Contribution of Nitrogen Vacancies to Ammonia Synthesis over Metal Nitride Catalysts

Abstract

Ammonia is one of the most important feedstocks for the production of fertilizer and as a potential energy carrier. Nitride compounds such as LaN have recently attracted considerable attention due to their nitrogen vacancy sites that can activate N 2 for ammonia synthesis. Here, we propose a general rule for the design of nitride-based catalysts for ammonia synthesis, in which the nitrogen vacancy formation energy (E NV ) dominates the catalytic performance. The relatively low E NV (ca. 1.3 eV) of CeN means it can serve as an efficient and stable catalyst upon Ni loading. The catalytic activity of Ni/CeN reached 6.5 mmol•g −1 •h −1 with an effluent NH 3 concentration (E NH3 ) of 0.45 vol %, reaching the thermodynamic equilibrium (E NH3 = 0.45 vol %) at 400 °C and 0.1 MPa, thereby circumventing the bottleneck for N 2 activation on Ni metal with an extremely weak nitrogen binding energy. The activity far exceeds those for other Co-and Ni-based catalysts, and is even comparable to those for Ru-based catalysts. It was determined that CeN itself can produce ammonia without Ni-loading at almost the same activation energy. Kinetic analysis and isotope experiments combined with density functional theory (DFT) calculations indicate that the nitrogen vacancies in CeN can activate both N 2 and H 2 during the reaction, which accounts for the much higher catalytic performance than other reported nonloaded catalysts for ammonia synthesis.

Development and Recent Progress on Ammonia Synthesis Catalysts for Haber–Bosch Process

“…Ye et al furthered their work in this area with Ni–CeN, gaining a better insight into the nature of the ammonia synthesis reaction on this new catalyst class. [ 150 ] An activity of 6500 μmol g −1 h −1 was achieved for Ni–CeN at 400 °C, 0.9 MPa, and a WHSV of 36 000 mL g −1 h −1 . [ 150 ] The nitrogen vacancies generated in the metal nitride, CeN, were responsible for the activation of N 2 in the catalyst system and the Ni in this system was responsible for H 2 activation.…”

“…[ 150 ] An activity of 6500 μmol g −1 h −1 was achieved for Ni–CeN at 400 °C, 0.9 MPa, and a WHSV of 36 000 mL g −1 h −1 . [ 150 ] The nitrogen vacancies generated in the metal nitride, CeN, were responsible for the activation of N 2 in the catalyst system and the Ni in this system was responsible for H 2 activation. This changes the system for the conventional catalyst and promoter to a cocatalyst system where each part performs its own task in ammonia synthesis.…”

“…[ 150 ] Ammonia is produced from the reaction of adsorbed hydrogen and lattice nitrogen in the CeN catalyst; the produced vacant nitrogen sites resulting from this are then used in the nitrogen gas activation step, thereby achieving continuous ammonia synthesis. [ 150 ] The mechanism proposed for this was further investigated through nitrogen isotope experiments in which ammonia synthesis was conducted with 15 N 2 and H 2 using the pure CeN catalyst. [ 150 ] This provided further confirmation that lattice nitrogen ( 14 N) in CeN takes place in the ammonia synthesis reaction.…”

“…The strong hydrogen adsorption energy of Ni compared to the nitrogen vacancies in the cocatalyst, calculated as −1.30 and−0.62 eV, respectively, by Ye et al enables the dissociate hydrogen adsorption step to take place preferentially on the Ni catalyst. [ 150 ] Ammonia is produced from the reaction of adsorbed hydrogen and lattice nitrogen in the CeN catalyst; the produced vacant nitrogen sites resulting from this are then used in the nitrogen gas activation step, thereby achieving continuous ammonia synthesis. [ 150 ] The mechanism proposed for this was further investigated through nitrogen isotope experiments in which ammonia synthesis was conducted with 15 N 2 and H 2 using the pure CeN catalyst.…”

“…[ 150 ] The mechanism proposed for this was further investigated through nitrogen isotope experiments in which ammonia synthesis was conducted with 15 N 2 and H 2 using the pure CeN catalyst. [ 150 ] This provided further confirmation that lattice nitrogen ( 14 N) in CeN takes place in the ammonia synthesis reaction. To further understand the activity of these catalysts, DFT calculations were performed with the nitrogen vacancy formation energy calculated.…”