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Within the literature, active CoRe4 catalysts have been prepared by pre-treatment of their precursors with NH3[8] and [9]. However, as discussed by Wise and Markel in relation to the binary molybdenum nitride system, ammonolysis is not practical on a large scale for the preparation of nitrides from oxide precursors [13]. A number of issues such as heat transfer mean nitridation from reactant H2:N2 mixtures would be preferable. Accordingly, in this study, reduction has been undertaken using H2:N2 (3:1) with H2:Ar (3:1) being employed for means of comparison to assess any potential role of possible nitride formation. For both resultant materials, the ambient pressure steady state ammonia synthesis activities at 400 °C are reported in Table 1. From the table it can be seen that ammonolysis is not a necessary step in the preparation of active catalysts and indeed the mass normalised rates determined exceed that of the material prepared by ammonolysis and run under comparable conditions as reported elsewhere (where rates of 600 μmol g− 1 h− 1[8] and ca. 470 μmol g− 1 h− 1[9] have been quoted). If thermodynamic equilibrium was attained under the reaction conditions, employed it would correspond to a mass normalised rate of ca. 2140 μmol g− 1 h− 1. Upon making comparisons with other systems reported to display high activity in the literature, rates of 652 μmol g− 1 h− 1 for Co3Mo3N [5], ca. 400 μmol g− 1 h− 1 for Ni2Mo3N [7] and 437 μmol g− 1 h− 1 for Re/MCM-41 [14] have been reported for conditions comparable to those applied in this study. In the case of Ru dispersed on electride, a rate of 3550 μmol gcat− 1 h− 1 has been reported at atmospheric pressure and 340 °C [3].
To obtain a more complete understanding of the role of pre-treatment and fundamental aspects of nitrogen activation, temperature programmed homomolecular exchange of a 14N2:15N2 mixture has been conducted. This process can be indicative of some of the fundamental N2 activation steps. As can be observed in Fig. 2, there are significant differences between the exchange profiles as a function of pre-treatment. In the case of the H2:Ar pre-treated sample, the 14N2–15N2 homoexchange reaction does not occur within the temperature range applied. However, in the case of the H2:N2 pre-treated system, there seems to be release of a very low amount of hydrogen (the maximum pressure recorded at 550 °C is about 0.1 mbar) followed by development of N2 homolytic exchange. This clearly indicates the importance of pre-treatment with the N2 containing mixture in the generation of an active surface. In addition, it has been observed that application of the secondary vacuum step at 600 °C yields materials which do not desorb hydrogen and which are unreactive for N2 exchange whereas replacing this step by a 30 min 600 °C N2 pre-treatment step does result in exchange which once again may follow evolution of a very small amount of H2. These observations may have interesting implications since it could be imagined that in vacuo at 600 °C H2 loss and/or removal of surface nitride may occur. It could also be the case that there are subtle changes in surface structure and/or composition which lead to the loss of N2 activation ability and the application of in-situ XPS studies to probe these possibilities will be explored. In addition to activity for nitrogen activation, the evolution of H2 suggests N2:H2 pre-treatment to lead to strong hydrogen adsorption. |
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