Unique structure of active platinum-bismuth site for oxidation of carbon monoxide

As the technology development, the future advanced combustion engines must be designed to perform at a low temperature. Thus, it is a great challenge to synthesize high active and stable catalysts to resolve exhaust below 100 °C. Here, we report that bismuth as a dopant is added to form platinum-bismuth cluster on silica for CO oxidation. The highly reducible oxygen species provided by surface metal-oxide (M-O) interface could be activated by CO at low temperature (~50 °C) with a high CO2 production rate of 487 μmolCO2·gPt−1·s−1 at 110 °C. Experiment data combined with density functional calculation (DFT) results demonstrate that Pt cluster with surface Pt−O−Bi structure is the active site for CO oxidation via providing moderate CO adsorption and activating CO molecules with electron transformation between platinum atom and carbon monoxide. These findings provide a unique and general approach towards design of potential excellent performance catalysts for redox reaction.

The paper of Nan et al. presents the synthesis of a novel highly active CO oxidation catalyst based on PtxBiyOz clusters. The paper is very comprehensive, rich of profound characterization of the complex atomic structure of the catalyst and convincing in the interpretation of the data. In a similar report recently published (Meunier F.C. et al. Angew. Chem. Int. Ed. 2020, 59,2-9) was already discussed the positive effect of oxidized Pt species and clearly showed the reaction mechanism of CO oxidation over these sites. Even though the presented paper shows very interesting results it does not reach unfortunately the high novelty standards of this journal, which need to fit to a broad audience. Therefore, I cannot recommend the publication on Nature Communications and I encourage the authors to submit their manuscript to a more specific journal (e.g. ACS Catalysis). Also, I suggest to improve the introductory part highlighting the major novelty of their work. So far, this does not come through.
Reviewer #2 (Remarks to the Author): Manuscript number:  In the manuscript by B. Nan et al., the experimental and theoretical results on the catalytic performances of platinum-bismuth cluster on silica under CO oxidation are presented. It was suggested that Pt surface with Pt−O−Bi structure is the ac ve site for CO oxida on via providing moderate CO adsorption and activating CO molecules with electron transformation between the platinum atom and carbon monoxide. I find that this is quite an intriguing result that shows the synergy between Pt and Bi, which can be useful for the design of novel bimetal catalysts. I suggest that the paper be accepted after major revision. Below are my detailed comments and questions that need to be addressed.  Figure 5a shows the presence of BiOx. In addition, XPS data shown in Figure 4i shows the moderate oxidation of Bi. Therefore, it can be assumed that Pt coexists with BiOx clusters, and the metal-oxide interfaces could be reactive species. DFT calculation shows that Pt−O−Bi structure is the ac ve site for CO oxida on. However, I don't see clear evidence of the formation of Pt−O−Bi structure. 4. Figure 1 shows high-resolution TEM images and EDS mapping of elements of PtBi nanoclusters. I am wondering if PtBi is core-shell-like structure because of the surface segregation of one element. After the reaction, how does EDS elemental mapping change?
Reviewer #3 (Remarks to the Author): The manuscript by Si and coworkers present the improved CO oxidation activity of Pt-Bi catalysts supported on SiO2. The authors claim that their atomically designed Pt-O-Bi species facilitates CO oxidation, recording the high activity of 487 molCO2•gPt-1•s-1 at 110 °C. Generating additional catalytic pathway in Pt particles supported on irreducible oxides is essential in designing active/efficient catalysts with inherently high structural robustness. It is well-known that the interfaces between Pt and reducible oxide supporting materials facilitate CO oxidation through the Mars-van Krevelen mechanism. On the other hand, the surface of Pt nanoparticles supported on irreducible oxide supports oxidizes CO through the Langmuir-Hinshelwood mechanism, which CO and O2 should be coadsorbed on Pt and O2 activation should be followed for subsequent oxidation of adjacent Pt-CO*, making this process quite sensitive to the environmental factors such as the CO/O2 concentration ratio in the reaction feedstock and the size of Pt particles, which is correlated with their surface area/volume ratio.
