# Pt/CeO2 as Catalyst for Nonoxidative Coupling of Methane: Oxidative Regeneration

Hao Zhang, Valery Muravev, Liang Liu, Anna Liutkova, Jérôme F. M. Simons, Blanka Detlefs, Huaizhou Yang, Nikolay Kosinov, Emiel J. M. Hensen

► To cite this version:

Hao Zhang, Valery Muravev, Liang Liu, Anna Liutkova, Jérôme F. M. Simons, et al., Pt/CeO2 as Catalyst for Nonoxidative Coupling of Methane: Oxidative Regeneration. Journal of Physical Chemistry Letters, 2023, 14 (30), pp.6778-6783. 10.1021/acs.jpclett.3c01179. hal-05095288

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# Pt/CeO₂ as Catalyst for Nonoxidative Coupling of Methane: Oxidative Regeneration

Hao Zhang, Valery Muravev, Liang Liu, Anna Liutkova, Jérôme F. M. Simons, Blanka Detlefs, Huaizhou Yang, Nikolay Kosinov,* and Emiel J. M. Hensen*

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Cite This: *J. Phys. Chem. Lett.* 2023, 14, 6778–6783

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**ABSTRACT:** Direct nonoxidative coupling is a promising route for methane upgrading, yet its commercialization is hindered by the lack of efficient catalysts. Pt/CeO₂ catalysts with isolated Pt species have attracted an increasing amount of interest in recent years. Herein, we studied the catalytic role and evolution of isolated Pt centers on CeO₂ prepared by flame spray pyrolysis under the harsh reaction conditions of nonoxidative methane coupling. During the reaction at 800 °C, the isolated Pt sites sinter, leading to a loss of the ethylene and ethane yield. The agglomerated Pt can be redispersed by using an *in situ* regeneration strategy in oxygen. We found that isolated Pt centers are able to activate methane only at the initial reaction stage, and the CePt₅ alloy acts as the active phase in the prolonged reaction.

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Nonoxidative coupling of methane to more valuable hydrocarbons is a potential route for natural gas and biogas valorization.¹⁻⁴ The pioneering study of a highly active and stable Fe©SiO₂ catalyst by Bao and co-workers led to a substantial research effort in this area.¹,⁵,⁶ The isolated Fe centers in the Fe©SiO₂ catalyst are argued to limit C–C coupling and, therefore, coke formation on the catalytic surface. Based on this insight, other catalysts containing isolated metal sites such as Pt/CeO₂,⁷⁻¹¹ Pt/C₃N₄,¹² and Ru/TiO₂¹³ have been explored for nonoxidative coupling of methane. Xie et al. were the first to report on the use of Pt/CeO₂ with isolated Pt sites for the NOCM reaction.⁷ A methane conversion of 14.4% with a C₂-hydrocarbon selectivity of 74.6% was obtained from a 1 vol % CH₄ feed. The authors reported that Pt remained isolated during a 40 h stability test. Later, Bajec et al. used microkinetic analysis of Pt/CeO₂ catalysts with isolated Pt sites to demonstrate that hydrogen abstraction from methane determines the overall rate, while surface C–C coupling reactions only slightly affect the overall CH₄ conversion.⁸ Eggart et al. investigated Pt-doped CeO₂ catalysts by operando X-ray absorption spectroscopy and synchrotron-based vacuum ultraviolet photoionization mass spectrometry.¹¹ Methyl radicals were detected as the main reaction intermediate, indicating that gas-phase reactions are involved in the formation of C₂-hydrocarbons. Different from the study by Xie et al.,⁷ these authors observed extensive sintering of the initially isolated Pt sites. The agglomeration of Pt was observed even after He treatment at this temperature. The authors also contended that methane activation can occur at the interface between Pt and the ceria support. Theoretical

studies have also been carried out to understand the reaction mechanism on such Pt/CeO₂ catalysts. Wen et al. performed a detailed density functional theory (DFT) study, showing that methane conversion strongly depends on the Pt particle size.⁹ Isolated Pt on CeO₂ was predicted to exhibit the highest C₂-hydrocarbon selectivity. Chang and co-workers reported that the active sites in Pt/CeO₂ involve two functionalities, namely, single Pt atoms close to a frustrated Lewis acid obtained upon oxygen removal from ceria, which are together involved in methane activation under NOCM conditions.¹⁰ Despite the growing interest in Pt/CeO₂ for the NOCM reaction, it remains unclear how isolated Pt sites behave under harsh reaction conditions.

