Experimental and theoretical investigations on the anti-perovskite nitrides Co3CuN, Ni3CuN and Co3MoN for ammonia synthesis.

The ammonia synthesis activities of the anti-perovskite nitrides Co3CuN and Ni3CuN have been compared to investigate the possible metal composition-activity relationship. Post-reaction elemental analysis showed that the activity for both nitrides was due to loss of lattice nitrogen rather than a catalytic process. Co3CuN was observed to convert a higher percentage of lattice nitrogen to ammonia than Ni3CuN and was active at a lower temperature. The loss of lattice nitrogen was revealed to be topotactic and Co3Cu and Ni3Cu were formed during the reaction. Therefore, the anti-perovskite nitrides may be of interest as reagents for the formation of ammonia through chemical looping. The regeneration of the nitrides was achieved by ammonolysis of the corresponding metal alloys. However, regeneration using N2 was shown to be challenging. In order to understand the difference in reactivity between the two nitrides, DFT techniques were applied to investigate the thermodynamics of the processes involved in the evolution of lattice nitrogen to the gas phase via conversion to N2 or NH3, revealing key differences in the energetics of bulk conversion of the anti-perovskite to the alloy phase, and in loss of surface N from the stable low-index N-terminated (111) and (100) facets. Computational modelling of the density of states (DOS) at the Fermi level was performed. It was shown that the Ni and Co d states contributed to the density of states and that the Cu d states only contributed to the DOS for Co3CuN. The anti-perovskite Co3MoN has been investigated as comparisons with Co3Mo3N may give an insight into the role structure type plays in the ammonia synthesis activity. The XRD pattern and elemental analysis for the synthesised material revealed that an amorphous phase was present that contained nitrogen. In contrast to Co3CuN and Ni3CuN, the material was shown to have steady state activity at 400 °C with a rate of 92 ± 15 µmol h-1 g-1. Therefore, it appears that metal composition has an influence on the stability and activity of the anti-perovskite nitrides.

Mechanism of ammonia synthesis on Fe3Mo3N.

Ammonia (NH3) synthesis is an essential yet energy-demanding industrial process. Hence, there is a need to develop NH3 synthesis catalysts that are highly active under milder conditions. Metal nitrides are promising candidates, with the η-carbide Co3Mo3N having been found to be more active than the industrial Fe-based catalyst. The isostructural Fe3Mo3N catalyst has also been identified as highly active for NH3 synthesis. In the present work, we investigate the catalytic ammonia synthesis mechanisms in Fe3Mo3N, which we compare and contrast with the previously studied Co3Mo3N. We apply plane-wave density functional theory (DFT) to investigate surface N vacancy formation in Fe3Mo3N, and two distinct ammonia synthesis mechanisms. The calculations reveal that whilst N vacancy formation on Fe3Mo3N is more thermodynamically demanding than for Co3Mo3N, the formation energies are comparable, suggesting that surface lattice N vacancies in Fe3Mo3N could facilitate NH3 synthesis. N2 activation was found to be enhanced on Fe3Mo3N compared to Co3Mo3N, for adsorption both at and adjacent to the vacancy. The calculated activation barriers suggest that, as for Co3Mo3N, the associative Mars van Krevelen mechanism affords a much less energy-demanding pathway for ammonia synthesis, especially for initial hydrogenation processes.

The effect of H2 : N2 ratio on the NH3 synthesis rate and on process economics over the Co3Mo3N catalyst.

In this study, the process economics of ammonia synthesis over Co3Mo3N was investigated by searching for an optimum feed stoichiometry. From ammonia synthesis rate measurements at atmospheric pressure and 400 °C over Co3Mo3N, it was found that the rate was independent of H2 : N2 stoichiometry for stoichiometries above 0.5 : 1. For H2 : N2 stoichiometries below 0.5 : 1, there was a linear dependency of ammonia synthesis rate on the H2 : N2 stoichiometry. Static measurements of hydrogen adsorption isotherms at 25, 50, and 100 °C revealed that the adsorbed amounts of strongly bound hydrogen over the Co3Mo3N surface were saturated at around 100 Torr hydrogen pressure. This pressure corresponds to the partial pressure of hydrogen when the H2 : N2 stoichiometry is around 0.5 : 1, confirming the role of strongly bound hydrogen in ammonia synthesis. These results were used to modify an existing kinetic expression to be used in a conceptual design, based on a late mixing strategy for the hydrogen stream. This conceptual design and its economic analysis revealed that using low hydrogen stoichiometries can cut the investment and operating costs by a factor of 2.

