Surpassing the single-atom catalytic activity limit through paired Pt-O-Pt ensemble built from isolated Pt1 atoms

Despite the maximized metal dispersion offered by single-atom catalysts, further improvement of intrinsic activity can be hindered by the lack of neighboring metal atoms in these systems. Here we report the use of isolated Pt1 atoms on ceria as “seeds” to develop a Pt-O-Pt ensemble, which is well-represented by a Pt8O14 model cluster that retains 100% metal dispersion. The Pt atom in the ensemble is 100–1000 times more active than their single-atom Pt1/CeO2 parent in catalyzing the low-temperature CO oxidation under oxygen-rich conditions. Rather than the Pt-O-Ce interfacial catalysis, the stable catalytic unit is the Pt-O-Pt site itself without participation of oxygen from the 10–30 nm-size ceria support. Similar Pt-O-Pt sites can be built on various ceria and even alumina, distinguishable by facile activation of oxygen through the paired Pt-O-Pt atoms. Extending this design to other reaction systems is a likely outcome of the findings reported here.

A n ideal supported metal catalyst will simultaneously maximize dispersion of the metal and display optimal intrinsic activity per metal atom. Recently, advanced techniques for synthesizing many heterogeneous catalysts as single atoms have addressed the former issue. Single-atom catalysis dramatically reduces the usage requirements of expensive and rare metals by stabilizing the supported metal atoms in a fully dispersed state as isolated bonded species that serve as active sites [1][2][3][4][5][6] . A general question regarding single-atom metal catalysts, despite the nearly 100% material efficiency of the supported metals, is whether a catalytic center designed in the form of one metal atom substituted or anchored on a support represents the optimal structure to deliver the highest intrinsic catalytic activity. Previous work answered this question for platinum and gold catalysts for the water-gas shift reaction, where the optimal catalytic center is the single-atom Pt 1 (or Au 1 )-O(OH) x species on a variety of catalyst supports [4][5][6] . Nonetheless, for other important reactions such as the low-temperature CO oxidation, these configurations as isolated atomic active sites may lack neighboring metal centers and the reactivities associated with the latter. This fundamental question remains unanswered and industrial needs for more active catalysts await.
The catalytic oxidation of CO to CO 2 involves classic molecular rearrangements with oxygen intermediates that make it an attractive probe reaction in catalytic systems to gain better mechanistic understanding, such as the identity of metal catalytic centers and the importance of metal-support interaction. The low-temperature CO oxidation is also important in the purification of vehicle emissions. To meet latest fuel-efficient engine designs and to reduce vehicle exhaust emissions, platinum group metals (PGMs) dispersed on ceria supports are needed to be much more active in eliminating CO emissions below 150°C during the engine cold start 7 . A group of Pt single-atom catalysts using CeO 2 8,9 , Al 2 O 3 10,11 , and KLTL zeolite 12 supports were developed and probed for CO oxidation. Compared with the conventional Pt nanoparticles where most of the metal atoms are buried inside the particle bulk without catalyzing the surface reaction, these single-atom catalysts certainly facilitate the full utilization of scarce platinum metal [4][5][6]8,9 . However, the properties of the Pt 1 may be suboptimal for certain reactions. Indeed, a closer examination of the activity per Pt atom shows that such Pt 1 catalysts are often similar to (or even worse than 3 ) the conventional Pt nanoparticles and clusters under comparable reaction conditions and catalyst formulations (Supplementary Table 1). In the context of oxygen-rich reaction conditions (oxygen in excess to fully oxidize all the reductants), which reflects the implementation of emerging fuel saving technologies such as lean-burn engines, hybrid powertrains, and dynamic fuel management, the known benefit of creating oxygen vacancies on ceria surfaces to promote CO oxidation under near stoichiometric oxygen concentrations [13][14][15] cannot be sustained because the surface oxygen vacancies associated with Ce(III) heal in seconds 16,17 . Consequently, the natural question arises whether a properly paired multi-atom catalytic site (ensemble of the single-atom M 1 -O x species) will increase the catalytic performance over Pt 1 /CeO 2 under oxygen-rich conditions, and how we can build such a site with an appreciable loading amount on a given support surface. In this work, by extending the concept of isolated atoms to the paired ensembles, we show that the paired Pt-O-Pt catalytic units can achieve higher activity through an oxygen migration mechanism.
