Following the fabrication of the first practical single-atom catalyst (SAC), Pt₁/FeO_x [1], the concept of SACs has been successfully extended to other FeO_x supported metals, including Ir₁/FeO_x [2, 3], Au₁/FeO_x [2, 4], and Ni₁/FeO_x [5]. Recently, considerable progress has also been made in the investigation of other single-atom catalysts [2, 6-31]. These SACs possess well-defined surface-supported single-atom active sites, and exhibit high activity and stability in various heterogeneous reactions, including the water-gas shift (WGS), CO oxidation or preferential oxidation (PROX), and hydrogenation. Among these various reactions, CO oxidation is a simple yet important reaction, which has been extensively investigated from the perspective of environmental protection, exhaust purification for motor vehicles, gas purification for closed-cycle CO₂ lasers, and for CO detectors [32, 33]. Moreover, this reaction is also an elementary step in the WGS reaction [2, 34], and the preferential oxidation of CO in H₂ (i.e., the PROX reaction) serves as a key step for removing CO from reforming gas in fuel cell applications [35]. Over the past few decades, a number of oxide-supported noble-metals (e.g., Pt, Au, Ir, and Pd) [1, 19, 36-38] have been identified as active catalysts for CO oxidation. In particular, as for FeO_x based SACs, the supported non-noble 3d-transition metal has been even found to be effective for the oxidation of CO, that is, our recent theoretical investigation indicated that FeO_x supported 3d-transition metals such as Ni₁/FeO_x, can catalyze CO oxidation with high activity, even at room temperature [5].

While Fe is a typical 3d-transition metal and the Fe atoms in the FeO_x support can also be considered to be a monodisperse metal center on the surface of the support. To be consistent with the categorization of Pt₁/FeO_x SAC, here we denote the FeO_x substrate with an oxygen vacancy as Fe₁/FeO_x. However, this base material has been found to have a much lower activity for CO oxidation than that of Pt₁/FeO_x [1]. Our previous density functional theory (DFT) calculations showed that the high catalytic activity of Pt₁/FeO_x correlates with the partially vacant 5d orbitals of the positively charged, high-valence single-atom Pt₁. This effect reduces the probability of CO poisoning and facilitates adsorption of oxygen. However, it remains unclear what factors give Pt₁/FeO_x such a high catalytic activity [1] and limit the activity of the FeO_x support as a catalyst in CO oxidation. To understand the differences between these two catalysts, we performed theoretical investigations on possible catalytic reaction pathways of CO oxidation on Fe₁/FeO_x. We examined the chemical interactions using DFT, with our previous calculated results of Pt₁/FeO_x as a reference.

On the basis of our theoretical calculations, we show that among the fundamental differences in the catalytic activity toward CO oxidation between Pt₁/FeO_x and Fe₁/FeO_x, the regeneration of single-atom active centers is the key factor, which determines the catalytic activity for CO oxidation. These findings help to develop an understanding of the high activity of Pt₁/FeO_x for CO oxidation, which will guide the design of new FeO_x supported single-atom catalysts for potential practical applications.

As used in our previous work [1], the (0001) surfaces of α-Fe₂O₃ were represented by a periodic slab model, constructed from bulk cell dimensions: a = b = 5.04 Å and c = 13.72 Å. Because α-Fe₂O₃ is antiferromagnetic and has an atomic magnetic moment on iron atoms, we used the primitive rhombohedral unit cell of Fe₂O₃ with a magnetic configuration (+ --+) to build the surface slab. This arrangement has previously been shown to be the most energetically favored magnetic configuration for α-Fe₂O₃ [39]. A vacuum distance 12 Å-width in the direction perpendicular to the surface was set to eliminate the interaction between layered nanostructures in adjacent cells. Considering the large relaxation typical of Fe₂O₃ surfaces [1, 40], we selected slabs containing 12 layers of Fe atoms, and 7 atomic layers of O₃ (see below) to model the O₃-terminated surfaces [1]. The 10 top-layer slabs of the surface were allowed to relax while the other layers beneath the surface were frozen during the geometry optimizations. On the basis of previous studies [1, 8], here the relatively favorable O₃-terminated surface of Fe₂O₃(0001) was selected, where each Fe₁ atom is coordinated by three surface oxygen atoms (i.e. the O₃ atoms), with a third-layer of Fe atoms lying below the Fe₁ atoms. The Fe₁ can be viewed as a single-atom adsorbed to the α-Fe₂O₃ substrate.

