The “vicinal nonmetallic sites” -promoted Hydrogenation Mechanism
Inspired by the low-coordinated iron complexes of the “Fe2N4” for the N2 reduction, 44-46 a diatomic catalyst, Fe2/mpg-C3N4, was developed for the thermal conversion of N2 to NH3. The favorable configuration of Fe2/mpg-C3N4 is shown in Figure 1a, where each Fe atom binds with two adjacent nitrogen atoms forming a flat-lying Fe2 right above mesoporous graphitic carbon nitride. The binding energy of Fe2 with mpg-C3N4 is thermodynamically stable at -7.53eV due to the back donation interactions of iron d orbitals to π* orbitals of the mpg-C3N4 support, substantiated by +1.08|e| Bader charges. The Fe2 moiety is calculated with magnetic moment of 6μB. Such low oxidation state and high spin polarization characteristics on Fe2 facilitate to activate N2.47
Our investigation started with the adsorption of N2 to understand the influence of the activation of nitrogen for the reaction mechanism. The adsorptions of N2 on Fe2/mpg-C3N4 were calculated in three models that were further verified by Ab Initio molecular dynamics (AIMD) simulations (Figure 1b and Figure S1-2). The most stable configuration is the side-on/side-on (μ-ŋ2:ŋ2) mode with Eads= -1.52eV, which is different from the known “Fe-NN-Fe” complexes, 28, 29, 46, probably due to the change in geometry impacting the Fe−NN-Fe bonding interaction. The calculated energies for side-on/end-on (μ-ŋ2:ŋ1) and terminal end-on (ŋ1:ŋ1) modes were 0.02 and 0.23eV less stable, respectively, see Table S1 for details. On close inspection of the μ-ŋ2:ŋ2 mode, the bond length of four Fe-N bonds varied from 1.92 to 1.98Å. The remarkably elongated N-N bond for chemisorbed N2 (from 1.11 Å of free N2 to 1.25 Å) can be ascribed to the back donation interactions of iron d orbitals to two π* orbitals of N2, supported by the increase of negative charge (-0.86|e|) on the *N2. Therefore, the μ-ŋ2:ŋ2 mode was used for the following mechanistic investigations of N2 hydrogenation and N-N dissociation.48
Since the calculated N-N dissociative mechanism in Fe2/mpg-C3N4 is particularly unfavorable via a 2.75eV energy barrier of N-N cleavage (Figure S3), the associative hydrogenation of N2 will be an alternative choice. In contrast to the electro-catalytic reaction where protons and electrons are transferred directly to the adsorbed nitrogen, H2 activation and transfer are important in the thermal catalytic hydrogenation of nitrogen. Due to lack of binding site on the Fe2 cluster, H2 molecules prefer to approach and absorb dissociatively on “C-N” site in the support rather than the Fe site with reaction energy -2.30eV versus -1.26eV, respectively (Figure 1c and Figure S4).49, 50 The dissociation of H2 tends to be hemolytic, to form C-H and N-H bonds, and the charges on each *H are 0.49|e| and 0.88|e|. Because of the H-H repulsion between the N1-H and N2-H, the activation of the second H2 at the C1-N1 site (Figure 1c and Figure S4, Eads= -2.18eV) is less stable. However, the H-H repulsion will make *H on the N1 site easily transfer to the Fe1 site with an energy barrier of 0.75eV. Then through an easily overcome 0.47eV energy barrier, this Fe1 bonded hydrogen (-0.17|e|) would attack the activated *N2 and transfer its electrons to the π* orbital of *N2 forming a *N2H intermediate (a2-a4 in Figure 2a). Subsequently, the Fe1 site favors accepting the second *H on the C1 site by overcoming an activation energy barrier (Ea) of 1.38eV rather than the *H on the N2 site (a4-a5, Ea=1.91eV), see Figure S5 for more details. The synergistic activation of H2 at “vicinal nonmetallic sites” not only reduces the adsorption competition between N2 and H2, but also promotes the subsequent hydrogenation process of N2 (A further discussion of hydrogen activation and transfer are in the last Section.).