Here we report a general approach-reassembling isolated Pt 1 atoms as the precursor to create a one-layer multi-atom oxo site (Pt-O-Pt) while keeping~100% Pt utilization. We use a variety of experimental and computational techniques (grand canonical Monte Carlo (GCMC) simulations combined with density functional theory (DFT) calculations (GCMC-DFT) 18 to elucidate the catalyst active site structure and the CO oxidation reaction mechanism that is responsible for the dramatically enhanced reactivity of this multi-atom oxo site. The Pt-O-Pt ensemble is shown to be the base unit in the high-performance catalyst, where the well-known Pt-CeO 2 metal-support interfacial catalysis no longer ranks as the favorable reaction path for this highly active low-temperature CO oxidation catalyst under oxygen-rich reaction conditions.

Results
Dramatic change of catalytic performance. We began this work by synthesizing a variety of single-atom Pt 1 /CeO 2 to serve as the baseline for catalytic performance. Next, the isolated Pt atoms in the Pt 1 /CeO 2 were used as seeds to generate a much more active Pt site through a facile activation treatment, where a mild H 2 reduction was followed with a CO plus O 2 treatment to trigger the restructuring of the platinum (see optimization of the activation treatment and the stable, high reaction rates in Supplementary Figs. 1-3. The optimized samples are labeled as Pt-O-Pt/ CeO 2 , see Table 1). The potential alternative activity contributors, such as creating persistent oxygen vacancies and additional -OH species on the catalyst surface during the activation treatment, have been excluded (Supplementary Figs. 4 and 5).
Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images show that the single Pt atoms are the majority species in the various Pt 1 /CeO 2 samples prepared in this work (see Fig. 1a and Supplementary Figs. 6-8 for Pt 1 /CeO 2 -a, Pt 1 /CeO 2 -b, and Pt 1 / CeO 2 -c samples), where Pt loadings of 0.27, 0.16, and 0.11 wt.% and ceria supports with diverse amounts of reducible oxygen species were used (Supplementary Figs. 9 and 10). A few pseudoclusters of platinum might be found in these single-atom catalyst samples, where these pseudo-clusters are composed of several nearby single-atom Pt 1 species embedded in the cerium columns. In the activated catalysts, namely the Pt-O-Pt/CeO 2 samples, the Pt atoms are fully transformed into another structure with a narrow size distribution of 1 ± 0.1 nm on the ceria surface (see Fig. 1b Fig. 17) with ceria nanoparticle sizes of 10-30 nm. There are some rounded edges, steps, and kinks on these rather typical industrial CeO 2 support particles, but no clear evidence of these locations as the preferred anchoring sites for the platinum species is found ( Supplementary Fig. 18). Therefore, we selected CeO 2 (111) to model the stable geometry and CO oxidation reaction path for Pt 1 /CeO 2 and Pt-O-Pt/CeO 2 .
Despite the apparently low conversion values on the Pt 1 /CeO 2 catalysts ( Fig.1c and Supplementary Fig. 19), which are due to the high gas flow rate compared to catalyst weight, the absolute (or intrinsic) activities of our single-atom Pt for low-temperature CO oxidation per Pt site (turnover frequencies, TOFs) are within the same order of magnitude as the activities reported recently for other single-atom Pt catalysts [8][9][10][11][12][19][20][21][22][23][24] , particularly when reducible oxide supports such as titania and ceria were used (Supplementary Table 1). Under the same reaction conditions, however, the Pt-O-Pt/CeO 2 catalysts have 2-3 orders of magnitude higher intrinsic activity than their Pt 1 /CeO 2 counterparts from 80 to 150°C. The reaction rates on the Pt 1 /CeO 2 -a catalyst are 1.7 × 10 -9 and 2.2 × 10 -7 mol CO 2 /(g cat ·s) at 80 and 150°C, respectively. In contrast, at the same total platinum loading, the reaction rates on the Pt-O-Pt/CeO 2 -a catalyst are 2.6 × 10 −6 and 2.5 × 10 −5 mol/(g cat ·s) at 80 and 150°C. Kinetic measurements (Fig. 1d) further reveal that the Pt-O-Pt/CeO 2 and Pt 1 /CeO 2 catalysts are also distinguishable from differences between their catalytic centers and reaction mechanisms. Specifically, the CO oxidation reaction catalyzed by the Pt-O-Pt/CeO 2 catalysts has a smaller measured apparent activation energy (E app = 