Atomically dispersed Pd-Mn dual-metal doped CeO_2 nanorods for efficient methane oxychlorination

Domain: chemistry
Published: 2026-01-06
Editorial outcome: major_revision

Abstract
The oxyhalogenation of methane to mono-halogenated methane CH3X (X=Cl, Br, or I) is one of the most feasible routes for the utilization of methane, but the current catalysts still suffer from limited product yield due to the over-oxidation of CH4 into CO and CO2 at high conversion levels. Herein, we demonstrate a CeO2 nanorod catalyst with the surface fabricated by atomically dispersed Pd and Mn for efficient methane oxychlorination (MOC). The optimum Pd-Mn/CeO2 catalyst offers an MOC performance with a CH3Cl selectivity of 72% at CH4 conversion of 33% at 450 °C, stably operating for over 500 h without deactivation. The state-of-the-art performance is attributed to the formation of two synergistic sites with complementary properties, i.e., Pd−O−Ce and Mn−O−Ce centers, which modulate the activation of HCl and O2, respectively. Both in situ spectroscopy and theoretical calculations identify the metal−O−Cl species as a key intermediate in the reaction network. The MOC reaction catalyzed by Pd-Mn/CeO2 achieves about 10% lower life-cycle carbon emissions than the traditional route and retains this advantage across platform-chemical-to-PVC conversion pathways.

Introduction
Methane (CH4) is well-known as the primary component of natural gas, CH4 hydrate, shale gas, and coal-bed gas with large reserves. Besides natural sources, CH4 also comes from human activities, such as landfills, the oil industry, gas extraction, wastewater treatment, and food production1–3. One should note that CH4 is a powerful greenhouse gas, trapping heat 28 times more than carbon dioxide and accounting for about 20% of climate change4,5. In the context of rapid climate and energy transitions, CH4 with renewable or clean energy properties has re-emerged as an extremely promising feedstock for the production of sustainable fuels and chemicals6–9. The current industrial route for CH4 utilization is still mediated by syngas (a mixture of CO and H2), which is subsequently used for methanol (CH3OH) synthesis or Fischer-Tropsch (FT) synthesis (Fig. 1a)10–12. However, CH4 reforming to syngas is quite expensive and energy-consuming due to its high operation temperatures, typically above 800 °C13,14. The direct transformation of CH4 to valuable compounds is conceptually more advantageous than the multi-step processes, but lacks implementable catalytic systems. The perfect tetrahedral symmetry and strong C−H bonds make the adsorption and activation of CH4 under mild conditions quite challenging15. Recently, considerable effort has been devoted to the direct conversion of CH4 with the aid of external fields, such as electricity, light, and plasma16–20, but the energy efficiency and potential for scale-up application still need further verification.

Halogen-mediated functionalization of CH4 is of significant interest as it allows selective one-step transformation of CH4 under relatively mild conditions21,22. Monohalogenomethanes (CH3X), a chemical analog of CH3OH, could serve as alternative platform molecules for further catalytic conversion into high-value chemicals22–24. Olah and co-worker reported a series of supported superacids and or platinum metal catalysts for the selective monohalogenation of CH4 to CH3Cl or CH3Br using Cl2 or Br2, followed by hydrolysis of methyl halides to CH3OH or dimethylether25,26. This process has been proven to be an alternative route for commercial CH4 conversion into hydrocarbons other than the syngas route27. Nevertheless, the generated HCl and HBr must be oxidized to Cl2 and Br2 to complete the catalytic cycle through the Deacon process28, which is quite expensive and environmentally unfriendly (Fig. 1b). In comparison, the methane oxychlorination (MOC) reaction offers a distinct advantage over direct CH4 chlorination by integrating CH4 conversion and HCl recovery feasibly (Fig. 1c), resulting in process intensification and a 100% Cl atom efficiency.

Various catalysts, including metal chlorine oxides29, metal chlorides30–32, noble-based metals33, and metal phosphates34,35 have been explored for the MOC reaction. For example, copper chloride catalysts are widely used in the industrial oxychlorination of ethylene and have also been applied to CH4 oxychlorination36–38, but their low melting point leads to poor structural stability, and the selectivity of monochloromethane (CH3Cl) is still unsatisfying. Lercher and co-workers discovered that the LaCl3 catalyst exhibited a CH4 conversion of 12% and a CH3Cl selectivity of 55% at 475 °C, whereas the vulnerable chemical stability limited its further development39. Our group first reported that the CeO2 nanorod is an efficient candidate for the MOC reaction, showing good stability and CH3Cl selectivity of 66% at a CH4 conversion of 12% at 480 °C40. However, it is difficult for individual phases or sites to satisfy the activation of C−H bonds, O2, and HCl simultaneously, restricting the improvement in MOC performance. Shalygin et al. reported that a composite K2Ru10OCl2/TiO2 catalyst could significantly decrease the MOC temperature to 327 °C, achieving a relatively high CH3Cl selectivity of 82% at a CH4 conversion level of ~8%41. Terlingen et al. recently found that the partial replacement of Eu3+ by La3+ in EuOCl could significantly enhance the CH3Cl selectivity from 54% to 66% compared to the individual LaOCl or EuOCl, owing to the improved degree of chlorination42. Overall, the MOC reaction is still severely restricted by the activity-selectivity trade-off, and the development of an efficient catalyst with high CH4 conversion levels, CH3Cl selectivity, and stability remains challenging.

