Abstract
In modern chemistry, the development of highly efficient room-temperature catalysts is of great significance and remains a long-standing challenge in various typical reactions. Here, we theoretically demonstrate that the two-dimensional (2D) multiferroic, Cr(half-fluoropyrazine)2 [Cr(h-fpyz)2], is a promising single-atom catalyst (SAC) operating at room temperature for CO oxidation. The rate-limiting barrier is merely 0.325 eV, leading to a reaction rate (i.e., 3.47 × 106 s−1) of six orders of magnitude higher than its monoferroic derivative [Cr(pyz)2], due to the synergetic effects of two aspects. First, the more flexible ligand rotations in Cr(h-fpyz)2 facilitate the activation of O2 molecule, simultaneously enhancing the charge transfer and spin-accommodation process. Second, on Cr(h-fpyz)2, O2 adsorption induces a distinctly lower local positive electric field, reducing the electrostatic repulsion of the polar CO molecule. These findings may also pave the way for establishing highly efficient SAC platforms based on 2D multiferroics where multidegree of freedom (e.g., spin, polarity) synergistically matter.
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Introduction
With the rapid advancement of modern industry and the intensifying energy crises, the development of highly efficient catalysts has become critically important for a range of complex chemical reactions. However, most catalysts encounter complex stability issues, and their practical applications are also generally constrained by the relatively high operating temperature1,2. The U.S. Department of Energy (US-DOE) has set the goal of achieving 90% conversion of all criteria pollutants at 150 °C—“the 150°C challenge”3. Nevertheless, such a temperature is still ~ 100 °C lower than current state-of-the-art commercial automotive catalysts. In general, charge4, spin5, orbital6, and polarity7 can simultaneously play significant roles in chemical processes. Correspondingly, if a catalytic platform possesses well-matched degrees of freedom, such as spin and polarity, with those of the reactants, high-performance selective catalysis can be achieved8,9,10. Recent reports have demonstrated that high spin states of the catalysts can significantly enhance various chemical processes8,9,10, which can be effectively explained by Wigner’s spin selection rule9,11,12,13,14,15. Moreover, recent advances have confirmed that electric field16,17 and ferroelectricity can play a crucial role in enhancing catalytic activities, such as CO2 electrocatalytic reduction18 and photocatalytic water splitting19. This enhancement is primarily due to the significantly improved charge transfer facilitated by ferroelectric materials20. Therefore, developing highly efficient room-temperature catalysts leveraging the synergetic effects of the aforementioned degrees of freedom is both anticipated and warranted.
Multiferroics, which simultaneously encompass ferroelectric, ferromagnetic, and ferroelastic orders, have attracted significant attention from researchers due to their promising applications in various fields21,22,23. These applications include nonvolatile multistate data storage, magnetoelectric sensors, and multifunctional and electrically controlled spin wave devices24,25. Moreover, due to the tunable orders and the synergies of different degrees of freedom, such as charge, spin, and orbital integrated within a single phase, it is reasonable to expect that multiferroics could serve as highly efficient room-temperature catalytic candidates. Very recently, a new type of two-dimensional (2D) multiferroic material, i.e., a 2D metal-organic framework (MOF) room-temperature multiferroicity (RT-MF), specifically Cr(h-fpyz)2 (h-fpyz = half-fluoropyrazine)26 has been predicted, with stable ferroelectric and ferrimagnetic phase up to around 400 and 600 K, respectively. This unique 2D material can be rationally designed by introducing relatively large ferroelectricity (4.8 μC/cm2) through fluorination27,28,29 in Cr(pyz)2, while retaining the ferromagnetic coupling (3.61 μB/Cr) between the evenly dispersed metal ions. Essentially, this 2D RT-MF structure can be understood as a derivative of the experimentally fabricated bulk metal-organic ferrimagnetic layered material (Li0.7[Cr(pyz)2]Cl0.7\(\bullet\)0.25(THF))30, exhibiting a high Curie critical temperature of up to 515 K and significant coercivity (7500 Oersted) at room temperature. In recent decades, MOF structures have been widely applied in various fields, such as ferroelectricity31, sensors32, and photocatalysis33. Moreover, from a geometric perspective, the present 2D RT-MF Cr(h-fpyz)2 MOF structure, featuring evenly dispersed Cr species in single-atomic form stabilized by surrounding organic ligands, represents a promising candidate for high-loading single-atomic catalyst (SAC). SAC represents an important enabling concept34 and has been intensively exploited in the broad field of chemical catalysis, such as CO2 reduction35, CO oxidation36,37, O2 reduction38, N2 reduction39, hydrogen evolution reaction40, and oxygen evolution reaction41. Furthermore, by probing the interactions between the single-atom reactive sites and the reactants, multiple potential physical mechanisms have been established from various perspectives. For example, insights from the aspects of geometric flexibility42,43, charge transfer44,45, orbital hybridizations46,47, and spin selection48,49 have been proposed. However, in these related investigations, the materials used as SAC platforms typically exhibit only one type of ferroic order, such as ferromagnetism50,51, or ferroelectricity52,53. To date, despite extensive research on the distinctive physical properties of both 2D multiferroic materials54,55,56, and the catalysis of numerous SAC systems6,34, the synergetic effect of these aspects on the room-temperature catalytic performance of 2D RT-MF SAC structures, such as Cr(h-fpyz)2, remains significantly underexplored.
