we propose potential electrochemical processes to run the loop, such as molten salt electrolytic cell and solid electrolyte electrolytic cell. Finally, we put forward some strategies, such as controllable synthesis of N carriers and adding efficient catalysts, to improve the kinetics of chemical looping reactions.

Searching for the Rules of Electrochemical Nitrogen Fixation

“…Nitrides conversion to ammonia was completed in specific conditions (high temperatures, plasmas, etc.). 23,54,60,61 We verify whether this limits ammonia synthesis in our conditions by trying to protonolyze commercial nitrides in different protic media (Figure 3D). The protonolysis yield scales with the nitrides formation energy and with their Gibbs free energies of hydrolysis calculated by Gao et al 23 This suggests that a nitride that is easy to make will be harder to take all the way to ammonia.…”

“…22 However, M x N y is likely to be a key intermediate considering its thermodynamic stability. 23 About half of the evaluated elements have a stable nitride phase (Figure 1A, blue elements), among which Li has the most negative reduction potential, 24−27 hence the highest intrinsic overpotential for ammonia synthesis (Figure 1B). Replacing Li with any element will likely result in high gains in energy efficiency, and some elements (e.g., Al, Ca, Mo, W, and Mg) are also produced globally at much higher rates, meaning better scalability.…”

“…This is debatable, even for Li where intermediates are unknown, although we expect it to be a mixture of Li (M x ), hydride (M x H y ), nitride (M x N y ), and nitride hydride (M x N y H z ) . However, M x N y is likely to be a key intermediate considering its thermodynamic stability . About half of the evaluated elements have a stable nitride phase (Figure A, blue elements), among which Li has the most negative reduction potential, − hence the highest intrinsic overpotential for ammonia synthesis (Figure B).…”

“…). ,,, We verify whether this limits ammonia synthesis in our conditions by trying to protonolyze commercial nitrides in different protic media (Figure D). The protonolysis yield scales with the nitrides formation energy and with their Gibbs free energies of hydrolysis calculated by Gao et al This suggests that a nitride that is easy to make will be harder to take all the way to ammonia. When repeating this experiment in water, proton activity is drastically increased; Ca and Mg nitrides fully hydrolyze, although still not enough to fully convert AlN – the most stable of them, to NH 3 .…”

Enroute to the Carbon-Neutrality Goals via the Targeted Development of Ammonia as a Potential Nitrogen-Based Energy Carrier

“…Chemical looping is another energy efficient and high product selectivity process, being employed in gasification, reforming, combustion, and air separation for oxygen supply that usually avoids the undesired characteristics of alternative synthesis techniques while utilizing the nitrogen carrier (NC) materials at mild reaction conditions. − Chemical looping ammonia synthesis (CLAS) by efficient fixation/release of NC at ambient pressure is currently a promising alternative route that could produce energy via several continuous reactions or the regeneration of chemical intermediates by simply decoupling the overall reaction scheme of NH 3 synthesis into various spatially separated subreactions mediated by a nitrogen carrier. − The overall scenario of intensive reaction conditions in H–B process (450 °C, 200 bar), primarily lies in entailing the grand challenge of overcoming the high energy barrier (941 kJ/mol) of the inert and nonpolar NN molecule, its sluggish reaction rate and the scaling coordination existing on the catalyst surface. − The CLAS process performed at atmospheric pressure is a simplified ammonia synthesis route, which enhances energy efficiency and reduces the carbon footprints as compared to that of H–B process, by circumventing the severe process conditions and scaling problems by avoiding the trade-offs in heterogeneous catalysis of competitive N 2 and H 2 (or H 2 O) adsorption. , The cyclic operation of storing fixed nitrogen is usually a way of encountering intermittent renewable power sources, energy requirements for process heating and compression, and relocation of the centralized and integrated large-scale ammonia synthesis units. CLAS processes could be a possible way of energy storage, making it feasible to synthesize ammonia on site at atmospheric pressure rather than transporting it long distances.…”