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Activating and Converting CH4 into CH3OH via CuPdO2/CuO Nanointerface Shuxing Bai, Yong Xu, Pengtang Wang, Qi Shao, and Xiaoqing Huang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00714 • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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Activating and Converting CH4 into CH3OH via CuPdO2/CuO Nanointerface Shuxing Bai,1† Yong Xu,2† Pengtang Wang,1 Qi Shao,1 and Xiaoqing Huang1*

1College

of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu, 215123, China.

2Institute

of Functional Nano&Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Jiangsu, 215123, China. †These *To

authors contributed equally to this work.

whom correspondence should be addressed. E-mail: [email protected]

Abstract: The activation and conversion of methane (CH4) is one of the most challenging processes due to the highly chemical inertness of CH4 and the subsequent uncontrollable over-oxidation. Herein, we report that the CuPdO2/CuO interface in PdxCu1-xO/C can efficiently activate and convert CH4 into CH3OH using H2O2 or O2 as the oxidant under mild conditions, where the CH3OH yield (4076.5 μmol g1)

and selectivity (93.9%) of the optimized Pd0.3Cu0.7O/C are much higher than those of PdO/C,

CuPdO2/C and the mixture of CuPdO2/C and CuO/C. Structural characterizations and mechanism studies reveal that the highly activity of Pd0.3Cu0.7O/C is attributed to the strong synergistic effects in

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PdxCu1-xO/C. The formation of Pd4+ species at the interfaces of CuPdO2 and CuO, which is promoted by the electron transfer from Cu to Pd, can selectively oxidize CH4 to produce CH3OH. This work highlights the importance of controlled interface in Pd-based catalysts for heterogeneous catalysis and beyond.

Keywords: CuPdO2, CuO, Interface, Methane conversion, Methanol

INTRODUCTION Methane (CH4) has been recognized as one of the most attractive feedstocks for producing high value-added products in chemical industry.13 Conventionally, CH4 is converted into liquid hydrocarbons and oxygenates through the formation of CO and H2, which is performed under high temperature.4,5 The increasing demand for energy requires chemists to develop new strategies for converting CH4 to high value-added products.6 Recently, the direct conversion of CH4 to methanol (CH3OH) has attracted increasing attention.79 For instance, Kwon et al. demonstrated that CH4 can be converted to CH3OH and methylhydroperoxide (CH3OOH) on the single-atom Rh catalyst in the presence of hydrogen peroxide (H2O2). They claim that single-atom Rh can selectively oxidize CH4 into *CH

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intermediates and suppress the over-oxidation to form CO2.7 Huang and co-workers reported the

Pd1O4 single sites on ZSM-5 with CuO as a cocatalyst can be used as highly selective catalysts for direct CH4 conversion with CH3OH yield of 3934 μmol g-1 and CH3OH selectivity of 86.4%.8 Hutchings et al. reported that colloidal AuPd nanoparticles can be used as highly selective catalysts to catalyze CH4 towards primary oxygenates (CH3OH, CH3OOH, and formic acid) with selectivity of 88.0% in the presence of H2O2 and oxygen (O2).9 Despite the tremendous progress, CH4 direct conversion to CH3OH is extremely challenging due to the high C-H bond energy (439 kJ mol-1) in CH4, which results in the