40 ± 2 kJ/mol) compared with the Pt 1 /CeO 2 catalysts (E app = 86 ± 3 kJ/mol). Oxygen atom migration at the Pt-O-Pt ensemble; CeO 2 is not involved in the catalytic cycle HAADF-STEM, high-angle annular dark-field scanning transmission electron microscopy, Exp., experiment measured, The., theory predicted a Based on spectroscopic observations from HAADF-STEM images and the chemical titrations of CO chemisorption b The catalyst activity is similar with many recently reported single-atom Pt1/CeO2 catalysts (see Supplementary Table 1 In agreement with earlier studies on metal nanoparticle catalysts 25,26 , recent reports have confirmed the benefit of H 2 O and its dissociated -OH species for promoting CO oxidation on Pt 1 /CeO 2 catalysts 9,19 . In our case, an activity improvement for both Pt-O-Pt/CeO 2 and Pt 1 /CeO 2 catalysts is observed after adding 3% H 2 O into the CO oxidation feed stream (Supplementary Fig. 20), but the large activity gap between the Pt-O-Pt/CeO 2 and Pt 1 /CeO 2 samples remained. This result proves that the beneficial H 2 O/-OH-rich environment 9,19,25,26 does not diminish the superior activity of the Pt-O-Pt/CeO 2 over its Pt 1 /CeO 2 counterpart. Despite the further complication of reaction routes due to the water-containing experiment, these data demonstrated that the relative activity of the Pt-O-Pt structure compared with Pt 1 /CeO 2 will not be negated even in the presence of water and -OH-enriched environment. The activated Pt-O-Pt/CeO 2 -a sample remains similarly active even after being hydrothermally aged at 750°C for 20 h (Supplementary Fig. 20). Further evidence of the generalizability of this synthetic approach to effectively construct the active Pt-O-Pt catalytic center is shown on two types of commercial platinum-ceria catalysts (Supplementary Figs. 21 and 22).
Atomic-level structural analyses. Our DFT calculations identified the stable Pt structures for the Pt 1 /CeO 2 and Pt-O-Pt/CeO 2 catalysts. We find that the isolated Pt 1 atom prefers to substitute the Ce atom rather than anchor on the CeO 2 (111) surface (the dominant facet on which~70% counts of the experimental Pt 1 and~1 nm Pt-O-Pt species were anchored, Supplementary Fig.  17). This Pt 1 anchoring site was proposed in a prior experimental study 27 . The Pt 1 anchored on top of the CeO 2 (111) is thermodynamically unstable due to a highly unsaturated coordination environment ( Supplementary Fig. 23a). In the identified Pt 1 /CeO 2 structure ( Fig. 2a and Supplementary Figs. 23b and 24), the Pt 1 substitutes the Ce atom on the ceria surface and is surrounded by up to six nearby oxygen atoms. In line with the recent findings from surface science and DFT calculations studies 28,29 , we noticed that four of the oxygen atoms prefer to bind directly to the Pt center to form a square-planar Pt 1 -O 4 structure, which is the starting structure of the Pt 1 /CeO 2 catalysts used here to model the CO oxidation reaction.
Computationally intensive GCMC-DFT simulations were performed to search the structure and composition of Pt 8 /CeO 2 in the presence of oxygen (p O 2 = 0.05 atm) at 350 K, which is consistent with the experimental reaction conditions. The computationally tractable Pt 8 cluster was selected here because the experimental Pt-O-Pt/CeO 2 sample has a similar platinum diameter of~1 nm on the CeO 2 (111) surfaces ( Supplementary  Fig. 25a). When exposed to an oxygen atmosphere as in our reaction tests, the Pt 8 /CeO 2 rearranges into Pt 8 O 14 (~0.92 nm), which contains solely Pt-O-Pt as the catalytic base unit and is only one layer thick ( Fig. 2b and Supplementary Fig. 26b). Our GCMC simulation is similar with the experimental activation procedure in that the reduced Pt x clusters, formed by assembling Pt 1 species under the first step of H 2 treatment, can be oxidized to the Pt-O-Pt ensemble by the introduction of an O 2 -rich environment. The Pt 8 O 14 cluster model identified by the GCMC algorithm is regarded as a representative structure of our experimental system, as in reality some experimental heterogeneity does exist. Nevertheless, as we show below it captures many of the salient features of this rather clean experimental catalytic system, as indicated by the overall agreement of experimental observations and computational predictions.