Here, employing state-of-the-art first-principles calculations and model analysis, taking spin-triplet O2 activation and polar molecule CO oxidation as a prototypical example, we demonstrate that the present 2D RT-MF Cr(h-fpyz)2 can serve as a highly efficient SAC candidate operating at room temperature. The calculated rate-limiting energy barrier is 0.325 eV, resulting in a reaction rate of 3.47 × 106 s−1 at 300 K. This rate is six orders of magnitude higher than its monoferroic derivative [Cr(pyz)2] counterpart, due to the synergetic effects of magnetism and ferroelectricity. Specifically, the flexible rotation of the organic ligand in the multiferroic Cr(h-fpyz)2 functions as a controllable knob, enhancing the charge transfer and spin accommodation during the activation of spin-triplet O2 molecule, while simultaneously lowering the local normal positive electric field. This synergetic mechanism results in the polar CO molecule experiencing relatively low repulsive electrostatic interactions and a significantly reduced reaction barrier, making the 2D RT-MF Cr(h-fpyz)2 a promising candidate for a room-temperature SAC system. Beyond their various applications due to their exotic physical properties, the present findings significantly broaden the potential applications of 2D multiferroics in the crucial field of chemical catalysis. Moreover, this work may direct new avenues toward developing highly efficient SAC platforms for various other important chemical processes where multiple degrees of freedom, such as spin and polarity, synergistically matter.
Results
Geometric and electronic structures of the 2D RT-MF Cr(h-fpyz)2
First, we briefly reexamined the optimized geometric structure of the 2D Cr(h-fpyz)226 metal-organic framework, simulated within a unit cell comprising four Cr metal atoms and eight h-fpyz organic ligands. Here, four of the latter are connected to the central Cr atoms in a clockwise manner, as indicated by the curved arrows in Fig. 1a. Furthermore, it has been identified that the 2D Cr(h-fpyz)2 system exhibits an antiferroelectric (AFE) state, due to the antiparallel arrangements of the eight h-fpyz ligand molecules in the primitive cell, as illustrated by the thick arrows in Fig. 1a. Our extensive spin-polarized density-functional theory (DFT) calculations confirmed that in the 2D Cr(h-fpyz)2 structure, Cr ions exhibit a preference for ferromagnetic order over antiferromagnetic order by about 0.25 eV/f.u., consistent with prior research findings26. Moreover, the calculated magnetic moment (MM) of each four-coordinated Cr ion is about 3.68 μB, which is antiparallel to that of its four adjacent h-fpyz ligand molecules (Fig. 1b), each possessing an MM of 0.84 μB. Thus, the ground state of 2D Cr(h-fpyz)2 is ferrimagnetic, with a total MM of approximately 8.0 μB per primitive unit cell. Figure 1c shows the side view of Cr(h-fpyz)2. The spin-polarized band structure and density of states (DOS) (Fig. 1d) demonstrate a direct energy band gap of approximately 0.55 eV, which aligns with previous DFT calculations26.
In (a), the curved red arrow indicates the tilted orientation of the h-fpyz ligands. Here, the center and edge Cr atoms of the primitive cell are termed CrI and CrII, respectively, and the rest of the Cr atoms are termed CrIII. The dipoles of h-fpyz molecules are also schematically indicated by the red thick arrow. In (b), the quantity and direction of the magnetic moments are illustrated. c Side view of Cr(h-fpyz)2. d Energy band structures and projected density of state (PDOS) of Cr(h-fpyz)2. e Side view of the optimal local structure of O2 adsorption on Cr(h-fpyz)2.
Activation of spin-triplet O2 on 2D Cr(h-fpyz)2
Subsequently, we investigate the adsorption and activation of spin-triplet O2 on 2D Cr(h-fpyz)2, which is the key step for CO oxidation. It is revealed that the O2 molecule preferentially adsorbs on the CrI site, with an adsorption energy (Eads) of 1.096 eV. This adsorption is accompanied by a significant increase in the O-O bond length from 1.24 to 1.44 Å (Fig. 1e). Here, we define Eads = −[E(O2-Cr(h-fpyz)2) − E(O2) − E(Cr(h-fpyz)2)]. On CrII and CrIII, the calculated Eads are relatively low, 0.676 and 0.353 eV, respectively. These distinct adsorption behaviors at the three different sites can be attributed to the contrasting local geometric environments of the antiferroelectric state in the 2D RT-MF Cr(h-fpyz)2 structure. Specifically, on CrI, CrII, and CrIII, the adsorbed O2 is surrounded by four H atoms, two H and two F atoms, and four F atoms of the four organic ligands, respectively (Supplementary Fig. 1). In other words, the incoming O2 molecule may experience disparate local electric field (LEF) nearby (~ 3 Å) above the CrI and CrIII atoms, as demonstrated in Fig. 2a, d and Supplementary Fig. 2. This directly influences the charge transfer as detailed shortly, as also supported by the density of states analysis performed in Supplementary Fig. 3, which accounts for the distinct activities of these two types of Cr single-atom reactive sites. These findings strongly indicate that the catalysis of ferroelectric materials25,26,27,28 can be effectively modulated by the ferroelectric polarization direction down to the single-atom scale, specifically in SAC systems. Note that, here the spin-orbit coupling (SOC) effect has a negligible influence on O2 adsorption and activation. Therefore, the subsequent calculations were performed without considering SOC.
