Despite more than a century of advances in catalyst and production plant design, the Haber-Bosch process for industrial ammonia (NH3) synthesis still requires energy-intensive high temperatures and pressures. We propose taking advantage of sunlight conversion into surface plasmon resonances in Au nanoparticles to enhance the rate of the N2 dissociation reaction, which is the bottleneck in NH3 production. We predict that this can be achieved through Mo doping of the Au surface based on embedded multireference correlated wave function calculations. The Au component serves as a light-harvesting antenna funneling energy onto the Mo active site, whereby excited-state channels (requiring 1.4 to 1.45 eV, near-infrared-to-visible plasmon resonances) may be accessed. This effectively lowers the energy barriers to 0.44 to 0.77 eV/N2 (43 to 74 kJ/mol N2) from 3.5 eV/N2 (335 kJ/mol N2) in the ground state. The overall process requires three successive surface excitation events, which could be facilitated by amplified resonance energy transfer due to plasmon local field enhancement.

https://advances.sciencemag.org/content/advances/3/12/eaao4710.full.pdf

Prediction of a low-temperature N2 dissociation catalyst exploiting near-IR–to–visible light nanoplasmonics

Der Einflus der Ruckdiffusion des Reaktionsproduktes auf die Kinetik der Ammoniaksynthese wird fur eine enge, lange (6 mm Durchmesser) sowie fur eine weite, flache (24 mm Durchmesser) Katalysatorschuttung untersucht. Es wird gezeigt, das im engen, langen Bett die Ruckdiffusion zu vernachlassigen ist und die kinetischen Messungen mit der entsprechenden integrierten Geschwindigkeitsgleichung auszuwerten sind. Im flachen, weiten Bett dagegen ist unter gewissen Bedingungen die Ruckdiffusion so schnell, das ein fast vollstandiger Konzentrationsausgleich uber den Katalysator stattfindet. Messungen in dieser Anordnung sind am zweckmasigsten wie die in einem gradientlosen Ruckflusreaktor auszuwerten.



The influence of back-diffusion is studied in the case of ammonia synthesis by means of two different arrangements of the catalyst: 1) a long narrow (6 mm O) and 2) a short broad (24 mm O) catalyst bed. For arrangement 1) the back-diffusion can be neglected. The kinetics follow the integrated rate equation. At arrangement 2) the back-diffusion may be so fast that there is almost no gradient of concentration along the bed. The kinetics in this case correspond to the ones in a reflux-reactor.

J. Hensel

Papers

Über den Einfluß der Längsdiffusion auf die Kinetik der Ammoniakbildung