Since our synthesis of the Pt 1 /CeO 2 and Pt-O-Pt/CeO 2 catalysts yields almost exclusively either single-atom Pt 1 species or the~1 nm Pt-O-Pt structure on the different ceria supports, the representative Pt 8 O 14 structure predicted by GCMC-DFT can be vetted by our experimental analysis. The x-ray absorption near-edge structure (XANES) and the x-ray photoelectron spectroscopy (XPS) results ( . Adsorbed CO can interchange between the bridge site and the top site. This evidence does not necessarily corroborate that these CO adsorption modes must belong to the reaction intermediates of the most energy-favored reaction pathways. Also, although the calculated frequencies of adsorbed CO on Pt 8 O 13 /CeO 2 have similar values compared with the experimental peak centers, we cannot exclude the other possible CO adsorption modes on any heterogeneous PtO x species in the Pt-O-Pt/CeO 2 catalyst. This limitation is also evidenced and discussed from the perspective of experimental DRIFTS results ( Supplementary Fig.  30). We emphasize that the C-O vibrational frequency alone cannot be used to definitively identify the Pt adsorption site 32 , and a combined analysis approach of HAADF-STEM, XPS, XANES, and EXAFS must be used to corroborate the theory predicted representative structure, as these characterization results together set the context of platinum species size, chemical valence, and coordination environment.
Identification of oxygen migration reaction mechanism. To understand how the Pt 1 /CeO 2 and Pt-O-Pt/CeO 2 catalysts catalyze CO oxidation so differently, DFT calculations combined with mean-field microkinetic simulations were conducted to study the CO oxidation mechanism (the partial pressure for CO and O 2 are 0.001 and 0.05 bar, respectively, consistent with our reaction studies). The schemes for the CO oxidation cycles and geometric and energetic information are shown in Fig. 3 Table 4 on the Pt site during the CO oxidation cycle due to stronger adsorption of CO compared with O 2 at the same Pt site (Supplementary Table 5). CO oxidation is predicted to follow the Mars-van Krevelen (MvK) mechanism 33-35 at the square-planar Pt 1 -O 4 unit in Pt 1 /CeO 2 ( Fig. 3a and Supplementary Based on microkinetic simulations, the predicted apparent activation energy (E app ) on Pt 1 /CeO 2 is 78 kJ/mol (Fig. 4), which is in close agreement (within typical DFT errors of ±15 kJ/mol) with the experimentally measured apparent activation energy of 86 ± 3 kJ/mol. Degree of rate control analysis shows that O 2 dissociation is the rate-determining step (RDS) for CO oxidation on Pt 1 /CeO 2 ( Supplementary Figs. 34a, 35a).   Table 4). The microkinetic simulations predict E app for the Pt-O-Pt/CeO 2 catalysts of 54 kJ/ mol, which is close to our experimentally measured result of 40 ± 2 kJ/mol (Fig. 4). The indirect catalytic role of ceria predicted in this proposed catalytic route corroborates our experimental findings. As shown by the H 2 temperature programmed reduction (TPR) results (Fig.  5a) of the reaction-spent catalysts, the more abundant reducible oxygen species from the platinum-ceria interface in the Pt 1 /CeO 2 catalysts did not count towards the superior catalytic performance of the Pt-O-Pt/CeO 2 catalysts. The ceria supports are essentially identical for the Pt 1 and Pt-O-Pt groups of reaction-spent catalysts according to Ce3d and O1s XPS spectra ( Supplementary  Fig. 4). Without the presence of the supported platinum, the [O] reduction in the ceria lattice by H 2 will not take place until above 200°C (Supplementary Fig. 9b). We attribute the major peak in the temperature range of 60-100°C to the immediate [O] depletion at the six nearest oxygen atoms in the Pt 1 -O-Ce unit. The next H 2 consumption peak in the temperature range of 100-160°C is related to the further depletion of the ceria lattice oxygen that can migrate to the Pt 1 -CeO 2 interfaces. In contrast,   (Fig.  5b). This finding shows that the highly active Pt-O-Pt catalytic unit overrides the influence of the different oxygen-supply capabilities from ceria in catalyzing the low-temperature CO oxidation reaction, and the uniformity of the catalytic units created by our activation procedure is remarkable. We also probed the possibility of creating similar Pt-O-Pt sites on an alumina support, in which the parent catalyst is the singleatom Pt 1 /La-Al 2 O 3 with a platinum loading of~0.5 wt.% that we recently reported 11 . Through a similar redox activation protocol that we have applied to the Pt 1 /CeO 2 catalysts, we created an activated Pt/La-Al 2 O 3 catalyst having a portion of its platinum as the active Pt-O-Pt catalytic sites, according to CO-DRIFTS studies (Supplementary Fig. 38) and kinetic measurements of E app (41 ± 2 kJ/mol, Supplementary Fig. 39) and reaction orders (0.3 and 0.1 for CO and O 2 , respectively, Supplementary Fig. 37). This evidence further supports our hypothesis about the indirect role of the ceria particles (10-30 nm) in influencing the intrinsic lowtemperature CO oxidation catalysis of Pt-O-Pt/CeO 2 under oxygen-rich conditions.

