Cooperative spin transition of monodispersed FeN3 sites within

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Cooperative spin transition of monodispersed FeN sites within graphene induced by CO adsorption Qin-Kun Li, Xiao-Fei Li, Guozhen Zhang, and Jun Jiang

J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07816 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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Cooperative spin transition of monodispersed FeN3 sites within graphene induced by CO adsorption Qin-Kun Li,†,¶ Xiao-Fei Li,‡ Guozhen Zhang,∗,† and Jun Jiang∗,†

†Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), CAS Center for Excellence in Nanoscience, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, China ‡School of Optoelectronic Science and Engineering, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, Sichuan, 610054, China ¶Present address: Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States Received October 31, 2018; E-mail: [email protected]; [email protected]

Abstract: The significance of identifying the fundamental mechanism of interactions between adjacent catalytic active centers has long been underestimated. Utilizing density functional theory calculations, we demonstrate controllable cooperative interaction between two nearby Fe centers embedded on nitrogenated graphene aided by CO adsorption. The interconnected adjacent Fe atoms respond cooperatively to CO molecules with communicative structural self-adaption and electronic transformation. The adsorbed CO changes not only the spin of the active site it is attached to, but also that of its adjacent site. Consequently, the two adjacent Fe atoms feature unique oscillatory long-range spin coupling. Our theoretical investigation suggests cooperative communication between adjacent active sites on a single-atom catalyst is non-trivial.

Atomic-level knowledge on the nature of active sites, are crucial for mechanistic understandings and performance enhancement of heterogeneous catalysis. 1–6 Especially, the interplay between active sites and adsorbates, like host-guest interaction, resulting in mutual impacts on their respective geometric and electronic structures, is well recognized and investigated. 7–11 Meanwhile, the communicative effect between active sites has also been claimed as common and indispensable mechanism in nature. 12 However, much less is recognized about whether similar communication specifically connecting two individual active sites occurs in heterogeneous catalysts. As emerging atomic-scale heterogeneous catalysts, single-atom catalysts (SAC) comprising transition metal (TM) atoms embedded in graphene matrix have drawn intensive spotlight. 11,13–16 Yet, the inter-site communication in SAC architecture is generally overlooked, although multiple adjacent sites exist under real synthesis condition. 17 Recently, Reed et al. reported that CO adsorption in a metal-organic framework of Fe2 Cl2 (bbta) undergoes an abrupt lifting via a cooperative spin transition mechanism. 18 In contrast, such phenomenon is absent in Fe-BTTri owing to distant arrangement of neighboring iron sites. 19 The authors ascribed this cooperative CO adsorption in Fe2 Cl2 (bbta) to the magnetoelectronic interaction of adjacent iron sites. Additionally, Ren et al. found that monodispersed Mn atoms loaded in graphene matrix exhibit distance-dependent spin coupling behavior via graphenemediated inter-Mn interactions. 20 These studies inspired us to examine whether a single-atom site in SAC communicates with its neighbouring sites and behaves cooperatively. Herein, we predict cooperative spin transition of adjacent singleatom sites upon stepwise CO adsorption in a Fe-based SAC, where

two adjacent Fe atoms were anchored onto nitrogenated graphene. The CO-induced local electronic transformation on one site triggers associated structural rearrangement and electronic transition of adjacent Fe atoms, rendering possible such communicative longrange spin transition of two individual active sites.

Figure 1. Top view of the optimized supercell structure of FeN3 (a) and FeN3 -FeN3 (b), (c)(d) and (e)(f) are the top and side view of net spin density of FeN3 , FeN3 -FeN3 at an isosurface value of 1 × 10−3 eÅ−3 .