After forming the *NNH intermediate (a5 in Figure 2), four different hydrogenation pathways are proposed to generate NH3, dependent on different combinations of alternative/distal hydrogenation on *N2H and the N-N bond dissociation of *NHxNHy(x=0-3, y=0-3) intermediates (Figure 2 and Schematic depiction in Figure S6). It is favorable for *NNH to continue hydrogenation rather than to break the N-N bond due to the relatively high dissociation energy barrier (Ea=1.10eV in a5→d6, AH3). The following hydrogenation favors the formation of *HN-NH via an alternate hydrogenation (AH) mode with a 0.36eV energy barrier (a5→b6 in AH1), which is 0.30eV lower than the formation of *N-NH2 via distal hydrogenation (DH) mode (a5→a6 in DH1). Then, the Fe1 site can accept and activate additional H2 by breaking the Fe-N bond of *HN-NH to form co-adsorption configuration c7, which is ready to further transfer H* to nitrogen bonded to Fe2 site (c7→c8 via 0.36eV energy barrier in AH2). The resulting *NHNH2 (c8) is unusual example in the thermal catalytic reaction, although it is very common in electro- and enzyme catalysis via the N2/H+/e− reaction system. 13, 39-41 With the continuous hydrogenation of *N2 intermediates (a5→b6→c8), the N-N bond becomes much weaker and its bond length is elongated from 1.33 Å to 1.44 Å. The *NHNH2 intermediate with an N–N single bond character ( stretch= 1007 cm−1) would undergo N-N bond cleavage to form bridged μ-*NH and terminal *NH2 as shown in c9 of Figure 2a. This N-N cleavage with 0.50eV energy barrier is facile and not a rate-limiting step, which is distinguished from direct cleavage of nitrogen in the Haber-Bosch process.6, 7 Subsequently, the remaining H* on the Fe1 center is further transferred to the μ-*NH forming the μ-*NH2 species. The last H2 would still be activated by the Fe1 center, and delivers the active H* species to generate two NH3. Since the hydrogenation mechanism of nitrogen in Fe2/mpg-C3N4 relies on synergistic catalysis of Fe2 active sites and “vicinal nonmetallic sites”, we call it “vicinal nonmetallic sites” promoted hydrogenation mechanism. The key intermediates of the preferred reaction pathway are illustrated in Figure S7. More details for four reaction pathways such as optimized intermediates, transition states and energy diagrams are also given in Figure S8-11.
In fact, the reaction pathways calculated above mainly include two types of reactions: N-N bond dissociation and N-H bond formation. Energy barriers for N-N bond dissociations will significantly decrease depending on the gradual hydrogenations to form different *NHxNHy (N-NH, NH-NH, N-NH2 and NH-NH2) intermediates in Figure 2b. This can be rationalized by electron accumulations on related *N2 and elongated N-N bond lengths during the hydrogenation (Table S2). Therefore, promoting the hydrogenation of *N2 would be an applicable way to lower the energy barrier of N-N dissociation. As shown in Figure 3, the complicated N-H bonds formation reactions in different pathways can be resolved to two kinds according to whether the N-N bond is dissociated or not. The nitrogen hydrogenation energy barriers on Fe2 clusters are low and vary from 0.22 to 0.66eV, probably due to the electronic structures of *N2 and the proper Fe-H bond strength. In addition, it is more feasible for the Fe2 cluster with low positive Bader charges to transfer their hydrogen (e.g. a11-a12/c9-c10 or a8-a9/b11-a12, etc. in Figure 3). Overall, Fe2/mpg-C3N4 could be a potential catalyst for N2-to-NH3 conversion with a lower hydrogenation barrier (0.36eV in TS-a5-b6 and 0.36eV in TS-c7-c8) and a lower N-N bond dissociation barrier (0.50eV in TS-c8-c9) in the AH2 reaction pathway.
The Mechanisms of Nitrogen Reduction on various Iron clusters
As diverse multinuclear systems in active sites can significantly change the adsorption mechanism and catalytic performance, we preformed more mechanistic studies of nitrogen reduction by using the newly designed Fe3/mpg-C3N4, 51 Fe4/mpg-C3N4 and the Fe (211) surface19 for the comparisons with the Fe2/mpg-C3N4 system. The reaction pathways and corresponding structures can be obtained in Figure S12-S13. Catalytic N-H bond formation and N-N bond dissociation as key reaction steps are illustrated in Figure 4 to probe the relationship between mechanisms and iron coordination numbers (Fen) of the adsorption sites. 52 Generally, the coordination numbers of the iron active sites have the opposite correlation with the energy barriers of N-N bond breakage (Figure 4a) and N-H bond formation (Figure 4b), as seen in the negative and positive slopes of the linear relationships, respectively. With the increase of coordination number of irons, N-N bond dissociation barriers of *NHxNHy (x=0-1, y=0-2) gradually decrease, while the energy barriers for N-H bond formation of *NHxNHy (x=0-1, y=0-2) tend to be unfavorable. Furthermore, with increasing hydrogen atoms in *NHxNHy (x=0-1, y=0-2) in the linear relation diagram, the N-N bond breaking energy barrier seems to be less sensitive to the Fe coordination number, which is supported by the flattening slope. That is in contrast to N-H bond formation with positive correlation. These findings intrigued us to further investigate the mechanistic details of the N2-to-NH3 thermal conversion.