The theoretical calculations were performed by DFT using the Vienna Ab-initio Simulation Package (VASP) [41, 42]. The core and valence electrons were represented by the projector augmented wave (PAW) [43] method and plane-wave basis functions with a kinetic energy cut-off of 400 eV [1]. Considering that both Pt and Fe have relativistic effects, we included the mass-velocity and Darwin relativistic effects through the PAW potentials. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) [44] exchange-correlation functional was used in the calculations. A Monkhorst-Pack grid of size of 3×3×1 was used to sample the surface Brillouin zone [1]. Ground-state atomic geometries were obtained by minimizing the forces on the atoms below 0.02 eV/Å. Because of the strong d-electron correlation effects for Fe, the calculations were performed with the DFT+U method, based on the formalism suggested by Liechtenstein and Dudarev et al. [45]. The parameters were set at U = 4 eV and J = 1 eV according to the previous study [1]. The transition states were obtained by relaxing the force below 0.05 eV/Å via the dimer method [46].

We begin our study using the previous calculated model of M₁/FeO_x (M = Pt, Ir, Au, Ni) [1, 3-5], which derives from the primitive structure of the α-Fe₂O₃ (0001) surface with an oxygen vacancy (O_vac) on its O-termination surface. The FeO_x substrate in this model is similar to other M₁ in M₁/FeO_x SACs with a single atom of Fe₁ anchored to the surface, denoted as Fe₁/FeO_x. After intensive searching for adsorption and transition states, the predicted catalytic cycle and calculated reaction energies of CO oxidation on the α-Fe₂O₃ substrate are shown in Fig. 1 (top view). The corresponding bond lengths of the species are presented in Fig. 2 (side view) and Table 1. In the initial step of the reaction (step ⅰ), an oxygen vacancy (O_vac) near the Fe atoms is constructed to represent the reduction of the hematite surface, and the optimized bond lengths Fe₁-O_A1 and Fe₁-O_A2 are found to be 1.78 and 1.77 Å, respectively. Owing to the presence of the oxygen vacancy, O₂ molecule prefers to dissociatively adsorb to the surface of FeO_x.That is, the calculated adsorption energy of O₂ dissociation (-3.75 eV) is much higher than molecule activation of O₂ on the oxygen vacancy (-2.14 eV). With the dissociative adsorption of O₂ on the surface (step ⅱ), the distance between the O_B atom (light green) and O_C atom (light green) is 2.70 Å (as seen Fig. 2), which indicates the formation of O adatoms on the surface. After this adsorption of O₂, the first CO molecule is co-adsorbed on the single-atom Fe₁(step ⅲ) through the C atom (pink) of CO, with a Fe-C distance of 2.18 Å. The optimized distance of the C and O_B atoms on the single-atom Fe₁ is 2.36 Å. As the C atom of CO approaches O_B to generate the first CO₂ molecule (CO + O_B → CO₂), only a negligibly barrier energy (0.08 eV) is needed to be overcome through the first transition state (TS-1) with an imaginary vibration frequency at 345i cm^-1. The activation barrier to the first formation of CO₂ on the single-atom Fe₁ is low enough for the reaction to occur at a relative low temperature. After climbing TS-1, the first CO₂ molecule desorbs from the surface of FeO_x, leaving behind the dissociated O_C atom to heal over the O_vac on the FeO_x surface (step ⅳ).

In addition to reactions of CO_ad and O_ad on Fe₁ single active site through the Langmuir-Hinshelwood (L-H) mechanism, the Eley-Rideal (E-R) mechanism might also be involved in the catalytic process as suggested by our previous study on SAC Ir₁/FeO_x [3]. The E-R mechanism was also considered for formation of the first CO₂on the Fe₁/FeO_x surface, owing to vertical adsorption of the dissociated O_B atom on the single-atom Fe₁. The calculated E-R reaction pathway and the selected distances of atoms in these species are presented in Fig. 3. The distance between the C atom of CO and the O_B atom vertically adsorbed on the single-atom Fe₁ is 3.33 Å, indicating only weak adsorption of the CO molecule in the initial reactant pattern (R), as shown in Fig. 3. By overcoming a barrier energy of 0.23 eV, the conversion of CO and O_B to CO₂ can be accomplished through a transition state (TS) on the surface of FeO_x to give the final product (P). Thus, both the L-H and E-R mechanisms might be involved in the formation of the first CO₂ molecule because the reaction activation energies of these two processes are low enough for the reaction of CO + O_B → CO₂ to occur at a low temperature.

