Towards stable single-atom catalysts : strong binding of atomically dispersed transition metals on the surface of nanostructured ceria †

The interaction of a series of different transition metal atoms with nanoparticulate CeO2 has been studied by means of density-functional calculations. Recently, we demonstrated the ability of sites exposed on {100} nanofacets of CeO2 to very strongly anchor atomic Pt, making the formed species exceptionally efficient single-atom anode catalysts for proton-exchange membrane fuel cells. Herein, we analyzed the capacity of these surface sites to accommodate all other group VIII–XI transition metal atoms M = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Cu, Ag, and Au. The interaction of the M atoms with {100} nanofacets of ceria leads to oxidation of the former and such interaction is calculated to be stronger than the binding of the atoms in the corresponding metal nanoparticles. Comparing the stability of metal–metal and metal-oxide bonds allows one to establish which metals would more strongly resist agglomeration and hence allows the proposal of promising candidates for the design of single-atom catalysts. Indeed, the remarkable stability of these adsorption complexes (particularly for Pt, Pd, Ni, Fe, Co, and Os) strongly suggests that atomically dispersed transition metals anchored as cations on {100} facets of nanostructured ceria are stable against agglomeration into metal particles. Therefore, these sites appear to be of immediate relevance to the preparation of stable catalysts featuring the highest possible metal efficiency in nanocatalysis.


Introduction
Reducing the amount of precious noble metals used in catalytic materials is one of the challenges in catalysis research. 1 The ever-increasing demand from the automobile industry along with limited supply has a critical effect on the price of such metals, which is an obstacle for the large-scale implementation of applications that require noble-metal-based catalysts. [2][3][4] It is well known, for example, that the very high cost of platinum is one of the stringent factors limiting the wider use of fuel cell technology. 3,4 Two strategies are commonly followed to cope with the challenge of reducing the content of precious metals in catalysts. The first strategy involves the partial or complete replacement of the precious metal by less-expensive materials. Following it, numerous alternatives have been proposed that can substantially decrease the cost of the catalyst. [5][6][7] However, the resulting catalytic performance is often inferior to that of the analogous noble-metal systems. The other strategy focuses on the more efficient utilization of the noble metal rather than its substitution. This approach aims to maximize the specific catalytic performance of the noble-metal phase, i.e. its per-atom activity. The latter has been customarily achieved by finely dispersing the metal on supportsa paradigm of nanocatalysis. 8,9 The limiting case of metal dispersion corresponds to catalytic systems with atomic metal species on the surface of the support, denoted as single-atom catalysts (SACs). [10][11][12] Notably, non-reducible metal-oxide supports exposing regular surfaces, such as MgO(100), 13 adsorb atomic transition metal species too weakly to prevent their clustering. A similar situation takes place on the most stable (111) surface of CeO 2 , a reducible oxide support widely used in catalysis. 14 This indicates that the formation of transition metal SACs that are sufficiently stable to counteract metal-metal bond formation upon sintering requires special strongly binding sites on the supports.
Various composites featuring atomically dispersed noble metals have been reported to be catalytically active for different reactions. For instance, cationic Pt and Au species on nanostructured ceria 15 and more inert oxides, such as zeolites and silica, 16,17 were found to catalyze the water-gas shift reaction at low temperatures. Materials formed by Pd cations anchored on alumina 18 and Pt cations on FeO x (ref. 19) were reported to be active towards CO oxidation. High catalytic hydrogenation activity has also been attributed to such SACs, as for instance Pd atoms anchored to cavities of mesoporous graphitic carbon nitride 20 and FeO x -supported Pt. 21 These and analogous systems represent a promising new generation of cost-effective catalytic materials. Most publications point to oxidized states of the supported noble-metal atoms as the active species in these catalysts. It should be noted, however, that the structure of SAC materials can change under the often harsh catalytic conditions, leading to partial loss of the specific activity of the metal due to its sintering or bulk diffusion. These phenomena can strongly reduce the number of active metal sites exposed to reactants. Thus, it is essential that the support provides sufficient concentration of surface sites that can anchor the metal atoms strongly enough to prevent agglomeration and bulk diffusion. To this end, {100} nanofacets of ceria nanoparticles (NPs) have been shown to be remarkably efficient in anchoring single Pt atoms as Pt 2+ cations. 