First-Principles Microkinetic Model for Ammonia Synthesis on Fe(110) under Nonequilibrium Programmable Heating and Quenching Operation

Abstract

Abstract Traditional thermochemical processes typically operate under relatively static conditions, where conventional reactors restrict the time scales over which key reaction parameters (such as temperature) can be varied. Electrification via Joule heating offers a way to overcome this limitation by producing pulsed temperature profiles with millisecond time resolution via programmable electric currents through a lightweight resistive catalyst support. The programmable heating and quenching (PHQ) approach enables access to nontraditional reaction dynamics and may significantly enhance product yield and selectivity with improved energy efficiency. Ammonia (NH3), produced by the Haber–Bosch (HB) process, plays a critical role in fertilizer production and is gaining attention as a potential hydrogen carrier. The conventional HB process requires high temperatures and pressures, limiting energy efficiency and decentralized production. Dong et al. demonstrated NH3 synthesis from H2 and N2 at ambient pressures using a PHQ reactor and an Fe catalyst, with yields higher than those obtained under isothermal conditions and stable output [Dong et al., Nature, 605, 470–476 (2022)]. Here, we used density-functional-theory-based microkinetic modeling under both steady-state (isothermal) and dynamic (PHQ) conditions to understand the effect of controlled thermal fluctuations on heterogeneous catalysis of NH3 synthesis on the most stable facet of Fe, namely, (110). We demonstrate how PHQ can alter reaction rates and surface species; specifically, we predict fluctuations in concentrations of adsorbed N and NH; we also investigate how variations in temperature pulsing, e.g., the maximum temperature allowed and cycle time, influence yield. We find that PHQ enhances NH3 yield only when the average of the fluctuating temperature deviates from the optimal isothermal temperature (especially when lower). Furthermore, increasing the time between high-temperature pulses could dramatically improve energy efficiency due to the slow temperature decay between pulses.