After the formation of a perfect Fe₁-terminated surface of α-Fe₂O₃ (0001) (step ⅳ), the second CO molecule can favorably adsorb on the single-atom Fe₁ as illustrated in step ⅴ. The optimized adsorption energy of CO is -0.73 eV, and the distance between the C atom from CO and the O_C atom dissociated from O₂ is 3.11 Å, as shown in Figs. 1 and 2. The adsorbed CO can approach O_C at O_vac, leading to a small barrier energy of 0.66 eV for the reaction of CO + O_C → CO₂ via the L-H mechanism on the surface of FeO_x(TS-2). This transition state features an imaginary vibration frequency of 391i cm^-1. The distance between the C atom and O_C is shortened from 3.11 to 1.69 Å in TS-2, indicating the formation of a new C-O bond and the generation of a second CO₂ analogue on the surface of FeO_x; however, the CO₂ molecule remains adsorbed to the FeO_x surface (step ⅵ). Before the CO₂ is released from the FeO_x surface into the gas phase, an energy barrier of 1.09 eV must be overcome in TS-3. Finally, the whole catalytic cycle for CO oxidation is complete and the original catalyst with O_vac (step ⅰ) of Fe₁/FeO_x is regenerated.

The reaction rate-determining step over the whole catalytic cycle is clearly the desorption of the second CO₂ molecule from the surface of FeO_x (TS-3). Thus, the FeO_x substrate without a supported single-atom Pt₁ has a much lower catalytic activity than that of Pt₁/FeO_x SAC in CO oxidation reaction, which agrees well with previous experimental results [1]. When the standard enthalpies of formation of the oxides are compared, the lower catalytic activity of Fe₁/FeO_x for CO oxidation is also likely related to the relatively high oxygen affinity of Fe (250-300 kJ/mol) [47].

To determine why SAC Pt₁/FeO_x has a much higher catalytic activity than that of the substrate α-Fe₂O₃ (Fe₁/FeO_x), and elucidate the nature of the binding of the single-atom Pt₁ to the FeO_x support, we compared the mechanism of catalysis and electronic structures in the proposed reaction pathways of CO oxidation. The relative binding energies for CO oxidation over these two catalysts are presented in Table 1. As shown in Fig. 1 and Table 1, there are several differences between these two different catalysts. First, O₂ is adsorbed in a molecular form onto the single Pt₁mechanism, but is dissociatively adsorbed on the single-atom Fe₁. Second, the difference in the mode of O₂ activation leads to a discrepancy in the energy barriers for formation of the first CO₂ molecules on Pt₁ and Fe₁ single-atom active sites. As shown in Figs. 1 and 2, the activation barrier for formation of the first CO₂ molecule on single-atom Fe₁ (0.07 eV, TS-1) is much lower than that on Pt₁ (0.59 eV, TS-1) because the dissociated O_B atom reacts with the adsorbed CO much more easily. Third, the activation barriers of the rate-determining step of these SACs for the second CO₂ formation are clearly different for the two SACs M₁/FeO_x (M = Fe, Pt). The activation energy of CO_ad + O_C → CO₂ (TS-3) on Pt₁/FeO_x (0.79 eV) is much lower than that of CO_ad + O_C → CO₂ (TS-3) on Fe₁/FeO_x (1.09 eV), indicating that the α-Fe₂O₃ substrate has a low catalytic activity for CO oxidation.