22,23 Density-functional calculations combined with X-ray photoemission spectroscopy (XPS) experiments were used to determine that Pt 2+ cations adsorbed on such sites are efficiently protected from reduction, aggregation or diffusion into the bulk, up to high temperatures. 22 This specific nanofacet site thus fulfills the stability requirements for a SAC. Importantly, this surface site consisting of four O atoms in a square-planar geometry (below referred to as an O 4 site), is not unique to ceria NPs. This structural motif can also be found on extended CeO 2 (100) surfaces 24 and on the step edges of low-energy CeO 2 (111) islands. 25 The extraordinary stability of Pt atoms on CeO 2 {100} nanofacets suggests that these sites may also strongly bind other transition metal atoms. To assess this peculiar binding propensity important for preparing stable SACs, we have investigated the interaction of these sites with transition metal atoms M = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Cu, Ag, and Au in groups VIII-XI of the periodic table. Our results reveal that the O 4 sites on {100} facets of ceria NPs strongly bind and oxidize single atoms of all studied metals. Comparison of the adsorption energies of the M 1 species with the binding energies of the corresponding atoms in metal NPs indicates high energetic stability of the anchored M 1 species against agglomeration. The oxide support acts in the anchoring according to coordination chemistry principlesas a polydentate ligand formed by surface oxygen anions. 26 This explains how the adsorption bonds are as strong as metal-ligand bonds in common transition metal complexes. Findings of the present study suggest general guidelines for the preparation of enduring transition metal SACs.

Computational details
The adsorption of single atoms of 11 different transition metals on the {100} O 4 site of a ceria NP has been investigated by means of periodic spin-polarized density-functional calculations. The generalized gradient approach (GGA) corrected with the on-site Coulomb interaction Hubbard term U (GGA+U) has been used. [27][28][29] Common exchangecorrelation potentials suffer from an incomplete cancellation of the self-interaction error, thus leading to a poor description of strongly correlated systems. The U-correction scheme offers a computationally feasible improvement, which enables an appropriate treatment of transition and rare-earth metal oxides featuring localized electrons in d and f orbitals, respectively. The GGA+U approach has been widely used to describe ceria-based systems, whose Ce 4f states can be occupied upon reduction. 30 However, there is no unique U value allowing simultaneous reproduction of all properties of oxidized and reduced ceria. 31 For example, calculations with larger values of U predict band gaps in better agreement with experiment, but overestimate interatomic distances in ceria. Following previous studies of ceria NPs, 32-35 present calculations have been performed using the PW91 (ref. 36) exchange-correlation potential corrected with a value of U = 4 eV (referred to as the PW91+4 scheme). The effect of using different U values on the calculated properties was assessed (see section 4).
Calculations have been carried out using the VASP code, [37][38][39] representing the valence states in plane-wave basis sets with a cut-off of 415 eV for the kinetic energy. The corevalence interaction has been described through the projector augmented wave method. 40 Only the Γ-point has been used to sample the reciprocal space. The electron density was selfconsistently converged with a 10 −4 eV total energy threshold and all geometric structures were optimized until forces acting on each atom became smaller than 0.02 eV Å −1 . Some adsorbed metal atoms can exhibit different oxidation states. This has been explored and the resulting oxidation states of the adsorbed M 1 species have been determined by the analysis of the localized magnetic moments on Ce cations. Bader charge analysis 41 has also been performed.

M 1 -CeO 2 and M n models
A cuboctahedral Ce 40 O 80 NP ( Fig. 1) has been chosen as a representative model of nanostructured CeO 2 , previously shown to be capable of reproducing various experimental observations 22,32,35 (effect of the NP size on the calculated observations is briefly addressed in section 4). The structure of this NP resulted from a global optimization using interionic potential and density-functional calculations. 33,34 The Ce 40 O 80 NP retains the cubic fluorite-type crystallinity of bulk CeO 2 and exposes small O-terminated {111} and {100} facets. The latter corresponds to the polar (100) surface, less stable than the (111) surface. 42 Yet, ceria nanosystems with abundant [100] terminations have also been prepared, ranging from nanocrystals with extended (100) terraces 24 to nanocubes exposing only {100} facets. 43 The size of the periodically repeated unit cell was chosen to keep distances between NPs in neighboring cells ≥700 pm to avoid significant spurious interactions of the NPs.
Similarly, the propensity of the anchored M 1 species to agglomerate can be assessed by comparing the E ad values with the experimental metal cohesive energies (see Table  S1 †).

