Selective Activation of Methane on Single-Atom Catalyst of Rhodium

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Selective Activation of Methane on Single-Atom Catalyst of Rhodium Dispersed on Zirconia for Direct Conversion Yongwoo Kwon, Tae Yong Kim, Gihun Kwon, Jongheop Yi, and Hyunjoo Lee J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11010 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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Journal of the American Chemical Society

Selective Activation of Methane on Single-Atom Catalyst of Rhodium Dispersed on Zirconia for Direct Conversion Yongwoo Kwona†, Tae Yong Kimb†, Gihun Kwona, Jongheop Yib*, and Hyunjoo Leea*

a

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science

and Technology, Daejeon 34141, Republic of Korea; bSchool of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea

KEYWORDS: methane, activation, direct conversion, single-atom catalyst, rhodium

ACS Paragon Plus Environment

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ABSTRACT Direct methane conversion into value-added products becomes more important. Due to inertness of methane, cleaving the first C-H bond has been very difficult requiring high reaction temperature on the heterogeneous catalysts. Once the first C-H bond becomes activated, the remaining C-H bonds are successively dissociated on the metal surface, hindering the direct methane conversion into chemicals. Here, a single-atom Rh catalyst dispersed on ZrO2 surface has been synthesized and used for selective activation of methane. The Rh single atomic nature was confirmed by extended X-ray fine structure analysis, electron microscopy images, and diffuse reflectance infrared Fourier transform spectroscopy. A model of the single-atom Rh/ZrO2 catalyst was constructed by density functional theory calculations, and it was shown that CH3 intermediates can be energetically stabilized on the single-atom catalyst. The direct conversion of methane was performed using H2O2 in the aqueous solution or using O2 in gas-phase as oxidants. Whereas Rh nanoparticles produced CO2 only, the single-atom Rh catalyst produced methanol in aqueous phase or ethane in gas-phase.


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1. INTRODUCTION

Methane has attracted much attention as a feedstock for chemicals and clean fossil energy. Shale gas, methane hydrates, and coal-bed methane have been actively developed,1-2 and there is a strong demand for upgrading methane into value-added products. Most of the chemicals such as methanol, olefins, and aromatics are produced from methane by an indirect pathway; methane is reformed to a synthesis gas (H2+CO) first, and then the synthesis gas is converted to various chemicals.3 The first step typically occurs at high temperature (>800 °C) and the second step occurs at high pressure (>10 atm), consuming massive energy. Recently, direct conversion of methane to chemicals has been studied as a potential route using much less energy. The direct pathway would circumvent the step for the expensive intermediates of synthesis gas.4-5 Methane has low electron and proton affinity, low polarity, high C-H bond energy of 439 kJ/mol, and high ionization energy.6 Because of the inertness of methane, the activation of first C-H bond is very difficult, and a rate-determining step of the methane activation is a cleavage of the first C-H bond.7-9 The activation energy of methane is usually higher than that of products such as ethane or methanol on most heterogeneous catalysts.10 Typically, the direct conversion of methane to chemicals has been suffered from non-selective activation of methane.6, 11 Zeolites containing Cu or Fe species have been reported to produce methanol from methane via multi-steps of activation, methane conversion, and extraction.12-14 Cu or Fe sites are activated into oxo species by O2 or N2O, then the active sites react with CH4 producing methoxy groups. Sequentially, the methoxy groups are extracted out as methanol by flowing steam. The active sites need to be re-activated again for the next catalytic cycle. Only a few closed catalytic system have been reported by using H2O2 as an oxidant for methane conversion.15-17 Direct conversion