Discussion
It is worth noting that the concept of "maximized atom efficiency" is different from "maximized activity per atom." Our results highlight that Pt 1 indeed has maximized its material utilization efficiency, but there is large room to improve the activity per Pt atom. The solution from this work is to tackle the issue of lacking neighboring Pt atoms in the typical Pt 1 /CeO 2 system. By forming the Pt-O-Pt catalytic unit in representative one-layer Pt 8 O 14 cluster, the Pt atoms can now effectively activate and utilize the oxygen intermediates to catalyze the low-temperature CO oxidation. A recent work adjusted the Pt-O coordination number between 2 and 3 in PtO x clusters by either reductive or oxidative treatment at 350°C to modify the catalytic activity of PtO x /CeO 2 nanowire catalysts within one order of magnitude for the CO oxidation under oxygen-rich conditions 24 . Despite that the platinum was not fully exposed in this prior work, as the dispersion ranged from 10 to 83% (Supplementary Table 1), the authors may have created a portion of similar sites as we did in this work (best-performing catalysts from the current work are on average six times more active by incorporating the possible impact of different reactant concentrations). Here, by maintaining 100% platinum dispersion, which means that all the Pt atoms are accountable for surface catalysis and there is a minimal amount of spectator Pt species to distort the averaged characterization results, we found that the coordination number of Pt-O may not be the most decisive factor for the much more dramatic change of the CO oxidation activity, because both our Pt 1 and Pt-O-Pt structures have four oxygen atoms directly bonded to the platinum center at the starting point of each catalytic cycle. More importantly, the synergistic effect of the two, paired, platinum atoms in the Pt-O-Pt ensemble provides an alternative oxygen supply route independent of ceria substrates. This intrinsic catalytic difference between the isolated Pt 1 and the paired Pt-O-Pt structure could likely not be overcome by merely changing the Pt-O coordination numbers and considering each Pt atom as an independent unit. The mechanistic importance of the Pt-O-Pt interaction is highlighted in this work, where most of the attention was on the Pt-O-Ce interaction in previous studies. As shown in Fig. 6, the rate-determining steps of the CO oxidation reaction by Pt 1 /CeO 2 and Pt-O-Pt/CeO 2 catalysts involve different sites and mechanisms for oxygen activation. The CO oxidation reaction proceeds through the MvK mechanism at the Pt 1 -O-Ce interface in the Pt 1 /CeO 2 catalyst, while the reaction is more efficiently catalyzed by the Pt-O-Pt/CeO 2 catalyst at its Pt-O-Pt unit with the bridge -O-participating. The similar feature might be shared with other oxide clusters having high metal dispersion and abundant undercoordinated metal sites.
The findings from this work should only be cautiously extended to a general type of clusters that retain a layered threedimensional structure, where the lower dispersion and more saturated coordination environment of platinum bring more uncertainties. However, we infer that the Pt-O-Pt catalytic unit defined in this work is a prototype that illustrates an important advantage of catalysts with neighboring metal centers for efficient oxygen activation. report a synthesis of a Ir-O-Ir structure on α-Fe 2 O 3 , and the synergistic effects between the two nearby iridium atoms were inferred to explain the 2.6 times higher activity of the dinuclear structure from its single-Ir atom counterpart for solar water oxidation. Jeong et al. 39 reported that the Rh 1 /CeO 2 catalyst has low activity for C 3 H 6 and C 3 H 8 oxidation, but another speculated Rh ensemble catalyst is highly active for C 3 H 6 and C 3 H 8 oxidation. The elusive Rh ensembles cannot be observed in HAADF-STEM images, and the k 3 -weighted EXAFS radial distribution indicates two sets of Rh-O-Rh bonds with coordination numbers near 0.6 (means not even a dinuclear structure-instrumental signal limitations). More recently, Dessal et al. 40 confirmed an enhancement of CO oxidation activity of the Pt/γ-Al 2 O 3 catalysts when the Pt 1 atoms were agglomerated into platinum oxide clusters with a Gaussian distribution of cluster sizes from 0.5 to 1.5 nm. These studies are using different PGM species, supports, PGM domain sizes, and different reactions, but the formation of the paired PGM-O-PGM bond is the common variable behind the improved catalytic performance. We believe this is where the general implication of this material development and mechanistic investigation work resides. One should also avoid oversimplifying this work as any dinuclear Pt-O-Pt structures can be more active than the singleatom Pt species in activating [O] intermediates. Notably, one of the platinum atoms in this highly active Pt-O-Pt catalytic unit does not have any direct -O-linkages to the ceria support, and it is in fact the enabler for facile O migration and fast CO oxidation. Indeed, the lattice oxygen of ceria is critical for the formation of stable Pt-O bonds according to our XAS analyses. However, the ceria is also shown to be largely a spectator species in the lowtemperature CO oxidation reaction catalyzed by the Pt-O-Pt/ CeO 2 systems. We envision that, if the Pt-O-Pt unit can be stabilized through similar oxygen linkages, a similar catalytic species may be created on various support substrates other than ceria. The formation of a Pt-O-Pt structure on activated Pt/La-Al 2 O 3 supports this hypothesis (Supplementary Figs. 37-39), but developing alternative preparation methods to build the exclusive catalytic sites on the alumina support needs further effort that is outside the scope of this work.