LEF distributions along the Z direction passing through the two representative types of Cr reactive atom (CrI and CrIII) in pristine 2D RT-MF Cr(h-fpyz)2, Δρ and ΔM are calculated on the optimized structure when an O2 molecule is adsorbed on the Cr reactive site performed by Bader analysis. The iso-surface value is 0.0198 e/Å3. a–c for the case of CrI, and (d–f) for the case of CrIII. In (a) and (d), Cr atoms are set as the position references.
Second, we conduct Bader charge analysis to examine the charge transfer (Δρ = ρ(O2-Cr(h-fpyz)2) − ρ(O2) − ρ(Cr(h-fpyz)2)) between the incoming O2 and the Cr(h-fpyz)2 catalytic substrate. Here, ρ(O2) and ρ(Cr(h-fpyz)2) represent the calculated charge obtained from the isolated ground-state O2 molecule and the 2D Cr(h-fpyz)2 substrate, respectively. To gain a clearer understanding of the charge transfer, we conducted a Bader charge analysis on the 2D Cr(h-fpyz)2 substrate (Supplementary Fig. 4). Briefly, in this pristine 2D RT-MF, both nitrogen (N) and fluorine (F) atoms accommodate charge transferred predominantly from their neighboring chromium (Cr) atoms, amounting to approximately 1.3 |e|, as well as from carbon (C) atoms. Here, C atoms can be classified into two types, which are coordinated by H and F atoms, termed CH and CF, respectively. The nearest neighboring CH and CF of CrI (CrII) donate about 0.46 (0.36) and 1.09 (0.99) |e|, respectively. Note that, when the incoming O2 molecule is positioned more than 10 Å above the designated CrI active site, as termed the initial state (IS), negligible charge transfer is observed between O2 and the substrate (Supplementary Fig. 5a). Upon O2 adsorption on the reactive CrI (CrIII) hosting site, approximately 1.02 (0.79) |e| is transferred from the substrate to O2 respectively (Fig. 2b, e). Such a distinct charge transfer between CrI and CrIII accounts for their contrasting catalytic behaviors in CO oxidation, as discussed later. In this process, the reactive CrI (CrIII) atom, along with the remaining C, H, and F (C, N, and F) atoms donate electrons by approximately 0.23, 0.69, 0.31, and 0.06 |e| (0.21, 0.39, 0.16, and 0.08 |e| ), respectively. Furthermore, CrII and CrIII (CrI and CrII), which are positioned approximately 7.02 and 9.92 Å (9.92 and 7.02 Å) away from CrI (CrIII), respectively contribute negligible charge transfer to the adsorbed O2. It is noteworthy that, although the CrI reactive site is directly linked by the N atom of the h-fpyz ligand, the second nearest neighboring C atoms of CrI, in other words, the nearest neighboring C atoms of CrII, dominate the charge transfer to the adsorbed O2 (Supplementary Fig. 6). In addition due to the significantly lower electronegativity of H than that of F, the nearby H atoms donate significantly more charge to the adsorbed O2 species than F atoms. This further rationalizes the contrasting Eads of the O2 on CrI, CrII, and CrIII sites.
In contrast to charge transfer, the reactive CrI atom, along with the substrate N, and C atoms, play a crucial synergetic role in accommodating the spin transferred from the adsorbed O2 molecule. This facilitates its spin triplet-to-singlet activation of the O2 molecule, which is the key step for CO oxidation. Similar to the definition of charge transfer Δρ, here we define the magnetic moment transfer as ΔM = M(O2 − Cr(h-fpyz)2) − M(O2) − M(Cr(h-fpyz)2). The initial state (IS) primarily reflects the intrinsic magnetic moment of the incoming spin-triplet O2 molecule and the substrate (Supplementary Fig. 5b), with negligible interactions between them. Upon O2 adsorption, the total magnetic moment of the optimized O2-Cr(h-fpyz)2 complex remains at 8 μB (spin quantum number \({S}_{{{{{\rm{O}}}}}_{2}{\mbox{-}}{{{\rm{Cr}}}}{({{{\rm{h}}}}{\mbox{-}}{{{\rm{fpyz}}}})}_{2}}=4\)). That is, the spin−spin coupling between the spin-triplet O2 molecule (\({S}_{{{{{\rm{O}}}}}_{2}}=1\)) and the Cr(h-fpyz)2 catalyst (\({S}_{{{{\rm{Cr}}}}{({{{\rm{h}}}}{\mbox{-}}{{{\rm{fpyz}}}})}_{2}}=4\)) follows the channel of “1 + 4 = 4”, which is a spin-allowed reaction according to Wigner’s spin selection rule9,11,12,13,14,15. Such a classic rule restricts that the total spin quantum number \({S}_{{{{{\rm{O}}}}}_{2}{\mbox{-}}{{{\rm{Cr}}}}{({{{\rm{h}}}}{\mbox{-}}{{{\rm{fpyz}}}})}_{2}}\) in the spin-allowed reaction should satisfy \({S}_{{{{{\rm{O}}}}}_{2}{\mbox{-}}{{{\rm{Cr}}}}{({{{\rm{h}}}}{\mbox{-}}{{{\rm{fpyz}}}})}_{2}}=|{S}_{{{{\rm{Cr}}}}{({{{\rm{h}}}}{\mbox{-}}{{{\rm{fpyz}}}})}_{2}}-{S}_{{{{{\rm{O}}}}}_{2}}|\), \(|{S}_{{{{\rm{Cr}}}}{({{{\rm{h}}}}{\mbox{-}}{{{\rm{fpyz}}}})}_{2}}-{S}_{{{{{\rm{O}}}}}_{2}}|+1,\ldots,|{S}_{{{{\rm{Cr}}}}{({{{\rm{h}}}}{\mbox{-}}{{{\rm{fpyz}}}})}_{2}}+{S}_{{{{{\rm{O}}}}}_{2}}|\), where \({S}_{{{{\rm{Cr}}}}{({{{\rm{h}}}}{\mbox{-}}{{{\rm{fpyz}}}})}_{2}}\) and \({S}_{{{{{\rm{O}}}}}_{2}}\) are the spin quantum numbers of the substrate Cr(h-fpyz)2 and an isolated ground state O2 molecule, respectively.