As seen from the calculated Bader charges for M₁ single-atoms in M₁/FeO_x (M = Fe, Pt) in Table 2, the valence electron loss of the Fe₁ single-atom (1.488 |e|) is larger than that of a single-atom Pt₁ species (1.045 |e|) after CO adsorption on these single atoms. This result indicates the different interaction strengths between the adsorbed CO molecules and the single atoms of Pt₁ and Fe₁. To further understand the chemical interactions between the CO molecule and Pt₁ or Fe₁ single-atoms, we analyzed the electronic properties of these two different surfaces before (step ⅳ) and after (step ⅴ) CO adsorption. The formation of the second CO₂ molecule in these two steps governs the major differences in the process of CO oxidation. According to the density of states (DOS) distributions in Figs. 4 and 5, both the 3d and 5d states of the single Fe₁ and Pt₁atoms mix well with the 2p states of the neighboring oxygen atoms, suggesting strong chemical interactions between the single metal atoms with the FeO_x support, and that the stability of Pt₁/FeO_x is equivalent to that of the FeO_x substrate. The C 2p states of the adsorbed CO also mix with the d-states of single-atom metal species, indicating chemical bonding of CO on both SACs. As shown in Table 1, the calculated CO adsorption energy on Pt₁/FeO_x is larger than that on the FeO_x substrate. From Fig. 2, the stronger CO adsorption on the Pt₁ center makes it easier for the adsorbed CO to bind to the surface O atom in the OC…O(surface) of TS-3. Indeed, for the TS-3 structure the OC…O(surface) distance is 0.05 Å shorter for Pt₁/FeO_x than that for Fe₁/FeO_x, which accounts for the lower activation barrier of CO oxidation on Pt₁/FeO_x.

Finally, we use a (MeO)₃M←CO (M = Fe, Pt) molecular system to model the bonding interactions between CO and the single metal atom at the surface. In this model, the M atom is clearly in the +3-oxidation state, while the real oxidation states of Pt and Fe in Pt₁/FeO_x and FeO_x are likely to be somewhat different. However, this molecular model has a similar coordination environment to that of the surface Pt and Fe atoms, and can be used to provide insight into the orbital interactions involved at these types of active sites with similar local structure. The structures optimized by the PBE functional and TZ2P basis sets as implemented in ADF 2016 are shown in Fig. 6 and the bond parameters are given in Table 3. The orbital energy-level correlation diagram is shown in Fig. 7, with the corresponding molecular orbitals listed in Table 4. As can be seen from Fig. 7 and Table 4, the 5s orbital of CO is stabilized by a donor interaction with (MeO)₃M, and the main interactions in the frontier MOs are back-donation to 2π orbitals of CO, which contribute to the HOMO-1 and LUMO+1 of (MeO)₃Fe←CO as well as the HOMO-2 and LUMO of (MeO)₃Pt←CO. The calculated energy-level splittings between the bonding Fe 3d or Pt5d and the anti-bonding orbitals CO (2π*) of (MeO)₃M←CO (M = Fe, Pt) are 4.73 and 5.24 eV, respectively. Thus, the orbital interactions between 2π(CO) and (MeO)₃Pt are stronger than those between 2p(CO) and (MeO)₃Fe owing to the larger extent of the Pt 5d orbital than that of the Fe 3d. This effect cancels the lower energy of the 5d atomic orbitals (AOs) for Pt than that of the 3d orbitals for Fe. Therefore, the stronger adsorption of CO on the Pt single-atom active center and the higher activity of Pt₁/FeO_x for CO oxidation are related to differences in the orbital interactions between the various species and Pt 5d and Fe 3d orbitals.

We have performed systematic theoretical investigations using relativistic density functional theory (DFT) to develop an understanding of why single-atom Pt₁ on FeO_x possesses excellent catalytic activity for CO oxidation, but the FeO_x substrate alone is not an effective catalyst. Our calculations indicate that the formation mechanisms of the first CO₂ molecules on the surfaces of these SACs are quite different owing to the different activation of O₂ at these two single-atom sites. Molecular adsorption occurs on the single-atom Pt₁ while dissociative adsorption is unavoidable at the single-atom Fe₁. More importantly, a relatively high activation energy (1.09 eV) is required to remove the O atom from the perfect (0001) surface of α-Fe₂O₃ through the formation of the second CO₂ molecule, owing to the weak Fe(OC…O_surface) interaction. A relatively high oxygen affinity for Fe (250-300 kJ/mol) and the weak strength of CO adsorption on the single-atom Fe₁ are the key factors contributing to the lower catalytic activity of Fe₁/FeO_x for CO oxidation. The present theoretical study offers an explanation for why the FeO_x support is effectively inert in CO oxidation compared with the SAC Pt₁/FeO_x. These findings provide insights that will be useful for the design of new FeO_x supported single-atom catalysts.

Acknowledgments

The calculations were done using supercomputers at Tsinghua National Laboratory for Information Science and Technology, the State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University), and Guizhou Provincial High-Performance Computing Center of Condensed Materials and Molecular Simulation.