This manuscript presents an interesting results on the promoting role of Bi dopants in two folds: stabilization of small Pt species and catalytic activation of the Pt surfaces. Although several aspects claimed by the authors should be supported by additional data or discussion, my first impression on this manuscript is quite positive, as the main message is clear and is supported by combinatorial spectroscopic analysis results, which are nicely integrated with other experimental findings. The revised version of the manuscript may qualify the publication criteria of Nat. Commun.
Comments: 1. Introduction should be revised to be more informative and persuasive. Overall, introduction of the current manuscript is not persuasive and lack of important information, making the audience difficult to understand the urgency and the scientific value of this work.
Several suggestions: 1-1) Deduce and present the motivation that drove the authors to design Pt-based catalysts supported on SiO2 for CO oxidation by discussing more precisely the advantages/disadvantages of using irreducible oxides as a supporting material for Pt. It is well known that Pt nanoparticles supported on such irreducible oxides catalyze CO oxidation through the Langmuir-Hinshelwood mechanism, rather than the interface-mediated Mars-van Krevelen mechanism. dispersed alloy like clusters of Pt and Bi, largely attributed to the disintegrating and stabilizing power of Bi. However, the subsequent structural analysis results showed that Bi-stabilized Pt was aggregated into the large particles (also suggested by DRIFTS spectra) and the residual Bi formed a sort of BiOx on the surface of Pt nanoparticles. If this is the case, I like to see high-resolution STEM images demonstrating the presence of the Bi species on the surface of Pt nanoparticles.

Response to reviewer's comments
To Reviewer 1 The paper of Nan et al. presents the synthesis of a novel highly active CO oxidation catalyst based on PtxBiyOz clusters. The paper is very comprehensive, rich of profound characterization of the complex atomic structure of the catalyst and convincing in the interpretation of the data.
In a similar report recently published (Meunier F.C. et al. Angew. Chem. Int. Ed. 2020, 59,2-9)  (1) Formation of uniform PtxBiyOz binary metal oxide clusters to improve the thermal resistance to sintering of platinum on irreducible support (SiO2). The Bi-promoted Pt/SiO2 catalysts with low content of platinum (0.8 wt.%) were prepared by incipient-wetness impregnation method and it exhibits excellent thermo-stability due to the formation of PtxBiyOz binary metal oxide cluster, as confirmed by the results of aberration-corrected HAADF-STEM, XAFS and in situ DRIFTS experiments. This binary oxide clusters could be maintained in small size (~2 nm) on the surface of inert support (SiO2) after calcination at 550 °C in air. Meanwhile, our DFT results also confirm the doping of Bi element could stabilize Pt atom and prevent the formation of huge Pt particles. As we all known, the inert oxides (such as SiO2 and Al2O3) show poor ability to stabilize noble metal at subnanometer scale and frequent aggregation into huge particles due to weak interaction between metal and support. Other researcher has prepared silica-platinum catalysts with core-shell structure to prevent the aggregation of active metal, which always tend to hinder the contact of active site with reaction gas resulting in low catalytic activity (Kim et al. ChemCatChem 2019, 11, 4653-4659 ). We believe this strategy of synthesis could provide new guideline for preparation of small-size bimetal catalysts on inert support.
(2) Our silica-supported platinum-bismuth catalysts catalyze CO oxidation efficiently with CO2 production rate of 487 μmolCO2·gPt −1 ·s −1 at 110 °C and low apparent activation energy (~52 kJ/mol), which is much higher than that of Pt/CeO2 (103 μmolCO2·gPt −1 ·s −1 at 130 °C, Nie et al. Science 358 (6369), 1419-1423). Thus, our Bi-promoted silica-supported platinum catalyst is a promising and high effective catalyst for CO oxidation. In another hand, the Ea value (~52 kJ/mol) of our PtBi-SiO2 catalysts is much lower than that (~98 kJ/mol) of conventional silicasupported platinum catalyst.   Figure   6b indicate that the Pt- [O]x-Bi structure could provide abundant surface-active oxygen species, which could react with CO molecule at ~50 °C through Mars-van Krevelen (MvK) mechanism rather than the conventional Langmuir-Hinshelwood mechanism for inert-supported metal catalysts.