As shown in Figure 5 and Figure S14, key reaction pathways of related iron clusters indicate the Fe3/mpg-C3N4 and Fe4/g-C3N4 systems tend to form *NNH via *N2 hydrogenation (0.95eV in Fe3/mpg-C3N4 and 1.32eV in Fe4/mpg-C3N4), followed by N-NH bond cleavage with energy barriers of 0.69eV and 0.31eV respectively. The continued hydrogenations to form *NH-NH or *N-NH2 were unfavorable due to high energy barrier (Figure S12-13). These mechanistic results in Fen/mpg-C3N4(n=3,4) are consistent with results in Fe3/Al2O3 clusters from Li’s group.19 There might be an energy crossover point in the Fe4/mpg-C3N4, because the energy barriers of N-N bond cleavage (1.34eV) and N-H bond formation (1.32eV) are quite close, indicating that Fe coordination number over four will cause the dissociative mechanism of nitrogen to dominate. For example, the energy barrier for the direct dissociation of the N2 molecule is only 0.50eV at the Fe (211) C7 site.19 On the contrary, the Fe2/mpg-C3N4 catalyzed system favors continuous hydrogenation of *NNH until the formation of *NHNH2, which can promote the breaking of the N-N bond. Such a synergistic associative mechanism can break the traditional BEP relationship with low N-N dissociation energy barrier and low NHx adsorption energy compared with metal surfaces 53, 54 such as Figure 5b. We can rationalize the results through charges of *N2 as shown in their relationship with energy barriers of N-N dissociation and N-H formation (Figure 5c). With more coordination of Fen, the negative charges of *N2 will increase that enable N-N bond cleavage to be facile, but inhibit *N2 hydrogenation via Fe-hydride intermediates. Therefore, the fewer electrons on nitrogen, the more favorable it is for the hydride to transfer hydrogen to adsorbed *NHxNHy (x=0-1, y=0-2) on the iron sites.
Electronic structure analysis
Besides the Bader charge analysis, projected density of states (PDOS) of adsorbed N2 on different clusters can provide further explanation. As shown in Figure 6a, the α-spin orbitals of the Fe2 clusters are much lower energy than the 2π* orbitals of N2, which cannot fulfill the orbital interaction. The well matched β-spin d orbital of Fe2 can partially donate its electron to the 2π*orbital of N2 forming a β-spin d-π* interaction, which leads to the strong spin polarization of the *N2. This result was further supported by differential charge density and spin density (Figure 6b). Through fragment orbital analysis between an isolated Fe2 cluster and N2 in Figure 6c and Figure S15, the occupied β-dxy-xy/β-dxz-xz orbitals of the Fe2 species can interact with empty nitrogen 2π* orbitals to form two bonding orbitals (β-dxy-xy+π* and β-dxz-xz+π*), which is consistent with molecular orbital interaction between nitrogen and Fe2/mpg-C3N4 (Figure S16) and indicate the electron transfer from Fe2 to nitrogen. Therefore the strong spin polarization of the activated *N2 will facilitate to accept the electron and therefore make hydrogen transfer process on the metal-hydride accessible. In terms of the Fe coordination number increase (Figure 6a), the α-spin orbitals of Fen clusters have shifted to relatively high energy levels and match with the energy level of the 2π* orbitals of N2. The additional electron transfer from α-spin orbitals of Fe clusters will lead to an increase in the electron and a decrease in the spin polarization for *N2, further weakening of the bond strength of *N2, which promotes the N-N bond dissociation. When the coordination numbers of Fe is over 4, such as C7 site of Fe (211), the energy barrier for direct cleavage of N-N bond is favorable. For Fen/g-C3N4 (n=2,3) catalysts, the N2 associative mechanism becomes dominant in nitrogen reduction to ammonia, supported by previous discussion in Figures 4 and 5.
To understand the unusual hydrogenation mechanism of N2 on the smallest cluster-Fe2/mpg-C3N4, the projected density of states (PDOS) and spin densities for key intermediates were shown in Figure 7. With continuous hydrogenation, both Fe2 clusters and the absorbed hydrogens transfer their electrons to the N2 2π* orbitals, which decreased the energy of the N-N anti-bond orbital and activates the N-N bond in Figure 7a. The formed *NNH has relatively high spin polarization to make the next hydrogenation favorable, forming *NHNH via an alternate pathway. Because the α-spin electrons of the N2 2π*orbital keep increasing from *NN to *NHNH2, the spin density for each intermediate is gradually eroded until *NHNH2 with no obvious spin density. After that, the following transformation would favor the dissociation of the N-N bond rather than additional hydrogenation. In contrast, the spin density of *NNH in Fe3/g-C3N4 is relatively low, hindering further hydrogenation and making N-N bond cleavage accessible.
