Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Electron-Hopping Brings Lattice Strain and High Catalytic Activity in the Low-Temperature Oxidative Coupling of Methane in an Electric Field Shuhei Ogo,*,†,‡ Hideaki Nakatsubo,† Kousei Iwasaki,† Ayaka Sato,† Kota Murakami,† Tomohiro Yabe,† Atsushi Ishikawa,‡,§,∥ Hiromi Nakai,§ and Yasushi Sekine† †
Department of Applied Chemistry and §Department of Chemistry and Biochemistry, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo, 169-8555 Japan ‡ PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ∥ Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *
ABSTRACT: Detailed reaction mechanisms for the oxidative coupling of methane (OCM) over Ce2(WO4)3 catalysts at low temperatures in an electric field were investigated. The influence of Ce cations in the Ce2(WO4)3 catalyst was evaluated by comparing the OCM activity over various Ln2(WO4)3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) catalysts in an electric field. The electronic states of Ln and W cations and the relationship between the distorted Ce2(WO4)3 structure and methane activation were examined using X-ray absorption fine structure (XAFS) measurements and first-principles calculations. The results reveal that the Ln2(WO4)3 catalysts with redoxactive Ln cations (Ce, Pr, Sm, Eu, and Tb) show OCM activity. First-principles calculations indicate that Ce3+ species in the Ce2(WO4)3 structure are oxidized to Ce4+ species in an electric field by extracting electrons from the Ce 4f orbitals near the Fermi level; as a result, its structure is distorted. The results indicate that the redox reaction of Ln cations in Ln2(WO4)3 induced by an electric field brings lattice strain and a high OCM activity in an electric field.
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an electric field,11−14 in anticipation of methane activation at low temperatures. This novel process is completely different from electrical heating of catalyst or plasma. Recent reports have described that the OCM proceeds selectively even at low temperatures (423 K) in an electric field over a Sr−La2O3 (Sr/ La = 1:20) catalyst14−16 and a Ce−W−O system catalyst.14,17,18 In particular, a Ce−W−O system catalyst with a Ce2(WO4)3 structure showed a high C2 yield (16.8%) in an electric field even at 423 K. We also reported that Ce2(WO4)3 is an active site for OCM in an electric field and that the active oxygen species played an important role in the selective oxidation of methane created by the distorted Ce2(WO4)3 structure in an electric field.17 In this work, we specifically strove to reveal the fundamental reaction mechanism of methane activation over Ce2(WO4)3 catalyst in an electric field. First, to evaluate the influence of the Ce cations in the Ce2(WO4)3 catalyst, we compared the OCM activities among various Ln2(WO4)3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) catalysts. The
INTRODUCTION Oxidative coupling of methane (OCM) is an effective means for utilizing natural gas: Useful C2 hydrocarbons are produced directly from methane using air as an oxidizing agent.1−10 The reaction can be described as CH4 +
1 1 O2 → C2H4 + H 2O 2 2
(ΔH °298 = −140.1 kJ mol−1)
(1)
Although OCM is a highly exothermic reaction, the reaction is usually conducted at higher reaction temperatures of >973 K because of the high stability of methane. At such high temperatures, gas-phase nonselective and sequential oxidation using molecular oxygen easily proceeds because the formed C2 hydrocarbons are more reactive than methane. Therefore, it is extremely difficult to obtain high C2 yield in the OCM. If methane could be activated at low temperature, then the OCM would proceed selectively at low temperatures without gasphase nonselective and sequential oxidation. To resolve the difficulties described above, we employed an unconventional catalytic system, namely, a catalytic reaction in © XXXX American Chemical Society
Received: September 9, 2017 Revised: December 25, 2017
A
DOI: 10.1021/acs.jpcc.7b08994 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C electronic states of the Ln and W cations were characterized using XAFS measurements. The relationship between the distorted Ce2(WO4)3 structure and methane activation was examined using first-principles calculations.
C2 selectivity (%, C‐based) =
(5)
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Characterization of Catalyst. Ex situ X-ray absorption fine structure (XAFS) measurements of the Ce K-edge and La K-edge were conducted on beamline BL14B2 at SPring-8 (Hyogo, Japan). Catalyst treated under the reaction conditions was ground into fine powder and pressed to form a pellet (ϕ 7 mm). Then, the pellet was packed into a gas-barrier bag. Pellets were diluted with BN to adjust for XAFS measurements. X-ray absorption near-edge structure (XANES) analyses were performed using Athena (version 0.9.25) software. The crystalline structure of the catalyst was characterized by powder X-ray diffraction (XRD) (SmartLab III; Rigaku Corp.) operating at 40 kV and 40 mA with Cu Kα radiation. The specific surface area of the catalyst was measured using N2 adsorption with the Brunauer−Emmett−Teller (BET) method (Gemini VII; Micromeritics Instrument Corp.) after pretreatment at 473 K in a N2 atmosphere for 2 h. In Situ W L3-Edge XAFS. In situ W L3-edge XAFS measurements in an electric field were conducted in a glass reactor with gold foil electrodes (see Figure S2, Supporting Information). The measurement sample was molded to form a disk (ϕ 7 mm) with BN (see Figure S2). The reactor temperature was set to 423 K. The reactant feed gases were methane, oxygen, and N2 (CH4:O2:N2 = 25:15:60, total flow rate = 100 SCCM). An electric field was imposed using a constant current at 10.0 mA. XANES analysis was conducted using Athena (version 0.9.25) software. Computational