In the present study, Pt/CeO₂ was prepared by the one-step flame spray pyrolysis (FSP), which is a proven method to synthesize catalysts with highly dispersed metal species.¹¹,¹⁴ Fourier and wavelet transformed EXAFS demonstrates the isolated nature of Pt in which only one clear Pt–O coordination shell at ~1.6 Å is observed (Figure 1a, Figure S1). XPS analysis shows the predominance of Pt²⁺ species, with a Pt 4f₇/₂ binding energy of ~72.9 eV on the surface of the fresh catalyst (Figure S2). Previous experimental and theoretical studies have supported the interpretation of such

Received: May 1, 2023

Accepted: July 18, 2023

Published: July 21, 2023

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<figure><img src="image_9.png" /><figcaption>**Figure 1.** (a) k³-weighted EXAFS measured at the Pt L₃ edge of fresh and used Pt/CeO₂ catalysts. The Pt⁰ and PtO₂ references are included for comparison. (b) Catalytic performance of the Pt/CeO₂ catalyst at 800 °C using 100 mg catalysts under 10 mL/min 95 vol % CH₄ with 5 vol % Ar. (c) Synchrotron XRD patterns of fresh and used Pt/CeO₂ catalysts. The inset shows the enlarged region highlighted by the blue dashed cube.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 2.** (a) Reaction-regeneration tests of the Pt/CeO₂ catalyst at 800 °C using 100 mg catalysts. A flow of 50 mL/min of He was purged to the reactor for 10 min between the NOCM reaction in 10 mL/min of 95 vol % CH₄ and the regeneration under 40 vol % O₂. (b) The ethane and ethylene yield obtained by GC during the reaction-regeneration cycles using 100 mg of Pt/CeO₂ catalyst at 800 °C. A total flow of 10 mL/min (95 vol % CH₄ + 5 vol % Ar for reaction and 40 vol % O₂ diluted by He for regeneration) was used for the measurements. The frequency of GC injection is every 2 min. The gray zone indicates the He purging, and the orange zone represents the regeneration treatment in O₂.</figcaption></figure>

a Pt feature in terms of isolated Pt²⁺ ions in a square planar configuration on the CeO₂ surface.¹⁵⁻¹⁸ The main products of the NOCM reaction with the Pt/CeO₂ catalyst at a reaction temperature of 800 °C were ethylene and ethane (Figure 1b).

At the given conditions, ethane is obtained in larger amounts than ethylene. Coke was also formed on the Pt/CeO₂ catalyst (Figure S3). During the initial stage of reaction, the methane conversion decreases from ~1% to ~0.2%, which goes together
<figure><img src="image_11.png" /><figcaption>**Figure 3.** (a) Quasi *in situ* synchrotron XRD patterns with the enlarged region. (b) Raman spectra measured using the 532 nm laser. (c) Pt 4f XPS results of Pt/CeO₂ catalysts at different reaction/regeneration stages. In CH₄_n and O₂_n, "n" refers to the nth reaction or regeneration. CH₄_1 indicates the catalyst obtained after the first reaction stage, and the corresponding O₂_1 suggests the regenerated CH₄_1 sample after the O₂ treatment.</figcaption></figure>

with a slow increase of the ethane and ethylene yield. The Pt/CeO₂ catalyst exhibits a C₂-hydrocarbon selectivity of 68% at a methane conversion rate of 0.85 mmolₐ₄ gPt⁻¹ min⁻¹, which is comparable with earlier reported activity data for Pt/CeO₂ tested under similar reaction conditions, namely, a C₂-hydrocarbon selectivity of 55% at a reaction rate of 0.34 mmolₐ₄ gPt⁻¹ min⁻¹ at 780 °C.⁸ The reaction involves an induction period, which is probably caused by the transformation of the initially isolated Pt²⁺ species. We characterized in detail the fresh and used Pt/CeO₂ catalysts. The synchrotron XRD pattern of fresh Pt/CeO₂ is dominated by diffraction lines from CeO₂ (PDF no. 00-004-0593). The pattern of the used sample contains additional features due to Ce₂O₃ (PDF no. 01-083-5456) and CePt₅ alloy (PDF no. 01-071-7052, Figure 1c, Figure S4). Diffraction peaks of Pt metal (PDF no. 00-004-0802) were not observed. The formation of CePt₅ in a Pt/CeO₂ sample reduced at high temperature in H₂ or CH₄ has been reported before.¹¹,¹⁹ Rietveld refinement points to the sintering of CeO₂ particles during the NOCM reaction (Figure S5 and Table S1), which is confirmed by TEM (Figure S6). The formation of CePt₅ is in line with XANES and EXAFS analysis (Figure 1a, Figures S1, S7, and S8, and Table S2). The reduction of Pt is also in keeping with the absence of Pt²⁺ in the XPS result of the used catalyst (Figure S2). Therefore, we can conclude that Pt on the catalyst surface was fully reduced into CePt₅. As such, it can be stated that the CePt₅ alloy is the active phase during a prolonged NOCM reaction, although the role of the initially present atomically dispersed Pt²⁺ sites needs to be further investigated.