The efficient catalyst by B doping mpg-C3N4
Although the iron diatomic cluster shows a catalytic advantage in the synergistic associative mechanism for ammonia synthesis, the hydrogen transfer from the C1 site of the mpg-C3N4 support to Fe active sites seems unfavorable (Ea(C-H) = 1.38eV in a4 a5 of Figure 8a and Figure 2), which might severely hinder the following transformation. To accelerate this step, we tried to modulate hydrogen adsorption by doping other heteroatoms (including B, O, and N) at the C1 site as shown in Figure S17 and Figure 8. Theoretical simulation indicates that doping with B atoms would not only maintain the stability of the structure, but also promote the activation and conversion of H2.
In B-doped Fe2/mpg-C3N4 (Fe2/B/mpg-C3N4), two H2 molecules were absorbed dissociatively on the C-N and B-N sites with adsorption energy of -2.33eV (a2 in Figure 8b). Different from the C-N site on the support, the dissociation of H2 in the B-N site tends to be heterolysis forming B-H- and N-H+ that are supported by the -0.57|e| and +0.45|e| charges on hydrogens, respectively. The *H stabilized by the electron-deficient B atom (as Lewis acid) will change the transfer order of *H on supports acting as a non-innocent ligand in homogenous catalysis. 55, 56 In the Fe2/B/mpg-C3N4, *H on the B site would preferentially transfer to the Fe1 site with an energy barrier of 0.75eV and continue to attack the activated *N2 to form the *N2H intermediate. After that, the *H on the N1 site will shift to the Fe1 site via a 0.86eV barrier that is slightly higher energy compared to that in the Fe2/mpg-C3N4 system (0.75eV). However, the overall hydrogen transfer process by doping the support with B would reduce the energy barrier from 1.38eV to 0.86eV. This designed B-N system can be regarded as a typical “Lewis pair” that enables heterolysis of the H-H bond and facilitates the hydrogen transfer.57
Stimulated by the excellent theoretical performance of Fe2/B/mpg-C3N4 for H2 activation and transformation, we recalculated the nitrogen reduction mechanism to further investigate whether doping with B will affect the conversion of N2. As shown in Figure S18 and Figure 9a, the doping of B atoms does not change the reaction mechanism, and the variation of energy barriers for the subsequent N2 reduction reaction can be ignored (±0.02eV). Therefore, B doping of the support was selectively targeted to optimize of the H transfer process without hampering other catalytic processes, it would be a good candidate to improve the catalytic performance of N2 reduction. Furthermore, Microkinetic analysis of Fe2/mpg-C3N4 and Fe2/B/mpg-C3N4 by the rate determining states method, proposed by Kozuch et al, 58 are conducted to further probe the reaction rate for the influence of B doping and the catalytic performance. The calculated TOF of ammonia synthesis on Fe2/B/mpg-C3N4 is 6.34 × 10−1 s−1 site−1 at 100 bar and 700 K, which is 2 orders of magnitude faster than the reaction rate of Fe2/mpg-C3N4 (3.07×10−3 s−1 site−1). The TOF of ammonia production on Fe2/B/mpg-C3N4 is less than 10 -10 s−1 site−1 below 400 K, which is due to the stable NHx adsorption species at low temperature. Due to entropy effects for free gases, the bare site will increase accordingly after desorption of NHx upon the temperature rise. Under the condition of constant pressure, the temperature increased from 300K to 700K, and the reaction rate increased by 13-14 orders of magnitude. At constant 700K temperature, the pressure has a relatively gentle effect on the reaction rate, so it is expected to realize the nitrogen conversion reaction at low pressure (Figure 9d in Fe2/B/g-C3N4). In order to achieve maximum TOF, our calculated partial pressure of N2 is 0.3 (PN2/P (N2+H2)), close to ideal ratio of 1/4 (Figure 9c), indicating that the Fe2/B/g-C3N4 catalyst can reduce the competitive adsorption of N2 and H2. That is distinct from Fe3/Al2O3 forming the co-adsorption of N2 and H2 on Fe3 clusters, which leads to a 0.44 partial pressure of N2 for maximum TOF. In contrast, it is necessary to remarkably change the partial pressure of nitrogen in classical metal catalysts, for example 0.06 (PN2/P (N2+H2)) in the Fe (211) C7 site and 0.78 in the Ru (001) B5 site. 19