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

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.

N onoxidative 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©SiO 2 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©SiO 2 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/CeO 2 , 7−11 Pt/C 3 N 4 , 12 and Ru/ TiO 2 13 have been explored for nonoxidative coupling of methane. Xie et al. were the first to report on the use of Pt/ CeO 2 with isolated Pt sites for the NOCM reaction. 7 A methane conversion of 14.4% with a C 2 -hydrocarbon selectivity of 74.6% was obtained from a 1 vol % CH 4 feed. The authors reported that Pt remained isolated during a 40 h stability test. Later, Bajec et al. used microkinetic analysis of Pt/CeO 2 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 CH 4 conversion. 8 Eggart et al. investigated Ptdoped CeO 2 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 C 2 -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/CeO 2 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 CeO 2 was predicted to exhibit the highest C 2hydrocarbon selectivity. Chang and co-workers reported that the active sites in Pt/CeO 2 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/CeO 2 for the NOCM reaction, it remains unclear how isolated Pt sites behave under harsh reaction conditions.
In the present study, Pt/CeO 2 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 Pt 2+ species, with a Pt 4f 7/2 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 a Pt feature in terms of isolated Pt 2+ ions in a square planar configuration in the CeO 2 surface. 15−18 The main products of the NOCM reaction with the Pt/CeO 2 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/CeO 2 catalyst ( Figure S3). During the initial stage of reaction, the methane conversion decreases from ∼1% to ∼0.2%, which goes together  with a slow increase of the ethane and ethylene yield. The Pt/ CeO 2 catalyst exhibits a C 2 -hydrocarbon selectivity of 68% at a methane conversion rate of 0.85 mmol CH4 g Pt −1 min −1 , which is comparable with earlier reported activity data for Pt/CeO 2 tested under similar reaction conditions, namely, a C 2hydrocarbon selectivity of 55% at a reaction rate of 0.34 mmol CH4 g Pt −1 min −1 at 780°C. 8 The reaction involves an induction period, which is probably caused by the transformation of the initially isolated Pt 2+ species. We characterized in detail the fresh and used Pt/CeO 2 catalysts. The synchrotron XRD pattern of fresh Pt/CeO 2 is dominated by diffraction lines from CeO 2 (PDF no. 00-004-0593). The pattern of the used sample contains additional features due to Ce 2 O 3 (PDF no. 01-083-5456) and CePt 5 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 CePt 5 in a Pt/CeO 2 sample reduced at high temperature in H 2 or CH 4 has been reported before. 11,19 Rietveld refinement points to the sintering of CeO 2 particles during the NOCM reaction ( Figure S5 and Table S1), which is confirmed by TEM ( Figure S6). The formation of CePt 5 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 Pt 2+ in the XPS result of the used catalyst ( Figure S2). Therefore, we can conclude that Pt on the catalyst surface was fully reduced into CePt 5 . As such, it can be stated that the CePt 5 alloy is the active phase during a prolonged NOCM reaction, although the role of the initially present atomically dispersed Pt 2+ 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 PtO 2 can be trapped by CeO 2 , leading to isolated Pt species. 20 We employed an in situ O 2 regeneration at high temperature, consisting of periodically switching between a CH 4 -containing reaction feed and an O 2 -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 C 2 -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/CeO 2 . By optimizing the length of the reaction and regeneration treatments, we found that a regeneration period of 10 min in O 2 can already restore the initial catalytic activity. Nevertheless, the reaction−regeneration cycles lead to a decrease of the maximum C 2hydrocarbon signals at m/z = 27 by nearly an order of magnitude after 4 cycles (Figure 2a). The higher formation rate of C 2 -hydrocarbons upon regeneration was confirmed by GC analysis (Figure 2b). Other products including H 2 , H 2 O, and CO 2 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 CeO 2 . Without Pt, the C 2 -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 CeO 2 support are likely due to homogeneous gas-phase reactions. Nevertheless, we cannot completely rule out the catalytic role of CeO 2 in methane activation. Chang and co-workers reported the reduced CeO 2 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 The Pt/CeO 2 sample was characterized in detail after each reaction and regeneration cycle. The formation of Ce 2 O 3 upon an NOCM cycle follows from the XRD patterns of CH 4 _1 and CH 4 _3 (in CH 4 _n, "n" refers to the nth reaction cycle) and is likely caused by the reduction of CeO 2 by CH 4 . However, the main phase present in the used materials is CeO 2 . Ce 2 O 3 was oxidized during regeneration as follows from the XRD patterns of the corresponding regenerated catalysts O 2 _1 and O 2 _3 (Figure 3a, in O 2 _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 O 2 _1 and the O 2 _3 samples. Upon reaction in CH 4 , diffraction lines of CePt 5 are observed. Rietveld refinement of the synchrotron XRD patterns indicates the sintering of CeO 2 from ∼7 to ∼120 nm during the three reaction−regeneration cycles. The sintering of CeO 2 slowed after the third reaction stage. Meanwhile, the crystallite size of the CePt 5 particles increased from ∼4 to ∼8 nm, while the Pt 0 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/CeO 2 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 O 2 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 O 2 . 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 Pt 2+ state. The fraction of Pt 2+ increased to 88% after O 2 regeneration, pointing to redispersion of Pt on the catalyst surface (Figure 3c and Table  S3). TEM analysis of Pt/CeO 2 catalysts also suggests the  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 CH 4 . However, this shell can be seen again after O 2 regeneration. There is also a second shell in the EXAFS after CH 4 or O 2 treatment, which can be linked to the formation of CePt 5 and/or Pt 0 particles based on the above-discussed synchrotron XRD data. As the second shell after the regeneration in O 2 cannot be assigned to bulk PtO 2 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 O 2 regeneration step. The coordination number of the Pt−O shell obtained after the third O 2 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/CeO 2 catalyst after O 2 regeneration is caused by the reappearance of isolated Pt centers.
According to XPS and EXAFS, the reoxidation of reduced Pt to Pt 2+ 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 L 3 -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 L 3 -edge was employed to investigate the evolution of Pt (Figure 4b,c). 23 Upon heating the fresh catalyst in 40 vol % of the O 2 to 800°C, the white line intensity decreased, indicating the reduction of Pt ( Figure S19). After switching to CH 4 , 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 O 2 , which implies redispersion of Pt centers. We continued the reaction− regeneration experiments and observed the periodic reduction and oxidation of Pt in CH 4 and O 2 , respectively (Figure 4c). We compared the HERFD-XANES spectra obtained after each reaction and regeneration stage with the spectra of Pt 0 and CePt 5 . The HERFD-XANES after the NOCM reaction is closer to that of CePt 5 , whereas the O 2 regeneration step induced the formation of Pt 0 ( 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/CeO 2 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 O 2 treatment, which can be ascribed to the Pt−O single scattering path. These in situ tests confirm the redispersion of Pt after the O 2 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/CeO 2 with isolated Pt sites was investigated as a catalyst for the methane nonoxidative coupling reaction. Sintering of the initially isolated Pt 2+ sites in CeO 2 is inevitable, involving the reduction not only of Pt but also of part of Ce, leading to CePt 5 particles. This CePt 5 alloy acts as the active phase during the prolonged NOCM reaction. An in situ O 2 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/ CeO 2 during the NOCM reaction and oxidative regeneration.

■ 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 CLAESS 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.