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poor activity and/or low selectivity.10,11 Hence, it is highly desired to develop active and selective catalysts for the direct conversion of CH4 to CH3OH. Composite nanomaterials have recently attracted great attentions in heterogeneous catalysis, because the synergies between each single component can significantly enhance the catalytic performance in terms of activity, stability and selectivity.12 Among those composite nanomaterials, the heterostructures are among the most important topic, as the interface in heterostructures may play a vital role in the activity and selectivity.1316 To date, the heterostructures have been widely used in CH3OH synthesis, alcohols selective oxidation, CO2 reduction, etc.1722 For instance, Graciani reported that the metal and oxide interface (Cu/CeO2) is present as the active site for CO2 hydrogenation to CH3OH.18 Huang and co-workers demonstrated that the Ag/Cu interface can selectively catalyze CO2 to ethylene.21 Our previous work showed that the high density of interfaces between platinum-nickel and nickel sulfide components significantly enhanced the activity and stability towards hydrogen evolution reaction.22 Despite plenty of reports, CH4 direct conversion to CH3OH on heterostructures has rarely been studied, which may open a new avenue for the applications of heterostructures in heterogeneous catalysis. Considering non-precious metal-based catalysts (Cu, Fe, Mo, et al.) are generally inactive for CH4 activation (90-150 kJ mol-1), while precious metal-based catalysts (PdO, RhOx, RuO2 and IrO2) are much more active CH4 activation (25-40 kJ mol-1) but suffering poor selectivity.2325 We here prepare PdxCu1-xO/C heterostructures with CuPdO2/CuO interface for CH4 direct conversion to CH3OH in the presence of H2O2. We expect the synergistic effects between CuPdO2 and CuO at the interface can significantly enhance both the activity and selectivity. Results show that the CH3OH yield and selectivity reach 4076.5 μmol g-1 and 93.9%, respectively on the optimized catalyst of Pd0.3Cu0.7O/C, which are much higher than that from these reference catalysts (PdO/C, CuPdO2/C and the mixture of CuPdO2/C and CuO/C). Structural characterizations and mechanism studies reveal that the highly activity of Pd0.3Cu0.7O/C is attributed to the strong synergistic effects in PdxCu1-xO/C. The formation of


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Pd4+ species at the interfaces of CuPdO2 and CuO, which is promoted by the electron transfer from Cu to Pd, can selectively oxidize CH4 to produce CH3OH in the presence of H2O2. RESULTS AND DISCUSSION Initially, the PdxCu1-x nanoparticles (NPs) were prepared using a simple wet-chemical method, which adopted palladium (II) acetylacetonate (Pd(acac)2) and copper (II) chloride dihydrate (CuCl2·2H2O) as precursors, ascorbic acid as reducing agent and oleylamine as solvent. Taking Pd0.3Cu0.7 NPs as example, the products are monodisperse with an average diameter of ~6.8 nm, as confirmed by the high angle annular dark field scanning transmission electron microscopy (HAADFSTEM, Figure 1a). The PdxCu1-x NPs with different molar ratios of Pd to Cu can be obtained by simply changing the amount of CuCl2·2H2O supplied (Figure S1). Pd NPs and Cu NPs were also synthesized as references (Figure S2). The X-ray diffraction patterns (XRD, Figure 1b&Figure S3) of PdxCu1-x NPs match well with the face-centered cubic (fcc) phase PdCu (PDF No. 481551).26 The (111) diffraction peaks of PdxCu1-x NPs shift from 41.4° (Pd0.5Cu0.5 NPs) to 42.0° (Pd0.25Cu0.75 NPs) with the increase of Cu content (Figure S3). The elemental compositions of these PdxCu1-x NPs are determined by the scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX, Figure S4), where the atom ratios of Pd/Cu in Pd0.5Cu0.5 NPs, Pd0.4Cu0.6 NPs, Pd0.3Cu0.7 NPs and Pd0.25Cu0.75 NPs are 51.5/48.5, 40.4/59.6, 31.3/68.7 and 25.7/74.3. The lattice space of 0.219 nm corresponding to the (111) facet of fcc phase PdCu is clearly observed, as revealed by the high-resolution TEM image (HRTEM, Figure S5) of Pd0.3Cu0.7 NPs.27


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Figure 1. Morphological and structural characterizations. (a) HAADF-STEM image of Pd0.3Cu0.7 NPs. (b) XRD patterns of Pd NPs, Pd0.3Cu0.7 NPs, PdO/C and Pd0.3Cu0.7O/C. (c) HRTEM image, (d) line-scan analysis, (e) elemental mapping images and (f) atom model of Pd0.3Cu0.7O/C.