However, the local spin moment of the optimized O2-Cr(h-fpyz)2 complex with O2 adsorbed on CrI (CrIII) reactive site changes intriguingly, as shown in Fig. 2c (f). Specifically, the magnetic moment of the adsorbed O2 on CrI (CrIII) is reduced from 2.0 μB to nearly zero, i.e., − 0.24 (− 0.02) μB, indicating that the O2 molecule has transitioned from a spin triplet state to singlet. Correspondingly, the magnetic moment of the CrI (CrIII) atom is reduced from 3.68 (3.69) to 3.01 (3.08) μB, with a reduction of 0.67 (0.61) μB. Somewhat unexpectedly, the nearby non-metal atoms, such as C and N, in the vicinity of the reactive CrI (CrIII) atom primarily accommodate the magnetic moments by 0.83 (0.60) μB. However, the relatively distant transitional metal atoms, CrII and CrIII (CrI and CrII) only accommodate about 0.04 (0.03) μB upon O2 adsorption and activation. These results demonstrate a significant synergetic spin communication mechanism among the O2 molecules, the substrate p-block C/N atoms, and the d-block Cr reactive site in the activation of spin-triplet O2. Collectively, comparing the cases of O2 adsorption on CrI and CrIII, it is evident that the reactivity of the Cr atom increases with the synergy of charge transfer and spin communication involving the p-block C/N atoms. Moreover, there is a significant linear scaling relationship9,57 between the Eads of the O2 molecule and the magnetic changes of different Cr reactive sites and the nonmetal atoms as well, as detailed in Supplementary Fig. 7. According to Wigner’s spin selection rule, the catalytic system in a low spin state (S = 0) typically exhibits high inertness toward spin-triplet O213,14. This observation convincingly demonstrates the crucial role of the magnetic order of the 2D RT-MF Cr(h-fpyz)2 in facilitating the activation of the ground state O2 molecule.
CO oxidation on 2D Cr(h-fpyz)2
Now, we continue to examine the minimum energy pathway (MEP) of CO oxidation on 2D Cr(h-fpyz)2 (step i, Fig. 3) via interacting with the adsorbed O2 molecule (step ii, Fig. 3). Note that, here we have also verified the CO adsorption on the pristine Cr(h-fpyz)2, and the optimized Eads(CO) is merely about 0.367 eV, which is much lower than that of Eads(O2) = 1.096 eV. Therefore, for the present system, CO oxidation should prefer the E-R reaction mechanism with O2 preferentially adsorbed. It is observed that the adsorbed O2 molecule is readily attacked by the incoming CO, releasing a CO2 molecule upon overcoming a low activation energy (Ebar) of 0.325 eV (TS1, Fig. 3), with an exothermic energy of 2.799 eV. In TS1, the distance between CO and O is 2.0 Å. Following the release of the generated CO2 molecule, an O species is left on the CrI site (step iv, Fig. 3). Consequently, the second incoming CO can also smoothly react with the protruding O atom, generating another CO2 precursor with an exothermic energy of 2.454 eV after overcoming an even lower Ebar of 0.298 eV (TS2, Fig. 3). Further calculations demonstrate that the desorption energy of the second CO2 (step vi, Fig. 3) is fairly low, ~ 0.2 eV. Correspondingly, the low rate-limiting energy barrier of 0.325 eV results in a high reaction rate of about 3.47 × 106 s−1 at room temperature (300 K), as estimated by using the Arrhenius form58 (R = R0×exp[−Ebar/(kBT)]) with a typical prefactor R0 of 1012 s−1. Such a low rate-limiting Ebar of 0.325 eV is significantly lower than those obtained for various other typical high-efficient noble SACs or nanocluster-based catalytic complexes, e.g., 0.79 eV for Pt1/FeOx34, 0.91 eV for Pt1/Fe2O359, 0.56 eV for Pt1/CeO260, 1.1 eV for Ir1-on-MgAl2O461, 0.66 eV for Pt4/CoNi@C62, and 1.71 eV for Pt-carbonyl63, respectively. Moreover, the calculated R (Ebar = 0.325 eV) reaches 1.34 × 108 s−1 at 423 K, implying that the present 2D RT-MF Cr(h-fpyz)2 is a promising high-performance catalyst subject to “the 150 °C challenge”.