The Pt-CeOx-TiO2 interface reported by Yoo et al. (Energy Environ. Sci., 2020 just provide the surface oxygen to react with CO molecule after 130 °C. In addition, we found an enormous difference in CO adsorption on active site. The in-situ DRIFTS results in Fig. 7 demonstrate that the formation of Pt- [O]x-Bi structure could effectively prevent the excessive adsorption of CO on platinum site and there is a red-shift in CO adsorption frequency (~20 cm -1 ) for 1Pt2Bi-SiO2 due to more electron transporting from Pt atom in Pt- [O]x-Bi structure to CO molecule to activate carbon monoxide as also confirmed by DFT results.
In conclusion, in our work, we provided a new strategy of synthesis for anti-sintered catalysts and identify the local coordination structure of PtxBiyOz binary metal oxide cluster. Furthermore, we discovered this PtBi-SiO2 catalyst showing excellent catalytic activity in CO oxidation and determined a new active site: metallic platinum clusters with surface Pt- [O]x-Bi structure. It is significant to identify this Pt-[O]x-Bi structure because this structure could provide superior active oxygen, which may be suitable for many oxidation reactions catalyzed by platinum or bismuth catalysts, such as oxidation of 1,6-hexanediol, ethanol oxidation and methane oxidation.
According to the reviewer's comment, we have made the careful modification about the introduction part to be more informative and persuasive (Page 3-4, Line 40-80).
The corrected description for introduction part in the revised manuscript: The CO oxidation reaction (CO + 1/2 O2 = CO2) is a well-known model reaction in heterogeneous catalysis, as well as a key step to resolve automobile exhaust containing CO, NO and hydrocarbons 1-5 . According to the previous reports, the platinum (Pt)-based catalysts exhibit excellent catalytic activity in CO oxidation. In one hand, these high-performance catalysts always require reducible oxides as supports, such as CeO2 2 , FeOx 3 , MnO2 4 and Co3O4 4 , due to their rich surface oxygen vacancy and the so-called "strong metal-support interaction" 6-8 . In another hand, the irreducible oxide (SiO2 and Al2O3)-supported platinum catalysts with the advantages of commercial production, low cost and extensive application frequently show poor catalytic activity in CO oxidation especially in low temperature (<150 °C) because of lack of surface activated oxygen and suitable active site 9,10 .
Meanwhile, SiO2 shows inferior ability to stabilize active platinum species in small size (< 2 nm) consequently resulting in the deactivation of silica-supported platinum catalysts. Therefore, it is a big challenge to prepare a kind of irreducible oxide-supported platinum catalyst with both excellent catalytic performance and thermo-stability to meet the demand of future exhaust-treatment system.
Many research groups have found that the addition of a secondary element, such as Sn 5 , K 11 , Co 12 and Bi 13,14 , distinctly improved the activity of silica-or alumina-supported platinum catalysts, no matter in the form of oxide clusters or metallic alloy for Pt. As for bismuth element, it has been widely used as the secondary element to improve the catalytic activity in various oxidative reactions 13,14 , due to providing high content of mobile oxygen 13 , preventing the overoxidation of noble metal 15 and suppressing adsorption of poisoning species 16 . Mondelli and co-workers has also confirmed that addition of Bi actually maintain Pt in a metallic state with the help of in-situ X-ray absorption spectroscopy (XAS) 15 . Meanwhile, Ding et al also reported metallic Pt nanoparticles show activity for CO oxidation 17 . However, the precise determination of active site (alloy or oxide solid solution?) and reaction mechanism (Langmuir-Hinshelwood or Mars-van Krevelen mechanism?) of Bi-promoted platinum catalysts are still in huge arguments 14,18 . So, it is significant to prepare silica-supported platinum-bismuth catalysts to realize the high efficiency catalysis of CO oxidation and employ the comprehensive characterization methods to investigate the precise local structure of active site.
Moreover, many researchers have identified that various interfaces in platinum-based catalysts play a key role in in many industrially important reactions, such as metal−support 3,19,20 , metal−oxide 21-23 and metal−metal hydroxide 24 . Chen et al. reported iron nickel hydroxide-platinum nanoparticles (Pt-OH-Fe/Ni) were highly efficient for CO oxidation owing to abundant sites of Pt-OH-M interfaces 24 . According to previous reports 25,26 , it is easy to build multifarious atomic interface between metal and reducible oxide to improve catalytic performance. However, it is extremely difficult to structure effective metal-support or metal-oxide interface due to the infertile oxygen, poor reducibility and over-stable surface composition of irreducible oxide (SiO2 and Al2O3). Therefore, it is great research interests to build stable and efficient interfaces on inert support to catalyze all kinds of heterogeneous reactions.