We report a strategy for fabricating CeO2 nanorods using Pd and Mn dual metals with atomic dispersion, which enhances the MOC reaction performance significantly. The Pd-Mn/CeO2 catalyst exhibited a remarkable CH4 conversion of up to 33% with a high CH3Cl selectivity of 72% and excellent stability for 500 h, outperforming most of the reported MOC catalysts. Through a series of characterizations, we propose a synergy between the formed Pd−O−Ce and Mn−O−Ce sites. The doping of hypervalent Pd species promotes the formation and reactivity of selective surface chlorine species. The Mn−O−Ce sites are conducive to oxygen activation, promoting the regeneration of hypervalent Pd and metal−O−Cl species, which catalyze the conversion of CH4 to CH3Cl. The MOC reaction catalyzed by Pd-Mn/CeO2 achieves about 10% lower life-cycle carbon emissions than the traditional route and retains this advantage across platform-chemical-to-PVC conversion pathways.

Results
Catalytic performance
We have previously demonstrated that CeO2 with nanorod morphology is the most efficient oxide catalyst for the MOC reaction40. In this work, it was found that the pristine CeO2 nanorods exhibited a CH3Cl selectivity of 55% at a CH4 conversion of 27% at 480 °C (Fig. 2a). Since noble metals have often been used for the activation of C−H bonds, which is considered to be the rate-determining step for selective CH4 conversion33, we employed Rh, Ru, Au, Pt, Ag, and Ir to improve the MOC performance of CeO2 nanorods. However, most of them primarily catalyzed over-oxidation reactions, producing CO and CO2, except for Pd (Supplementary Fig. 1 and Supplementary Table 1). Pd is often employed for the direct halogenation of C−H bonds in organic chemistry43. Then, we investigated the effect of Pd weight loading on the MOC performance of CeO2 nanorods (Fig. 2a and Supplementary Table 2). With the fabrication of Pd, the selectivity of the CH3Cl steadily increased from 55% (CeO2 nanorods) to 84% (1.5Pd/CeO2), and the formation of byproduct CO and CO2 was significantly suppressed. However, the conversion levels of O2 and CH4 were also suppressed, especially at high Pd loadings, resulting in a lower yield of CH3Cl.

To promote the activation of O2 and CH4, we introduced a second metal with redox features into 1Pd/CeO2 (Fig. 2b and Supplementary Table 3). The modification of 1Pd/CeO2 by Co, V, Cr, and Cu was found to significantly increase the CH4 conversion but decrease the CH3Cl selectivity, whereas the modification of Ni, Zn, and Fe promoted CH3Cl formation but did not significantly affect the CH4 conversion. Notably, the Pd and Mn co-promoted catalyst exhibited promising performance for the selective conversion of CH4 to CH3Cl. The CH3Cl selectivity was found to be independent of Mn weight loadings, but CH4 and O2 conversion increased with Mn contents from 1 wt% to 3 wt% and then reached a plateau until Mn loading was 15 wt% (Fig. 2c and Supplementary Table 4). However, increasing the Mn content to 20 wt% was detrimental to the activities. Mn oxides are supposed to catalyze the deep oxidation in alkane conversion under high reaction temperatures44,45, but the pristine MnO only exhibited a low CH4 conversion of 1.8% in the MOC reaction (Supplementary Table 4). Therefore, we can speculate that only fractional Mn species doped onto the CeO2 surface contributed to the enhanced MOC performance, while excessive Mn loadings may cover the active sites on the CeO2 surface. The 1Pd-3Mn/CeO2 exhibited a CH3Cl selectivity of 72% at a CH4 conversion of 33%, with a CH3Cl yield of 24%. Furthermore, the 1Pd-3Mn/CeO2 catalyst was stable, and no catalyst deactivation was observed during the 500 h reaction (Fig. 2d). When catalyst stability, low-temperature activity, and product yield are considered comprehensively, the Pd-Mn/CeO2 catalyst exhibits superior performance compared with previously reported catalysts, thereby serving as a benchmark for MOC reactions (Fig. 2e). The control Mn/CeO2 catalyst without Pd fabrication achieved only a 15% yield of CH3Cl, despite a high CH4 conversion of 32% (Supplementary Table 4). The above findings indicate a synergistic effect existing between Pd and Mn in enhancing the oxychlorination performance.

Catalyst structure
The doping of Pd and Mn into CeO2 nanorods was executed by an impregnation method using a solution containing inorganic metal salts, followed by calcination. The representative 1Pd-3Mn/CeO2 exhibited a uniform nanorod morphology, predominantly exposing the (200) and (111) facets, revealed by transmission electron microscope (TEM) (Fig. 3a and Supplementary Fig. 2). The inductively coupled plasma emission spectrometry (ICP-OES) shows that the actual loadings of Pd and Mn in 1Pd-3Mn/CeO2 were 0.89 and 2.8 wt%, respectively, close to the nominal values (Supplementary Table 6). X-ray diffraction (XRD) analysis revealed that only the crystalline structure of cubic CeO2 was observed over xPd-yMn/CeO2 series catalysts with no detectable peaks corresponding to PdO or MnOx phases (Supplementary Fig. 3). Even after 500 h of reaction, no characteristic peaks of PdO and MnOx were observed over 1Pd-3Mn/CeO2 (Supplementary Fig. 4), indicating the structural robustness of the catalyst. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging also confirmed the absence of PdO and MnOx particles on the CeO2 surface and the richness of defective voids inside CeO2 crystals (Fig. 3b). Energy-dispersive X-ray spectroscopy (EDS) mapping suggested a homogeneous distribution of Pd and Mn on CeO2 nanorods (Supplementary Fig. 5). The Pd and Mn weight loadings analyzed by EDS mapping are close to those measured by ICP-OES (Supplementary Table 7).