Minimum energy pathway (MEP) for O2 activation and CO oxidation via the Eley-Rideal (E-R) mechanism on CrI reactive site in Cr(h-fpyz)2, with the energy activation barrier (Ebar) indicated. Here, *O2(O) denotes the adsorption of O2(O) on the catalytic substrate. CO--*O2(O) and CO-*O2(O) correspond to the relatively weak and strong interactions between the CO and the adsorbed O2(O) species.
As a comparison, we have also examined CO oxidation via interacting with the adsorbed O2 on CrII and CrIII single-atom reactive sites, as detailed in Supplementary Fig. 8. Briefly, on CrII and CrIII, the relatively high rate-limiting Ebar of 0.582 and 1.098 eV result in low reaction rates of about 1.67 × 102 and 3.57 × 10−7 s−1 at room temperature, respectively. Especially compared to CrI, reversing the LEF direction around the CrIII single-atom reactive site significantly (see Fig. 2a and d) reduces the reaction rate, by approximately thirteen orders of amplitude. Note also that, in consideration of the zero-point energy and entropy contributions, on the two representative reactive sites of CrI and CrIII, the Ebar is increased by 38 and 115 meV, respectively. Correspondingly, the calculated reaction rates of these two cases become 7.97 × 105 and 4.18 × 10−9 s−1 at 300 K, respectively. These findings convincingly confirm that an on-off switching for the room-temperature catalysis of SAC can be achieved by tuning the polar direction of the 2D RT-MF Cr(h-fpyz)2. The CrII reactive site positions intermediately between CrI and CrIII due to its intermediate LEF feature as compared to the latter two cases, see also Supplementary Fig. 2.
Figure 4a presents the Bader charge analysis for the key steps in CO oxidation depicted in Fig. 3. When CO is weakly adsorbed in the vicinity of the adsorbed O2 (step iii, Fig. 3), the O2-CO species acquires a charge of approximately 1.04 |e|. Specifically, the non-metal atoms in the substrate contribute about 0.81 |e|, whereas the metal Cr atoms contribute only about 0.23 |e|, indicating that the non-metal atoms dominate the charge transfer. This trend is also observed in TS1 and all other representative steps up to TS2 (Fig. 3), as also demonstrated in Fig. 4a and Supplementary Fig. 9a.
The changes in the magnetic moment (ΔM) of the corresponding key steps are analyzed in Fig. 4b. From step iii to step vi, there are substantial reductions in the local magnetic moments of both the O2-CO species and the Cr metal atoms. Notably, as in step iii, in almost all the other key steps, the p-block non-metal C and N atoms accommodate more MM than that of the d-block Cr reactive atom (Supplementary Fig. 9b). These findings demonstrate a delicate synergetic charge and spin accommodation mechanism occurred among the O2/CO molecule, d-block reactive Cr atom, and the surrounding p-block C and N atoms, occurring during both O2 activation and CO oxidation on the multiferroic Cr(h-fpyz)2 catalytic platform.
Comparison of CO oxidation on monoferroic Cr(pyz)2
To more clearly demonstrate the crucial role of the multiferroic orders in high-performance CO oxidation, we conducted a comparative examination of O2 activation and CO oxidation on Cr(pyz)2 (pyz = pyrazine). This compound was created by substituting all the F atoms in Cr(h-fpyz)2 with H atoms (Supplementary Fig. 10a, b). Consequently, the substrate undergoes a transition from multiferroic to monoferroic, remaining only ferrimagnetic properties. Based on this, we further explored the adsorption of O2 on the monoferroic Cr(pyz)2 substrate (Supplementary Fig. 10c). In comparison to the adsorption on the CrI reactive site of multiferroic Cr(h-fpyz)2, the optimized Eads(O2) increases slightly by 0.040 eV to reach 1.136 eV. In addition, the O-O bond length is enlarged to 1.44 Å. Similarly, CO exhibits weak adsorption on the Cr atom, characterized by an Eads(CO) of 0.383 eV, suggesting that only an E-R mechanism is viable for CO oxidation. Surprisingly, despite comparable activation of O2 on both catalytic platforms in terms of bond length and adsorption energy, a significantly elevated rate-limiting Ebar of 0.659 eV is observed for CO oxidation on the monoferroic Cr(pyz)2 (Supplementary Fig. 11). Using the same approach, the calculated CO oxidation rate is 8.48 × 100 s−1, which is about six orders of magnitude lower than that observed on the 2D RT-MF Cr(h-fpyz)2 at the room temperature. These findings further underscore the pivotal role of multiferroicity in enhancing the catalytic performance of the present 2D material for CO oxidation.