We developed an oxidative regeneration strategy based on the work of Datye and co-workers. They reported that gaseous PtO₂ can be trapped by CeO₂, leading to isolated Pt species.²⁰

We employed an *in situ* O₂ regeneration at high temperature, consisting of periodically switching between a CH₄-containing reaction feed and an O₂-containing regeneration feed, aiming at the redispersion of agglomerated Pt. The evolution of the MS signals of 27 (ethylene and ethane) and 30 (ethane) in Figure 2a during consecutive reaction-regeneration cycles indicates the rapid decline of the C₂-hydrocarbon yield after the initial period (i.e., the *m/z* = 27 signal drops by 1 order of magnitude). This deactivation is likely caused by the rapid sintering of Pt in Pt/CeO₂. By optimizing the length of the reaction and regeneration treatments, we found that a regeneration period of 10 min in O₂ can already restore the initial catalytic activity. Nevertheless, the reaction-regeneration cycles lead to a decrease of the maximum C₂-hydrocarbon signals at *m/z* = 27 by nearly an order of magnitude after 4 cycles (Figure 2a). The higher formation rate of C₂-hydrocarbons upon regeneration was confirmed by GC analysis (Figure 2b). Other products including H₂, H₂O, and CO₂ and small amounts of benzene and toluene were also detected (Figure S9). To confirm the pivotal role of Pt, we also studied the reaction-regeneration cycles for CeO₂. Without Pt, the C₂-hydrocarbon signal at *m/z* = 27 is about 3 times lower in the first reaction cycle (Figure S10). The hydrocarbon products obtained with the bare CeO₂ support are likely due to homogeneous gas-phase reactions. Nevertheless, we cannot completely rule out the catalytic role of CeO₂ in methane activation. Chang and co-workers reported the reduced CeO₂ surface to be active in the NOCM reaction. Based on theoretical modeling, they proposed that frustrated Lewis pairs, involving surface Ce cations and oxygen vacancies, are candidate active sites for methane activation.²¹ Nevertheless, the catalytic activity is substantially enhanced when Pt is
<figure><img src="image_12.png" /><figcaption>**Figure 4.** (a) Quasi *in situ* *k*²-weighted EXAFS at the Pt L₃-edge of the Pt/CeO₂ catalyst at different reaction and regeneration stages. The raw data are shown using open circles, and the gray lines indicate the fitting results. (b) Pt L₃-edge HERFD-XANES spectra of the Pt/CeO₂ catalyst at different reaction and regeneration stages. (c) The evolution of white line intensity during the *in situ* HERFD-XANES measurements at the Pt L₃-edge. The measurements were performed at 800 °C using 30 mg of Pt/CeO₂ catalysts under 5 mL/min CH₄ for reaction and 15 mL/min 40 vol % of O₂ for regeneration. A flow of 20 mL/min of He was purged to the *in situ* cell between the NOCM reaction and the catalyst regeneration. The position of the white line intensity of Pt⁰ is labeled. The He-purging period is labeled in gray.</figcaption></figure>

introduced, showing that Pt plays an important role in the catalytic activity of Pt/CeO₂ catalysts. This conclusion is also in line with other studies of the role of Pt in Pt/CeO₂ for the NOCM reaction.⁷,¹¹

