Low-T Mechanisms of Ammonia Synthesis on Co3Mo3N

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 Co3Mo3N 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

Nitrogen Activation in a Mars-van Krevelen Mechanism for Ammonia Synthesis on Co3Mo3N

Co3Mo3N is one of the most active catalysts for ammonia synthesis; however, little is known about the atomistic details of N2 adsorption and activation. Here we examine whether N2 can adsorb and activate at nitrogen surface vacancies. We have identified the most favorable sites for surface nitrogen vacancy formation and have calculated vacancy formation free energies (and concentrations) taking into account vacancy configurational entropy and the entropy of N2 at temperature and pressure conditions relevant to ammonia synthesis (380–550 °C, 100 atm) via a semiempirical approach. We show that 3-fold hollow bound nitrogen-containing (111)-surfaces have surprisingly high concentrations (1.6 × 1016 to 3.7 × 1016 cm–2) of nitrogen vacancies in the temperature range for ammonia synthesis. It is shown that these vacancy sites can adsorb and activate N2 demonstrating the potential of a Mars–van Krevelen type mechanism on Co3Mo3N. The catalytically active surface is one where 3f-hollow-nitrogens are bound to the molybdenum framework with a hexagonal array of embedded Co8 cobalt nanoclusters. We find that the vacancy-formation energy (VFE) combined with the adsorption energy can be used as a descriptor in the screening of materials that activate doubly and triply bonded molecules that are bound end-on at surface vacancies

The potential of manganese nitride based materials as nitrogen transfer reagents for nitrogen chemical looping

A systematic study was carried out to investigate the potential of manganese nitride related materials for ammonia production. A-Mn-N (A = Fe, Co, K, Li) materials were synthesised by nitriding their oxide counterparts at low temperature using NaNH2 as a source of reactive nitrogen. The reactivity of lattice nitrogen was assessed using ammonia synthesis as a model reaction. In the case of Mn3N2, limited reactivity was observed and only 3.1% of the available lattice nitrogen was found to be reactive towards hydrogen to yield ammonia while most of the lattice nitrogen was lost as N2. However, the presence of a co-metal played a key role in shaping the nitrogen transfer properties of manganese nitride and impacted strongly upon its reactivity. In particular, doping manganese nitride with low levels of lithium resulted in enhanced reactivity at low temperature. In the case of the Li-Mn-N system, the fraction of ammonia formed at 400 °C corresponded to the reaction of 15% of the total available lattice nitrogen towards hydrogen. Li-Mn-N presented high thermochemical stability after reduction with hydrogen which limited the regeneration step using N2 from the gas phase. However, the results presented herein demonstrate the Li-Mn-N system to be worthy of further attention

DFT-D3 study of H-2 and N-2 chemisorption over cobalt promoted Ta3N5-(100),(010) and (001) surfaces

The reactants for ammonia synthesis have been studied, employing density functional theory (DFT), with respect to their adsorption on tantalum nitride surfaces. The adsorption of nitrogen was found to be mostly molecular and non-activated with side-on, end-on and tilt configurations. At bridging nitrogen sites (Ta–N–Ta) it results in an azide functional group formation with a formation energy of 205 kJ mol−1. H2 was found also to chemisorb molecularly with an adsorption energy in the range −81 to −91 kJ mol−1. At bridging nitrogen sites it adsorbs dissociatively forming >NH groups with an exothermic formation energy of −175 kJ mol−1 per H2. The nitrogen vacancy formation energies were relatively high compared to other metal nitrides found to be 2.89 eV, 2.32 eV and 1.95 eV for plain, surface co-adsorbed cobalt and sub-surface co-adsorbed cobalt Ta3N5-(010). Co-adsorption of cobalt was found to occur mostly at nitrogen rich sites of the surface with an adsorption energy that ranged between −200 to −400 kJ mol−1. The co-adsorption of cobalt was found to enhance the dissociation of molecular hydrogen on the surface of Ta3N5. The studies offer significant new insight with respect to the chemistry of N2 and H2 with tantalum nitride surfaces in the presence of cobalt promoters