We have anchored two single Fe atoms, in uniform distance (∼11 Å) to their nearest neighbors, separately onto nitrogen-doped single vacancy (SV) of graphene as shown in Figure 1(b). For comparison, we have also depicted the completely isolated single Fe site in Figure 1(a), and refer these two structures as FeN3 and FeN3 FeN3 , respectively. Experimentally, pyridinic-N, which exhibits high stabilization effect on TM atoms, can be achieved in graphene with very high doping levels and facile tunability. 21,22 A series of TM atoms have been precisely embedded in SV of nitrogenated graphene, namely, TMN3 , 23 and the spatial distance between two adjacent TM atoms can also be controlled accurately by high energy electron beam technique. 24 In light of such advanced synthesis techniques, FeN3 -FeN3 will also be created under reasonable conditions. In FeN3 -FeN3 , both of single Fe atoms were stabilized by pyridinic-N with an overall binding energy of -8.58 eV, verifying

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the stabilization of anchoring Fe atoms onto SV of graphene. Compared to FeN3 , FeN3 -FeN3 retains a non-planar structure with C2v symmetry but Fe-N bond length is slightly shortened by 0.02 or 0.03 Å. The ground state of FeN3 -FeN3 is ferromagnetic (FM) with a total spin moments of 8.47 µB equally distributed on two Fe atoms, a little smaller than twice that of FeN3 (4.35 µB ). Such spin moment reduction on each individual Fe atom (3.41 µB ) have been deemed highly related with shortening of Fe-N bond length in analogous Fe-porphyrin molecule. 25 We have also found that AFM state is less stable than FM by 10.05 meV, indicating noticeable ferromagnetic coupling interactions between two adjacent Fe atoms. 20,26 Further, Figure 1(c)-(f) show that most of net spinpolarized electrons distribute at the vicinity of Fe atoms, indicating large spin polarization. With such uniform spin alignment on single Fe atom, the FeN3 -FeN3 @graphene has illustrated 2D long-range ferromagnetic ordering.

Figure 2. Top view of the optimized structure of FeN3 (a) and FeN3 FeN3 (c) after first CO molecule is adsorbed, (b) and (d) are the side view of net spin density of FeN3 and FeN3 -FeN3 at an isosurface value of 1 × 10−3 eÅ−3 .

Initially, we consider the first CO molecule adsorption on one Fe site of FeN3 -FeN3 shown in Figure 2. CO molecule bonds to Fe atom in end-on fashion and C-O bond length is elongated by 0.03 Å. The calculated adsorption energy is 1.86 eV, 0.11 eV higher than that in FeN3 system (1.75 eV) and indicating considerable adsorption enhancement compared to single Fe site. Upon CO adsorption, the geometrical structure of FeN3 -FeN3 adjusted accordingly. The Fe-N bond length of corresponding Fe site increases by 0.09 or 0.04 Å whereas that in neighbouring Fe site shortens by 0.01 or 0.02 Å. Such associated structural transformation upon molecule adsorption cannot be observed in single FeN3 framework. Moreover, the spin moment of CO-attached Fe atom is decreased to 2.81 µB . The reduction of net spin charge localized on Fe atom is accompanied by the spin polarization of the adsorbed CO, which is similar to spin-polarized N2 activation by FeN3 @graphene. 14 More interestingly, the structural transformation induced by CO adsorption leads to dramatic spin transition on nearby Fe site as illustrated in Figure 2(d), namely, the most energy-favorable spin ordering of FeN3 -FeN3 is shifted from ferromagnetic to ferrimagnetic with total spin moment of 1.12 µB . Such drastic total spin moment reduction is mainly due to the completely spin reversal to 3.39 µB of the neighbouring Fe site. This spin transition process is further verified by the calculated charge density difference (see Figure S4 of Supplemental Information). These two evident structural and electronic changes of nearby Fe site signify the interconnectedness of adjacent Fe centers and inter-site communication. The long-range cooperative spin transition upon CO adsorption indicates that adjacent Fe atoms interact indirectly via the graphene matrix, 26 and this indirect exchange coupling between adjacent Fe atoms can be effectively mediated by the surface electron transfer of graphene 20 (see Figure S4). Figure 3(c) depicts the optimized structure of a second CO molecule adsorption on the neighbouring Fe site. The second CO binding energy (1.70 eV) is 0.16 eV weaker than the first one, which suggests that FeN3 -FeN3 may not possess similar cooperative CO

