Evidence of bifunctionality of carbons and metal atoms in catalyzed acetylene hydrochlorination

Carbon supports are ubiquitous components of heterogeneous catalysts for acetylene hydrochlorination to vinyl chloride, from commercial mercury-based systems to more sustainable metal single-atom alternatives. Their potential co-catalytic role has long been postulated but never unequivocally demonstrated. Herein, we evidence the bifunctionality of carbons and metal sites in the acetylene hydrochlorination catalytic cycle. Combining operando X-ray absorption spectroscopy with other spectroscopic and kinetic analyses, we monitor the structure of single metal atoms (Pt, Au, Ru) and carbon supports (activated, non-activated, and nitrogen-doped) from catalyst synthesis, using various procedures, to operation at different conditions. Metal atoms exclusively activate hydrogen chloride, while metal-neighboring sites in the support bind acetylene. Resolving the coordination environment of working metal atoms guides theoretical simulations in proposing potential binding sites for acetylene in the support and a viable reaction profile. Expanding from single-atom to ensemble catalysis, these results reinforce the importance of optimizing both metal and support components to leverage the distinct functions of each for advancing catalyst design.

1.In the second paragraph of page 10, the authors suggested that "The acetylene adsorption on the metal sites might be prevented by the presence of chloride ligands".However, in the second graph of page 8, the authors presented that "On the contrary, upon exposure to only acetylene, chloride ligands are removed."Is there any experimental phenomenon indicating that acetylene has bonded on the uncoordinated metal.
2. the authors proposed role of carbon as an "acetylene reservoir."and also presented that "The lack of metal-acetylene interactions in all the examined Pt SACs supported on the diverse carbons (AC, NC, and C), indicates that the acetylene activation step occurs over the carbon, irrespective of its properties."I wonder what is the identity of the active site that can bind acetylene so strong (even stronger than metal-acetylene interaction) and activate acetylene to undergo VCM formation.What is the mechanism that involved in above process?
Reviewer #2 (Remarks to the Author): In this manuscript, combining the operando X-ray absorption spectroscopy with other spectroscopic and kinetic analyses, the effect of single metal atom and carbon support in a supported Pt catalyst for the acetylene hydrochlorination has been discussed.The dissociation of HCl owes to the single metal atom, which is in accordance with the work published by Hutchings et al [10.1126/science.aal3439].On the other hand, C2H2 is adsorbed and activated by the activated carbon support, and then the dissociated HCl molecule migrates to activated C2H2 to form VCM product, which has not been proposed before.This work can help explain the effect of carbon support in acetylene hydrochlorination, but there are some results that need to be clarified.Before considering accepting the work published in Nature Communications, the authors need to issue the following questions: Minor questions: 1.There are some distinguishable peaks surrounding C(101) surface in Supplementary Figure S1.Are they related to metals or metallic compounds?What are they?2. As shown in Supplementary Figure S2, there might be some metal clusters rather than single-atom Pt on the PtSA/NC-a-473 catalyst.The authors need to explain it.
3. On Page 6, line 16: "In line with previous reports, mild activation temperatures (i.e., 473 K) yield single atoms, while harsher thermal treatments (i.e., 673 K) lead to metal sintering on AC and the formation of chloride-free isolated Pt sites on NC that are four-fold coordinated with the support (Figure 2c), as corroborated by extended X-ray absorption fine structure (EXAFS, Supplementary Table S6)."The authors considered the N-doped carbon support but without presenting the coordination results of Pt-N.Please explain it.4. In Figure 2c, the authors mentioned that Pt species were sintered under the high temperature of 673 K, and the model diagram shows the Pt cluster rather than the single dispersed Pt species in PtSA/AC-w-873 catalyst.According to the caption and model diagram, should PtSA/AC-w-873 be PtNP/AC-w-1073?or 1023?673?If so, why Pt-Pt peak could not be found in R space?In addition, should PtSA/AC-w-1023 be PtSA/AC-w-1073, please clarify it.S11, why do the fitting parameters of PtSA/AC-w-473-4h catalyst show a significant difference with other catalysts?6.The ratio of C2H2 and HCl reactants in the description of catalytic evaluation is different from that in the caption of Figure 4a.Please clarify it.