To probe the valence state and coordination environment of the active sites, we performed X-ray absorption spectroscopy (XAS) studies over Pd-Mn/CeO2 catalysts. The X-ray absorption near-edge structure (XANES) spectra of 1Pd-3Mn/CeO2 suggested that the valence state of Pd was between 0 (Pd foil) and +2 (PdO) but closer to +2 (Fig. 3c). The extended X-ray absorption fine structure (EXAFS) spectra and wavelet transform (WT) plots also suggest the absence of Pd0. The peaks located at 1.50 and 2.95 Å could be assigned to the Pd−O and Pd−O−Ce coordinations, respectively (Fig. 3e and Supplementary Fig. 6)46,47. The XANES spectra of Mn indicate that the valence state of Mn was close to that of Mn2O3 reference, implying that the Mn mainly existed in the +3 oxidation state (Fig. 3d). The EXAFS spectra of Mn exhibited a prominent peak at 1.41 Å, attributed to Mn−O first shell scattering (Fig. 3f and Supplementary Fig. 7)48. These findings explicitly demonstrate the high dispersion and oxidative state of Pd and Mn on CeO2 nanorods.

X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy studies were performed to further investigate the nature of the active centers. The XPS spectra of both 1Pd/CeO2 and 1Pd-3Mn/CeO2 catalysts exhibited two signals with binding energy (BE) values at approximately 337.8 eV (Pd 3d5/2) and 343.1 eV (Pd 3d3/2) corrected by Ce 3d3/2 at 916.8 eV (Fig. 4a). The introduction of Mn did not cause a significant shift in the binding energy of Pd, indicating a weak interaction between Pd and Mn. As the Pd loading increased to 2 wt%, an additional weak peak located at approximately 336.8 eV was observed and could be attributed to the formation of PdO clusters (Supplementary Fig. 9a). Some groups attributed the peaks at 337.8 and 343.1 eV to the atomically dispersed Pd species stabilized in the form of the Pd−O−Ce species with hypervalent Pd46,47,49. In the Raman spectra, the weak bands at 189 and 668 cm–1, associated with Pd−O−Ce vibrations50, were also observed in both xPd/CeO2 and 1Pd-3Mn/CeO2 catalysts (Fig. 4b and Supplementary Fig. 9b), in line with XPS analysis. The Raman bands at 460 and 590 cm–1 were assignable to the Ce−O−Ce vibration and the defect-induced (D) models, respectively (Fig. 4b)51. Additionally, no characteristic vibrations assigned to Mn2O3 and MnO2 were observed over yMn/CeO2 and 1Pd-yMn/CeO2 catalysts with Mn contents ≤ 5 wt% (Supplementary Fig. 10), suggesting that the Mn was probably in the form of Mn−O−Ce species dispersed on the CeO2 surface.

By the deconvolution of the Ce 3d XPS spectra, we can observe the presence of Ce4+ together with Ce3+, and the ratio of Ce3+/Ce4+ increased as the fabrication of Mn (Fig. 4c), indicating that Mn could promote the redox properties of Pd-Mn/CeO2 catalysts. The existence of Ce3+ also suggests the formation of oxygen vacancies (OVs), which relates to the Raman vibration peak at 590 cm–1 (Fig. 4b). Previous studies theoretically demonstrated that the Pd doping could decrease the formation energy of OVs on CeO2 by emerging Pd-induced gap states above the valence band and below the empty Ce 4f states52. We further investigated the chemisorption of O2 over each catalyst using O2 temperature-programmed desorption (O2-TPD) experiments (Fig. 4d). No desorption peak was observed for the pristine CeO2, and the promotion of Pd led to a weak desorption peak at 150 °C. The promotion of Mn had a more significant influence on the adsorption of O2 than that of Pd. Significant amounts of adsorbed O2 above 500 °C were observed, which may enhance the reactivity of the MOC reaction.

Catalytic kinetics
Kinetic studies were performed to gain insight into the activation mechanism of CH4, HCl, and O2 (Fig. 5 and Supplementary Fig. 11). At a constant temperature of 450 °C, O2 partial pressure of 10 kPa, HCl partial pressure of 20 kPa, and He as the balance gas, the formation rates of CH3Cl over CeO2 and 1Pd-3Mn/CeO2 showed reaction orders of 0.75 and 0.70 with respect to CH4, respectively. This indicates that CH4 was weakly adsorbed on these catalysts under reaction conditions. We speculate that fractional CH4 adsorbed on the catalysts would eventually be over oxidized to CO and CO2. Additionally, at a constant O2 partial pressure of 10 kPa and CH4 partial pressure of 40 kPa, the formation rates of CH3Cl over both CeO2 and 1Pd-3Mn/CeO2 tended to reach a plateau with increased HCl partial pressure. Based on the above, it can be inferred that the reaction of CH4 and HCl to CH3Cl may follow an Eley-Rideal mechanism, involving a reaction between gaseous CH4 and adsorbed chlorine species. Furthermore, with a constant CH4 partial pressure of 40 kPa and HCl partial pressure of 20 kPa, the formation rates of CH3Cl over 1Pd-3Mn/CeO2 showed a reaction order of zero with respect to O2. Thus, we conclude that lattice oxygen or adsorbed oxygen of 1Pd-3Mn/CeO2 is the reactive oxygen species, and the reaction mainly proceeds through the Mars-van Krevelen mechanism. The reaction order of O2 over CeO2 is about 0.40, indicating that 1Pd-3Mn/CeO2 could activate O2 more strongly than CeO2.