A comparative analysis of the mechanisms of ammonia synthesis on various catalysts using density functional theory

In this review, we present the recent progress in ammonia synthesis research using density functional theory (DFT) calculations on various industrial catalysts, metal nitrides and nano-cluster-supported catalysts. The mechanism of ammonia synthesis on the industrial Fe catalyst is generally accepted to be a dissociative mechanism. We have recently found, using DFT techniques, that on Co_{3}Mo_{3}N (111) surfaces, an associative mechanism in the synthesis of ammonia can offer a new low-energy pathway that was previously unknown. In particular, we have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies which can activate dinitrogen, thereby promoting the associative mechanism. These fundamental studies suggest that a promising route to the discovery of low-temperature ammonia synthesis catalysts will be to identify systems that proceed via the associative mechanism, which is closer to the nitrogen-fixation mechanism occurring in nitrogenases

Combination of theoretical and in situ experimental investigations of the role of lithium dopant in manganese nitride: a two-stage reagent for ammonia synthesis

Manganese nitride related materials are of interest as two-stage reagents for ammonia synthesis via nitrogen chemical looping. However, unless they are doped with a co-cation, manganese nitrides are thermochemically stable and a high temperature is required to produce ammonia under reducing conditions, thereby hindering their use as nitrogen transfer materials. Nevertheless, when lithium is used as dopant, ammonia generation can be observed at a reaction temperature as low as 300 °C. In order to develop strategies for the improvement of the reactivity of nitride materials in the context of two-stage reagents, it is necessary to understand the intrinsic role of the dopant in the mechanism of ammonia synthesis. To this end, we have investigated the role of lithium in increasing the manganese nitride reactivity by in situ neutron diffraction studies and N2 and H2 isotopic exchange reactions supplemented by DFT calculations

Nitrogen transfer properties in tantalum nitride based materials

Ta3-xMxNy (M=Re, Fe, Co; x=0, 0.25, 0.5, 1) materials with different microstructural features (e.g. surface area) were successfully prepared using different synthesis techniques. The dependence of nitrogen transfer properties upon tantalum nitride microstructure and its chemical composition was evaluated using the ammonia synthesis with a H2/Ar feedstream (a reaction involving lattice nitrogen transfer). It was shown that nitrogen reactivity for tantalum nitride is more dominated by lattice nitrogen stability rather than microstructural properties. In the case of non-doped tantalum nitride, only a limited improvement of reactivity with enhanced surface area was observed which demonstrates the limited impact of microstructure upon reactivity. However, the nature of the transition metal dopant as well as its content was observed to play a key role in the nitrogen transfer properties of tantalum nitride and to impact strongly upon its reactivity. In fact, doping tantalum nitride with low levels of Co resulted in enhanced reactivity at lower temperature

The integration of experiment and computational modelling in heterogeneously catalysed ammonia synthesis over metal nitrides

In this perspective we present recent experimental and computational progress in catalytic ammonia synthesis research on metal nitrides involving a combined approach. On this basis, it suggested that the consideration of nitrogen vacancies in the synthesis of ammonia can offer new low energy pathways that were previously unknown. We have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies on their surfaces. These vacancies adsorb dinitrogen much more strongly than the defect-free surfaces and can efficiently activate the strong N-N triple bond. These fundamental studies suggest that heterogeneously catalysed ammonia synthesis over metal nitrides is strongly affected by bulk and surface defects and that further progress in the discovery of low temperature catalysts relies on more careful consideration of nitrogen vacancies. The potential occurrence of an associative pathway in the case of the Co3Mo3N catalytic system provides a possible link with enzymatic catalysis, which will be of importance in the design of heterogeneous catalytic systems operational under process conditions of reduced severity which are necessary for the development of localised facilities for the production of more sustainable "green" ammonia