Results and discussion
We performed a comparative study of the adsorption of 3d (Fe, Co, Ni, Cu), 4d (Ru, Rh, Pd, Ag) and 5d (Os, Ir, Pt, 22 Au) metal atoms on O 4 sites of {100} nanofacets of the Ce 40 O 80 NP. This is not the only site on the NP where metal atoms can be anchored, but the adsorption of Pt on other NP sites was calculated to be much weaker than on the O 4 site 22,23 and similar in strength to the adsorption on CeO 2 (111) sur-faces. 44 Since this site preference is expected for the adsorption of all metal atoms under scrutiny, we focused on interactions with the O 4 site.
The optimized structures for the M 1 -Ce 40 O 80 systems are shown in Fig. 3. Via the analysis of spin moments, we were able to quantify the charge transfer taking place upon atom deposition on the {100} nanofacet. Such charge transfer results in the oxidation of the M 1 adsorbate with the concomitant reduction of a certain number of Ce 4+ cations to Ce 3+ . The number of reduced Ce cations depends on the M atom and equals its oxidation state. Corner Ce 4+ cations in the Ce 40 O 80 NP are the easiest to reduce due to their lower coordination number and an accordingly less destabilizing electrostatic environment. 34,45 These corner Ce 4+ cations accepted electrons from M atoms, which were concomitantly oxidized to +1 or +2 oxidation states. In oxidation states higher than +2, a search for the most stable locations of the additionally formed Ce 3+ cations has not been performed in view of a very high number of possible configurations. The location of the Ce 3+ cations in less stable positions of the NP could induce a destabilization of the adsorption by up to 40 kJ mol −1 per Ce 3+ cation. 34 Yet, this difference does not affect the upcoming discussion of the adsorption energy values of M atoms. Note that the appearance of Ce 3+ cations (larger than Ce 4+ ) significantly elongates the corresponding Ce-O distances, to 228 pm from 213 pm in the pristine CeO 2 NP (see Fig. S1 and S2 †).
Herein, we discuss the oxidation states for each adsorbed metal atom and the M-O coordination modes (see for details Fig. S1 and S2 †). We also comment on how our results compare to pertinent experimental data.  22 A similar coordination mode of Fe was detected by Mössbauer spectroscopy for Fe 3+ cations inserted into the lattice of ceria-zirconia catalysts. 46 In turn, the self-dispersion of Ru metal powder on ceria in the form of small particles and single atoms in a highly oxidized state was demonstrated to be very stable by X-ray absorption near edge structure (XANES) and Raman spectroscopic techniques. 47

Group IX metals
Atoms of this group manifested more diverse oxidation states. Co is the only metal atom in this group that features a +3 state. Despite the well-known stability of the +3 state for    52 There is experimental evidence that Pd 2+ cations in the square-planar coordination by oxygen are highly stable in PdO x -CeO 2 solid solutions. 53 The presence of atomic platinum as Pt 2+ cations on ceria surfaces has been documented by several theoretical and experimental studies. Formation of these species was often related to the exposure of (100)-terminated facets. 22,23,[54][55][56] The present GGA+U data depend on the chosen U value. In particular, larger U values stabilize the presence of localized f electrons favoring processes that involve the reduction of Ce 4+ to Ce 3+ . To benchmark this dependence we calculated E ad with different U values for a single Pd atom adsorbed on the Ce 40 O 80 NP (see Fig. 4). The use of U = 3 eV decreases E ad by 53 kJ mol −1 compared to that obtained with U = 4 eV chosen in this work. The use of a larger U value of 5 eV has an opposite effect on the E ad increasing its magnitude by 54 kJ mol −1 . A similar trend was found for the binding of supported atomic Pt species on ceria. 22,57 In general, the usage of smaller, i.e. less interfering U values seems to be preferable. Yet, U < 4 eV values often do not allow complete localization of 4f electrons on the Ce 3+ cations. The U = 4 eV employed throughout this work seems to be an adequate compromise providing a physically correct description of localized Ce 4f states and minimizing spurious stabilization of reduced Ce 3+ species. One can also question, to what extent

Group XI metals