Reaction mechanism
To illustrate the reaction mechanism, we followed the evolution of the surface species by in situ Raman spectroscopy. The vibrational bands observed at 273 cm–1 and 302 cm–1 can be ascribed to the asymmetric and symmetric stretching vibrations of the Pd−Cl bonds, and the ones at around 176 cm–1 and 207 cm–1 can be attributed to the formation of Ce−Cl bonds53,54. First, we observed the formation of Pd−Cl species on the Pd-containing catalysts but not on CeO2 after the MOC reaction (Fig. 6a). For pristine CeO2 catalyst, the formation of Ce−Cl species were observed at ≥150 °C upon heating under HCl atmosphere, and these signals persisted up to 450 °C (Fig. 6b). Interestingly, the formation of Pd−Cl species was detected even at 25 °C over 1Pd/CeO2 catalyst under HCl atmosphere (Fig. 6c), indicating that doping Pd is conducive to the adsorption of chlorine species. However, at temperatures of ≥200 °C, the vibration peaks of Pd−Cl species disappeared, and the intensity of Ce−Cl species decreased at ≥300 °C. This implies that the Pd doping may electronically fabricate the Ce sites, thus affecting the strength of Ce−Cl bonds, or there exists *Cl2 exchange between Pd and Ce sites. The formation of Ce−Cl species was also observed over the Mn/CeO2 catalyst, but the peak intensity was weaker than that of pristine CeO2 (Fig. 6d).

The evolution of chlorine species was further investigated under a mixture of O2 and HCl atmospheres. Compared with the high thermal stability of Ce−Cl species over CeO2 under HCl atmosphere (Fig. 6b), the vibration peaks of Ce−Cl disappeared when the temperature was raised to 300 °C (Fig. 6e). In the case of Mn-containing catalysts with high redox activity, the Ce−Cl vibration peak disappeared at even lower temperatures of ≤200 °C (Supplementary Fig. 12). Therefore, it is reasonable to speculate that oxygen may react with or consume the adsorbed Ce−Cl species, leading to the formation of other intermediates. The Pd doping to CeO2 led to the formation of more types of chlorine species, especially at temperatures of ≤150 °C (Fig. 6f). Except for Ce−Cl species, suspicious vibration peaks at 138 cm–1 and 166 cm–1, which could be attributed to the metal−O−Cl species54, were also observed. It is noteworthy that metal oxychloride (M−OCl) catalysts have been applied in the MOC reaction32,41,42. Quasi-in situ electron paramagnetic resonance (EPR) was performed to investigate the defective sites of the CeO2-based catalyst. Both 1Pd/CeO2 and 1Pd-3Mn/CeO2 catalysts, before and after reactions, exhibited a sharp signal at g=2.003 (Supplementary Fig. 13), which can be identified as the unpaired electrons trapped on OVs55. The O2 molecules have been evidenced to be feasibly adsorbed at OVs and charged to form active oxygen species56. The 1Pd/CeO2 after the reaction shows a higher signal intensity than that of the fresh catalysts, indicating that lattice oxygen is involved in the reaction. In addition, the introduction of Mn is conducive to the activation of O2 and the formation of OVs.

Density functional theory (DFT) calculations were performed to gain in-depth insights into the reaction mechanism. We compared the dissociative adsorption energies of the HCl molecule on CeO2, Pd/CeO2, and Mn/CeO2 to evaluate the HCl activation capability of different catalysts (Fig. 7a). The results showed that the dissociative adsorption energies of HCl are −1.97 eV, −2.41 eV, and −1.08 eV, respectively. It was shown that Mn/CeO2 possesses weaker HCl activation ability than pristine CeO2, indicating that, after CH4 undergoes C−H bond activation, the resulting CHx intermediates (e.g., *CH3 and *CH2) are less likely to react with Cl species to form CH3Cl and CH2Cl2. Instead, the CHx intermediates tend to undergo deep oxidation, yielding CO and CO2. In contrast, Pd/CeO2 displays a higher HCl dissociative adsorption energy, indicating that the presence of the Pd active site strengthens HCl activation, thereby suppressing the over-oxidation of CH4, and improving the selectivity of CH3Cl and CH2Cl2. The in situ Raman spectroscopy implies that the adsorbed oxygen may react with the adsorbed Cl species to form metal−O−Cl species (Fig. 6f). We further computationally examined O2 activation and followed methane reaction behavior on surface metal−Cl species (Fig. 7b). After surface Cl species are formed according to Equations (1) and (2), O2 adsorbs at oxygen vacancies on the catalyst surface, leading to O−O bond cleavage. The resulting O species then inserts into the M−Cl bond (M=Ce, Pd, and Mn), forming M−O−Cl species (Fig. 7b). The energy barriers for this O2 activation process follow the trend of Pd/CeO2 (2.18 eV) > CeO2 (1.72 eV) > Mn/CeO2 (1.63 eV), which is consistent with the observed oxygen conversion rates in the catalytic reaction over these three materials (Fig. 2), and further confirming the promoting effect of Mn on O2 activation. In addition, we also examined the charge distribution of Cl species before and after oxygen insertion. In this process, the charge of the Cl species in M−Cl (M=Ce, Pd, and Mn) and M−O−Cl fragment is opposite. For instance, in the case of CeO2, the Cl species has a charge of −0.423 |e| in Ce−Cl, whereas in the Ce−O−Cl fragment, it is +0.269 |e| (Fig. 7c). It may facilitate the coupling of Cδ−H species, generated from activating CH4 via H abstraction, with the Clδ+ species. Our DFT calculations reveal that the reaction between Metal−O−Cl and CH4 leads to the formation of CH3Cl. The activation barriers follow the order of Mn/CeO2 (1.36 eV) > Pd/CeO2 (1.13 eV) > CeO2 (0.77 eV). Notably, the activation barriers of this step on all three models are lower than the O2 activation step, implying that the O2 activation step is likely the rate-determining step. This observation partially accounts for the enhanced catalytic activity of the Pd-Mn/CeO2 catalyst, which can be attributed to the improved capacity for O2 activation upon the incorporation of the Mn component.

