Low ‑ T Mechanisms of Ammonia Synthesis on Co 3 Mo 3 N

: Dispersion-corrected periodic DFT calculations have been applied to elucidate the Langmuir − Hinshelwood (dissociative) and an Eley − Rideal/Mars − van Krevelen (associative) mechanism for ammonia synthesis over Co 3 Mo 3 N surfaces, in the presence of surface defects. Comparison of the two distinct mechanisms clearly suggests that apart from the conventional dissociative mechanism, there is another mechanism that proceeds via hydrazine and diazane intermediates that are formed by Eley − Rideal type chemistry, where hydrogen reacts directly with surface activated nitrogen, in order to form ammonia at considerably milder conditions. This result clearly suggests that via surface defects ammonia synthesis activity can be enhanced at milder conditions on one of the most active catalysts for ammonia synthesis.


■ INTRODUCTION
The development of alternative nitrogen-fixation processes could have a profound economic and environmental impact, as more that 50% of ammonia for soil fertilizers is produced by man.−4 Currently ammonia synthesis industrially is achieved mostly via the classical Haber−Bosch (H-B) process, 5−7 with a Fe−K 2 O−Al 2 O 3 catalyst, which operates under high temperatures (>400 °C) and pressures (150−200 atm).Some industrial plants have changed to the Kellogg advanced ammonia process that uses a graphite-supported alkali/alkaline-earth promoted Ru catalyst, which operates at milder conditions but is expensive. 8Researchers are now seeking catalytic materials that could potentially produce ammonia at low temperatures (T = 200−300 °C) in order to save energy.The generally accepted mechanism for ammonia synthesis on the iron K-promoted Fe catalyst is a Langmuir− Hinshelwood (L-H) mechanism which is dissociative. 9,10An L−H mechanism has also been found on the Ru(0001) surfaces, via density functional theory (DFT) calculations in order to model the Ru−graphite catalyst. 11,12The ratedetermining step (RDS) was found to be the activation step for N 2 , 13 due to the high bond dissociation enthalpy of N 2 (945 kJ mol −1 ) compared to H 2 (436 kJ mol −1 ). 14−20 Although the high activity of this and related materials has been established, there is a need to understand its origin and to elucidate the mechanism of ammonia synthesis.Only a few previous DFT studies have attempted to model the kinetics of ammonia synthesis on this catalyst, based on linearscaling relationships, that correlated the dissociation barrier to the adsorption energy of N 2 , 16,21 where an L−H mechanism was assumed. 13However, a detailed study of the elementary mechanistic steps and the role of surface defects in particular is currently lacking.In two recent DFT studies we have identified possible sites for the adsorption and activation of the reactants of the ammonia synthesis reaction on a model Co 3 Mo 3 N surface with heterogeneity due to surface nitrogen vacancies. 22uch vacancies are present in large concentrations even at ambient temperatures (i.e., 10 13 cm −2 ) and can efficiently activate N 2 . 23Our earlier work found that there are two activation sites for N 2 , the first is a surface cavity, where N 2 is bound side-on aCo 8 cluster at the 16d Wyckoff site, where N 2 is activated 21% (as measured by the percentage change in the N−N bond length); the second, a somewhat weaker activation site (11%) that was found to be located at surface 3f-nitrogen vacancies for which we have suggested that nitrogen vacancies can participate in the elementary reaction steps toward the formation of ammonia. 22Indeed, recent DFT studies show that N-vacancies can participate in the mechanism for the electrochemical reduction of ammonia on Zr, Nb, Cr, and V mononitrides 24,25 and in the two-step solar-energy driven ammonia synthesis on metal-nitrides. 26,27Additionally, in a recent DFT study, the electrochemical synthesis of ammonia was studied for the Mars−van Krevelen mechanism on group III−VII transition metal nitrides and a N−N was found to be suitable, whereas the other metal nitrides decompose to their pure metals. 28