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of methane to ethylene, aromatics, and hydrogen has also been reported using single Fe sites embedded in a silica matrix, but the reaction temperature is very high above 1000 °C. Catalysts that can activate methane selectively at low temperature with a closed catalytic cycle should be developed for the direct conversion of methane to value-added chemicals. Among various types of catalysts, precious metal-based heterogeneous catalysts have not been considered seriously for the direct methane conversion due to their inherent ‘overoxidation’ feature. Because the C-H bond of a CH4 molecule tends to dissociate successively on the surface of metals such as Pd, Rh, Pt, Ir, and Mo, the CH4 would be completely dehydrogenated to C+4H, and complete oxidation into CO2 would occur. The removal of the first hydrogen is typically a rate-determining step with the highest activation energy, so the intermediate species of CHx– would keep being dissociated without a chance to form valuable chemicals. In the case of IB metals such as Cu, Ag, and Au, the activation energy of the first C-H bond dissociation is too high to activate methane.18-19 Recently, single-atom catalysts have been realized where precious metal atoms are located separately on the solid supports, and their distinct catalytic properties have been demonstrated comparing with nanoparticle catalysts for many heterogeneous reactions such as CO oxidation, allylic alcohol oxidation, electrocatalytic reactions.20-24 Density functional theory (DFT) calculations have predicted that Pt clusters or Rh with a low coordination number can prohibit the successive dehydrogenation of methane, stabilizing the CH3 intermediate.25-28 Because the CH3 species is a key intermediate for the selective conversion of methane, the single-atom catalyst can be a promising candidate, unlike metal nanoparticles. In this work, a single atomic Rh was stabilized on a tetragonal ZrO2 surface (denoted as Rh1/ZrO2 below), and the single atomic nature was investigated by various characterizations. The


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Rh1/ZrO2 catalyst could activate methane selectively to form CH3 on the catalyst surface. Direct methane conversion was tested using H2O2 as an oxidant in aqueous phase or O2 in gas phase on the Rh1/ZrO2 catalyst.

2. EXPERIMENTAL SECTION

2.1 Catalyst synthesis A zirconia support was synthesized via a sol-gel method.29 Zirconium n-propoxide 14.4 ml (70 wt% in n-propanol; Sigma-Aldrich) was diluted by adding 38.1 ml of n-propanol (>99.5%; Sigma-Aldrich). Aqueous ammonia (20 ml, Duksan) was added dropwise into the zirconia precursor solution with vigorous stirring and kept for 1 h. The gel was dried at 80 °C for 12 h. The dried sample was calcined at 400 °C for 4 h in air. Rh was deposited on the zirconia support by a wet impregnation method. The Rh precursors (RhCl3; Sigma-Aldrich) were dissolved in 10 ml of anhydrous ethanol (99.9%; Samcheon). Then, 300 mg of metal oxide supports, which are the synthesized zirconia, titania (P25, Degussa), ceria (nanopowder, < 25 nm; Sigma-Aldrich), or silica (nanopowder, ~12 nm; Sigma-Aldrich), were dispersed in 20 ml of ethanol. The two solutions were mixed with vigorous stirring and dried at 60 °C. The dried sample was calcined at 400 °C for 1 h in air. In the case of 5 wt% Rh/SiO2, the additional reduction was carried out under 10% H2/N2 flow at 200 °C for 1 h.

2.2 Characterizations X-ray photoelectron spectroscopy (XPS; K-alpha, Thermo VG Scientific) with an Al Kα X-ray source (12 kV, 3 mA) was used to analyze the surface property of the catalysts. Binding energies


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were estimated by locating the maximum intensity of adventitious C 1s peak at 284.8 eV as a reference. The crystalline structure was analyzed by powder X-ray diffractometer (XRD; SmartLab, RIGAKU). Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were taken with Titan cubed G2 600-300 (FEI) at 300 kV with a spherical aberration Cs corrector. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS; Nicolet iS50, Thermo Scientific) was carried out with an MCT detector and a diffuse reflectance assembly chamber having a KBr window. The sample was pre-treated at 100 °C for 1 h under Ar flow, cooled to room temperature, and a background spectrum was obtained. For CO adsorption, 1% CO/Ar gas flowed over the sample for 10 min. Then the spectra were obtained during CO desorption by Ar flow with evacuation for 20 min at room temperature. For CH4 adsorption, 5% CH4/Ar gas was used instead. The CH4 adsorption and desorption temperature were kept at 70 °C. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of Rh K-edge was measured using the 10C XAFS beamline of Pohang Light Source (PLS). The energy of the storage ring electron beam was 2.5 GeV with a ring current of ~360 mA. The incident X-ray was monochromatized by Si(111)/Si(311) double-crystal. The Rh K-edge spectra were obtained in a fluorescence mode using a passivated implanted planar silicon (PIPS) detector (Canberra). The spectrum for a reference Rh foil was also measured concurrently to calibrate each sample. The EXAFS and XANES data were processed and fitted with ARTEMIS and ATHENA software. A coordination number was calculated by fixing the S02 to the values obtained from the reference Rh foil.