Life-cycle environmental impacts and economic costs
To quantify the environmental impacts and economic performance of the MOC reaction over the Pd-Mn/CeO2 catalyst, we developed and executed an integrated framework that coupled life cycle assessment (LCA), material flow analysis (MFA), scenario analysis (SA), and techno-economic assessment (TEA). MFA results indicate that for the MOC reaction with 72% selectivity of CH3Cl at the CH4 conversion of 33%, producing 1000 kg CH3Cl requires 3532 kg of main raw materials and 1 kg of Pd-Mn/CeO2 catalyst, yielding 1889 kg of by-products and 634 kg of wastewater (Fig. 9a). The LCA and SA results show that by integrating strategies such as by-product recycling, CO2 capture, heat recovery, selectivity and conversion improvements, and clean energy use, the carbon emissions of the MOC reaction can be reduced to as low as 2.2 kg CO2-eq/kg (Fig. 9b and Supplementary Table 10). Key measures for cleaner production include recycling CH4 and HCl by-products and improving selectivity and conversion. The MOC reaction shows low sensitivity to the origin of CH4 (11 scenarios), the CH4 supply location (15 scenarios), and the energy type (6 scenarios), with a standard deviation of 8−14% across scenarios after excluding one outlier (Fig. 9c and Supplementary Table 11).

Furthermore, we compared our optimized MOC process (S5) with the CH3OH-fed solid-catalyst route (functional unit: 1 kg CH3Cl). The MOC reaction delivers 10.1% lower carbon emissions (Fig. 9d and Supplementary Table 12) but incurs higher costs due to palladium chloride in the Pd-Mn/CeO2 catalyst (Fig. 9e and Supplementary Table 12), underscoring the need for low-cost catalyst alternatives. In addition, the MOC reaction shows advantages in more than 88% of life cycle impact assessment (LCIA) midpoints, with disadvantages only in material resources (caused by the consumption of cerium nitrate hexahydrate) and ionizing radiation (caused by HCl-related processes). To evaluate the environmental benefits of different platform chemicals (functional unit: 1 kg PVC resin), we further assessed five platform-chemical-to-PVC conversion paths (R1-R5). Among them, the pathway of CH3Cl from the MOC reaction exhibited lower carbon emissions than the other two emerging chemical routes, and the commercialized and technically mature CaC2 route (Fig. 9f and Supplementary Table 10).

Methods
Catalysts preparation
All the chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd, and used without further purification. The CeO2 nanorods were synthesized using a hydrothermal method, as reported earlier. In brief, 3.47 g of Ce(NO3)3∙6H2O was dissolved in 20 mL of deionized water. Then, 140 mL of 9 M NaOH solution was quickly added to the former solution under vigorous stirring for 0.5 h to form a homogeneous solution. The slurry was subsequently transferred into a 200 mL Teflon liner, and the hydrothermal synthesis was subjected to a stainless steel autoclave at 100 °C for 24 h. The formed solids were washed several times with deionized water until a PH of ~7 was obtained. The CeO2 nanorods were obtained by calcining the dried powders in air at 550 °C (with a heating rate of 2 °C min–1) for 6 h.

The noble metals (Pd, Rh, Ru, Au, Pt, Ag, and Ir) modified CeO2 nanorods were prepared by an impregnation method. Taking the 1Pd/CeO2 catalyst as an example, an appropriate amount of H2PdCl4 solution was impregnated onto the CeO2 powder. The resulting mixture was stirred continuously for 12 h at room temperature and then slowly heated to 80 °C until dry. The obtained solid was dried at 60 °C overnight, followed by calcining at 550 °C for 6 h. The catalysts thus prepared were denoted as 1Pd/CeO2, 1Rh/CeO2, 1Ru/CeO2, 1Au/CeO2, 1Pt/CeO2, 1Ag/CeO2, 1Ir/CeO2, respectively. The Pd/CeO2 samples with different weight percentages were denoted as 0.5Pd/CeO2, 1Pd/CeO2, 1.5Pd/CeO2, and 2Pd/CeO2, depending on the intended Pd loading. The noble and transition metals co-modified (xPd-yMn/CeO2, where x and y denote weight percent loadings) CeO2 nanorods were prepared by an impregnation method, where x and y denote the weight percentage of CeO2 nanorods, respectively. Taking the 1Pd-3Mn/CeO2 catalyst as an example, the CeO2 powder was first dispersed in an appropriate amount of H2PdCl4 solution and stirred for 0.5 h at room temperature. Mn(NO3)2∙xH2O solution was then added to the former solution under vigorous stirring for 0.5 h. The resulting mixture was stirred continuously for 12 h at room temperature and then slowly heated to 80 °C until dry. The obtained solid was dried at 60 °C for 12 h and calcined at 550 °C for 6 h.

