The Denitridation of Nitrides of Iron Cobalt and

Authors: AM Alexander J S J Hargreaves C Mitchell

Publish Date: 2013/07/17

Volume: 56, Issue: 18-20, Pages: 1963-1969

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Abstract

The denitridation behaviour of binary iron cobalt and rehnium nitrides under H2 /Ar has been investigated The iron nitride was found to lose over 70  of its as prepared nitrogen content at 400 °C The cobalt nitride was completely denitrided at 250 °C Rhenium nitride lost close to 90  of its nitrogen at 350 °C In addition CoRe4 prepared by ammonolyis was investigated whilst only traces of NH3 were lost from this material under H2/Ar at 400 °C with H2/N2 it proved to be an active ambient pressure ammonia synthesis catalyst in accordance with previous literatureMetal nitrides constitute an interesting class of heterogeneous catalyst 1 2 3 4 In some cases their activities reportedly rival or even exceed those of commercial catalysts and comparisons between the efficacy of certain metal nitrides with that of noble metals have frequently been drawn in the literature With the aim of developing novel nitrogen transfer pathways it is of interest to explore the reactivity of the lattice nitrogen within nitrides This idea has its origins in the Marsvan Krevelen mechanism wherein oxidation over metal oxide catalysts is accomplished by the direct reaction of lattice oxygen with the substrate and its subsequent replenishment from the gas phase oxidant 5 This mechanism can also be developed into a two step process where the oxidation of substrate and the reoxidation of reduced catalyst are performed in separate steps which can deliver selectivity and heat transfer advantages when the target product is eg susceptible to further oxidation in the gasphase The selective oxidation of butane to yield maleic anhydride provides an example of a catalytic process for which such a twostage procedure has been investigated 6 In addition to oxidation reactions catalysed by metal oxides Marsvan Krevelenlike reaction mechanisms are documented for other types of reaction 7 for example those involving sulfide 8 and carbide 9 10 catalysts In the case of carbides it is interesting to note that a two stage process methane decomposition to yield intermediate carbide and subsequent hydrogenation to yield higher hydrocarbons at lower temperature has been employed for methane homologation 11 12It is of interest to explore the possibility of utilising the reactivity of lattice nitrogen to develop novel nitrogen transfer pathways to generate products of industrial importance If the replenishment of nitrogen depleted phases can be accomplished by N2 directly more direct routes bypassing ammonia which is often employed in large scale processes may become available This would be a significant development especially when the energy intensive nature of ammonia synthesis 13 is taken into account To date such approaches seem to have been relatively little explored Itoh et al 14 have reported the production of ammonia by reduction from nitrogencontaining intermetallic phases generated by ammonolysis Ley et al 15 16 17 have employed magnesium nitride as a source of ammonia for organic synthesis in the presence of protic solvents Aluminium nitride has also been used for this purpose 18 The reactivity of lattice nitrogen to hydrolysis has also been investigated with the aim of applying solid phase metal nitride reactants to solar ammonia production 19 The reactivity of binary and ternary molybdenum nitrides to hydrogen has been explored 20 21 Of particular note is the observation that 50  of the lattice nitrogen can be removed from Co3Mo3N to yield Co6Mo6N which possesses a previously unprecedented structure wherein the residual nitrogen relocates from the 16c to 8a Wyckoff site 22 23 Furthermore it is possible to regenerate Co3Mo3N by treatment of Co6Mo6N with N2 alone 24 Similar experiments undertaken with molybdenum oxycarbonitrides have demonstrated that the reactivity of their lattice nitrogen is much greater than that of their lattice carbon 25Previously we have reported upon the reactivity of the lattice nitrogen within Cu3N Ni3N Zn3N2 and Ta3N5 26 Cu3N and Ni3N were found to be relatively unstable with up to 30  of their lattice nitrogen generating NH3 at 250 °C 15  of the total lattice N available within Zn3N2 yielded NH3 at 400 °C and in the case of Ta3N5 reactivity of lattice N was definitely established with 13  yielding NH3 up to 700 °C In the present manuscript we detail the reactivity of binary nitrides of iron cobalt and rhenium to hydrogen as an initial screen to indentify systems/conditions of potential interest for application Renitridation of systems of interest ideally by N2 alone would be a necessary step in any envisaged applicationNitrides were prepared by ammonolysis Approximately 05 g of each precursor was in a vertical quartz glass reactor into which 94 ml min−1 of NH3 BOC 9998  was introduced The furnace was programmed to heat the material in accordance with the conditions detailed below Following reaction the material was cooled under the flow of ammonia and upon reaching ambient temperature the system was purged with 100 ml min−1 of N2 gas BOC 99995  for 30 min In order to limit the possibility of their decomposition upon storage both iron nitride and cobalt rhenium nitride samples were passivated using a mixture of 2  O2/Ar flowing at 5 ml min−1 and N2 flowing at 95 ml min−1Cobalt nitride was prepared by reaction of ammonia with Co3O4 at 700 °C for 2 h The temperature was increased from room temperature to 300 °C over 30 min after which it was increased to 450 °C at a rate of 07 °C min−1 and then up to 700 °C at a rate of 167 °C min−1

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