Role of ligand rotation in LEF: A knob for on-off switching room-temperature catalysis
To further elucidate the observed phenomena, we conducted a comparative analysis of the local geometric structures around the Cr reactive site in multiferroic Cr(h-fpyz)2 and monoferroic Cr(pyz)2, both with and without O2 adsorption (Fig. 5). Initially, in the case of 2D RT-MF Cr(h-fpyz)2, prior to O2 adsorption, each of the four h-fpyz ligands surrounding the Cr reactive site tilts approximately 37.8° away from the (100) plane (Fig. 5a) or the (010) plane (Fig. 5b). However, upon O2 adsorption, the angles undergo symmetry-breaking evolution into two distinct classes. Specifically, the angles between the two h-fpyz rings connecting the reactive Cr atoms along [010] ([100]) direction and the (100) ((010)) plane are significantly reduced (slightly enlarged) from 37.8° to 19.3° (37.8° to 38.5°), (Fig. 5a, b). This change in orientation can be attributed in part to the electrostatic attractive interactions between the negatively charged O2 and the nearby H cations, and the smooth tunability and adaptability of the organic ligands as well, as detailed in Supplementary Fig. 12.
a, b Local geometric structures around the Cr reactive site of the multiferroic Cr(h-fpyz)2 and (c, d) Monoferroic Cr(pyz)2 with (step i) and without (step ii, the first transition state (TS1)) O2 adsorption viewed from the (a, c) [010] and (b, d) [100] directions, respectively. The lower panels illustrate the displacements of the Cr atoms along the [001] direction relative to the distinct mass centers of the N atoms along the (a, c) [010] and (b, d) [100] directions, respectively.
Meanwhile, as illustrated in the corresponding lower panels, the position of the reactive Cr atom undergoes spontaneous changes along the [001] direction relative to the pristine Cr(h-fpyz)2 substrate. Specifically, the Cr atom shifts upward, displaying significant displacements relative to the mass centers of the N atom along the [010] and [100] directions (Fig. 5a, b). For instance, following O2 adsorption (structure ii), the Cr atoms shifted modestly (considerably) upward by approximately 0.24 (0.81) Å. Similar contrasting phenomena are also observed during other critical stages of the CO oxidation process, including TS1, iv, TS2, and vi (Supplementary Fig. 13). Furthermore, qualitatively similar findings are confirmed for O2 adsorption and CO oxidation on the monoferroic Cr(pyz)2. Specifically, the angles (about 47.5°) with respect to (100) ((010)) plane are slightly reduced (enlarged) to 33.6° (47.8°) (Fig. 5c, d). Meanwhile, the Cr reactive site exhibits modest (considerable) upward shifts along the [010] (0.22 Å) and [100] (0.76 Å) direction.
However, quantitatively, the changes in relative angles of the organic ligands during the key steps of the CO oxidation on the multiferroic Cr(h-fpyz)2 are more pronounced compared to those observed in the monoferroic Cr(pyz)2 system. For instance, in the TS1 state, the angle change reaches approximately 51.3% (from 37.8° to 18.4°) for Cr(h-fpyz)2, whereas it is about 39.2% (from 47.5° to 28.9°) for Cr(pyz)2. Simultaneously, the upward shift of the Cr reactive site is more significant in Cr(pyz)2 (0.90 Å) than in Cr(h-fpyz)2 (0.82 Å), which may correspond to the relatively higher Ebar of the monoferroic Cr(pyz)2 (0.659 eV) compared to multiferroic Cr(h-fpyz)2 (0.325 eV). This difference is attributed to the relatively large energy cost associated with the upward shift of the Cr atom compared to the rotation of the organic ligands (Supplementary Fig. 13).
The rotations of the organic ligands and the upward shift of the Cr reactive site effectively promote charge transfer from the substrate, specifically involving nearby p-block elements H and C, and d-block Cr atoms, to the O2 molecule (Fig. 6). To investigate this phenomenon, we performed Bader charge analysis on the optimized structures of O2 adsorption on both 2D multiferroic Cr(h-fpyz)2 and monoferroic Cr(pyz)2 systems, considering scenarios with and without allowing the rotations of the organic ligands and the accompanied upward shifts of the Cr atom. Specifically, when an O2 molecule adsorbs freely on multiferroic Cr(h-fpyz)2, the O2 species acquires a charge of approximately 1.02 |e|, with the underlying Cr reactive site, the nearby C and H atoms donating about 0.23, −0.05, and 0.11 |e| per atom, respectively. However, for the case without rotation of the organic ligands and the accompanied upward shift of the Cr atom, the charge on the adsorbed O2 molecule decreases to about 0.70 |e|, with the reactive Cr atom and the nearby H atoms at relatively greater distances donating about 0.09 |e| and 0.08 |e| per H atom, respectively (see Fig. 6a). A similar trend is also evident in the case of O2 adsorption on the monoferroic Cr(pyz)2. Specifically, for the case of with (without) free rotations of the ligands as shown in Fig. 6b, the adsorbed O2 is charged by about 1.0 (0.68) |e|, with the underlying Cr reactive site, the nearby C and H atoms donating about 0.29 (0.10) |e|, −0.11 (−0.03) and 0.07 (0.03) |e| per atom, respectively. Moreover, the analysis of the local spin moment of the optimized O2-Cr(pyz)2 complex is also performed. Specifically, in the monoferroic Cr(pyz)2, the magnetic moment of the adsorbed O2 on Cr is reduced from 2.0 to −0.08 μB. Correspondingly, the magnetic moment of the Cr atom is reduced from 3.67 to 2.95 μB, with a reduction of 0.72 μB. The non-metal atoms, majorly the C and N in the vicinity of the reactive Cr atom, accommodate a magnetic moment of 0.77 μB, which is lower than the value of 0.83 μB in the case of O2-Cr(h-fpyz)2. Moreover, the effect of ligands rotation on the spin accommodations are further comparatively analyzed in the Supplementary Fig. 14. Collectively, the contrasting geometric structure changes between the multiferroic Cr(h-fpyz)2 and monoferroic Cr(pyz)2 systems upon O2 adsorption result in quantitively distinct charge transfers and spin accommodations between the O2 molecule and the substrate atoms. Importantly, such disparate charge transfers upon O2 adsorptions result in distinct LEF surrounding the O2 species, which are destined to significantly influence the oxidation of the polar molecule CO, as confirmed in the following section.