The Pt/CeO₂ sample was characterized in detail after each reaction and regeneration cycle. The formation of Ce₂O₃ upon an NOCM cycle follows from the XRD patterns of CH₄-1 and CH₄-3 (in CH₄_n, “n” refers to the nth reaction cycle) and is likely caused by the reduction of CeO₂ by CH₄. However, the main phase present in the used materials is CeO₂. Ce₂O₃ was oxidized during regeneration as follows from the XRD patterns of the corresponding regenerated catalysts O₂_1 and O₂_3 (Figure 3a, in O₂_n, “n” refers to the nth regeneration cycle). With respect to Pt, the diffraction line at ~2.79 Å⁻¹ can be linked to the Pt metal in the O₂_1 and the O₂_3 samples. Upon reaction in CH₄, diffraction lines of CePt₅ are observed. Rietveld refinement of the synchrotron XRD patterns indicates the sintering of CeO₂ from ~7 to ~120 nm during the three reaction-regeneration cycles. The sintering of CeO₂ slowed

after the third reaction stage. Meanwhile, the crystallite size of the CePt₅ particles increased from ~4 to ~8 nm, while the Pt⁰ species observed after the oxidative regeneration have a size of ~20 nm (Table S1 and Figure S5). We further studied these samples by Raman spectroscopy (Figure 3b). The Raman bands at ~550 and ~690 cm⁻¹, which can be clearly observed in the fresh Pt/CeO₂ catalyst, are due to the asymmetric and symmetric stretching modes of the Pt-O-Ce moiety, respectively.²² These two bands are not present anymore after the NOCM reaction but reappeared upon regeneration in O₂ albeit with a weaker intensity. The Raman results can be explained by the aggregation of Pt during the NOCM reaction and, subsequent, Pt redispersion during regeneration in the presence of O₂. XPS analysis reveals that 75% of surface Pt species are present in the reduced state after the first reaction cycle with the remaining 25% in the Pt²⁺ state. The fraction of Pt²⁺ increased to 88% after O₂ regeneration, pointing to redispersion of Pt on the catalyst surface (Figure 3c and Table S3). TEM analysis of Pt/CeO₂ catalysts also suggests the
agglomeration and partial redispersion of Pt species upon CH₄ and O₂ treatments (Figure S11).

Quasi *in situ* EXAFS analysis was carried out to follow the changes in the Pt phase (Figures S12 and S13). The Pt–O coordination shell at ~1.6 Å disappeared after the NOCM reaction in CH₄. However, this shell can be seen again after O₂ regeneration. There is also a second shell in the EXAFS after CH₄ or O₂ treatment, which can be linked to the formation of CePt₅ and/or Pt⁰ particles based on the above-discussed synchrotron XRD data. As the second shell after the regeneration in O₂ cannot be assigned to bulk PtO₂ or PtO particles based on reference compounds, the presence of a Pt–O shell can be related to the partial redispersion of sintered Pt (Figure S12). EXAFS fitting shows that the coordination number of the Pt–O shell decreased from ~3.8 to ~0.4 during the first NOCM reaction and increased again to ~2.8 during the first O₂ regeneration step. The coordination number of the Pt–O shell obtained after the third O₂ regeneration was lower at a value of ~1, indicating a lower extent of redispersion (Figure 4a, Figure S14, and Table S4). Based on the above analysis, we infer that the enhanced catalytic performance of the Pt/CeO₂ catalyst after O₂ regeneration is caused by the reappearance of isolated Pt centers.

According to XPS and EXAFS, the reoxidation of reduced Pt to Pt²⁺ occurs during the redispersion of the sintered Pt species. Therefore, the redispersion of Pt can be tracked by following the oxidation state of Pt via the white line intensity of the Pt L₃-edge (Figure S15).²³ For this purpose, we carried out *in situ* HERFD-XAS to confirm the Pt evolution under actual reaction conditions. HERFD-XANES can provide enhanced energy resolution compared with the conventional total fluorescence yield detection method, allowing better monitoring of the changes in the XANES region (Figure S16). A home-built high-temperature *in situ* XAS cell with quartz tube reactor was used for the HERFD-XANES experiments at 800 °C (Figures S17 and S18). The white line intensity recorded at the Pt L₃-edge was employed to investigate the evolution of Pt (Figure 4b,c).²³ Upon heating the fresh catalyst in 40 vol % of the O₂ to 800 °C, the white line intensity decreased, indicating the reduction of Pt (Figure S19). After switching to CH₄, Pt reduction proceeded, as revealed by the further decrease of the white line intensity. This reflects the sintering of Pt. The Pt species were reoxidized after exposure to O₂, which implies redispersion of Pt centers. We continued the reaction–regeneration experiments and observed the periodic reduction and oxidation of Pt in CH₄ and O₂, respectively (Figure 4c). We compared the HERFD-XANES spectra obtained after each reaction and regeneration stage with the spectra of Pt⁰ and CePt₅. The HERFD-XANES after the NOCM reaction is closer to that of CePt₅, whereas the O₂ regeneration step induced the formation of Pt⁰ (Figure S20). These findings are consistent with the synchrotron XRD results. To determine the structure of the Pt phase, we measured HERFD-EXAFS of the Pt/CeO₂ catalyst at different reaction and regeneration stages at room temperature (Figure S21). It is clear that an additional shell at ~1.6 Å appears after the O₂ treatment, which can be ascribed to the Pt–O single scattering path. These *in situ* tests confirm the redispersion of Pt after the O₂ regeneration. We verified that X-ray beam damage of the samples was negligible during the *in situ* HERFD-XAS measurements (Figure S22).