The exception from the general assessment of the present work is most likely to happen when quantum-size effects become evident in small ceria nanoclusters (especially <5 nm). Elegant studies showed that the oxygen vacancy formation in CeO 2 nanoclusters exposing small O-terminated (111) and (100) facets is more probable compared to the extended CeO 2 surfaces [41][42][43][44] . Theoretical evidence shows that the oxygen reverse spillover from these small ceria nanoparticles to the supported Pt species can generally be a more favorable process compared with Pt species supported on larger ceria particles, the latter of which is typically represented by slab computational models 9,45,46 . Such an unusually high oxygen mobility from the ceria have been predicted by modeling studies on CeO 2 nanoclusters comprising from about 60 to over 200 atoms, where the largest Ce 140 O 180 has an approximate size of~2.4 nm 42,47,48 . The supporting experimental evidence was provided by carefully growing small Pt/CeO 2 nanoparticles on a CeO 2 (111) film under ultrahigh vacuumthese grown ceria nanoclusters are only about~3 nm in diameter and 0.4 nm in height 29,43 . We emphasize that the these CeO 2 nanoclusters (up to~3-4 nm) with extraordinary capability for generating mobile oxygen species are intrinsically different from the 10-30 nm ceria particles (homemade and commercial) used in this work. For example, small ceria nanoparticles of 3-4 nm were shown to improve the catalytic activity of gold species for CO oxidation by about two magnitudes 49,50 , but this highly active oxygen supply disappears quickly for regular ceria nanoparticles that are larger than 10 nm in automotive applications [51][52][53] . Typical calcination treatment from 300 to 800°C to fully decompose cerium precursors and to form stable ceria structures usually leads to the CeO 2 particle size from 10 to 30 nm 51,52 . These ceria particles have abundant stable CeO 2 (111) facets 54,55 . These differences lead to the observation that our single-atom Pt 1 (IV)-O 4 is mostly observed on the stable CeO 2 (111) surfaces of a 10-30 nm ceria particle, although a sinter-resistant single-atom structure Pt 1 (II)-O 4 sits at the less stable CeO 2 (100) nanofacets of the small ceria nanoclusters (e.g., 1-3 nm) 29,56 . Of course, the small fraction of rounded edges, steps and kinks in our 10-30 nm ceria particles may well have led to the presence of a tiny portion of active sites as Pt-O-Pt plus active ceria substrates. These nonuniformities could be part of the reason for the deviations of kinetics between the computational results and experimental measurements. A success of synthesizing and stabilizing tiny ceria nanoclusters and anchoring an appreciable amount of the targeted platinum structure (e.g., the Pt-O-Pt) onto them bodes the ultimate solution of best using both the platinum and ceria substrate.
However, the bottom line is, when widely available ceria particles with larger size (>10 nm) and mediocre oxygen mobility are being adopted as an industrial catalyst support, activating the single-atom Pt to form the paired Pt-O-Pt ensemble is an effective way to create an alternative oxidative reaction pathway to benefit the low-temperature reactions. These tunable catalytic systems, either at a single-atom form or a Pt-O-Pt structure, may serve as powerful platforms for future studies of many other reactions.

