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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. In order to further elucidate the effect of the pre-treatment, a range of characterisation studies has been undertaken. Post-reaction powder X-ray diffraction patterns for materials activated under both H2:Ar and H2:N2, presented in Fig. 3, are consistent with the presence of rhenium, although it should be noted that the high backgrounds could disguise a significant degree of X-ray amorphous material. In addition, no cobalt containing phases are evident despite its relatively high concentration. This is contrary to the situation which pertains for the XRD patterns for CoRe4 catalysts prepared by ammonolysis where a mixture of both Co and Re reflections is apparent [8] and [9]. Most interestingly, the background of the H2:N2 pre-treated material in the range ca 37.5–50° 2θ shows a possible broad feature distinctly difference with respect to its H2:Ar pre-treated counterpart. Such a feature may be attributed to rhenium nitride, as reported elsewhere [10]. However, post-reaction nitrogen analyses undertaken by combustion upon both samples returned a nil result suggesting a very low (if any) content of N, which could still be consistent with the presence of a surface and/or stable nitride phase in the case of the H2:N2 activated material. TEM studies of both materials, both directly after the activation process and also after reaction, have been undertaken to determine potential differences in morphology and to provide a more detailed insight into any phases not apparent in the XRD studies. Representative micrographs are shown in Fig. 4. In these studies, a pronounced amorphous surface film, which is present to a lesser degree in the H2:Ar pre-treated system, was evident in the H2:N2 pre-treated material. From the image contrast pattern, which is different from that of amorphous carbon and also from the fact that double amorphous layers were sometimes observed, it is believed that this amorphous layer is likely formed due to surface reaction of the particles. Whilst this is a very apparent difference, a degree of caution has to be exercised as to its significance since, although it is clear that the materials behave differently upon pre-treatment, exposure to air has occurred prior to TEM measurement which could result in different degrees of surface oxidation. Overall, the H2:Ar pre-treated sample appears to be less ordered than its H2:N2 counterpart. The lattice spacings marked in Fig. 4(c) and (f) are consistent with the (101) plane of Re metal which is evident in the near surface region of samples prepared by both activation procedures, whilst that evident in Fig. 4(i) could correspond to the (111) plane of Co3O4. Co3O4 could either indicate incomplete reduction of the sample upon pre-treatment and subsequent reaction or could otherwise be formed by oxidation of reduced Co phase(s) upon the discharge of the catalyst from the reactor and/or its storage in air. Taken together, a number of the above results provide tentative support for the significance of a nitride phase in relation to the activity of CoRe4 ammonia synthesis catalyst, as detailed elsewhere [8], although such a phase is not directly evident from N analyses of active materials. However, indirect support for its formation is provided by the induction period evident upon the H2:Ar pre-treatment and the isotopic N2 exchange behaviour, as well as from the background of the XRD pattern of the H2:N2 pre-treated catalyst. Alternatively, the different pre-treatment procedures may also exert effects by influencing the Co:Re surface ratio. Accordingly, it is our intention to address these aspects in comparative in-situ pre-treated depth profile XPS studies. It is also planned to undertake in-situ XRD/XAS measurements to provide a level of structural detail currently not possible in laboratory XRD investigations. The application of advanced techniques to more directly determine surface structure, also of potential significance, will be investigated. Despite the current uncertainties about the exact nature of the active surface phase, from the results presented herein, it has been shown that ammonolysis is an unnecessary step for the preparation of CoRe4 catalysts and also that such catalysts may be more active than non-promoted ternary nitride systems which have been the subject of some attention in the literature. These interesting findings indicate the system to be of further potential interest. |
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