Figure 3. (a) and (b) are the top and side view of CO-induced charge density difference at an isosurface value of 5 × 10−4 eÅ−3 . Yellow and blue bubbles represent charge accumulation and depletion, respectively. (c) Top view of the optimized structure of FeN3 -FeN3 after two CO molecules are adsorbed on the two individual Fe sites, (d) the side view of net spin density at an isosurface value of 1 × 10−3 eÅ−3 .

adsorption to Fe2 Cl2 (bbta). We attribute this to the robust configuration of graphene matrix, unlike the flexible structure and readily occurrence of notable geometric rearrangements triggered by gas uptake in MOF or biological enzymes. 18,27 Since adsorption energy is deemed as a crucial and fundamental descriptor in catalysis, 28 we expect that the density of active site in SAC can affect its catalytic performance via inter-site communication. Upon the second CO adsorption, the Fe-N bond lengths readjust slightly and C2v symmetry is restored. The Fe-N bond length of first CO-attached Fe site is shortened by 0.01 or 0.02 Å whereas that is elongated by 0.03 or 0.09 Å in the second CO-attached Fe site. In the ground state after two CO adsorption, the spin moment of corresponding Fe sites decreases from 3.39 to 2.89 µB whereas that of the neighbouring site with preadsorbed CO arises a little by 0.08 µB . The equal (2.89 µB ) but antiparallel spin moment on each individual Fe site unambiguously indicates the 2D long-range antiferromagnetic (AFM) orders on Fe sites. This AFM spin ordering is completely different from the FM spin ordering when Fe active sites are free from CO adsorption, although both structures possess the same symmetry. More specifically, this subsequent CO adsorption in turn leads to electron transfer (Figure 3(a)(b)) and changes the spin of its neighbouring site (Figure 3(d)) and tailors their local spin to be equal and antiparallel, further demonstrating the existence of inter-site communication.

Figure 4. (a)(c) Top view of the optimized structure of three and four CO molecules, respectively, adsorbed on the two individual Fe sites embedded at graphene, (b) and (d) are the corresponding side view of net spin density at an isosurface of 1 × 10−3 eÅ−3 .

Considering the coordinatively unsaturated Fe centers of FeN3 FeN3 , we further probe a third CO molecule adsorption on one of Fe centers and the optimized structure is shown in Figure 4(a). A third CO adsorption energy is 1.23 eV, ∼0.60 eV weakening than first CO adsorption, resulting from the less residual coordination number of Fe center and the repulsion with the preadsorbed CO. The third CO adsorption shortens the Fe-N bond length by 0.01 or 0.06 Å in corresponding Fe site and 0.01 or 0.02 Å in nearby Fe site. This transformation is ensued by the notable spin transition depicted in Figure 4(b). FeN3 -FeN3 is in FM spin ordering with a total spin moment of 4.36 µB , in which 2 CO- and 1 CO-attached Fe