Catalyst characterization
Powder X-ray diffraction (XRD) was conducted using a Rigaku SmartLab-SE diffractometer with Cu Kα radiation (40 kV, 30 mA). Raman spectra were recorded on a confocal Renishaw inVia instrument equipped with a CCD detector using a 514 nm laser as the excitation source. The metal loading was measured by ICP-OES on a Thermo Scientific instrument (iCAP 7000). The X-ray photoelectron spectroscopy (XPS), which could provide information on the surface elemental compositions, was performed on a PHI Quantum 2000 Scanning ESCA spectrometer using a monochromatized Al-Kα as the X-ray source. The X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) of catalysts at the Pd K-edge and Mn K-edge were performed on the BL141W1 beamline of the Shanghai Synchrotron Radiation Facility, operated at 3.5 GeV with a constant electron current of 260 mA. All measurements were performed in fluorescence excitation mode using Pd foil, PdO, Mn foil, MnO, Mn2O3, and Mn3O4 as references. The collected spectra were background-subtracted and normalized using the Athena software package. EXAFS fitting of the k3-weighted data processed with Artemis software. Transmission electron microscope (TEM) images and element mapping measurements were performed on a Talos F200X instrument operated at an acceleration voltage of 200 kV. The electron paramagnetic resonance (EPR) was performed at room temperature on a Bruker EMX-10/12 EPR spectrometer. O2 temperature-programmed desorption (O2-TPD) was carried out on a Micromeritics AutoChem Ⅱ 2920 instrument equipped with a thermal conductivity detector (TCD). Typically, 100 mg of catalyst was pretreated in a 10 vol% O2 in Ar flow at 500 °C for 2 h, followed by cooling down to 50 °C. Then, the Ar flow was switched for 0.5 h to level off the baseline. The desorption was performed using Ar temperature, which was heated to 900 °C at a heating rate of 10 °C min–1.

Catalyst evaluation
The oxidative chlorination of CH4 was performed on a fixed-bed flow reactor with an inner diameter of 10 mm at atmospheric pressure. Before the reaction, 0.5 g of powdery catalyst was pretreated for 0.5 h at 550 °C under an O2/Ar mixture flow. After cooling down to 100 °C, the reactant gas mixture with a CH4/HCl/O2/N2 molar ratio of 4/2/1/3 and a total flow rate of 40 mL min–1 was fed to the reactor, and then the system temperature was heated to 450 °C. The gaseous products were analyzed online using a gas chromatograph. 5A Molecular sieve column was coupled with TCD to analyze O2, N2, CH4, and CO. KB-Al2O3/Na2SO4 column was coupled with FID to analyze CH4, CH3Cl, and CH2Cl2.

Computational method
All spin-polarized calculations were performed with the Vienna ab initio simulation package (VASP 5.4.4)57,58. The electron exchange was described by generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) functional59, and the projector augmented-wave (PAW)59,60 method was used to describe the ion-electron interaction with a plane wave basis set using a cut-off energy of 400 eV. In order to describe the Ce 4f electrons, the DFT+U method61 with an effective Hubbard U value (Ueff) of 4.5 eV was employed. Van der Waals interaction was described by means of Grimme’s DFT-D3BJ approach62. The CeO2 (110) surface was modeled using a four-layer slab with (2×3) surface supercell (Ce48O96). Pd/CeO2 and Mn/CeO2 were modeled by replacing one of the surface Ce atoms with a Pd or Mn atom on the CeO2 (110) face, while one or two H atoms were added to the CeO2 surface to balance the oxidation states of Pd2+ and Mn3+ (Supplementary Figs. 15–17). A vacuum layer of 30 Å in the z direction was used to avoid artificial self-interactions between slabs due to periodic boundary conditions. Brillouin zone sampling was restricted to the Γ point, and Gaussian smearing was employed with a width of 0.05 eV to determine electron occupancy. For the geometry optimization, the convergence criteria for energy and force were set to 10−6 eV/atom and 0.05 eV/Å, respectively. Transition states were located by climbing nudged elastic band (CI-NEB)63 and dimer methods64, and the force residue was set to 0.05 eV/Å.