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To study the catalytic performance on CH4 direct conversion, the as-prepared PdxCu1-x NPs were then loaded on carbon (C, Vulcan XC72R) with the loading amount of Pd at 1 wt%, where C as the support for nanoparticles to prevent them from agglomeration and exclude the influence of the complicated interface originating from inorganic oxides (Al2O3, SiO2, TiO2, etc). Afterwards, these samples were calcined at 350 °C in air for 1 h to form heterostructures (Figure S6). The XRD patterns (Figure 1b red curve & Figure S7) of PdxCu1-xO/C show obvious peak at 34.6°, 35.5°, and 38.9°, corresponding to (101) of CuPdO2 (PDF No. 440185) and (111) and (200) of CuO (PDF No. 450937).28 With the decrease of x in PdxCu1-xO/C, the diffractions peaks of CuO become more and more obvious, while the diffractions peaks of CuPdO2 are maintained (Figure S7). The slight preserved metal phases (~ 40.5°) indicated that Pd or PdCux were partially oxidized, which should be located within the NPs, because the oxidation of metal NPs usually starts from the surface. The lattice fringes of 0.164 nm and 0.253 nm correspond to the (112) facet of CuPdO2 and the (002) facet of CuO (Figure 1c). After Fourier transformation, the diffraction pattern of Pd0.3Cu0.7O/C (Figure S8) shows two types of diffraction spots, CuPdO2 and CuO. This segregation structure of Pd0.3Cu0.7O/C has been also confirmed by line-scan analysis (Figure 1d) and elemental mapping (Figure 1e), in which Pd mainly distribute unilaterally of the NPs, while Cu distribute evenly in the whole NPs. These above results implied that CuPdO2/CuO heterostructures with CuPdO2 and CuO interfaces were formed in Pd0.3Cu0.7O/C after calcination (Figure 1f).


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Figure 2. Products yields and oxygenates selectivity of (a) Cu/C, Pd/C and Pd0.3Cu0.7/C, (b) CuO/C, PdO/C and Pd0.3Cu0.7O/C, (c) PdxCu1-xO/C and (d) Pd0.3Cu0.7O/C, CuPdO2/C and CuPdO2/C+CuO/C. Reaction conditions: 10 mg catalyst, 0.5 MPa CH4, 20 mL 1 mol L-1 H2O2, 50 °C, 1 h.

To explore the catalytic behaviour of these Pd-based NPs for CH4 direct conversion, we used H2O2 (1 mol L-1) as oxidant in a pressurized reactor with CH4 (0.5 MPa) and N2 (2.5 MPa) at 50 ºC for 1 h. After reaction, liquid and gas mixtures were detected by 1H-NMR and gas chromatograph (Figure S9S13), where CH3OH, CH3OOH, CO, and CO2 were detected. Since the chemical state of the active component is vital for catalytic performances,29 we first explored the catalytic performances of active component with metallic state (Pd/C, Cu/C, and Pd0.3Cu0.7/C, Figure 2a) and oxidation state (PdO/C, CuO/C, and Pd0.3Cu0.7O/C, Figure 2b). It was found that the Pd/C, Cu/C, and Pd0.3Cu0.7/C exhibit poor activity and low selectivity of oxygenates for direct CH4 conversion, where the overall yields are 85.1,


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689.0, and 702.5 μmol g-1 and the oxygenates’ selectivities are 33.0%, 42.2%, and 56.0%, respectively. Interestingly, the CuO/C, PdO/C, and Pd0.3Cu0.7O/C display improved activity and oxygenate selectivity, with the overall yields of 173.4, 1198.0, and 2710.2 μmol g-1 and the oxygenates’ selectivities of 53.9%, 79.9%, and 88.3%, indicating that the catalytic performance was strongly enhanced over the CuPdO2/CuO heterostructures. To further reveal the contribution of interface, we have systematically studied the effects of PdxCu1-xO/C compositions on CH4 direct conversion. As shown in Figure 2c, the decrease of Pd/Cu ratio in PdxCu1-xO/C leads to a volcano-shape of oxygenates’ yield and selectivity, indicating that the interface of CuPdO2 and CuO plays a vital role in CH4 direct conversion.30 To further confirm this point, additional experiments were performed by etching CuO from Pd0.3Cu0.7O/C. As shown in Figure S14 and S15, it is found that CuO was completely etched by acetic acid, leaving only quasi-spherical CuPdO2 NPs. CuO/C is inactive for CH4 direct conversion, giving a poor oxygenates’ yield of 93.4 μmol g-1 (Figure 2b). When CuPdO2/C was used as catalyst for CH4 direct conversion, the yield of oxygenates strongly decreases to 970.6 μmol g-1, while the selectivity decreases to ~ 82.1% (Figure 2d). When the mixture of CuPdO2/C and CuO/C were used as catalyst, the yield of oxygenates slightly increases to 1133.0 μmol g-1, while the selectivity of oxygenates is kept identical at 82.2%. It should be noted that all the catalysts above exhibit much poorer catalytic performance than Pd0.3Cu0.7O/C, indicating that the interface of CuPdO2 and CuO can significantly enhance the CH4 direct conversion in terms of activity and selectivity. In addition, the effects of reaction temperature, reaction time, H2O2 concentration and CH4 partial pressure on CH4 direct conversion have also been investigated (Figure S16). Pd0.3Cu0.7O/C exhibits the oxygenates’ yield of 5466.3 μmol g-1 at a selectivity of 93.9% under the optimized conditions, which outperforms most previously reported catalysts (Table S1).