Authors need to improve the quality of figures to improve the readability of data.
Author reply: we thank the reviewer's suggestion. In the revised, we have replaced all figures with higher resolution pictures. Chem. Int. Ed. 2020, 59, 21736-21744) or in-situ XAFS (Hao et al. Nat. Catal. 2019, 2, 448-456;Guo et al. Topics in Catal. 2009, 52, 1517-1524 experiment to study the variation of valance state and active site structure in different reaction. We have tried to carried out the in-situ XAFS experiment to detect the local structure of Pt species. However, we only acquired a bad signal-noise ratio XAFS spectrum due to the low content of platinum (0.8 wt. %) (Phys. Chem. Chem. Phys. 2013, 15, 18827-18834) and using powder sample. In another hand, we can press powder sample into pellet to improve the signal-noise ratio of in-situ XAFS, but only a few of Pt species on the surface of pellet can be reduced in hydrogen or participate in CO oxidation. It is difficult to acquire the real and reliable local structure of whole platinum species with in-situ XAFS experiment in this work. On the contrary, for quasi in-situ XAFS experiments, we can pretreat catalysts in powder state to acquire real active structure under different gas condition in a stainless reactor with two globe valves and then press powder into solid pellet to guarantee the quality of XAFS signal. Therefore, the quasi in-situ XAFS both can monitor the real active structure and guarantee the signal-noise ratio of XAFS spectrum. Before quasi in situ XAFS experiment, the sample was pretreated in a stainless reactor with two globe valves and tabletting in glove box under nitrogen atmosphere (see   According to the catalytic performance in CO oxidation with hydrogen reduction pretreatment, an obvious structural evolution occurred in Bi-promoted catalysts. Recently, more and more reports indicate that in-situ or quasi in-situ techniques cloud seize the evolution of active site and the synergistic effect on bimetal catalysts 33-36 . The quasi in situ XAFS experiment could acquire a good signal-noise ratio of XAFS spectrum for low content of metal and retain the real structure of active site under different condition. In order to elucidate the real active site structure in reductive and CO oxidation atmosphere, XAFS spectrums of 1Pt-SiO2 and 1Pt2Bi-SiO2 were collected after hydrogen reduction at different temperature (150 and 210 °C) and 1 h of time-on-stream in CO oxidation (100 °C) in a stainless reactor with two globe valves and tabletting in glove box under nitrogen atmosphere and ambient temperature for further XAFS experiments without exposure to air.

Recent studies suggest that the synergistic catalytic effect on bimetal catalysts is
Combination with the catalytic performance in Fig. 3a and previous reported 29,37 , the lower oxidized state of Pt species is appropriate for lower temperature CO oxidation. XANES data in Fig.   5a,c indicated that the valance state of platinum species is decreasing (+1.2 to +0.2 and +2.5 to +0.4 for 1Pt-SiO2 and 1Pt2Bi-SiO2 respective) as the increasing of hydrogen reduction temperature (150 to 210 °C) (Supplementary Table 5). The corresponding XANES profiles during CO oxidation following the hydrogen reduction at 210 °C were collected in Fig. 5a,c, the average valance of platinum slightly increases from +0.2 to +1.0 and +0.4 to +1.3 for 1Pt-SiO2 and 1Pt2Bi-SiO2 respective compared with that in reduced state at 210 °C. It demonstrated that the oxygen-rich reaction gas could make platinum species slight oxidative. Meanwhile, the similar valance of platinum species about +1 indicates that the difference in CO oxidation activity for 1Pt-SiO2 and 1Pt2Bi-SiO2 is not mainly due to platinum valance. Furthermore, the relevant EXAFS profiles were exhibited in Fig. 5c,d. For 1Pt-SiO2, after hydrogen reduction, a main metallic Pt−Pt shell (R ≈ 2.75 Å, CN ≈ 7.0-9.2) was acquired and only a minor Pt−O shell (R ≈ 2.00 Å, CN ≈ 0.6) can be fitted at lower reduction temperature (150 °C), which may result in low activity due to no surface-active oxygen to participate in CO oxidation 38 . For 1Pt2Bi-SiO2, in order to require more reliable local coordination structure, we conducted the EXAFS fitting process with or without PtOBi shell in Supplementary Fig. 13. Obviously, PtOBi shell is an essential composition to acquire the most reasonable fitting results. In Fig. 5d and Supplementary  Fig. 4a is due to incomplete evolution of active site at 150 °C.