In Table
Major concerns: 7.In the previous work [10.1021[10. /acssuschemeng.2c07478, 10.1039[10. /D1NJ05120B, 10.1016[10. /j.mcat.2023.113158.113158],C2H2-TPD results showed obvious interactions between active metal and C2H2 on the Au-, Ru-and Pt-based catalysts, which seem to conflict with this work.The authors need to compare the C2H2-TPD results for supported metal catalysts and supports.For example, PtSA/AC-w-473 and AC-w-473.8. To further explore the effect of support on the activity, contrast experiments need to be performed.Since the catalyst prepared by the aqua regia will cause the oxidation of support, the catalyst with the aqua regia modified support and then the water-impregnated Pt should be prepared.Catalytic evaluation experiments and C2H2-TPD characterizations need to be conducted.9. On page 21, the authors mentioned that "EXAFS analysis evidences two contributions: an unaltered short-bonding one, at 1.85 ± 0.03 Å, and a growing long-bonding one, at 2.04 ± 0.02 Å (Supplementary Table S19).Though oxygen and carbon atoms exhibit similar scattering properties in XAS, the short-and long-bonding contributions are in agreement with Pt-C and Pt-O references, respectively (Figure 4b)".Why not use the Pt-C and Pt-O instead of Pt-C/O short and Pt-C/O long, respectively?The same problem is shown in the two types of Pt-Cl coordination with obviously different values of CN and R. Is it caused by the first and second shells?10.Since the activated carbon support can adsorb C2H2, what kind of carbon sites show unique activity?In addition, if the reaction follows the Eley-Rideal mechanism, which means that the gas C2H2 molecule could react with the adsorbed HCl molecule, the Pt-C coordination will not show obvious change during the reaction.In this case, it is also consistent with the results in Figure 5a.How the authors evidence that both C2H2 and HCl participate in the reaction with the adsorbed or activated state.
11.The Pt-Cl coordination in the PtSA/NC-w-1073 catalyst did not exist after annealing at 1073 K, is it replaced by Pt-N coordination?Will it affect the HCl dissociation by active metal sites and the catalytic activity for acetylene hydrochlorination?
Reviewer #3 (Remarks to the Author): In this manuscript, Giulimondi et al. reported bifunctional role of carbon and metal atoms (Pt, Ru and Au) for the acetylene hydrochlorination reaction.The catalysts were characterized by XRD, HAADF-STEM, XPS and XAS spectroscopy and the catalytic tests were carried out in a continuousflow fixed-bed reactor under relevant conditions.The study is very concrete and the results obtained are very coherent and supported by many evidences such as operando XAS which is challenging and at the same time also risky and I am glad that proper safety assessments were carried out.The EPR (HYSCORE) study is also interesting as coking is a huge challenge for this industrially relevant reaction.I am sure that in the near future this process will be conducted based on the transition metals instead of hazardous supported HgCl2 as catalyst.The manuscript is written very nicely and the presentation of figures and graphics are very clear.
The manuscript can be further improved for general readers by implementing the following points.
1.Since the manuscript shows lots of results based on XAS, it will be helpful for the readers if the authors also include the spectra of PtO2, PtCl2 and Pt metal in the figure 2d as they did in the figure S4.
2.The table related to the EXAFS fitting parameters missed two important things (a) R-factor and (b) Amplitude reduction factor.Is there any reason that the authors did not include these?3.EXAFS spectra are usually recorded at room temperature to have minimum thermal disorder which influences the debye waller factor.It seems that majority of the spectra are collected at 473 K and I am bit surprised that the Debye-Waller parameters are still showed relatively lower values.In the fitting, what kind of model were taken into taken into account?4.I am little bit worried about the assignment of Pt-Pt shell in figure 2c.The Pt-Pt shell that is assigned at 2.2 Å seems to be shorter (even without phase correction) than usual as the authors correctly showed in table S15.May be the authors also compare the R space of Pt foil in the same figures to make it clear.
5.In page 6, in the sentence 11, instead of "gradually resembling the PtO2 reference", I think the features of H2PtCl6 resembles the most of the features.Hence, the authors should consider changing it.6.It is not clear what is step ν in the page 9 refers to (sentence 10, "results, analysis of the Cl 2p XPS spectra after exposure to acetylene only (step ν) reveals a" 7.Line 14, page 9, it is mentioned that "Furthermore, analysis of the O 1s spectra shows that the oxygen functionalities undergo a slight reduction, pointing toward their participation in the reductive formation of VCM".Is it under the reaction conditions?8.It will be very informative to the general reader to know the parameters of MCR analysis.The authors can consider including this information in the ESI.9.For the future scope (beyond this work), studies like FTIR (DRIFT) might be very informative even though I can understand that to design cell will be extremely challenging under such corroding environment.However, to observe interactions between M-acetylene might be useful.