Life-cycle environmental impacts and economic costs evaluation
The integrated framework centers on LCA with a functional unit of 1 kg CH3Cl or 1 kg PVC and a cradle-to-gate system boundary. MFA was used to construct the lifecycle inventory (LCI) for the MOC reaction, while LCIs for other chemicals and processes were compiled from the literature (Supplementary Table 9). The SA comprised six forward-looking technology cases for the MOC reaction (S1-S6) and 32 scenarios varying the origin of CH4, the location of CH4, and the energy type. LCIs for all scenarios were determined from literature data and stoichiometric calculations. Background data were sourced from the Ecoinvent65 and HiQLCD66 databases. Under the ReCiPe 2016 method, 18 LCIA midpoints were applied, with global warming potential (GWP 100) representing carbon emissions. The TEA covered raw material, energy, and waste-disposal costs, while excluding feedstock transport and fixed capital due to data gaps.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, a Pd-Mn/CeO2 catalyst is presented to carry out methane oxychlorination to methyl chloride with stable behavior for 100 hours on stream. The authors have previously worked on this reaction, specifically on CeO2 nanorods, so this study can be considered as a follow-up study to prepare a material with improved efficiency. The manuscript presents improvements over their previous CeO2 system, but I am not convinced with the final catalytic performance, the fundamental insights into the catalytically active assembly and reaction mechanism compared to studies reported by other groups. More importantly, I am shocked that there is no economic and environmental assessment of this technology to justify the overall study.\nThe paper provides valuable data for a specialized catalysis journal such as J. Catal. or ACS Catal. but in my opinion, the lack of conceptual breakthrough, more evidence of the reaction mechanism, as well the absence of economic and environmental impacts on the oxychlorination technology do not warrant publication in Nature Communications. Let me elaborate on four major comments that justify my opinion on this piece.\n1. The authors ‘over-sell’ a record performance in Fig 2e but the CH3Cl yield is very similar or worse than reported systems. They justify the superiority of Pd-Mn/CeO2 based on a 100-h test but other systems reported were stable for 50 h and they are considered inferior. If they want to make a difference in terms of stability, tests should run for at least 1000 h. 50 and 100 h do not make a difference to discriminate catalysts as the difference is anecdotical.\n2. The characterization of the catalysts use conventional methods and the resulting interpretations are speculative. The authors have not been able to prove experimentally the proximity between Pd-O-Ce and Mn-O-Ce centers. The use of operando spectroscopies beyond Raman would make headway. Also the microscopy of the material is pretty conventional and does not provide true insights on the catalytically active ensemble.\n3. The design of the catalyst from CeO2 to Pd-Mn/CeO2 is obvious and therefore I see the concept incremental and expected, since Pd has been reported as active phase in methane oxychlorination and Mn is now to increase the number of oxygen vacancies on ceria.\n4. Mechanistic interpretations based on steady-state kinetic measurements (Eley-Rideal with respect to CH4 and Mars-van Krevelen with respect to oxygen) is speculative. Transient kinetic methods and techniques that are sensitive to radicals in the gas phase should have been used to strengthen the mechanistic part of the paper. An in-depth DFT analysis would be required too in order to support experimental observations and gain further molecular level insights.\n5. A detailed techno-economic analysis (TEA) and life cycle analysis (LCA), including a sensitivity analysis on origin, location, of methane should have been conducted (a) to evaluate the potential of methane oxychlorination to methyl chloride with respect to e.g. methanol coming from biogas as well as (b) to determine what threshold of CH3Cl yield (selectivity, conversion) and lifetime makes this technology economic and environmentally attractive. Otherwise I find no justification to incrementally improve the catalytic performance. How can methyl chloride compare and beat to methanol in economic and environmental terms?\nWith these weaknesses related novelty and impact and taking into account the general scope of Nature Communications, I cannot recommend acceptance.", "reviewer_2": "Methane oxychlorination (MOC) represents a promising pathway for the valorization of methane into functionalized intermediates, such as methyl chloride (CH₃Cl), which can subsequently be transformed into valuable hydrocarbon chemicals, like methanol. Although several redox-active oxide catalysts have been reported, the rational design of highly efficient oxychlorination catalysts remains in its infancy. This manuscript presents a Pd-Mn-modified CeO₂ nanorod catalyst featuring atomically dispersed active sites for MOC. The catalyst achieves a CH₃Cl selectivity of 72% at a CH₄ conversion of 33%, surpassing the performance of most catalysts reported to date. The authors have thoroughly investigated the relationship between catalyst structure and catalytic behavior. Overall, the study provides a meaningful contribution to the development of MOC catalysis. However, the following comments should be considered.\n1. The authors mention the incorporation of a secondary metal to enhance O₂ activation in catalysts containing 1.0 wt% Pd. However, it is noted that the 1.5Pd/CeO₂ catalyst exhibits even higher CH₃Cl selectivity. Could the authors elaborate on why the 1.5Pd/CeO₂ composition was not chosen as the baseline for introducing the second metal?\n2. Interestingly, upon Mn incorporation, the catalytic performance appears to plateau with increasing Mn content, from 1Pd-3Mn/CeO₂ to 1Pd-10Mn/CeO₂ and further to 1Pd-15Mn/CeO₂. Is this saturation effect in activity linked to changes in the oxidation state or dispersion of Mn species? Further clarification would be appreciated.\n3. The manuscript proposes an Eley–Rideal (E–R) mechanism for CH₃Cl formation based on kinetic studies. To further substantiate this mechanistic hypothesis, additional experiments under elevated CH₃Cl partial pressure would be valuable. Specifically, the authors are encouraged to examine the effects of systematically increasing the HCl partial pressure on the reaction rate. Such data could reveal whether a distinct rate plateau emerges at high HCl concentrations, thereby supporting the E–R model. How well do the observed kinetics conform to the characteristic signatures of an E–R mechanism?", "reviewer_3": "Selective functionalization of methane into other chemical intermediates is a holy grail in catalysis. Currently, the chemical transformation of methane is primarily limited to its reforming into syngas, which has to be operated above 1000 °C and is very costly and energy-intensive. This manuscript investigates methane oxychlorination, which operates under mild conditions of around 450 °C. Several groups have previously several types of oxide catalysts, but the product yield and catalytic stability are limited. Han et al. reported a well-designed Pd-Mn dual-metal doped CeO2 nanorod catalyst for methane oxychlorination, offering excellent performance with a CH3Cl selectivity of 72% at CH4 conversion of 33% at 450 °C, with stability maintained for over 100 hours. This performance is better than the reported results. Besides, the catalyst structure and reaction network have been thoroughly characterized through a series of techniques, and this manuscript is well-written and organized. I would like to suggest its acceptance after addressing the following comments.