Methods
Catalyst preparation. Platinum was loaded onto the ceria supports by a strong electrostatic adsorption method 57 . H 2 PtCl 6 was chosen as the platinum precursor, because the platinum-ligands complex anions can evenly adsorb on the positively charged -O-Ce-OH 2 + surface sites on ceria as single-atom layers when the pH of the solution is below the point of zero charge of ceria. To begin, the pH value of the H 2 PtCl 6 solution was adjusted to pH ≈ 9 by ammonia. The transparent solution was stirred at 70°C overnight to allow the substitution of -Cl in [PtCl 6 ] 2− by -OH in solution. The as-prepared ceria powder was then added into the solution to adsorb the preformed [Pt(OH) 6 ] 2− as a single-atom Pt layer. The concentrations of the platinum precursor and the amount of ceria were varied to keep the same solid-liquid contact interface of 500 m 2 /L. We washed the filtration cake (filtered catalyst) with a total of 2 L distilled water at 80°C during each sample filtration. The obtained samples were dried at 100°C overnight, then calcined in air at 500°C for 3 h, followed by H 2 reduction at 250°C for 0.5 h to further remove any possible residual -Cl, and finally calcined in air at 500°C for 1 h. These Pt 1 /CeO 2 samples are hereafter referred to as the "as-prepared" catalysts, and designated as "Pt 1 / CeO 2 -a, Pt 1 /CeO 2 -b, and Pt 1 /CeO 2 -c". None of the Pt-containing components were detected as crystallized structures (11)(12)(13)(14)(15)(16), and limited changes happened to the BET surface areas, that is, they changed to 74, 60, and 44 m 2 /g from 80, 64, and 51 m 2 /g, respectively. Our Pt 1 /CeO 2 samples were calcined at 500°C, so the bulk diffusion of the Pt into CeO 2 that usually occurs above 700°C is limited 58,59 . The nearly 100% Pt dispersion (measured at room temperature by a CO chemisorption method that passivates the ceria support in directly contributing to CO adsorption 39,60,61 ) (Fig. 5a). During the optimized H 2 reduction phase, we expect the single-atom Pt 1 /CeO 2 catalysts to generate abundant undercoordinated Pt atoms as nanorafts under the rather mild reduction condition and relatively short reduction time 30,31,62 . During the phase of reoxidation using a mixture of CO plus O 2 , both CO and O 2 can induce a Pt restructuring depending on their respective pressure [63][64][65][66] . In general, O 2 molecules tend to coordinate with Pt to form nano islands of multilayered α-PtO 2 -like oxides 67 . This is an unwanted outcome for the scope of this work, because the formation of the multi-layer spherical platinum oxide structures may result in the creation of Pt atoms with nonuniform chemical environment depending on their relative location in the platinum particle and from the ceria support. Characterizing such a mixed batch of catalytic species will generate average quantities and even distorted results, which will mask the characteristics of the active species 4,6 . Some Pt atoms may also be buried in the particle bulk, losing their ability to catalyze surface reactions. To prevent the formation of bulk particles, CO molecules were added to attach to Pt surfaces as ligands to cause CO-CO repulsion between nearby Pt sites 65 . As shown in Supplementary Fig. 2, having a trace amount of CO in the diluted oxygen feed stream is indeed helpful to activate the catalysts. These treatment steps lead to the formation of the Pt-O-Pt structure on ceria according to our characterization studies. Therefore, these activated samples are denoted as "Pt-O-Pt/CeO 2 ." We excluded the impact of chloride on the change of catalytic activities, as all our Pt 1 / CeO 2 and Pt-O-Pt/CeO 2 catalysts show a minimal and similar chloride concentration of 70-90 ppm according to ion chromatography analysis. The Ptrelated catalytic sites were characterized by STEM, CO chemisorption, XPS, XAS, and H 2 TPR after exposure to reaction conditions as the working catalysts.