Additional questions
What are the noteworthy results?The results presented here are very insightful for such an industrially relevant process Will the work be of significance to the field and related fields?How does it compare to the established literature?Very significant Does the work support the conclusions and claims, or is additional evidence needed?Yes, very coherent study with evidences Are there any flaws in the data analysis, interpretation and conclusions?Do these prohibit publication or require revision?Is the methodology sound?Does the work meet the expected standards in your field?Yes, the methodology applied is commendable Is there enough detail provided in the methods for the work to be reproduced?Yes

Manuscript NCOMMS-23-20207 -Response to Reviewers
Comments in blue -Replies in black -Actions in bold Important note: Indicated page, line, or figure numbers refer to the revised manuscript and/or Supplementary Information with changes highlighted.

Reviewer #1
The paper presents evidence of bifunctionality of carbons and metal atoms in catalyzed acetylene hydrochlorination.It is a topic of interest to the researchers in the related areas.But the paper needs very significant improvement before acceptance for publication.
We thank the Reviewer for recognizing the significance of our study, relevant to the fields of acetylene hydrochlorination and single-atom catalysis.Their thoughtful comments, addressed below, prompted us to conduct additional kinetic tests and experimentally-guided density functional theory (DFT) simulations to further elaborate on the active site structure and the function of each component partaking in the catalytic cycle.