2.3 Computational details


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Periodic density functional theory (DFT) calculations were carried out using a Vienna ab initio simulation package (VASP).30 A generalized gradient approximation (GGA) and exchangecorrelation functions parameterized by Perdew-Burke-Ernzerhof (PBE) were used,31 in conjunction with the projector augmented wave (PAW) method to describe ionic cores.32 DFTD2 Grimme’s empirical method was employed to include dispersion forces.33 An energy cut-off of 400 eV and a 1 × 1 × 1 Monkhorst-Pack k-point mesh was used throughout the calculations, except for the bulk optimization, which used a cut-off energy of 520 eV and higher k-point mesh (8 × 8 × 6 for tetragonal ZrO2, 6 × 6 × 6 for Rh2O3 and RhO2). All calculations were converged until the forces on all atoms were less than 0.03 eV/Å. The electronic optimization steps were converged self-consistently to fewer than 2 × 10-6 eV. Initial atomic positions for bulk optimization of tetragonal ZrO2 (t-ZrO2) were obtained from the experimentally identified structure with a P42/NMCZ space group.34 An asymmetric slab with 3 atomic layers and the p(2×2) supercells were constructed to model the (101) surface of t-ZrO2, using the optimized parameters from the bulk optimization. The bottom layer was fixed while the remaining top layers were allowed to relax. Vacuum gaps that were thicker than 20 Å were included to prevent non-physical electronic interaction. The Rh atomic charge was calculated via Bader charge analysis.35

2.4 Direct methane conversion The direct methane oxidation into methanol was carried out using a Teflon-coated stainless steel autoclave with a total volume of 60 ml. The catalyst 30 mg was dispersed in 10 ml of 0.5 M H2O2 aqueous solution. Then the autoclave was pressurized to 30 bar using methane gas (95% methane and 5% helium; the helium was added as an internal standard for analysis of the gas


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phase products). The autoclave was heated to 70 °C and kept for 30 min with stirring at 800 rpm. After the reaction, the autoclave was cooled in a refrigerator to a temperature below 10 °C to minimize the volatilization of products. The gas phase products were measured by a gas chromatograph (GC-6100 series; Yonglin) with Molsieve 5A and Porapak N columns (SigmaAldrich). The signal was detected by thermal conductivity detector (TCD) and flame ionized detector (FID) with methanizer. The liquid phase was centrifuged and the supernatant was analyzed by 1H NMR spectroscopy (400 MHz; Agilent) with solvent suppression. The product solution 0.7 ml was mixed with 0.1 ml of D2O (99%; Sigma-Aldrich) and 0.01 ml of 0.3 mM 3(trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS, 97%; Sigma Aldrich). In order to quantify methanol and methyl hydroperoxide accurately, their calibration curves were made for 1

H NMR data as shown in Figure S1. Catalyst performance was also examined for gas-phase reaction in a quartz glass fixed-bed

flow reactor at atmospheric pressure and 260 °C using O2 as an oxidant. A total of 0.15 g of the catalyst was placed inside the reactor. The reactant gas was introduced with a flow rate of 0.85 mL/min of 3% O2/N2 mixed gas and 9 mL/min of methane (99.999%). A total flow rate was 9.85 mL/min of 91% CH4, 0.3% O2, and 8.7% N2. The nitrogen was used as an internal standard. The product gas was analyzed with gas chromatograph (GC-6100 series, Yonglin) with Molsieve 5A and Porapak N columns (Sigma-Aldrich) which was equipped with thermal conductivity detector (TCD) and flame ionization detector (FID) with methanizer.

