Article pubs.acs.org/JACS
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*,† †
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea ‡ School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea
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ABSTRACT: Direct methane conversion into value-added products has become increasingly important. Because of 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 singleatom 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. methane conversion, and extraction.12−14 Cu or Fe sites are activated into oxo species by O2 or N2O, and 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 reactivated again for the next catalytic cycle. Only a few closed catalytic systems have been reported by using H 2 O 2 as an oxidant for methane conversion.15−17 Direct conversion 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 the 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
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 valueadded 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 the 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 suffered from nonselective activation of methane.6,11 Zeolites containing Cu or Fe species have been reported to produce methanol from methane via multisteps of activation, © 2017 American Chemical Society
Received: October 15, 2017 Published: November 10, 2017 17694
DOI: 10.1021/jacs.7b11010 J. Am. Chem. Soc. 2017, 139, 17694−17699
Article
Journal of the American Chemical Society 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 as compared to 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), and the single atomic nature was investigated by various characterizations. The 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.
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. Periodic density functional theory (DFT) calculations were carried out using a Vienna ab initio simulation package (VASP).30 A generalized gradient approximation (GGA) and exchange-correlation functions parametrized by Perdew− Burke−Ernzerhof (PBE) were used,31 in conjunction with the projector augmented wave (PAW) method to describe ionic cores.32 DFT-D2 Grimme’s empirical method was employed to include dispersion forces.33 An energy cutoff 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 cutoff 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 nonphysical 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. The autoclave then 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-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 (Sigma-Aldrich). 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). To quantify methanol and methyl hydroperoxide accurately, their calibration curves were made for 1H 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.
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 npropanol; Sigma-Aldrich) was diluted by adding 38.1 mL of npropanol (>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). Next, 300 mg of metal oxide supports, which are the synthesized zirconia, titania (P25, Degussa), ceria (nanopowder,