My detailed comments are as follows:
1.In the second paragraph of page 10, the authors suggested that "The acetylene adsorption on the metal sites might be prevented by the presence of chloride ligands".However, in the second graph of page 8, the authors presented that "On the contrary, upon exposure to only acetylene, chloride ligands are removed."Is there any experimental phenomenon indicating that acetylene has bonded on the uncoordinated metal.This is a central point, which relates to comment #10 of Reviewer #2.The operando XAS analysis presented in our contribution does not evidence any metal-acetylene interactions under reaction conditions, as discussed in detail for the PtSA/AC-w-473 catalyst (page 8, lines 24-26; page 9, lines 1-3 and 16-22).This has been corroborated by extensive computational studies (page 33, Figure 7a; page 16, lines 8-19).DFT results evidence that highly chlorinated metal atoms present endergonic adsorption (Gibbs energies of acetylene > 0.5 eV, page S37, Supplementary Table S22).This is exemplified by di-chlorinated Pt atoms, the most likely metal speciation in the optimal working catalyst (page S37, Supplementary Table S22).
To further validate the lack of metal-acetylene interaction, we exposed as-prepared Pt SACs to C2H2 only from room temperature, to maximize adsorption, to reaction temperature (i.e., 473 K).Operando XAS analysis shows that metal dechlorination occurs only upon increasing the temperature, suggesting that the dechlorination process is linked to VCM formation.More specifically, EXAFS analysis of the as-prepared and C2H2-exposed catalysts shows only a partial loss in the Pt-Cl contribution (coordination number, CN = 3.1 and 0.7, respectively).This is counterbalanced by a stronger long-bonding Pt-C/O contribution (CN = 0.2 and 2.0, respectively), while the short-bonding Pt-C/O one remains unaltered (CN = 0.9 and 1.0, respectively).Indeed, the total coordination number of the metal atoms determined by operando XAS, encompassing Pt-Cl and Pt-C/O contributions, remains ca. 4 prior and after exposure to C2H2 (page S34, Supplementary Table S19).The attribution of the increased Pt-C/O interaction to metal-acetylene interactions cannot be ruled out, as it has been clarified in the manuscript (page 13, lines 9-10).However, the long-bonding Pt-C/O contribution is in agreement with the Pt-O references rather than Pt-C ones (J.Phys.Chem. B 104, 1998Chem. B 104, , (2000)); Catal.Lett. 8, 283, (1991)).This suggests that the partial removal of chloride ligands leads the uncoordinated metal atom to re-coordinate to the support rather than adsorbing C2H2 as a ligand.In line with this, DFT simulations predict an increase in the metal-support binding energy of at least 1.2 eV upon partial metal dechlorination (i.e., average loss of two chloride ligands; page S36, Supplementary Table S21), irrespective of the metal site structure.These considerations relating to both experimental and computational investigations are now discussed in the manuscript and Supplementary Information (page 33, Figure 7a; page 13, lines 9-10; page 16, lines 8-19; pages S36-S38, Supplementary Tables S21-S23).

The authors proposed role of carbon as an "acetylene reservoir." and also presented that "The lack of metal-acetylene interactions in all the examined Pt SACs supported on the diverse carbons (AC, NC, and C), indicates that the acetylene activation step occurs over the carbon, irrespective of its properties." I wonder what is the identity of the active site that can bind acetylene so strong (even stronger than metal-acetylene interaction) and activate acetylene to undergo VCM formation. What is the mechanism that involved in above process?
We thank the Reviewer for the insightful question, which also relate to comment #10 of Reviewer #2.We clarify that the presented spectroscopic and kinetic investigations point to the carbon supports as responsible for adsorbing and activating C2H2, but no conclusion could be drawn on the nature of the sites, requiring further investigations, as stated in the manuscript (page 15, lines 1-5).Indeed, carbon-based materials present different functionalities that can bind C2H2 (Nature 436, 238 (2005); Chem.Mater.31, 4919 ( 2019)).
To gain further insights into the active site structure and the function of each component in fulfilling the catalytic cycle, we have conducted additional kinetic and computational investigations.Efforts were focused on the PtSA/AC-w-473 catalyst, selected for its high activity and unparalleled stability (Nat.Catal.3, 376 (2020)).Derivation of partial reaction orders corroborated the adsorption of both reactants over the catalyst surface (please see our reply to comment #10 of Reviewer #2, page 15, lines 6-12; page S66, Supplementary Figure S23).To identify the reactant binding sites, DFT simulations were performed on active structures featuring chlorinated Pt atoms anchored over diverse potential O-functionalities in AC.Operando XAS analysis indicates coordination of Pt atoms with at least two chloride ligands, which preferentially bind HCl over C2H2, as detailed in our reply to comment #1.Conversely, C2H2 adsorption on the chlorinated Pt atoms is endergonic (> ca.0.5 eV; page S37, Supplementary Table S22).Thereafter, the adsorption of C2H2 was computed on different functionalities in the support that neighbor the di-chlorinated Pt species, resulting in Gibbs energy values ranging from −0.9 to 0.0 eV.This exergonic character can be assigned to the ability of C2H2 to undergo addition reactions with the carbon support by forming five-and six-membered rings (page 16, lines 16-19; page 33, Figure 7a).Finally, guided by the operando XAS and kinetic analyses, a reaction profile was proposed and found to be energetically viable, as detailed in our reply to comment #10 of Reviewer #2.

