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New information from XANES/ EXAFS for the development of novel ammonia decomposition catalysts On-demand production of hydrogen from ammonia can be accomplished by combining cobalt and rhenium, resulting in comparable activity to ruthenium, the best catalyst. In-situ EXAFS/XANES analyses demonstrate that bimetallic Co-Re species are responsible for the activity. The re-reduction of Co partially oxidised by NH3 in the presence of Re coincides with the on-set of activity. Ammonia is an attractive sustainable energy carrier due to its high hydrogen content and narrow flammability range  enabling the long-term energy storage in chemical bonds versus the short-term storage offered by electrochemical means. Despite its potential, the implementation of ammonia in the energy landscape relies on the capability of releasing hydrogen on demand, preferably at temperatures aligned to those of fuel cells. Considerable scientific effort is currently focused on developing catalysts to attain this goal. The strategy employed here was the development of bi-metallic ammonia decomposition catalysts combining metals possessing different N-adatom adsorption energies following the DFT simulations by Hansgen et al.  to achieve an optimum binding energy for catalytic performance mimicking the N-adsorption energy of ruthenium, the benchmark catalyst in this process.
CoRe1.6 had comparable hydrogen production to 7 wt.% Ru/CNT (Figure 120a) . While 3-5 nm supported Ru nanoparticles (7 wt.% Ru/CNT) present a considerably higher activity than the unsupported CoRe1.6 per mol of metal (Figure 120b), a rate orders of magnitude higher is shown by the CoRe1.6 based upon metallic surface area. Therefore the active sites in Co-Re might be considerably more active than those in Ru. The low temperature activity is directly related to the intimate Co-Re interaction (Figure 121a and 121b) with the activity onset related to the contraction of the Re-Co bond distance.
In-situ XAS undertaken at beamline BM31 revealed reduction of the cobalt precursor to Co0 occurred over a narrow temperature range from 350°C - 400°C. Reduction of Re occurred in a single step from 300°C. XANES upon NH3 treatment show partial oxidation of cobalt, evident by the increased white line intensity  (Figure 121c). Although changes were observed in the Co XANES, EXAFS revealed no light atom scattering pairs (i.e., Co-N) were formed during low-temperature NH3 treatment. The white line intensity decreased from 400°C during NH3 treatment (Figure 121d and 121e) and reached a similar intensity to CoRe1.6 during pre-treatment at 600°C, corresponding to a partial reduction of Co coinciding with the onset of NH3 decomposition activity for CoRe1.6 .
Despite the changes observed, the Co-Co backscattering pair was stable when the temperature was increased from 300°C to 600°C under NH3. The average Co-Co multiplicity remained at 4 with a bond distance of 2.45 Å. The Re-Re backscattering pair elongated from 2.65 Å to 2.73 Å during the pre-treatment while a contraction of the bond distance to 2.65 Å occurred after switching from pre-treatment conditions to NH3 at 200°C. During NH3 decomposition, these distances changed: Re-Re increased to 2.73 Å, while Re-Co was shortened (2.56 Å) coinciding with activity onset. While the change in bond distance of Re-Re approached that of Re-foil (2.74 Å), it did not seem to be associated with sintering of a pure Re-Re phase as the average multiplicity was not changed.
These observations indicate local restructuring of both monometallic and bimetallic particles occurs between 400-600°C, after CoRe1.6 is fully reduced. The major contributing species were Co-Co and Re-Re with only ~20% of the bimetallic Re-Co pair. Co-Re is clearly a complex system with interesting NH3 decomposition activity, where XANES/EXAFS-derived information is critical for enhanced understanding and further development.
Fig. 120: a) Activity of n CoRe1.6 s Ni2Mo3N u Co3Mo3N l 7% Ru/ CNT. b) Reaction rate of 7 wt.% Ru/CNT (orange) and CoRe1.6 (black).