■ RESULTS AND DISCUSSION
In this communication we have considered a Langmuir− Hinshelwood (L-H; see Schemes 1 and 3) and an Eley−Rideal/ Mars−van Krevelen (E-R/MvK; see Schemes 2 and 4) mechanism for ammonia synthesis on Co 3 Mo 3 N-(111) surfaces.
In particular, we have applied dispersion-corrected (D3) DFT in order to calculate the potential energy diagram for ammonia synthesis for two mechanistic pathways for ammonia synthesis on the (111)-surfaces of Co 3 Mo 3 N in the presence of surface defects, such as nitrogen vacancies and intrinsic surface cavities.The computational methodology has been described in detail elsewhere. 22,23Activation barriers were obtained by the nudged elastic band (NEB) method in which the barrier was modeled first by 10 images to acquire the details of the potential energy surface.Once the various intermediates were found and fully optimized, the transition states were located by a separate NEB run of 3 images.Remarkably, we find that there are two possible mechanisms for ammonia synthesis; the first is the commonly known L-H mechanism 8  E-R/MvK (Associative) Ammonia Synthesis Mechanism.In the E-R/MvK mechanism, ammonia is synthesized at the 3f-nitrogen vacancy sites on the Mo 3 N framework.The active site is shown in the box of Scheme 4 and has some similarities with FeMo-cofactor in nitrogenases: A′: nitrogen adsorbs in an end-on configuration at a 3-fold N-vacancy on the Mo 3 N framework with 11% activation; B′: gas phase hydrogen reacts directly in an Eley−Rideal mechanism with surface activated nitrogen forming a trans hydrazine intermediate (>NNH 2 , similar to MoNNH 2 29 and Fe−NNH 2 30 ), in a low barrier process; C′: hydrogen adsorbs molecularly onto the Mo 3 N framework adjacent to the nitrogen vacancy site, while trans hydrazine isomerizes into a conformation that can react with H 2 ; D′:H 2 reacts with the hydrazine-like intermediate forming diazane and −H; E′: Diazane readily decomposes due to its positive charge into a tertiary amine and −H; F′: −H reacts with the tertiary amine forming an azane bound to a Mo 3 hollow; G′: hydrogen adsorbs molecularly at Mo 3 N framework adjacent to where previously the nitrogen vacancy site was located; H′: the precursor-mediated adsorbed state of H 2 reacts with azane forming primary amine, through a high barrier step; I′: the second −H reacts with the primary amine forming surface chemisorbed ammonia that is bound to a Mo 3 hollow site; J′: finally, the second NH 3 desorbs through a high barrier step >200 kJ/mol.
Comparison of the two distinct mechanisms shows that the barriers for the corresponding hydrogenation steps are lower for the associative E-R/MvK (associative) mechanism and the barrier of the rate-determining step (RDS) is 90 kJ mol −1 smaller in the E-R/MvK mechanism.Additionally, the E-R/ MvK mechanism lacks the second and third activation barrier in the L-H mechanism that are high.These results clearly suggest that apart from the conventional L-H (dissociative) mechanism, there is also a mechanism which procceds via diazane and hydrazine intermediates that are formed by a direct associative Eley−Rideal/Mars−van Krevelen mechanism, where molecular hydrogen reacts directly with surface activated nitrogen, in order to form ammonia at considerably milder conditions.Intriguingly, if the mechanism is cycled in two phases, phase I: steps A′ through D′ as depicted in Scheme 4, and phase II: the catalyst is activated to form nitrogen vacancies, then barriers are <70 kJ/mol for phase I which produces half the stoichiometric amount of ammonia.The second half would be generated by annealing the catalyst to cause desoprtion of NH 3 (ΔH des = 204 kJ mol −1 ) and regenerate the amount of nitrogen vacancies on the catalyst surface.

■ CONCLUSIONS
We have studied with dispersion-corrected DFT two complete mechanisms for ammonia synthesis on cobalt molybdenum nitride.The first that occurs at surface cavities is a Langmuir− Hinshelwood (dissociative) mechanism, and the second that occurs at surface nitrogen vacancies is an Eley−Rideal/Mars− van Krevelen (associative) mechanism.We show that, in the Eley−Rideal/Mars van Krevelen mechanism, hydrogen reacts directly with surface activated nitrogen, in order to form ammonia, at considerably milder conditions.The mechanism proceeds via hydrazine and diazane intermediates.This result clearly suggests that, via surface defects, ammonia synthesis activity can be enhanced at milder conditions on one of the most active catalysts for ammonia synthesis.

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12364.
The coordinates of the structure of the various intermediates and transition states (Figure S2) and the detailed barrier calculations (Figure S1) (PDF)
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Scheme 1. Elementary Reaction Steps for the L-H (Dissociative) Mechanism