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Figure 3. Products yields and oxygenates selectivity for Pd0.3Cu0.7O/C with (a) O2 (0.5 MPa O2, 50 °C) and (b) H2O2 and molecular oxygen (0.5 MPa O2, 20 mL 0.05 mol L-1 H2O2, 50 °C) as oxidant. Reaction conditions: 10 mg catalyst, 3 MPa CH4, 20 mL H2O, 50 °C. (c) Products yields and oxygenates selectivity for Pd0.3Cu0.7O/C over ten rounds of successive reactions. Reaction conditions: 10 mg catalyst, 3 MPa CH4, 20 mL 1 mol L-1 H2O2, 50 °C, 1h. (d) TEM image of Pd0.3Cu0.7O/C after ten rounds of successive reactions.

In addition to H2O2, O2 can also be used as an oxidant for CH4 direct conversion to CH3OH.9 We further investigated the catalytic performance of CH4 direct conversion over Pd0.3Cu0.7O/C in the presence of O2. Figure 3a shows that the Pd0.3Cu0.7O/C displays the oxygenates’ yield and selectivity of 356.3 μmol g-1 and 18.9%, which are 4.1-fold and 2.7-fold higher than those of PdO/C, respectively. Both the yield and selectivity of oxygenates are much lower than those when H2O2 is used as the oxidant. These phenomena are similar with Hutchings and coworkers’ work, which suggests that the activation of CH4 is triggered by radicals.9 Therefore, we added additional H2O2 as radical initiating agent (0.05 mol


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L-1) in the presence of O2 (0.5 MPa). As shown in Figure 3b, both the yield and selectivity of oxygenates significantly increase on PdO/C and Pd0.3Cu0.7O/C catalyst. In particular, for Pd0.3Cu0.7O/C catalyst, the oxygenates’ yield and selectivity increase to 3161.1 μmol g-1 and 91.9%, respectively. Even though the enhancement in catalytic performance by adding H2O2, the yield and selectivity of oxygenates from Pd0.3Cu0.7O/C are 3.3-fold and 2.7-fold higher than those of the PdO/C catalyst. In addition, no obvious decays in the yield and selectivity of oxygenates were observed in ten consecutive rounds, indicating that the Pd0.3Cu0.7O/C could be used as highly active, selective and stable catalyst for CH4 direct conversion (Figure 3cd).


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Figure 4. (a) TPRS spectra of CH4 and CO2 obtained after adsorbing CH4 on PdO/C and Pd0.3Cu0.7O/C. (b) DRIFTS spectra and (c) Pd 3d XPS curves of PdO/C and Pd0.3Cu0.7O/C. (d) Schematic showing the proposed mechanism for CH4 activation and direct conversion.

CH4 temperature-programmed reaction spectroscopy (TPRS) was performed to further study the mechanism of CH4 direct conversion on PdCuO/CuO heterostructures.31 As shown in Figure 4a and Figure S17, to the contrast of free desorption on CuO/C, obvious features of CH4, CO2 and H2O are observed in TPRS patterns, suggesting that CH4 molecules can strongly adsorb on PdO/C and PdxCu1xO/C.

The TPRS pattern of PdO/C exhibits the feature of CO2 at 185250 °C, indicating that PdO/C

favors to oxidize CH4 to CO2 (Figure 4a). Interestingly, in addition to CO2, the feature of CH4 appears in the TPRS patterns of PdxCu1-xO/C at 200450 °C, which is formed via the hydrogenation of *CH3 on the catalyst (Figure S17a). These results imply that PdxCu1-xO/C may tend to selective activate CH4 to *CH

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on PdxCu1-xO/C, leading to the formation of oxygenates in the presence of H2O2 and/or O2.