Furthermore, there is similar coordination structure for 1Pt2Bi-SiO2-210 °C and 1Pt2Bi-SiO2-CO oxidation, indicating the reduction temperature at 210 °C is appropriate for construction of optimal active sites. The EXAFS fitting results for Pt−Pt shell with CN ≈ 7.4 also confirmed the average grain size of platinum cluster was ~2 nm for 1Pt2Bi-SiO2 39 as observed in HAADF-STEM images ( Fig. 4a-c). In addition, we found that the Pt−O (R ≈ 2.00 Å, CN ≈ 1.5) of 1Pt2Bi-SiO2-CO oxidation is higher than that (R ≈ 1.98 Å, CN ≈ 0.6) of 1Pt2Bi-SiO2-210 °C, may due to the formation of more  Figure 5a shows the presence of BiOx. In addition, XPS data shown in Figure 4i shows the moderate oxidation of Bi.
Therefore, it can be assumed that Pt coexists with BiOx clusters, and the metal-oxide interfaces during the data-analysis process. Therefore, we determined that the oxide clusters in Bi-promoted samples are PtxBiyOz clusters. In addition, there is another reduction peak in Fig. 6a (in the revised) for fresh 1Pt2Bi-SiO2 centered at 350 °C, which can be attributed to isolated BiOx cluster or Bi 3+ single atoms on the surface of silica due to excessive dopant of bismuth. This part of bismuth species has no interaction with platinum species and this reduction peak is similar to results of Bi2O3 in ref.

13.
After CO oxidation, the low oxidation state Pt δ+ -O-Bi structure (0 < δ < +2) is formed due to hydrogen reduction. As shown in Fig. 6a (in the revised), there is a weak reduction peak at ~250 °C, which is attributed to the BiOx clusters adjoining to platinum cluster. However, the oxygen provided by this BiOx cluster only can be activated at high temperature (> 200 °C), which is quite inconsistent with the low temperature CO oxidation activity. Meanwhile, the EXAFS fitting results of used 1Pt2Bi-SiO2 in Fig. 5d (in the revised) confirm the existence of Pt-O-Bi structure with R = 2.99±0.02, CN = 2.2±0.8. Therefore, the surface Pt- [O]x-Bi interface is the main active site for CO oxidation rather the BiOx cluster around platinum cluster. We have corrected the corresponding description to explain the reduction peak at 250 and 350 °C in Page 18, Line 334-339 in the revised version: "In addition, there are two reduction peaks at 250 and 350 °C , which are attributed to the high dispersion BiOx cluster adjoining to platinum cluster and isolated BiOx deposited on the surface of SiO2.
However, the oxygen provided by these BiOx clusters only can be activated at high temperature (> 200 °C), which makes few contributions to low temperature CO oxidation activity."

Figure 1 shows high-resolution TEM images and EDS mapping of elements of PtBi nanoclusters.
I am wondering if PtBi is core-shell-like structure because of the surface segregation of one element.

After the reaction, how does EDS elemental mapping change?