CO oxidation tests and kinetics. The CO oxidation reaction was conducted in a packed-bed tubular reactor. A 25 mg powder sample was diluted with 200 mg quartz sand in the catalyst bed. The catalysts were tested in an O 2 -rich gas atmosphere to reflect the lean-burn gasoline and diesel engine conditions. The test procedures were as follows: first, we ramped up the reactor temperature to 500°C in 20% O 2 balanced with N 2 at a heating rate of 10°C/min, and held for 30 min. Next, we cooled down the reactor to near-ambient temperature with an N 2 purge until the temperature of the catalyst bed was stable. After the CO oxidation reaction feed stream ([CO] = 1000 ppm, [O 2 ] = 5%, balanced with N 2 at a flow rate of 1000 mL/min) was switched in. The steady-state and light-off conversion rates were measured at elevated temperatures after the baseline readings became stable at near-ambient temperature. To activate the as-prepared sample, a treatment including a reduction at 200°C in 5% H 2 for 15 min and a subsequent exposure to the CO plus O 2 atmosphere at ambient temperature was used before the reaction. Wet CO oxidation followed the same test procedure with a feed stream containing 3% H 2 O ([CO] = 1000 ppm, [O 2 ] = 5%, [H 2 O] = 3%, balanced with N 2 at a flow rate of 1000 mL/min). The concentrations of CO and CO 2 were monitored by an MKS 2030 gas cell Fourier-transform infrared spectroscopy, and the O 2 concentration was measured by mass spectroscopy (Hiden HPR20). Kinetic measurements were carried out on the same equipment setups. A typical flow rate ranging from 1200 to 1500 mL/min was used for 10 to 20 mg of catalyst in each test to ensure the catalytic reaction was free of heat and mass transfer effects. The CO conversion was therefore kept below 20% for all the reaction rate measurements.
DFT modeling approach. Spin-polarized DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) 68,69 with the projector augmented-wave method to treat electron-ion interactions 70 . The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional 71 was used as the density functional approximation for all genetic algorithm (GA) and GCMC calculations. The revised Perdew-Burke-Ernzerhof (RPBE) exchange-correlation functional 72 was used to calculate the CO oxidation catalytic cycle because of its superior ability to predict accurate adsorption energies over PBE as compared with the experiment 72 . The strongly correlated 4f electron of cerium was treated with the DFT + U correction, using a U eff = 5 eV for both PBE and RPBE calculations 73 . Brillouin zone sampling was restricted to the Γ point for all DFT calculations. To avoid artificial self-interactions between slabs due to periodic boundary conditions, the surface slabs were separated by a vacuum layer of 15 Å. A GA was used to find the global minimum structure of a Pt 8 cluster supported on ceria (Pt 8 /CeO 2 ). The most stable Pt 8 /CeO 2 structure is used as the initial configuration for the GCMC simulations. For the GA and GCMC simulations, a p(4 × 4) CeO 2 (111) surface with one O-Ce-O layer was used as the support model without geometry relaxation to improve the calculation efficiency. We did not include the presence of persistent oxygen vacancies on CeO 2 (111) surfaces during our CO oxidation mechanistic studies, as their rapid healing has been demonstrated by both theory and experiments [16][17][18] , especially under oxygen-rich reaction conditions 17 . The plane-wave cutoff energy was set to 300 eV and the convergence threshold for geometry optimizations was specified to 10 -3 eV for both GA and GCMC calculations.
A p(4 × 4) CeO 2 (111) surface with two O-Ce-O layers was used as the support for studying the CO oxidation catalytic cycle on both the Pt 1 /CeO 2 (111) and Pt 8 O 14 /CeO 2 (111) systems. To analyze the impact of the exposed ceria facet on CO oxidation, the representative Pt 8 O 14 structure searched by GCMC 18 simulations on CeO 2 (111) was deposited on two O-Ce-O-layered (110) and (100) surfaces with each Pt atom binding four oxygen atoms according to our XPS and EXAFS data. Experimental observations 74,75 suggest that the (100) surface is terminated by 0.5 monolayer of oxygen. To obtain a consistent model, we constructed the oxygen terminated (100) surface by removing half of the oxygen atoms on the top and bottom surfaces (Supplementary Fig. 36). The top O-Ce-O layer, adsorbates, Pt single atom, and Pt 8 O 14 cluster could relax during geometry optimization and transition state searches. A plane-wave basis with a cutoff energy of 400 eV was chosen. The climbing-image nudged elastic band (CI-NEB) method 76,77 was used to find the transition states involved in the CO oxidation mechanism. The CI-NEB force tolerance was set to 0.05 eV/Å. CO vibrational frequency was calculated within the harmonic approximation. The calculated vibrational frequency of gaseous CO by DFT-RPBE was 2105 cm −1 , which is 65 cm −1 smaller than the true value measured in our experiment (2170 cm −1 ). Thus, we applied a 65 cm −1 rigid shift to all the calculated CO vibrational frequencies on Pt 1 /CeO 2 and Pt 8 O 14 /CeO 2 catalysts to compare with our experimental DRIFTS spectra. The zero-point energy correction was not considered. Details on GA and GCMC simulations as well as the mean-field microkinetic simulations approach are presented in the Supplementary Information.