Compared to CuO/C and PdO/C, the peak intensities and areas are obviously different, indicating that the chemisorption and activation are different among different catalysts. Furthermore, we used diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to investigate the intermediate species after exposing these catalysts in CH4. When CH4 was introduced, two groups of peaks are observed at 3018 and 1304 cm-1, which can be ascribed to the physical absorption of CH4 on catalysts (Figure S18). After flushing with Ar to remove the physical absorption of CH4, no obvious peaks are observed in the spectrum of CuO/C, suggesting that CuO/C is inactive for CH4 activation under the indicated conditions (Figure S19). However, the bands at 11001700 cm-1 appeared on PdO/C and Pd0.3Cu0.7O/C (Figure 4b), illustrating the CH dissociation of CH4 on Pd oxide species.32,33 On PdO/C, various intermediate species were detected at 1590 cm-1 (*CO32-), 1538 cm-1 (*CO), 1453 cm-1 (*CH2O), 1378 cm-1 (*CH3), 1240 cm-1 (*CO) and 1108 cm-1 (*CH3O), and the extremely weak peak of *CH3O is consistent with the


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result of low CH3OH yield (77.1 μmol g-1) using O2 as oxidant (Figure 3a). Notably, the strong bands appeared on Pd0.3Cu0.7O/C at 1538 cm-1 and 1108 cm-1 corresponding to the *CH3O species, indicating that Pd0.3Cu0.7O/C can selectively oxidize the strongly adsorbed CH4 to CH3OH.

Figure 5. (a) Pd K-edge XANES spectra of Pd foil, PdO, and Pd0.3Cu0.7O/C. (b) Cu K-edge XANES spectra of Cu foil, CuO, and Pd0.3Cu0.7O/C. (c) Pd K-edge Fourier transform EXAFS spectra of Pd foil, PdO, and Pd0.3Cu0.7O/C. (d) Cu K-edge Fourier transform EXAFS spectra of Cu foil, CuO, and Pd0.3Cu0.7O/C.

Afterwards, X-ray photoelectron spectroscopy (XPS) measurements were performed on different catalysts. As shown in Figure 4c, two peaks at ~337.5 eV and ~342.7 eV were observed in the XPS spectrum of PdO/C, which could be assigned to Pd2+ 3d5/2 and Pd2+ 3d3/2, respectively.34 For the PdxCu1xO/C,

in addition to the Pd2+, two peaks appeared at ~338.6 eV and ~343.7 eV in the XPS spectrum,


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which could be indexed as Pd4+ 3d5/2 and Pd4+ 3d3/2, respectively.3537 Similar results were observed in other PdxCu1-xO/C catalysts with different compositions (Figure S20). No peaks corresponding to N indicate that the surface ligands have been removed (Figure S21). In order to exclude the influence of the support, the Pd4+ species and Pd2+ species were also observed in the Pd 3d XPS of Pd0.3Cu0.7O without support (Figure S22). More interestingly, when Pd0.3Cu0.7O/C heterostructures were etched by acetic acid to remove CuO, only two peaks corresponding to Pd2+ 3d5/2 and Pd2+ 3d3/2 were observed in the XPS spectrum (Figure S22). We thus speculated that the Pd4+ species might play a vital role in the enhancement of catalytic performance. In addition, we have further studied the effects of Pd4+ ratio on the TCH4 and CH3OH yield over PdxCu1-xO/C catalysts. As depicted in Figure S23, it is shown that both the TCH4 and CH3OH yield were strongly relevant to the Pd4+/(Pd2+ + Pd4+). The optimized catalyst of Pd0.3Cu0.7O/C, who gives the best DMC perforamcne, has the highest ratio of Pd4+ (Figure 4d). X-ray absorption near edge structure (XANES) measurement has further been performed at Pd K-edge and Cu K-edge to study the synergistic effects in Pd0.3Cu0.7O/C. As shown in Figure 5a, compared to the references of Pd foil and PdO, the XANES spectrum from Pd0.3Cu0.7O/C gives similar features with PdO foil. However, the Pd K-edge obviously shifts to higher energy (inset of Figure 5a), indicating that the Pd2+ and Pd4+ species exist in Pd0.3Cu0.7O/C. The XANES spectrum of Pd0.3Cu0.7O/C at Cu K-edeg indicates that Cu in the catalyst has been oxidized into Cu2+ after calcination, which is in good agreement with XPS results (Figure 5b). Interestingly, Cu K-edeg in Pd0.3Cu0.7O/C shifts to higher energy position in comparison to CuO reference, giving a higher intensity of white line (inset of Figure 5b). These results imply that electrons may transfer from Cu to Pd.38 The coordinations of Cu and Pd in Pd0.3Cu0.7O/C are analyzed by extended X-ray absorption fine structure (EXAFS) spectra. As depicted in Figure 5c5d, Pd foil and Cu foil gives a strong feature of PdPd and CuCu coordination at ~2.49 and 2.23 Å, respectively. For PdO and CuO, PdO and CuO feature is observed at ~1.56 and 1.57 Å, while the features of PdPd and CuCu appear at 2.293.56 and 2.243.18 Å, respectively. Despite the very