Author reply: We thank the reviewer's comment. We have carried out the HAADF-STEM images and the corresponding EDS mapping of single oxide clusters for fresh 1Pt2Bi-SiO2. These experimental findings demonstrate the uniform dispersion of platinum and bismuth element rather than core-shell structure (Fig. 1c). After the CO oxidation, it is also found that the platinum and bismuth elements are distributed uniformly, according to the STEM-EDS mapping results for individual cluster in Fig. 4d. The elemental mapping in the previous manuscript is not clear because of the demagnification of the image. In the revised manuscript, the enlarged mapping images are used to display clearly the elemental distribution (Fig. 4d). Due to the amorphous nature of PtBi nanocluster as well as similar atomic number (Z) of Pt and Bi, it is almost not able to identify the core-shell structure in TEM/STEM images. However, the EXAFS fitting results (Fig. 5d) Fig. 4d and corrected the corresponding description in Page 7, Line 120-122: "When the EDS mapping was conducted for the individual cluster, no obvious core-shell structure can be observed ( Supplementary Fig. 3c)." and Page 13, Line 231-233: "Meanwhile, the related STEM-EDS mapping results of cluster indicated that the Pt and Bi elements still distribute together, not core-shell structure at the same area ( Fig. 4d and Supplementary Fig. 11)." 1-1) Deduce and present the motivation that drove the authors to design Pt-based catalysts supported on SiO2 for CO oxidation by discussing more precisely the advantages/disadvantages of using irreducible oxides as a supporting material for Pt. It is well known that Pt nanoparticles supported on such irreducible oxides catalyze CO oxidation through the Langmuir-Hinshelwood mechanism, rather than the interface-mediated Mars-van Krevelen mechanism. Nat. Catal. 2, 955-970 (2019) andACS Catal. 8, 7368-7387 (2018) and references therein. The added description in the revised manuscript for introduction part: The CO oxidation reaction (CO + 1/2 O2 = CO2) is a well-known model reaction in heterogeneous catalysis, as well as a key step to resolve automobile exhaust containing CO, NO and hydrocarbons 1-5 . According to the previous reports, the platinum (Pt)-based catalysts exhibit excellent catalytic activity in CO oxidation. In one hand, these high-performance catalysts always require reducible oxides as supports, such as CeO2 2 , FeOx 3 , MnO2 4 and Co3O4 4 , due to their rich surface oxygen vacancy and the so-called "strong metal-support interaction" 6-8 . In another hand, the irreducible oxide (SiO2 and Al2O3)-supported platinum catalysts with the advantages of commercial production, low cost and extensive application frequently show poor catalytic activity in CO oxidation especially in low temperature (<150 °C) because of lack of surface activated oxygen and suitable active site 9,10 .

1-3) In the 3rd paragraph, provide in-depth discussion on the relevant recent studies on the catalytic role of various interfaces. Refer to
Meanwhile, SiO2 shows inferior ability to stabilize active platinum species in small size (< 2 nm) consequently resulting in the deactivation of silica-supported platinum catalysts. Therefore, it is a big challenge to prepare a kind of irreducible oxide-supported platinum catalyst with both excellent catalytic performance and thermo-stability to meet the demand of future exhaust-treatment system.
Many research groups have found that the addition of a secondary element, such as Sn 5 , K 11 , Co 12 and Bi 13,14 , distinctly improved the activity of silica-or alumina-supported platinum catalysts, no matter in the form of oxide clusters or metallic alloy for Pt. As for bismuth element, it has been widely used as the secondary element to improve the catalytic activity in various oxidative reactions 13,14 , due to providing high content of mobile oxygen 13 , preventing the overoxidation of noble metal 15 and suppressing adsorption of poisoning species 16 . Mondelli and co-workers has also confirmed that addition of Bi actually maintain Pt in a metallic state with the help of in-situ X-ray absorption spectroscopy (XAS) 15 . Meanwhile, Ding et al also reported metallic Pt nanoparticles show activity for CO oxidation 17 . However, the precise determination of active site (alloy or oxide solid solution?) and reaction mechanism (Langmuir-Hinshelwood or Mars-van Krevelen mechanism?) of Bi-promoted platinum catalysts are still in huge arguments 14,18 . So, it is significant to prepare silica-supported platinum-bismuth catalysts to realize the high efficiency catalysis of CO oxidation and employ the comprehensive characterization methods to investigate the precise local structure of active site.
Moreover, many researchers have identified that various interfaces in platinum-based catalysts play a key role in in many industrially important reactions, such as metal−support 3,19,20 , metal−oxide 21-23 and metal−metal hydroxide 24 . Chen et al. reported iron nickel hydroxide-platinum nanoparticles (Pt-OH-Fe/Ni) were highly efficient for CO oxidation owing to abundant sites of Pt-OH-M interfaces 24 . According to previous reports 25,26 , it is easy to build multifarious atomic interface between metal and reducible oxide to improve catalytic performance. However, it is extremely difficult to structure effective metal-support or metal-oxide interface due to the infertile oxygen, poor reducibility and stable surface composition of irreducible oxide (SiO2 and Al2O3). Therefore, it is great research interests to build stable and efficient interfaces on inert support to catalyze all kinds of heterogeneous reactions.