3. RESULTS AND DISCUSSION

3.1 Characterizations of a single atomic Rh catalyst


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The synthesized ZrO2 support had a BET surface area of 94.3 m2/g. Figure S2 shows a TEM image and XRD pattern of the ZrO2 support. Its crystalline structure was mainly tetragonal ZrO2 with a small portion of monoclinic ZrO2 phase. The Rh was deposited on the ZrO2 support using a wet impregnation method. Figure 1a shows EXAFS data for 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2, and 5 wt% Rh/SiO2. The detailed fitting results are showed in Table 1 and k-range oscillations are shown in Figure S3. The 0.3 wt% Rh/ZrO2 had a dominant peak of Rh-O at 1.5 Å without a Rh-Rh peak at 2.4 Å. The coordination number for the neighboring oxygen was 4.9 and the data were not fitted with Rh-Rh interaction included. The 2 wt% Rh/ZrO2 had a strong Rh-O peak with small Rh-Cl and Rh-Rh peaks located at 2.0 and 2.4 Å, respectively. The coordination number was 4.3 for Rh-O, 1.0 for Rh-Cl, and 0.6 for Rh-Rh. The 5 wt% Rh/SiO2 showed a weak Rh-O peak at 1.5 Å and a strong Rh-Rh peak at 2.4 Å. The coordination number was 2.3 for Rh-O and 5.6 for Rh-Rh. The EXAFS data confirm that the 0.3 wt% Rh/ZrO2 has single atomic Rh sites. XANES data in Figure 1b show that the single atomic Rh catalyst has very oxidic character. The 0.3wt% Rh/ZrO2 has higher white line intensity than 5 wt% Rh/SiO2, Rh foil, RhCl3, and even Rh2O3. The positive shift in the XANES peak also indicates that Rh on the 0.3wt% Rh/ZrO2 has more oxidic state.36 Figure S4a shows a HAADF-STEM image of the 0.3 wt% Rh/ZrO2 catalyst. No metal nanoparticles were observed even at lower magnifications. It is hard to distinguish Rh single atoms on the ZrO2 support because of the similar atomic numbers of Rh (ZRh = 45) and Zr (ZZr = 40). However, the EDS mapping image in Figure S4b clearly showed that Rh was distributed on the ZrO2 surface.


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Figure 1. (a) Rh K edge k3-weighted Fourier transformed EXAFS spectra of the 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2, 5 wt% Rh/SiO2, and Rh foil. The dots indicate experimental data, and lines indicate fitted results. (b) Rh K edge XANES spectra.

Table 1. EXAFS fitting results for the 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2, and 5 wt% Rh/SiO2 R(Å)

Debye-Waller factor (σ2/Å2)

R-factor

0.035

Sample

Path

Coordination number

0.3 wt% Rh/ZrO2

Rh-O

4.9

1.995

0.001

Rh-O

4.3

2.025

0.003

Rh-Cl

1.0

2.373

0.003*

Rh-Rh

0.6

2.731

0.003*

Rh-O

2.3

2.027

0.004

2 wt% Rh/ZrO2

5 wt% Rh/SiO2

0.025

0.010


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Rh-Rh

5.6

2.687

0.006

*fitted with fixed parameters

The single atomic nature of the Rh dispersed on ZrO2 was further investigated by DRIFTS with CO as a probe molecule. Figure 2 shows the FT-IR data of the CO molecules adsorbed on the 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2, and 5 wt% Rh/SiO2 samples. The peak intensities were adjusted for easier comparison. TEM images of these three samples are shown in Figure S5. Whereas many Rh nanoparticles were clearly observed on the 5 wt% Rh/SiO2, Rh nanoparticles were rarely observed on the ZrO2 support. Instead, HAADF-STEM images in Figure S6 show that there exist many Rh clusters in 2 wt% Rh/ZrO2. The linear-bound CO peak at 2047 cm-1 and bridge-bound CO peak at 1850 cm-1 were observed on the Rh nanoparticles of the 5 wt% Rh/SiO2 sample.37-39 However, these two peaks disappeared on the 0.3 wt% Rh/ZrO2 sample, and new peaks appeared at 2087 cm-1 and 2017 cm-1, which correspond to symmetric and asymmetric stretching vibration of geminal dicarbonyl species adsorbed on the same Rh atom. The geminal peaks were observed at atomically dispersed Rh or very small Rh clusters.38, 40-41 The 2 wt% Rh/ZrO2 sample showed the geminal peaks and a weak bridge-bound CO peak at 1826 cm-1, indicating the presence of small Rh clusters.