Finally, adsorbed atoms of group XI, like those of group IX, exhibit several oxidation states. Moreover, each adsorbed group XI element can be in different oxidation states. The presence of Cu as Cu 2+ ions in the ceria lattice was recently reported. 58 We calculated Cu + to be coordinated in a distorted linear fashion with two short (189 pm) and two long Cu-O distances (231 and 262 pm). Cu 2+ adsorbs in a squareplanar coordination mode with four equal Cu-O distances (199 pm). The Cu 2+ state is found to be by 54 kJ mol −1 more stable than the Cu + state (Table 1). Atomic Ag can be adsorbed as either Ag + or Ag 3+ , the latter was calculated to be somewhat more stable. The not very usual Ag 3+ forms four Ag-O distances of 205-206 pm, in line with its square-planar coordination in oxoargentates. 59 Ag + is in a distorted squareplanar coordination, where the metal atom is 90 pm above the O 4 plane. This distortion elongates Ag-O distances to 239-241 pm. Atomic Au can also be adsorbed as Au + and Au 3+ , the latter being more stable by 70 kJ mol −1 (Table 1). Au, similar to the other presently investigated 5d metals, forms the highest oxidation state (+3) within the group. A linear coordination of Au + cations is preferred, with two short (206 pm) and two long distances (274 and 276 pm), while Au 3+ prefers square-planar coordination (Au-O distances are 204-205 pm). Ag adsorbed as single atoms on microporous hollandite manganese oxide was found by XANES to feature the +1 oxidation state. 60 Meanwhile, gold can be stabilized both as Au 3+ and Au + surface cations in ceria-supported gold catalysts. 61 The adsorption energies in Table 1 reveal that the {100} nanofacet of ceria can strongly anchor not only atomic Pt, but other transition metal atoms as well. Furthermore, atoms of all considered transition metals are oxidized upon adsorption on the O 4 site. The Bader charges of adsorbed M 1 species reflect the formal oxidation states only qualitatively, being notably smaller than the latter and manifesting significant covalence in the M-O interactions. Despite the simplicity of the presently considered O 4 sites exposed by the Ce 40 O 80 model NP, they are expected to be representative for a variety of experimental coordination environments of different transition metal cations. In particular, we are confident that our model NP allows us to quantify the strength of the M-O(-Ce) bonds formed by oxidized metal atoms. To assess whether these metal centers are resistant to agglomeration and sintering processes, which is a crucial issue in the design of metal-efficient SACs, one can compare the binding energies of transition metal atoms on the ceria NP surface and in the corresponding M 79 NPs (Table 1).
For all M atoms under scrutiny, the adsorption energy E ad on the ceria NP is larger in magnitude (more negative) than the binding energy E ad79 of an edge atom in the M 79 NPs. This indicates that the metal dispersion as single atoms on the O 4 sites of ceria nanostructures is energetically favored over the formation of metallic particles. Therefore, very stable adsorption complexes on the nanoparticulate oxide support should resist sintering processes, especially for metals featuring substantial energy differences ΔE = E ad79 − E ad ( Table 1). The ΔE value defines the propensity of anchored metal atoms to form particles: the larger ΔE, the less prone the metal center is to sintering. We predict Fe 3+ , Os 6+ , Co 3+ and group X metals to be particularly resistant to agglomeration in oxidative media. For group X metals, the extraordinary stability of the square-planar coordination for d 8 metal centers explains the particularly strong interaction with the ceria NP. Small ΔE values calculated for cationic Ag, Au, and Ir species  Clear trends in calculated adsorption energies emerge along the rows and groups of the periodic table (Fig. 5). Both E ad and E ad79 generally decrease in magnitude, when moving from the left to the right of the period. This indicates that metals with less occupied d bands form stronger metal-metal bonds and also bind more strongly to the oxide support. For 4d and 5d metals, such decreases in the E ad and E ad79 are quite monotonous, with more pronounced differences for Au. The trend along the period for 3d metals is less linear and both E ad and E ad79 values are crossed with those of the 4d and 5d metals. These trends also indicate that, except for Au, 5d metals form the strongest bonds with the O 4 site of the ceria NP, whereas 4d metals form the weakest bonds, with the exception of Ru.
The agglomeration of the transition metal content into the corresponding oxide phase under an oxidative environment would also lead to the destruction of the single-atom sites. Similar to sintering into metal NPs, the formation of the oxide phase would lead to less exposed metal atoms, with different properties and local environment. The propensity of the single-atom sites to such restructuring can be assessed from the experimental standard heats of formation (ΔH f ) for the most stable oxide phases of the metals under study (see Table S2 †). In general, 3d metals are more prone to the formation of oxides than 4d and 5d metals, and the oxides of metals situated at the beginning of the period are more stable than those at the end of the period. 63 Comparing E ad and ΔH f values for each studied transition metal one expects generally high resistance against decomposition via the formation of a metal oxide phase except for Fe, which is quite susceptible to oxide formation.