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Journal of the American Chemical Society centers bear parallel-aligned spin moments of 1.03 and 2.96 µB , respectively. Noteworthily, the third CO molecule again reverses the spin moment of adjacent Fe center, which differs from the scenario of one CO adsorption. The contrast between 1 CO and 3 CO adsorption on FeN3 -FeN3 suggests that cooperative electronic transformation via inter-site communication can modulate long-range magnetic orderings on magnetic TMN3 framework. As a fourth CO molecule binds to the neighbouring Fe site (Figure 4(c) and (d)), it recovers the geometric symmetry of FeN3 -FeN3 and the calculated adsorption energy is 1.20 eV. Along with the increasing binding with CO molecules, the residual spin moment of each individual Fe center is partially screened to 1.02 µB . And the spin on two individual Fe atoms is in antiparallel alignment with total spin moment of 0 µB . It throws the overall spin ordering back to AFM state, similar to the case of two CO adsorption. This oscillatory behavior of spin transition between FM and AFM states might be accounted by Ruderman-Kittel-KasuyaYosida (RKKY) interaction between Fe centers via the graphene surface. 20,26,29 The CO adsorption leads to electron depletion and inter-site charge redistribution on graphene matrix, which is expected to modify the inter-site exchange coupling strength and spin ordering of adjacent Fe atoms (see Supplemental Note 3 and Fig. S4). A similar long-range ferrimagnetic order has been observed between iron phthalocyanine (FeFPc) and manganese phthalocyanine (MnPc) molecules co-assembled onto Au(111) substrates. It has been ascribed to RKKY interaction mediated by surface-state electrons of the Au(111) substrate. 26 Furthermore, FM-AFM oscillation of magnetic adatoms caused by RKKY interaction can be tuned by charge-carrier concentration on substrate. 20 We have also investigated O2 adsorption on FeN3 -FeN3 and found similar cooperative spin transitions to CO adsorption. Intriguingly, O-O bond is stretched to a larger extent in the presence of an adjacent Fe site (more details in Supplemental Information), suggesting a potentially alternative strategy of oxygen activation by inter-site communication of active sites. In summary, we have demonstrated, from theoretical perspectives, cooperative spin transition between adjacent Fe sites in SAC induced by small molecule adsorption. CO-induced local electronic transition on one Fe active site can trigger adjoint structural adaption of adjacent Fe sites through inter-site communication. Consequently, communicative Fe sites anchored onto graphene matrix establish tunable long-range magnetic ordering in the presence of external stimulus. This is different from previous works 26,30 where the spin-bearing molecules are assembled in specific pattern onto conductive substrates. Most importantly, our results indicate the impact of proximity of active sites of SACs on their catalytic activities is non-negligible. Since high-density single-atom sites are regarded as a key factor for higher activity of SACs, it will naturally lead to the proximity of active sites. 31 Hence it’s necessary to carefully estimate the effect of inter-site cooperativity when constructing high-density SACs. Noticeably, when preparing this manuscript, three different groups have reported their observations of cooperative communications in SAC 17 and metal nanocatalysts, 32,33 respectively. These exciting discoveries strongly support our vision on cooperative effects of neighboring active sites in catalysis and prompt us to continue explorations. Acknowledgment: This work was financially supported by MOST (No. 2014CB848900, 2018YFA0208702), NSFC (No. 21790351, 21703221, 21633006), the Fundamental Research Funds for the Central Universities (WK2060030027). Supercomputing Center of University of Science and Technology of China is acknowledged for the computing resource. ZGZ is grateful to Prof. George Schatz and Dr. Jia Chen for helpful discussion and comments.

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A New Type of Strong Metal-Support Interaction and the Production of H2 through the Transformation of Water on Pt/CeO2 (111) and Pt/CeOx /TiO2 (110) Catalysts. J. Am. Chem. Soc. 2012, 134, 8968. (5) Pfisterer, J. H.; Liang, Y.; Schneider, O.; Bandarenka, A. S. Direct instrumental identification of catalytically active surface sites. Nature 2017, 549, 74. (6) Wu, Y.; Jiang, J.; Weng, Z.; Wang, M.; Broere, D. L.; Zhong, Y.; Brudvig, G. W.; Feng, Z.; Wang, H. Electroreduction of CO2 Catalyzed by a Heterogenized Zn-Porphyrin Complex with a Redox-Innocent Metal Center. ACS Cent. Sci. 2017, 3, 847. (7) Parent, L. R.; Pham, C. H.; Patterson, J. P.; Denny Jr, M. S.; Cohen, S. M.; Gianneschi, N. C.; Paesani, F. Pore Breathing of Metal-Organic Frameworks by Environmental Transmission Electron Microscopy. J. Am. Chem. Soc. 2017, 139, 13973. (8) Schneemann, A.; Bon, V.; Schwedler, I.; Senkovska, I.; Kaskel, S.; Fischer, R. A. Flexible metal-organic frameworks. Chem. Soc. Rev. 2014, 43, 6062. 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