Bader charges analysis for the cases of (a) Cr(h-fpyz)2 and (b) Cr(pyz)2 systems in the left panels shaded in gray and those with O2 adsorption. In the central panels shaded in shallow green with O2 adsorption, the structures are optimized without any constraints, i.e., ligand rotation is turned on. In the right panels shaded in pink, the angles of the organic ligands and the positions of Cr atoms are fixed to that in the pristine Cr(h-fpyz)2 and Cr(pyz)2, i.e., ligand rotation is turned off upon O2 adsorption. Positive and negative values represent charge depletion and accommodation, respectively.
Crucial role of LEF surrounding the adsorbed O2 in CO oxidation rate
The contrasting charge transfer and spin accommodations, due to the disparate rotations of the ligands within the two typical catalytic substrates, result in significant contrast LEF surrounding the O2 species (Fig. 7a). Specifically, we present comparative electric field analyses for the optimized structures of O2 adsorption on both catalytic complexes along the normal, indicated by the yellow solid arrow (Fig. 7a). The yellow dashed line marks the path through which the arrow runs, representing the position of the CO molecule in the transition state. Our DFT calculations demonstrate a significantly higher positive electric field for monoferroic Cr(pyz)2 (indicated by the red solid line in Fig. 7a) compared to multiferroic Cr(h-fpyz)2 (black solid line in Fig. 7a). Notably, in the transition state, the CO molecule is positioned nearly vertically above the O2 molecule at similar heights for both Cr(pyz)2 and Cr(h-fpyz)2 cases. During CO oxidation, the CO molecule prefers a downward orientation towards the C atom when reacting with the activated O2 molecule. This orientation results in stronger repulsive interactions between the polar CO molecule and the positive electric field surrounding the adsorbed O2 molecule on monoferroic Cr(pyz)2 compared to multiferroic Cr(h-fpyz)2. Consequently, the significantly lower rate-limiting Ebar observed in the multiferroic Cr(h-fpyz)2 underscores its enhanced catalytic efficiency compared to monoferroic Cr(pyz)2. Note that, our DFT calculations can be further supported by a simple point-charge model analysis with and without considering the screening effects (Supplementary Fig. 16), where an asymmetric charge distribution between Cr and O2 molecule can lead to an unexpected positive electric field (Supplementary Fig. 17).
a One-dimensional LEF profiles (solid lines) around the adsorbed O2 along the normal directions of the substrates, as navigated by the yellow solid lines of the insets. The yellow dashed line marks the path through which the arrow runs, representing the position of the CO molecule in the transition states shown in Fig. 5a and Supplementary Fig. 15. The dashed lines correspond to the cases without O2 adsorption, i.e., the pristine substrates. The green shaded part indicates the zone that covers the CO molecule. The insets illustrate the two-dimensional electrostatic potential contours right above the adsorbed O2 and the CO-O2 species of the TS state. b LEF obtained with the same approach as that in (a) with allowing the organic ligands rotations (w/ R), nevertheless without allowing the organic ligands rotations and upward shift of the Cr atoms, as simply termed w/o R. c Rate-limiting activation energy barriers (Ebar) for CO oxidation with (without) allowing the organic ligands rotations and upward shift of the Cr atoms (w/ R (w/o R)). d Logarithms of reaction rate of the CO oxidation on the multiferroic Cr(h-fpyz)2 and monoferroic Cr(pyz)2 for the cases of w/ R and w/o R.
Furthermore, for comparison, we have also calculated the rate-limiting Ebar of the CO oxidation on both multiferroic Cr(h-fpyz)2 and monoferroic Cr(pyz)2, without allowing the rotations of the organic ligands and upward shift of the Cr atoms (w/o R), i.e., fixing them the same as that in the pristine substrate. When the organic ligands do not rotate, the CO molecule experiences stronger electrostatic repulsion (Fig. 7b). As a consequence, the barrier rises significantly from 0.325 (0.659) eV to 0.802 (0.900) eV in Cr(h-fpyz)2 (Cr(pyz)2) (Fig. 7c). Such an elevated Ebar results in a dramatically reduced reaction rate of 3.36 × 10−2 s−1 (7.60 × 10−4 s−1). To more clearly demonstrate the relative importance of the flexibility of the organic ligand and the metal reactive site in the multiferroic Cr(h-fpyz)2 and monoferroic Cr(pyz)2 for CO oxidation, in Fig. 7d, we have further represented the reaction rates in the form of logarithms for the cases of both w/ R and w/o R. These findings further demonstrate that, the flexible rotation of the organic ligand and the metal reactive site can function as a tuning knob, lowing the rate-limiting Ebar for CO oxidation and enabling on-off switching the room-temperature catalysis in single-atom catalysts.