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Figure 2. DRIFT spectra of CO molecules adsorbed on bare ZrO2 support, 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2, and 5.0 wt% Rh/SiO2 samples. The peak intensities were adjusted for easier comparison.

In the geminal dicarbonyl species of Rh(CO)2, the angle between two adsorbed carbonyl groups is related to the ratio of the IR peak areas for symmetric and asymmetric stretches. This technique was previously reported for detecting single atomic Rh dispersed on ZnO.42 Table S1 lists the angles between the carbonyl groups estimated from the IR data. The 0.3 wt% Rh/ZrO2 samples had an angle of ~90°, whereas a physically mixed sample of RhCl3+ZrO2 showed an angle of ~120°, and 2 wt% or 5 wt% Rh/ZrO2 samples had the angle of ~100°. This suggests that the geometry of CO adsorbed on the single atomic Rh sites is different from the CO on the Rh-Cl species or the CO on the Rh clusters. The IR data, showing the absence of a bridge-bound CO peak, the presence of geminal CO adsorption peaks, and an angle of ~90° between the geminal CO molecules, indicate that the Rh on the 0.3 wt% Rh/ZrO2 samples has a single atomic nature. Combining EXAFS and DRIFTS data, it could be deduced that the 0.3 wt% Rh/ZrO2 has Rh


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single atoms, the 2 wt% Rh/ZrO2 has sub-nanometer Rh clusters, and the 5 wt% Rh/SiO2 has Rh nanoparticles.

3.2 Stabilization of CH3 intermediates on the single atomic Rh1/ZrO2 catalyst The models of the single atomic Rh on the ZrO2 surface were constructed by density functional theory (DFT) calculations. The adsorption of Rh1Ox on the ZrO2 surface or the substitution of Zr into Rh was considered as shown in Table S2. Figure S7 shows that hydroxylated ZrO2 surface can be more stable than clean surface, so both of clean ZrO2(101) and hydroxylated ZrO2(101) were considered. Their detailed structures are provided in Figure S8 and Figure 3. The Rh oxidation state was estimated by Bader charge analysis. Figure S9 shows that there is a linear relationship between Bader charges and oxidation states for Rh metal, RhO2, and Rh2O3. Using this line for calibration, the Rh oxidation state of each model in Table S2 was estimated by calculating the Bader charge by the DFT. The Rh oxidation state of the synthesized Rh/ZrO2 catalysts was measured by XPS as shown in Figure 3a. The binding energy of the XPS Rh 3d5/2 peak was 309.4, 309.7, and 308.3 eV for the 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2, and 5 wt% Rh/SiO2, respectively. Considering that the XPS Rh0 peak appears at 307.0 ~ 307.3 eV, the Rh3+ peak at 308.1 ~ 308.6 eV, and the Rh4+ peak at 309.4 ~ 310.0 eV,43-45 the single atomic Rh species seem to have an oxidation state between +3 and +4. XANES data in Figure 1b also support that the Rh in the 0.3 wt% Rh/ZrO2 catalysts is very oxidic. Among the various DFT models in Table S2, Zr substitution models showed the oxidation state of +3.6 for a RhZr-Clean model, where Rh substitutes Zr site on clean ZrO2(101) surface as shown in Figure 3b, or +3.7 for a RhZr-Hyd model, where Rh substitutes Zr site on hydroxylated ZrO2(101) surface as shown in Figure 3c. These values in Rh oxidation state


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are similar to that determined by XPS analysis. Furthermore, the coordination number of Rh in these models is in accord with that estimated from EXAFS fitting in Table 1. When the relative energy was estimated based on bulk Rh2O3 phase, it became smaller after surface hydroxylation, implying that the surface hydroxyl groups help to stabilize the single atomic Rh species. Thus, the RhZr-Hyd model was chosen as the most realistic site for the single atomic Rh on ZrO2 surface.