Reviewer #2
In this manuscript, combining the operando X-ray absorption spectroscopy with other spectroscopic and kinetic analyses, the effect of single metal atom and carbon support in a supported Pt catalyst for the acetylene hydrochlorination has been discussed.The dissociation of HCl owes to the single metal atom, which is in accordance with the work published by Hutchings et al [10.1126/science.aal3439].On the other hand, C2H2 is adsorbed and activated by the activated carbon support, and then the dissociated HCl molecule migrates to activated C2H2 to form VCM product, which has not been proposed before.This work can help explain the effect of carbon support in acetylene hydrochlorination, but there are some results that need to be clarified.
We thank the Reviewer for appreciating the novelty of our findings regarding the uncovering of the co-catalytic role of carbon supports in metal-catalyzed acetylene hydrochlorination.By addressing their constructive criticism, we were able to further strengthen the quality and impact of our study, as well as the clarity of the messages.Specifically, additional kinetic and experimentally-guided computational investigations, as well as C2H2 adsorption analyses, have been conducted to further support our conclusions and gain deeper insights into the active site structure and functions in fulfilling the catalytic cycle.Each point is addressed below with a description of the actions taken upon revision.

Before considering accepting the work published in Nature Communications, the authors need to issue the following questions: Minor questions:
1.There are some distinguishable peaks surrounding C( 101) surface in Supplementary Figure S1.Are they related to metals or metallic compounds?What are they?
The diffraction peaks in question are attributed to incomplete activation of the commercial AC support employed in the study (Sci.Rep. 10, 2563 (2020)), as detailed in the Supplementary Information (page S41, lines 5-6).In line with this, the diffraction peaks are not observed in NC-based catalysts.2c, the authors mentioned that Pt species were sintered under the high temperature of 673 K, and the model diagram shows the Pt cluster rather than the single dispersed Pt species in PtSA/AC-w-873 catalyst.According to the caption and model diagram, should PtSA/AC-w-873 be PtNP/AC-w-1073?or 1023?673?If so, why Pt-Pt peak could not be found in R space?In addition, should PtSA/AC-w-1023 be PtSA/AC-w-1073, please clarify it.

In Figure
We thank the Reviewer for pointing out the typo in the sample code in Figure 2c, which should denote "PtNP/AC-w-873" instead of "PtSA/AC-w-873", as now corrected (page 25, Figure 2c).As clarified in the revised manuscript (page 6, lines 21-23), the formation of clusters is observed under temperatures above 673 K (Nat.Catal.3, 376 (2020)), which is attributable to the partial loss in chloride ligands that stabilize the Pt species as single atoms.In line with this, our operando XAS investigation of AC-supported Pt species undergoing thermal treatment at 873 K, as indicated by the sample code, evidences the formation of metallic clusters reflected in a Pt-Pt CN of 1.3 and a low Pt-Cl CN of 0.4 (page S19, Supplementary Table S6).Furthermore, the HAADF-STEM investigation of PtNP/AC-w-873 shows mixed metal speciation encompassing nanoparticles and low-nuclearity species (page S43, Supplementary Figure S2), indicating the uncontrolled nature of the sintering process, as highlighted in the Supplementary Information (page S43, Supplementary Figure S2, lines 3-5).S11, why do the fitting parameters of PtSA/AC-w-473-4h catalyst show a significant difference with other catalysts?