\n[1] The authors evaluated the stability of the 1Pd-3Mn/CeO2 catalyst, demonstrating nearly constant CH4 conversion and CH3Cl selectivity. Whether the authors further characterize the post-reaction catalyst's properties, such as differences in crystal structure or variations in Raman spectra before and after the reaction?\n[2] In Figure 6a, the authors assigned both the peaks at 273 cm-1 and 302 cm-1 to Pd-Cl vibrations. Could these two peaks correspond to distinct Pd−Cl coordination geometries?\n[3] What is the rate-determining step (RDS) in the methane oxychlorination reaction over this catalyst? Did the authors perform relevant in-situ characterization of the CeO2 catalyst (e.g., monitoring dynamic changes in surface oxygen vacancies or chlorine adsorption states)? A critical experiment to supplement would involve tracking chlorine species (e.g., adsorbed Cl coverage, chlorination depth) on Pd-Mn/CeO2, as this could provide direct evidence linking catalyst surface evolution to the proposed RDS.\n[4] Please ensure all catalysts discussed in the manuscript have corresponding ICP-OES entries in the Supporting Information (in Table S6)."}, "author_response": "Responses to Reviewers\nResponse to Reviewer 1\nGeneral comment: In this work, a Pd-Mn/CeO catalyst is presented to carry out methane\n2\noxychlorination to methyl chloride with stable behavior for 100 hours on stream. The authors have\npreviously worked on this reaction, specifically on CeO nanorods, so this study can be considered as a\n2\nfollow-up study to prepare a material with improved efficiency. The manuscript presents improvements\nover their previous CeO system, but I am not convinced with the final catalytic performance, the\n2\nfundamental insights into the catalytically active assembly and reaction mechanism compared to studies\nreported by other groups. More importantly, I am shocked that there is no economic and environmental\nassessment of this technology to justify the overall study.\nThe paper provides valuable data for a specialized catalysis journal such as J. Catal. or ACS Catal.\nbut in my opinion, the lack of conceptual breakthrough, more evidence of the reaction mechanism, as\nwell the absence of economic and environmental impacts on the oxychlorination technology do not\nwarrant publication in Nature Communications. Let me elaborate on four major comments that justify\nmy opinion on this piece.\nAuthor reply: We sincerely thank the reviewer for the careful evaluation of our work and for raising\nthese important points. We fully understand the concerns regarding the catalytic performance,\nmechanistic insights, and the broader economic and environmental relevance of methane\noxychlorination. In the revised version, we have carefully addressed these issues: (1) We performed\nadditional experiments and analyses to clarify the superiority of catalytic performance and provide\ndeeper insights into the structure–activity relationship of the Pd-Mn/CeO catalyst. Especially, this\n2\ncatalyst exhibited a stable performance of 500 hours, far superior to reported catalysts. (2) We expanded\nthe mechanistic discussion with new supporting data, including in situ characterizations and DFT\ncalculations, based on which we have demonstrated the roles of Mn and Pd in catalysis. (3) We\nconducted techno-economic analysis (TEA) and life cycle analysis (LCA) to highlight the potential\nadvantages and applicability of the MOC process. It is evidenced that the MOC process may reduce\ncarbon emissions by integrating strategies such as by-product recycling, CO capture, heat recovery,\n2\nand clean energy use. We hope these revisions and additional results will enhance the manuscript's depth\nand meet the high standards of this journal.\n3\nor worse than reported systems. They justify the superiority of Pd-Mn/CeO based on a 100-h test but\n2\nother systems reported were stable for 50 h and they are considered inferior. If they want to make a\ndifference in terms of stability, tests should run for at least 1000 h. 50 and 100 h do not make a difference\nto discriminate catalysts as the difference is anecdotical.\nReply and actions taken: We sincerely appreciate the reviewer’s comments. We have now removed\nthe statement of \"record performance\" and made fair comparisons with the reported results. Regarding\nthe stability evaluation, we have conducted an additional 500-hour stability test. The new data (Updated\nFig. 2d and Fig. 2e, also see below) show that our Pd-Mn/CeO catalyst was quite stable during 500\n2\nhours of reaction, without observable decay in both CH Cl selectivity and CH conversion. The reason\n3 4\nwe didn’t run the reaction for 1000 hours is due to the safety regulations of our institute. This stability\nfar exceeds the typical lifetime of 24–100 hours reported in the literature, rendering our catalyst one of\nthe best catalysts considering catalytic performance, catalyst cost, and operation conditions. We have\nupdated the relevant data and analysis in the manuscript.\n...\n(Full text of rebuttal continues as provided in the source)"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors convincingly addressed my main concerns through extensive additional analyses. I am happy to support its acceptance.", "reviewer_2": "The authours have well answered my questions. The revsied manuscrtipt can be considered for publication in NC.", "reviewer_3": "The authors have done an excellent job in revising the manuscript, carefully addressing all comments point-by-point. I have no further concerns and recommend acceptance."}, "author_response": "Nature Communications manuscript NCOMMS-25-07020A\nResponse to Reviewers\nResponse to Reviewer 1\nComment: The authors convincingly addressed my main concerns through extensive additional\nanalyses. I am happy to support its acceptance.\nReply: We thank the reviewer for the positive evaluation and are pleased to hear that our revisions\nhave satisfactorily addressed all concerns.\nResponse to Reviewer 2\nComment: The authors have well answered my questions. The revised manuscript can be considered\nfor publication in NC.\nReply: We thank the reviewer for the encouraging feedback and are delighted that the revisions meet\nthe reviewer’s expectations.\nResponse to Reviewer 3\nComment: The authors have done an excellent job in revising the manuscript, carefully addressing\nall comments point-by-point. I have no further concerns and recommend acceptance.\nReply: We sincerely appreciate the reviewer’s supportive comments and are glad that the additional\nanalyses fully resolved the raised issues."}]

Decision letter