Figure 3. (a) XPS Rh 3d spectra of 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2, and 5 wt% Rh/SiO2. Top view of the DFT models, (b) RhZr-Clean and (c) RhZr-Hyd, for the single atomic Rh on ZrO2. The numbers in an inset indicate Rh oxidation state estimated by Bader charge analysis.

Relative energies of CHx intermediates on the RhZr-Hyd model were calculated as shown in Figure 4a. On the single atomic Rh site, the adsorbed CH3 intermediate (CH3*) has the lowest energy during the methane dehydrogenation. Further dehydrogenated species of the adsorbed CH2 (CH2*) have a higher energy than CH3*. The single atomic Rh1/ZrO2 catalyst could surely stabilize the CH3 intermediate, helping the dissociative adsorption of methane. The adsorption of methane on the Rh1/ZrO2 was experimentally monitored by DRIFTS. As shown in Figure 4b, Rh


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nanoparticles on 5 wt% Rh/SiO2 or bare ZrO2 support did not show a distinct methane adsorption peak. The 0.3 wt% Rh/ZrO2 catalyst, however, clearly presented peaks at 2955, 2921, and 2852 cm-1. The peaks at 2955 and 2852 cm-1 correspond to the C-H bond stretching vibration in the adsorbed methoxy group (CH3O-), and the peak at 2921 cm-1 is attributed to the C-H bond stretching vibration in the adsorbed methyl group (CH3-).46-47 The 2 wt% Rh/ZrO2 catalyst showed the peaks of the adsorbed methoxy or methyl groups, although the peak height is much smaller. Both the DFT calculation and DRIFT experimental data confirmed that CH3 intermediate can stably exist on the single atomic Rh on ZrO2.

Figure 4. (a) Energy diagram of methane activation on the RhZr-Hyd (4.3 OH/nm2) model with optimized geometries of the intermediates. In inset figures, red balls indicate oxygen, cyan zirconium, turquoise rhodium, white hydrogen, gray carbon. (b) DRIFT spectra of CH4 adsorbed on bare ZrO2, 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2 and 5 wt% Rh/SiO2. The methane adsorption was performed at 70 °C.


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3.3 Direct conversion of methane First, methane was oxidized with H2O2 in aqueous phase using 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2, and 5 wt% Rh/SiO2 catalysts. The partially oxygenated products of methanol and methyl hydroperoxide were detected in the liquid phase and CO2 was detected in the gas phase. When the ZrO2 support itself was used for the methane conversion without any metal deposition, no oxygenates were formed. Among various kinds of metals such as Rh, Pd, Pt, and Ir, 0.3 wt% Rh/ZrO2 showed the highest methanol production as shown in Table S3. Figure 5a shows that the property of Rh active sites affects the methane oxidation significantly. The single atomic Rh could make methanol with the highest productivity, whereas Rh nanoparticles on SiO2 produced only CO2 without the formation of C1 oxygenates. The recyclability of the 0.3 wt% Rh/ZrO2 catalyst was checked as shown in Figure 5b. The solid catalyst was collected by centrifugation after one batch of reaction, and used for the next batch of reaction after washing the catalyst with deionized water. The amount of the oxygenated products showed only a little difference up to the 5th cycle. When the liquid supernatant was used at the same reaction condition without the solid catalyst, no methane conversion was observed. These results confirm that the single atomic Rh catalyst actually catalyzed the methane conversion producing methanol. When other supports of CeO2, TiO2, and SiO2 were used instead of ZrO2, they produced lower amounts of methanol as shown in Table S4. It was reported that the ZrO2 has the highest H2O2 decomposition rate among ZrO2, CeO2, TiO2, and SiO2.48 The O-O bond in H2O2 or MeOOH might be cleaved more efficiently on the ZrO2 surface.