To summarize, Pt/CeO₂ with isolated Pt sites was investigated as a catalyst for the methane nonoxidative coupling reaction. Sintering of the initially isolated Pt²⁺ sites

in CeO₂ is inevitable, involving the reduction not only of Pt but also of part of Ce, leading to CePt₅ particles. This CePt₅ alloy acts as the active phase during the prolonged NOCM reaction. An *in situ* O₂ regeneration strategy was developed, in which the Pt-containing particles can partially redisperse. This work highlights the dynamic nature of the Pt species in Pt/CeO₂ during the NOCM reaction and oxidative regeneration.

## ■ ASSOCIATED CONTENT

### ■ Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.3c01179.

Experimental details; EXAFS, XANES, Raman, and XPS results of fresh and used Pt/CeO₂ catalysts and the catalyst after different reaction or regeneration stages; crystal structure; Rietveld refinement results of synchrotron XRD patterns; TEM images of Pt/CeO₂ catalysts; the results of the switching experiments over CeO₂; EXAFS fitting results of Pt/CeO₂ catalysts; MS data; surface fraction data; the procedure, experimental setup, and results of *in situ* HERFD-XANES measurements (PDF)

## ■ AUTHOR INFORMATION

### Corresponding Authors

Nikolay Kosinov – Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, S600 MB Eindhoven, The Netherlands; ● orcid.org/0000-0001-8520-4886; Email: n.a.kosinov@tue.nl

Emiel J. M. Hensen – Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, S600 MB Eindhoven, The Netherlands; ● orcid.org/0000-0002-9754-2417; Email: e.j.m.hensen@tue.nl

### Authors

Hao Zhang – Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, S600 MB Eindhoven, The Netherlands

Valery Muravev – Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, S600 MB Eindhoven, The Netherlands; ● orcid.org/0000-0002-1357-1086

Liang Liu – Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, S600 MB Eindhoven, The Netherlands

Anna Liutkova – Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, S600 MB Eindhoven, The Netherlands

Jérôme F. M. Simons – Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, S600 MB Eindhoven, The Netherlands

Blanka Detlefs – European Synchrotron Radiation Facility, 38043 Grenoble, France

Huaizhou Yang – Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and
Chemistry, Eindhoven University of Technology, S600 MB Eindhoven, The Netherlands; orcid.org/0000-0001-7489-0948

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jpclett.3c01179

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work is supported by the Advanced Research Center for Chemical Building Blocks, which is cofounded and cofinanced by The Netherlands Organization for Scientific Research and The Netherlands Ministry of Economic Affairs. We acknowledge Dr. Carlo Marini for the assistance in using the CLÆSS beamline at ALBA Synchrotron under proposal no. 2022025616. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and we would like to thank Dr. Marta Mirolo for assistance in using beamline ID31 under the proposal no. MA5228. We acknowledge Timothy Bohdan and Dr. Pieter Glatzel for their help in using beamline ID26 under the proposal no. MA5121. We acknowledge SOLEIL for provision of synchrotron radiation facilities related to the experiments at the ROCK beamline under project no. 20210495. We thank Dr. Stephanie Belin for the assistance at ROCK. The experiments performed at ROCK were supported by a public grant overseen by the French National Research Agency (ANR) as part of the "Investissements d'Avenir" program (reference: ANR-10-EQPX-45). Rim C. J. van de Poll is acknowledged for performing the TEM measurements. We acknowledge Adelheid M. Elemans-Mehring and Thijs H. T. Moerkens for the assistance in ICP-OES measurements. Bianca Ligt is acknowledged for the assistance in FSP synthesis.

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