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.

XAS investigation of silica aerogel supported cobalt rhenium catalysts for ammonia decomposition.

The implementation of ammonia as a hydrogen vector relies on the development of active catalysts to release hydrogen on-demand at low temperatures. As an alternative to ruthenium-based catalysts, herein we report the high activity of silica aerogel supported cobalt rhenium catalysts. XANES/EXAFS studies undertaken at reaction conditions in the presence of the ammonia feed reveal that the cobalt and rhenium components of the catalyst which had been pre-reduced are initially re-oxidised prior to their subsequent reduction to metallic and bimetallic species before catalytic activity is observed. A synergistic effect is apparent in which this re-reduction step occurs at considerably lower temperatures than for the corresponding monometallic counterpart materials. The rate of hydrogen production via ammonia decomposition was determined to be 0.007 molH2 gcat-1 h-1 at 450 °C. The current study indicates that reduced Co species are crucial for the development of catalytic activity.

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.

DFT-D3 study of H2 and N2 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.

Iron ochre - a pre-catalyst for the cracking of methane.

BACKGROUND: Iron ochres are gelatinous sludges that can cause problems in terms of water management. In this work, the application of iron ochre obtained from a river has been applied to catalytically crack methane - another potential waste product - into two useful products, hydrogen and a magnetic carbon-containing composite. RESULTS: The powder X-ray diffraction (XRD) pattern of the iron ochre was found to be consistent with the expected 2-line ferrihydrite, and energy dispersive X-ray (EDX) analysis showed Fe to be a major component although some Si and Ca were present. The sample was observed to contain a fraction with a tubular morphology consistent with the presence of extra-cellular biogenic iron oxide formed by leptothrix. Upon exposure to methane at elevated temperatures, the material was found to transform into an active catalyst for hydrogen production yielding a magnetic carbon-containing composite material comprising filamentous carbon and encapsulating graphite. CONCLUSION: The application of two waste products - iron ochre and methane - to generate two useful products - hydrogen and a magnetic carbon-containing composite - has been demonstrated. Furthermore, the ochre has been shown to comprise tubular morphology extra-cellular biogenic iron oxide which may be of interest in terms of other applications. © 2014 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

A Comparison of the Reactivity of the Lattice Nitrogen in Tungsten Substituted Co3 Mo3 N and Ni2 Mo3 N.

The effect of the partial substitution of Mo with W in Co3 Mo3 N and Ni2 Mo3 N on ammonia synthesis activity and lattice nitrogen reactivity has been investigated. This is of interest as the coordination environment of lattice N is changed by this process. When tungsten was introduced into the metal nitrides by substitution of Mo atoms, the catalytic performance was observed to have decreased. As expected, Co3 Mo3 N was reduced to Co6 Mo6 N under a 3 : 1 ratio of H2 /Ar. Co3 Mo2.6 W0.4 N was also shown to lose a large percentage of lattice nitrogen under these conditions. The bulk lattice nitrogen in Ni2 Mo3 N and Ni2 Mo2.8 W0.2 N was unreactive, demonstrating that substitution with tungsten does not have a significant effect on lattice N reactivity. Computational calculations reveal that the vacancy formation energy for Ni2 Mo3 N is more endothermic than Co3 Mo3 N. Furthermore, calculations suggest that the inclusion of W does not have a substantial impact on the surface N vacancy formation energy or the N2 adsorption and activation at the vacancy site.

Alternative catalytic materials: carbides, nitrides, phosphides and amorphous boron alloys.

Catalysts generated by the addition of carbon, nitrogen or phosphorus to transition metals have interesting properties and potential applications. The addition of carbon, nitrogen or phosphorus can lead to substantial modification of the catalytic efficacy of the parent metal and some carbides and nitrides are claimed to be comparable to noble metals in their behaviour. Amorphous boron transition metal alloys are also a class of interesting catalyst, although their structures and phase composition are more difficult to define. In this critical review, the preparation of these catalysts is described and brief details of their application given. To date, attention has largely centred upon the application of these materials as alternatives for existing catalysts. However, novel approaches towards their utilisation can be envisaged. For example, the extent to which it is possible to utilise the "activated" carbon and nitrogen species within the host lattices of carbides and nitrides, respectively, as a reactant remains largely unexplored (195 references).