In Table
We thank the Reviewer for pointing out this inconsistency.We deeply regret that the different fitting parameters for the PtSA/AC-w-473-4h catalyst were misreported, and they have been corrected (page S24, Supplementary Table S11).
6.The ratio of C2H2 and HCl reactants in the description of catalytic evaluation is different from that in the caption of Figure 4a.Please clarify it.
We thank the Reviewer for noting the inconsistency between the caption of Figure 4a and the description of the catalytic evaluation description in the Methods section (page 20, line 5).The reporting of the C2H2 and HCl volume percentages in the caption of Figure 4a was reversed and has been corrected (page 28, line 5).
The conflicting results likely arise from the ex situ versus operando conditions of the C2H2-TPD and XAS analyses, respectively.C2H2-TPD is a valuable approach to probing the interaction of a reactant with the catalyst surface 'off-line'.However, the technique cannot resolve either the nature of the adsorption site or the catalyst dynamic behavior under reaction conditions, as surface saturation by C2H2 is carried out (i) at room temperature and (ii) in the absence of HCl, thus excluding reactant competitive adsorption.Therefore, species such as chloride-free metal atoms, as in the case of the mentioned Ru SACs (Mol. Catal. 543, 113158 (2023)), might bind C2H2 in the TPD analysis but might show greater affinity for HCl under reaction conditions and prevent the competitive adsorption of C2H2.Additionally, the presence of metal clusters and nanoparticles, as respectively evidenced by the EXAFS and microscopy analyses of the mentioned Au and Pt catalysts (New J. Chem., 46, 3738 (2022); ACS Sustainable Chem.Eng. 11, 3103 (2023)), further complicates the analysis of acetylene-metal atom interactions.As a result, the different metal speciation hinders a direct comparison with our work.We highlight that our contribution focuses on understanding the catalytic roles of single metal atoms and carbon supports in catalyzing acetylene hydrochlorination, and does not encompass carbon-supported metal clusters or nanoparticles for which no conclusions are drawn as they would require dedicated studies, as clarified in the amended manuscript (page 18, lines 14-17).
Despite these limitations, we agree with the Reviewer that the comparison of C2H2-TPD profiles of the supported chlorinated metal single atoms and the bare support can offer insights into the C2H2 binding sites.Accordingly, we have conducted C2H2-TPD analysis of the bare AC-w-473, whose profile shows comparable adsorption properties as those of PtSA/AC-w-473.Though not fully reflective of acetylene adsorption over the catalyst surface under reaction conditions, these results suggest that the integration of chlorinated Pt atoms does not affect the ability of the carbon support to bind acetylene reinforcing the role of the carbon in the adsorption process.The additional C2H2-TPD analysis and related discussion have been included in the manuscript and Supplementary Information (page 8, lines 12-15; page S50, Supplementary Figure S8).8. To further explore the effect of support on the activity, contrast experiments need to be performed.Since the catalyst prepared by the aqua regia will cause the oxidation of support, the catalyst with the aqua regia modified support and then the water-impregnated Pt should be prepared.Catalytic evaluation experiments and C2H2-TPD characterizations need to be conducted.mid-term is unfeasible.Nevertheless, such an endeavor would be highly valuable for resolving the chemical state of both carbon functionalities and metal species, and should be pursued in future studies, as discussed in the manuscript (page 18, lines 10-14).

It will be very informative to the general reader to know the parameters of MCR analysis. The authors can consider including this information in the ESI.
The description of the MCR analysis method has been included in the Supplementary Information (page S6, lines 7-11).9.For the future scope (beyond this work), studies like FTIR (DRIFT) might be very informative even though I can understand that to design cell will be extremely challenging under such corroding environment.However, to observe interactions between M-acetylene might be useful.
Operando infrared spectroscopy methods could, indeed, probe the nature and dynamic behavior of the acetylene binding sites in the catalyst.Not only could they enable distinction between metal and support sites but also resolution of the carbon functionalities.Nevertheless, as pointed out by the Reviewer, the highly corrosive nature of the reaction requires ad hoc cell design.This is further complicated by the reduced signal-to-noise ratio that carbons exhibit due to their opaque nature (Carbon 26, 889 (1988)).Therefore, we agree with the Reviewer that adequate execution of these experiments will need dedicated time and effort in a future investigation.As this undertaking goes beyond the scope of the herein-presented first study on the role of carbon and metal atoms in catalyzing acetylene hydrochlorination, the potential insights that future studies employing operando infrared spectroscopy methods could offer have been discussed in the manuscript (page 18, lines 10-14).