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Figure 5. (a) Direct methane oxidation results using H2O2 as an oxidant in an aqueous solution on the 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2, and 5 wt% Rh/SiO2 catalysts. (b) Recyclability test results performed with the 0.3 wt% Rh/ZrO2 catalyst. Reaction condition: 30 bar of 95% CH4/He, 70 °C, 1 h, 0.5 M H2O2, and catalyst 30 mg. (c) Direct methane conversion to ethane using O2 as an oxidant in gas-phase on the 0.3 wt% Rh/ZrO2, 2 wt% Rh/ZrO2, and 5 wt% Rh/SiO2 catalysts. Reaction condition: 9.85 sccm of 91%/0.3%/8.7% CH4/O2/N2, 260°C, atmospheric pressure, and catalyst 0.15 g.

Time-resolved reaction was carried out on the 0.3 wt% Rh/ZrO2 catalyst as shown in Figure S10. Initially, much MeOOH was observed. As reaction time increased, the amount of MeOOH decreased, instead the amounts of MeOH and CO2 increased. The decomposition reactions of MeOOH and MeOH were also performed as shown in Figure S11. The MeOOH was decomposed into MeOH and CO2 in the presence of the 0.3 wt% Rh/ZrO2, and the MeOH was converted into CO2 only at the same condition. The Rh nanoparticles on the 5 wt% Rh/SiO2 catalyst converted MeOH into CO2 much more than the single atomic Rh catalyst. The mechanistic study was also carried out by DFT calculation. Optimized structural models are shown in Figure S12a and the energy diagram is shown in Figure S12b. CH4 is activated on the


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Rh site and is dissociatively adsorbed as CH3* and H*. The adsorbed CH3 can easily react with adsorbed H2O2 to form MeOOH. Then, deoxygenation of MeOOH into MeOH is energetically favored, which is in accord with our time-resolved reaction results. Furthermore, ethane was observed for direct methane oxidation when a small amount of O2 was flown together with methane at 260 °C and atmospheric pressure. Figure 5c showed that while 5 wt% Rh/SiO2 did not produce any ethane, 0.3 wt% Rh/ZrO2 and 2 wt% Rh/ZrO2 could produce ethane stably over 12 h. DFT calculations also showed that dissociative adsorption of two CH4 molecules and subsequent formation of ethane are energetically plausible on the singleatomic Rh site as shown in Figure S13. CH4 is favorably adsorbed as CH3* and H* species and then migration of CH3* to nearby O atom permits dissociative adsorption of another CH4 molecule. The adsorption of 2nd CH4 is slightly endothermic, but coupling of two CH3* into C2H6 is fairly exothermic. Therefore, ethane can be favorably formed on Rh1/ZrO2.

4. CONCLUSION

Direct conversion of methane was performed using a single atomic Rh catalyst dispersed on ZrO2 support. The single-atomic nature was investigated by EXAFS, XANES, HAADFSTEM/EDS images, and CO-adsorption using DRIFTS measurements. A DFT model of the single atomic Rh1/ZrO2 catalyst was chosen by comparing the prediction of the DFT models and the experimentally determined property. The stabilization of CH3 intermediates on the singleatomic Rh1/ZrO2 upon methane adsorption was confirmed by the DRIFTS measurement and DFT calculation. The single atomic Rh catalysts could activate methane at the mild condition, and convert methane to methanol using H2O2 in the aqueous solution with a closed catalytic


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cycle or convert methane to ethane using O2 in the gas phase below 300 °C. This work clearly shows that selective activation of methane and direct conversion of methane to chemicals can occur on the heterogeneous single-atom catalyst, and it would provide new insight into the development of heterogeneous catalysts for methane utilization.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge: additional data, Table S1~S4 and Figure S1~S13.

AUTHOR INFORMATION Corresponding Author * Hyunjoo Lee ([email protected]), Jongheop Yi ([email protected]) Author Contributions †These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was financially supported by the National Research Foundation of Korea (NRF2016M3D3A1A01913255 and NRF-2015R1A2A2A01004467). The experiments at PLS were supported in part by MSIP and POSTECH.


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Table of Contents


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Selective Activation of Methane on Single-Atom Catalyst of Rhodium

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