Discussion
Employing first-principles calculations based on density functional theory, we demonstrate that the 2D multiferroic structure of Cr(h-fpyz)2 functions effectively as a room-temperature single-atom catalyst. This is evidenced by its highly enhanced catalytic activity, as highlighted by the following key findings:
-
(i)
The calculated rate-limiting energy barrier for CO oxidation on the Cr single-atom reactive site within the 2D multiferroic catalytic platform is approximately 0.325 eV, resulting in a reaction rate of 3.47 × 106 s−1 at room temperature and 1.34 × 108 s−1 at 423 K, respectively, implying that the present 2D RT-MF Cr(h-fpyz)2 is a promising high-performance catalytic candidate subject to “the 150 °C challenge” of US-DOE. This rate is six orders of magnitude higher than its monoferroic derivative, [Cr(pyz)2]. Moreover, reversing the polar direction (rotating the organic ligands) of the 2D multiferroic Cr(h-fpyz)2 structure reduces the reaction rate by eleven (eight) orders of the magnitude, enabling on-off switching of room-temperature catalysis of the 2D multiferroic system. Moreover, it seems that the present conclusion could also be generalized to some three-dimensional (3D) multiferroic catalytic platforms, such as the 3D Cr(h-fpyz)2 counterpart, as detailed in Supplementary Fig. 18.
-
(ii)
The underlying microscopic mechanism involves the activation of spin-triplet O2 molecule, facilitated by more flexible rotations of the organic ligand around the Cr single-atom reactive site within the multiferroic Cr(h-fpyz)2 complex compared to its monoferroic Cr(pyz)2 derivative. This flexibility enhances the charge transfer and spin-accommodation processes, predominantly influenced by spin-polarized p-block substrate atoms and the confined d-block Cr single-atom reactive site.
-
(iii)
Intriguingly, on the multiferroic Cr(h-fpyz)2, O2 adsorption induces a significantly lower local positive electric field normal to the surface compared to the Cr(pyz)2, as supported by a simple point-charge model calculation. Consequently, the polar CO molecule experiences reduced repulsive electrostatic interactions and a considerably lower reaction energy barrier. Further simulations confirm that, without allowing the rotations of the organic ligands, the CO oxidation rate is dramatically reduced due to an increased rate-limiting energy barrier, by at least eight orders of magnitude.
-
(iv)
The present findings are expected to offer new insights into establishing highly efficient single-atom catalyst platforms based on 2D multiferroics for a variety of other important chemical processes, such as NO oxidation, oxygen evolution reaction, oxygen reduction reaction, and photocatalysis on water splitting, wherein the charge, spin, and polarity degrees of freedom universally play significant roles.
Methods
Our calculations were carried out by spin-polarized first-principles calculations based on density functional theory (DFT)64,65, as implemented in the Vienna ab initio simulation package (VASP)66,67 with the version of VASP. 5. 4. 4, with the projector augmented wave (PAW)68,69 method and the revised Perdew-Burke-Ernzerhof (RPBE)70 for the exchange-correlation functional. DFT + U method71,72 with Ueff = 4 eV is employed to treat the localization of 3 d orbitals of Cr atoms, as suggested by previous theoretical calculations26. The electronic wave functions were expanded in a plane wave basis with an energy cutoff of 550 eV, and the k-space integration was performed with a 2 × 2 × 1 Monkhorst-Pack k-point mesh in the Brillouin zone for the relatively large simulation cell. A vacuum space beyond 12 Å along the c-axis is incorporated to prevent interactions between neighboring slabs. To attain the optimized structures, all the atoms were allowed to relax until all the residual force components were less than 0.02 eV/Å. The kinetic properties of CO oxidation were investigated using the climbing-image nudged elastic band (CI-NEB) method73,74, with the convergence criterion for electronic step within 10−5 eV and the total energy within 1 meV.
Data availability
The data supporting the findings of the study are included in the main text and supplementary information files.
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Acknowledgements
F.X.Z. and P.S.W. contributed equally to this work. We thank Prof. Zhenyu Zhang for the helpful discussion. This work was supported by the NSF of China (Grants No. U23A2072 (S.F.L.), 12074345 (S.F.L.), 12174349 (X.Y.R.), 12204431 (J. L.S.). The calculations were performed at the National Supercomputing Center in Zhengzhou, Henan.
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S.F.L. and X.Y.R. conceived the idea and designed the work; F.X.Z. performed the calculations; P.S.W. conducted the mechanism analysis and model calculations; S.F.L., X.Y.R., F.X.Z., and P.S.W. wrote the manuscript; Y.D.Z., J.L.S., and R.P. participated in the discussion on the data and mechanism analysis. All authors contributed to the preparation of the manuscript.
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Zhang, F., Wang, P., Zhu, Y. et al. Highly enhanced room-temperature single-atom catalysis of two-dimensional organic-inorganic multiferroics Cr(half-fluoropyrazine)2 for CO oxidation. Nat Commun 16, 1580 (2025). https://doi.org/10.1038/s41467-025-56863-1
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DOI: https://doi.org/10.1038/s41467-025-56863-1