Doped Co3Mo3N is an ammonia synthesis catalyst with an activity which may exceed that of the catalyst normally applied in the Haber Bosch Process [1]. A computational study proposes that the origin of the high activity lies in the expression of the (111) surface plane where both Mo and Co are exposed leading to an average N2 adsorption enthalpy close to that of Ru [2]. Lattice N was stated to have no role in the reaction other than a structural effect. A contrary view of the reaction pathway is the possibility that the lattice N itself is active via a Mars-van Krevelen mechanism wherein it is directly hydrogenated to yield ammonia and then replenished. Co3Mo3N has been shown to undergo nitrogen exchange directly with gas-phase 15N2 [3] and the lattice nitrogen reactivity is dramatically affected by pre-treatment. N2 pretreatment yields materials possessing significantly more reactive lattice N than materials comparably pretreated with Ar. To investigate the role of lattice N and also the role of pre-treatment, computational modelling and depth profiling XPS studies have been undertaken. Computational studies were used to investigate the role of N vacancy formation and replenishment. These results will be presented along with considerations of nitrogen diffusion coefficients through to the surface from the bulk. As demonstrated by the spectra in Fig 1, XPS shows a pronounced effect of pre-treatment atmosphere upon the surface composition of Co3Mo3N. This will be discussed and the N diffusivity through segregated surface layers considered. [1] Kojima, R, Aika, K-I, Appl Catal A: Gen 2001, 215, 149 [2] Jacobsen, C J H et al,, JACS 2001, 123, 8404 [3] Hunter, SM et al, ACS Catal 2013, 3, 1719

Lattice nitrogen reactivity in Co3Mo3N catalysts: Understanding the role of surface structure and composition through application of depth resolved XPS and computational modelling

251st National Meeting (2016)

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