Understanding the Structure/Activity Relationships of Interstitial Nitrides for Ammonia Synthesis
A. R. McFarlane1*, J.S.J. Hargreaves1, J. Cook2, A. L. Hector2, W. Levason2, K. Sardar2, N.Bion3, F.Can3, M. Richard3, W. Flavell4, M. Leontiadou4, H.F. Greer5, and W.Z. Zhou5
1. School of Chemistry, Joseph Black Building, University of Glasgow, Glasgow, U.K.
2. Department of Chemistry, University of Southampton, Highfield , Southampton, U.K.
3. CNRS UMR 7285, Institut des Milieux et Materiaux de Poitiers, University of Poitiers, Poitiers, France
4. Institute of Photon Science, Alan Turing Building University of Manchester, England, U.K.
5. School of Chemistry, University of St Andrews, St Andrews, Scotland, U.K.
The importance of catalytic industrial ammonia synthesis cannot be overstated. Ammonia as a precursor to fertilizer sustains the global population and the scale of production is such that it is responsible for greater than 1 % of the world’s total energy consumption. Any improvement over the current industrial process would therefore have significant environmental impact.
Traditional ammonia synthesis is performed using the Haber – Bosch process, technology which has remained largely unchanged for almost a century and which generally utilises promoted Fe catalysts operating under high pressures. A number of alternative catalytic materials have been studied for this reaction and interstitial metal nitrides are a range of materials that are increasingly attracting interest both within the field of heterogeneous catalysis. This is in no small part due to claims that their activity can be similar to that observed for noble metals . For ammonia synthesis, a body of literature exists on the application of ternary metal nitrides including Co3Mo3N and Ni2Mo3N which have been reported to display high efficacy under industrially relevant conditions [2,3]. Of these two systems, it seems that Co3Mo3N is the more active, however there has been little investigation into rationalising these findings within the context of structure/activity relationships. One theoretical study by Nørskov et al. has invoked a synergy between Co and Mo for N2 adsorption where one species binds N too strongly and the other too weakly – resulting in a material that has the optimal N2 adsorption enthalpy – and also suggests a degree of surface sensitivity as it is the (111) plane that appears to be the source of the activity . Further increasing the understanding of the relationship between structure and function of interstitial nitrides should allow for improved catalyst design.
Along these lines, recent studies have begun to examine the role of the lattice nitrogen within interstices of both binary and ternary nitrides. Homomolecular and heterolytic nitrogen exchange mechanisms have been examined using 14N/15N and it has been demonstrated that the lattice nitrogen present in the nitride is exchangeable . This raises the possibility that ammonia synthesis using this material could be occurring via a Mars-van Krevelen type mechanism which could allow competitive reaction rates to occur under more benign operating conditions. This observation of lattice nitrogen exchangeability also suggests that interstitial nitrides could be useful for a range of nitrogen insertion reactions. It is important to note however, that the ammonia synthesis rates and the extent of lattice nitrogen exchange in ternary interstitial nitrides have been shown to vary greatly within the same material and both the synthesis method employed to obtain the nitride and any gas pretreatments that may be used prior to ammonia synthesis have been studied to this effect.
One example of this apparent sensitivity is Ni2Mo3N which has been prepared by both ammonolysis of a nickel molybdate precursor using NH3 and by reacting N2/H2 with a citrate-gel precursor. XRD confirms that whilst both routes generate the same phase of Ni2Mo3N, the former results in a Ni2Mo3N with a residual nickel metal impurity, as dictated by stoichiometry. When examined for both ammonia synthesis and lattice nitrogen exchange, only the Ni2Mo3N prepared via citrate-gel synthesis shows any activity whilst the nitride with the impurity is inactive for both. Investigations into reasons for the differences in activity will be presented.
A second example that will be discussed is that of Co3Mo3N. Generally, before any ammonia synthesis experiments are performed, nitrides are first pretreated under reducing atmospheres to remove any surface oxide that may have formed over time. Lattice nitrogen exchange experiments have shown that the extent of exchange is highly dependent on the pretreatment conditions employed prior to testing. Pretreating Co3Mo3N with N2 allows 25% of the lattice nitrogen to be exchanged whereas only 6% is available after an otherwise equivalent Ar pretreatment . X-ray photoelectron spectroscopy has been employed to attempt to understand any potential differences in oxidation state that may occur with the different pretreatments. Synchrotron-excited XPS has a number of advantages such as being able to tune the incident x-ray energy, and hence the photoelectron inelastic mean free path length, which allows depth profiling studies to be undertaken as well as being able to quantify the relative concentrations of each element present. Analysis of the nitride before and after various pretreatments has shown that segregation of molybdenum to the surface of the material occurs and that this molybdenum exists either in a metallic state or that of a binary nitride depending on the pretreatment gas. It appears that it is this restructuring that is responsible for the extent of lattice nitrogen exchangeability.
The two examples above highlight the importance of developing a better understanding of the structure/function relationships of interstitial nitrides and how this information could in turn lead to improved materials for ammonia synthesis.
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