Ambient Carbon-Neutral Ammonia Generation via a Cyclic Microwave Plasma Process

Abstract

A novel reactor methodology was developed for chemical looping ammonia synthesis processes using microwave plasma for pre-activation of the stable dinitrogen molecule before reaching the catalyst surface. Microwave plasma-enhanced reactions benefit from higher production of activated species, modularity, quick startup, and lower voltage input than competing plasma-catalysis technologies. Simple, economical, and environmentally benign metallic iron catalysts were used in a cyclical atmospheric pressure synthesis of ammonia. Rates of up to 420.9 μmol min–1 g–1 were observed under mild nitriding conditions. Reaction studies showed that both surface-mediated and bulk-mediated reaction domains were found to exist depending on the time under plasma treatment. The associated density functional theory (DFT) calculations indicated that a higher temperature promoted more nitrogen species in the bulk of iron catalysts but the equilibrium limited the nitrogen converion to ammonia, and vice versa. Generation of vibrationally active N2 and, N2 + ions is associated with lower bulk nitridation temperatures and increased nitrogen contents versus thermal-only systems. Additionally, the kinetics of other transition metal chemical looping ammonia synthesis catalysts (Mn and CoMo) were evaluated by high-resolution time-on-stream kinetic analysis and optical plasma characterization. This study sheds new light on phenomena arising in transient nitrogen storage, kinetics, effect of plasma treatment, apparent activation energies, and rate-limiting reaction steps.

Towards green and efficient chemical looping ammonia synthesis: design principles and advanced redox catalysts

“…46 Therefore, there are also some redox cycling processes for ammonia production similar to CLAS including electrolytic-chemical cycle, 48,49 as well as some external field-assisted chemical loops for ammonia production, such as plasma-assisted CLAS. 20,50,51…”

“…The results revealed that the K diff and K r increase with the increase of temperature and the potential rate-determining step of the reduction process is the surface reaction. 50 Achieving the reasonable match between the surface reaction rate and the bulk diffusion ability of redox catalysts is an essential approach to improve the reaction performance of redox catalysts in chemical looping processes. It is found by Xiao and co-workers that the introduction of an appropriate amount of Ni can achieve the optimal kinetic matching between surface reaction and bulk diffusion of lattice nitrogen in Mn-based redox catalysts, which ultimately improves the selectivity and yield of Ni–Mn–N composite in the hydrogenation step of CLAS.…”

“…3 of ref. 50kThe unit of ammonia production rate is μmol h −1 cm geo −2 where ‘geo’ indicates that it is the geometric surface area that is stated. 26 lThe ammonia production rate is 4.5 μg mg −1 Pd h −1 (calculated to be ∼265 μmol g −1 Pd h −1 ).…”

“…Subsequently, NH 3 was obtained by thermal-hydrogenation of NaNH 2 . 51 Recently, Brown et al 50 developed a microwave plasma-assisted reactor for H 2 -CLAS (Fig. 9c), whereby enhanced reaction kinetics and NH 3 yield were achieved due to the pre-activation of the stable N 2 molecule before reaching the redox catalyst surface.…”

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

“…Increasing efforts have been devoted to improving the efficiency of CLAS process, using the electric or photon energy, ,, structure modulation of NC, ,,,, and kinetics enhancement by adding catalyst. , Similarly, the strong cooperative reactivity of the composite Mn based nitrogen carrier (Mn 4 N and LiH or BaH 2 Mn nitride), could result in the exceptional catalytic activities for ammonia synthesis under mild conditions as compared to that of their parent elements by catalyzing the N 2 fixation and hydrogenation reactions. ,, Suggesting new chemical looping reaction pathways could be another strategy of improving the CLAS process, ,,,,− where the easy alloying characteristics of lithium and group 14 elements (Li x M) could provide some interesting results in N 2 fixation through the generation of ternary nitrides. − …”