2. I see that large Pt particles were developed during calcination (also evidence by the XRD spectrum) in 1Pt-SiO2. Presumably, the most Pt-like XANES profile of 1Pt-SiO2 compared with the others containing Bi suggests that Pt particles were formed in 1Pt-SiO2. Based on their EXAFS fitting profile result, the authors stated that platinum oxide clusters (line 108, on page 6) were formed together with large Pt particles in 1Pt-SiO2. However, I note that the XAS spectra of small Pt species (clusters) or Pt single atoms, which make close and strong contact between Pt and the oxygen ion of the supporting oxide, could be largely shifted toward that of PtO2. Therefore, it is cautious to state that Pt oxide clusters (rather than single atoms or small sub-nanometer sized Pt clusters) were formed on SiO2.
Author reply: We thank the reviewer's comment. According to previous reports, the platinum valance state is determined by the intensity of white peak (Langmuir 2001, 17, 3047-3050) rather than the shift of adsorption edge in XANES profiles. The average valance state of platinum is +1.8 (supplementary Table 2), according to the linear combination fitting of XANES profiles, about 45% in Pt 0 and 55% in Pt 4+ (Fig. R3). Furthermore, our previous report (J. Phys. Chem. C 2017, 121, 25805−25817) have identified that the fraction of different Pt species could be calculated by coordination number (as shown in Table R1) about 50% in cluster and 50% in particle based on the CN of metallic PtPt and PtOPt shell. Therefore, combination of ratio of cluster (50%) and Pt 4+ (55%), we can confirm that the small size cluster is almost platinum oxide cluster rather than Pt 0 cluster. Besides, as a reference, we also prepared a sample called 1Pt-SiO2-400, which was calcinated at 400 °C under air without huge Pt particle and the same synthetic method with 1Pt-SiO2. For 1Pt-SiO2-400, the XRD ( Supplementary Fig. 7c ), TEM ( Supplementary Fig. 7a,b) and XANES (Fig. R4, 100% in Pt 4+ ) results show that the small-size platinum species is totally in the form of PtxOz oxide clusters rather than Pt 0 cluster after calcination in air. Therefore, as the increase of calcination temperature, the sintering of platinum species results in the generation of huge metallic platinum particles and according to the previous reports (Jones et al. Science 2016, 353, 150-154;Zhai et al. Science 2010, 329, 1633-1636Ke et al. ACS Catal. 2015, 5, 5164-5173), no matter on the reducible (CeO2) or irreducible (SiO2, Al2O3) oxide supports, the small-size platinum species is still in the form of PtxOz oxide clusters or Pt δ+ single atom after calcination under air.
Hence, we can conclude that the small-size PtxOz oxide clusters and huge metallic platinum particles simultaneously exist in fresh 1Pt-SiO2.   Moreover, it is highly likely that the Pt particles presented in Figure S2a were not physically separated from the sample before conducting further analyses. If this is the case, the authors may centrifuge the sample and collect the inherent XAS spectra of the small Pt species supported on SiO2, which are not biased from Pt particles.
Author reply: We thank the reviewer's suggestion. We have conducted the centrifuged experiment for 1Pt-SiO2 at 10,000 r/min for 60 mins with TG16-WS Where is the total energy of the whole system, ( ) ,   Author reply: We thank the reviewer's comment and suggestion. First, the DRIFTS spectra in Fig.   2 and Supplementary Fig. 6 are attributed to the comparison of CO adsorption of 1Pt-SiO2-400 and 1Pt2Bi-SiO2. The reason why we compare the CO adsorption of 1Pt2Bi-SiO2 with 1Pt-SiO2-400 rather than 1Pt-SiO2 is to exclude the size effect of active site on DRIFTS spectra. The average size of oxide cluster is 1.70.4 nm for 1Pt2Bi-SiO2 and 1.80.3 nm for 1Pt-SiO2-400. As our reply in comment 2, only PtxOz clusters and some Pt δ + single atom exist in 1Pt-SiO2-400 without metallic platinum particles. Therefore, the two CO adsorption peaks centered at 2093 and 2075 cm -1 must be attributed to the CO molecule adsorbed on Pt δ+ single atom and PtxOz clusters. Meanwhile, we agree with the reviewer's viewpoint: "The location of the CO-probe peak is dependent to the supporting material and pre-or post-treatment condition." In addition, the collection temperature of spectra also effects the CO adsorption frequency. Therefore, according to wt.%), species composition (Pt single atoms and nanoparticles) and collection temperature (100 ºC).
