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Thus, the surface-anchored Fe 3 single cluster serves as an electron reservoir that regulates the charge variation of the whole process. Changes of N—N bond length and charges of Fe 3 and the adsorbates. Black, blue, and red curves represent N—N bond lengths, Bader charges of Fe 3 cluster, and Bader charges of dinitrogen at every step of the catalytic cycle with the associative mechanism. Therefore, the large spin polarization on Fe 3 cluster is responsible for the activation of N 2. Electronic structure analysis. Fragment orbital analysis is further performed to provide more details of such interaction between isolated Fe 3 and N 2 Fig. Supplementary Fig. With the increase of temperature, the concentrations of NH x decrease and that of bare site increases, since the entropy of free gas molecules becomes dominant.
While on the Ru step surface, the TOF of the associative mechanism is calculated to be three orders of magnitude lower than that of the dissociative mechanism Microkinetic simulations. The solid blue line represents a least-squared interpolation between the points, the red line depicts the cases with addition of alkali potassium promotion. Note that the energy data for the metal surfaces in e are taken from ref. The coverages and free energy diagrams for surface species are shown in Supplementary Fig. It is thus possible to bypass the BEP relation with the associative mechanism, and the limitation underlying one side of the volcano curve is then removed.
As a counterpart, very recently Chen et al. With these strategies of breaking the BEP relation, ammonia synthesis with ambient conditions does not seem to be impossible. Especially, the procedure of N 2 activation and hydrogenation can be affected by moisture or solvents if existing. Further study of the optimal surface metal clusters and support for N 2 -to-NH 3 conversion under different chemical conditions will be interesting. The nature of the surface SCCs will likely offer high selectivity like single-atom catalysts. We propose a surface-anchored Fe 3 single-cluster active center for ammonia synthesis by first-principle calculations and microkinetic analysis.
Remarkably, the associative mechanism bypasses the BEP relation and thus the limitation underlying one side of the volcano curve. Such surface-anchored Fe 3 center represents a class of new catalyst—single-cluster catalyst, which features identical yet isolated active centers on support and thus bridges the gap between heterogeneous and homogeneous catalysis, serving as a heterogeneous catalyst design that enables the associative mechanism for ammonia synthesis from dinitrogen.
Thus, the anchored Fe 3 center is a promising candidate heterogeneous catalyst for highly selective ammonia synthesis via the associative mechanism. In the future work, we will conduct extensive investigation of various surface-anchored metal trimer and polynuclear clusters, in order to find the optimum of such design for ammonia synthesis toward high TOF at low temperature and low pressure. Highly stable single-cluster catalysts with well-accommodating support hold promises for rational design of highly selective and active catalysts for complicated catalytic reactions such as N 2 -to-NH 3 conversion. Atomic positions were optimized until the forces were less than 0.
Transition states were searched by climbing image nudged elastic-band method CI-NEB and further confirmed by vibrational frequency analysis The trinuclear Fe clusters were anchored by coordinating with surface oxygen as shown in Fig. So the formation energy includes both the formation energy of gas phase Fe n cluster and its binding energy on surface. Therefore, we can directly evaluate the stability by E f. Fe 3 and Fe 3 N 2 were optimized under D 3h and C s point-group symmetries, respectively, at all possible spin configurations. Microkinetic modeling was carried out using the CatMAP software package The model was constructed by numerically solving the differential equations that describe the coverage of each surface intermediates under the steady-state approximation.
The rate constant of each elementary step was calculated by using harmonic transition state theory. The free energies for gas molecules were estimated using the ideal gas approximation considering the vibrational, rotational, and translational contributions to both entropy and enthalpy, while the free energies for surface adsorbates were approximated using the harmonic approximation that treats all degrees of freedom as vibrational modes. The steady-state TOFs were calculated based on the steady-state surface coverages.
All other data supporting the findings of this study are available in the article and its Supplementary Information files and from the corresponding author on request. Haber, F. Article Google Scholar. Ertl, G. Cherkasov, N. A review of the existing and alternative methods for greener nitrogen fixation. Process Process Intensif. Hammer, B. Theoretical surface science and catalysis—calculations and concepts. CAS Google Scholar. Honkala, K. Ammonia synthesis from first-principles calculations. Science , — Ammonia synthesis over a Ru surface studied by density functional calculations.
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