Details. All calculations were conducted with the Vienna ab initio simulation package (VASP, version 5.3.3).22,23 Core electrons were represented by the projectoraugmented wave (PAW) method.23−25 The valence parts of the wave functions were expanded in plane-wave basis sets with kinetic energies lower than 500 eV. The Perdew−Burke− Ernzerhof (PBE)26 exchange-correlation functional was used for spin-polarized density functional theory (DFT) calculations. The generalized gradient approximation considering on-site Coulomb interactions (GGA + U) approach was employed to describe highly localized electrons such as f electrons. It has also been reported that various U parameters have been used for rare-earth elements27 and O atoms.28 For the present work, the simplified but rotationally invariant method of Dudarev et al.29 was employed. We applied Ueff as U (equivalent to U − J) on the Ce 4f electrons. Ueff = 3.5 eV was employed throughout the present study, and this value was confirmed to reproduce the experimental band gap. The structure of cerium tungstate, Ce2(WO4)3, is monoclinic C2/c, as reported by Gressling and Müller-Buschbaum.30 Lattice parameters were optimized, and errors of lattice constant with experimental values were less than 2%. For the calculation of the unit cell of cerium tungstate, Brillouin zone integration was done by taking 3 × 2 × 2 kpoints for the geometry optimization. For the density of states (DOS) and structure distortion calculations, we increased the k-point mesh to 6 × 4 × 4 because DOS calculations generally require much larger numbers of k-points. For smearing near the Fermi level, the Gaussian method with σ = 0.05 eV was employed for the geometry optimization, and the tetrahedron method with the Blöchl correction31 was used for the DOS calculations. The effect of electric field was considered by adding and removing electrons from the unit cell. In these calculations, the monopole correction was included.32
METHODS Catalyst Preparation. Ce2(WO4)3 catalyst was prepared using a complex polymerization method combining ethylenediamine tetraacetic acid and citrate ions, as described in earlier reports.19,20 Details of the preparation methods are provided in the Supporting Information. Ln2(WO4)3 (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) catalysts were prepared as described in a previous report,21 with some modification. A detailed preparation method is presented in the Supporting Information. Activity Tests. Catalytic oxidative coupling of methane was conducted in a fixed-bed flow-type reactor equipped with a quartz tube (4.0-mm i.d.). A schematic diagram of the reaction system is presented in Figure S1 of the Supporting Information. The catalyst was sieved to 355−500 μm. Then, 100 mg of it was charged into the reactor. The reactant feed gases were methane, oxygen, and Ar (CH4:O2:Ar = 25:15:60, total flow rate = 100 SCCM). The contact time (W/FCH4) was 1.5 gcat h mol−1. For the reaction in an electric field, two stainless steel electrodes (2.0-mm o.d.) were inserted contiguously into the catalyst bed in the reactor. Additionally, a thermocouple was inserted into the catalyst bed to measure the reaction temperature. An electric field was imposed using a constant current (3 mA) with a DC power supply. The imposed voltage depended on the electric properties of the catalyst. Current and voltage profiles were measured using an oscilloscope (TDS 2001C; Tektronix Inc.). The reactor temperature was set to 423 K to avoid the condensation of water produced by the reaction, except for reactions without an electric field. After passing a cold trap, product gases were analyzed using a gas chromatograph with a flame ionization detector (GC-14B; Shimadzu Corp.) with a Porapak N packed column and a methanizer (Ru/Al2O3 catalyst), as well as a gas chromatograph with a thermal conductivity detector (GC-2014; Shimadzu Corp.) with a molecular sieve 5A packed column. The respective equations for the calculations of conversions, C2 yields, and C2 selectivities were as follows CH4 conversion (%, C‐based) = {[number of moles of carbon in (CO, CO2 , C2H6, C2H4 , and C2H 2)]/(number of moles of carbon in input CH4)} × 100
O2 conversion (%) number of moles of O2 consumed = × 100 number of moles of input O2
(2)
(3)
C2 yield (%, C‐based) = {[number of moles of carbon in (C2H6, C2H4 , and C2H 2)]/(number of moles of carbon in input methane)} × 100
C2 yield × 100 CH4 conversion
(4) B
DOI: 10.1021/acs.jpcc.7b08994 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
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RESULTS AND DISCUSSION Activity Tests over Ln2(WO4)3 Catalyst in an Electric Field. To elucidate the role of Ce in the Ce2(WO4)3 catalyst in the OCM activity, catalytic oxidative coupling of methane over various Ln2(WO4)3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) catalysts was conducted with an electric field at 423 K (Table 1). Ln2(WO4)3 catalysts with Ce, Pr, Sm, Eu, and Tb
Ln2(WO4)3 is induced by the redox reaction of Ln cations in an electric field, probably contributing to the creation of reactive oxygen species suitable for OCM. Next, to clarify whether the valence state of the Ln cations changes upon imposition of an electric field, XANES measurements before and after the reaction with an electric field were conducted using La2(WO4)3 and Ce2(WO4)3 (Figures 1, S4, and S5). As shown in Figures S4 and S5a, no
Table 1. Oxidative Coupling of Methane over Various Ln2(WO4)3 (Ln = La, Ce, Pr, Nd, Eu, Gd, Tb, and Dy) Catalysts in an Electric Fielda conversion (%) catalyst
voltage (V)
Ttcb (K)
CH4
O2
C2 selectivity (%)
C2 yield (%)
La2(WO4)3 Ce2(WO4)3 Pr2(WO4)3 Nd2(WO4)3 Sm2(WO4)3 Eu2(WO4)3 Gd2(WO4)3 Tb2(WO4)3 Dy2(WO4)3
400 700 600 400 800 600 500 600 400
525 659 543 550 628 576 560 506 517
0.0 9.7 3.0 0.0 6.6 8.2 0.0 5.4 0.0