In figure S8 of Ding's work (Ding et al. Science,350 (6257),(189)(190)(191)(192), the band at ~2092 cm -1 is attributed to CO molecule adsorbed on Pt δ+ single atom, which is identical with the band at 2093 cm -1 in our work. In addition, Ding et al. also reported that the CO adsorption peak on oxide Pt nanoparticles (XPS results in Figure S7) is between 2050 and 2080 cm -1 (Figure 2A) and we have confirmed that the CO adsorption on Pt 0 particle is at 2060 cm -1 in Fig. 7a Supplementary Fig. 9b), though possessing similar cluster size (1.8±0.1 nm) to 1Pt2Bi-SiO2 ( Supplementary Fig. 7).".
Supplementary Fig. 9 (b) the catalytic performance of 1Pt-SiO2 and 1Pt-SiO2-400.  Figure S7A). However, we found that the CO conversion is always maintained at 100 % at 500 °C even with 3 mg 1Pt2Bi-SiO2 catalyst, which cannot distinguish the excellent durability or excess mass of catalysts. Therefore, we employ the reaction gas at stoichiometric condition (2%CO + 1%O2) with super high gas hourly space velocity (300,000 mL gcat -1 h -1 ) to control the CO conversion at 95% in Supplementary Fig. 9c. Therefore, in the revised manuscript we have added the corresponding description in Page 10, Line 182-185: "To mimic lean-burn diesel engine exhaust and acquire the best catalytic performance, we used excess O2 in the reactant (CO/O2 = 1/20) 28 . A gas hourly space velocity of 134,000 ml gcat -1 hour -1 was tried to match standard vehicle exhaust conditions." and Page 12, Line 215-219: "Additionally, 1Pt2Bi-SiO2 showed remarkable thermo-stability for ~70 hours at 150 C and 200,000 mL g -1 h -1 in 1%CO/20%O2/N2 (Fig. 3d) and catalytic stability at high temperature (500 C) under the extremely high space velocity to maintain the CO conversion at 95% (300,000 mL·gcat −1 ·h −1 , Supplementary Fig. 9c) (83), it is almost impossible to distinguish Bi from Pt atoms, so it is difficult to demonstrate the presence of Bi species on the surface of Pt cluster from the atomic-resolution HAADF-STEM images. Here, the core-shell structure is confirmed by the EXAFS fitting and in situ DRIFTS results. In the revised manuscript, we have changed the EDS mapping images of used 1Pt2Bi-SiO2 in Fig. 4d and made modification about description in Page 13, Line 231-233: "Meanwhile, the related STEM-EDS mapping results of cluster indicated that the Pt and Bi elements still distribute together at the same area ( Fig. 4d and Supplementary Fig. 11)." Author reply: we thank the reviewer's suggestion for the discussion about the single atom catalysts.
In the revised manuscript, we have added the discussion about single atom catalysts for introduction and in-situ DRIFTS part in Page 4, Line 74-80, and Page 20, Line 360-364 and the above literatures as ref. 25,26,30,46, as below: According to previous reports 25,26 , it is easy to build multifarious atomic interface between metal and reducible oxide to improve catalytic performance. On the contrary, it is extremely difficult to structure effective metal-support interface due to the infertile oxygen, poor reducibility and stable surface composition of irreducible oxide (SiO2 and Al2O3). Therefore, it is great research interests to build stable and efficient interfaces to catalyze all kinds of heterogeneous reactions.
Because, recent reports have indicated that a lot of atomically dispersed platinum catalysts shows low catalytic activity in CO oxidation due to over strong adsorption of CO, even with