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similar PdO and CuO cooridination in Pd0.3Cu0.7O/C comparing to the PdO and CuO reference, it should be noted that the radial distance of PdO (1.58 Å) in Pd0.3Cu0.7O/C is slightly longer than that of PdO, while the the radial distance of PdO in Pd0.3Cu0.7O/C is slightly shorter than that of CuO (1.52 Å). This can be interpreted as electrons transfer from Cu to Pd leads to electron-rich Pd environment, as a result of longer PdO bond. On the contrary, the electron-deficient Cu will lead to a shrunken CuO bond. Therefore, the electron transfer from Cu to Pd in the Pd0.3Cu0.7O/C catalyst, which can significantly enhance catalytic performance of CH4 selective conversion into CH3OH and CH3OOH, has been well revealed by XANES and EXAFS analysis. These strong synergies can not only promote the selective activation of CH4 to *CH3 species at the interface, but also suppress the dehydrogenation of CH3OH to *CHO,39 leading to a higher yield and selectivity of CH3OH. CONCLUSIONS To summarize, a class of PdxCu1-xO/C serving as highly active, selective and stable catalysts for direct CH4 conversion to CH3OH have been demonstrated. These PdxCu1-xO/C catalysts possess heterojunction structure with the interfaces between CuPdO2 and CuO, and exhibit much higher activity and selectivity for direct CH4 conversion to CH3OH than those of PdO/C, CuPdO2/C and the mixture of CuPdO2/C and CuO/C. Especially, the high activity and oxygenates’ selectivity of the Pd0.3Cu0.7O/C outperform those of most previously reported catalysts. CH4-TPRS and DRIFTS results demonstrate that Pd0.3Cu0.7O/C can selectively oxidize CH4 to CH3OH. Detailed characterizations show that the electron transfer from Cu to Pd will result in a high ratio of Pd4+ species at the interface. This work highlights the importance of designing Pd-based catalysts with precisely modulated interfaces for heterogeneous catalysis and beyond. ASSOCIATED CONTENT


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Supporting Information. Figure S1Figure S23&Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author [email protected] ACKNOWLEDGMENT This work was financially supported by the Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), the National Natural Science Foundation of China (21571135, 51802206), Young Thousand Talented Program, Jiangsu Province Natural Science Fund for Distinguished Young Scholars (BK20170003), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the start-up supports from Soochow University. References 1. Wang, V. C.-C.; Maji, S.; Chen, P. P.-Y.; Lee, H. K.; Yu, S. S.-F.; Chan, S. I. Alkane oxidation: methane monooxygenases, related enzymes, and their biomimetics. Chem. Rev. 2017, 117, 85748621. 2. Crabtree, R. H. Aspects of methane chemistry. Chem. Rev. 1995, 95, 9871007. 3. Zhu, Q.; Wegener, S. L.; Xie, C.; Uche, O.; Neurock, M.; Marks, T. J. Sulfur as a selective 'soft' oxidant for catalytic methane conversion probed by experiment and theory. Nat. Chem. 2013, 5, 104109. 4. Bar-Nahum, I.; Khenkin, A. M.; Neumann, R. Mild, aqueous, aerobic, catalytic oxidation of methane to methanol and acetaldehyde catalyzed by a supported bipyrimidinylplatinum-polyoxometalate hybrid. J. Am. Chem. Soc. 2004, 126, 1023610237. 5. Hickman, D. A.; Schmidt, L. D. Production of syngas by direct catalytic oxidation of methane. Science 1993, 259, 343346. 6.

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