All results of the present study correspond to stoichiometric ceria NPs, which are mostly relevant to conditions of ultra-high vacuum or very low oxygen pressure. 23 Yet, under ambient atmosphere and oxidative catalytic conditions ceria NPs can be stabilized by an excess of oxygen. [64][65][66] There, a variety of oxygen-containing surface species is expected to create a pool of additional adsorption sites capable of stabilizing oxidized transition metal atoms as potential SACs. We expect the binding properties of such sites to be quite similar to those of the sites on the {100} O-terminated facets of the stoichiometric NPs. This assumption is supported by the finding that oxygen atoms of the {100} O 4 sites are loosely bound to the ceria NPs, 33,34 which is reminiscent to the binding of the species adsorbed on the NP faces under excess O 2 . Therefore, the present theoretical prediction that surface oxygen sites of nanostructured ceria are able to make diverse single-atom metal catalysts resistant to sintering probably can also be generalized to different experimental conditions.
The above results on the extreme stability of supported single metal atoms are expected to provide a guideline establishing suitable candidates for the design of SACs. This work opens a way to examine the catalytic function of the proposed materials individually for each reaction of interest. Co-sputtering of metals under an oxygen atmosphere, allowing the preparation of nanocomposites of atomically dispersed Pt on ceria, 22,67 can also be used to disperse other metals, the atoms of which are strongly bound to the squareplanar O 4 sites. This has been demonstrated for model catalysts prepared according to the guidelines provided by the present calculations and comparing the stability and reactivity of Pt 2+ , Pd 2+ , and Ni 2+ species on nanostructured ceria. 68 Interestingly, well-characterized steps on extended CeO 2 (111) surfaces 25,69 also appear to efficiently anchor Pt 2+ by forming PtO 4 moieties, indicating that the preparation of ceria surfaces with very abundant steps also facilitates metal dispersion in the form of atoms. 70 In addition, the detailed structural data calculated in this work (e.g. metal coordination and metal-oxygen distances, see Fig. S1 and S2 †) provide a benchmark for the characterization of atomically dispersed metal sites in SACs supported on CeO 2 NPs. For example, Pt coordination and Pt-O bond length measured by means of extended X-ray absorption fine structure (EXAFS) 54 experiments fully agree with the calculated structure of Pt 2+ on the {100} sites of the ceria NPs. 22,23 Another implication of the present findings for nanocatalysis is related to the ability of some M 1 -ceria SACs to undergo agglomeration and redispersion cycles under certain reaction conditions, forming  Fig. 2 (filled points). Results for 3d, 4d and 5d metals are shown by diamonds, squares, and circles, respectively. metal clusters during the reaction and re-dispersing to the M 1 -ceria state upon termination of the reaction. 71 The estimated stability ΔE = E ad79 − E ad of the M 1 -ceria materials with respect to the agglomeration in metal clusters (Table 1) controls that clusters remain small enough (possibly, subnano) and can readily transform back into single-atom species.

Summary
The interaction of 11 different transition metal atoms M = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Cu, Ag, Au with the representative model Ce 40 O 80 NP has been studied by means of GGA+U density-functional calculations. We have shown that adsorption sites of the ceria {100} nanofacets can very effectively anchor all considered metal atoms in the form of M n+ cations. The oxidation of the M centers takes place with the accompanying reduction of Ce 4+ cations to Ce 3+ . Calculated data indicate that higher oxidation states are favored by transition metals in later periods and in groups more to the left in the periodic table. The deposition of the M atoms leads to a significant structural reconstruction of the oxygen atoms constituting the adsorption site, which coordinate differently to the M n+ cation depending on its oxidation state. Remarkably, the adsorption for every M 1 -ceria system studied is stronger than the binding of the corresponding M atom in a metal NP M 79 . These adsorption energy differences are particularly large for the group X metals (Ni, Pd, and Pt) and for Fe, Co, and Os. This suggests that, especially for these metals, CeO 2 -based materials exposing O 4 sites such as those on {100} nanofacets could provide suitable architectures for preparing singlemetal atom catalysts with very high resistance to sintering. In general, the present calculated results are expected to be helpful in the preparation and atomic-level characterization of efficient single-atom catalysts, the limiting case in nanocatalysis featuring the smallest active species.